Study On Microstructure Refinement And Properties Of Inconel 718 Nickel-based Alloy Welds

Nickel based alloy has excellent performance due to its unique composition design and strengthen manner,so is widely applied in high temperature and corrosive environment.However,the microstructure of weld would become coarser after welding which will reduce the performance of the weld.Therefore,a new type of nickel based alloy welding wire was developed to refine the microstructure and improve service performance.In this paper,Inconel 718 nickel-based alloy was used as the research material,and nano-TiO₂was selected as the refiner to refine the weld microstructure by chemical method.The influence of the addition content of nano-TiO₂on the refinement effect on the weld microstructure was studied.Moreover,the relationship between the grain refinement and the high-temperature oxidation performance and mechanical properties of the weld were studied.(1)The content of nano-TiO₂has an obvious effect on the grain refinement,and with the increase of nano-TiO₂the refinement effect become fine firstly and then getting worse.When the addition of nano-TiO₂was 0.6%,the grain refinement reached to the best and the secondary dendrite spacing of the weld was 5.96?m.Subsequently,with the further increase of the addition of nano-TiO₂,the refiner polymerization was caused and the secondary dendrite spacing of the weld reached to 8.53?m when the addition of nano-TiO₂was 1.0%.What's more,the number,size,distribution and morphology of the precipitated phases also changed.As a result of the grain refinement,the precipitated phase that was deviated along the grain boundary was distributed uniformly in the weld microstructure in the form of small blocks and small short bars.In addition,the number of Laves phase decreased due to the grain refinement,while the number of(Nb,Ti)C increased.(2)The nano-TiO2 transformed to Ti₂O₃when adding to the weld,and the mismatch between Ti₂O₃and?was 1.41%,which could effectively refine the weld microstructure.What's more,with the grain refinement of the weld microstructure,the elements distribution become uniform,so the number of precipitated phases increase and the size decrease,and the distribution of them become homogeneous.Therefore,the microstructure of the weld is further refined.(3)The high temperature oxidation of the weld increases with grain refinement.For the weld with nano-TiO₂addition of 0.6%,the oxidation weight gain was the minimum after oxidating at 650? for 120 hours,and the oxidation weight gain was 0.74mg/cm².The oxidation weight gain coefficient was obtained by fitting the oxidation weight gain of the weld with different addition of nano-TiO₂.The oxidation weight gain coefficient of the weld with 0.6%addition of nano-TiO₂was smallest,and the oxidation weight gain coefficient k_0.6=0.0047.Therefore,the high temperature oxidation resistance of the weld reaches to the best when the addition of nano-TiO₂is 0.6%.(4)It was found that the hardness of the weld changed with the addition of nano-TiO₂.The hardness reached to the largest 304HV when the addition of nano-TiO₂was 0.6%.The tensile strength of the weld at room temperature was different with the addition of refiner,and the tensile strength of the weld was highest when the addition of refiner was 0.6%.(5)The high temperature tensile strength of the weld with different addition of nano-TiO₂was higher than that of the base metal,and the tensile strength of weld with0.4%and 0.6%nano-TiO₂reached to the highest 370.60MPa and 373.36MPa respectively.However,after the weld was processed at 650? for 120 hours,the high temperature tensile strength of the weld with 0.4%nano-TiO₂was the highest.(6)No matter at low temperature or room temperature,the impact toughness of the weld reached to the best when the addition of nano-TiO₂was 0.6%.The test results showed that the weld still had good impact toughness even at the low temperature of-190?.