Friction Stir Welding of Thin Sheets – An Overview

Friction Stir Welding of Thin Sheets – An Overview

Yogesha C , Suresha C N, Sarala Upadhy Department of Mechanical Engineering, Jyothy Institute of Technology,

Bangalore-INDIA

Rajaprakash B M

Department of Mechanical Engineering, University Visvesvaraya College of Engineering, Bangalore University, Bangalore-INDIA

Abstract: Friction-stir welding (FSW) is a solid-state joining process that uses a third body tool to join two facing surfaces. FSW has been proven successful on numerous of alloys and materials, including high-strength steels, stainless steel and titanium. FSW produces welds of high quality in difficult to weld materials such as aluminium and is fast becoming the process of choice for manufacturing light weight transport structures such as boats, trains and aeroplanes. FSW is considered to offer advantages over fusion welding in terms of dissimilar jointing. Improvements on the existing methods and materials as well as new technological development, an expansion are expected. Research towards the further extension of the process to join thin sheet of similar and dissimilar metal combinations like aluminium alloys, aluminium to magnesium, steel and titanium is currently underway. In this paper an attempt has been made to review the current state of FSW in joining of thin sheets.

Keywords: Friction Stir Welding, Thin sheet, Tool, Mechanical Properties

1. INTRODUCTION

   Increasing in the need to design for lightweight structures in aircraft panels and vehicle body shells has lead invention of advanced joining technology which is an integral part of the manufacturing processes. Considerable effort has been expended to develop various joining processes and assess their suitability for use in light weight structures [12].

   FSW is one of the solid-state joining techniques which was invented at The Welding Institute (TWI), UK, in 1991. The FSW has been found to be effective for joining hard-to- weld metals and for joining plates with different thicknesses and different materials [3].

   Fig.1. Schematic drawing of FSW process [4]

   Fig.2. Joint configurations for FSW [5]

   The working principle of FSW as shown in Fig 1. In FSW the parts intended for joining are usually arranged in a butt configuration. The rotating tool is then brought into contact with the work pieces. The tool has two basic components: the probe, which protrudes from the lower surface of the tool, and the shoulder, which is relatively large diameter. The length of the probe is typically designed to match closely the thickness of the work pieces. Welding is initiated by first plunging the rotating probe into the work pieces until the shoulder is in close contact with the component top surface. Friction heat is generated as the rotating shoulder rubs on the top surface under an applied force. Once sufficient heat is generated and conducted into the work piece, the rotating tool is propelled forward. Material is softened by the heating action of the shoulder, and transported by the probe across the bond line, facilitating the joint [4]. FSW can be used in a variety of joint configurations as shown in Fig 2. Including butt welds, dissimilar thickness butt welds, lap penetration, and lap fillet configurations. This can often require redesign of the product, to take full advantage of the FSW process [5].

   The objective of this paper is to study the attempts that have been made by different researchers in the interest of joining thin sheets by FSW process.

2. THE REVIEW WORK IS CLASSIFIED INTO SIX MAJOR GROUPS FOR EASE OF

   UNDERSTANDING AND DISSEMINATION

   1. FSW of thin sheets

      P Staron et. al. studied the residual stress states of FSW joints of 3.2 mm thick AA2024 Al-sheets that have been welded under mechanical tensioning were analyzed, they found that large compressive stress can be induced in the weld by applying mechanical tensioning during welding [6].

      A. Scialpi et. al. studied thin aluminium alloy 2024-T3 and 6082-T6 sheets of 0.8 mm thick, have been welded in the rolling direction by FSW (FSW for ultra-thin sheets). Micro-hardness and residual stress measurements have been executed on welded sheets for each joint typology [7].

      R.K. Kesharwani et. al. joined 2.0-mm thick AA5754-H22 and AA5052-H32 sheet metals with optimized tool design and process parameters to maximize the weld strength and total elongation reducing the surface roughness and energy consumption, they found that the formability was improved by 27% using the modified conical tractrix die [8].

      Shuja Ahmed et. al. joined AA6XXX series aluminium alloys of thickness 0.44 mm have been welded together in both butt and lap fashion. They established the measure of joints tensile strength (by conducting both transverse and longitudinal tension tests) and the micro-hardness [9].

      1. Heidarzadeh et. al. a thermal model was developed and applied to simulate the friction stir welding of pure copper plates with the thickness of 2 mm. The different traverse speeds of 100, 200, 300, and 400 mm /min and rotational speeds of 400, 700, 900 rev min1 were considered as welding parameters, they found that the peak temperature was the dominant factor controlling the grain size and mechanical properties, where the fine grains could be achieved at low rotational speed as well as high traverse speed [10].

      2. FSW of special materials

         High-strength of M190 steel sheets of 1.1 mm thickness were joined by friction-stir welded by Ghosh et. al [11]. It was found from the optical microstructure of the joints that, at the weld nugget depended on the cooling rate during welding. They observed the effect on joint efficiency were more than 50 pct of that of parent alloy [11].

         AZ31B magnesium alloy of 2 mm thickness were joined by F SW with various welding parameters. By the three-point method experimentally studied that the transverse rigidities of the friction stir welded samples were lower of those of the base material. Furthermore, reduction of transverse rigidities was induced by the decrease of Youngs moduli [12].

         D.G. SANDERS et. al. joined two sheets of Ti-6Al-4V alloy of 2.54 mm thickness were butt welded by FSW to have an ultra-fine grain size. They conducted the experiments in

         accordance with industry standard testing specifications. It is reported that the macrostructures revealed a good quality weld joint with no inclusions, no voids, and complete fusion [13].

      3. Tool geometry used for FSW of thin sheets

      2-mm-thick of 7B04-T74 aluminum alloys were joined by Friction stir lap welding. Min Wang et. al. [14] investigated the effect of pin length on hook size and joint properties. They found that the process parameters, characteristics of pin length on joint properties are dependent on the heat input levels.

      1. Simoncini et. al. [15] investigated the effect of tool geometry, tool configuration and process parameters on 1 mm thick of AA5754 Al alloy by FSW. They found that the joints welded by PIN tool exhibits more grain refinement and mechanical properties than joints welded by PINLESS tool.

         The fracture behavior and intermetallic formation were investigated by joining of 2.8mm thickness of Al-Cu bi- layer sheets by FSW with help of tapered threaded pin. They found that the FSW of Al-Cu laminates demonstrate important role of contact condition on joint quality [16].

         1. Process parameters in FSW of thin sheets

            Ghosh et. al [11] joined high-strength of M190 steel sheets of 1.1 mm thickness by FSW under different tool rotational and traversing speeds. They concluded tha adequate joint efficiency (60 pct) can be obtained by FSW through controlling the tool rotational and traversing speed.

            Tool material, configuration and geometry, which affects on macrostructure, microstructure, and mechanical properties of pure titanium weld joint were investigated by K. Reshad Seighalani et. al. [17]. They found that Ti can be joined by the FSW, using a tool with a shoulder made of tungsten (W) and simple pin made of tungsten carbide (WC). Furthermore, the tensile strength and the yield strength of the welded joint were similar to the corresponding strengths of the base metal.

            FS welds of very thin plates of the AA 6016-T4 aluminium alloy has been done by D.M. Rodrigues et. al.[18] with two different tools to analyze the microstructure and mechanical properties. They concluded that the differences in tool geometry and welding parameters induced significant changes in the material flow path during welding as well as in the microstructure in the weld nugget.

         2. Significant microstructures of FS Welded material 2 mm thick of AA2024-T3Cu10100 dissimilar joints were made by FSW offsetting the tool probe and selected processing parameters. They analyzed that the Joint microstructure by conventional optic microscopy and scanning electron microscopy also found that microstructure variation significantly affects the micro-hardness distribution in the cross-section of the joint [19].

            Amir Abbas Zadpoor et. al.[20] studied the FS welded joints with dissimilar aluminum alloys of 2024-T3 and 7075-T6 of

            1.2 mm thicknesses were welded in different combinations of alloys and thicknesses. They found that the dissimilar alloys has no significant diffusion of alloying elements between the alloys and the fracture mechanism were mixture of ductile and brittle fractures and to qualify as quasi- cleavage.

            Investigated the internal features, defects and initial gap between the two parent sheets of AA2198-T851 of 3.1 mm thickness were joined by FSW. Based on the experimental data micro-structural, mechanical characterization of base material and Failure location is sensitive to the presence of some of the considered weld flaws [21].

         3. Challenges faced in FSW of thin sheets

      Jiye Wang et. al.[22] joined the Ti-6Al-4V alloy of rolled sheet with a thickness of 2.5 mm were welded with a position-controlled FSW machine. They found that the tool debris particles became the void nucleation sites, which significantly lowered the total ductility, although the particles had little effect on the uniform plastic strain.

      Dissimilar lap joints of Ti-6Al-4 V titanium alloy sheet of

      1.6 mm thick and an AISI 304 stainless steel sheet of 1 mm thickness were welded by FSW process. Based on the experimental and numerical analysis, both hooking defects and tunnels may occur with incorrect choice of process parameters. [23].

      1. mm thick 2060 Al-Li alloy plates were FS welded, they investigated the effects of welding speed and rotation speed on formation quality, microstructure, phase particles transformation, and mechanical properties of the joints. They found that the joints fails with a shear fracture due to welding parameters, where there exist f layered ledges and river patterns, joints welded using high heat input fracture from the heat-affected zones close to the thermo- mechanically affected zones [24].

         Muthukrishnan et. al. studied the effects of process parameters in butt welded of Al-6082-T6 plates Of 3mm thickness plates by FSW method. They concluded that tensile strength of FSW welds is directly proportional to the travel / welding speed. Hardness drop were observed in the weld region [25].

         FSW of 2024-T3and 6082-T6 Al rolled sheets of 0.8 mm thickness have been tried by Emanuela Cerri et. al. used a cylindrical, non-threaded probe. They found that the specimens fractured in the middle of the weld zone in a ductile mode due to the micro-void size distribution and coalescence of void nucleation mechanism [26].

3. CONCLUSIONS

This paper provides some useful insights into the use of friction stir welding for joining configuration of thin sheets. It reviews the latest developments in the study of the FSW process parameters, microstructures of FSWelded joints and the properties of FSWelded structures. Several important key problems and opportunities for further research are also observed. Also in this paper, the study on the effects of the tool shape and surface condition on the microstructure, micro hardness and grain size for thin aluminum alloy, magnesium alloy, titanium alloy, etc,. FSW were observed.

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