Foseco Non-Ferrous Foundryman’s Handbook Part 2 doc

Originally this was done by making use of specially engraved rules, known as “contraction rules”, the dimensions of which incorporated a contraction allowance such as 1 in 75 for alumini

The physical properties of metals (Continued)

Element Thermal

conductivity

(W/m·K)

Resistivity

( ohm·cm

at 20°C)

Vol· change

on melting

(%)

Density

(g/cm 3 ) Coeff· of

expansion

(  10 –6 /K)

Brinell hardness no·

Densities of casting alloys

Stainless

Approximate bulk densities of common materials

Material kg/m 3 lb/ft 3 Material kg/m 3 lb/ft 3

Patternmakers’ contraction allowances

Castings are always smaller in dimensions than the pattern from which they are made, because as the metal cools from its solidification temperature to room temperature, thermal contraction occurs Patternmakers allow for this contraction by making patterns larger in dimensions than the required castings by an amount known as the “contraction allowance” Originally this was done by making use of specially engraved rules, known as

“contraction rules”, the dimensions of which incorporated a contraction allowance such as 1 in 75 for aluminium alloys, or 1 in 96 for iron castings Nowadays, most patterns and coreboxes are made using computer-controlled machine tools and it is more convenient to express the contraction

as a percentage allowance

Predicting casting contraction can never be precise, since many factors are involved in determining the exact amount of contraction that occurs For example, when iron castings are made in greensand moulds, the mould walls may move under the pressure of the liquid metal, causing expansion

of the mould cavity, thus compensating for some of the metal contraction Cored castings may not contract as much as expected, because the presence

of a strong core may restrict movement of the casting as it is cooling Some core binders expand with the heat of the cast metal causing the casting to be larger than otherwise expected For these reasons, and others, it is only possible to predict contractions approximately, but if a patternmaker works with a particular foundry for a long period, he will gain experience with the foundry’s design of castings and with the casting methods used in the foundry Based on such experience, more precise contraction allowances can

be built into the patterns

The usually accepted contraction allowances for different alloys are given

in the following table

Aluminium alloys

Al–Si8Cu3Fe LM24

Al–Si12 LM6

Volume shrinkage of principal casting alloys

Most alloys shrink in volume when they solidify, the shrinkage can cause voids in castings unless steps are taken to “feed” the shrinkage by the use of feeders

Comparison of sieve sizes

Sieves used for sand grading are of 200 mm diameter and are now usually metric sizes, designated by their aperture size in micrometres (m) The table lists sieve sizes in the British Standard Metric series (BS410:1976) together with other sieve types

Sieve aperture, micrometres and sieve numbers

ISO/R.565 series

(BS410:1976)

180

Notes: The 1000 and 45 sieves are optional.

The 212 and 150 sieves are also optional, but may be included to give better separation between the 250 and 125 sieves

Calculation of average grain size

The adoption of the ISO metric sieves means that the old AFS grain fineness number can no longer be calculated Instead, the average grain size, expressed as micrometres (m) is now used This is determined as follows:

1 Weigh a 100 g sample of dry sand

2 Place the sample into the top sieve of a nest of ISO sieves on a vibrator Vibrate for 15 minutes

3 Remove the sieves and, beginning with the top sieve, weigh the quantity

of sand remaining on each sieve

4 Calculate the percentage of the sample weight retained on each sieve, and arrange in a column as shown in the example

5 Multiply the percentage retained by the appropriate multiplier and add the products

6 Divide by the total of the percentages retained to give the average grain size

Example

ISO aperture

Average grain size = 20 335/99.6

Calculation of AFS grain fineness number

Using either the old BS sieves or AFS sieves, follow, steps 1–4 above

5 Arrange the results as shown in the example below

6 Multiply each percentage weight by the preceding sieve mesh number

7 Divide by the total of the percentages to give the AFS grain fineness number

Example

BS sieve

number

% sand retained

on sieve

Multiplied by previous sieve no·

Product

AFS grain fineness number = 6854.2/100

= 68.5 or 68 AFS

being the most commonly used Direct conversion between average grain size and AFS grain fineness number is not possible, but an approximate relation is shown below:

AFS grain

Average

grain size (m) 390 340 300 280 240 220 210 195 170 150 While average grain size and AFS grain fineness number are useful parameters, choice of sand should be based on particle size distribution

Recommended standard colours for patterns

Core prints for unmachined openings and end prints

on black

Seats of and for loose pieces

and loose core prints

Green

stripes with clear varnish

Dust control in foundries

Air extraction is used in foundries to remove silica dust from areas occupied

by operators The following table indicates the approximate air velocities needed to entrain sand particles

Terminal velocities of spherical particles of density 2.5 g/cm3(approx.)

For the comfort and safety of operators, air flows of around 0.5 m/sec are needed to carry away silica dust If air flow rate is too high, around the shake-out for example, there is a danger that the grading of the returned sand will be altered

Buoyancy forces on cores

When liquid metal fills a mould containing sand cores, the cores tend to float and must be held in position by the core prints or by chaplets The following table lists the buoyancy forces experienced by silica sand cores in various liquid metals, expressed as a proportion of the weight of the core:

Core print support

(21 psi) So the core print can support the following load:

1 kN = 100 kgf (approx.)

Support (kgf) = Core print area (m2)  15 000

Example: A core weighing 50 kg has a core print area of 10  10 cm (the area

of the upper, support surface), i.e 0.1  0.1 = 0.01 m2 The print support is

150  0.01 = 1.5 kN = 150 kgf

If the mould is cast in iron, the buoyancy force is 50  3.5 = 175 kgf so chaplets may be necessary to support the core unless the print area can be increased

Opening forces on moulds

Unless a mould is adequately clamped or weighted, the force exerted by the molten metal will open the boxes and cause run-outs If there are insufficient box bars in a cope box, this same force can cause other problems like distortion and sand lift It is important therefore to be able to calculate the opening force so that correct weighting or clamping systems can be used The major force lifting the cope of the mould is due to the metallostatic pressure of the molten metal This pressure is due to the height, or head, of metal in the sprue above the top of the mould (H in Fig 1.1) Additional

Figure 1.1 Opening forces of moulds.

forces exist from the momentum of the metal as it fills the mould and from forces transmitted to the cope via the core prints as the cores in cored castings try to float

The momentum force is difficult to calculate but can be taken into account

by adding a 50% safety factor to the metallostatic force

The opening metallostatic force is calculated from the total upward-facing area of the cope mould in contact with the metal (this includes the area of all the mould cavities in the box) The force is:

F(kgf) = A  H  d  1.5

1000

A is the upward facing area in cm2

H (cm) is the height of the top of the sprue above the average height of the

upward facing surface

d is the density of the molten metal (g/cm3)

1.5 is the “safety factor”

For ferrous metals, d is about 7.5, so:

F(kgf) = 11 A  H

1000

For aluminium alloys, d is about 2.7, so:

F(kgf) = 4  A  H

1000

Forces on cores

The core tends to float in the liquid metal and exerts a further upward force (see page 18)

In the case of ferrous castings, this force is

3.5  W (kgf) where W is the weight of the cores in the mould (in kg)

In aluminium, the floating force can be neglected

The total resultant force on the cope is (for ferrous metals)

(11  A  H)/1000 + 3.5 W kgf

Example: Consider a furane resin mould for a large steel valve body casting

with a core weighing 40 kg The opening force is

11  2500  30/1000 + 3.5  40 = 825 + 140

= 965 kgf

It is easy to see why such large weights are needed to support moulds in jobbing foundries

Dimensional tolerances and consistency achieved in castings

Errors in dimensions of castings are of two kinds:

the design dimension given on the drawing Consistency: statistical errors, comprising the dimensional variability

round the mean dimension

Dimensional accuracy

The major causes of deviations of the mean dimension from the target value are contraction uncertainty and errors in pattern dimensions It is usually possible to improve accuracy considerably by alternations to pattern equipment after the first sample castings have been made

Dimensional consistency

Changes in process variables during casting give rise to a statistical spread

of measurements about a mean value If the mean can be made to coincide with the nominal dimension by pattern modification, the characteristics of this statistical distribution determine the tolerances feasible during a production run

The consistency of casting dimensions is dependent on the casting process used and the degree of process control achieved in the foundry Fig 1.2 illustrates the average tolerance exhibited by various casting processes The tolerance is expressed as 2.5 (2.5 standard deviations), meaning that only 1 casting in 80 can be expected to have dimensions outside the tolerance

There is an International Standard, ISO 8062–1984(E) Castings – System of dimensional tolerances, which is applicable to the dimensions of cast metals

and their alloys produced by sand moulding, gravity diecasting, low

pressure diecasting, high pressure dicasting and investment casting The Standard defines 16 tolerance grades, designated CT1 to CT16, listing the total casting tolerance for each grade on raw casting dimensions from 10 to

10 000 mm The Standard also indicates the tolerance grades which can be expected for both long and short series production castings made by various processes from investment casting to hand-moulded sand cast

Reference should be made to ISO 8062 or the equivalent British Standard BS6615:1985 for details

Figure 1.2 The average tolerance (taken as 2.5 ) exhibited by various casting processes (From Campbell, J (1991) Castings, Butterworth-Heinemann,

reproduced by permission of the publishers.)

Chapter 2

Aluminium casting alloys

Introduction Aluminium casting is dominated by the automotive industry Roughly two thirds of all aluminium castings are automotive where the use of aluminum castings continues to grow at the expense of iron castings Although aluminium castings are significantly more expensive than ferrous castings, there is a continuing market requirement to reduce vehicle weight and to increase fuel efficiency It is this requirement which drives the replacement

of ferrous parts by aluminium

Aluminium castings are widely used in cars for engine blocks, heads, pistons, rocker covers, inlet manifolds, differential casings, steering boxes, brackets, wheels etc The potential for further use of aluminium in automotive applications is considerable European cars in 1992 had 50–60 kg

Al castings and this is expected to double by year 2000

When aluminium alloys are cast, there are many potential sources of defects which can harm the quality of the cast part All aluminium alloys are subject to:

Shrinkage defects Al alloys shrink by 3.5–6.0% during solidification

(depending on alloy type)

which is expelled during solidification giving rise

to porosity

forming a skin of oxide which may be entrained into the casting

Because of these potential problems aluminium castings, like all castings, suffer from variable mechanical properties which can be described by a distribution curve The mechanical properties used by the designer of the casting must take the distribution curve into account If, for example, the process mean tensile strength for a cast alloy is 200 MPa, the designer must use a lower figure, say 150 MPa, as the strength of the alloy to take into account the variability of properties If the spread of the distribution curve can be reduced, then a higher design strength, say 170 MPa can be used, even though the process mean for the alloy and the casting process stays the same

160 200 240 280 320

Top filled filtered

Range

0

5

10

15

20

25

30

Tensile strength: MPs

Top filled unfiltered

Range

0 5 10 15 20 25

30 Tensile strength: MPs

Bottom filled filtered

Range

0

5

10

15

20

25

30

Tensile strength: MPs

Bottom filled unfiltered

Range

0 5 10 15 20 25

30 Tensile strength: MPs

The strength of castings does not follow the normal bell-shaped distribution curve Figure 2.1 shows the range of tensile strengths found in 12.5 mm diameter test bars cast in an Al–Si7 Mg alloy into resin bonded sand moulds using various pouring methods: top or bottom filled, filtered

or unfiltered In all cases the process mean tensile strength is about 260 MPa, but the distribution is different

The unfiltered castings show a few but very significant low strength test pieces, known as outliers

For each filling category the plots show two distinct bands of tensile strength

A design strength below 200 MPa would have to be used for unfiltered castings because of the occasional outliers

Examination of the fracture surface of the low strength outliers showed massive oxide fragments indicating that inclusions in the unfiltered castings were responsible for the low tensile strength Filtration of the metal eliminates the inclusions allowing the design strength to be increased to around 230 MPa

Figure 2.1 Histograms showing the distribution of tensile strength of Al–Si7Mg

alloy test bars cast in various ways (From Foseco Foundry Practice, 226, July

1995.)

The double band appearance of the histograms is interpreted as indicating that more than one defect type is acting to control the behaviour at fracture

It is by reducing the variability of properties of castings that the greatest progress has been made in recent years This has allowed designers to have greater confidence in castings so that thinner sections and lower weight components can be used The stages in the aluminium casting process where the greatest improvements have been made are:

Efficient degassing

Grain refinement

Modification of structure

Metal filtration

Non-turbulent filling of moulds

Chill casting (into metal moulds) has inherently a greater possibility of producing higher quality than sand casting because the higher rate of solidification reduces pore size and refines grain size The highest quality components are produced using filtered metal, non-turbulently introduced into metal moulds and solidified under high external pressure to minimise

or totally avoid porosity While it is not always possible to use high external pressure during solidification (castings using sand cores will suffer from metal penetration), the understanding of the origins of defects in aluminium castings and their reduction by attention to degassing, metal treatment and filtration has greatly improved the general quality of castings in recent years There is little doubt that improvements will continue to be made in the future

Casting alloys

There is a large and confusing range of casting alloys in use worldwide, defined by the National Specifications of the major industrial countries Unfortunately there is little correspondence between the Standard Alloys used in different countries

A European Standard for Aluminium Casting Alloys, EN 1706, was approved in August 1997 and the English language version BS EN 1706:1998 was published in March 1998 Along with the following standards, it partially supersedes BS 1490:1988 which will be withdrawn when EN 1559–4 is published

BS EN 1559–1:1997 Founding Technical conditions of delivery

General

for remelting Specifications

Addi-tional requirements for aluminium castings