Study On The Regulation Of Microcrack,Microstructure And Properties Of GH3536 Fabricated By Selective Laser Melting

Nickel-based superalloys have excellent comprehensive properties at high temperature and act as an indispensable material in the fields of aeronautics industry.With the urgent demand of advanced engines,the service temperature of some key hot-end components approach or even exceed the limit of materials,it is necessary to manufacture complex cooling channels on the surface and inside the parts.Thus,it offers great challenge to the manufacturing of these components with traditional methods,which is the Achilles’heel for their industrial applications.Selective laser melting（SLM）,a promising metal processing technique in the additive manufacturing（AM）family,is able to realize the rapid manufacturing of high-temperature alloy parts with complex structures.However,due to a high thermal expansion coefficient of nickel-based superalloys,it is prone to microcrack under the condition of rapid melting and solidification during SLM process.In this paper,GH3536 with good high temperature mechanical properties was selected as the raw material,and the process parameter optimization,heat treatments and alloy composition optimization were studied to regulate microcrack,microstructure and mechanical properties.The specific research work and results are listed as follows:The effect of SLM process parameters on density and microcrack of GH3536 alloy,as well as its microstructure and high temperature mechanical properties,were systematically analyzed.With the increase of laser energy density,the relative density of SLM GH3536alloy increased in a logarithmic form,while the density of microcrack defects did not show significantly fluctuation.GH3536 alloy with a defect volume fraction of 0.96%was obtained by SLM when the body energy density in the range of 90 to 115 J/mm³.Typical fine columnar and cellular sub-grains with high density of dislocation networks and similar crystal orientations were generated within the melt pools,and nanoscale spherical carbides were embedded at the sub-grain boundaries.Besides,a large number of microcracks with a length of 10～100μm were found in heat affected zone of molten pool and propagated along the high-angle grain boundaries.According to the EPMA analysis,it was found that low melting point carbides rich in Cr Mo exists at grain boundaries in the heat affected zone,which provided a channel for crack propagation.Tensile experiments show that SLM GH3536 were anisotropic,the ultimate tensile strength along the building direction is higher than that of horizontal direction.The maximum tensile strength at room temperature was 860 MPa and the elongation was 26.8%.The maximum tensile strength at 900℃was 247 MPa,and the elongation was only 5.7%.A two-step heat treatment,including hot isostatic pressing（HIP）and solid solution heat treatment（SSHT）,was proposed to obtain crack-free SLM GH3536 components with desirable mechanical performance.After HIP treatment,microcracks in the SLM alloy were eliminated.while after HIP+SSHT treatment,the volume fraction of microcracks and pore defects decreased from 0.96%to 0.11%.Due to the fact that SSHT treatment increases the solid solution degree of alloying elements,the XRD diffraction peaks shifted to left obviously.After the treatment of HIP+SSHT,the average grain size increases to 21.8μm,and the dentritic grains were transformed into equiaxed grains.At the same time,the sub-grains and the dislocation network disappeared,and the size of dispersed carbide increased to micron scale.The tensile strength and elongation at break of heat treatment specimens at room temperature were 3.6%and 143.0%,respectively,which is higher than that of as-fabricated ones.Furthermore,tensile strength and ductility at 900℃after the two-step treatment were improved by 11.9%and 412.0%,respectively.The effect of alloy composition on microcrack and microstructure of GH3536 fabricated by SLM was investigated.The content of Mo,Cr,Co and other elements were increased,and the content of C element was decreased.A modified GH3536M alloy with a defect volume fraction of 0.07%and nearly crack-free was obtained by SLM.The ultimate strength and elongation at 900℃of SLM GH3536M specimens along the building direction were increased by 20.0%and 370.0%respectively,which were equivalent to the alloy performance after the two-step heat treatment mentioned above.TEM shows that the solution strengthening effect of GH3536M was improved,and spacing in the（2 0 0）crystal planes increased from 0.178 nm to 0.182 nm.The thermal expansion test show that the thermal expansion coefficient of GH3536M was significantly reduced,specifically,the differential thermal expansion coefficient at 1150℃was reduced from 22.61×10^-6 K^-1 to 18.79×10^-6 K^-1.DSC test showed that the solidification temperature range was reduced from 41.8℃to33.0℃.The improvement of solution strengthening effect increased the ultimate tensile strength,the reduction of the thermal expansion coefficient improved the thermal stress,and the narrowing of the solidification temperature range improved the segregation of Mo,Cr,and C elements.Thereby,it reducing the hot crack sensitivity of GH3536.