Motorsport Fabrication Fundamentals: Effect of Heat on Metal
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Effect of Heat on Metal
05.36
| 00:00 | - In motorsports, we're often working to control heat and trying to manage the temperature of components to improve performance and reliability. |
| 00:07 | It's not just heat management in the engine bay that we need to consider though but also how the heat affects the material during fabrication, particularly when it comes to welding. |
| 00:17 | This is all because metal expands when heated. |
| 00:20 | Its length, surface area and volume increase with temperature, which is a process known as thermal expansion. |
| 00:26 | The degree of thermal expansion depends on the temperature of the component as well as the thermal expansion coefficient of the metal and this can vary dramatically from one material to another. |
| 00:37 | Accounting for this thermal expansion and controlling it is essential when working with metals. |
| 00:42 | It's especially apparent in fabrication due to the localised input of heat when we're welding. |
| 00:46 | Because hot materials expand and then contract again once cooled. |
| 00:51 | A large temperature variation between the molten weld pool and the cooled base material can result in internal stresses as the weld area cools after the weld is complete. |
| 01:01 | While not immediately obvious, this can cause problems with the weld strength and reliability of some materials like the chromoly we discussed in the materials section. |
| 01:10 | Uniformally heating the metal slowly and cooling it back down at the same rate allows metals to expand and contract evenly which can help circumvent some of the problems associated with welding. |
| 01:22 | This process of pre heating, welding and slow cooling of metals minimises the temperature differences between the weld and the base material and lessens stresses that can lead to cracking and distortion. |
| 01:33 | This is especially handy when welding thick sections of metal because pre heating the weld area will also allow for better penetration and therefore more strength. |
| 01:43 | If we look at the other end of the scale and consider extremely thin sections of sheet metal, we need to employ a different type of thermal control. |
| 01:51 | Thin sheet metals can expand extremely quickly and without the necessary clamps, heat sinks or weld settings, can end up severely bent or distorted which may then be impossible to straighten out, rendering the component that we've just made useless. |
| 02:05 | While not necessarily a thin sheet metal, a perfect example of this situation would be welding the exhaust manifold runners to a header flange that's just sitting unsupported on our workbench. |
| 02:17 | Without proper support and heat sinking, the end result will be a header flange that's about as straight as a banana. |
| 02:23 | Not ideal for sealing to the cylinder head without serious exhaust leaks. |
| 02:27 | So let's take a look at some of the clamps that can be used to control this thermal expansion. |
| 02:32 | We should clarify that we can't eliminate thermal expansion but what we can do is limit it and lessen its impact on our finished products. |
| 02:40 | By clamping our workpiece with a series of G clamps or in the case of the exhaust manifold flange, simply bolting it firmly to a cylinder head, we can prevent the heated area from retracting and pulling our component out of alignment, making the finished product significantly more precise. |
| 02:56 | Using something called a heat sink is very common when it comes to TIG welding. |
| 03:00 | This is a material with a very high thermal conductivity like a block of aluminium for example that can be clamped to the workpiece. |
| 03:07 | The heat sink essentially absorbs the heat that's been applied to the workpiece, lowering the temperature variation and stabilising the expansion and contraction of the material. |
| 03:16 | In our example of the manifold flange, bolting the flange to a cylinder head actually gives us the benefit of both techniques. |
| 03:23 | The flange is clamped which helps prevent distortion but the alloy head also acts as a great heat sink at the same time. |
| 03:30 | Another aspect of thermal management that we should always be keeping in mind is simply good weld technique and part fit up. |
| 03:37 | By minimising gaps in the construction of our component, we essentially spend less time welding and filling gaps which means less heat is input into the material resulting in reduced thermal expansion. |
| 03:48 | Good weld management of course comes with experience but we can fast track this experience by equipping you with a few quick pointers to get you started. |
| 03:56 | First of all, let's discuss tack welds which are a key part of the welding process. |
| 04:01 | These can be used to temporarily tack our components together while we confirm proper orientation before we commit to fully welding the part. |
| 04:09 | They also ensure parts can't pull or move when the component is fully welded. |
| 04:14 | When we apply tack welds, we want to ensure that we tack the component in multiple places and locate the tacks evenly around the part. |
| 04:22 | This is especially important with things like headers and exhaust systems, since if we only tack one or two positions on each section of the exhaust, then we're really playing a guessing game as to how good the exhaust alignment will be once we get it all welded up. |
| 04:35 | By tacking uniformly around each joint, we minimise distortion and the exhaust ends up going where we intended. |
| 04:41 | Like tacking, stitch welding can be done in sections and slows down the rate of thermal expansion. |
| 04:48 | Maybe you're fabricating something that doesn't need to be fully welded or you're just being careful. |
| 04:53 | Stitch welding is a great way to space out the heat generated and can play a vital role in making sheet metal behave itself during the welding process. |
| 05:01 | In summary, when it comes to welding metals, we don't need to understand what's happening at an atomic level or even the science of mettalurgy. |
| 05:08 | But we do need to understand the effects of thermal expansion and how we can minimise the harmful results that this can have on our welding process. |
| 05:16 | The next time you pick up a welding torch, just take a look at the job and have a think about the effect that your weld is about to have on the part. |
| 05:22 | Take a moment to decide if you can clamp, heat sink or change a style of weld to improve the finished product because it's a lot less work to do it now, rather than after the weld is completed. |
