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5/31/2010

DMME

Bangladesh University of Engineering and Technology g y g g gy

MME 6203 Advanced Topics in Foundry Engineering

Lecture 3 Lecture 3

Casting Defects

1. Oxide Films

A.K.M.B. Rashid Professor, Department of MME BUET, Dhaka

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Today's Topics

Summary of casting defects 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 Deactivation of entrained film Elimination of entrained film

© B. Rashid, DMME, BUET

Lec #3: Casting defects ­ oxide films

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Casting Defects

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

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Lec #3: Casting defects ­ oxide films

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Film Forming Reactions

Two most important filmforming reactions: 1. Formation of oxide film by decomposition of moisture 1 Formation of oxide film by decomposition of moisture M + H2O = MO + 2[H] 2. Formation of graphite film by decomposition of hydrocarbon CxHy = xC + y[H] Both reactions result in the increase of hydrogen in liquid metal, causing an increased tendency to pore formation.

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Lec #3: Casting defects ­ oxide films

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Surface Film Formation

Copper and its alloys

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 environment In reducing environment No oxide is formed. Thus, liquid copper is free from film problem in most cases.

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Surface Film Formation

Zinc and its alloys

Most zincbased casting are made from pressure die g p 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. and casting temperatures Other Znbased alloys containing higher amount of aluminium (e.g., ZA series; contain 827 % Al) and are highly problematic alloys because of severe film formation.

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Surface Film Formation

Aluminium and its alloys

A solid Al A solid Al2O3 film is formed from the liquid surface atom by atom film is formed from the liquid surface, atom by atom. For pure Al, the film is initially Al2O3, which inhibits further oxidation. After an incubation period, it transforms into Al2O3, which allows oxidation at a faster rate (7x107 kg/m2/s). Since the oxide is impervious to the diffusion of both metal and oxygen, how can further growth occur after the first molecular thickness? The 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).

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Lec #3: Casting defects ­ oxide films

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Surface Film Formation

Aluminium and its alloys

In presence of Mg, the film changes into spinel (MgO.Al ) In presence of Mg the film changes into spinel (MgO Al2O3) first , then purely magnesia (for Mg >2%). AlMg alloys containing 510 % Mg are especially difficult to cast due to severe film problem and known as the world's most uncastable casting alloys !! Additions of alkaline earth metals (Be, Ca, Sr, Ba) and alkali metals (Li, Na, K) have similar effects. metals (Li Na K) have similar effects

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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Surface Film Formation

Films on cast iron

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 q , quickly spheroidize into compact droplets and y p p p 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.

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Surface Film Formation

Films on cast iron

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 q ick copio s gro th 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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Lec #3: Casting defects ­ oxide films

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Surface Film Formation

Films on cast iron

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 en ironment h drocarbons 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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Lec #3: Casting defects ­ oxide films

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Incorporation of Surface Films into the Bulk Liquid

Surface film is not harmful when 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).

Ways of mixing of surface film into the bulk: 1. 2. 3. 4. 5. 6. Melt charge materials Pouring Surface flooding Surface turbulence Confluence weld Bubble trails

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

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Lec #3: Casting defects ­ oxide films

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Incorporation of Surface Films into the Bulk Liquid

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 g , , g 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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Incorporation of Surface Films into the Bulk Liquid

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 p p g q 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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Lec #3: Casting defects ­ oxide films

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Incorporation of Surface Films into the Bulk Liquid

3. Surface Flooding and Formation of Bifilm g

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.

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Incorporation of Surface Films into the Bulk Liquid

3. Surface Flooding and Formation of Bifilm g

In case of unstable advance of film forming alloy during pouring, double oxide layer, called the "bifilm" 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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Incorporation of Surface Films into the Bulk Liquid

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 = Vd / n

Internal pressure = V2 / 2Cd2 Viscous pressure = nV / d

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.

Incorporation of Surface Films into the Bulk Liquid

4. Surface Turbulence

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 = V2 / 2Cd2 Pressure against surface tension g to create new surface = 2 / r The Weber number: We = 0.2 ­ 0.8, free from surface turbulence We = 100, surface turbulence becomes problematic We = 100000, creates atmisaztion !!

We = V2r /

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Incorporation of Surface Films into the Bulk Liquid

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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Incorporation of Surface Films into the Bulk Liquid

6. Bubble Trail 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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Lec #3: Casting defects ­ oxide films

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Effect of Entrained Films

General Problem due to Submerged Films

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

Effect of Entrained Films

Machining Problem

Oxides are much harder than the metal itself, causing d hh d h h l lf dragging out during machining, leaving unsightly grooves. The cutting edge of tool is often chipped or blunted by encounters with such problems.

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Lec #3: Casting defects ­ oxide films

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Effect of Entrained Films

Leak Tightness

For thin-sectioned castings (5 mm and below), film defects can be extended from wall to wall across the mould cavity and so connect the casting cavity, 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.

Fluidity

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)

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Effect of Entrained Films

Mechanical Properties

Castings made using processes which reduce surface turbulence have been found to have uniform good mechanicall properties with low scatter. They also f dt h if d h i ti ith l tt Th l show excellent fatigue resistance.

Al4.5Cu fractured surfaces (a) Oxide covered (0.3 % elongation) (b) Ductile fracture (3.0 % elongation)

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© B. Rashid, DMME, BUET

Lec #3: Casting defects ­ oxide films

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Deactivation of Entrained Films

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 liquid well wetted 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 liquid 2. Microscopically rough surface retains additional gases 3. When folded, they form bifilm defects, which stores even more gases

It is the film of air which is the most damaging.

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Lec #3: Casting defects ­ oxide films

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Deactivation of Entrained Films

When submerged, the film continues to grow by consuming the stored gases (oxygen to form oxides, nitrogen to form nitirdes, other goes in solution). E t ll all the gases are consumed and the most damaging l ti ) Eventually ll th d d th td i effects of the film (causing leaks, nucleating bubbles, cavities, cracks) will have been removed. This 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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Lec #3: Casting defects ­ oxide films

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Elimination of Entrained Films

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 use filter in the running system, after pouring, but before the t b f th metall enters th mould, t t the ld to ensure that the flow conditions into the mould cavity do not reintroduce new oxide.

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Lec #3: Casting defects ­ oxide films

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