Metal Spinning Deformation 1

[Shahyan]

This shell made by metal spinning keeps deforming and I do not know why. There is no problem in the tooling. It is an issue with the material. I do not which mechanical property is the root cause. % elongation 38.4%, maximum tensile strength 426 MPa and yield strength 266.9 MPa. All three properties concur with the standard values. Any help in this matter will be highly appreciated.

 

[Tmoose]

Is the issue some un-identified "deformation" in the picture, or what looks like cracking at the inner ends of the 2 small crimps at 12:00 o'clock on the small diameter?

is the entire part spun from plate or sheet metal ?

Based on virtually no information I think I'd warm up the small diameter to anneal it before forming the crimps.

 

[SnTMan]

I thought deformation was the purpose of metal-spinning...

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[Compositepro]

I'm certainly not an expert in spinning but to me it appear to be a problem with the spinning technique. The material buckled because too much deformation was attempted in one pass. Another issue could be lubrication. Poor lubrication during forming could promote buckling. The poor surface finish tends to point to this. Spinning is primarily a stretching process. These are compression buckles. You must stretch the material lengthwise while avoiding excessive compression radially (or circumferentially).

 

[bobjustbob]

If you don't know what is causing the issue, how do you know that it is a material issue? Have you compared material certifications for lots of material that did not have the problem to lots that showed the problem?

 

[Shahyan]

Sorry guys if I wasn't too elaborative. The crimps you see in the picture are the abnormal deformation. I know there is something wrong with this material because I have changed machines and this same problem arises and when I change the material this problem goes away. There is no issue with the spinning technique and lubrication.

 

[3DDave]

Four numerical place values on a yield strength is suspicious. Try checking the material hardness.

 

[IRstuff]

You're essentially saying that the "different" materials have the same specs?

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[btrueblood]

Short answer - dunno, I let our spinning shop worry about the details, I just know that spinning is more of a black art than a science. But before blaming the material, I'd be sure to check:

Is this manual spinning? Same operator?
If machine (NC) spinning, same tool and buck (form), same rpm profile, checked the lubricant delivery? Do you see any signs of the material slipping/galling on the buck or at the clamp interface?

For the material I wouild check yield strength and elongation, initial material thickness, and rolling/forming grain direction of the two materials. Also confirm condition (as rolled, annealed, pickled...etc.)

 

[Compositepro]

What is the material you are working with? What do you mean by "change material".

 

[Shahyan]

By material change I mean I checked the mechanical properties of the material and the young's modulus varies from 15000 MPa to 22000 MPa. All else remains same. I haven't checked the strain hardening of the material. I just want to confirm whether my hunch is right and whether I should contact my supplier to replace the material.

 

[btrueblood]

" the young's modulus varies from 15000 MPa to 22000 MPa"

The 15 GPa modulus is a fictitious number, no metal alloy I've heard of has a normal value near that. 22 GPa is near typical for a stainless steel. How were the moduli determined?

It looks like you are doing a diameter reduction of a tube? That's a pretty tricky process, and local buckling due to high tangential forces (friction) is an expected failure mode. Try doing in-process anneals at finer reduction ratios, changing lubricant, using a roller rather than a spoon-type tool, or using multiple tools to reduce/distribute the tangential forces.

 

[Compositepro]

What is the metal alloy.

 

[IRstuff]

when I change the material this problem goes away.

You've not yet explained this part. What is the material change? How do you know you've change the material if you can't tell them apart?

If you look at the YouTube videos for spin forming, you'll see people using additional tools to constrain the free end, and something like that might prevent the buckling. What seems odd is the sharpness of the buckle, so possibly your tool is too small?

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[Compositepro]

The sharpness of the buckle is as should be expected. It is the lowest energy mode of buckling failure. Once the buckle starts, all of the elastic energy of the compressive stresses in the circumference will be released at the one point. There are other constraints like the fact that the compressive stresses are due to being attached to adjacent material that has tensile stresses and the metal has a shear strength. So a buckle can only relieve stress over a limited distance. This is why there are several buckles.

 

[IRstuff]

Thanks, CP

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[CWEng]

"I just want to confirm whether my hunch is right and whether I should contact my supplier to replace the material. "

What part of your purchase order/specifications did the material supplier violate that they should replace it? Or did you just mean buy new material?

 

[IRstuff]

Have you looked it who is doing the forming? Is there any correlation between former and buckling? From what I've seen spin forming is very much an art.

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[Shahyan]

The tests were conducted using a universal testing machine. Two rollers placed either side of the shell reduce the diameter. The materials come in different pallets. The first pallets material gets formed correctly, except for the other three.

1) Yield Strength= 244 MPa
Ultimate tensile strength= 408 MPa
Elongation= 39%
Young's modulus= 15.9 GPa

2) Yield Strength= 263.5 MPa
Ultimate tensile strength= 422.8 MPa
Elongation= 37%
Young's modulus= 18.5 GPa

3) Yield Strength= 263 MPa
Ultimate tensile strength= 421 MPa
Elongation= 37%
Young's modulus= 18.5 GPa

4) Yield Strength= 267 MPa
Ultimate tensile strength= 426 MPa
Elongation= 38.4%
Young's modulus= 22.2 GPa

All four materials are in within the standard range except there is no standard for young's modulus. The first material gets formed correctly only.

 

[bobjustbob]

If the material meets your specification then its on you. You cannot expect the supplier to be responsible for criteria that is not in the purchase order.

Do the properties match the material test reports from your supplier? If so have you reviewed past certifications to see if you have had material with similar mechanical properties? If you have had material with similar properties that did not show this type of failure then you need to start asking yourself what is different about this batch or what has changed in your process. If not then you need more data points for failure before changing your material specifications.

First I would look in my inventory to see what material I have on hand that I could test to ensure that I can turn the problem on and off. Barring that I would work with your supplier to see if they can provide you with some sample materials of different strengths to ensure that you have indeed spotted the problems.

Then comes the negotiations with your supplier, as you will need to tighten up your material standards. Anytime you tighten up your standards, your supplier will want to increase the cost and justifiably so.