Tech Talk: Successfully Welding Sheet Metal With MIG and TIG
(As seen in Applied Welding, Issue 1, 2003)
Q: How can I avoid warping, burn-through and excessive heat affected zones while still ensuring the weld has sufficient mechanical strength?
A: Start by using a weld process that provides more control over heat input: short circuit transfer GMAW (“short arc”), pulsed GMAW, GTAW and pulsed GTAW.
Q: Where do I direct the arc to minimize heat input?
A: Direct the arc at the middle of the weld puddle. This enables the puddle to insulate the base metal from the arc’s full force. Do not whip or weave the torch; the more time you keep the arc in an area the hotter it becomes. Always travel in a straight line and use the fastest travel speeds possible that maintains a good bead profile.
Q: How can I avoid warping?
A: Distribute the heat as evenly as possible by using a skip welding technique. To dissipate heat faster, place the heat affected zone (HAZ) in contact with a “backing bar” or “chill bar.” A backing bar can be as simple as a metal bar (usually copper or aluminum because they dissipate heat best) clamped to the back of the weldment. In higher duty-cycle applications, you may need to consider a water-cooled backing bar.
Q: Is part fit-up really that critical on thin metal?
A: Yes. Imagine a butt weld on 20 ga. metal. If the parts fail to touch for even 1/16 in., you have just left a gap that cannot absorb the heat and burn-through will occur. To avoid rework, adhere to the old saying “measure twice, cut once.”
Q: What is a good power source for short circuit GMAW?
A: One with good voltage control, good arc starts and good arc stability at the low end of its voltage range. In the “all-in-one MIG” category, the Millermatic® Passport™ and Millermatic 350/350P can’t be beat. They feature inverter technology that responds thousands of times faster to changing arc conditions than conventional machines, which in turn produces smoother arc starts (less “popping”) and weld beads with less spatter. For MIG power sources that require a separate feeder, the XMT® 350, Axcess™ Series and Dimension™ NT 450 provide outstanding short circuit MIG characteristics. Using solid wire and 75/25 argon/CO2 shielding gas, operators can weld at low amperages without experiencing arc outages, excess spatter or burn-through. Note that the Axcess and Millermatic 350P also feature built-in pulsing capabilities.
Q: Which TIG welder works best on thin metal?
A: For unbeatable results on aluminum, use an inverter with extended AC balance control and an adjustable output frequency, such as Miller’s Dynasty®. It lets you precisely direct the arc, establish the weld puddle faster and place the filler wire right where you want it, all of which prevents burn-through.
GMAW (short arc)
|Type of metal it weld||Steel, stainless, aluminum||Steel, stainless, aluminum||Steel, stainless, aluminum||Steel, stainless (rarely used on thin aluminum|
|Metal thickness||24 gauge and up||1/16 in. and up||.020 in. and up||.020 in. and up|
|Welding speed||Very fast||Very fast||Slow||Slow|
|Bead Appearance||Good—some spatter||Better—almost no spatter||Better— no spatter||Best— no spatter|
|Skill required||Some skill||Some skill||Most skill||Most skill|
|Operating cost (inc. labor cost)||Low||Low||High||High|
Figure 1: This is a brief comparison of welding processes for sheet metal. The right process for you depends on the relative influence of the factors in your operation.