The Research Of Friction Stir Lap Welding Of 304 Stainless Steel Sheets And The Microstructure And Properties Of The Lap Joint

Friction stir lap welding of 304 stainless steel sheets were completed successfully in this article.This article was divided into two parts.The first part was friction stir lap welding of 304 stainless steel sheets whose thickness is 1mm.Another part was using Sn as a filler metal in Steel/Steel friction stir lap welding which can promote the diffusion and dissolution of elements and improve the mechanical properties of lap joint.For the first part,the influence of welding parameters on the appearance of welding joints,the deformation of sheets and the wear condition of tools was studied.On the basis of a large number of early exploratory experiments,the influence of different welding speed was selected to be studied.The range of optional welding speeds was narrow for thin plates.So,the emphases of this part was analysing the flow of plastic metals and microstructure of joint which was got by the best welding parameter.The surface of the welds was smooth without any macroscopic cracks and cavities only when the rotation speed is 50 mm/min.The stir zone(SZ)had equiaxed grains with austentic structure due to dynamic recrystallization.There was not any evidence for the presence of sigma phases being found in this study.Meanwhile,there was a weak region in the advancing side because of the flow of plastic metals.However,compared with the base material,mechanical properties of the joint are maintained in a comparatively high level.It was reasonable for using FSW to weld 304 stainless steel sheets based on the first part.However,shorter pin was chosen to protect from deformation for thinner sheets.In order to improve the properties of joints,tin layer was added between the sheets.Under the condition of constant stirring speed(1200r/min)and constant pressure drop(0.1mm),the influence of different welding speeds on the microstructure,the diffusion of intermediate elements and the mechanical properties of joints were investigated.It was found that,when then welding speed was slow,the heat input was sufficient and the diffusion of the elements was more sufficient.With the welding speed increasing,the heat input reduced and the there was obvious defect between the layer and interface.During the welding process,the tin layer melted and the interdiffusion between the matrix and the interlayer elements occured.Compared with the joint without intermediate layer,tin layer promoted the diffusion and dissolution of elements.For the joint of stainless steel sheets with tin layers,ductile fracture occured in the subsequent shear tests at the welding parameters of v=30mm/min,n=1200r/min and d=0.1mm.With the increase of welding speed,the fracture mode gradually transformed from ductile fracture to brittle fracture.Moreover,compared with the lap joint without tin layer,the mechanical properties of the welded joint had been greatly improved by adding the tin layer.The final study showed that the diffusion and dissolution of elements was changed by adding tin layer to the stainless steel sheet and the joint properties was enhanced effectively.