INTRODUCTION

Aircraft landing gears, structures, and turbine components (“aircraft critical components”) are subjected to severe loading, corrosion, and adverse environmental conditions and have complex shapes varying from thin to thick sections. High-strength titanium alloys and steels are widely used for aircraft critical, high-stressed components. These materials are chosen because of their specific strength, tensile strength-to-density ratio, and fatigue strength and toughness.

High-strength titanium alloys such as Ti-6Al-4V, Ti-10V-2Fe-3Al, and Ti-5Al-2Sn-2Zr-4Cr-4Mo are excellent candidates for aircraft landing gears and structures due to their high specific and fatigue strengths, good toughness, and excellent corrosion resistance. However, high cost limits their applications.

This article proposes quenched and tempered high-strength, corrosion-resistant (HSCR) steel as a suitable replacement for manufacturing aircraft critical, high-stressed components due to its high specific and fatigue strengths, good toughness, and corrosions resistance [1].

 

DESCRIPTION

HSCR (i.e., stainless) steel and high-strength titanium alloy are two options for critical high-stressed aircraft landing gears and structures. These two materials have their advantages and shortcomings.

 

METHODS AND PROCESSES

Premium-quality HSCR steel ingots are produced by vacuum melting processes. HSCR steel powder can be produced by atomization processes, including vaccum atomization.

Aircraft critical components can be manufactured from the HSCR steel or high-strength titanium alloy by the following three methods:

1. Hot working (HW) – hot working the melted ingots, including forging, rolling, and pressing, followed by machining and hardening.
2. Powder metallurgically-based, hot isostatic pressing (PM HIP) – hot isostatic pressing of powder followed by machining and hardening.
3. Additive manufacturing (AM) – additive manufacturing powder followed by finish machining and heat treatment.

HSCR steel hardening consists of austenitizing and rapid cooling, optional refrigeration, and tempering at low, medium, and high temperatures (secondary hardening) that depend on the required properties.

 

MANUFACTURING THE COMPONENTS

Formation of near net shape (NNS) by PM HIP allows manufacturing various types of the complex-shaped, aircraft critical components [2]. The process supplies precise geometry of the articles and properties close to the forgings. PM HIP article cost is generally higher than the cost of HW articles; however, small batches of the large-section articles are economically feasible to produce by PM HIP rather than HW of the melted ingots. Figure 1 shows an example NNS part made by PM HIP.

Manufacturing the aircraft components by PM HIP from the HSCR steel powder achieves a higher quality at a reasonable cost.  Components made by PM HIP from the HSCR steel powder have the same lifetime and durability at lower cost than the same weight components made by PM HIP from high-strength titanium alloy powder.

AM allows manufacturing NNS articles of high-strength titanium alloys. The NNS article made by AM is cost-effective due to minimizing waste. The “buy-to-fly” ratio (the ratio of the mass of raw material to the mass of the product) is significantly lower than the hot-worked articles. However, extremely high costs of titanium powder and manufacturing, high oxidation, and issues with heat treatment and machining limit AM of titanium alloys.

Aircraft critical components made by AM from the HSCR steel powder are a lower cost alternative to the same weight components made by AM from the titanium alloy powder.

 

COMPARING PROPERTIES

Table 1 shows the room-temperature mechanical properties of samples of the HSCR steel and Ti-6Al-4V alloy made by the following three different processes:

1. HW + hardening – hot-worked HSCR steel hardened by quenching, refrigerating, and tempering (QRT) and the hot-worked Ti-6-4 alloy hardened by solution treating and aging (STA) [3].
2. PM HIP + hardening – consolidated by HIP of HSCR steel powder hardened by QRT and the Ti-6-4 alloy powder hardened by STA.
3. Selective laser melting (SLM) + annealing – built with the SLM using the HSCR steel powder, followed by annealing and the built-by SLM using the Ti-6-4 alloy powder [4].

Figure 2 shows the comparison of the mechanical properties. In the figure, HSCR steel made by three different processes has slightly higher E/ρ and UTS/ρ, higher S, the same K_1C, and higher CVN than the Ti-6-4 alloy manufactured by the same processes. Also, the HSCR steel has higher elevated temperature strength, better workability and machinability, and better wear resistance, although the Ti-6-4 alloy has better corrosion resistance.

Given the superior material properties of the HSCR over the Ti-6Al-4V aircraft critical components manufactured by HW, PM HIP, and SLM processes, the Ti-6Al-4V alloy can be substituted with the same weight aircraft components, manufactured by the same processes from the HSCR steel, and without sacrificing stiffness, durability, and lifetime.

The aircraft components manufactured by HW, PM HIP, and SLM processes have at least 25% projected cost reductions compared to the same weight components from the Ti-6Al-4V alloy.

LNT PM Inc. is planning a pilot production for the NNS, aircraft critical components made from HSCR steel powder by PM HIP as a lower cost substitution of the high-strength titanium alloys. The HSCR steel is suited for defense industry applications such as missiles, artillery barrels, military land vehicles, and other applications, wherein high-strength and fatigue limits, good toughness, and corrosion resistance at reasonable cost are required.

 

CONCLUSIONS

HSCR steel possesses a specific strength that is slightly higher than the Ti-6Al-4V alloy at the same toughness. Therefore, HSCR steel can be substituted for the Ti-6Al-4V alloy in manufacturing aircraft critical components.

Projected cost of manufacturing the aircraft critical components from the corrosion-resistant steel is significantly lower than the Ti-6Al-4V alloy.

Additionally, manufacturing the aircraft critical components made from the HSCR steel reduces the energy consumption by 25% or more compared to the Ti-6Al-4V alloy. Using HSCR steel reduces the aircraft manufacturers’ dependency of titanium from the monopoly of suppliers on the world market.

 

REFERENCES

1. Vartanov, G. “High Strength CRA Ideal for Landing Gear.” Stainless Steel World, vol. 29, p. 42, April 2017.
2. Samarov, V., D. Seliverstov, and F. H. (Sam) Froes. “Fabrication of Near-Net Shape Cost-Effective Titanium Components by Use of Prealloyed Powder and Hot Isostatic Pressing.” ASM Handbook, vol. 7: Powder Metallurgy, pp. 660–670, 2015.
3. Carpenter Technology Corp. “Titanium Alloy Ti-6Al-4V.” Technical datasheet, https://cartech.ides.com/datasheet.aspx?i=101&E=268, 1 July 2000.
4. Liu, S., and Y. C. Shin. “Additive Manufacturing of Ti6Al4V Alloy: A Review.” Materials and Design, vol. 164, no. 107552, 2019.

 

BIOGRAPHY

GREGORY VARTANOV is the chief engineer of Advanced Materials Development Corp., a Toronto-based company that develops high-strength steels and alloys focusing on aerospace and defense applications. His interests include developing high-strength steels, alloys, and composites, as well as designing critical components for aerospace and defense applications. He has five U.S. patents and more than 10 publications in high-strength steels and alloys. Dr. Vartanov holds an M.S. and Ph.D. in materials science and metallurgy.