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MME 345, Lecture 43

Casting Defects1. Common casting defectsRef: J. Campbell, Castings, Butterworth-Heniemanne, 1992

Summary of casting defects

Oxide film defects Formation of surface film Incorporation of surface film into the bulk liquid Effect of entrained film Deactivation of entrained film Elimination of entrained film

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Oxide films and bubble trails Segregation, inclusion and gas porosity Shrinkage cavity Hot tear and cold crack Residual stress

See “Defects Fact Sheet”

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Two most important film-forming reactions:

M + H2O = MO + 2[H]

CxHy = xC + y[H]

1. Formation of oxide film by decomposition of moisture

2. Formation of graphite film by decomposition of hydrocarbon

Both reactions result in the increase of hydrogen in liquid metal, causing an increased tendency to pore formation.

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In moist, oxidising environment

[O] diffuses and reacts with copper to form Cu2O, which goes into

solution (for up to 0.14 % oxygen) and the excess amount dispersed into liquid as precipitates. So no oxide “film” is produced.

In reducing environmentNo oxide is formed.

Thus, liquid copper is free from film problem in most cases.

Copper and its alloys

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Zinc and its alloys

Most zinc-based casting are made from pressure die casting alloys, which contains up to 4 % Al.

This Al creates a thin film of Al2O3

protects the iron and steel machine parts from zinc attack.

in case of extreme turbulence, this film causes air entrapment and reduces casting quality.

But these problems remained tolerable due to low melting and casting temperatures.

Other Zn-based alloys containing higher amount of aluminium (e.g., ZA series; contain 8-27 % Al) and are highly problematic alloys because of severe film formation.

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Aluminium and its alloys

A solid Al2O3 film is formed from the liquid surface, atom by atom.

For pure Al, the film is initially g-Al2O3, which inhibits further oxidation. After an incubation period, it transforms into a-Al2O3, which allows oxidation at a faster rate (7x10-7 kg/m2/s).

a-Al2O3 film is porous and fresh supplies of liquid metal arrive at the surface of the film, not by diffusion (which is slow), but by flow of liquid along the capillary channels (which is very fast).

Aluminium bronze contains approximately 10 % Al. The high casting temperature and high aluminium level combine to produce a thick and tenacious film, which makes aluminium bronze one of the most difficult of all foundry alloys.

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In presence of Mg, the film changes into spinel (MgO.Al2O3) first, then purely magnesia (for Mg >2%).

Al-Mg alloys containing 5-10 % Mg are especially difficult to cast due to severe film problem.

Additions of alkaline earth metals (Be, Ca, Sr, Ba) and alkali metals (Li, Na, K):

When strontium is added, it oxidizes very rapidly by the moisture in the environment and release hydrogen into the melt.

Thus, Sr addition always increases porosity in aluminium casting.

Sodium addition has less porosity problem, but it increases the rate of oxidation of melt.

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Oxide films in grey iron

At high temperatures, CO is more stable than SiO2, so no film is formed. Below 1400 C, stability is inverted and SiO2 appears on the surface as dry, solid, grey coloured film.

At lower temperatures, complex oxides containing FeO, MnO, SiO2 and MnS form low-melting eutectic film, which floats on top. If it becomes entrained, it quickly spheroidize into compact droplets and float out rapidly due to lower density.

The harmless disposal of the oxide film in this way is reason for which cast irons are cast effectively in greensand moulds without having much trouble with turbulence.

Films on cast iron

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Oxide films in ductile iron

Presence of Mg changes the nature of oxide film.

Below 1454 C, a film starts to form, increasing its thickness to 1350 C, at which point the surface exhibits solidified crusty particles.

By 1290 C, the entire surface is covered with dry dross. Oxidation of Mg vapour to powdery oxide at the upper surface of the dross causes the quick and copious growth of the dross.

Because of this dry, non-wetting heap of dross, ductile irons are difficult to cast cleanly without such unsightly dross defects.

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Graphite films

When cast irons are poured in resin-bonded mould, a new defect, known as “lustrous carbon” is formed.

It is a graphite film, deposited from the hydrocarbon gas on to the liquid surface. This film is very similar to the appearance of alumina film on aluminium (shiny, wrinkled like elephant skin).

In a high concentrated environment of hydrocarbons, the rate of arrival of C on to the surface is higher than the diffusion of C into the bulk liquid. Thus, C becomes concentrated on the surface and may exceed saturation, allowing carbon to build up at the surface as solid.

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The problem with a surface film only occurs when it becomes a submerged film

Ways of mixing of surface film into the bulk:

1. Melt charge materials 4. Surface turbulence2. Pouring 5. Confluence weld3. Surface flooding 6. Bubble trails

Surface film is not harmful when it remains on top of the surface

in case of aluminium, the surface film protects the liquid from catastrophic oxidation (as in the case with Mg)

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1. Melt Charge Materials

Introduction of films from charge materials is most common during melting of aluminium.

When the charge is introduced directly into a liquid pool, oxide layer on the charge becomes immediately submerged, to float freely later when the underlying solid melts.

For chill cast ingots, the film is thin, but for sand cast ingots the skin grows very thick. This way, the melt can become so bad as to resemble a slurry of old sacks.

When 99.5 % pure aluminium was melted repeatedly, after only 8 times, the elongation value was reduced to 20-30 %.

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2. Pouring

During pouring, the surface film grows very quickly and form a tube around the falling stream.

If the length of the falling stream is high, this oxide tube can be broken due to shear force and, together with entrained air, becomes submerged into the liquid.

If the pouring head is low and the liquid is poured in a weir basin, which is full of liquid, the oxide film may not enter the casting.

If the pouring head is high or a conical cup is used during pouring, then practically all of the oxide formed will enter the casting.

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The underside of film is well wetted by the liquid. The top surface is, however, dry.

During flooding of liquid metal over the surface of a film, air is trapped in between the dry sides of two overlapping films.

3. Surface Flooding and Formation of Bi-film

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In case of unstable advance of film forming alloy during pouring, double oxide layer, called the “bi-film” is formed between the first and second layers of advancing liquid.

This problem can be avoided by increasing the rate of filling, or by reducing the film forming condition

(for example, by using vacuum, or by maintaining a reducing atmosphere).

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4. Surface Turbulence

In fluid dynamics, turbulence is measured by using Reynold’s number:

V = velocity of melt

r = density of melt

d = linear dimension of flow path

n = viscosity of melt

Cd = drag coefficient

Re = Vrd / n

Re < 2000, smooth, laminar, turbulent-free flow

Re > 2000, turbulent flow

The flow behaviour of liquid considered here takes place in the bulk of the liquid, underneath the surface. During such turbulence, the surface of liquid may remain quite tranquil.

Turbulence as predicted and measured by Reynold’s number is therefore strictly bulk turbulence.

Internal pressure = V2r / 2Cd2

Viscous pressure = nV / d

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The surface films are submerged into the bulk liquid is due to the turbulence occurred at the surface of the liquid.

Pressure on surface by bulk liquid to disrupt the surface

= V2r / 2Cd2

Pressure against surface tension to create new surface

= 2g / r

The Weber number:

We = V2rr / g

We = 0.2 – 0.8, free from surface turbulence

We = 100, surface turbulence becomes problematic

We = 100000, creates atomization !!18/30

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5. Confluence Weld

The separation and rejoining of metal stream involves the formation of films at the advancing front of liquid with the consequent danger of the stream having difficulty in rejoining successfully.

Number of conditions that may happen for confluence weld:

1. Two streams do not meet at all.

2. Two streams touch, but the join has no strength

3. The joint has partial strength

4. The joint has full strength, because the streams have successfully fused, resulting in a joint which is indistinguishable from bulk material.

Clearly, condition 4 is the target

Defects as in conditions 1 and 2 are undesirable

But condition 3 is the most serious, because it is difficult to detect

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6. Bubble Trail

It is the defect which remains in film-forming alloy after the passage of bubble (of air, water vapour, core gases) through the melt.

The inner wall of this oxide (or graphite) tube is dry and non-adherent, forming an excellent route for the escape of further bubbles, which will help to strengthen the film formed.

The trail is a serious threat to mechanical strength and integrity of the casting.

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The submerged films are always associated with air or other gas, trapped on the non-wetted dry surface of the film, or trapped between the folded film.

General Problem due to Submerged Films

The gaseous films floated around the liquid constitute cracks in the liquid and, after freezing, constitute cracks in the finished products.

The gas-coated film acts as excellent nucleating sites for the subsequent growth of bubbles or shrinkage cavity.

Higher-melting-point heavy phases may be precipitated on to the floating oxides, which form defects with large, coarse crystals of heavy intermetallic phase, together with entrained oxide film and associated porosity.

Oxides are much harder than the metal itself, causing dragging out during machining, leaving unsightly grooves.

The cutting edge of tool is often chipped or blunted by encounters with such problems.

Machining Problem

For thin-sectioned castings (< 5 mm), film defects can be extended from wall to wall across the mould cavity, and so connect the casting surfaces with a leak path.

Bubble defects are specially troublesome with respect to leak tightness, since they necessarily start at one casting surface and connect to the surface above. A leak path is almost guaranteed.

Leak Tightness

The fluidity of clean melt is always higher than that of dirty melt, and can be cast at a lower temperature.

The cumulative benefits are valuable. (oxide free casting high properties; low porosity high properties;

low pouring temperature finer grain size, high properties

Fluidity

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Castings made using processes which reduce surface turbulence have been

found to have uniform good mechanical properties with low scatter. They also

show excellent fatigue resistance.

Mechanical Properties

Al-4.5Cu fractured surfaces(a) Oxide covered (0.3 % elongation)(b) Ductile fracture (3.0 % elongation)

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In case of submerged film, the bottom side of the film, which is in contact with liquid, is well-wetted and is an unfavourable nucleating site for volume defects such as cracks, gas bubbles or shrinkage cavities.

The side of the film in contact with air causes the most problem:

1. They are non-wetted by the liquid2. Microscopically rough surface retains additional gases3. When folded, they form bi-film defects, which stores even more gases

It is the film of air (or other gas) which is the most damaging

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When submerged, gases on the dry side of film react with the liquid metal (oxygen to form oxides, nitrogen to form nitirdes, argon goes in solution)

eventually all the gases are consumed and the gas film will disappeared

and the most damaging effects of the film (causing leaks, nucleating bubbles, cavities, cracks) will have been removed

This natural or automatic deactivation of entrained film usually occurs in cases where the metal is subjected to pressure

squeeze casting @ 50-150 MPa

hot isostatic pressing @ 200 MPa

even some benefits are obtained in sand casting with moderate pressure of only 0.7 MPa

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Natural or automatic deactivation of film is not occurred in general practices.

So it is important to try to reduce the formation of oxide at all stages of melt preparation and handling.

Ultimately, the only way to succeed in reducing the oxide in casting is to usefilter in the running system, after pouring, but before the metal enters the mould, to ensure that the flow conditions into the mould cavity do not reintroduce new oxide.

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Next ClassMME 345, Lecture 44

Casting Defects2. Effect of defects on properties