Study On Heat Flow Transfer And Defect Evolution Of Laser Powder Bed Fusion Of Aluminum Alloys

The build quality of laser Powder Bed Fusion(PBF)components largely depends on the possible defects in the printing process.The complex heat transfer and defect evolution of high strength aluminum alloy Al2024 were researched by numerical simulation and experiments.A multi-physical field numerical model based on discrete powder was developed to calculate the critical metallurgical variables such as the molten pool characteristics and thermal cycle during the laser moving process.Meanwhile,the single track,multi-track and multi-layer samples were prepared by experiment to validate the simulation model and the crack characteristics were further studied.Two types of inter-track pores were found.According to its formation mechanism,the first type appears as an irregular long strip,and the second type appears as subspherical.The former was due to the incomplete melting of powders,and the latter was due to the gas bubbles which were not removed in time in the overlapping areas of adjacent tracks.Compared with Al Si10 Mg,Al2024 has a wider temperature range and higher thermal conductivity.More heat needs to be absorbed during the melting process.Improper energy input was more likely to result in unfused defects in Al2024.The mode of solidification crack generation and propagation was analyzed by using different scanning strategies and the overlap ratio of adjacent tracks.With the increase of the overlap rate of adjacent tracks,the crack propagation patterns transited from the longitudinaldominant to the transverse-dominant mode.Besides,the lengths and propagation angles of the transverse cracks were increased with the overlap rate of adjacent tracks.For the dominant mode of transverse crack,the bidirectional laser scanning cases produced zigzag crack,while the unidirectional laser scanning cases produced similar crack propagation direction.These diverse crack propagation patterns resulted from the localized solidification conditions when the molten pool moves along the scanning direction.A method to eliminate the defects of LPBF builds was proposed considering the heat input,the overlap rate,and the laser scanning strategies.The research in this paper is conducive to a deeper understanding of the defect generation and evolution of Al2024 in the LPBF process,and the research method can be extended to other materials.