The effect of heat treatment on microstructure, microhardness, and pitting corrosion of Ti6Al4V produced by electron beam melting additive manufacturing process

Mingzhen Xiu,Srinivasan Raghavan,Min Hao Goh,Yong Teck Tan,Mui Ling Sharon Nai

doi:10.1007/s00170-022-08839-4

Abstract

There has been limited studies on corrosion behaviour of post-processed electron beam melted (EBM) Ti6Al4V, given that the factors affecting corrosion resistance of AM Ti6Al4V remain unclear. This paper proposes using heat treatment method to improve the pitting corrosion resistance of EBM Ti6Al4V. Different treatment profiles alter the microstructure of EBM Ti6Al4V. A clear trend is observed between microhardness and α lath width. As-printed EBM Ti6Al4V exhibits an inferior pitting potential, while heat treatment provided a significant improvement in the corrosion resistance. This study finds that the β phase fraction is a better indicator than the α lath width for pitting corrosion resistance. Solution air-cooled and ageing heat-treated EBM Ti6Al4V exhibits good mechanical and corrosion properties, and even performs better than commercial cast Ti6Al4V.

Highlights

Additive manufacturing (AM) is an attractive and effective manufacturing technique used in producing complex components or prototypes
The Scanning Electron Microscope (SEM) image (Fig. 1(c)) presents a lamellar phases microstructure that the dark background is α phase and the bright strip is β phase, it is a typical as-printed Electron beam melting (EBM) Ti6Al4V microstructure pattern [34]. α lath is identified as the spacing between the two bright β phases that is an important factor to determine the mechanical properties of EBM Ti6Al4V
Since the α phase will not completely transform into the β phase below the transus temperature according to the phase diagram, the presence of the α phase will hinder the growth of β grains given that the columnar morphology remains unchanged in the sub -transus heat treatment

Summary

Introduction

Additive manufacturing (AM) is an attractive and effective manufacturing technique used in producing complex components or prototypes. AM processing has a shortcoming of rapid inherent directional solidification, leading to highly non-equilibrium microstructure of the as-printed metal part [3] Such unique solidification process gives that the formation of martensite phase and columnar grains in titanium alloys [2, 4, 5]. EBM generates a high energy electron beam from a tungsten filament electron gun which can accelerate to 60 kV and selectively melt the titanium powders. It operates in a controlled vacuum (~2×10-3 mbar) and elevated temperature condition to avoid oxidation and internal pressure during processing. EBM parts can be found in multiple applications in orthopaedic [6], aeronautics, and motorsports [7], with the mechanical behaviour of as-printed EBM metal parts comparable or even better than traditional processed metal parts [4, 8]

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