Lecture 8: Casting Technology

Lecture 8: Casting Technology

MT321: Principles of Materials Processing Lecture 8:Casting Technology 1


Design of Gating Systems

Functions of a gating system:

To deliver liquid metal to mould cavity within a short time.

To minimise turbulent flow.

To keep dross and/or inclusion particles from entering mould cavity.



How to deliver liquid metal fast? By using a sufficient large cross sectional area; By using multiple runners.

How to minimise turbulent flow? By using tapered sprue and runners. By bottom filling of the liquid into the mould cavity. By regulating the change of cross sectional area of

the channels according to fluid dynamics principles.


How to keep dross and inclusion particles from entering mould cavity?

By using dross traps.

By using filters.


Various types of ceramic filters that may be inserted into the gating systems of metal castings



Solidification Shrinkage The liquid of most metals and alloys

shrinks during solidification.

Solidification shrinkage (percent) of some common engineering metals and alloys


Two considerations must be made in designing a casting mould, due to the solidification shrinkage: A riser, which is a reservoir of liquid, is needed to

compensate for the shrinkage of the whole casting.

For every location of a casting, when the liquid solidifies, liquid from the surrounding of that location is needed in order to compensate for the instantaneous shrinkage.


Microstructure of the hub section of a Mg alloy casting, showing pores caused by lack of compensation for the solidification shrinkage


Riser Design Risers are added reservoirs designed to

compensate for the solidification shrinkage of casting.

An Al casting produced without using a riser

An Al casting produced with a riser

Riser


Criteria for riser design: The volume of a riser must be adequate to

compensate for all shrinkage volume.

Riser must be the last to finish solidification.

The liquid flowing channels between the riser and the solidifying metal must be kept open at all times.

The minimum size of a riser needs to satisfies the following equation:

triser = 1.25tcasting (1)

triser is the solidification time of the risertcasting is the solidification time of the casting

triser and tcasting can be calculated using simulations software or Chrovinov’s rule which will be learnt later.



Types of riser: open riser and blind riser


Risering Aid The general function of risering aid: to promote

directional solidification of casting towards riser.

Types of risering aid: Chills Insulation materials Exothermic materials


The function of chills: to affect the direction of solidification.

Types of chills:

External chills: materials with high heat capacity and high thermal conductivity. Placed in mould, adjacent to casting.

Internal chills: pieces of metals placed within the the mould cavity. Form part of casting.

Chills


The function of insulation or exothermic materials: to slow down the liquid solidification in a riser.

They can reduce the required riser size --> increase materials yield.

Insulation or Exothermic Materials


The Five Feeding Mechanisms After mould filling, the liquid needs to continue to

flow to compensate for the instantaneous shrinkage caused by cooling and solidification of the liquid.

This process is called feeding.

There are five feeding mechanisms: Liquid feeding Mass feeding Interdendritic feeding Burst feeding Solid feeding


Schematic diagram showing the five feeding mechanisms in a solidifying casting (From “Casting”, by John Campbell, 1990)


Liquid Feeding

This is the feeding mechanism in which shrinkage is compensated through movement of liquid in volume.

It often precedes other forms of feeding.

The liquid should be largely free of solid grains.


Mass Feeding

This is the feeding mechanism in which the shrinkage is compensated through the movement of a slurry consisting of a mixture of free-moving solid grains and liquid.


Interdendritic Feeding

This is the feeding mechanism in which the shrinkage is compensated by liquid flowing through a porous solid network formed by dendrites impinging on each other.

In this situation, the solid dendrites are not free to move.


Burst Feeding

This is the feeding mechanism in which solidification shrinkage in a confined region is compensated by liquid breaking the surrounding solid barrier (bursting) and flowing to the solidifying region.


Solid Feeding This is the feeding mechanism in which the

solidification shrinkage in a confined region is compensated by the yielding (plastic deformation) of the surrounding solid caused by the hydrostatic stress due to the solidification shrinkage.

This feeding mechanism often causes surface slumping.

Schematic diagram showing plastic deformation zones spreading from isolated volumes of residual liquid in a casting, illustrating solid feeding in action (from “Castings”, by John Campbell, 1993)


Pressure (or stress) required to drive different feeding mechanisms

Different pressure is required to drive different feeding mechanisms.

The pressure required increases with decreasing the diameter of the liquid channels.

Hydrostatic pressure in the residual liquid calculated for various feeding regimes during the freezing of a 20mm diameter Al alloy cylinder (from “Casting” by John Campbell, 1990)


Gas (mostly O2 and H2) originally dissolved in liquid will come out during solidification.

This is because the solubility of gas in solid is generally much lower than in liquid. (e.g. H2 in Al)

Dissolved gas


When coming out of the liquid solution, the gas forms bubbles which then turn into gas pores if they are trapped in the solid.


How to prevent formation of gas pores?

Keep effective channels during casting for gas bubbles to come out.

Perform degassing: Flush other gas bubbles through liquid to bring dissolved gas out.

Keep the liquid at as low temperature as posisble. (Not strongly recommended).


Pattern Design When an expendable mould is used, it is

necessary to design a pattern that is used to make the cavity for the casting.

Pattern is a rough duplicate of the casting to be made.


Considerations to be made in designing a pattern:

Shrinkage allowance must be given: the size of pattern is ~2% larger than the size of casting.

Position of parting line must allow removal of the pattern after moulding.

Draft allowance (1-2o) is needed to assist removal of pattern.

Machining allowance is needed if the casing is machined.




Note: In many cases, a slight change of the design of a casting can make the casting easier to make or improve its quality.


Major Casting Processes Sand casting Investment casting Lost-foam casting Gravity die casting (permanent mould casting) Low pressure die casting High pressure die casting Centrifugal casting


Sand Casting

Sand casting process makes use of sand as the moulding material.

The sand that is often used: silica, zircon or olivine.



Requirements for sand moulds:

Refractoriness: the ability to withstand high temperatures. (sufficient for high melting point metals such as steel)

Cohesiveness: the ability to retain a given shape when

packed into a mould

Permeability: the ability to permit gas to escape through the walls

Collapsibility: the ability to permit the metal to shrink after it solidifies, and be broken to remove casting


Where does the refractoriness come from?

From the nature of sand: High melting point.


How to obtain high cohesiveness? By using binders:

Clay and water --> green sand mould

Sodium silicate + CO2 (CO2 is hardener) --> Na2SiO3 + CO2 -->Na2CO3 + SiO2 (colloidal)

Resin --> resin bonded sand mould.


How to achieve sufficient permeability? By controlling the following factors:

Sand particle size distribution The amount and type of binder The moisture content The compacting pressure


How to achieve collapsibility?

By controlling the binding strength of the binder

By adding some organic materials which burn out when in contact with hot metals: e.g. cereals or cellulose.


Steps of Sand Casting(a) A pattern board is placed between the bottom (drag) and top (cope) halves of a flask, with the bottom side up. (b) Sand is then packed into the drag half of the mould. (c) A bottom board is position on top of the packed sand, and the mould is turned over, showing the cope half of pattern. The patterns for sprue and riser are also in place. (d) The cope half of the mould is then packed with sand. (e) and (e’) The mould is opened and the pattern board is removed. The runner and gate are cut. (f) The mould is assembled, and molten metal is poured. (g) The casting is removed from the mould.


Applications of sand casting process Sand casting is one of the most versatile

manufacturing processes. Size of casting: small to huge Melting point of casting: low to high (up to

1800oC) Number of duplicate castings: 1 to thousands.

When a large number of duplicate castings are made, the sand moulding process is normally automated.


Two ways of compacting sand in automatic sand moulding


An automatic sand moulding process using a match plate


Investment Casting Video clip: http://v.youku.com/v_show/id_XNTc5NDExOTMy.html?from=y1.2-1-103.3.7-1.1-1-1-6


Investment Casting Steps:

Produce wax patterns using a master die.

Assemble the wax patterns onto a common wax sprue to make a pattern cluster.

Dip the pattern cluster into a ceramic slurry and sift refractory particles on it. Repeat this for a few time times to produce an investment shell.

Allow the investment shell to harden.


Steps (continued): Melt or dissolve the wax pattern cluster to

remove it from the mould.

Preheat the mould to an elevated temperature (550-1100oC) to sinter the ceramic shell mould.

Pour the molten metal into the mould and let it solidify.

Break the ceramic shell mould and remove the casting.


Applications of investment casting

To produce castings with complex shapes.


Lost-foam Casting Features:

Pattern is made of polystyrene.

The pattern remains in the mould while hot metal is poured

During pouring, the polystyrene melts and burns, leaving space for metal to fill.


Applications of lost foam casting:

To produce castings with complex shapes.


Permanent Mould Casting Features:

Mould (die) is machined from cast iron, steel, bronze or graphite.

Making die contributes to a large fraction of process cost --> die life is very important.

Shape of the casting to be made should not be too complex for low cost.

The number of duplicate castings to be produced is normally large.


Advantages:

High production rate. Castings have good surface finish. Some time castings have high strength.

Limitation: mostly used to produce castings of low melting point alloys.


Major Types of Permanent Mould Casting Gravity die casting (often referred as permanent

mould casting): use gravity to assist mould filling

Low pressure die casting: use gas pressure to drive mould filling and metal feeding

High pressure die casting: use high mechanical pressure to drive mould filling and metal feeding



Low pressure die casting process: •Liquid metal is pressurized using air or inert gas to drive the metal to fill the die cavity.

•Gauge pressure: 0.2-0.5bar.

•The liquid metal solidifies under this pressure.

air or other gas


Hot chamber high pressure die casting process: •Liquid metal is pumped into the die cavity.•Liquid solidifies under high pressure.•Pressure > 100 bar


Cold chamber high pressure die casting process: •Liquid metal is poured into a short sleeve.•The liquid metal in the short sleeve is pushed into the die cavity by piston.•The liquid metal solidifies under high pressure.•Pressure > 500 bar (50MPa)



Centrifugal Casting Features:

Liquid metal is poured while the die is being rotated.

Liquid solidifies under both gravity and centrifugal force.


Applications of centrifugal casting:

Producing pipes (currently major application).

Producing shaped castings (future trend).

Documents

Lecture 8: Casting Technology