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 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.
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:
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.
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.
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.
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.
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!