Since its invention by TWI in the UK in 1991, Friction Stir Welding (FSW) has been extensively researched and applied in the manufacturing industry, including the aerospace industry. Unlike fusion welding, FSW is a solid-state process that avoids common defects such as porosity and cracking, making it possible to achieve defect-free welding of metals like Al 2xxx and 7xxx, which is widely used in aerospace manufacturing. While FSW has matured as a manufacturing process, there are still many issues that need to be addressed, particularly the avoidance of welding defects in the FSW process. To avoid defects, it is crucial to understand their causes and the process of their formation. Therefore, this presentation will focus on the discussion of welding defects during the FSW process. A thorough literature review will be conducted to introduce different types of defects and their formation. Understanding the causes and formation of defects will help to improve the welding process and avoid defects. This presentation will also discuss the possible mechanism of defect formation, based on our experiments and literature. By gaining a better understanding of FSW defects and their mechanisms, researchers and manufacturers can further improve the FSW process and develop better strategies for avoiding defects in the future.
Quantitatively measuring FSW defects is challenging, so the types of defects are not strictly defined. However, after a thorough investigation, the most common types of defects in FSW for square butt joints are voids (tunneling and cavity), kissing bonds, and surface defects (including flash and surface groove). Voids are the most common defect type in FSW and can be various shapes and formed by different processes. Some voids are extended along the traveling direction and referred to as tunneling defects. Other voids have no particular shape and can be variable in direction. Voids are typically found to result from high traveling speed. A kissing bond is a defect where two layers are combined extremely close but not enough for the establishment of metallic bonds. It is usually formed due to insufficient stirring and arises mainly from the presence of an oxide layer. The broken oxide layer is included in the joint, weakening its mechanical properties. Flash is formed due to high pressure from the shoulder and excessive friction heat. A surface groove is a groove-like cavity on the joint's surface that might result from inadequate plunge depth. After identifying the types of defects, experiments are conducted to determine their possible causes. Different welding parameters are tested, and the temperature and torque are measured for each experiment.
Most of the existing research on FSW defects remains qualitative in nature. The difficulty in observing some defects has resulted in a lack of experimental data, rendering some researchers' views unconvincing. Other researchers have attempted to model the defect formation process through numerical simulation of material flow using finite element analysis or CFD. However, these models are not practical or universally applicable for industry use. Therefore, it is crucial to study the influence of state variables, such as temperature, on the formation of defects. Although welding speed, rotation speed, and plunge force are important, the dominant factor in defect formation is the mechanical and thermal internal variables that are determined by these welding parameters. Temperature, in particular, has been identified as a critical internal variable for defect formation, and a boundary for proper temperature can be predicted based on previous work. By establishing the connection between state variables and defects, a better understanding of the real process of defect formation can be achieved. Future work should involve developing a scaled model for defect formation that is not limited to specific materials and has universal properties.
The issue of defects in FSW is a critical concern in the manufacturing industry, particularly in aerospace applications. These defects typically include voids, kissing bonds, and surface defects, and their formation process and impact on welding joints have been the subject of much research. However, most of these studies have only been qualitative in nature, and the lack of quantitative analysis makes it difficult to determine optimal welding parameters without extensive trial and error. This can lead to significant financial losses for the industry. Fortunately, temperature has been identified as a key indicator of defect formation, and previous research, along with our own experiments, have confirmed its importance. Going forward, more quantitative analysis and investigation into temperature as a factor in defect formation will be crucial for improving the reliability and quality of FSW in industry applications.