African Fusion March 2018

GMAW versus hybrid plasma arc welding

the base metal and welded joints with GMAW and HPAW of weldox960 plates are listed in Table 4. It can be seen that the average ultimate tensile strength of HPAW welding joint is 1 040MPa, which is between the averageUTS of the basemetal and the GMAW joint. Furthermore, the average elongation of the welding joint performed with HPAW is 8%, which is lower than those of base metal and the GMAW joint. Impact test: The average impact values of the base metal and weld joints with GMAW and HPAW are shown in Table 5. It can be seen from Table 5 that the average impact energies of weld zone, heat-affected zone and fusion line of HPAW weld- ing joints are lower than those of the base metals and GMAW welding joints. The results of micro Vicker’s hardness test are shown in Table 6. It canbe seen fromthe table that the average hardness values at weld zone and heat affected zone of HPAW joint are higher than those of GMAW joint. Furthermore, comparedwith the average hardness value of 343 Hv of base metal, the aver- age hardness value of weld zone andheat affect zone hardness of HPAW joint reached to 375.7 Hv and 412 Hv, respectively. However, the average hardness value of WZ and HAZ of GMAW joint are lower: 338.3 Hv and 315 Hv, respectively. The formulation of the microstructures in the weld metal and heat-affected zone is strongly dependant on the cooling time of the weld joint. Thus it is important to investigate the cooling time for HPAW and compare it with that for GMAW. When the workpieces were joined using HPAW, the gap was filled using only one pass, and the workpiece was not pre- heated. This increases the cooling rate and shortens the cooling time. According to literatures, the time for a high strength steel weld joint to change froma temperature of 800 °C to 500 °C (t 8/5 ) via two dimensional heat flow is calculated from(2) as follows: Where d is the thickness (mm); Q is the heat input (kJ/mm), T 0 is the initial temperature of workpiece (°C); and F is the shape factor, which is equal to 0.9 in the case of butt-welding with V grooves. From the above equation, the t 8/5 of HPAW process for butt-welding of 12 mm thickness AHSS plates is 10.23 s. As a comparison, the t 8/5 for the GMAW process for final pass of the butt weld is 20.5 s, which is twice as long as that from the HPAW process. That is to say, the quenching effect of the HPAWprocess in this study is stronger than for GMAW, while the tempering effect of the GMAW process in this study is stronger. Figure 5 shows themicrostructures of the HAZ of both GMAW joints andHPAW joints. Compared with the microstructures of the GMAW joint shown in Figure 5(a), larger amount of lath martensite in the t T Q d T T 8 5 / 0 5 ) × × × 2 2 0 2 0 2   4300 4 5 . 10 1 500 1 800 - - = − × (     −         × F (2)

distance of 150 mm away from the centre of weld seam and along the direction parallel to weld seam. This is shown in Figure 4. It can be seen from Figure 4 that the maximum post- weld deformation for GMAW is about 3.6 mm on one side. In comparison, the maximum post-weld deformation of the HPAW specimen is only 1.6 mm on one side. In other words, compared with GMAW, the post-weld plate deformation with HPAW is decreased by about 55.6%. The main reason for this phenomenon is that the heat input of HPAW is about 1/3 of that of GMAW, resulting less deformation. Mechanical properties Tensilestrengthandextensionrate: AccordingtoISO15614‑1, the test specimens were sectioned fromthe test plate and pol- ished. Then the specimenswere clamped to a hydraulic tensile testing machine with the maximum tension force capacity of 1 000 kN. The average tensile strength and extension rate of Figure 4. The post-weld deflection deformation of the test specimens for GMAW and HPAW.

Base metal

GMAW HPAW

Tensile strength (MPa)

1075

960

1040

Elongation (%)

11

9

8

Table 4: The average tensile strengths and elongation percentages of the base metals, the GMAW weld joints and the HPAW joints.

Weld zone Heat affected zone

Fusion line

GMAW Charpy impact values (J) HPAW Charpy impact values (J)

51.8

45.9

46.6

34.3

34.5

33.6

Table 5: The average impact test values in the base metal; the joints welded with GMAW; the joints welded with HPAW; and the manufacturer’s standards taken from the weld zones, heat-affect zones and fusion zones. (Reference minimum impact toughness of the base metal: 27 J at -40 °C).

grains of the HPAW joint can be observed. The overall width of martensite laths in the grains of the GMAW joint is larger and the distribution of laths within the grains show random character- istics (Fig. 5a). As a comparison, the width of lath martensite reveals a parallel pattern within the grain. The difference in the amount of martensite, the

Weld zone Fusion line Test value Average value Test value Average value Test value Average value Heat affected zone

395 365 367 327 359 329

414 402 420 307 320 326

335 341 355 368 349 324

HPAW

375.7

412

343.7

GMAW

338.3

317.7

347

Table 6: A comparison of the average Vickers hardness test values of the GMAW and HPAW joints taken from the weld zones, heat-affect zones and the base metal.

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March 2018

AFRICAN FUSION