| Al-Mg alloy is widely used in vehicle manufacturing,electronic and electrical engineering,aerospace and other fields because of its high specific strength,excellent plasticity and weldability.However,it is difficult to obtain Al-Mg alloy with complex shapes or high strength and toughness by traditional processing methods.As one of the metal 3D printing technologies,selective laser melting（SLM）technology can not only prepare geometrically complex metal components fast and accurately to achieve"free manufacturing",but also shorten processing time and save processing costs.At the same time,in order to meet the demand for higher strength A1-Mg alloys in the aerospace field,the alloy properties were improved by introducing in-situ nanophases（co-addition of Sc and Zr）,and the relationship between master ingot alloy,vacuum gas atomized powder and as-built alloy was explored.In this work,the Al-Mg-Sc-Zr alloy with high strength and toughness was prepared by the routine of master ingot alloy melting-powder producing by vacuum gas atomization-SLM forming.The characteristics of this work are verifying the improvement of the properties of Al-Mg-Sc-Zr alloy with co-addition of Sc and Zr by casting method,completing the preliminary screening of the master ingot alloy;producing the powder with more uniform composition and better compatibility with 3D printing compared to directly melting of intermediate alloys for vacuum gas atomization;using SLM technology to print the powder into alloy components afterwards;and finally carrying out the aging treatment of the as-built alloy to improve the strength of the alloy.The microstructure and mechanical properties of Al-Mg alloy and Al-Mg-Sc-Zr alloy were compared and analyzed.The phase structure evolution of Al-Mg-Sc-Zr alloy was studied by X-ray diffraction（XRD）.The grain morphologies and precipitation phases evolution of Al-Mg-Sc-Zr alloy were characterized by field emission scanning electron microscopy（FESEM）,electron backscatter diffraction（EBSD）and transmission electron microscopy（TEM）,and the strengthening mechanism of the alloy was discussed.The primary Al3（Sc,Zr）phases precipitated during the melt solidification process of Al-Mg-Sc-Zr master ingot alloy has a similar lattice structure toα-Al,which provides effective nucleation particles forα-Al and enables the refinement of grains,the average grain size of Al-Mg master ingot alloy and Al-Mg-Sc-Zr master ingot alloy being 33.16μm and 23.70μm,respectively.The tensile test results showed that the ultimate tensile strength（UTS）,yield strength（YS）and elongation（EL）of Al-Mg-Sc-Zr ingot alloy are 249.2MPa,180.3MPa and2.3%,respectively,which are 16.2%and 44.4%higher than UTS and YS of Al-Mg master ingot alloy,respectively.Sc and Zr additions eliminate the intergranular non-equilibrium eutectic phases in the Al-Mg-Sc-Zr master ingot alloy for a more homogeneous alloy composition.Al-Mg-Sc-Zr alloy powder prepared by vacuum gas atomization method has excellent properties,with loose density of 1.25g/cm3,beating density of 1.56g/cm3,N/O content（wt.%）of0.020/0.002,sphericity of 0.93and rest angle of 33.0°.Due to the homogeneous composition of the master ingot alloy and the high cooling rate of vacuum gas atomization,all the alloying elements are dissolved in theα-Al matrix,resulting in uniform powder composition.As-built Al-Mg-Sc-Zr alloy has a bimodal grain structure,consisting of equiaxed crystals in the fine grain area and columnar crystals in the coarse grain area.The rapid cooling during SLM technology processing and the heterogeneous nucleation ofα-Al on Al3（Sc,Zr）significantly refines the grains.The EBSD analysis results showed that the average grain size of as-built Al-Mg-Sc-Zr alloy decreased compared to that of as-built Al-Mg alloy:the average grain size of the fine grain area decreased from 1.37μm to 1.34μm,and the proportion of fine grain area increased from 33.74%to 50.51%;the average grain size of the coarse grain area decreased from 6.43μm to 4.33μm,and the proportion of coarse grain area decreased from66.26%to 49.49%.The addition of Sc and Zr increased the solid solubility of Al-Mg-Sc-Zr alloy and the density of nano-precipitated phases in the alloy.Due to the combined effects of fine-grained strengthening,solution strengthening and Orowan strengthening,the UTS,YS and EL of Al-Mg-Sc-Zr alloy in the as-built state were 345.0MPa,282.3MPa and 12.0%,respectively.However,the coarse grains and high defect density of Al-Mg alloy in the as-built state led to poor mechanical properties,and the UTS,YS and EL were 96.8MPa,95.2MPa and0.72%,respectively.After aging treatment at 325℃/4h,the secondary Al3（Sc,Zr）phases precipitated from the aged Al-Mg-Sc-Zr alloy,and the primary Al3（Sc,Zr）phases grew.The average size of the precipitated phases was 58.3nm,slightly bigger compared to that of the precipitated phases（40.6nm）in the as-built Al-Mg-Sc-Zr alloy,the corresponding volume fractions being 2.220%and 0.932%,respectively.The nano-Al3（Sc,Zr）co-latticed withα-Al has a strong pinning effect on the grain boundary of the alloy,effectively inhibiting the grain growth during aging.statistical results show that for the fine grain area of Al-Mg-Sc-Zr alloy aged at 325℃/4h,the average grain size is 1.35μm,and the fine grain area accounts for 50.41%;The average grain size of the coarse grain area was 4.99μm,and the coarse grain area accounted for 49.59%.Compared with the as-built Al-Mg-Sc-Zr alloy,the aging treatment significantly improves the strength of Al-Mg-Sc-Zr alloy.With The duration of the aging process fixed at 4h,and as aging temperature（275℃,325℃ and 375℃）rises,the YS first increases and then decreases,the optimal aging temperature being 325℃.With the extension of aging time（2h,4h,6h,8h）,the UTS first increases and then decreases,and optimal mechanical performance was obtained in 6h sample.The UTS,YS and EL of said sample were 453.7MPa,418.4MPa and 5.7%respectively,whereas compared with that of as-built Al-Mg-Sc-Zr alloy,the UTS and YS increased by 31.5%and 48.2%respectively. |
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