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Old 02-16-2018, 09:03 PM
cliffrod cliffrod is offline
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Default Work hardening

I've been looking but haven't found the answers or info I think might be in the smarter minds here-

Comparing work hardening rates (not sure how to refer to it) of various materials- CRS, aluminum alloys, copper, stainless- is it most related to

1. the number of cycles of manipulation (hammer strikes, flexes or similar)
2. The molecular composition or crystalline structure of the media
3. A point of yield (fragile/crack) or obstruction (thickening)
4. Some combination of these things, possibly still with one predominant aspect

As a specific example, I'm Trying to understand how something like stainless, which work hardens significantly and quickly, is made into something like a deep pot or bowl.

If a large tonnage press accomplishes the task with a single stamping motion or one that only uses a very few stamping cycles, what happens at the molecular level? Does the metal become as work hardened during that single stamping action as it would if it was struck 100 times or more with a regular hammer swung by hand? If it does or doesn't in stainless, what about other metals?

And if the metal does work harden as much during that single stamping action, how is a failure such as tearing or cracking avoided? Does the blank simply need to be of an adequate initial thickness so that it is not stretched too thin- maybe some limit to draw or thinning? Would a failure (crack or tear) occur because of the metal work hardening and becoming resistant to further movement or because it was simply stretched too thin even if it was still not work hardened enough to fail for that reason?

I'm not talking about annealing during this forming process, which would impact the overall action & reaction as well as what might be possible in terms of form realization. Just trying to better understand the actual process of work hardening alone.
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Old 02-17-2018, 12:31 AM
leoitch leoitch is offline
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Cliffrod,
firstly, i am no metallurgy expert by any stretch of imagination but..

i think for a small studio with mostly hand tools and small power tools the main concern is concentration of stress when thinking about cracking due to hardening.
if you can work the metal evenly then you can form pretty extensively without cracking. of course once the metal work hardens it just takes more effort to work it.
unfortunately, when hand working it is quite hard to make sure you are hammering at same strength at every blow and blows are spread evenly. even working with power hammers it is difficult to make sure blows are spread evenly.
once you have a spot that has been overworked then that becomes a weak spot and any further work will concentrate stresses there.
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Last edited by leoitch; 02-17-2018 at 12:33 AM. Reason: vocabulary
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Old 02-17-2018, 01:31 PM
billfunk29 billfunk29 is offline
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Default Work hardening

There are a lot of variables in work hardening. Strain rate is important. Metals will behave "superplastic" when formed at a very high rate. Explosion forming, for instance.
https://www.youtube.com/watch?v=tm9V...9OQ-0z4hlwwb_M
Stretching by hammering and making the metal squirt out the sides is rather inefficient. Pulling in the direction you want to stretch is more efficient and will work harden slower.
These pop cans have a lot of stretch, very fast, with no annealing.
https://www.youtube.com/watch?v=V7Y0zAzoggY
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Old 02-17-2018, 01:41 PM
Ken Hosford Ken Hosford is offline
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In Bills posthe mentions pop cans , they have very special aluminum Very largecrystal structure [my father is a metalurgist and did extensive reshearch on subject ] The diameter of the top has gotten a lot smaller over the years , the reason is it is thick . Cans have reduced in weight almost to 1/3 of originals
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Old 02-17-2018, 11:31 PM
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https://www.engineering.com/Advanced...roforming.aspx
A good overview of two methods for the controlled forming of ductile metals with predictable and consistent results.
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Old 02-18-2018, 07:18 AM
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Link to a slideshow on sheet metal forming processes. This simple to view and straightforward presentation of 34 different processes for metal forming takes just a few minutes. The illustrations a professional and easy to understand.

https://www.slideshare.net/sahilslid...rmingprocesses

Those of you interested in making tooling, jigs, and bead roller or press brake dies or hammerforms will get some ideas from this show.

Note on draw forming (making parts such as a kettle in a punch press). The natural inclination is to assume the metal has been stretched to shape. That is most often not the case. Draw forming pulls metal into the die as the punch descends. The "draw ring" retaining the flat blank of metal is engineering to allow the material to slip or be "drawn" (pulled) into the die cavity as it is formed. Stretching of the formed part is avoided as tearing and weakening of parts is often the result.
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Old 02-18-2018, 08:02 AM
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Listed below are numerous videos on "raising processes" to form vessels. Raising is a hand forming process that is similar to "draw forming" on a punch press. Similar in that the metal being formed is not thinned by stretching, but moved by repeated hammering to alter the metal's form.

https://www.youtube.com/watch?v=kKCca2Fug4E
www.youtube.com/watch?v=7XE23W2SvJI
https://www.youtube.com/watch?v=JrQ7pOoE__U
https://www.youtube.com/watch?v=hJOx7c_qwys
https://www.youtube.com/watch?v=hJOx7c_qwys

The below two videos are in Japanese. However, observation of the work being accomplished tells the whole story.
https://www.youtube.com/watch?v=ZrATLKHfWP0
https://www.youtube.com/watch?v=7L2V_Qes3Hg

I realize this raising process is of most interest to the "artsy" metalworking craftsmen. Many of the techniques crossover and will be useful to those with automotive interests.
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Old 02-18-2018, 09:58 AM
Ken Hosford Ken Hosford is offline
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I am able to form both stainless steel and titanium successfully with doming dies by bottoming them out with very small amount of difference in radii , basicily forming only slight ripple to be squashed out not a tuck as in forming with a hammer. Doing it this way I do not over work the material , it puts it into a very low angle deflection and leaves vertually no marks. I also can for brass this way without annealing . With the titanium I was a so many die steps into it before it move at all I thought it was not going to move because I would have had a lot of shape in steel by that point . By doing it this way it put the metal in compression and seem not to work harden as badly
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Old 02-18-2018, 12:04 PM
leoitch leoitch is offline
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Richard,
speaking of raising...
i studied under David Pimentel of Arizona State University and he was THE MAN when it comes to raising, especially copper.
he pioneered raising with modern plastic hammers and stakes (UHMW and such). unfortunately he passed away of cancer couple of years after i graduated.
he was not an ambitious man and did not try to get his name out beyond those in the know. there's not much about him in Google but here is a link to a blog from someone who knew him. you can see examples of his work.

http://lynetteandreasen.blogspot.my/2008/10/dave.html
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Last edited by leoitch; 02-18-2018 at 12:18 PM. Reason: grammar
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