A356-T6 internal stresses 1

[KeithSeeley]

We purchase tubular A356-T6 castings, nothing special about their geometry, and experience movement when we bandsaw cut through the casting wall (ie. .015" collapse in a 11-1/4 OD x 8-1/4 ID). We've used 713 previously and did not have this problem.

Is A356-T6 known to have internal stresses? I'm trying to determine if it's a processing issue on the part of our vendor or if it's just the nature of the alloy.

TIA

 

[tbuelna]

KeithSeeley,

You mention this is a "tubular" casting. Does this mean it was produced using a continuous casting process? Or is it just a sand casting in the shape of a tube?

A356 aluminum is a very castable alloy, and usually is very stable dimensionally if stress relieved and heat treated properly after casting.

Good luck.
Terry

 

[arunmrao]

0.38mm is the wall movement. Are you certain,that this was not a pre existing defect and has formed only during the process of cutting?

 

[KeithSeeley]

Thanks for the responses.

This is a sand casting made using a wooden pattern. It is a simple OD x ID x Length, for example 11-1/4" x 8-1/4" x 3", with no unmachined surfaces on the finished part. After the part is machined it is checked on a mandrel, then band saw cut in half for installation. When we attempt to put each half back onto the mandrel we find that the part is collapsed smaller than the machined diameter and thus won't fit without forcing it.

In order to determine if the dimension change was due to stressed induced by machining we parted off 1" and bandsaw cut just one side. The kerf of the bandsaw blade is about 1/32" and we measured it on the part at .015", therfore we think it's due to internal stresses in the casting/tempering process.

I really just want to make sure that I'm not chasing a problem that is inherent with the A356-T6 material. As I stated previously, we've successfully used 713 in the past with no issues.

 

[arunmrao]

A few curious questions. Mold and core materials . Was the core collapsible,if it was an organic binder as the core volume is large compared to metal thickness,heat liberated due to burning could have an effect. Finally did you factor for the blade thickness while sawing ?

 

[blacksmith37]

What about the T6? Solution/quench anneal and age;I would think would change /remove residule stress from casting. Or,if it is going to get T6 later , give it a slow cool anneal now to remove casting stress.

 

[swall]

In my experience, castings given a T6 treatment will have internal stresses that manifest themselves during machining, cutting, etc. Have had many a 355-T6 fuel body pinch the bandsaw during sectioning for metallogaphic examination.

 

[CoryPad]

I second what swall said. Band saw blade pinching when cutting heat treated aluminum castings has been frequent.

 

[KeithSeeley]

arunmrao, balcksmith: I do not know the precise casting/treatment methods used by our vendor so I don't know the answer to your questions.

swall, CoryPad: It's funny you mentioned "blade pinching" as that's exactly what our machinist called it. I guess my question becomes: Is there something I can suggest to our vendor to reduce this issue or will I simply have to live with it?

Thanks for your responses.

 

[CoryPad]

I think your options are limited unless you spend a lot of time and money. You can control the entire process more precisely. For example, dimensions and placement of the pattern/insert; pouring and gating of the liquid metal; part orientation during quenching; ratio of part mass to quenchant mass. You could add a thermal stress relief step, but that could lower the mechanical properties.

 

[MikeHalloran]

A356-T4 may distort less. Check the properties, of course.

If bandsawing is the only issue, use a larger blade and/or more set in the blade's teeth.





Mike Halloran
Pembroke Pines, FL, USA

 

[MikeHalloran]

If the reported pinching is actually from part distortion, then more set is indicated.

If it's from overheating of the part, or the sawblade getting clogged, you can probably double or quadruple the blade speed to good effect. At least measure the blade speed; machinists like to run their tools really slow, so they'll last a lifetime. If the machinist is buying the sawblades, that's understandable. But if the company is buying the sawblades, they should be treated as a consumable, and consumed.



Mike Halloran
Pembroke Pines, FL, USA

 

[KeithSeeley]

I'm getting some samples in a T7 temper. Material "book" properties look close enough so this could resolve the issue.

Thanks all. Appreciate the assistance.