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OutlawSmithy
09-27-2003, 01:11 AM
Can anyone tell by looking at this pic, whether this is cast aluminum or 7075?? The fella that makes 'em claims they are 7075! After I saw this, I put a small piece of 7075, .250"x1" in a vise and tried knocking off the corner with a 32oz ball peen hammer. Eventually, it did break, but, it bent some first. Now, just by looking at the pic, there are no signs of distortion at the break, which is why I suspect this is cast. Nor does it look like it was cracked, making the break possible. Any opinions?

http://www.rcpics.net/media/7075-broken.jpg

Evan
09-27-2003, 01:28 AM
7075 will bend, very reluctantly, with a very large bend radius. It is not exactly brittle but when bent too sharp starts to check on the outside radius with microcracks. It does depend on the temper. According to my Kaiser Sheet and Plate product information handbook, the minimum bend radius for 7075-T6 alloy is 3-5 thicknesses. I don't think that is 7075. It looks like cast aluminum to me. I don't know what alloy it might be.

WJHartson
09-27-2003, 02:24 AM
Could this be a forging? It will act differently than a casting. Can't tell much from the photo except to say that it doesn't look like it bent or distorted prior to breaking. Forging can do this depending on the heat treat.

Thrud
09-27-2003, 02:50 AM
Could be a forged part. The photos do not show all sides of the main or the snapped off part, but it appears that it has failed from a stress riser either at an inside corner, machined edge, or hole (probably threaded).

The part looks overstressed from the rather large hole (signs of hammering inside the failed hole mostly on the small broken piece).



[This message has been edited by Thrud (edited 09-27-2003).]

SJorgensen
09-27-2003, 03:30 AM
I have no real expertise here. I am a simple hobbyist that is learning what he can, and I am doing some simple castings. I use these castings and my CNC BP Rigid Ram milling machine to make my parts. I found some tiny inclusions in my metal so I looked for solutions. I found Russ Larsen at Larsen Foundry Supply in Salt Lake City and he was so very generous with his time, and also in showing me his warehouse and his products. I learned a lot about the different types of aluminum alloy ingots that are available and also the tempering ability of each of them. One thing that he said was that 6061 is not a casting alloy but an extrusion alloy. The alloys for casting are quite different from the ones for casting. Anyway these people don't advertise and they are a great resource. So I had to give them a plug. Russ Larsen gave me a bag full of ceramic aluminum filters and told me how to use them. This was at no charge, and so I am very grateful and very appreciative. I'm sure that he could easily see how thin my shoe leather is, and yet he spent 30 min. with me at no profit to him. When I am ready to buy a crucible or commercial furnace or casting sand, that is where I will look first.

My impression of the break that I see in the casting you have is that the grain is pretty course for a piece of aluminum that was expected to have a high degree of strength. I don't know that much about it yet, but I think that it looks like some of the castings that I make from the scrap aluminum that I cast. I don't have much experience with high purity aluminum or aluminum alloys. I would expect that a high temper aluminum alloy would have a finer grain and a finer break in failure than what I see in your picture.

Keep me informed,

Spence

Evan
09-27-2003, 03:35 AM
If you have access to a hardness tester you can probably determine what it isn't.

http://www.matweb.com/search/SpecificMaterial.asp?bassnum=MA7076

Doc Nickel
09-27-2003, 04:32 AM
There's almost no sign of bending prior to failure, and the exposed surfaces don't show any "tearing". It's a casting. No way it's 7075.

Doc.