A new insight on the analysis of residual stresses related distortions in selective laser melting of Ti-6Al-4V using the improved bridge curvature method

Abstract

Residual stresses are a major issue in the Selective Laser Melting process, which allows to build near net shape parts by stacking several thousand weld beads. This experimental study aims to investigate the relationships between process parameters and the part distortion. Based on the Bridge Curvature Method (BCM) and 3D topographic measurements of the top surface of the specimens, the effect of the volumetric energy density, the geometry and the scanning strategy are analyzed. The method and the proposed distortion criteria clearly highlight the anisotropy of the residual stress state, which depends on the direction and the speed of the scanning. The main curvature is oriented along scan direction for scanning speed higher than 1000 mm/s. The decrease in volumetric energy increases distortion, below a threshold value. The scan vector length, which can be varied by changing the scanning strategy and the specimen geometry, plays a primordial role on the residual stresses. The distortion amplitude decreases by shortening the scan vectors. Finally, the prospect of an adimensional analytical model taking into account the transient thermal gradient is given.