Microstructure and Mechanical Properties of Al-Mn-Mg-Sc-Zr Alloy Prepared by Selective Laser Melting with Different Heat Treatment
In recent years,the preparation of high strength aluminium alloys using selective laser melting(SLM)process has re-ceived increasing attention.In order to obtain SLM-formed Al-Mn-Mg-Sc-Zr alloys with good mechanical properties,the best heat treat-ment process was obtained to understand the changes in microstructure and mechanical properties before and after heat treatment.Al-Mn-Mg-Sc-Zr alloy was prepared by SLM,the heat treatment process was optimized based on microhardness and tensile properties tests,and the effect of the optimized heat treatment process on the microstructure and properties of the alloy was analyzed using scan-ning electron microscopy(SEM)and transmission electron microscopy(TEM).The microstructure of the as-deposited samples and the alloy samples obtained from the optimum heat treatment process were compared.SEM was used for microstructure observation and the accompanying electron backscattering spectrometer(EBSD)and electron emission channel correlation(ECC)were used for grain size and phase distribution analysis.TEM and the accompanying energy spectrometer(EDS)were used to analyze the internal grain and in-tergranular precipitation phases composition.The microstructure of heat-treated alloy samples was observed and compared with as-de-posited samples.It could be determined that the characteristic columnar-equiaxed crystal structure was preserved,but the second phase particles were coarsened and precipitated more.Combining the characteristics of crystal morphology and the effect of heat on crystal growth,the reasons for the changes in the morphology and quantity of the second phase were proposed.Combined with EBSD analysis,the phenomenon that second-phase particles existed both at grain boundaries and in specific directions within grains was ex-plained.The second phase particles in the columnar crystal region were observed using TEM.Combined with EDS analysis,it was de-termined that the second phase particles in the as-deposited alloy was Al6Mn phase.The second phase particles in the equiaxed crystal region were observed and analyzed,and the existence of Sc-rich phase was found.Further,the diffraction lattice obtained by TEM was measured.This enabled the lattice constant of 0.417 nm of Sc-rich phase to be obtained,and became the basis for the determination of Al3Sc phase.Comparing the tensile stress-strain curves of the as-deposited and heat-treated samples at room temperature,it could be clearly seen that the strength of the alloy after heat treatment was significantly improved,and the plasticity did not decrease.Through further observation,it could be found that there was a certain fluctuation in the curve of the plastic deformation stage of the as-deposit-ed sample,which had obvious subsidence after heat treatment,and the similar phenomena were widely found in similar alloy systems.To explain this phenomenon,TEM images of the sample before and after heat treatment were compared,and it was found that the heat treatment reduced the dislocation density by reducing the internal stress.The tensile strength of the alloy specimen in the as-deposited sample was(458±0.7)MPa and the elongation after fracture was 15.4％±0.3％,while the tensile strength of the specimen after heat treatment was(556±2.3)MPa and the elongation after fracture was 14.2％±1.2％.The grain size of the alloy was relatively small,with columnar crystal size of 4～8 μm and equiaxed crystal less than 3 μm,with no significant grain growth before and after heat treatment.As-deposited alloy contained a large number of sub-micron short rod-shaped Al6Mn phases between the grains and a large number of point-like particles within the grains,which were mainly Al6Mn and Al3Sc phases,with Al3Sc precipitating around Al6Mn phase.It was believed that the reduction of internal stress and dislocation density ensured the plasticity of the alloy.At the same time,the secondary precipitation of the second phase particles destroyed the anisotropy of the columnar crystals on the one hand,and aggravated the effect of the second phase strengthening on the other hand,which was believed to be the reason for the increase in strength after heat treat-ment without affecting the plasticity.
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