of the tool or bobbin tool may result in excess heat due to the dual friction on upper and lower surfaces of the workpieces, eventually increasing the flash. The preweld heating, performed by Aydin (Ref. 7) on 4-mm-thick ultrahigh molecular weight (UHMW) polyethylene sheets, may not be suitable for polymers with low melt viscosity and can intensify the flash formation due to increasing heat. The influence of pin shape, studied on PE sheets by Bilici et al. (Ref. 8) and Ahmadi et al. (Ref. 9) made them conclude that the truncated cone pin has the highest tensile strength as compared to other pin shapes. However, due to large rheological and property differences among polymers, one pin profile cannot be assumed as optimum for all polymers. Another tool consisting of the “hot shoe” was investigated by Bagheri et al. (Ref. 10) on ABS sheets. The main reason to use this shoe was to heat the polymer through the shoulder during the welding process. Their results were comparable with the work carried out by Mendes et al. (Ref. 11), who used a stationary shoulder tool on the same material without external heating. However, squeezing out of the plasticized material below the shoulder, particularly in low melt viscosity polymers still remained a problem. Inaniwa et al. (Ref. 12) and Panneerselvam et al. (Ref. 13) joined Nylon 6 sheets with thicknesses of 5 and 10 mm, respectively. In their studies, they eliminated the primary heat source by keeping a small opening between the shoulder and workpiece top surface. Comparing their approaches, it was found that Panneerselvam et al. (Ref. 13) joined Nylon 6 at a quite higher revolution pitch (ratio between welding speed and rotation rate) compared to Inaniwa et al. (Ref. 12) work. However, considering the Nylon 6 properties’ especially low melt viscosity behavior, it is believed that higher revolution pitch will produce enormous flash. Flash formation has been reported by Panneerselvam et al. (Ref. 13) as well. On the other hand, the gap between shoulder and workpiece will certainly lead to the formation of a crown above the weld zone. Material flow, due to its direct relation with weld quality, has been thoroughly investigated on metals by various means. Lorrain et al. (Ref. 14) and Li et al. (Ref. 15) used foil insert technique, Edwards and Ramulu (Ref. 16) used powder as a tracer material, Seidel and Reynolds (Ref. 17) utilized the marker material insert technique, whereas Colligan (Ref. 18) inserted small steel balls in aluminum to study material flow. Colligan (Ref. 18) concluded that material moved behind the pin and deposited on the retreating side. In another study on aluminum 6061, Guerra et al. (Ref. 19) reported different flow on advancing side (AS) and retreating side (RS). Seidel and Reynolds (Ref. 17) observed that the majority of the material in the weld nugget simply moved around the pin and displaced behind the pin. The material flow in polymeric materials has been studied on poly methyl methacrylate (PMMA) by Simoes et al. (Ref. 20). They compared their flow study with the Arbegast (Ref. 21) flow model and observed that the pin-affected zone remained isolated and straight along the pin. Their results showed no cross flow WELDING RESEARCH from the weld zone to the base material. Similarly, a clear distinction between the shoulder-affected zone and pin-affected zone could be seen. Current work involved studying the process on 16-mm-thick Nylon 6 plates by investigating the temperature development, micromechanical, and thermal properties of the joint. With the aim to reduce the flash formation, a small-diameter shoulder tool with right-hand threaded pin was used. Moreover, marker material insert technique was utilized to examine the flow phenomenon and stirring uniformity in the weld. Materials and Method In the present investigation, 180- mm-long weld passes were made on 16-mm-thick Nylon 6 (Polyamide-6) plates in butt joint configuration at room temperature. Bridgeport VMC 2216 CNC machine was utilized for FSW of specimens welded at a 0-deg tilt angle and FSW-TS-F16 FSW machine was used to prepare welds at a 3- deg tilt angle. The FSW tool used in this study was machined from H13 tool steel rod — Fig. 1. The pin of the tool was made right-hand threaded for uniform stirring, while rotating in a clockwise direction (Ref. 13). The tool was heat treated before being used for welding and, therefore, its hardness was increased to 56 HRC from 24 HRC. Rotational speed, due to its main contribution in the FSW process, has been studied (Ref. 22). It was there- JUNE 2016 / WELDING JOURNAL 211-s Fig. 1 — Friction stir welding tool with a schematic of dimensions. Fig. 2 — A schematic of the thermocouple locations. Fig. 3 — Configuration of the tensile test specimen.
Welding Journal | June 2016
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