High Yield in welded 6061-T6

Date 05-05-2015 15:40 Edited 02-01-2016 23:14

My question is very general and many. We are doing tensile testing on some 6061-T6. We have 3 PQR's one done with AC, one with DC-, and one with AC/DC-. These plates have been heat treated back to T-6 condition. Now before we did these plate we were sent samples to pull and we could not reach the yield the customer was aiming for in the 34K+ range. We did a set of tensiles exactly like D1.2 states to cut them and got into the mid 24K range for yield. We are using a a united testing tensile machine computer aided with extensometer. After we got those results we changed the dimensions to narrow up the sample. From a reduced section thickness of 1.5" to 1" and another to 3/4". The 1 inch and 3/4 both yielded above 34K. Why is that? Also will a shorter reduced section cause higher yield? Lastly if the whole plate is theoretically welded the same, should they all be proportional to their area in the reduced section?

A bit towards metallurgy question: If aluminum is welded hot and gets too hot it can become brittle in the weld zone. Now if its taken back up T6 in heat treat, will that phase change "fix" the over heated weld? or will the weld still hold some qualities of the less than favorable welding? Basically can heat treat fix anything structurally that would normally fail in the as welded condition.

Thanks
-Jay

By kcd616

Date 05-07-2015 16:43

I will let Lawrence take this
have my ideas, but only my ideas
because I will be wrong :red:

always let the best answer things
sincerely,
Kent

By MRWeldSoCal (***)

Date 05-08-2015 18:53

Thanks for looking at it Kent!

By Lawrence (*****)

Date 05-11-2015 18:21

Typically when aluminum is "overheated" brittleness is not the issue... Rather it's opposite.

"Lastly if the whole plate is theoretically welded the same, should they all be proportional to their area in the reduced section?"

It's doubtful that your whole plate is welded the same. Heat travels QUICKLY through aluminum... The heat input and HAZ at the beginning of your PQR sample will probably have a much different performance than the end of the same weld.... This is especially prominent with GTAW.. GMAW travels much more quickly, so the material performance can be a bit more consistent.

I cannot speak to the difference in sizing of your tensile samples...

Heat treatment on your T6......... Unless you used a heat treatable filler, the weld zone will not respond to the heat treatment... Your engineer should already know this and plan for it.

If the weld is "overheated" to the point that it is filled with Hydrogen (a rough pebbly surface is an indicator) no amount of treatment will fix this... remove the bad material.....

Also... One last admonishment.... You can take all kinds of precautions to get a PQR to obtain the required mechanical properties (in a lab).... If you jettison any of those precautions in production... You PQR is a lie and you might as well put it in the trash.

If you expect your production work to have mechanicals that comply with your PQR... It's important to plan you production work with the same care you plan your PQR.

By MRWeldSoCal (***)

Date 05-12-2015 16:32

Thank you!

Now this PQR is on a 1/2" plate so wouldnt the heat still be the same across that plate? I could see that being a point on a single or maybe even a 2 pass plate but these have many passes.

Would then an over heated weld hold up longer in tension then if its the opposite of brittle? Therefore adding yield?

Lastly curious where I can read more about the Hydrogen in the weld creating the bumpy surface. Ive seen it before and didnt know that was a sign of that. Would be great to know more!

Thank you very much for your insight

Jordan

By Lawrence (*****)

Date 05-12-2015 18:11

Now this PQR is on a 1/2" plate so wouldnt the heat still be the same across that plate? I could see that being a point on a single or maybe even a 2 pass plate but these have many passes.

Controlling heat over many passes is a different challenge than on a single pass... I'm not sure how to explain to you the thermal conductivity with aluminum... Heat builds and grows in the direction of travel.. The end of the weld will be hotter than the start. When you get to the end of a joint, no matter if it is thick or thin, there is less mass for the heat to run to. Increased travel speed can mitigate this to some degree.

Would then an over heated weld hold up longer in tension then if its the opposite of brittle? Therefore adding yield?
NO ! Brittle material (typically) has greater tensile strength (holds up longer in tension) than softer (more ductile) materials.

Lastly curious where I can read more about the Hydrogen in the weld creating the bumpy surface. Ive seen it before and didnt know that was a sign of that. Would be great to know more

Aluminum being soluble to hydrogen when it is molten is no real news.

Way back machine... 1974 Welding Journal
https://app.aws.org/wj/supplement/WJ_1974_03_s97.pdf

ASM Handbook Vol 6 has more data.

By MRWeldSoCal (***)

Date 05-12-2015 18:36

Well thank you for all the information! I have been welding for 12 years or so and I understand the thermal conductivity. I have experienced it many times, lightening up on the pedal to avoid washing out of the puddle. Again, I just though that a thick plate would be pretty evenly heated on the last few passes.

Ill read up more, Thank you

Jordan

By 803056

Date 05-14-2015 21:36 Edited 05-15-2015 15:15

6061-T6 is a heat treatable alloy. It will overage rather quickly if held at temperature during the welding operation. Over aging will reduce the mechanical properties rather quickly.

Remember: "Aluminum, almost a metal."

The "rules" that apply to most metals do not always hold true for aluminum alloys. In this case, while the tensile strength and yield strength of the aluminum are degraded when it is over aged, the ductility does not increase. Instead, the UTS, YS, and ductility are degraded. Ductility, as gauged by elongation, is only on the order of 7% if everything is done properly. It can be worse when things are not done properly.

It is due to over aging that preheat is not recommended when welding heat treatable aluminum alloys. Instead, it is best to cool the weld joint between weld beads to maximize the mechanical properties by minimizing time at temperature and thereby minimize the chance of over aging.

Best regards - Al