Editorial Feature

Titanium for Automotive Applications

Automotive applications of titanium follow logically from the high strength, low density and low modulus of titanium and its alloys, and their excellent resistance to corrosion and oxidation. Prime components for titanium on the internal combustion engine are valves, valve springs and retainers and connecting rods. On vehicle bodies, springing remains the principal application, but exhausts capable of surviving a 100,000-mile warranty, wheels, high strength fastenings and corrosion-resistant, damage-tolerant underpanels are also under evaluation.

The performance benefits of titanium in automotive applications have been established and widely reported over many years, and are not an issue - titanium works! However, use of the metal has been limited to racing cars and a few top-of-the-range road models because of cost. Increasingly, the titanium industry has attempted to meet the tough financial and life cycle targets set by the automotive industry. Attention has concentrated on three areas:

  • Development of lower cost alloys
  • Exploration of low cost manufacturing methods
  • Where required, evaluation of treatments to enhance wear resistance.

Progress for the Titanium Industry

Positive progress is now being made towards achieving cost effective application of titanium in vehicles for the mass market. The leading applications are cold wound springs manufactured from low cost beta alloy, and exhaust systems manufactured from commercially pure titanium. These two classes of components are currently being manufactured for the automotive industry in titanium, using processes and tooling designed for manufacturing parts in steel.

Automotive suppliers and manufacturers look at total system cost, and while the titanium industry is working to develop low-cost alloys, it is clear that less expensive titanium alone will not guarantee an automotive application, but a competitively priced titanium component will, hence, the need to establish low cost manufacturing methods for titanium parts. In working with automotive designers, the titanium industry continually monitors the cost benefit of using titanium, and seeks to be made aware of all design and production considerations in order to attack the cost and manufacturing challenges with its full resources.

Requirements of the Automotive Industry

To the automotive producers, style and performance remain key sales factors, but environmentally aware customers with increasingly educated social consciences want not only these, but also greater safety, minimum noise, maximum fuel economy and the continued reduction of harmful emissions. Pressure of legislation, typically the US Corporate Average Fuel Efficiency (CAFE), adds to market competition and growing customer demand, forcing design and materials changes. As demands increase for more fuel-efficient and environmentally friendly cars, the gap between affordability and cost is narrowing and more improvements are being incorporated into engines, suspension and bodywork. Increases in the cost of fuel serve to hasten this process, while enabling more to be spent on corrosion resistance, weight and fuel saving. Current affordable US figures are shown in Table 1.

Table 1. Estimate of savings on fuel costs due to reduced weight in manufactured vehicles.

Type of vehicle

$/car/kg of weight saved

Mass production

$2.20

CAFÉ limited

$4.50-$7.70

Specialty and luxury

$8.50+

Titanium Springs

Uniquely among engineering alloys, titanium possesses the strength, density and modulus to make the ‘ideal’ spring for almost every application. The key to successful spring design is to optimise the saving of weight and space. Titanium springs are smaller and typically 60-70% lighter than steel equivalents. Several design concepts for cars, across the range of future models, have space constraints that cannot be met using steel springs.

Spring weight for a given load and spring rate is proportional to the product of the shear modulus and density of the alloy divided by the square of the allowable stress. Weight is minimised when titanium is used because of its low shear modulus and density combined with high allowable stress. At the same time, spring deflection is inversely proportional to shear modulus and is therefore high for titanium, so fewer active coils are needed, permitting a reduction of free height (by 50-80% of a comparable steel spring), with further weight reduction and a higher natural frequency.

Valve Springs

Valve springs indeed represent an attractive entry point for titanium into the automotive engines for the mass market. The potential for reduction of overall engine height offers a benefit where space is constrained. Combined with titanium valves and valve retainers further savings are possible, less spring power being required to prevent ‘valve bounce’ at high engine speeds. Alternatively, titanium can be used without incurring a weight penalty to increase the spring load and permit a more rapid valve motion and greater rpm. The use of lower spring loads with lighter valves reduces the friction of the valve system that is typically about 20-25% of the total mechanical friction of the engine. Lowering friction without compromise to engine power output gives improved engine efficiency, less noise and reduced fuel consumption. Estimates of fuel savings vary from 2% to 4% according to engine type, with greater levels of improvement possible in engines with four valves per cylinder, where twice the number of moving parts and sliding faces generates a greater part of the overall engine friction. Surface treatment to improve the wear resistance of titanium alloys is a key to successful application overall in the valve train. Exhaust valves present a particular challenge to provide oxidation and creep resistance at high operating temperatures.

Suspension Springs

Titanium suspension springs offer significant opportunity for savings of weight and space. A typical helical spring design in titanium when compared to a steel version will reduce the weight by upwards of 70% (4.12 kg to 1.36 kg in one example). Rigorous schedules of on and off-road testing are being conducted. Although the mass market of private cars is a clear target for the wider use of titanium suspension springs, applications of comparable importance exist for public service vehicles and freight haulage. Additional carrying space, reduction of weight, and saving of fuel may permit increasing payload or greater operating range and more revenue. A case study conducted in the US on a long haul freight vehicle showed a weight saving with titanium springs of 140 kg, for an additional outlay of £1000 (US$1500) on the cost of the truck. Turning the weight saved into cargo over a typical 600 trips produces added revenue of £8000 (US$12000) and a net annual return on investment of some 20%! The corrosion resistance of titanium alloys is an added benefit to service life for springs on equipment, even if it is only occasionally operated in aggressive conditions. Corrosion fatigue is the probable cause of the majority of steel suspension spring failures in motor vehicles.

Titanium Spring Materials

A wide range of titanium alloys are suitable for making springs. Good performance has been achieved with Ti-6Al-4V and other alpha-beta alloys, but much better results are obtained from the higher strength beta alloys, which are more easily drawn into wire and cold fabricated to springs. Of these the Ti-4.5Fe-6.8Mo-1.5Al alloy (Timetal LCB) offers the best combination of desirable properties at an economical price. This was developed primarily for automotive springs and can be formulated at as little as half the cost of typical existing beta alloys. Given sufficient production volume, its price should ultimately achieve the target for cost-effective use in automotive springs and other components.

Properties of Titanium Alloys

Table 2 highlights the outstanding combination of properties available from this alloy in comparison with other titanium alloys. Characterisation of springs made from Timetal LCB is in hand to establish the safe upper limit for allowable stress. This will be closer to the tensile strength than for other spring alloys, an essential requirement for the springs of lowest weight and least cost.

Table 2. Properties of Timetal versus Ti-6Al-4V.

Property

Ti-6Al-4V Bar STA

Timetal 10-2-3

Timetal LCB

RT yield strength (N.mm-2)

1100

1200

1350

RT UTS (N.mm-2)

1185

1275

1420

Elongation (%)

13

9

10

R of A (%)

24

21

35

Density (g.cm-3)

4.43

4.64

4.79

Tensile Modulus (103 N.mm-2)

112

106

110

Fatigue endurance limit (N.mm-2)

 

 

 

RT Smooth (kt=1)

895

950

1000

RT Notched (kt=3)

240

290

330

The strength, density, shear modulus and relative weight of this alloy are compared with those of steel of similar tensile strength in Table 3. This table represents the optimum comparison for steel, because it is not uncommon to apply an allowance for corrosion to the diameter of the steel spring, making it even heavier and more bulky. No such allowance is required for titanium, nor is there normally any need to apply paints or other protective coatings or anticorrosion treatments. Beta alloys as a class offer designers many options to select a final combination of properties for specific applications. Consultation with the alloy producer and the spring manufacturer is recommended to ensure that the appropriate method of manufacture and finishing is used.

Table 3. Properties of Timetal springs versus steel equivalents.

Property

Steel

Timetal LCB

Allowable Stress (N.mm-2)

1000

1000

Shear Modulus (103 N.mm-2)

80

43

Density (g.cm-3)

7.82

4.78

Relative weight

100

33

 

Timetal LCB spring wire can be supplied hot rolled, or hot rolled and cold drawn, (normally up to 20% cold reduction), or hot rolled and solution treated. LCB can be either cold wound or hot wound at 700-760°C. If hot wound, springs would be fan air cooled and aged. Cold wound springs may pass directly for aging typically at 510-540°C for up to two hours. Finishing of springs manufactured by either method would be blasting and pickling followed by shot peening, typically to 16-18A intensity.

Production Economics

The manufacturing methods for LCB springs enable production costs and production rates to be achieved which are similar to those for steel springs, employing standard steel spring making equipment, standard dry coatings for cold coiling, with similar aging (tempering) furnaces, temperatures and times. In this way capital costs for re-equipment normally associated with the implementation of new materials and technologies are entirely avoided. Production rates equivalent to those for steel springs have been achieved.

Titanium Exhaust Systems

In the US, the environmental agencies now require guaranteed corrosion resistance on exhaust systems to upwards of 100,000 miles. Titanium easily achieves this requirement and surpasses the stainless steel (409) systems currently in place. A typical expansion box and tail pipe in 409 steel weighed 10kg. A redesign with titanium reduced this to 3.2 kg! This 6-8 kg weight saving beneficially reclassified the vehicle within the CAFE system.

Commercially pure titanium sheet and tube are the materials of choice for silencers, and pipework. Reduction of both weight and cost are addressed by selecting the thinnest gauges of materials consistent with the engineering and acoustic requirements of the exhaust system. Titanium may not be suitable for the entire system, and in practice will most probably be limited to components in which the metal temperature does not exceed 400°C for sustained periods of time. US units coupled immediately behind the catalytic converter continue to perform well. The use of titanium lugs welded to the pipe will almost certainly prove to be the most effective way to attach the exhaust to the chassis, (via rubber isolators), but other methods are being tested.

Suitability of Titanium Alloys

Material conforming to ASTM Grade 2, (e.g. Timetal 50A) offers the optimum in terms of cost, availability, fabricability, weldability and mechanical properties. Grade 45A is slightly less strong and more ductile than 50A and may be required where extensive forming is part of the manufacturing process e.g. lock seaming (Table 4). Both alloys are fully weldable, require no intermediate or post-forming heat treatment, and are available in wide sheet coil. These alloys are also used to manufacture low-cost continuously welded tube.

Table 4. Properties of commercially pure titanium Timetal 45A and 50A.

Grade

45A

50A (ASTM 2)

Proof Stress min (N.mm-2)

200

275

UTS min (N.mm-2)

290

345

Elongation min (%)

23

20

RA min (%)

40

35

Hardness (Hv)

140-170

160-200

Tensile modulus (103 N.mm-2)

103

103

Torsion modulus (103 N.mm-2)

45

45

Density (g.cm-3)

4.51

4.51

Thermal expansion (10-6 °C)

8.9

8.9

Thermal conductivity (W.m-1.°C-1)

21.6

21.6

Specific heat (J.kg-1.°C-1)

519

519

Titanium Production Allowances

Commercially pure titanium is cold formable, and sheet and tube can be shaped readily at room temperature using techniques and equipment suitable for steel. Attention to three points of detail ensures trouble-free production:

  • The ductility of titanium is generally less than that of steel. More generous bend radii may be required or Timetal 45A specified to meet essential tight bending and seaming requirements.
  • The modulus of elasticity of titanium is about half that of steel. This means that titanium will spring back after forming. Compensation for this is made by slight overforming.
  • Titanium tends to gall against unlubricated forming tools. Clean, properly lubricated tooling presents no difficulty.

Both lock seaming and resistance welding are suitable to close the expansion box, and autogenous TIG is suitable to seal the ends and pipe joints with a torch trailing shield to the external surface of the box end joints. Argon shielding is not required for resistance welding.

Production Economics

In the US, a leading exhaust system manufacturer, Arvin, successfully rolled and seam welded a run of titanium boxes at one of its plants. This was done with no cost penalty compared with steel production. The company has also found that no extra costs apply when bending titanium tubes through plants that are currently producing steel components.

Summary

Current practice, made possible by cold hearth melting enables ‘ingot’ for strip production to be cast as slab and then hot rolled directly to coil, the feedstock for final cold reduction to strip. Timet is well equipped for strip production and finishing, and with the combined Sendzimir mill and continuous vacuum annealing line, is a long established low cost producer of both strip and tube.

Rolling of commercially pure titanium is an uncomplicated process, with high yield and excellent reproducibility. Additional capacity to accommodate a substantial growth of demand in one product sector, (e.g. strip for exhaust manufacture, almost certainly exists outside the titanium industry, with companies that supply automotive steels. It is not unreasonable to anticipate a time when titanium will be a part of the normal delivery programme of these companies, who will seek to retain their share of the market as titanium gains a more significant place in mass market vehicles.

Primary author: David Peacock

Source: Materials World, Vol. 5 no. 10 pp. 580-83 October 1997.

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