Annealing 6061-T6

Date 07-13-2006 14:18

We do alot of aluiminum welding also. mostly plate. our attachments to our vessel is 6061 and it cracks everytime we bend it at a 90 degree angle. never thought about annealing it prefab, but i will try that. sometthing about the ductility in the 6 series doesnt allow it to bend at 90 degrees. however, the 5 series (5083), can be bent at a 90 without annealing. very helpful, thanks.

Shaun

By 803056

Date 07-23-2006 16:05 Edited 07-23-2006 16:56

Hello Fellas;

Don't you just love working with aluminum?

While heating the 6061 aluminum will soften it and allowing for easier bending and forming, consider what is happening to the mechanical properties. You may be overaging the 6061-T6 and reducing the mechanical properties of the part being manufacturered. To what extent is the big mystery.

Another consideration is wall thinning due to the bending operation. The extrados (outside radius) of the bend will elongate (stretch) and become thinner, while the intrados (inside radius) will compress and get thicker. ASME B31.1 and B31.3 has formulas for calculating the maximum permissible thinning of fabricated bends for pressure piping applications. Clearly the ASME codes may not be applicable in your situation, but the degree of thinning is sometimes overlooked by designers and workers that are not familiar with ASME pressure piping codes where such consideration is required. Too tight a bend radius can result in excessive wall thinning , the possibility of cracking the extrados, or crushing and corrogating the intrados. Increasing the bend radius will usually alleviate the problems mentioned.

Regarding the bending of plate materials, it is true that the 60XX aluminum alloys are not as ductile as the 50xx alloys, but it can be bent to 90 degrees without cracking. There are two things that come to mind immediately. make sure your bends are perpendicular to the direction of rolling. Second, make sure you are not exceeding the limits of elongation for the material being bent. Materials with high ductility can be bent using a small bend radius. Those materials with poor ductility (I consider 6061 aluminum to fall into this category) requires a large bend radius.

The ductility of a material even comes into play when qualifying a welder or procedure and bend tests are performed. The bend radius is a function of the thickness and ductility (elongation) of the material being tested, be it filler metal or base metal. AWS B2.1, figures II-5A, 5B, and 5C, includes footnotes that provide formulas for determining the proper bend radius when the elongation is known. Aluminum alloys, high strength steels, etc. may require bend radi that are different than the standard 3/4 bend radius used for most 3/8 inch thick test samples of carbon steels listed in AWS D1.1.

One of my first aluminum projects involved a window washer unit for a high rise structure. Two men were killed as a result of the window washer failing while they were using it. The bottom line was that I was hired to witness and report to the engineers and lawyers how it was fabricated and welded by the manufacturer. They too (the fabricator) used an acetylene torch to "blacken" the aluminum (6061-T6), preheated it to burn off the soot, then welded the components using short circuiting GMAW. They didn't preclean the components properly and a host of other practices that I considered to be unacceptable.

My prediction was that the test unit to be load tested to failure would not carry the design load, never mind the proof load (which includes the safety factor). The unit failed at 48% of the "safe working load".

Our investigation didn't bring the dead workers back to life, but the widows and children didn't go hungry or homeless once the laywers got done with the fabricator and the insurance carriers.

Best regards - Al

By cryogenicshaun (**)

Date 07-24-2006 11:26

Very well put Al. What if a large bend radius is not possible? Would it be safe to say we would probably need to go with the 5 series? We are bending 6061 at a very short radius 90, thus making them all crack. These are various gussets on our transport vessel. We are repairing them as we go, but the down time and extra time is killing us as far as schedule. The info you have give really helps my debate with the owner. You touched on some very good points. As far as your first question, "Don't you just love working with aluminum?", my answer would have to be NO! HA! Thats probably my least favorite out of the materials we use. but hopefully it'll get better.

thanks,

Shaun

By 803056

Date 07-24-2006 15:42

I enjoy working with it because it does challenge me.

I've trained many welders to weld aluminum and the same problem comes up time after time, cleaning and keeping everything used to work the aluminum clean.

I would suggest looking into the mechanical properties of the alloy being considered. Elongation as well as the tensile and yield strengths are important. However, due consideration must be given to the type of alloy to be used; heat treatable verus nonheat treatable alloying systems. The alloying elements used can affect weldability. Some of the high strength aluminum alloys, 7075 for example utilizes zinc as it alloying consituant, resulting in a non-weldable alloying system.

Some mechanical properties of alloys such as 6061 degrade in the weld and heat affected zones. Consider the as wrought condition of 6061-T6, the tensile strength is on the order of 42,000 psi. Once it is welded, the materials properties are considered acceptable if the tensile strength is 24,000 psi.

The AWS Welding Handbooks has some good information on aluminum, but my favorite reference is WELDING Kaiser Aluminum. I use the Second Edition published in 1978. Dated, perhaps, but I had a heck of a time tracking down a copy when I found this one a few years ago. Maybe E-bay or Amazon are two possible places to start a search if you're interested.

Here's a few examples of the mechanical properties pulled from my Kaiser reference book:

1100 welded to 1100 aluminum using 4043 filler metal TS - 13,400 psi YS - 6,000 psi Elongation - 21%    compared to unwelded 1100-H16 - TS - 19,000, YS - 17,000   Elong. 4%
3003 welded to 3003 aluminum using 4043 filler metal TS - 17,000 psi   YS - 8,300 psi Elongation - 19%   compared to unwelded 3003-H16 - TS - 24,000, YS - 21,000   Elong. 4%
5050 welded to 5050 alumnium using 4043 filler metal TS - 23,000 psi   YS - 12,000 psi Elongation - 16% compared to unwelded 5050-0 - TS - 14,000, YS - 5,000   Elong. 22%
5052 welded to 5052 alumium using 5356 filler metal TS - 30,000 psi   YS - 15,000 psi Elongation - 18% compared to unwelded 5052-0 - TS - 25,000, YS - 9500   Elong. 20%
5086 welded to 5086 aluminum using 5183 filler metal TS - 39,000 psi   YS - 16,000 psi Elongation - 21% compared to unwelded 5086-0 - TS - 35,000, YS - 14,000   Elong. 18%
6061 welded to 6061 aluminum using 5356 filler metal TS - 30,000 psi   YS - 19,000 psi Elongation - 13% compared to unwelded 6061-T6- TS - 30,000, YS - 25,000   Elong. 8%

The values reported by Kaiser are on the high side and are for welds that are "perfect". The values of the welded tensile strength and yield strength seem to be higher than the requirements of AWS B2.1. However, my point being that the mechanical properties of the weld are related to the filler metal used, the welding process (time at temperature for some alloys), the alloying system, and whether the aluminum alloy is heat treatable. It difficult to predict how the mechanical properties with change when the base metal is welded. So, bottom line, you need to have a source of good information and use your references before making you decission on what alloy to use and what filler metal to use. Personnally, I use AWS B2.1 and D1.2 for welded aluminum alloys and I use Kaiser as a starting point when working with aluminum in general. The AWS literature does not always agree with the information published by Kaiser, but that's not entirely unexpected in the wonderfull world of metallurgy.

Nonheat treatable alloys are rolled to increase their strength. Generally, they lose that added strength when welded. Heat treatable alloys are generally precipitation hardened. Welding usually overages the heat treatable alloys and degrades the mechanical properties. But don't bet your house on it unless you know exactly how it is being welded, what filler metal is being used, how it is being cleaned before welding, what post weld heat treatment is going to be performed, etc.

As for bending aluminum tubes, look at the elongation values. Those alloys having low elongation values are going to crack when bent to a small radius. You may have to experiment to find a radius and bending method or equipment that will work for your application. I have to imagine that different bending machines that utilize different mandrels and shoes will produce different results. If your application involves a large number of bends, an investment in equipment is warrented. If you are a job shop with only a few parts to be produced on a short run basis, it may be wise to look at a change in material to accommodate the equipment you have. As for bending plate materials, the formula given in B2.1 for bend testing welded aluminum samples should work for both parent material and welded materials. The formula allows you to use the thickness and elongation values to calculate a bend radius. Theres a lot of variables to consider, more than can be considered in this forum.

Good luck - Al

By Lawrence (*****)

Date 07-24-2006 16:12

Here is a nice bend allowence chart that provides data for 6061-T6 radii
http://www.precisionsheetmetal.com/home/materials.htm

Here is another article from "The Fabricator" that echos some of Al's advice
http://www.precisionsheetmetal.com/home/materials.htm

Besides, if you are weld repairing all bends on your 6061, none of the repaired brackets are T6 any longer.

Better is the advice to select an alloy that is more ductile. You will also gain strength via the work hardening of the bends as you fabricate them.

By jwright650 (*****)

Date 07-24-2006 17:56

For bends that are too close to 90* or beyond couldn't you just make up the joint with two plates vs repairing all of the cracked bends? Just a thought.

By vacuumbikes (*)

Date 07-24-2006 19:57

how wise would it be to attempt to only anneal the aera to be bent in hopes that the loss of strength from grain growth would be cancelled from the dislocations formed in cold working? Aluminium is a bit conductive.

By 803056

Date 07-25-2006 01:59

Some concise information in one location. I like it.

Al

By rebekah (**)

Date 07-25-2006 14:31

I am working overtime on a 6061 Aluminum battery frame (9 days straight so far) that ships out in 5 days. I was just browsing when I clicked "Tricks of the Trade"--I wasn't expecting a dissertation on the inner workings of Aluminum! Wonderful and very helpful information ya'll, definitely appreciate it.

Rebekah