Elsevier

Materials & Design

Volume 32, Issue 7, August 2011, Pages 4074-4079
Materials & Design

Technical Report
The optimization of welding parameters for friction stir spot welding of high density polyethylene sheets

https://doi.org/10.1016/j.matdes.2011.03.014Get rights and content

Abstract

Friction stir spot welding parameters affect the weld strength of thermoplastics, such as high density polyethylene (HDPE) sheets. The strength of a friction stir spot weld is usually determined by a lap-shear test. For maximizing the weld strength, the selection of welding parameters is very important. This paper presents an application of Taguchi method to friction stir spot welding strength of HDPE sheets. An orthogonal array, the signal to noise ratio (S/N), and the analysis of variance (ANOVA) are employed to investigate friction stir welding parameter effects on the weld strength. From the ANOVA and the S/N ratio response graphs, the significant parameters and the optimal combination level of welding parameters were obtained. Experimental results confirmed the effectiveness of the method.

Graphical abstract

Three phases of friction stir spot welding process, (a) plunging, (b) stirring and (c) retracting [8] and ANOVA analyses, the optimal welding parameters (dwell time and plunge depth) for weld strength.

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Highlights

► Friction stir spot welding parameters affect the weld strength of polyethylene sheets. ► A minitab statistical software was used to explain the welding parameter effect. ► Tables, diagrams were drawn to display on the weld strength. ► An application of Taguchi method was obtained optimum welding parameters.

Introduction

Spot welding is a very common joining technique in the automotive industry [1]. This welding process is widely used in the joining of sheet metal assemblies due to its advantages in welding efficiency and suitability for automation [2]. Global trends force the automotive industry to manufacture lighter, safer, more environmentally friendly and ultimately cheaper vehicles [3]. Reduction in vehicle weight can be obtained by replacing conventional steels and cast irons with advanced high strength steels and light weight materials, such as aluminium, magnesium and reinforced polymer composites [4], [5]. These new automotive materials, however, have limited weldability characteristics which require improvements both in conventional spot welding processes and new welding techniques [6].

In 2001, friction stir spot welding (FSSW) was developed in the automotive industry to replace resistance spot welding for aluminium sheets [7]. The FSSW process consists of three phases; plunging, stirring and retracting as shown in Fig. 1 [8]. The process starts with spinning the tool with a high rotational speed. Then the tool is forced into the weld spot until the shoulder of the tool enters the surface of the upper workpiece. The plunge movement of the tool causes material to expel as shown in Fig. 1a and b. When the tool reaches the predetermined depth, the plunge motion ends and the stirring phase starts. In this phase, the tool rotates in the workpieces without plunging. Frictional heat is generated in the plunging and the stirring phase and, thus, the material adjacent to the tool is heated and softened. The softened upper and lower workpiece materials mix together in the stirring phase. The shoulder of the tool creates a compressional stress on the softened material. A solid-state joint is formed in the stirring phase. When a predetermined bonding is obtained, the process stops and the tool is retracted from the workpieces. The resulting weld has a characteristic keyhole in the middle of the joint as shown in Fig. 1c. The tool geometry and welding parameters (tool rotational speed, tool plunge depth and dwell time) effect heat production, joint formation and strength of welds [9].

There are very few publications about polymer FSSW applications [10], [11]. No publication has been found on FSSW of high density polyethylene (HDPE) sheets, thus this study was intended to explain the effects of welding parameters on FSSW strength of HDPE sheets. Basically, classical experimental design methods are too complex and not easy to use. A large number of experiments have to be carried out when the number of the welding parameters increases. To solve this problem, the Taguchi method uses a special design of orthogonal arrays to study the entire parameter space with only a small number of experiments [12], [13], [14], [15]. The Taguchi design method has been found to be a simple and robust technique for optimizing the welding parameters [16].

The present study is performed to fulfil the following two objectives:

  • 1.

    To use the Taguchi method for determining the optimum FSSW parameters.

  • 2.

    To estimate the contribution of individual welding parameters to the strength of the weld joint.

Section snippets

Materials and experimental procedure

From the preliminary experimental results, three levels of welding parameters were selected as shown in Table 1. In this study an L9 orthogonal array with four columns and nine rows was used [14]. The experimental layout for the three welding parameters using the L9 orthogonal array is shown in Table 2. Since the L9 orthogonal array has four columns, each welding parameter is assigned to a column, and the last column is left empty for the error in experiments [14]. The orthogonality is not lost

Observation of the joint appearance and broken specimen appearance

Fig. 4 shows the appearance of the test specimen after joining. As shown in the figure, the joining is completed without any deformation of the upper or lower sheet. As shown in Fig. 4, the shape of the tool is transferred onto the joint on the side into which the tool is pressed and the base metal that is pushed out by the tool forms a ring of excess metal around the weld. In lap-shear tensile tests, mainly two different fracture morphologies were observed, namely, the cross nugget failure and

Conclusions

The effect of friction stir spot welding parameters of HDPE sheet weld strength was evaluated with the help of the Taguchi method. The following results were obtained by the experimental and the analytic results:

  • 1.

    The dwell time was the most dominant welding parameter for weld strength followed by the tool rotation speed.

  • 2.

    The optimum welding parameters for the weld strength are the tool rotation speed of 700 rpm, the dwell time of 60 s and the tool plunge depth of 6.2 mm.

  • 3.

    The improvement in the weld

Acknowledgements

This work was supported by Scientific Research Project Program of Marmara University (Project No. FEN-C-DRP–010710–230). The authors are grateful to Marmara University for their financial support and the provision of laboratory facilities.

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