Aluminum GMAW Hot Cracking

By TimGary (****)

Date 05-31-2013 16:46

I'm in the process of experimenting with parameters in preparation of qualifying a full penetration GMAW Spray transfer weld to AWS D1.2.
One weld immediately developed a center line crack (see pics).
I have my suspicions as to what caused the crack, but would ask your learned diagnosis for clarification.

Specifics:
Base Metal - ASTM B221 6061 T6511 x 1/4" thick flat bar
Filler Metal - 4043 x .047" dia wire
Joint Prep- 60 degree V groove with 1/8" land and 0" root opening @ 1G, cleaned and brushed
Weld Parameters - GMAW Spray @ 24V and 350 IPM WFS with 30 cfh 100% Argon
Technique - Straight push with 20 degree travel angle, 0 Degree work angle, 5/8" stickout and approx 25 IPM Travel Speed
Process - Single pass backing weld applied first, opposite from bevel side, followed by single pass fill and cap on bevel side. No cleaning or gouging between passes.
There was no protrusion of weld metal into the bevel following the backing weld.
The final result was a full penetration weld with no evidence of incomplete fusion inside the joint.
The weld cracked all the way through both passes while making the front side pass.

Thanks in advance for your consideration,
Tim Gary

Attachment: AlumFrontSideWeld.JPG (0B)

Attachment: AlumBackingWeld.jpg (0B)

By 99205

Date 05-31-2013 17:39

If all of your parameters are correct you may want to check the groove type your using. There are times when a "J" or "U" groove is a much better choice when it comes to aluminum. There are issues with the way aluminum welds freeze in a "V" groove.

By jwright650 (*****)

Date 05-31-2013 18:19

Any idea what the interpass temps were before starting the last pass?

By TimGary (****)

Date 05-31-2013 18:59

Hi John,

I didn't check the interpass temp on the practice piece, but "Smoking Hot" comes to mind.
I made the second pass immediately after the first.
I'd guess around 400 deg?

Tim

By jwright650 (*****)

Date 05-31-2013 19:26

from AWS' welding handbook Vol 4, 7th edition, page 370/Aluminum Alloys/Metallurgical Effects/Weld Metal

"The weld metal tends to crack if the solidification range is wide."

By tazmannusa (**)

Date 06-01-2013 17:46 Edited 06-02-2013 13:34

lincoln c8100 pdf is what I use for a good starting point, I will try to attach it here . From the color of the welds looks to hot to me

The pdf is to large to attach

http://www.lincolnelectric.com/assets/US/EN/literature/C8100.pdf

By 803056

Date 05-31-2013 20:39 Edited 06-03-2013 15:12

Were the plates clamped securely while you welded them?

If they were clamped securely, they could not slide as the plates contract during cooling. You might want to loosen the clamps on one side to allow some slippage as the plates cool.

6061-T6 alloy is precipitation strengthened. Allowing the material to stay hot between passes allows the material to overage, i.e., the alloying constituents will precipitate to the grain boundaries. In this case the last region to solidify would be the centerline of the weld, right where the crack is. To make matters worse, you have an unfilled crater, the best place in the world for a crack to initiate. Once the crater crack is there, it propagates through to the final weld bead.

Best regards- Al

By MMyers

Date 06-03-2013 14:58

This is a great suggestion to pass the test, but unless it's indicative of the fabrication conditions, it's also a great way to find out you have a cracking problem in production.

By TimGary (****)

Date 06-03-2013 17:06

Thanks to all for your replies.

My suspicion for the cracking was excess heat input, compounded with no root opening to allow for contraction.
There was no restraint on the test plates, they were simply lying free on a table.

However, I've found that sheilding gas may have been a contributing factor.
After I made the initial post, I checked the equipment and found that the gas regulating flow nut that we use to set the gas CFH from our manifold,
( 90/10 shielding manifold at 85 psi, reduced to 42 cfh by an orifice type flow nut)
had not been removed from the gas line when the 100% argon cylinder with flow meter had been installed.
I believe this limited the sheilding gas flow to less than optimum.
After the flow nut had been removed, I tried to replicate the cracking with all parameters the same except for gas pressure, but was unable to.

My intent is to qualify procedures for:
GMAW-P, CJP of 6061 thicknesses 1/8" thru 1/2", using 4043 filler.
GTAW, CJP of 6061 thicknesses 16 ga thru 3/16", using 4043 filler.
GTAW, CJP of 5052 thicknesses 18 ga thru 1/8", using 5356 filler.

I intend to set a max interpass temp of 250 F.

If anyone has some good advice for me, I would appreciate your input.

Thanks,
Tim Gary

By jwright650 (*****)

Date 06-03-2013 17:44

>I intend to set a max interpass temp of 250 F.

I think that is wise, Tim. I thought the picture looked like it was cooked to death, but had nothing to go on other than a gut feeling from the picture.

By 803056

Date 06-03-2013 20:46

The problem with some of the heat treatable aluminum alloys is related to time at temperature, i.e., overaging.

I limit the time at temperature by dispensing with preheat, it isn't needed if you use sufficient current. I also cool the joint between beads with a blower. Do not use compressed shop air, use a high velocity blower and heat sinks. I have also hastened cooling by wiping the areas adjacent o the joint with a water soaked rag.

You may have difficulty getting the thicker plates to attain the required tensile strength when welding the joint with GTAW. Again, the problem is related to the time at temperature and the resulting overaging of the base metal in the HAZ.

Best regards - Al