激光沉积制备316L-IN625梯度材料的组织与力学性能

Microstructure and Mechanical Properties of 316L-IN625 Gradient Material Prepared via Laser Deposition

采用激光沉积工艺制备了材料成分呈梯度变化的316L-IN625梯度材料,通过扫描电镜观察、X射线衍射、拉伸测试等分析技术研究了梯度材料不同区域的显微组织形态,以及连接试样、梯度试样的力学性能。结果表明:梯度材料不同区域的显微组织随着316L成分的减少依次呈现为胞状枝晶、柱状晶、粗糙枝晶与近等轴晶;与连接试样相比,316L-IN625梯度试样的屈服强度(σ_(0.2))升高至289 MPa,但由于高的热应力与脆性析出相的存在,其抗拉强度与延展性均有所降低;脆性析出相随着拉伸应力的增大发生不均匀的塑性变形,导致梯度试样发生脆性解离。IN625合金的固溶强化与析出强化,使得316L-IN625梯度材料的显微硬度沿沉积建造方向逐渐升高。

Objective Dissimilar metal joints composed of ferritic steel and austenitic alloy are widely used in the heat exchange tubes and pressure vessels of nuclear power generation facilities.However,there is a mismatch in the thermal expansion coefficients(CTE)of the materials at both ends of these joints.In addition,the high thermal stress inside the joints caused by a huge temperature gradient results in performance degradation.To reduce the CTE mismatch,the joints can be filled with nickel-based superalloys.Nonetheless,the problem of component failure due to mutations in the microstructure at the joints still exists.This problem can be resolved by a typical characteristic of the gradient materials,i.e.,layer-by-layer or continuous change in their microstructure and performance with the changing material composition.Laser deposition(LD)is equipped with a flexible powder supply system,which makes it suitable for the preparation of gradient materials with gradual composition.However,this composition is not beneficial to gradient materials in all situations because gradient materials composed of 316L and IN625 still produce defects,such as unmelted particles and cracks,during the LD process.In view of this background,to select the optimal composition in the intermediate regions,316L-IN625 gradient material was prepared by LD in this study.The microstructures of different regions of the gradient material and the mechanical properties,including joint and gradient samples,were studied.The results reveal the relationship between the microstructure and mechanical properties of the 316L-IN625 gradient material,and provide a reference for further study.Methods The 316L stainless steel and IN625 alloy powders with a particle size of 45--105μm were produced through vacuum atomization furnace(Fig.1),and LD experiment was performed using the LD-8060 powder feeding metal3Dprinting equipment.During the LD process,keeping the total powder supply rate unchanged,316L-IN625 gradient material was prepared by changing the composition ratio of 316Lto IN625 along the deposition building direction[Fig.2(a)].The powder particles were injected into the molten pool through the 4-path radially symmetric nozzles of the coaxial laser processing head[Fig.2(b)],and a serpentine scanning trajectory along the short-side direction(SD)was formed.After making the metallographic samples,the microstructural observations were recorded using the BX53 Moptical microscope(OM)and SU5000scanning electron microscope(SEM).The phase composition of the different regions was analyzed through the Ultima IV X-ray diffractometer(XRD).In addition,the tensile and microhardness tests were performed using the Z250universal testing machine and HM-200Vickers hardness tester,respectively.Results and Discussions In the 316Lregion,the segregation of composition occurred at the junction of the grain boundaries,resulting in the generation of irregular inclusions due to the weak bonding force between the inclusions and grain boundaries,where it eventually evolved into microvoids.As the building height increased(70%316L region),the grain boundaries of cellular dendrites became wider,and the microvoids at the grain boundaries disappeared.Later,epitaxially grown columnar grains were discovered in 40%316L region.Further,the microstructure transitioned to coarse dendrites,and its primary dendrites spacing decreased(30%316Lregion).Finally,the nearly equiaxed grains were formed in the IN625region,which caused by the faster cooling rate with the alleviation of heat accumulation(Fig.3).Compared with the joint sample,the 0.2%yield strength of the gradient sample increased with the combination of the high strength IN625.However,the high thermal stress and the possible presence of brittle precipitates(Laves phase)in the gradient regions were prone to causing sample to crack in the early stages of the tensile test,causing a reduction in the tensile strength and the ductility of the gradient sample(Fig.5).The fracture morphology of the gradient sample showed small holes at the bottom and top of the pits containing the precipitates[Fig.6(d)].With the increase of tensile stress,these brittle precipitates separated from the matrix,resulting in brittle dissociation due to uneven plastic deformation.IN625 mainly achieved solid solution strengthening by the refractory metals(Nb and Mo)that were retained in the austenite matrix during the cooling process.Moreover,Nb and Mo located in the previously deposited layer accumulated and eventually precipitated on the matrix as intermetallic phases and carbides under continuous thermal cycling conditions for precipitation strengthening.Due to the combined effect of solid solution and precipitation strengthening,the microhardness of 316L-IN625 gradient material gradually increased with the decrease of the 316L content(Fig.7).Conclusions In this work,316L-IN625 gradient material with the gradient change in the material composition was prepared by LD.The results show that with the reduction of 316L composition,the microstructures of different regions of the gradient material displayed cellular dendrites,columnar grains,coarse dendrites,and nearly equiaxed grains in sequence.Compared with the joint sample,the 0.2%yield strength of the 316L-IN625 gradient sample increased to 289 MPa.However,the tensile strength and the ductility reduced due to the presence of high thermal stress and brittle precipitates.In addition,the tensile deformation mechanisms of joint and gradient samples are ductile cavities and brittle dissociation,respectively.The microhardness of the 316L-IN625 gradient material gradually increases along the deposition building direction,which is related to the solid solution and the precipitation strengthening of IN625.