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This article was first published in 2007.
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Whenever you want to permanently join metals,
welding is usually the best choice from the perspectives of strength, durability
and appearance.
(Of course, that hasn’t always been so: just take
a look at big old steel bridges, many of them still in use. And the relevance?
Just try counting the rivets – welding wasn’t then being widely used!)
Cars have been welded since all-steel monocoque
bodies started dominating around World War II, while welding in one or more of
its different forms is used in nearly every metal product now made. A factory
producing metal goods that doesn’t use at least some form of welding or brazing
– whether that’s arc welding, MIG welding, TIG welding, gas welding, resistance
welding, spot welding, furnace brazing or one of the other types – is almost
unknown.
But what of the average person working on cars and
building things in his shed? (And ditto of course for ‘she’.) Robotic spot
welders might look great on a production line, but the cost and application of
these machines makes them irrelevant to most of us. So in this series we’re
going to concentrate on welders and welding techniques right at the cheap end of
the market – the sort you can justify buying for occasional use.
That ‘occasional use’ tag also needs further
discussion. With the proliferation of large industrial welders in workshops and
at welding specialists, if you need something welded only once or twice a year,
don’t even dream of doing it yourself. Simply take the parts to the specialist
and pay the nominal sum. By the time you take into account the skill level they
display and the cost of their equipment, you’d be mad to do it your self.
TIG’ing aluminium is a good example of this.
If on the other hand you have a really major
project that requires welding, for example building a tube frame car, we’d
suggest a different approach again. Major structural welding of this sort
requires both a high level of skill and good quality equipment. To my way of
thinking, that means a formal course in welding and several thousand dollars
worth of equipment.
So this series is unapologetically aimed at those
who can afford only a small amount of cash to get the welding equipment they
need for the jobs they do.
Arc Welding
Arc welding is called arc welding simply because
it uses an arc. (Unlike MIG and TIG, ‘arc’ is not an acronym.)
Buying an arc welder is the cheapest way to get a
welder. New arc welders are available from about AUD$150, while secondhand units
– often complete with gloves, a helmet and slag chipping hammer – can be found
from around fifty bucks. For general purpose welding – making workbench frames,
building shelving, making a heavy-duty underbonnet bracket or building a trailer
– nothing beats an arc welder for price.
However, this type of welder is best suited for
welding mild steel that’s in the range of 2-8mm in thickness. (Yes, thicker
steels can be welded, but normally it requires higher currents than are
available in the cheaper welders.)
An arc welder is basically just a big AC
transformer. Like any transformer, two windings are used. In Australia, mains
voltage of 240V at 10 or 15 amps is applied to one set of windings, while the
others generate a much lower voltage at a much higher current – say 120 or more
amps. (Inverter welders use different technology to achieve the lower
voltage/higher current.) The welding current is adjustable and is usually marked
in ‘amps’. In the simplest of welders there are no other controls but for an
on/off switch.
To operate an arc welder, one wire (the earth or
ground wire) must be attached to the material to be welded, or to a metal table
on which the material is placed. The other cable runs to the hand-piece, in
which the welding electrode is clamped. When the electrode touches the work,
electrical current flows from the welder through the rod and work-piece back to
the welding machine. That doesn’t sound very helpful, but if the current flow is
established and then the electrode quickly pulled back a little from the
work-piece, a very high temperature arc forms between the end of the rod and the
work-piece. This melts both the welding rod (which becomes filler material) and
the materials being welded.
The welding electrode (the stick or rod) has an
inner core of metal similar to the material that is being welded. This core also
has a diameter that is proportional to the material – as the work-piece gets
thicker, so too should the rod. The inner of the rod is surrounded by a welding
flux. When the molten material solidifies, the flux forms a sperate layer on top
that can later be knocked away with the chipping hammer.
Welding Rods
In addition to the differences in diameter
described above, welding rods vary in other characteristics. (Despite “general
purpose” welding rods being sold at every hardware store, in fact rods should
always be matched to the application.) The flux on welding rods serves these
functions:
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Provides a gas shield
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Gives a steady arc by providing a ‘current
bridge’
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Cleans the surface and slows the cooling of the
weld
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Introduces appropriate alloys into the
weld
Specific electrodes are available for welding:
Don’t underestimate the importance of using the
correct electrode. On a cast iron bracket I once had, even a welding rod
designed for cast iron gave a result so poor that a single hammer blow broke off
the (apparently sound) weld. On a high tensile steel tube, a tack made with
welding rod designed for mild steel had less strength than a blob of glue.
Welding Joints
Joint preparation is very important in gaining a
high quality, strong weld. (This applies for all welding and brazing techniques,
not just arc welding.) A joint that has an overly wide gap or is dirty, rusting
or greasy will not give a good result. That may be pretty obvious, but the way
in which the work piece is designed will also have an affect on the structural
integrity of the finished item. So what’s that about then? Let’s take a look at
the different types of commonly welded joints.
A butt joint occurs where two pieces of metal are
offered up to one another. They are on the same level and a small gap is left in
between. The weld fills the gap, penetrating through the thickness of the
sheet.
Where either access is more difficult and/or the
material is thicker, the edge should be ground away to allow better penetration.
This is called a single V butt joint.
Material that is thicker again, and which can be
accessed from both sides, can use a double V butt joint, where material is
bevelled away on both sides.
A lap joint, where the two pieces of material
overlap each other, can be a much stronger join than a simple butt joint. Not
only is the weld guaranteed full penetration on the exposed end of the material
but...
...holes or slots can also be cut to allow plug
welds (left) or slot welds (right) to be made. With appropriate penetration,
these welds can be very strong.
Another type of join is a T-joint. As with butt
joints, the material can be bevelled to allow better access and weld
penetration.
A fillet weld is used where surfaces join at 90
degrees. This type of weld can have another bead added at the arrow, the latter
being especially effective if the surfaces are bevelled to allow better
penetration.
Finally in common welds, there are edge welds.
In addition to showing the common joints, these
diagrams show how the types of welding needs to be considered when the design is
first being organised. For example, if the design is such that access can be
achieved only to one side of the work-piece, some joins may be unnecessarily
weak.
Welding
One of the hardest aspects that a beginner finds
in arc welding is striking the arc. The arc must be struck rather like a match
is lit. Scraping the end of the electrode across the work and then rapidly
lifting it slightly is how it’s done – but it’s often not as easy to do as it is
to write! Practicing striking the arc on a piece of scrap, and adjusting the
current and seeing the affect this has on arc striking, are the best two
approaches. The arc should be the shortest that will produce a good weld (eg
1.5mm), so be careful not to lift it too far after the arc has started.
The textbooks define the rate of forward process
and the angle that the rod should be held to the work, but much of the fine
detail of this depends on the material being welded, the current and rod being
used, and the skill level of the operator. One trick is to learn to look beyond
the arc to the pool of molten metal – that tells you a lot more about what is
going on than the appearance of the arc itself.
It’s also important to know what good and bad
welds look like, and the reasons why bad ones are like they are. This diagram
provides a good start in recognising errors – note that both plan and
cross-sectional views are shown. Cutting a practice piece in half and assessing
the penetration of the bead is worthwhile.
Another way to see what is happening is to
deliberately weld in error. What does a weld look like when the current is way
too high, or far too low? What is the bead’s appearance when welding
excruciatingly slowly, or far too rapidly? If you deliberately create the error,
you’ll be able to recognise what’s going on when the bad weld appears, even when
you’re trying hard!
Running multiple beads, one over the top of the
other, and weaving from side to side are often suggested in welding textbooks.
If you’re a competent welder (and in the case of the multiple beads, make
scrupulously sure that the slag is removed after running each bead), no problem
exists in doing these things. But if you’re a beginner, multiple beads and
weaving is a recipe for slag inclusions and voids.
Practice!
An
excellent practice at holding the handpiece steady and at a constant distance
from the work-piece can be achieved by using a sharp pencil, a washer and a
piece of paper. Practice pushing the washer along without letting the washer
slip out from under the pencil or the pencil touch the paper. When doing the
exercise, position yourself in your normal welding position, eg sitting at a
bench.
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In Use
Like all forms of welding, perfection comes with
practice – a lot of practice. The guy who welded a supercharger mount for me,
and who also welded my turbo manifold together, used an inverter arc welder – so
it was a little more sophisticated unit than the cheapy you can pick up for $50.
However, the quality of the resulting welds on the
supercharger bracket (10mm thick steel plate)...
... and the turbo exhaust manifold (3.6mm wall
thickness) were a lot more related to his day-in day-out experience with his
welding machine and having welded literally kilometres of steel plate!
Initially, I was taken aback that he didn’t get out a MIG; when I saw the
results, I knew it wasn’t needed. (However, and it’s a significant point, his
gas brazing of copper tube was pretty woeful – he simply hadn’t had the
practice.)
Using my $50 secondhand welder I’ve built this
very strong and sturdy workbench and, perhaps more unusually....
...I also tacked together all the exhaust tube used
to form the intercooler plumbing of this turbo conversion. I wouldn’t have
wanted to fully arc weld the thin tube, but tacking it together and then taking
it elsewhere to be MIG welded was infinitely easier than trying to hold
everything in the right positions with adhesive tape or a jig.
Conclusion
Look at some fancy inverter arc welders and you
could be forgiven for thinking that you need to spend a lot of money to get a
welder. But a secondhand older unit, or the simplest of today’s arc welders,
will allow you to make good welded joints in steel ranging in thickness from
about 2-8mm. The main keys to success are configuring the joint to allow for the
strongest weld, selecting the right rod – and practicing!
Safety
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Always wear an appropriate welding visor/helmet,
clothing that covers arms and legs, and gloves
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Wear goggles when chipping slag or wire-brushing
welds
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Keep all electrical cables in good condition
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Make sure contact cannot be made with hot
items
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Keep inflammable items well away
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Ensure there is plenty of
ventilation
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