Aluminum Casting and Some Common Defects

 Here is our 900,000 BTU #40 Speedy Melt metal furnace right before we pulled the silicon carbide crucible of molten aluminum.  Preheating the ingot molds and a few ingots of aluminum.  Everything has to be hot when casting metal.

Metal has pours that store moister and if tools or metal to be melted isn't preheated the water will turn to steam upon contact with molten metal causing all kinds of mayhem in the foundry.

It always comes down to the last 10 seconds!

We have a 12" deep sand pit.  I use 55gal steel barrels sections as flasks to hold up the shells for the pour.
These shells were preheated to 1200F and poured hot.  A mistake.  After this pour I realized the shells were too hot and kept the metal molten too long.  I now preheat my shells to 900F for Aluminum and 1400F for Bronze. This allows the metal to flow sufficiently, yet freezes off the metal quickly.  A good thing if a shell is leaking!

I had a terrible time casting these.  There were leaks, bubbling, core breakage, cold shut.
   

 As the aluminum cools it shrinks and as it shrinks it cracks the shells.  When the shells crack like this I begin chipping out the metal casting.

 The Defects

 Exhibit A:
Hydrogen porosity in the center of the pattern.  Molten Aluminum reacts with water vapor (H20) in the atmosphere.  The molten aluminum combines with the Oxygen and leave 2 Hydrogens roaming freely in the aluminum.  That's why aluminum must be degassed prior to pouring.
I did degass this pot, just not enough.  We degas with Dry Nitrogen or Pure Nitrogen.  The nitrogen creates a low pressure gas bubble in the aluminum.  Hydrogen is a high pressure gas and is tricked into entering the low pressure nitrogen bubble and thereby exiting the aluminum within the nitrogen.
Next time I cast aluminum I degassed for a longer period as the aluminum cooled in the crucible prior to pouring and got rid of the hydrogen porosity.

I also poured the aluminum too hot so it sat molten in the shells too long allowing the hydrogen to migrate and concentrate into patches. 
 As aluminum solidifies it rejects the hydrogen. The center of my piece stayed molten the longest and solidified last allowing the hydrogen to concentrate into a patch of porous metal.

Here is a closeup of the hydrogen gas patch.  If you look closely you can see the crystal formation of the metal.

 Exhibit B:
Cold Shut.  Never walk away from a shell right after it gets poured.  I forgot this detail and began pouring ingots then realized my shell had a leak and then didn't have enough metal to refill the shell.  I had to remelt new metal and cast over the cooled metal.  I sucessfully filled the shell, however the metals did not fuse together.

 Exhibit C:
Here is my worst porosity cavity.  As I poured this shell the aluminum in the cup was burping and bubbling.  When I saw that I knew I was going to be greeted by this sight.  This shell again received aluminum that had alot of hydrogen disolved it the metal.  As the aluminum solidified it pushed it out and up the shell towards the cup.

Exhibit D:
Here's another closeup of the skin layer porosity I received in a narrow section of one of the shells.  It is actually located just below the picture above on the casting.



These defects were all fixed by welding and sanding the surface smooth.

Lost Wax Casting and Wax Working; Sprues and Gating

 To cast this piece, I had to paint in multiple layers of molten wax to achieve the thickness I wanted.  For bronze or aluminum casting I make my wax pattern wall thickness between 3/16" to 1/4".

A trick I do to check wax for proper thickness is hold the wax up to a light and if you can see light thru the wax it's too thin.  I also use a wax mixture  between Red Casting Wax and Brown Microcrystalline Wax, so this ruke may not apply to lighter colored waxes

 This is a photo of the other two wax sections as they are coming out of the plaster molds.









A closeup after I patched the bubbles with utility patch wax and then polished the wax using a nylon window screen and mineral spirits.  Lighter fluid works great too.

 Another shot of the polished and patched section pre-gating.

 A few views of the piece as it will be in the end.  It really looks like melted chocolate which is dripping down a wall and shelf.

 The top view of the "butt crack" middle section.

The back view showing the gating system ans sprues.  This system is a mix of direct and indirect metal casting gating systems.









This is an article in the Tahoe Daily Tribune, which features our foundry:
Hard work Metal casting and fabricating classes at college turn out works of art

Instructions for Multi-Section Plaster Mold-Making

Clay Pattern:
This clay sculpture began with an aluminum armature.

The aluminum armature is covered in clay and the surface is refined.



This piece is a wall/shelf piece and will ultimately be cast in aluminum and polished.

First establish the seam lines:

Draw the seam lines using a tool on clay or a sharpie pen on other materials.
 
Roll out a 3/8" thick clay slab and cut 1"-1 1/4" wide strips of clay.

Wet the edge of the clay strips (shim) and attach the strips along the established seam lines. Refine and smooth the strips edge to get rid of any texture.   Seams should be as smooth as possible. 

Plaster does not stick to wet (leather hard) clay, so no release is needed.  On any other surface a release is required.

• For porous surfaces- Murphy's oil soap or 50/50 mix of petrolatum/mineral spirits works well. 
• For non-porous surfaces: silicon spray lubricant, Smooth-On Universal Release, WD40, baby oil, and pam are a few good options.

Pottery Plaster #1 is specially formulated for mold-making and has a mix ratio of 70 parts water to 100 parts plaster by weight.  This chart is the water (qts) to plaster (lbs) ratio chart I use.

Use a scale to weight the correct amount of powder plaster, and measure the total amount of water needed before mixing.


Water temperature dictates set time of the plaster.  The hotter the water, the faster the set time.  Cold to lukewarm water is best.

Add powdered plaster to water by sifting in by hand and allow it to set for a minute or three before blending.

Thoroughly mix plaster slurry and double check the mix ratio with your finger.  Dip your finger into the plaster and pull it out.  Proper mix ratio should coat finger with a cream-like layer that you can not see the color of your skin through.

But a scale never fails!

Mix plaster and water slurry in bucket until it begins to thicken.  Run you tool or finger across the surface of plaster, a trail should be left for half a second.  This is the indication the plaster is setting up. 

Using a paint brush begin applying plaster.  This first layer is critical, as it is capturing the detail.  Be careful not to capture any air bubbles in this first coat.

Continue to apply the plaster until you have the proper plaster mold thickness.  Once the plaster slurry is in the "frosting" state, I move to applying with a frosting knife. And like frosting a cake, evenly apply the plaster to the pattern following the contours of the object as well as using the clay shims as a guide to establish even mold wall thickness.
For most small to medium plaster molds a wall thickness of 1" - 1 1/2" is ideal.

Once the plaster slurry has moved past the frosting stage it enters the carving stage.  During this stage the plaster is easily tooled into a smooth and uniform surface.

Pictured is the back side of the mold with the clay shim still in place.

 As soon as the plaster gets hot (not warm), but hot to the touch, remove the clay shim and clean up the seam.  Remove any imperfection, as mold seams need to be as flat and uniform as possible.

 Using the rounded end of a frosting knife or a spoon cut round keys into the plaster every 4" to 6" along the seam.  Be careful not to create undercuts by making them too deep.  As a rule, a key should be wider then it is deep.

Next apply multiple coats of release (Murphy's oil soap or 50/50 vasoline/mineral spirits blend) on the plaster seam line.  Be careful not to apply too much so that it puddles.

Using the same mixing and application steps as above apply plaster slurry to the next section of the mold.

Using the frosting knife shape the contours of the mold again.

The bottom cap with clay shim.

Cut keys and shape seam clean.

Apply the release on the plaster seam again;
Fill section #3.  Keep track of the seams.

In an inconspicuous place, in my case the back top, make a pour spout.   Draft the pour spout in all directions so it will release from the mold easily.

 Again, clean up the seam and cut the keys;
apply 2-3 coats of release;

Mix plaster, apply and shape the final section of the mold.

Repeat this until you have effectively encasing the pattern completely.

Lastly, after the last section sets (heats up) use putty knives and a hammer to pop open the seams.  This step is tricky, start at the top around the pour spout and work your way down the seam until it pops.  Be careful, the plaster is easily broken during this stage.

To dry the mold for its final cure strap the sections in place and dry them together.  Plaster may warp if dried separately.

Place mold in front of a fan until dry.

Now it is ready to use for casting or press molding!