You’re watching a car chase movie. The hero is losing, until he remembers he has another gear in the ‘box. He shifts, he wins. It’s a tired old movie trope, but we all love it. Another favorite is, “Well this changes everything.” Of course, it’s pretty rare that something comes along to change everything, especially in the metal fabrication world. Cold metal transfer may however be that something.
Cold metal transfer (CMT) is new form of gas metal arc welding (GMAW.) It’s not exactly cold, but it is lower temperature than regular GMAW processes like MIG. That’s a big deal in welding because, as you’ll know if you’ve read our earlier Tiki Talk posts, heat causes distortion. (“Stitch Welding VS. Seam Welding” is just one example.) Here’s how it compares with MIG.
MIG welding basics
The workpiece and welding torch are part of the same electrical circuit. Filler wire is fed through the torch and when it touches the workpiece current flows. High amperage heats the wire rapidly, making it melt. As the metal drop separates an arc forms that’s about 6,500°F. This heats the shielding gas flowing through the torch, and also melts the workpiece, allowing the metals to fuse. At the same time, the filler wire feeds forward to touch the workpiece and stop the arc. This process repeats several times each second.
Making bigger welds with deeper penetration means turning up the voltage and the wire feed rate. Now the arc is continuous and the filler melts before reaching the workpiece. Technically, this is known as “globular transfer” and it causes weld spatter – those metal droplets scattered over the workpiece.
The heat problem
Much of the skill in welding is about managing all this heat. Metals, especially aluminum, conduct heat well, so heat from the weld spreads quickly through the workpiece and makes it expand. Then, when it cools back down, because the pieces are fused together they distort.
When welding thin gauge metal it’s very easy to put in too much heat. That causes burn-through, and instead of joining pieces you’ve just made a big hole.
CMT welding is a big advance in dealing with these challenges.
Less arc, less heat
Like MIG, CMT welding is a GMAW process. The big difference is in the wire feed. Rather than continuously moving forward into the weld pool, with CMT the wire is retracted the instant current flows. That breaks the arc. The metal droplet detaches from the filler and fuses with the – still molten – base metal. Then the wire moves forward to create another arc, and all this happens many times each second.
The difference from conventional MIG welding is fairly subtle, although it needs a lot of sophisticated control technology. The benefit is that it reduces heat input significantly. The developer of CMT welding, Fronius, describes it as, “Hot, cold, hot, cold, hot cold.”
CMT welding applications
Less heat makes CMT ideal for welding thin gauge material. It reduces distortion in welded metal fabrications, and it eliminates spatter, so there’s less clean-up. More intriguingly, it makes possible welding of aluminum to steel. That’s always been difficult because iron and aluminum don’t care to mix, which of course is how welding works. But the smaller heat input of CMT welding is opening the door to steel-aluminum welding.
Lots of potential
Not every welded fabrication will need CMT, but it’s definitely an interesting advance in welding technology. Less clean-up, less distortion and easier butt welding of thin gauge sheet will definitely benefit our customers. That’s why we’re watching CMT closely. Perhaps it will change everything, (in the world of fabrication anyway.) Now, about the sound those explosions make in the vacuum of space …