African Fusion November 2017

Helium and narrow gap GMA welding

The influence of helium content in shielding gas on metal melting characteristics for narrow gap GMA welding

Xiaoyu Cai, Chenglei Fan, Sanbao Lin, Chunli Yang

Presented in Shanghai at the IIW 2017 International Conference in June this year, this work details research done at the Harbin Institute of Technology in China on the use of helium in shielding gases for narrow gap gas metal arc welding (GMAW).

S hielding gas composition plays an important role in welding process. Tanaka et al studied arc properties in different gases and pointed out that the anode and arc temperatures increased when heliumwas added to argon [1]. J Zähr et al studied the influence of helium-addition to argon TIG-arc properties, and the investigations show that helium additions cause an increased heat input into the workpiece and a decreased arc pressure [2]. Huang found that the pen- etration and cross-sectional area of the weld increased with increasing additions of nitrogen to argon-base shielding gas in TIG welding for stainless steel [3]. Compared with GTA welding, the GMA welding process is more complicated due to the melting electrode. It is neces- sary to understand the role that the shielding gas plays in the plasma arc of GMAwelding in order to obtain satisfactoryweld quality. Some researchers havemade studies on the influence of shielding composition in GMAwelding, but the composition is usually unitary or binary. Wang et al investigated the effects of the gas composition on the temperature distribution in the arc and on the molten pool dynamics in GMA welding of steels with a self-consistent 3D process simulation tool, and the results showed that as the CO 2 content increases, amore constricted arc plasmawith

higher-energy density is generated and more heat transfers from the arc to the workpiece, which results in a wider and deeper weld pool [4]. Rao et al developed a 2D model to study the effects of argon-helium mixtures on the distribution of temperature, fluid flow, current density and electromagnetic force. The research showed that increasing helium levels in argon lead to an increased droplet size and a decreased droplet transfer frequency [5-6]. In conventional narrow gap MAG welding for steel, the shielding gas is the Ar and CO 2 mixture. Urmston pointed out that the weld bead geometry was improved and a more roundedpenetrationprofilewas producedwith the additionof helium in shielding gas [7]. Theocharis used a ternary shield- ing gas mixture (82.5% Ar; 2.5% CO 2 ; 5% He) and it provided good results [8]. The lack of sidewall fusion is the most frequent defect in narrowgapwelding. In this study, the heliumwas added to the shielding gas (Ar/CO 2 ) to assure sidewall fusion. It is expected that the addition of helium will lead to significant changes in the structure and characteristics of the plasma arc, which affects metal melting and transfer, weld bead formation and, ultimately, the weld quality. The effects of shielding gas composition on narrow gap GMAwelding process are investigated in terms of (1) arc shape; (2) wire melting characteristics; (3) weld bead profile. This research provides the essential knowledge needed for the selection of shielding gas composition to obtain the optimum narrow gap GMA welding process and weld quality. Experiments The schematic of the experimental apparatus is presented in Figure 1. The CLOOS 503 power source was operated in DCEP mode. The end of the contact tip was bent to direct the wire toward the sidewall to ensure sufficient sidewall penetration. The U-groove with the bottom width of 10 mm, top width of 12 mm and depth of 25 mm was applied. A high-speed video

Weld No

Ar (%)

CO 2

(%)

He (%)

1 2 3 4 5 6

90 85 80 75 70 65

10 10 10 10 10 10

0 5

Figure: 1. A schematic of the experimental apparatus.

10 15 20 25

Wire feed speed (m/min)

Original wire extension (mm)

Welding voltage (V)

Welding speed (mm/min)

Wire-sidewall distance (mm)

9

18

26

580

2

Table 1: Welding parameters.

Table 2: Experimental design.

18

November 2017

AFRICAN FUSION