Effect of Microstructure on Fatigue Properties of Several Ti-Alloys for Aerospace Application

Abstract

A wide range of available Ti-alloys is used at Liebherr-Aerospace Lindenberg GmbH for several aeronautical applications in flight controls and landing gear systems. For these applications, the mechanical properties of conventionally manufactured Ti-alloys (α+β, near β) as well as additive manufactured Ti-alloy were optimized. Modification of the heat treatment parameters of a near-β titanium alloy leads to optimization of the hardening process of large cross-sections. This modification allows the adjustment of an optimum volume fraction of the primary α-phase resulting in enhancing of the elongation, fracture toughness and fatigue properties. For a fatigue critical forging part from (α+β)-alloy a slight modification of the chemical composition combined with an additional heat treatment step during the forging process was performed. The adjusted microstructure of the modified process exhibits better fatigue behavior when compared to the conventional microstructure. Ti6Al4V parts produced by Additive Manufacturing, printed with optimized parameters and followed by heat treatment will result in reasonable fatigue properties in all printing directions, reducing the anisotropy of the printed parts. These improvements bring Liebherr-Aerospace Lindenberg GmbH in the position to adapt the used titanium alloys for the needs in a wide range. For the evaluation of the microstructure, light and scanning electron microscopes were used. Furthermore a model described in the “Metallic Materials Properties Development and Standardization” (MMPDS) was modified and used for the evaluation of the fatigue results.