Prediction study on in-situ reduction of thermal stress using combined laser beams in laser powder bed fusion

High thermal stress induced by extremely high temperature gradient significantly hinders the development of the laser powder bed fusion (LPBF). In this study, an in-situ stress control method based on the combined laser beams was proposed, in which a second laser beam was added to release the thermal stress during the process. The effect of the combined laser beams on the temperature distribution and thermal stress in LPBF was studied. A definition of the equivalent scan length was adopted to simulate the effect of the scan length. It was found that when the second beam was used, the post-heating strategy can reduce the residual stress better than the preheating strategy. The stress relief effect was close to the scan direction of the second beam and the vertical post-heating pattern was recommended to in-situ reduce the thermal stress. More obvious thermal stress cycles and the thermal stress peaks appeared in the long scan length. The stress release effect caused by the second beam decreased with the increase of the scan length, indicating that the scan length should be also reduced even if the second beam was used. The simulated results were indirectly verified by the experimental data with different re-scanning strategies and scan lengths. The purpose of this paper is expected to significantly in-situ reduce the thermal stress during the process without increasing the manufacturing time.