【作者】 丁建；

【导师】 李桂中；

【作者基本信息】 吉林大学 ， 材料加工工程， 2012， 硕士

【摘要】 低碳钢多层板点焊随着接头板材层数的增加，使得接头的结构愈加复杂，因而熔核区电流场和散热条件与两层板接头相比更为复杂，分流和飞溅现象更加严重。这就增大了获得优质熔核的难度。本文借助对低碳钢多层板点焊动态过程重要信息的测试与分析，并结合最终熔核质量的评价，力求推出一种较为理想的监控机制，即采用一套控制参数来同时适应或满足多层板点焊的实际工况，以使点焊操作更为便捷。本文首先通过工艺试验，获得低碳钢二层板点焊最佳工艺参数；在此基础上，运用多种动态信息的对比试验进一步探讨了不同控制模式对多层板点焊的适应性；进而深入探讨了采用不同监控方法时，运用相同控制参数实现多层板点焊热过程实时监控的可能性。最后本文还对多层板点焊过程熔核生长形态进行了较为细致的研究，为揭示多层板点焊熔核形成规律以及防止多层板点焊的飞溅问题提供了可靠依据。研究结果表明，简单定时法用一套工艺参数无法实现多层板优质点焊，熔核质量随着接头板材层数的增加而明显下降。恒流法和IDRC法在无分流的条件下，能够用一套控制参数较好地实现二层板或三层板的优质焊接，却无法同时保证四层板的点焊质量。这主要因为四层板点焊中间界面的熔核生长过快且尺寸极易超出电极直径的压合范围而产生后期飞溅，严重影响焊接质量。但在有焊点分流的情况下，恒流法由于其监控机制的局限性，不能对分流现象做出有效补偿，根本无法保证多层板点焊的焊接质量，且随着板材层数的增加分流影响也更加严重。而IDRC法凭借对不同时刻熔核质量信息的控制优势，可有效地对分流影响做出及时而有效热量补偿，即使存在分流的情况下仍可适应二层板或三层板点焊热过程的实时控制；但无法切实保证四层板点焊的焊接质量。为有效地抑制四层板点焊的后期飞溅问题，可将电极直径增大到7.5mm，以增大电极的压合尺寸，实现四层板的优质焊接。对熔核形态的研究结果表明，三层板点焊熔核虽然为一个整体熔核，但其生长过程较为复杂。在焊接前期，熔核在板材界面处优先生长，且熔核在界面处的尺寸要大于在中间板材处的尺寸；到焊接中期，熔核在中间板材处尺寸的增长速度逐渐加快，并逐渐超过熔核在界面处的尺寸；并最终在中间板材处达到最大熔核尺寸。四层板点焊熔核也是一个整体熔核，但其生长过程更为复杂。在焊接初期，熔核只在中间界面处生长，而在上下两界面处并没有形成熔核。随着焊接的进行，中间界面母材不断熔化接触电阻逐渐降低，直至中间界面的接触电阻小于上下界面接触电阻时，熔核便开始在上下界面处快速生长，且很快赶上并超过熔核在中间界面处的尺寸。随着上下界面母材不断熔化接触电阻逐渐降低，熔核又开始在中间界面处快速的生长并很快赶上并超过上下界面的熔核尺寸，并最终在中间界面处达到最大熔核尺寸。更多还原

【Abstract】 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.更多还原