Evolutions of distortion and residual stress in laser powder bed fusion based on assumption of constraining force

Abstract

Laser powder bed fusion is a promising technique that can produce complex-shaped and integrated part. However, distortion and residual stress are two issues that may decrease the precision and performance of built parts. Classical thermal gradient mechanism offers a basic interpretation toward distortion and residual stress. The evolutions of distortion and residual stress in laser powder bed fusion remain unclear. In this study, we used a physical model with an assumption of constraining force to illustrate the evolutions of distortion and residual stress during the additive process. Based on the model, we are able to understand the phenomenon of X-directional shrinkage, Z-directional distortion, and “tensile-compressive-tensile” distribution of X-directional stresses at the same time. It can be concluded that the shrinkage, distortion, and X-directional residual stress all result from the constraint between previously deposited layer and newly deposited layer, which has a strong shrinkage tendency when cooling. The distortion of part increases with deposition height, especially during first several layers. The “tensile-compressive-tensile” distribution of X-directional stresses can be maintained during the additive process. The magnitude of top tensile stress remains stable, while the tensile stress at bottom increases with the deposition height. This work provides a comprehensive understanding toward the evolutions of distortion and residual stress in laser powder bed fusion.