| Laser powder bed fusion(LPBF)is one of the most potential additive manufacturing techniques for metal components.It has numerous advantages over traditional subtractive manufacturing processes,such as short lead time,low materials wastage,and high design freedom without usage of tools.High-performance,lightweight alloys have become an integral component of the modern strategies for sustainable development.7××× series aluminium alloys have become popular in aerospace,automobile,and military industries due to their low density,high strength,and good corrosion resistance.With the rapid development of these industries,the requirements for complex and integral products that are difficult to be realized by traditional manufacturing processes have also increased greatly.The potential use of LPBF manufactured aluminum parts as structural materials has attracted widespread attention.However,due to the wide solidification freezing range and poor liquid flowability 7075 aluminum alloy is easily to form solidification cracks during the forming process,which limits its applications.In this present work,7075-Er alloy powder,7075-Zr alloy powder prepared by mechanical ball milling method and 7075-Er-Zr alloy powder prepared by gas atomization method were used to study the effects of Er and Zr elements on the microstructure and properties of LPBF fabricated 7075 aluminum alloy.By adjusting process parameters reduction of solidification cracks was achieve,and the corresponding mechanisms of crack inhibition were discussed.The main conclusions are as follows:(1)Simulation of temperature field during LPBF of 7075 aluminum alloy was performed using the ANSYS APDL software.The influence of laser power and scanning speed on the LPBF thermal cycling,molten pool size,cooling rate and temperature gradient was investigated.The simulation results show that the size and temperature gradient of the molten pool increase with the increase of laser power or the decrease of scanning speed,and the cooling rate of the molten pool increases with the increase of laser power or scanning speed.The temperature gradient along the vertical direction on the cross section of the melt pool is maximum.The simulation results can help to explain the distribution of solidification cracks in the molten pool.(2)The influence of Er element on the processing quality of LPBF fabricated 7075 aluminum alloy was investigated.The evolution of Er and the microstructure in a molten pool was characterized by a scanning electron microscope(SEM),an X-ray diffractometer(XRD)and high-resolution transmission electron microscope(HRTEM).The mechanism of solidification crack inhibition was revealed.The results show that Er exists in two forms:submicron Al3 Er compounds and unmelted particles.Al3 Er acts as a nucleation site which facilitates grain refinement and thus reduces cracking.The presence of Er particles in the molten pool reduces the fluidity of the melt and leads to an increase in the porosity.(3)The influence of Zr elements on the processing quality of LPBF fabricated 7075 aluminum alloy was investigated.The microstructure in the melt pool and the evolution of Zr elements were revealed by SEM and TEM.The results show that the addition of 1 w.t.% Zr element significantly reduces the crack density and increase the relative density with high laser energy density.Due to the presence of Al3 Zr compound,the nucleation of aluminum grains is facilitated and fine equiaxed grains are formed at the bottom of the melt pool boundary for the purpose of crack elimination.However,due to the unmelted Zr particles,the fluidity of the molten pool is reduced,which increased the porosity of the Zr-modified 7075 aluminum alloy.(4)The influence of Er/Zr elements on the density,microstructure,and mechanical properties of pre-alloyed 7075-Er-Zr alloy prepared by gas atomization during LPBF process was investigated.The results show that compared with 7075 aluminum alloy,the relative density of 7075-Ee-Zr alloy was improved from 94.1%～97.3% to 95.2%～97.8%.The formability of7075-Er-Zr alloy was improved,and the processing window was extended to a wider range.The mechanical properties were enhanced.After Er/Zr modification,the number and size of coarse solidification cracks were reduced to an extent,and the number of micro-solidification cracks was significantly reduced.The hardness of 7075-Er-Zr alloy was increased from 119.5 HV to126.9 HV and the compressive strength was increased from 466 MPa to 546 MPa under optimal parameters.Compared with the alloys prepared with ball milling,no unmelted particles in the printed parts were observed in the pre-alloyed modified alloy prepared by the gas atomization,and the structure was more uniform with higher processing quality.The present study will provide a theoretical basis and technical guidance for improving the performance of LPBF fabricated 7075 aluminum alloy. |
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