Casting Aluminum Brackets for Kayak
Posted: Mon Nov 04, 2013 7:24 am
The Foundrymans Resource
http://founddreaming.com/metal_casting/forum/phpBB3/
http://founddreaming.com/metal_casting/forum/phpBB3/viewtopic.php?f=32&t=455
Frank,F.C. wrote:INDEED.... wow.... First and foremost I need to see how you gated the piece. Second, I need to know what your sand mix was... not just composition but percentages of binder, water and sand as a ratio of various componets of solution. BRING IT!!! Inquiring minds need to know... I also need to know what fuel you use to fire your furnace, how your furnace works (i.e., where the flame enters the crucible and how it interacts with the inside of the furnace wall, gas, oil or coal, and the list goes on, and on, and on..... heh, heh... Don't panic though.... as much as this information is critical it's also somewhat overboard with regard to getting a decent cast product on a small scale. It all plays a factor in the final product thought... REALLY. What it looks like to me, from the initial view of the picture, is that your melt has been too exposed to the FLAME and your furnace interior is not designed as it needs to be. There are ways to subtract "some" of the flame effect by use of purging chemicals prior to pulling the crucible and pouring the mold. In essense... de'gass your melt before pouring. I'll bet this is the problem you have in that photo. Fix the furnace, then Iron out the other issues and you'll cast far better product time after time therafter (assuming, of course, your sand mix is proper).
FrankF.C. wrote:Couple or three things you can do...
From what I see, the picture speaks clearly that the pepper flake gas bubbles throughout your piece (i.e., soluble hydrogen) is a sure sign of overheating of the melt in conjunction with an improper flame in the furnace. All aluminum will develop a certain amount of gas when melted. Melting contaminated scrap or aluminum with high contents of magnesium or manganese will aide in creating and compiling gas problems. The ideal is to keep your furnace as close to the melting temp of the aluminum as possible and not overheat. Also, you need an "oxidizing" flame, not a "reducing" flame. A reducing flame will force the metal to take on more gas in its matrix than it normally would. Also, degassing does not work well "outside" the furnace. I do mine inside the furnace when I know I'm slightly above pouring temp (with the flame off). After skimming, I lift the crucible and pour. You want your metal to pour like cream milk, not like mercury. There should be a sleeve formed at the lip of your crucible down to the pour cup that the aluminum flows through, this is evident you at that most perfect pouring temp.
Proper gating is also critical in eliminating gas or air bubble entrapment. If your sand mix has too much binder then the sand mold cannot absorb any off gassing of the melt introduced and becoming chilled. Situations like this will generally cause surface gas inclusions. What you have in that picture, however, is evident of soluble hydrogen spread throughout your entire melt. You get that occurring with a reducing flame and heating too hot.
Not sure if you are aware (and hopefully you're not color blind) but aluminum gives off a very subtle (barely noticeable) pinkish hue when the melt is at it's maximum temp you dare bring it to inside the furnace. Also, do not skim your melt while the furnace is running, nor skim after each ingot of scrap you drop into it once it melts. Leave the skin on till you're ready to degas and remove the skin... i.e., right before you pull the crucible and pour. When you pull the crucible there should still be that pinkish hue to the melt (not talking about the crucible here, only the melt). If it's still got that hue, sloooow down. Relax... take your time. That hue will disappear quick enough and at that point make your pour. Still, don't rush. If your gating and sand are right, and you've had a proper flame inside the furnace during the melt, chances are you will not need to degas. If you feel you have to degas due to a variety of scrap you've been melting use this... go to the grocery store and buy a couple of them "blue" toilet bowl cakes that turn your toilet water blue. Break off a thumbnail size piece and insert it into the end of a perforated length of steel thin wall pipe attached to sturdy metal rod stock. While the melt sits in a glowing hot furnace, insert the perforated pipe through the melt's surface skin and gently swirl it about. It will boil vigourously and emit smoke and the smell of spent firecracker powder. Breath shallow, or wear a respirator, or do this with some cross ventilation. When the boiling and bubbling stops, skim, lift the crucible and, if there's no pinkish hue, go ahead and pour.
Other than that, I recommend you determine whether you have too much binder and/or oil, in your sand. Often people with limited experience in making sand molds will keep adding binder till their sand packs firmly like clay. Yes, it gives them nice molds that don't damage that easily but it plays hell with the melt once entered inside the mold cavity.
Frank