Problem on 1 inch Thich alm 6061 T6 welding and pull testing

Date 10-16-2008 20:28

Need some direction: This is what we are welding it is a 4foot by 4foot frame grid. Made from 1 inch thick x 6 inch wide 6061 T6 Aluminum plate. We are welding the 4 corners on this frame both sides with a bevel. Then welding in a 1 inch thick x 6 inch corner block 4 times for a caster. Then a set of 4, 1/2 inch thick x 6 inch gussets going around the inside of the square frame in a diamond pattern for support. All welds are beveled. These welds are 3/8-1/2 fillets. We are GMAW these frames at 29 volts 650 inches of wire feed, we are useing 100% Argon, Filler rod is ER5356 .047. We are getting what I believe is good results we are cleaning the metal every pass and the welds look like they are getting good penetration. we are keeping the interpass tempature below 275 degrees.

Problem: The Customer wants to achieve 24,000LB structual load before weld failure. When they are sent out to the lab for pull testing we are only getting about 19,700lbs before the welds would fail! When I went up and actualy watched the test, I thought is was being tested incorrectly. They are not testing the frame the way it will be used in the field. They are pulling the frame standing up where the welds are horizontal and act as hinges when the frame is pulled appart.

I know that ER5356 rod is Stronger and harder structualy but the way they are testing this frame I am thinking the rod is to hard and because the welds act as hinges during the pull test they might be cracking and then shattering because it is to stiff. We are going to try to use 4043 rod to see if there might be more forgiveness in the welds during the pull test.

Any help thoughts, suggestions or whatever would be greatly appreciated!

By Lawrence (*****)

Date 10-16-2008 20:58

I'm not an engineer by any stretch... But my thinking was going another way.

Perhaps the multiple passes of .047 wire is making for a softened joint and HAZ.

Perhaps a wire diameter the next size up would be more suitable for the application. Less overall heat input via fewer weld passes (stringers, no weaves) to achieve the required weld size might add to the pull strength.

Not to mention the large diameter wire will bird nest less often and costs less if your buying bulk.

By 1fastguy (**)

Date 10-17-2008 12:23

Hey Lawrence, What about my opionion that the filler rod might actually be to hard and cracking then shattering because it has no forgivenes in it. From what I understand another vendor welded these frames before useing ER4043 rod and they passed pull test.

By Lawrence (*****)

Date 10-17-2008 15:01

Paul,

I dunno about that so much... Keep in mind this question is a bit above my paygrade.

Even the 5356 looses considerable strength during welding.. The weld and HAZ are much softer than the surrounding parent material so I don't think the failures are due to hardened weld deposit. Would a switch to 4043 make for an even more ductile weld and HAZ? I think it would, but I can't speak to it solving your problem.

But the way the testing is conducted may be an issue as MDG pointed out and so did you.

By Kix

Date 10-17-2008 17:24

Are there any groove welds on this particular part? When the guy says he wants failure to be above 24ksi it sounds like he is specing the requirements for a PQR out of D1.2 structural aluminum. 24,000psi is the minimum failure load in tensile pulls for 6061-T6. If you are going buy code and there are groove welds on this part then all you need to be testing is a test plate to qualify a procedure. Then you can do fillet breaks or etching for the fillet weld procedures. Now if your just load testing the part and he wants it to fail above 24,000psi, then i'd say the way you test it is going to determine when it breaks. You might need to go with a whole new joint design. If you have procedures for the 5356 wire and you switch to the 4043, you will have to qualify new procedures.
We had trouble with our tensiles on 6061-T6 in that they pulled at well below 24ksi (18 and 19ksi) and we had to do them over. We were using 4643 filler on 1/4" thick 6061-t6 base material. I found the proublem to be hydrogen porosity in the root and that we got the pipe to hot for to long even with 250deg interpass temps. I also switched to 4043 for sits and giggles and they came out this time. I'm pretty sure it was because I let it cool way below 250 deg for interpass temp and got rid of the hydrogen pockets in the root. Are your failures in the weld or in the base material? If they are in the weld, do you see any shiny little pockets in the broken weld?

By MDG Custom Weld (***)

Date 10-16-2008 21:07

From what you describe, I agree that the testing may be the problem. If the customer wants the weld to pull 24k in a particular direction, the testing should be in the same direction. Any partial pen fillet weld could fail if loaded in tension or shear long before it would fail in compression. Even if the weld is a gusset where half of it is in tension and half it in compression it will carry a much higher load. Having the joint in tension and acting like a hinge is surly going to net lower than desired results. I'm not sure that changing filler will gain you anything either. Go larger with your filler wire, that will reduce the size of the HAZ too.

By Bob Garner (***)

Date 10-17-2008 19:07

1fastguy,

I'm looking at the 2005 Edition of the Aluminum Association Design Manual. Table 2-21 Gives the allowable stresses for 6061-T6 fabrications. The allowable tensile stress on welded material is only 12.5 ksi (thousands of pounds per square inch). The allowable tensile stress on unwelded 6061-T6 material is only 21 ksi. This is the design code for aluminum structures. Keep in mind, these are stress values (per square inch) and not total load values (pounds). And they are for tension stress, not bending stress.

If your customer is talking in terms of stress values and not total load values, which I think he is because 24 ksi is the old code value for aluminum stress in bending, then your client may not know the appropriate code values. Also, bending stress values are always higher than tensile stress values.

Can you ask your client what code he is following? His "dream" values may not be attainable per the Aluminum Code.

Bob Garner

By Kix

Date 10-22-2008 18:05

D1.2 still requires 24ksi for tensile pulls with 6061-t6 and it is achievable in the as welded condition, but it's tuff. D1.2's last adition was back in 2003, so sounds like they need to update to what the aluminum association is specing because they know their stuff!