Aluminum Fillet Weld Design

[mpdst11]

I am using the Aluminum Association's Aluminum Design Manual to design an aluminum frame with extruded tube shapes and welded connections, and I am a little out of my element working with aluminum instead of steel.

What is the maximum allowable size of a fillet weld based on the thickness of the section that is being welded?

This doesn't appear to be anywhere in the manual, and I can't assume it is that same as the values listed in the AISC manual. Please help! Thanks.

 

[desertfox]

hi mpdst11

A rule for fillet welds is that the fillet cannot be larger than the thinnest section of the two welded components.

desertfox

 

[Ron]

You're right that it is not limited by ADM 2000 or 2005. If you were dealing with open sections (angles, flat bar, sheet), then it serves no purpose to make the fillet larger than the thickness of the thinner part joined. When you have closed sections, such as the extruded tubes you mentioned, there is no forced limit to the fillet weld size, but there is a practical limit to the size. The fillet weld should not be larger than the shear capacity of the member in the direction of the analysis. As an example, let's consider that you are using a 2" square tube welded longitudinally to a 4" square tube. What is the shear area of 2" tube at shear failure? The fillet weld need not be any larger than that (factored for weld strength reduction, of course).

 

[MartinSr00]

Though not a structural engineer (I'm an ME). Maybe you know this already. I didn't at first when I reviewed some aluminum structural designs prior to possible manufacture.

Do not assume with aluminum that the weld metal has a strength equal to or greater than the parent materials. If you have a common T6 temper aluminum and you weld it, even after heat treating, you may be able to re-temper the parent material, but common weld rods for aluminum cannot be tempered to this strength. Also, aluminum is subject to over-aging and things like that.

Unless your very well versed in this, and I'm telling you something you already know, you need the counsel of a metallurgist.

One thing is sure, you cannot take all that you find in the AISC manual and generalize it for aluminum -- you will get in big trouble.

 

[Ron]

MartinSr00

The Aluminum Association puts a limit on the allowable stress and factoring (for LRFD) for welded sections. The reduction applies to areas within one-inch of the weld, in the base metal. It is a mandated and pronounced heat affected zone.

The reductions are quite pronounced. As an example, for 6063-T6 the allowable stress in bending is just over 15 ksi. For areas within one inch of the weld, the allowable stress drops to 6.7 ksi....no need for a metallurgist.

Ron

 

[mpdst11]

desertfox,

is that rule for aluminum, or just in general?

for example, in the AISC manual, the maximum fillet weld size for steel is the thickness of the material only for materials up to 3/16" thick. for sections 1/4" or thicker, the maximum size is 1/16" less than the thickness.

so i guess i am looking for something a little more specific.

 

[desertfox]

Hi mpdst11

Well you can always go smaller with the fillet weld so long as its not thicker than the thinnest section.
Its a general rule of thumb which as served me well over the years, really I suppose one should calculate the stress in the weld before deciding on the fillet size and try to use the smallest size fillet weld possible with a good margin of safety.

http://faculty.delhi.edu/hultendc/AECT250-Lecture 14.pdf

http://www.eng-tips.com/viewthread.cfm?qid=23184&page=104

http://www.roymech.co.uk/Useful_Tables/Form/Weld_strength.html

Here are some links which may help the middle one being a thread in here.

desertfox

 

[Bagman2524]

Welding aluminum reduces the strength of the base material to about 1/3 of its' previous strength.

 

[ash060]

Check table 7.1-1 of the 2005 Manual for the correct type of filler metal for welding, it varies with the Alloy.

I would use what ever size you need to make it work, but because of the reduction in member strength it may be the weld does not have to be very large.

Also, you should check with the local aluminum supplier in your area to see what extrusions they have and in what grade. Because you may spec 6061 or 6005, but get 6063, and aluminum varies alot

 

[mpdst11]

Thanks for all your posts. I still haven't really gotten an answer to my original question, though.

If I was working with steel, and I had a tube with a quarter inch wall thickness, 3/16" would be my maximum fillet weld size according to AISC.

What about aluminum? Could I go up to 1/4" fillet weld? What is the rule? Is there a cutoff like with steel? Can thinner sections receive a full thickness weld, where thicker sections the weld size is reduced? Where can I find this information?

 

[gtaw]

Consider referring to AWS D1.2 Structural Welding Code/Aluminum.

Make sure your design calculation take into consideration the "as welded" strength of the aluminum. Many of the common heat treatable alloys suffer from degraded properties when welded. Example:

6061-T6 - as wrought and HT'd - TS 42 ksi
as welded - TS - 24 ksi



Best regards - Al

 

[mpdst11]

I should have known I'd have to buy another book.

I found this document online from the Iowa Department of Transportation regarding their aluminum welding qualification procedures.

One of the tests involved fillet welding two plates of the same thickness, and it said that if the plate was 1/4" thick, 5/16" would be the maximum weld size.

Does anyone actually design aluminum fillet welds to be thicker than the base material? Or is this just an exercise for the welder?

I am working on a connection right now, and I have taken into account the reduced capacity of the welded aluminum, and the aluminum stresses are fine, but the minimum weld size that I am getting to work is 5/16" for a 1/4" material thickness.

So I can change the connection, which my boss doesn't really want me to do unless I absolutely have to, or maybe the 5/16" weld is actually ok.

Maybe when I tell him we have to buy a new book, he will just let me change the connection.

Thanks anyway.

MPD

 

[gtaw]

The 5/16 weld leg will only provide a throat that is 88% (7/32 inch) of the plate thickness, thus it will still be the weak link.

Best regards - Al