Semi-Solid Die Casting (SSDC) involves injecting a metal slurry (20-60% solid) at 580-620°C under 50-100 MPa pressure. This forms a globular microstructure, enhancing strength by up to 20%. Shear rates are typically below 10 s⁻¹, ensuring controlled mold filling for precise parts. Common alloys include Aluminum A356 and Magnesium AZ91D.

Learn why SSDC is preferred to get precision results. Discover how it works and what its key methods, applications, and advantages are.

What is Semi-Solid Die Casting?

Semi-solid die casting is a manufacturing process. It is used to form metal into a semi-solid state. These parts are dough-like, equally containing both liquid and solid shapes. This kind of slurry helps the manufacturer to produce top-quality, intricate parts. It is widely used in industries, whether it is automotive or consumer electronics. SSDC parts are stronger and have almost zero defects compared to traditional casting.

 Thixotropy and Rheopexy

1.   Rheocasting (Thixotropic Behavior)

The process of rheocasting is initiated after having a standard metal ingot. That can be aluminum alloy A356. Metalworkers melt these ingots at the initial temperature of 650°C in a furnace.

When this molten material begins to cool down at a semi-solid temperature of 580°C, the workers use a mechanical stirrer, rotating it at 500 rpm. By doing this, the solid particles break into tiny globules. It is 50-100 microns in size.

The stirring makes thixotropic behavior. That’s the semi-solid state casting with 40% solid particles. It flows very smoothly when you push.

This slurry is later on used for casting versatile components of industries such as car suspension components.

2.   Thixocasting (Rheopectic Behavior)

In thixocasting, the manufacturers use premade billets. They are kind of alloys such as magnesium AZ91D. Usually, there is already a proper globular microstructure in this material.

The metalworkers first saw these billets. Their lengths vary in many cases, but they usually range from 150 mm. They remelted these billets, applying a 575°C temperature. That aligns with standard thixocasting. The process lasts for 15 minutes in an induction furnace until the material takes on an ideal semi-solid form.

The manufacturers push this material into the mold at 1 m/s. This is quite different from thixotropic because it is not thinner. Instead, it is a kind of rheopexy.

This process increases viscosity under shear (10%). This results in low risks of turbulent flow and air entrapment. When they cast components like laptop casings, these characteristics produce smoother surfaces.

How does Shear Rate Controls Flow?

The semi-solid metals have shear rates below the typical range of 10 s⁻¹. Being a thick state, it moves slowly into a mold cavity, filling each section properly, especially in need of thin casting.

The shear increases when narrow gates push the metal at high speeds. It fills the mold within 0.5 seconds, fabricating precise products.

The shear thinning is the parameter. That enables the workers to tackle metal flow during various stages of casting.

Solid Fraction

The solid fraction (fₛ) is the proportion of the solid content in SSDC. That’s what manufacturers keep within a feasible range of 20% and 60%.

Lower conditions below 20% make the metal runny and too high, more than 60%, results in a harder form. It causes trouble in mold.

Microstructure Evolution

In comparison to the old dendritic structure, semi-solid metal has a globular or rosette-like microstructure. Particles are round-shaped, enabling smoother flow and minimizing defects. Improved strength and fine quality are the basic results of this process in the final outputs.

Semi-Solid Die Casting Process

Slurry Generation Methods

1.   Strain-Induced Melt Activation (SIMA):

After buying standard billets, manufacturers strain them at 300° C. They then put them into furnaces for reheating at 580°C. In this liquid matrix, spherical particles form (50μm). That’s good enough for semi-solid processing.

2.   Magnetohydrodynamic (MHD) Stirring

Electromagnetic coils use 500A current. This is work to make 600 rpm stirring without contact. 40% ideal solid state is formed as a result with this process, avoiding contamination.

3.   Cooling Slope Casting

The manufacturer flows down the molten state of metal at 620°C on the slope. That’s usually a copper slope with a 60°position.

They use faster cooling to get a semi-solid slurry in more than 3 seconds.

Injection Unit Modifications

Use a special type of short sleeve with ceramic thermal coatings. They keep the temperature of the slurry at 570°C. So that you can cast it without worrying throughout the injection phase.

In addition to this, precision-engineered plungers ensure the mold fills evenly. It operates with a critical velocity of 0.3-0.8 m/s. This feature balances operations and reduces detrimental phase separation. That happens between the slurry’s liquid and solid particles.

Die Design Considerations

You must use a 30% greater cross-sectional area than conventional dies in the case of gate systems. This will aid in maintaining the proper flow of semi-solid metals.

For runner systems, incorporate gradual curvatures. That has a minimum 20mm radius. By doing this, the techniques will maintain laminar metal flow and minimize turbulence.

Speaking about venting slots, they are precisely machined to 0.1mm width. It will control air entrapment during casting. Wherein they also deal with leakage issues.

Advantages of Semi-Solid Die Casting

Smoother Surfaces and Precise Sizes

Semi-solid metal flows into die sections at a controlled speed. This is much slower than liquid metal. That also lessens the air bubbles by up to 90 %.

In comparison to regular casting, this process reduces shrinkage (0.5%), which is less than the 1.2% when cooling. That also helps in making parts that are snap-fit.

Stronger Metal Structure

The small, round, shaped particles of semi-solid state pack together tight enough. This means they have a denser structure with 20% more strength when stretched.

You can bend it with more force 15% before fracture. These parts last 30% longer with endurance of repeated stress.

Fewer Holes and Defects

There are almost zero empty spaces (1 to 2 %) inside  SSDC. Meanwhile, regular die casting has a 5 to 8 percent chance.

Air bubbles, holes, shrinkage, cracks, and rough surface spots are eliminated because of this process.

Energy Savings

You must know another feature or benefit of this process is energy saving. It reduces energy usage in many ways. For instance:

• The metal heats at 580°C instead of 680°C.
• Faster flows (25%) use less energy.
• It reduces material waste by up to 15 percent during operation.

Lower Production Costs

Because the semi-solid parts need fewer polishing machines, it saves up to 40% on it.

Among their production output, 5% of parts can be rejected. That’s less than 15 % of those normal processes.

You can use the SSDC molds 50000 more times instead of only 30000 times.

Applications of Semi-Solid Die Casting

Critical Automotive Components

The parts of semi-solid die casting in the automotive industry are:

• Steering knuckles
• Engine mounts
• Transmission cases
• Brake calipers

This process produces highly complex parts with structural details. For instance, subframes and suspension arms with hollow designs.

In EVs (electric vehicles), the parts are battery housings and motor enclosures. They are lightweight and durable. Also, it handles constant vibration and thermal stresses.

High-Performance Aerospace Parts

The semi-solid state produces aerospace components with precision quality. These are:

• Wing brackets
• Landing gear components
• Turbine engine parts
• Radar housings
• Avionics enclosures
• Satellite

They are durable and have less weight. The missile guidance system casing leverages it for its ability. They keep tight tolerances over critical environments.

Measured Performance Gains

In field tests, semi-solid cast brake calipers last 80000 km before they wear out with the old method. Furthermore, aircraft cast parts get 25% more fatigue resistance.

The parts of automotive sectors gain better impact resistance (15%) while operating in crash tests.

Growing Market Applications

Over time, growing market applications leverage SSDC to create:

• Precise 5G antenna housings with 0.05 mm waveguides.
• Medical implant tray containing bacteria-resistant surfaces.
• Drone motor housings with better heat dissipation.

Moreover, electric vehicles use this process in terms of getting 0.2 mm flatness. That’s across 300 mm spans in cooling plates of the battery.

Materials Used in Semi-Solid Die Casting

Specific Alloy Designations

Aluminum A356 (AlSi7Mg) and magnesium AZ91D are the alloys that work best in semi-solid die casting. They melt faster and more evenly, creating an ideal texture.

Because there is high strength in A356 alloy, which is why automotive companies usually use it 70%. Meanwhile, the alloy AZ91D goes well with the casting of lightweight electronics cases.

Rheological Properties

The charts depict how the alloys of SSDC work at different stages and varying solid fractions (Fs). At Fs=0.37. It shows a decline in viscosity when the shear rate raises from 1 to 10 s⁻¹.

Fractions like Fs=0.48, which are higher, keep the flow thickening. They use more force to fill dies. This graph demonstrates the situation of why manufacturers use Fs between 0.40 and 0.45 to get the best output.

Solidification Behavior

A356 casting cooldowns in the 50°C range. They allow you enough time to spread metal inside the mold.

To increase this range, you can add 0.3% magnesium. That’s to increase it up to 15°C for better flow.

Conversely, the alloy AZ91D takes on a fully solid form more quickly. It, however, makes parts with stronger, thinner walls. They are up to 2mm thicker.

Secondary Processing

The parts often need fewer steps of secondary processing. This is because it needs 0.1mm surface removal versus 0.5mm for conventional castings.

Also, passing A356 alloy through heat treatment brings improved strength up to 20% without warping.

Semi-solid die Casting vs. Traditional Die Casting

Process Parameter Comparison

Material Structure

The globular structure in semi-solid casting gives 20 percent higher toughness than the old casting’s structure. It contains about 2 % porosity; conversely, 5.8%.

Cost Factors

The initial cost, however, is higher up to 20%. Regardless, it can be cost-effective as it reduces material waste by around 15% and machining costs by up to 40%. That offsets its upfront expense.

When to Choose

Opt for semi-solid when needing:

Choose a semi-solid die-casting process when you produce:

• Thin walls (<3mm)
• High strength (>250 MPa)
• Volumes >20,000 units/year
• Smooth finishes (<3.2μm Ra)

Conclusion:

Semi-solid die casting is made with a higher quality of toughness. It also adds excellent surface finishing with minimal porosity, around 30% less than the regular process.

Although the technique uses specific alloys and expensive initial setups, it becomes cost-effective for fabricating parts exceeding 20000 units.

The upcoming market is looking forward to expanding SSDC applications across automotive, aerospace, and emerging technologies. It focuses on discovering advancements in process control and tooling as well.