P. LICHÝ et al.: REFINING ALUMINIUM-ALLOY MELTS WITH GRAPHITE ROTORS
263–265
REFINING ALUMINIUM-ALLOY MELTS WITH GRAPHITE
ROTORS
UPORABA GRAFITNIH ROTORJEV ZA REFINACIJO TALIN IZ
ZLITIN NA OSNOVI ALUMINIJA
Petr Lichý
1
, Markéta Bajerová
2
, Ivana Kroupová
1
, Tomá{ Obzina
1
1
V[B – Technical University of Ostrava, Faculty of Materials Science and Technology, Department of Metallurgy and Foundry, 17. listopadu
2172/15, 708 00 Ostrava-Poruba, Czech Republic
2
JAP Industries s.r.o., Bystøice 1260, 739 95 Bystøice, Czech Republic
Prejem rokopisa – received: 2019-07-12; sprejem za objavo – accepted for publication: 2019-11-08
doi:10.17222/mit.2019.147
The presented paper is focused on the study of refining aluminium-alloy melts. The quality of castings is determined by the
basic material, i.e., the input material (furnace charge), its subsequent melting and subsequent metallurgical modifications. To
allow the profitability of production, low-quality input materials are currently used, which, although they have the required
chemical composition, may contain a large amount of impurities, coming not only from the original input materials, but also
from their processing. To allow a cost reduction, in addition to rejected castings, the residues of gating and chipping from the
machining are used as charge materials in foundries. However, the melt thus prepared is heavily contaminated with gases and
inclusions.The aim of the research was, therefore, to verify the use of graphite rotors functioning as degassing units for refining
aluminium melts. The experiments were conducted under laboratory and pilot-plant conditions using the plant equipment. The
refining efficiency was monitored with respect to the type and shape of the graphite rotor and the refining medium used. The
achieved purity of the resulting melt was evaluated to establish the density index and the final cast microstructure.
Keywords: aluminium alloy, degassing, microstructure, density index
^lanek opisuje rafinacijo talin iz Al zlitin. Kvaliteta ulitkov je odvisna od osnovnega materiala (sestave, ~istosti itd.), postopka
taljenja in metalur{kih modifikacij. S pridobitvenega stali{~a proizvodnje se trenutno uporabljajo manj kakovostni vhodni
materiali, ki sicer ustrezajo glede zahtevane osnovne kemijske sestave, vendar lahko vsebujejo veliko ne~isto~. Te se ni nujno,
da nahajao v originalnem izhodnem materialu, lahko pa nastajajo tudi med samim proizvodnim procesom. Zaradi zmanj{anja
stro{kov se v livarnah kot vlo`ek uporabljajo tudi s strani kontrole zavrnjeni ulitki (izmet) in ostanki mehanske obdelave
(odrezki, ostru`ki). Zaradi tega je pripravljena Al talina mo~no kontaminirana s plini in nekovinskimi vklju~ki. Cilj avtorjev
pri~ujo~e raziskave je bil verificirati rafinacijo Al talin z uporabo grafitnih rotorjev v enotah za razplinjevanje. Preizkuse so
izvajali na laboratorijskih in pilotnih napravah. U~inkovitost rafinacije so zasledovali glede na tip in obliko izbranega grafitnega
rotorja in uporabljeno rafinacijsko sredstvo. Dose`eno ~istost talin so ovrednotili z dolo~anjem indeksa gostote (naprava Dichte
Index) in analizo kon~ne mikrostrukture ulitkov.
Klju~ne besede: zlitina na osnovi aluminija, razplinjanje, mikrostruktura, indeks gostote
1 INTRODUCTION
One of the most important steps in the production of
aluminium-alloy castings is the preparation of the liquid
metal. Pigs with the required chemical composition and
residues (especially the remains of the gating including
risers and mismatched castings) are used as the input
materials. Many foundries also add drillings left after the
machining of aluminium castings but these are very
heavily contaminated with organic compounds (e.g., re-
sidues of oily emulsions). Due to economic require-
ments, these contaminated materials together with
lower-quality input materials (aluminium scrap) are used
more and more. This then leads to subsequent problems,
which include an increased degree of gassing of the melt,
a higher proportion of inclusions and related waste
casting rate.
1
This situation then requires the use of a
more efficient method of refining the melt. Still, the most
commonly used way of refining is the use of the refining
equipment whose most important part is the refining
rotor. It is most often made of silicon carbide or graphite.
Graphite rotors are characterized by a low price, high
strength and resistance to temperature changes.
The use of the refining equipment should be based on
an assumption that the entire refining process, i.e., the
elimination of gas and inclusions from the melt, should
be as fast as possible, but also as effective as possible, in
order to avoid unnecessary downtimes in production.
From the point of view of energy loss, this fact is very
important, too.
2 EXPERIMENTAL PART
The practical part of the study deals with comparing
the efficiency of the degassing of chosen types of rotors
using nitrogen as the degassing medium. Two rotors
from the company JAP INDUSTRIES s.r.o. (Ltd.)
Materiali in tehnologije / Materials and technology 54 (2020) 2, 263–265 263
UDK 620.1/.2:67.017:669.715:536.411 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(2)263(2020)
*Corresponding author's e-mail:
petr.lichy@vsb.cz (Petr Lichý)

marked as J8 and F2A (Figure 1)w e r et e s t e da t
500 min
–1
.
Both rotors were tested with the AlSi7Mg0.3 alloy.
2
The flow rate of the degassing medium was 10.5 L/min,
the melt temperature 710 °C, the ambient temperature
21 °C and air humidity of 28 %. The rotor was located in
the melt 150 mm above the crucible bottom. The cycle
for evaluating the refining effectiveness is given in
Table 1. After each step, the samples were taken
(marked with J for the J8 rotor and marked with F for the
F2A rotor) and evaluated for purity with respect to their
microstructure and density index (DI). The DI is the
number characteristic for the melt quality of an alumi-
nium melt sample.
3
The melt-sample pair densities – the
one resulting from the vacuum-density test (D
80mbar
) and
the one resulting from the hardening under atmospheric
pressure (D
atm
) – are related to each other based on
Equation (1).
DI=(( D
atm
-D
80mbar
)/D
atm
)*100 [%] (1)
Table 1: Test refining cycle
Order Step
Operation
time
(min)
Sample markings
Rotor
J8
Rotor
F2A
1 After melting - J1 F1
2 Refining with N
2
3J 2F 2
3 Gassing with H
2
+N
2
6J 3F 3
4 Refining with N
2
6J 4F 4
5 Gassing with H
2
+N
2
6J 5F 5
6 Refining with N
2
3J 6F 6
7 Gassing with air 5 J7 F7
8 Refining with N
2
6J 8F 8
9 Gassing with air 5 J9 F9
10 Refining with N
2
3 J10 F10
11 Gassing with H
2
+N
2
5 J11 F11
3 RESULTS
The microstructures of the samples (J1 and J2)
obtained with the use of rotor J8 is shown in Figure 2
(sample J1) and Figure 3 (sample J2). It is evident from
the figures that in the case of the refining cycle lasting 3
min there is a significant clean-up of the melt.
The values of gassing the melt obtained by evaluating
the DI are given for individual samples in Figure 4.
4 DISCUSSION
With respect to the final value after the 10
th
step, both
types of rotor showed virtually the same efficiency.
However, with respect to the shortest possible time of
P. LICHÝ et al.: REFINING ALUMINIUM-ALLOY MELTS WITH GRAPHITE ROTORS
264 Materiali in tehnologije / Materials and technology 54 (2020) 2, 263–265
Figure 1: Used graphite rotors (J8 – left, F2A – right)
Figure 3: Microstructure of the J2 sample
Figure 2: Microstructure of the J1 sample

reaching the lowest DI value there is a noticeable diffe-
rence. The J8 rotor was more efficient because after the
first refining lasting 3 min, the DI value was reduced by
42 %, whereas for the F2A rotor, this reduction amount-
ed to only 7 %.
5 CONCLUSIONS
Achieving a high quality of cast parts made of alumi-
nium alloys is conditioned, inter alia, by the reduction of
the hydrogen content. With regard to the production
efficiency, it is thus important to achieve this state in the
shortest possible time. When comparing the selected
rotors, the J8 rotor meets this condition. The use of
appropriate types of rotors in combination with inert
gases (argon and nitrogen) will allow us in future to
reduce the amount of refining salts, thus eliminating the
economic and ecological burdens of foundry plants.
Acknowledgment
The contribution was worked out with the support of
the project MPO TRIO FV 10080 and the support of the
projects within "the Student Grant Competition", with
numbers SP2020/39 and SP2020/64.
6 REFERENCES
1
M. Cagala, M. Bruska, P. Lichy, J. Beno, N. [pirutová, Influence of
Aluminium-Alloy Remelting on The Structure and Mechanical
Properties, Mater. Tehnol., 47 (2013) 2, 239–243
2
R. Pastir~ák, J. [~ury, Effect of Pressure on the Crystallisation of
AlSi7Mg Alloy, Arch. Metall. Mater., 62 (2017) 4, 2193–2198,
doi:10.1515/amm-2017-0323
3
E. Czekaj, J. Nykiel, Z. Kwak, A. Garbacz-Klempka, M. Nykiel, The
Influence of Selected Refining Methods of AlSi7Mg0.3 Silumin on
its Quality Index, Archives of Foundry Engineering, 18 (2018)2 ,
72–78, doi:10.24425/122505
P. LICHÝ et al.: REFINING ALUMINIUM-ALLOY MELTS WITH GRAPHITE ROTORS
Materiali in tehnologije / Materials and technology 54 (2020) 2, 263–265 265
Figure 4: DI determined in relation to the refining step and rotor type