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<strong>Influence</strong> <strong>of</strong> raw material constitution on <strong>the</strong> quality <strong>of</strong>
gloss alloy-type extrusion billets – a report from practice
Klaus H<strong>of</strong>fmann, Ralf Munk, Marcel Rosefort and Dietmar Bramh<strong>of</strong>f, Trimet Aluminium AG
The Trimet group provides <strong>the</strong> <strong>aluminium</strong>
processing industry with primary metal,
alloys, semi-finished products, castings
and recycling. The competences <strong>of</strong> <strong>the</strong>
business unit Primary Products are electrolysis
and continuous casting. Products
are rolling slabs, extrusion and forging
billets, hot dip-coating alloys, initial products
for safety parts and liquid <strong>aluminium</strong>.
Due to <strong>the</strong> demand for high quality
and services regarding <strong>the</strong>se products in
particular <strong>of</strong> <strong>the</strong> automotive industry it
is essential to enforce and secure quality
control. All <strong>the</strong>se high quality products
from Trimet, especially <strong>the</strong> safety parts,
<strong>the</strong> gloss alloys and parts with functional
surfaces for automotive applications, are
characterized by an extremely homogenous
microstructure without
internal defects or inclusions.
Within our permanent quality
improvement programme an examination
has been carried out to
measure <strong>the</strong> influence <strong>of</strong> raw material
constitution on <strong>the</strong> product
quality. Therefore <strong>the</strong> use <strong>of</strong> liquid
metal, ingot metal and mixtures <strong>of</strong>
<strong>the</strong>se has been tested. The paper
covers <strong>the</strong> essential production
steps for such challenging high
quality billets. It gives a report
on <strong>the</strong> quality requirements, <strong>the</strong>
qualifying <strong>of</strong> <strong>the</strong> alloying material and <strong>the</strong>
process parameters. Especially <strong>the</strong> methods
<strong>of</strong> analysis, like PoDFa and inclusion
identification, and <strong>the</strong> correlations between
<strong>the</strong> process parameters, <strong>the</strong> feedstock
and metal quality are described.
The market for extrusion billets for bright surface
products in Europe is bigger than 15,000
tonnes a year. The highest demand for this
special material comes from <strong>the</strong> automotive
industry. There it is used for trim parts, ro<strong>of</strong>
racks and cover plates. But <strong>the</strong>re is also a
market for this material in <strong>the</strong> lighting and audio
industry. One can say that <strong>the</strong> time <strong>of</strong> <strong>the</strong>
black coated trims is over – for <strong>the</strong> moment.
To fulfil <strong>the</strong> very challenging specifications
for <strong>the</strong> final parts, <strong>the</strong> whole process from <strong>the</strong>
alumina for <strong>the</strong> electrolyses up to <strong>the</strong> anodising
<strong>of</strong> <strong>the</strong> final part has to be fully tuned and
straight designed. All changes in this process
chain have to be scrutinised, and all changes
have to be discussed and well co-ordinated
between <strong>the</strong> partners <strong>of</strong> this sensitive process
chain.
So, why change such a sensitive process?
If we want to improve <strong>the</strong> quality <strong>of</strong> <strong>the</strong> end
product, we will have to change <strong>the</strong> process
from time to time, or <strong>the</strong>re are technical needs
to change a step in <strong>the</strong> process. This paper discusses
<strong>the</strong> influence <strong>of</strong> <strong>the</strong> solid/liquid metal
ratio.
The trimal BQ process
Trimet developed a special process named BQ
(Brilliant Quality) to fulfil all <strong>the</strong>se requirements
for a brilliant end product.
The trimal BQ process is a special process
for producing extrusion billets, to fulfil high
requirements and to cover all <strong>the</strong> production
steps and parameters that are adjusted for <strong>the</strong>
end product: <strong>the</strong> alumina, melting, casting, homogenizing,
testing.
This process includes all parts <strong>of</strong> <strong>the</strong> pro-
Production flow in <strong>the</strong> casthouse
Images: Trimet
EXTRUSION
duction chain starting with <strong>the</strong> alumina and
anodes for <strong>the</strong> electrolysis at <strong>the</strong> Essen or
Hamburg smelter to produce <strong>the</strong> liquid and
solid raw material. Followed by alloying, metal
treatment, grain refining and casting, finished
up with <strong>the</strong> customised homogenizing process
unique for <strong>the</strong>se billets.
The electrolysis process
At <strong>the</strong> electrolysis <strong>the</strong>re are several pots operated
as high purity pots. For <strong>the</strong>se pots <strong>the</strong>
alumina is not used in <strong>the</strong> waste gas purification
process to avoid <strong>the</strong> enrichment <strong>of</strong> o<strong>the</strong>r
elements. The anodes are hand selected according
to <strong>the</strong>ir metal content and <strong>the</strong>ir operating
life in <strong>the</strong> cells is shorter than in <strong>the</strong>
standard pots.
The control <strong>of</strong> <strong>the</strong> pots is done by
<strong>the</strong> Trimet 9-box matrix model [1]. This
is increasing current efficiency and reducing
energy consumption. After <strong>the</strong>
transport, crucible with <strong>the</strong> pot room
metal is arriving at <strong>the</strong> casthouse <strong>the</strong>
dross is accurately removed from <strong>the</strong>
metal surface. Now <strong>the</strong> metal is transferred
to one <strong>of</strong> <strong>the</strong> melting furnaces or
casted into sow forms.
Melting and casting
The process in <strong>the</strong> casthouse (see flow
chart) starts with <strong>the</strong> loading <strong>of</strong> solid metal into
<strong>the</strong> gas fired melting furnace. When <strong>the</strong> solid
metal is heated up to <strong>the</strong> required temperature
<strong>the</strong> hot pot room metal (approx. 930°C)
is added to <strong>the</strong> furnace. In this step we use <strong>the</strong>
energy which is stored in <strong>the</strong> pot room metal
to bring <strong>the</strong> sows into <strong>the</strong> liquid state. The liq-
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