CMT Surfi Texturing Of Invar Alloy And The Process And Mechanism Of Pressure Welding With Pure Aluminium

Aluminum-steel composite joints are used to connect aluminum superstructurewith steel main deck, which have a high strength requirements. Invar alloy hasstable coefficient of linear expansion, high mechanical properties, and strongcorrosion resistance. Therefore, comparing with common aluminum-steel compositestructure, invar-aluminum composite structure has a better integrated performance.After using CMT technology textured the surface of invar alloy, pressure weldingprocess was carried out. CMT texturing technology, plastic flow of pure aluminumin heating stage, interfacial reaction in thermal insulation stage and residual stress ofjoints in cooling stage were analyzed in this investigation. Change rules of eachstage is also involved in this paper.Since there are no systematic references about the weldability of this system,diffusion welding of aluminum/invar is carried out firstly. There are two inevitableproblems in the welding process, one is the formation of intermetallic layer in thejoining, the other is the high residual stress.Texturing the surface of invar alloy was carried out secondly.12kinds ofsurface texturing sample have been accomplished. Welding nails had goodmicrostructure. Shear force between the nails and the invar reached250MPa.Thirdly, pressure welding of surface-texturing-invar/aluminum was carried out.Influence of welding temperature and holding time on the joint interfacialmicrostructure and mechanical properties of joints were analyzed. Pressure weldingof12kinds of surface texturing sample were carried out respectively. There are twofracture mechanism of the joints, one is occuring on the outermost interface betweenthe aluminum and the nails, the other is occuring on the interfacial microstructure.When texturing morphology parameter is2mm,3mm×3mm, the tensile shearstrength reach the maximum,64MPa,which is more than90%of the strength of1060aluminum.With finite element method for temperature stage pure aluminium internal flowregularity for the numerical simulation analysis. Pure aluminium plasticity flow wasinvestigated with finite element method when welding nail pierced into the purealuminium in the stage of temperature up. There is a critical pressure to make tinycompartments closed completely in the interface of Inconel625/pure aluminium andInvar/pure aluminium. There are large size holes in the edge of the joint, which areunable to avoid. Boundary constraints in the edge of the joint can solve this problemwell.The residual stress of joint was investigated with finite element method in the stage of temperature down. The results show that the triaxial residual stresses canaccumulate to relatively high values on the pure aluminium between the weldingnail, which result in the embrittlement in this area. The peel stress and shear stresshave low influence on interface reaction layer. So the welding technique solve theproblem that the residual stress of interface reaction layer are large in the method ofdirect diffusion welding.