Semi Solid Metal Casting

Semi-solid metal Casting (SSM)

Casting and Semisolid Casting

• The conventional casting often contains internal structural defects that lead to poor

mechanical properties.

• The dendritic morphology in conventional casting is also responsible for low

mechanical properties and fracture toughness. The tips of the dendrites act as a stress

raiser.

• Thus, to improve the mechanical properties it is required to modify the dendritic

microstructure of the alloys.

• The dendritic morphology can be altered by grain refinement, rapid solidification, etc.

• Dendritic morphology can also be broken during its initial growth by some means in

the agitation of melt itself. That means the alloys are processed in the partially solid

and partially liquid state, which is known as semi-solid processing.

• Different semi-solid processing routes are rheocasting, thixocasting, strain induced

melt activation (SIMA), spray deposition technique and squeeze casting.

Compiled by Prof Amruta A. Rane 2

Semi-solid metal casting (SSM)

• SSM is done at a temperature that puts the metal between its liquidus and solidus

temperature. Ideally, the metal should be 30 to 65% solid.

• Semi-solid metal casting (SSM) is a near net shape variant of die casting.

• The process is used with non-ferrous metals, such as aluminium, copper and

magnesium.

• The process combines the advantages of casting and forging.

• Semi-solid casting is typically used for high-end castings. For aluminum alloys

typical parts include engine suspension mounts, engine blocks and oil pump filter

housing, etc.


(a) Dendrite structure (b) Globular structure

Typical microstructure of an Al-7%Si alloy after:

(a) liquid casting and (b) semi-solid casting


Semi-solid metal casting (SSM)

• There are four different processes: thixocasting, rheocasting, thixomolding, and

SIMA.

• There are a number of different techniques to produce semisolid castings. For

aluminum alloys the more common processes are thixocasting and rheocasting.

• With magnesium alloys, the most common process is thixomolding.

• Discovered at MIT in 1971

• During study of hot tearing of steel during solidification

• Modeled by Sn-15Pb alloy using Coeutte Rheometer


LIQUID

SEMI-SOLID

SOLID

HOT FORMING

SEMI-SOLID METAL

CASTING/FORMING

Known since 3000 BC

Known since 3500 BC

Ambient temperature

COLD FORMING Known since 4000 BC

Known since 1973 AD

THIXO RHEO

Globular structure

formation

Liquidus temperature

Solidus

temperature

CASTING


Rheocasting & Thixocasting

• Rheocasting: Shearing the liquid while cooling from liquid to liquid – solid

range and casting a net-shape product.

• Thixocasting: Shearing a partially melted suitable precursor (a forerunner) in

semi-solid range.


Liquidus and Solidus Temperature • In chemistry, materials science and physics, the solidus is the locus of temperatures (a curve on a phase diagram) below which a given

substance is completely solid (crystallized).

• The solidus quantifies the temperature at which melting of a substance begins, but the substance is not necessarily melted completely,

i.e., the solidus is not necessarily a melting point.

• The solidus is always less than or equal to the liquidus.

• If a gap exists between the solidus and liquidus, it is called the freezing range, and within that gap, the substance consists of a mixture of

solid and liquid phases (like a slurry).

• The liquidus temperature, TL or Tliq specifies the temperature above which a material is completely liquid and the maximum temperature

at which crystals can co-exist with the melt in thermodynamic equilibrium.

• It is mostly used for impure substances (mixtures) such as glasses, alloys and rocks.

• Above the liquidus temperature the material is homogeneous and liquid at equilibrium.

• Below the liquidus temperature, more and more crystals will form in the melt if one waits a sufficiently long time, depending on the

material.

• For pure substances, e.g. pure metal, pure water, etc. the liquidus and solidus are at the same temperature and the term "melting point"

may be used.


Metal Shaping Processes at various temperatures

Sand Casting,

Die Casting, etc.

L

α + L

α + Si

α


Squeeze Casting


Squeeze Casting

• Combination of casting and forging.

• Also known as liquid-metal forging.

• Near net shape process - the initial production of the item is very close to the final (net)

shape, reducing the need for surface finishing.

• Squeeze casting is simple and economical, efficient in its use of raw material.

• The process generates the highest mechanical properties attainable in a cast product.

• Casting has been around for approximately 6000 years, but squeeze casting is a relatively

new development; being introduced along with other pressurized casting techniques

during the mid 1800's.

• The process was introduced in the United States in 1960 and has since gained widespread

acceptance.


Squeeze Casting: Process

• The squeeze casting process uses an accurately measured or metered quantity of molten metal

which is poured into a heated mold via a launder.

• The mold is closed to produce an internal cavity in the shape of the required component.

• The molten metal is forced/displaced into the available space of the die cavity. As with most

casting processes, using a permanent pattern.

• The mold is coated with a suitable release agent and for squeeze casting it is usually in the form

of a graphite coating.

• Pressure continues to be applied to the molten metal until it has solidified and forms the

required component. The press is then withdrawn and the component is ejected.

• Squeeze casting is most suited to the production of light alloy components in large production

quantities.

• Retractable and disposable cores can be used to create complex internal features.



• Squeeze casting consists of

pouring of liquid metal into a

preheated, lubricated die and

forging the metal while it

solidifies.

• The load is applied shortly after the

metal begins to freeze and is

maintained until the entire casting

has solidified.



• Casting temperatures depend on the alloy and the part geometry. The starting

point is normally 6°C to 55°C above the liquidus temperature.

• Tooling temperatures ranging from 190°C to 315°C are normally used.

• Pressure levels: 50 to 140 Mpa.

• Lubrication: For aluminum, magnesium and copper alloys, a good grade of

graphite spray lubricant has proved satisfactory when sprayed on the warm dies

prior to casting


Squeeze Casting

• Non ferrous alloys like aluminum, magnesium and copper alloy components are

readily manufactured using this process.

• The squeeze casting process, combining the advantages of the casting and forging

processes, has been widely used to produce quality castings.

• The cooling rate of the casting can be increased by applying high pressure during

solidification.

• Contact between the casting and the die is improved by pressurization which

also results in the foundation of fine-grained structures.


Squeeze Casting: Advantages

• Offers a broader range of shapes and components than other manufacturing

methods.

• Little or no machining required post casting process.

• Because of the high pressure applied during solidification, porosities caused by

both gas and shrinkage can be prevented or eliminated.

• Good surface texture.

• Fine micro-structures with higher strength components.

• No waste material, 100% utilization.


Squeeze Casting: Limitations

• Costs are very high due to complex tooling.

• No flexibility as tooling is dedicated to specific components.

• Process needs to be accurately controlled which slows the cycle time down and

increases process costs.

• High costs mean high production volumes are necessary to justify equipment

investment.

• The casting can be affected by premature chemical reaction, air entrapment

and failure to fill the cavity resulting in rejection of components.


• Squeeze casting is most suited to the production of light alloy components in

large production quantities.

• Both ferrous and non-ferrous materials can be produced using this method.

• It has found extensive application in automotive industry in producing aluminium

front steering knuckles, chassis frames, etc.

• Making of High capacity propellers for boat-engine.

Piston, made by

squeeze casting

Applications

Car rim

Thixocasting


Thixocasting • Thixo casting utilizes a pre-cast billet that is normally produced by vigorously

stirring the melt as the bar is being cast.

• Induction heating is normally used to re-heat the billets to the semi-solid

temperature range and die casting machines are used to inject the semi-solid

material into hardened steel dies.

• Thixo casting has the ability to produce extremely high quality components due to

the product consistency that results from using pre-cast billet that is manufactured

under continuous processing conditions that are employed to make forging or

rolling stock.

• The main disadvantage is that it is expensive due to the special billets that must be

used. Other disadvantages include a limited number of alloys that can be

manufactured and scrap produced in this process cannot be directly reused. Compiled by Prof Amruta A. Rane 24

Rheocasting

High Pressure Die Casting

Liquid Metal Semi-solid metal with globular grain structure

Cool and stir

Transfer

Diagram of the one-step Rheo route of SSM casting method


Rheocasting – Stepwise Process


Rheocasting

• Unlike Thixo casting, which re-heats a billet, Rheocasting develops the semi-solid

slurry from the molten metal produced in a typical die casting furnace.

• This is a big advantage over Thixocasting because it results in less expensive

feedstock, in the form of typical die casting alloys and allows for direct recycling.


The advantages of Semisolid Casting

• Due to the lower pressures and temperatures required to die cast semisolid metal

the die material does not need to be exotic. Often graphite or softer stainless

steels may be used.

• Even nonferrous dies can be used for one time shots.

• Because of this the process can be applied to rapid prototyping needs and mass

production.

• This also allows for the casting of high melting point metals, such as tool steel and

Stellite, if a higher temperature die material is used.

• Other advantages include: easily automated, consistent, production rates are

equal to or better than die casting rates, no air entrapment, low shrinkage rates,

and a uniform microstructure


The advantages of Semisolid Casting

• Complex parts produced net shape

• Porosity free - Smooth die filling with no air entrapment favouring product soundness

• Excellent mechanical performance

• Tight tolerances

• Thin walls

• Energy efficient

• Production rate compete that for pressure die casting

• Lower processing temperature

• Lower Impact to the die

• Reduced solidification shrinkage


The disadvantages of Semisolid Casting

• Because thixotropy (semisolid state) is a middle state in physical or rheological

sense process conditions form a band so narrow even environmental

temperature differences have to be considered.

• Thus production facilities need a high level of technology and operators require

similar knowledge and training.

• Relatively higher feedstock material cost

• Precise control of operating condition is required.

• Liquid segregation may occur as a result of non-uniform heating.


Applications

• Potential Replacement of permanent mould parts to eliminate machining and

finishing

• Pressure tight parts such as master brake cylinders, fuel rails, air conditioner

compressor housing etc.

• High strength parts such as engine mounts, tie rods etc.

• Wear resistant parts made from hypereutectic alloys such as compressor piston,

brake drums, gear shift levers etc.


Current Applications/Products

Hydraulic Brake Valve

Automobile Wheels

Brake master Cylinder Compiled by Prof Amruta A. Rane 32

Current Applications/Products

Door Pillars for Audi

Control arms for Steering Fiat Engine bracket Compiled by Prof Amruta A. Rane 33

Semi Solid Metal Casting

Engineering

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