13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- performed at a welding speed of 5.2 mm/s. In the experiments of Sivaprasad et al. [11] the beneficial effect of current pulsation was observed for a welding speed of 1.11 mm/s. Overall, 4 specimens were welded (Pulse 1 to 4 in Table 3) and the fusion zone microstructure was observed as will be presented in section 3.1. Table 3. Welding parameters for the current pulsing experiments Pulse 1 Pulse 2 Pulse 3 Pulse 4 Voltage, U (V) 8.0 8.0 8.0 8.0 Current parameters Current ratio 0.66 0.42 0.35 0.21 Low Current, LC (A) 54.0 40.0 30.0 20.0 Low time, LT (s) 0.15 0.15 0.15 0.15 High Current, HC (A) 82.0 96.0 86.0 96.0 High time, HT (s) 0.15 0.15 0.15 0.15 Welding speed, WS (mm/s) 3.39 3.39 3.39 3.39 Filler wire speed (mm/s) 4.66 4.66 4.66 4.66 Heat Input1, HI (J/mm) 160.6 160.6 137.0 137.0 Welding Power2, W (Watt) 544 544 464 464 1Calculated by ۶۷ ൌ܃∗ሺ ۺ ۱∗ ܂ۺ ା۶۱∗۶܂ሻ ሺ ܂ۺ ା۶܂ሻ∗܅܁ 2 Calculated by ܅ൌ܃∗ሺ ۺ ۱∗ ܂ۺ ሺ ା۶۱∗۶܂ሻ ܂ۺ ା۶܂ሻ 2.2.2. Constitutional supercooling Theoretically, equiaxe grains can form spontaneously in supercooling conditions. This should be achieved by welding in conditions of high solidification rate (R) but low temperature gradient (G) as proposed by Kou et al. [12]. To produce such conditions within the weld pool, the welding speed and the welding power were increased as presented in Table 4. Three trials (SC1 to SC3) were conducted at a high welding speed (8.47 mm/s) and with an increasing welding power as reported in Table 4. The filler wire speed was also increased in order to reproduce comparable weld geometries. According to the theoretical solidification map of Gaümann et al. [13] obtained for nickel superalloy CMSX-4, these welding conditions should result in the formation of equiaxe dendrites. The resulting microstructures will be presented in section 3.2. Table 4. Welding parameters for constitutional supercooling SC 1 SC 2 SC 3 Voltage, U (V) 7.8 7.8 7.8 Current (A) 121.9 137.5 147.5 Welding speed, WS (mm/s) 8.47 8.47 8.47 Filler wire speed (mm/s) 9.53 9.53 9.53 Heat Input1, HI (J/mm) 112.2 124.0 135.8 Welding power2, W (Watt) 951 1072 1150 1Calculated by ۶۷ ൌ܃∗ ۯ ܁܅ 2 Calculated by W = U*A
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