Internal Stress Evolution and Subsurface Phase Transformation in Titanium Parts Manufactured by Laser Powder Bed Fusion - An In Situ X-Ray Diffraction Study

Laser powder bed fusion (LPBF) is a metal additive manufacturing technology, which enables the manufacturing of complex geometries for various metals and alloys. Herein, parts made from commercially pure titanium are studied using in situ synchrotron radiation diffraction experiments. Both the phase transformation and the internal stress buildup are evaluated depending on the processing parameters. For this purpose, evaluation approaches for both temperature and internal stresses from in situ diffraction patterns are presented. Four different parameter sets with varying energy inputs and laser scanning strategies are investigated. A combination of a low laser power and scanning speed leads to a more homogeneous stress distribution in the observed gauge volumes. The results show that the phase transformation is triggered during the primary melting and solidification of the powder and subsurface layers. Furthermore, the stress buildup as a function of the part height during the manufacturing process is clarified. A stress maximum is formed below the part surface, extending into deeper layers with increasing laser power. A temperature evaluation approach for absolute internal stresses shows that directional stresses decrease sharply during laser impact and reach their previous magnitude again during cooling.