Numerical simulation and experimental study of laser powder bed fusion for directional DZ125 alloy repair

Laser powder bed fusion （LPBF） technology offers significant advantages， including high flexibility， no mold requirements， and rapid manufacturing capabilities， so it is well-suited for repairing complex and precision components such as aero-engine blades. It is difficult to efficiently and accurately reveal the evolution rules of defects and microstructures in the multi-scale and multi-physical field coupling LPBF process through experimental methods only. The finite volume method and a cellular automaton model were used to simulate the morphological evolution of the powder bed melt pool and the corresponding microstructure formation process. Combined with experimental observations， the evolution rules of metallurgical defects in the alloy and grain growth under different printing parameters are revealed. The results indicate that during the LPBF repair process， energy density significantly affects the morphology of the melt pool. When the energy density is less than 87.9 J/mm³， the powder particles are not completely melted， accompanied by the formation of defects such as pores and unmelted areas. When the energy density is greater than 137.4 J/mm³， the surface smoothness of the solidified melt pool is significantly reduced. The increase in energy density enhances the horizontal thermal flow disturbance in the melt pool， and the crystals are affected by shear forces， leading to a greater orientation difference with the substrate. Additionally， the laser power significantly affects the microstructure of the alloy. As the laser power increases， the temperature gradient gradually decreases. The low temperature gradient promotes the formation of the supercooled liquid region， which in turn facilitates the formation of new crystal nuclei. When the laser power increases from 150 W to 250 W， the epitaxial growth grains change from columnar crystals to a large number of polycrystalline grains. Based on the numerical simulation methods， the optimal process parameters for repairing DZ125 alloy by LPBF are determined as follows：P=200 W， V=1000 mm/s， and H=65 μm. This method helps to reduce experimental costs and accelerate the acquisition of reasonable process parameters for LPBF repair of alloys.