0

I'm trying to cast an impeller but I have a problem. The melted aluminium doesn't go inside the thin parts of the mould. Here is the mould before casting:

enter image description here

After the casting:

enter image description here

After breaking the mould:

enter image description here

As you can see the vanes of the impeller are not produced. It should be something like this:

enter image description here

What's the problem? How to figure it out?

Roh
  • 113
  • 5
  • I'd guess that as the aluminium starts to flow into the thin bits, the molten metal loses heat to the mould and solidifies, and then the solid bit stops more liquid going in. Maybe see if you can preheat the mould? Long thin stuff like that is very hard to cast, there's not much hot metal and a lot of cold mould in contact, and viscosity means it won't flow fast, and there's not room for air bubbles to get out – sqek Jun 09 '22 at 11:52
  • @sqek But the "pouring" process is too much quick. I doubt. – Roh Jun 09 '22 at 11:58
  • @Roh so did you try preheating? – Solar Mike Jun 09 '22 at 12:03
  • 1
    @Roh from https://www.dynacast.com/en-gb/resources/blog/cast-aluminum-with-thin-walls : "the fill time needs to be less than 30 milliseconds for a thin-wall (0.5mm – 1.0mm) feature to be created." That probably isn't doable with a non-industrial setup – sqek Jun 09 '22 at 12:11
  • @SolarMike No, I didn't – Roh Jun 09 '22 at 12:18
  • @sqek Wow. I was not aware of that. – Roh Jun 09 '22 at 12:19
  • 2
    @Roh if you look at cast objects in general they're normally big and chunky, when there is a thin web it's normally surrounded on all sides. You might be able to make that impeller by bending sheet metal for the vanes, the putting the pre-made vanes in your mold and casting the base around them - if they poke out enough and everything's hot enough it might weld or bond (but might not because oxide layers on aluminium are a pain) – sqek Jun 09 '22 at 12:28
  • the impeller in the last picture appears to be machine finished – jsotola Jun 09 '22 at 16:02

2 Answers2

2

The aluminum was not hot enough, you need more super heat. Possible poor alloy choice, a high silicon ( such as 12% ) is common for Al castings. Shell molds are normally heated ; I would use at least 500 F. Surface finish looks rough, a finer grain first layer of sand is needed. The good example required years for many experienced engineers to develop.

blacksmith37
  • 6,144
  • 1
  • 9
  • 15
  • 2
    Is the second picture really cast? – DKNguyen Jun 09 '22 at 17:53
  • @blacksmith37 I've used a combine of 6000 and 3000 alloy series. Is there really any aluminium alloy with 12% silicon!? I use a furnace with LNG/LPG gas source and a big torch. What furnace do you use? for mold I printed the model using a 3D printer and it doesn't work very well in my opinion. – Roh Jun 09 '22 at 18:09
  • I was reading this article: https://www.researchgate.net/publication/354352931_Manufacture_of_centrifugal_compressor_impeller_using_FDM_and_investment_casting before reading it, I jut thought that I need to use die casting to cast such things with thin parts. – Roh Jun 09 '22 at 18:09
  • @Roh There's thin parts, then there's thin and deep parts. – DKNguyen Jun 09 '22 at 19:34
  • Th alloy designation 6000 is for wrought ; 3003 is nearly pure Al. Cast alloys use 3 digits like 356 , a very common casting alloy for Al, with 6 % Si. Alloy 132 has 12% Si.. – blacksmith37 Jun 09 '22 at 20:09
  • That mold could be shell or investment , both are normally heated. For investment mold heated to 1700 F steel sections of 1 mm are no problem for at least 10 mm. I am not very familiar with aluminum casting , although we made one part with 356 with a short wall with thickness of 0.5 mm. As an old person I have no clue what a 3D mold is. – blacksmith37 Jun 09 '22 at 20:20
  • American Society for Metals International have many informative books on about any metal topic. – blacksmith37 Jun 09 '22 at 20:23
1

Your objective seems to be a cast aluminum turbocompressor wheel. The techniques for casting such thin sections are highly specialized- you just can't achieve the desired result when gravity-casting aluminum in a fused sand mold in your back yard.

There's also a number of good reasons why turbocompressor wheels are not made of cast aluminum in the first place: as-cast aluminum is simply not strong enough to stand up to the stresses in that application. Superalloys high in chrome, nickel, etc. are required.

niels nielsen
  • 14,708
  • 1
  • 13
  • 30