Projects 1st Alum Casting - What Went Wrong?

A quick update - I cut the second attempt and am happy with the pour. No occlusions or porosity!



The shrinkage on the bottom is very noticeable, now! I did some research on parting dust and found my use of diatomaceous earth to be a poor choice. Especially in a humid environment. It absorbs moisture very easily and mine was a couple of years old (used it in the garden). Probably why my casting surfaces were so bad. So, I’m going to try calcium carbonate on the next pours!

 

Shrink is usually a factor with an under- gated pattern. As someone suggested, having a well after the part is good for both contamination and shrink. If ur not doing production over gate the shit out of it, whether vertical or horizontal. Contact size can also effect flow(shrink) during cooling. Lotta factors involved in making a good casting.

Fyi- molds left overnight will usually accumulate moisture.

 

I don’t know much about casting, but have been interested in the process most of my life. My grandfather was a pattern maker and talked about some of the things in this thread. I believe that advice about a horizontal poor is good.

 

Hey, rsc;
Don't know for sure that the following will actually help, but when I was pouring eons ago, I found something that helped get better castings. What I noticed, was that when pouring, as I was pouring into the well, I watched the level of the Al in the well. I noticed that it kinda rose as if to overflow, & then sank(almost like a bubble - but it was metal temps changing & volume shrinking). I noticed that if I stopped at that point, the casting had some troubles. If I kept on pouring, but slowly enough to keep filling, but not overflow the well, stopping when it actually did fill up(looked like a small flat muffing-), the pours n casting were much better. IIRC, we degassed after running the temps to ~ 1300f, slicked off the scum & letting it cool just abit to ~ 1150-1200f. Seemed to work ok. This was on oil-sand, & while we used dusting powder for the mating sand surfaces of the cope n drag, I don't remember using it on the mold. Just screened the sand ultra-fine, & tamped the tight areas w/small tools/objects/wire/etc, until a thin base/covering was gotten, then rammed up w/the normal screened sand. Did lightly blow or even vac-out the cavities to get rid of loose sand, before assembling the cope/drag. & you do need at least two wells(depends on part-complexity) to slowdown shrinkage. You're on the right track. FWIW.
Marcus...

 

Good stuff, really enjoying this thread

 

My shop teacher back in high school explained to us the purpose of the vent. As has been mentioned already, shrinkage and if it’s still gassing off. A third thing he was talking about is since our sand molds are a bit cold compared to the aluminum, a properly sized vent allows the first bit of aluminum to heat up the mold cavity closer to the casting temp, which in turn does cool the aluminum below the ideal flow temp. The volume of the vent then allows room to keep pouring material to push the colder material out, allowing the correct temp material to flow correctly. He showed us two different castings, one with a good vent and one without. The surface finish was much closer to the sand texture, without the vent it was closer to the texture you have. If I remember right we were using green sand.

 

I've only cast low temp metals like Pewter but does anyone suggest baby powder. It was suggested to me and really helped.

 

Johnson and Johnson is being sued in a class action lawsuit because their talcum baby powder had asbestos in it. You can only get cornstarch based stuff now. Anyone know of a good source of the dangerous kind?..... Mexico?

 

https://www.amazon.com/Fiberglass-Supply-Depot-Inc-Powder/dp/B09PRQX8FD
You can still get Talc, just can't put it on your beaver.
Mike

 

Right away the discussion has to head there, typical for the HAMB.

 

Good stuff thanks for sharing your project and progress . I’ve never cast anything but this looks interesting.

 

Great thread Chris. Aluminum casting was the cornerstone of early hot rodding and it’s good to see someone taking on the challenge to make what they need. Following along and learning.

 

Well pour #3 was pretty much a failure! I’m still getting some surface porosity. Especially on the tops of the bosses and on the bottom of the casting.


Also, a lot of shrinkage through the body!



I guess I did improve the parting line. The surface texture would be quite nice if it wasn’t for the porosity. So, my ramming technique has improved, too. I think on my next attempt, I’ll have the input and exit sprues on the left and right bosses and continue with a riser on the middle boss. Adding some pinholes on the bottom might help, too.

 

Doesn't look too bad from here for an amateur attempt. . Try sanding it, maybe buffing. It will look even better.

 

Great to see you still around Rocket Scientist Chris.

 

Another update - I cut #3 at the two worst shrinkage areas on the body of the casting. There is some visible shrinkage on the tops of each boss next to the risers. But, I chose to concentrate on the two large areas. So, again, no occlusions and no visible porosity!



The cylindrical shape of the body is horribly distorted. The surface is the same as the two previous attempts: pocked and rough. My problem seems to be pointing more towards too low of a pouring temperature. Each of my pours has been at 700 degrees C. But, that temp has been according to my cheapy electronic thermometer. I’m not sure what the resolution is, but it wouldn’t surpise me if it was off 20+ degrees. I may need to invest in a real pyrometer!
From what I have read, increasing the pour temp will help reduce shrinkage and improve surface texture. So, for my next pour, I’m going to shoot for 750 C to see if that helps improve the results!

 

I'm not sure that is shrinkage . Maybe it's the picture but it may be sand collapse. It looks like the sand sloughed a bit as you poured. Shrinkage to me looks porous and has a different look like no sheen just flat.

 

Hmmm - I guess the only way to diagnose shrinkage vs mold sand collapse would be to very carefully separate the flask when removing the cast so the mold cavities can be examined. Something I didn’t do!

 

I think there is just too much mass to avoid the shrinkage conditions you are seeing. What you have done is improve shrink in some areas (evident in how the tops of the bosses appear flat). You may want to try adding feeder sprues in the shrink areas on the cope side of the mold. Remember as the part cools the metal is shrinking thus there needs to be extra molten alum that can be pulled into those thick sections. If you look at the caps in my avatar the wing sections are very thick at their ends compared to the the 1/4" thickness on the main part of the cap. I have two large feeder sprues that feed these wings otherwise the top surface of them would have big sink marks.

 

I think something like this may work for you

 

Another thing you could do/try, is in addition to needing to add another gate as an outlet, is to create a well ~ twice the dia of the gate & maybe one dia deeper at & below the 90* turn into the pattern entrance, same for the outlet. Does use more Al in the pour, but allows more shrinkage to take place there. Jethro's pic helps w/the visual idea, his is just taller. & place the cope/drag up near the furnace while melting, to heat it up, just not so close you trip over it handling the hot stuff.
Marcus...

 

I’ll be honest with you I have never cast anything in my life but I’m loving this thread! Just the fact that you guys can do this and all the science behind it is amazing, tweaking here and there; the inner nerd is coming out and I’m soaking it all in. A friend of my dad’s , Paul Kosma cast his own heads and fuelie intakes for his flathead Bonneville car “The ‘49er” and that blew my mind! What you are doing is truly going back to the root of hot rodding when you had to make what you thought would get you faster and lighter; the true innovators

 

I have not cast anything since high school back in the late '70's so I have no idea what I'm talking about but, could the aluminum be the problem? With different alloys out there could one shrink less than another?
And to tell the truth, it looks like the first couple of attempts you had less shrinkage, the body looked round and had the correct shape. If you are remelting over and over could this "cook out" some of the properties that control the shrinkage?
I have no idea on any of this, just thinking. Good luck on your project.

 

This is awesome ... dross, sprue, occlusion, diatomaceous ... the wife doesn't stand a chance next time we play Scrabble

 

This shrink is caused by the gate freezing before the casting. The correction is to have a riser with a greater volume offset a short distance away and fed from a separate choked sprue so the flow is controlled and doesn't wash sand. The riser should be the the hottest metal and the last to freeze so it feeds the casting.
I have been able to improve surface finish by dusting the mold cavity with locksmith's graphite.
If your Petrobond is a little tired, it can be refreshed with a little denatured alcohol.
I believe your pouring temperature is OK, I usually pour at 1250F.

 

Increase the size of the feed gate, make it the full width of the casting body, no reason to step it down. Add clear vents to each of the 3 bosses - dont just add a closed off nub like attempt 3. Make sure you have a good head of molten metal in your riser, and stop pouring once metal comes out of the 3 boss vents.

Good Luck!

 

Thanks, everyone, for your input! I definitely have a combination of issues (and a lack of knowledge!) My biggest problem seems to be getting molten aluminum into the mold quick enough and maintain heat so cooling is controlled. I’m starting to understand the issues with my #3 pour. I need more input flow and head pressure. This is evident by the lack of fill in the risers on the bosses. The risers were open to the surface of the cope, but only filled about a third of the way. Hence the “nubs”.

So, for the next pour, I’m going to rethink the input and exit sprues. Both will get taller riser tubes. And, I’ll keep the three risers on the bosses. I think I’ll bump up the temp, too. I place the flask faily close to my furnace. But, I have a feeling I’m loosing some temp during the crucible move and pour.

Oh! I’m going to try to get some graphite to line my mold cavity, too. It’ll be worth the potential mess if it helps smooth my casting surfaces.

 

Nothing to contribute, but The Choked Sprue's would make a great band name...

 

That is gas, you can buy a sintered graphite rod, partially drill it for a steel tube and thread for a hose fitting. You can dip that into the aluminum and purge with argon to degas the melt. Also when you pour, pour it like a beer to reduce the cavitation. In a sand casting maybe angle the main sprue.

 

You want the out sides of the pattern to solidify first and fast to maintain shape, if you are getting “shrink” (can also look like gas) then increase the size of the sprue runners for the solidification to pull from fresh molten metal.