Extreme shrinkage of tin alloy?

[yondering]

I have a mystery metal alloy, which I thought was approximately 2/3 tin, based on density. No idea what else is in it, or what it came from. I got it from a pile of scrap lead somebody had melted into frying pan sized ingots.

It gives my ww alloy the right flow properties, like tin should, when mixed for 2% tin content, BUT, my boolits are sometimes showing small pinholes in the bases, even though I pour a very generous sprue puddle. When the puddle frosts over, it looks normal, but when I cut the sprue I find pinholes. No idea how deep the pinholes go or how big the cavities are inside; weight is still within a grain or two and they shoot OK, so it can't be too bad. Anybody know what could be in this alloy causing this shrinkage? These boolits do drop a little smaller from the mold, by almost a full .001", compared to normal WW+2% mix.

Here's a picture of the cooled melt of this "tin" alloy in my 4 lb Lee pot. Notice the large void in the middle? It goes about 1/2 to 2/3 down into the pot. It leaves a void similar to this every time it cools off. I've never seen this before. Tin doesn't do this, does it? I'd like to figure it out, because I've got at least 20 lbs of this stuff.

http://i24.photobucket.com/albums/c26/zthang43/molds/IMG_1315.jpg

http://i24.photobucket.com/albums/c26/zthang43/molds/IMG_1318.jpg

[Bob Krack]

Dunno, but my Lee Pro-4 20# pot always has a small dimple in the center after the alloy cools - even what I believe to be pure lead does also.

Vic

[Lloyd Smale]

high tin alloys not only melt at a lower temp but harden at a lower temp. What can happen is the lead hits the mold and sets up almost imediately not allowing the spruce to suck down into the mold. Its a very fine balancing act but you need to increase heat. Another thing that helps is to keep pouring the lead to it even after you have a spruce puddle. Let it run right down the mold. If its a torn spruce and not an actuall hole you just need to cut your spruce sooner.

[yondering]

Loyd, I really don't think it's a heat issue, because it's specific to this alloy. I cast my boolits frosty, especially in my Lee molds, and the heat is up there at 800 to 850.

This mystery alloy does have a low melting point like tin. Of course, I'm not mixing enough of it in my WW metal to significantly change the melting point of the bullet alloy.

The sprues are cut cleanly, not torn. The pinholes are not necessarily in the sprue cut, but anywhere on the base of the bullet. I found a few with pinholes in the driving bands and lube grooves.

[John Boy]

When the puddle frosts over, it looks normal, but when I cut the sprue I find pinholes.
Your cutting the sprue too soon. Let the puddle set for 5 seconds and then count another 15 seconds before you cut the sprue. The metal of the base has to harden to obtain a base bottom that is cut flush with no pinholes

http://www.longrangebpcr.com/8Phases.htm

[KYCaster]

I have the same problem with some alloy from an indoor range. It also freezes faster and is considerably harder than WW. It doesn't have anything to do with the temp. I get the same results no matter how hot.

I know this stuff has steel jackets, aluminum jackets and non-toxic shot in it. The steel jackets and steel shot shouldn't be a problem, but I wonder if the Al or the Bi may be the culprit.

It probably has some Zn in it also. When smelting it there's often some grainy looking crud that floats.

Mixed 3:1 WW/range scrap, it's managable, but it sure is a PITA. If it (I) wasn't so cheap I wouldn't mess with it.

So, Yondering, if you figger out how to use it let me know.

Jerry

[yondering]

John Boy, thanks for the link, it's interesting. I guess you didn't read my second post though; the pinholes aren't necessarily in the sprue hole, and I wait long enough that the sprue cuts cleanly.

KYCaster, your alloy sounds similar to mine, except there's no Zinc in mine. I'll post here if I figure anything out.

[runfiverun]

think copper guy's, it cools faster than lead.
and is readily available in range scrap.

[felix]

Copper does not melt. ... felix

[NuJudge]

Shrinkage porosity is something you will find in castings. The more Tin you have in a Lead alloy, the smaller the voids should be.

As any metallic alloy freezes, there is not a smooth wave front where the freezing is happening, there are a group of 'Dendrites' growing into the liquid, looking very much like trees, growing new branches. The thicker the branches, the more inhibited the liquid is in filling the vacuum left by the liquid metal shrinking on freezing. Tin operates to do a number of things, amongst others to allow liquid to reach the voids longer. With Tin, the porosity should be less, but it will still be there.

Putting pressure behind the liquid metal is one way Industry reduces porosity in commercial castings. They don't use ounces or pounds of pressure, they use tons.

All of this being said, I believe you are just opening the mold too soon.