UDK 669.715:621.74	ISSN 1580-2949
Professional article/Strokovni članek	MTAEC9, 48(6)991(2014)
PROPERTIES OF AlSi5Cu1Mg MODIFIED WITH Sb, Sr AND Na
LASTNOSTI ZLITINE AlSi5Cu1Mg PO MODIFIKACIJI
S Sb, Sr IN Na
Žarko Rajic1, Matjaž Torkar2, Irena Paulin2, Borut Žužek2, Črtomir Donik2,
Milan Bizjak3
1CIMOS, d. d., C. Marežganskega upora 2, 6000 Koper, Slovenia 2Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia 3University of Ljubljana, Faculty of Natural Sciences and Engineering, Department of Materials and Metallurgy, Aškerčeva cesta 12,
1000 Ljubljana, Slovenia matjaz.torkar@imt.si
Prejem rokopisa - received: 2014-06-05; sprejem za objavo - accepted for publication: 2014-07-25
The properties of the secondary AlSi5Cu1Mg alloy, modified with Sb, Sr or Na, used as a cast housing for the automotive industry were investigated. The alloy was prepared from secondary aluminium and the properties of the material from the castings were determined. Presented here are the results of the mechanical tests of the as-cast and heat-treated material (T6), metallographic investigations and EDS analyses of the intermetallic phases based on Fe as well as electrochemical corrosion tests in a solution of 3 % NaCl. In the microstructure and with respect to the corrosion resistance of the alloys, modified with Sb, Sr or Na, no important differences were observed. The tests confirmed that the mechanical properties of cast-on bars from secondary aluminium AlSi5Cu1Mg were suitable for the application, only the microporosity of the castings needs to be eliminated through the selection of the proper casting parameters.
Keywords: secondary aluminium, modifiers, AlSi5Cu1Mg, mechanical properties, intermetallic phases, EDS analysis, microporosity, corrosion resistance
Opravljene so bile aktivnosti za uporabo sekundarne aluminijeve zlitine AlSi5Cu1Mg, modificirane s Sb, Sr ali Na, za lita ohišja za avtomobilsko industrijo. Zlitina, izdelana iz sekundarnega aluminija, je bila preizkušena in določene so bile lastnosti materiala, vzetega iz stene ulitkov. Predstavljeni so rezultati mehanskih preizkusov materiala v litem in v toplotno obdelanem stanju (T6), rezultati metalografskih preiskav in EDS-analize intermetalne faze na osnovi Fe, kot tudi elektrokemijski korozijski preizkusi v 3-odstotni raztopini NaCl. Niso bile ugotovljene pomembne razlike pri vplivu modifikatorjev Sb, Sr ali Na na mikro-strukturo in korozijsko odpornost zlitin. Preizkusi so potrdili, da so mehanske lastnosti prilitih palic iz sekundarne aluminijeve zlitine AlSi5Cu1Mg primerne za uporabo, potrebna pa je odprava mikroporoznosti ulitkov z izbiro primernih parametrov pri litju.
Ključne besede: sekundarni aluminij, modifikatorji, AlSi5Cu1Mg, mehanske lastnosti, intermetalne faze, EDS-analiza, mikroporoznost, korozijska odpornost
1 INTRODUCTION	The recycling of aluminium alloys7-9 is also import-
ant from the environmental point of view. The availability of primary and secondary raw mate- The quality of secondary aluminium mostly depends rials and energy are of great concern in the metallurgical on its residual Fe content and the formation of interindustry and in the other base-materials industries.1 Less metallic phases based on Fe and other trace elements. tha^ a quarter of the current aluminium demand is co- Iron is a common impurity element in secondary alumi-
vered by scrap from used products. However the recy- nium alloys. In Al-Si foundry alloys, iron forms inter-cled content of aluminium products is not low because of
metallic compounds that are detrimental to the mecha-inefficient recycling, but because of increasing demand nical ro ertigs 10
for long-life products.1 This recycling will also increase	p p .
in the future with more sophisticated recycling methods.2 It has been proven that the modification of eutectic
Remelting aluminium requires about 5-10 % of the silicon plays an important role in improving the mecha-
energy required for primary production, so that recycling	nical properties of hypoeutectic Al-Si alloys. Elements
is very attractive from an energy point of view.3 To pro-	that produce a refined' flake-like structure are antimony
duce primary aluminium the largest amount of electricity	(Sb), arsenic (As) and selenium (Se). Only Sb, Sr and Na
is required for electrolysis, while for the mining, trans-	produce a significant modification at low levels of
port and production of alumina and anodes only minor	addition.11
amounts of energy have to be supplied.	The aim of the presented research was to evaluate the
In the process of melting aluminium scrap, about	applicability of a secondary AlSi5Cu1Mg alloy, mo-
12 % of the metal is burnt and about 10 % of it is lost	dified with Sb, Sr or Na, for the casting of housings from
because aluminium mixes with the slag removed from	the point of view of microstructure, mechanical proper-
the surface of the molten metal.4-6	ties and corrosion resistance.
2 EXPERIMENTAL PROCEDURE
The melt of AlSi5Cu1Mg was prepared in a melting furnace from a mixture of secondary aluminium alloy ingots. The melt was alloyed with the necessary elements, degassed, modified with a Sb, Sr or Na modifier and cast. The required composition of the test alloys is presented in Table 1.
Table 1: Demanded range of elements in AlSi5Cu1Mg alloy, including modifier (mass fractions, w/%)
Tabela 1. Zahtevana okvirna sestava zlitine AlSi5Cu1Mg, vklju~no z modifikatorjem (masni deleži, w/%)
Element	Min. w/%	Max. w/%
Si	4.5	5.5
Cu	1.0	1.5
Mn		< 0.1
Mg	0.4	0.6
Zn		< 0.1
Ti		< 0.2
Fe		< 0.20
Ni		< 0.2
Individual impurity		< 0.05
Impurities Total		< 0.15
Modifier Sb	0.10	0.20
Modifier Sr	0.015	0.030
Modifier Na	0.0060	
The charge consists of a combination of ingots with close to the demanded chemical composition. The melt was prepared in a KEMOTERM melting furnace 600 kg. The melt was, prior to casting, degassed with argon for
about 9 min. During the degassing the AlTi5B1 was added. The modifier in the form of AlSb10, AlSr10 or Na in the form of pills (Simodal 77) was added before the casting. The temperature of the melt during casting was 730 °C.
The castings were gravity cast into iron or sand forms. At the same time, cast-on bars were cast for comparison with tensile tests. After cooling the casting system was removed and from the wall of the castings the samples for the tensile tests were cut. The samples were treated using the T6 procedure: solution annealing: (527 ± 6) °C, 8 h, water cooling and artificial aging: (154 ± 5) °C, 8 h, cooling in air. For the tensile tests, as-cast samples were prepared as well as samples treated with the T6 procedure. The tensile tests were performed on an Instron 8802, tensile-testing machine 250 kN. The contact extensometer, mounted on samples, measured the stress-strain during the tensile tests.
The samples for the metallography were taken from both the thinner and thicker walls of the castings (Figures 1a and 1b). The samples for metallography were prepared by a standard procedure. The intermetallic phases rich in Fe were observed with a light microscope (Microphot FXA, Nikon) using a 3CCD video camera (Hitachi HV-C20A) and computer software for the analysis. The intermetallic phases were also observed by scanning electron microscopy (JSM-6500F) with a field-emission source of electrons, INCA ENERGY Oxford Instruments, and analysed by energy-dispersive spectroscopy (EDS). The electrochemical corrosion tests of the material modified with Sb, Sr or Na were
Table 2: Chemical composition of tested alloys, modified with Sb, Sr or Na and cast into sand form and cast into an iron mould (mass fractions, w/%)
Tabela 2: Kemijska sestava preizkusnih zlitin, modificiranih s Sb, Sr ali Na in ulitih v peš~eno formo ali v železno kokilo (masni deleži, w/%)
Element	Cast in sand form			Cast in iron mould		
Sample and modifier	59-Sb	44 - Sr	62-Na	237 - Sb	147 - Sr	333 - Na
Si	5.06	5.10	5.29	5.05	5.21	5.20
Fe	0.1394	0.1528	0.145	0.151	0.153	0.1611
Cu	1.326	1.622	1.40	1.188	1.112	1.376
Mn	0.0643	0.0541	0.0471	0.0378	0.0274	0.0562
Mg	0.4901	0.508	0.481	0.519	0.4438	0.527
Zn	0.0601	0.0662	0.0575	0.0489	0.0375	0.0658
Ni	0.0865	0.1761	0.107	0.0233	0.0071	0.0159
Cr	0.0036	0.0041	0.0033	0.0023	0.0020	0.0029
Pb	0.0020	0.0029	0.0022	0.0020	0.0012	0.0038
Sn	< 0.0010	< 0.0010	< 0.0010	< 0.0010	< 0.0010	< 0.0010
Ti	0.1015	0.171	0.110	0.0870	0.0935	0.1137
Be	< 0.0005	< 0.0005	< 0.0005	< 0.0005	< 0.0005	< 0.0005
Bi	< 0.0010	< 0.0010	< 0.0010	< 0.0010	< 0.0010	< 0.0010
Ca	< 0.0010	< 0.0010	< 0.0010	< 0.0010	< 0.0010	0.0010
Na	0.0015	0.0014	0.0091	< 0.0010	< 0.0010	0.0089
P	< 0.0010	< 0.0010	< 0.0010	< 0.0010	< 0.0010	< 0.0010
Sb	0.0729	0.0011	0.0037	0.0984	< 0.0010	< 0.0010
Sr	0.0017	0.0195	0.0020	0.0007	0.0173	0.0023
Zr	0.0050	0.0050	0.0109	< 0.0050	< 0.0050	< 0.0050
Co	< 0.0010	< 0.0010	0.0012	< 0.0010	< 0.0010	< 0.0010
Al	92.6	92.1	92.3	92.8	92.9	92.5
performed in a 3 % solution of NaCl. The test specimens were cut into discs of 15 mm diameter. The specimens were ground with SiC emery paper down to 4000 grit and rinsed with distilled water, prior to the electrochemical studies. The specimens were then embedded in a Teflon holder and employed as the working electrode. The reference electrode was a saturated calomel electrode (SCE, 0.242 V vs. SHE) and the counter electrode was a flat platinum mesh. All the potentials described in the text are relative to the SCE, unless stated otherwise.
The potentiodynamic measurements were recorded using a BioLogic SP-300 Modular Research Grade Po-tentiostat/Galvanostat/FRA and EC-LAB software 10.37 computer programs. In the case of the potentiodynamic measurements the specimens were immersed in the solution 1 h prior to the measurement in order to stabilize the surface at the OCP. The potentiodynamic curves were recorded, starting at 250 mV (SCE) more negative than the OCP. The potential was then increased, using a scan rate of 1 mV/s, until the transpassive region was reached.
3 RESULTS AND DISCUSSION
The chemical compositions of the tested alloys, cast in sand or cast in an iron mould, are presented in Table 2.
From Table 2 it is evident that the content of the main impurity, i.e., iron, in the tested alloys is in the range of w = 0.13 % to w = 0.16 %. Presented here are also the contents of the elements for the modification of the alloys (Sb, Sr and Na).
Figure 1: Schematic presentation of the location of the specimens O = (4 ± 0.05) mm, Lq = (20.00 ± 0.04) mm, Lc = 55.0 mm for tensile tests (ASTM B 557 M-07): a) thinner wall, b) thicker wall Slika 1: Shematski prikaz položaja vzorcev O = (4 ± 0,05) mm, Lq = (20,00 ± 0,04) mm, Lc = 55,0 mm za natezni preizkus (ASTM B 557 M-07): a) tanka stena, b) debela stena
Figure 2: Tensile-test samples
Slika 2: Preizkušanca za natezni preizkus
The demanded mechanical properties for this type of AlSi5Cu1Mg alloy are presented in Table 3.
Table 3: Demanded mechanical properties of AlSi5Cu1Mg alloy, heat treated T6
Tabela 3: Zahtevane mehanske lastnosti zlitine AlSi5Cu1Mg, toplotno obdelane (T6)
RpO.2	Rm	HB
> 185 MPa	> 285 MPa	80-90
Tensile test specimens were cut from the thin and thick walls of the housing, as schematically presented in Figures 1a and 1b, heat-treated T6 and tensile tests were made (Figures 2 and 3). The results of the tensile tests of the samples cast in the mould and cast in the sand form and heat treated T6 are collected in Tables 4 and 5.
For comparison, the tensile test samples were also prepared from the bars of diameter 18 mm, cast in the iron mould. The tensile-test samples were machined and heat treated T6. The results of the tensile tests are presented in Table 6.
From Tables 4 and 5 it is evident that the tensile properties Äpo,2 and Rm of the samples taken from the wall of the casting are below the demanded values. Better
Figure 3: Extensometer Instron, installed on tensile-test sample Slika 3: Ekstenzometer Instron, nameščen na trgalnem preizkušancu
mechanical properties are shown by the tensile samples prepared from the cast on bars.
Table 4: Mechanical properties of T6 heat-treated samples from housing, cast in sand form, thicker wall
Tabela 4: Mehanske lastnosti po toplotni obdelavi T6 vzorcev iz ohi{ja, ulitega v pesek, debela stena
Sample	Äp0.2/MPa	Äm/MPa	A/%	Z/%
59/1 Sb	170	186	1.5	4
59/2 Sb	159	191	1.8	3
59/3 Sb	163	201	2.6	3
44/1 Sr	173	216	2.0	5
44/2 Sr	169	197	1.6	3
44/3 Sr	168	174	1.6	3
62/1 Na	158	187	2.8	4
62/2 Na	161	176	2.6	4
62/3 Na	186	192	2.2	4
Mean value	167	191	2.1	3.6
Table 5: Mechanical properties of T6 heat-treated samples from housing, cast in iron mould, thicker wall
Tabela 5: Mehanske lastnosti po toplotni obdelavi T6 vzorcev iz ohi{ja turbo kompresorja, ulitega v kokilo, debela stena
Sample	Äp0,2/MPa	Äm/MPa	A/%	Z/%
237/1Sb	178	260	4.5	8
237/2Sb	203	246	3.0	6
237/3Sb	183	266	5.3	8
147/1Sr	172	247	6.1	9
147/2Sr	187	242	5.6	7
147/3Sr	174	253	5.8	9
333/1Na	180	211	3.2	4
333/2Na	189	234	4.2	5
333/3Na	155	228	3.1	6
Mean value	180	243	4.5	6.8
Table 6: Mechanical properties of cast-on bars, diameter 18 mm, heat-treated T6, cast in iron mould
Tabela 6: Mehanske lastnosti prilitih palic premera 18 mm po toplotni obdelavi T6, ulito v kokilo
Sample	Äp0,2/MPa	Äm/MPa	A/%	Z/%
1/1 - Sr	285	354	3.9	7
1/2 - Sr	273	337	3.1	5
1/3 - Sr	282	354	4.2	7
1/4 - Sr	278	351	4.6	8
2/1 - Sb	284	361	4.4	7
2/2 - Sb	287	356	4.0	6
2/3 - Sb	285	364	5.1	7
2/4 - Sb	284	352	3.8	6
3/1 - Na	279	365	6.1	7
3/2 - Na	288	333	2.0	4
3/3 - Na	285	327	1.4	4
3/4 - Na	291	356	3.5	5
Mean value	283	351	3.8	6.1
A comparison of the data from Tables 4, 5 and 6 reveals the difference in the mechanical properties of the castings and the cast-on bars. The main reason for the lower values of the samples from the castings was revealed by the metallography (Figures 4 and 5). Shrinkage
Figure 4: Shrinkage porosity in housing, thinner wall. The alloy was modified with Na.
Slika 4: Kr~ilna poroznost ohi{ja, tanka stena. Zlitina je bila modificirana z Na.
Figure 5: Microstructure of the cast on bars, after T6. Alloy, modified with: a) Sb, b) Sr and c) Na.
Slika 5: Mikrostruktura prilitih palic po T6. Zlitina, modificirana s: a) Sb, b) Sr in c) Na.
porosity was observed only in the castings. A similar porosity was observed at all three alloys, modified with Sb, Sr or Na. The appearance of the shrinkage porosity should be prevented by applying the appropriate measures involving melt treatment, gating/risering design and the effective use of the principles of directional and layered feeding.12
Besides the shrinkage porosity, also the presence of a needle-like Fe-based intermetallic phase was observed (Figure 6). To reduce the formation of the Fe-based intermetallic phase it is necessary to keep the Fe content in the secondary aluminium alloy as low as possible.
The basic microstructure of the alloy modified with up to 0.098 % Sb, 0.0182 % Sr and 0.010 % Na is presented in Figure 5. The microstructure of all three alloys is very similar.
Fe-based intermetallic phase are present in Figures 6 and 7.
Besides the light microscopy, also EDS analyses were performed on the Fe-rich intermetallic phases, as the most detrimental for the mechanical properties of secondary alloys. The results of the EDS analyses are presented in Tables 7 and 8. The as-cast sample (Figure 6, Table 7) was compared with the heat-treated T6 sample (Figure 7, Table 8).
Table 7: EDS analyses of areas marked on Figure 6 (mass fractions, w/%)
Tabela 7: EDS-analize področij, označenih na sliki 6 (masni deleži, w/%)
Spectrum	Al	Si	Mn	Fe	Cu	Mg
1	61.13	14.13	2.62	17.90	2.78	1.44
2	39.65	49.92	1.21	9.22		
3	5.75	94.25				
4	97.37	1.06			1.57	
As follows from Figure 6 and Table 7, the Fe-rich intermetallic phase is plate- or needle-like and contains up to 17.9 % Fe in some cases up to 24 % of Fe. The
Figure 6: Fe-rich intermetallic, needle-like phase in the as-cast alloy modified with Sb. Marked are the positions of the EDS analyses. Slika 6: Intermetalna faza, bogata z Fe, igličaste oblike v zlitini, modificirani s Sb. Označena so področja EDS-analiz.
EDS analyses confirmed, besides Fe, also other elements, such as Al, Si, Mn, Cu and Mg are present in the Fe-rich intermetallic phase.
Table 8: EDS analyses of areas marked on Figure 7 (mass fractions, w/%)
Tabela 8: EDS-analize na mestih, označenih na sliki 7 (masni deleži, w/%)
Spectrum	Al	Si	Mn	Fe	Cu	Ni
1	70.25	5.95		10.93		12.87
2	58.54	16.66	2.98	21.83		
3	83.20	8.59		8.22		
4	7.84	92.16				
5	61.57	9.67	2.43	21.02	3.33	1.98
6	58.20	16.46	3.27	22.07		
7	97.61	1.13			1.26	
Tables 7 and 8 present the EDS analyses of the different phases in the as-cast and heat-treated samples. Most of the Fe-rich intermetallics are plate like. The refinement of the intermetallic compounds that crystallize as the primary phase from the Al-Si-Fe alloy is possible by applying ultrasonic vibration at their nucleation tempe-rature.13
In most cases the EDS analysis did not reveal the elements (Sb, Sr, Na) added for modification of the alloy. It looks as if the elements for modification are well distributed in the solid solution of the Al matrix and are not associated with the various phases in the microstructure of the alloy.
For the practical application of the alloys, the corrosion properties of the material are also important. As the most detrimental to the material is the influence of chloride ions, the electrochemical corrosion tests of alloys, modified with Sb, Sr or Na, were performed in a 3 % solution of NaCl. The results of the electrochemical corrosion test are presented in Figure 8. From the poten-tiodynamic measurements it can be concluded that the
Figure 7: Fe-rich intermetallic, plate- and needle-like phase in T6 heat-treated alloy, modified with Sb. Marked are the positions of the EDS analyses.
Slika 7: Z Fe bogata intermetalna faza v obliki ploščic in igel v T6 toplotno obdelani zlitini, modificirani s Sb. Označena so področja EDS-analiz.
Figure 8: Potentiodynamic curves for AlSi5Cu1Mg alloys modified with Sb, Sr or Na in a solution of NaCl 3 %
Slika 8: Potenciodinamske krivulje za zlitino AlSi5Cu1Mg, modificirano s Sb, Sr ali Na v raztopini NaCl 3 %
tion of housings. The main concern is to hold the level of residual Fe as low as possible. The casting of housings should be made with proper process parameters (preparation of the melt, cast at proper temperature, suitable mould form), so as to eliminate the shrinkage porosity in the castings.
Acknowledgement
The authors would like to thank The Ministry for Economic Development and Technology of the Republic of Slovenia for financial support within the RC SIMIT project.
corrosion properties in the Tafel area (± 250 mV around the Ecorr) do not show any significant differences between the studied materials. From the icorr and Ecorr we can conclude that no major difference exists in the corrosion resistance of the tested alloys of AlSi5Cu1Mg. The addition of a different modifier (Sb, Sr or Na) has no remarkable impact on the electrochemical corrosion behaviour of the AlSi5Cu1Mg alloy in a 3 % solution of NaCl.
4 CONCLUSIONS
Based on the investigation and a comparison of secondary AlSi5Cu1Mg alloys, modified with Sb, Sr or Na, the following conclusions can be drawn:
•	A comparison of the microstructures of the AlSi5Cu1Mg alloys, modified with Sb, Sr or Na, did not reveal any noticeable differences.
•	The microporosity was observed only in castings for all three modified alloys.
•	The Fe-rich intermetallic phase is plate- and needlelike and contains up to w = 24 % of Fe.
•	The poor mechanical properties of the castings in the as-cast or in the T6 state were a consequence of the shrinkage porosity.
•	Higher values of Äpc.a and Rm, as demanded, were only obtained for tensile tests of the cast-on bars after the T6 heat treatment. The reason is the absence of shrinkage porosity in the cast-on bars.
•	The modification of the AlSi5Cu1Mg alloy with Sb, Sr or Na did not influence the corrosion resistance of the alloy in a 3 % solution of NaCl.
•	The investigations confirmed the secondary alloy AlSi5Cu1Mg is conditionally suitable for the produc-
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