Practical TIG Welding: Purging
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Purging
06.18
| 00:00 | - At this point in the course, we've already spent a fair amount of time talking about the importance of proper shielding gas coverage if you want to produce a quality weld. |
| 00:08 | We've also mentioned that it's not just the top of the weld that needs to be shielded but also the reverse side. |
| 00:14 | At least when you're dealing with more reactive materials like stainless steel and titanium. |
| 00:19 | In this module, we're going to have a detailed look at the actual process of purge welding and see how you can correctly apply it. |
| 00:26 | Purging of course refers to shielding the back of the weld with the same argon gas that runs through our TIG torch. |
| 00:32 | By introducing this to the back of the weld, usually by flooding the internal volume of the weld area with the gas, we can ensure a quality weld with no porosity or inclusions, thus ensuring maximum strength and long term reliability. |
| 00:46 | While we normally purge to ensure strength and reduce the likelihood of fatigue related failures, due to the smoother internal weld finish compared to welding without purging, the internal surface also offers less resistance to air or fluid flow as well. |
| 01:00 | For this demonstration, we'll be welding up a section of 2.5 inch stainless tube and we'll purge one half and leave the other half unpurged so the differences become obvious. |
| 01:11 | Fit up is always important but when we're purging, this takes on another level of importance because poor fit up results in more potential gaps where our argon gas can easily flow and escape. |
| 01:22 | With this in mind, minimising gaps is critical here. |
| 01:25 | We also want to ensure we take the normal material preparation precautions, including deburring the cut ends of the tube then cleaning the surfaces with scotch brite before a final wipe with acetone and a clean rag. |
| 01:38 | This needs to be done on the inside as well as the outside of each weld join too. |
| 01:42 | Next we need to tack our parts together prior to fully welding them and as discussed in the tacking section, we're using sections of Kapton tape to help us support the exhaust tube prior to tack welding. |
| 01:54 | The advantage of the Kapton tape is that we can leave sections in place while we're welding elsewhere and this will help seal the unwelded joint and prevent loss of our purging gas through the join. |
| 02:05 | A common questions we hear is with regard to the need for purging during the tacking process. |
| 02:10 | In general, provided the tacks are kept small and the penetration is minimal, purging isn't necessary for this process and won't adversely affect the completed weld. |
| 02:20 | With the parts tacked, we need to seal the open ends of the tube ready for the supply of our purging gas. |
| 02:27 | As discussed within the course, there's plenty of options here including aluminium foil, masking tape, aluminium bungs or dedicated silicon purging plugs. |
| 02:36 | If the weld seam is more than about 25 mm away from the open end then a silicon plug is a viable option. |
| 02:44 | Much closer and you risk damaging the silicon material from the heat generated by the weld. |
| 02:49 | If the weld is a long way from the end of the tube then masking tape will suffice as a cheap solution. |
| 02:54 | We need to remember that the argon is heavier than air and this needs to be factored into the location of the filler and vent. |
| 03:01 | We want to fill at the lowest point and vent from the highest point on the part that we're welding. |
| 03:06 | For our demonstration we're using a separate argon bottle that we dedicate to purging, however you can also purchase a second regulator for your main bottle. |
| 03:15 | The best option for you will depend on the amount of purging you're doing but this can use quite a large volume of argon and can therefore empty your main bottle quite quickly. |
| 03:25 | As a guide, the gas flow for the purging should be set approximately the same as the gas flow you're using for the torch however if you're welding a part with lots of potential areas for the argon to leak out of, then you may need to raise the gas flow considerably higher. |
| 03:40 | For multiple unwelded pie cuts for example, a gas flow of 25 litres per minute may be necessary to initially flood the part and remove atmospheric air before reducing the flow somewhat for the actual welding process. |
| 03:53 | We do need to allow time for the argon to flood the part internally and the larger the volume of the part, or the more joins there are, the longer this process is going to take. |
| 04:03 | If you want to get technical, then you can work out the internal volume and then allow the argon to flow for around 20 seconds per litre of capacity prior to beginning welding. |
| 04:13 | The reality is that very few welders will go to this trouble and usually allowing between about 20 seconds of gas flow for smaller parts through to around 60 seconds of gas flow for larger parts will be sufficient to displace atmospheric gases and allow you to start welding. |
| 04:29 | Any longer is usually just going to waste your expensive argon. |
| 04:33 | Some common sense does need to be applied and if your weld is at the very top of the component you're purging then understandably it's going to take a little longer to purge the internal volume to this point compared to welding lower down on the same part. |
| 04:46 | Ultimately you'll start to get a feel for what's required once you've completed a few purge welds and can inspect the result on the inside of the component. |
| 04:56 | It's important to make sure that you have adequate venting of the argon from the workpiece so that pressure can't build up within the part. |
| 05:03 | This can result in the last part of your weld blowing out, which can make a mess of an otherwise tidy weld. |
| 05:09 | This becomes more of an issue with higher argon flow rates too. |
| 05:12 | The process of welding stays the same regardless of what's happening on the back side of the weld. |
| 05:19 | Although a little additional air with the purging gas inlet and outlet, may be required as the part is moved or relocated to allow it to be fully welded. |
| 05:28 | With our two samples welded and then opened up, you can clearly see the improvements to the quality of the internal weld when purging was used. |
| 05:36 | And hopefully it should be easy to understand the effect this can have on the weld strength and reliability in particular. |
| 05:43 | We can also see the dull and porous weld that makes up the back side of the part we failed to purge. |
| 05:48 | There's a good chance that this will present some strength and cracking issues over time when we introduce vibration and thermal expansion and contraction. |
| 05:56 | If you put a lot of time and energy into making a part then why not finish it off to the best of your ability by applying the purging process. |
| 06:05 | For the price of a small amount of gas, and a little additional time, we can complete a weld that will last the test of time and lift your fabrication level up a notch. |
