| 7075 aluminum alloy is one of the main structural materials in aerospace industry because of its high strength,high specific stiffness,easy processing and good corrosion resistance.Traditional manufacturing process is difficult to produce complex three-dimensional shape,achieve personalized customization for engineering parts in a shorter cycle.And it is limited for its long production cycle and high cost of die manufacturing.Therefore,the process parameters,microstructures and mechanical properties of 7075 aluminium alloy by selective laser melting(SLM)were studied in this paper.The causes of cracks in 7075 aluminium alloy by selective laser melting were aslo analyzed.Aiming at the causes of cracks in 7075 aluminium alloy by SLM,Ti B2 submicron powder was mixed into7075 aluminium alloy powder to prepare aluminum matrix composites,then forming by SLM to eliminate cracks and improve properties.Firstly,we researched the effect of SLM process parameters on SLMed samples' s density,microstructure and mechanical property.The research suggest that the density of 7075 aluminium alloy SLMed samples increases rapidly with the increase of laser energy density,but when the laser energy density reaches 118J/mm3,the density of SLMed sample tends to a stable value of 97.2%.In addition,remelting scanning strategies with different process parameters can improve the surface quality of SLMed samples and obtain samples with continuous surface molten channel.It is found that the smaller length of the chesss scanning strategy,the better the surface flatness and the higher the density.When the length of the chess is 0.5mm,the density can reach 97.8%.The analysis shows that the crack of 7075 aluminum alloy SLMed sample is caused by the component segregation during solidification,which changes the temperature of the local equilibrium liquidus and causes the crack at the solid-liquid interface.The segregation results in the precipitation of the second phase S(Al2Cu Mg).Subsequently,we carry out the study on 7075/Ti B2 by SLM.The results show that the SLMed samples without thermal crack can be obtained by mixing 1.5wt%Ti B2 powder.Thegrain size of the samples is obviously refined compared with that of 7075 aluminium alloy SLMed sample,and the average grain size is reduced from 6.8?m to 3.8?m(laser power320 W,scanning speed 1000mm/s).Comparative analysis shows that 7075 aluminum alloy and 7075/Ti B2 SLM test sample texture components showed {001}<100> cubic texture,and7075/Ti B2 SLMed samples micro hardness was higher than 7075 aluminum alloy SLMed samples.7075/Ti B2 composite powder SLMed samples' the highest microhardness can reachs228 HV.However,the compressive properties of 7075/Ti B2 composite powder SLMed samples are lower than that of 7075 aluminum alloy SLMes samples.The reason is that there is a non-coherent relationship between Ti B2 and Al matrix,which shows the interface bonding is poor.Finally,the effect of heat treatment on the microstructure,mechanical properties and texture of the SLMed samples was studied.After solid solution aging(2h + 18h)treatment,there are disc,needle ?' phases and long needle ? phases in SLMed samples,while large scale irregular and long plate-shaped precipitates are dissolved without heat treatment.Except for7075 aluminium alloy SLMed samples,the strength of the SLMed samples after solution aging treatment has been greatly improved,and the fracture strength of 7075/1.5wt.% Ti B2 SLMed samples reaches 601 MPa.After heat treatment,the plasticity of the SLMed samples has been improved.The plasticity of 7075/1.5wt.% Ti B2 SLM ed samples increased the most,and the fracture strain increased from 15.9% to 35.5%.The increase of plasticity of the sample is positively correlated with the increase of large angle grain boundary in the samples.The texture of the SLMed sample changed from cubic texture {001} < 100 > to Gauss texture{110} <001> after solution aging treatment by EBSD.And the grains grew after solution aging treatment,but the preferred growth direction remained unchanged. |
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