Handout 2: Casting 1

ENGINEERING TRIPOS PART IIA, 2012-13 MODULE 3C1: Materials Processing and Design

MANUFACTURING ENGINEERING TRIPOS PART IIA, 2012-13 PAPER 3P1: Materials into products

HANDOUT 2: CASTING 1. Processes, selection and design 1.1 Casting processes 1.2 Process selection 1.3 Design of castings 2. Solidification theory 2.1 Revision of nucleation theory 2.2 Solidification mechanisms 2.3 Solidification of alloys 3. Microstructure of castings 3.1 Grain structure 3.2 Chemical inhomogeneity 3.3 Porosity 3.4 Casting alloys

Essential Revision: Phase diagrams, phase transformations, shaping processes IB Materials notes + Teach Yourself Phase Diagrams (www-materials.eng.cam.ac.uk/typd) Ashby, Shercliff & Cebon: Materials: engineering, science, processing and design (Ch. 18, 19) Ashby & Jones: Engineering Materials II

Other References for Casting: Edwards L and Endean M. Manufacturing with Materials (CUED JA146) Waters, TF. Manufacturing for Engineers (CUED BN204) Campbell, J. Castings (Lots of technical detail) (CUED JO41)

H.R Shercliff (C.Y. Barlow) October 2012

1 1. PROCESSES, SELECTION AND DESIGN

1.1 Casting processes

Typical casting process and terminology: Sand Casting

1. A solid re-usable pattern (often wooden) is made of the component.

2. Sand with a small amount of resin binder is packed around the pattern in a box called a drag.

3. The drag is inverted and the pattern is lifted out, leaving a cavity. In-gates and runners may be carved or moulded into the sand.

4. Interior detail may be produced by inserting a core (also moulded out of sand) into the cavity.

The upper part of the mould (the cope) is formed from sand, incorporating a pouring basin, a sprue, vents, risers/feeder heads (either moulded from patterns – e.g. runner pin and riser pin shown – or which may be carved in.

5. Mould bolted together, metal poured in. Once the casting has solidified, the mould is removed and the sand mould and any cores broken up and brushed out. The casting is fettled: cutting off the runner, ingate, sprue, risers and feeder head. The parting line of the mould may also leave a ridge which must be ground off.

2 Overview of Casting Processes

Melt Transfer into mould Remove from mould Refractory (ceramic) crucible Pour under gravity Permanent mould (e.g. ingot casting, “Clean” heat sources: or continuous casting, Electric furnace, or RF Force under high pressure die casting): induction into mould open mould, remove (can be under vacuum or in an or inert gas atmosphere) part, clean mould and Use inert gas pressure re-use or (controlled atmosphere) to Oil/gas-fuelled furnace force metal into mould Permanent pattern (e.g. sand casting): one-off moulds, destroy on removal Check composition Solidify: a few before casting; additives seconds for small to refine grain size or parts; days for large modify structures

Classification of casting processes

Ingot or Continuous Shaped Casting Casting (i.e. solidify to near net-shape) Permanent mould casting Permanent pattern casting Simple shapes: no re-entrant More intricate shapes: mould surfaces (need to be able to for each individual casting is Continuous casting remove parts from mould). created around a pattern and for most high-volume Moulds expensive (tool steel; mould is destroyed as the steel; “direct chill” may make 103 – 106 castings), casting is removed. (DC) casting for production rates high. Low setup costs and wrought aluminium production rates. alloys. Typically used for large numbers of small parts. Used for larger parts, or Ingot casting when small production runs. (permanent mould) Examples: Examples: used for lower Gravity die casting Sand casting volume alloys. Pressure die casting Investment casting Centrifugal casting Evaporative mould casting

Post-processing: Post-processing: Homogenisation “Fettling” (trim solidified feeder channels) + Thermomechanical Machine/grind critical areas (improve tolerances and surface processing, e.g. finish around joints, seals, contact surfaces) hot/cold rolling, Machine/drill features, holes etc. forging, extrusion Some castings heat-treated to improve properties. + heat treatment

3 Examples of permanent pattern (expendable mould) casting

Sand Casting (details above) Advantages: Versatile, low material and equipment costs, OK for large simple parts; internal detail possible. Disadvantages: Poor dimensional accuracy and surface finish; not good for thin sections; relatively high labour costs; “dirty” process; can’t be used for refractory metals because sand undergoes phase change.

Investment casting Something of a hybrid process: by “permanent pattern” we mean that a permanent mould is made to make an expendable pattern. This pattern is covered in a disposable ceramic/ refractory shell in which the casting itself is produced. Advantages: Excellent accuracy and surface finish Disadvantages: Limited to small parts; much more expensive.

4 Full / evaporative mould casting Closely related variant, using a polystyrene foam pattern. Advantages: High accuracy and surface finish (especially with small-bead polystyrene); lighter patterns than wax, so suitable for large parts. Disadvantages: Labour still quite high

Examples of permanent mould casting

Pressure die casting Externally applied pressure permits use of higher viscosity fluid, thinner sections, and minimises waste from runners, risers etc). Susceptible to entrapped bubbles due to turbulence, which can be detrimental to properties – see later.

A common, important process – often just called ‘die casting’. Limited to low-melting point alloys (because the dies must not distort or wear whilst making many thousands of castings).

Common example: zinc die-casting alloy (a low-melting point alloy, Zn + 4Al, 1Cu, 0.05Mg), chosen for ease of processing and cheapness, rather than for good mechanical properties, e.g. toy models.

(Note equivalent polymer process: injection moulding, the commonest polymer process. It uses a rotating screw to plasticise the polymer, but the whole screw is translated along the feeder tube to force the polymer rapidly into the mould.)

Gravity die casting Variant process using gravity feed (as in sand casting) but with permanent mould in separable parts, as in pressure die casting.

5 Centrifugal casting

Used for axisymmetric hollow parts (e.g. pipes)

(A related process for polymers is Rotational Moulding)

1.2 Process selection The choice of process depends on a range of factors including: material size, shape complexity, section thickness These are covered by CES (below) dimensional accuracy, surface finish number of parts required

mechanical (and other) properties Properties sensitive to the combination of material, process and design parameters

Technical attributes

6 Quality attributes

Economic attribute Process attribute charts from CES, for metal shaping processes.

The same factors influence the choice of process class (cast vs. deformation vs. powder) and the choice of process variant within a class (e.g. sand cast

Observations (both comparing process classes, and on variants within casting): - considerable overlap between process classes on mass and section thickness; - wide variation in precision and surface finish; - competition between process classes largely driven by economics – but remember that it may be cheaper to use an inexpensive process (e.g. sand casting) followed by a local machining), rather than a more expensive process.

Casting Alloys versus Wrought Alloys Casting and thermomechanical (wrought) processes use different, dedicated alloys, as casting alloys must satisfy separate requirements relating to fluidity, lower melting point, and solidification microstructure. For example: - carbon steels (Fe + 0.1-0.8wt% C): hot/cold formed; cast iron (Fe + 4wt% C): only cast. - wrought Al alloys: Al + Mg + (Cu, Zn or Si) ( 1-5wt%): hot/cold rolled, extruded; cast Al alloys: Al +Si (typically 12%) (+ Cu, Mg): only cast.

Castings have historically had poorer mechanical properties than their forged counterparts, largely because of a tendency to contain porosity, and a high second phase content. Casting (plus heat treatment) can be the route to high-quality property-critical components (e.g. internal Al alloy frame of Airbus doors, jet engine nickel alloy turbine blades).

Remember that the properties achieved in a casting are dependent on the coupling between material, process and design parameters.

7 1.3 Design of castings

Solidification rate Solidification time is important in casting because it affects: - production rate, and hence process economics - the resulting microstructure, and hence properties Chvorinov’s rule states that: solidification time of a section is proportional to [Volume/Surface area]2

Physical basis:

One application of this is in the design of the feeder heads for (e.g.) sand castings. Most metals shrink on cooling and solidifying, so moulds are designed to hold a reserve of molten metal to allow metal to be fed in during solidification. Hence the metal in the feeder head must solidify last, otherwise parts of the casting may be starved, leading to porosity:

Solidification rate also affects the scale of the microstructure, which may have implications for mechanical properties. What matters physically is not the total time for solidification, but the local velocity of the solidification interface. The length-scale of the microstructure (e.g. the spacing of the plates in eutectics) is inversely proportional to this velocity.

Casting Defects Defects include cavities, and internal or surface cracks. Non-destructive testing (NDT) of component integrity is routine for larger parts, by such methods as: Surface: visual examination, die penetrant, magnetic particles Internal: X-rays, sonic (e.g. C-scan, damping properties)

The formation of defects in casting depends on many factors in the design of the gating system and the mould, the shape of casting, and the alloy used. The main physical origins of defects are: porosity and turbulence, fluidity, and shrinkage.

(i) Porosity and Turbulence Porosity in casting can be on a macroscopic scale (cavities on millimeter scale) or on the scale of the microstructure (micron scale). The source of porosity is either dissolved gas released during solidification, or bubbles entrapped by turbulence. Gases may be absorbed from the atmosphere, or may be injected as part of process (e.g. oxygen is injected in steel- making to burn off the excess carbon in pig iron, to the lower level needed in the steel, leaving residual oxygen, CO and CO2 dissolved in the steel). More details in section 3.3.

8 Turbulence may arise close to where metal is poured into mould, or within the mould if there are changes in section. (a) Air may be entrapped, leading to the formation of large-scale porosity (blowholes), or smaller pores. Pressure die-casting always results in turbulence and porosity because the metal enters the mould so quickly: the mechanical properties of such castings are notably poor as a result. Vacuum die-casting, though expensive, improves casting quality – the metal is degassed by putting it under vacuum before casting.

(b) In sand casting, the mould may be damaged by rapid metal flow, releasing sand into the casting and causing loss of dimensional accuracy of the casting. Types of mould damage:

(c) The surface of the liquid metal is often oxide covered: with turbulent flow the oxide can become entrapped in the casting.

Solutions: Well-designed gating system. Avoid sudden changes in section thickness. Vacuum degassing before casting.

(ii) Fluidity: Misruns and Cold Shuts

Solutions: Redesign running and gating systems (position, size and number of ingates and vents). Increase fluidity by raising pouring temperature or preheating mould.

9 (iii) Shrinkage Moulds must be designed to allow the semi-solid casting to shrink safely (particularly when there is a large semi-solid temperature range): semi-solid has very little strength.

When section changes are unavoidable, the solidification pattern may be altered by the use of chills, to cause early solidification in vulnerable regions (e.g. metal insert in sand mould).

2 SOLIDIFICATION THEORY 2.1 Revision of nucleation theory A system is characterised by the Gibbs free G energy G = H – TS (H is enthalpy; T absolute temperature; S is entropy). For any phase, G Liquid varies with T as shown. Phase transformations occur when the lines Solid for two states cross: a system can reduce its free energy G by shifting to the lower line. The energy difference between the lines, the

volume free energy, is the driving force. Temperature, T

Homogeneous Nucleation Crystals form spontaneously (by statistical fluctuations) within a liquid which is below its melting temperature. The system reduces its internal energy U by producing solid: volumetric (chemical) free energy Gv (which will be a negative number) is the driving force for solidification. Surface energy  per unit area must be supplied by the system. SL

10 For a spherical nucleus: Net energy change = Surface energy + volume energy U  4 r 2   4  r3 G SL 3 v H T Volume free energy Gv  , where undercooling T = Tm – T Tm (Tm = melting temperature).

For a nucleus to be stable its energy must fall as r increases: differentiate U to find maximum: U 2  0, from which the critical radius r*  SL r Gv For a typical nucleus of radius r* = 1nm, undercooling = 100K (very large).

Heterogeneous nucleation Instead of the nuclei atoms forming a spherical cluster, they are attached to a nucleating surface, e.g. mould wall, impurity (details later).

liquid, L contact or interface energy SL ‘wetting’ angle 

solid, S

nucleating surface, N

Note that there are actually three surface energies interacting: SL (as before), and also those between the nucleating surface and both liquid and solid, NL and NS. The solid forms a circular area on the nucleating surface, but has displaced the same area that was previously there between the liquid and the nucleating surface. There is therefore a net energy change given by the difference in the surface energies (NS – NL), multiplied by the circular area of contact. The interaction between the competing surface energies is conveniently captured by the contact or wetting angle, .  Volume of spherical cap = 1  r3 ( 2  3cos  cos3 ) 3 2 r*  SL (as before) Gv

Undercooling to achieve critical radius is much smaller – virtually all nucleation in casting is heterogeneous.

11 Temperature dependence of nucleation rate: Nucleation rate

T = Tm – T

Influence of contact angle : Graph shows ratio of net energy changes at critical radius, U*, for heterogeneous to homogeneous nucleation.

Good wetting (i.e. low ) is needed for easy heterogeneous nucleation. Very specific particles are used in industrial solidification

U*HET processes to stimulate heterogeneous U*HOM nucleation. These are known as inoculants (more detail later).

 2.2 Solidification mechanisms

Stable growth If molten metal is poured into a mould to cast an ingot, solidification will start as a result of heterogeneous nucleation on the mould walls. Latent heat is released at the solid-liquid interface as solid forms. Heat is removed from the melt by conduction through the mould wall, and the solidification front moves in towards the centre of the ingot. Consequences:

Temperature of interface is TM Solidification rate proportional to rate of removal of latent heat Growth rate determined by rate at which heat is lost from system

Metals solidify easily because solid-liquid interfaces are rough on an atomic scale (even though they may be smooth on a microscopic scale). This means that there are many sites available for atoms to attach themselves firmly to the new solid. Leads to rounded, ‘blobby’ solid particles.

Non-metals (e.g. graphite, silicon) generally have an atomically smooth solid-liquid interface. Growth takes place by steps growing across the solid in an orderly way, and is much slower than for metals. Leads to more angular particles: needles; ‘crystals’, snowflakes

The same applies when these non-metals form as a second phase in a casting alloy (e.g. graphite in cast iron, silicon in Al alloys). This is not good news for toughness – flakes of brittle phases behave like cracks. The growth can be modified by poisoning: atoms of an alloy addition which halt the growth of steps, leading to smaller more rounded “morphology” (size and shape) of the brittle second phase (examples later).

Unstable growth: dendrites Consider first the solidification of a pure liquid (alloys are considered in the next section). Nucleation requires supercooling of the liquid, with the first growth of nuclei into liquid below the equilibrium solidification temperature, Tm. The nuclei release latent heat which warms up the solid, and the liquid immediately around them, to the melting point. Overall cooling history: Away from the nucleus, there is a negative temperature gradient into the supercooled liquid:

time

13 As a result, there is a greater driving force for solidification away from the solid-liquid interface than at the interface itself. Growth is therefore unstable – if part of the nucleus extends into the surrounding liquid, it can continue to grow rapidly in that direction. This occurs spontaneously in particular crystallographic directions, leading to long thin crystals. The same behaviour may then be repeated with secondary side arms forming on the side of these crystals.

The distinctive crystal shape formed during solidification is known as a dendrite.

In pure liquids, these are known as thermal dendrites (as they are the result of a thermal gradient leading to supercooling). Dendrite growth stops when the residual liquid has all been warmed to the melting point – further solidification is then by stable growth (thickening of the dendrite arms), governed by the remote loss of heat to the surroundings. Once the arms of a given dendrite meet, they form a single grain (since all arms of the dendrite have a common crystallographic orientation), and their characteristic structure is not visible – the final structure only reveals the grain boundaries where the crystal orientation changes. Each nucleus has a random orientation of the crystallographic planes, and becomes a single grain.

14 2.3 Solidification of Alloys Concentration gradients in solidification Consider the solidification of liquid with initial concentration Co. Idealised phase diagram: Definitions: k = partition coefficient (k < 1)

CS /CL = k

where CS and CL are equilibrium concentrations

Solidification of a long bath of liquid with diffusion in the liquid:

(a) Formation of initial transient

Solidification starts at temperature T0.

For typical solidification timescales, there is essentially no diffusion in the solid, hence the non-uniform concentration gradient is “locked in”: this is known as segregation.

15 (b) Steady state

Solidification temperature is now T1

Solidification temperature Long bar now undefined, but can be well below T1

Now consider solidification from both sides of a mould towards the centre (two of these curves back-to-back). The excess solute from both sides concentrates into the last part to solidify. This macrosegregation (i.e. on the scale of the whole casting) may lead to poor mechanical properties (high concentrations of brittle second phases in the grain boundaries). And if the segregating solute is a gas, this may lead to the formation of macroporosity in the centre of the casting. More details later.

Constitutional supercooling of alloys

Recall from Part IB Teach Yourself Phase Diagrams: “Constitution” = phases present, their proportions and compositions Consider again the steady-state distribution of composition, away from the initial transient region (figure below). From the phase diagram, the higher the concentration, the lower the solidus temperature. Hence there is a gradient in the local solidification temperature with position, which essentially has the same form of the concentration curve, but inverted.

Now we superimpose the actual temperature gradient in the casting (decreasing from right to left, as heat is conducted out through the mould in this direction). The temperature is equal to the local solidification temperature at the interface, but there are then two possibilities, depending on the gradient in the temperature (dT/dx) compared to the gradient in the local solidification temperature (dTm/dx).

(Make sure that you distinguish the gradient, with x, in Tm from the gradient in T).

16 (i) (dT/dx) > (dTm/dx): T in liquid everywhere above its (local) melting point; solidification is stable at the interface (leading to a planar solidification front and columnar grain growth).

(ii) (dT/dx) < (dTm/dx): there is a region ahead of the interface for which T in liquid is below its (local) melting point; solidification is therefore unstable – regions extending into this liquid can rapidly solidify ahead, i.e. dendrite formation.

This is known as constitutional supercooling, since in this case it is a composition gradient coupled with a thermal gradient that leads to supercooling.

This condition can remain throughout steady-state solidification, but only in the small region just ahead of the growing solid – the “bow wave” of elevated concentration translates ahead of the interface, at the same speed. A constitutionally supercooled region is commonly found in the solidification of alloys, so dendritic growth is the usual solidification mode.

The imposed cooling gradient, dT/dx, depends on many factors: - the casting alloy (i.e. its thermal properties, and its phase diagram) - type of mould (i.e. its thermal properties and its size), pouring temperature - size and shape of the casting

This is a good example of how casting properties depend in a complex coupled way on the combination of material, process and design parameters.

17 3 MICROSTRUCTURE OF CASTINGS 3.1 Grain Structure In general, heterogeneous nucleation occurs first on the cool mould walls, with a dense array of small grains forming the chill zone. As cooling continues, favourably-oriented nuclei grow inwards to form a columnar zone. The central part may be occupied by an equiaxed zone, heterogeneously nucleated within the melt. The proportions of the three structures depend on the alloy and the cooling conditions:

Horizontal Section

Vertical Section

No nucleation within Nucleation both on No nucleation on walls melt walls and within melt

Columnar zone growth mechanism: Nuclei in the chill zone are randomly oriented. Competitive growth takes place – those with their fast growth direction normal to the mould wall survive. These form the columnar zone.

18 It is often desirable to ensure that equiaxed grains form the major part of the casting:  Finer grain size.  Reducing the extent of macrosegregation (see below).  Interfaces between columnar grains also contain high proportions of impurities due to lateral segregation. Feeding liquid metal into these regions is difficult, so fully columnar castings can contain interconnected porosity.

Nuclei for grains in the equiaxed zone can arise from various sources: (a) Oxide or solid metal nuclei formed on surface of melt during pouring (b) Inoculants (grain refiners): small amounts of specific solid (fine powder) added just before pouring. Inoculants must have a very low wetting angle to allow easy nucleation.

e.g. addition of 0.05 – 0.1%TiB2 is standard for cast Al alloys. (c) Turbulence in melt breaks off dendrites, which form stable nuclei in body of melt. Increase turbulence by vibrating mould – though this is not practical for many processes.

Special process: casting single-crystal jet engine turbine blades

Grain boundaries accelerate creep (e.g. providing short circuit diffusion paths). So components designed to resist stress under high-temperature conditions are sometimes made as single crystals. To achieve this, we need to make one 'seed' crystal grow. Rather than placing a seed crystal into the casting mould, we can force the system to perform the selection of a single seed itself. Rolls Royce Trent 800 turbine blades are made from creep-resistant nickel-based superalloy MAR-M200. These “nimonic” alloys are fcc, with fast growth directions at 90º to each other. The blades are solidified by slowly withdrawing downwards from a furnace, so solidification starts in the helical grain selector, or ‘pigtail’. This guarantees that only one crystal orientation survives as the casting solidifies from the bottom. Tight control of the thermal conditions around the casting is required to ensure that the blade solidifies from this end only, without further nucleation from the mould. Interior channels are also created by using a "core", held in place before and during casting by positioning pins (P’Pins). The core is dissolved out after casting.

19 3.2 Chemical inhomogeneity Cast structures in alloys all have some degree of partitioning of elements (i.e. segregation). (a) Macro-scale: If the columnar zone extends through the whole casting, there will be inhomogeneity on the scale of the casting. Impurities accumulate on the centre-line of the casting, leading to a plane of weakness. Solution: rather than removing the impurities (expensive) the solution is further alloying, to trap the impurities in a harmless form throughout the casting. e.g. all C steels contain sulphur as an impurity; the addition of Mn leads to the formation of a dispersion of MnS particles thoughout the casting, rather than letting the S segregate to the grain boundaries, forming brittle FeS.

(b) Medium scale: Equiaxed grains will have some edge-to-centre segregation (typical grain sizes 100µm up to several mm). Dendritic or columnar grains will show compositional variation across their width: the interior of the grains being purer than the grain boundaries. Composition gradients can sometimes be revealed by etching (see figure).

The ‘dendrite arm spacing’ refers to the length scale of the secondary dendrite arms (typically a few µm), and is often cited as a key length-scale of the cast microstructure (i.e. the scale of inhomogeneity that can be influenced by heat treatment – see below).

Secondary dendrite arms

Segregation leads to different parts of the casting having different mechanical and physical properties (e.g. yield strength, melting point). The significance of segregation depends on the length size, and what happens subsequently to the casting: Large-scale: central impurities may end up as a weak mid-plane when an ingot is rolled. Medium-scale: grain boundaries may corrode more rapidly, or form coarse precipitates at grain boundaries, reducing toughness or ductility. Micro-scale: non-uniform distribution of precipitates in subsequent age-hardening heat treatments.

20 Homogenisation heat treatment Homogenisation is an immediate, prolonged “soak” of a casting at high temperature in the single-phase field (e.g. 24 hours at 530-580oC for Al alloys, with melting point c.630oC). This requires bulk diffusion of the components that are distributed inhomogeneously. In most cases the alloying additions form substitutional solid solutions (rather than interstitial), and are therefore inherently slow to diffuse – hence high temperatures and long hold times are needed. The degree of homogenisation can be estimated using the rule-of-thumb diffusion equation: x2 = Dt where diffusion of species with diffusion rate D takes place over a characteristic distance x in

time t, and D = Do exp (-Q/RT) (cf. Materials Databook). The practical limits to temperature and time set a limit on the length-scale that can reasonably be homogenised (see Examples Paper 1).

3.3 Porosity Porosity stems from entrapped bubbles (due to turbulent flow of the melt), or from dissolved gas segregating and coming out of solution during solidification (the bubbles effectively being “precipitates” of gas). Porosity is more severe if liquid metal cannot feed into the last regions to solidify, leading to large cavities. Interconnected 'sponge porosity' can cause castings to leak in service. Dissolved gas can be partially extracted by holding the melt under vacuum (expensive). Alternatively, it can be converted into solid particles (e.g. oxides) during solidification,

e.g. ‘killing’ a steel means adding Al powder (or other oxide former), to produce Al2O3 particles, effectively removing oxygen from solution. Large scale: mainly relevant to bulk cast ingots, subsequently rolled to stock sizes. In some cases, a controlled amount of porosity can be beneficial:

(a) Degassed steel (b) Balanced “semi- (vacuum-treated or killed” steel: killed): Distributed bubbles Ingot contracts without within the interior, further metal feeding, balancing the volumetric leading to macroporosity contraction. Whole ingot and an internal “pipe”. can be rolled, and porosity Ingot must be cropped, is flattened out during leading to high scrap subsequent hot rolling. fraction.

21 Over 90% of steel in Britain is produced by continuous casting, in which macroporosity problems are eliminated. The steel is vacuum degassed before casting. degassed before casting.

Ingot structure:

- part columnar, part equiaxed.

- no macroporosity (liquid “sump” connected to incoming liquid metal).

Micro scale: Porosity on the scale of the dendrite arms is a real “fingerprint” of cast structures, and can be important in limiting mechanical properties.

Example cast microstructures: e.g. Al-4% Cu As-cast : Homogenised:

Dark regions are Grain size inter-dendritic unchanged – but regions and grain grains don’t look like dendrites any boundaries. more: impurities Dark colour arises have redistributed. from etching of Dark spots within impurities, and the grains are the from porosity. porosity trapped Each dendrite makes between the one grain. dendrite arms. Note that the porosity remains after homogenization: the pores are filled with gas, and for the pores to close the gas has to diffuse away through the solid. Most of the gas is in the form of molecules rather than single atoms, and the diffusion rate for these is very small (since molecules are much bigger). Porosity can be removed by deformation e.g. rolling: OK for an ingot; not for a near-net shape casting! Hot isostatic pressing (HIPing) can be used to remove porosity (see Powder Processing lectures) – costly, but provides marked improvement in mechanical properties. Easiest for Al and Mg alloys, where HIPing temperature required is comparatively low (so relatively economical).

22 3.4 Casting Alloys For near-net shape castings, material considerations include: Melting temperature, solidification temperature range, shrinkage. Common casting alloys are often close to eutectic composition. Advantages: Low melting temperature: Cheaper (lower heating cost, faster production rate)

Low solidification (“freezing”) range: (i.e. from liquidus  solidus temperatures) High fluidity Easier to feed: lower shrinkage, lower porosity

The “freezing range” is important for casting integrity: liquid feeding will be slow in the semi-solid “mushy zone”, and the mixture of liquid and solid has very little strength so tears easily due to contraction stresses.

Two-phase material so scope for improved strength (fine-scale hard second phase)

Example cast microstructures: Al-13%Si eutectic (basis of many Al casting alloys) (Phase Diagram: see Materials Databook) The silicon forms a network structure of hard and brittle needles in the soft Al matrix. Considerably improved strength, toughness and ductility are achieved by poisoning the growth mechanism of the brittle non-metallic phase: adding 0.01% sodium immediately before casting to modify the growth to give finer, more rounded particles of silicon.

Unmodified Al-Si eutectic Na modified (Al removed by etch): much finer, rounded Si phase

23 Grey Cast Iron Cast irons still have high-volume markets. Example near-eutectic alloys: Fe + 3-4%C (eutectic at 1130ºC). Microstructure: ferrite + graphite (+ iron carbide, depending on composition/heat treatment).

Graphite ‘morphology’: interconnected network of plates

(a) Schematic of graphite (b) Scanning Electron Micrograph of deep-etched sample (Fe removed)

The graphite plates are weak, and act like cracks – so unmodified cast iron has poor tensile properties. The flake size is dependent on the cooling rate: faster cooling gives more nucleation, smaller flakes and higher strength.

The graphite flakes are good for damping mechanical vibration (so useful for machine tool mountings), and cast iron can be machined without lubricant (as the

graphite provides internal lubrication). (c) Optical micrograph (polished surface)

24 Improved mechanical properties can again be achieved by poisoning: changing the growth mechanism of the (non-metallic) graphite. In this case, the addition of a small amount (approx. 0.5wt%) of magnesium or cerium causes the graphite to form as nodules, significantly improving the strength and toughness.

As-cast structure of modified cast iron: ‘Nodular’ or ‘Spheroidal Graphite (SG)’ cast iron.

Dimensional changes on solidification of cast iron

In carbon steels and “white” cast irons, the carbon forms cementite Fe3C. However, in grey cast irons, much of the carbon forms graphite (as above). Graphite is a low density phase, so this counteracts the contraction of the iron on solidification. For a specific composition of grey cast iron (just hypo-eutectic) there is a zero volume change on solidification – useful for producing castings directly to final size and shape.

% volume change on solidification

Fe3C