What is Semi-Solid Die Casting?
In semi-solid die casting, the metal used for casting is pushed into the die cavity in semi-solid form rather than liquidus form. In this process, the metal has to be up to 65% solid. This semi-solid form is called the “slurry”.
Semi-solid metal casting (SSM) is used for casting non-ferrous metals like aluminum, magnesium, copper, etc. Different types of semi-solid metal casting processes are:
- Rheocasting
- Thixocasting
- Thixomolding
Aluminum alloy parts mostly require the rheocasting or thixocasting process, but magnesium alloy parts require the thixomolding process for manufacturing.
Rheocasting
In the rheocasting process, the liquidus metal is first passed through a controlled chamber. In this chamber, it is cooled to a semi-solid temperature to make the slurry. After that, the plunger forces the slurry into the die cavity where it turns into the final part after solidification.
Thixocasting
In the thixocasting process, manufacturers cut solid metal bars into small billets. Then heat the billets to a semi-solid temperature to make the slurry. Finally, push the slurry into the die cavity using a plunger.
There is a specialized process for preparing aluminum alloys using cold and hot working in thixocasting. This process is called SIMA (Strain-induced Melt-activated).
Here, the metal is continuously stirred while it is in semi-solid form. This makes a globular structure in the semi-solid metal (slurry) or the slurry obtain a non-dendritic morphology. Only small parts (less than 37-mm or 1.5-inch diameter) are cast using this process.
Thixomolding
The thixomolding process is the same as injection molding. Here, you have to provide sufficient heat to the magnesium chips in a barrel so that they can reach the semi-solid temperature and turn into the slurry.
A screw conveyor then pushes the slurry into the cavity. While pushing, the screw conveyor rotates and transfers shear force to the slurry. This force generates a globular structure in the slurry for SSM casting.
What Are the Differences between Die Casting and Semi-Solid Die Casting?
The differences between conventional die casting and semi-solid die casting are:
| Key Point | Die Casting | Semi-Solid Metal Casting |
| Material State | In conventional die casting, you have to use molten metal to produce the parts. | In semi-solid die casting, you have to use slurry is instead of molten metal. |
| Shrinkage | The casting parts will shrink more in die casting than SSM casting. | The final casting part will shrink less in semi solid metal casting than regular die casting. |
| Production Cost | For mass production, the overall cost is less than SSM casting. But the overall cost is higher for small production. | The overall cost is less than die casting for small production. But mass production of parts will cost more in SSM. |
| Quality | In regular die casting, the microstructure of the casting parts is less consistent than SSM casting. So the parts’ quality is not as good as SSM casting parts. | You will get better consistency in the microstructure with semi-solid die casting, and so the parts have better quality. |
What Are the Benefits and Drawbacks of Semi-Solid Die Casting?
The semi-solid casting process has a lot of benefits over regular die casting. But there are also a few drawbacks.
Benefits
The SSM casting parts-
- Have less porosity than regular die casting parts.
- It shrinks less, and the final dimension is more precise than normal casting parts.
- Have thinner walls and better mechanical properties than regular die-cast parts.
- Have a better surface finish than conventional casting parts.
- Requires less machining than normal casting parts after production.
Drawbacks
- Controlling the temperature is a great drawback in the SSM casting process.
- The equipment used in this process is more expensive than the usual die casting process.
Which Process of the Semi-Solid Casting Method is the Cheapest?
When it comes to rheocasting vs thixocasting, rheocasting is cheaper. You will use the metal directly in rheocasting, but you have to use metal billets in thixocasting.
Making billets is an extra step, and so it adds a little cost to the whole process. However, if you want to buy and use readymade billets, it will still cost you more than regular metal chunks.
Rheocasting is also cheaper than thixomolding. Thixomolding is used for magnesium alloys which cost more than aluminum alloys. So the overall production cost of thixomolding is more than rheocasting.
What Are the Applications of Semi-Solid Die Casting?
Medical, Aerospace and Automotive industries use Semi-solid die casting parts for high-end applications. Here is a list of semi-solid metal casting parts:
- Hydraulic brake valves and wheels
- Brake master cylinders and oil filter housings
- Car door pillars and steering control arms
- Engine brackets, blocks, mounts, etc.
SSM casting aluminum alloy parts have better mechanical properties than regular casting parts. The parts are also stronger and more precise. That is why manufacturers mostly use aluminum alloys in SSM casting for making these parts.