| Low carbon steel unequal-thickness multilayer sheets due to the overall thickness andthe different way of packing, makes the form of the joint structure is very complex, Whenthick plate position is different,due to the factors of thickness will make the electrodepressure, current density and heat changed,and lead to serious shunt and splash phenomenon.This case increases the difficulty of access to high quality nugget and the uncertainty ofnugget growth. With the help of unequal-thickness multilayer sheets spot welding nuggetresearch of microscopic growth way and combined with the evaluation of the final nuggetsquality, Finally,tries to find out the effect law of nugget growth morphology in the case ofdifferent control mode, overall thickness and pile methods of joint. In addition, compare theinfluence on welding result after increased preheat process with constant current and IDRCcontrol mode. Find out the most reasonable technology way in spot welding process.The research illustrate that under the total thickness have been fixed, different ways tocombine sheets will not have a big impact on overall heat input, but the nugget order anddistribution have obvious changes. It is because that the constant current control willmaintain a constant total loop current, the total current through three layer sheets is remainsunchanged, and then the overall heat also will not happen obvious change. Nugget growthlevel are more similar. But, when thick plate position is different will cause the resistancechanged between two contact surfaces of thick plate, meanwhile the required power of eachcontact surface will redistribute, Eventually lead to nugget size and initial nugget position ischanged. Comparison with different control modes in similar parameters is found thatSimple-timing method is worst for different thickness three layers sheets welding, Constantcurrent method and IDRC method are more outstanding, especially the IDRC method canachieve the high quality of welding joint in a relatively weak technological parameters. Afterincreased the weld preheating found that preheating is very effective for improve joint abilityof dissipation and production heat. Get bigger nugget diameter in the same parameters andavoided the splash, but make nugget partial phenomenon more outstanding for theasymmetric joint; For the IDRC method, preheating although never significant increasing thenugget diameter, but shortened the welding time, obtain the high quality welded joint in theweak technology, improved welding efficient. In addition, preheating is effective to improvethe nugget migration phenomenon in the asymmetric structure, nugget diameter is morecloser in two side, it also illustrates that the preheating improved the heat distribution and uniformed the welding heat.The result show that the length of incubation period is related to the overall thickness,the greater total thickness, the more energy needed to form nugget, and the longer incubationperiod. It was also found that for the symmetric spot welding joints of three equal-thicknessand unequal-thickness sheets, the middle sheet size have an important influence on thenugget initial form position. Besides, for the asymmetric spot welding joints of threeunequal-thickness sheets, the nugget will firstly appear in the interface of the lateral sheetand the middle sheet. At last, for all kinds of three sheet stack, if the nugget can fully grow,the ultimate shape of the whole nugget will be symmetrical, and its will basically locate atthe place on the half of the overall stack. |
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