My friend Pierre (an electrician) was keen to see the hovercraft get going and he fitted a new power cable for the garage. This is rated to 64 Amps, so it shouldn’t have any problems powering my MIG welder. So now that the heavy duty cable is wired in I can finally try welding aluminium!!!
I had scoured the internet for advice on "domestic" MIG welding and found almost nothing. I did find an excellent book though – "Aluminium boatbuilding" by Ernest Sims, which gave some great tips (available from Amazon).
All the experts said the same thing – Aluminium MIG welding needs pure Argon gas, not an Argon & CO2 mix. Machine Mart sold small bottles, so I bought three disposable bottles of Argon and two small reels of wire to practise. As aluminium is softer and therefore more difficult to push down the wire tube, they a also recommended that I should replace the standard wire liner supplied with the machine with a Teflon one.
The instruction book that came with the MIG machine said to turn up the wire speed to max and set the current to almost max. I gave it a go. I welded a thin straight(ish) bead, then over-welded a slower zig-zag. I was actually surprised at the quality of my first weld. OK, so it wasn’t anywhere near as good as what I could achieve with steel MIG, but all those gloomy forecasts were wrong. It looked like it had reasonable penetration, but the result was too wide really. I showed the wife who was also quite impressed. Now I reckon that wives don’t normally know much about welding, but mine does – she once worked on a professional welding magazine so she knows more than the average wife. This welding lark might be possible!
I took these practise welds in to work to show fellow engineers (yes I know, sad or what). They were quite impressed too. I spent the next weekend using up more gas and more wire, doing more practising. The results were very variable, with many different problems, and generally far worse than the first run I did. What happened to those nice welds that I started off with - was that beginner's luck? Apart from the quality of the weld, the tip would often jam, as the wire melted back. Then this caused a birds nest inside the machine at the wire feed point. And when this happened I had to stop, cut out the tangled wire, pull the rest through, throw that all in the bin and feed through fresh wire. This was getting to be a real pain in the neck. But gradually things got better and I worked out where I was going wrong.
To prevent the birds nest problem the wire feed pressure needs to be just right – although the recommendation is to use a low pressure on the grip roller I found that as aluminium is so soft, it needs almost no pressure.
The wire speed is also really critical. Too fast and the weld bead will be poor, too slow and the tip will clog. For me, 9½ is the best speed. 10 is too fast and 9 is too slow.
The other problem was the way aluminium melts. As aluminium conducts heat very well, much of the heat from the weld pool is immediately absorbed into the surrounding metal. After striking the arc you need to pause at the start of the weld to build up the heat, then move off steadily. This means the start is a bit untidy but you can always grind it out if you don’t like it. However, when the metal is still hot from a previous weld it will be easier to weld.
When welding a relatively thin piece , you need to get the balance between good weld penetration, and not melting a hole through it. A great tip that I picked up from the "Aluminium Boatbuilding" book is to use of a backing piece made of steel which is clamped to the underside of the weld area. This supports the weld and prevents the molten aluminium from falling away from the weld to leave a hole. As steel has a much higher melting point than aluminium, the backing piece does not get welded into the aluminium and is easily removed afterwards – usually it just falls away when the clamp is removed..
Another tip I read about was to only use a stainless steel wire brush to prepare the area to be welded, and not to use these brushes on anything but aluminium. Mild steel brushes would apparently trap steel in the surface of the weld, leading to problems. I thought this probably made little difference unless you were welding something critical, but to be on the safe side I bought a couple of stainless steel brushes.
This test piece represents the construction of the space frame - a flat strip welded to the side of a tube. These small bottles of Argon are cheap but they don't last long - and they can run out at an inconvemient time. If you're into serious welding, get a big bottle.
More expense and more practising
Having done a fair amount of practising, it was clear that my welding wasn’t perfect, but I had convinced myself that with more practise I could weld up the hovercraft myself. But so far I had used four gas bottles and three reels of wire and I hadn’t touched the hovercraft yet! To weld the entire frame together I was going to need around 50 of those small bottles of gas, which at around £10 each was clearly not the way to go. Not only were the small reels and bottles less cost effective, but it was a pain running out of gas or wire half way through the weld run. What I needed was bigger reels of wire, and a much bigger bottle of Argon.
Home Dec2002 Jan2003 Feb Mar April May JuneJuly Aug Sept Oct Nov Dec2003 Jan2004 Feb Mar April