Practical MIG Welding: Worked Example - Mild Steel Exhaust

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Worked Example - Mild Steel Exhaust

07.15

00:00	In this worked example we're going to walk through the process of MIG welding a section of 2.5 inch mild steel exhaust using the HPA 6 step MIG welding process.
00:09	This exhaust is for our Toyota FJ40 project and it'll replace a small section of the existing exhaust.
00:17	We've sourced a combination of parts including a 2.5 inch mild steel 2 bolt flange to connect to our existing exhaust, a few 90 degree bends, a section of straight 2.5 inch tube and a stainless 2.5 inch to 3.5 inch cone shape to adapt to our 3 inch stainless steel V band flange.
00:38	Due to real world limitations on what we could source locally, we've ended up with a mix of mild steel and stainless components.
00:46	This makes the job a little more interesting and it gives us a good chance to show how MIG welding handles dissimilar metals and the real world limitations that you're also likely to run into.
00:58	The first step in the process is part preparation.
01:02	As always, good prep is the foundation of a quality weld.
01:06	We've cleaned all of the raw material we'll be using down with acetone to remove the protective coat of oil left by the supplier so that we don't transfer this oil onto our hands, gloves, cutting tools or work surfaces as we move through the process.
01:22	Making sure to deburr and clean each cut of our 1.6mm thick tubes inside and out, we work our way along the chassis using our bandsaw, a ruler and an angle gauge to fit our exhaust into place.
01:36	Fabricating an exhaust like this is a matter of measuring, marking, cutting, test fitting and tacking repeatedly until we've worked our way along the chassis and connected the dots between our start and our end points so we'll be revisiting this part preparation step regularly throughout the process.
01:54	MIG welding isn't as sensitive to contamination as TIG but surface rust, dirt and oil can still introduce porosity so this is a step that you do not want to skip.
02:04	Every mating surface is treated to a thorough scotch brite followed by acetone.
02:09	In our case we used a selection of clamps and a purpose built jig to hold the parts in position.
02:15	If you're not working on a bench, a few magnets and a flat surface will usually get the job done.
02:20	Now, we can move onto welder setup.
02:22	For this job we're using a combination of our Lincoln Speedtech MIG 215 welder in synergic mode which automatically adjusts voltage based on wire diameter, shielding gas and material thickness.
02:34	And we're also using our Lincoln PowerMIG 180 with manual wire feed and voltage dials to demonstrate that the process works regardless of your MIG machine as long as you know how to dial in your settings.
02:47	These machines are both loaded with 0.9mm ER70S -6 wire.
02:53	While we are joining a few stainless pieces into this exhaust, using mild steel filler here will be acceptably strong for the job.
03:01	We'll just keep in mind that this section may corrode faster over time if left uncoated.
03:06	We're running Argo Shield Universal gas which is a standard 86% argon, 12% CO2 and 2% O2 mix.
03:16	Flow rate is set at 10 litres per minute which is suitable for our enclosed fabrication space but if you're welding in a drafty environment or outdoors, you may need to bump this up slightly.
03:26	The welder settings were dialed in using the built in synergic presets for 1.6mm mild steel and 0.9mm wire on some test welds before moving to the actual part and we found that the default wire speed and voltage recommendations gave good penetration without excessive spatter.
03:43	It's always worth doing a couple of practice beads to check that the settings suit your material and technique.
03:51	Before welding we need to address our personal setup.
03:54	As we touched on in the PPE section of the course, using a set of air plugs and an auto darkening welding helmet will save us from the painful side effects of the all too common safety squint method and increased welding comfort.
04:07	MIG welding produces UV radiation and weld spatter.
04:12	Even short tacks can launch molten metal so gloves and long sleeves are essential.
04:17	Additionally, making sure that your clothing and footwear is appropriate and won't let glowing red balls of molten steel drop down the back of your collar or into the top of your boots is a hard learned tip that I can personally recommend.
04:29	Now, we can move into the actual welding process.
04:33	We begin by tacking our V band to our cone and working our way along the chassis, measuring, cutting and tacking, being careful to ensure gaps won't open in our joints between tacks.
04:44	Once our part is assembled and loosely tacked together, we can add a few more tacks, spaced evenly around each joint.
04:51	This helps to control distortion and ensures that parts won't pull away as we complete our longer weld beads.
04:59	In our case we decided to fully weld this section of the exhaust on our bench for 2 reasons.
05:04	Firstly, this allows us to focus on getting the part oriented comfortably so that we can perform a nice controlled weld every time.
05:12	Secondly, we don't have to worry about protecting our chassis and other components from the weld heat and spatter produced during the process.
05:20	Each joint was welded in short stitches using a standard push technique with a 10-15 degree torch angle.
05:26	We moved around the joint in an alternating pattern, similar to tightening the nuts on a wheel.
05:32	This may not always be essential but it helps to reduce the chance of warping the tube by heating one side of the tube excessively while the other is still cool.
05:42	You'll notice that MIG doesn't give the same refined weld appearance as TIG but if you control stick out, travel speed and torch angle, you can still get solid consistent welds.
05:54	When welding our flanges at either end of the part, we made sure to bias the MIG torch angle towards the thicker material to ensure good penetration into both sides of the joint without blowing through the thinner tube walls.
06:07	Once all of the welds were complete, we inspected the inside and outside of the joints.
06:11	MIG welds typically show a slightly convex bead.
06:15	If your settings are right, the inside of the tube should show penetration without heavy build up or spatter.
06:21	If you're seeing burn through, undercut or irregular bead shape, revisit your wire feed speed and technique.
06:29	With welding complete, the exhaust was left to cool slowly on the bench.
06:33	Trying to quench or rapidly cool any section of the exhaust at this stage could lead to warping along the entire length of the tube or introduce cracks into our weld joints, leaving the entire part weak or unusable.
06:46	So, fight the temptation to rush here and let the metal cool naturally.
06:51	Finally, once cooled, the exhaust was reinstalled and clamped in place.
06:55	From a structural standpoint these welds are strong and with a high temp coating, they'll hold up well to the elements and the abuse of everyday driving for years to come.
07:04	If you've got any questions about this process or want to share your own results, let us know in the forums, we'd love to see what you're working on.