We are currently welding prototype Aluminium rocker covers for road and race engines using the Tig process.
We are using 1.2mm or 1.6mm 5005 sheet welded to a 6mm 5083 flange. The sheet is plastic coated which is removed prior to welding to keep the appearance in A1 condition.
Our problem is an "etching" of the sheet adjacent to the weld of approximately 3mm to 5mm which I believe to be the oxide removed during welding. We are using the latest hybrid wave OTC pulse Tig machine which is giving us a great weld appearance it's just this oxide removal which tends to stand out on the finished product.
We are using the AC balance on the machine to give minimum penetration, but a neat flat weld.
Does anyone know of a method which minimises this oxide removal?
We are unable to wire brush prior to welding as this would look worse than what we already have and we don't want to go to the added expense of polishing after welding.
Any comments would be appreciated.
Regards Andrew.
when welding aluminum, the welding machine is designed to "burn off" that oxide, because that thin oxide has a higher melting point than the aluminum itself. if the oxide isn't burned off, you'll end up w/ poor fusion, burn-throughs, and some porosity. as far as i know, there's no way to get around it. i don't think u want to get around it. if the oxide isn't burned off your weld would look pretty bad and inconsistent. but either way, i don't think there's a TIG machine out there that won't burn off the oxide. MIG machines are the same way. whichever way you look at it, you'll be sacrificing weld quality, or post welding appearance.
When welding is performed with AC, then these oxides are removed during the electrode-positive portion of the cycle.
DC- will minimize much of the appearance your trying to avoid, but you may end up sacrificing the weld quality. Al oxide is detrimental to this material when joined by welding. In other words, the weld area must be very clean to DC weld a joint. Depends on your weld quality requirements. DC- does not have the cleaning action of AC.
Most companies that I work with who fabricate aluminum do all the final cleaning after the part has been completed. The weld looks very nice after chemical cleaning (usually an etching). Look at the welds on most new motorcycles with aluminum frames (the spars and swingarm welds are usually visible), very clean, no etched areas that your trying to avoid. A clear coat is often applied to maintain that finish. Some clear coats may withstand the high temperatures and chemicals (oil, gas, etc.) present in the engine compartment.
I'm not familiar with this machine, but with Miller's equipment or ThermalArc the key to narrowing the "frosted" width of the oxide removal is in the frequency you are operating at, the torch angle and holding a tight consistent arc. Typically you want from 180Hz to 220Hz for your frequency setting. If you go above 220 the arc tends to be too forceful. One last thing, the AC balance setting should be about 80% of maximum. You should be going for the maximum penetration side of your machine function. Make sure you degrease, without leaving any hydro-carbons on the surface.
All the best, Brad
By -
Date 01-21-2002 18:39
Are you sure that the "etching" you are seeing is due to oxide removal? You say that it is up to 5mm on either side of the weld. I find it difficult to believe that the oxide removal can be that wide on either side of the weld.
I believe that possibly what you are seeing is a high temperature oxide scale. It will be particularly visible because of the good finish of your parent metal.
If my assessment is correct, shielding the weld untill the temperature is low should solve or reduce the problem.
Regards
Niekie
Andrew
Unfortunately the same parameters that give you the flat bead appearance and minimum penetration is exactly the same thing that will maximize your etching. Having said that, your situation is more controllable due to the fact you're using an asymmetric power source. Assuming you can control both your Balance (dwell time on each side of the half/cycle) and your amperage on each side of the half/cycle, your possibilities are nearly limitless.
Only trials on your specific component will tell the truth about the visual quality of etch your gonna get. This is why operators either love or hate Asymmetric power sources. For aluminum between 1 and 2 millimeters thickness you really do have a lot of flexibility. Remember the better the prep the less Reverse polarity is required for etch. You may minimize Reverse polarity in two ways, Balance Control adjustment and Amperage reduction on the Reverse (EP) side of the Half cycle. Still this can allow you to run many combinations. Unfortunately none of the manufacturers have had the nerve to publish extensively on this matter. This may be because even with modern technology breakthroughs the old Aluminum Tig rules still apply.
Mike Sammons from Miller Electric says
"There are no hard rules about setting balance control,
but the typical error involves over-balancing the cycle".
"Too much cleaning action (EP duration) causes excess heat
buildup on the tungsten, which creates a large ball on the end.
Subsequently, the arc loses stability, and the operator loses the
ability to control the arc's direction and the weld puddle. Arc
starts begin to degrade as well".
"Too much penetration (or, more precisely, insufficient EP current)
results in a scummy weld puddle. If the puddle looks like it has
black pepper flakes floating on it, adding more cleaning action
will remove these impurities"
Now I have one more thing to say about frequency,.. It sounds like your more interested in a wider flat bead profile rather than maximum penetration. The higher your AC frequency goes the more conical your arc shape will become and the more narrow your bead. If your requirements are for a flatter bead, than lower your frequency in Hz, This might help a tiney bit in your weld appearence and profile if you also lower your Reverse polarity innput one way or another.
(Frequency adjustments under 200 Hz is irrelevant unless you are suffering from arc wander or lack of directional control). I have found no use for Frequencies over 220 hz with aluminum.
Ps. If your using Max Reverse polarity/Cleaning for thin gauge aluminum fillet welds, you might want to do a destructive exam to be sure your getting complete fusion at the root.
Send me an Email if you would like to further discuss some of the trials we have done with asymmetric power sources.
Lawrence
Thank you gentlemen, for your input with regards to my problem. I think that a few more test pieces may be in order before we finally start production.
I think our first try is a bit higher AC balance than we currently are using as this will probably narrow the bead. We don't want to go to far though, as the weld appearance we have so far is very good.
Also as Lawrence suggests, perhaps some amperage controls on each half/cycle are in order and some hz adjustment as well.
I think we'll start changing one parameter at a time just to make sure.
Well, at least we've got plenty of scrap material to play with. I think we'll need it. :-)
Many thanks for all on this board. It's people like you that make this board worthwhile.
Regards Andrew.
