Billet Aluminium in Your Garden

The above is a 210g Aluminium-Lead-Antimony cast billet with an approximate composition of:

Lead (Pb): ~ 1.1%

Antimony (Sb): ~ >0.1%

Aluminium (Al): remainder by weight

None of these values were planned or calculated - the estimates are based on residual material from a previous melt using the same crucible.

Later, I learned that the lead may improve the machining characteristics of the aluminium and that the antimony acts as a substitute for bismuth in preventing hot cracking, which both sound most desirable for affordable hobby grade materials, win win? However, in these proportions, who knows what difference their inclusion will make.

The point of this exercise was to quickly produce a machineable cast-aluminium block for a second project and get a feel for casting aluminum in a permanent steel mold.

• Scrap Aluminium
• Antimony or Bismuth - Optional
• Lead (not suitable for consumer products) - Optional
• river sand or something
• heat proof tape or something
• A short length of steel box section, square tubing or pipe to suit dimensional requirements.
• Small home foundry - noting special, but must be able to manage 700 degrees Celsius
• Steel, cast iron or other aluminium friendly crucible
• Assorted workshop tools
• Safety supplies: ear, eye, face and body protection, fire controls - fire extinguisher, fire bucket (sand/fire retardant powder - not water) etc.

The concept is simple: build a controlled, oxygen ( 'air' ) rich fire and heat up a steel container to around 660-690 degrees and hold it there. In my case, this was achieved by lining an old pressure cooker pot with ceramic blanket and blasting a jet of air in through a hole in the side, the furnace is then charged with charcoal.

Next, add scrap aluminium to the container and allow it to liquefy. A build up of impurities is expected - when this build up becomes quite excessive and before pouring, it is good practice to skim it off the surface.

Sufficient fuel must be added regularly to keep the heat and provide surplus energy to melt the aluminium. If your heat drops too rapidly, the aluminium will begin to solidify on the surface, in the coolest places and on any new materials added. Of course, too much heat and you risk burning through your crucible and consequently, writing off the melt.

With some trial and error the correct temperature range is easily maintained. ( as a guide, red hot steel/cast iron is about 460-500 degrees [900 °F] ). PRO TIP: Infrared thermometers work well too.

After a suitable quantity of liquid alu has been produced, a billet can be formed. Obviously (I hope).

Important Safety Notes:

Fire is Dangerous. Remember to observe every precaution in preventing damage to property, life, the environment, yourself and others.

Leaf blowers produce a dangerously loud sound and hearing protection must be worn if one is used.

Keep at least 20 metres away from anything flammable and don't attempt this project if winds having the potential to carrying sparks any lengthily distance are blowing.

This is an OUTDOOR activity only. Unless you have access to a foundry environment.

Sadly I don't have access to a suitable energy source to make this a 'green' project - but, if at all possible, please refrain from the burning of non-renewable and dirty fuels (charcoal, oil, wood and most gasses) in the best case, use an electric furnace supplied by a clean power source (I.e. solar, wind, water, etc.)

Now for the fun bit!

The method used here is known as 'permanent mold casting' and in this case, was really simple and fast to slap together.

As aluminium is known to shrink by about 6% (1) when solidifying, materials with a higher melting point and lesser coefficient of thermal expansion (2) are suitable for the mold.

Worked example:

Inside diameter/width of tube = nominal dimension - (2 X wall thickness)

ID = 39.5 - [ 2(1.6) ]

ID = 36.3mm

final dimension of cast block = ID - (ID * 6/100)

= 34.122mm (theoretical)

The actual value ended up being: 34.05mm +- 0.01mm

It is worth noting that the gap between the aluminium and steel walls is not 1.089mm as the example suggests, I'm assuming this is because the steel also expands and contracts but at a slower rate of change.

River sand is used to plug the bottom of the tube and the aluminium tape is applied in such a way as to prevent the sand from pouring out again. The aluminium tape should be placed lower than the level of the sand to prevent it from softening when the molten aluminium is poured and all mold surfaces and sand must be free of moisture to prevent steam.

Once heated well over melting temperature (680°C to 750°C), the molten aluminium can be poured into the mold without it going cold during the pour. Begin slowly and increase the pouring speed steadily until the mold is full or the crucible empties. For best results, don't fill the pipe or tube right to the top as there is an increased chance of spillage.

Always remember to place the foundry set up and mold on flame proof surfaces away from combustible materials.

(1) source

(2) The coefficient of thermal expansion describes how the size of an object changes with a change in temperature - source

Once cooled, a few gentle taps from a hammer and a block of wood will make light work of freeing the billet. A nice property of the cast aluminium is that it doesn't pick up much sand or rust from the mold cavity and will end up very clean indeed. Of course any textures will be transferred to the final piece.

Degasing:

After a few billet castings with and without degasing the aluminium with candle wax, no real advantage seemed evident, but its probably still a good idea. What worked the best to prevent impurities was to first melt the aluminium scrap into small ingots and then melt the ingots for casting into a billet.

Gas bubbles:

More experienced foundry men may ask about the lack of air escape channels, and you'd be right to do so, I was hoping that the loosely packed sand and a few needle holes in the aluminium tape would suffice - which it does, the result is acceptable, but there were still a few small air pockets at the base of the material (perhaps pouring form a grater height might help [high pour]). Ideally the mold cavity should be filled from the bottom with a sprue styled goodie graphed into the base.

End:

The mold can be reused as many times as you like and size and shape are only limited by the size and shape of the material you can get your hands on. You probably have a suitable length of scrap to use already.

I'll and some notes on the machining process as soon as its completed.

Thanks for checking in, chow