The Microstructure And Properties Of TiB₂/Al-Mg-Sc Composites By Selective Laser Melting

Aluminum and its composite materials have the advantages of aluminum matrix and reinforced materials at the same time,and they have become important engineering materials that can be used in various fields.Selective laser melting is based on the principle of discrete-stacking,and adopts layered and superimposed forming methods,which can form parts with complex structures,significantly reduce processing costs,improve material utilization,and develop rapidly.According to research in recent years,using aluminum-based composite materials as raw materials and selective laser melting as a processing method can produce high-performance and complex structural parts,which provides a solution to the high-standard requirements in the aerospace field.This paper takes TiB₂/Al-Mg-Sc composites with different reinforcement contents as the research object,explores the evolution of the microstructure and mechanical properties with TiB₂ particles and their content,and optimizes and customizes heat treatment process according to their solidification characteristics and microstructure characteristics.And the performance evolution mechanism of the composite material under different post-treatment conditions are analyzed.Studies have shown that due to the rapid cooling characteristics of the SLM forming process,a large amount of Mg and Si elements are dissolved in the forming material,and they are concentrated near the grain boundaries.These solute atoms are enriched in the stacking fault zone to form a Suzuki atmosphere.TiB₂ has a good wetting relationship with the matrix,and can show a coherent orientation relationship with the matrix.TiB₂that is coherent with the matrix is swallowed during the forming process and acts as a nucleation point to significantly refine the grains and increase the volume fraction of the ultra-fine crystal region.When the reinforcement content is 0%,1%,and 2%,the grain sizes are 0.84?m,0.65?m,0.57?m,respectively,and the volume fraction of the ultrafine crystal region accounts for 67.5%,84.0%,and 88.7%,respectively.TiB₂ that is not coherent with the matrix will be pushed forward by the front edge of the interface,and finally fixed at the final solidified position,that is,the grain boundary.This part of the particles will circulate the heat flow transfer,hinder the growth of the grains along the heat flow,and reduce the aspect ratio of the grains.It can also pin the grain boundaries,prevent the formed grains from growing up by heating,and reduce the recrystallization fraction.When the reinforcement content is 0%,1%,and 2%,the grain length-to-diameter ratio is 1.72,1.56,and 1.54,respectively,and the volume fraction of the completely recrystallized area is 40.2%,17.1%,and 15.6%,respectively.In addition to the changes in the structure,TiB₂ reinforcement may also form a coherent interface with Al₃Sc and Al₃Zr precipitation phases,so it can promote the precipitation of these two precipitation phases to varying degrees,increase the volume fraction of the second phase in the composite material.This second phase interacts with dislocations to enhance performanceIn terms of mechanical properties,the density of 1%TiB₂ composite material is higher than that of the base alloy,which is 99.7%,its tensile strength reaches 483MPa,and its elongation rate is 15.8%.Because TiB₂ particles can also hinder the heat flow transfer,the composite material contains more Mg elements than the matrix alloy,so the Suzuki atmosphere effect is more significant,and the grain size in the composite material is smaller,and the dislocation deposition effect is obvious,so During the deformation process,the Portevin-Le Chatelier effect and discontinuous strain phenomenon are more pronounced than in the base alloy.In the deformation process,the PLC effect will cause impact,which will cause the crack to be impacted,and the crack will expand in the tensile direction,which reduces the strengthening effect of TiB₂.Through the research of the heat treatment process,the heat treatment system determined in this experiment is 325?/2h,the tensile strength after heat treatment reaches 583MPa,the elongation rate is 13.1%,and the PLC effect and discontinuous strain phenomenon are reduced,and the comprehensive mechanical properties exceed most aluminum alloys and their composite materials;although 2%TiB₂ can continue to optimize the microstructure of the sample,it reduces the density of the sample and there are more agglomerations.On the one hand,the sharp corners of these agglomerates will become the source of cracks.Initiation of micro-cracks,on the other hand,will attract nearby Sc and Zr elements,reducing the strengthening effect of their dispersion and precipitation.The grains in the 2%TiB₂composite also show a bimodal distribution,so the PLC effect is more obvious,Which reduces the mechanical properties of composite materials.According to the phase analysis and microstructure characterization,the grains of TiB₂/Al-Mg-Sc composites formed by SLM are greatly refined,TiB₂ is dispersed and distributed,and there are multiple second-phase strengthening mechanisms with the matrix,and promote the precipitation of Al₃Sc and Al₃Zr particles,and further The strengthening mechanism of the second phase in the material is improved;after the heat treatment,more Al₃Sc,Al₃Zr,Al₆Mn second phases are precipitated,and the size of Al₃Sc and Al₃Zr is controlled to be less than 15nm,and the coherent relationship with the matrix is maintained.The boundary Mg and Si elements are also precipitated during the heat treatment process,forming the second phase of Mg₂Si,thus reducing the PLC effect and discontinuous strain phenomenon during the deformation process.