Aircraft Landing Gears and Structures Made from Powder of High Strength Corrosion Resistant Steel
Aircraft landing gears and structures are subjected to severe loading, corrosion, adverse environmental conditions and have complex shapes which vary from thin to thick sections. High strength titanium alloys and high strength steels are widely used for the highly stressed aircraft landing gears and structures. The main criteria for choosing these high strength materials include their specific strength, a ratio of tensile strength to density as well as fatigue strength and toughness. High strength titanium alloys such as Ti-6Al-4V, Ti-10V-2Fe-3Al, Ti-5Al-2Sn-2Zr-4Cr-4Mo are excellent candidates for aircraft landing gears and structures due to their high specific strength, high fatigue strength, good toughness, and excellent corrosion resistance. However, their high cost limit the applications of these alloys. Powder of the high strength corrosion resistant (HSCR) steel is a suitable material for manufacturing critical high stressed aircraft landing gears and structures due to high specific strength, high fatigue strength, good toughness, and corrosion resistance .
HSCR steel and the high strength titanium alloy are two options for critical high stressed aircraft landing gears and structures due to their high specific strength. These two materials have their advantages as well as shortcomings. Aircraft landing gears and structures can be manufactured from the powder of the HSCR steel or high strength titanium alloy through at least 2 methods:
1. Hot Isostatic Pressing (HIP) - hot isostatic pressing of powder followed by machining and hardening
2. Additive Manufacturing (AM) – additive manufacturing of powder followed by finish machining and heat treatment.
Hardening of the HSCR steel consists of austenitizing and rapid cooling, optional refrigerating, and tempering at low, medium, and high temperature (secondary hardening) that depends on the required properties. Formation of near net shape (NNS) by HIP allows for the manufacturing of various types of the complex shape articles . The process supplies precise geometry of the articles and properties that are close to the properties of forgings. The cost of HIP articles is generally higher than the cost of the forged articles; however small batches of the large section articles are economically feasible to produce by HIP rather than forging of the melted ingots. Manufacturing of the aircraft components by HIP from the HSCR steel powder allows for high quality products to be achieved at a reasonable cost. The components made by HIP from the HSCR steel powder have the same life-time and durability at a cost lower than the same weight components made by forging or HIP from high strength titanium alloy powder. Additive manufacturing (AM) allows for the manufacturing of the NNS articles of high strength titanium alloys. The NNS article manufactured by AM is cost-effective due to its waste minimization. The “buy-to-fly” ratio (the ratio the mass of raw material to the mass of the product) is significantly lower than hot worked articles. However, the extremely high cost of titanium powder, high cost of manufacturing, high oxidation, and issues with heat treatment and machining limits the applications of AM of titanium alloys. Aircraft components made by AM from the HSCR steel powder are a lower cost alternative of the same weight components made by AM from the titanium alloy powder. The table below shows the room temperature mechanical properties of the HSCR steel and the Ti-6Al-4V alloy made by two different manufacturing processes: 1. PM HIP + Hardening - consolidated by HIP of HSCR steel powder hardened by QRT and the consolidated by HIP the Ti-6Al-4V alloy powder hardened by STA 2. Selective Laser Melting (SLM) + Annealing - built with SLM using the HSCR steel powder followed by annealing and built with SLM using the Ti-6Al-4V alloy powder followed by annealing.
The HSCR steel also has higher elevated temperature strength, better workability and machinability, lower galling, and better wear resistance, although the Ti-6Al-4V alloy has better corrosion resistance. Aircraft components manufactured by HIP and SLM processes from the Ti-6Al-4V alloy can be substituted with the same weight aircraft components manufactured by the same processes from the HSCR steel without sacrificing stiffness, durability, and life-time. The aircraft components manufactured by HIP and SLM processes have 40-60% projected cost reductions compared to the same weight components manufactured by the same processes from the Ti-6Al-4V alloy. The HSCR steel is also suited for defense industry applications such as missiles, artillery barrels, military land vehicles, and other applications wherein high strength, high fatigue limits, good toughness, and corrosion resistance at reasonable cost are required.
HSCR steel possesses specific strength that is slightly higher than the Ti-6Al-4V alloy at the same toughness. Therefore HSCR steel can substitute the Ti-6Al-4V alloy in the manufacturing of aircraft landing gears and structures. Projected cost of manufacturing of aircraft landing gears and structures from the corrosion resistant steel is significantly lower than the cost of the same weight components from the Ti-6Al-4V alloy. Manufacturing of the aircraft landing gears and structures made from the HSCR steel reduces the consumption of energy by 25% or more compared to the same weight products made from the Ti-6Al-4V alloy. HSCR steel reduces the titanium dependency of the aircraft manufacturers from the monopoly of suppliers of titanium on the world market.
 Gregory Vartanov “High strength CRA ideal for landing gear”: Stainless Steel World, Volume 29, April 2017, p. 42  V. Samarov ;D. Seliverstov ;F.H. (Sam) Froes “Fabrication of Near-Net Shape Cost-Effective Titanium Components by Use of Prealloyed Powder and Hot Isostatic Pressing”: ASM Handbook, Volume 7, Powder Metallurgy, p.p. 660-670, 2015