Thixocasting or Semi Solid Metal Casting

contents by: CB.EN.P2MFG15006 : Ilyas Hussain CB.EN.P2MFG15007 : Jithin Jose

Thixocasting Thixo is a general term used to describe the near-net

shape forming process from a partially melted non dendrite alloy within a metal die.

Thixotropy- viscosity decreases with time due to constant agitation or strain.

If component shaping is performed in a close die it is called thixocasting.

If component shaping is performed in an open die it is called thixoforming.

process

Thixocasting consists of three separate stages:1. The production of a pre-cast billet having special

globular microstructure.2. The re-heating of these billets to the semi solid

casting temperature.3. Casting of component by pressing these billets to

die cavity.

Production of billet Non-dendritic billets of A356.2 aluminum alloy were

produced in the laboratory to manufacture SSM components.

Around 20 kg A356.2 aluminum alloy ingots were loaded into a furnace and melted at 850 °C.

Once the metal reached molten stage, the grain refiner of Ti-B was added at 0.1% of the total metal being melted, and the modifier of aluminum-strontium of 0.06 % of the total metal being melted was added to the molten metal and allowed to soak for 10 min.

Then, around 1.5 kg molten metal was transferred into a crucible at about 750 °C, and when the temperature reached 620 °C, the metal was poured into the preheated metal mould kept inside the electromagnetic stirrer.

The electromagnetic stirrer was operated at 350A, 50 Hz. Once the metal temperature reached 610 °C in metal mould, the water supply for the metal mould was switched on and immediately allowed to pass through the cooling channels to quench the metal in the metal mould.

The static mould billets were 230mm in length and 75 mm in diameter.

Metallic mould Electromagnetic stirrer

process In Thixocasting, a solid billet with a globular grain

structure is prepared by stirring the melt first while the billet is being cast, and then supplied to the die caster.

This billet is then reheated to the semi-solid temperature (between the solidus and liquidus temperatures of the alloy) by the die caster, using an induction furnace.

It is then placed in a shot sleeve and injected into the die cast machine.

Since the melt is partly solidified, proper thermal control of the die is essential to ensure proper filling of the die cavity.

Dendritic microstructure Non dendritic microstructure

Casting of non-dendritic billets using electromagnetic

stirring depends on many parameters, such as the

stirring intensity, excitation frequency and cooling

rate.

A minimum shear rate should be produced so that

dendrites can be detached from their roots. The position of the stirrer in the mould is also very

important.

Subsequent to the production of non-dendritic microstructure billets, the process of re-heating of the billet to the desired semisolid temperature is also a challenging task.

Induction heating is generally used to reheat the billets in semisolid state.

Major objective is to minimize non-uniformity in temperature distribution.

All these parameters affect the overall solidification process which will determine the final microstructure.

advantages Energy efficiency: metal is not being held in the liquid

state over long periods of time. Production rates are similar to pressure die casting or

better.

Smooth filling of the die with no air entrapment and

low shrinkage porosity gives parts of high integrity

(including thin-walled sections) and allows application

of the process to higher-strength heat-treatable alloys.

Lower processing temperatures reduce the thermal

shock on the die, promoting die life and allowing the

use of non-traditional die materials. Fine, uniform microstructures give enhanced

properties.

Reduced solidification shrinkage gives dimensions

closer to near net shape and justifies the elimination

of machining steps. Surface quality is suitable for plating.

Thixocasting has quite a low yield strength, high

fluidity, low forming load and low surface roughness.

Especially in the thixocasting process, a complex

geometry product can be obtained by only one step

forming.

This technology has been widely applied in

nonferrous metal forming and satisfactory results

were derived, but not with ferrous metal.

disadvantages The cost of raw material for thixoforming can be high

and the number of suppliers small. Process knowledge and experience has to be

continuously built up in order to facilitate application of the process to new components.

Personnel requires a higher level of training and skill than with more traditional processes.

Temperature control: the solid fraction and viscosity in the semi-solid state are very dependent on temperature.

applications Semi-solid processing is ideally suited for the

production of large volume die casting components, including light weight, high strength components for automobiles.

For aluminum alloys typical parts include engine suspension mounts, air manifold sensor harness, engine blocks and oil pump filter housing.

For magnesium alloys, semi-solid casting is typically used to produce extremely thin walled castings, such as computer and camera bodies. Because of this the process can be applied to rapid prototyping needs and mass production.

applications Automotive wheel.

Antilock brake valve.

Disc brake caliper.

Engine piston.

Anti lock brake valve Disc brake caliper

Thank you