| In the process of friction stir welding,the axis of the welding tool is generally inclined to a certain angle with the plate vertical direction,so as to ensure the sufficient force of the forging ahead of the metal in the welding process,which limits the variety of the form of the weld?such as the curve weld?.Welding with zero tilt angle is conducive to the diversification of weld form,which is very suitable for automatic welding.However,in many cases,due to the limited structure space,the small diameter size shoulder should be adopted to ensure the relatively weld depth.Therefore,it is great economic benefit to carry out the research of FSW welding technology zero tilt angle and small diameter size shoulder to solve the problems in industrial production.In this paper,the microstructure and interface structure of the joint was analyzed by SEM and EDS,and the macroscopic formation,the mechanism of the defect formation and the mechanical properties of the joint are studied.The result shown that:A deeper weld can be obtained in the case of small diameter size shoulder,the ratio of shaft shoulder diameter to weld depth can be up to 2.35.The design of the pin shape with the flow guide groove and the outer convex height of the shoulder profile of 0.3mm can effectively improve the forming condition of the joint.The original lap interface is migrated during welding,and the"Hook"defect can be observed in the cross section of the joint.The grain size of the heat affected zone does not grow obviously coMPared with the parent material,which is related to the smaller size of the shoulder;the influence area of the advanced side is very obvious with the nugget zone interface;when the diameter of the shoulder reduced,the lower the ability to obtain the hole defect joint,but the maximum height of the hook defect of the original interface is greatly reduced;when the rotating speed increased or the welding speed reduced,the HD value increased.when the original lap position is set on the forward side,the HD value is greatly reduced,the maximum reduction can be up to 87%;the surface cleaning before welding can not reduce the HD value effectively,but it can reduce the migration length of the"Hook"defect in the nugget zone.The highest hardness is the parent material,the maximum softening area of the joint is at the junction of the heat affected zone near the retreating side thermai mechanical affected zone.The softening degree of the joint is reduced when the welding speed is increased or the rotation speed of the tool and the diameter of the shoulder reduced;the hardness loss of the advanced side heat affected zone is less than the hardness loss of the retreating side heat affected zone;the hardness of the nugget zone is not affected by the process parameters changed.The tensile strength of the joint increased when the rotating speed reduced or the welding speed increased;the tensile strength of the joint obtained with the 5mm diameter shoulder tool is higher than that obtained by using the 6mm diameter shoulder tool;the tensile strength of the joint is not affected by the surface cleaning before the welding;in the range of the process parameters.the weld joint had a maximum tensile strength of 386MPa at a processing parameter of 3000rpm,240mm·min-1,represent 83.9%of the basic metal,in addition,the failure ouuurred in the weld zone,and the tensile strength of the joint is determined by the morphology of the"Hook"defect and the HD value. |
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