Modeling Phase Transformation for Residual Stress Study in Selective Laser Melting Processes by Finite Element/Matlab Integration

Abstract

Selective laser melting (SLM) has been identified as a promising manufacturing technology for modern additive fabrication. However, this process results in powder sintering/melting during fabrication. Due to enormous thermo-mechanical mismatch, consequently, significant structural distortion and thermal stress would be expected.  Without carefully analyzing the stress and deformation during SLS, the final product could suffer from excessive distortion or strong residual stress. However, the thermo-mechanical properties of powder and sintered materials has significant difference and the phase transformation depends directly with processing parameters. Current finite element analysis usually needs to pre-assign the material properties and lacks the ability for performing effective material change during simulation. Without this capability, the achieved stress analysis results could be questionable due to incorrect constraints on structural deformation attributed to material Young’s modulus, for example. In this work, a rational flow for solving the above problem is proposed by integrating finite element package ABAQUS, Matlab, FORTRAN, and Python. The entire program is constructed as a Matlab code, where ABAQUS FE analysis is a major subroutine. In together with FORTRAN, it is responsible for obtaining the temperature and stress of each node/element. For each step, the Matlab main program makes judgement to determine if the material of elements should be changed from powder to sintered state based on the temperature distribution and other judgement criteria. These decisions are then used to re-compose the FE input files using Python and the new input files are then served as the FE simulation for the next step. By this approach, it is possible to adequately describe the phase change behavior and leads to a more reliable subsequent stress analysis for solving the residual stress and distortion problems commonly faced in SLS processes. Technically, this approach can also be applied to other ad hoc problems such as wafer polishing or wearing of mechanical probes.