| Selective laser melting（SLM）,an emerging additive manufacturing technology,is widely used in the processing of complex shaped parts.Al-Si alloy is the most commonly used system for selective laser melting.With the increasing demand for lightweight and high strength aluminum matrix composites in the manufacturing industry,traditional Al-Si alloys produced by SLM face defects such as pores and hot cracks,which make it difficult to meet industrial needs.Developing the Al-Si alloy system with excellent mechanical property and processability has become an important task in solving the problem in application of Al-Si alloys produced by SLM.In this paper,the design of introducing external nano phases and in-suit nano phases was selected.Firstly,the AlSi7Mg alloy master ingots were prepared by stirring casting method,which realized the optimization of matrix composition and pre-dispersion of nano phases.Subsequently,AlSi7Mg powders with high sphericity and without deformation were prepared by gas atomization.AlSi7Mg alloys with fine grains,dispersed precipitates,and excellent mechanical properties were prepared by SLM process.Release the residual stress and reduce brittleness of as-built alloy by annealing.The phase structures of powders and alloys were characterized by X-ray diffraction（XRD）.The micrographs of powders and alloys were characterized by optical microscope（OM）and scanning electron microscope（SEM）.The nano precipitates,dislocations and twins of as-built alloys were characterized by transmission electron microscope（TEM）.The recrystallization behaviors in annealing were characterized by electron backscatter diffraction（EBSD）.The addition of Cu effectively improved the microstructure and mechanical properties of AlSi7Mg alloy ingots.The dendrites in the as-cast AlSi7Mg master ingots transformed into equiaxed grains,and the Si phase transformed into uniform distribution with the amount of Cu increasing from 0 to 0.09 wt.%.The tensile strength of T6 state AlSi7Mg master alloy with0.09 wt.%Cu addition reached 341 MPa.The addition of graphene and Er introduced external and in-suit nano phase respectively,which promoted the uniform distribution of Si phase and the precipitation of second phase,and improve the tensile strength of T6 state AlSi7Mg master alloy to 380 MPa and 375 MPa respectively.Segregation free solidification during gas atomization process resulted in uniform distribution of elements and single phase.The solid solubility of Si in matrix reached 2.01 at.%.With the addition of graphene and Er,the average particle size of powders decreased from 41.13μm to 34.48μm and 32.92μm respectively and the sphericity increased from 0.978 to 0.983and 0.980 respectively.The decreasing of particle size and increasing of sphericity resulted from the improvement of melt flowability.The addition of graphene and Er resulted in the apparent density decreasing at same time.The microstructures of as-built alloys were molten pool and cellular structures.The distributions of elements were uniform.The solid solubility of Si in matrix reached 1.34 at.%.The addition of graphene and Er promoted the precipitation of Si,which widening the heat affected zones in molten pool boundaries.The graphene and Er in-suit reacted with matrix and produced Al4Si2C5 and Al3Er nano phases respectively,and promoted the formation of dislocation tangles and nano twins.SLM processing produced significant fine grain strengthening,solid solution strengthening,and Orowan strengthening,which improved the yield strength,tensile strength and elongation of as-built AlSi7Mg alloys to 272 MPa,396 MPa and 5.8%.The addition of graphene and Er promoted the mechanical properties of as-built AlSi7Mg alloys additionally,which improved the yield strength,tensile strength and elongation of as-built AlSi7Mg alloys to 275 MPa,430 MPa and 9.8%,and 288 MPa,441 MPa and 8.7%,respectively.And the strengthening mechanism of graphene and Er is Orowan strengthening resulted from Al4Si2C5 and Al3Er precipitations.After 275℃/2h anneal treatment,the AlSi7Mg remain the molten pool boundaries,and a certain degree of recrystallization transformation occurred.The graphene and Er significantly hindered the recrystallization transformation of AlSi7Mg alloy,which decreased the fraction of recrystallization grains from 25.9%to 10.4%and 4.8%respectively,and increased the fraction of residual low angle grain boundaries from 54.2%to 61.0%and 62.1%respectively.The inhibition of recrystallization by the addition of graphene and Er is attributed to the pinning of dislocations by Gr,Al4Si2C5,and Al3Er nanoparticles. |
