Titanium and Its Alloys

Selection of Materials and Applications
Source: http://www.keytometals.com

Abstract:
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.

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.

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