Filling small holes in Aluminum

Gentlemen
I am in the process of a rebuild on 20" aluminum boat. There are a scattered holes in my cabin and hardtop were the previous owner had fittings that I have removed, These are not structural or have anything to do with the hull or water.
I plan to weld them closed and grind so when the new paint goes over they will be gone.
I only have access to a mig with spool-gun. I have heard if you place a piece of copper or brass plate on the other side of the hole while welding you can fill the hole with a half decent finish already to that side. Can somebody please verify this or if they have another method I would like to here about it.
Thanks
Katoh
Sorry forgot to add holes are 3-4mm in diameter

 

Use a copper back up plate & argon as a sheilding gas.

 

Just fill with epoxy if non structural.

NEVER allow copper to come in contact with ally....it will eat the ally!

 

Fishhawk
Do you polish up the copper plate prior to welding, I take it you definitely would not use an oil on it as it would contaminate the Al.
Ad Hoc
The theory is to use a copper plate as a backing just to fill the hole, The Al. wont (should not) bond with it and it just comes off, Then you use it for the next hole, say like form-work. Not unless they bond? I know you use brass on mild steel as a backing to fill in holes with the mig, that works well. I don't like using epoxies, just seems to be a cheap, 1/2 hartted job to me. Reminds me of patching cars with "Bog" car putty, this is just my opinion.
I notice your location is Japan have you been effected badly by the earthquake?
katoh

 

Once you place the copper in direct contact, you'll have microscopic shavings that will remain. These shall cause terrible corrosion. I strongly advise you NOT to use copper as a backing plate. You'll never clean it all off..

Yes, I am in Japan, and thanks, we are fine. We are some 600+km from the epicentre.

 

Good to hear you're OK, 600k is not much, you must have had some real rumbling going on.
Is there something else you can use as a backing, What about just mild steel or even a piece of galvanized steel? other option I was thinking was to use a Al. Rod but your not going to get it in the same grade, being a rod will be 6xxx series.

 

Use stainless steel. If you feel like making them permenent backing bars, then use ally.

 

Thanks for the input, If I make a stainless plate with a handle I should be able to fill all those little holes in. I take it doesn't matter on the grade of steel.
Excellent Ill do that then.
Cheers
Katoh

 

ad hoc

ad hoc, as a retired aircraft welder (45 yrs) we have always used copper to back up alum. because on complex shapes, usually .020-.035 in the annealed condition as its much softer to form. stainless steel is also used mostly on ling flat parts, but only use type 300, never type 400. I would use 4043 or 5356 rod for filler.

 

Hi Fishhawk
The only wire I use on the boat is 5356, Ill make a stainless jig with a flat side and handle on the other with a wooden handle on top. Hard to explain but like a miniature plasterer's hawk. Someone can get in and hold it against the hole while I zap it from the outside.
Corrosion scare's the hell out of me, as that's what I'm trying to repair on the hull, just cant work it out I have two 1/2" holes perfect round through the hull the same distance up from the keel 6". I cut an access in the floor over one of the holes and found nothing, good as the day it was built, no corrosion internally at all! There's a hole starting on the opposite side but has not gone through yet, got me bamboozled! Sorry I'm raving on about something else, but if Ad Hoc reckons bits of copper will be in the weld and I start having more corrosion troubles I will find a way to make Aluminum burn. There will be nothing left of the bloody thing by the time I'm done with it!.
Cheers
Katoh

 

with a temp backing bar any size hole can be filled, if you have no pulse, just stop and start (trigger the weld) I often use steel with a groove cut in the surface to one side full penetration weld plates. Leaves a nice bead on the inner This techique has been around for 60 years, It is Even in the Lincoln Bible a tomb 3 inches thick THESE days one can buy pulse on pulse machimes that,ll weld ,8 mm that 32 thou)

 

I always thought aircraft were riveted, not welded

Aircraft generally uses 2000 and 7000 series alloys. These contain high quantities of copper. The 2000 can have as much as 6.0% as in 2011, or as 2.0% in the 7010/7075 alloys. Aircraft are also in a non-corrosive environment, like seawater. They operate in one medium, air... Aircraft do suffer corrosion, but not the same degree as aluminium boats, not by a longgggggggg way!

Marine grade 5000 series alloys such as 5083 has just 0.10% copper, as has 6082.

Copper, as shown on the attached galvanic chart is low down.



You can see that copper is -360mV and ally nominally -760mV.

The max potential difference for safe working is approximately 50mV...for "general" other use 100mV. Previously values of 250mV were used, but this has shown to be not sufficient to prevent corrosion.

Once you place bear copper against ally, small shavings will remain. In an atmosphere of seawater...the result is a serious attack on the ally.

As for 5356....this is a very good around filler. BUT, you do need to know what grades you are welding together.

 

G'Day Dean
Those holes in the hull Ive got other plans for, I plan to drill them to 20mm or3/4" then with the CNC machine route some 10mm plate down 5mm circle on one side same dia as the hole, but leave it longer length then tack a rod on it so you can hold it. Do not know if I am explaining it right, its like patching a hole in a hollow wall. Then with all the extra material there I can really give it some, get penetration and fill from one side only. I only have access to one hole not all through the hull.
Cheers
Katoh

 

Ad Hoc
Without doing any major test I would definitely say were dealing with 5086, No guarantee, but the builder was quite professional from what I can see, and that's nearly 20 years ago, I can not check this as he is long retired. From what I have found about its history , it is an ex rescue boat, so must have been in survey at some stage. It was the previous owner that did things to it that caused all these problem's!
How would you check the plate grade?
Cheers
Katofh

 

Given the above info....your only choice is to get a sample tested chemically by a Lab. Otherwise you can trust the guy telling you it is 5086. In which case 5356 is fine.