Study On SSFSW Pross And Properties Of Welded Joints

Stationary shoulder friction stir welding(SSFSW)is a new welding pro cess based on conventional friction stir welding(FSW).This method not only has the advantages of FSW,but also improves the temperature gradient and flange defects of FSW.The development of SSFSW greatly promotes the application of light metals such as Al,Mg,Cu and their alloys.It has high research and application value and has attracted the attention of scholars and experts at home and abroad.In this paper,3 mm thick pure aluminum plate is effectively joined by self-developed SSFSW tool.Firstly,the size and shape of stirring needle,the size and shape of stationary shoulder and the gap fit between stirring needle and stationary shoulder are analyzed.Secondly,the effects of stirring needle speed and welding speed on weld formation,tensile strength,elongation and microhardness of welded joints were analyzed,and the surface forming,tensile strength,elongation and microhardness of SSFSW and FSW were compared and analyzed.Finally,the EBSD analysis and corrosion performance analysis of the welde d joints obtained under the optimal welding parameters are carried out.Firstly,under certain welding parameters,SSFSW welded joints with good shape,no surface defects and internal defects were obtained.The weld seam manifested "bowl" shape.And the welded joints are mainly divided into four areas: stir zone(SZ),thermomechanically affected zone(TMAZ),heat affected zone(HAZ)and base metal zone(BM).Secondly,the performance of welded joints under different process parameters was analyzed: By comparing the hardness of SSFSW welded joint with that of base metal,the former is obviously lower.The hardness of SSFSW welded joint shows a U-shaped distribution.The hardness of base metal is the highest and that of stir zone is the lowest.The hardness of the TMAZ and HAZ has little change compared with that of base metal.The microhardness value of the forward welded joint is slightly larger than that of the backward side.The properties of the welded joints were analyzed under different parameters.Firstly,the influence of rotational speed on weld is studied.The tensile strength,elongation and hardness of welded joints obtained at different welding speeds were analyzed at the speed of 1400 rpm.The tensile strength and elongation increased first and then decreased with the increase of welding speed.When the welding speed was 70 mm/min,the tensile strength and elongation of welded joints reached the maximum.To be specific,the tensile strength and elongation reached 80.56% and 86.9% of the base metal respectively.Secondly,the influence of the rotation speed of stirring needle on the properties of welded joints was studied.The welding speed was fixed at 30 mm/min and the rotation speed of stirring needle ranged from 800 rpm to 1400 rpm.With the increase of the rotation speed of stirring needle,the tensile strength of welded joints increased first and then decreased.The tensile strength of welded joints with the rotation speed of stirring needle of 1000 rpm reached the maximum,reaching 81.56% of base metal,and the elongation 76% of the base metal.Lastly,the corrosion behavior of SSFSW was studied.The soak corrosion test found that the corrosion of pure aluminum welded joint in 3.5% NaCl solution was mainly pitting corrosion.The corrosion resistance of the SZ position is the best,followed by BM.The worst corrosion resistance is the TMAZ.Combined with electrochemical corrosion,the different areas of the welded joint are affected by the mixing effect of the welding process and the degree of th ermal cycling.There is also a difference in corrosivity,and the order of corrosion rate is: SZ>BM>HAZ>TMAZ.The corrosion mechanism of pure aluminum includes two stages: the first stage is the beginning stage of pure aluminum immersion into 3.5% NaCl sol ution,and the 3.5% NaCl solution contacts the pure aluminum matrix through the micro-void of the passivation layer,causing corrosion of the substrate;In the second stage,as the corrosion time prolongs,the corrosion products at the micro-voids increase,and the corrosion of the base metal increases.In addition,the passivation layer is slowly destroyed by chloride ions,and the passivation layer loses the protective layer,forming a corrosion pit,which is called pitting corrosion.