Welding Variables – Talking Voltage and Amperage

When it comes to weld penetration, the current (amperage) has the upper hand. When it comes to welding, penetration is more technically known as “depth of fusion,” which is a measure of how far a weld extends into the base metal from the surface melted during welding. The correlation between current and depth of fusion is direct: increase the amperage and the depth of fusion increases; decrease the amperage and the depth of fusion decreases proportionally.

Voltage just doesn’t have the same impact. It does affect arc length, and arc length affects the arc cone (the width of the arc when it reaches the weld), which in turn has a minimal effect on weld penetration or depth of fusion. The narrower arc cone (lower voltage) has more focus and thus creates a slightly deeper penetration. However, this effect is minimal compared to that provided by altering the amperage, and it can be compensated for by the speed of travel. The effect of speed of travel is fairly intuitively predictable. The longer the arc remains over one spot, the more heat is generated in that one spot and the deeper the weld can penetrate.

Taken to an extreme, depth of weld penetration can lead to weld blowout, where the base metal becomes molten throughout and the shielding gases blow bubbles through it. The optimum weld is an appropriate middle ground between one where inadequate penetration creates structural weakness due to insufficient depth of fusion and one where excessive penetration creates structural weakness due to fusion adulterated by weld defects. Moderation in all things, as they say.

The type of machine used determines the type of control over these factors. Constant current (CC) machines have current as of the primary presettable output. With Constant Voltage (CV) machines, voltage and Wire Feed Speed (WFS) are the presettable factors, and current is a consequence of Wire Feed Speed. This is another direct correlation, whereas WFS increases, current increases, and as WFS decreases current decreases. By extrapolation, as WFS increases, depth of fusion increases, and as WFS decreases, the depth of fusion decreases.

There are a number of other variables, too numerous to detail here, that can affect weld penetration: the welding process used, the type (and size) of electrode used, the contact-tip-to-work distance (CTWD), travel angle and travel speed. In many of these factors, too, the current is the underlying agent of the depth of fusion. For example, the greater the CTWD, the greater the resistance to the flow of electricity, which in turn reduces the current, while reduced CTWD also reduces the resistance, which in turn increases the current.

The proper admixture of all these factors can contribute to the creation of the optimum weld, contingent on the base metals being joined. Consult your friends at Eureka Oxygen for suggestions as to the appropriate tools and methods for your application.