The study investigates the effects of tool pin length, rotation speed, and dwell time on the microstructure and tensile strength of dissimilar joints between low-carbon galvanized steel and aluminium T6-6061 through friction stir spot welding

• Published in: Engineering failure analysis 2024-03, Vol.157, p.107925, Article 107925
• Main Authors: Feizollahi, Vahid, Heidary Moghadam, Ali
• Format: Article
• Language: English
• Subjects: Micro-hardness; Microstructure; Pin length; Tensile strength; Tool dwell time; Tool rotational speed
• ISSN: 1350-6307; 1873-1961
• DOI: 10.1016/j.engfailanal.2023.107925

•Aluminum 6061-T6 plates were joined with low-carbon galvanized steel by FSSW.•Due to the low process temperature, the steel microstructure did not change.•The intermetallic layer has a direct effect on the quality and strength of the joint.•By increasing the length of pins the tensile strength was improved. Friction stir welding is an efficient method for joining steel and aluminum plates at a point of disturbance. In many studies, the tool is only introduced into the aluminum plate, avoiding penetration into the steel plate due to its high strength and the need for a robust tool. Under such conditions, the created connection usually possesses low mechanical properties. In this research, by increasing the strength of the H13 steel tool through thermal operations, the possibility of penetration and bonding of aluminum 6061 with galvanized low-carbon steel has been provided. Moreover, by penetrating the steel plate, the integration of galvanized layers in the disturbance region has been facilitated. The examined variable parameters include pin length, rotational speed, and tool dwell time. Mechanical properties such as tensile strength, fracture force, and Vickers hardness were investigated. Furthermore, the weld microstructure and intermetallic layers were examined using scanning electron microscopy (SEM) and EDS analysis. Tensile and shear strength increase with an increase in pin length, rotational speed, and tool dwell time. Shear fracture loading showed a decrease with increasing pin length, then reached its maximum value when using long pins, while continuously increased with higher rotational speed and longer tool dwell time. As we approach the center of the weld in both the steel and aluminum parts, smaller grain sizes are observed due to increased plastic deformation, more material mixing, and increased mechanical tool pressure, while in the heat-affected zone, a coarse grain structure forms. Hardness in the aluminum plate decreased in the heat-affected zone and reached its minimum value in the thermo-mechanical-affected zone, then increased to the maximum value in the nugget zone, while hardness changes in the steel plate were uncertain. The most influential parameter on mechanical and microstructural properties was pin length, followed by rotational speed and tool dwell time.