ARCHIVES
of
ISSN (1897-3310)
Volume 9
Issue 2/2009
21-24
FOUNDRY ENGINEERING
5/2
Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences
Effect of dispersion hardening on impact
resistance of EN AC-AlSi12Cu2Fe silumin
J. Pezda *
Faculty of Chipless Forming Technology, University of Bielsko-Biaáa, Willowa 2, 43- 300 Bielsko - Biaáa, Poland
*Contact for correspondence: e-mail: jpezda@ath.bielsko.pl
Received 26.02.2009; accepted in revised form: 30.03.2009
Abstract
Development of modern technology have generated supply of better and better, more resistant structural materials not attainable earlier.
Weight of metal structures is of a great importance, and as a consequence, also weight of materials used for a given structure. More often,
for metal structures are used lightweight metals and their alloys, from which aluminum and its alloys have become the most widespread.
These alloys, based on Al-Si equilibrium system, contain additional constituents (e.g.: Mg, Cu) enabling, except modification,
improvement of mechanical properties obtained in result of heat treatment. The paper presents an effect of modification process and heat
treatment on impact resistance of EN AC-AlSi12Cu2Fe alloy. Solutioning and ageing temperatures were selected on base of registered
curves of the ATD method. For the neareutectic EN AC-AlSi12Cu2Fe silumin one obtained growth of the impact resistance both due to
performed modification treatment and performed heat treatments of the alloy.
Keywords: Al-Si alloys, crystallization, heat treatment, ATD
1. Introduction
Pure aluminum is used rarely, the most often in foundry
industry are used silumins (Al-Si alloys) with neareutectic
contents of silumin (about 12%), and with various, other alloying
additions. These alloys are used in various branches of
engineering industries, e.g. automotive, precise, aircraft,
household equipment industry, ect. Such broad application of
these alloys results from their very good physical-chemical and
technological properties. These alloys feature relatively low
density, relatively low melting temperature, good thermal and
electric conductivity, high mechanical properties (some silumins
can be heat treated), and are characterized by good casting
properties (good castability, small solidyfying shrinkage), good
machinability and considerable corrosion resistance [1,2].
Susceptibility to generation of coarse grain structure
constitutes a significant disadvantage of these alloys (it concerns
mainly alloys made in ceramic moulds and thick-walled castings
from metal moulds), what disadvantageously effects on
mechanical properties of the alloys.
Return to normal eutectic mixture (fine-grained one) is
obtained by means of its refinement (modification), adding
inoculants to the metallic bath.
Modification of alloys can effect, depending on type and kind
of alloy and inoculants, in many phenomena, e.g.: generation of
additional nucleuses of crystallization, generation of inclusions
which restrict growth of crystals (structure of alloys is of
polycrystalline character), local changes in concentration of
elementary substances and surface tension, change of conditions
of generation of super-cooled alloy, deoxidation and degassing of
the bath, ect. Due to fact that the modification can induce various
effects; none general theory of modification was developed up-tonow, whereas numerous theories attempting to describe
phenomena connected with modification of the alloys are
functioning now [3-8].
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Obtained results connected with change of the structure, and
hence, change of mechanical and technological properties of the
alloy depend of selection of a suitable modifier, its correct dosing
and maintenance of proper temperature of the metal in course of
modification process [2-3, 6, 9].
Carrying out modification treatment is very important from
alloy’s heat treatment point of view, because form of the eutectics
mixture effects on suspectibility of eutectic silicon to coalescence
and spheroidizing during the heat treatment – phenomena of
coalescence and spheroidizing of the eutectic silicon proceed
more easily and more quickly when molecules of the eutectic
silicon are more dispergated before the heat treatment [10].
Heat treatment used in case of alloys based on Al-Si system
with additives of Mg and Cu enables to obtain further
improvement of their mechanical properties, comparing to
modified alloys.
Temperatures of solutioning and ageing, as well as duration of
these treatments constitute main parameters of the heat treatment
process (dispersion hardening) of aluminum alloys.
In case of temperature, to determine temperature ranges one
can make use of the ATD thermal method of crystallization
process analysis [11, 12]. The ATD method, based on analysis of
temperature changes course, enables registration of a phenomena
arisen in result of melting and solution heat treatment processes of
alloys. Implementation of that method enables to determine
melting temperature of the material, and the same, permits to
determine maximal temperature of the solutioning treatment with
elimination of possibility of partial melting of the material
(casting). The ATD method can be also used to estimation of
mechanical properties of alloys [13, 14].
2. Methodology of the research
In course of the testing one used EN AC-AlSi12Cu2Fe alloy
(classified among multicomponent alloys commonly implemented
in foundry industry), which was melted in electric resistance
furnace.
Investigated alloy was refined with Rafal 1 in quantity of
0,4% of mass of the metallic charge, and next modified with
AlSr10 master alloy in quantity of 0,6% of mass of the metallic
charge.
Registration of heating and melting of the investigated alloy
was performed with use of the ATD Cristaldimat analyser.
Heat treatment was performed for the modified alloy.
Temperatures of solutioning and ageing treatments were
determined on base of registered heating and solutioning curves
of the ATD method.
One assumed the following temperature ranges of heat
treatments of the investigated alloy:
- solutioning temperature: 525 - 555oC,
- ageing temperature: 180 - 340oC.
Duration of solutioning and ageing treatment amounted to from
0,5 up to 3 hour.
In the Fig. 1 are shown registered curves of the ATD method
for the investigated alloy with marked temperature ranges of the
solutioning and ageing treatments.
Fig. 1. Curves of the ATD method for the investigated alloy
In course of the solutioning treatment, temperature of the
treatment was monitored through permanent recording of
temperature inside the furnace and temperature of the sample.
For refined alloy, refined and modified alloy, and alloy after
heat treatment one performed measurement of impact resistance
on Charpy impact tester.
3. Description of achieved results of own
researches
Impact resistance of raw alloy (from pig saw) amounted to 2,6
y 4,0 kJ/m2. After refining there occurred a slight change of
impact resistance of the alloy, which amounted to 2,7 y 4,5 kJ/m2.
refined and modified alloy was characterized by growth of the
impact resistance amounted to 5,7 y 7,0 kJ/m2. After the heat
treatment the impact resistance amounted to 4,2 y 13,8 kJ/m2. In
the Fig. 2 is shown an impact of refining treatments and heat
treatment on change of impact resistance of the EN ACAlSi12Cu2Fe silumin.
Fig. 2. Effect of performed heat treatments of EN AC-AlSi12Cu2Fe
silumin
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In the Figs. 3-5 is shown an effect of solutioning and ageing
treatment’s duration and temperature on change of impact
resistance of investigated alloy.
a)
Fig. 5. Effect of temperature and solutioning duration on KCV
impact resistance of EN AC-AlSi12Cu2Fe alloy – isohypse
diagram
Change of material’s structure after performed heat treatment
is also directly connected with change of its tensile strength and
elongation. In the Fig. 6 is shown an effect of tensile strength
change Rm and elongation A5 change on KCV impact resistance
of the investigated alloy.
a)
b)
Fig. 3. Effect of temperature and solutioning duration on KCV
impact resistance of EN AC-AlSi12Cu2Fe alloy: a) threedimensional diagram, b) isohypse diagram
b)
Fig. 4. Effect of temperature and solutioning duration on KCV
impact resistance of EN AC-AlSi12Cu2Fe alloy
Fig. 6. KCV impact resistance of EN AC-AlSi12Cu2Fe silumin in
function of tensile strength Rm and elongation A5: a) threedimensional diagram, b) isohypse diagram
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High impact resistance of the investigated alloy, obtained in
result of performed heat treatment is simultaneously connected
with its tensile strength Rm having value of 270 to 320 MPa, and
elongation A5 having level of 3-4,5% (Fig. 6).
References
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4. Conclusions
[3]
Modification and heat treatment have effect on improvement
of impact resistance of EN AC-AlSi12Cu2Fe silumin.
The highest impact resistance, KCV = 13,8 kJ/m2 , was
obtained for specimens of the alloy which was solution heat
treated in temperature of 555oC for 1,5 hour, and next ageing
treated in temperature of 250oC during period of 3 hour.
Impact resistance of the investigated alloy reached its the highest
values for:
- solutioning temperature amounted to from 540 up to 555oC,
- solutioning duration from 0,5 up to 1,5 hour,
- ageing temperature from 180 to 250oC,
- ageing duration from 1 do 2 hour.
The lowest value of the KCV impact resistance of the alloy after
heat treatment was obtained for specimens of the alloy solution
heat treated in temperature of 555oC for 3 hour, and next ageing
treated in temperature of 180oC for the period of 3 hour.
Impact resistance of the investigated alloy attains its lowest values
for solutioning temperature of 555oC and solutioning duration of
3 hour. In this case, change of temperature and duration of the
solutioning did not cause any significant change of the impact
resistance, which amounted to 4,2 – 4,7 kJ/m2.
Maintaining suitable ranges of temperatures and durations of
solutioning and ageing treatments is a precondition of optimal
impact resistance of the alloy, enabling simultaneously to shorten
time of the heat treatment to an indispensable minimum.
Performance of a further investigations shall be connected
with necessity of determination of an effect of selected parameters
of dispersion hardening on technological properties of ACAlSi12Cu2Fe silumin.
24
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