Study Of Materials Flow In Process Of Non-keyhole FSW And Tool Optimization

Traditional friction stir welding will form a keyhole at the end of the weld,which will affect the appearance of the joint and reduce its mechanical properties.Non-keyhole friction stir welding used in this paper is an effective combination of refill friction stir spot welding and friction stir welding,which can obtain non-keyhole weld.In this paper,the material flow behavior in the welding process was studied by numerical simulation and experiment,and the tool was optimized.In this paper,the formability and mechanical properties of the non-keyhole friction stir welded joint are explored.The tracer experiment showed that the copper foil in the stir zone is completely broken to form copper particles dispersed in the stir zone,and the copper foil in the heat affected zone is not broken but obviously deformed.The numerical simulation results showed that the high speed flow area of material is mainly concentrated on the outer side of the sleeve,and the effect of the hollow sleeve driving material flow is better.In the study on optimization of stirring tools,it is found that the sleeve with screw thread on the side has stronger stirring effect,and the width of stir zone is larger.When the right screw thread rotates counterclockwise,it promotes the downward flow of material,which is helpful to eliminate root defects.The bottom six spiral groove can increase the driving force of the stirring tool to the material,and the driving effect is better when the angle between the six spiral groove and the inner wall of the sleeve is larger,which is beneficial to eliminate the defect of root kissing bond.Therefore,tools with side threads and six spiral grooves at a large angle on the bottom surface have a stronger driving effect to the material.High quality welds can be obtained by using optimized welding tools,but too high welding velocity will lead to groove defects.Increasing the rotational velocity and decreasing the welding velocity will cause the heat input to be too high and make the width of stir zone and heat affected zone become larger.Higher heat input also leads to grain growth in the stirred zone and the heat affected zone.The hardness of the cross section of the joint is distributed in the shape of "W",the hardness of the base material is the highest,followed by the stirring zone,and the heat affected zone is the lowest.When the welding parameters are 2000 rpm-190 mm/min,the maximum strength of the joint is 250.4 MPa,reaching 70.5%of the strength of the base material.At low welding velocity,the joint is easy to break in the heat affected zone,while at high welding velocity,the joint is more likely to break in the mixing zone due to kissing bond.