MME 345, Lecture 44

Casting Defects2. Effect of defects on propertiesRef: J. Campbell, Castings, Butterworth-Heniemanne, 1992

Summary of casting defects

Effect of defects on Yield strength

Fracture toughness

Fatigue

Ductility

Ultimate tensile strength

Leak tightness

Residual strength

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Compact defects Inclusions

Gas porosity

Core blow

Planar defects Layer porosity

Tears and cracks

Films

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In the past, some castings containing large undetected defects

have gone on to serve useful lives in critical applications

Others, however, have failed (e.g., the Tay Bridge disaster in Scotland, UK)

The mixed performance arises because of the elementary point that

the size of the defect is often of much less importance than its form and position.

a large pore in a low-stressed area of the casting may be far less detrimental

than a small region of layer porosity in a sharp corner subject to a high tensile stress

To have blanket specifications requiring the elimination of all types of defect from

every area of the casting is therefore not appropriate

• may result in the scrapping of many serviceable castings

The most logical and effective control over casting performance is achieved by

• specifying separate designated regions of the casting, and then

• each separate region being required to contain no defects above the critical size

appropriate for that location4/23

The list of properties which a casting may be required to possess can be long.

It might include, for instance, high-temperature properties such as creep resistance,

high-temperature fatigue resistance, or oxidation resistance.

Alternately, room temperature properties such as resistance to corrosion

in specific environments may be required.

In this lecture, we shall confine ourselves to the more usually specified properties

such as strength, toughness, ductility, and leak-tightness.

Different defects have different effects on each of these properties

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Yield Strength or Proof Stress

Because no substantial deformation has

taken place, it is logical to assume that the

YS / 0.2PS will be unaffected by most defects

The only effect will be that due to the

reduction in area

but since most defects occupy at most only a few

per cent of the area of the casting, this effect is

usually hardly detectable

UTS and E are, however, noticeably affected

The 1 % or so of reduction in 0.2PS because of the

1 % or so loss of area of layer porosity cannot be detected in the scatter of the results.

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A notable exception to the constancy of the

yield stress is found for the embrittlement of

steels by grain boundary precipitates of

sulphides

• The large effect on yield strength is

consistent with the large area which the

sulphides cover on the grain boundaries,

and the weakness of the sulphide phase

• intergrannular brittle failure by a fast-running

crack through the grain boundaries becomes

favoured over extensive plastic deformation

of the matrix

Steel embrittlement by grain boundary precipitation of sulphides

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Fracture Toughness

Fracture toughness is a material property, independent of the size of gross defects.

useful in the prediction of the shapes and sizes of defects which might lead to failure

For an Al alloy

KIC = 32 MPa m1/2

sy = 240 MPad = 11 mm (inside crack)d = 9 mm (surface initiated crack)

For inside crack of diameter d, the critical defect size

According to the fracture mechanics, fracture may begin when the stress-intensity

factor K exceeds a critical value, the fracture toughness K1C.

edge cracks

are somewhat more serious than

centre cracks

• These are comparatively large defects (11 mm / 9 mm)

can be detected relatively easily by non-destructive tests

prior to the casting going into service

• Most current radiography standards which state “no linear

defects of any size” or which reject aluminium alloy

castings for flaws more than approximately 1 mm in size,

may not be logical

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Steels have high fracture toughness,

with correspondingly good tolerance to large

defects at low applied stress

However, when they are used in highly

stressed applications, the permissible defect

size is reduced again

When the applied stress equals the yield stress, the greatest resistance to

crack extension offered by the materials is controlled by the value of (K1C/sYS)

this parameter is a valuable measure of the defect tolerance of a material

d = 2K1C2/ps2

Permissible defect size diminishes with increasing strength

as fracture toughness falls with increasing strength

For stronger irons, the permissible defect size is below 1 mm

this is particularly difficult to detect, and sets a limit to the stress at

which a strong ductile iron casting may be used with safety

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• Poor properties of flake irons and the excellent

properties of some steels and titanium alloys;

ductile irons occupy an interesting middle ground

• In general, it seems that most groups of alloys can

exhibit either high strength and low toughness, or

high toughness and low strength.

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For brittle materials, the use of fracture toughness is

more appropriate measure of the reliability of the casting

than ductility.

• Any crack would have to exceed certain critical value before

failure, even if such failure is eventually of a brittle failure.

properties of Al-7Si-0.4Mg alloy as a function of iron

intermetallic particles

little affected by ageing

and retains a respectable

value regardless of low ductility

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Fatigue

The fatigue performance of cast alloys is generally poor.

• The initiation of fatigue crack during stage 1 of the fatigue process is short.

This is due to the presence of defects (pores, oxide films).

The growth and propagation of crack in stage 2 is, however, remarkably slow.

• This is due to the result of irregular branched nature of the crack, which appeared to have

follow a zigzag path through the as-cast structure.

• This contrasts with the wrought alloys where the fine-grained structure allows the crack to

spread unchecked along a straight path.

Thus, if crack initiation can be slowed, then the fatigue lives of cast metals

might be extended considerably, perhaps in excess of those of wrought alloys.

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The following factors appeared to

have beneficial effect on fatigue

performance of cast materials:

• Shot blasting due to the introduction

of compressive stress at the surface

• Filtering the liquid prior to casting

and/or improving casting methods

(e.g., low pressure, and uphill casting)

due to reduced defects

• Metal mould casting due to reduced

size in defects

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Ductility

Why should an assembly of holes in a

matrix affect the ductility so remarkably ?

All second phase additions (including pore) has deleterious effect on ductility.

This is due to the lack of cohesion of the second phase with the matrix when deformation starts.

Thus all particles are soon acting as holes.for only 1 vol.% pore,

ductility falls from a

theoretical maximum of

100 % to an approx. 10 %

1%

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Since failure occurs at 45°, the theoretical

elongation is equal to the linear dimension of the cross-section, l.

For test piece with single pore of size d,

the elongation is equal to the half of the remaining sound length, i.e., (l -d)/2.

In general, for a spacing S in an array of micropores, we have

Elongation = s – d

= 1/n1/2 – (f/n)1/2

= (1 – f1/2) / n1/2

n = no. of pores per unit area = 1/s2

f = area fraction of pore on fracture surface = nd2

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Film defects were less important than an equal area fraction of compact inclusions in lowering the ductility of casting.

Micro-inclusions are more effective than

maco-inclusions in lowering ductility.

When f = 1, ductility falls to zero.

This situation can easily happen when turbulence

during filling can generate oxide films, which

occupy the whole of the cross-section of the test

piece.

there were between 100 and 1000 times the

number of microinclusions compared to film-type

defects in a given area of fracture surface

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Ultimate Tensile Strength

Ultimate tensile strength comprises

1. yield strength plus

2. additional work strengthening during plastic deformation before failure.

The behaviour becomes more complicated to understand

than the behaviour of yield stress or ductility alone.

UTS = proof stress, when

1. ductility = 0, or

2. work strengthening effect = 0

(mainly occurs at high temperature when rate of recovery exceeds rate of hardening;

ductility can be high for this case)

At room temperature, UTS increases with

1. increasing ductility (since YS/0.2PS is relatively insensitive to many variables), and

2. work hardening.

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For aluminium alloys, UTS increases

due to a reduction in defects.

mainly due to the increase in ductility

When cracks or films occupy the

majority of the cross-section,

the casting will be highly injurious.

the UTS falls to zero as the crack

occupies progressively more of

the area under test

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Layer porosity has similar effect on UTS as it has on ductility.

Have larger effect when porosity is layered perpendicular to the applied stress and “stitched”

together to form a single crack.

even this marked reduction in

UTS is not as serious as would

be expected if the layers had

been cracks

Leaks are seldom caused by gas porosity.

because the distribution of gas porosity is not random; they are distributed at specific distances,

and are kept apart by the presence of dendrite arms.

Leakage due to gas porosity occurs

only when the sample contains an impossibly amount of high gas content (~20 – 30 vol.%.)

Leak Tightness

Most leaks in light alloys and al-bronze castings are the result of oxide inclusions.

They fall into 2 categories:

1. Old, thick oxide films that are in suspension are jamming/bridging together as the metal rises

2. Folding of new oxide films into the metal due to turbulence. These poorly wetted, folded over

dry side to dry side bi-films constitute the major leak paths through the walls of casting.

Sometimes results due to shrinkage porosity.

especially in long-freezing-range alloys where porosity adopts sponge/layer morphology;

porous metals resulted due to poorly fed shrinkage will produce leak, especially after machining

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Residual Stress

Unseen and unsuspected,

residual stresses can be the most damaging defects of all.

the stress can be so large that it can outweighing the effect of all other defects

Never specified to be low in any standards.

Practically impossible to measure in any NDT test.

Residual stress is added up to the applied stress, and put the casting

near to its point of failure even at a relatively small applied loads.

The remedy is, of course, the application of stress relief by thermal treatment.

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Next ClassMME 345, Lecture 45

Casting Design Considerations