Study On The Microstructure And High Temperature Mechanical Properties Of Inconel718 Alloy Processed By SLM

Inconel718 alloy has been widely used in aviation,aerospace,metallurgy,machinery,chemical engineering and nuclear power industries known for its ability of superior microstructure stability and comprehensive mechanical properties at elevated temperature up to 650°C.However,there are many problems such as harmful phases,defects,segregation and residual stress in Inconel718 alloy parts processed by traditional methods that including casting,forging and powder metallurgy,which seriously affect the performance of the Inconel718 alloy.And there are also technical problems in the components manufacturing with complex shapes.Selective laser melting(SLM)technology is the most potential additive manufacturing method for forming Inconel718 parts with low cost,short production cycle and possibility for manufacturing complex shapes.Therefore,it is of great significance to study the microstructure and high temperature mechanical properties of Inconel718 alloy processed by SLM.In this study,the Inconel718 alloy processed by SLM was by solution heat treatment that involves three different temperatures of 1080°C,980°C and1080°C+980°C,respectively,and then was carried out by same double aging treatment.By comparing and analyzing the microstructure,hardness,high temperature stress rupture and high temperature tensile properties of the Inconel718 alloy processed by SLM before and after solution treatment,the effect of solution treatment temperature on the microstructure and high temperature mechanical properties of the Inconel718alloy processed by SLM was revealed.Therefore,this study lay the foundation of the regulation for the microstructure and high temperature mechanical properties of Inconel718 alloy processed by SLM.The results show that there are columnar grains epitaxially grown along the building direction in the as-deposited Inconel718 alloy processed by SLM,which have the characteristics of directional solidification structure.And the shape of grains along the scanning direction shows the regular pattern of the alternately distribution of the coarse and fine grain.Driven by the ultra-high temperature gradient(1.5×10~7~4×10~7K/m)and ultra-fast cooling rate(1.4×10~6 K/s)at the solid-liquid interface of the molten pool during the SLM process,the average sizes of as-deposited grains are only 48?m and 32?m in longitudinal and transverse cross section,respectively.Because the dislocations formed during SLM process can move freely within the crystal,and there are also twin structures existed.Therefore,the as-deposited sample has good high temperature plasticity.However,due to the inhibition of precipitation of the strengthening phase during the rapid solidification process and the presence of brittle Laves phases,the as-deposited sample has lower hardness,high temperature stress rupture property and high temperature tensile strength.And the tensile fracture mechanism at high temperature is ductile transgranular fracture.After heat treatment at different solution temperatures,due to different degrees of precipitation of the??and??phases of the Inconel718 alloy,the hardness,high temperature stress rupture property and high temperature tensile strength of the Inconel718 alloy processed by SLM are increased in varying degrees.However,the elongation at high temperature decreases in varying degrees,and the decrease range is between 45%and 55%.The reason is that the temperature of solution treatment is different,and the degree of precipitation degree and morphology of??,??and?phases are different,which will hinder dislocation movement in varying degrees,and thus shows different plasticity.In addition,during the high temperature stretching process,the long needle-shaped and short rod-shaped?phases precipitated with different shapes and positions are pulled apart into short rods and granules along the tensile direction,showing different hardness,high temperature stress rupture property and tensile strength.And the tensile fracture mechanisms at high temperature are also not the same.After single solution treatment at 980°C,the grains of Inconel718 alloy processed by SLM are still very fine.The short rod-shaped and fine needle-shaped?phases distributed along the grain boundary and grain interior seriously impede the dislocation movement.Thus 980°C heat treated sample has the worst high temperature mechanical properties.And the tensile fracture mechanism at high temperature is mixed crystal ductile fracture.After solution treatment at 1080°C and 1080°C+980°C,the grains of Inconel718 alloy processed by SLM become significantly larger.The mechanical properties all exceed the standard of the forging,showing good mechanical properties at high temperature.The tensile fracture mechanisms at high temperature are all typical ductile fractures.However,after double solution treatment at 1080°C+980°C,an appropriate amount of short rod-shaped?phases are precipitated at the grain boundary,which increase the strength of the grain boundary under high temperature conditions.And after single solution treatment at 1080°C,only a small amount of carbides precipitated along the grain boundary,reducing the creep resistance of the alloy.It's considered that the Inconel718 alloy processed by SLM after double solution treatment at 1080°C+980°C has the best comprehensive mechanical properties.The above research establishes the mapping relationship between solution treatment temperature and its corresponding typical microstructures,precipitated phase and high temperature mechanical properties of the Inconel718 alloy processed by SLM.Therefore,this study will provide reference for developing the heat treatment system of SLM manufacturing metal materials.