Welding Thin Wall Aluminum

[weeeds]

We are TIG welding 6061T6 6" OD x 1/8" wall tube to a 1" thick fabricated slip-on type flange using double fillet welds.
Over the years we have performed a good deal of welding using heavier wall aluminum construction so we are not completely inexperienced.
We have tried pre-heat from 200 F (as per our WPS) to as high as 800 F.
At hydotest we have experienced regular failures in the heat affected zone with the tube cracking in the circumerential direction. The cracks are about 1 - 2" long.
None of our people have seen anything like this and we don't know what is causing the failure.
Has anyone experienced a similar problem? If so how did you deal with it? Any recommendations?
Thank you

 

[metengr]

Could you explain the exact location of the cracking? My reason for asking is to determine if the actual fillet weld deposit is experiencing cracks or the base metal - tube side.

Second question - what is your welding sequence for the double fillet welds. Do you deposit the inner weld and then deposit the outer fillet weld? Do you tack weld the flange into position?

 

[stanweld]

In addition to metengr's questions, are you welding in the flat position and the weld made continuously or are you welding 180 degrees at a time; e.g. from 6:00 to 12:00? Are the cracks at the tie-in locations?

 

[deanc]

Slip-on flange? Tack and weld the inside joint first. Try to leave the outside tack free.

Keep the preheat and cool slowly,try helium,also 5356 for filler.

Make sure the coatings and oxides are cleaned from the OD of the pipe. Do not use a carbon steel wire brush or a grinder used for carbon steel.

 

[weeeds]

Thank you for the questions.
The cracks are in the heat affected zone in the parent metal.
The flange is tack welded and the inner weld is done first, followed by the outer weld.
Welding is done in the flat and is continuous through the full circumerence without stopping.
We can not see a pattern to the location of the cracks other than that they are in the tube wall, the heat affected zone, and run circumferentially. It appears that we may be tearing the material but have not been able to prevent it.
Thank you.

 

[metengr]

widla;
It sounds like you are tearing the thinner gage material. Try dropping the preheat entirely. It may be that preheat is providing added thermal stresses resulting in ductile tearing after the joint cools. This thin gage of aluminum does not require preheat despite welding to the thicker aluminum flange.

 

[deanc]

Hmmm-metengr gave me a thought. How much gap do you have between the ID of the flange and the OD of the pipe? If it is alittle to big,preheat would close it and then the shrike may be doing the tearing.

So-no preheat may leave a larger gap and help in some areas but with a continuous weld there could be problems at the end. Try decreasing the gap.

 

[RikuY]

My company welds a lot of 6061-T6 and have run into the same problem. We don't preheat any material, and have found that the tears are majorly from the heat build-up mentioned above. To further help, we also weld half of the perimeter at a time, depending on thicknesses of the materials. Move on to another weldment and weld half and so on. After the batch is cycled, we return to do the other half. Our seams are oriented to avoid loading.

If this happens persistently, we move to GTAW.

I hope this helps...

 

[weeeds]

We have tried the posted suggestions with little success. The TIG welds look great but we have a 50% failure rate. In one case the tube cracked during cooling at the toe of the weld and you could hear the noise. In anothoer case you could not see the defect but water was leaking out in one location at the HAZ during hydrotest. We used no preheat and cleaned the heck out of the materials.
Any other comments?

 

[metengr]

widla;
Double check the composition and heat treat condition of your tube material. If everything looks acceptable, it sounds like the weld joint is still under considerable restraint during welding. Are you fixturing the assembly to make the fillet welds? What is your welding position? As deanc pointed out, make sure your joint gap or clearance between the OD of the tube and the ID of the flange is minimal.

You could try to alternate the weld sequence by depositing half of the inner fillet weld on the slip on flange, and move on to the outer fillet weld. Come back and complete the inner fillet and finish the outer fillet weld.