Weld Nugget Formation Research Articles

Weld growth mechanisms during resistance spot welding of two and three thickness lap joints

Understanding the mechanisms of weld growth during resistance spot welding is a prerequisite for the development of optimum welding conditions which ensure high levels of joint quality in autobody manufacture. This study compared the heat development/weld growth of double and triple sheet lap joints as a means of understanding the role of welding conditions, sheet thickness configuration, and physical material properties on joint microstructure and performance. High temperature heat patterns have been determined using metallographic and thermographic techniques for double and triple sheet lap joints in uncoated low carbon steel. In the case of double sheet joints, initial heat generation and weld nugget formation was observed to occur where the resistance to the flow of current was the greatest, i.e. at the sheet/sheet interface. This was also the case for heat generation in triple sheet joints, but in this more complex case weld nugget formation was dependent on sheet thickness and joint configuration. Furthermore, the incubation period to weld nugget formation was dependent on total joint thickness with a longer weld time required for joints of greater thickness. For non-uniform triple sheet joints, heating was observed to be concentrated around the thinnest sheet as a result of the way heat was dissipated by the steel substrate.

Resistance spot welding of galvanized steel: Part II. Mechanisms of spot weld nugget formation

Dynamic inspection monitoring of the weld current, voltage, resistance, electrode displacement, and force was performed in conjunction with a detailed study of the effects of material variations and weld process parameter modifications on resistance spot welding of coated and uncoated steels. In order to determine the mechanisms of weld nugget formation and growth, scanning electron microscopy photos were taken of the developing nugget. These physical changes were then related to the dynamic inspection curves and the welding current lobe. The effects of material variations and weld process modifications, the results of which were presented in Part I, can be explained through an understanding of these mechanisms.