SPECIAL CASTING PROCESSES

FOR LIGHT METALS

PREPARED BY :-SWAPNIL NIGAM

PROCESSES

SEMI-SOLID PROCESSING

SQUEEZE CASTING

COSWORTH PROCESS

ILP CASTING PROCESS

SEMI-SOLID PROCESSING (SSP) Initiated at MIT in 1971, by Spencer & Flemings, during

their experiment with Sn-15%Pb alloy.

PRINCIPLE OF PROCESS – Thixotropy [thixis (means shaking) + trepis (means changing)], i.e. reduction in the “rheopectic properties” of a system (eg. viscosity, yield stress etc.), isothermally and reversibly, due to application of shear strain, such that the reduction is distinctly dependent on time.

Thixotropic behavior of a system

Metals and alloy systems , generally, when they are solidified such that, their solidification temperature lies in between the liquidus & solidus, then they exist in the SLURRY state. On applying shear stress (by stirring the system) at this temperature, the system will show Thixotropic behavior, i.e.,

• Their viscosity is in inverse to the shear rate.

• Their viscosity increases with time, at constant shear rate.

Earlier, based on the principle of SSP, two processes were used for producing finished products,

• Rheocasting - A slurry produced by agitating an alloy in a narrow, annular region between the furnace wall and a central cylindrical stirrer was transferred and cast in a pressure die casting machine.

• Thixoforging - Slugs of the above prepared slurry, can be removed and forged in a die.

Time-dependent thixotropic behavior

But the above systems presented the following problems,

• Design of furnace-stirrer system, with adequate quantities of slurry.

• Rapid chemical attack & erosion of stirrer material.

THIXOMOULDING process developed by Dow Chemical Company, overshadowed the above limitations.

Thixomoulding is a viable technology for forming alloys in semisolid state to near net-shaped products. It involves a combination of plastic injection moulding technology with metal pressure die casting.

The feed stock required is solid alloy pellet, which are heated such that they form slurry with 0.5-0.6% solid fraction.

The feed stock is heated to the semi-solid temperature and sheared into a thixotropic state in a screw feeder before passing directly into the diecasting machine.

Schematic of THIXOMOLDING machine

Original THIXOMOLDING machine

HDPC Machine component prepared

by thixomolding

BENEFITS of SSP –• Energy requirements are less because

complete melting is not required. Energy needed to heat aluminium alloys for casting is 35% greater than that required to heat the same aluminium alloy for the conditions needed for semisolid forming

• Shrinkage and cracking within the mould are reduced because the alloy is already partly solidified when cast.

• Lower operating and pouring temperatures lead to an increase in the lives of metal dies.

• Foundry cleanliness is easy to maintain, cycle times are reduced and scrap is minimized.

• Composite materials can be readily produced

• Significantly lower capital investment & operating costs.

SQUEEZE CASTING Idea suggested by CHERNOV in 1878 to apply steam pressure to molten metal while being solidified.

PRINCIPLE OF PROCESS – High pressure solidification of the system within a reusable die.

Squeeze casting process is a generic term to specify a fabrication technique, which combines the permanent mold casting with die forging, into a single operation, where molten metal is solidified under high hydrostatic pressure.

It has become known as ‘squeeze’ casting because the casting is squeezed in a controlled fashion under high pressure to complete the filling of the die

The process is now-a-days the most popular fabricaiton route for MMC (Metal Matrix Composites) artifacts

Schematic of SQUEEZE FORMING

OPERATIONAL STEPS OF PROCESS –A. Pouring of predefined amount of molten

metal into a preheated die cavity of a hydraulic press.

B. Activation of the press to close die cavity and pressurize the liquid metal, until the solidification is complete.

C. Final, withdrawal of the pump, and ejection of the component.

MODES OF PROCESS – 1. On basis of the way of pressure application,

• Direct SC mode• Indirect SC mode

2. On basis of the timing of pressure application,• Before beginning of crystallization• After beginning of crystallization

In Direct casting process, metered amounts of molten metal are poured into a die and then pressure is applied to the solidifying metal via the second moving half of the die. It is also divided into 2 types,

• Forward process – No metal movement is involved. Suitable for ingot type components.

• Backward process – Metal movement is involved. Versatile process & used for wide variety shaped components.

In Indirect casting process, molten metal is first poured into a shot sleeve and then injected vertically into the die by a piston which sustains the pressure during solidification. It is also divided into 4 types,

• Vertical die closing & injection• Horizontal die closing & injection• Vertical die closing & horizontal injection• Horizontal die closing & vertical injection

Squeeze casting in general is employed for Low melting temperature alloys of Al & Mg.

The most dominant process parameters are die temperature, pouring temperature & superheat. Other parameters involve cleanliness of the metal, metal movement inside the die, die coating, & Dead Time (the interval for which pressure is applied).

In general for Al & Mg alloys, following operational parameters are used,

• Pressure = 50-150 MPa• Die temperature = 200-300°C• Die coating = Graphite based

SQUEEZE Casting machine setup

BENEFITS of SC –• Using this method it is possible to produce

heat treatable castings.

• Yields a pore-free, fine-grained casting with wrought product type mechanical properties & casting has isotropic properties. The S–N curves obtained in fatigue tests also lie within the scatter band for wrought materials.

• Offers high metal yield, minimum gas or shrinkage, low porosity and an excellent surface finish, combined with lower operating costs.

• Possibility of using pre-forms (high-porosity bodies made from specially selected materials).

DEMERITS of SC –• High costs due to complex tooling & thus, high

production volumes.• No flexibility as tooling is dedicated to specific

components & increased process costs.

COSWORTH PROCESS Process was developed in 1978 to meet the need for highly specialized components for the Formula One racing car engines manufactured by Cosworth Engineering, Ltd., in England.

PRINCIPLE OF PROCESS – Counter-gravity pumping of the molten metal, into the mould through the base using pressure assisted feeding. Schematic of COSWORTH

PROCESS

STEPS INVOLVED IN PROCESS :-• Mold preparation :

1. Accurate pattern & core box equipment.2. Zircon sand replaces silica sand, due to

former’s greater expansion predictability. 3. Gas hardening by SO2 .

• Metal preparation & transfer :1. Metal is melt & held in electrical-resistance

heated “Holding Furnace”.2. Metal is drawn from the middle-zone.3. Controlled degassing at the ingot station &

use of a protective atmosphere, to minimize the risk of gas absorption during holding.

4. Metal is transferred using an electromagnetic pump into the mold, in quiescent manner.

A recent development has been to roll over the mould under an applied positive pressure immediately after filling. This further improves the quality of castings and allow higher production rates to be achieved.

PROCESS CHARACTERISTICS:• Alloy range – Al alloys.• Casting size = for Al-alloys (0.2-50 kg) , max. size

used is 915 x 610 mm.• Thin section capabilities – 2.5 mm with overall

size of 200mm.

• Cast holes – dia. of 6mm.• Surface finish = 3.5-6.3 microns.

LIMITATION: Is not suitable for a wide range of metals and casting sizes.

BENEFITS of Cosworth Process :• Castings are pressure-tight due to the absence of

porosity and inclusions.• Use of reclaimable zircon sand, avoids the volume

change associated with the phase transition from α- to β-quartz that occurs in silica at temperatures close to those used for melting aluminium alloys, thus improving the stability of both moulds and cores, which allows close dimensional tolerances of castings to be obtained and repeated.

• Gives high dimensional accuracy, resulting in minimum fettling and machining.

• Fatigue properties of casting at both room and elevated temperatures are also superior and scatter in the test results is much reduced.

• Yields of 85% or better, castings that are typically 10 to 12% lighter than those produced by other methods excellent mechanical and physical properties

• Can cast thinner sections, allowing the design of lighter, more robust components and resulting in considerable weight saving.

ILP PROCESS It is the Improved Low Pressure process of casting, developed in Australia. PRINCIPLE OF PROCESS – utilizes transfer of molten metal vertically through a riser tube into the bottom of the mould cavity. SPECIAL FEATURES :

• The use of a combination of metal cores and resin-bonded silica sand for the mould which promotes rapid unidirectional solidification in those regions of the casting where optimal properties are required.

• The mould can be inverted to facilitate this controlled solidification, which may provide dendrite arm spacings < 20 m adjacent to the metal cores, and < 0.5% microshrinkage overall.

Schematic of ILP PROCESS

The process has been adapted to allow robotic handling of the moulds so that movements are precisely repeatable.

It is being used in Mexico for the mass production of cylinder heads and blocks for engines of some models of automobiles that are manufactured for the North American market.

REFERENCES FOR SEMI-SOLID CASTING;

• http://www.hindawi.com/isrn/ms/2013/679820/• http://

www.docstoc.com/docs/100692285/Semi-Solid-Processing

FOR SQUEEZE CASTING;• http://

issinstitute.org.au/wp-content/media/2011/05/ISS-FEL-REPORT-A-IYER-low-res.pdf

• http://www.cmse.sdu.edu.cn/clcx/jxkj/clcxjc//%CF%C8%BD%F8%D6%FD%D4%EC%BC%BC%CA%F5%BF%CE%BC%FE2010/squeeze%20casting.pdf

• http://www.jim.or.jp/journal/e/pdf3/50/12/2820.pdf FOR COSWORTH PROCESS;

• http://core.materials.ac.uk/repository/eaa/talat/3201.pdf

• http://www.fkm.utm.my/~zulkepli/notescast/cosworth.pdf

• http://www.ecoefficiency.com.au/portals/56/factsheets/foundry/ecofndry_process.pdf