To solve environmental and energy problems, light materials are increasingly being adopted as structural constituents in different transportation industries, such as the automotive, aeronautic and train industries. The various parts of light materials are combined and joined to assemble such structures and components for cars, airplanes and trains. There are several joining technologies [1
] including welding, bolt joining adhesive joining, and so on. Tungsten inert gas (TIG) welding and laser welding are commonly used to join similar and dissimilar light metal sheets.
Friction stir welding (FSW) has been recently developed and successfully applied to join light materials such as aluminum alloys and titanium alloys. The welding method is a promising ecological welding method that enables workers to diminish material waste and avoid radiation and harmful gas emissions that often occur from fusion welding [2
]. The main process parameters in controlling the quality of joints are tool rotation speed, tool traverse speed, vertical pressure on the specimen, the tilt angle of the tool, tool geometry, and others [4
]. During welding, a non-consumable tool attached with a specially designed pin rotates and pushes the butting edges of the two plates to be joined. The friction heat causes the material to soften, allowing the tool to penetrate into the material surface. The tool shoulder sits on the specimen’s surface during penetration. Under this condition, the rotating tool traverses along the joining line. Thus, generated frictional heat causes both materials to soften under the tool where joining is achieved. This process is suitable for joining plates and sheets. However, it can also be employed for joining pipes and hollow sections [6
]. Although the FSW process was initially developed for aluminum alloys [7
], it also has a great potential for the welding of copper [10
], titanium [11
], steel [12
], magnesium [13
], metal matrix composites [14
], and different material combinations [15
Recently, studies have shown that applying FSW welding to thermoplastics is successful and various factors influencing its joinability were investigated. The effect of tool tilt angle and welding speed on the tensile strength in FSW of polyethylene [16
] was studied using different tool dimensions and pre-heated pins [17
]. Taguchi’s approach to parameter design and analysis of variance were utilized with further experimental confirmation for FSW of polyethylene. It was shown that the optimum welding parameters were tool rotational speed of 3000 rpm, traverse of 115 mm/min and tilt angle of three degrees [18
]. In a novel study by Vijendra, a new hybrid friction stir welding process, i-FSW, was designed. In i-FSW, the tool is heated during welding by an induction coil, and the temperature is precisely maintained through feedback control [19
]. Another study on tool design where a new self-reacting tool with a convex pin was utilized showed the greatest effects on tensile strength in welding of acrylonitrile butadiene styrene (ABS) sheets [20
]. Recent research that combines experimental and analysis models for optimization is popular to reduce experimental cost and for the ability to predict optimal conditions precisely. Factors that influence FSW were optimized using a factorial design and analyzed using Artificial Neural Networks (ANN) to compare the experimental and the model analysis, which has demonstrated that tool plunge rate, dwell time and waiting time, plunging force, and torque were discussed as the most influential factors [21
]. A study of an ABS sheet optimized by Analysis of Variance (ANOVA) and Response Surface Methodology (RSM) demonstrated high diameter ratio and low rotational speed, which are optimal. A comparison indicated the more accurate prediction of a corresponding model for a conical pinned tool than a cylindrical probe tool. The recommended conditions identified are two degrees for tilt angle, 900 rpm rotational speed, a tool with diameter ratio of 20/6 and linear speed of 25 mm/min, which generated a weld joint with equal yield strength to the base material [23
]. In another study, Simoes analysed the material flow and thermo-mechanical phenomena taking place during FSW of polymers. Polymethylmethacrylate (PMMA) was used owing to the high transparency so that polarization during tool penetration could be observed clearly. It has been reported that due to the polymers’ rheological and physical properties, the thermo-mechanical conditions during FSW are very different from those registered during the welding of metals. The material flow and temperature distribution between metallic and polymeric materials were compared based on the Arbegast flow-partitioned deformation zone model for FSW in metals. The formation of discontinuities was indicated as one of the main weldability problems for polymers [24
There are very few reports on the friction stir spot welding of polymer-polymer as well as polymer-metal combinations, which have great applicability and are in high demand, especially in the automotive industry [25
]. In the author’s previous work [27
], the dissimilar joining of aluminum alloys (A5052) and polyethylene terephthalate (PET) was attempted using the frictional energy generated from friction spot welding. In the joining conditions shown for plunge depth of 0.7 mm, the lower plunge speed exhibited higher tensile strength, which was the result of longer contact time and more generated heat. The process yielded the dissimilar joining of the two materials despite the low joining strength. In the dissimilar friction stir butt joining of aluminum and Polycarbonate (PC), the feasibility was achieved, but the concerns remain regarding lower tensile strength due to fracture induced voids [28
]. The dissimilar joining of aluminum and thermoplastics by adopting the hole-clinching method has been reported in several studies. Lee investigated tool shapes such as punch diameter, punch corner radius and die depth on hole-clinching for dissimilar materials [29
]. Studies on the joinability of rigid thermoplastic polymers with aluminum AA6082-T6 alloy sheets by mechanical clinching have revealed that fracture at the metal or polymer sheet was the main factor contributing to unsuccessful joinability. Joinability has been examined by studying mechanical interlocking manipulated by tool geometry [30
], tool shapes [31
] or temperature [32
]. These studies focused on tool shapes that directly influence mechanical interlocking at the microstructural level. An analysis study by Wirth on the bonding behavior and joining mechanism of aluminum and thermoplastics recommended optimal conditions such as holding time, axial force, etc.
]. High lap joint quality with shear strength of 5–8 MPa was reported in a case study of aluminum and laser transmission joints of nylon [34
] and PMMA [35
] where in both cases the temperature reached the melting temperature of the thermoplastics.
In the present study, the effect of surface roughness on the joining strength of an FSW-ed PET-A5052 dissimilar joint is investigated with the aim to increase the joining strength. The joining mechanism and effects of surface roughness are discussed in detail.