| :The benefits of conventional friction stir welding(FSW) which includes bobbin tool friciton stir welding(BT-FSW),while the benefits of the BT-FSW,except FSW,are well known and include joints that with low distortion,reduction in process loads,elimination of the possibility of penetration defect on the root side of the weld. The process is the ability to eliminate the large stiff structure behind the parts to be welded, this makes BT-FSW an effective method for jioning extruded cross-sections and tank structures.For some jiont configurations and desired applications bobbin tool welding is a process variant that can circumvent limitations arising from the high process forces in conventional FSW. As bobbin tools are highly mechanically loaded, in-depth understanding of the evolution of material flow behavior is essential to avoid tool failure and extend the range of applicable process parameters.There are urgent needs of bobbin tool technology for the longitudinal welding and circular welding to the cylinderical tank of large rockets, which thickness are mainly between4-8mm.Thus, Numerical simulations and experiments of6mm6061-T6aluminum alloy are researched in this paper.One transient temperature field model for BT-FSW is presented in this paper, a moving coordinate is introduced to replace the moving tool.Another is a thermal-flow coupling static model of6061aluminum alloy based on computational fluid dynamics and elastic-plastic mechanics theory.Both models’temperature fields validate each other and the numerical models are confirmed accurately by temperature testing and metallography.According to the temperature field simulation,it is found that sufficient plasticity of the material can be guarantee at high welding speed or low rotation due to the additional heat providing by the low shoulder, thus incressing the welding processing window. It is higher at the advancing side than that at the retracting side, and steeper before the welding tool than that after, which is asymmetric distribution at the shoulder zone. The highest temperature of515℃is located at front of the welding tool closed to advancing side. When it comes to flow field is found that threaded probe is benifical to material flow and the3flats of the pin results in reduction which are too large to material flow positively and defects are easy to form, that is consistent to the phenomena appearing in the welding processing experiment.Through the analysis of the heat model, the numeriacal simulations and the processing experiment, it can derive the structural design requirements and the size of the welding tool,The paper focuses on the influence of welded pressured deepth, it showed that only did the deepth of both top and bottom surfaces are appropriate, a good weld will be able to form. It can be determined by the analysis experiment that the tesile fracture is ductile due to there are some second phase particles in a lot of dimples. The tensile strength of the welded joints is235MPa as to pct76of the base material.while, it somewhat softens at the nugget zone and it pressents a typical W-type of the hardness.it is lightly smaller than conventional FSW.From the metallographic analysis,it can be concluded that the grain shape of the advancing side transites to nugget zone is relative sharply,however, it is more smoothly at the retracting side. Meanwhile both lazy S and line B are discovered in the welded. |
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