| Being the lightest and high-strength deformed magnesium alloy,ZK60 magnesium alloy would be widely preferred for multiple applications,such as electronics,transportation and the aerospace.Bobbin Tool Friction Stir Welding,a new solid-state joining technology developed on the basis of friction stir welding,can not only solve the hot cracks,pores,oxide inclusions and loss of metallurgical elements that are associated with conventional fusion welding process of magnesium alloy joint.Thus,it is effective in eliminating the incomplete penetration defects that are likely to occur when is used for joining medium and thick plate via ordinary friction stir welding.But also accurately is applied to the welding of hollow and spatial unsupported structures by manufacturing the equipped with the lower shoulder.In this paper,the bobbin tool friction stir welding test of 4mm thick ZK60 magnesium alloy was carried out.Combined with the inherent advantages of numerical simulation methods,the optimal design of the pins applied to magnesium alloy was carried out.The regulation mechanism of the pins on the temperature field distribution and the plastic metal flow state of the weld were systematically explored.The effect of process parameters on weld formation and material flow behavior was studied.Moreover,the regional microstructure characteristics of ZK60 magnesium alloy bobbin tool friction stir welding joint were analyzed by the welding process.The influence of the process parameters on the weld formation,the evolution of the microstructure and the mechanical properties of the joint were analyzed,and the fracture mechanism of the joint under typical parameters was revealed.This research does lay a foundation for obtaining high-quality ZK60 magnesium alloy welding joint process specifications and meeting its engineering applications.The numerical calculation model of the bobbin tool friction stir welding process was established to investigate the flow behavior of of ZK60 magnesium alloy,and the thermal cycles of the characteristic points is basically consistent with the experiment results measured by the thermocouple.By simulating the optimized design of the welding tools,it was found that the heat generation of the conical 3flats pin was higher than that of the cylindrical and the conical pin,and this pin accelerated the material flow behind the stirring tool along advanced side during the welding process,and reduced the material loss in the thickness direction.The flow field shows that the formations of the joints were sound and no additional defects such as grooves and unwelded joints under the parameters of 250mm/min,300mm/min.The cross-section of bobbin tool friction stir welding joint showed a compressed " hourglass " shape,and the cross-section of the joint can be delineated four distinct zones basing on the differentiation of microstructure,i.e.,the weld nugget zone,the heat mechanical affected zone,the heat affected zone,and base metal.The grain structure in the weld nugget zone exhibited continuous dynamic recrystallization,and the equiaxed grain size decreased with the increase of welding speed.The partially fine grains in the heat mechanical affected zone can be surveyed due to the incomplete recrystallization nucleation rate and the effective strain.The heat-affected zone and base metal indicated coarse lath-shaped rolling structures,and the grain size of the heat-affected zone decreased with the increase of welding speed.During the transition from the weld nugget to the base metal,the plastic deformation degree of the regional organization decreases successively,the number of dislocations and the low angle grain boundaries gradually increased,and the average size of the precipitated phase in the matrix increased.The grain orientation tended to be more pronounced,and the maximum pole density of the texture also increased.The microhardness of the ZK60 magnesium alloy bobbin tool friction stir welding joints presented a "convex" shape distribution,and the hardness reached the maximum in the weld nugget zone.The hardness distribution is mainly affected by the fine-grain strengthening and the solid solution and precipitation of the precipitation phase.The tensile strength and elongation of the joints both increased first and then decreased with the increase of the welding speed.The tensile strength of the joint reached the maximum value at 300 mm/min,which was 195 MPa,and the highest elongation reached 9.71% at 350 mm/min,which was 64.7% of the base material.The fracture positions were all located at the edge of the weld nugget zone in the advancing side of the joint closing to the heat-mechanical affected zone.The microscopic fractures showed that the defective joints were induced to crack at the hole defects,the joints presented a mixed fracture mode of brittle-ductile,and non-defective joints presented a ductile fracture mode. |
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