African Fusion June 2018

Figure 3: Schematic diagram of welding sequence and the distribution of weld passes.

Results and Discussion Microstructure analysis

Figure 5(a) shows the cross-sectionmac- rographs of the LAMIG-welded joint. It was clear that the weld seampresented adumbbell shape andwelddefects such as full penetration and low distortion were not observed. The detailed microstructure of the cosmetic weld (marked as A) is shown in Figure 5(b). It could be seen from Figure 5(b) that coarsening acicular α ' phase traversed the grain to form a basket-like widmannstatten structure.

Figure 2: Schematic diagram of laser-MIG hybrid welding system.

Power (kW) Arc current (A) Arc voltage (V) Welding speed (m/min) 4.0 50 17 0.8

Table 3: Backing weld parameters.

Number Power (kW)

Arc current (A)

Arc voltage (V)

Welding speed (m/min)

Heat input (J/m)

1 2 3

2.0 2.5 3.0

80 63 55

20 20 17

0.7 0.5 0.3

2.2×10 5 3.2×10 5 5.5×10 5

Table 4: Cosmetic weld parameters (Thermal efficiency=0.7).

a four-layer welding process was adopted. Figure 3 shows the welding sequence and distribution of weld passes. The backingweld and cosmeticweld parameters are listed in Table 3 and Table 4, respectively and the backing weld was carried out at fixed welding parameters. Figure 4(a) shows the shape and size of the non-standard butt joint tensile sample for the tensile test. The tensile tests were performed by an Instron-1186 mechanical test- ing machine. In addition, Figure 4(b) shows the GATA in situ tensile equipment that was adopted to stretch pre-notched samples and investigate the effect of microstructure on crack propagation. The weld seam was required to be located in the 2.0 mm region in themiddle of in situ test samples. (2.0mmaway from the top surface). As the tensile of theweld seam is quite high, a notch is cut in the bottomof the weld seam to ensure the pro- cess of the in situ tensile test. Themicrostructure and fracture surface of the joints were analysed using a scanning electron microscope (SEM). Moreover, the morphology and diffraction patterns of the phases were characterized by transmission electron microscope (TEM).

Figure 4: Shape and size of tensile samples (a) and the butt-joint tensile sample (b).

Figure 5: The microstructure of the Laser-MIG joint: (a) cross-sectional profile of a weld joint; (b) the microstructure of the cosmetic welding zone; and (c) the microstructure of the backing weld zone.

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June 2018

AFRICAN FUSION