Applying thermal post-processing techniques to engineer the microstructure and mechanical performanceof selective-laser-melted Ti-6Al-4V ELI

Abstract

As an additive manufacturing technology, selective laser melting (SLM) affords the creation of customisable parts at high resolutions to within the micron range. Among the materials being processed by SLM systems, the Ti-6Al-4V titanium alloys (grade 5 and grade 23) continue to be heavily researched. In the as-printed state, SLM'd grade 23 parts suffer from brittle microstructures and poor mechanical performance which often does not conform to minimum standard specifications. This can largely be attributed to the martensitic microstructures rendered in the as-built state. Researchers in the space have employed post-processing strategies in attempting to ameliorate such microstructures. Heat treatments for printed Ti-6Al-4V components are designed to transform the microstructure and tailor the mechanical properties of SLM'd Ti-6Al-4V parts.

While there is a gamut of published works prescribing so-called optimal heat treatment profiles, there is a lack of thorough investigations in to how SLM'd Ti-6Al-4V material responds to heat treatment. This work was concerned with better understanding how SLM'd Ti-6Al-4V material responds to heat treatment, principally in terms of microstructure evolution and the resulting mechanical performance of such microstructures.

The first experiments involved a detailed examination of how printed components respond to annealing treatment profiles of a broad range of target temperatures and soak times. The evolved alpha-phase microstructural features and volume fraction of beta-phase were quantified and related to the observed tensile test performance.

As a large variety of, sometimes conflicting, SLM'd Ti-6Al-4V microstructures and mechanical performance data has been reported in the published literature, the next experiments focused on determining the impact of microstructural heritage. Specifically, how Ti-6Al-4V components printed with different SLM process parameters respond to a number of annealing treatment profiles. It was found that the interaction of SLM process parameters, printed part density and impurity content wielded considerable influence over the exhibited tensile performance of SLM'd and annealed material.

The final set of experiments investigated how printed material responded to an unconventional heat treatment strategy. At the time of writing, there are many recent papers exploring the approach of unconventional treatment profiles. Through cycling the target temperature during heat treatment, bimodal microstructure morphologies can be rendered and tailored in printed parts. Such bimodal microstructure morphologies display different mechanical performances to lamellar microstructure (the typical morphology rendered) counterparts. While the bimodal feature sizes were shown to be readily tuneable, it was ultimately shown that lamellar microstructures outperform bimodal microstructures from both a tensile test and high-cycle fatigue performance perspective.