Abstract
The concept of sustainable manufacturing has compelled the automotive industries to produce vehicles that are fuel-efficient and emit low greenhouse gas emissions. For this purpose, the car manufacturers have been reducing the overall weight of vehicles without compromising quality and passenger safety. In the past, steel was the major constituent in car bodies, whereas car bodies are also fabricated with other high-strength-to-low-weight ratio materials in the present age, including aluminum, magnesium, and composites. Car bodies with multi-material components involve the need for joining different materials together since the production of a complex part as a single unit is not possible. The conventional welding methods involve the melting of the base materials. Hence, they are not normally suitable for joining dissimilar materials due to their very different chemical, physical, and mechanical properties. Furthermore, the addition of a filler material could alter the weldment properties. For these reasons, new solid-state welding technologies have been introduced in the past decades to overcome the limitations of conventional welding methods. Friction stir welding (FSW), invented in 1991 by Thomas in The Welding Institute (TWI), is one of the solid-state joining processes that uses a non-consumable tool to weld two metals. FSW is popular for joining dissimilar materials ranging from low to high melting points and with different thicknesses.
This thesis aims to comprehensively study the FSW of aluminum and steel sheets with a low thickness (≤ 1 mm) for possible use for automotive applications. The effect of the most important FSW parameters on the microstructure and mechanical properties of the weld joints will be evaluated. The formation of different types of defects and the possible presence of Al/Fe intermetallic (IMC) layers will be thoroughly investigated. In addition, the effect of these phenomena on the mechanical performance of the weld joints will be discussed in detail.
In the preliminary stage of experiments, AA6016 with 1 mm thickness and DC05 steel with 0.8 mm thickness were welded in a lap joint configuration. The FSW experiments were carried out in force control and position control settings with varying rotational and welding speeds. In force control, a constant vertical force was maintained over the aluminum surface; in position control, a constant plunge depth was retained during the FSW experiments. It has been discovered that the joints obtained with the position control setting were not properly welded. Hence, they were not qualified for microstructure and mechanical investigations. On the other hand, the joints realized with the force control setting were properly welded and, therefore, microstructure and mechanical tests were performed to evaluate the weld joint properties.
Similar to a typical FSW joint, the welds obtained with the force control setting consisted of a weld nugget (WN), heat affected zone (HAZ), and thermo-mechanically affected zone (TMAZ). Defects, such as voids and tunnels, were found in the weld joints due to an improper intermixing of the workpiece materials. The weld joints were categorized into three morphologies: incomplete penetration, complete penetration with defects, and complete penetration without defects. The mechanical properties of the weld joints were evaluated by Vickers hardness and shear tension
tests. In the second stage of FSW experiments, AA6016 and DC05 both with 1 mm thickness were joined with a force control setting. The results showed that defects were significantly reduced due to the adequate heat generation and proper intermixing of workpiece materials. However, some IMC layers of about 1 μm thickness were observed at the Al/steel interface. The mechanical properties were evaluated by hardness mapping and shear tension tests.
From the experimental results, it can be concluded that FSW performed with the force control setting allows to obtain sound quality weld joints as compared to the position control setting. Overall, very low rotational and high welding speeds normally generate insufficient heat input and an improper intermix of the workpiece materials. As a result, defects form in the weld joint, which reduces weld strength. On the other hand, very high rotational and low welding speeds often generate excessive heat input, so promoting the formation of thick IMCs, which have a deleterious effect on the weld strength. Therefore, improper heat input and intermixing of the materials are the main critical factors influencing quality of weld joints.
A variety of aluminum and steel sheets with low thicknesses are used in the manufacturing of car bodies. Therefore, the findings of this research provide some guidelines to effectively employ the FSW technology to join aluminum- and steel-based sheets. In the future, the current research work could be extended to evaluate the effect of other FSW parameters, such as tool pin profile and tilt angle, and of tool wear on the microstructure and mechanical properties of the weld joints. This activity could be conducted with the support of thermographic analysis in order to monitor the welding process and to predict joint quality (as a non-destructive control technique) and tool wear through the analysis of the temperature field on the metal and tool surfaces.