| 316L stainless steel and In718 superalloy alloy are important metal materials,and are widely used in aerospace,military,automotive and other industrial fields.With the continuous progress of science and technology,the structure composed of a single material(316L or In718)has been unable to meet the requirements of use,the functional 316L-In718 bimetal structure can achieve the same part in different regions corresponding to different working conditions,becoming an urgent need in the modern industrial field.This topic uses the selective laser melting(SLM)technology to realize the overall block printing of the 316L-In718 bimetal structure.First,the functional lightweight design of the structural parts is carried out,and then the 316 L and In718 are divided into regions according to the performance requirements of different parts of the same part.That is,after the 316 L printing is completed on the substrate,In718 printing is formed on the 316 L forming sample,thereby obtaining an integrated functional bimetal structure.Finally,the residual tensile stress generated in the SLM forming process is eliminated by laser shock peening and heat treatment process,at the same time to achieve micro-organization control,thereby improving the overall quality and mechanical properties of the formed part.The main research contents are as follows:(1)Through orthogonal experiments and density testing,the optimal process parameters of SLM forming 316L-In718 bimetallic structure were explored.In the orthogonal test,the best density of 316 L is 99.91%,and the worst is 98.77%.The optimal process parameters corresponding to the best density of 316 L are laser power of 265 W,scanning speed of 910mm/s,and filling distance of 0.1mm;The best density of In718 is 99.9%,the worst is 99.5%,and the best density corresponds to the optimal process parameters: laser power is 285 W,scanning speed is 960mm/s,and the filling interval is 0.11 mm.The voids in the micro-topography with good density are small and shallow,and there are many large and deep voids in the micro-topography with poor density,which is due to the mismatch of the process parameters that leads to over-fired or unmelted metal powder.(2)Inspire simulation analysis and experimental verification were carried out,and the influence of lattice structure on the mechanical properties of SLM forming 316 L and In718 was studied.Compression is easy to cause tensile stress fracture at the 45 ° inclined connecting rod structure.If there is a deviation in the upper and lower loading positions,shear fracture is likely to occur at the intermediate position;stretching is likely to occur at the vertical connecting rod.Bending easily breaks and breaks in the section from the center to both ends of the fixed restraint;shearing easily breaks and breaks in the section from the center of the restraint to the two ends.The maximum force that the 316 L lattice structure can withstand in the tensile,compression,bending and shear tests are 6.62 kN,9.14 kN,7.6kN and 21.2kN,the maximum force that the In718 lattice structure can withstand in the tensile,compression,bending and shear tests are 32.3kN,42.98 kN,34.73 kN,and 94.85 kN,respectively.The experimental verification found that the lattice structure has a mechanical performance lower than that of the solid structure by about 20%,due to the cushioning effect of the lattice structure,the toughness becomes better and the fracture position Consistent with simulation results.(3)The effect mechanism of laser shock and heat treatment on the microstructure and microhardness of SLM forming 316L-In718 bimetallic structure was studied.Laser shock can refine the crystal grains,the melt channel is relatively smooth at the 316L-In718 fusion line,the organization has a significant displacement,the tissue penetration nailing phenomenon is weakened;Heat treatment re-forms and grows the nucleation of the structure,316 L corrosion resistance becomes better,a large amount of intergranular strengthening phases precipitated in the In718 sample,and the ? phase precipitated after solid solution plus double aging treatment was pinned at the grain boundary to suppress grain coarsening.In addition,the hardness values of the four states of untreated,laser shock,heat treatment and heat treatment plus laser shock are as follows: The average hardness values of 316 L samples are 250 HV,285HV,225 HV and 270 HV,which is increased by 14% after laser shock and decreased by 10% after heat treatment;The average hardness values of In718 samples are 333 HV,360HV,503 HV and 520 HV,which are increased by 8.1% after laser shock and 51.1% after heat treatment;The hardness value of the 316L-In718 fusion line is between the two,280 HV,316HV,355 HV and 378 HV,respectively.(4)The effect mechanism of laser shock and heat treatment on the mechanical properties and fracture morphology of SLM formed 316L-In718 bimetallic structure was studied.The tensile strength of SLM formed 316 L specimens was increased by 8.6% and 9.4% after laser shock in the XY and Z forming directions,elongation rate in XY direction was increased by 5.8%,elongation rate in Z direction was decreased by 18.5%;Tensile strength was reduced by 2.6% and 2.8% after heat treatment,elongation rate was increased by 15.5% and 20.8%,the tensile strength of the untreated sample in the Z direction is lower than that in the XY direction by 15.2%;the fracture morphology in the XY direction was all ductile fracture,and the fracture morphology untreated in the Z direction and after laser shock is a composite fracture between ductile fracture and brittle fracture,and it is ductile fracture after heat treatment.The tensile strength of the In718 specimen after laser shock in the XY and Z forming directions was increased by 17.2% and 12.8%,and the elongation rate was increased by 12.4% and 6%,respectively;The tensile strength after heat treatment was increased by 50.1% and 41.1%,the elongation rate decreased by 48.2% and 42.7%,respectively;The untreated tensile strength in the Z direction was reduced by 2.5% compared with the XY direction,and the fracture morphology was all ductile fracture.The above data shows that the tensile strength of the 316 L and In718 samples is better than the Z direction in the XY direction.The 316L-In718 sample only studies the change of the tensile strength in the XY forming direction.The laser shock improves the tensile strength by 10.4%,and the elongation rate increase by 31.5%,heat treatment reduces the tensile strength by 3.3%,elongation rate increases by 11.5%,the fracture morphology of the untreated specimen is cleavage fracture,and the heat treatment specimen is a composite fracture between ductile fracture and brittle fracture.In summary,the research of this subject provides a theoretical basis and experimental basis for reference for the impact mechanism of laser shock and heat treatment on the microstructure and mechanical properties of SLM forming 316L-In718 bimetallic structure. |
PDF Full Text Request