Purpose The purpose of this study is to develop a novel comprehensive three-dimensional computational model to predict the transient thermal behavior and residual stresses resulting from the layer-by-layer deposition in the direct metal laser sintering process. Design/methodology/approach In the proposed model, time integration is performed with an implicit scheme. The equations for heat transfer are discretized by a finite volume method with thermophysical properties of the metal powder and an updated convection coefficient at each time step. The model includes convective and radiative boundary conditions for the exposed surfaces of the part and constant temperatures for the bottom surface on the build plate. The laser source is modeled as a moving radiative heat flux along the scanning pattern, while the thermal gradients are used to calculate directional and von Mises residual thermal stresses by using a quasi-steady state assumption. Findings In this study, four different scanning patterns are analyzed, and the transient temperature and residual thermal stress fields are evaluated from these patterns. It is found that the highest stresses occur where the laser last leaves off on its scanning pattern for each layer. Originality/value The proposed model is designed to capture the layer-by-layer deposition for a three-dimensional geometry while considering the effect of the instantaneous melting of the powder, melt pool, dynamic calculation of thermophysical properties, ease of parametrization of various process parameters and the vectorization of the code for computational efficiency. This versatile model can be used for process parameter optimization of other laser powder bed fusion additive manufacturing techniques. Furthermore, the proposed approach can be used for analyzing different scanning patterns.

Laser powder bed fusion (LPBF) is an Additive Manufacturing (AM) process that uses a laser beam to solidify powder particles following a predefined pattern on a powder bed to build a part layer-by-layer as per a CAD model. In LPBF, the moving heat source and the rapid solidification cause nonuniform variations in temperature along the build part. This transient and moving heating and cooling process causes uneven expansion and shrinkage of the part that leads to the development of residual stresses in the part. The residual stresses depend on the thermal history of the part and may eventually lead to part distortion, crack initiation, warpage, etc. The present study represents the effect of altering the scan pattern layer-by-layer on the residual stress. Furthermore, a novel alternating double pass spiral scan pattern is introduced and compared with alternating island zigzag, alternating zigzag, regular zigzag, and spiral out-center patterns on the basis of thermal distribution and residual stress. Numerical approach is used to solve the governing equations. It is observed that residual stress greatly depends on thermal distribution. The variation in inherent residual stress is found to be lower for alternating zigzag pattern than regular zigzag pattern due to more even thermal distribution. Furthermore, the novel pattern also effectively distributes the heat which contributes to the reduction of inherent residual stress from the first layer. On the other hand, it is found that the alternating island zigzag scan pattern increases island temperature that can prevent rapid solidification. Overall, the findings of this study can be helpful in understanding the effects of altering scan directions layer-by-layer and in identifying a scan strategy that can enhance the usability of the powder bed fusion additive manufacturing technology.

In this paper, maximum stresses from the Direct Metal Laser Sintering (DMLS) process are numerically calculated for each layer using a novel computational model that has been developed to capture the layer-by-layer deposition. The computational domain with all layers is modeled numerically with conduction, while using convection and radiation on the model boundaries. The phase change of the material between liquid and solid states is accounted for and the residual thermal stresses are obtained from the temperature gradient data in conjunction with Hooke’s law. The resulting maximum stress versus time behavior and maximum stress distribution patterns on each layer are complex and do not always match the scanning path. However, there is direct correspondence between the stress distribution and the scanning patterns.