Microstructural evolution during post heat treatment of the Ti–6Al–4V alloy manufactured by laser powder bed fusion

Abstract

The purpose of the present study is to investigate the effect of post heat treatment as well as the manufacturing strategy on the microstructure evolution during martensite decomposition of Ti–6Al–4V manufactured by laser powder bed fusion (LPBF). The microstructural evolution was tracked using in situ high-energy synchrotron X-ray diffraction and differential thermal analysis. The phase fraction, the mean lattice parameters, d-spacing and the Full Width at Half Maximum (FWHM) variations were determined by Rietveld refinement from XRD patterns during continuous heating up to the single β phase domain with different heating rates and manufacturing strategies. The complementarity of characterization tools clearly evidences a shift of the α/α’ → β transformation kinetics toward higher temperatures as the heating rate increases. The variations in transformation kinetics are discussed with regard to the predicted values at thermodynamic equilibrium. In addition, the combined analysis of d-spacing and FWHM shows the deviations from linearity at intermediate and high temperatures that are related to the heating and manufacturing strategy. These variations were analyzed in regard of internal stress relaxation or changes of chemical composition. Moreover, the anisotropic elastic distortions in martensite α’ caused by the manufacturing strategy were examined.

Keywords

• Laser powder bed fusion
• Ti–6Al–4V titanium Alloy
• Phase transformation kinetics
• Martensite decomposition
• High-energy synchrotron diffraction