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.
Purging of course refers to shielding the back of the weld with the same argon gas that runs
through our TIG torch.
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.
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.
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.
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.
With this in mind, minimising gaps is critical here.
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.
This needs to be done on the inside as well as the outside of each weld join too.
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.
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.
A common questions we hear is with regard to the need for purging during the tacking process.
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.
With the parts tacked, we need to seal the open ends of the tube ready for the supply of our
purging gas.
As discussed within the course, there's plenty of options here including aluminium foil,
masking tape, aluminium bungs or dedicated silicon purging plugs.
If the weld seam is more than about 25 mm away from the open end then a silicon plug is a viable
option.
Much closer and you risk damaging the silicon material from the heat generated by the weld.
If the weld is a long way from the end of the tube then masking tape will suffice as a cheap solution.
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.
We want to fill at the lowest point and vent from the highest point on the part that we're welding.
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.
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.
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.
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.
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.
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.
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.
Any longer is usually just going to waste your expensive argon.
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.
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.
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.
This can result in the last part of your weld blowing out, which can make a mess of an
otherwise tidy weld.
This becomes more of an issue with higher argon flow rates too.
The process of welding stays the same regardless of what's happening on the back
side of the weld.
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.
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.
And hopefully it should be easy to understand the effect this can have on the weld strength
and reliability in particular.
We can also see the dull and porous weld that makes up the back side of the part we
failed to purge.
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.
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.
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.
That was just one of over 50 modules taken from HPA's Practical Motorsport TIG Welding
course.
If you want to learn how to lay down those perfect stacked dime welds every time you
pick up the TIG torch, this course is perfect for you.
In this course, you'll learn how to get the most out of your TIG welder and correctly
adjust it to suit your specific welding task.
You'll also learn about correct part preparation and how to work with
different materials, everything from mild steel and stainless through to chromoly,
aluminium and even titanium.
Regardless whether you're completely new to TIG welding or maybe you've had
some experience and want to take your skills to the next level, this course is
perfect for you.
For more information and to purchase this course, click the link to enrol now.