Aluminum 7075 home made alloy, possible?

[lukewarm]

Anyone make their own aluminum alloys? I am interested in casting an aluminum part requiring high strength with little need for corrosion resistance. Something along the lines of 7075 sounds perfect. The data sheets for 7075 give the ratios for zinc (5.1-6.1%), copper (2%) and magnesium (3%). There are lots of other elements, obviously, but I think the bulk of them are for corrosion resistance and other things I'm not too concerned with. Does anyone see a flaw in heating up a crucible full of aluminum, and adding in these three elements (well ventilated and with respirators for zinc fumes).

Thanks in advance!

[kineticstep]

Melting point of copper is 1083C. Use an inert atmosphere to avoid a fire. A microwave oven and a silicon carbide crucible would be a quick starting point for an experiment, but it isn't easy to regulate the temperature. Don't look at the crucible contents without welding goggles.

[lukewarm]

After doing a bit of research I've come to the conclusion that even after casting under a protective atmosphere Magnesiums ignition temperature is likely to be reached during precipitation hardening. I will try substituting the Magnesium in the alloy by increasing the levels of Cu and Zn by 1.5% each and hope for the best.


For a Zinc and Copper based alloy do I need an inert atmosphere still?

I plan on degassing with a lance type degasser bubbling argon through the melt. Any problems anybody can see?

[kineticstep]

Once the magnesium is alloyed it may not be a problem.

[lukewarm]

You might be right but the possibility of a raging magnesium fire in my crucible scares me quite frankly

[T3sl4]

6061 has almost as much magnesium and I'm guessing you've melted that plenty of times already without a fire. Under 20% I would guess, magnesium alloys just don't burn, not unless you seriously overheat them anyway.

You could start with aluminum from the 1000 or 5000 series. The former is pure aluminum (give or take; 1199 is 99.99%), the latter is magnesium based (just add copper and zinc).

I recommend preparing a copper master alloy beforehand. Weigh equal parts copper and aluminum (both should be >90% pure, but beyond that, it doesn't matter much). Melt the copper and slowly add the aluminum. Wait for the charge to return to full temperature after each addition, until the aluminum is half gone. If you do not, you will be left with undissolved intermetallics and a poor alloy. Past the halfway point, the melting point drops steeply and overheating is no longer necessary. 50% master alloy consists almost entirely of "theta" intermetallic (Al2Cu), which is as brittle as glass, quite weak, and melts at 1186F, slightly less than pure aluminum.

Zinc can be added whole. As long as the melt is less than 1600F, it will not fume, though you will still notice a skin of yellow zinc oxide forming, characteristic of zinc-aluminum alloys.

More to the point, cast 7075, or 6061 or 2024 for that matter, are very weak. These alloys are meant for extrusion and rolling. Without any cold working, they will never attain the high strength they are known for. The best as-cast alloys are 300 series silicon alloys, 320 and 390 being the strongest, as strong as wrought 6061. The casting performance (fluidity) is far in excess of any other alloy, which is why the 300 series is completely dominant in cast aluminum applications.

Tim

[lukewarm]

Thanks T3sl4 for a obviously knowledgeable reply!

I'm having a hard time finding much info about 390 honestly. At least the proper ratio of silicion is eluding me. I was hoping to use the 7075 and age harden it but if its casted strength is indeed low then whats the point. I will continue to look for more info on 390 aluminum, thanks!

[T3sl4]

380, 390 and others in that range, and the associated hot-forging alloys in the 4000 series, are sometimes used for pistons and connecting rods. Names include hypersil, hypereutectic, etc. Since these have almost twice the silicon of typical casting alloys, they can be mixed half-and-half with other alloys (like 6061) to make something close to 320 (at your own risk of course).

The best alloy database I know is MatWeb:
http://www.matweb.com/index.aspx
You can even search by alloy composition, so if you look at >70% aluminum, >15% silicon, you'll see all the hypereutectic alloys. If you instead searched for >75% aluminum, 5-15% silicon, you'll find most of the hypoeutectic alloys. Many of the entries have physical data, so you can see how most aluminum alloys are weaker than iron, while the strongest (a lithium bearing 8000-series number, IIRC) is as strong as mild steel.

Tim

[lukewarm]

Thanks for the link Tim. Now I guess its time to make silicon... I have no idea where to even start looking for it to purchase.

[paprjam]

Copper will easily disolve into molten Aluminum. No need to heat it to Copper's melting point. I have made Aluminum Bronze, but not anything with Mg.

[motordude]

Hello,
about adding Mg, Zn and Cu. The melting of Cu is about 900degC, but that doesn't matter. You don't need to heat the Al to that high at temperature, normal casting temperature is enough. It's like adding sugar to water, the melting point of sugar is quite high, but it still dissolves in cold water. You can add copper as 100% pure copper if you like, it dissolves easily in molten Al. Zn drops to the bottom and dissolves fast. I normally add Mg as AZ91 alloy, but that is because I have 500 kg's of it in my shop, you can use any alloy. It will not start to burn and I don't use any protective flux. I have added the Mg along with the cold Al in the beginning of the heat, and they melt down together, or you can add it to the molten Al. I clean the melt with a Nitrogen lance.

I have also made Zn castings alloyed with Mg, I then add the Mg and Al when the Zn is molten, no fires!