Welding speed is the linear rate at which the arc moves with respect to plate along the weld joint. Welding speed generally conforms to a given combination of welding current and arc voltage.
If welding speed is more than required
• Heat input to the joint decreases.
• Less filler metal is deposited than requires, less weld reinforcement height
• Undercut, arc blow, porosity and uneven bead shape may result.
If welding speed is slow
• Filler metal deposition rate increases, more weld reinforcement
• Heat input rate increases
• Weld width increases and reinforcement height also increases more convexity.
• Penetration decreases beyond a certain decrease in speed.
• A large weld pool, rough bead and possible slag inclusion.
With all variables held constant, weld penetration depth attains a maximum at a certain intermediate welding speed. At excessively low welding speeds the arc strikes a large molten pool, the penetrating force gets cushioned by the molten pool. With excessively high welding speeds, there is substantial drop in thermal energy per unit length of welded joint resulting in undercutting along the edges of the weld bead because of insufficient backflow of filler metal to fill the path melted by the arc. Welding speed is to be adjusted within limits to control weld size and depth of penetration.
3.9.3 Electrode Feed Speed
Electrode feed rate determines the amount of metal deposited per unit length or per unit time. In most welding machines the welding current adjusts itself with electrode feed speed to maintain proper arc length.
3.9.4 Electrode Extension
Electrode extension, also known as length of stick out, is the distance between the end of the contact tube and the end of the electrode as shown in Fig. 3.25. An increase in electrode extension results in an increase in electrical resistance.
This causes resistance heating of electrode extended length, resulting in additional heat generation and increase of electrode melting rate. But the energy so consumed reduces the power delivered to the arc. This reduces arc voltage and thus decreases bead width and penetration depth.
To maintain proper head geometry alongwith a desired penetration and higher melting rate (i. e., large electrode extension), the machine voltage setting must be increased to maintain proper arc length. At current densities above 125 A/mm2, electrode extension becomes important. An increase of upto 50% in deposition rate can be achieved by using long electrode
Fig. 3.25 GMA welding terminology
extensions without increasing welding current. This increase in deposition rate is accompanied with decrease in penetration.
Thus when deep penetration is desired long electrode extension is not desirable. On the other hand, for thinner plates, to avoid the possibility of melting through, a longer electrode extension becomes beneficial. It is also important to note that the increase in arc extension make it more difficult to maintain correct position of electrode tip with respect weld centreline.
3.9.5 Electrode Diameter
Electrode affects bead configuration, affecting penetration and deposition rate. (Fig. 3.26). At any given current, a smaller diameter electrode will give higher current density causing a higher deposition rate compared to large diameter electrode. A larger diameter electrode, however requires a higher minimum current to achieve the same metal transfer characteristics. Thus larger electrode will produce higher deposition rate at higher current. If a desired feed rate is higher than the feed-moter can deliver changing to larger size electrode will permit desired deposition rate and vice versa. In case of poor fit-up or thick plates welding larger electrode size is better to bridge large root openings then smaller ones.
600 A, 30 V, 13 mm/s
3.15 mm 4 mm 5.6 mm
Fig. 3.26 Effect of electrode size on bead geometry