Influence of the natural aluminium oxide layer on ... - ALU-WEB.DE

EXTRUSION
Grain refiner rod (AlTixBx )
• Chemical analysis
• Efficiency
• Metallography
Oxide, <strong>oxide</strong> lines
TiB2 agglomerate
Al3Ti - phases
Grain refiner input [2]
uid metal will <strong>the</strong>n be mixed thoroughly.
For <strong>the</strong> quality <strong>of</strong> <strong>the</strong> end product <strong>the</strong> quality
<strong>of</strong> <strong>the</strong> metal input is essentially important. If
low quality base material is used, <strong>the</strong> production
<strong>of</strong> a clean melt will be impossible. At Trimet
<strong>the</strong> hot metal is analysed and melt-dross
is skimmed <strong>of</strong>f; when cold metal is used, only
ingots from in-house production are added to
<strong>the</strong> pot room metal.
After skimming <strong>the</strong> surface <strong>of</strong> <strong>the</strong> bath,
a sample will be taken and <strong>the</strong> required elements
will be brought into <strong>the</strong> batch. This
addition will be done in <strong>the</strong> appropriate sequence
and at <strong>the</strong> needed temperature. For <strong>the</strong>
alloying we use master alloys from certified
suppliers. The melt will be stirred, skimmed
and a sample will be taken. If <strong>the</strong> analysis is
Filter box for <strong>the</strong> continuous casting [3]
Unacceptable <strong>oxide</strong> lines and TiB 2
agglomerates in a grain refiner rod
within <strong>the</strong> range given by <strong>the</strong> customer and
<strong>the</strong> temperature at <strong>the</strong> demanded level, <strong>the</strong>
melt will be transferred into <strong>the</strong> inductive
heated casting furnace. From this point on
<strong>the</strong>re is no fur<strong>the</strong>r addition <strong>of</strong> any alloying
agent. The only exception is <strong>the</strong> grain refiner
rod, which is added process-related during <strong>the</strong>
casting. All delivered batches <strong>of</strong> grain refiner
are tested for chemical analysis, <strong>oxide</strong> lines,
TiB 2 agglomerates and Al 3Ti particle size.
In <strong>the</strong> following it is very important to prevent
any turbulence to avoid <strong>the</strong> forming <strong>of</strong>
dross in <strong>the</strong> melt. The melt cleaning process is
divided into three separate steps.
In <strong>the</strong> casting furnace <strong>the</strong> first step <strong>of</strong> <strong>the</strong>
metal cleaning is
done. The melt is
cleaned by a mixture
<strong>of</strong> Ar and Cl,
this is dispersed
by a rotary gas
injector into <strong>the</strong>
melt. During this
step sodium, calcium,
hydrogen
and non-metallic
inclusions are removed.
After <strong>the</strong>
second step, a suf-
Automated casting using <strong>the</strong> spout and floater technique at <strong>the</strong> Trimet plant in Essen [4]
ficient holding time, <strong>the</strong> casting is started and
during casting <strong>the</strong> third step <strong>of</strong> <strong>the</strong> metal cleaning
is operated in-line by a CFF (see Fig. 3).
Usually we use 40 ppi CCFs for <strong>the</strong> trimal BQ
process. For <strong>the</strong> automated casting we use <strong>the</strong>
spout and floater technique, which has some
advantages concerning <strong>the</strong> inclusion content.
With this technique we are able to cast all our
alloys in <strong>the</strong> desired quality.
Quality control
First a short overview <strong>of</strong> <strong>the</strong> inspections and
tests which have to be done before, during and
after <strong>the</strong> BQ process to secure <strong>the</strong> expected
brilliant quality.
Melting: analysis <strong>of</strong> hot and cold metal and <strong>of</strong>
master alloys.
Metal treatment: analysis, quantities, times,
filter.
Casting: H 2 measurement, analysis at casting
start, casting parameter, analysis at end casting,
PoDFA samples.
First inspection: ultrasonic test, length, shape,
surface defects.
Homogenization: recipe, temperature, time,
cooling.
Structure: cracks, inclusions, porosity, coarse
grain, surface defects.
Of particular importance during quality
control in <strong>the</strong> cast shop are <strong>the</strong> melt analysis
concerning H 2 and inclusions (PoDFA-measurements),
<strong>the</strong> continuous control <strong>of</strong> all casting
parameters and <strong>the</strong> ultrasonic testing after
casting.
These checks are supplemented by metallographic
quality control in <strong>the</strong> Trimet laboratory.
This means detailed controlling <strong>of</strong> possible
cracks, inclusions, porosity, grain structure,
etc.
Trials about <strong>the</strong> solid / liquid ratio
So why should we change a stable and successful
process? One <strong>of</strong> <strong>the</strong> reasons was <strong>the</strong>
crisis that started in 2009. Trimet had to reduce
its output <strong>of</strong> <strong>the</strong> casthouse and to switch
<strong>of</strong>f around two thirds <strong>of</strong> <strong>the</strong> electrolyses capacity.
This implied that <strong>the</strong> number <strong>of</strong> high
purity pots was decreased, too. This resulted
in a lack <strong>of</strong> high purity metal for a trimal BQ
campaign.
The trials were supposed to answer <strong>the</strong>
question whe<strong>the</strong>r <strong>the</strong>re is a difference in <strong>the</strong>
quality <strong>of</strong> <strong>the</strong> billets when <strong>the</strong>y are produced
from 100% solid metal or 100% liquid metal
or 50/50%.
The melt quality was checked with <strong>the</strong>
PoDFA method. The Porous Disc Filtration
Apparatus is a method by which liquid metal
32 ALUMINIUM · EAC CONGRESS 2011