African Fusion November 2017

Helium and narrow gap GMA welding

wire melting static characteristics equation: v m = k i I - k u U k i ∝ α + β e k u ∝ µ L a U Where v m is the wire melting rate, k i

Compared with zero%helium content, the current increases when the helium content is 5%. Helium has a higher thermal conductivity than argon, with the addition of helium, more heat transfers to the periphery of the arc, and the arc temper- ature increases. More particles are ionised and the charged particles increase due to the thermal ionisation, and the welding voltage is constant, so the welding current is higher. However, as the helium content increases further (>5%), the welding current decreases. This can be explained by the

is wire melting coefficient

with current, k u is wire melting coefficient with voltage; I is welding current; U is welding voltage; α , β and µ are propor- tionality constant; L a is arc length: and L e is wire extension length. When the arc length is stable, thewire feed speed v f is equal to the wire melting rate. v f = v m It can, therefore, be deduced that: I = v f /k i + Uk u /k i Thewirefeed rate, vf is fixed at 9m/min andU is constant. With an increase in helium content, the La decreases and the Le increases. The ki increases and ku decreases, thereby resulting in a decrease in current, I. So it can be concluded that, at a constant wire feed speed, the wire constant melting curve ‘moves left’ as shown in Figure 4. Furthermore, the shielding gas composition also directly influences α , β and µ, but the influencing mechanism is com- plex and has not been well understood. As shown in Figure 5, line A is the power source output characteristic, line B is arc voltage-current relationship and line C is the wire constant melting curve with a low helium content. For a stablewelding process, A, BandCmust intersect at a single point, Q 1 . When the helium content in shielding gas increases, line B moves up to line b, and line C moves left to line c. After the change, A, b and c intersect at the single point, Q 2 , which is the newequilibriumposition, where thewiremelting rate is equal to the wire feed speed. Based on the above results, the shielding gas composi- tion affects the wire melting characteristics in MAG welding. For constant welding voltage, the current decreases with an increase of helium (I 2 < I 1 ), but the wire melting rate remains constant. Base metal melting characteristics for different helium content In order to produce completedweld beads, twin-wire welding

Figure 4: Wire constant melting characteristics for different helium content.

Figure 5: Wire melting characteristics for different helium content.

experiments were carried out. The two wires were controlled independently by two power sources. The ends of the two contact tips were angled to direct the two wires toward opposite sidewalls. The dis- tance between the two wires was 30 mm. The weld profiles for different helium content are presented in Figure 6. As he- lium content increases, the finger-shaped profile transforms to bowl-shaped. Side fusion depths are measured and given in Figure 7. These show that the depth of sidewall fusion increases signifi- cantly with the addition of helium in the shielding gas, and it is improved by about 40% when a mixture of 80% Ar; 10% CO 2 ; 10% He is used. When the helium content is 20%, the

Figure 6: Weld fusion profiles for different helium content.

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November 2017

AFRICAN FUSION