Bronze Metallurgy

[AidanMc]

I have some questions related to the heat treating of a silicon iron bronze alloy (AMS 4616, similar to C65620). It has been an ongoing discussion at our facility for many years now. In fact, I asked a question on here a number of years ago relating to it (thread330-242181) and was provided a fine answer. However, it has recently become an issue again and I'm looking for more help.

When we turn this material from barstock, we run into roundness issues. Despite our best efforts at minimizing turning forces, we still see parts that will be out of round by up to about 0.006" diametrically. We've found that stress relieving the material can reduce this out of round below 0.001". What we're struggling with is the soak temperature for the stress relieving process. Based on my research, including the above mentioned thread, I've found that these silicon bronze alloys should be stress relieved at 500-600°F. This is the temperature that I've stuck to when asked about the process. I'm fairly competent with steel alloys but lack enough knowledge about copper based alloys to feel comfortable deviating from the recommended temperatures.

This is where we run into issues. We've had some people start stress relieving this material at 750°F. I've argued against this based on our lack of expertise in copper alloys: we don't know what unintended consequences there are from going to this higher temperature. It works better from the distortion perspective but I'm hesitant to use it because it may have other effects that we're not looking for.

So now I'm trying to educate myself more on copper based alloys and I'm hoping that the community here can help me out. Will there be any problems with going to this higher temperature stress relieve? Is there anything that we need to be looking out for other than the distortion and hardness (which we can easily check)? I appreciate any expertise that this fine community can help me and look forward to hearing any responses.

-Aidan

Aidan McAllister
Metallurgical Engineer
Automotive Enthusiast

 

[EdStainless]

I would suggest that you look at some basics.
Compare micros, and tensile properties. While you are at it check to see if any of these samples are magnetic at all.
If the 750F material has any odd secondary phases (especially along grain boundaries) there is your answer.

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P.E. Metallurgy, Plymouth Tube

 

[Jughandle]

Practically speaking, I would tell those who want the higher temperature (750F vs 600F) that they would be wasting company resources if the final requirements of the part can be met using less energy. (And I would document it in writing).

As to the more academic reasons, I am practically a novice with copper alloys so I would start by researching the strengthening mechanism for this alloy and what effect temperature has. Is there a phase change, precip coarsening, diffusion, vacancy effects, specific volume changes, plus the aforementioned characterizations, what is happening? Also find out what mill produces the stuff and talk to them.

 

[AidanMc]

Ed,
Thanks for the ideas. That's sort of what I was hoping to hear. I don't really know what the failure mechanisms would be here so I didn't really know what to look for. We're used to steel metallography though so it should be a big deal for us to make up a new etchant and try it out on this bronze. I wasn't sure that would be an effective technique because I didn't know what we should be looking for, thanks for the help.

Jughandle,
The issue with your first suggestion is that the higher temperature seems to produce better results, i.e. lower roundness after turn. Both stress relieve cycles help but the higher temperature helps more. As to your second suggestion, I've been trying to learn more about the alloys but can't find a lot of information on this specific alloy. It has a fairly significant (compared to steel alloying elements) addition of iron so I wasn't sure if anything I find for regular silicon bronzes (like C65500) would be applicable or not. I am also trying to get in contact with the mill that makes the bars but I have to go through our purchasing department and that's slowing things down so I'm trying to research anywhere that I can.

Thank you both for the time and the responses. Hopefully I'll get some more insight on here and be able to report back with results when I can find some things out.

Aidan McAllister
Metallurgical Engineer

 

[WKTaylor]

Aidan MC...

Based on the spec AMS4616... this bronze is so soft it's like trying to machining butter. OH Yes... and it is already supposed to be stress-relieved so the reasoning for a second SR heat-treat is baffling to me.

Questions...

Have You confirmed that the material certifications supplied with the bar in question are accurate and not bogus [junk]?? A second test by a qualified lab [wet chemistry, grain, etc... Plus basic 'mechanical tests' [FTu, FTy, E and e etc ... could verify that the material in hand is 'honest-to-god' what You paid for.

I cannot conceive of any application where this material would be machined. I suspect that AMS4616 it is intended for parts made by die-impact, extrusion or impact-extrusion processes.

Is the any absolute reason this material has to be AMS4616? We have an excellent track record using AMS4640 Al-Ni-Fe rod/bar/etc TQ50 or HR50 for precision small/large machined parts where wear/bearing/galling are critical issues. Also, for that matter, any conventional cold worked silicon bronze would far better during machining, than AMS4616.

Regards, Wil Taylor

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[AidanMc]

Will,
You are correct, this is supposed to be stress relieved. That's one of the reasons that I'm trying to go back and get in touch with the barstock supplier: I want to understand what they're doing and when the stress relieve is in their process. We started using the stress relieve before my time here but probably we were having some issues and someone thought "Well, let's try this" and it worked. After it worked once, we just started applying it across the board. To be sure, it's a band-aid of a practice and remedying that is the long term goal of my project. Short term though, I just need to make sure we're not going to do something detrimental to the material.

In response to your questions:

This is an ongoing problem that isn't limited to a single bar or heat of material. We check on an audit basis by sending things to an outside lab to verify properties and have never had an issue. I don't think we're getting duped with the material we're buying. I think it's just a poor choice for the application.

Speaking of the application, there's no chance we can make a change. This is an aerospace part that has already been approved with engine testing and goes to one of our very picky customers who (to put it politely) are not amenable to changes. They have the design authority so we cannot make the changes to the design of the part and we get stuck making it. It's not a good choice but I have the unenviable task of trying to make it work anyway.

I appreciate the suggestions to help me diagnose our problems, thanks!

-Aidan

Aidan McAllister
Metallurgical Engineer