A small bit on a big bit

[Member 66061]

I didnt know what else to title this thread.
Anyways, time to start begging for advice...
I had a hard time building up a couple of small tabs about 10mm x 10mm x 3mm...ish on a fairly intricately machined fist size piece of decent quality cast aluminium. Why does this forum insist im spelling aluminium wrong?
I built said tabs up but decided they needed to be slightly wider at the base after filing them to shape but i couldnt get the metal at the bottom of the tab to pool without the tab starting to melt and pool first, so i was at risk of melting the tab id worked so hard on into a messy useless puddle.
I had my frequency turned right up so i was using a tight little ark, amps at 68, balance at about 40%, 2.4 white tungsten, standard gas lens and no.6 ceramic.
I tried dwelling on the bigger part of the cast but it just plain refused to pool and the work got too hot.
To be succinct, how do you weld something small to something big without melting the small thing into a blob. Thanks

 

[BrokenBiker]

68 amps seems very low for a fist sized bit of aluminium...ali is a huge heat sink so im not surprised you couldnt get a puddle at 68 amps.

the general rule of thumb for welding thick to thin is to set your amps to the thick section, strike an arc on the thick section and then focus the arc on the thicker section and just wash the puddle over to the thin section in order to melt enough of it to get the required joint strength.

im not a tig welder but 68 amps struck me as being very low

 

[Tigman]

a bit of pre heat on the larger piece would help

 

[Member 66061]

I was welding at about 75 amps but struggled with building up the small tab without it melting. the tab sticks up at right angle from the main piece.
The main piece was too hot to hold when i attempted to add more weld at the tabs bases.

 

[Richard.]

I see what your saying. Your trying to weld onto thick stuff but without ruining the small tab that your trying to build up. Naturally you would think "go steady on the current" but that's what is causing the problem. 70 amps is probably too much over a period of time for that poor little tab yet it's a fraction of the current you need to get your pool going on the base. Your problem is an easy one to resolve. First warm up the thick section to eliminate the heat been sucked away then a very short sharp blast at high current and get the filler in quick then stop. Repeat if needed. High freq (150 plus) very sharp tungsten and set your balance well into the high en to keep that tungsten shape maintained for a focused arc* Too long time spent at low current is going to ruin the job. Give it a good whack of current get in the fill them get off the current. (80% en or 20%ep) depending on how you set it on the machine is where I would be setting it.

 

[Member 66061]

You understand entirely, thanks for your articulate and informed reply.
I am good enough to have been able to build up these small tabs, but i will have to admit defeat with this after reading your solution.
It it just above my skill level to be able to successfully do what you say. I cant afford to ruin this job after getting so far.
I will have to go see a skilled welder. Atleast ive trimmed the cost of the job down a bit by doing most of it myself.
Sheesh........i need waaaay more practice.

 

[Richard.]

You understand entirely, thanks for your articulate and informed reply.
I am good enough to have been able to build up these small tabs, but i will have to admit defeat with this after reading your solution.
It it just above my skill level to be able to successfully do what you say. I cant afford to ruin this job after getting so far.
I will have to go see a skilled welder. Atleast ive trimmed the cost of the job down a bit by doing most of it myself.
Sheesh........i need waaaay more practice.

Do you have a foot pedal? It would be easier then you wouldn't need to worry about numbers you'd just stamp on it far enough till it felt right. If not what thickness roughly is the base your trying to focus on. I could give you some guideline settings. I don't think you'll ruin it you just need to get filling in quick then back off even quicker.

 

[chevy2]

Concentrate the heat on the thicker piece but close to the thin piece and as the thick piece starts to pool you can then add filler to it to build the pool across to the thinner piece and then continue to run a bead, you'll still need to prioritise the heat more so on the thicker piece as you run the bead due to differing thicknesses.

 

[Member 66061]

Yes i have a foot pedal, when i say i was welding at 68/75 amps i mean max setting.
My machine only goes between 30% - 70% ep i had it set at 40ish.
Seriously though, I cant afford to attempt this on this piece.
I could only afford to practice this kind of thing on scrap, and i need waaay more practice

 

[Richard.]

Can we see a photo of it mate.

 

[Member 66061]

Well i went out this morning and measured the tabs, 2.8mm and 3.4mm thick. It turns out this is actually the proper thickness.
Ive just been judging by eye and for some reason they just looked thinner,should have known better and measured them after id finished filing them to shape.
Ive got a real bad habit of being over zealous and putting way more weld on things than they need.
Richard, thank you for your help and understanding.