Machining Question

good idea.

I make some hillarious measurement mistakes. For whatever reason, the various number of decimal places on measuring tools is what throws me off.

Not that 7075 is corrosion-resistant , just that it can indeed be cast by a home shop foundry . I've cast a couple of pieces with no problem - in fact it casts nicely and machines well if you chill it in water straight out of the mold then let it age for a couple of weeks . 6061 is very gummy especially when freshly cast , and 356 is sweet to work with . As far as copper in aluminum , we try to avoid it . It does indeed cause problems , mostly in the machinibility of the items . A trace is no problem , but get too much in the melt and you'll find that it's nearly impossible to machine , even with carbide tooling . Some of the best bronzes are mostly copper with small amounts of al , ampco 45 for example and they too are difficult to machine . I'm not set up to weld aluminum , and cannot speak to the weldability of any of the alloys . When I need that knowledge I'll be looking for info ! -- Snag

Ok, a little deeper digging: 7075 develops brittle microstructure and intermetallics when cast. There have been many professional experiments with it, including rapid chilling and squeeze-casting (not really full-melt casting), but they have been abandoned as far as I can tell.

An application that requires wrought 7075 requires yield strength on the order of 73,000 psi or higher. The best figure I've found for cast alloys in the 7xxx series is 30,000 psi.

I have some more checking to do when I get a chance. But what I think you're getting with your castings is an alloy that may machine well, but which has low strength compared to any wrought 7075, plus 7075's limitations: poor corrosion resistance and low ductility. Intermetallics frequently aid machining. They also destroy strength.

The alloy was developed by Sumitomo for making fighter aircraft in the

1930s, similar to our earlier development of 4130 steel. Since then, other aluminum alloys in the 7xxx series (and 7xx.x for casting alloys) have been developed, and they deal better with individual limitations of 7075.

That's not to say you can't make useful castings with it. If it works for you, great. The copper level is not high, so it may avoid the hot-short issue and internal voids that make 2024 so difficult. Porosity is an open question. But it seems unlikely that you're getting a result with any more strength than 6061, and with less ductility.

I'll let you know if I find anything else. Not today, though. The trouble with the information available is that 7075 is not a casting alloy so you aren't going to find a lot of scientific information on it. And I don't think most hobby casters do enough testing to know what they have.

California prohibits (most) detachable magazines, so people work around this by using a "fixed" magazine that requires a "tool" to detach the magazine.

It so happens that the mechanism some people use fix the magazine is with a fastener that requires a tool that is of the same shape as a typical bullet, ergo a "bullet button".

Jon

Half a pound isnt shit.