13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- 2.2.3. Weld pool shape control According to the work of Savage [14] an elliptical weld pool shape promotes the formation of a microstructure composed of columnar dendrites but free of CLGB. CLGB forms when columnar grains growing perpendicular to the solid walls meet at the center of the weld. This type of microstructure is typically observed at high welding speed when a tear drop shape weld pool forms. Welding conditions can be modified to favor grain curvature and avoid the formation of a CLGB. Reducing the welding speed favors the formation of an elliptical weld pool and the curvature of grains within the longitudinal plane as experimented by Kou and Le [12]. Moreover, reducing the heat input reduces the depth to width ratio of a weld which favors the formation of a radial grain structure [3] which is also beneficial to the microstructure. It was attempted to produce a weld with an elliptical pool shape and with a low depth to with ratio by decreasing the weld speed (WS) and the heat input (HI) as proposed in Table 5. The filler wire speed was also decreased in order to reproduce comparable weld geometries. Overall, 5 specimens were welded (a combination of 3 WS and 3 HI) and their microstructures were observed as will be presented in section 3.3. Table 5. Welding parameters for the elliptical weld WS 1, HI 1 WS 1, HI 2 WS 1, HI 3 WS 2, HI 1 WS 3, HI 1 Voltage, U (V) 8.0 8.0 8.0 8.0 8.0 Current (A) 94.0 85.0 74.0 60.0 44.6 Welding speed (mm/s) 5.27 5.27 5.27 3.39 2.54 Filler wire speed (mm/s) 5.72 5.72 5.72 4.66 2.79 Heat Input1 (J/mm) 142.7 129.0 112.3 141.6 140.5 Welding power2, W (Watt) 752 544 592 480 357 1Calculated by ۶۷ ൌ܃∗ ۯ ܁܅ 2 Calculated by W = U*A 3. Results 3.1. Current pulsing All welds produced with pulsed current have a similar microstructure which is shown in Fig. 2. Current pulsing did not have the expected effect on the microstructure and changing the current ratio had no significant influence. The most noticeable feature observed by comparing Fig. 2 to Fig. 1, is the existence of a longitudinal grain at the centerline of the welds produced with pulsed current. This was possibly caused by the reduction in welding speed rather than by the current pulsing as will be shown in section 3.3. The formation of a longitudinal grain is not beneficial to the weld microstructure. The grain is coarse and it is bounded by two weak planes oriented perpendicularly to the transversal stresses. According to the work of Sivaprasad and al. [11], current pulsing is more effective for welds performed at low heat input. Their experiments were done on 3 mm thick sheets at a heat input of 180 J/mm. The experiments presented in this document were performed on 1.2 mm thick sheets using a heat input as low as 137.0 J/mm. The heat input range is comparable considering that our
RkJQdWJsaXNoZXIy MjM0NDE=