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AC / DC WELDING - CONTINUE
AC welding is also the preferred method for:
• TIG welding aluminum, because the current supports welding at a higher temperature.
• Making repairs on machinery because the machinery usually has a magnetized field and is older and may
have rusty areas where there is concern about the higher heat penetration that can occur with DC welding.
• Seam welding in shipbuilding because the current settings can often be higher than those used in DC
welding and a deeper penetration of plate metals can be obtained.
The biggest drawback to using AC welding is the quality of the weld. It is not as smooth as DC welding
because of the continuous change in directional flow and there is likely to be more spatter.
Different waveforms for AC welding on TIG welding machines
There will be different waveforms depending on the parameter settings. This gives the welder even more precise
control of the weld pool and the seam’s appearance. The flickering of the pulse arc can also be affected, reducing
the stress on the welder’s eyes in the process. There are also acoustic differences as noise is reduced.
Applications of DC Welding
Welding with DC is best used for:
• Hard facing
• Overhead or vertical welding
• Single carbon brazing
• Build-up of heavy deposits
• Stainless steel TIG welding
• Cutting tap
As a rule of thumb, DC is preferred for welding because:
It produces a smoother weld and there is less spatter because of the constant linear direction of the current.
It maintains a constant and stable arc and is thus is easier to handle and more reliable than AC current.
Machinery that uses DC current is generally cheaper and easier to use.
It welds thinner metals better than AC current.
Overall Strength of Welds
Overall the strength of the weld can be determined by many factors, such as:
• Proper electrode, welding apparatus and procedures;
• Properties of the materials being welded-magnetic vs. non-magnetic;
• Proper edge preparation-the cleaner the edge, the better the weld;
• Current settings – DC vs. AC;
• Speed of travel – the angle of the electrode needs to be maintained throughout the length of the joint as it is
being welded.
A strong weld can be achieved in both AC welding and DC welding so long as you weld with the current and
polarity appropriate for the material being welded. One doesn’t necessarily, always and everywhere, produce
a better weld than the other. It’s a matter of choosing the right one for your job.
AC
AC
AC
Waveform: Hard rectangle
Waveform: Soft rectangle
Waveform: Triangle
Switching between the plus and
minus pole is very fast.
Effect:
The goal is an extremely fast zero
crossing of the half-wave. The
arc becomes highly stable while
a very loud, “hard” arc noise is
produced at the same time.
Applications:
This “hard” arc is still the standard
today for many welders as they
have always been used to it and
it offers good welding properties.
There is also the advantage that
the arc can be used on very thick
oxide layers.
The switching between plus and
minus is therefore not quite as
abrupt as it is with the hard
rectangle function, and not as
delayed as in sine mode.
Effect:
The arc combines the advantage
of the sine and the hard rectangle
modes. The result is a relatively
stable arc, with relatively low/quiet
background noise.
Applications:
Due to these properties, the option
“soft rectangle” for TIG welding
systems is also the universal set-
ting– for a wide range of welding
tasks.
The variation of the current between
plus and minus is distributed
entirely evenly over time.
Effect:
The zero crossing is therefore
rather slow in comparison to
the other waveforms. However,
the even distribution of the
current-time area achieves a
higher peak current, although the
mean current remains the same.
This in turn results in a particularly
high arc pressure. Acoustically, the
arc is only slightly louder than the
sine setting.
Applications:
The high arc pressure means the
triangle setting is particularly well
suited for root passes.
