Steel Metal

The importance of titanium, or any other material for that matter, can be no greater than the use to which it is put. In selecting titanium and its alloys for any particular application, the engineer must consider both the economic and technological justification for the utilization of this metal in specific components.

Even at the current premium price of titanium, many items for civilian and military uses are justifiable in titanium. In many items the initial high cost of the material is compensated for either by the advantages of weight reduction due to the low density of the metal or by the increased life of the component due to high corrosion resistance of the metal.

Since it is generally anticipated that the price of this metal will no doubt decrease with increasing production and improvement in processing, it is not intended here to treat fully, by any means, the economic considerations. Rather, it is intended to consider the technological justification required in utilization of titanium for the various components desired.

Two basic considerations must be appreciated: one stemming from the specifications of the component and the other from design. Specifications usually require the meeting of certain mechanical properties desirable in the end item. Such properties may include one or several of the following: yield strength, tensile strength, elongation, reduction in area, bend ductility, impact strength, hardness, fatigue strength, creep, and elevated temperature properties.

Upon selection of a material by the materials engineer which meets the basic minimum requirements specified, it then becomes the product engineer’s responsibility to consider the fabrication problems which are peculiar to the design. Here the capabilities of the materials to undergo the required fabrication methods to produce the desired end-product must be evaluated.

Selection of Materials
Good purity unalloyed titanium is cast, formed, joined, and machined with relative ease as compared with the alloy grades. In view of this, wherever the properties desired in the end item can be satisfied by the employment of unalloyed titanium grades, the selection should be made on this basis.

There is considerable variation in the properties offered by the unalloyed grades of commercial producers. Even with the same producer, variation has been noticed among heats of the same grade. As melting techniques are continually improved, greater homogeneity can be expected. Significant improvement has been made in this direction in the last few decades.

To insure the ease of fabricability, consistent with that of unalloyed titanium, the materials engineer should acquaint himself with the contaminating interstitial content of the metal in order that the material used will not exceed the maximum tolerable limits of these elements.

Where higher strengths are required or where special applications necessitate specific alloying elements, an alloy grade of titanium must be considered. Increasing the alloy content will increase, to a point, the strength, usually with an accompanying loss of ductility. This lowering of ductility indicates a lessening in the ease of formability.

In selecting an alloy, therefore, it is generally desirable to choose that alloy which offers the maximum formability for the strength level desired whether the strength requirement be tensile, fatigue, or creep strength. Where high strength and hardness are prime requirements, it may be desirable to select an alloy which exhibits a good response to heat-treatment. In this way the material can be heat-treated to obtain maximum ductility, rendering ease of formability. Subsequently, formed products can be heat-treated to the required strength or hardness.

Manganese and chromium binaries have generally not been found desirable as casting materials. Aluminum additions to these binary alloys improve the quality of the casting produced. Multicomponent alloys containing aluminum as the major addition have been found to offer better elevated temperature properties. It appears now that aluminum ternary alloys with either manganese, chromium, or vanadium will become the most useful titanium materials.

As a general guideline, employ unalloyed titanium wherever possible. Where alloy grades are required, the material which offers the best formability at the required strength level should be selected. Where possible, heat-treatment should be employed either to obtain the best ductility for ease of forming or to obtain maximum strength in the end product.

For adequate commercial utilization of titanium, it is necessary that the particular component be justifiable both from the standpoint of economics and technology. Designers and engineers have already found wide utilization for this lightweight, high strength, corrosion resistant metal encompassing many diversified applications.

Aircraft Applications
Aeronautical design engineers find in titanium and its alloys a metal whose light weight and high strength, particularly at elevated temperatures, render it a highly desirable material in aircraft construction.

Titanium is finding increasingly greater preference over aluminum and stainless steel in aircraft utilization. Aluminum loses its strength rapidly at elevated temperatures. Titanium, on the other hand, has a distinct high temperature strength advantage at temperatures up to 800°F (426°C); such elevated temperatures occur at high speeds due to aerodynamic heating.

The advantage of titanium substitution for steel in aircraft stems from its accompanying weight reduction with no loss in strength. The overall reduction of weight and the increased elevated temperature performance allowed by the utilization of titanium permit increased pay loads, as well as an increase in range and maneuverability. In view of this, effort is being applied to utilize this metal in aircraft construction from engines and airframes to skins and fasteners.

In jet engines titanium is chiefly used in compressor blades, turbine disks, and many other forged parts. The materials replaced in these applications are stainless and heat-treated alloy steels.

Marine Applications
The corrosion resistance of titanium and its alloys makes this metal a prime consideration for use in marine environments. The Navy is thoroughly investigating titanium’s corrosion resistance to stack gases, steam, and oil as well as sea water. Of almost equal importance in these applications is the high strength-weight ratio.

The light weight of the metal, in conjunction with the corrosion resistance, offers in naval vessels improved maneuverability, increased range, less preventative maintenance, and reduced power cost.

Naval investigations cover applications such as wet exhaust mufflers for submarine diesel engines, meter disks, and thin wall condenser and heat exchanger tubes. In the case of the exhaust mufflers, titanium may offer greater service life than is offered by most materials. Titanium as applied to meter disks should offer improved service in salt water, gasoline, or oil where present materials are inferior in one or more of these environments.

Also being investigated for possible utilization are heat exchanger tubes which must be resistant to corrosion by sea water on the outer walls and at the same time give equal resistance to exhaust condensate on the inner walls. Items such as antennas and exposed radar components, which require resistance to stack gases as well as to marine atmospheres, are also being considered.

Ordnance
Perhaps the largest potential military consumer of titanium products will be the Army Ordnance Corps. Much of the sponsorship of the early research and development on titanium stemmed from Army Ordnance. Many prototype components are currently being investigated by ordnance engineers. However, few production applications of the metal are standardized. The vast amount of development work and the few production items are indicative of the great interest shown by Ordnance and the limits imposed on production items by high cost.

Early investigation of titanium and its alloys indicated that the metal had promising armor plate applications. Tests on early titanium armor permitted a 25% weight saving by substitution of titanium for steel armor with equal resistance to ballistic attack. This was accomplished by replacing 1/2-inch armor plate with 5/8-inch titanium armor. With improved alloys an inch-for-inch substitution does not seem unreasonable. This would allow up to a 44% weight saving.

Employment of titanium on a production basis would allow greater maneuverability, wider traveling range, and greater useful life. For airborne transportation, the advantage of lightweight vehicles fabricated from titanium is obvious. The first standard application of titanium by Ordnance has been in the manufacture of a titanium alloy gas piston for use in some automatic weapons.

Transportation
Many of the advantages indicated for armored vehicles also apply to the transportation industry.

Decreased fuel consumption or increased pay load and better fatigue strength in piston rods and transmissions are possible advantages offered by the substitution of titanium for materials used in transportation industries today. In railway equipment applications, dead weight considerations are of utmost importance. Where the overall weight of a railway car can be substantially decreased by the application of titanium, it follows that the horsepower required to pull this lighter car will be markedly reduced, as will be the size required for the journals and the journal boxes.

Another application where load is a major consideration is in trailer trucks. Here, also, increased pay load can be achieved by the replacement of steel with titanium in such items as axles and wheels.

Chemical
In the chemical industry the corrosion resistance of a metal plays the most important part. However, light weight and strength are desirable. The advantages described there indicate utilization in many industries once the price is reduced to a competitive level.

Production equipment which facilitates transportation of corrosive materials such as acid, alkali, and inorganic salts are logical applications for titanium. Manufacturing equipment such as vats, reflux towers, filters, and pressure vessels give additional opportunities for the utilization of titanium.

Titanium tubing can improve the performance of heating coils in laboratory autoclaves and heat exchangers.

Miscellaneous Applications
The food, petroleum and electrical industries, as well as the field of surgical instruments and surgery itself, are representative of the diverse fields in which application of titanium has been found desirable.

Food processing tables as well as steam tables, where titanium has been substitute for stainless steel, have been evaluated and results indicate superior performance and potential utilization.

In oil and gas drilling applications, the corrosion problem is serious, and titanium substitution will permit less frequent replacement of corroding underground shafts. In catalytic processing applications and fuel pipe lines, titanium’s high temperature properties and corrosion resistance are desirable. Increased utilization is again dependent upon increased supply of the metal at reduced prices.

The electrical industry is equally desirous of taking advantage of the metal’s high strength-lightweight ratio and, in addition, its high electrical resistance and nonmagnetic properties for utilization as cable armor material.

Most industries employ fasteners in some form or other, and the production of titanium fasteners on a commercial basis has not been lacking over the conventional surgical instruments.

Hot Working

Titanium and its alloys can be readily hot worked at temperatures generally somewhat lower than those used for steels. To minimize surface contamination, titanium should be held at high temperatures for only a short time before forging. The rate of contamination, relatively low up to 700°C, increases rapidly with increase of temperature.

All forging furnace atmospheres contain free or combined oxygen, and some absorption of this element inevitably occurs. In addition to visible scaling, diffusion of oxygen results in hardening of a relatively shallow underlying layer. The effect of nitrogen is not usually significant at preheating temperatures. Subsequent operations such as machining will remove the hardened surface layer, and the final product will have hardness similar to forging stock.

Hydrogen, however, diffuses more rapidly than oxygen and may penetrate the full section of the work piece, which can have a serious effect on properties. Such material can only be recovered by prolonged vacuum annealing. Hydrogen is absorbed from both reducing and oxidizing gas- and oil-fired furnaces, but at a tolerably slow rate under strongly oxidizing conditions.

The order of preference of preheating atmospheres is therefore dried air (electric heating), undried air (electric heating), oxidizing oil- or gas-fired furnaces. Direct flame impingement must be avoided.

Forging. Techniques for press and hammer forging of titanium are essentially the same as for low-alloy steels. Good handling methods and plant layout will reduce the number of reheats necessary, minimizing contamination during forging. Because of the rapid cooling and the fairly narrow hot working range, the chilling effect of tools should be reduced to a minimum by keeping contact time as short as possible. Preheating the tools also helps. Repeated light blows, or attempts to continue forging at too low a temperature, may promote internal cracking and should be avoided. Moreover, a large number of reheats with only a small amount of deformation between heats is also detrimental, because it leads to a coarsening of the microstructure and consequently poor mechanical properties.

In drop forging, die contours should have larger radii and fillets than those used for steel; the lower thermal contraction of titanium requires a smaller shrinkage allowance. Trimming should be carried out hot; furnace, drop hammer and trimming press should be as close together as possible to minimize preheating and avoid wasting time and heat. A final stress-relief anneal is recommended.
Forming

Annealed and solution treated sheet can be pressed, stretch-formed, spun and dimpled, but maximum deformation depends upon the load being applied slowly. Good results are achieved with hydraulic presses, the rubber-pad method being useful for forming light-gauge parts. Drop hammer forming, with heated blanks, is widely used for sheet metal parts of complex contour. Punch presses, which should be slowed down to half or one-third their normal speed, can also be used.

Blanks may be prepared for forming by shearing, sawing, nibbling or blanking, using slow cutting speeds. Edge condition is important, and edge cracking may be minimized by keeping the guillotine blade sharp and close fitting or by heating metal before shearing. All burrs must be removed and, for more difficult forming operations, cut edges may need filing or polishing.

Simple shapes can be formed at room temperature, deformation being limited by the strength and springiness of the material. Solid lubricants such as soap, molybdenum disulphide or graphite are preferred to mineral oils and greases. ICI "Trilac" coating and polythene sheeting have been found to effect considerable improvement in difficult pressing operations.

For more complicated designs, the work piece and, where possible, the dies should be heated to facilitate forming. The use of heat in forming increases ductility, which is reflected in lower minimum bend radii and reduces both the load required to effect deformation and subsequent spring-back, thus ensuring greater accuracy.

Furthermore, at elevated temperatures, the spread between yield and ultimate strengths is increased, which also aids forming. The temperature to select for hot forming depends upon the alloy and the severity of the shape to be produced. Good results can be expected using temperatures of about 200-300oC for commercially pure titanium and IMI Titanium 230, and 550-650oC for IMI Titanium 317 and 318.
Heat Treatment

With heating in conventional furnaces there is always some surface contamination and a risk of hydrogen absorption. Vacuum treatment, though ideal, is rarely practicable, so it is customary to use ordinary electric furnaces; hydrogen pick-up is not usually excessive. Fuel-fired furnaces should be avoided if at all possible; titanium rapidly absorbs free or combined hydrogen from the surrounding atmosphere, and this can be serious, particularly with thin sections.

Superficial hardening by oxygen diffusion is almost inevitable at the higher annealing and preheating temperatures suggested for some titanium alloys. The hardening effect is insignificant at low annealing temperatures but above 600°C may lead to surface embrittlement. Both the oxide film and the underlying oxygen-rich layer should therefore be removed by one of the methods of surface treatment; this is particularly important for high-strength alloys.
Machining and Grinding

Titanium and its alloys can be machined successfully on conventional machine tools provided that certain requirements are satisfied. In all machining operations rigidity of both work piece and cutting tool is desirable. Best results will be obtained if the cutting tools have a good surface finish. If the cutting tools are in good condition, it is no more difficult to machine titanium than an alloy steel of equivalent strength.

Titanium has a tendency to gall or smear on to other metals. Sliding contact between the work piece and its support should be avoided, and the use of roller steadies and running centres is recommended.

Turning. In general, cutting speeds should be low and feeds as coarse as practicable. A good surface finish can be obtained with very coarse feeds by using suitably shaped tools with a large nose radius. This will, however, be limited by work piece rigidity as a large nose radius causes increased tool loads and work piece deflection. Due to the lower elastic modulus of titanium, these deflections are greater than would occur on steel workplaces.

Tool materials may be high-speed steel, cast alloy, or tungsten carbide. The "super" grades of high-speed steel are satisfactory, giving good results in turning where large feeds can be employed, and particularly where the surface is rough or the cut intermittent. Tungsten carbide may be necessary for heavy work on certain harder alloys or for intermittent cutting, but in general its use is confined to lighter, more continuous cuts. For economic use of carbide tools it is essential to regrind before wear becomes excessive, and mechanically clamped tips are an obvious advantage.

Threading. Single-point screw cutting is preferable to threading with a die. Conventional methods of screw-cutting can be used, but success can also be achieved when increments of cut of 0.25-0.50 mm are applied at right angles to the axis of the component. Cuts of less than 0.13 mm should be avoided. Machine tapping with cutting speeds up to 6 m/min is preferable to hand manipulation. Tapping of full threads should be avoided: a thread of 80% depth is much easier to tap and loses little strength.

Planing. Shaping and planing of titanium are not difficult, provided that the foregoing requirements of rigidity, speed and feed are satisfied. Tungsten carbide tools with a large radius, producing a broad and relatively thin chip, are most successful. As in all cutting operations, it is essential to use sharp tools and replace them before appreciable wear occurs. For planing, clamped circular buttons of tungsten carbide have obvious advantages.

Milling. In milling, the chief problem arises from chips welding on to the teeth, resulting in cutter chipping and breakage. This is minimized with climb milling, in which the tooth finishes its cutting stroke when moving parallel to the feed. Absolute rigidity is necessary to avoid chatter, but the chip is only attached to the tooth by a thin sliver which is easily broken off.

Drilling. Titanium may be drilled with short high-speed-steel drills; the holes should be as shallow as possible. A 140o point is best for sizes below 6-5 mm and a 90° or double-angle point for larger sizes. For holes of a depth greater than five diameters, it is helpful to retract the drill at intervals and clear the swarf. Flood lubrication with a heavily chlorinated cutting oil reduces frictional troubles.

Grinding. A reduction in wheel speed to a half or a third of the conventional speed, together with the use of a suitable coolant, will usually achieve an acceptable grinding ratio. Water-base soluble oils result in poor wheel life, but some chlorinated or sulphurised grinding oils, and solutions of vapour-phase rust inhibitors of the nitrite-amine type, are satisfactory.

Polishing. Titanium can be mechanically polished by techniques similar to those used for stainless steel; reductions in wheel or mop speeds are often beneficial. If a high polish is required, light pressures are necessary during the final operations. Good results have been obtained with a canvas wheel coated with 240E1 `Alundum` grit, which can be blended with stearic acid for a finer finish.
Descaling and Surface Treatment

When titanium and its alloys are heated in air, absorption of oxygen and, to a lesser extent, nitrogen, results in the formation of an outer layer of oxide and nitride and an underlying thin layer into which oxygen and nitrogen have diffused. Removal of this hardened metal layer is essential for optimum mechanical properties, and an integral part of any descaling process.

All types of scale can be removed in fused caustic soda, but use of an unmodified bath leads to hydrogen contamination and poor surface quality. The sodium hydride process results in good surfaces and efficient scale removal but, again, hydrogen contamination occurs. Consequently, neither process is suitable for thin sections.

Caustic soda with about 10% oxidizing additions can be used for slightly thicker material, descaling conditions being 20-30 minutes immersion (longer for very heavy scale) at 425°C. Reaction between titanium and any fused caustic soda bath may lead to a dangerous build-up of heat if a stack of thin sheet is descaled. Thin-gauge material should, therefore, be handled in small batches, at a temperature not exceeding 425°C.

Anti-galling Treatments. The tendency for titanium to gall when in sliding contact with itself or with other materials can be reduced by some form of surface treatment. This is particularly desirable for bearing surfaces and for threads of bolts. Both anodizing and `Sulfinuz` treatments reduce the galling tendency, while adherent nickel and chromium deposits provide good wear resistant surfaces. Cadmium plating or the use of anti-galling paints are effective in preventing seizure of bolt threads. Details of electro deposition and anodizing procedures are given in the following paragraphs.

Electrodeposition. Adherent metallic coatings can only be electrodeposited on to titanium if the surface is suitably prepared. A procedure which has been found successful for depositing nickel, chromium, zinc and cadmium on to some titanium alloys uses a pretreatment comprising: (1) Vapour degrease, (2) Hydrochloric acid etch, 5 min in concentrated HCl at 90-110°C, (3) Cold water rinse, (4) Nickel strike for 3 min, (5) Cold water rinse.

Anodizing. Surface properties of titanium and its alloys can be modified by anodic oxidation treatment, which covers the entire surface with a thin but compact oxide film. Almost any aqueous solution can be used, but immersion in a solution of 80% phosphoric acid, 10% sulphuric acid and 10% water gives a particularly coherent film. A potential increasing from 0 to 110 volts over ten minutes should be applied.

Anodized titanium has no affinity for dyestuffs, but the film itself shows interference colors, determined by the final anodizing potential.

Chemical Properties
Titanium, like other elements, is a composite of several isotopes, which range in atomic weight from 46 to 50. The proportions of these isotopes have been computed from spectrographic analysis. Mathematical calculations employing the proportions and mass numbers have assigned titanium a mean atomic weight of 47.88.

Titanium has a large capture cross-section, and five other isotopes of titanium have been identified. Titanium 43 has a half-life of 0.58 second and is a beta positive emitter. Titanium 45 has two forms, one a beta positive and gamma emitter with a half-life of 3.08 hours and a second form with a half-life of 21 days. Titanium 51 has a half-life of 72 days and is a beta negative and gamma emitter. There is also a meta stable form of titanium 51, which has a half-life of 6 minutes and is also a gamma and beta negative emitter.

Valence. As is characteristic of transition elements, titanium has a variable valence and occurs commonly in the bi-, tri-, and tetra-valent states. Literature has reported valences of five and higher, but no substantiation of these has ever been shown.

Gases. The chemical reactivity of titanium is dependent upon temperature. The metal’s action with other substances proceeds more readily at elevated temperatures. This property is especially exemplified by the metal’s extreme reactivity to the gases of the atmosphere at high temperatures.

This necessitates the use of inert atmospheres for hot working and surface protection for high temperature applications. The rapid combination of titanium with the reactive gases of the atmosphere above 950°F produces surface scale. With larger intervals of time and increase in temperature, the gases diffuse into the lattice.

The metal combines with oxygen to form a long series of oxides from TiO to Ti7O12, each of which exhibits a different hue and, at short time exposures, a rainbow-colored surface film is produced. Although this surface oxidation proceeds at 950°F, no appreciable diffusion into the lattice occurs below 1300°F. Ignition of the metal occurs in air at 2200°F, and a pure oxygen atmosphere reduces this temperature to 1130°F.

The reactivity of titanium with nitrogen is similar to its action with oxygen where a yellow-brown scale is formed on the surface as the nitride. Nitrogen will diffuse into the lattice with a restricted depth of penetration. This property has been employed in the nitride casing of the metal.

Most unique of the gas-titanium reactions is that between hydrogen and the metal. The reaction proceeds at temperatures slightly above room temperature, and as much as 400 cc of the gas can be absorbed by one gram of titanium. In small amounts the gas adds as an interstitial, but at higher concentrations the hydride TiH is formed. The addition of hydrogen to titanium is only stable, however, below 680°F; above this temperature the gas is evolved and burns.

All of these gas-titanium reactions are accelerated by decreasing vapor pressures and complete protection from the atmosphere is required.

Water vapor and carbon dioxide are decomposed by hot titanium metal. Above 1500°F water vapor and the metal combine to form the oxide and evolve hydrogen. At higher temperatures the hot metal will absorb CO2 and may form the oxide and the carbide.

Acids. The chemical reactivity of titanium to the halides is similarly exhibited by its combination with their acids. The most rapid reaction is again with the fluoride. This reaction has various applications; it is one of the basic agents for dissolution of the metal and its alloys for chemical analysis; it is used as a general etchant on both a macro and micro scale, in metallographic work; and it is also employed as a descaling agent.

The action of hydrochloric acid and, in a similar manner, that of sulfuric acid proceed slowly at room temperatures. However, a small input of heat accelerates the attack, which results in the formation of the lower chlorides and the mono-sulfate. These reactions are utilized in a similar manner to that of hydrofluoric acid and, because they are less toxic and corrosive, they are gradually replacing the acid fluoride.

Organics. The chemical reactivity of titanium with organic material has been exploited by the metal industry only to a slight extent. Organic acid-titanium reactions produce colored films on the metal’s surface and are being used by the metallographer to stain-etch microspecimens.

Solids. In the molten state titanium combines with many metals, metalloids, and carbonaceous matter to form systems of much importance. In the oxide state it reacts with the alkali, alkaline earth, and heavy base metals to form titanates, a few of which are being studied in conjunction with cheaper methods of production.

The reactivity to the metalloids, especially the metal oxides, has been extremely disturbing to the foundryman since molten titanium severely attacks most of the known refractories to form metal-metalloid systems. Such refractory materials as silicon dioxide and aluminum oxide are so severely attacked that their use is hazardous. Of all the metalloids only beryllium oxide and thorium oxide have shown any appreciable resistance to the liquid metal.

Another reaction with great import is that of carbon and titanium. The metal in the molten state has a great affinity for carbon, and because of its detrimental effect on the properties of titanium, extreme care must be taken to minimize its presence in fabricated items.

Electrochemistry. The metal may be electrodeposited by various complex methods, none of which gives industrially applicable films. Electrolytic means have been used to reduce the metal from its tetravalent state to both the bi- and trivalent forms by employing acid electrolytes and electrodes of cither lead, copper, platinum, or mercury jet.

Safety. The chemical reactivity of titanium is generally nonhazardous. With the exception of finely divided particles, and metal exposed to fuming nitric acid for a prolonged time, it has not been found to be either explosive or flammable.

Mechanical properties
Tensile Properties. Unalloyed titanium may have tensile strengths ranging from 35,000 psi (250 MPa) for high purity metal produced by the iodide reduction process to 100,000 psi (690 MPa) for metal produced with sponge titanium of high hardness. The arc-melted unalloyed titanium products are reasonably ductile.

Ductility. The arc-melted commercially pure titanium products range in ductility from 20% to 40% elongation and from 45% to 65% reduction in area, depending upon the interstitial content. The iodide process titanium yields a product possessing 55% elongation with 80% reduction in area.

As is the case with steel, titanium is alloyed with other metals to increase its strength. Such metallic additions as Al, V, Cr, Fe, Mn, Sn are employed either as binary additions or as complex systems. The resulting increase in strength is accomplished, however, with a lowering of ductility.

Modulus of Elasticity. Unalloyed titanium has a modulus of about 15x106 psi and can be increased to about 18x106 psi by alloying. Titanium’s modulus compares favorably with those of aluminum (10.4x106) and magnesium (6.4x106) but poorly with that of steel (29x106).

Like the modulus of elasticity the modulus in shear, modulus of rigidity, of titanium falls between that of aluminum and that of steel.

Hardness. Titanium is a much harder metal than aluminum and approaches the high hardness possessed by some of the heat-treated alloy steels. Iodide purity titanium has a hardness of 90 VHN (Vickers), unalloyed commercial titanium has a hardness of about 160 VHN and when alloyed and heat-treated, titanium can attain hardnesses in the range of 250 to 500 VHN. A typical commercial alloy of 130,000 psi yield strength might be expected to have a hardness of about 320 VHN or 34 Rockwell C.

Impact Resistance. Knowledge of tensile strength and ductility of a metal is insufficient for many engineering applications without the knowledge of toughness. Titanium falls among the few metals capable of possessing good toughness along with high strength and ductility.

Titanium may have impact strengths ranging from more than 100 foot pounds Charpy for the higher purity iodide product and 30 foot pounds for the commercial unalloyed product to 1 or 2 foot pounds for some of the high strength but brittle alloys.

Few researches have been carried out on the influence of environment on the fatigue propagation of short cracks apart from those specific ally related to the so called corrosive environments. Previous studies on 7075 aluminum alloys and a construction steel have shown that the initial growth of short cracks occurs at a much lower stress intensity range (AK) in an active environment (ambient air or nitrogen containing traces of water vapor) than in vacuum. However, Gerdes have shown that there is no significant difference in the initial stress level in air and in vacuum for small surface cracks initiated in a Ti-8.6 Al alloy.

Moreover, crack closure is known to play a dominant role in influencing near threshold growth rates. Through the removal of material from the wake of long crack arrested at threshold, several authors have studied the location and the origin of crack closure. It has been concluded that the behavior of short and long bidimensional cracks can be rationalized in terms of the effective stress intensity factor range for Al alloys and steels.

To get more information on the fatigue behavior of Ti-alloys, the growth of long cracks and of physically short bidimensional cracks has been studied on a TA6V alloy. This article presents some results obtained on the influence of the crack wake on the propagation of fatigue cracks in specimens tested in ambient air and in high vacuum.

The material used for this investigation was a forged Ti-6AI-4V alloy (wt % 6.27 Al, 3.86 V, 0.12 Fe, 0.18 O2). After forging the alloy was heat treated at 955°C for 1 h 30 min. and water quenched, followed by 2 h at 700°C and air-cooling.

Fatigue crack propagation tests were carried out under ambient air and high vacuum (< 5.104 Pa) on CT specimens 5 or 12 mm thick and 24 mm wide, at a test frequency of 35 Hz and a load ratio R = 0.1. Crack advance was optically monitored and crack closure was detected by mean of the compliance technique with a back face strain gauge.

A long crack was first obtained at a/w ~0.5 using a load shedding procedure down to threshold or at constant ΔK. Then the plastic wake of the crack was progressively removed by spark erosion to obtain a crack length about 0.1 mm. Then the bidimensional short through cracks were propagated at increasing ΔK.

Near threshold propagation of long cracks
The relationship between the propagation rate da/dN and the nominal (ΔK) and the effective (ΔKeff) stress intensity factor ranges are plotted for tests performed in ambient air at R = 0.1 and 35 Hz on the Ti 6AI-4V alloy. Decreasing the specimen thickness from 12 to 5.5 mm lowers the nominal threshold range but does not affect the effective threshold range (ΔKeff)th. Consequently the thinner specimen corresponds to a lower Kop level.

Figure 1. Crack opening stress intensity factor Kop versus the remaining crack length Δa at different steps of crack wake removing procedure

The 5.5 thick specimens were tested in air and in high vacuum. A strong detrimental influence of the ambient environment on the resistance to crack propagation at low rates can be observed. The effective data confirm that the ΔKeff concept cannot account for the influence of environment.

Consequently, the crack growth mechanism in moist ambient air (about 50% R.H.) must be different from the intrinsic mechanism governing crack propagation in vacuum. Similar behaviors have previously been observed on Al-alloys and steels, for which materials an embrittling effect of water vapor adsorbed at the crack tip has been proposed. A similar mechanism could be suggested for Ti-alloys.

Crack closure location to analyze the location of crack closure, the crack wake was progressively removed and Kop measurements were performed at each step of this procedure.

The evolution of Kop versus the remaining crack length Δa is plotted for cracks pregrown at threshold ambient air on 5.5 mm and 12 mm thick specimens. The lowering of Kop at decreasing Δa is more pronounced on the thinner specimen. At Δa ~0.1 mm no closure was detected on the 5.5 mm thick specimen while a substantial remaining closure effect was measured with the thicker specimen. The length, along which the decrease in Kop was observed, i.e. where crack closure is localized, is about 1.5 mm for the thinner specimen thickness and 0.7 mm for the thicker one.

A complementary test on a 12 mm thick specimen was performed at a constant ΔK of 7.5 MpaV/m corresponding to a constant growth rate about 3x10-9 m/cycle. Removing the crack wake did not affect Kop except when Δa is 0.1 mm.

The same measurements were made on cracks pregrown at threshold in vacuum. It was seen that, as in air, Kop is lower for the smaller thickness but it is independent of Δa. Similar observations have been made on a construction steel type E460.

As a consequence of these observations, it appears that the short crack effect (defined as the decrease in Kop with Δa) depends upon the embrittling effect of environment for cracks pregrown near threshold in air. However, for the crack pregrown at a constant rate of 3x10-9 m/cycle, Kop is, as in vacuum, independent of Δa, which suggests that this embrittling effect in air occurs only at ultra low rates. But an environmental influence is also observed at rates higher than 10-9 m/cycle. This suggests the existence of two distinct regimes in environmentally assisted crack growth. Similar behaviors have been observed on Al-alloys and steels.

Short crack propagation
The short cracks here obtained on 5.5 mm specimens were propagated at increasing ΔK. In vacuum, there was no difference between long and short cracks, which was consistent with the independence of Kop upon Δa. Initially, the short crack grew for about 0.3 mm at ultra low rates (about 3 to 6x10-11 m/cycle) without any detectable closure. There after, there was an abrupt acceleration of the crack rate and crack closure was detected; and then progressively, a behavior similar to the one of a long crack was reached for a crack length about 1 mm.

The effective data confirm the existence of a typical near threshold mechanism without closure, which is different from the mechanism governing the propagation above 2x10-10 m/cycle. The microfractographic aspects of the fracture surfaces show that, compared to vacuum, the fades in air appear more brittle with crystallographic facets corresponding to a grains and lamellae. This crystallographic aspect is more developed at ultra low rates. On going experiments will hopefully give new information, which may permit a deeper analysis of these crack growth mechanisms.

The main conclusions of the present study on short cracks obtained artificially from long cracks in a TA6V alloy are:

* The following conditions are required to obtain a short crack effect:
o a pregrown long crack near threshold in air
o removal of crack wake.
* Under such condition a propagation regime typical of ultra low rates, without detectable closure, is observed for short cracks grown in ambient air.
* No short crack effect is observed in other studied conditions, i.e.:
o short cracks obtained from long crack pregrown in air at a constant rate of 3 x 10-9 m/cycle (AK = 7.5 MPa/m);
o short cracks in vacuum.

To be a versatile engineering material, a metal must offer a wide range in its mechanical properties to meet industrial requirements. Such a variation is accomplished in the mechanical properties of titanium by alloying and heat-treatment. Since neither one alloy nor one heat-treatment alone is capable of perfecting a metal with properties meeting all the demands of industry, alloying and heat-treatment have become major tools in the production of titanium materials.

Alloying
Research and development have shown that alloying can raise the tensile strength of titanium metal to more than 200000 psi (1380 MPa) while still maintaining useful ductility. The presence of interstitial elements, mainly carbon and the reactive gases of the atmosphere, also add strength to the metal but at the expense of severe loss in ductility. For simplicity the interstitial elements carbon, nitrogen, oxygen, boron, and hydrogen will be referred to as contaminants, and the substitutional elements, intentionally added, will be referred to as alloying elements.

Contaminants. Contaminants remain in titanium metal from incomplete purification in the reduction process or are absorbed in the melting practices employed. Iron in small qualities, 0.5 to 1%, appears to have a contaminating effect on ductility, but in larger quantities acts to influence the ductility no worse than other good substitutional alloying elements. The interstitials and iron are introduced in the sponge production reactions; in the melting practice, carbon, nitrogen, and oxygen contents may be further increased.

One major problem confronting both the sponge and wrought producer is to eliminate these impurities or at least keep them at a minimum. Present production of titanium metal has kept nitrogen and hydrogen contents, in general, below the critical combining quantity where reduction in ductility becomes marked. Hydrogen, carbon, oxygen, and, in some cases, iron, however, are still found frequently in titanium in proportions which are intolerable to the user.

Carbon and oxygen appear to have a combined effect on ductility and toughness. A low quantity of one will allow a greater toleration of the other. Large variations (0.03%-0.20%) have been noted in oxygen contents of commercially produced metal, and carbon contents up to 0.2% still result, although most titanium metal currently arc-melted is below 0.1% in carbon.

Nitrogen has been maintained generally below 0.05% in commercial production, and this quantity does not influence severely tile strength or ductility. With nitrogen contents above this, strength rises sharply and ductility falls off as severely as occurs with oxygen. The effect of boron, which is only slightly soluble in titanium, has not been thoroughly investigated.

Recently the titanium industry has become aware that hydrogen is a major factor in embrittling titanium. Most alloys cannot tolerate more than 200 parts per million of hydrogen, particularly if the material is to be subjected to fatigue or creep loading. Hydrogen can be substantially reduced by vacuum annealing, but this process does not lend itself to production economics.

Carbon, oxygen, nitrogen, and hydrogen, although they increase the strength of titanium, adversely affect the ductility and toughness so severely that these elements are kept at a minimum and are rarely employed as alloy additives.

Alloy Additives. To increase the strength of titanium metal and still maintain useful ductility, substitutional elements are employed. These elements replace titanium atoms in the lattice structure rather than situate in the voids between them, as do the interstitials.

By the utilization of basic physical metallurgical studies such as equilibrium diagrams of various alloy systems and by practical alloy development work, several substitutional elements have emerged as promising alloy additions. Manganese, aluminum, chromium, tin, iron, vanadium, and molybdenum in various combinations have been shown to lend versatility to tile mechanical properties of titanium metal.

These alloying elements increase the strength of titanium with an accompanying loss in ductility and toughness. The ductility and toughness, however, are far less influenced by these elements than by the contaminants, where increased strength is gained at a great sacrifice of ductility and toughness.

Heat-treatment
The mechanical properties of titanium are more dependent on the phases present than they are on the actual composition of the alloy. Substitutional elements partially replace the titanium atoms in the lattice and in this manner alter the properties. In actuality, the amount of any and all phases present is better governed by the heating and cooling cycles than by this atom alteration.

Most alloy additives stabilize the body centered beta phase and lower the temperature of transformation to such an extent that at room temperature the alloys are a mixture of both the alpha and beta. The hexagonal alpha is relatively soft, tough, and ductile; whereas the beta is harder, stronger, but less ductile.

From this it can be seen that by changing the proportions of these phases, the mechanical properties can be varied. Many methods have been employed to produce the desired phase proportions, and from these have emerged five basic methods of heat-treatment: quenching, tempering, continuous cooling, isothermal transformation, and solutionizing and aging.

Quenching. If alloys are rapidly cooled by water quenching from the all beta region, the tendency of the alpha phase to form is suppressed, and the beta phase is retained. Certain alloy compositions, however, exhibit a peculiar transformation on quenching. This mechanism of martensitic or shear-like transformation is not completely understood. The formation of this structure, the so-called alpha prime, causes some distortion of the lattice. This distortion and the resulting strain produce a material, which is hard and tough, and possesses better fatigue properties than alpha. This quenching process is also the initial point for tempering.

Tempering. When titanium is quenched from an elevated temperature, reheated to a temperature below the beta transus, held for a length of time and again quenched, it is said to have been tempered. Three variables exist in tempering: the phases present, the time held, and the tempering temperature.

When the initial structure contains alpha prime, two changes occur: the alpha prime transforms to alpha, and at longer times the alpha becomes serrated. The result is a loss of hardness and strength and an increase in ductility and impact. Alpha-beta structures, however, do not follow this pattern. The alpha primarily remains unchanged; the beta decomposes to form more alpha at the expense of the beta phase. At low temperatures more alpha, will be formed; thus, low tempering temperatures result in a greater decrease in strength and hardness and a larger increase in ductility than the high temperature tempering over identical time intervals.

Solutionizing and Aging. If a titanium alloy is held in the beta or high in the alpha-beta region, quenched, and then reheated again to the alpha-beta region, it is said to have been solution-treated and aged. This treatment on titanium alloys produces much the same effect as tempering, with the exception that the initial structure is, for the most part, beta. Maximum hardness can he achieved in short-time aging, which is associated with the formation of a phase, referred to as beta prime. With longer times this beta prime is dissipated and alpha-precipitated, decreasing the hardness and resulting in better ductility.

Isothermal Transformation. On hot-quenching an alloy from the all beta region to temperatures in the alpha-beta field and holding for a period of time and then further quenching to room temperature, the material is transformed isothermally. Treatment in this way causes precipitation of the alpha phase from the beta. At high temperatures the alpha precipitates first at grain boundaries and later within the beta grains themselves.

This treatment, when holding at temperatures just below the transformation temperature, at first gives a very hard material due to formation of beta prime. If the time of holding is extended, the hardness and strength decrease with an accompanying increase in ductility and toughness. At lower temperatures a gradual rise in hardness and brittleness takes place, and at prolonged times a higher hardness may be obtained than by short time high temperature treatments.

Continuous Cooling. Continuous cooling is the lowering of the temperature of an alloy from the all beta field at any rate without interruption or subsequent reheating. Quenching, already discussed, is a specialized form of continuous cooling. The cooling rate, although not associated with one temperature, governs the interval of the transformation period. Rapid cooling rates suppress alpha formation and result in the beta phase being at least partially retained, which gives a moderately hard material. Slightly slower cooling rates result in a much harder and brittle metal of the same type previously referred to as beta prime. Slower rates give alpha-beta structures. The slower the rate, the greater the amount of alpha formed. As alpha increases, the ductility and toughness increase and hardness falls off.

When high hardness is the ultimate need, the material must be treated in such a way that the peak of the curve is reached. High hardness throughout the piece is best obtained by quenching a rich alloy, which falls to the left of the peak, and then tempering at low temperatures until the peak is reached.

When toughness is the prime factor, it is best obtained by quenching the lean alloys, which fall far to the right, from just below the beta transus. Such treatment gives low yield strength, but high impact strength. Some increase in yield strength can be obtained if these alloys are hot-worked in the alpha-beta region prior to quenching.

Since the introduction of titanium and titanium alloys in the early 1950s, these materials have in a relatively short time become backbone materials for the aerospace, energy, and chemical industries.

The combination of high strength-to-weight ratio, excellent mechanical properties, and corrosion resistance makes titanium the best material choice for many critical applications. Today, titanium alloys are used for demanding applications such as static and rotating gas turbine engine components. Some of the most critical and highly-stressed civilian and military airframe parts are made of these alloys.

The use of titanium has expanded in recent years to include applications in nuclear power plants, food processing plants, oil refinery heat exchangers, marine components and medical protheses.

The high cost of titanium alloy components may limit their use to applications for which lower-cost alloys, such as aluminium and stainless steels. The relatively high cost is often the result of the intristic raw material cost of metal, fabricating costs and the metal removal costs incurred in obtaining the desired final shape.

These titanium net shape technologies include powder metallurgy (P/M), superplastic forming (SPF), precision forging, and precision casting. Precision casting is by far the most fully developed and the most widely used titanium net shape technology. The annual shipment of titanium castings in the United States increased by 260% between 1979 and 1989.

As aircraft engine manufactures seek to use cast titanium at higher operating temperatures, Ti-6Al-2Sn-4Zr-2Mo and
Ti-6Al-2Sn-4Zr-6Mo are being specified more frequently. Other advanced high-temperature titanium alloys for service up to 595^oC, such as Ti-1100 and IMI-834 are being developed as castings. The alloys mentioned above exhibit the same degree of elevated-temperature superiority, as do their wrought counterparts over the more commonly
used Ti-6Al-4V.

The wrought product forms of titanium and titanium-base alloys, which include forgings and typical mill products, constitute more than 70% of the market in titanium and titanium alloy production. The wrought products are the most readily available product form of titanium-base materials, although cast and powder metallurgy (P/M) products are also available for applications that require complex shapes or the use of P/M techniques to obtain microstructures not achievable by conventional ingot metallurgy.

Powder metallurgy of titanium has not gained wide acceptance and is restricted to space and missile applications. The primary reasons for using titanium-base products are its outstanding corrosion resistance of titanium and its useful combination of low density (4.5 g/cm³) and high strength. The strengths vary from 480 MPa for some grades of commercial titanium to about 1100 MPa for structural titanium alloy products and over 1725 MPa for special forms such as wires and springs.

Another important characteristic of titanium- base materials is the reversible transformation of the crystal structure from alpha (a, hexagonal close-packed) structure to beta (b, body-centered cubic) structure when the temperatures exceed certain level. This allotropic behavior, which depends on the type and amount of alloy contents, allows complex variations in microstructure and more diverse strengthening opportunities than those of other nonferrous alloys such as copper or aluminum.

Pure titanium wrought products, which have minimum titanium contents ranging from about 98,635 to 99,5 wt%, are used primarily for corrosion resistance. Titanium products are also useful for fabrication but have relatively low strength in service.

Titanium has the following advantages:

1. Good strength
2. Resistance to erosion and erosion-corrosion
3. Very thin, conductive oxide surface film
4. Hard, smooth surface that limits adhesion of foreign materials
5. Surface promotes dropwise condensation

Commercially pure titanium with minor alloy contents include various titanium-palladium grades and alloy Ti-0,3Mo-0,8Ni (ASTM grade 12 or UNS R533400). The alloy contents allow improvements in corrosion resistance and/or strength.

Titanium-palladium alloys with nominal palladium contents of about 0,2% Pd are used in applications requiring excellent corrosion resistance in chemical processing or storage applications where the environment is mildly reducing or fluctuates between oxidizing and reducing.

Alloy Ti-0,3Mo-0,8Ni (UNS R533400, or ASTM grade 12) has applications similar to those for unalloyed titanium but has better strength and corrosion resistance. However, the corrosion resistance of this alloy is not as good as the titanium-palladium alloys. The ASTM grade 12 alloy is particularly resistant to crevice corrosion in hot brines.

Titanium alloy compositions of various titanium alloys. Because the allotropic behavior of titanium allows diverse changes in microstructures by variations in thermomechanical processing, a broad range of properties and applications can be served with a minimum number of grades. This is especially true of the alloys with a two-phase, a+b, crystal structure.

The most widely used titanium alloy is the Ti-6Al-4V alpha-beta alloy. This alloy is well understood and is also very tolerant on variations in fabrication operations, despite its relatively poor room-temperature shaping and forming characteristics compared to steel and aluminium. Alloy Ti-6Al-4V, which has limited section size hardenability, is most commonly used in the annealed condition.

Other titanium alloys are designed for particular application areas. For example:

1. Alloys Ti-5Al-2Sn-2Zr-4Mo-4Cr (commonly called Ti-17) and Ti-6Al-2Sn-4Zr-6Mo for high strength in heavy sections at elevated temperatures.
2. Alloys Ti-6242S, IMI 829, and Ti-6242 (Ti-6Al-2Sn-4Zr-2Mo) for creep resistance
3. Alloys Ti-6Al-2Nb-ITa-Imo and Ti-6Al-4V-ELI are designed both to resist stress corrosion in aqueous salt solutions and for high fracture toughness
4. Alloy Ti-5Al-2,5Sn is designed for weldability, and the ELI grade is used extensively for cryogenic applications
5. Alloys Ti-6Al-6V-2Sn, Ti-6Al-4V and Ti-10V-2Fe-3Al for high strength at low-to-moderate temperatures.

Welding has the greatest potential for affecting material properties. In all types of welds, contamination by interstitial impurities such as oxygen and nitrogen must be minimized to maintain useful ductility in the weldment. Alloy composition, welding procedure, and subsequent heat treatment are highly important in determining the final properties of welded joints.

Some general principles can be summarized as follows:

1. Welding generally increases strength and hardness
2. Welding generally decreases tensile and bend ductility
3. Welds in unalloyed titanium grades 1, 2 and 3 do not require post-weld treatment unless the material will be highly stressed in a strongly reducing atmosphere
4. Welds in more beta-rich alpha-beta alloys such as Ti-6Al-6V-2Sn have a high likelihood of fracturing with little or no plastic straining.

Titanium and titanium alloys are heat treated for the following purposes:

1. To reduce residual stresses developed during fabrication
2. To produce an optimal combination of ductility, machinability, and dimensional and structural stability (annealing)
3. To increase strength (solution treating and aging)
4. To optimise special properties such as fracture toughness, fatigue strength, and high-temperature creep strength.

Titanium alloy Ti-6%Al-4%V has good tensile properties at room temperature, annealed material having a typical tensile strength of 1000-1100 MPa (145-160 ksi), and a useful creep resistance up to 300°C of about 570 MPa (83 ksi) for 0-1% total plastic strain in 100 hours. Heat treatment will give a guaranteed minimum tensile of 1100 MPa (160 ksi) for such applications as springs, bolts or other fasteners.

Resistance to fatigue and crack propagation is excellent. Like most titanium alloys and grades, titanium alloy Ti-6%Al-4%V has outstanding resistance to corrosion in most natural and many industrial process environments. Its density of 4.0-4.2 g/cm³ is even lower than that of pure titanium. It can readily be formed or forged; many welding operations are possible.

Ti-6%Al-4%V is an alpha+beta alloy, containing 6% aluminum and 4% vanadium. The aluminum stabilizes and strengthens the alpha phase, so raising the beta-transus temperature, as well as reducing the density of the alloy. The vanadium is a beta stabilizer, and provides a greater amount of the more ductile beta phase during hot working.

On solution treatment high in the alpha+beta field, followed by rapid cooling to room temperature, the beta phase transforms to a structure which can subsequently be tempered to a fine dispersion of beta in an alpha matrix, with consequent strengthening of the alloy. The chemical composition of Ti-6%Al-4%V given in the Table 1.

Table1. Chemical composition of Ti-6%Al-4%V

Element	Al	V	Fe	H2	Ti
Wt%	5,5-6,75	3,5-4,5	0,30 max	0.0125 max	Remainder

Titanium alloy Ti-6%Al-4%V is available as annealed plate and sheet, as hot worked rod, bar and billet for further working, or as annealed rod and bar for machining. Heat-treatable rod is available for fastener manufacturing and bard-drawn wire can be supplied for spring applications. Pipes can be supplied as extrusions or formed and welded from plate. More complex sections are available, made by extrusion or forming or rolling.

Grades of Ti-Alloy of extra low interstitial content can be made available for specific applications, which demand special ductility, fracture toughness, or resistance to crack propagation in aqueous environments. Typical specifications for these grades are AMS 4907 for sheet and strip, and AMS 4930 for bars, forgings, and rings.

Specification properties

Sheet is supplied in accordance with British Standard TA 10 or TA 59, and plate in accordance with British Standard TA 56.

Rod, bar and billet are supplied in accordance with British Standards TA11 and 12. BS TA 11 refers to bar for machining, which has the specification properties in ruling sections up to 150 mm. BS TA 12 covers forging stock in which the specification properties are only developed after annealing; in material greater than 150 mm ruling section, a transverse slice may be upset forged 3:1 before annealing and testing. BS TA 13 gives properties on annealed forgings.

Bolt stock is supplied in accordance with BS TA 28.

Forging

The beta transus temperature of Ti-6%Al-4%V is higher than that of many other alloys of titanium, which allows a somewhat higher forging temperature to be used. To get the optimum combination of strength and ductility in a finished component, however, it is necessary to carry out at least a 4:1 reduction in the alpha+beta field and it is recommended that the maximum temperature reached during preheating and forging should not exceed 975°C. In order to guard against internal overheating by kinetic work during rapid forging, it may be safer to use a preheating temperature of 950°C.

For initial cogging of large or complicated pieces, or where heavy reductions can thereby be achieved, it may be permissible to use higher temperatures in the early stages. Some non-critical components may even be beta-forged to the finished shape, with relatively little loss of strength, ductility or fatigue resistance; fracture toughness may even be improved. Much depends on the power of the forging plant available.

Oxidation becomes progressively more serious as the temperature is raised. For this reason, time and temperature should be kept to a minimum consistent with thorough heating and, provided that the furnace capacity is adequate for the mass of metal involved, a total heating time of 1 hour per 50 mm of section should be adequate.

Forming

One of the advantages of Ti-6%Al-4%V is its availability as sheet and plate as well as rod, bar and billet. It can thus be used for sheet-metal fabrications or composite sheet/forging assemblies.

Limiting factors, when carrying out work at room temperature, are its minimum bend radius of 5t and the relatively narrow proof/tensile gap. Both these are improved by moderate heating, and temperatures up to 700°C are commonly used in warm-working the alloy. This has the added advantage that spring-back is less and dimensional accuracy thereby improved.

Fine-grained Ti-6%Al-4%V sheet can be superplastically formed, giving very high elongations, tight radii and negligible springback. The temperatures (900-950°C), pressures and times required are the same as those needed for diffusion bonding, and very complex parts can be made by combining these two processes. The equipment required generally includes metal tools with integral heaters, and means for evacuating the die cavities and applying argon gas pressure to deform the metal.

Heat treatment

Most of the applications of Ti-6%Al-4%V call for it in the annealed state, and the properties specified in British Standards TA 10, 11, 12 and 13 refer to a heat treatment at 700°C, followed by air cooling to room temperature. For sheet, it is sufficient to soak for 20 min at temperature; for rod or forgings, normal practice is 1 h per 25 mm of section with a minimum time of 1 h at temperature.

Annealing at 700°C gives the best combination of softening with little oxidation temperature of 850-900°C will provide maximum ductility and proof-stress/tensile-strength gap but with increased oxidation.

For disc quality material to yield optimum structure and properties, the heat treatment recommended is 960°C, water quench, followed by annealing at 700°C. The objective in the first stage of the treatment is to reach a structure containing between 15 and 45 per cent retained alpha.

For stress relieving of, for example, complex fabrications, it is often possible to obtain sufficient relaxation at a lower temperature, such as 500 or 600°C. As a rough guide 1 h at 600°C may prove adequate in most cases. The stress-relieving treatment can also act as an ageing treatment if the part has previously been solution treated at a higher temperature as described in the following paragraph.

The tensile strength of small sections such as bolts and other fasteners can be improved by heat treatment high in the alpha+beta field, followed by water quenching. The high-temperature beta phase is thereby transformed into a martensitic structure, which responds to controlled ageing, giving a useful increase in strength. This avoids problems due to excessive contamination and the possibility of gram growth, which are encountered at higher temperatures.

Elevated-temperature tensile tests have shown that the strength increase is proportionally retained at temperatures up to 540°C. The strengthening effects referred to earlier can only be obtained following a rapid quench from the solution-treatment temperature. Solution treatment and ageing is usually restricted, therefore, to small sections such as aircraft fasteners.

Mechanical properties

Room-temperature tensile properties

Various forms of Ti-6%Al-4%V, such as rod, bar and billet, are sold as forging stock and are therefore left in the hot-rolled or forged condition. It is only guaranteed to have the specification properties after annealing at 700°C; material greater than 150 mm ruling section may be tested on an upset-forged and annealed slice.

Elevated-temperature and sub-zero tensile properties

The properties of Ti-6%Al-4%V vary smoothly with temperature, which covers the range from minus 196°C up to 750°C. Although it retains useful short-term properties up to 500°C, its properties over the longer term tend to limit its useful range to 300°C, as suggested by the stress-rupture and creep curves.

Creep and Stability

Creep testing has shown that heat-treated material both metallurgical stability and surface stability under conditions of stressed exposure for up to 500 h at 450°C.

Fatigue properties

Rotating bending fatigue tests on specimens machined from 20 mm annealed rod have shown lives of >107 cycles at ±560 MPa (81 ksi). Samples of larger bar, i.e. 60 mm dia., have given slightly lower values of ±430 MPa (62 ksi) on smooth specimens, both plain and after anodizing. Notched specimens (Kt = 2,7) gave values of ±230 and 210 MPa (33 and 30 ksi) respectively.

Direct-stress zero minimum fatigue tests on 25 mm diameter bar gave fatigue limits of 690 MPa (100 ksi) on smooth specimens and 260 MPa (39 ksi) on notched specimens (Kt = 3).

Fracture toughness

Titanium alloy Ti-6%Al-4%V has good fracture toughness, as shown by the following properties obtained on 75 mm diameter bar.

Table2. Effect of heat treatment on tensile and fracture toughness properties of 75 mm diameter bar.

Heat treatment	0-2% proof stress Mpa	Tensile strength Mpa	Elongation on 50 mm %	Reduction in area %	Fracture toughness MPa√m
Annealed 2 h/700°C	890	980	17	39	84
1h/900°C.WQ+8h/500°C	970	1080	16	42	69
1h/960°C.WQ+2h/700°C	950	1030	14	37	57

Impact properties

A typical room-temperature Izod value of IMI 318 is 20 J, toughness varying quite smoothly with temperature from 95 J at 500°C down to 15 J at minus 196°C.

Properties of welds

Electron-beam welds

Alloy Ti-6%Al-4%V is an excellent material for joining by electron-beam welding techniques. Electron-beam welding has been widely adopted for critical components such as the center wing-box for the Tornado, Concorde engine thrust struts, and the engine spool assembly for the Rolls-Royce Gem.

Flash-butt welds

Alloy Ti-6%Al-4%V is readily joined by flash-butt welding, a process, which is widely used for the manufacture of engine rings.

A hardness survey showed small peaks on either side of the weld, in the as-welded condition, but these disappeared on heat treatment.

Tungsten-inert-gas welds

Ti-6%Al-4%V is less suited to TIG welding. There is little change in tensile strength, but the tensile elongation of, for example, TIG welded 1.6 mm sheet measured over 50 mm can drop from 14 to 5 per cent. Post-weld heat treatment in the range 700-800°C improves ductility, but can cause surface oxidation and distortion of sheet-based structures.

Applications

Titanium alloy Ti-6%Al-4%V is perhaps the most fully evaluated of all titanium alloys and has been used in the widest range of finished parts. Originally developed for the aircraft industry, it has been used as sheet fabrications, brackets and fasteners where lightness and high strength are required.

Its easy forgeability and strength at moderate temperature has led to extensive use as compressor blades and discs in gas-turbine engines and as fan blades in the most recent turbofan engines. An entirely new range of cost and weight saving components for both airframes and engines are now being developed using superplastic forming and diffusion bonding processes, for which this alloy is ideal.

Industries other than the aircraft industry have used for steam-turbine blades and lacing wire, axial and radial-flow gas compressor discs, springs for corrosion resistance, data logging capsules for oil and mineral exploration, etc.

A growing use of Ti-6%Al-4%V is as an implant material. Its excellent biocompatibility and good fatigue strength in body fluids make it ideal for the replacement of hip and knee joints, for bone screws, and for other surgical devices.

Other uses include reciprocating and rotating parts such as compressor valve plates, internal-combustion-engine connecting rods, rocker arms, valve springs and retaining caps, road springs and drive shafts for racing cars, and rotors for centrifuges and ultracentrifuges. Marine uses include armament, sonar equipment, deep-submergence applications, hydrofoils, telephone cable repeater station capsules, etc.

Although one of the earliest titanium alloys is studied so widely, many fresh uses are still being found for this versatile material.

Titanium Alloy Ti-2.5Cu, a binary alloy containing 2.5% copper, combines the formability and weldability of unalloyed titanium with improved mechanical properties, particularly at elevated temperatures. It can be used at temperatures up to 350°C. Chemical composition of Ti-2.5Cu is given in the Table 1.

Table 1. Chemical composition of Ti - 2.5Cu alloy

Element	Cu	Fe	H2	Ti
Wt %	2-3	≤ 0.2	≤ 0.01	Remainder

It has been used since 1959 as sheet, forgings and extrusions for fabricating components such as bypass ducts of gas-turbine engines. In these applications it has been used in the annealed condition, but since 1965 its use has spread to the airframe industry, following the development of an ageing treatment that raises room-temperature tensile properties by about 25% and nearly doubles the elevated-temperature properties (e.g. creep at 200°C). Such a material is particularly attractive since it can be formed in the soft condition, thus lowering fabrication costs.

British Standards TA 21, 22 and 23 refer to sheet, bar and forging stock in the annealed condition. DTD specifications 5233, 5243 and 5253 cover the same three forms of material, but in the ageable or aged condition. A different heat treatment is given to solution-treated material (suitable for ageing) from that for annealed material. It is therefore necessary to decide, before ordering, in which condition it is required. Specifications BS TA 24 and DTD 5263 refer to annealed and aged forgings respectively.

Alloy metallurgy. The diagram for the binary titanium-copper system is shown in Figure 1. The α+β / β transus temperature for a pure Ti-2.5Cu alloy is around 850°C. The difference from the 895°C found in commercial material is due to the oxygen and nitrogen content in the latter.

Figure 1. Binary titanium-copper diagram

There is a maximum solid solubility of 2.1 wt% of copper in alpha titanium at 798°C, and about 0.7 wt% at 600°C. This gives the possibility of an age-hardening reaction by precipitation of a compound, Ti₂Cu, from a supersaturated alpha solid solution. It is also possible to get age hardening from a martensitic alpha structure, obtained by rapid quenching of the alloy from the beta field, but there is some loss of ductility in this condition.

In material of commercial purity, X-ray diffraction studies indicate that maximum supersaturation of the alpha phase with copper is obtained by solution treatment at 790°C. However, to obtain consistent results under normal production conditions, a solution-treatment temperature of 805 ± 10°C is used.

Decomposition of the supersaturated solid solution occurs, as in all age-hardening systems, by a nucleation-and-growth mechanism. Precipitation is controlled firstly by the number of nuclei available, which decreases with rise in temperature, and secondly by the rate of growth of nuclei by diffusion of copper, which increases with rise in temperature.

Early work indicated that maximum age hardening occurred after treatment at 400°C, but the ageing reaction was very slow and ageing times as long as 140 h were needed to achieve satisfactory response. Higher temperatures produce a more rapid reaction. The optimum combination is an initial treatment of 8-24 h at 400°C to develop the maximum number of well distributed nuclei, followed by further ageing for 8 h at 475°C to give a more rapid growth to the optimum size for maximum strength. This duplex treatment gives a consistent response without risk of over-ageing.

Heat treatment. The full heat treatment consists of solution treatment at 805 ± 10°C followed by a rapid cool and a duplex ageing process of 8-24 h at 400°C and 8 h at 475°C. Many current specifications were evolved at a period when it was thought that 24 h at 400°C was necessary to ensure adequate nucleation. Experience has shown that this is unnecessarily long, and that 8h is quite sufficient. This accounts for the wide tolerance in time (8-24 h) for the first part of the duplex ageing process. All heating may be done in air, provided that normal precautions are taken to restrict contamination by hydrogen.

For sheet, 1/2 h at the solution-treatment temperature is sufficient. Rapid cooling is usually achieved by using a forced air blast, which causes less distortion than a water quench. For thicker sections such as rod, bar, extrusions or forgings, 1 h per 25 mm of section is recommended for thorough heating.

Ageing, for 8-24 h at 400°C and 8 h at 475°C, should be followed by air-cooling in each case. It may be necessary to guard against distortion during the ageing treatment by, for instance, supporting sheet on a flat base or by holding a complex fabrication in a suitable jig. The linear dimensional change on ageing is very small.

On occasions, when subsequent development of the fully aged properties is not necessary, a simple anneal at 790 ± 10°C followed by air cooling is sufficient for both sheet and thicker sections. This treatment may also be used to give a full recrystallisation anneal between heavy cold working operations (e.g. during flow turning).

Stress relieving of annealed material after forming and welding is achieved by a subsequent treatment at 600°C for 5h. For solution-treated material, however, the duplex ageing treatment acts as a satisfactory stress-relieving operation, and the 600°C treatment should not be used since it will cause over-ageing.

The oxide scale formed during annealing or solution treatment should be removed by vapor blasting or other scale-modifying treatment, followed by light pickling in a bath containing 4% commercial hydrofluoric acid and 20% nitric acid in water.

Creep properties. Aged Ti-2.5Cu Alloy (IMI Titanium 230) is more creep-resistant than was IMI Titanium 317 over the range 150-320°C and more creep-resistant than the hardest grade of commercially pure titanium at all temperatures.

It is important that any material whose properties make it technically attractive for operation for long periods at elevated temperatures should be metallurgically and mechanically stable under operating conditions. The room-temperature tensile properties of aged Ti-2.5Cu sheet are not affected by creep exposure for 100 h at temperatures in the range 200-350°C. Slight increase in strength and precipitate particle size is apparent after 5000 h at 350°C but there is no evidence of surface instability or loss of ductility, even though the post-creep tensile tests were carried out with no further surface preparation.

Fatigue properties. Like the steels and most other titanium alloys, Ti-2.5Cu Alloy has a fairly well defined fatigue limit and the S/N curve becomes horizontal at 10⁷ or 10⁸ reversals of stress. The fatigue ratio is particularly good; in most cases, the fatigue limit is more than 0.6 times the static tensile strength.

Forging. Ti-2.5Cu Alloy is very easy to forge. A certain amount of forging in the α+β field is required to develop optimum properties. The ideal forging preheating temperature is 800-820°C, though a preheating temperature of 850°C is commonly used. It may, on occasions, be permissible to go as high as 875°C for initial roughing operations, provided that a reduction of at least 2:1 or 4:1 is subsequently carried out at the lower temperature.

Forming. As the mechanical properties indicate, Ti-2.5Cu Alloy in the annealed or solution-treated condition is capable of undergoing considerable cold deformation without cracking. It is thus amenable to cold forming by conventional methods, such as pressing, stretch forming, spinning, etc., as applied to the stainless steels.

Welding. Ti-2.5Cu Alloy can be joined by fusion, resistance, flash-butt and pressure welding. Fusion welds can be made by both argon-arc and electron-beam welding. Joining techniques are governed by the metal`s affinity for atmospheric gases. At its melting point, titanium rapidly dissolves oxygen, nitrogen, hydrogen and carbon. Oxy-acetylene, metal-arc, carbon-arc and atomic-hydrogen welding processes are therefore unsuitable for titanium.

With adequate control of welding techniques, Ti-2.5Cu Alloy is one of the easiest metals to join. Welds of 100% strength can be obtained, with only a slight loss in tensile or bend ductility. When making a welded fabrication in an age-hardenable alloy such as Ti-2.5Cu Alloy, it is best to carry out the forming and welding on sheet already in the solution-treated condition. Ageing can then be carried out on the fabricated component. If the alloy is to be used in the annealed condition, then welding should be followed by stress relieving for 1/2 h at 600°C.

Casting and powder metallurgy are related only in that both are methods by which metal products are fabricated from materials that are not easily machined or joined, or fabricated into parts which, by their shape and size, are not economically feasible with conventional processes. These practices employing titanium have been studied extensively; however, commercial exploitation of the developmental work performed has been rather limited.

The various refractory materials employed in casting are attacked by titanium with such severity that sounds castings, possessing good mechanical properties are difficult to obtain. Powder metallurgical studies have met with equal difficulties. Most titanium metal powders currently available in commercial quantities do not have sufficient purity to produce ductile metal compacts.

Casting
The pouring of molten metal into a mold in which solidification takes place is termed casting. Although the term casting has been applied to ingot production, and certain components such as welds have a cast structure, it is intended here to deal with the casting of finished or semi-finished products.

The difficulties of casting titanium stem from inherent characteristics, such as its high chemical reactivity, and the flow properties of the molten metal. Conventional methods which employ such refractoriness as silica, magnesia, or alumina and which have been successfully applied to other metals are not practical with titanium.

Since titanium will likewise attack the furnace crucible material during melting of the metal prior to casting, it has been found necessary to prevent the impingement of the molten metal on the crucible wall. This has been accomplished by using skull melting techniques.

The method requires the maintenance of a solid layer of titanium metal between the crucible wall and the molten metal. This is accomplished by directing the arc at the center of the charge, and the careful maintenance of a temperature gradient between the molten metal and the wall.

As in ingot production, the molten metal and, in turn, the hot casting are susceptible to atmospheric contamination. To prevent this contamination an argon atmosphere is sustained in the crucible and the mold. To further eliminate the contamination, the furnace is designed to allow tapping of the melt at the bottom or side, and the sealing of the mold to the furnace over these vents. Thus, the pouring of the casting is also accomplished under an inert atmosphere.

This method of casting requires some skill in its operation. Not only does the skull technique require careful control in the melting, but ventilation must also be supplied simultaneously with pouring to allow escape of the gases present to minimize gas porosity. At present it is the only satisfactory technique for casting titanium.

A second difficulty, one which is peculiar to titanium, is in the maintenance of good flow over severe changes of dimension or direction within the mold. This requires, in many instances, the redesign of the mold or the cast component. Fillets or tapers, where dimensional or directional changes occur, have proved quite satisfactory in minimizing the difficulty.

Shell casting. Shell casting or shell molding utilizes the bonding of a refractory with a thermosetting plastic resin to form the mold. Stepwise, the procedure consists of forming two metal cavities, usually from aluminum, which are patterns of the part to be cast. One pattern coincides with one half of the part and the other pattern with the other half. These patterns, of course, must contain the necessary gates and risers.

One of the patterns is heated and clamped on top of a dump box or hopper, which contains the resin-refractory mixture. The refractory material can be cither graphite or zirconium, and the resin any one of several commercial phenolic casting materials. The mesh size of the refractory is variable, as is the ratio of resin to refractory, and is experimentally determined for the shape being cast.

In general the amount of resin employed should be slightly higher than that employed in the shell castings of iron alloys. When the pattern has been securely fastened lo the dump box, the hopper and pattern arc inverted for 30 to 60 seconds, after which time they are returned to their original position. The pattern which is coated with the resin-refractory mixture is removed and placed in a furnace for curing.

The curing time and temperature are dependent on the resin employed. After curing, the mold is stripped from the pattern. The other half of the mold is prepared in a similar manner using the second pattern.

Investment casting. Investment casting, also termed precision casting or lost wax casting, like shell molding is employed to cast small parts. This method is not as adaptable to assembly line speed as shell molding, but it is capable of producing the intricate shapes not possible with the shell technique.

To prepare an investment mold, a pattern with the necessary gates and risers is formed in the image of the component to be cast. The material used to form the pattern should be something which can be melted, volatilized, or burned off, such as wax or plastic. This pattern is coated with slurry consisting of the refractory and a binding agent. For titanium, zirconium is used as the refractory, and a silicate or zirconium compound such as zirconium nitrate serves as the binder.

The coated pattern is further built up by backing with coarse-mesh zirconium and the investment is then air-dried. The air-dried investment is recoated with slurry of coarse zirconium, the binder, and a hardening agent. The investment is again air-dried, after which it is heated to remove the pattern. The mold is fired at 1500 to 1600°F (810 to 870°C) for one to two hours and air-cooled.

Some work has also been carried out with graphite as the refractory. This method omits the backing and uses lower firing temperatures. Results are inconclusive, and the superiority or inferiority of graphite to zirconium is not readily evident.

In either case the mold is sealed to the furnace and the casting poured and allowed to solidify and cool. As in shell molding, the mold is expendable and is stripped from the casting, after which the necessary machine work is performed.

Titanium end products have been produced by this method and have been found to possess good mechanical properties.

Powder Metallurgy
Powder metallurgy is a method of fabrication in which metal powders are produced and further utilized by compacting and sintering to form useful products. Although the powder metallurgy branch of the metallurgical industry is limited, utilization of titanium metal powders from scrap some day may render this field important to the titanium industry. Powder metallurgy is employed today primarily to produce simple shapes with good dimensional stability, to form shapes with material of extremely high melting temperatures, and to produce parts not feasible by other means.

Both titanium scrap and sponge are being utilized as source material for titanium powder. The production of titanium carbide as a cutting tool and other titanium intermetallics such as refractory materials utilizes to advantage powder metallurgical processes.

Powder preparation. With most other metals to which powder metallurgical techniques are applied, the prime consideration is the compacting procedure rather than the preparation of the powder. The major powder preparation for other metals is accomplished by the relatively simple hydriding process in which the metal is embrittled with hydrogen and therefore more easily pulverized.

Application of this hydriding process in the preparation of titanium powders has not been successful in producing ductile compacts, since complete removal of hydrogen is economically impracticable. Therefore, with titanium the powder preparation becomes the major concern, since compacting and sintering, as will be shown, follow conventional procedures. Titanium powders are prepared by milling sponge or scrap.

The sponge is pulverized in an attrition mill employing titanium plates to minimize contamination which would result from conventional cast-iron plates. To prevent galling of the titanium sponge to the titanium plates, it has been found advisable to employ water at 40°F (4°C) as a protective medium. The final particle size of the powder should range from 30 to 100 meshes.

Additional problems are introduced in the preparation of metal powders when scrap materials rather than sponge are used as a source for powder. Scrap material usually contains some form of surface contamination such as grease, lubricating oil, or oxide film which may adhere to the surface.

Compacting and sintering. Compacting of the titanium powders is carried out in a die under extremely high pressures. The apparatus usually consists of a die body with upper and lower movable punches. These are shaped to the desired contours and are usually made of hardened tool steel ground and lapped to a mirror finish. The correct volume of powder predetermined from the size, shape, and density of the piece desired is placed in the die body.

Pressure is applied to the punches and the powder compacted to the desired shape. The pressures employed are approximately 20 tons per square inch. The compact is then sintered in a vacuum furnace at temperatures ranging from 1900 to 2000°F (1040 to 1100°C) for one to two hours.

The properties of powder metallurgy compacts produced by the techniques described above are not comparable to the properties obtained in the wrought metal. In general at equivalent strength levels the sintered metal will have a somewhat lower ductility than the wrought product. This is true of both the unalloyed and alloyed titanium.

Contamination in the final product should be kept to a minimum by careful selection of material and preparation of the powders. The maximum contamination tolerable will depend on the minimum specifications of the part required.

Titanium is lightweight, strong, corrosion resistant and abundant in nature. Titanium and its alloys possess tensile strengths from 30,000 psi to 200,000 psi (210-1380 MPa), which are equivalent to those strengths found in most of alloy steels. The density of titanium is only 56 percent that of steel, and its corrosion resistance compares well with that of platinum. Of all the elements in the earth’s crust, titanium is the ninth most plentiful. Physical Properties If all the elements are assembled in order of atomic number, it can he noticed that there is a relationship in properties corresponding to the atomic number. Titanium is found in column four along with chemically similar zirconium, hafnium, and thorium. Therefore, it was not unexpected that titanium would possess some properties similar to those found in these metals. Titanium has two electrons in the third shell and two electrons in the fourth shell. When this arrangement of electrons, where outer shells are filled before the inner shells are completely occupied, occurs in a metal, it is known as a transition metal. This arrangement of electrons is responsible for the unique physical properties of titanium. To mention a few, chromium, manganese, iron, cobalt, and nickel are found in the transition series. The atomic weight of titanium is 47.88, while aluminum has an atomic weight of 26.97, and iron 55.84. A crystal structure may he thought of as a physically homogeneous solid in which the atoms are arranged in a repeating pattern. This arrangement is instrumental in the physical behavior of a metal. Most metals have either a body-centered cubic, face-centered cubic, or a hexagonal-close-packed structure. Titanium has a high melting point of 3135°F (1725°C). This melting point is approximately 400°F above the melting point of steel and approximately 2000°F above that of aluminum. Thermal Conductivity. The ability of a metal to conduct or transfer heat is called its thermal conductivity. Thus, a material, to be a good insulator, would have a low thermal conductivity, whereas a radiator would have a high rate of conductivity to dissipate the heat. The physicist would define this phenomenon as the time rate of transfer by conduction, through unit thickness, across unit area for unit temperature gradient. Linear Coefficient of Expansion. Heating a metal to temperatures below its melting point causes it to expand or increase in length. If a bar or rod is uniformly heated along its length, every unit of length of the bar increases. This increase per unit length per degree rise in temperature is called the coefficient of linear expansion. Where a metal will be alternately subjected to beating and cooling cycles and must maintain a certain tolerance of dimensions, a low coefficient of thermal expansion is desirable. When in contact with a metal of a different coefficient, this consideration assumes greater importance. Titanium has a low coefficient of linear expansion which is equal to 5.0x10-6 inch per inch/°F, whereas that of stainless steel is 7.8x10-6, copper 16.5x10-6, and aluminum 12.9x10-6. Electrical Conductivity and Resistivity. The flow of electrons through a metal due to a drop in potential is known as electrical conductivity. The atomic structure of a metal strongly influences its electrical behavior. Titanium is not a good conductor of electricity. If the conductivity of copper is considered to be 100%, titanium would have a conductivity of 3.1%. From this it follows that titanium would not be used where good conductivity is a prime factor. For comparison, stainless steel has a conductivity of 3.5% and aluminum has a conductivity of 30%. Electrical resistance is the opposition a material presents to the flow of electrons. Since titanium is a poor conductor, it follows that it is a fair resistor. Magnetic Properties. If a metal is placed in a magnetic field, a force is exerted on it. The intensity of the magnetization, called M, can be measured in terms of the force exerted and its relation to the magnetic field strength, H, depending upon the susceptibility, K, which is a property of the metal. Metals have a wide variance in susceptibility and can be classified in three groups: * The diamagnetic substances in which K is small and negative, and thus are feebly repelled by a magnetic field; examples are copper, silver, gold and bismuth. * The paramagnetic substances in which K is small and positive, and thus are slightly attracted by a magnetic field; the alkali, alkaline and the nonferromagnetic transition metals fall in this group (it can be seen that titanium is slightly paramagnetic). * The ferromagnetic substances, which have a large K value and are positive; iron, cobalt, nickel, and gallium fall under this heading. An important feature of Group 3, besides the strong attraction in a magnetic field, is the fact that these metals retain their magnetization after being removed from the magnetic field. Most of the more important physical properties of titanium have now been indicated.