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I have a 120mmx120mm brushless fan with an Aluminum frame, which as you can imagine, isn't very thick. I want to thread the base, so I can mount it with a fairly large screw. For that I need to add more Aluminum so the screw has more to grip on.

I'm thinking of creating a casting mold around the exterior side of the Aluminum frame (proof sealed) and then pouring molten Aluminum in it. I've seen videos of people pouring Aluminum and it seems to have a relatively low Heat Capacity compared to other metals (i.e. it will cool down and solidify before it melts the surface of Aluminum frame), as I've seen Anthill Art being made without so much as scorching the earth. Also, as Aluminum is a good Thermal Conductor, the heat would dissipate throughout the frame pretty quickly.

How would I alleviate the two problems of low Heat Capacity and being a good Thermal Conductor? I know that one always have to pour excess molten metal to the block being added to, but do you guys have better ideas??

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    $\begingroup$ This, uh, sounds like a bad idea. Also, be very sure the fan frame isn't actually a zinc alloy (Mazak etc) with a much lower melting point than Al before you pour... $\endgroup$ – Brian Drummond Apr 17 '17 at 8:47
  • $\begingroup$ "I've seen Anthill Art being made without so much as scorching the earth" - that is because the water content of the soil needs about 2500 times the heat capacity of Aluminium to vaporize it into steam (for equal masses of Aluminum and water), and the temperature won't rise about 100C until that has happened. $\endgroup$ – alephzero Apr 17 '17 at 9:42
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This is unlikely to work very well.

The biggest issue is that you won't get good adhesion between the two surfaces. aluminium has a particularly resilient and inert surface oxide layer so molten aluminium won't just stick to the surface of solid aluminium. Welding it requires an inert atmosphere and reverse polarity current or alternating current to clean the surface. Pre-cleaning won't work either as the oxide layer forms very quickly. Equally the aluminium will shrink substantially as it cools so anything you cast onto the surface will in all likelihood just pop off.

In practice, casting a new part from scratch using a known castable alloy is far more likely to work and not likely to be significantly more effort. Note also that cast aluminium is prone to porosity and needs to be de-gassed to produce reliable mechanical parts.

Secondly without knowing the alloy you are working with and its initial condition there is no way of knowing how the heat will affect its mechanical properties. Aluminium anneals as a fairly low temperature and if the existing part reaches that temperature it will be drastically weakened and there is a good chance that it will distort.

A better solution would be to fabricate the extra part you need and either get it TIG welded on (even then there is some risk of distortion and not all aluminium alloys are readily weldable) or to bond it on with adhesive, which is likely to be the only practical solution which is reasonably likely to be problem free.

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  • $\begingroup$ Thanks for the input. Alright, I see now how difficult it can be to accomplish this. Nobody mentioned brazing this, so I guess it's just that it's destructive as the fan frame will have to be heated first. With Aluminum being a good thermal conductor, the whole frame would heat first before the contact area can be at the right temperature, unlike when pouring that instantly heat the contact point. $\endgroup$ – Dehbop Apr 18 '17 at 9:02
  • $\begingroup$ I propose another solution. What about excessively soldering the contact area first with a strong flux? This will prevent oxidation and will provide a good area for the molten Aluminum to weld in. If the soldered are were to be heated further (say lightly torching it), then an intermetallic layer will form. Then I brush off the oxidized and excess solder. Then I pour molten Aluminum. I only need a little extra Aluminum as the heat from the poured Aluminum will be enough to remelt the intermetallic layer, rather than having to remelt a larger portion of the contact, as originally proposed. $\endgroup$ – Dehbop Apr 18 '17 at 9:39
  • $\begingroup$ Then I can even solder (not braze) a piece of zinc-based braze in an area very close to the mold (by using a strong flux as above). If this piece of brazing were to melt during the pouring, then the frame is getting too hot and I better stop pouring. If it's perfectly alright, then I can probably go ahead and pour a little bit more in excess than intended to add the threading. $\endgroup$ – Dehbop Apr 18 '17 at 9:39
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Your ultimate goal is not clear - you've asked how to do a task rather than how to achieve a result.

For example: if your fan is not terribly massive, then what matters is the surface area of contact between the screw head and the frame. This is more easily achieved with a wide flat washer or two (under the screw head and perhaps under the matching nut on the far side).

Another possibility is to follow ChrisJ's answer and bond a piece on, using an epoxy rated for metal-to-metal.

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