Ti-2007 Science and Technology, edited by M. Ninomi, S. Akiyama, M. Ikeda, M. Hagiwara, K. Maruyama The Japan Institute of Metals (2007) Progress on Titanium Spring

Baoliang Bai, Jianchao Yang, Quan Hong

Northwest Institute for Nonferrous Metal Research, Xi'an 710016, China

For years the automotive industry has shown interest in titanium alloys for many automotive components. One of the limiting factors in titanium's acceptance over the years has been its relatively high price compared to standard materials. However, as the design engineers strive for lighter weight vehicles with improved performance and the materials manufacturers developed ways to reduce the cost and streamline processing, titanium has become more attractive. Titanium alloys offer weight savings, strength, corrosion resistance and modulus advantages over steels used for automotive applications. This paper addresses the current and potential uses for titanium alloys in spring application. Selected comparisons with conventional alloys are presented. Typical weight and performance advantages as well as a discussion relative to cost will also be made. Attention should be concentrated on the following three areas in the near future: (1) development of lower cost alloys; (2) exploration of low cost manufacturing methods; (3) evaluation of treatments to enhance wear resistance if necessary.

Keywords: titanium(Ti), spring, progress, low cost

1. Introduction engineers can reduce the free height of the spring to 50-80% Uniquely among engineering alloys, titanium and titanium that of a comparable steel spring. This can translate into greater alloys possess the strength, density and modulus to make the styling and structural design freedom as well as improved ‘ideal’ spring for almost every application1~3). The key to passenger compartment and payload space design flexibility. successful spring design is to optimize the saving of weight and Several design concepts for cars, across the range of future space. Modulus of Ti is only about 36.9GPa, and which of steel models, have space constraints that cannot be met using steel is about 79.3GPa, so Ti is approximately twice as "springy" as springs. steel. Since the density of titanium is about half that of steel, These properties combine to allow a spring designer to utilize a titanium springs are smaller and typically 60-70% lighter than larger wire diameter and many fewer coils to provide a spring steel equivalents, and the lower weight improves suspension rate that is equivalent to the steel spring being replaced. The dynamics and response. At the same time titanium has 75% of resulting titanium spring is both significantly lighter and will the strength of steel, tensile strength of Renton Spring Grade provide 1.2 in more travel due to having fewer coils (the travel Titanium is around 1270MPa, and which of Chrome Silicon of Ti spring is 6.4 in., and that of steel spring is only 5.2 in.). Spring is 1720MPa or so4). This gives the titanium springs the characteristic "open spaced Spring weight for a given load and spring rate is proportional to coil" look. The corrosion resistance of titanium alloys is an the product of the shear modulus and density of the alloy added benefit to service life for springs on equipment, even if it divided by the square of the allowable stress. Weight is is only occasionally operated in aggressive conditions5). minimized when titanium is used because of its low shear Since the Ti alloys are very proper to manufacture springs, modulus and density combined with high allowable stress. At more and more attention has been paid on these kind of alloys the same time, spring deflection is inversely proportional to [1~3]. In this paper, progress on Ti springs are reviewed, shear modulus and is therefore high for titanium, so fewer including Ti spring materials, application of Ti springs, and the active coils are needed, permitting a reduction of free height way to lower the cost are also presented. (by 50-80% of a comparable steel spring), with further weight reduction and a 44% higher natural frequency, from 2. Titanium Spring Materials 127Hz to 88Hz, as shown in Table 1. A wide range of titanium alloys are suitable for making springs Table l. Comparison between Titanium and steel spring4) for virtually any weight- or space-sensitive application. 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. Beta-C (Ti-3Al-8V-6Cr- 4Mo-4Zr), which has been used in the aerospace industry over ten years in spring applications6,7). The expense of producing Beta-C has precluded its use in automotive applications. By altering the alloy formulation through the use of a much less expensive Fe-Mo master alloy, Timetal LCB (Low Cost Beta, Ti-6.8Mo-4.5Fe-1.5Al) was developed primarily for automotive springs and can be formulated at as little as half the cost of typical existing beta alloys, e. g. Beta-C8,9). Ti-B20 was one kind of beta Ti alloys developed in The low modulus and low mass of titanium springs enable them to be designed for smaller spaces so that

1411 Northwest Institute for Nonferrous Metal Research (NIN), available from Timetal LCB alloy in comparison with other China. The ultimate strength for the alloy treated at 780°C is titanium alloys at ambient temperature. From the table, it can 919 ~1003 MPa, the yield strength equals to 875 ~ 990 MPa, be seen that Timetal LCB and Ti-B20 alloy possess favorite and the elongation varies from 16% to 18%, the area of comprehensive properties, which are quite suit for reduction varies from 48% to 53%. The properties after aging is manufacturing high performance springs. shown in Table 2, Table 2 also shows the properties.

Table 2. Mechanical properties of several Ti alloys at ambient temperature (STA)

Springs made with Timet LCB do not require as many turns as Table 3. Properties of Timetal springs vs. steel equivalents4) a steel spring because of its low shear modulus, and low density12), as shown in Figure 1. 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 Timetal LCB spring wire can be supplied hot rolled, or hot any need to apply paints or other protective coatings or rolled and cold drawn, (normally up to 20% cold reduction), or anticorrosion treatments. Beta alloys as a class offer designers hot rolled and solution treated9). LCB can be either cold wound many options to select a final combination of properties for or hot wound at 700-760°C. If hot wound, springs would be fan specific applications. Characterization of springs made from air cooled and aged. Cold wound springs may pass directly for Timetal LCB is in hand to establish the safe upper limit for aging typically at 510-540°C for up to two hours. Finishing of allowable stress. This will be closer to the tensile strength than springs manufactured by either method would be blasting and for other spring alloys, an essential requirement for the springs pickling followed by shot peening, typically to 16-18A of lowest weight and least cost. From the table, it can be also intensity7). seen that the density of titanium is about half that of steel, Fatigue13) is a likely failure mechanism for all types of springs titanium can perform the same task as steel springs in most (compression, extension, torsion, leaf, presswork, spiral, applications while weighing 60-70% less. Even if the two constant force, disc, etc.) as well as for all spring sizes(fatigue materials have the same density, the titanium spring would be occurs in springs made from materials 0.1 mm thick to 80 mm lighter because it does not use as much material. diameter). In standard salt-spray exposure fatigue tests, the typical fatigue strength of a steel spring, coated or uncoated, is reduced by up to 50% over that of the same spring tested in air. In the same tests, the life of the titanium springs in salt spray was reduced by less than 4% over the tests done in air5). Unlike steel springs, titanium springs do not require protective coatings. Corrosion fatigue is the probable cause of the majority of steel suspension spring failures in car, motor vehicles. Springs are invariably manufactured from high-strength materials, the most important of which are carbon steels, low- alloy steel, stainless steels, and nickel, cobalt, copper, and Figure 1. Comparison of mass difference of Timetal LCB springs with a titanium alloys, and all are susceptible to fatigue failure. Table conventional steel spring of the Volkswagen Lupo FSI 4 shows the fatigue endurance limit for several Ti alloys. From the table, it can be seen that Timetal LCB alloy possesses favorite fatigue strength both for smooth specimen and notched ones.

1412 Table 4. Fatigue endurance limit for several Ti alloys, MPa

3. Application Springs made by the Timetal LCB are typically 30 percent Titanium has found many application for springs. The use of lighter than the steel springs on the comparable 2005 model TIMET Exhaust Grade titanium for the exhaust system of the and weigh 1.1 lb (500 g) less than their steel counterparts. 2001 Corvette Z6 was the first significant appearance of Ultimate Performance Grade (UPGTM) Beta titanium material titanium on a volume production car (covered in AEI in made by Dynamet Incorporated has been used in the drive October 2000). The first production cars to ride on Timetal clutch, torsion and suspension springs on the 2004 Arctic Cat LCB springs are now on the road, with the rear suspension of King Cat mountain snowmobiles -- believed to be the first time the 2001 Volkswagen Lupo FSI fitted with these springs13). that titanium springs will be standard on a manufacturer's Valve springs indeed represent an attractive entry point for original equipment snowmobile15). Coil springs used in Arctic titanium into the automotive engines for the mass market5). The Cat's newest standard equipment King Cat shaves precious potential for reduction of overall engine height offers a benefit pounds from the snowmobile. The titanium coil springs, made where space is constrained. Combined with titanium valves and by Renton Coil Springs, also are designed to improve steering, valve retainers further savings are possible, less spring power ride and pitch control. 21b weigh savings will be got when Ti being required to prevent ‘valve bounce’ at high engine speeds. coil springs made from UPG Beta material for high- Alternatively, titanium can be used without incurring a weight performance motorsports and recreational penalty to increase the spring load and permit a more rapid applications. valve motion and greater rpm. The use of lower spring loads with lighter valves reduces the friction of the valve system that 4. The Way to Low the Cost is typically about 20-25% of the total mechanical friction of the The cost is the threshold for Ti spring. Aftermarket steel engine. Lowering friction without compromise to engine power springs typically retail for $80. The titanium springs retail for output gives improved engine efficiency, less noise and about $500. This is $216 per pound of weight saved. Compare reduced fuel consumption. this cost with other methods of saving weight. Combined with Titanium suspension springs offer significant opportunity for the improved performance, titanium springs are a must have for savings of weight and space. A typical helical spring design in the performance minded enthusiast. A case study conducted in titanium when compared to a steel version will reduce the the US on a long haul freight vehicle showed a weight saving weight by upwards of 70% (4.12 kg to 1.36 kg in one example). with titanium springs of 140 kg, for an additional outlay of Rigorous schedules of on and off-road testing are being £1000 (US$1500) on the cost of the truck. Turning the weight conducted. Although the mass market of private cars is a clear saved into cargo over a typical 600 trips produces added target for the wider use of titanium suspension springs, revenue of £8000 (US$12000) and a net annual return on applications of comparable importance exist for public service investment of some 20%4). vehicles and freight haulage. Additional carrying space, The ways to solve the problem may be as follows: (1) to reduction of weight, and saving of fuel may permit increasing develop low cost Ti alloys, e.g. Timetal LCB; (2) to explore payload or greater operating range and more revenue. Titanium low cost manufacturing methods; (3) to evaluate the treatments springs are lighter than steel and have less mass to accelerate to enhance wear resistance; (4) to increase the production each time the suspension activates. The lower mass system volume, when given sufficient production volume, the price of takes less energy to accelerate and decelerate. This allows the Ti springs should ultimately achieve the target for cost- wheel to track terrain more accurately by reducing the “spring effective use in automotive springs and other components. surge” which contributes to wheel hop. Many riders describe Beta C is a high-strength metastable titanium alloy widely used improved traction and control with the use of Titanium for spring and fastener applications. Input stock of these suspension springs. springs and fasteners is currently made by rolling billets forged For model year 2006, the Yamaha YZ125 and YZ250 two- from conventional double-melt VAR (2 X VAR) ingots. Recent stroke and the YZ250F and YZ450F four-stroke motocross advances in plasma arc melting (PAM) single-melt technology motorcycles all carry titanium shock springs, marking the first offer the potential to reduce the input stock cost by directly time production motorcycles from any manufacturer have been rolling as-cast near-net shape PAM ingots instead of the fitted with titanium suspension springs as original equipment14), conventional forged billets. A 127 mm (5in.) diameter as-cast the new springs offer "outstanding strength and fatigue Beta C PAM ingot was rolled into 15 mm (0.6 in.) bars for tolerance" and reduce unsprung weight for improved springs and fasteners6). suspension performance.

1413 5. Prospect 15) http://www.cartech.com/news/wr_news_arctic_cat.html. Positive progress is now being made towards achieving cost 16) J. Kiese, W. Walz, B. Skrotzki: Ti-2003 Science and Technology, Proc. effective application of titanium in vehicles for the mass 10th World Conference on Titanium, ed. by G. Lütjering and J. Albrecht, market. The leading applications are cold wound springs (Druckhaus Darmstadt GmbH, Darmstadt, 2004) pp.3043-3050. manufactured from low cost beta alloy, and exhaust systems 17) Nippon Steel and TIMET Joining Hands to Market Automotive Titanium manufactured from commercially pure titanium. Alloy. Japan Metal Bulletin, 2001; V.49-43, no.6164, pp.5-6. The cost-effective substitution of metastable beta-titanium for steel will require that titanium suspension springs be manufactured using existing steel suspension fabrication equipment and techniques. These techniques involve coil winding followed by short-time aging and shot peening15,16). The cold spring winding, cold setting (blocking), and shot peening process steps are basically the same for titanium springs as for steel. The learned process differences included minor adjustments - such as determining the optimum heat treatment approach and the optimum aging time - rather than substantial changes, which were largely necessitated by low production volumes. Cooperation between springs manufactures and parts makers through their own sales channels should be strengthened. For example, Nippon Steel Corp. and Titanium Metals Corp. (TIMET) have made a cooperative agreement to market titanium alloy for automotive springs in Japan17). Under the agreement, TIMET is to supply Nippon Steel with intermediate stock of "TIMET LCB (low cost beta)", spring alloy of its own development and Nippon to process the stock at the bar and wire mills in its Hikari Works into finished products for sales to Japanese automakers.

REFERENCES 1) M. Hayes: Wire Journal International. 39(2006) pp.66-69. 2) M. Kocan, B. Yazgan-Kokuoz and H. J. Rack: JOM.57(2005) pp.66-68.

3) P. Ponticel: Automotive engineering international. 112(2004) pp. 83-85. 4) http: // www.coilspring.com / performance / why_titanium / 5) D. Peacock: Materials World. 5(1997) pp.580-583. 6) K.O. Yu, E.M. Crist and R. Pesa: Journal of Materials Engineering and Performance. 14(2005) pp.697-702. 7) V. R. Jablokov, J. R. Wood and B.G. Drummond: Ti-2003 Science and Technology, Proc. 10th World Conference on Titanium, ed. by G. Lütjering and J. Albrecht, (Druckhaus Darmstadt GmbH, Darmstadt, 2004) pp.3035-3042. 8) Y. Kosaka and K. Faller: Wire Journal International. 39(2006) pp. 87-93. 9) Y. Kosaka, S. P. Fox and K. Faller: Journal of Materials Engineering and Performance. 14(2005) pp.792-798. 10) R.Boyer, G.Welsch and E.W.Collings: Materials Properties Handbook: Titanium Alloys, (ASM International, Materials Park, 1994) pp. 493-885. 11) P. Ge, Y.Q. Zhao and L. Zhou: Rare Metal Materials and Engineering. 35 (2006) pp. 707-710. 12) D. Helm, O. Roder and S. Lüetjering: Ti-2003 Science and Technology, Proc. 10th World Conference on Titanium, ed. by G. Lütjering and J. Albrecht, (Druckhaus Darmstadt GmbH, Darmstadt, 2004) pp.69~80. 13) M. Kocan, H. J. Rack, and L. Wagner: Journal of Materials Engineering and Performance, 14(2005) pp.765-772. 14) K. Faller: Springs, 45(2006) pp.49-50.

1414