Fig. 6 — SEM microstructures of welds, observed in weld nugget, at rotational speed of 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. tages over other techniques, can be suitable for this material. Fracture Analysis The fractured area in any place of the welded joint is a direct indication of the weakest part of that joint. In the present investigation it was noted that all specimens of each set of parameters exhibited identical fracture location, which is at the interface of the WZ on the retreating side (IW-RS). However, interface of the WZ on the advancing side (IW-AS) remained intact. One fractured specimen for each rotation rate is shown in Fig. 8. It is also important to note here the specimen welded at 500 rev/min (Fig. 8C) also showed fracture at the IW-RS, despite the fact it contains a tunnel defect. It indicates that the IW-RS is weaker than the tunnel defect. Inaniwa et al. (Ref. 12) and N. Mendes et al. (Ref. 11) also observed the same fracture locations in their study on FSW of different polymers. This prefered fracture location in these specimens can be related to the formation of flash preferrentially on the retreating side. In general, flash formation causes the lack of material, which ultimately results in cavities and blowholes. In addition to it, other phenomena in the WZ make the RS weaker than the AS include higher temperature on the RS (Ref. 23), low shear velocity on the RS (Ref. 12), and difference in flow on both sides (Ref. 10). Scanning electron microscopy results of fractured specimens observed toward the WZ, shown in Fig. 9, exhibit the same fracture phenomenon in all specimens. Temperature Analysis Temperatures measured at 1 mm below the pin tip are shown graphically in Fig. 10. An increase in temperature with the increase of rotation speed is obvious and can be observed in the graph. Although the temperature differences at low rotation rates are not significant, noticeable differences in weld quality were observed. Cavities, tunnel defect, and smoke during the process were seen above 400 rev/min rotation speeds. A maximum temperature of 167°C at 1000 rev/min showed large amounts of flash formation with the emission of smoke. Smoke is a combination of different volatiles, majorly carbon dioxide (CO2), water (H2O), and ammonia (NH3), evolved due to endothermic reaction during the FSW process. Evolution of volatiles is directly linked to the weight, suggesting a decrease in polymer weight (Ref. 26). Therefore, it is believed that at relatively higher rotation rates, a small decrease in weight also occurred. Considering the melting point of Nylon 6 is 220°C, it can be assumed all rotation rates used in the present study are applicable for Nylon 6, as thermal degradation of polymer occurs at 350°C (Ref. 27). It was observed that temperature for optimum weld parameter 300 rev/min with 0- deg angle is 119°C. There was low flash formation and no visible defect that could be attributed to the welding in plasticized condition due to low heat input avoiding low melt viscosity. Low heat input results in avoiding the deterioration in the properties of base material. Increase in temperature above this point may cause the formation of excess flash due to lower melt viscosity, which eventually results in lower weld quality. Analysis of Crystallinity In order to analyze the postweld thermal conditions of the joint and to investigate the reason of identical fracture locations, the degree of crystallinity of different sections of the WZ was analyzed using DSC curves. The specimens, for this purpose, were taken from the base material (BM), weld center (WC), AS, and RS. The degree of crystallinity of any polymer has a direct relation to its mechanical and physical properties. Increase in the degree of crystallinity has shown increase in tensile strength, stiffness, WELDING RESEARCH 214-s WELDING JOURNAL / JUNE 2016, VOL. 95 Fig. 7 — Effect of rotational speed on tensile strength of Nylon 6 FSW specimens. A C B D
Welding Journal | June 2016
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