A Numerical and Experimental Study for Residual Stress Evolution in Low Alloy Steel during Laser Aided Additive Manufacturing Process.

Abstract

One of the challenges in laser aided Direct Metal Deposition (DMD) process is control of the residual stress generated during the process due to thermal loads and solid state phase transformation. However, in situ residual stress monitoring in DMD process, used as an on-line sensor for a feedback control system, is difficult and also requires relatively high cost to accurately monitor mechanical deformations. Therefore, a fundamental understanding of the correlations between processing variables and the thermal and mechanical behaviors of material in DMD process is essential, because the residual stress field can be controlled in the stage of developing laser tool paths and the corresponding processing parameters. Mathematical models in DMD process are developed and utilized to obtain the correlations, rather than performing a series of experiments in the study. A self-consistent transient 3-D model is adopted to predict thermal behaviors and the model is experimentally validated by comparing temperature history, melt pool flow, and deposition geometry with different processing parameters. The results from the thermal model are used to predict mechanical deformations in DMD process using a commercial software package ABAQUS with proper user subroutines. X-ray diffraction residual stress measurements are conducted for validation purpose, and the validated mathematical model is utilized to explain the evolution of stress in DMD process and to investigate the effects of processing parameters on the residual stress. The considered processing variables are metal powder flow rate, laser power, scanning speed, scanning direction, and deposition layer thickness. The residual stress is determined in three stages: thermal expansion by a heat source, restoration by melting, and thermal contraction by cooling, and the residual stress can be controlled by altering melt pool geometry with processing variables due to the dependence of residual stress on melt pool geometry. The most significant factor to determine the magnitude of the residual stress is the melt pool penetration to the substrate and the most influential parameter defining the residual stress profile along the depth direction is the amount of energy density.