Thursday, June 21, 2012
Aluminum Casting and Some Common Defects
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!
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.
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.
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.
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.
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.