Experiment Research On Heat Source Assisted Friction Stir Welding Of Dissimilar Stainless Steel

Super austenitic stainless steel AISI 904 L has a good performance to prevent pitting and crack corrosion.SAF 2507 Super duplex stainless steel has great behavior in mechanical and preventing stress corrosion.Those two stainless steels are usually used in shipbuilding,petroleum and petrochemical and other engineering fields.In oil and gas fields or offshore projects,in order to control costs and improve performance in complex environments,dissimilar welds of two stainless steels have become the preferred solution and have been widely used.However,due to the huge differences in performance between dissimilar materials,traditional fusion welding is often accompanied by problems such as element segregation and brittle intermetallic phase,resulting in loss of service performance.Friction stir welding(FSW)is an emerging technology aim to avoid some of the problems in fusion welding since it was invented in 1991,and its unique advantages are conducive to the realization of dissimilar welding of metals.However,because of the heat input relies on friction,the problems such as large friction loss of tool and low welding efficiency limit its application in high melting point alloys.In this paper,a heat-source-assisted FSW device was constructed,and the dissimilar FSW of super duplex stainless steel with super austenitic stainless steel and the FSW process with heat source assistance were carried out,and the dissimilar weld with reliable quality was obtained.The research contents are as follows:Firstly,carrying out the dissimilar FSW of 2507 and 904 L under different process parameters to study how the process influence on the welding.The results show that heat is one of the most influential affecting the quality of welds.Reducing the welding speed or increasing the spindle speed will make much more heat,promote the diffusion and mixing of materials,and make the weld better.Exchanging the relative position of the two steel has almost no effects on the quality and mechanical performance of the welds,but it is conducive to the two base metals crossing the weld midline and increasing the diffusion range.Under the process parameters of welding speed of20mm/min,spindle speed of 560 rpm,duplex stainless steel placed on the forward side and the tool biased to the backward side by 0.7mm,a dissimilar stainless steel weld with good surface quality,no defects inside the weld and good mechanical performance was obtained.On the basis of process research,the microstructure evolution and mechanical performance of the welds under optimal process parameters were analyzed in detail.Sufficient material flow occurs in the mixing zone,with two typical zones: austenitic zones with minimal ferrite content and duplex zones with uneven biphasic distribution.Mechanical mixing and diffusion of alloying elements lead to microstructural evolution and complex phase transitions in the stirring zone,and the ferrite and austenite grains in the weld undergo dynamic recrystallization.Affected by the difference in the crystal structure,those two materials have different recrystallization methods,and the grain boundary distribution characteristics and grain size are also quite different.The grain size in the stirring zone has been refined.Both ferrite and austenite grain sizes in the duplex region are smaller than those in the 2507 base material,and austenite grain sizes in the austenitic region are smaller than those in the 904 L base material,but larger than the austenite grains in the duplex region.The grain size refinement makes the microhardness of the duplex zone higher than the 2507 and the 904 L base metal.The transverse tensile test shows that the mechanical performance of the dissimilar welds is better than that of the 904 L base metal,and the fracture location is near the boundary of the stirring zones on the retreating side.In view of the low FSW efficiency of dissimilar stainless steel,a heat sourceassisted FSW test was carried out to study its influence on the dissimilar welding process and weld forming quality.The role of heat source assistance is mainly reflected in preheating the weld,increasing the peak temperature,increasing the welding heat input.This method can significantly reduce the load,promote the flow of material,and improve the performance.When the welding parameters are reasonable,the heat source assistance cannot significantly improve the quality.It is found that with the assistance of induction heating with an induction power supply of 25 A,an induction frequency of12.4k Hz and a heating power of 1.6k W,the quality of the weld with a welding speed of40mm/min is significantly better than the conventional weld,and is similar to the weld in 20mm/min speed.The results show that heat source assistance can double the welding efficiency of 2507 duplex stainless steel and 904 L austenitic stainless steel dissimilar FSW.