Control Of Bicrystal Microstructure And Mechanical Properties Of GH4169 Superalloy By Laser Hybrid Manufacturing

Laser Hybrid Manufacturing(LHM)technology is a kind of hybrid fabricating method(HF)through laser additive manufacturing on castings or forgings with complex structure.This technology can not only solve the problem to produce large integral structural parts which is difficult to achieve by traditional processing technology,but also take advantage of the advantages of short production cycle,low cost,good microstructure and excellent mechanical performance of laser additive manufacturing technology.In this paper,the low temperature heat treatment of eliminating Laves phase by means of ? aging + solution treatment is proposed,because high temperature solution treatment can not be used to avoid the heat damage of the forged substrate of LHFed GH4169 superalloy.Then the effect of the grain size in forged substrate on the microstructure and mechanical properties of LHMed GH4169 superalloy was explored.The research can provide theoretical and experimental basis for the preparation of integral parts with double crystal structure by LHM.The specific research work and main conclusions are as follows.Low temperature solution treatment of brittle Laves phase in the additive region of LHMed GH4169 superalloy was studied.The results show that the low temperature solution treatment consumes Nb in Laves phase through the precipitation of ? phase,then can "cut" the long-stripe Laves phase into block or granule shaped,and finally can dissolve ? phase and Laves phase.Low temperature solution treatment can effectively promote the diffusion of Nb from the inter dendrite to the core dendrite,and weaken the micro-segregation of alloy elements(especially Nb)during rapid solidification.Moreover,the tensile strength and plasticity of LHMed GH4169 superalloy after low temperature solution treatment are improved.The matching relationship between the equiaxed crystal in the substrate and the columnar crystal in the additive area was researched.The results show that when the average grain size in the substrate of LHMed GH4169 superalloy is small(about 20?m),the coarse columnar crystal in the additive region grow on the substrate with fine equiaxed crystal,which have a wide grain transition region between them.When the average grain size of the substrate is large(100?200 ?m),the matching growth of equiaxed crystal structure and columnar crystal is easier,which have a narrow transition area.And even there is no obvious transition area in some grains,which shows that the columnar crystal grow on the coarse equiaxed crystal directly.The mechanical properties of LHMed parts with different grain sizes in the substrate are measured.It can be found that the fracture position of the tensile specimen is additive area basically when the average grain size of the substrate is between 17.48 ?m and 124.88 ?m.However,the fracture occurs in the substrate when the average grain size of the substrate reaches between 182.59 ?m and 196.07 ?m.In addition,it is also found that the grain size in the substrate of the LHMed parts after low temperature solution treatment has no obvious coarsening phenomenon.Meanwhile,it can get the result that the tensile strength of the LHFed parts increases obviously,and the plasticity decreases sharply with the increase of the average grain size in the substrate.The experimental results show that the mechanical properties of the bicrystal microstructure with different grain morphology are different in different parts.Then,the molecular dynamics method is used to explored the high temperature tensile property and the change of crystalline structure further.The results show that columnar grains and equiaxed grains have thte different grain morphology.The interface is the weak area and many holes tend to form easily in the interface in the effect of low temperature tensile.When the tensile temperature is changed in the range of 700 K ?1100K,a large number of orderly FCC atoms in the model are transformed into disordered Other atoms.Meanwhile,the degree of material amorphization is serious,and the phenomenon of interface atom doping is obvious,which makes the interface difficult to form holes.