Application of Titanium Powder Metallurgy Structural Alloy Products Abroad

Titanium powder metallurgy technology began in the early 1950s, but it was not until the 1970s that a significant breakthrough was made. Since then, developed countries around the world have invested a lot of effort in researching this technology and developing its metallurgical products, making it widely used in many industrial fields.

In recent years, titanium products in the aviation field, in addition to high-temperature performance structural parts, are developing towards high-temperature titanium alloys; in the non-aviation field, the demand for titanium products is also increasing. Experts from Japan's Kobo Steel Company predict that by the end of the century, the application of titanium in the non-aviation industry in the Western industrial market will increase from the current one-third of the output of titanium processing products to at least one-half. Similar to titanium casting metallurgy, in the development of titanium powder metallurgy applications, each country has its own focus according to its needs. The United States is mostly used in military and aviation engineering, and Western European industrial countries (mainly Germany, Britain and France) are committed to developing aviation and some civilian titanium powder metallurgy products, especially in the field of aviation structural parts. There are many technologies and products. Japan has been vigorously developing titanium civilian products, especially titanium powder metallurgy products in the fields of ocean, chemical industry, energy, etc. Russia has also shown a high level of application of titanium powder metallurgy products in the fields of petroleum, chemical industry, and metallurgy.

Commonly used structural titanium alloys in industry include: industrial pure titanium, Ti-6 Al-4 V, Ti-5 Al- 25 Sn, Ti-6 Al-6 V-2 Sn- (CuFe), Ti-8 Al-l Mo-l V. Powder metallurgy titanium structural alloys often use similar alloys according to actual needs, and Ti-6 Al-4 V is the most widely used.

1. Powder metallurgy titanium structural alloy products for aviation

1.1 Aviation products

As early as 1956, the American Kitchen Appliance Company used the method of hot pressing sponge titanium powder to produce a large number of GET73 turbojet engine bearing seats, and then only through finishing machining to form the final product, the cost was reduced by 25% to 30% compared with the same product made by machining forged bars. At that time, the US aircraft manufacturing industry used hot-pressed Ti-6 Al-4 V alloy powder to manufacture rivets, locking rings for C-5 A transport aircraft, and compressor blades for TF39 engines, and used non-alloyed powder to make discs and valve seats.

Dynamet of the United States used Ti-6 Al-4 V alloy powder to preform blanks and forge them into gas turbine blades. The US Nuclear Materials and Equipment Company used the preformed blank-forging-refining process to manufacture the guide vanes and cargo rings of gas turbine engines. In a naval project costing $2 million, Grumman Aerospace of the United States used the ceramic mold hot isostatic pressing method to produce the inner support rod (0.77kg) and engine nacelle frame (23.85kg) of the F-14 fighter. The material utilization rate increased from 15% to 30% to 50% to 60%, and some even reached 90%, and the cost was reduced by 25% to 37%. It is said that the cost of the fuselage struts made of hot isostatically pressed (HIP) Ti-6 Al-4 V powder for the F-14 aircraft has been reduced from $400 per forged piece to $245.

MBB of Germany uses pre-alloyed powder (PA) Ti-6 Al-4 V hot isostatically pressed and forged to produce helicopter blade connecting rod joints and connecting arms of air buses, saving 40% of materials. The cost can be reduced by 25% by using the HIP process alone, and further selection of the best process can reduce the cost to 34%.

1.2 Economic Benefit Evaluation of Aviation Products

Generally speaking, for large and complex parts, powder metallurgy is more attractive than casting. According to analysis, powder metallurgy can reduce costs by 20% to 50% depending on the size and complexity of the component.

Grene H. of the Kruup Research Institute in Germany used Ti-6Al-4V and Ti-10V-2Fe-3Al pre-alloyed powders and ceramic mold hot isostatic pressing to produce complex-shaped impellers with mechanical properties comparable to those of cast alloys, while reducing costs by 40%. The cost structure is mainly distributed in forging and machining processes. Therefore, if a closer net shaping technology is used for complex-shaped components, the economic benefits will be more obvious.

The Crucible Materials Company in the United States produces a fixed support frame for the F-18 fighter engine. The shape is relatively simple, and the cost depends mainly on the powder cost and production scale [7]. When mass-produced, the cost of this component is one-third of that of conventional methods. Obviously, the use of powder metallurgy to produce large and complex components can compete with conventional processes. The main factors that determine production costs are powder cost, production scale, and true near-net shaping.

1.3  Problems in the development of aviation products

In the production of complex powder metallurgy titanium alloy aviation structural parts, the BE method can meet the performance requirements for non-fatigue application parts and greatly reduce costs; as for parts produced by the PA method, their performance (especially fatigue performance) is comparable to that of products produced by the casting method; for complex large parts, their production costs are also low, but their application in the aviation field is relatively small, and their slow progress is difficult to understand. Froes F.H believes that the reason lies in the inherent conservatism of the aviation industry. Therefore, in order to develop the application of PA products, on the one hand, it is necessary to insist on low product costs, and on the other hand, the product design should reach the level required by users. At the same time, the production scale should be increased, and complex components should be produced using technologies closer to net forming, and chemical surface treatment should be used to improve surface properties, or shot peening should be used to improve the fatigue performance of components.

2 Powder metallurgy titanium structural alloy products for automobiles

2.1 Titanium parts for automobiles

The light weight and high strength of titanium have long been noticed by automobile manufacturers. Titanium has been used in racing cars for more than 20 years. Currently, almost all racing cars use titanium, and some sports cars also use various titanium parts. Using titanium in racing car engines can reduce the weight of running parts and improve their acceleration performance. In the United States, titanium exhaust valves, valve retainers, connecting rods and other parts for racing cars have been produced and sold on the market. The first Japanese car to use titanium was the Nissan R382. At present, the main titanium parts used in automobiles include:
(1) Valves. In the United States, professional manufacturers that use titanium alloys to make intake and exhaust valves are relatively common. The intake valve uses Ti-6Al-4V alloy, and the exhaust valve uses Ti-6Al-2Sn-4Zn-2Mo alloy. The intake valve made of titanium alloy weighs 55g, which is 35g lighter than the steel valve, and the high-speed performance is improved by 10% to 15%. A titanium exhaust valve can be 50g lighter than a steel valve, and it has high reliability and a service life that is 2 to 3 times longer. In addition, it is reported that the use of titanium alloys for intake and exhaust valves can also save 2% of fuel. These valves are now used in various types of automotive engines.

(2) Valve baffles. Valve baffles made of Ti-6A1-4V alloy are widely used in racing cars and sports cars, with an annual production of more than 250,000 pieces. They are cheap and do not require surface treatment (titanium materials for automobiles generally require special surface hardening treatment to solve the problem of titanium's easy adhesion). They are also 10 to 12 grams lighter than steel valves. Japan uses Ti-5A1-2Cr-Fe alloy to make valve baffles.

(3) Connecting rods. Using titanium alloy to make connecting rods is the most effective way to reduce engine weight. Groth K. [10] calculated that when steel connecting rods are replaced with titanium alloys, the performance is improved by 43% due to the weight reduction; if the maximum load application part is considered and partial strengthening is required, the performance can also be improved by 27%. Even if the low elastic modulus of titanium alloy is considered and strengthening is required, the performance can still be improved by 17%. The connecting rod is made of Ti-6A1-4V, and other alloy materials such as Ti-4A1-4Mn0-2Si and Ti-7AI-4Mo are also under development.

(4) Crankshaft and other engine parts. Japan and other countries are trial-producing Ti-5AI-2Cr-Fe alloy crankshafts. This type of crankshaft needs to be treated to prevent adhesion and has not yet been put into practical use.
Other engine parts include Ti-6A1-4V alloy rocker arms, valve springs and lower bolts of connecting rods.

(5) Car body and other parts. Using Ti-6AI-4V pipes and flanges for welding in the exhaust system can reduce weight by 35% to 45%. In the Prosche 908, pure titanium pipes and plates are welded into a full titanium exhaust system, reducing weight by 450 kg. In addition, there are titanium bolts, nuts and other connectors and transmission parts such as clutch discs and pressure plates. Compared with steel housing, titanium clutch housing made by rotational molding can more easily mitigate the destructive impact of flywheel.

In short, the use of lightweight and high-strength titanium alloy can reduce the noise, impact and vibration of automobile engines and improve fuel efficiency. Therefore, as titanium products are increasingly moving towards non-aviation markets, its entry into the automotive industry is of great significance. A report from the American Refractory Metals Co., Ltd. pointed out that Ford used titanium alloy parts for cylinder components in its 1989 models; and Mitsubishi Corporation of Japan has already used titanium alloy intake and exhaust valves in its 1986 models. The super-loaded sports car FZR 750R produced by Japan's YAMAHA Engine Company uses titanium alloy connecting rods. The product is small and has a well-balanced center of gravity device, which can ensure continuous and durable use at high speeds. The price is 2 million yen per vehicle. Mitsubishi Metal Corporation Central Research Institute of Japan uses Ti-6AI-2Sn-42r-2Mo alloy scraps to make valve heads through casting, and uses Ti-6 Al-4V alloy scraps to make valve stems, which are connected by friction welding to make the intake and exhaust valves of automobile engines.

2.2 Powder metallurgy titanium structural alloy parts for automobiles

The above-mentioned titanium alloy structural parts for automobiles are all cast or forged products. It is reported that the former Soviet Union applied titanium powder metallurgy products to automobile engines in the 1970s. For example, powder metallurgy connecting rods were successfully used in the 0.5 t Zaporozhers-969 car, and it is estimated that 16,450 rubles can be saved for every 100,000 titanium powder hot forged connecting rods produced. Japan used sintered titanium as wheel nuts and covers in 1975.

In 1987, Clevite Company of the United States used the mixed element method (BE) to produce titanium valve spring retaining cups and connecting rods. The valve spring retaining cup of Ti-6AI-4V alloy produced by the company's patented MR-9 method, after cold pressing and vacuum sintering, fully meets the mechanical performance requirements (especially fatigue performance). The weight is reduced from 28.5 g of conventional forged steel parts to 8.5 g of powder metallurgy parts. The tensile strength and fatigue performance both exceed the specified values, the weight is reduced by 70%, the material utilization rate is 100%, and the cost is lower than that of the 7.8 g cast titanium parts. It has passed the 10 million cycle fatigue test conducted by Honda Motor Company in the Amsler fatigue test. Clevite uses cold isostatic pressing and vacuum sintering of Ti-6A1-4V alloy powder connecting rod blanks, which are then machined and inspected by Honda R&D Co., Ltd. according to their design. The results show that the connecting rods produced by the MR-9 process have good performance, and the bending strength is 21% to 43% higher than the ultimate strength (108MPa) of the steel connecting rods with the same cross-section. The connecting rods and surface polished samples are repeatedly pulled and compressed, and their endurance strengths are 212MPa and 254MPa respectively. The material utilization rate is 80%. Honda estimates that at least 33% of the weight can be reduced.

2.3 Possible development

As titanium tends to be used in the civilian field and the characteristics of titanium bring significant improvements in automobile performance, titanium has great potential for large-scale production in the automotive industry. Although the high price is the reason why titanium alloys have not been widely used in automobiles, it is also obviously due to its poor publicity and insufficient development. Although the use of titanium to reduce 1kg of weight will increase the cost by 600 to 800 yen, so it is still difficult to use in ordinary cars, but with the increasingly serious energy problem, high fuel efficiency cars have become a common concern of general automakers and customers. Each car only needs 1kg of titanium parts, and the amount of titanium used is considerable according to the current scale of automobile production. Obviously, as long as the parts in the car are correctly selected, the product design is optimized, the mixed element powder metallurgy technology that can meet the performance of the car and is low in cost is adopted, and the scale production is reasonably organized, it is possible that titanium alloy structural materials will be widely used in ordinary cars.

The use of powder metallurgy to produce Ti3Al and TiAl intermetallic compound-based titanium alloys can replace heat-resistant steel in automobile engines, reduce the weight of the car and improve fuel efficiency. It may be easier to put into practical use than engineering ceramics for automobiles.

3. Other applications

Japanese experts predict that powder metallurgy sintered titanium has the following uses in precision machinery: camera parts (shutters, etc.), watch parts (watch cases), measuring instruments, testing machine parts, copier, printing machine parts, etc.; in the electronics industry, it has the following uses: tape recorder guide rollers, magnetic head materials, communication machinery (vibration membrane materials), household appliance parts, bearings, etc. In addition, it is also used in daily necessities, sporting goods, handicrafts, and medical machinery. In the civil industry, the use of titanium powder metallurgy is also profitable compared with titanium alloy casting. In fact, some special-shaped parts, nuts, valve rings, cylinder brackets and other accessories of some hydraulic systems have been mass-produced. In recent years, Japan has paid great attention to the development of titanium applications in sporting goods, construction, decoration, furniture, cooking utensils, etc., and its products are basically cast and processed products. Obviously, for the civil industry that attaches more importance to economic benefits, the application of titanium powder metallurgy in some products is very attractive. Japan has developed titanium bicycles, including titanium rims, frames, beams, headstocks, handlebars, guide rings, gears and other parts, which are made by rolling, shaping or forging. Each vehicle weighs 8.5 to 9.5 kg, and its demand is quite large. The price of each high-end racing car is 250,000 to 300,000 yen.

For example, Mori Industries and Shimano and other companies in Japan have tried to produce a group of high-end bicycle parts called DURA-ACE, which include sprocket crankshaft devices, flywheel guide devices, pedals, guide wheels, card brakes and other parts that have been sold on the market. After replacing iron parts, it can reduce the weight of the vehicle by 50% to 60%. After improving the hardness of titanium alloys through heat treatment and surface treatment, satisfactory performance can be obtained. In such applications, it is technically and economically feasible to use powder metallurgy titanium alloys similar to the above-mentioned parts and other nut parts.