Semi-solid aluminum die casting is an advanced manufacturing process that injects partially solid (slurry) aluminum into a precision mold. It operates in a mushy region between the solid and liquid phases, enabling smooth flow with reduced turbulence. This results in reduced porosity, improved structural integrity, and improved dimensional accuracy. The process is generally divided into two methods: thixocasting and rheocasting.
This article will delve into the technical fundamentals, process types, process flows, material selection, main advantages and limitations of semi-solid die casting. Let’s keep reading to see why it is being adopted by more and more manufacturers.
Technical Fundamentals
Mushy Zone Explained
Semi-solid die casting is carried out in a narrow temperature zone between the metal’s solid phase line and the liquid phase line.
In this state, aluminum presents a “slurry-like” mixture of solid-liquid coexistence.
This unique consistency allows the metal to flow smoothly and less turbulently during mold filling, thereby achieving a more stable casting process and reducing internal defects.
Thixotropic Behavior
The so-called thixotropy refers to the ability of a material to flow under pressure but maintain its shape at rest.
This feature is particularly critical for filling complex molds, as it can prevent splashing and gas inclusions.
Metal can adapt to mold geometry under stress, improve part accuracy, and reduce the need for high pressure or additional exhaust systems.
Semi-Solid Die Casting Types
Thixocasting
Thixocasting involves using a specially cast, non-dendritic aluminum alloy billet that is reheated to a semi-solid state.
This method allows for excellent control of the solid fraction and microstructure, resulting in high part consistency, minimal shrinkage, and low porosity.
Rheocasting
Rheocasting produces a semi-solid slurry directly from molten aluminum through controlled stirring and cooling.
It eliminates the need for billet preparation, reducing material handling and inventory costs.
While rheocasting is slightly less consistent than thixocasting, rheocasting is more flexible and cost-effective for high-volume production.
Thixocasting Process Flow
Raw Material Preparation
Preformed, non-dendritic billets (usually A356 or A357) are selected because they have stable semi-solid behavior.
Billet Reheating
The billet is heated in a controlled furnace to reach the desired semi-solid temperature, typically within the mushy zone.
The goal is to achieve a specific solid fraction (e.g., 40-60%) with a uniform microstructure.
Mold Preparation
The mold is preheated and maintained at a stable temperature to promote smooth filling and uniform solidification.
Slurry Injection
The reheated billet (now in slurry form) is injected into the mold at high speed but low turbulence to maintain its thixotropic flow behavior.
Solidification and Ejection
The part solidifies under pressure and cools rapidly in the mold. It is then ejected with minimal shrinkage or deformation.
Rheocasting Process Flow
Raw Material Preparation
Standard aluminum alloy ingots are melted in a temperature-controlled furnace. Impurities are removed to ensure clean metal.
Slurry Generation
The molten aluminum is mechanically stirred while cooling to produce a semi-solid slurry with the desired solid fraction. This step is critical for flowability and part quality.
Mold Preparation
The mold is heated to an optimal temperature to avoid cold shut-off and achieve consistent flow.
Slurry Injection
The newly generated slurry is injected into the mold at a controlled speed and pressure, using its thixotropy to accurately fill complex geometries.
Curing and Ejection
As with thixotropic casting, the parts are rapidly cured under pressure and ejected with low defect rates and excellent dimensional stability.
Material Selection
Semi-solid die casting mainly uses specialized aluminum alloys with good thixotropic properties, the most widely used being A356, A357, and 319.
These alloys have excellent fluidity, mechanical strength, and thermal stability.
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A356: In a semi-solid state, the flowability is stable, and after forming, it has good toughness and ductility.
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A357: By optimizing the magnesium content, it exhibits higher strength and heat resistance in semi-solid die casting.
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A319: It has excellent dimensional stability and processing performance in semi-solid die casting.
Core Advantages
Excellent Mechanical Properties
Semi-solid die castings have higher tensile strength and ductility. Its dense microstructure can resist fatigue and impact, making it suitable for critical safety components.
Its mechanical properties are usually superior to traditional castings.
The table below compares the typical mechanical properties of A356 aluminum alloy in conventional die casting and semi-solid die casting:
|
Process |
Conventional Die Casting | Semi-Solid Die Casting |
|
Tensile Strength (MPa) |
180–250 | 280–350 |
|
Yield Strength (MPa) |
120–160 |
200–250 |
| Elongation (%) | 1%–3% |
6%–10% |
| Hardness (HB) | 60–80 |
85–100 |
High-Dimensional Accuracy
The product has high dimensional stability, small deformation, and significantly reduces subsequent processing or manual debugging.
Precision-fitting parts can be directly assembled, especially suitable for complex structural components.
Good Surface Quality
The surface of the casting is smooth, with very few burrs, low porosity, and both aesthetics and structural strength.
Subsequent painting, electroplating, or polishing are easier, and common cold insulation or indentation defects rarely occur.
High Production Efficiency
Strong near-net forming ability, significantly reducing post-machining time. Due to rapid solidification, the production cycle is short.
The process can be highly automated in large-scale production.
Energy and Resource Efficiency
Compared to traditional aluminum alloy die casting, this process has a lower working temperature, less energy consumption, and longer mold life.
High material utilization rate, almost no flow channel or overflow loss, in line with lean manufacturing requirements.
Challenges and Limitations
High Initial Investment
Establishing a semi-solid die-casting line requires a significant investment in specialized equipment and tools, and increased costs for employee training and process adjustments.
Complex Process
Accurate control of temperature, pressure, and slurry state is necessary. Any small fluctuations may result in the entire batch of products being scrapped.
Low Fault Tolerance
Semi-solid die casting requires extremely high precision in controlling process parameters such as temperature and solid phase ratio.
A slight deviation may lead to poor mold filling or a decrease in mechanical properties.
Critical Mold Design
Due to the different flowability of materials in the semi-solid state compared to traditional liquid metals, mold design needs to be specifically optimized.
For example, the gating, exhaust, and temperature control systems must be readjusted according to the thixotropic characteristics to ensure smooth mold filling and component quality.
Industry Applications
Automotive
Semi-solid die casting is used for suspension arms and engine brackets that must be lightweight yet durable under vibration and fatigue.
It provides high strength with minimal defects, ideal for safety-critical components.
Aerospace
Applied in satellite mounts and UAV frames, where low porosity, tight tolerances, and fatigue resistance are crucial.
The process ensures structural integrity with reduced machining needs.
Electronics
Used for heat sinks and connector housings that demand precise geometry and efficient thermal performance.
The process supports compact, high-density designs with excellent surface finish.
Medical Equipment
Ideal for medical casings and surgical equipment requiring high dimensional accuracy and biocompatible-grade surface quality.
Semi-solid casting enables the consistent production of complex forms.
Industrial Equipment
Employed in robotic joints and actuator housings where high mechanical strength and precision are key.
The process allows for the efficient forming of intricate, load-bearing parts.
Conclusion
Semi-solid die casting has distinct technical advantages. It enables better control of microstructure, reduced porosity, shorter cycle times, and minimizes the need for post-processing.
The process effectively overcomes many of the defects found in traditional die casting and supports the production of complex parts with tight tolerances.
As a leading aluminum die casting company in China, CEX Casting provides fully customized, end-to-end casting solutions.
Contact us today to learn how CEX Casting can make your next project into reality.