Best temperature when casting aluminium

Discussion in 'Castings, finishing/ repair/ and patina's' started by Thronborg, Feb 22, 2021.

  1. Thronborg

    Thronborg Copper

    I understand that the best aluminium parts to use is from earlier casted parts as, aluminum wheels, casted hubs, carburators, pistons etc. Since its impossible to know what other metals included, what is the golden temperature when pouring. I have seen between 640-750 C.

    It is a wide span, in there any negatives effects to go for 750C?

    And is a IR-termometer working?

    Thanks in avvance.
    Ole Thronborg
  2. It depends on how you melt your metal, is your furnace an electric or induction furnace or fuel heated?. An ideal aluminium furnace would be an electric heated furnace with no water from combustion gases. Water vapour can cause hydrogen porosity problems and the aluminium will absorb more over time from the combustion gasses. Also the size of your crucible matters as a small crucible can cool faster so you have no time to pour the metal. The sand mould will absorb some heat during pouring so having the aluminium hotter will compensate for a small crucible and the cooling effect of the mould.

    So the metal temperature depends on your crucible size with bigger crucibles cooling slower as they have less surface area for a given volume. The longer you have the aluminium in a combustion fuel powered furnace the more hydrogen it can dissolve which is bad. Electric induction or resistive heating furnaces minimize hydrogen absorbtion which can still occur in humid environments.
  3. oldironfarmer

    oldironfarmer Silver Banner Member

    An IR thermometer does not work very well. The emissivity of the molten surface changes based on how thick the oxide layer is, dross present, and the angle you are shooting at. I have tried to calibrate by IR gun in combination with an immersed thermocouple and gave up. It does pretty good on the refractory wall of the furnace.

    An optical pyrometer is best but a k-thermocouple in a SS sleeve does ok and lasts a long time at aluminum temperatures. I've also had good luck immersing a bare k-thermocouple. Of course that has the quickest response. I was surprised I got good reading with the thermocouple in a conductive liquid. I also use a k-thermocouple for brass, bronze, and copper. You are working close to it's limit so you really need to pull it out quickly when you approach the limit.

    The negative effect of hot aluminum is gas absorption but I pour as hot as 850C in an oil fired furnace.
  4. Thronborg

    Thronborg Copper

    I got this old electric furnance, i hope its good enough. It goes to 1400C and can be programmed in different time and temperature. I think its for hardening steel. But its small wide 30cm x dept 40cm x height 10cm. I hoped to find a bigger but i will only make small parts so it will due for now.

    I se in a video by Myfordboy he measured the temterature to 640C when he degassing, so i assume he got it higher when he pouring.

    It seemes that casting is really easy befor you dig in to it. But its not, thats why long times experience and skills is the trick for sucess. So i just have to learn and i am happy i have you guys here to guide me.

    Thanks a lot.

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  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    The biggest challenge you have is with the vertical dimension of your furnace being only 10cm as this will limit your crucible size. It may also prove inconvenient for measuring your melt temperature as you will nee to remove the crucible to measure it. In any case, you will never need to use the 1400c capability of your furnace to melt aluminum. A preheat at 1000c set point will rapidly melt in a small crucible. If you have relatively clean netal I wouldn't worry about degassing in an electric furnace.

  6. metallab

    metallab Silver

    An optical pyrometer works on the same principle as an IR gun pyrometer, with emissivity. The dross has a higher emissivity than the bare metal at the same temperature, hence the bare metal shows a lower value with both an optical pyrometer and an IR gun.

    What is more important is that aluminum produces lots of dross and when all is molten, remove as much as possible (while the crucible still in the furnace) and adding some table salt might dissolve some dross.
  7. oldironfarmer

    oldironfarmer Silver Banner Member

    Maybe eutectic NaCl/KCl mixture help in the crucible but I don't think salt will do any good and may introduce moisture.

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