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.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 2 MT321: Principles of Materials Processing Lecture 8:Casting Technology 3 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.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 4 How to keep dross and inclusion particles from entering mould cavity?
By using dross traps.
By using filters.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 5 Various types of ceramic filters that may be inserted into the gating systems of metal castings
MT321: Principles of Materials Processing Lecture 8:Casting Technology 6 MT321: Principles of Materials Processing Lecture 8:Casting Technology 7 Solidification Shrinkage
The liquid of most metals and alloys shrinks during solidification.
Solidification shrinkage (percent) of some common engineering metals and alloys
MT321: Principles of Materials Processing Lecture 8:Casting Technology 8 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.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 9 Microstructure of the hub section of a Mg alloy casting, showing pores caused by lack of compensation for the solidification shrinkage
MT321: Principles of Materials Processing Lecture 8:Casting Technology 10 Riser Design Risers are added reservoirs designed to compensate for the solidification shrinkage of casting.
Riser
An Al casting produced without An Al casting produced with a riser using a riser
MT321: Principles of Materials Processing Lecture 8:Casting Technology 11 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.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 12 The minimum size of a riser needs to satisfies the following equation:
triser = 1.25tcasting (1)
triser is the solidification time of the riser tcasting 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.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 13 Types of riser: open riser and blind riser
MT321: Principles of Materials Processing Lecture 8:Casting Technology 14 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
MT321: Principles of Materials Processing Lecture 8:Casting Technology 15 Chills
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.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 16 Insulation or Exothermic
Materials 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.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 17 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
MT321: Principles of Materials Processing Lecture 8:Casting Technology 18 Schematic diagram showing the five feeding mechanisms in a solidifying casting (From “Casting”, by John Campbell, 1990) MT321: Principles of Materials Processing Lecture 8:Casting Technology 19 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.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 20 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.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 21 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.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 22 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.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 23 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) MT321: Principles of Materials Processing Lecture 8:Casting Technology 24 Pressure (or stress) required to drive Hydrostatic pressure in the different feeding mechanismsresidual liquid calculated for various feeding regimes during the freezing of a 20mm Different pressure diameter Al alloy cylinder (from “Casting” by John is required to drive Campbell, 1990) different feeding mechanisms.
The pressure required increases with decreasing the diameter of the liquid channels.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 25 Dissolved gas
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) MT321: Principles of Materials Processing Lecture 8:Casting Technology 26 When coming out of the liquid solution, the gas forms bubbles which then turn into gas pores if they are trapped in the solid.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 27 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).
MT321: Principles of Materials Processing Lecture 8:Casting Technology 28 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.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 29 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.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 30 MT321: Principles of Materials Processing Lecture 8:Casting Technology 31 MT321: Principles of Materials Processing Lecture 8:Casting Technology 32 Note: In many cases, a slight change of the design of a casting can make the casting easier to make or improve its quality.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 33 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
MT321: Principles of Materials Processing Lecture 8:Casting Technology 34 Sand Casting
Sand casting process makes use of sand as the moulding material.
The sand that is often used: silica, zircon or olivine.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 35 MT321: Principles of Materials Processing Lecture 8:Casting Technology 36 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
MT321: Principles of Materials Processing Lecture 8:Casting Technology 37 Where does the refractoriness come from? From the nature of sand: High melting point.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 38 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.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 39 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
MT321: Principles of Materials Processing Lecture 8:Casting Technology 40 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.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 41 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.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 42 Applications of sand casting
processSand 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.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 43 Two ways of compacting sand in automatic sand moulding
MT321: Principles of Materials Processing Lecture 8:Casting Technology 44 An automatic sand moulding process using a match plate MT321: Principles of Materials Processing Lecture 8:Casting Technology 45 Video clip: http://v.youku.com/v_show/id_XNTc5NDExO Investment CastingTMy.html?from=y1.2-1-103.3.7-1.1-1-1-6
MT321: Principles of Materials Processing Lecture 8:Casting Technology 46 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.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 47 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.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 48 Applications of investment
castingTo produce castings with complex shapes.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 49 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.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 50 Applications of lost foam casting:
To produce castings with complex shapes.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 51 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.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 52 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.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 53 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
MT321: Principles of Materials Processing Lecture 8:Casting Technology 54 MT321: Principles of Materials Processing Lecture 8:Casting Technology 55 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
MT321: Principles of Materials Processing Lecture 8:Casting Technology 56 Hot chamber high pressure die casting process: •Liquid metal is pumped into the die cavity. •Liquid solidifies under high pressure. •Pressure > 100 bar
MT321: Principles of Materials Processing Lecture 8:Casting Technology 57 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)
MT321: Principles of Materials Processing Lecture 8:Casting Technology 58 MT321: Principles of Materials Processing Lecture 8:Casting Technology 59 Centrifugal Casting Features: Liquid metal is poured while the die is being rotated. Liquid solidifies under both gravity and centrifugal force.
MT321: Principles of Materials Processing Lecture 8:Casting Technology 60 Applications of centrifugal casting:
Producing pipes (currently major application).
Producing shaped castings (future trend).
MT321: Principles of Materials Processing Lecture 8:Casting Technology 61