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Evaluation of the hot forming capability of an IMC FeAl-based alloy
Wojciech Szkliniarz1, Eugeniusz Hadasik1
Ivo Schindler2
1
SILESIAN UNIVERSITY OF TECHNOLOGY, ul. Krasińskiego 8, 40-019 Katowice, Poland,
[email protected], [email protected]
2
VŠB - TECHNICAL UNIVERSITY OF OSTRAVA, Institute of Modelling and Control of
Forming Processes, 17. listopadu 15, 708 33 Ostrava, Czech Republic, [email protected]
Abstract
Alloy with chemical composition of Fe-38Al-0,2Mo-0,05Zr-0,1C (in at. %) was studied.
The alloy was melted in a vacuum induction furnace using a vacuum of 0.5 Pa and a spinel
MgO·Al2O3 melting pot. The melt was cast to the preheated graphitic mould. Threefold
refining re-melting for the purpose of the alloy’s homogenization and removing of
contamination was applied. Thus obtained content of oxygen and hydrogen of 38 ppm, resp.
0.2 ppm enabled to apply hot forming on this alloy.
The as-cast microstructure comprises the FeAl-phase grains of mean diameter about
100 µm with occurrence of dispersive phases at grain boundaries as well as inside the grains.
Following annealing 1273 K/24 hours yields in the chemical composition homogenization and
annihilation of former dispersive phases. The hardening and dynamic softening processes
were studied using the hot rolling tests at two different laboratory rolling mills. Based on the
measurement of rolling forces by forming specimens with graduated thickness in a wide range
of strain and strain rates, the mathematical model was developed which enables to predict the
mean equivalent stress values.
1. INTRODUCTION
The interest in alloys with matrix of ordered intermetallic phase FeAl, constituting the
matrix of potential structural materials, is due to unique combination of their excellent
resistance to the action of oxidizing atmosphere, carbonizing atmosphere or containing sulfur
compounds, showing a high abrasion resistance or relatively low density and high resistance
in elevated temperatures [1-4].
Phase FeAl occurs in alloys containing 36-48% of aluminium. The main defect of alloys
with FeAl phase matrix is their low plasticity in room temperature caused by environment
effect [5], and in particular the water steam in air, as well as low technological plasticity in
hot working processes.
The alloys on the matrix of FeAl intermetallic phase are produced usually in the method
of classic melting and casting into a form of ingots or cast pieces [2, 6-7]. Prior to plastic
working the ingots are subject to homogenizing treatment. After mechanical working
(pressing, forging, rolling, extrusion, hot working) the semi-products are subjected to surface
thermal treatment or machining, and then control of quality, structure and properties.
The alloys on FeAl intermetallic phase matrix are belonging to materials creating no
serious technical problems during melting and casting. However, the technical requirements
made to these alloys are raised up in case when they are processed into semi-products
destined for plastic working, which characterize with a set of properties assuring their
workability, including in the first place a high purity, homogeneous and fine grain structure.
These requirements could be fulfilled by application of vacuum melting with subsequent
multiple refining remelting and casting into hot moulds with a forced, non-uniform oriented
evacuation of heat [2, 6].
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The aim of the research was to study hot deformation behaviour (i.e. formability as well as
flow stress) of an IMC Fe-Al based alloy by laboratory rolling. Model of the mean equivalent
stress should be obtained in the mathematical form.
2. MATERIALS FOR TESTS
For examinations was selected the alloy with chemical composition as specified in
Table 1. This alloy contains recommended amount of aluminium and molybdenum which
provides a solution strengthening of alloy, as well as microadditives of zirconium, carbon and
boron, which should produce the presence of fine-dispersion carbides and borides in the
structure, and finally the grain size reduction.
Table 1. Assumption of the chemical composition of the alloy
Component
% at.
% weight
Fe
61,64
76,62
Al
38,00
22,82
Mo
0,20
0,43
Zr
0,05
0,10
C
0,10
0,03
B
0,01
0,002
The proposed chemical composition ought to enable production of mono-phase alloy with
FeAl phase structure, good mechanical properties in elevated temperatures, satisfactory
plasticity and ability for plastic working.
In form of charging components were used: Armco iron, aluminium with purity min.
99.98%, molybdenum in form of compressed powder and technical purity, zirconium of
technical purity over 99%, carbon in form of anthracite and amorphous boron of technical
purity.
All melts were performed in induction vacuum furnace IS5/III made by LeyboldHeraeus, equipped with crucible of rammed spinel magnesite mix Al2O3·MgO. The melts
were executed in vacuum of 0.5 – 1.0 Pa. After melting the alloy was remelted twice, and
each time it was heated up to temperature 1823 K, then cast into cold graphite moulds
obtaining ingots of 25 mm dia. and length 300 mm. Prior to each fusion the ingot was
mechanically cleaned, removing the external surface with slag intrusions. Next, the ingots
were cut and used as charge for subsequent, third melting in a laboratory, induction vacuum
furnace VSG-02 of Balzers Co. with working chamber under approx. 1.0 Pa pressure. During
this fusion were used crucibles made of compressed and sintered Al2O3.
After third melting in laboratory, induction vacuum furnace the alloy being pre-heated to
temperature 1823 K was cast to the graphite, pre-heated to 673 K temperature moulds, in form
of flat bars 125 x 30 x 8 mm. From these plates were made samples for rolling test, having
dimensions as in Fig.1.
Fig. 1 Initial shape of the graded sample
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In result of three times repeated melting, the obtained value of oxygen and hydrogen at
the level of 38 ppm and 0.2 ppm respectively, enables utilization of alloy as material suitable
for hot plastic working.
The microstructure of alloy after casting, as proven by X-ray examinations, is composed
of the grains of ordered phase FeAl having structure B2 and mean diameter approx. 100 µ m.
Around grains, and in smaller extent at the grain boundaries are appearing the fine-dispersion
phase precipitates like carbides and borides, which are creating microadditives of zirconium,
carbon and boron (Fig. 2a).
100 µm
200 µm
a)
b)
Fig. 2. Microstructure of investigated alloy in the initial state (a) and after homogenization
(1273 K/24h/furnace cooling) (b)
A homogenizing treatment carried trough 24 h in temperature 1273 K, while not
changing the character of microstructure, is causing considerable reduction of segregation
degree around grains and participation of dispersion precipitates in the microstructure
(Fig. 2b).
3. METHODOLOGY APPLIED
Determination of mean equivalent stress values was based on forces measured during
rolling of flat samples graded in thickness [8, 9]. It starts from rolling of samples with shape
and dimensions as in Fig. 1.
Each sample is carefully measured and afterwards directly heated in the electric
resistance furnace to the forming temperature. After extraction from the furnace the heated
sample is immediately rolled down in stand A of the laboratory mill Tandem [10, 11]. For
each sample the following parameters are changed: temperature, roll gap adjustment (i.e. total
deformation of the particular step of the sample) and nominal revolutions of rolls – they
determine achieved strain rate. Rolling forces F and instantaneous revolutions of rolls N
(decreasing in relation to nominal speed in dependence on the total rolling force or torque) are
recorded by computer. The Fig. 3 shows an example of recorded variables.
For each step of the given sample, the total rolling force F [N] and corresponding variable
N [min-1] are determined after rolling. After cooling down of the rolling stock, width and
thickness of individual steps are also measured; spread is dependant mainly on the amount of
height draught, thickness is influenced by the amount of rolling force (springing of rolls).
An advantage of the sample with thickness graded in size is four times higher quantity of data
achieved by its rolling at exactly defined temperature as compared with rolling of a flat
sample with the constant thickness. All variables stated above are put down in the Excel table
and recalculated on values of logarithmic height strain e (normally ca 0.1 – 0.7) and strain rate
é [12] (mostly 10 – 160 s-1).
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Fig. 3 Example of measured total rolling force and actual revolutions of rolls at forming
of flat sample with variable thickness (4 steps)
Mean equivalent stress σs [MPa] is calculated according to the relation [13, 14]
σs =
F
(1)
QFv ⋅ ld ⋅ Bs
where QFv is forming factor corresponding to the particular mill stand, ld [mm] is roll bite
length and Bs [mm] is mean width in the given place of the rolling stock (the average of
widths before and after rolling). Reliability of calculation of σs is most influenced by accuracy
of the estimate of the forming factor, which actually transfers pertinent values of deformation
resistance to values of equivalent stress. Values of QFv for both stands of the mill Tandem
were acquired by previous research [15] and described in relationship to geometric factor
ld/Hs by equation of type


l 
H 
QFv = J − K ⋅ exp − L ⋅ d  + exp M ⋅ s 
Hs 
ld 


(2)
where J ... M are constants for the given facility, Hs [mm] is mean thickness of the rolling
stock in the given place (the average of thickness values of the given step before and after
rolling). Benefits of the laboratory rolling for description of deformation resistance against,
for example, the torsion test [16] consist above all in possibility to achieve higher strain rates
and lower cost for the experiment (first of all in the stage of preparation of samples).
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4. EXPERIMENTAL DATA PROCESSING
The above mentioned samples were rolled down in stand A of the mill Tandem (roll
diameter ca 159 mm) at temperature ranging from 1100 to 1325 °C. The forming conditions
varied from 0.2 to 0.8 (strain) and from 18 to 130 s-1 (strain rate), respectively. Standard
program Microsoft Excel or Access was used for automatic collective recalculations of
laboratory measured rolling forces to values of deformation resistance. By means of the
statistic software Unistat 4.53 the methodology for development of models for deformation
resistance with three independent variables was managed, based on non-linear regression.
Basic type of the resulting model will have the previously selected and verified form [8, 9]
σ s = A ⋅ eB ⋅ exp(− C ⋅ e ) ⋅ éD ⋅ exp( −G ⋅ T )
(3)
where σs [MPa] is mean equivalent stress, A ... G are material constants, T is temperature
[°C]. The model contains similar members as the physically more substantiated equations (see
e.g. [17-22]), but in less complex form – see the member of softening and strain rate. The
simplification is possible above all by the fact that the model calculates with mean values of
deformation resistance and thus it is less sensitive to change of deformation as independent
variable. The influence of dynamic softening is involved that significantly extends the range
of applied strains.
The mathematical processing of experimental data was very complicated by
heterogeneity of the input cast material as well as by frequent cracking (Fig. 4). It is
considerably hard to determine the optimum plasticity conditions in such a case of forming
relatively low number (altogether 18) of samples with so poor formability. The best results
(limited cracking) were obtained after high-temperature rolling (categorically above 1200 °C)
with some medium rolling speed (say at about 120 min-1 in our conditions). Low rolling speed
yielded in fast cooling of the specimen’s surface layers and marked transverse cracking. On
the other hand, its seems to be noteworthy and hopeful that this IMC tolerated so huge
reductions in single pass occasionally.
Fig. 4 Some rolled samples in comparison with the initial graded sample (right)
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Extreme cracking occurred at low temperatures – see the devastated samples 3
(1100 °C) and 8 (1150 °C) at Fig. 4. The formability was growing up with temperature – see
samples 5 (1200 °C) or 16 (1170 °C – related with Fig. 3). Quite good was the situation at
high-temperature rolling – see samples 10 (1250 °C) and 13 (1325 °C). Notches in the first
(upper) step of every sample were used for their marking and following identification after
heavy rolling.
There was absolutely no chance to describe deformation resistance of the studied alloy by
one relation only. Different deformation behaviour at temperatures over or below 1250 °C
was recognizable. Probably due to the ordering ferrite A2/intermetallic B2 occurring in the
vicinity of that temperature (see Fig. 5), it was necessary to derive two models separately for
two deformation temperatures interval:
For T < 1250 °C:
σ s = 197108 ⋅ e 0.435 ⋅ exp(− 1.064 ⋅ e ) ⋅ é 0.055 ⋅ exp( −0.00545 ⋅ T )
(4)
For T ≥ 1250 °C:
σ s = 94767 ⋅ e 0.490 ⋅ exp(− 0.833 ⋅ e ) ⋅ é 0.015 ⋅ exp(−0.00499 ⋅ T )
(5)
For evaluating accuracy of the models developed, the relative error [%] of mean
equivalent stress was defined as residuum over “measured” value. The graphs in Fig. 6
demonstrate a quite good consistency of “measured” and “predicted” (according to Eqs. 4 and
5 calculated) values of σs. It is important that no evident tendency of relative error depending
on any independent variable occurs.
Fig. 5 Iron-aluminium equilibrium diagram [2]
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Fig. 6 Relative errors of mean equivalent stress according to Eqs. 4 and 5, depending on
temperature, strain and strain rate
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5. SUMMARY
Applying the laboratory mill Tandem, high-speed hot rolling of an IMC Fe-Al based alloy
was feasible even by high reductions in single pass. Rolling conditions for he optimum
formability have been ascertained. Models for deformation resistance prediction were
developed individually for ferritic A2 structure (above 1250 °C) as well as ordered
intermetallic B2 structure (below 1250 °C). The models are suitable for high strain rate region
(over ca 10 s-1) and wide range of strain as they simply reflect dynamic softening processes.
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Acknowledgements
This work was supported by Polish Commitee of Scientific Research (grant No. PBZ/KBN-041/T08/11-02)
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