Influence of Traverse Speed in Self-Reacting FSW of AA6061-T6

Self-reacting friction stir welding (SRFSW) is a variant of friction stir welding in which a small modification of the tool design, i.e., the addition of a lower shoulder eliminates the need for a backing plate. In the present work, SRFSW was carried out on 4-mm-thick AA6061-T6 aluminum, which is one of the most extensively used of the 6000 series aluminum alloy at different traverse speeds. The focus of this research work is to understand the effect of tool traverse speed on the mechanical properties and the microstructural zones created in the joints, keeping all other parameters constant. From the tensile test, it was observed that the yield strength and tensile strength of the joints increased with the increase in traverse speed. From the fractography study, ductility was found to increase with the increase in traverse speed. From the Vickers microhardness test, it was clear that the hardness distribution across the thickness indicated uniformity in mechanical properties across the thickness. From the microstructural study, it was observed that with the increase in welding traverse speed, the elliptical swirl zone size and shape changed significantly, which affects the joint quality significantly. It was also observed that traverse speed has significant effect on the flashing out of material from weld zone. With lower traverse speed, chances of flashing out of material is more, which results in the formation of wormhole defect (main weld defect in friction stir welding). A saddle-shaped macrostructure of the bobbin tool friction stir welding joint was observed, which was prominent in the advancing side and was found to have a banded microstructure. 1. Introduction It has been reported that with the aim of increasing payload and reducing fuel consumption, shipbuilders are turning toward aluminum alloys for their potential to reduce the weight of ship structures considerably (Aluminium Insider 2016). Austal Ships built the 127-m Benchijigua Express, which is the world's largest aluminum ship commissioned in 2005 (Marine Insight 2013; Aluminium Insider 2016). The other advantage of using aluminum alloys is the considerably higher corrosion resistance as compared with low-carbon steels, which reduce the need for maintenance (painting, coatings etc.), resulting in reduced costs during service (Aluminium Insider 2016). The use of friction stir-welded aluminum has also been reported in manufacturing of naval warships (Aluminiumindustry.org 2017). The inventors of friction stir welding (FSW), The Welding Institute (TWI), have reported the application of FSW of aluminum in shipbuilding. Some of them are used for manufacturing freezer panels for fishing boats, honeycomb panels and corrosion-resistant panels, panels for decks, and in wall construction of high-speed ferries and hulls (Delany et al. 2007). The most commonly used aluminum alloys in the shipbuilding industry are the non-heat-treatable 5000 and heat-treatable 6000 series alloys (Aluminium Insider 2016). The 6000 series alloys are the most commonly used aluminum alloys for structural applications. These alloys can be joined using fusion welding techniques such as gas metal arc welding and gas tungsten arc welding but are crack sensitive and require appropriate filler material in sufficient quantity to eliminate cracking. Also, because of the significant heat input, the fusion-welded joints suffer from considerable distortion. There is considerable loss of strength in fusion-welded joints as the initial temper of the alloy is lost and the heat-affected zone (HAZ) has the properties of O temper-annealed materials (Lincoln Electric 2017). Unlike fusion welding techniques, the FSW, which is a solid state welding process, requires no shielding gases and filler materials to produce the joints. Because of low heat input, very low distortion is observed in the FSW of aluminum alloys. The residual stresses of friction stir-welded aluminum panels are very low. FSW produces joints with excellent mechanical properties and good surface finish. The mechanical properties of the joints produced by FSW have been found comparable with the parent material (PM). Also, the corrosion resistance of FSWjoints was found better than fusion-welded joints (Delany et al. 2007). Bobbin tool friction stir welding (BTFSW) or self-reacting friction stir welding (SRFSW) is a variant of FSW, which can potentially increase the application of FSW to curved, closed, and large structures. The SRFSW variant uses a bobbin tool, which has two shoulders and eliminates the need for a backing plate. Elrefaey et al. (2014) welded a rotary engine housing made of aluminum alloy using conventional friction stir welding (CFSW), which can be done with BTFSW without the need of any special arrangements. This small modification brings about significant changes in the process such as the fixtures, load experienced by the machine tool, weld cycle, tool wear rate, forces on the tool, mechanical properties, microstructure of the welded joints etc. In addition to this, there are differences in the process parameters in the two techniques. The effect of the process parameters in the two techniques is not necessarily the same. Detailed studies on the effect of tool design on the CFSW process have been carried out (Biswas & Mandal 2011; Venkateshwarlu et al. 2013). However, Sued et al. (2014) reported that the effect of tool features in CFSW is not the same for BTFSW and, hence, the tool design is not directly transferable and separate studies are required to understand the effect of different tool features. Lot of efforts have been put to understand the effect of process parameters on the mechanical properties and microstructure in CFSW (Liu et al. 1997; Rhodes et al. 1997; Li et al. 1999; Fujii et al. 2006; Ouyang et al. 2006; Cao & Jahazi 2009; Cavaliere et al. 2009; Kostka et al. 2009; Tanaka et al. 2009; Rajakumar et al. 2011; Xue et al. 2011; Barlas&Ozsarac 2012), but only a few investigations have been carried out on the effect of process parameters on the mechanical properties and microstructure of SRFSW joints. Lafly et al. (2006) carried out a comparative study between CFSW and BTFSW of AA6056 sheets. A microstructural study revealed that the two techniques produce similar grain structure, which can be related to the thermomechanical conditions induced by the process. Thomas et al. (2009) performed welding of 12% Cr steel plates of 12 and 8 mm thickness by CFSW and SRFSW, respectively. The tool used was a composite tool with the shoulder and probe made from different materials (Refractory alloys with different levels of Tungsten) using a matching taper drive coupling. The feasibility of successful defect-free joints in 12% Cr steel using SRFSW was demonstrated. A microstructural examination revealed that the heat input was similar on both sides of the weld. However, a detailed study of the process was lacking. Threadgill et al. (2010) studied BTFSW in 25-mm-thick AA6082-T6 and also compared the results with the data available for CFSW. Okamoto et al. (2012) studied bead-on-plate welds in 6XXX aluminum alloys using BTFSW. Several tool designs were applied at different rotational speeds and welding speeds to study their effect on mechanical properties. Liu et al. (2013) reported the presence of band patterns whose shape differed with change in welding speed in SRFSW joints in AA6061-T6. However, the study lacked a detailed description of the various features of these patterns and the reasons for the formation of these patterns. It is important to understand the various features of the joints and the reasons behind it to improve the tool design and optimize the process for wider application. In the present study, SRFSW using a fixed-gap bobbin tool has been carried out for AA6061-T6, which is one of the most extensively used alloy of the 6000 series aluminum alloy. AA6061 is a medium-to-high strength heat-treatable alloy with very good corrosion resistance and has applications in marine fittings, shipbuilding, motor boats etc. The experiments have been carried out at a constant rotational speed, and attention has been given to understand the effect of traverse speed on the microstructure and mechanical properties of the joints. Traverse speed is a very important parameter from production point of view and application of this process in industry. Microstructural studies have been used to understand and explain the formation of different zones of the joints.