How to weld, Part 4
PART FOUR IN A SERIES ON HOW TO WELD
How to weld aluminium
Aluminium alloys are used in so many different applications in our modern world, from motorbike frames and boats, big and small, to super yachts. The list is endless. So when it comes to building, manufacturing, and repairs, welding is often the best solution. Most aluminium alloys are weldable, but it is important to understand the special aspects and quirks of working and welding aluminium and to figure out the best welding technique.
Clean it
Easily the most overlooked problem in welding aluminium is not having a clean welding preparation. So first up, cleanliness is of the utmost importance. When faced with repairs in the shed or workshop, you will find that aluminium that is old, dirty, or with an oxidized surface will need to be cleaned. Clean it up so you get rid of any paint or oxide film on the surface. Sometimes with cast aluminium, oil and grease can embed themselves into the parent metal. Use white spirit to clean the oil off before you use an abrasive. Aluminium oxide is that
dull, matte, silvery-grey coating on the surface of all aluminium caused by contact with oxygen. This oxide melts at more than three times the temperature of the aluminium, so you can’t break through the skin if it’s oxidized. To remove all oxide, you need to clean it off, either by grinding or hand-sanding with an abrasive such as emery cloth.
TIG welding
TIG welding uses a non-consumable tungsten electrode surrounded by a ceramic shield that controls the flow of the protecting argon gas. Argon gas shields the molten metal from the oxygen and nitrogen in the atmosphere that would otherwise oxidize the metal and rod. The argon also acts as a carrier for the current which is the heat source. Gas flow is around eight to 10 litres per minute. The electrode tip in the TIG torch for steel or stainless steel is very pointed and made of thoriated tungsten. For aluminium welding, the TIG torch tungsten electrode should be blunt, with the corners ground off, and made of zirconiated tungsten. The tungsten electrode has a white tip for aluminium and a red tip for steel.
TIG advantages
• Weld deposits are generally smooth, with no splatter or slag coverage.
• The weld pool can be controlled and manipulated easily.
• The filler rod can be controlled separately.
• Very low fume generation.
• Improved fatigue performance.
• Extremely strong fusion welds.
• Very low hydrogen deposits.
Aluminium TIG
Aluminium is welded with AC power with a high-frequency signal. Aluminium TIG welding needs more power. Stainless steel needs 60–120A
The cleanliness of the aluminium surface is important; the less oxide and dirt on the surface, the less power needed
but aluminium can start at 150A for similar thicknesses, depending on the size of the machine.
This can become a problem for single-phase TIG welding machines. But for thinner sections, say up to 6mm thick, the smaller 200A machines are ideal. Technology has come a long way where single-phase AC/DC TIG machines are concerned. Preheating the job can help if there is insufficient power. Or the Lincoln Powercraft 200 AC/DC has a Mix mode that alternates between AC and DC, creating a slightly hotter weld pool.
The current setting depends on the thickness of aluminium and size of the tungsten electrode being used. If it’s heavy aluminium, you may need a bigger torch and tungsten. As previously mentioned, the cleanliness of the aluminium surface is important; the less oxide and dirt on the surface, the less power is needed to burn through to create a weld pool. More heat is required because of aluminium’s high conductivity.
Pay attention to using the correct TIG rod thickness:
• for aluminium thicknesses from 1.0 to 4.4mm: use a 1.6mm rod
• for aluminium thicknesses from 3 to 8mm: use a 2.4mm rod.
Weld pool
When you strike an arc with TIG, do not expect to see a molten pool immediately. Be patient and let the weld pool form, because at the start everything is very cold and it’s necessary to warm the work. Once the weld pool has formed you can start adding your filler rod. Adding the filler rod at just the right amount will cool and stabilize the weld you are creating. If the pool feels too cold, stop feeding. Otherwise continue dabbing the rod in and out.
It is crucial not to touch the electrode to the work, otherwise the electrode ends up with molten metal stuck to it. The welding has to stop while you
The less oxide and dirt on the surface, the less power is needed to burn through to create a weld pool
reshape the tip or weld on a piece of scrap until it is clean again.
Technique
When you are TIG welding, you need to hold the torch in one hand a little bit like a pencil, with the second finger over the top of the torch. The filler rod is held in the other hand and fed in as for gas welding. Adding filler rod takes place in front of the TIG torch as you move forward. The torch and the filler rod should be in a 90-degree configuration to each other. The torch is held about 60–70 degrees from the work and the rod held at about 20–30 degrees.
Always try to push the TIG torch, not drag it. Always add the filler metal on the leading or front edge of the weld pool. Don’t be afraid to add too much; it is easy to cut back if necessary.
When starting out, most learners have issues getting their hands working independently. While the torch does move slowly, most of the action is with the TIG rod and the weld pool. Beginners usually end up moving both hands at the same time, and the tungsten dips into the weld pool too, which usually results in touching the filler metal to the tungsten. We have all done it.
So with the power off, hold the TIG torch still and concentrate on sliding the TIG rod underneath the tungsten without touching it until each hand performs its task independently. When you’ve mastered these movements, you’re ready to strike an arc.
All welding should be done in a comfortable position, so if possible rest the hand with the torch on something firm. Calculate where you will finish the weld and position your start to reach that position.
When finishing an aluminium weld, take you finger off the button, but don’t pull the torch away. The protective argon post-flow keeps on going after you have stopped, so leave the torch there for five seconds to allow the weld to cool without it oxidizing immediately. This also prevents crater cracks.
Points to note
Make definite movements with the rod in and out, and don’t leave the rod too close in. If the rod is close, the metal gets very hot and can vaporize as you bring the rod in again, especially if the rod is too small. Too far away and the molten end of the rod will oxidize.
It’s worth experimenting with the controls — you will learn a lot about your machine this way
The best spot is 5–6mm outside the pool but still under the argon shield.
You should try to obtain a series of overlapping circles, like cent pieces. If you are not using enough power, you get an ugly lumpy build-up on the plate.
Keep the torch at an angle of about 60–70 degrees to the work.
If the weld is too thin, chances are you are not adding enough metal to the weld pool from the rod, or the TIG rod is too thin. Don’t be afraid to put too much weld down when starting out.
Beginners usually end up moving both hands at the same time, and the tungsten dips into the weld pool too
Setting
Most TIG machines have a balance control, often confusing for a newbie. Where do you set it? What does it do? This gives you either penetration or cleaning action.
‘AC’ is alternating current, which alternates at 50 cycles per second (50Hz). The electrode positive side of the AC cycle is where the current flows from the work to the tungsten electrode. This half of the cycle burns off surface oxides and allows the electrode negative side of the cycle — where the arc current flows from the tungsten back to the work — to melt the base metal and fuse the aluminium, giving you penetration.
The balance control can give you more positive or negative control, meaning penetration can be preferred over cleaning.
Balance-control benefits
Effects of increasing the electrode negative:
• greater penetration can be achieved on thicker sections
• travel speed can increase
• the weld bead can be narrowed
• tungsten electrode life can be increased and balling action reduced
• a smaller diameter tungsten can be used for more precise work.
Effects of increasing the electrode positive:
• greater cleaning action can be achieved to remove heavy oxides (this negative side is what breaks up aluminium oxides that DC cannot burn through)
• minimal penetration, which helps prevent burn-through on thinner aluminium alloys
• bead profile can be widened, excellent for scalloped welds on hot rods and cold-air intakes
• tungsten electrode life can be decreased and balling action increased.
The rules about setting balance control are dependent on what you need in an aluminium TIG weld. A typical error involves too much cleaning action (electrode positive), which creates heat build-up in the tungsten. A large ball will form on the end of the tungsten.
The arc can lose stability and this makes it difficult to control the direction of the arc and the weld pool.
Too much penetration creates insufficient cleaning action, resulting in a dirty weld pool. Often your weld will have black, burnt particles set into it. These are burnt oxides and other impurities.
All machines are different, so it’s worth experimenting with the controls — you will learn a lot about your machine this way.
Welding aluminium with small MIG welders
There are thousands of small MIG welding machines in sheds everywhere
Stable, clean
and numerous brands available. To give instructions about MIG welding aluminium for every machine type, model, and size would become a marathon. But with the right wire, gas, and power settings there is no reason why anyone can’t make a successful job of welding an aluminium project.
Many of these small machines are more than capable of putting down a nice-looking weld. But there is a set of basic rules that are relevant to every MIG owner wanting to MIG weld aluminium.
Basics
Let’s start by listing some of the basics. To avoid irregular feeding and burn back, ensure you are using:
1. a U-groove roller: this allows pressure to be put on the wire without squashing it out of shape and reduces the likelihood for tangled wire or ‘bird nesting’ in the rollers
2. a clean liner: when was the last time you checked your liner? A clogged liner can cause so many headaches
3. the correct MIG tip: one size bigger is the rule for aluminium; e.g., if using 0.9mm wire, use a 1.0mm tip. This way you are less likely to get burnbacks into the tip
4. a straight torch: when welding, keep your MIG torch as straight as possible, so there are no kinks in the wire. The straighter the feed, the less drama when welding
5. the correct MIG wire: magnesiumbased aluminiums such as extrusions and plate are normally magnesiumbased marine-grade 5356. Most aluminium boats are welded with 5356 wire. Other aluminium material such as castings are siliconbased grade 4043. This is very soft and is often too thick to weld with these small machines anyway. The 4043 wire can be hard to feed
6. a suitable wire size: a wire of 0.9mm is the favoured size as it is easier to push through the liner than 0.8mm and the amount of extra power needed is negligible
7. an appropriate feed-roll pressure: set the feed rolls as light as possible. Too much pressure will spiral the wire and cause problems.
For a stable arc and clean welds, use:
1. argon gas: no other generally used gas mixture will do (apart from expensive helium mixtures). Not argon mixes, not carbon dioxide (CO2). Straight argon
2. the right gas flow: the correct gas flow should be 18–20 litres per minute. Having the gas flow too low is a common problem and this will create black, smoky-looking welds and porosity
3. negative earth: it is surprising how many welders get this wrong.
4. good earth: always earth onto the material you are welding
5. clean material: don’t try to weld over-oxidized or coated material. Giving the parent material a quick brush with a stainless wire brush can save you so many problems later
6. the appropriate torch angle. The torch angle should be forward approximately 20–30 degrees. This will give better gas coverage and also keep the heat moving forward.
If you are not using enough power, you get an ugly lumpy build-up on the plate
Thickness v. power
Having to weld aluminium that is under 2mm thick can be a big ask for the inexperienced operator. The best advice I can give is:
• tune your machine to weld a little bit hotter than you normally would
• weld a bit faster than, say, if you were welding mild steel
• set up and practise on scrap of similar thickness before starting on the good stuff.
Welding aluminium more than 2mm thick shouldn’t be too difficult as long as you don’t expect to weld gigantic lumps of aluminium. Aluminium between 4mm and 6mm thick is about the maximum you can expect to weld successfully with your small MIG machine, again depending on the size and brand.
Common problems
1. Burn through. Moving too slowly? Speed up. Don’t be afraid to move a bit faster. Your welding current may be too high and you need to find a happy medium. Again, use trial and error on scrap pieces.
2. Lack of fusion, commonly known as ‘seagull poo’. Those round blobs usually mean that the voltage setting is too low for the amount of wire set on the machine. Set the amps and volts up higher and work your way down. You may be surprised how hot you need to go before a weld with good edgewetting is produced.
3. Black or dirty welds. These can be caused by several different factors:
• inadequate gas coverage
• dirty parent material, with heavy aluminium oxides not cleaned off
• the wrong torch angle
• the wrong gas
• leaks in the MIG torch and hose
• holding too long an arc
• a loose nozzle
• an unstable arc from a poor earth connection
• a worn contact tip that needs changing.
Amps and volts
Ask 10 experts what the secret is for a beginner to set the amps and volts on a small MIG welding machine and you could get 10 different answers. I believe the best advice I can give is to conduct a practical test on a piece of scrap aluminium 3–4mm thick.
Set your wire speed on max and voltage on max. If the wire in the arc is stubbing and spattering, turn the wire speed down. If the arc has a hissing sound and the wire wants to burn back into the tip, turn the voltage/power setting back. You will hear when the arc is playing the right tune. From this point, both settings can be adjusted down to the required setting. You will also have a better idea of what your machine is capable of. Don’t forget to write down your settings as you go. Many machines come with settings in the instruction book, so as a last resort read the instructions. The inside of the wire-feed units often has good info, depending on the brand.
Aluminium v. mild steel
When aluminium welding with a MIG welder compared with using one for mild steel, there are some differences that should be noted:
• torch angle: use plenty of forward angle. I use about a 40- to 45-degree forward angle — too straight an angle will give a sooty weld
• speed up: keep moving about a quarter faster than for steel welding. It looks better and keeps the heat out
• stick out: use stick out of 15–20mm. The tip should be back into the nozzle by 4–6mm. If the tip is flush with the nozzle, this can give you a sooty finish.
Summary
If all else fails, try your local welding shop. Most welding agents and welding supply chains have people who can help, especially when it comes to things like tips, nozzles, and liners. Keep your torch in good clean condition. Change the parts before the problems start. Don’t be afraid to experiment with power settings and material thickness. Once you get to know your machine, welding aluminium will be a breeze.