Tips & Tricks
Welder Buyer’s Guide
Welding is defined as “a joint where pieces or parts have been fused together”, usually by heat. Most commonly welded materials are metals, but the process can include many plastics too. The heat is generated by an electrical spark or a gaseous flame.
With minimal training, pretty much anyone can join bits of metal together, but really specialised skills are learned – and practised – over years.
There are various ways of creating welded joints – and when you’re making metal hot enough to melt together, you need some specialist equipment. Yes, we’ve seen the bush mechanics who can weld using a car battery and some fence wire, but they’re seldom permanent repairs!
There’s no single machine that can really do it all, but depending upon the materials being joined together, the most commonly used equipment will fall into one of three main types:
Arc welders join two metals by generating an electric arc (spark) between a covered sacrificial metal electrode and the metals to be joined using high voltage. Think of it as handheld lightning and you’ll have an idea of the process.
The heat produced by the arc between the positive and negative electrical charge melts the parent metal, which mixes with the molten deposits of the coated electrode – or the wire and gas shield.
The most common arc welder is the good old ‘stick’ welder: the electrodes – the sticks – are about 250mm long and look like stout bits of wire that were dipped in cement.
More correctly called ‘shielded metal arc’ welding, stick welding works better than most other methods on dirty or rusty materials and in less-than-perfect conditions.
Depending on the metals to be joined, there are many types and diameters of welding electrodes, but with age the flux (the ‘cement’) can absorb moisture or crumble off, making fresh rods desirable.
Welding rods are kept in ‘hot boxes’ on some big jobsites to prevent moisture in the flux.
For big welding jobs, stick welding is comparatively slow, since the consumable electrodes must be replaced every couple of minutes and because slag (the residue from the flux) must be chipped away after the weld is complete.
Arc welding current
Stick welders can be of AC (alternating current) or DC (direct current) types.
DC welding offers advantages such as easier starts, fewer arc outages, less sticking, less spatter, better-looking welds, and easier vertical up and overhead welding. Plus, with DC it’s easier to learn how to weld, and you get a smoother arc.
DC reverse polarity (electrode positive) provides about 10 per cent more penetration at any given amperage than AC, while DC straight polarity (electrode negative) welds thinner metals better.
AC has advantages if welding on material that has become magnetised from friction, such as when hay, feed, grain or water constantly rubs against a steel part.
In that case, a DC output doesn’t work well because of
‘arc blow’, where the magnetic field blows the molten filler metal out of the weld puddle. The AC succeeds because AC output alternates between polarities, thus enabling welding of magnetised parts.
A machine rated at between 225 to 300 amps will be capable of handling most jobs that a decent mechanic will need to weld: beyond that, you might be in for a trip to a specialist.
In fact, most stick-welding procedures require less than 200 amps. To weld material thicker than about 10mm, simply make multiple passes – this is what the pros do, even when welding on 1-inch (25mm) structural steel.
When buying a machine, the welder’s ‘duty cycle’ is important. This refers to the number of minutes out of a 10-minute cycle a welder can operate.
For example, Miller’s Thunderbolt XL creates a 200-amp DC output at 20 per cent duty cycle. It can weld continuously at 200 amps for two minutes, and then must cool for eight minutes to avoid overheating.
Duty cycle and amperage are inversely proportional. Operating at only 90 amps, the Thunderbolt has a 100 per cent duty cycle, meaning you can weld without stopping.
Exceeding the rated duty cycle can damage the machine or cause it to overload and trip out.
Arc welding rods
The details for welding rods are printed on the end of the stick if you no longer have the packet. These designations were standardised in the United States by ASTM and WIA, with the first two digits representing the metal’s tensile strength. For example, 60 series rods have a maximum tensile of 60,000psi (415MPa) and the 70 series 70,000psi (485MPa).
Common electrodes used for general work include 6010, 6011, 6013, 7018 and 7024, each of which has specific properties: 6010 electrodes penetrate deeply, while 6013s penetrate less.
For much better bead appearance and work on higher strength steels – say for an implement hitch – use a 7018 rod. For better penetration on thick material, grind open the joint to a 30-degree bevel (leave a 2mm width vertical land on the bottom of the groove) and make multiple passes.
Alternatively, make the first pass with a 6010 rod, then make a ‘cap’ with a 7018.
The 7024 rod is perhaps the easiest to use.
This is also known as a ‘drag rod’, meaning the electrode’s thick flux automatically maintains the correct arc length, which allows you to drag the rod directly along the work piece. That’s pretty handy for better-looking welds.
Hard-facing rods can provide impact resistant or abrasion-resistant welds – or both, depending on the application.
Because the type of rod required depends on the type of environment the tool will exposed to, it’s probably better to ask your local welding supplier for advice.
Inverter welders have some distinct advantages over more traditional welders.
Both require a transformer to convert incoming current to suitable welding current, but an inverter welder does this far more efficiently and is much smaller and lighter — and uses less power too.
Because they’re more efficient, their duty cycle is also higher. Using modern solid-state electronics, Inverter welders lose very little heat in comparison to traditional machines, giving the smaller inverter machine the ability to use nearly all of its input current – where an older-style transformer could lose up to 20 per cent efficiency to heat loss.
The electronics make it possible to maintain an arc where a conventional transformer welder would allow the electrode stick to the work.
Big 5mm diameter rods can be run on an inverter machine and it’s practically impossible to make them stick to the job.
The higher frequency of the output current and computer software to monitor and adjust current and voltage while welding produces a consistent, smooth arc that’s easier to strike and to maintain.
When running on domestic single-phase power, most inverter machines do need a 15-amp power outlet.
Welding supplies such as electrodes, welding wire and shielding gas typically last longer than when using a traditional welding power supply.
Adjustments to current and voltage can be made for different materials and thicknesses, giving the operator tighter control over the welding process.
The small size and weight of these welders make them popular – where a traditional transformer welder is too bulky or uses too much power – so they are popular in maintenance facilities, general fabrication shops, construction sites and farms as portable, lightweight units for on-site repairs.
Tungsten inert gas (TIG) welding, sometimes known as gas tungsten arc welding (GTAW), requires more skill than most other welding methods but is extremely versatile, capable of joining a huge range of metals in varying thicknesses.
The result is neat and precise, giving a beautifully finished weld bead where appearance is important.
In the TIG process, the arc jumps between a non-consumable tungsten electrode and the work piece, generating high temperature. The inert gas shield, usually including argon or another noble gas, prevents weld oxidation.
There is no spatter or slag, and thin metals are readily welded, and with less distortion.
The most obvious difference between a TIG unit and an AC/DC stick welder is the non-consumable tungsten electrode which is held in the handle.
The shielding gas is directed around the weld by a ceramic nozzle or cup designed to withstand the heat of welding.
Different sized cups are used: smaller cups for smaller welds and reaching difficult areas; larger cups for improved gas flow and coverage for bigger weld beads.
While there’s no slag to chip off and clean up, TIG requires a clean prepared surface for quality welds. Any type of foreign matter, including moisture or cleaning solvents, will contaminate the weld.
Apart from the higher skill required with TIG, there are some other considerations, such as cost and speed.
Although there are cheaper units available, a good TIG welder can cost several thousand dollars; in contrast, an inverter TIG can be bought for around $400.
That’s still twice more than a beginner needs to spend on a modest stick welder, but its compactness and the ease of striking and running a smooth bead are impressive once you’re used to having both hands engaged.
Finally, TIG welding is a bit slower than other arc welding methods, so the time spent TIG welding is relatively high.
Where time equates money, another process might be more cost effective.
MIG stands for metal inert gas, and this process is also known as gas (shielded) metal arc welding (GMAW).
This process has a continuously fed wire electrode that melts in the arc to form a weld bead. Like TIG, the weld is protected from surrounding air by a shielding mixture of gasses, usually including argon.
Alternatively, tubular flux-cored ‘self-shielding’ wires are available, which need no external gas. Ventilation is needed because of fumes, and the slag formed on the weld has to be removed between passes or before painting.
MIG welders with solid wire are more commonly used with industrial robots and on heavy, thicker work because the continuously fed electrode wire and arc length are automatically controlled.
MIG weld joints are stronger, more ductile, have less distortion than other welding processes, and are ideal for thin metal work, alloys and aluminium.
However, welding wire isn’t cheap and there’s a bit more to consider with MIG, like welding meter amps and volts, wire speed and position.
There’s a gasless wire option with MIG that’s a lot cheaper than bottle rental but it isn’t as nice as running a MIG on gas, and the power supply must be considered – ideally a consistent current with very little drop in voltage is needed.
Duty cycle is also a consideration: the machine will need to be watched to avoid overheating. For example, a 200-amp welder with 30 per cent duty cycle should be operated for no more than three minutes in 10, allowing seven minutes for cooling.
More often than not, gas welding uses a combination of (usually) oxygen and acetylene as the heat source, which is why it’s also called oxy welding or oxyacetylene welding.
As for technique, if you’re learning to weld using the stick process, remembering these five points will improve your technique. Think CLAMS: Current, Length of arc, Angle, Manipulation and Speed.
1 & 2. Arc welding 3. Gas welding
4. Inverter welding