Study On Interlaminar Fusion Characteristics And Defect Suppression Measures Of Multi-layer And Multi-pass Welding For Invar Alloy

Invar alloy has been widely used in the manufacture of aircraft composite material molds because of its low thermal expansion coefficient,which is similar to that of composite materials.However,due to the large size of Invar alloy molds,it is difficult to form directly.Usually,the multi-layer and multi-pass welding(MLMPW)technology is adopted for thick plate molds.Nevertheless,in the MLMPW process,the complex thermal process in the interlayer zone among weld seams affects its microstructure and defect morphology,thus impacting the quality of welded joints.Therefore,experiment and simulation methods are used to investigate Invar alloy MLMPW process,welding thermal cycling,interlayer fusion characteristics,microstructure morphology,defect formation mechanism and restraining measures in this paper.This lays a foundation for solving Invar alloy aircraft composite mold welding quality assurance and defect precise control.Firstly,considering the influence of groove shape,pre-welding preparation measures and welding parameters on weld formation,the MLMPW experiment of 19.05 mm(0.75 inch)thick Invar alloy is carried out,in which TIG welding is used for backing layer and MIG welding is adopted for filling layer and cosmetic layer.Quantitative study is inplemented on the macro-forming of weld seam,the morphological characteristics and the weld width of different layers.The effects of different welding heat input on weld width are discussed,and the preliminary optimized welding parameters are acquired.Secondly,the three-dimensional finite element mesh model of Invar alloy MLMPW is constructed.And the welding path and boundary conditions are set up.Double ellipsoid heat source is considered as the welding heat source and checked by experimental results.The thermal process of Invar alloy MLMPW is simulated by finite element simulation software.According to the simulation results,the distribution characteristics of temperature field in different welding layers and the thermal cycle curves of different layers are analyzed.Thirdly,the macromorphology,microstructure and element distribution among different layers of the welded joints are analyzed.The microstructure of interlayer remelting zone is investigated emphatically.The relationship between welding thermal cycle curve and interlayer microstructures is explored according to the distribution characteristics of temperature field between different layers.It is found that the microstructure in remelting zone suffers different peak temperatures because of multiple thermal cycles of subsequent weld seams,which results in the difference of the final microstructure morphology at room temperature.Finally,focusing on the welding defects of Invar alloy MLMPW joints,e.g.,porosities,cracks and lack of fusion,the formation mechanism of welding defects and the distribution characteristics of surrounding elements have been thoroughly studied.Moreover,the corresponding restraining measures for different welding defects are put forward.Meanwhile,in order to verify the results of this study,the mechanical properties of different joints,such as microhardness and tensile strength,are tested and analyzed for obtaining optimal process parameters and good quality welded joints.