Causes of Cracks in Mid-Frequency Spot Welds

An analysis of the reasons for cracks in certain structural welds is conducted from four aspects: macroscopic morphology of the welding joint, microscopic morphology, energy spectrum analysis, and metallographic analysis of the mid-frequency spot welding machine weldment. The observations and analysis indicate that the welding cracks are caused by external forces, primarily due to the presence of extensive welding defects, with improper welding processes and inadequate cleaning of the welding surfaces being the main contributing factors to these defects. Below are several problems that lead to joint cracking:

Crystalline cracks:
During the solidification and crystallization of the welding pool, cracks form along the grain boundaries of the weld metal due to crystallization segregation and shrinkage stress and strain. These cracks occur only within the weld.

Liquation cracks:
During welding, under the influence of peak temperatures in the welding heat cycle, intergranular metal near the weld seam in the interlayers of multi-layer welds may re-melt due to heating. Under certain shrinkage stress, cracks develop along the austenite grain boundaries, a phenomenon sometimes referred to as hot tearing.
High-temperature low-ductility cracks:
After the completion of liquid phase crystallization, as the welded joint metal begins to cool from the material’s ductile recovery temperature, for certain materials, when cooled to a certain temperature range, the ductility decreases due to the interaction of strain rate and metallurgical factors, leading to cracking along the grain boundaries of the welded joint metal. This type of cracking generally occurs in the heat-affected zone farther from the fusion line than liquation cracks.
Reheat cracks:
After welding, during stress relief heat treatment or without any heat treatment, cracks develop along the austenite grain boundaries of the weld metal at a certain temperature under specific conditions. Reheat cracks are a significant issue in the welding of low-alloy high-strength steels, especially in the thick plate welds of low-alloy high-carbon steels and heat-resistant steels containing a large amount of carbide-forming elements (such as Cr, Mo, V). Dealing with these defects is time-consuming and has a significant impact on production.
Suzhou Agera Automation Equipment Co., Ltd. specializes in the development of automated assembly, welding, testing equipment, and production lines, primarily serving industries such as household appliances, automotive manufacturing, sheet metal, and 3C electronics. We offer customized welding machines, automated welding equipment, and assembly welding production lines according to customer needs, providing suitable overall automation solutions to assist companies in quickly transitioning from traditional production methods to high-end production methods. If you are interested in our automation equipment and production lines, please contact us: leo@agerawelder.com