13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- a) Longitudinal section b) Cross section Figure 1. Typical microstructure of a cracked specimen revealed with etchant composed of 5% H202 (30%), 47.5% HCl (40%) and 47.5% methanol 2.2 Welding experiments Experiments were performed to identify a range of welding parameters that creates a weld free of CLGB. The ideal microstructure would be fully or partly equiaxe and fine. If it is impossible to form equiaxe grains, a more tortuous central grain boundary would be an improvement of the actual microstructure. To reach the objective, theoretical considerations were reviewed and the following approaches were tested: 2.2.1 Current pulsing, 2.2.2 Constitutional supercooling, 2.2.3 Weld pool shape control. Tests were done on rectangular sheet coupons having a thickness of 1.27 mm. Two coupons were firmly clamped together and a semi-automated TIG machine was used to produce a straight 10 cm long weld. Longitudinal and cross sections of the welds were prepared for metallographic observations. 2.2.1. Current pulsing According to the results of Ram and Reddy [8], current pulsing can produce a weld with a fine and homogeneous microstructure which considerably improve the weld ductility at 650oC. Their study was performed for autogeneous welding of 2 mm thick sheets of IN718. In another paper from the same research team, it was explained that current pulsing increases the cooling rate resulting in significant refinement of the fusion zone structure [11]. In our study, current pulsation was attempted at a frequency of 3.3 hertz using four different current ratios (low current/high current) as reported in Table 3. The welding speed and the filler wire speed were kept constant at values of 3.39 mm/s and 4.66 mm/s respectively. This welding speed is lower than the nominal welding speed for which the cracking problem was initially observed. It was decided to reduce the welding speed following unsuccessful current pulsation welding tests 1000 m 1000 m
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