Practical MIG Welding: Preventing Heat Distortion
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Preventing Heat Distortion
03.17
| 00:00 | At its core, MIG welding involves the process of heating and melting our base material while filler wire forms the weld bead as it joins with the base metal to bridge the gap, creating a weld seam. |
| 00:11 | As you can probably guess, this welding process creates a lot of heat and MIG welding is known as the most aggressive method in terms of the amount of heat input into the workpiece out of all the welding processes. |
| 00:21 | Unfortunately the heat generated can result in some less than desirable changes to the parts that we're welding and we need to have at least a basic understanding of these metals and the changes that occur when they're welded. |
| 00:32 | In short, metal expands when heated. |
| 00:35 | The length, surface area and volume all increase with temperature. |
| 00:39 | The scientific term for this is thermal expansion and it occurs at different rates in different materials and will affect anything that we weld. |
| 00:47 | There's not a whole lot we can do to control the amount of heat that will be applied to the part. |
| 00:51 | After all, we need enough heat to properly fuse the workpiece together and produce a strong weld. |
| 00:56 | However, if left unsupported, our weld will be strong but our part will be dimensionally all over the place, potentially rendering it completely useless. |
| 01:05 | Keeping a part dimensionally correct is possible with the use of a heatsink or clamping aid or a combination of the two. |
| 01:12 | A heatsink, as its name suggests, is anything that removes the heat out of the part by drawing it into itself. |
| 01:18 | By using a material with a higher thermal conductivity than the base material that we're welding, we're removing the heat faster than it can accumulate and preventing movement of the part we're trying to keep supported. |
| 01:30 | Aluminium and copper are two of the best thermal conductors that we have and because aluminium is much more common in the workshop, it's the go to metal of choice for heatsinks. |
| 01:39 | A large piece of aluminium clamped or bolted to our workpiece will do a great job of removing heat and the closer it is to the weld, the more effective it'll be. |
| 01:47 | Large aluminium plates are perfect for pulling heat out of the welding process and jigging or clamping our part to the heatsink will ensure it stays dimensionally stable. |
| 01:56 | If we didn't have our part secured to a heatsink like this, then we'd be letting the steel expand and contract unrestricted which would almost certainly end up in a part that won't remain flat once completed. |
| 02:07 | This could be a real problem with parts such as exhaust manifold flanges which rely on a flat surface to seal properly. |
| 02:14 | By leaving the part secured until it's completely cool, we're lengthening the entire heating and cooling process, minimising movement in the metal that would normally happen if it wasn't supported. |
| 02:24 | Stainless steel has a very low rate of thermal conductivity whereas mild steels have a little more, followed by aluminium which has the highest rate of metals we're likely to be using in automotive fabrication. |
| 02:34 | Understanding the effects of heat in metals and employing some controls over them will dramatically improve the quality and consistency of your finished parts. |
| 02:43 | Before moving on, let's quickly run back over the main points covered in this module. |
| 02:47 | MIG welding generates significant heat in our workpiece and if this heat is not controlled and removed, it can warp the part to a point that it's completely useless to us. |
| 02:57 | To control this, heat sinks which are materials with high thermal conductivity like aluminium can be used to draw heat away from the workpiece, preventing distortion. |
| 03:06 | Clamping or jigging the part and allowing it to cool while supported minimises the metal movement during the heating and cooling process. |
