Can we make an engine out of titanium

Compared to lightweight materials such as aluminum, magnesium alloys and carbon fiber reinforced plastics, titanium has high specific strength, heat resistance and corrosion resistance, making it an ideal metal for transportation equipment in the aerospace and automotive industries. On the one hand, titanium is considered to be a widely used aerospace material. On the other hand, however, in the automotive industry, steel is better known for its broad versatility. This is because automotive engines made from titanium are usually too costly for the automotive consumer market. The high cost of manufacturing titanium parts has hindered its expansion to mass production applications. Instead, it has been identified primarily as a specialty material, for example, for automotive and motorcycle racing parts where low weight and high strength and stiffness are critical.

Reciprocating engine components are the most effective applications for titanium, including intake valves, exhaust valves, valve spring seats, valve springs, valve tappets, connecting rods and piston pins, as shown in Figure 3. Among them, intake valves, valve top rods, connecting rods are usually designed using typical structural α+β phase titanium alloys, such as Ti-6Al-4V, which operate up to 300°C. This is because the big end of the connecting rod must feature optimal geometry shape and sufficient stiffness to suppress the strength reduction caused by abrasion fatigue. The weight reduction of the piston pin greatly reduces noise, vibration, acoustic roughness and improves engine performance including fuel consumption. Instead, valve spring seats are made of β-phase titanium alloys such as Ti-22V-4Al (DAT51), which has cold deformability. In addition, Ti-6.8Mo-4.5Fe-1.5Al (or low-cost beta,LCB) alloys are also suitable for manufacturing valve spring seats. LCB alloy boasts low Young's modulus, high strength and reduced manufacturing cost compared to Ti-6Al-4V alloys.Details regarding spring performance for automotive applications will be addressed later. The crankshaft is the only reciprocating component in an engine that is difficult to manufacture effectively from titanium. Since the role of the crankshaft is to convert the reciprocating motion of the piston into torque, which generates the driving force, weight reduction of the material would cause it to lack the necessary stiffness to perform the appropriate function.

From a manufacturing cost perspective, titanium exhaust valves may be more cost-efficient when replaced with reciprocating engine components made of more expensive alloy composition, for example in high temperature conditions around 800°C. Since iron-based exhaust valves are mainly composed of Fe-21Cr-0.4N (21-4N), they have high austenitic heat resistance but are expensive. Therefore, since the 1980s, high-temperature near-alpha-phase titanium alloys Ti-5.8Al-4Sn-3.5Zr-0.5Mo-0.7Nb-0.35Si-0.06C (IMI834) and Ti-6Al-2.7Sn-4Zr-0.4Mo-0.45Si (Ti-1100) have been developed by IMI and TIMET. By the 1990s, the increased power of reciprocating engines and stricter emission regulations led to higher exhaust valve temperatures and the adoption of a large number of high-temperature alpha-phase titanium alloys for exhaust valves. These trends gave rise to new titanium technologies, such as unique alloy designs and microstructure control, as shown in Figure 3. Alloys such as 5vol% TiB/Ti-6Al-4Sn-4Zr-1Mo-1Nb-0.2Si (TiB MMC), Ti-5.8Al-4Sn-3.5Zr-2.8Mo-0.7Nb-0.35Si-0.06C (DAT54) and Ti-6Al-4Sn-3.5Zr-0.5Mo-1Ta-0.35 Si , are typical titanium alloys used in the development of exhaust valves. In particular, TiB MMC has unique properties such as ultra-high strength and heat resistance up to about 900°C, as well as corrosion resistance and high wear resistance. This heat resistance is due to the presence of TiB particles in Ti- MMC, but after in situ precipitation of TiB2 in reaction with Ti during high temperature sintering, TiB particles are only stable in titanium-based alloys. In contrast, DAT54 obtained a balance between creep resistance and low circumferential fatigue resistance at high temperature because the addition of 2.8% Mo gave it a unique bimodal structure. ti-1100 with β-turn needle-like organization and high creep strength was also applied to exhaust valves.

In addition, to promote the use of titanium in reciprocating engine components, it is important to combine the required processing conditions to obtain the desired properties for each engine component. For example, as previously mentioned, mixed element (BE) powder metallurgy processes and high temperature extrusion processes have been successful in achieving 100% material yields. Figure 4 shows a test product for manufacturing engine components using the BE powder metallurgy process. Cold forming is also important for manufacturing low-cost titanium engine parts. For example, valve spring seats using alloy Ti-22V-4V have better formability than Ti-6Al-4V alloy. Surface treatments such as shot blasting to cope with high circumferential fatigue strength, metal plating, thermal spraying, and hardening by interstitial elements increase value and also effectively improve reliability, especially oxidation as a typical low-cost treatment applied to engine valves. tiB MMC engine valves with low-cost surface treatments meet stringent wear resistance criteria after engine durability testing, as described later. .