Page 1 of 1
Strength of tigging al.
Posted: Wed Sep 21, 2016 9:45 pm
by Farmwelding
I've heard that tig welding aluminum cuts its strength in half if done properly. Is this true. And if so why build vehicle frames of a pickup out of them. It doesn't seem practical to me other than weight. Maybe I'm wrong.
Re: Strength of tigging al.
Posted: Thu Sep 22, 2016 1:57 am
by Arno
It varies a lot between different grades of alu how much strength the HAZ and weld loses over the parent material, so that's one consideration. But also if the complete construction can be heat treated or hardened after completion it may be possible to return some of the lost strength.
But still, weight saving over steel and corrosion resistance can be a big plus, and if the design of the joints is correct to take the expected weakening in consideration then it's really not an issue.
Where people tend to go wrong is to take a design made for a steel structure and copy that 1:1 into alu and then get surprised that it fails. A good designer/engineer would take the different properties of alu in consideration and adjust the design of any weld joints to compensate (eg. not use butt-joints in corners, but using overlapping pieces to get a bigger load path) and probably beef up some other parts to get the same properties as the original steel design.
And then there's even the option of not even welding structures but going to adhesives or other mechanical means of fastening when maximum weight reduction is wanted (aka. can't use thicker/oversized joints to compensate for the strength loss in welds) or alu alloys are used that are not weldable (eg. many aircraft frame parts or some car parts)
And in some cases it simply turns out that alu may not be the best material for the job, or that the weight saving (after proper redeisng) ends up being too small to matter.
Basically like in many cases it's choosing the right tool/material and design for the job and not just for the 'bling' factor
Bye, Arno.
Re: Strength of tigging al.
Posted: Thu Sep 22, 2016 7:21 am
by exnailpounder
To expand on Arno's post, a good engineer, even a good fabricator, doesn't rely on the strength of a weld alone in their design. A weld should always be viewed as a possible failure point. I do alot of repairs and see many failed welds and one of the main reasons of failure, IMO, is relying on the strength of a weld instead of using a mechanical fastener or re-designing to take the stress off of a weld. Sometimes that's not possible. There is a reason that aluminum, in any of it's alloys, isn't used in applications such as race car frames or anything subject to extreme forces but manufacturers cheap out and weld in areas that really should be bolted or fastened some other way. Others might disagree but I have seen a lot of really good welds that have cracked because of stress, in all metals.
Re: Strength of tigging al.
Posted: Thu Sep 22, 2016 7:17 pm
by MosquitoMoto
Arno and Nailer gave viewpoints based on experience and engineering, so I'll wade in from the newbie viewpoint.
I've been welding for around a year and I very often stare at a weld - even one of my better ones - and think 'Yeah, it looks okay, but how strong is it?"
So using a very unscientific testing method that is still strangely satisfying, I go and flog the daylights out of the part with a big hammer. And without exception I have always been surprised at just how tough my aluminium welds are. I generally see the parent metal failing before the weld does.
Mind you, vibration is a different kettle of fish altogether. And of course this opens up a whole other conversation regarding aluminium types, filler types, etc.
Kym
Re: Strength of tigging al.
Posted: Thu Sep 22, 2016 7:50 pm
by LtBadd
MosquitoMoto wrote:
So using a very unscientific testing method that is still strangely satisfying, I go and flog the daylights out of the part with a big hammer.
Kym
Kym, you seem a bit stressed out, why not beat the crap out of that weld
Re: Strength of tigging al.
Posted: Thu Sep 22, 2016 7:59 pm
by exnailpounder
MosquitoMoto wrote:Arno and Nailer gave viewpoints based on experience and engineering, so I'll wade in from the newbie viewpoint.
I've been welding for around a year and I very often stare at a weld - even one of my better ones - and think 'Yeah, it looks okay, but how strong is it?"
So using a very unscientific testing method that is still strangely satisfying, I go and flog the daylights out of the part with a big hammer. And without exception I have always been surprised at just how tough my aluminium welds are. I generally see the parent metal failing before the weld does.
Mind you, vibration is a different kettle of fish altogether. And of course this opens up a whole other conversation regarding aluminium types, filler types, etc.
Kym...the vast majority of broken welds is from vibration..at least thats what I see. Butt welds are famous for splitting because they split just like welding over a crack and not getting full pen. What are people thinking when they weld something like that? ...get it out the door and let it be someone elses problem? I bet thats it..
Kym
Re: Strength of tigging al.
Posted: Thu Sep 22, 2016 8:26 pm
by MosquitoMoto
LtBadd wrote:MosquitoMoto wrote:
So using a very unscientific testing method that is still strangely satisfying, I go and flog the daylights out of the part with a big hammer.
Kym
Kym, you seem a bit stressed out, why not beat the crap out of that weld
Me stressed? Heck no!
But I do enjoy destruction testing. I think I was originally inspired by the video in which Jody puts huge effort into welding up a coped bicycle frame...then goes out into the parking lot and wails on it with a sledge hammer.
As for vibes, I hear you, exnailpounder. I am about to mount up a front subframe for a race motorcycle of mine - I will be interested to see how it copes. Have built a couple of failsafes into it so that it can't just collapse like a box of broken biscuits.
Kym
Re: Strength of tigging al.
Posted: Thu Sep 22, 2016 8:38 pm
by exnailpounder
MosquitoMoto wrote:LtBadd wrote:MosquitoMoto wrote:
So using a very unscientific testing method that is still strangely satisfying, I go and flog the daylights out of the part with a big hammer.
Kym
Kym, you seem a bit stressed out, why not beat the crap out of that weld
Me stressed? Heck no!
But I do enjoy destruction testing. I think I was originally inspired by the video in which Jody puts huge effort into welding up a coped bicycle frame...then goes out into the parking lot and wails on it with a sledge hammer.
As for vibes, I hear you, exnailpounder. I am about to mount up a front subframe for a race motorcycle of mine - I will be interested to see how it copes. Have built a couple of failsafes into it so that it can't just collapse like a box of broken biscuits.
Just be careful...of course if you WERE careful..you wouldn't be racing motorcycles in the first place...
I sure miss racing...
Kym
Re: Strength of tigging al.
Posted: Thu Sep 22, 2016 8:43 pm
by MosquitoMoto
To continue the threadjack...
Creating a front subframe first is my way of being 'careful'. If it fails, it is held together at multiple points and a failure at one point can't possibly drop the whole fairing onto the front wheel, it will just sag a little.
This is a nice way of doing some testing before building a rear subframe that will have to support my skinny but.
Hey, maybe dunkster could contribute here? He creates beautiful aluminium subframes for race bikes and I can hardly think of another application that is subjected to so many vibes. I would love to hear what he has to say about material and filler types as well as HAZ behaviour and I'm sure it would be interesting for the OP.
Kym