How long to wait to machine for newly cast aluminum?
So, I've been working to make this idea a success. I'm getting to the point where I'm having some fairly good successes, but I'm noticing that the small test pieces I've been making have been fairly brittle when I go to machine them.
I learned fairly early that the legs need to be tied together within the log or they'll work themselves out due to the interior of the wood being (essentially) charcoal, so I ensure there's an internal channel of at least .375" diameter. Today I was cleaning up a test piece I cast yesterday and when the belt sander grabbed it I snagged it by one of the legs. As I caught it I felt the channel inside twist and yield, now I no longer have a leg attached to what was going to be a stool for my niece.
Now, even a relatively weak aluminum alloy should be able to withstand as much torque as I put on this, but it broke like it was balsa wood.
The aluminum was obtained from a transmission case and a cast oil pan, still dialing in this furnace so we ended up pouring a little hotter than we wanted to at about 1600*F instead of 1400*F, however previous tests were in a different smaller furnace that has a better control of temp and we were hitting 1400 within about a 25* +/- window and still noticed it was fairly brittle when machining. Both drilling and milling, and in one case when I pressed a brass rod into one piece
I'm wondering if I need to age the aluminum, or do something else to strengthen it.
Last edited by Mr. Asa; 03-10-2017 at 01:22 AM.
Do you have a picture of the break or is it buried in the wood? You're right, the metal shouldn't have snapped like that- if it wasn't compromised. I would think 3/8 is a pretty small channel for the metal to travel through wood for any distance so you might not have completely filled it. Even if it filled completely the charring wood may introduce a lot of gas into the metal making it porous.
Sorry I can't address the main question of your post, others here are more knowledgeable about it, but I'm interested and didn't want to see thread get buried.
I haven't removed it completely as I want to try and repair it if possible, so no I don't have a picture but I can see where it is. Up to that point I had an open channel on the surface of the bench seat with roughly a minimum area of .5" wide x .75" deep and then for aesthetics it went down to a channel about .375" x .5" then a hole of .375" diameter as it went through the wood (maybe for an inch or so of length.)
So where it broke is probably that minimum size, but upstream of that it's fairly hefty.
From what I've seen of previous pieces I've cut apart, you can find a small amount of pourousness at the edge of the cast. It's a very small amount, though. Maybe one bubble about the size of a pencil point, max depth of 1/16" away from the surface of where the aluminum froze against the wood, and dispersed maybe 1 per every 1/4" or so. The worse your venting is in this style project, the more porous your cast is, but on this one every vent was filled so I don't think that was an issue. If I were engineering with a piece of metal of this porousness, I'd assume there would be issues there, but not for a stool.