Microstructures And Mechanical Properties Of Aluminum Matrix Composites Reinforced With Hybrid TiB₂ And SiC Particles Prepared By Selective Laser Melting

Particle reinforced aluminum matrix composites are widely used in automobile,aerospace,and other fields because of their light weight,high specific strength,good heat and abrasion resistance.Compared with single particle reinforced AMCs,hybrid particles reinforced AMCs can not only keep the advantages of all reinforcing phases,the synergistic hybrid effect of multiphase simultaneous enhancement,and excellent mechanical properties,but also reduce the cost of materials.However,the traditional manufacturing process has some problems,such as long production cycle,high costs of die manufacturing,limited shape of parts.Up to now no study has been reported on hybrid particles reinforced AMCs prepared by selective laser melting?SLM?.TiB₂ particles have low thermal conductivity and high laser absorption,which can be used not only as dispersion enhancement phase of aluminum alloy,but also as heterogeneous nucleating agent.SiC particles with high hardness,good wear resistance and low cost can be used as dispersion reinforcing phase of aluminum alloy.In this work,?TiB₂+SiC?/AlSi10Mg composite powders were prepared by low-energy ball milling.Processing parameters,microstructures and mechanical properties of AMCs reinforced with hybrid TiB₂and SiC particles prepared by SLM were studied,and the heat treatments were also discussed,which aims to provide technological guidance for the production of hybrid particles reinforced AMCs parts with high density and excellent mechanical properties by SLM.The optimum ball milling parameters for?TiB₂+SiC?/AlSi10Mg composite powders is 135r/min-3h-5:1,with which nano-TiB₂ particles and submicron-SiC particles are uniformly dispersed on the surface of AlSi10Mg powders and the sphericity of powders can be well preserved.The effect of 2wt.%TiB₂ particles on reducing the laser reflectance of powders exceeds that of 1wt.%SiC particles on increasing the laser reflectance of powders,thus increasing the laser absorption of powders,which is good for SLM forming.The effects of SLM processing parameters on density,roughness,microstructure and microhardness of AMCs reinforced with hybrid TiB₂and SiC particles were investigated.In high-speed scanning,the greater the laser energy density,the greater the density is and the smaller the surface roughness is.When the laser power is 240W,the density of samples reaches more than 97.5%.Meanwhile,with the increase of scan speed,the density of samples increases first then decreases and the microhardness increases.Within a proper range,with the increase of the laser energy density,Marangoni convection is enhanced,which can accelerate the uniform distribution of reinforced particles.The optimized SLM processing parameters of AMCs were determined,Laser power is 240 W,scan speed is 1400 mm/s and scan spacing is 110?m.The density of samples can reach a maximum of 98.9%.Microstructures and mechanical properties of SLMed AlSi10Mg alloy and AMCs reinforced with hybrid TiB₂and SiC particles were compared and analyzed.The interface between the reinforced particles and aluminum matrix is clean,and there is no interface reaction product.Compared with SLMed AlSi10Mg alloy,the addition of 2wt.%nano-TiB₂ particles has a good heterogeneous nucleation effect,which can not only change the grain microstructure of the XOZ surface in AMCs samples from coarse columnar to fine equiaxed grains,but also decrease the average grain size from 7.88?m to 2.48?m.As a result,<001>preferred orientation and{001}<100>cubic texture are greatly weakened.AMCs with excellent comprehensive mechanical properties can be successfully prepared by SLM.Their tensile strength,ductility increase to 509.8 MPa,10.6%,respectively.The enhancement of strength is mainly attributed to load-bearing strengthening,fine grain strengthening and Orowan strengthening.The improvement of plasticity is mainly attributed to grain refinement.Different heat treatments were applied for SLMed AMCs reinforced with hybrid TiB₂ and SiC particles.Results show that the optimum direct aging parameters are aging at 180? for12 h.After direct aging,a small number of Si phases precipitate,which can soften the Al matrix.Meanwhile,needle-like?"phases and rod-like?'phases precipitate on the Al matrix,which can make up for the reduction of solid solution strengthening.As a result,yield strength and hardness increase,the tensile strength increases slightly but the ductility decreases.After annealing,a considerable number of Si phases precipitate.With the increase of annealing temperature,the eutectic Si network dissolves and breaks up gradually,the precipitation of Si increases gradually and the Si phase gathers and grows.The solid solution strengthening effect is weakened,thus hardness and strength decrease and plasticity increase.