Microstructure And Mechanical Properties Of Al-Fe-Sc-Zr Heat-resistant Al Alloys Printed By Laser Powder Bed Melting

Laser additive manufacturing of heat resistant aluminum alloy has important demand and application prospect in aviation,aerospace and rail transportation fields.However,extreme non-equilibrium solidification and repeated thermal cycling in additive manufacturing process can easily lead to the cracking of formed samples,so it is urgent to develop special heat resistant aluminum alloy for additive manufacturing.It should not only meet the"printability",that is,no cracking;And to meet the"high heat resistance",that is,excellent high temperature performance（200-400℃）.Based on this,a heat-resistant Al-5Fe-1Mg-0.8Sc-0.7Zr alloy for laser powder bed melting（LPBF）additive manufacturing was designed in this study.The design of this non-equilibrium alloy is based on:（1）During the laser rapid solidification process,the solid solubility of Fe element in aluminum alloy will increase significantly,forming supersaturated solid solution;（2）Using Sc and Zr to refine the microstructure can inhibit the generation of microcracks and improve the strength of the alloy.The main conclusions and innovations of this paper are as follows.（1）Optimized the printing parameters,it is found that when the laser energy density is 83.3 J/mm³,a better density of 99.2%can be obtained.The microstructure is characterized by ultrafine equiaxed grains at the molten pool boundary（0.2-1μm）and coarse columnar grains（1-2μm in width and 10-20μm in length）.At the molten pool boundary,Al₆Fe,Al₁₃Fe₄and primary Al₃（Sc,Zr）precipitates are abundant aroundα-Al grains.The eutectic structure ofα-Al and Al6Fe is found in the molten pool.The maximum solid solubility of Fe in aluminum matrix is 5.58%.（2）The microstructure evolution of LPBF alloy during non-equilibrium solidification and high temperature thermal exposure was revealed.During the non-equilibrium solidification process,Al₁₃Fe₄phase and primary Al₃（Sc,Zr）phase are firstly formed at the solid-liquid interface,whileα-Al and Al₆Fe eutectic structures are gradually formed in the center of the molten pool in the remaining liquid phase.However,under high temperature thermal exposure,Al₆Fe particles gradually coarsened and began to dissolve in the matrix above 300℃,and the steady Al₁₃Fe₄phase would precipitate and grow in the supersaturated matrix above 350℃.After 200 hours of thermal exposure at 400℃,the microstructure of the sample is completely thermal stabilized,and the inner and boundary of the molten pool are Al₁₃Fe₄needle-like microstructure.（3）Different strengthening mechanisms at room temperature and high temperature were analyzed,and their contribution values were calculated.The yield strength of the sample at room temperature is 396MPa,the tensile strength is 489 MPa and the elongation is 3.7%.At200℃,the yield strength is 300 MPa,the tensile strength is 376 MPa and the elongation is 5.8%;at 300℃,the yield strength is 237 MPa,the tensile strength is 260 MPa and the elongation is 7.6%.The strength at room temperature and high temperature is 20-50 MPa higher than that of LPBF Al-2.5Fe and Al-15Fe alloys.The strengthening mechanism at room temperature is solid solution strengthening,fine grain strengthening and precipitation strengthening.The calculated total contribution strength is 450 MPa,which is slightly higher than the actual yield strength of 396MPa.Because the fine grain strengthening almost fails and the solid solution strengthening partially fails at high temperature,the coarsening and growth of precipitated particles can make up for most of the matrix softening caused by high temperature.The calculated total contribution strength at 300℃is 227 MPa,which is slightly lower than the actual strength of 237 MPa.