Projects 1st Alum Casting - What Went Wrong?

My first two attempts at casting aluminum didn’t go as well as I would have liked. Both attempt #1 and #2 have what looks like porosity. I went with a higher temp ( about 680 degrees C) thinking I may have a flow issue (cooling too fast and not filling the mold). Not an issue, the molds filled just fine. But, lots of pocking, little craters and rough surface occurred on both attempts. My sand is Petrobond and the casting flask is a pre-made cast iron set.
The pictures:
Attempt #1


Closer pic of Attempt #1


Attempt #2


Closer pic of Attempt #2


Any ideas of what I’m doing wrong?

 

It looks like you need to degas the aluminum before pouring.
Are there voids inside the casting or all of the holes on the surface?

 

I don’t know if the porosity/voids are in the castings themselves. I guess I should commit to cutting one open. If the casting is solid on the inside, would I still need to degas?

 

For a first attempt they really don't look too bad. Like 51 Norm said maybe degass with a flux then skim the dross and pour at about 1300 - 1350 F that's about 700 C.
Your patterns look good , nice and smooth and no undercuts.
It's fun isn't it! It's satisfying to make cool stuff outa really hot stuff.
Keep up the good work!

 

Thanks, Jethro! Making the pattern was a Herculean task in itself. Lots of filling, sanding and priming. Over and over, until I was happy with it! Next time, I’ll use a softer wood dowel. But, oak was all they had at Home Depot.

 

Check out myfordboy on youtube. He has done a lot of casting at home videos.

 

You may have to add sprues (3) were the high points of the casting is.

 

Thanks, Mart! Myfordboy has a bunch of great videos. I’m going to need to dedicate an afternoon to go through them.

 

I use MDF to make most of my patterns. It's not hard like oak , it machines and sands easily and no splinters or grain to worry about. I use epoxy primer to seal them and clearcoat for the finish.
Catdad has a point about sprues. If you lay the pattern horizontally and have a vertical sprue on each end and some small vents on your risers that might release some of the bubbles you are experiencing.

 

I don't like to use the word "patina" when it comes to engines. But if we use the word "history" to describe many of the HAMB-friendly engine rooms that look like they were there at the beginning and have survived the test of time, your castings could be right at home. Certainly, do a bit of clean-up sanding but leave the character of having been cast in the old ways....... unless everything else has a high polish.
Of course, if the "porosity" you mentioned means that fuel under pressure will leak, then that's definitely a problem.

 

That was one of my concerns when I was making the sand molds. But, I felt extra sprues wouldn’t be needed if the mold filled adequately. The extra length in the fill sprue gave a “stand pipe” effect to add a little pressure during the fill. I think my fill process is OK for now. I just need to tackle the texture issue.

 

I didn’t consider a horizontal pour. I could still have my stand pipe with two rammed tubes on both ends and some air vent pin holes for each riser. That will be one of the next “tries”!

 

Sand castings usually use a sprue and a riser. Don't see that on yours.
When I was teaching sand casting I always tried to pour the aluminum at 1200 degrees F.
You also have to degas the alum. and remove the slag before you pour it. I also used Petrobond sand.
Look here.
sprue and riser - Search (bing.com)
The melting point of aluminum is about 660 degrees Celsius or 1220 degrees Fahrenheit1

 

I’m definitely going to cut one of my castings length wise to see if there are any voids or porosity. Plus, it’ll help me decide what needs to be done for the next attempts. I don’t mind it looking a bit like a shop class project. But, I really do want it to be smoother!

 

My buddy Crafty B does a LOT of sand casting, and has a couple of classes coming up. He's on IG, "Crafty B".

 

There are no mistakes , just learning opportunities.....unless of course you get hurt or die that'd be bad.
What doesn't turn out goes back into the soup pot and becomes the next learning opportunity.

 

These were my first bad attempts:
https://www.jalopyjournal.com/forum/threads/reinventing-the-wheel.1104945/

 

Hey, RSC;
Jethro nailed in, post9, but another possibility is that you didn't screen(screed) the sand fine enough, & also didn't ram it hard enough. Either of those will leave that kind of surface texture, although you do know sand is sorta rough - you'll never get it to appear as smooth as die-cast. When pouring in school decades ago, I could get some serious detail, but never "smooth". The worst was the green sand, set by CO2 - that was course stuff & resulting texture, but then that was also the only grit green sand available at school. Most of the courseness will sand out. Lots of minor voids back in the day from cheap speed equip vendors, that weren't concerned about hi-quality finishes. Compare surface finishes of Bugatti &/or Miller to Edmunds or somesuch.
Marcus...

 

I worked in a foundry for about 7 years back in the 1970's, and they never poured a mold without pre-heating them first. Just put a torch or weed burner over the sprue opening and let it pre-heat the mold for about 10 minutes. Then try the pour again, and see if the porosity disappears.

 

Check the American Foundry Society web page, they may be able to help.

 

I would run a vent off ur furthest high point from the spru or a riser of some sort. You could literally drill a 1/4 hole thru the sand in the appropriate place for use as a vent. That would be the easiest way to test whether it's still gassing during the pour after uv worked out the proper prep on ur aluminum b4 pouring.

 

I agree. Have only cast lower temp metals but they need a vent to allow gases to escape.

 

I went back and looked at your photos and noticed this shrinkage....



Ideally you want this type of shrinkage to happen in your sprues or vents , so this is another reason for a horizontal pour. Make sure there is enough volume so there will be plenty of molten metal to shrink back.

 

thats Awesome, Chris! keep trying

 

Thanks, Jethro! When I first saw that low spot, I thought it might have been caused by the mold coming apart (lack of firm ramming). After seeing similar low spots in other peoples casting, I have to agree with your assessment of metal shrinkage. Hopefully, this weekend, I’ll be able to set up my flasks for some horizontal pours.

 

Thanks, Bruce!

 

A few things that stick out to me:

The top of your sprue is about level with the top of your casting, so you essentially have no head pressure at the top of your part. You want volume of molten metal above your part feeding the part as it cools.

Your in gate is pretty long and narrow. I would probably have your sprue run right into the top of the left bump on your casting, and have the vent coming out the right bump.

As for the porosity (I'm not convinced you are have porosity, sand castings are always a little rough) your sand might be a little wet? Too much moisture in the sand causes the water to boil off and the steam can cause porosity.

 

The two pours were completely vertical. The blob on the right is actually the resevoir feeding the mold. I wanted to make sure my mold would fill. Which it did, but without a nice surface. My molds may have picked up some condensation. I rammed the molds the day before the pour and let them sit on the garage floor overnight. I did the pours the next morning (and didn’t check the humidity) on the back porch, out of direct sunlight. I should have let the flasks sit in the sun for awhile to burn off any possible moisture.
The next attempts will be with horizontal flasks, raised feed reservoirs and some proper ventilation.

 

Please keep us informed. A guy here was making parts for his 1928 Hudson

 

Lots of good advice here. I noticed a few things that could use improvement. One is that your molten metal is coming to a dead stop at the end of your part. That means that any sand pulled in with the molten metal has no escape. If you continue a runner off the end of the part and end it with a well that should help with the sand inclusion. Also pay attention to make sure your runners are very smooth and blow away any loose sand. That is a pretty thick part which means as it cools and shrinks there needs to be feeders off of the thick sections (in this case the bosses). These feeders do exactly that - allow metal to be fed to the thick sections as they shrink to minimize that puckered surface.