INDUSTRIAL PERSPECTIVE
Casting technology is developing rapidly, driven by a combination of
improvements in understanding of underlying solidification theory,
increased computer capacity for design simulation and on-line control, and
enhanced industrial competition in increasingly globalized world markets.
Wrought products are often dominated by the quality and yield of primary
solidification of ingot and billet feedstock. Moreover, many industrial sectors
are developing rapid prototyping and agile manufacturing processes to
reduce time to market and speed up responsiveness to customer demands.
Casting has much to offer as a near net shape technology, but important
issues of reproducibility and quality need to be improved. The overall industrial
perspective on casting technology is discussed in this section, concentrating
on recent innovations and challenges for the future.
Chapters 1 and 2 discuss respectively semi-continuous and continuous
methods of casting aluminium alloys, and chapter 3 discusses continuous
casting of steel. Chapters 4 and 5 discuss different aspects of the importance
of casting in the important automotive sector.
Chapter 1
Direct chill billet casting of
aluminium alloys
Martin Jarrett, Bill Neilson and
Estelle Manson- Whitton
Introduction
This chapter details the operational and technological developments of the
direct chill (DC) casting process for a high volume commercial extrusion
business. Other continuous casting processes are discussed in chapters 2
and 3. Modelling of DC casting is discussed in chapter 6. Improved extrusion
manufacturing efficiency is driving the need for better and more consistent
billet quality. This has necessitated significant technological and process
development of DC billet production, in order to produce extrusion ingots
of predictable performance. A thorough understanding is required of the
interaction of the equipment and process technologies that impact on the
metallurgical macro- and microstructure of the DC cast ingot, and its
subsequent performance in the extrusion process.
The first commercial semi-continuous casting machine for aluminium
alloys was opened in Germany in 1936, following development of vertical
continuous casting techniques for other metals, such as lead, since the
mid-nineteenth century [1]. Over the past 50 years, the DC casting process
has been developed to become the predominant process for extrusion
billet production [2, 3], with 6xxx alloy extrusion billet accounting for a
large percentage of the throughput of worldwide aluminium DC casting
production.
Together with market driven advancement, environmental considerations
are driving significant change in DC casting technology. The provision
of consistent high quality DC cast billets of predictable performance is
of fundamental importance in operating extrusion presses at maximum
efficiency, while meeting the stringent quality requirements of the
market place. Several key factors affecting billet quality, that impact on
extrusion performance, have been previously described by Weaver [4] and
3
4 Direct chill billet casting of aluminium alloys

Langerweger [5], and more recently by Bryant and Fielding [6]. These
emphasize the importance of characterization and control of the total
process. A schematic representation of a typical DC casting process is
shown in figure 1.1. The key process stages form the basis of the subsequent
sections of this chapter, which describe the critical aspects of both equipment
and process technology.
Direct chill billet production
The DC casting process from melting, through melt in-line treatment, casting
and homogenization is discussed in terms of the present best practice
and promising novel techniques. The impact of best practice on billet
quality and ultimately the impact on extrusions are also then discussed,
utilizing the extrusion limit diagram concept, the critical aspects of which
have been summarized by Parson et al [7]. All discussion is in the context
Melting 5
of a high throughput DC casting operation, concentrating on 6xxx
series alloys.
Melting
Current standard DC casting facilities use melting furnaces which are
charged with a mixture of primary and secondary scrap aluminium depending
on target billet specification. The molten metal is transferred to a
holding furnace before casting. The use of a holding furnace maximizes
efficiency by fully utilizing melting time. The ideal configuration balances
the melting capacity of the furnaces with the casting capacity of the DC
casting machine. Alloying additions are made in the melting furnace, in
the holding furnace, or during laundering to the holding furnace. Better
mixing is achieved if alloying additions are made earlier, although there is
a danger of significant loss if additions are made to the melting furnace,
and, unless additions can be made with the furnace door closed, production
time may be lost.
Melting and holding furnaces can be either induction or gas heated.
Gas is preferred for efficiency, and for melt cleanliness as the churning
resulting from induction heating can drag oxide particles from the dross
back into the melt. More recently oxyfuel furnaces, burning gas with
approximately 10% oxygen, have been introduced, although a wellcontrolled
gas furnace, incorporating either regenerative or recuperative
air heating systems to maximize fuel efficiency, remains the industry
preference.
Furnaces can be fixed hearth or tilting. Fixed hearth furnaces have lower
capital cost but tilting furnaces are preferred for metal cleanliness, process
control, and safety, as at any point the flow can be stopped by resetting
the furnace, whereas a fixed hearth furnace requires manual plugging.
Older fixed hearth furnaces are generally being replaced with tilting furnaces.
A further advantage of tilting furnaces is that they can be fully drained,
allowing greater flexibility for alloy changes.
Together with other areas of the DC casting process, the environmental
impact of furnaces is being minimized by the reduction of particulate
emissions through more efficient furnace design (for example, the use of
regenerative burners) and, where necessary, the use of equipment for the
capture of both particulate and noxious gaseous emissions.
Temperature control is of paramount importance in casting, having a
direct bearing on product quality and production efficiency. The optimum
melt delivery temperature for 6xxx alloys is in the range 690-750°C, and is
product and plant specific. Temperature measurement is predominantly
through the use of thermocouples. Other methods of temperature measurement
such as optical pyrometry are used, but encounter problems with
6 Direct chill billet casting of aluminium alloys
aluminium due to oxide skin formation. Immersed thermocouples, however,
remain the most accurate, robust and reliable technology. Heating
equipment is universally thermostatically controlled, to maintain thermal
efficiency and process control.
Molten metal pre-treatment
Molten metal pre-treatments, carried out in the melting or holding furnace,
can be distinguished from in-line treatments given during laundering of the
melt to the casting machine. The main purpose of fluxing [8] is to clean the
melt by degassing and to remove oxides and other inclusions. Other advantages
of fluxing include the production of a dry dross, which minimizes metal
losses during skimming. Fluxing, together with more recent developments in
dross reprocessing (pressing and recycling), has led to improved recoveries in
the industry, whilst maintaining cleaner and more efficient melting units.
Currently, two methods of fluxing are available, using chlorine gas or
chlorine-based salt. Injection of gas (sometimes using a spinning nozzle)
below the surface of the melt has until recently been the preferred method.
However, environmental legislation is now necessitating the phasing out of
chlorine gas use at the melting stage. The use of fluxing salts, although a
more mature technology than gas fluxing, is now being re-evaluated as a
replacement for chlorine gas. However, there are still environmental
concerns over the chlorine and fluorine reaction products produced which
remain in the dross, and the uncertain determination of potentially hazardous
products of reaction emitted to the environment from the use of
these fluxes. The need to reduce and eventually eliminate the use of chlorine
in fluxes has led to a gap in the market for an alternative environmentally
friendly fluxing method. Currently there are no processes yet capable of
commercial operation.
In-line metal treatment
Certain melt treatments, namely grain refinement, degassing and filtration,
must be given in-line during laundering to the casting machine to accrue
maximum benefit. Grain refining inoculants have in the past been added to
the holding or even melting furnace. The disadvantages of this method
include fade (where the effectiveness of the grain refiner decreases with
time), and the formation of a boron-particle-rich sludge in the bottom of
the furnace which contaminates the metal, and leads to through-length
variation in cleanliness and composition of the DC cast log. Degassing and
filtration are performed in the launder such that there is minimum turbulent
flow which can cause the reintroduction of oxides before the casting
Grain refinement 7
machine. Recent technological developments of these techniques have been
reviewed by Fielding and Kavanaugh [9], who emphasize the criticality of
degassing and filtration in the DC casting process.
Grain refinement
The purpose of grain refinement is to produce a refined, equiaxed grain structure
with modified second phase particle morphology through the thickness
of the DC cast log. Nucleation and grain refinement are discussed in detail
in chapters 12 and 13. Current practice is to use approximately l0 ppm of
titanium to give the most desirable grain size of approximately 80-150 mm.
There are a number of alternative grain refiners available, all based on Al,
the most common being 6%Ti, 3%Ti, 5%Ti-0.1%B, 5%Ti-l%B and
5%Ti-0.2%B. All these compositions are in commercial use for a variety
of applications and products. Best metal cleanliness is achieved if grain
refiner is added before de-gassing or filtration. Figure 1.2 shows a system
for injection of grain refiner rod into the launder, which can be controlled
automatically to give the desired rate of addition.
Universally the 5%Ti-l%B grain refiner is the preferred inoculant [10].
As a result of this it has been extensively researched, leading to a number of
publications discussing in detail its capability as a grain refiner under various
conditions [11,12]. Its difficulty, however, is the high boron content. Boron is
an insoluble element and boron particles are prone to flocculate, forming
large clusters which can be deleterious to products in the form of pick-up,
and as an abrasive to dies. To counter the problem of boron inclusions, it
is advised to inoculate at a reduced rate that accounts for the boron content
of the recycled aluminium.
Figure 1.3 shows the effect of titanium content on grain size for a typical
5:1 grain refiner achieving the target grain size. In addition to grain size
modification [13], experiments have shown that the morphology of the
insoluble iron-rich phase can be influenced, not only by the grain refiner
composition, but also by the manufacturing route used by different suppliers.
(The presence of the iron-rich phase in the form of a-script has been shown to
affect adversely extrusion surface quality by increasing pick-up [14]). This
would indicate that the nucleation mechanism of the iron-rich particles is
strongly influenced by the grain refinement process. This effect is currently
being investigated to gain an understanding of the process. Figure 1.4 is a
plot of a-script occurrence for different grain refiners, and shows that the
choice of grain refiner can have a significant effect on the amount of ascript
in the microstructure.
The most promising new grain refiner currently available is titanium
carbide [15-17], which, in addition to eliminating boron contamination,
is reported to overcome problems of poisoning by zirconium- and
8 Direct chill billet casting of aluminium alloys
Figure 1.2. System lor automatic injection ot grain renner rod into trie melt during
laundering to the casting machine.
Figure 1.3. Average grain size (determined at mid-radius) as a function of total melt
titanium content for a 180mm diameter 6063 billet following in-line inoculation with
5.%Ti-l% Brod.
Degassing 9
Figure 1.4. Number of a-script particles per mm2 for different grain refiners produced by
three different suppliers.
chromium-containing alloys, yet is as effective in reducing grain size as titanium
boride [15]. Closer temperature control is, however, required in using
titanium carbide and thus commercial implementation may require adjustment
to processing control. Individual plants are currently evaluating the
use of titanium carbide. Other novel methods of grain refinement include
Nb additions [18], and physical methods (which retard formation of and
break up dendrites) such as ultrasonic vibration [19], sump displacement
and electromagnetic stirring [20].
Degassing
Most high-quality cast houses now use in-line spinning nozzle degassing
systems for the removal of hydrogen. The most common systems are the
Alpur (Pechinney) and SNIP (Foseco). However, some patents are now
ending, and enterprising companies are designing their own systems based
on existing and new technology [9]. An example of an Alpur degasser is
shown in figure 1.5, and a schematic of a SNIP degasser is shown in figure
1.6. Efficiencies of spinning nozzle type degassers are very high, and they
can deliver hydrogen levels of less than approximately 0.1 cc/100 g, compared
to previous levels of approximately 0.3-0.4cc/100 g. Figure 1.7, for example,
shows the hydrogen content of a melt measured before and after passing
through the Alpur degasser shown in figure 1.5.
Both Alpur and SNIP systems, when using chlorine gas, have also been
shown to reduce oxide inclusions by approximately 50%. They will reduce
the overall volume fraction of inclusions, and through the stirring action
10 Direct chill billet casting of aluminium alloys
Figure 1.5. The Alpur degasser in use at British Aluminium Extrusions. Banbury. UK.
Figure 1.6. Schematic of the SNIP R-140 degasser showing two chambers.
Degassing 11
Figure 1.7. Hydrogen content of 6xxx alloys measured before and after passing through an
Alpur degasser.
will break up coarse borides. However, as a result of this, the number density
of particles can increase. Improved process control using multi-chamber
SNIF and Alpur systems, incorporating two-way metal flow and controlled
argon and chlorine gas mixtures have, however, demonstrated significantly
improved levels of particle removal