A B C D A B C D yield point, and hardness, but a reduction in impact strength (Refs. 28–30). Figure 11 shows the DSC curves of a specially chosen 300 rev/min with 0- WELDING RESEARCH deg-angle specimen, due to its comparatively good micro-mechanical results. The peak value of the curve gives the melting enthalpy (Hm) of the polymer, which is directly proportional to the crystallinity according to this formula (Ref. 31) Wc = Hm/Hm 100% (1) where Hm is the enthalpy of fusion and Hm is the enthalpy of fusion of 100% crystalline Nylon 6, of which the value is 230 J/g (Ref. 32). Keeping in view Equation 1 and Fig. 11, it can be concluded the crystallinity of the specimen is reduced in the WZ compared to the base material. It reduced further in the RS and became lowest in the WC by making the AS the comparatively highest crystalline region in the WZ. However, low crystallinity along with some defects on the borderline of the RS made the RS the preferred fracture location. It is also important to mention here the reduction in crystallinity is basically linked to the rapid cooling of the material (Ref. 32). In this case, the cooling rate for the whole joint was the same, as postweld specimens were placed in an open environment at room temperature. Therefore, it is believed that it is mainly the temperature differences at different regions of the WZ that lead to the cooling difference and, therefore, the difference in crystallinity. Material Flow during Friction Stir Welding Material flow during the welding process was studied by employing a 1.5-mm-thin ABS sheet as a marker material. For this purpose, specimens were friction stir welded at optimum parameters of 300 rev/min rotation speed, 0-deg tilt angle, and 25- mm/min feed rate, selected based on previous results. The set of parameters is shown in Table 1. Postweld specimens were cut in different sections, such as longitudinal-section (parallel to welding direction), vertical cross section (perpendicular to welding direction), and horizontal cross section, to visually analyze the displacement of marker material in x, y, and z directions. The sectioning scheme is also shown in Fig. 12. JUNE 2016 / WELDING JOURNAL 215-s Fig. 8 — Fractured specimens during tensile tests welded at 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. Fig. 9 — SEM micrographs of fractured surfaces, observed at the interface of the RS toward the weld zone, welded at 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
To see the actual publication please follow the link above