Microstructure of additive layer manufactured Ti–6Al–4V after exceptional post heat treatments

The microstructure of Ti–6Al–4V components, which were produced by an additive layer manufacturing (ALM) process, typically consists of columnar prior β-grains containing colony and/or basket-weave α + β with martensite. The microstructure is equivalent to microstructures neither of wrought nor of cast parts. However, the post heat treatments applied for the present investigation are derived from heat treatment specifications of those materials. This paper presents the microstructure and hardness of additive manufactured Ti–6Al–4V after heat treatments, which are not standardized or commonly applied, in order to suggest new ways forward. The experiments show that columnar prior β-grains can be transformed to globular prior β-grains without cold or hot working. The hardness and microstructure in a prior β-grain is a function of the cooling rate through β transus temperature rather than of the duration at above β transus. A heating cycle was determined which leads to a microstructure that is homogeneous in each prior β-grain and does not significantly differ from grain to grain.

► Exceptional heating cycles are applied to investigate microstructural effects.
► Columnar prior β-grains can be transformed to globular prior β-grains.
► It was not possible to induce a recrystallization which results in fine grains.
► Hardness and microstructure is a function of the cooling rate through beta transus.