A B C D higher heat input, leading to excess flash in the form of bubbles, which appeared at some intervals — Fig. 4C. 300 rev/min with a 3-deg-angle specimen also showed bubble-like flash, which indicates that tilting of the angle at 3 deg generated high heat during the process — Fig. 4E. It may be due to the tilted tool trailing edge of the shoulder that applies higher compressive load in the surface of material increasing friction and resulting in higher heat generation. The formation of a crown-like shape was also observed in all specimens except the 1000-rev/min specimen. Crown peak was less in 300 rev/min with 0-deg specimen angle compared to 400, 500, and 300 rev/min with 3-deg-angle specimens. A specimen welded at 1000 rev/min showed a different morphology — Fig. 4D. Excess semimolten Nylon 6 squeezed out of the WZ, and after swirling around the shoulder, appeared to have solidified on the shoulder. As a result, the WZ decreased in thickness, which would eventually reduce the tensile strength. It is believed the appearance of weld defects at higher rotation rates is due to overheat generation, which eases the plasticized polymer to flow out. Low magnification SEM images of weld zones, taken on cross sections, are shown in Fig. 5. The specimen welded at 1000 rev/min was excluded from further results and discussion due to its poor weld quality. The weld in Fig. 5A, which was made with the lowest rotation speed of 300 rev/min with 0-deg angle, presented an excellent superficial appearance, unlike welds produced at higher rotation rates (Fig. 5B–D), which showed different defects, likely caused by excess heat input. However, a minor defect in the form of improper bonding was observed at the bottom of the RS interface. A specimen welded at 400 rev/min rotation rate showed various small porosities with improper bonding on the RS border line — Fig. 5B. Likewise, increasing the rotational speed to 500 rev/min resulted in a major defect called tunnel defect with poor bonding on the RS border line — Fig. 5C. Considering the good weld appearance of 300 rev/min with a 0-deg-angle specimen, and in order to remove the minor weld defect at the bottom, it was investigated using a 3-deg tilt angle as well. However, the micrograph result WELDING RESEARCH showed a large cavity with a slight improper bond on the RS in Fig. 5D. Moreover, weld zones were also analyzed at higher magnification. Micrographs shown in Fig. 6A–D are the higher magnifications of Fig. 5A–D, respectively. It is clear rotation speed has a significant effect on the microstructure of the welds. The 300 rev/min with 0-deg angle specimen in Fig. 6A showed uniform and perfect surface quality, whereas the specimens at higher rotation rates or a 3-deg tilt angle showed a relatively rough surface — Fig. 6B–D. It is assumed that fracture is easy to occur in rough surface compared to smooth surface, as it can provide stress concentration points. From the above detailed description, it can be deduced that, due to low melt viscosity compared to other polymers, such as ABS (Ref. 11), PE (Ref. 22), PP (Ref. 24), and PMMA (Ref. 25), Nylon 6 is weldable only at lower rotation speeds. Tensile Test Results Figure 7 exhibits the peak stress values of tensile specimens, obtained from stress strain curves. The trend indicates the increase in tool rotational speed leads to a decrease in tensile strength. However, considering the visible aforementioned defects, Fig. 7 shows the difference in strength values is not that large. The tensile strength obtained at 500 rev/min is also comparable to that of the specimen at 400 rev/min, even though it comprises tunnel defect in it. Moreover, the strength of each specimen, in terms of value, was quite less than that of the base material. The highest tensile strength, obtained at 300 rev/min, 0-deg tilt angle, was 27.21 MPa, which corresponds only to about 32% of base material. At the same rotational rate when the angle is tilted to 3 deg, the strength decreased to its lowest value of 18.08 MPa. Compared with Panneerselvam et al. (Ref. 13) and Inaniwa et al. (Ref. 12), who studied 10- and 5-mm-thick Nylon 6, respectively, almost similar results were found. Although Nylon 6 FSW results are lower in tensile strength compared to other polymers, such as PE (Ref. 4), ABS (Ref. 10), and polypropylene (PP) (Ref. 24), it is believed that this process, due to its certain advan- JUNE 2016 / WELDING JOURNAL 213-s Fig. 5 — Micrographs of welds at a rotational speed of the following: A — 300 rev/min with 0deg angle; B — 400 rev/min with 0deg angle; C — 500 rev/min with 0deg angle; D — 300 rev/min with 3deg angle.
Welding Journal | June 2016
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