Dissimilar friction stir welding (DFSW) is the application of friction stir welding (FSW), invented in The Welding Institute (TWI) in 1991, to join different base metals including aluminum, copper, steel, titanium, magnesium and other materials. It is based on solid state welding that means there is no melting. DFSW is based on a frictional heat generated by a simple tool in order to soften the materials and stir them together using both tool rotational and tool traverse movements. In the beginning, it is mainly used for joining of aluminum base metals due to existence of solidification defects in joining them by fusion welding methods such as porosity along with thick Intermetallic compounds. DFSW is taken into account as an efficient method to join dissimilar materials in the last decade. There are many advantages for DFSW in compare with other welding methods including low-cost, user-friendly, and easy operation procedure resulting in enormous usages of friction stir welding for dissimilar joints. Welding tool, base materials, backing plate (fixture), and a milling machine are required materials and equipment for DFSW. On the other hand, other welding methods, such as Shielded Metal Arc Welding (SMAW) typically need highly professional operator as well as quite expensive equipment.
Principle of operation
The mechanism of DFSW is very simple. A rotating tool plunges into the interface of parent metals, and heat input generated by the friction between the tool shoulder surface and top surface of the base metals lead to softening of the base materials. In other words, the rotational movement of the tool mixes and stirs the parent metals and create a softened pasty mixture. Afterwards, the tool's traverse movement along the interface creates a joint. This results in a final bond that combines both mechanical and metallurgical bonding at the interface. These two bondings are critical in order to achieve proper mechanical properties. Butt and lap designs are the most common joint types in dissimilar friction stir welding (DFSW). Likewise, one material is generally harder than the other. In general, hard and soft materials are placed in advancing and retreating sides respectively during welding.
Tool Geometry
Tool configuration is an important factor to achieve a sound joint. The tool consists of two parts including tool shoulder and tool pin, as shown in below figure. The tool shoulder generates frictional heat, while the tool pin stirs the softened materials. Various pin and shoulder configurations may be used for DFSW. "Cylindrical", "rectangular", "triangular" and "threaded-cylindrical" are the most common tool pin profiles, while "featureless" and "scrolled" are the most common tool shoulder configurations. Tool material selection is dependent on the base materials to be joined. For example, for aluminum/copper joints, hot working alloy steel is generally used, while for harder metals such as titanium/aluminum joints, tungsten carbide is common.
Welding Parameters
In DFSW, mechanical properties mainly include tensile strength, hardness, yield strength, elongation. Selecting optimum welding parameters results in achieving proper mechanical properties of the joint. Tool rotational speed (rpm), tool traverse speed (mm/min), tool tilt angle (degree), tool offset (mm), tool penetration (mm), and tool geometry are most important welding parameters in DFSW. The tool center is typically placed in the centerline of the joint for similar joints such as aluminum/aluminium or copper/copper joints; in contrast, it is shifted towards the softer materials in DFSW called tool offset. It is a significant factor to achieve a joint possessing smaller welding defect and higher mechanical properties. Generally, harder and softer materials are placed in Advancing Side (AS) and Retreating Side (RT) respectively. Regardless of the tool geometry, which plays a critical role on final mechanical and metallurgical properties of the weldment, the effect of the tool rotational speed and tool offset are taken into account as the most important welding parameters during DFSW.