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Practical TIG Welding: Preventing Heat Distortion

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Preventing Heat Distortion


00:00 - While we're not here to get too deep into the weeds with metallurgy, we do need to have a basic understanding of metals and the changes that occur when they're welded.
00:08 In short, metal expands when heated and its length, surface area and volume all increase with temperature.
00:15 The term for this is thermal expansion and it occurs at different rates in different metals and will affect everything that we weld.
00:23 The TIG welding process is the best form of welding to minimise this thermal expansion due to the small arc size and the fine control over the amount of amperage we use and the time spent welding the part.
00:35 This means that all things being equal, we input less heat into the work piece when TIG welding compared to MIG welding and in turn, the part will experience less thermal expansion.
00:45 Even TIG welding still applies substantial heat to the material though.
00:49 After all, we need enough heat to produce a strong weld.
00:53 This means we need to consider the effect of this heat on our parts and take measures to mitigate the problems heat can cause.
01:00 Otherwise our welded part will be dimensionally all over the place when we're finished.
01:04 Keeping a part dimensionally stable is possible with the use of a heat sink or a clamping aid or a combination of the two.
01:12 A heat sink, as its name suggests is anything that sinks the heat out of the part by drawing it into itself.
01:19 By using a material with a high thermal conductivity we're removing the heat faster than it can accumulate and preventing the movement of the part we're trying to keep supported.
01:29 Aluminium and copper are ideal materials for use as heat sinks since they both have very high thermal conductivity but since aluminium is much more common in the workshop, it's the go to metal of choice for heat sinks.
01:42 For our demonstration we're going to weld two samples of stainless steel and we'll be welding a right angle onto the flat plate.
01:49 In this instance, what we want to do is see the effect of heat on the flat section of plate when the part is being welded and allowed to cool.
01:57 We'll start by welding the first example without any heat sinking or clamping to just see how bad this distortion can get.
02:05 To do this, we're just going to set up on our welding bench and run a weld down each side of the sample part.
02:11 Once the part is cooled we can see just how much the flat plate has distorted.
02:15 Obviously if the flatness of this part was critical to its fitment, it may now be completely useless.
02:21 For our second demonstration, we'll see how we can improve the situation by using a thick aluminium plate as a heat sink and we'll then clamp our part to the heat sink and to our welding bench to keep it in position during the welding and cooling process.
02:36 The actual process of welding is of course no different but we have the benefit of the clamps preventing the part from distorting and the alloy plate would help pull heat out of the stainless and dissipate it.
02:47 It's important when using clamps like this to leave the part clamped until it is completely cooled to room temperature.
02:54 Once it has, we can remove the clamps and it's straight away obvious how little distortion is now evident in this part.
03:00 If we look at the back of the welded section, we can also see how effective the heat sink has been in drawing heat out of the work piece.
03:07 The part welded without using the heat sink is showing significant discolouration which is the reaction of the stainless material with atmospheric air.
03:16 On the other hand, this is greatly diminished when the part was clamped to the heat sink.
03:21 One of the more common situations where clamping and heat sinking are critical is in the fabrication of exhaust headers.
03:28 As we weld the primary runners to the header flange, the heat can easily cause severe bending to the point that the header can't be recovered and is useless.
03:37 Fortunately, a simple solution for header fabrication is the use of a spare cylinder head.
03:43 With the majority of modern heads being aluminium, they act as an excellent heat sink and they offer the benefit of being able to use the manifold studs to tightly clamp the flange in place during the welding and cooling process.

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