Optimization Method And Mechanism Of High Strength And High Plasticity Of Al-Zn-Mg-Cu Arc Additive Manufacturing Components

Al-Zn-Mg-Cu alloy is widely used in the aerospace field.With the development of advanced aircraft in terms of reducing weight and improving mobility,which produced components of large aircraft with combined high reliability and functional structure with a low cost and a high efficiency,which has become a new problem in the development of aviation manufacturing technology.As large and complex aluminum alloy structures are more and more widely used in aircraft,it is very inconvenient to adopt the conventional plasticity processing methods,mainly because of its long forming cycle,complex process and high requirements for processing equipment.Additive manufacturing technology is a technology of manufacturing solid components by using the method of layer by layer accumulation of materials via CAD design data.Compared with the conventional manufacturing technology,computer software was adopted in additive manufacturing technology to build the corresponding threedimensional model and directly produced the deposited parts after planning the path,which quickly and accurately manufacture the various complex parts,greatly reducing the production cycles,manufacturing cost.However,there are few papers on arc additive manufacturing of Al-Zn-Mg-Cu alloy.At present,most of the research focused on the forming,but few on the improvement of properties of Al-Zn-Mg-Cu alloy arc additive deposited parts.On the one hand,because of the particularity of heat source in additive manufacturing,the grain size of deposited parts was difficult to control,thus the strength was improved slightly.Furthermore,after heat treatment,although the strength of material was greatly improved,the plasticity was decreased.The mechanical characteristics of the strengthplasticity trade off also poses a great challenge to the strengthening of metal materials.In the present study,the comprehensive strengthening method of heterogeneous particles and heat treatment was used to modify the microstructure and properties of Al-Zn-Mg-Cu alloy arc additive deposited parts,and the improvement mechanism of the strength-plasticity of deposited parts was discussed in depth.Firstly,the effects of size and concentrations of TiN particles on the microstructure and properties of Al-ZnMg-Cu arc additive deposited parts were investigated,and the influence mechanism of grain morphology and properties of deposited parts was discussed.It was found that the grain size of TiN(μm)deposited parts decreased from 692.6 μm to 79.1 μm,and the refinement effect reached 88.6 %.The grain size of TiN(nm)TiN deposition was reduced to 62.9 μm,which was reduced by 92.9 %,and the refining effect was similar.Although the strength of deposited parts was not significantly improved,the horizontal and vertical elongations were increased by 160% and 98%,respectively.Furthermore,by investigating the solidification process and precipitation of Al-Zn-Mg-Cu alloy,the heterogeneous nucleation and fine grain strengthening are the main mechanisms of grain refinement and property strengthening of the deposited parts,and fine grain strengthening and texture weakening can simutaneously improve the mechanical properties of deposited parts to a certain extent.Secondly,the strengthening methods via a variety of heat treatment(T4、T6 and T74)process were used to optimize the mechanical properties of deposited parts,the differences of microstructure and mechanical properties of deposited parts with various heat treatment processes were analyzed,and the mechanism of performance strengthening was investigated.It was found that after heat treatment,the segregation of solutes and impurity elements at the grain boundary and dendrite gap disappeared,the second phase characteristics of different shapes were inhibited,and the Zn and Mg elements were fully dissolved into the Al matrix.T6 heat treatment has the most noticeable effect on the mechanical properties of deposited parts.Compared with normal deposited parts,the horizontal tensile strength was increased from 243 Mpa to 513 Mpa,the vertical tensile strength was increased from 230 MPa to 504 Mpa,the microhardness was increased from 119 HV to 205 HV,and the distribution in the vertical direction was more uniform.The solid solution strengthening and precipitation strengthening are the main mechanisms for the improvement of the material properties,and the increase of the proportion of large angle grain boundaries is found to contribute to the improvement of the elongation of the deposited parts by T4 heat treatment.Finally,by combining the heterogeneous particles and heat treatment,the deposited parts with simultaneously improvement of strength and plasticity were successfully fabricated.While ensuring the dense equiaxed crystal matrix,a large number of nano-precipitation phases were precipitated inside the grains.The tensile strength of deposited parts in both directions increased from 219.4Mpa to 487.9Mpa(horizontal)and 252.2Mpa to 469.3Mpa(vertical),an increase of 122.3% and 86.1%,respectively,which is basically close to the level of forgings.Precipitation strengthening is the main strengthening mechanism of strength.The elongation increased from 4.6% to 12.6%(horizontal),6.2% to 10.0%(vertical),173.9% and 61.3%respectively.And the fluctuation was small.The hardness increased from 105.3HV to192.2HV,an increase of 82.5%.In addition,by investigating the solidification process and precipitation of Al-Zn-Mg-Cu alloy,it was concluded that the comprehensive effect of fine grain strengthening,solid solution strengthening and precipitation strengthening are the main mechanisms of performance strengthening of deposited parts.