| The process of preparing magnesium alloy by selective laser melting combines the characteristics of rapid solidification and the advantages of lightweight of magnesium alloy.It can effectively solve the limitations of traditional magnesium alloy technology in the production,and meet the high requirements for the increasing development of magnesium alloy materials in aviation,aerospace,medical and other fields,so as to produce high-performance magnesium alloys with a wider application range and greater difficulty in forming product.This is undoubtedly of great significance.This work systematically studied the processing parameters,balling behavior,mechanical properties of selective laser melting of AZ61 magnesium alloys,and applied hot isostatic pressing(HIP),solution heat treatment,etc.to improve the performance of SLM magnesium alloy.In addition,quantum genetic algorithm optimized support vector regression algorithm(QGA-SVR)was used to establish a quantitative prediction model for surface roughness.Finally,the optimal forming conditions of SLM AZ61 magnesium alloy were obtained,and the problems of poor toughness and surface roughness of SLM AZ61 magnesium alloy were solved.The following research results have been achieved:(1)The effects of processing parameters on the forming quality and balling behavior of SLM magnesium alloy were studied.With the increase of scanning speed and hatch spacing(i.e.the decrease of energy density),the internal pores increase significantly,and the balling occurs.This is because the energy density is too low,the temperature of the molten pool decreases,and the viscosity of the melt increases,which hinders the smooth flow of the melt to the edge of the molten pool.Increasing the energy density helps to reduce defects such as balling and porosity.However,the Marangoni effect and recoil pressure will affect the surface quality when the surface gradually becomes flat.In order to solve the contradiction between energy density and surface quality,a model of the corresponding relationship between different forming quality and energy density range was established.The optimal forming energy density of SLM AZ61 magnesium alloy ranges from 125 to 250J/mm3,and the corresponding optimal surface roughness is 7.5μm.(2)Through the analysis of the thermodynamics and kinetics of magnesium alloy balling behavior and the competitive behavior of magnesium droplets spreading/solidification on the substrate,the spreading/solidification model of the SLM AZ61 magnesium alloy droplets was established.Studies have shown that in the SLM process,Mg and several other metal droplets spread under the action of capillary force,while inertial force hinders spreading.The solidification process of the droplet is controlled by the diffusion of elements and the temperature gradient between the droplet temperature,the substrate temperature and the solidus temperature.The key to inhibiting balling is to control the solidification time longer than the spreading time so that the molten metal has sufficient time to spread before solidification.The theoretically optimal temperature for controlling the balling of SLM AZ61 is 900℃(1173K)by calculation.The solidification time at this temperature is longer than the wetting time,which can minimize balling,reduce surface roughness,and achieve compact molding.The shape and size of the aperture of SLM AZ61 is closely related to the scanning speed and hatch spacing.The relative density increases as the scanning speed and hatch spacing decrease.The best processing parameters of SLM AZ61 are laser power P=150W,scanning speed v=400mm/s,hatch spacing H=0.06mm and layer thickness T=0.04mm.At this time,the relative density of the sample is up to 99.4%;the relative density of SLM AZ61 is related to the solid solution of Al element.Adjusting the energy density to control the solute capture effect can increase the relative density of the magnesium alloy.In addition,the support vector regression algorithm optimized by quantum genetic algorithm(QGA-SVR)was used to establish the surface roughness prediction model of SLM AZ61 magnesium alloy,and the prediction accuracy rate reached 94%,which laid a theoretical foundation for engineering applications and effectively solved the problem of high cost of preliminary experiments.(3)The grain size of SLM AZ61 magnesium alloy is refined to 1.61～2.46μm.The microstructure is composed of equiaxed α-Mg grains and net-like β-Mg17Al12 distributed along the grain boundaries.Under the best processing parameters,the ultimate tensile strength is 287MPa,the yield strength is 233MPa,which are respectively increased by 93%and 135%compared with the as-cast state,and the elongation is 3.12%.Through the analysis of microstructure and fracture morphology,it is considered that internal pores and β-Mg17Al12 precipitated along the grain boundary are the reasons for poor plasticity.(4)Hot isostatic pressing(HIP)plays an important role in closing the internal pores of SLM magnesium alloy and improving plasticity.The density of SLM AZ61 magnesium alloy is close to 100%after HIP,and the dissolution ofβ-Mg17Al12 occurs during the heat preservation process.The results show that after HIP treatment at 350℃,103MPa and 3h,the network β-Mg17Al12 is decomposed,and a large amount of block β-Mg17Al12 is precipitated.The second phase is completely dissolved at 450℃,103MPa and 3h.Under 350℃ and 450℃,the ultimate tensile strength is 279MPa,274 MPa,the yield strength is 198 MPa and 126MPa,and the elongation is 5.5%and 8.2%,respectively,which are 77%and 165%higher than SLM magnesium alloy.The plasticity has been greatly improved while retaining the ultimate tensile strength.Studies have found that after the same pressure is used to close the pores,the more completely theβ-Mg17Al12 is dissolved,the higher the plasticity.Moreover,the main strengthening mechanism of AZ61 under SLM+HIP is clarified through calculation.(5)Solution heat treatment can improve the plasticity of SLM AZ61 magnesium alloy.The best solution heat treatment method is the segmented heating mode.Through solution heat treatment at 330℃,350℃,380℃,and 410℃ for different time,β-Mg17Al12 decomposes from the network precipitated along the grain boundary to form a block,and the dissolution does not change much with time when the temperature is lower than 410℃.The β-Mg17Al12 is almost completely dissolved at 410℃,2h,and the dissolution rate is very fast,indicating that the best solution temperature is 410℃.Solution heat treatment causes grain coarsening.The grain size increased from 3.3±1.3μm to 29.2±3.7μm at 330℃～410℃,but still much smaller than the as-cast level.As the solution temperature increases,the strength of SLM AZ61 decreases and the plasticity increases.The tensile strength at 410℃ is 240±5MPa,the yield strength is reduced to 124±6MPa.However the elongation is increased to 5.9%,which is 84%higher than that of SLM AZ61 magnesium alloy.The dissolution behavior of the β-Mg17Al12 after 5min,10min,15min,20min,30min,1h,2h,15h at the optimal solution heat treatment temperature of 410℃ was studied,and the kinetic model of the β-Mg17Al12 decomposition was established.(6)Comparing the solution heat treatment of SLM AZ61 magnesium alloy with the HIP process,the results show that HIP can not only close the internal pores,but also dissolve the β-Mg17Al12,which eliminates the influence of pores and β-Mg17Al12 on the plasticity of SLM magnesium alloy and significantly improves the plasticity.The best post-treatment method for plasticity improvement is HIP treatment at 450℃ and 103MPa for 3 hours.Finally,using the best processing parameters of SLM AZ61 obtained from the research,the actual parts of SLM AZ61 magnesium alloy for aviation equipment were trial-produced.The results show that compared with the traditional cutting process,SLM magnesium alloy has simplified the whole process by about 40%,the processing time has been reduced by about 80%,and the cost has been reduced by about 20%,which shows that SLM magnesium alloy has great development potential in major fields such as aerospace. |
PDF Full Text Request