Selective Laser Melting Produced Ti-6Al-4V: Post-Process Heat Treatments to Achieve Superior Tensile Properties.

Gerrit Ter Haar,Thorsten Becker

doi:10.3390/ma11010146

Abstract

Current post-process heat treatments applied to selective laser melting produced Ti-6Al-4V do not achieve the same microstructure and therefore superior tensile behaviour of thermomechanical processed wrought Ti-6Al-4V. Due to the growing demand for selective laser melting produced parts in industry, research and development towards improved mechanical properties is ongoing. This study is aimed at developing post-process annealing strategies to improve tensile behaviour of selective laser melting produced Ti-6Al-4V parts. Optical and electron microscopy was used to study α grain morphology as a function of annealing temperature, hold time and cooling rate. Quasi-static uniaxial tensile tests were used to measure tensile behaviour of different annealed parts. It was found that elongated α’/α grains can be fragmented into equiaxial grains through applying a high temperature annealing strategy. It is shown that bi-modal microstructures achieve a superior tensile ductility to current heat treated selective laser melting produced Ti-6Al-4V samples.

Highlights

Since the emergence of selective laser melting (SLM) as a humble rapid prototyping technology in2006, the process has advanced exponentially to a point where it has become the most broadly used powder-bed fusion manufacturing process in industry [1]
This study aims to explain grain morphological transformation as a function of annealing temperature, holding time and cooling rate of SLM produced Ti-6Al-4V to contribute to the understanding of the mechanism of grain morphology transformation in the solid solution temperature region (SSTR)
Results obtained for group D agree with ductility achieved by solid solution heat treatments Tion

Summary

Introduction

Since the emergence of selective laser melting (SLM) as a humble rapid prototyping technology in2006, the process has advanced exponentially to a point where it has become the most broadly used powder-bed fusion manufacturing process in industry [1]. Considerable attention has been directed toward Ti-6Al-4V, a 6% aluminium, 4% vanadium titanium alloy that is considered the ‘workhorse’ in the titanium industry due to its excellent material properties. SLM produced Ti-6Al-4V parts in an as-built state are unable to achieve the high material performance of its wrought counterparts [3,4]. The major major contribution contribution being being the the entropy entropy or or chemical chemical potential potential energy energy of of alloying alloying elementals elementals in in the the respective α and β phases [26]. Change in α/β phase fraction through phase transformation occurs respective α β phases α/β phase fraction through phase transformation occurs as nucleation and growth of the phase until a stable/equilibrium alloy concentration is reached [40] Gibbsfree free energy energy [26,37,38,39]. [26,37,38,39].

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