Is semi-solid die casting the right choice for your components? This guide explains how the process works and outlines its strengths and weaknesses, helping you to make an informed decision.
What Is Semi-Solid Die Casting?
Semi-solid die casting is a near-net-shape manufacturing process for forming metal components that uses a semi-solid metal slurry instead of fully liquid metal.
During the process, a slurry containing both liquid and solid phases is created by partially solidifying metals such as aluminum, magnesium or copper alloys. Depending on the material and process requirements, the solid fraction is typically controlled between 30% and 65%.
Unlike in conventional die casting, where the metal is fully molten and flows rapidly into the mold cavity, in semi-solid die casting, a controlled solid-liquid structure is used. The solid particles alter the way the slurry flows through the mold, enabling more precise filling and reducing your casting’s common defects.
Best Materials for Semi-Solid Die Casting
Magnesium Alloys
Magnesium alloys are primarily used in semi-solid forming applications where the aim is to reduce weight further. Common materials include AZ91D and AM60B. AZ91D offers good castability and relatively high strength, while AM60B provides better elongation and impact resistance.
With a density of around 1.8 g/cm³, magnesium is much lighter than aluminum. This makes it ideal for use in lightweight structures, electronic housings and thin-walled, complex components. Semi-solid forming uses the flow properties of magnesium slurries to create more complex shapes, helping you overcome some of the limitations of forming thin walls and intricate sections.
Aluminum Alloys
Aluminum is the most well-established and widely used material for semi-solid die casting. Popular options include A356, A357, AlSi7Mg and AlSi10Mg. These alloys have suitable solid-liquid phase ranges and good semi-solid forming characteristics. This makes them ideal for producing metal slurries that flow well.
If you need to balance low density, mechanical performance and complex geometry, Aluminum alloys are usually the best option. For example, A356 and A357 can achieve higher strength and elongation after T6 heat treatment, making them suitable for structural components, automotive parts and pump housings where weight and performance are important factors.
Copper Alloys
Copper alloys can also be processed using semi-solid forming, although their application is much less common than aluminum and magnesium alloys. Materials that may be considered include certain Cu-Zn, Cu-Sn and Cu-Ni alloys.
Copper alloys have higher melting and forming temperatures, placing greater demands on slurry preparation, die materials and equipment. Material costs are also usually higher than for aluminum alloys. If your component requires a combination of electrical or thermal conductivity, wear resistance or high strength and its geometry makes conventional machining impractical, the semi-solid processing of copper alloys can provide an effective forming solution.
How Does Semi-Solid Die Casting Work?
In semi-solid die casting, the alloy is first converted into a slurry containing both solid and liquid phases. The slurry’s flow characteristics are then used to complete mold filling and solidification. You can think of it as first making the metal suitable for forming, and then controlling how it flows and where it solidifies. These three stages determine the internal quality, filling performance and dimensional stability of your final casting.
Slurry Preparation
The key to preparing a slurry is not just to make the metal partially solid, but to achieve the right balance of flowability and stability for injection. The solid fraction, temperature and the morphology and distribution of the solid particles all need to be controlled.
If the slurry condition fluctuates, its flow behavior during injection will also change. If you require thin-walled, complex-cavity or high-volume parts, please note that these characteristics can directly impact mold filling and consistency from batch to batch.
Thixotropic Flow Behavior
When subjected to shear, the internal structure of a semi-solid slurry changes, causing a decrease in its apparent viscosity and allowing it to flow more easily under pressure. This is a key reason why semi-solid die casting is well-suited to thin-walled and complex geometries.
Rather than simply increasing the speed of the injection, the semi-solid process controls the flow behavior of the slurry to fill the mold. This can help to reduce the risk of internal defects associated with turbulent flow and air entrapment in your complex components.
Injection and Solidification
The slurry enters the die and fills the cavity. It then continues to solidify under pressure. The key at this stage is achieving the right balance between mold filling and solidification. If the slurry begins to solidify too early, this can affect the filling of your castings’ thin walled areas. Excessive differences in cooling rates between thick and thin sections can also increase the risk of shrinkage defects and dimensional variation.
Main Types of Semi-Solid Die Casting
The semi-solid metal processing method involves several technical processes, which are primarily distinguished by the way the semi-solid slurry is prepared and fed into the forming operations. The most widely adopted techniques are rheocasting, thixocasting, thixomolding, and the SIMA process.
| Process | How It Works | Common Materials | Key Characteristics |
| Rheocasting | Semi-solid slurry is generated directly from molten alloy before injection into the mold. | Aluminum alloys | Direct slurry production with fewer reheating steps; ideal for complex components requiring high structural integrity |
| Thixocasting | Pre-cast billets are reheated to the semi-solid temperature window before forming. | Aluminum alloys | Stable and consistent slurry quality; suitable for precision components |
| Thixomolding | Magnesium alloy pellets or chips are heated and sheared into semi-solid slurry for mold injection. | Magnesium alloys | Optimised for lightweight, thin-wall and intricate magnesium parts |
| SIMA Process | Specially treated billets undergo thermal conditioning to form the required semi-solid microstructure before forming. | Mainly aluminum alloys | Used for smaller wrought-style components demanding high integrity, where higher billet costs are acceptable |
Advantages of Semi-Solid Die Casting
Higher Mechanical Performance
The semi-solid die casting process results in lower internal porosity and a more uniform microstructure. T6 heat treatment enables aluminum alloys such as A356 and A357 to achieve tensile strengths of around 280–320 MPa, yield strengths of 200–250 MPa and an elongation of approximately 8–12%, depending on the specific temper and location of the test sample. These strength and elongation levels are well suited to structural applications if your component needs to withstand continuous loads, vibration or impact.
Lower Porosity for Pressure-Tight Parts
The internal porosity of semi-solid die castings can typically be controlled within a range of 1–2% when the process conditions are suitable. Lower internal porosity can help reduce leakage risks for valve bodies, pump housings, hydraulic manifolds and other components requiring gas or liquid tightness, particularly where machining may expose subsurface defects.
Thin Walls & Complex Geometries
Semi-solid die casting is suitable for thin walls, deep cavities, internal ribs and complex curved surfaces. If you need to reduce the weight of your design while retaining internal ribs, mounting features or complex cavities, this forming capability can eliminate the need for additional parts, assembly and machining.
Better Dimensional Stability
The stable as-cast dimensions provided by semi-solid die casting make it suitable for components with demanding assembly requirements and CNC machining references. If your part includes bearing seats, mounting holes, locating surfaces or other critical dimensions, maintaining stable as-cast dimensions can minimize the need for dimensional adjustments and variations in machining allowances during subsequent CNC machining.
Better Surface Finish
Semi-solid die castings usually have a more uniform surface texture when cast, and this can be further controlled by adjusting the alloy, tooling conditions and process parameters. For parts that require painting, anodizing or other surface treatments, achieving a consistent as-cast surface can help reduce the need for surface finishing and ensure more consistent results in subsequent processes.
Reduced Thermal Fatigue & Extended Tool Life
Semi-solid die casting typically involves lower forming temperatures than fully molten metal die casting. This reduces the thermal shock experienced by the die. In the long term, this can help you reduce die damage caused by thermal fatigue and heat checking, as well as reducing the frequency of maintenance and replacement.
Applications of Semi-Solid Die Casting
- Semi-solid die casting is widely used to manufacture structural components for the automotive and EV industries, including steering knuckles, engine mounts, brake calipers, EV battery housings and motor housings. It delivers improved impact resistance compared to conventional high-pressure die casting.
- High-precision aerospace parts, including aircraft brackets, satellite housings and guidance components. Depending on the alloy and heat treatment conditions, it provides improved fatigue resistance compared to conventional casting methods.
- Electronics and UAV assemblies, including 5G antenna housings and UAV motor casings. As-cast dimensional accuracy eliminates the need for secondary machining.
- Medical and industrial components: EV battery cooling plates achieving 0.2 mm flatness over 300 mm in suitable part designs, medical trays and precision industrial parts.
- Hydraulic and fluid control components, such as valve bodies and pump housings. Low porosity prevents leaks under cyclic pressure.
Semi-Solid Die Casting vs Traditional Die Casting vs Forging
| Process | Semi-Solid Die Casting | Traditional Die Casting | Forging |
| Internal Quality | Low porosity and minimal shrinkage defects | Prone to gas porosity and shrinkage cavities; strict defect control mandatory | Exceptionally dense microstructure |
| Mechanical Properties | Superior to conventional die casting; supports T4/T5/T6 heat treatment for suitable aluminum alloys, depending on alloy composition and part design | Moderate performance; heat treatment may cause blistering in alloys with higher porosity levels | Highest mechanical strength and fatigue resistance |
| Complex Geometry | Excellent for complex thin-wall, near-net-shape designs | Good for intricate shapes | Limited; difficult to form thin walls and internal cavities |
| Dimensional Accuracy | High, stable dimensions after cooling & heat treatment | High as-cast precision | Near-net shape rarely achievable; extensive machining required |
| Surface Quality | Clean as-cast surface with minimal flow defects | Acceptable surface quality with possible flow lines | Surface quality relies on subsequent finishing |
| Production Efficiency | High throughput | Very high, optimal for mass production | Relatively low cycle output |
| Secondary Machining | Reduced CNC machining allowance | Machining workload varies by design | Machining requirements vary; near-net shapes are generally not achievable in a single forging step |
| Initial Investment | Higher equipment & process costs | Mature process, lower entry investment | High tooling and equipment expenditure |
| Typical Applications | High-performance, complex lightweight structural parts | High-volume standard cast components | Heavy-duty, highly loaded safety-critical parts |
When Should You Choose Semi-Solid Die Casting?
Consider semi-solid die casting if your component meets several of the following criteria.
| If Your Part Requires | Semi-Solid Die Casting May Fit |
| Pressure-tight performance | Valve bodies, pump housings, EV drive housings, hydraulic manifolds |
| Higher mechanical performance | Steering system, suspension components, chassis brackets, structural mounts |
| Thin walls or complex geometry | Lightweight housings, UAV frames, components with internal ribs or cavities |
| Tight tolerances and near-net shape | Parts that currently require substantial CNC machining |
| Part consolidation | Assemblies that currently use multiple brackets, housings or mounting components |
If your design meets the above requirements, it would be worthwhile to compare semi-solid die casting with your current manufacturing process. Conventional die casting, squeeze casting, machining or forging may offer better value for money if only a few criteria apply.
Challenges and Limitations of Semi-Solid Die Casting
- Higher Initial Investment — The costs of equipment and tooling are higher than for conventional die casting. The investment is generally better justified at medium-to-high production volumes to lower per-unit costs.
- Narrow Process Window — Temperature, solid fraction and injection parameters all require tight control. Even minor deviations can result in inconsistent batch quality.
- Limited Alloy Selection — Only certain aluminum, magnesium and copper alloys are suitable. This process is not suitable for steel components.
- Tooling Must Be Redesigned — Standard die casting dies cannot be reused. You must optimize your gating, venting and thermal layout for semi-solid flow.
- Longer Cycle Times — Slurry preparation and controlled filling increase cycle duration. However, if you are manufacturing simple, high-volume parts, throughput may become a constraint.
- Performance Ceilings — The process has difficulty with thick-section castings. Do not approve a safety-critical part based on the process name alone. Your final route must meet the tensile, fatigue, impact, porosity and validation requirements set out in the drawing.
FAQ
Can Conventional Die Casting Molds Be Reused for Semi-Solid Die Casting?
Standard die-casting molds cannot be used directly without modification. A route-specific DFM review is required for your existing tooling. Modifications to the gating, venting, shot sleeve, thermal balance and fill path may be necessary; however, new tooling is only required if the original die cannot support your selected slurry and filling strategy.
Which Quality Standards Apply to Semi-Solid Die Casting Components?
No single standard covers all the requirements for semi-solid die casting. For dimensional tolerances and machining allowances, you can reference ISO 8062-3 on your drawing. However, general casting standards do not define critical performance requirements such as porosity limits, microstructure, tensile properties, leak tightness and fatigue strength. You must specify these on the component drawing and link them to the material specification. You must also develop a test plan for your specific application to confirm them.
What Inspection Methods Are Used for Semi-Solid Die Castings?
Core tests iclude three processes: CMM size verification, X-ray porosity scanning and metallographic microstructure analysis. Tensile and fatigue tests must be performed on all load-bearing components. If your part requires sealing, you need to perform a pressure leakage test to confirm its integrity and prevent hidden leakage following CNC machining.
What Is the Minimum Wall Thickness for Semi-Solid Die Casting?
The typical minimum wall thickness for semi-solid die casting of aluminum alloys is around 1.5–2.0 mm. Under suitable process conditions, however, this figure can be reduced to around 1.5 mm for magnesium alloys. The minimum wall thickness required also depends on factors such as the type of alloy used, the distance of the fill, the temperature of the slurry, the design of the pouring system and the local cooling conditions. If the design includes a part with a wall thickness of less than 1.5 mm, its filling capacity should be confirmed at the DFM stage. The focus should be on the filling distance and local solidification in the thin-walled area.
How Does Semi-Solid Die Casting Compare With Squeeze Casting?
Both squeeze casting and semi-solid die casting result in high-density castings. However, the specific process chosen depends on the required geometry and performance. In squeeze casting, the metal is completely molten and formed under high pressure. This process results in low porosity in the castings, but makes it difficult to achieve thin walls or complex internal features. By contrast, semi-solid die casting uses a semi-solid slurry in a laminar state. This enables thinner wall thicknesses and more complex shapes to be produced while maintaining lower porosity. Squeeze casting is a better choice for producing parts with relatively simple structures and thick walls. However, if your project involves complex geometries, thin-walled structures or applications that require heat treatment, a semi-solid die casting process is worth considering.
How Does Semi-Solid Die Casting Handle Internal Cavities and Undercuts?
Thanks to laminar flow, semi-solid slurry can smoothly fill moderately complex internal cavities. Even if your design contains undercuts, slide cores and lifters are still required in the same way as in conventional die casting. This process does not eliminate the need for moving mold components. Forming the extremely narrow, lengthy internal channels is extremely challenging. During the DFM review, you should therefore simplify such features to avoid incomplete filling and persistent internal defects.
What Causes Cold Shut Defects in Semi-Solid Die Casting?
Most cold shuts in semi-solid die casting are caused by an unstable slurry temperature and an inconsistent solid fraction. Poor runner layouts can split the flow front, creating gaps where separate streams do not fully fuse. Insufficient venting can also exacerbate this issue. Unlike in conventional die casting, turbulent air entrapment is not usually the main cause. If you notice cold shuts on your components, you should first review these parameters.
How Does Semi-Solid Die Casting Compare With Permanent Mold Casting?
Compared to permanent mold casting, semi-solid die casting produces parts with thinner walls, lower porosity, and greater dimensional stability. Although permanent mould casting has lower initial costs, the production cycle is long and manufacturing parts with complex structures and thin wall thicknesses is difficult. Additionally, the mechanical properties of semi-solid castings can be enhanced through heat treatment. If your goal is to manufacture lightweight yet complex parts that can withstand high loads, semi-solid-state moulding would be a more suitable option.
Final Thoughts
Semi-solid die casting offers unique advantages for the production of complex, high-performance components. To evaluate whether this process is suitable for your design, consider your geometry, performance targets and production volume. Contact Inoxcast to discuss your part specifications.