EUREKA'S May 1997 Cover Feature Story

AUTOMOTIVE MATERIALS

Titanium alloys come down to earth

Super strong and corrosion-resistant alloys of titanium, well established in aerospace, are finding increasing usefulness in cars. Tom Shelley reports

As strong as steel but only 60 per cent of the weight, titanium alloys have always had attractions for engineers, not least because of their extreme corrosion resistances.

While their use in aerospace, motor sports and certain luxury cars is generally accepted, a slow but steady reduction in base alloy and fabrication costs is making them increasingly attractive to the makers of volume vehicles.

Valves, valve springs, retainers and connecting rods offer opportunities, as one might expect, but serious consideration and field trials are under way with titanium exhausts, wheels, fasteners, and damage tolerant under panels.

Titanium alloy automotive components typically cost about three to five times as much as those fabricated in steel, but are considerably less expensive than those made of fibre-reinforced composite. In the past, users of titanium components have been put off by fabrication costs and concerns about welding. While there are some aspects of working with titanium which require care, there are now many companies which have mastered the techniques - 14 in Norway alone, for example. Titanium alloys have half the modulus of elasticity of steel, which at the same time makes titanium springs one third the weight of their steel equivalents, but requires over forming to ensure right final shape. Ductility is less than that of steel, requiring more generous bend radii and annealing reheats, but these can be undertaken in air, and overall, titanium forming and forging is less difficult than with some stainless steels.

Welding requires exclusion of oxygen, but argon shielding is quite sufficient. Welding in vacuum chambers is totally unnecessary in most cases.

One of the strong interest areas in the US is in using titanium for exhaust systems. The goals are to reduce weights to meet Corporate Average Fuel Efficiency goals, and to extend useful life. There is some concern in the US, backed by legislation, to the effect that exhaust systems, and the catalytic converters which form part of them should be able to last at least 100,000 miles. Commercially-pure titanium and titanium alloy sheet and tube easily achieve this requirement, and substantially surpasses the 409 series stainless steel systems currently in place. Weight saving is a bonus. Because of its superior corrosion resistance, titanium can be used much thinner than stainless steel. Hence a typical expansion box and tail pipe in 409 stainless weighs 10kg whereas a redesign with titanium weighs 3.2kg.

Titanium may not be suitable for entire exhaust systems. In practice, it is likely to be limited to components whose metal temperature does not exceed 400 OC for sustained periods of time. Titanium units immediately behind catalytic converters in the US perform well. Titanium lugs welded to pipes are likely to prove the most effective way of attaching exhaust systems to vehicle chassis by means of rubber isolators. Trials are under way with Chrysler, which has surpassed 80,000 km; General Motors, which has reached 40,000 km, and with Ford. Exhaust boxes have been made by Arvin and Calsonics.

The other potentially attractive area for titanium alloys is in vehicle suspension springs. Titanium bends much more than steel before it breaks, which means that in addition to being lighter per unit volume of metal, a titanium spring also has fewer turns. Timet has developed a reduced cost alloy, Timetal LCB, which is specifically aimed at the spring market. LCB is a beta alloy of composition: 4.5 per cent iron, 6.8 per cent molybdenum, and 1.5 per cent aluminium, in which iron replaces the much more expensive vanadium used in previous beta alloys.

In a case study conducted in the US on a long-haul freight vehicle, titanium springs gave a weight saving of 140kg for an additional outlay of $1,500 (1,000 pounds). Turning the weight saving into additional cargo over 600 trips produced additional revenue of $12,000 or 8,000 pounds. And because they are corrosion resistant, the titanium springs are likely to last longer too.

Valve springs, and whole valve trains, offer multiple advantages for titanium. With lightweight titanium valves and valve retainers, less spring power is required to prevent valve bounce at high engine speeds, further reducing spring size and weight. Alternatively, the same-sized spring can be used to permit more rapid valve motion and higher engine speed. Lower spring loads and lighter valves reduce the friction of the valve system, which is typically 20 to 25 per cent of the total mechanical friction of the engine. Lowering friction gives improved efficiency, less noise and reduced fuel consumption. Estimates of fuel savings vary from two to four per cent with greater levels of improvement in engines with four valves per cylinder with twice the number of moving parts and sliding faces. Surface treatment to improve wear resistance is a key to successful application. Exhaust valves present a particular challenge to provide oxidation and creep resistance at high operating temperatures.

Titanium connecting rods are already used in Lexus cars and in the Honda 280 HP, 3 litre, V6 NSX sports cars. Reducing reciprocating mass allows increased engine speed, reduced crankshaft stress, and/or a lighter crankshaft, and generation of less noise, vibration, and sideways thrust on pistons. In the NSX, titanium is said to give a 700 rpm increase in engine speed over steel connecting rods, a 30 per cent reduction in weight and enhanced fatigue resistance. The alloy developed in Japan for this application is a free machining alloy with three per cent aluminium and 2.5 per cent vanadium.

Nor should humble fasteners be neglected. Modern cars are still mostly made of steel and other alloys and subject to severe corrosion stresses. If components made of different alloys have to be joined together, much potential trouble from galvanic corrosion can sometimes be saved by judicious use of a few titanium screws and rivets. Titanium owes its corrosion resistance to the formation of a tenacious oxide covering film which is highly electrically insulating.

As more users realise the benefits of titanium, this will in turn drive the producers onto new fabrication techniques. Forging and welding are well established, but powder metallurgy and hot isostatic pressing offer opportunities for further cost reductions.

Timet

Design Pointers

* Titanium and its alloys have similar strengths to steels, but with about 60 per cent of the weight. Modulus of elasticity is about half

* Weights of systems and components is often much less than 60 per cent of their steel equivalents. Titanium exhausts can be made thinner and springs with fewer coils

* Titanium can be forged, with reheats in air. Exhaust boxes may be closed by lock seaming or resistance welding. Ends and pipe joints may be sealed by tungsten inert gas welding

* Its ductility is less than steel, so more generous bend radii are required. It also has a modulus of elasticity about half that of steels so that it tends to spring back when formed. This can be compensated for by over forming. Tooling must be clean and properly lubricated

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