Aluminum Casting Alloys English PV 2012-11-30

Aluminium Casting Alloys


Content

Introduction 5

Recycled aluminium 6

Technology and service

for our customers

Quality Management 7

Work safety and health 8

protection

Environmental protection

Aluminium and aluminium 9

casting alloys

Aluminium Material properties

Recycling of aluminium

Shaping by casting 10

Product range and 11

form of delivery

Technical consultancy 12

service

Selecting aluminium 13

casting alloys

Criteria for the selection of 14

aluminium casting alloys

Infl uence of the 18

most important alloying

elements on aluminium

casting alloys

Infl uencing the 19

microstructural formation of

aluminium castings

Grain refi nement 20

Modifi cation of AlSi eutectic 21

Refi nement of 23

primary silicon

Melt quality and melt cleaning 24

Avoiding impurities 25

Melt testing and 28

inspection procedure

Thermal analysis 30

Selecting the casting process 31

Pressure die casting 32

process

Gravity die casting process

Sand casting process 34

Casting-compliant design 35

Solidifi cation simulation 37

and thermography

Avoiding casting defects 38

Heat treatment of 40

aluminium castings

Metallurgy

fundamental principles

Solution annealing 41

Quenching

Ageing 42

Mechanical machining of 44

aluminium castings

Welding and joining 45

aluminium castings

Suitability and behaviour

Applications in the

aluminium sector

Welding processes

Weld preparation 47

Weld fi ller materials

Surface treatment: corrosion 48

and corrosion protection

Information on physical data, 50

strength properties and

strength calculations

Notes on the casting 51

alloy tables

Overview: Aluminium casting 52

alloys by alloy group

Eutectic aluminium-silicon 59

casting alloys

Near-eutectic wheel 63

casting alloys

The 10 per cent aluminium- 66

silicon casting alloys

The 7 and 5 per cent 71

aluminium-silicon

casting alloys

Al SiCu casting alloys 76

AlMg casting alloys 81

Casting alloys for special 87

applications

4


In the second part, all technical aspects

which have to be taken into account in

the selection of an aluminium casting al-

loy are explained in detail. All details are

based on the DIN EN 1676: 2010 standard.

The third part begins with notes on the

physical data, tensile strength charac-

teristics and strength calculations of

aluminium casting alloys. Subsequently,

all standardised aluminium casting alloys

in accordance with DIN EN 1676 as well

as common, non-standardised casting

alloys are depicted in a summary table

together with their casting/technical and

other typical similarities in alloy families.

The aim of this new, revised and rede-

signed Aluminium Casting Alloys Cata-

logue is to give the user of aluminium

Many of you have most certainly worked

with the old Aluminium Casting Alloys

Catalogue over the years in thousands

of workplaces in the aluminium indus-

try, it has become a standard reference

book, a reliable source of advice about

all matters relating to the selection and

processing of aluminium casting alloys.

Even if you are holding this Aluminium

Casting Alloys Catalogue in your hands

for the fi rst time, you will quickly fi nd your

way around with the help of the following

notes and the catalogues detailed index.

How is this Aluminium Casting Alloys

Catalogue structured? The catalogue

consists of three separate parts. In the

fi rst part, we provide details on our com-

pany a proven supplier of aluminium

casting alloys.

Introduction

casting alloys a clear, well laid-out com-

panion for practical application. Should

you have any questions concerning the

selection and use of aluminium casting

alloys, please contact our foundry con-

sultants or our sales staff.

You can also refer to www.aleris.com.

We would be pleased to advise

you and wish you every success

in your dealings with aluminium

casting alloys!

5


Recycled aluminiumTechnology and service for our customers

Employing approx. 600 people, Aleris

Recycling produces high-quality cast-

ing and wrought alloys from recycled

aluminium. The companys headquar-

ters are represented by the Erftwerk

in Grevenbroich near Dsseldorf which

is also the largest production facility in

the group. Other production facilities

in Germany (Deizisau, Tging), Norway

(Eidsvg, Raudsand) and Great Britain

(Swansea) are managed from here. With

up to 550,000 mt, Aleris Recycling avails

of the largest production capacities in

Europe and is also one of the worlds

leading suppliers of technology and

services relating to aluminium casting

alloys. Aleris Recycling also offers a wide

range of high-quality magnesium alloys.

Aluminium recycled from scrap and

dross has developed to become a

highly-complex technical market of the

future. This is attributable to the steady

increase in demand for raw materials,

the sustainability issue, increased envi-

ronmental awareness among producers

and consumers alike and, not least, the

necessity to keep production costs as

low as possible.

This is where aluminium offers some es-

sential advantages. Recycled aluminium

can be generated at only a fraction of the

energy costs (approx. 5%) compared to

primary aluminium manufactured from

bauxite with the result that it makes a

signifi cant contribution towards reduc-

ing CO2 emissions. This light-alloy metal

can be recycled any number of times

and good segregation even guarantees

no quality losses.

Its properties are not impaired when

used in products. The metallic value is

retained which represents a huge eco-

nomic incentive to collect, treat and melt

the metal in order to reuse it at the end

of its useful life.

For this reason, casting alloys from Aleris

Recycling can be used for manufacturing

new high-quality cast products such as

crankcases, cylinder heads or aluminium

wheels while wrought materials can be

used for manufacturing rolled and pressed

products, for example. Key industries

supplied include:

Rolling mills and extrusion plants

Automotive industry

Transport sector

Packaging industry

Engineering

Building and construction

Electronics industry

as well as other companies in the

Aleris Group.

State-of-the-art production facilities and

an extensive range of products made of

aluminium in the form of scrap, chips or

dross are collected and treated by Aleris

Recycling before melting in tilting rotary

furnaces with melting salt, for example,

whereby the salt prevents the aluminium

from oxidising while binding contami-

nants (salt slag). Modern processing and

melting plants at Aleris Recycling enable

effi cient yet environmentally-friendly re-

cycling of aluminium scrap and dross.

The technology used is largely based on

our own developments and in terms of

yield and melt quality works signifi cantly

more effi ciently than fi xed axis rotary

furnaces and hearth furnaces. The melt

gleaned from these furnaces has a very

low gas content thanks to the special gas

purging technique we use as well as

being homogeneous and largely free of

oxide inclusions and/or contaminants.

The resulting high quality of Aleris alloys

enables our customers to open up an in-

creasing number of possible applications.

All management processes and the en-

tire process chain from procurement

through production to sale are subject to

systematic Quality Management. Com-

bined with Quality Management certifi ed

to ISO/TS 16949 and DIN EN ISO 9001,

this guarantees that our clients maximum

requirements and increasing demands

can be fulfi lled.

The product range offered by Aleris Re-

cycling comprises more than 250 differ-

ent casting and wrought alloys. They can

be supplied as ingots with unit weights

of approx. 6 kg (in stacks of up to 1,300

kg) as well as pigs of up to 1,400 kg or

as liquid metal. Based on our sophisti-

cated crucible technology and optimised

transport logistics, Aleris Recycling sup-

plies customers with liquid aluminium in

a just-in-time process and at the appro-

priate temperature.

6


Quality Management

We believe that our most important cor-

porate goal is to meet in full our custom-

ers requirements and expectations in

terms of providing them with products

and services of consistent quality. In or-

der to meet this goal, our guidelines and

integrated management system specifi -

cations outline rules and regulations that

are binding for all staff.

As a manufacturer of aluminium casting

alloys, we are certifi ed according to ISO/

TS 16949. In addition, we operate ac-

cording to DIN EN ISO 9001 standards.

Due to its future-oriented corporate

structure, Aleris Recycling supplies the

market with an increasing number of

applications involving high-quality sec-

ondary aluminium. This service is not re-

stricted to the area of casting alloys but

also applies for 3000- and 5000-grade

wrought alloys, for example. Aleris Re-

cycling is also capable of offering some

6000-grade secondary aluminium alloys

largely required by the automotive sector.

For this so-called upgrade, Aleris applies

special production technologies when

it comes to manufacturing high-quality

alloys from scrap.

Recycled aluminium is increasingly be-

coming a complex range at the interface

between high-tech production, trade and

service. In addition, customers demand

intensive consulting as well as individual

service. Aleris Recycling enjoys an ex-

cellent position in this regard.

At its various locations, the company

units offer a high degree of recycling ex-

pertise, manufacturing competence and

delivery reliability for its customers. With

the result that Aleris Recycling guarantees

its customers a high level of effi ciency

and added value while supporting their

success on the market.

The principle of avoiding errors is para-

mount in all our individual procedures and

regulations. In other words, our priority

is to strive to achieve a zero-error target.

By effectively combating the sources of

errors, we create the right conditions for

reliability and high quality standards.

We have also established a comprehen-

sive process of continuous improvement

(PMO, Best Practice, Six Sigma etc.) in

our plants in response to the demands

being placed on our company by the

increasing trend towards business glo-

balisation. This creates the right cli-

mate for creative thinking and action.

All members of staff, within their own

area of responsibility, endeavour to en-

sure that operational procedures are

constantly improved, even if in small,

gradual stages, with a clear focus on

our customers needs.

7


Work safety and health protection

Our staff are our most valuable asset. Work

safety and health protection, therefore,

have top priority for us, and also make

a valuable contribution to the success

of our company. Our Work safety and

health protection programme is geared

towards achieving a zero accident rate,

and towards avoiding occupational ill-

nesses. Depending on the respective

location, we are certifi ed to OHSAS

18001 or OHRIS.

All management members and staff are

obliged to comply with legal regulations

and company rules at all times, to pro-

tect their own health and the health of

other members of staff and, when en-

gaged in any company operations, to

do their utmost to ensure that accidents

and work-related illnesses are avoided,

as well as anything that might have a

negative impact on the general company

environment. Management provides the

appropriate level of resources required

to achieve these goals.

There are regular internal and external

training seminars on the topic of work

safety, and detailed programmes to im-

prove health protection. These help to

maintain our comparatively low accident

and illness rates.

Environmental protection

Following the validation of our environ-

mental management system in conformity

with EMAS II and certifi cation to DIN EN

ISO 14001, we have undertaken not only

to meet all the required environmental

standards, but also to work towards a

fundamental, systematic and continual

improvement in the level of environmental

protection within the company.

Our management system and environ-

mental policy are documented in the

company manual which describes all

the elements of the system in easily

understood terms, while serving as a

reference for all regulations concerning

the environment.

The environmental impacts of our com-

pany operations in terms of air purity,

protection of water bodies, noise and

waste are checked at regular intervals.

By modifying procedures, reusing mate-

rials and recycling residues, we optimise

the use of raw materials and energy in

order to conserve resources as effi ciently

as possible.

We pursue a policy of open information

and provide interested members of the

public with comprehensive details of

the companys activities in a particu-

lar location, and an explanation of the

environmental issues involved. For us,

open dialogue with the general pub-

lic, our suppliers, customers and other

contractual partners is as much a part

of routine operations as reliable co-op-

eration with the relevant authorities and

trade associations.

Likewise, ecological standards are in-

corporated in development and planning

processes for new products and produc-

tion processes, as are other standards

required by the market or society at large.

Our staff is fully conscious of all environ-

mental protection issues and is keen to

ensure that the environmental policy is

reliably implemented in day-to-day op-

erations within the company.

8


Aluminium and aluminium casting alloys

Recycling of aluminium

Long before the term recycling became

popular, recycling circuits already exist-

ed in the aluminium sector. Used parts

made from aluminium or aluminium alloys

as well as aluminium residue materials

arising from production and fabrication

are far too valuable to end up as land-

fi ll. One of the great advantages of this

metal, and an added plus for its use as a

construction material, is that aluminium

parts, no matter the type, are extremely

well suited to remelting.

The energy savings made in

recycling aluminium are

considerable. Remelting requires

only about 5 % of the energy

initially required to produce

primary aluminium.

As a rule, aluminium recycling

retains the value added to the

metal. Aluminium can be recycled

to the same quality level as the

original metal.

Aluminium recycling safeguards

and supplements the supply of

raw materials while saving

resources, protecting the

environment and conserving

energy. Recycling is therefore also

a dictate of economic reason.

Aluminium is light; its specifi c weight

is substantially lower than other

common metals and, at the same

time, it is so strong that it can with

stand high stress.

Aluminium is very corrosion-

resistant and durable. A thin,

natural oxide layer protects

aluminium against decomposition

from oxygen, water or chemicals.

Aluminium is an excellent

conductor of electricity,

heat and cold.

Aluminium is non-toxic, hygienic

and physiologically harmless.

Aluminium is non-magnetic.

Aluminium is decorative and

displays high refl ectivity.

Aluminium has outstanding

formability and can be

processed in a variety of ways.

Aluminium alloys are easy to cast

as well as being suitable for all known

casting processes.

Aluminium alloys are

distinguished by an excellent

degree of homogeneity.

Aluminium and aluminium

alloys are easy to machine.

Castings made from aluminium

alloys can be given an artifi cial,

wear-resistant oxide layer

using the ELOXAL process.

Aluminium is an outstanding

recycling material.

Aluminium Material properties

Aluminium has become the most widely

used non-ferrous metal. It is used in the

transport sector, construction, the pack-

aging industry, mechanical engineering,

electrical engineering and design. New

fi elds of application are constantly open-

ing up as the advantages of this material

speak for themselves:

9


Shaping by casting

Casting represents the shortest route

from raw materials to fi nished parts a

fact which has been known for fi ve thou-

sand years. Through continuous further

development and, in part, by a selective

return to classic methods such as the

lost-form process, casting has remained

at the forefront of technical progress.

The most important advantage of the

casting process is that the possibilities

of shaping the part are practically limit-

less. Castings are, therefore, easier and

cheaper to produce than machined and/

or joined components. The general waiv-

ing of subsequent machining not only

results in a good density and path of

force lines but also in high form strength.

Furthermore, waste is also avoided. As a

rule, the casting surface displays a tight,

fi ne-grained structure and, consequently,

is also resistant to wear and corrosion.

The experience accumulated over ma-

ny decades, the use of state-of-the-art

technology in scrap preparation, remelt-

ing and exhaust gas cleaning as well

as our constant efforts to develop new,

environmentally-sound manufactur-

ing technology puts us in a position to

achieve the best possible and effi cient

recycling rates. At the same time, they

also help us to make the most effi cient

use of energy and auxiliary materials.

The variety of modern casting process-

es makes it possible to face up to the

economic realities, i.e. the optimisation

of investment expenditure and costs

in relation to the number of units. With

casting, the variable weighting of pro-

duction costs and quality requirements

are also possible.

When designing the shape of the cast-

ing, further possibilities arise from the

use of inserts and/or from joining the

part to other castings or workpieces.

In the last decade, aluminium has at-

tained a leading position among cast

metals because, in addition to its other

positive material properties, this light

metal offers the greatest possible variety

of casting and joining processes.

10


Product range and form of delivery

Our casting alloys are delivered in the

form of ingots with a unit weight of ap-

prox. 6 kg or as liquid metal.

We distinguish between ingots cast in

open moulds and horizontal continu-

ously cast ingots (so-called HGM). In-

gots are dispatched in bundles of up to

approx. 1,300 kg.

The delivery of liquid or molten metal is

useful and economic when large quanti-

ties of one homogeneous casting alloy are

required and the equipment for tapping

and holding the molten metal containers

is available. Supplying molten metal can

lead to a substantial reduction in costs

as a result of saving melting costs and

a reduction in melting losses. The sup-

ply of liquid metal also provides a viable

alternative in cases where new melting

capacities need to be built to comply

with emission standards or where space

is a problem.

As ecological and economic trends sen-

sibly move towards the development of

closed material circuits, the clear dividing

lines between the three classic quality

grades of aluminium casting alloys are

ever-decreasing. In future, people will

simply talk about casting alloys. In

practice, this is already the case. Metal

from used parts is converted back into

the same fi eld of application. The DIN

EN 1676 and 1706 standards with their

rather fl uid quality transitions take this

trend into account.

Aleris is one of only a few companies

to produce a wide range of aluminium

alloys; our product spectrum extends

from classic secondary alloys to high-

purity alloys for special applications.

Production is in full compliance with

the European DIN EN 1676 standard

or international standards and in many

cases, manufactured to specifi c cus-

tomer requirements. We have also been

offering several aluminium casting al-

loys as protected brand-name alloys

for many years, e.g. Silumin, Pantal

and Autodur.

11


Technical consultancy service

The technical consultancy service is

the address for questions relating to

foundry technology. We provide assis-

tance in clarifying aluminium casting alloy

designations as stated in German and

international standards or the temper

conditions for castings. We also offer

advice on the selection of alloys and can

provide aluminium foundries or users of

castings with information on:

Aluminium casting alloys

Chemical and physical properties

Casting and solidifi cation

behaviour

Casting processes and details

regarding foundry technology

Melt treatment possibilities, such as

cleaning, degassing, modifi cation

or grain refi nement

Possibilities of infl uencing the

strength of castings by means

of alloying elements or heat

treatment

Questions relating to surface

fi nish and surface protection.

Technical consultants also provide as-

sistance in evaluating casting defects or

surface fl aws and offer suggestions with

regard to eliminating defects. They sup-

ply advice on the design of castings, the

construction of dies, the casting system

and the confi guration of feeders.

Technical consultants also provide tech-

nical support to aluminium foundries in

the preparation of chemical analyses,

microsections and structural analyses.

Customer feedback coupled with exten-

sive experience in the foundry sector fa-

cilitates the continuous optimisation and

quality improvement of our aluminium

casting alloys.

In co-operation with our customers, we

are working on gaining wider acceptance

of our aluminium casting alloys in new

fi elds of application.

Where required and especially where

fundamental problems arise, we arrange

contracts with leading research institutes

in Europe and North America.

12


As far as possible, the use of common

aluminium casting alloys is recommended.

These involve well-known and proven

casting alloys and we stand fully behind

the quality properties of these casting

alloys which are often manufactured in

large quantities, are more cost-effective

than special alloys and, in most cases,

can be delivered at short notice.

In the European DIN EN 1676 and DIN

EN 1706 standards, the most important

aluminium casting alloys have been col-

lated in a version which is valid Europe-

wide. Consequently, there are already

more than 41 standard aluminium casting

alloys available.

Aluminium foundries should according

to their respective structure limit them-

selves to as small a number of casting

alloys as possible in order to use their

melting equipment economically, to keep

inventories as low as possible and to re-

duce the risk of mixing alloys.

With regard to the quality of a casting,

it is more sensible to process a casting

alloy which is operational in use than one

which displays slightly better properties

on paper but is actually more diffi cult to

process. The quality potential of a cast-

ing alloy is only exploited in a casting if

the cast piece is as free as possible of

casting defects and is suitable for subse-

quent process steps (e.g. heat treatment).

Our sales team and technicians are on

hand to provide foundries and users

of castings with assistance in select-

ing the correct aluminium casting alloy.

To supplement and provide greater depth

to our technical explanations, we refer

you to standard works on aluminium

and aluminium casting alloys. Further

details on other specialist literature are

available and can be requested at any

time. We would be delighted to advise

you in such matters.

Should you have any queries or com-

ments, which are always welcome,

please contact our technical service.

Standard works on aluminium and alu-

minium casting alloys:

Aluminium-Taschenbuch, Verlag

Beuth, Dsseldorf

Aluminium viewed from within -

Profi le of a modern metal, Prof.

Dr. D. G. Altenpohl, Verlag Beuth,

Dsseldorf.

Once the requirements of a casting

have been determined, the selection of

the correct casting alloy from the mul-

titude of possibilities often represents

a problem for the designer and also for

the foundryman. In this case, the Alu-

minium-Taschenbuch can be of great

assistance.

Selecting aluminium casting alloys

13


different casting alloys are compared.

These casting alloys are used for high-

grade construction components, espe-

cially for critical parts.

hard

The casting alloys of this group must

display a certain tensile strength and

hardness without particular requirements

being placed on the metals elongation.

First of all, Al SiCu alloys belong to this

group. Due to their Cu, Mg and Zn con-

tent, these casting alloys experience a

certain amount of self-hardening after

casting (approx. 1 week). These alloys

are particularly important for pressure

die casting since it is in pressure die

casting except for special processes

such as vacuum die casting that pro-

cess-induced structural defects occur,

preventing high elongation values. Due

to its particularly strong self-hardening

characteristics, the Autodur casting al-

Criteria for the selection of

aluminium casting alloys

In the following section, we provide an

insight into the chemical and physical

potentials of aluminium casting alloys by

describing their various properties. The

standardisation provided here helps to

establish whether a casting alloy is suit-

able for the specifi c demands placed

on a casting.

Degree of purity

One important selection criteria is the de-

gree of purity of a casting alloy. With the

increasing purity of a casting alloy family,

the corrosion resistance and ductility of

the as-cast structure also increase; the

selection of pure feedstock for making

casting alloys, however, will necessarily

cause costs to rise.

The increasing importance of the closed-

circuit economy means that, for the pro-

ducer of aluminium casting alloys, the

transition between the previous quality

grades for aluminium casting alloys is

becoming ever more fl uid.

Due to their high purity, casting alloys

made from primary aluminium display the

best corrosion resistance as well as high

ductility. By way of example, Silumin-Beta

with max. 0.15 % Fe, max. 0.03 % Cu

and max. 0.07 % Zn can be mentioned.

In many countries, the Silumin trademark

has already become a synonym for alu-

minium-silicon casting alloys.

Casting alloys made from scrap are,

with regard to ductility and corrosion

resistance, inferior to other casting alloy

groups due to their lower purity. They are,

however, widely applicable and meet the

set performance requirements.

Strength properties

Strength properties should be discussed

as a further selection criterion (Table 1).

A rough subdivision into four groups is

practical:

strong and ductile

The most important age-hardenable

casting alloys belong to this group. By

means of different kinds of heat treat-

ment, their properties can be adjusted

either in favour of high tensile strength

or high elongation. In Table 1, the typi-

cal combinations of Rm and A values for

Classifi cation of casting alloys acc. to strength properties 1)

Casting alloy Temper Tensile Elongation Brinell strength hardness Rm A5 [MPa] [%] HB

Strong Al Cu4Ti T6 330 7 95and ductile Silumin-Beta T6 290 4 90 Al Si10Mg(a) T6 260 1 90

Hard Al Si8Cu3 F 170 1 75 Al Si18CuNiMg F 180 1 90

Ductile Silumin F 170 7 45

Other Al Mg3 F 150 5 50

1) Typical values for permanent mould casting, established on separately-cast test bars.

Table 1

14


Casting properties

Further selection criteria comprise cast-

ing properties such as the fl uidity or

solidifi cation behaviour which sets the

foundryman certain limits. Not every

ideally-shaped casting can be cast in

every casting alloy.

A simplifi ed summary of the casting prop-

erties associated with the most impor-

tant casting alloys is shown in Table 2.

Co-operation between the technical de-

signer and an experienced foundryman

works to great advantage when looking

for the optimum casting alloy for a par-

ticular application.

Given constant conditions, the fl uidity

of a metallic melt is established by de-

termining the fl ow length of a test piece.

Theoretically, low fl uidity can be offset

by a higher casting temperature; this is,

however, linked with disadvantages such

as oxidation and hydrogen absorption as

well as increased mould wear. Eutectic

AlSi casting alloys such as Silumin or

Al Si12 display high fl uidity. Hypoeutectic

AlSi casting alloys such as Pantal 7 have

medium values. AlCu and AlMg casting

alloys display low fl uidity.

Hypereutectic AlSi casting alloys such

as Al Si17Cu4Mg occupy a special posi-

tion. In their case, very long fl ow paths

are observed. This does not however

necessarily lead to a drop in the melt

temperature since primary silicon crys-

tals already form in the melt. The melt

still fl ows well because the latent heat

of solidifi cation of the primary silicon

ductile

Casting alloys which display particu-

larly high ductility, e.g. Silumin-Kappa

(Al Si11Mg), come under this general

heading. This casting alloy is frequently

used for the manufacture of automobile

wheels.

In this particular application, a high elon-

gation value is required for safety reasons.

other

Casting alloys for more decorative pur-

poses with lower strength properties, e.g.

Al Mg3, belong to this category.

loy represents a special case allowing

hardness values of approx. 100 HB and

a corresponding strength albeit at very

low ductility in all casting processes.

Hypereutectic AlSi casting alloys such

as Al Si18CuNiMg and Al Si17Cu4Mg,

for example, which display particularly

high wear resistance due to their high

silicon content, can also be classifi ed

in this group.

Classifi cation of casting alloys acc. to casting properties

Fluidity Thermal Casting alloy Type of solidifi cation crack susceptibility

High Low Silumin Exogenous-shell forming

Al Si12

Al S12(Cu) Exogenous-rough wall

Al Si10Mg Endogenous-dendritic

Silumin-Beta

Al Si8Cu3

Pantal 7

Al Si5Mg

Al Cu4Ti

Al Mg3 Endogenous-globular

Low High Al Mg5 Mushy

Table 2

15


heats up the remainder of the melt. The

already solidifi ed silicon, however, causes

increased mould wear and very uneven

distribution in the castings. In these

casting alloys, high melting and holding

temperatures are necessary so that a

casting temperature of at least 720 C

for pressure die casting and 740 C

for sand and gravity die casting has to

be attained.

The susceptibility to hot tearing is almost

the opposite of fl uidity (Tables 2 and 3).

By hot tearing, we mean a separation of

the already crystallised phases during

solidifi cation, e.g. under the infl uence of

shrinkage or other tensions which can

be transmitted via the casting moulds.

The cracks or tears arising can be healed

by, among other things, the feeding of

residual melt. Eutectic and near-eutectic

AlSi casting alloys also behave particularly

well in this case, while AlCu and AlMg

casting alloys behave particularly badly.

In practice, there are mixed forms and

transitional forms of these solidifi cation

modes. The solidifi cation behaviour is

responsible for the formation of shrink-

age cavities and porosity, for example,

or other defects in the cast structure

as it determines the distribution of the

volume defi cit in the casting. To curb

the aforementioned casting defects,

casting/technical measures need to be

taken: e.g. by making adjustments to

the sprue system, the thermal balance

of the mould or by controlling the gas

content of the melt. A volume defi cit

occurs during transition from liquid to

solid state. This is quite small in high

silicon casting alloys since the silicon

increases in volume during solidifi cation.

In any case, the volume defi cit incurred

Selection criteria for aluminium casting alloys

Casting properties Strength characteristics Corrosion resistance*Shrinkage Fluidity Thermal crack High strength Strong Ductile Hard formation susceptibility and ductile (T6) and ductile

Coarse High Low Silumin

Silumin-Kappa

Silumin-Delta

Al Si12

Al Si12(Cu) Al Si12CuNiMg

Al Si17Cu4Mg

Al Si18CuNiMg

Autodur

Silumin-Beta

Al Si10Mg

Al Si10Mg(Cu)

Al Si8Cu3

Pantal 7

Al Cu4Ti

Al Mg3Si

Al Mg3

Al Mg5

Fine Low High Al Mg9

* Analogue to DIN EN 1706

Table 3

16


needs to be offset as far as possible by

casting/technical means (see also the

section on Avoiding casting defects).

Figure 1 indicates the main types of so-

lidifi cation; each type is shown at two

successive points in time. With regard

to aluminium, only high-purity aluminium

belongs to Solidifi cation Type A (exog-

enous-shell forming). The only casting

alloy which corresponds to this type is

the eutectic silicon alloy or Al Si12 with

approx. 13 % silicon.

The hypoeutectic AlSi casting alloys

solidify according to Type C (spongy),

AlMg casting alloys according to a mix-

ture of Types D and E (mushy or shell-

forming). The remaining casting alloys

also represent intermediate types. At high

solidifi cation speeds, the solidifi cation

types move upwards, i.e. in the direction

of exogenous-rough wall.

Shell-forming casting alloys with smooth-

wall or rough-wall solidifi cation are sus-

ceptible to the formation of macroshrink-

age which can only be prevented to a

limited extent by feeding. Casting alloys

of a spongy-mushy type are susceptible

to shrinkage porosity which can only be

avoided to a limited extent by feeding.

In castings which demand feeding by

material accumulation in particular and

which should be extensively pore-free

as well as pressure-tight the preferred

casting alloys are to be found at the top

of Table 3.

For complex castings whose geometry

does not allow each material accumu-

lation to be achieved with a feeder, the

casting alloys listed in Table 3 offer ad-

vantages provided that a certain amount

of microporosity is taken into account.

Picture 1

A Smooth wall B Rough wall C Spongy

Exogenous solidifi cation types

D Mushy E Shell forming

Endogenous solidifi cation types

Mould

Fluid

Strong

17

SiSiFe


Copper

increases the strength, also at

high temperatures (high-

temperature strength)

produces age-hardenability

impairs corrosion resistance

in binary AlCu casting alloys, the

large solidifi cation range needs to

be taken into account from a

casting/technical point of view.

Manganese

partially offsets irons negative

effect on ductility when iron

content is > 0.15 %

segregates in combination with

iron and chromium

reduces the tendency to stickiness

in pressure die casting.

Magnesium

produces age-hardenability in

combination with silicon,

copper or zinc; with zinc also

self-hardening

improves corrosion resistance

increases the tendency towards

oxidation and hydrogen

absorption

binary AlMg casting alloys are

diffi cult to cast owing to their large

solidifi cation range.

Zinc

increases strength

produces (self) age-hardenability

in conjunction with magnesium.

Infl uence of the most important

alloying elements on aluminium

casting alloys

Silicon

improves the casting properties

produces age-hardenability in

combination with magnesium but

causes a grey colour during anodi-

sation

in pure AlCu casting alloys (e.g.

Al Cu4Ti), silicon is a harmful im-

purity and leads to hot tearing

susceptibility.

Iron

at a content of approx. 0.2 % and

above, has a decidedly negative

infl uence on the ductility (elonga-

tion at fracture); this results in a

very brittle AlFe(Si) compound in

the form of plates which appear in

micrographs as needles; these

plates act like large-scale micro-

structural separations and lead to

fracture when the slightest strain

is applied

at a content of approx. 0.4 % and

above, reduces the tendency to

stickiness in pressure die casting.

Nickel

increases high-temperature

strength.

Titanium

increases strength (solid-solution

hardening)

produces grain refi nement on its

own and together with boron.

18


Infl uencing the microstructural formation of aluminium castings

The marked areas in Figure 1 denote

where it makes sense to carry out the

respective types of treatment on AlSi

casting alloys.

Some of these measures are explained

in more detail in the following section.

Common treatment measures include:

grain refi nement of the solid

solution with Ti and/or B

transformation of the eutectic Si

from lamellar into granular form

modifi cation of the eutectic Si

with Na or Sr

refi nement of the eutectic

Si with Sb

refi nement of the Si primary

phase with P or Sb.

Measures infl uencing microstructural

formation are aimed at improving the

mechanical and casting properties. In

practice, apart from varying the cool-

ing speed by means of different mould

materials, additions to the melt are usu-

ally used.

Types of treatment to infl uence grain structure Figure 1

Temperature [C]

700

600

500

400

Primary Si refi nement

Grain refi nement

0 2 4 6 8 10 12 14 16 18 20 22 24

Modifi cation

Eutectic temperature 577 C

Melt + Si

Melt

Melt + Al 660 C

Al Al + Si

Al Si5 Al Si7 Al Si9 Al Si12 Al Si18

Silicon [wt. %]

19


Grain refi nement

The solidifi cation of many aluminium

casting alloys begins with the formation

of aluminium-rich dendritic or equiaxed

crystals. In the beginning, these solidifi ed

crystallites are surrounded by the remain-

ing melt and, starting from nucleation

sites, grow on all sides until they touch

the neighbouring grain or the mould wall.

The characterisation of a grain is the

equiaxed spatial arrangement on the

lattice level. For casting/technical or

optical/decorative reasons as well as

for reasons of chemical resistance, it is

often desirable to set the size of these

grains as uniformly as possible or as fi nely

as technically possible. To achieve this,

so-called grain refi nement is frequently

carried out. The idea is to offer the so-

lidifying aluminium as many nucleating

agents as possible.

Since grain refi nement only affects the

-solid solution, it is more effective when the casting alloy contains little silicon,

i.e. a lower fraction of eutectic (Figure 2).

Grain refi nement is particularly important

in AlMg and AlCu casting alloys in order

to reduce their tendency to hot tearing.

From a technical and smelting perspec-

tive, grain refi nement mostly takes place

by adding special Al TiB master alloys.

We pre-treat the appropriate casting al-

loys when producing the alloys so that

grain refi nement in the foundry is either

unnecessary or only needs a freshen-

up. The latter can be done in the form of

salts, pellets or preferably with titanium

master alloy wire, following the manu-

facturers instructions.

Since every alloying operation means

more contaminants in the melt, grain

refi nement should only be carried out

for the reasons referred to above.

To make a qualitative assessment of a

particular grain refi nement treatment,

thermal analysis can be carried out (see

section on Melt testing and inspection

procedure).

Effect of silicon content on grain refi nement with Al Ti5B1 master alloy

Mean grain diameter Casting temperature 720 C[m] holding time 5 min

1400

1200

1000

800

600

400

200

0

Silicon [%]

Columnar and equiaxed crystals

Without grain refi nement

With grain refi nementAl Ti5B1: 2,0 kg/mt

0 2 4 6 8 10 12

Figure 2

20


Figures 3 and 4 depict the formation of

microstructural conditions or the degree

of modifi cation as a result of interaction

between sodium and strontium and the

phosphorous element. It can be ascer-

tained that the disruption of modifi cation

due to small amounts of phosphorous

is relatively slight. In Sr modifi cation, a

high phosphorous content can be offset

by an increased amount of modifying

agent. In aluminium casting alloys with a

silicon content exceeding 7 %, eutectic,

silicon takes up a larger part of the area

of a metallographic specimen. From a

silicon content of approx. 7 to 13 %,

the type of eutectic formation, e.g.

grained or modifi ed, thus plays a key

role in determining the performance

characteristics, especially the ductility

or elongation. When higher elongation is

required in a workpiece, aluminium cast-

ing alloys containing approx. 7 to 13 %

silicon will thus be modifi ed by adding

approx. 0.0040 to 0.0100 % sodium (40

to 100 ppm).

In casting alloys with approx. 11 % silicon,

particularly for use in low-pressure die

casting, strontium can also be used as a

long-term modifi er since the melting loss

behaviour of this element is substantially

better than that of sodium. In this case,

the recommended addition is approx.

0.014 to 0.04 % Sr (140 to 400 ppm).

With suitable casting alloys, the required

amount of strontium can be added

during alloy manufacture so that, as

a rule, the modifi cation process step

Modifi cation of AlSi eutectic

(refi nement)

By modifi cation, we mean the use

of a specifi c melt treatment to set a

fi ne-grained eutectic silicon in the cast

structure which improves the mechanical

properties (and elongation in particular)

as well as the casting properties in many

cases. As a general rule, modifi cation

is carried out by adding small amounts

of sodium or strontium. To facilitate an

understanding of the possible forms of

eutectic silicon, these are depicted in

Figure 2 (a-e) for Al Si11 with a varying

Na content:

a) The lamellar condition only

appears in casting alloys which

are virtually free of phosphorous

or modifi cation agents, e.g.

Na or Sr.

b) In granular condition which

appears in the presence of

phosphorous without Na or Sr, the

silicon crystals exist in the form of

coarse grains or plates.

c) In undermodifi ed and

d) to a great extent in fully-modifi ed

microstructural condition, e.g.

by adding Na or Sr, they are

signifi cantly reduced in size,

rounded and evenly distributed

which has a particularly positive

effect on elongation.

e) In the case of overmodifi cation

with sodium, vein-like bands with

coarse Si crystals appear.

Overmodifi cation can therefore

mean deterioration as regards

mechanical properties.

a) Lamellar b) Granular

e) Overmodifi ed

c) Undermodifi ed

d) Modifi ed

Picture 2Types of grain structure

21


can be omitted in the foundry. At low

cooling rates, strontium modifi cation is

less effective so that it is not advisable

to use this in sand casting processes.

To avoid the burn-off of strontium, any

cleaning and degassing of Sr-modifi ed

melts should be carried out with chlorine-

free preparations only, preferably using

argon or nitrogen. Strontium modifi ca-

tion is not greatly impaired even when

remelting revert material. Larger losses

can be offset by adding Sr master alloy

wire in accordance with the respective

manufacturers instructions. At the right

temperature, the addition of sodium to

the melt is best done by charging stand-

ard portions. For easy handling, storage

and proportioning, the manufacturers

recommendations and safety instruc-

tions should be followed.

Since sodium burns off from the melt

relatively quickly, subsequent modifi -

cation must take place in the foundry

at regular intervals. In melts modifi ed

with sodium, any requested cleaning

and degassing should be carried out

with chlorine-free compounds only

(argon or nitrogen). A certain amount

of sodium burn-off is to be reckoned

with, however, and needs to be taken

into account in the subsequent addition

of sodium. When absolutely necessary,

the melt can be treated with chlorine-

releasing compounds long before the

Phosphorous [ppm]

Overmodifi ed

Granular

Modifi ed

Lamellar

Undermodifi ed

Microstructural formation in relation to the content of phosphorous and sodium Al Si7Mg

Sodium Sand casting[ppm] cooling rate 0.1 K/s

140

120

100

80

60

40

20

0

0 5 10 15 20 25 30 35 40 45 50 55 60

Figure 3

Phosphorous [ppm]

Modifi ed Undermodifi ed Granular Lamellar

Microstructural formation in relation to the content of phosphorous and strontium Al Si7Mg

Strontium Gravity die casting[ppm] gravity die cast test bar cooling rate 2.5 K/s

450

400

350

300

250

200

150

100

50

0

0 10 20 30 40 50 60 70 80 90 100

Figure 4

22


fi rst addition of sodium. If such treat-

ment is carried out after adding sodium

or strontium, chlorine may react with

these elements and remove them from

the melt, thereby preventing any further

modifi cation.

Modifi cation with sodium or strontium

increases the tendency to gas absorp-

tion in the melt. As a result of the reac-

tion of the precipitating hydrogen with

the rapidly-forming oxides, defects can

occur in the casting, especially cumulant

microporosity. In many practical cases,

this potential for micropore formation

is even desirable. Then, the purpose

of modification is also to offset the

expected macroshrinkage by forming

many micropores.

An accurate assessment of the effects

of modifi cation can only be made by

means of metallographic examination.

As a quick test, thermal analysis can be

carried out if it is possible to establish by

means of a preliminary metallographic

examination which depression value is

necessary to attain a suffi ciently-modi-

fi ed grain structure (for more information

on thermal analysis, please refer to the

section on Methods for monitoring the

melt). Under the same conditions, rapid

determination of the modifi ed condition

is also possible by measuring the elec-

trical conductance of a sample.

In aluminium casting alloys of the type

Al Si7Mg, a refi nement of the eutectic

silicon with antimony (Sb) is possible.

A Sb content of at least 0.1 % is required.

This treatment, however, only produces

a fi ner formation of the lamellar eutec-

tic silicon and is not really modifi cation

in the traditional sense. The danger of

contamination of other melts by closed-

circuit material containing Sb exists as

even a Sb content of approx. 100 ppm

can disturb normal sodium or strontium

modifi cation. Whats more, refi nement

with antimony can be easily disturbed

by only a low level of phosphorous (a

few ppm) (Figure 5). In contrast to modi-

fi cation, refi nement with antimony can

not be checked by means of thermal

analysis of a melt sample.

Refi nement of primary silicon

In hypereutectic AlSi casting alloys

(e.g. Al Si18CuNiMg), the silicon-rich,

polygonal primary crystals solidify fi rst.

To produce as many fi ne crystals as pos-

sible in the as-cast structure, nucleating

agents need to be provided.

This is done with the aid of prepara-

tions or master alloys which contain

phosphorous-aluminium compounds.

This treatment can also be carried out

when the alloy is being manufactured

and, in most cases, the foundryman

does not need to repeat the process.

If required, the quality of such primary

refi nement can be checked by means

of thermal analysis.

Phosphorous [ppm]

Infl uence of antimony and phosphorous content on the form of the eutectic silicon of Al Si7Mg

Antimony[%]

0.30

0.20

0.10

0.00

0 2 4 6 8 10

Coarse-lamellar

Acceptable Coarse-lamellarto granular

High-purity base

Figure 5

23


Melt quality and melt cleaning

To achieve good melt quality, the for-

mation of oxides and the absorption

of hydrogen have to be suppressed as

much as possible on the one hand, while

other hydrogen and oxides have to be

removed from the melt as far as pos-

sible on the other, although this is only

possible to a certain extent.

All factors which come under the gen-

eral term of melt quality have a direct

effect on the quality of the casting to be

produced. Inversely, according to DIN EN

1706, the cast samples play a valuable

role in checking the quality of the melt.

Most problems in casting are caused by

two natural properties of liquid melts, i.e.

their marked tendency to form oxides

and their tendency towards hydrogen

absorption. Furthermore, other insolu-

ble impurities, such as Al-carbides or

refractory particles as well as impurities

with iron, play an important role.

As mentioned in other sections, the

larger oxide fi lm can lead to a material

separation in the microstructure and,

consequently, to a reduction in the load-

bearing cross-section of the casting.

The solubility of hydrogen in aluminium

decreases discontinuously during the

transition from liquid to solid so that as

solidifi cation takes place, precipitating

gaseous hydrogen reacting with exist-

ing oxides can cause voids which can

in turn take various forms ranging from

large pipe-like blisters to fi nely-distrib-

uted micro-porosity.

Segregation factor [(Fe)+2(Mn)+3(Cr)]

Al Si8Cu3 Al Si6Cu4 Al Si12(Cu)

Critical melting temperatures in relation to the segregation factor

Temperature[C]

650

640

630

620

610

600

590

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

Figure 6

24


Avoiding impurities

Ingot quality

An essential prerequisite for a good

casting is good ingot quality. The metal

should be cleaned effectively and the in-

gots should display neither metallic nor

non-metallic inclusions. The ingots must

be dry (there is a risk of explosion when

damp) and no oil or paint residue should

be present on their surface. When using

revert material, this should be in lumps,

if possible, and well cleaned.

Melting

When melting ingots or revert material,

it must be ensured that the metal is not

exposed unnecessarily to the fl ame or

furnace atmosphere. The pieces of metal

should be melted down swiftly, i.e. follow-

ing short preheating, immersed directly

in the liquid melt.

Large-volume hearth or crucible furnaces

are best suited to melting. Furnaces with

melting bridges are oxide producers and

they lead to expensive, unnecessary and

irretrievable metal losses.

The type and state of the melt in contact

with refractory materials are of particular

importance in the melting and holding

of aluminium.

Aluminium and aluminium casting alloys

in a molten state are very aggressive, es-

pecially when AlSi melts contain sodium

or strontium as modifying agents. With

an eye to quality, reactions, adherences,

infi ltrations, abrasive wear and decompo-

sition have to be kept within limits when

using melting crucibles and refractory

materials as well as during subsequent

processing. The care and maintenance

as well as cleanliness of equipment are

equally important. Adhering materials

can very easily lead to the undesired

redissolving of oxides in the melt and

cause casting defects.

Melting temperature

The temperature of the melt must be set

individually for each alloy.

Too low melting temperatures lead to

longer residence times and, as a result,

to greater oxidation of the pieces jut-

ting out of the melt. The melt becomes

homogeneous too slowly, i.e. local un-

dercooling allows segregation to take

place, even as far as tenacious gravity

segregation of the FeMnCrSi type phases.

The mathematical interrelationship for

the segregation of heavy intermetallic

phases is depicted in Figure 6.

Furthermore, at too low temperatures,

autopurifi cation of the melt (oxides ris-

ing) can not take place.

When the temperature of the melt is too

high, increased oxide formation and

gassing can occur. Lighter alloying ele-

ments, e.g. magnesium, are subject to

burn-off in any case; this must be off-

set by appropriate additions. Too high

melting temperatures aggravate this loss

by burning.

25


Conducting the melting operation

As long as the melt is in a liquid condi-

tion, it has a tendency to oxidise and

absorb hydrogen. Critical points during

subsequent processing include decanta-

tion, the condition or maintenance of the

transfer vessel, possible reactions with

refractory materials as well as transport

or metal tapping. The addition of grain

refi ners and modifying agents above the

required amount can lead to an increase

in non-metallic impurities and greater

hydrogen absorption.

To minimise an enrichment of iron in the

melt, direct contact between ferrous

materials and the melt is to be avoided.

For this reason, steel tools and contain-

ers (casting ladles) must be carefully

dressed. Similarly, but also on economic

grounds, the feed tubes for low-pressure

die casting made from cast iron up to

now should be replaced by ceramic

feed tubes.

Even during the casting process itself

and especially due to turbulence in the

fl ow channel, oxide skins can once again

form which in turn can lead to casting

defects. Casting technology is thus re-

quired to fi nd ways of preventing the

excessive oxidation of the melt, e.g. by

means of intelligent runners and gating

systems (please refer to the section on

Selecting the casting process).

Type of melt treatment

Al Si8Cu3 Pantal 7 Al Mg5

Hydrogen content of various casting alloy melts after different types of treatment

Hydrogen[ml/100g]

0.50

0.40

0.30

0.20

0.10

0.00

10 20 30 0.5 2 4 24 10 20

Aft

er m

eltin

g

Rot

ary

deg

assi

ng

[min

]

Rot

ary

deg

assi

ng

[min

]

Gas

sing 24

h

Hol

din

g in

[h]

Figure 7

26


Cleaning and degassing the melt

Our casting alloys consist of effectively

cleaned metal. Since reoxidation always

takes place during smelting, and in

practice revert material is always used,

a thorough cleaning of the melt is nec-

essary prior to casting.

Holding the aluminium melt at the cor-

rect temperature for a long time is an ef-

fective cleaning method. It is, however,

very time-intensive and not carried out

that often as a result. Foundrymen are

thus left with only intensive methods, i.e.

using technical equipment or the usual

commercially available mixture of salts.

In principle, melt cleaning is a physical

process: the gas bubbles rising through

the liquid metal attach oxide fi lms to their

outer surfaces and allow hydrogen to dif-

fuse into the bubbles from the melt. Both

are transported to the bath surface by the

bubbles. It is therefore clear that in order

for cleaning of the melt to be effective, it

is desirable to have as many small gas

bubbles as possible distributed across

the entire cross-section of the bath.

Dross can be removed from the surface

of the bath, possibly with the aid of ox-

ide-binding salts.

Inert-gas fl ushing by means of an im-

peller is a widely-used, economical and

environmentally-sound cleaning process.

The gas stream is dispersed in the form

of very small bubbles by the rapid turn-

ing of a rotor and, in conjunction with the

good intermixing of the melt, this leads

to very effi cient degassing. To achieve

an optimum degassing effect, the vari-

ous parameters such as rotor diameter

and revolutions per minute, gas fl ow

rate, treatment time, geometry and size

of the crucible used as well as the alloy,

have to be co-ordinated. The course of

degassing and reabsorption of hydrogen

is depicted for various casting alloys

in Figure 7.

When using commercially available salt

preparations, the manufacturers instruc-

tions concerning use, proportioning,

storage and safety should be followed.

Apart from this, attention should also be

paid to the quality and care of tools and

auxiliary materials used for cleaning so

that the cleaning effect is not impaired.

If practically feasible, it is also possible

to fi lter the melt using a ceramic foam

fi lter. In the precision casting of high-

grade castings, especially in the sand

casting process, the use of ceramic

fi lters in the runner to the sand mould

has proved to be a success. Above all,

such a fi lter leads to an even fl ow and

can retain coarse impurities and oxides.

In the gravity die casting of sensitive

hydraulic parts, or when casting sub-

sequently anodised decorative fi ttings

in Al Mg3, ladling out of a device which

is fi tted with in-line fi lter elements and

separated from the remaining melt bath

is very common.

27


Melt testing and inspection procedure

To assess the effectiveness of the clean-

ing process or the quality of the melt, the

following test and inspection methods

can be used to monitor the melt:

Reduced pressure test

This method serves to determine the

tendency to pore formation in the melt

during solidifi cation. A sample, which

can contain a varying number of gas

bubbles depending on the gas content,

is allowed to solidify at an underpressure

of 80 mbar. The apparent density is then

compared with that of a sample which

is solidifi ed at atmospheric pressure.

The so-called Density Index is then

calculated using the following equation:

DI = (dA - d80)/dA x 100 %

DI = Density Index

dA = density of the sample solidifi ed

at atmospheric pressure

d80 = density of the sample solidifi ed

at under 80 mbar

The Density Index allows a certain infer-

ence to be drawn about the hydrogen

content of the melt. It is, however, strongly

infl uenced by the alloying elements and,

above all, by varying content of impurities

so that the hydrogen content must not

on any account be stated as a Density

Index value (Figure 8).

The assessment of melt quality by means

of an underpressure density sample there-

fore demands the specifi c determination

of a critical Density Index value for each

casting alloy and for each application.

The underpressure density method is,

however, a swift and inexpensive meth-

od with the result that it is already used

in many foundries for quality control.

To keep results comparable, sampling

should always be carried out according

to set parameters.

Determination of the hydrogen

content in the melt

Reliable instruments have been in opera-

tion for years for measuring the hydrogen

content in aluminium melts. They work

according to the principle of establish-

ing equilibration between the melt and a

measuring probe so that the actual gas

content in the melt is determined and not

in the solid sample. In this way, the effec-

tiveness of the degassing treatment can

be assessed quickly. The procurement of

such an instrument for continuous quality

monitoring is only worthwhile when it is

used frequently; in small foundries, the

hiring of an instrument to solve problems

is suffi cient.

28


Determination of insoluble

non-metallic impurities

For determining the number and type

of insoluble non-metallic impurities in

aluminium melts, the Porous Disc Filtra-

tion Apparatus (PoDFA) method, among

others, can be used. In this particular

method, a precise amount of the melt

is squeezed through a fi ne fi lter and

the trapped impurities are investigated

metallographically with respect to their

type and number. The PoDFA method

is one of the determination procedures

which facilitates the acquisition, both

qualitatively and quantitatively, of the

impurity content. It is used primarily for

evaluating the fi ltration and other clean-

ing treatments employed and, in casting

alloys production, is utilised at regular

intervals for the purpose of quality control.

This method is not suitable for making

constant routine checks since it is very

time-consuming and entails high costs.

Hydrogen content [ml/100g]

Correlation between the hydrogen content and density index in unmodifi ed Al Si9Mg alloy

Density index Measurement acc. to Chapel [%] at vacum 30 mbar

35

30

25

20

15

10

5

0

0 0.1 0.2 0.3 0.4 0.5 0.6

Figure 8

29


Thermal analysis

To evaluate the effectiveness of melt

treatment measures, e.g. modifi cation,

grain refi nement and primary silicon re-

fi ning, thermal analysis has proved itself

to be a fast and relatively inexpensive

method in many foundries. The test

method is based on the comparison of

two cooling curves of the investigated

melts (Figures 9 and 10).

The undercooling effect (recalescence)

occurring during primary solidifi cation

allows conclusions to be made about

the effectiveness of a grain refi nement

treatment, whereby the recalescence

values do not however allow conclusions

to be drawn as regards the later grain

size in the microstructure. Modifi cation is

shown in thermal analysis by a decrease

in the eutectic temperature (depression)

in comparison to the unmodifi ed state.

Here too, the level of the depression

values depend strongly on the content

of accompanying and alloying elements

(e.g. Mg) and, consequently, the de-

pression values required for suffi cient

modifi cation must be established case

by case, by means of parallel microstruc-

tural investigations.

Time [t]

Thermal analysis for monitoring the grain refi nement of Al casting alloys

Temperature [T]

With grain refi nement Without grain refi nement

Liquidus temperature [TL]

TL

TL

Figure 9

Time [sec]

Thermal analysis for monitoring the modifi cation of Al casting alloys

Temperature [C]

585

580

577575

570

565

560

0 10 20 30 40 50

Modifi ed Undermodifi ed Eutectic temperature

Figure 10

30


Selecting the casting process

Squeeze-casting is another casting pro-

cess to be mentioned; here, solidifi cation

takes place at high pressure. In this way,

an almost defect-free microstructure

can be produced even where there are

large transitions in the cross-section

and insuffi cient feeding.

Other special casting processes include:

Precision casting

Evaporative pattern casting

Plaster mould casting

Vacuum sand casting

Centrifugal casting.

The considerations above concern cast-

ing as an overall process.

In the following notes on casting prac-

tice, the actual pouring of the molten

metal into prepared moulds and the

subsequent solidifi cation control are

looked at in more detail.

From the numerous casting processes,

which differ from one another in the type

of mould material (sand casting, per-

manent dies etc.) or by pressurisation

(pressure die casting, low-pressure die

casting etc.), a few notes are provided

here on the most important processes.

nesses can be favourably infl uenced

with the help of risers. Cylinder heads

for water-cooled engines represent a

typical application.

In the low-pressure gravity die process

with its upward and controllable cavity

fi lling, the formation of air pockets is re-

duced to a minimum and, consequently,

high casting quality can be achieved. In

addition to uphill fi lling, the overpressure

of approx. 0.5 bar has a positive effect

on balancing out defects caused by

shrinkage. The low-pressure die casting

process is particularly advantageous in

the casting of rotationally symmetrical

parts, e.g. in the manufacture of pas-

senger vehicle wheels.

Pressure die casting is the most widely

used casting process for aluminium

casting alloys. Pressure die casting is

of particular advantage in the volume

production of parts where the require-

ment is on high surface quality and the

least possible machining. Special ap-

plications (e.g. vacuum) during casting

enable castings to be welded followed

by heat treatment which fully exploits

the property potential displayed by the

casting alloy.

In addition to conventional pressure die

casting, thixocasting is worthy of men-

tion since heat-treatable parts can also

be manufactured using this process.

The special properties are achieved

by shaping the metal during the solid-

liquid phase.

As mentioned in the introduction, the

entire casting process is the shortest

route from molten metal to a part which

is almost ready for use. All sections of

this catalogue contain advice on how the

entire experience should be carried out.

The casting process is selected ac-

cording to various criteria such as batch

size, degree of complexity or requisite

mechanical properties of the casting.

Some examples:

The sand casting process is used

predominantly in two fi elds of appli-

cation: for prototypes and small-scale

production on the one hand and for the

volume production of castings with a

very complex geometry on the other.

For the casting of prototypes, the main

arguments in favour of the sand casting

process are its high degree of fl exibility

in the case of design changes and the

comparably low cost of the model. In vol-

ume production, the level of complexity

and precision achieved in the castings

are its main advantages.

When higher mechanical properties are

required in the cast piece, such as higher

elongation or strength, gravity die cast-

ing, and to a limited extent pressure die

casting, are used. In gravity die casting,

there is the possibility of using sand

cores. Large differences in wall thick-

31


Gravity die casting process

The gravity die casting which includes

the well-known low-pressure die casting

process is applied. The main fi elds of

application are medium- or high-volume

production using high-grade alloys, and

also low to medium component weight

using heat-treatable alloys. Compared

with sand casting, the aluminium cast-

ings display very good microstructural

properties as well as good to very good

mechanical properties which result from

the rapid cooling times and the other

easily-controlled operating parameters.

The castings have high dimensional ac-

curacy and stability as well as a good

surface fi nish, are heat-treatable and

can also be anodised.

The basis for good quality castings is,

not least, the right melt treatment and

the appropriate casting temperature (see

section on Melt quality and melt clean-

ing). For castings with high surface or

microstructural quality requirements,

such as in decorative or subsequently

anodised components or in pressure-

tight hydraulic parts, it is useful to fi lter

the melt before casting.

Parts generated using the horizontal

pressure die casting process are light-

weight as low wall thicknesses can be

achieved. They have a good surface

fi nish, high dimensional accuracy and

only require a low machining allowance

in their design. Many bore holes can be

pre-cast.

The melting and casting temperatures

should not be too low and should be

checked constantly. Pre-melting alu-

minium casting alloys is useful. The melt

can thus be given a good clean in order

to keep the melt homogeneous and to

avoid undesirable gravity segregation

(see Figure 6). From a statistical point

of view, more casting defects arise from

cold metal than from hot. It is particu-

larly important to keep a suffi ciently high

melting temperature, even with hypere-

utectic alloys. These comments are also

valid for other casting processes.

Pressure die casting process

This process takes up the largest share.

The hydraulically-controlled pressure

die casting machine and the in-built

die make up the central element of the

process. The performance, the precise

control of the hydraulic machine, the

quality of the relatively expensive tools

made from hot work steel are the deci-

sive factors in this process. In contrast,

the fl ow properties and solidifi cation

of the aluminium casting alloys play a

rather subordinate role in this forced

casting process.

The pouring operation in horizontal pres-

sure die casting begins with the casting

chamber being fi lled with metal. The

fi rst movement, i.e. the slow advance of

the plunger and the consequent pile-up

of metal until the sleeve is completely

fi lled, is the most important operation.

In doing this, no fl ashover of the metal

or other turbulence may occur until all of

the air in the sleeve has been squeezed

out. Immediately afterwards, the actual

casting operation begins with the rapid

casting phase. High injection pressure is

essential to achieve high fl ow velocities

in the metal. In this way, the die can be

fi lled in a few hundredths of a second.

Throughout the casting operation, the

liquid metal streams are subject to the

laws of hydrodynamics. Sharp turns

and collisions with the die walls lead

to a clear division of the metal stream.

32


Demands on the casting system

To keep disadvantages and defects

which constantly arise from an oxide

skin forming on the melt within limits,

the gating system must guarantee low

turbulence in the metal stream and also

a smooth, controlled fi lling of the die

cavity. With the transition from a liquid

to a solid condition, volume contraction

occurs; this can amount to up to 7 %

of the volume. This shrinkage is con-

trollable when the solid-liquid interface

runs controlled or directed through

the casting, mostly from the bottom to

the top. This task, namely to effect a

directed solidifi cation, can be achieved

with a good pouring system.

The castings are usually arranged up-

right in the die. The greatest mass can

thus be placed in the bottom of the die.

Quality requirements can be, for example,

high strength, high-pressure tightness or

decorative anodising quality.

One example of an ideal gating system

which meets the highest casting require-

ments is the so-called slit gate system.

Here, the metal is conducted upwards

continuously or discontinuously to the

casting via a main runner. During mould

fi lling, the melt is thus superimposed layer

upon layer with the hotter metal always

fl owing over the already solidifying metal.

The standpipe ends in the top riser and

supplies it with hot metal. This way, the

solidifi cation can be directed from below,

possibly supported by cooling, towards

the top running through the casting and

safeguarding the continuous supply of

hot metal. When there is a wide fl are in

the casting, the gating system has to be

laid out on both sides. This symmetry en-

sures a division of the metal and also an

even distribution of the heat in the die.

In low-pressure die casting, directing

the solidifi cation by means of the gat-

ing system is not possible. Nor is there

any great possibility of classic feeding.

Directional solidifi cation is only possible

by controlling the thermal balance of the

die during casting. This mostly requires

the installation of an expensive cooling-

heating system.

Simulation calculations for die fi lling and

solidifi cation can be useful when laying

out and designing the die and possibly

the cooling. In actual production, the

cooling and cycle time can be optimised

by means of thermography (see section

on Solidifi cation simulation and ther-

mography).

33


Sand casting process

This process is used especially for in-

dividual castings, prototypes and small

batch production. It is, however, also

used for the volume production of cast-

ings with a very complex geometry (e.g.

inlet manifolds, cylinder heads or crank-

cases for passenger vehicle engines).

During shaping and casting, most large

sand castings display in-plane expan-

sion. With this fl at casting method, gating

systems like those which are normal in

gravity die casting for directing solidifi ca-

tion are often not applicable. If possible,

a superimposed fi lling of the die cavity

should be attempted here.

Another generally valid casting rule for

correct solidifi cation is to arrange risers

above the thick-walled parts, cooling (e.g.

by means of chills) at opposite ends. This

way, the risers can perform their main

task longer, namely to conduct the sup-

ply of molten metal into the contracted

end. Insulated dies are often helpful.

The cross-section ratio in the sprue system

should be something like the following:

Sprue :

Sum of the runner cross-section :

Sum of the gates:

like 1 : 4 : 4.

This facilitates keeping the run-in laun-

der full and leads to a smoother fl ow

of the metal. This way, the formation of

oxides due to turbulence can be kept

within limits. The main runner must lie

in the drag, the gates in the cope. In the

production of high-grade castings, it is

normal to install ceramic fi lters or sieves

made from glass fi bre. The selection of

the casting process and the layout of

the casting system should be carried

out in close co-operation between the

customer, designer and foundryman (see

section on Casting-compliant design).

34


Casting-compliant design

Only through good cast quality can the

technical requirements be met and the

full potential of the casting alloy be ex-

ploited. Every effort and consideration

must be made therefore to design a light,

functionally effi cient part whose manu-

facture and machining can be carried out

as effi ciently as possible. For this and

subsequent considerations, the use of

solidifi cation simulation is available (see

section on Solidifi cation simulation and

thermography).

Casting alloys shrink during solidifi ca-

tion, i.e. their volume is reduced. This

increases the risk of defects in the cast

structure, such as cavities, pores or

shrinkage holes, tears or similar. The

most important requirement is thus to

avoid material accumulations by hav-

ing as even a wall thickness as possible.

In specialist literature, the following lower

limits for wall thickness are given:

Sand castings: 3-4 mm

Gravity die castings: 2-3 mm

Pressure die castings: 1-1.5 mm.

In the valid European standard, DIN EN

1706 for aluminium castings, there are

strength values only for separately-cast

bars using sand and gravity die casting.

For samples cut from the cast piece,

a reduction in the 0.2 % proof stress

and ultimate tensile strength values of

up to 70 % and a decrease in elonga-

tion of up to 50 % from the test bar can

be anticipated. When the alloy and the

casting process are specifi ed, so too is

the next point within the framework of

the design, i.e. determination of the die

parting line. Die parting on one level is

not only the cheapest for patterns and

dies but also for subsequent working and

machining. Likewise, every effort should

be made to produce a casting without

undercuts. This is followed by designing

and determining the actual dimensions

of the part. The constant guideline must

be to achieve a defect-free cast structure

wherever possible.

The following notes on the design of

aluminium castings are provided to help

exploit in full the advantages and design

possibilities of near net shape casting.

They also align practical requirements

with material suitability.

Aluminium casting alloys can be pro-

cessed in practically all conventional

casting processes, whereby pressure die

casting accounts for the largest volume,

followed by gravity die casting and sand

casting. The most useful casting process

is not only dependent on the number and

weight of pieces but also on other tech-

nical and economic conditions (see sec-

tion on Selecting the casting process).

To fi nd the optimum solution and produce

a light part as cheaply and rationally as

possible, co-operation between the de-

signer, caster and materials engineer is

always necessary. Knowledge concern-

ing the loads applied, the distribution of

stress, the range of chemical loading and

operation temperatures is important.

35


The minimum values are also dependent

on the casting alloy and the elongation of

the casting. In pressure die casting, the

minimum wall thickness also depends

on the position of and distance to the

gate system.

Generally speaking, the wall thickness

should be as thin as possible and only

as thick as necessary. With increasing

wall thickness, the specifi c strength of

the cast structure deteriorates.

Determining casting-compliant wall

thicknesses also means, especially with

sand and gravity die casting, that the die

must fi rst of all be fi lled perfectly. During

subsequent solidifi cation, a dense cast

structure can only occur if the shrinkage

is offset by feeding from liquid melt. Here,

a wall thickness extending up

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Aluminum Casting Alloys English PV 2012-11-30

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