African Fusion March 2018

the workpiece. A tungsten electrode with 2.4 mm in diameter was used for the plasma arc. The E-FK1000 welding wire with a diameter of 1.2 mm was used in these experiments. Both plasma torch and GMAW torch were aligned in the welding direction, with the plasma arc leading. The angle between the hybrid plasma and the GMAW arc was set at 30°. It is worth mentioning that the plasma arc and GMAW arc were assembled in the same hybrid-welding torch, whichwas designed by Plasma Laser Technologies Co. Ltd. The molten pool was shielded using argon with the purity of 99.99% flow- ing from the plasma torch at the flow rate of 20 ℓ/min and 80% argon/20% CO 2 flowing from the nozzle of the GMAW torch at the flow rate of 15 ℓ/min, respectively. The distance between the GMAW arc and plasma arc was set at 2.0 mm. After thewelding process, deformation of bothworkpieces wasmeasured. Toget themechanical properties, theworkpiec- es were cut into standard test pieces according to ISO15614-1. Uniaxial tension tests were carried out at room temperature with the tensile direction perpendicular to the weld seam. For impact tests, the test specimens were sectioned from the weldedplate,machined to 5×10×55mmsections andpolished, then V-notched with a depth of 2.0 mm. A series of Charpy im- pact testswere conductedon three different regions – theweld  zone, fusion line and heat-affected zone – at -40 °C using an impact testingmachinewithmaximum impact energy of 300 J. Cross-weld Vickers micro-hardness tests were conducted on the polished and etched specimens transverse to theweld- ing direction at a load of 500 g, with loading time of 30 s. The hardness traverses were located 0.8 mm below the weld top for bothHPAWand conventional GMAWwelds; and the spacing between indentations was 0.5 mm. Also, the cross sections of theweldmetal andheat-affected zonewere observedusing scanning electronmicroscopy (SEM) performed on an FEI Sirion200. Theweld profilewas observed at 500× and 1000×magnification to check themicrostructures. The front and back appearances of weld bead with HPAW are shown in Figure 2. It can be seen from Figure 2 that the front welding joint is smooth, with successive welding appearance and without any spatter or undercut. Moreover, the back of the joint has suitable welding reinforcement and without any lack of fusion. As indicated in Figure 3, the weld seam is perpendicular to the welding direction. The weld bead shape has been smoothed using sandpaper, etched with 5% nitric acid/95% ethanol and observed and photographed using a metallo- graphic microscope. It can be seen from Figure 3 that the ratio of welding pen- etration toweldingwidth of theHPAWweld is larger than those of GMAW. WithHPAW, because of the high power density of the plasma arc, the surfacemetal melted quickly, thereby formed the keyhole easily. On the other hand, due to the plasma atmosphere at the top of the weld pool, the evolving vapour from the surface exerts a backpressure on the surface, so that the coordinated affect leads to a deep keyhole with deeper penetration and narrow width [7]. Keyholes obtained using high power density arcs such as laser or electron beam welders result in deeper penetration with a narrower bead, which result in a thinner HAZ and less Results and discussions Weld bead appearances

Figure 1: The HPAW system, SuperMIG, patented by Plasma Laser Technologies, was utilised for keyhole welding. The following elements detail the process: (1) workpiece; (2) plasma jet; (3) plasma nozzle; (4) melting metal; (5) plasma arc electrode axis; (6) wire axis; (7) angle between electrode’s axes; (8) tungsten electrode; (9) consumable electrode (wire); (10) GMAW arc; (11) plasma; (12) wire current (Iw) direction; (13) plasma current (Ip) direction; (14) magnetic forces (F) applied to plasma arc; (15) magnetic forces (F) applied to GMAW arc [6].

Figure 2: The appearances of (a) the top surface bead and (b) the root bead for the HPAW process.

Figure 3: Cross section of weld and heat-affected zones seam with (a) GMAW and (b) HPAW. residual stresses due to lower heat input. The heat input can be calculated by equation (1).

n

i ⋅ ⋅ ( ) i i

i l = ∑ η

=

Q

U I v /

(1)

i

Where, Q (kJ/mm) is heat input; η is efficiency factor, for a specific welding method – assumed at 0.85 for GMAW and 0.4 for plasma arc key hole welding; U(V) is welding voltage; I(A) is welding current; and v(mm/s) is welding speed. According to equation (1), it was calculated that the heat input per unit length of the HPAW process was 1.264 kJ/mm and the heat input of all three passes of GMAW was 3.404 kJ/ mmwith the final pass absorbing 1.72 kJ/mm. Post weld deformation Figure 4 shows the comparison of the post-weld deformation of both HPAWand GMAW. Due to the large heat input and slow welding speed of GMAW, the groove angle is 60° and the gap must be filled with a backing weld, a back sealing weld and a cosmetic weld pass, which leads to the large deformation. As a comparison, the low heat input, high welding speed of HPAW and the 30° groove angle, the weld gap is filled in only one weld pass, which gives rise to the smaller deformation. The post-weld side deflection of the test specimens of both GMAW and HPAW was measured at the edges and at a

15

March 2018

AFRICAN FUSION