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Motorsport Fabrication Fundamentals: Gas Shielding

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Gas Shielding


00:00 - In this module, we'll be taking a more detailed look at the shielding gas used on our welding process as well as why it's so important.
00:07 As we already discussed earlier in the course, shielding gas is an essential part of both MIG and TIG welding and knowing why it does what it does will help us to understand the role it plays.
00:17 The primary purpose of shielding gas is to prevent exposure of the molten weld pool to the oxygen, nitrogen and hydrogen contained in atmospheric air.
00:26 The reaction of these elements with a weld pool that's molten can create a variety of problems including porosity and excessive spatter which will negatively impact the strength and reliability of our weld, not to mention its finished appearance.
00:40 Different shielding gases all play an important roll in determining weld penetration profiles, arc stability, mechanical properties and weld appearance.
00:48 Let's begin with the MIG welding process and take a look at the most common shielding gases used.
00:54 Argon, helium, carbon dioxide and oxygen, with each providing unique benefits and drawbacks in any given application.
01:01 Carbon dioxide, also known as CO2 is the most common form of shielding gas used in MIG welding and can provide a big cost saving compared to other options.
01:10 At high welding temperatures, CO2 turns from an inert gas to a reactive gas which can provide a few key advantages.
01:17 Like improved weld penetration and weld speed which is especially useful for welding thick materials.
01:24 This can however make welding thin material trickier and pure CO2 has low electrical conductivity that results in a less stable arc and more spatter than when it's mixed with other gases.
01:36 By adding argon, we get a more desirable combination of arc stability, molten weld pool control and reduced spatter.
01:42 Oxygen is also often added and can improve weld pool fluidity, penetration and arc stability when working with mild carbon, low alloy and stainless steels.
01:51 It does also cause oxidation of the weld metal though so it's not recommended for use with aluminium, magnesium, copper or other exotic metals.
02:00 Next we have helium which produces a deep penetration profile and works really well with thick metals.
02:06 It's also usually mixed with argon and CO2 which creates a hotter arc that allows for faster weld travel speeds.
02:13 Pure argon is used in the MIG welder for aluminium fabrication but it's most commonly found hooked up to the TIG welder to weld all metals including stainless, titanium, aluminium and steel.
02:25 With all of that said, there's no point in having the right gas if it isn't getting to where we need it and that's the job of the regulator.
02:31 This controls our bottle's pressure and meters the flow rate to ensure the right amount of gas is shielding our weld.
02:37 The MIG welder should be flowing around 15 litres per minute and the TIG should be set usually around 6-8 litres per minute but this will depend on the particular welding task.
02:47 These settings should also be seen as a base for working indoors so if you're welding outdoors or have a breeze running through the workshop, it's best to up the gas flow or better still, create a wind block around your work area.
02:59 We've set up a demonstration by running smoke through three different TIG torch setups.
03:04 The first is a traditional TIG setup that uses a series of holes to expell the gas through the collet body into the nozzle.
03:11 As you can see, this gives a very narrow range of shielding and makes the technique of getting the torch angle and arc length correct, extremely important.
03:19 The second is a gas lens that utilises a number 6 cup.
03:22 This uses stainless mesh to reduce the shielding gas turbulence and provide a longer, undisturbed laminar flow of gas to the weld pool.
03:30 The gas lens also allows the welder to move the nozzle further away from the joint and extend the tungsten electrode past the nozzle to get into tighter weld areas such as merge collector on an exhaust.
03:41 When welding extremely reactive metals like titanium, we need to ensure complete gas coverage to the heat affected zone.
03:48 This is handled by a large cup that doubles the amount of lens mesh and further increases the laminar flow of the gas shield, both to the long tungsten stick out and the weld area itself.
03:59 This will require you to almost double the amount of flow to compensate for the larger diameter of the cup.
04:05 At this point we should take some time to understand that shielding gas isn't just require to shield the visible weld pool and the arc.
04:11 It's also needed for the back side of our heat affected zone and this is particularly important on tubular materials.
04:18 We shield the back side by plugging both ends of the tube and introducing a flow of argon into the workpiece assembly in a process called purging.
04:26 This allows the back side of the weld to seal itself and create a strong and smooth seam of weld around the join.
04:32 Argon is heavier than air, meaning that a hole in the top of the workpiece allows the air to release as the tube fills with argon.
04:39 This argon can continue to flow and should be felt exiting from the hole in the opposite end of the inlet until the weld process is complete.
04:46 A professional welder will have all types of jigs and fixtures on hand to set up all types of purging operations but for those of us just working from the home garage, there's still plenty of cost effective methods we can use to plug and purge our part and get excellent professional results that'll stand the test of time.
05:05 To summarise, shielding gas is used to ensure that the molten weld pool isn't exposed to the elements found in atmospheric air.
05:12 These elements can cause issues with weld quality and result in an ugly weld at best and a weak unsafe weld at worst.
05:20 There are different mixes of shielding gases available and what's going to work for you depends on the job you're doing and the material you're working with.
05:27 The amount of gas you need also depends on what and where you're welding as well as how your welder is set up.
05:34 Lastly purging is the key to creating a strong and uniform weld.
05:38 If you're fabricating anything from tubular material, this is a definite requirement in order to end up with a part that you can rely on.

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