بررسي اثر نيروي الكترود و زمان اعمال آن بر تنش پسماند ناشي از جوشكاري مقاومتي نقطه‌يي ورق‌هاي آلومينيوم

Investigation into the Effect of Electrode Force and Time on Residual Stress Resulting from Resistance Spot Welding of Aluminum Sheets

هدف از اين پژوهش بررسي اثر نيروي الكترود و زمان اعمال آن بر تنش‌هاي پسماند ناشي از جوشكاري مقاومتي نقطه‌يي آلياژ 6T-6061 آلومينيوم است. در اين مطالعه از يك مدل المان محدود مكانيكي ـ الكتريكي ـ حرارتي دو بعدي با تقارن محوري براي پيش‌بيني توزيع دما، اندازه‌ي دكمه‌ي جوش و تنش‌هاي پسماند استفاده شده است. نتايج به دست آمده از شبيه‌سازي با آزمايش‌هاي عملي كه تنش‌هاي پسماند در آنها با استفاده از تفرق اشعه‌ي x اندازه‌گيري شده، مقايسه شده است. مطالعات نشان مي‌دهد بالاترين مقدار تنش پسماند كششي در فصل مشترك ورق‌ها و مركز دكمه‌ي جوش رخ مي‌دهد و در راستاي شعاع جوش مقدار تنش‌ها كاهش مي‌يابند. نتايج حاصله نشان‌گر آن است كه با افزايش نيروي الكترود و زمان اعمال آن تنش‌هاي پسماند افزايش مي‌يابند اما تاثير زمان اعمال نيرو بر تنش‌هاي پسماند بيشتر از مقدار نيرو است.

Nowadays, application of light materials, such as aluminum sheeting, plays a very important role in various automotive and aerospace industries. Resistance spot welding is a widely used method in assembly lines of such industries. The main purpose of this study is to investigate the effect of electrode force and time on residual stresses produced by the resistance spot welding of aluminum alloy, 6061-T6. In this study, a two-dimensional electro-thermo-mechanical finite element model with axial symmetry has been employed to predict the temperature, nugget size and distribution of residual stress in the resistance spot welding process. For more realistic analysis, temperature dependent physical and mechanical properties have been defined for the electrodes and sheets. The simulation results have been compared with the results obtained from experimental tests. For validation, the nugget size and residual stresses were compared. The diameter of the nugget was measured by metallographical observation, and the residual stresses were measured by an X-ray diffraction method at three points of the nugget in three equal samples. The studies show that the highest amount of tensile residual stress occurs at the intersection of the sheets and at the center of the nugget; by moving away from the center along the nugget radius, residual stresses diminish. To study, numerically, the electrode force effect and its time, a set of parametric studies was designed. These parameters are current, current time, and either force or its time. The results indicate that by increasing the electrode force and its time, the residual stresses increase. But the effect of electrode force is less than its time on residual stresses. This fact refers to higher heat transfer and cooling rate at the welding zone. An increase in forcing time causes a higher cooling rate, and more columnar dendrite structure can be observed in the nugget area. This will create concentrated residual stresses at the nugget zone.