Microstructure Control And Strengthening And Toughening Of Inconel 718 Alloy Manufactured By Selective Laser Melting

Selective Laser Melting(SLM)is a high-precision and rapid prototyping additive manufacturing process.Due to the unique "point-line-plane-bulk" step-by-step processing method and highly free manufacturing parameters,it has outstanding advantages in the integration of parts manufacturing,lightweight structure design,gradient of material composition and complexity of shape and function.On the other hand,Inconel 718(IN718)nickel-based superalloy is widely used in aerospace,energy and chemical industries due to its excellent high-temperature mechanical properties,good weldability,oxidation resistance and corrosion resistance.SLM provides a new solution for the direct forming of complex parts of nickel-based alloys.However,the characteristic structure of the dislocation subgrain structure and columnar grain formed by the SLM process is obviously different from the traditional forming IN718 alloy microstructure.At present,the formation and evolution of the SLM fabricated IN718 alloy microstructure and its mechanical properties insufficient understanding of the relevance of SLM has severely restricted the application and promotion of SLM manufacturing IN718 alloy parts in the aerospace and energy chemical fields.In this context,this article compares and analyzes the microstructure and mechanical properties of IN718 alloy prepared by different SLM manufacturing processes and subsequent heat treatment systems,and aims to explore the formation and evolution of the characteristic structure of IN718 fabricated by SLM,and understand the internal relationship between characteristic organization and mechanical properties.The specific research content and conclusions are as follows:The influence of laser energy density on the epitaxial growth and cellular subgrain size of IN718 manufactured by SLM is analyzed.The results show that as the energy density of the laser decreases,the epitaxial growth of grains is inhibited,and columnar grains tend to transform to equiaxed grains;at the same time,the size of celluar subgrains gradually decreases,and the Laves segregation phase changes from a dot-like distribution to a network distributed.The evolution behavior of the cellular subgrain structure in the SLM fabricated IN718 alloy during the subsequent solution heat treatment was explored,and it was found that with the extension of the solution time,the initial dislocations in the alloy reacted with the dislocations released from the subgrain boundaries.The subgrains are merged through small-angle deflection,which is intuitively manifested as the decomposition of the subgrain boundaries into dislocations,and the dislocations are deflected and annihilated until the sub-grain boundaries disappear completely.With this guide,the celluar subgrain can be retained by solution treatment at 1080 ℃ for a short time for 1 hour,and the retention of the subgrain structure can simultaneously improve the strength and plasticity of IN718 alloy manufactured by SLM.The precipitation behavior of the second phase at the grain boundary and subgrain boundary of IN718 alloy manufactured by SLM during the subsequent solution heat treatment was further studied.When solution treatment is performed at980℃,the δ phase will nucleate and grow at the remaining Laves phase along the grain boundary and subgrain boundary,and finally form composite particles of the δphase and the Laves phase;when the solution treatment is performed at 1080 ℃,the Laves phase will be completely eliminated,but there are still two types of carbides,which Ti C at the subgrain boundary is formed during the SLM manufacturing process,and the Nb C at the grain boundary is precipitated during the solution treatment process.The difference in the second phase particles caused by the above-mentioned different solution temperature treatments has little effect on the tensile properties of IN718 alloy fabricated by SLM at room temperature,but it has a significant effect on high temperature tensile properties.The precipitation strengthening phase precipitation behavior in the microstructure of SLM fabricated IN718 alloy and wrought IN718 alloy was compared and studied.It is found that due to the existence of dislocation subgrain structure,the grain stability of IN718 alloy fabricated by SLM in the solution treatment stage is higher than that of wrought IN718 alloy.In the subsequent double aging stage SLM manufacturing IN718 alloy γ’’ phase precipitation is faster than wrought IN718 alloy.Considering the above factors,it is finally determined that the best heat treatment system for SLM fabricated IN718 alloy is 1080 °C,1 h /air cooling,720 °C,4h/furnace cooling(cooling rate is 55 °C/h)to 620 °C,8 h/air cooling.