African Fusion March 2018

GMAW versus hybrid plasma arc welding

This paper from the proceedings of the IIW 2017 International Conference of June 29-30 in Shanghai details a comparison between the gas metal arc welding (GMAW) process and the hybrid plasma arc welding (HPAW) process when applied to the welding of advanced high strength steels (AHSS) for the manufacture of construction equipment. A comparison of thick AHSS plate joined using HPAW and GMAW Xiaoteng Wang; Ling Fu; Huili Ren; Xing Tan; and E. Yong Guan

A dvanced high strength steels (AHSS) have been widely used in modern industries such as the automotive, construction machinery and offshore engineering in- dustries. Themain advantages of AHSS are not only their high strength and toughness values, but also the good formability. However, due to their relatively higher carbon equivalent value, the conventional gas-metal arc welding (GMAW) of advanced high strength steels is more difficult than ordinary low carbon steels. Cracks, especially hydrogen induced cold cracks, are more likely to appear in GMAW of AHSS. To prevent cracks fromoccurring during and after conven- tional GMAWprocesses, lowheat input is of great importance. Besides lowering the heat input, special treatment such as pre-heating, inter-pass temperature control and post weld heat treatment are also necessary. Lower heat input always leads to low efficiency multi-pass welding, while the costs of pre- and post-weld heat treatments have to be taken into ac- count when using this welding process. These factors lead to time and cost increases for GMAW of AHSS. Thus new joining methods that can weld AHSS at higher efficiency and without the need for pre- and post-weld heating is needed. As a high energy density weld method, hybrid laser-arc welding (HLAW) can be applied to different material, such as steel, aluminiumandmagnesiumalloys [1-5]. But the high cost of hybrid laser-arc welding is always a big concern for large industrial manufacturing activities. Another concern is the relatively poor gap tolerance of HLAW, which requires small Fe Weldox960 0.17 0.22 1.20 0.005 0.004 0.45 0.50 1.00 Bal E-FK1000 0.13 0.81 1.73 0.012 0.024 0.024 0.59 2.27 Bal Alloy C Si Mn P S Cr Mo Ni

or zero gaps to avoid lack of fusion and sagging. Another hybrid welding process, hybrid plasma arc weld- ing (HPAW), was commercially introduced in 2005. The HPAW method was proved to offer lower costs and higher gap tolerances when compared with HLAW. In comparison with traditional GMAW, HPAW provides deeper weld penetration, higher welding speed and less deformation. In this paper, an application of HPAW in welding AHSS in the construction machinery industry is investigated. As a comparison, bothHPAWandGMAWare used toweld advanced high strength steel. Mechanical properties including tensile strength, impact energy andmicro hardness are investigated. Also, the microstructures of the welds produced are studied using a scanning electron microscope (SEM). Experimental setup Advanced high strength steel plate Weldox960 of 12 mm thickness was obtained and sectioned to dimensions of 800×150 mm. The chemical compositions and mechanical properties of weldox960 and E-FK1000 welding wire are listed in Table 1 and Table 2. Plates were joined using Hybrid Plasma Arc Welding, (HPAW) and Gas Metal Arc Welding (GMAW) in the flat position (1G/PA). Before welding, the plates were brushed with stainless steel wire and cleaned with acetone to remove oxides and residues. During the welding process, the welding samples were clamped on a platform. The experiments were performed at room temperature and standard atmospheric pressure. The mainwelding parameters are shown in Table 3. For GMAW, the groove angle was set to 60° and three passes were needed to fill the groove. As a comparison, a 30° groove anglewas chosen for HPAW, enabling the weld gap to be filled in only one pass. Figure 1 shows the schematic drawing of HPAW. During the HPAW process, two different arcs exist simultaneously: a positive polarity direct current plasma arc between the workpiece and a tungsten electrode and a reversed polarity direct current GMAW arc between the filler wire electrode and

Table1: The chemical compositions of Weldox960 and E-FK1000 welding wire (wt %).

Alloy

Tensile strength Yield strength AKV (-40°C)

Weldox960 980 MPa E-FK1000 960 MPa

1010 MPa 965 MPa

27 J

-

Table 2: Mechanical properties of weldox960 and E-FK1000 welding wire.

I 1

/U 1

(root pass) I 2

/U 2

(cover pass) I 3

/U 3 (final pass) Welding speed Preheat temperature

GMAW

150 A, 22 V

150 A, 22 V

250 A, 27 V

200 mm/min -1

100~120 °C

MAG current I M

MAG voltage U M

Welding speed Preheat temperature

Plasma current I P

HPAW

240 A

360 A

30.5 V

500 mm/min -1

-

Table 3: Welding parameters of the welding process of Weldox960.

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March 2018

AFRICAN FUSION