DISSIMILAR METAL JOINING OF ALUMINIUM AND STEEL USING FRICTION STIR WELDING

Abstract

The present work is focused on the study of friction stir welding (FSW) of dissimilar metals, specifically aluminium and steel, in various joint configurations. The study investigates the effects of tool geometry, process parameters, and sheet placements on the joint formation and failure types for a variety of aluminium and steel alloys. The primary objective is to achieve sound weld joints of aluminium and steel using different approaches. In the first part of the study, the author investigated the effects of tool geometry on FSW of aluminium alloy and mild steel in a butt joint configuration using three different tools of tungsten carbide (WC) material. The study shows that the heat generated in FSW directly depends on the area of contact between the tool surface and the plate. Therefore, the tool with the maximum diameter of the shoulder (Tool 1) produces the maximum heat and causes a crack near the weld interface. Excessive heating due to a large rubbing area makes this tool inappropriate for dissimilar Al-steel FSW joining. Additionally, the study finds that a tapered cylindrical pin causes excessive steel rubbing and produces many steel fragments on a similar offset compared to a straight cylindrical pin. Therefore, the straight cylindrical pin tool with a lower area of rubbing (Tool 3) is suitable for dissimilar Al-steel joining. Moreover, the study shows that the intermetallic layer between the Al alloy and steel is of variable thickness ranging from 1 μm to 4 μm. The formation of the IMC layer is reduced on joining with FSW. The tensile strength of joints formed using Tool 3 is comparable to the base metal strength of the weaker material. The weld made by Tool 1 and Tool 2 (having intermediate shoulder diameter) failed at a much lower strength. Therefore, the joint obtained by tool 3 is assumed to be optimized on strength criteria. Finally, a mixed type of fracture mode is visible in fracturing the sample in tension, where the top layer of the forged aluminium failed in a completely ductile way, and at the bottom part, the decohesion of the Al-steel interface took place. The weld made by Tool 3 showed a larger fraction of ductile failure than the weld made by the other two tools. Thereafter, the author investigates the effects of process parameters on the FSW of aluminium alloy (Al5052) with low-carbon mild steel. A novel approach was used to quantify discontinuities (steel fragments and voids) in the weld stir zone to illustrate the outcome of varying process parameters, like tool rotational and traverse speeds, on the ultimate tensile strength. Various characterization techniques and analyses are performed, such as FESEM, optical microscopy, temperature measurements, image analysis of weld micrographs, and fracture analysis. The study conceptualizes a heat input factor ω2/v (ω is tool rotational speed and v is welding speed) to represent the heat generation in FSW, which is found to be dependent on the process parameters. The study finds that the joint strength exhibited an inverse relation with the area of steel fragments and voids in the weld nugget zone, as well as with the heat input factor (ω2/v). The combined result of the heat input factor and the area of steel fragments and voids determines the soundness and strength of the weld joints. An optimized parameter window is attained at a lesser heat input factor (ω2/v) with minimum void-causing steel fragments in the aluminium alloy-steel weld stir zone. The weld joints formed at a higher heat input factor also exhibit more area of steel fragments and voids in the weld stir zone. Furthermore, the author translated the optimized tools and parameters to different dissimilar Al-steel alloys. The first metal system to validate the applicability of the tool and process parameters is Al6061 and Austenitic stainless steel ASS304. Another dissimilar Al-steel system consists of high-temperature P22 steel and Al5052. The second part describes two studies related to the joining of aluminium with steel through the lap seam and lap spot configurations using Friction Stir Welding. The first study investigates the failure types and mechanisms of lap seam joints with different parameters. The author observed that the presence of a geometrical feature called a "hook" near the stir zone boundary was critical in determining the strength and failure type of the joint. The hook is a deformed steel metal formed due to the rubbing action of the WC tool pin against the steel surface, and its size varies with different welding parameters. While the presence of hooks was beneficial for bonding, larger hooks reduced joint strength considerably. The author also found that the advancing and retreating hook size dictated the type of joint failure in the shear-tensile test, and the optimum height of the advancing hook was from 8 to 15 per cent of the Al sheet thickness with a retreating to advancing hook height ratio of 1.5. The later part of the study investigates various parameters affecting the joint quality of lap spot joints. The author studied the effects of sheet placement and tool pin length on the joint structure, mechanical strength, hardness, thermal cycles, and fracture characteristics. It was found that the placement of steel plates on the upper side led to the development of a distinctive double hook geometry, while the placement of aluminium plates on top resulted in an excessive amount of flash. The author also observed that the availability of Zn due to galvanization coating on steel led to the formation of Al-Zn eutectic, which caused liquation cracking along the outer surface of the hooks, and the fracture of FSSW joints started from the edge of the welded area and propagated up to the exit hole to fail the joints. It was also found that the pin length was a crucial parameter in the fabrication of Al-steel friction stir spot joints, and a short pin length that was unable to reach the half thickness of the bottom plate resulted in either no joint formation or joint formation with extremely limited mechanical interlocking. Positioning a steel plate on top and a pin length that reached half the thickness of the bottom plate resulted in the best joint in terms of load-bearing capability. The author also investigated tool geometry modifications to improve joint strength and found that increasing the tool shoulder diameter greatly improved joint strength, as the broader area of the steel plate was squeezed beneath the shoulder and interacted with the bottom aluminium plate. Two techniques of exit hole refilling were investigated. It was observed that using a pinless tool to fill and stir the Fe-Zn powder in the exit hole and then plugging the exit hole with aluminium filler wire improved the joint strength of lap FSSW joints.