Basics of Welding Aluminium: Part 4

Fourth in a series of how to weld
By Greg Holster

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 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 working out the best welding technique is important.

Clean
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 aluminium that is old, dirty, or with an oxidised surface will need to be cleaned. Clean it up so you get rid of any paint or oxide lm on the surface. Sometimes with cast aluminium, oil and grease can embed itself into the parent metal. Use white spirits to clean the oil off before you use an abrasive. Aluminium oxide is that dull, matt silvery, grey coating on the surface of all aluminium. This is caused by the 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 oxidised. Clean the material to remove all oxide 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 which controls the ow of the protecting argon gas. Argon gas shields the molten metal from the oxygen and nitrogen in the atmosphere which would otherwise oxidise the metal and rod. The argon also acts as a carrier for the current which is the heat source. Gas ow is around 8-10 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, the corners ground off and is 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 to 120 amps but aluminium can start at 150 amps 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 6 mm thick, the smaller 200 amp machines are ideal. Technology has come a long way where single phase AC/DC TIG machines are concerned. Pre-heating the job can help if there is insufficient power.

Or in the case of the Lincoln Powercraft 200 AC/DC which 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. The cleanliness of the aluminium surface is important; the less oxide and dirt on the surface, the less power 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 thickness from 1 to 4-5mm: use a 1.6mm rod;

  • For aluminium thickness from 3-8mm: 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, 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 stabilise 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 reshape the tip or weld on a piece of scrap until it is clean again.

Technique

When you are TIG welding, it is necessary 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° from the work and the rod held at about 20°-30°. 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 easier to cut back if necessary. Most learners have issues when starting out, 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- ow keeps on going after you have stopped, so leave the torch there for five seconds to allow the weld to cool without it oxidising 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 vaporise 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 oxidise. Best spot is 5-6 mm 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.

Setting
Most TIG machines have a balance control, often confusing for a newbie. Where do I 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. This then 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.

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Balance control benefits

Effects of increasing the electrode negative:
-
Greater penetration can be achieved, on thicker sections
- Can increase travel speeds.
- Ability to narrow the weld bead
- Can increase tungsten electrode life and reduces balling action
- The ability to use a smaller diameter tungsten to give more precise work

Effects of increasing the electrode negative:
-
Greater cleaning action to remove heavy oxides. This negative side is what breaks up the aluminium oxides that DC cannot burn through.
- Minimal penetration, which helps prevent burn through on thinner aluminium alloys.
- Can widen the bead profile, excellent for scalloped welds on hot rods and cold air intakes.
- Can decrease tungsten electrode life and increases balling action.

The rules about setting balance control are dependent on what you need in an aluminium TIG weld. A typical error, involves too much cleaning. Too much cleaning action (electrode positive) creates heat build-up in the tungsten. A large ball on the end of the tungsten will form. The arc can lose stability. 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 its worth experimenting with the controls you will learn a lot about your machine this way.

Welding Aluminium with small MIG welders
There are literary thousands of small MIG welding machines in sheds everywhere and numerous brands available. To give instruction about MIG welding aluminium on 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 a aluminium project.

Many of these small machines are more than capable of putting down a nice- looking weld. But there are 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. “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. Clean liner. When was the last time you checked your liner? A clogged liner can cause so many headaches.
3. 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. 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. Correct MIG wire. Magnesium-based aluminiums such as extrusions and plate are normally magnesium-based marine grade 5356. Most aluminium boats are welded with 5356 wire. Other aluminium material such as castings are silicon- based 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. Wire size. A wire of 0.9 mm 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. Feed roll pressure. Set the feed rolls as light as possible. Too much pressure will spiral the wire and cause problems.

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Stable, clean
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. Correct gas ow. The correct gas ow should be 18-20 litres per minute. Having the gas- ow too low is a common problem and this will create black, smokey-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 the material. Don’t try and weld over oxidised or coated material. Giving the parent material a quick brush with a stainless wire brush can save you so many problems later.
6. Torch angle. The torch angle should be forward approximately 20-30 degrees. This will give better gas coverage and also keeps the heat moving forward.

Thickness vs. power
Having to weld aluminium that is under 2 mm 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, and
• 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 2 mm thick shouldn’t be too dif cult as long as you don’t expect to weld gigantic lumps of aluminium. Aluminium between 4 mm to 6 mm 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 know as seagull poo. Those round blobs usually mean 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 edge-wetting 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; wrong torch angle;
    • wrong gas;
    • leaks in the MIG torch and hose; holding too long an arc;
    • a loose nozzle;
    • unstable arc from a poor earth connection;
    • worn contact tip which needs changing.

Amps and volts
Ask ten experts what the secret is for a beginner to set the amps and volts on a small MIG welding machine and you could get ten different answers. I believe the best advice I can give is to conduct a practical test on a piece of scrap aluminium three or four millimetres 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 on 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 have good info depending on the brand.

Aluminium vs mild steel
With aluminium welding with a MIG welder compared to using one for mild steel, there are some differences that should be noted: TORCH ANGLE. Use plenty of forward angle. I use about 40-45 degrees forward angle; too straight an angle will give a sooty weld.
SPEED UP. Keep moving about a quarter faster than for steel welding. Looks better and keeps the heat out. STICKOUT. Use stickout of 15-20 mm. The tip should be back into the nozzle by 4-6 mm. 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.