Study On Microstructure And Mechanical Properties Of Al-Mn-Mg-Sc-Zr Alloys Fabricated By Selective Laser Melting

As an engineering structural material that can be produced on an industrial scale at present,aluminum alloy has the advantages of high specific strength,high specific stiffness,excellent casting performance and recycling,and is widely used in aerospace,automobile manufacturing and other fields.Selective laser melting（SLM）technology has the advantages of high processing freedom,short manufacturing cycle,direct forming of parts and simple post-processing process,which can expand the application range of aluminum alloy.SLM technology has a fast cooling rate,and the internal stress of the formed alloy is large,which is prone to deformation,cracks and other defects.At present,there are few reports on SLM Al-Mn alloys,and no systematic research has been formed.Therefore,it is of great significance to carry out research on related microstructure and properties.In this paper,Al-Mn-Mg-Sc-Zr alloy was prepared by selective laser melting.The microstructure of Al-Mn-Mg-Sc-Zr alloy before and after heat treatment was characterized by X-ray diffractometer（XRD）,scanning electron microscope（SEM）,energy dispersive spectrometer（EDS）and transmission electron microscope（TEM）.The mechanical properties of the experimental alloy were tested,and the existence form and mechanism of Sc element in the alloy were analyzed.The evolution behavior of precipitated phases in the microstructure was studied by in-situ TEM experiment,and the morphology,size,number of precipitated phases and their relationship with the precipitated interface were analyzed.The stability,electronic properties,mechanical properties and thermodynamic properties of the main second phases in the experimental alloys were calculated by first-principles.The results show that:（1）The microstructure of the as-deposited alloy is mainly composed ofα-Al,Al₆Mn and primary Al₃Sc.The microstructure of the alloy molten pool formed by SLM is’fish scale’,and a large number of fine equiaxed grains are formed near the fusion line in the molten pool.The average grain size is about 0.57μm,and the core of the molten pool is composed of columnar crystals.The rod-like Al₆Mn is mainly distributed along the grain boundary,and a small amount of granular primary Al₃Sc exists inside the grain,which indicates that the primary Al₃Sc can be used as heterogeneous nucleation particles to refine the matrixα-Al.（2）With the extension of aging time,the hardness of the alloy increases first and then decreases under different temperature conditions.After 325°C×180 min treatment,the hardness of the experimental alloy is about 30%higher than that of the as-deposited alloy,reaching 185.0 HV.After 375°C×40 min treatment,the tensile strength and yield strength of the experimental alloy reached 536 MPa and 511 MPa,respectively,which were 28.2%and 42.3%higher than those of the as-deposited experimental alloy,and the elongation decreased to 11.0%.After heat treatment,the size of equiaxed grains and the width of columnar grains increased,and a large number of fine and dispersed secondary Al₃Sc particles precipitated in the microstructure.（3）The in-situ heating TEM study of the experimental alloy shows that the lower aging temperature is difficult to promote the precipitation of Al₃Sc and Al₆Mn precipitates.During the two-stage aging process,the needle-like Al₆Mn precipitated phase begins to precipitate from the matrix,and its size and length are mostly in the range of 100～500 nm and evenly distributed in the matrix.With the extension of aging time,the number and size of Al₆Mn precipitates increased in different degrees,and the recrystallized grain size also increased in this process.（4）The first-principles calculation results show that both Al₃Sc and Al₆Mn phases can stably exist in the alloy structure.At the same time,both of them are hard phases,which are helpful to improve the mechanical properties of the alloy.The free energy of Al₆Mn is lower than that of Al₃Sc in the temperature range of 85 K～800 K,which is easier to form.