Study On Dynamic Process And Nugget Form Of Resistance Spot Welding For Multilayer Low-Carbon Steel Sheets

With the increase of the connector sheet layers, the joint structure of low carbon steelmultilayer sheets spot welding would be more complex, thus the current field and thermalconditions of the nugget zone are more complex than two sheets joint, so that streaming andsplashing phenomenon are more serious. This case increases the difficulty of access toquality nugget. With the help of test and analysis of the important information of thedynamic process of low carbon steel multilayer spot welding and combined with theevaluation of the final nuggets quality, an ideal monitoring mechanism would be introduced,which uses a set of control parameters to adapt or to meet the actual condition of themultilayer sheets spot welding, in order to make the operation of the spot welding moreconvenient.Firstly, based on the process testing, the best process parameters of the low-carbon steeltwo sheets spot welding would be obtained. And then, by means of the comparative test ofdifferent dynamic information,the adaptability of the different control modes for multilayersheets spot welding is explored; further, the possibility of real-time monitoring of thedifferent monitoring methods is discussed, using the same control parameters to achieve ofthe multilayer sheets spot welding. Finally, It was also studied in this paper that the nuggetgrow morphology of the multilayer sheets spot welding, which provide a reliable basis forrevealing the nugget formation law of multilayer sheets spot welding and preventing thespatter of the multilayer sheets spot welding.The results show that the simple-timing method can’t achieve high-quality welding witha set of welding parameters in the process of multilayer sheets spot welding. With theincrease of the sheet layers, the quality of the nugget decreases. Without the conditions ofshunting, the constant current method and the IDRC method can use a set of controlparameters to achieve high quality welding of the two sheets or three sheets, but which can’tensure the quality of four sheets spot welding at the same time. This is mainly because thenugget of four sheets spot welding grows excessively fast in the middle of the interface andthe size of nugget can easily exceed the crimping area of the electrode diameter to producethe late splash, and seriously affect the quality of welding. But in the case of shunting, due tothe limitations of the monitoring mechanism, the constant current method can’t makeeffective compensation for the diversion phenomenon, and can’t guarantee the quality ofmultilayer sheets spot welding, and the impaction of the diversion phenomenon is more serious with the increase of the sheet layers. Owing to the advantages of controlling of thenugget quality information on the different moments, the IDRC method can effectively maketimely and effective heat compensation to diversion influence. In the presence of thediversion, it can still meet the two sheets or three sheets spot welding thermal processreal-time control; but it also can’t ensure the quality of four sheets spot welding. Foreffectively suppressing the late splash of the four sheets spot welding, the electrode diameteris increased to7.5mm to increase crimping area of the electrode, and high-quality welding ofthe four sheets can be achieve.The study on the nugget morphology shows that the nugget of three sheets is a wholenugget, but its growth process is quite complex. In the previous stage of the welding, thenugget grows preferentially at the interface of the sheets, and the size of the nugget at theinterfaces is greater than that in the middle of the sheet; at the welding medium-term, thegrowth speed of the nugget in the middle of the sheet rapidly increased, and the size of thenugget gradually exceed its size at the interface; and ultimately, maximum nugget size isappear at the middle sheet. The nugget of four sheets spot welding is a whole nugget, but itsgrowth process is more complex. Early in the welding, the nugget grows only in the middleinterface, and the nugget don’t form in the upper interface and lower interface. With theprocess of the welding, the parent material of the intermediate interface continues to meltand the contact resistance gradually decreases. The nugget at the upper and lower interfacesdon’t grow fast until the contact resistance of the middle interface is less than the contactresistance of upper interface and lower interface, and soon catches up and exceeds the sizeof the nugget in the middle of the interface. As the parent material of the upper and lowerinterfaces continues to melt, the contact resistance decreases, the nugget in the middleinterface grows quickly, and catches up and exceeds the nugget size of the upper and lowerinterfaces, ultimately, the maximum nugget size is appear in the middle interface.