Aluminium MIG Welding over an existing weld, does it affect the strength of the weld?

[KJ222]

Hi, I am in Sydney Australia, new to this site, and have read some excellent tips here. Thanks to all the contributors. I have been teaching myself to MIG weld on 3mm aluminium rectangle hollow and have a few blow holes here and there on the joins. After reading other threads I feel confident I have enough info to attempt to fill them. My questions are,

If I have welded some joins without cleaning the oxide off the aluminium first, provided the welds have penetrated the metal, will the welds still be sound?

Does MIG welding over an existing MIG weld onto aluminium affect the strength of the weld at all or make the weld brittle?

If I have a few welds that are a bit rough, once I clean them up, is it OK to go over them and strengthen them?

Thanks,

KJ

 

[Engineer1964]

Hi, I am in Sydney Australia, new to this site, and have read some excellent tips here. Thanks to all the contributors. I have been teaching myself to MIG weld on 3mm aluminium rectangle hollow and have a few blow holes here and there on the joins. After reading other threads I feel confident I have enough info to attempt to fill them. My questions are,

If I have welded some joins without cleaning the oxide off the aluminium first, provided the welds have penetrated the metal, will the welds still be sound?

Does MIG welding over an existing MIG weld onto aluminium affect the strength of the weld at all or make the weld brittle?

If I have a few welds that are a bit rough, once I clean them up, is it OK to go over them and strengthen them?

Thanks,

KJ

Hi KJ and welcome to the forum.

if i was you for the sake of a few sanding pad in a grinder i would sand back the welds and even go as far as prepping it with a VEE, then simply re weld, thats the most surefast way of knowing you did right.

You could go over the top but the weld wouldn't sit right and i doubt if you'll get the same level of penetration. With ally its really quick to sand back the old welds, and at least that way you'll have a stronger joint with no contamination

Aluminum needs to be clean when welding. This is another golden rule when welding ally. Once again it is the nature of the beast that you are dealing with. They say that cleanliness is next to godliness, and nothing is further from the truth when trying to weld ally.

Use a stainless steel wire brush to clean off the oxide layer before you weld. As soon as aluminium is made it reacts with the oxygen in the air. It does what is called oxidizing. Thus the name of the oxide layer. It is a thin layer of aluminium oxide that needs to be brushed off for the best welding results. Never use a mild steel welding brush, only use a dedicated stainless steel wire brush for aluminum jobs. Get a marker pen and write the word "aluminum" or "aluminum only" on it. Keep it separate from the normal wire brush. You do not want to contaminate the brush or use the mild steel brush on ally as it will impregnate impurities into the soft aluminium surface.

 

[KJ222]

Thanks Gerry,
So rewelding over what's left of the other weld doesn't weaken the strength of the weld or make the aluminium or old weld brittle at all?
KJ

 

[Engineer1964]

Thanks Gerry,
So rewelding over what's left of the other weld doesn't weaken the strength of the weld or make the aluminium or old weld brittle at all?
KJ

Well welding over an old weld with out at least making a bit of an effort preparing or cleaning the old weld is a bit of a lost cause, You will find that ally welds are usally much stronger than the ally they are welded to this is because of the heat, so welding over an un prepared weld wouldn't be adviseable, i wouldn't go as far to say it'll make the weld brittle but it won't do it any favors.

 

[KJ222]

I understand, I will definitely take your advice and do the correct prep before re-welding. My question was more related to whether the properties of the aluminium or the original weld had been altered during the first weld and whether the process of re-welding (after the correct prep) over the top created other side effects like a making the aluminium brittle.

I suppose as aluminium is such an easily recyclable material, it is one of those materials that can withstand heating and re heating without much effect on it's actual structural strength.

I hope that makes sense. I have heard that with some materials, re-welding them can alter the strength of the material & weld. Perhaps I got that wrong or it's an urban myth!

Cheers KJ

 

[hotrodder]

Heating aluminium softens (anneals) it, typical annealing temps are in the the 300-400C range. Extrusions (tube, angle, strip etc) are typically a 6xxx series alloy which gets a lot of it's strength via heat treatment. Heat treatable alloys are a bit more complicated in that they take longer to fully anneal than say pure aluminium so rewelding can soften them more or result in a larger HAZ (heat affected zone). Most of the 'damage' is inevitable when welding ally but reheating/overheating causes more

 

[julieanncharters]

as has been said above yer can't polish a turd.
if a weld looks shocking quickest way of removal is to back chip /grind it out with a tct blade in 4"grinder and re- do.
laying crap on crap will produce ....................... you guessed it.
scary bit of kit,both hands on machine,welders gauntlets and full face shield for above,skimp at yer own peril.