How do I increase uniform elongation without losing yield strength in 5754-O? 2

[metals4j]

I'm trying to get 1.5 mm AA5754-O sheet to 19% minimum uniform elongation (consistently) while maintaining 105 MPa yield minimum. I can easily get one without the other, of course. I can get both, on occasion, but it's not consistent from sample to sample. At the very best, the yield strength is always borderline on the low side and the uniform elongation is borderline as well. Discontinuous yielding complicates the matter by adding to the variability of the uniform elongation results. I'd like to be "safely" within the specification. I've tried chemistry changes and rolling practice changes, and I've experimented with final anneal temperatures (within the limits of a batch furnace), but so far nothing has worked. I'm sure there are many things to consider, and I know I'm probably not giving enough info for specific answers at this stage, but I need to get some guidance and suggestions on which direction to take. Unfortunately, I don't have an R&D department or an extensive list of in-house technical resources on-hand, so I'm hoping there are some knowledgeable folks out there who can help! I'm certain someone out there is very familiar with this issue.

 

[TVP]

Are you purchasing 1.5 mm AA5754 already in the annealed condition? From a distributor or a mill?

 

[metals4j]

No, I'm not purchasing this material, I'm making the material. I work for the mill. I'm trying to create a 1.5 mm AA5754 in the annealed condition with the specified properties (215 MPa TS min, 105 MPa YS min, 19% uniform elongation min) for a customer. I don't have the experience or technical resources to answer the questions I have, and the current path I'm taking is full-scale experimentation using 50,000 lbs of material per trial run... After five or six iterations with little success, it's getting expensive!

 

[Maui]

What is your average grain size? Refining the grain structure through the addition of nucleasting additions to the melt or by adjusting the subsequent hot working practices and thermal treatments may help improve both the yield strength and the elongation simultaneously. For example, see

http://en.wikipedia.org/wiki/Grain_boundary_strengthening

Maui

http://www.EngineeringMetallurgy.com

 

[metals4j]

Thank you, Maui. So far my trials have produced final grain size of ASTM 7.5-8.0. We use TiB2 for grain refinement during casting, but I have not adjusted hot rolling practices beyond "standard" so far. I don't know which direction to go there - aim for a hotter coiling temperature? I've tried introducing a full anneal between cold mill passes, but that gave me the worst results. Both yield and uniform elongation were too low. Perhaps the subsequent reduced amount of total cold work (lower dislocation density) before the final anneal hurt my odds of getting good yield strength. But as for uniform elongation, I don't understand the mechanisms available that would allow me to achieve my goals. I had assumed that good total elongation leads to good uniform elongation. Is that incorrect, or at least, not necessarily true?
As for thermal treatments, my anneal practices are limited to batch process (continuous annealing would be ideal, I believe), but perhaps there is more I can do there. I've thought about raising the anneal temperatures, but will it help? Isn't there an increased risk of grain growth at higher temperatures (say above 650F)? Would there be a benefit?

 

[metengr]

metals4j;
In addition to the above recommendation regarding reducing grain size, the technical paper below may be of interest to you especially if you are making your own heats of AA 5754, and can optimize your alloying to achieve desired results;

http://iranscholars.org/wiki/images/8/87/RezaRouminaThesis.pdf

 

[TVP]

Maui is correct, you need to have a very fine grain size in the final annealed condition in order to have good uniform elongation together with a relatively higher yield strength. The way to do this is as follows:

1. Increase the amount of cold reduction prior to annealing. The increased dislocation density due to cold working will improve the recrystallization behavior. You already noted the problem of splitting the cold reduction over two passes with an intermediate anneal.

2. Recrystallization is also affected by the size, distribution, and volume fraction of dispersoid particles that are present in the hot-rolled coil. I'm assuming that you are using direct chill cast ingots (Aleris Lewisport, KY?) rather than strip cast product. What are you doing for homogenization prior to hot rolling? Have you evaluated the microstructure at this stage? Non-uniformity of the dispersoids will create excessive microstructural variation.

3. What are the composition levels of Mg, Cr, Mn, Sc, and Zr? All of these exert an influence either on dispersoid formation or solid solution strengthening, and hence final formability. I couldn't open metengr's link, but I am assuming it addresses some or all of these elements.