Q: For what jobs can Gas Metal Arc welding be used?
A: Let’s set up two simple examples of possible welding
jobs: replacement of a quarter panel (18-gauge steel) and replacement
of a front-frame horn (5/32-inch-thick steel). How would you weld
both jobs with one machine?
You could immediately rule out stick welding because of its tendency
to blow holes through thin materials and because it generates
too large of a heat-affected zone, which can cause loss of strength.
As for spot welders, domestic automakers don’t recommend them
for making structural repairs. Both welds could be made using
a TIG welder, but the process would be time consuming and require
a highly skilled tech. A MIG welder, however, can produce sound
welds on both jobs.
A MIG welder can: weld steel and aluminum; make plug, butt, fillet
and lap welds; and make welds in all positions. Almost every collision
repair made on a unitized car, truck or van can be accomplished
with a MIG welder.
Q: What size do I need?
A: The primary criteria for determining the size welder
needed is the thickness of the metal you want to weld. In MIG
welding, generally the thicker the material, the thicker the welding
wire you need. And the thicker the welding wire, the higher amperage
needed.
Therefore, the first step in determining size is to define what
type of work you want to do. If you only work on unitized vehicles
(no pickup trucks or full-size vans), the majority of your welds
will be made on upperbody cosmetic areas around 18-gauge and on
structural members less than 1/8-inch thick. The recommended welding-wire
diameter for these repairs is commonly .023 inch to .025 inch.
A welder that can produce about 90 amps at a 20 percent duty cycle
would enable you to make nearly every repair required on a unitized
vehicle.
If you plan to work on trucks, vans and sport-utility vehicles,
however, you’ll need a larger welder. For example, to replace
the front-frame horn on a small pickup truck, you need to make
a butt weld on mild steel 5/32-inch thick with a 5/32-inch root
gap and use a backer plate to control the flow of filler material.
To obtain optimum results, you need to completely fill in the
joint with one continuous weld pass. In this situation, commonly
used wire diameters are .035 inch or .045 inch. When using welding
wire this size on material this thick, you should use a welder
that can generate about 200 amps at a 60 percent duty cycle.
Q: Can high-amperage welders use small-diameter wires?
A: Not always. Not all high-amperage welders have the versatility
to handle both .023- and .045-inch wire. However, some units are
designed to run a broad range of wire sizes, and they make excellent
welds on both thick and thin metals.
Q: Why is duty cycle important?
A: The duty cycle of a welder is the amount of time you
can use it at a given output without it overheating or burning
up. In the United States, duty cycle is based on a 10-minute period.
If you buy a machine with a 40 percent duty cycle at 250 amps,
you can weld at 250 amps for four minutes. For the remaining six
minutes, the welder would need to idle and cool.
Since most collision-repair welds are short (compared to production
welds) or made using a skip-welding technique to avoid warpage,
100 percent duty-cycle welders are not needed. Also, note that
if the welder is operated below the rated output, the duty cycle
increases. For example, the 250-amp welder noted above can weld
1/8-inch steel without stopping.
Q: Can I MIG weld aluminum?
A: Yes, but not every MIG welder can run aluminum wire
effectively. Because it melts so quickly, aluminum requires a
wire-feed speed two or three times faster than steel. Welding
aluminum at 150 amps using .035-inch diameter, type ER5356 wire
requires a wire-feed speed of about 500 inches per minute.
If you plan to MIG weld aluminum, look for a welder that has a
top-end wire-feed speed close to 700 inches per minute and make
sure it has a good wire-braking system to prevent backlash (overspool).
Note: The thinnest possible aluminum most people can MIG weld
is about 1/16 inch.
Q: What equipment do I need?
A: To start: a work cable and clamp to complete the weld
circuit, a MIG-gun assembly (includes hose, nozzle and contact
tips sized for the wire diameter you plan to use), a shielding-gas
cylinder (a 75 percent Argon/25 percent C02 mix is a typical choice
for MIG welding, but 100 percent C02 can sometimes be used), a
gas regulator/flow meter to adjust the proper flow of shielding
gas and safety equipment (gloves, helmet, goggles and appropriate
welding clothes).
Q: What is the most common mistake?
A: Not practicing. Before you make the repair weld, practice
welding on similar material. This will allow you to fine tune
your wire-feed speed and voltage settings, enabling you to make
quality welds right at the start.
Writer John Helgren is manager of Miller Electric Mfg. Co.
Check It Out
If you’re considering a MIG welder, consider the following:
- A MIG welder can weld steel and aluminum; make plug, butt,
fillet and lap welds; and make welds in all positions. - In MIG welding, generally the thicker the material, the thicker
the welding wire you need. The thicker the welding wire, the higher
amperage needed. - Not all high-amperage welders have the versatility to handle
both .023- and .045-inch wire. - Since most collision-repair welds are short (compared to production
welds) or made using a skip-welding technique, a 100 percent duty
cycle isn’t needed. - To MIG weld aluminum, look for a welder that has a top-end
wire-feed speed close to 700 inches per minute, and make sure
it has a good wire-braking system to prevent backlash. - Most high-quality “all-in-one” MIG welders come
with everything you need to start welding except the gas cylinder
and safety equipment. - Before you make the repair weld, practice welding on material
similar to the repair you’ll be making.
