Current research status on Ti–6Al–4V heat treatment through laser powder bed fusion: microstructure evolution and corrosion resistance

Abstract

Laser powder bed fusion (LPBF), an additive manufacturing technology, is extensively applied in various fields. In the building process, one or more laser beams scan and melt the powder previously deposited on the build platform, following a prescribed scanning path to achieve the designed three-dimensional geometry. After the current powder layer is selectively melted and solidified, another layer of powder is spread, and the laser scanning continues. Such repetitive operations exhibit inherent rapid heating and cooling characteristics, resulting in uneven microstructures and the accumulation of internal stresses for the produced parts. For the most widely used Ti–6Al–4V, this LPBF characteristic forms fine needle-like α′ martensite and small amounts of beta phase. There is a phase potential difference between the α′ and the β phases, which constitutes a corrosive galvanic battery, and the corrosion reaction preferentially occurs at the phase interface of the α′ and the β phases. However, as a metastable structure, the thermodynamic stability of the α′ phase is lower than that of the β phase, which makes the α′ phase preferentially corroded. Moreover, the V element enrichment in the β phase imparts stability, exerting a significant inhibitory effect on the corrosion and dissolution of the Ti–6Al–4V alloy. This is evident in the denser and more stable passive film formed on the β phase compared with that on the α′ or α phase, accompanied by a corresponding increase in corrosion resistance. To better meet different service conditions, it is imperative to regulate its microstructure and alleviate residual stresses through heat treatment as a heat-treatable α + β type titanium alloy. However, the optimal heat treatment conditions for LPBF Ti–6Al–4V are being determined. We aim to provide a comprehensive overview of the current literature, focusing on the perspective that the properties of a material are inherently influenced by its microstructure. Specifically, the correlation between microstructure and corrosion resistance in LPBF-produced Ti–6Al–4V before and after heat treatment is explored. The influence of post-heat treatment on the microstructure and corrosion behavior of LPBF-produced Ti–6Al–4V is also discussed. Results reveal that appropriate heat treatment can facilitate the decomposition of fine needle-like α′ martensite and the formation of β phases, enhancing the corrosion resistance of LPBF-produced Ti–6Al–4V. However, excessive heat treatment will increase the grain size of the LPBF Ti–6Al–4V and deteriorate the corrosion resistance. Finally, conclusions are drawn, which is expected to bring some support to researchers.

Keywords

• laser powder bed fusion
• heat treatment
• titanium alloy
• microstructure
• corrosion resistance