Hydrogen as a Temporary Alloying Element for Establishing Specific Microstructural Gradients in Ti-6Al-4V

Abstract

Parts of vehicles, such as landing gear components of aircrafts, are subject to growing demands in terms of sustainability via lightweight design and durability. To fulfill these requirements, the development of thermochemical processes is auspicious. Titanium alloys allow a heat treatment in hydrogen-containing atmosphere for temporary hydrogen alloying, often called thermohydrogen treatment (THT). The investigation presented intends to realize a local microstructure modification of Ti-6Al-4V by means of THT. The study aims to use hydrogen (H) as a promoter for changing the local distribution and morphology of strengthening precipitates during THT as well as the local grain size (microstructural gradient). Both shall improve the fatigue properties of the material after hydrogen degassing. To derive suitable thermohydrogen treatment process parameters, the resulting fatigue crack propagation resistance and fracture toughness after different solution heat treatments are determined experimentally and compared to each other. Moreover, various graded microstructures are evaluated after hydrogen uptake (hydrogenation) and hydrogen degassing (dehydrogenation) using numerically simulated hydrogen concentration profiles, observed hardness curves, metallographically determined microstructure gradients and the corresponding results of the phase analysis by means of X-ray diffraction. The study shows that hydrogenation at 500 °C and dehydrogenation at 750 °C enables the generation of a promising microstructural gradient.

Feasibility of THT-Induced Microstructure Gradients and Their Effects on Fatigue Properties in Additively Manufactured Cylindrical and in Conventionally Manufactured Tube-Like Ti-6Al-4V Specimens

“…The study was carried out by using the (α+β)-Ti alloy Ti-6Al-4V as it is described in [15] (α lamella width: 10-80 µm, averaged 30 µm; α colony width: 600-800 µ). This alloy is widely used e.g.…”

“…Latter was carried out by Liebherr-Aerospace Lindenberg GmbH, Germany. Suitable time and temperature parameters for the THT process to generate microstructure gradients are taken from previous work [37]. The initial microstructure was lamellar characterized by a α lamella width of 2.3±0.8 µm.…”

“…Additionally, the locally increased dislocation density left by the former hydrides stimulates the recrystallization kinetics during subsequent annealing (dehydrogenation), potentially leading to a refined microstructure [13]. Previous studies on THT have demonstrated that this technique can yield to increased strength under cyclic and static loading conditions by generating a homogeneous microstructure with finer characteristics compared to conventionally heat-treated Ti alloys [9,14,15]. THT was also applied to additively via LPBF manufactured specimens, leading to a homogeneous microstructure with improved mechanical properties at quasistatic loading [16,17].…”

“…[35]), the goal is to establish a fine, equiaxed microstructure in the near-surface region, leading to a significant extension of the fatigue crack initiation phase. Simultaneously, a coarse, lamellar microstructure is targeted in the core, which decelerates the propagation of occurring long fatigue cracks [15,34]. Previous studies by Berg and Wagner [36] successfully achieved microstructure gradients in metastable β-Ti alloys using thermomechanical treatments to prolong fatigue life.…”

“…As described in [15,37], for conventionally manufactured cylindrical Ti-6Al-4V specimens, microstructure gradients in the surface near region, characterized by a fine and globular morphology, have already been succeeded by applying THT even without subsequent aging. The promotion of THT-induced microstructure grading with regard to industrial manufacturing and usage application requires further feasibility studies.…”