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Extrusion of silicon-rich AlMgSi alloys
Journal of Materials Processing Technology 212 (2012) 1437–1442
Contents lists available at SciVerse ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Extrusion of silicon-rich AlMgSi alloys Stanka Tomovic-Petrovic ∗ , Ola Jensrud Department of Materials Technology, SINTEF Raufoss manufacturing AS, B 163, N-2831 Raufoss, Norway
a r t i c l e
i n f o
Article history: Received 18 October 2011 Received in revised form 31 January 2012 Accepted 5 February 2012 Available online 14 February 2012 Keywords: Aluminum alloys Bulk deformation Light microscope Mechanical characterization Recrystallization
a b s t r a c t As a part of on-going research on phase transformations during the deformation of light alloys, the effect of silicon excess on the extrudability and mechanical properties of the standard AlMgSi1 alloy within AA6082 alloy is investigated in this study. The AlMgSi1 alloy and three experimental aluminum alloys with a silicon content of 1.98%, 3.73% and 5.51% were direct-chilled cast into billets 95 mm in diameter, homogenized at 540 ◦ C for 4 h and extruded into 12 mm diameter rods at different extrusion speeds. The results showed that an increase in the silicon content reduced the extrudability of the AlMgSi1 alloy by lowering the limiting extrusion speed. However, the extruded alloys with 3.73% and 5.51% silicon, generally characterized by a fine grained microstructure, exhibited higher strength levels compared with the 1.98% silicon alloy. Nonetheless, the mechanical properties of these alloys, in the T6 temper condition, were below those of the AlMgSi1 base alloy. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The influence of silicon and magnesium on the extrudability and mechanical properties of AA 6xxx alloys is a topic that has been discussed in numerous papers. The general conclusion is that the extrudability of these alloys is highly influenced by the amount of silicon and magnesium in the solid solution: i.e., by the amount and distribution of the precipitated Mg2 Si. Increasing the alloying levels with an aim to meet the required product strength always raises the extrusion pressure and limits the extrusion speed. Espedal et al. (1989) has shown that the extrusion pressure increases 8% per 0.1 wt% increase of Mg2 Si in 6xxx alloys. Reiso (2004) has found that the extrudability of the same alloys is reduced by 1–2% per 0.01 wt% increase in the silicon amount. He has noticed the same strong effect on extrudability from variations in the magnesium content. This effect is maintained up to approximately 0.55 wt% magnesium content in the alloy. Magnesium content beyond this level has an even more deteriorating effect. Clearly, both elements, and in particular, magnesium, should be kept as low as possible to allow the use of a high extrusion speed. In contrast, a minimum level of these elements is required to achieve a high precipitation potential and the specified mechanical properties following age hardening. Industrial alloys are usually designed with a balance of magnesium and silicon or with an excess of silicon above that required to form stoichiometric Mg2 Si. Gupta et al. (2001) have confirmed that this solute ratio (Si/Mg) will reduce the time needed to initiate
∗ Corresponding author. Tel.: +47 45280636; fax: +47 61153625. E-mail address: [email protected] (S. Tomovic-Petrovic). 0924-0136/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2012.02.004
strengthening, increasing the strength in both the T4 and artificially aged tempers. Langerweger (1988) claimed that the silicon excess in industrial alloys will assure a lower drop in extrudability for a given increment in strength. Nevertheless, the silicon amount in the 6xxx alloys is limited to a value of 1.3 wt%. The effect of higher silicon amounts (in the range from 2 to 6 wt%) on the extrudability and mechanical properties of the AA 6xxx alloys has not been investigated as often. The low-meltingpoint silicon-rich eutectic phase composition, which can appear locally in the structure of these alloys under rapid heating as during extrusion, makes the extrusion process complicated. This is the reason why the wrought AA 6xxx alloys with high silicon content have been marked as “non-extrudable alloys”. The present study was initiated to clarify the possibility that these alloys are, in some way, qualified as wrought alloys of interest. This issue could be of great importance when considering the increased interest in the recycling of light metals. Designation of new wrought alloys with a wider tolerance in composition aimed toward different applications could contribute to the redirection of the current wrought-to-cast to wrought-to-wrought recycling process (partially, at least). The primary objective of this study is to obtain a better understanding of how large additions of silicon influence the extrudability and mechanical properties of the AlMgSi1 base alloy. In particular, the effect of the intermetallic phase formation on the limiting extrusion speed and the mechanical properties following age hardening is investigated in more detail, based on a series of exploratory experiments carried out under controlled conditions in the laboratory. The effect of the high silicon additions on the extrusion pressure and structure evolution is also some of the issues discussed in this paper.
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Fig. 1. Sketch of the system used for measuring the maximum temperature achieved on the extrudate surface.
were performed using a long travel contact extensometer with a fixed original-gauge length of 30 mm. Finally, the Vickers hardness (HV10) of the different T6 heat treated alloys was measured in the cross section of the bars, employing 5 indentations per sample.
2.1. Materials and processing The standard AlMgSi1 alloy, as well as three experimental alloys having the same standard chemical composition but with different silicon contents (Table 1), was used in the study. The examined alloys were direct-chill (DC) cast, in a laboratory casting machine at Hydro R&D Materials Technology, Sunndalsøra, Norway, into billets with a diameter of 95 mm. After homogenization at 540 ◦ C for 4 h, the 200 mm long billets were extruded, employing an extrusion ratio of 70, into 12 mm rods using a vertical 8MN hydraulic press at SINTEF-Material and Chemistry, Trondheim, Norway. The applied homogenizing process, adjusted to the silicon-rich alloys, was defined in the test performed previously at SINTEF Raufoss manufacturing, Raufoss, Norway. The process consisted of heating the material to 540 ◦ C with an average heating rate of 90 ◦ C h−1 , solutionizing it at that temperature for 4 h and quenching it in water. Prior to extrusion, the billets were heated in an induction unit for 5 min to achieve the desired temperature of approximately 500 ◦ C at the front end and 490 ◦ C at the rear end of the billet. The extrusion speed was chosen as a variable in the extrusion trials. The maximum temperature developed in the material passing through the die was measured by means of a thermocouple integrated into the die bearing, a method established by Lefstad (1994), schematically presented in Fig. 1. Because the temperature gradient in the die region was very steep, an uncertainty of approximately ±5 ◦ C in the measurements was to be expected. 2.2. Mechanical testing Prior to mechanical testing, the materials were solution-heattreated at 530 ◦ C for 30 min, quenched in water, stored at room temperature for 1 h and then artificially aged at 150 ◦ C for 24 h. Four round tensile specimens (6 mm in diameter) were prepared from each alloy. These center rod specimens were taken from the middle of the extrusion press with their axes oriented in the direction of extrusion. The tensile tests were carried out with a constant strain rate of 0.004 s−1 using the standard equipment available at SINTEF Raufoss manufacturing, Raufoss, Norway. The strain measurements Table 1 Chemical composition of the alloys investigated [wt%]. Alloy
Si
Mg
Fe
Mn
Cr
Al
AlMgSi1 AlSi2 AlSi4 AlSi6
1.01 1.98 3.73 5.51
0.82 0.88 0.84 0.84
0.22 0.23 0.23 0.23
0.56 0.59 0.58 0.58
<0.03 <0.03 <0.03 <0.03
Balance Balance Balance Balance
2.3. Microstructure The microstructure of the as-homogenized billets was predicted by use of the microstructure model AlStruc® . Dons (2002) established this model, which can be used for predicting the size and type of the intermetallic particles in industrial commercial alloys and the micro-segregations in the matrix at various stages both during and after solidification and homogenization. The results obtained by means of AlStruc® were subsequently validated by experiments based on a combined optical microscope (OM) and SEM-EDS analysis. In addition, the microstructures of the alloys, in both the as-extruded and artificially aged (T6 temper) conditions, were analyzed in an optical microscope using polarized light. 3. Results and discussion 3.1. Effects of silicon on extrudability Extrudability is a measure of how fast an alloy can be extruded before some of the surface defects (die lines, pick-ups, tearing or hot shortness and spalling) occur at the extrusion surface. Data about the limiting extrusion speed in this investigation are based on the visual inspection of the extrusion surface with regard to tearing. The results obtained are summarized in Fig. 2. It is evident that the extrudability of the AlMgSi1 alloy is highly influenced by the silicon content. The limiting extrusion speed for this standard alloy, in prevailing circumstances, is 12 mm/s. The value is referred to the ram speed. An increase in the silicon content 14
Extrudability, [mm/s]
2. Experimental procedures
12
12 10 8
6
6
5
5
3.73
5.51
4 2 0 1.01
1.98
Si, [wt%] Fig. 2. Effects of silicon on the limiting extrusion speed as evaluated from the extrusion experiments.
S. Tomovic-Petrovic, O. Jensrud / Journal of Materials Processing Technology 212 (2012) 1437–1442 Table 2 The eutectics identified in the AlMgSi alloys.
Vmax -limiting speed; Melting point, ◦ C
Al + Mg2 Si + ␣(AlFeMnSi) + Si Al + Mg2 Si + Si Al + ␣(AlFeMnSi) + Si Al + Si
537 559 570 577
580 570
556
560
548
548
543
546
3.73
5.51
550 540
545
530
from 1.01% to 1.98% leads to a considerable reduction of the extrudability to 6 mm/s. In contrast, increased silicon content beyond this level will only marginally influence the limiting extrusion speed. Lefstad and Reiso (1996) have explained that the hot tears observed at the extrudate surface, when the limiting extrusion speed is exceeded, are caused by the cracking phenomena occurring due to local melting in the eutectic phase composition sites. Mondolfo (1967) gave a list of the eutectic phase compositions that can occur in the AlMgSi alloys with an important silicon excess (in this case the alloys 2, 3 and 4), together with their melting points (Table 2). Based on the measured maximum temperature developed in the material passing through the die shown in Fig. 3, it is reasonable to assume that the tearing in the silicon-rich alloys is related to the presence of the Al + Mg2 Si + Si eutectic composition. According to Mondolfo, the melting point of this eutectic is at 559 ◦ C. Considering that the uncertainty in the temperature measurement was typically ±5 ◦ C, the maximum temperature reached for the AlSi2, AlSi4 and AlSi6 alloys when the limiting extrusion speed was approached corresponds precisely with the melting point of this eutectic. As presented in Table 3, the volume percent of the Mg2 Si particles increases with the silicon content in the alloy. Moreover, the same Mg2 Si particles are primarily coupled with the silicon particles and not with the iron bearing particles (Fig. 4). These facts
V - 4mm/s
590 580
Tmax, °C
Eutectic
1439
520 1.01
1.98
Si, [wt%] Fig. 3. Effects of the silicon on the maximum temperature developed in the material passing through the die.
Table 3 Volume fraction of intermetallic particles in the as-homogenized alloys, based on the output from AlStruc® . Particle of type
Primary silicon Mg2 Si
Alloy AlMgSi1 (vol.%)
AlSi2 (vol.%)
AlSi4 (vol.%)
AlSi6 (vol.%)
0 0.414
0.701 0.423
2.961 0.496
5.214 0.577
confirm the assumption about the Al + Mg2 Si + Si eutectic composition causing the tearing in the examined alloys. The analysis of the influence of the eutectic Al + Mg2 Si + ␣(AlFeMnSi) + Si is also of interest (see Table 2). The rods produced in the silicon rich alloys (AlSi2-6) with a lower extrusion speed (4 mm/s) were found tearing free. In this instance, the maximum temperature developed in the material was above 537 ◦ C (see Fig. 2), which is the melting temperature
Fig. 4. Microstructures of the as-homogenized alloys (OM, bright field images). Mg2 Si – white arrow; primary Al12 (Mn,Fe)3 Si2 – red arrow; primary silicon – blue arrow (the output from Alstruc + EDS analysis).
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Breakthrough pressure
7
Minimum (forming) pressure PCG, [mm]
Extrusion pressure, kN
4500 4000 3500 3000
6 AlSi6 5
AlSi4
4 3 AlSi2
2 1 0
2500
1
2
3
4
5
6
7
8
9
10
11
12
Ram speed, [mm/s]
2000 1500 1.01
AlMgSi1
1.98
3.73
5.51
Fig. 7. Effects of the silicon excess and extrusion speed on the PCG zone in the as-extruded material. PCG of 6 mm – fully recrystallized.
Si, [wt %] 3.3. Grain size evolution Fig. 5. Breakthrough and forming pressure vs. silicon content. These curves apply to a constant extrusion (ram) speed of 5 mm/s.
of the mentioned eutectic. This result indicates that this eutectic composition does not appear in these alloys under the prevailing circumstances. 3.2. Effects of silicon additions on the applied extrusion pressure The increased silicon content has only a marginal effect on the applied extrusion pressure (Fig. 5). This is to be expected because the results in Table 3 show that most of the excess silicon already precipitates out in the form of intermetallic phases during homogenizing, thereby providing no significant increase in the flow-stress of the materials. If the silicon had been retained in solid solution instead, it would have led to a large increase in the deformation resistance and thus the applied extrusion pressure.
As shown in Fig. 6a, the AlMgSi1 and AlSi2 alloys extruded at a limiting extrusion speed have a fibrous structure with a peripheral coarse grain (PCG) zone. In contrast, the silicon-rich AlSi4 and AlSi6 alloys have an overall recrystallized structure, which becomes finer with higher silicon excess. It was observed that the PCG zone becomes wider when increasing both the silicon content and the extrusion speed (Fig. 7). The obvious relationship between the silicon content: i.e., the volume percentage of the primary silicon particles (see Table 3), and the recrystallization level in the investigated alloys shows the possibility of particle stimulated nucleation (PSN) of recrystallization. Humphreys and Hatherli (2004) asserts that the PSN of recrystallization has been observed in many alloys, including those of aluminum, and is usually only found to occur at particles of diameter greater than approximately 1 m. Furthermore, they claim that because the interaction of dislocations and particles is temperature
Fig. 6. Microstructures of the alloys investigated: (a) as-extruded, following extrusion at limiting extrusion speed, (b) in the T6 condition, and (c) longitudinal section of the extruded rod with position where the micro samples were taken out.
S. Tomovic-Petrovic, O. Jensrud / Journal of Materials Processing Technology 212 (2012) 1437–1442
The mechanical properties of the alloys in the T6 temper condition are summarized in Fig. 8. The AlMgSi1 alloy has the highest yield and tensile strength. When the silicon is in excess, compared with the standard one, the strength will first decrease and then slightly increase again, whereas the elongation shows the opposite trend. The increased silicon content is also accompanied by a continuous decrease in the fracture strain (computed by measurements of the area reduction) as well as an increase in hardness for the AlSi4 and AlSi6 alloys, but not for the AlSi2 alloy. The explanation of the results given above should be examined in the alloys structure in the T6 temper condition. The fibrous structure found in the AlMgSi1 alloy (see Fig. 6b) indicates that the strengthening of this alloy comes from both the fiber texture formed during the material extrusion and its subsequent precipitation hardening. However, the AlSi2 alloy has a fully recrystallized structure. This could mean that the strengthening of this alloy comes only from precipitation hardening and not from the fiber texture, as it does
Rp0.2
Rm
Elongation
Fracture strain 100.00
360 310
10.00
260 210
1.00
160 110
0.10 1
3
5
Elongation A, [%]; Fracture strain
3.4. Mechanical properties in the T6 temper condition
HV10
Rp0.2, [Mpa]; Rm, [Mpa]; HV10
dependent, PSN will only occur if the prior deformation is carried out below a critical temperature or above a critical strain rate. Because the size of the primary silicon particles in investigated alloys was found to be in the range of 1.5–4.0 m, the particle size requirement was fulfilled. Conversely, the high reduction ratio employed during extrusion causes the material to undergo both heat generation and extensive plastic deformation. This is the additional driving force required to initialize the PSN of recrystallization.
1441
Si, [wt%] Fig. 8. Mechanical properties vs. silicon content.
not exist. As a result, the strengthening level of the AlSi2 alloy is lower than that of the AlMgSi1 alloy. In addition, the AlSi4 and AlSi6 alloys contain recrystallized structures that become finer when the silicon addition is higher (grain size: 300 and 150 m, respectively). Consequently, an increase in material strengthening based on grain boundary strengthening was achieved. Thus, the experimental AlSi6 alloy with the finest structure achieved the highest strength. Further investigation of the precipitation strengthening will provide us more data about the strengthening mechanism of these alloys. It seems that the differences in precipitation strengthening
Fig. 9. Fracture surfaces showing the intermetallic particles precipitated along the grain boundaries.
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really exist. The matrix micro-hardness confirms this assumption as it shows the same trend as the macro-hardness. Finally, the silicon-rich alloys expressed a high brittleness (see Fig. 8). This was expected because the most of the intermetallic particles precipitated along the grain boundaries (Fig. 9). Regardless, an increase in the addition of silicon favored a more smooth-dimpled fracture surface without secondary inter-crystalline fractures as were observed in the AlSi2 alloy (indicated by the white arrow – Fig. 9). 4. Conclusions Based on the results obtained in the experimental study, the following conclusions can be drawn: 1. The experiments show that enhanced silicon levels will reduce the extrudability of the standard AlMgSi1 wrought aluminum alloy by lowering the limiting extrusion speed at which hot tears start to form at the surface. It follows that a 50% drop in the limiting extrusion speed is observed following a 1% silicon addition to the AlMgSi1 base alloy. On the other hand, increased silicon additions beyond this level will only marginally influence the limiting extrusion speed. 2. The increased silicon content has no significant effect on the applied extrusion pressure. 3. The as-extruded AlSi2 alloy reveals a fibrous structure with a PCG zone that is somewhat wider compared with the one found in the AlMgSi1 base alloy. Higher silicon additions result in the development of a structure with equiaxed grains, which tends to become finer with an increase in silicon content. 4. When excess silicon is added to the AlMgSi1 base alloy, the yield strength will first decrease and then slightly increase again, whereas the elongation shows the opposite trend. The increased silicon additions are further accompanied by a continuous decrease in the fracture strain as well as an increase in hardness for the AlSi4 and AlSi6 alloys, but not for the AlSi2 alloy. Nevertheless, the mechanical properties in the T6 temper condition are well below those of the AlMgSi1 base alloy. The experimental results have shown that the silicon-rich alloys had lower extrudability as well as mechanical properties compared
with the standard AlMgSi1 alloy. However, this study aimed to highlight the fact that the alloys containing silicon in the range of 2–6% can be used as wrought alloys. Further work will be focused on increasing the extrudability of the silicon-rich alloys as well as improving their mechanical properties. Variations in the extrusion parameters, such as billet preprocessing (preheating procedure), can be used to increase extrudability. Evidently, extrusion of silicon rich AlMgSi alloys requires a lower billet preheating temperature. Reiso (1988) suggested the procedure for maximizing the extrudability of the 6xxx alloys, which consists of solutionizing the billets followed by cooling them down to the working temperature, prior to extrusion. On the other hand, the post-processing of the final products (an adjusted heat treatment) could be a useful tool to improve the mechanical requirements. Acknowledgement This work was financed by the Norwegian Research Council. References Dons, A.L., 2002. Alstruc – a model for solidification and homogenization of industrial aluminum alloys. PhD thesis, Norwegian University of Science and Technology. Espedal, A., Gjestland, H., Ryum, N., McQueen, H.J., 1989. Hot deformation of Al–Mg–Si alloys. Scandinavian Journal of Metallurgy 18, 131–136. Gupta, A.K., Lloyd, D.J., Court, S.A., 2001. Precipitation hardening in Al–Mg–Si alloys with and without excess Si. Materials Science and Engineering A316, 11–17. Humphreys, H.J., Hatherli, M., 2004. Recrystallisation and Related Annealing Phenomena, second ed. Elsevier, Oxford, pp. 293–304. Langerweger, J., 1988. How casting billet methods can affect the quality of 6063 extrusion billets. In: Proceedings of the 4th International Aluminum Extrusion Technology Seminar, vol. 2, Washington, USA, pp. 381–384. Lefstad, M., 1994. Metallurgical speed limitations during the extrusion of AlMgSialloys. PhD thesis, Norwegian University of Science and Technology. Lefstad, M., Reiso, O., 1996. Metallurgical speed limitations during the extrusion of AlMgSi-alloys. In: Proceedings of the 6th International Aluminum Extrusion Technology Seminar, vol. 2, Chicago, USA, pp. 11–21. Mondolfo, L.F., 1967. Metallography of Aluminium Alloys. J. Wiley & Sons, Inc./Chapman & Hall, Limited, New York/London, pp. 245–247. Reiso, O., 1988. The effect of billet preheating practice on extrudability of AlMgSi alloys. In: Proceedings of the 4th International Aluminum Extrusion Technology Seminar, Washington, USA, pp. 287–295. Reiso, O., 2004. Extrusion of AlMgSi alloys. In: Nie, J.F., Morton, A.J., Muddle, B.C. (Eds.), Proceedings of the 9th International Conference on Aluminum Alloys. Brisbane, Australia, pp. 32–46.
Contents lists available at SciVerse ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Extrusion of silicon-rich AlMgSi alloys Stanka Tomovic-Petrovic ∗ , Ola Jensrud Department of Materials Technology, SINTEF Raufoss manufacturing AS, B 163, N-2831 Raufoss, Norway
a r t i c l e
i n f o
Article history: Received 18 October 2011 Received in revised form 31 January 2012 Accepted 5 February 2012 Available online 14 February 2012 Keywords: Aluminum alloys Bulk deformation Light microscope Mechanical characterization Recrystallization
a b s t r a c t As a part of on-going research on phase transformations during the deformation of light alloys, the effect of silicon excess on the extrudability and mechanical properties of the standard AlMgSi1 alloy within AA6082 alloy is investigated in this study. The AlMgSi1 alloy and three experimental aluminum alloys with a silicon content of 1.98%, 3.73% and 5.51% were direct-chilled cast into billets 95 mm in diameter, homogenized at 540 ◦ C for 4 h and extruded into 12 mm diameter rods at different extrusion speeds. The results showed that an increase in the silicon content reduced the extrudability of the AlMgSi1 alloy by lowering the limiting extrusion speed. However, the extruded alloys with 3.73% and 5.51% silicon, generally characterized by a fine grained microstructure, exhibited higher strength levels compared with the 1.98% silicon alloy. Nonetheless, the mechanical properties of these alloys, in the T6 temper condition, were below those of the AlMgSi1 base alloy. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The influence of silicon and magnesium on the extrudability and mechanical properties of AA 6xxx alloys is a topic that has been discussed in numerous papers. The general conclusion is that the extrudability of these alloys is highly influenced by the amount of silicon and magnesium in the solid solution: i.e., by the amount and distribution of the precipitated Mg2 Si. Increasing the alloying levels with an aim to meet the required product strength always raises the extrusion pressure and limits the extrusion speed. Espedal et al. (1989) has shown that the extrusion pressure increases 8% per 0.1 wt% increase of Mg2 Si in 6xxx alloys. Reiso (2004) has found that the extrudability of the same alloys is reduced by 1–2% per 0.01 wt% increase in the silicon amount. He has noticed the same strong effect on extrudability from variations in the magnesium content. This effect is maintained up to approximately 0.55 wt% magnesium content in the alloy. Magnesium content beyond this level has an even more deteriorating effect. Clearly, both elements, and in particular, magnesium, should be kept as low as possible to allow the use of a high extrusion speed. In contrast, a minimum level of these elements is required to achieve a high precipitation potential and the specified mechanical properties following age hardening. Industrial alloys are usually designed with a balance of magnesium and silicon or with an excess of silicon above that required to form stoichiometric Mg2 Si. Gupta et al. (2001) have confirmed that this solute ratio (Si/Mg) will reduce the time needed to initiate
∗ Corresponding author. Tel.: +47 45280636; fax: +47 61153625. E-mail address: [email protected] (S. Tomovic-Petrovic). 0924-0136/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2012.02.004
strengthening, increasing the strength in both the T4 and artificially aged tempers. Langerweger (1988) claimed that the silicon excess in industrial alloys will assure a lower drop in extrudability for a given increment in strength. Nevertheless, the silicon amount in the 6xxx alloys is limited to a value of 1.3 wt%. The effect of higher silicon amounts (in the range from 2 to 6 wt%) on the extrudability and mechanical properties of the AA 6xxx alloys has not been investigated as often. The low-meltingpoint silicon-rich eutectic phase composition, which can appear locally in the structure of these alloys under rapid heating as during extrusion, makes the extrusion process complicated. This is the reason why the wrought AA 6xxx alloys with high silicon content have been marked as “non-extrudable alloys”. The present study was initiated to clarify the possibility that these alloys are, in some way, qualified as wrought alloys of interest. This issue could be of great importance when considering the increased interest in the recycling of light metals. Designation of new wrought alloys with a wider tolerance in composition aimed toward different applications could contribute to the redirection of the current wrought-to-cast to wrought-to-wrought recycling process (partially, at least). The primary objective of this study is to obtain a better understanding of how large additions of silicon influence the extrudability and mechanical properties of the AlMgSi1 base alloy. In particular, the effect of the intermetallic phase formation on the limiting extrusion speed and the mechanical properties following age hardening is investigated in more detail, based on a series of exploratory experiments carried out under controlled conditions in the laboratory. The effect of the high silicon additions on the extrusion pressure and structure evolution is also some of the issues discussed in this paper.
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S. Tomovic-Petrovic, O. Jensrud / Journal of Materials Processing Technology 212 (2012) 1437–1442
Fig. 1. Sketch of the system used for measuring the maximum temperature achieved on the extrudate surface.
were performed using a long travel contact extensometer with a fixed original-gauge length of 30 mm. Finally, the Vickers hardness (HV10) of the different T6 heat treated alloys was measured in the cross section of the bars, employing 5 indentations per sample.
2.1. Materials and processing The standard AlMgSi1 alloy, as well as three experimental alloys having the same standard chemical composition but with different silicon contents (Table 1), was used in the study. The examined alloys were direct-chill (DC) cast, in a laboratory casting machine at Hydro R&D Materials Technology, Sunndalsøra, Norway, into billets with a diameter of 95 mm. After homogenization at 540 ◦ C for 4 h, the 200 mm long billets were extruded, employing an extrusion ratio of 70, into 12 mm rods using a vertical 8MN hydraulic press at SINTEF-Material and Chemistry, Trondheim, Norway. The applied homogenizing process, adjusted to the silicon-rich alloys, was defined in the test performed previously at SINTEF Raufoss manufacturing, Raufoss, Norway. The process consisted of heating the material to 540 ◦ C with an average heating rate of 90 ◦ C h−1 , solutionizing it at that temperature for 4 h and quenching it in water. Prior to extrusion, the billets were heated in an induction unit for 5 min to achieve the desired temperature of approximately 500 ◦ C at the front end and 490 ◦ C at the rear end of the billet. The extrusion speed was chosen as a variable in the extrusion trials. The maximum temperature developed in the material passing through the die was measured by means of a thermocouple integrated into the die bearing, a method established by Lefstad (1994), schematically presented in Fig. 1. Because the temperature gradient in the die region was very steep, an uncertainty of approximately ±5 ◦ C in the measurements was to be expected. 2.2. Mechanical testing Prior to mechanical testing, the materials were solution-heattreated at 530 ◦ C for 30 min, quenched in water, stored at room temperature for 1 h and then artificially aged at 150 ◦ C for 24 h. Four round tensile specimens (6 mm in diameter) were prepared from each alloy. These center rod specimens were taken from the middle of the extrusion press with their axes oriented in the direction of extrusion. The tensile tests were carried out with a constant strain rate of 0.004 s−1 using the standard equipment available at SINTEF Raufoss manufacturing, Raufoss, Norway. The strain measurements Table 1 Chemical composition of the alloys investigated [wt%]. Alloy
Si
Mg
Fe
Mn
Cr
Al
AlMgSi1 AlSi2 AlSi4 AlSi6
1.01 1.98 3.73 5.51
0.82 0.88 0.84 0.84
0.22 0.23 0.23 0.23
0.56 0.59 0.58 0.58
<0.03 <0.03 <0.03 <0.03
Balance Balance Balance Balance
2.3. Microstructure The microstructure of the as-homogenized billets was predicted by use of the microstructure model AlStruc® . Dons (2002) established this model, which can be used for predicting the size and type of the intermetallic particles in industrial commercial alloys and the micro-segregations in the matrix at various stages both during and after solidification and homogenization. The results obtained by means of AlStruc® were subsequently validated by experiments based on a combined optical microscope (OM) and SEM-EDS analysis. In addition, the microstructures of the alloys, in both the as-extruded and artificially aged (T6 temper) conditions, were analyzed in an optical microscope using polarized light. 3. Results and discussion 3.1. Effects of silicon on extrudability Extrudability is a measure of how fast an alloy can be extruded before some of the surface defects (die lines, pick-ups, tearing or hot shortness and spalling) occur at the extrusion surface. Data about the limiting extrusion speed in this investigation are based on the visual inspection of the extrusion surface with regard to tearing. The results obtained are summarized in Fig. 2. It is evident that the extrudability of the AlMgSi1 alloy is highly influenced by the silicon content. The limiting extrusion speed for this standard alloy, in prevailing circumstances, is 12 mm/s. The value is referred to the ram speed. An increase in the silicon content 14
Extrudability, [mm/s]
2. Experimental procedures
12
12 10 8
6
6
5
5
3.73
5.51
4 2 0 1.01
1.98
Si, [wt%] Fig. 2. Effects of silicon on the limiting extrusion speed as evaluated from the extrusion experiments.
S. Tomovic-Petrovic, O. Jensrud / Journal of Materials Processing Technology 212 (2012) 1437–1442 Table 2 The eutectics identified in the AlMgSi alloys.
Vmax -limiting speed; Melting point, ◦ C
Al + Mg2 Si + ␣(AlFeMnSi) + Si Al + Mg2 Si + Si Al + ␣(AlFeMnSi) + Si Al + Si
537 559 570 577
580 570
556
560
548
548
543
546
3.73
5.51
550 540
545
530
from 1.01% to 1.98% leads to a considerable reduction of the extrudability to 6 mm/s. In contrast, increased silicon content beyond this level will only marginally influence the limiting extrusion speed. Lefstad and Reiso (1996) have explained that the hot tears observed at the extrudate surface, when the limiting extrusion speed is exceeded, are caused by the cracking phenomena occurring due to local melting in the eutectic phase composition sites. Mondolfo (1967) gave a list of the eutectic phase compositions that can occur in the AlMgSi alloys with an important silicon excess (in this case the alloys 2, 3 and 4), together with their melting points (Table 2). Based on the measured maximum temperature developed in the material passing through the die shown in Fig. 3, it is reasonable to assume that the tearing in the silicon-rich alloys is related to the presence of the Al + Mg2 Si + Si eutectic composition. According to Mondolfo, the melting point of this eutectic is at 559 ◦ C. Considering that the uncertainty in the temperature measurement was typically ±5 ◦ C, the maximum temperature reached for the AlSi2, AlSi4 and AlSi6 alloys when the limiting extrusion speed was approached corresponds precisely with the melting point of this eutectic. As presented in Table 3, the volume percent of the Mg2 Si particles increases with the silicon content in the alloy. Moreover, the same Mg2 Si particles are primarily coupled with the silicon particles and not with the iron bearing particles (Fig. 4). These facts
V - 4mm/s
590 580
Tmax, °C
Eutectic
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520 1.01
1.98
Si, [wt%] Fig. 3. Effects of the silicon on the maximum temperature developed in the material passing through the die.
Table 3 Volume fraction of intermetallic particles in the as-homogenized alloys, based on the output from AlStruc® . Particle of type
Primary silicon Mg2 Si
Alloy AlMgSi1 (vol.%)
AlSi2 (vol.%)
AlSi4 (vol.%)
AlSi6 (vol.%)
0 0.414
0.701 0.423
2.961 0.496
5.214 0.577
confirm the assumption about the Al + Mg2 Si + Si eutectic composition causing the tearing in the examined alloys. The analysis of the influence of the eutectic Al + Mg2 Si + ␣(AlFeMnSi) + Si is also of interest (see Table 2). The rods produced in the silicon rich alloys (AlSi2-6) with a lower extrusion speed (4 mm/s) were found tearing free. In this instance, the maximum temperature developed in the material was above 537 ◦ C (see Fig. 2), which is the melting temperature
Fig. 4. Microstructures of the as-homogenized alloys (OM, bright field images). Mg2 Si – white arrow; primary Al12 (Mn,Fe)3 Si2 – red arrow; primary silicon – blue arrow (the output from Alstruc + EDS analysis).
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Breakthrough pressure
7
Minimum (forming) pressure PCG, [mm]
Extrusion pressure, kN
4500 4000 3500 3000
6 AlSi6 5
AlSi4
4 3 AlSi2
2 1 0
2500
1
2
3
4
5
6
7
8
9
10
11
12
Ram speed, [mm/s]
2000 1500 1.01
AlMgSi1
1.98
3.73
5.51
Fig. 7. Effects of the silicon excess and extrusion speed on the PCG zone in the as-extruded material. PCG of 6 mm – fully recrystallized.
Si, [wt %] 3.3. Grain size evolution Fig. 5. Breakthrough and forming pressure vs. silicon content. These curves apply to a constant extrusion (ram) speed of 5 mm/s.
of the mentioned eutectic. This result indicates that this eutectic composition does not appear in these alloys under the prevailing circumstances. 3.2. Effects of silicon additions on the applied extrusion pressure The increased silicon content has only a marginal effect on the applied extrusion pressure (Fig. 5). This is to be expected because the results in Table 3 show that most of the excess silicon already precipitates out in the form of intermetallic phases during homogenizing, thereby providing no significant increase in the flow-stress of the materials. If the silicon had been retained in solid solution instead, it would have led to a large increase in the deformation resistance and thus the applied extrusion pressure.
As shown in Fig. 6a, the AlMgSi1 and AlSi2 alloys extruded at a limiting extrusion speed have a fibrous structure with a peripheral coarse grain (PCG) zone. In contrast, the silicon-rich AlSi4 and AlSi6 alloys have an overall recrystallized structure, which becomes finer with higher silicon excess. It was observed that the PCG zone becomes wider when increasing both the silicon content and the extrusion speed (Fig. 7). The obvious relationship between the silicon content: i.e., the volume percentage of the primary silicon particles (see Table 3), and the recrystallization level in the investigated alloys shows the possibility of particle stimulated nucleation (PSN) of recrystallization. Humphreys and Hatherli (2004) asserts that the PSN of recrystallization has been observed in many alloys, including those of aluminum, and is usually only found to occur at particles of diameter greater than approximately 1 m. Furthermore, they claim that because the interaction of dislocations and particles is temperature
Fig. 6. Microstructures of the alloys investigated: (a) as-extruded, following extrusion at limiting extrusion speed, (b) in the T6 condition, and (c) longitudinal section of the extruded rod with position where the micro samples were taken out.
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The mechanical properties of the alloys in the T6 temper condition are summarized in Fig. 8. The AlMgSi1 alloy has the highest yield and tensile strength. When the silicon is in excess, compared with the standard one, the strength will first decrease and then slightly increase again, whereas the elongation shows the opposite trend. The increased silicon content is also accompanied by a continuous decrease in the fracture strain (computed by measurements of the area reduction) as well as an increase in hardness for the AlSi4 and AlSi6 alloys, but not for the AlSi2 alloy. The explanation of the results given above should be examined in the alloys structure in the T6 temper condition. The fibrous structure found in the AlMgSi1 alloy (see Fig. 6b) indicates that the strengthening of this alloy comes from both the fiber texture formed during the material extrusion and its subsequent precipitation hardening. However, the AlSi2 alloy has a fully recrystallized structure. This could mean that the strengthening of this alloy comes only from precipitation hardening and not from the fiber texture, as it does
Rp0.2
Rm
Elongation
Fracture strain 100.00
360 310
10.00
260 210
1.00
160 110
0.10 1
3
5
Elongation A, [%]; Fracture strain
3.4. Mechanical properties in the T6 temper condition
HV10
Rp0.2, [Mpa]; Rm, [Mpa]; HV10
dependent, PSN will only occur if the prior deformation is carried out below a critical temperature or above a critical strain rate. Because the size of the primary silicon particles in investigated alloys was found to be in the range of 1.5–4.0 m, the particle size requirement was fulfilled. Conversely, the high reduction ratio employed during extrusion causes the material to undergo both heat generation and extensive plastic deformation. This is the additional driving force required to initialize the PSN of recrystallization.
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Si, [wt%] Fig. 8. Mechanical properties vs. silicon content.
not exist. As a result, the strengthening level of the AlSi2 alloy is lower than that of the AlMgSi1 alloy. In addition, the AlSi4 and AlSi6 alloys contain recrystallized structures that become finer when the silicon addition is higher (grain size: 300 and 150 m, respectively). Consequently, an increase in material strengthening based on grain boundary strengthening was achieved. Thus, the experimental AlSi6 alloy with the finest structure achieved the highest strength. Further investigation of the precipitation strengthening will provide us more data about the strengthening mechanism of these alloys. It seems that the differences in precipitation strengthening
Fig. 9. Fracture surfaces showing the intermetallic particles precipitated along the grain boundaries.
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really exist. The matrix micro-hardness confirms this assumption as it shows the same trend as the macro-hardness. Finally, the silicon-rich alloys expressed a high brittleness (see Fig. 8). This was expected because the most of the intermetallic particles precipitated along the grain boundaries (Fig. 9). Regardless, an increase in the addition of silicon favored a more smooth-dimpled fracture surface without secondary inter-crystalline fractures as were observed in the AlSi2 alloy (indicated by the white arrow – Fig. 9). 4. Conclusions Based on the results obtained in the experimental study, the following conclusions can be drawn: 1. The experiments show that enhanced silicon levels will reduce the extrudability of the standard AlMgSi1 wrought aluminum alloy by lowering the limiting extrusion speed at which hot tears start to form at the surface. It follows that a 50% drop in the limiting extrusion speed is observed following a 1% silicon addition to the AlMgSi1 base alloy. On the other hand, increased silicon additions beyond this level will only marginally influence the limiting extrusion speed. 2. The increased silicon content has no significant effect on the applied extrusion pressure. 3. The as-extruded AlSi2 alloy reveals a fibrous structure with a PCG zone that is somewhat wider compared with the one found in the AlMgSi1 base alloy. Higher silicon additions result in the development of a structure with equiaxed grains, which tends to become finer with an increase in silicon content. 4. When excess silicon is added to the AlMgSi1 base alloy, the yield strength will first decrease and then slightly increase again, whereas the elongation shows the opposite trend. The increased silicon additions are further accompanied by a continuous decrease in the fracture strain as well as an increase in hardness for the AlSi4 and AlSi6 alloys, but not for the AlSi2 alloy. Nevertheless, the mechanical properties in the T6 temper condition are well below those of the AlMgSi1 base alloy. The experimental results have shown that the silicon-rich alloys had lower extrudability as well as mechanical properties compared
with the standard AlMgSi1 alloy. However, this study aimed to highlight the fact that the alloys containing silicon in the range of 2–6% can be used as wrought alloys. Further work will be focused on increasing the extrudability of the silicon-rich alloys as well as improving their mechanical properties. Variations in the extrusion parameters, such as billet preprocessing (preheating procedure), can be used to increase extrudability. Evidently, extrusion of silicon rich AlMgSi alloys requires a lower billet preheating temperature. Reiso (1988) suggested the procedure for maximizing the extrudability of the 6xxx alloys, which consists of solutionizing the billets followed by cooling them down to the working temperature, prior to extrusion. On the other hand, the post-processing of the final products (an adjusted heat treatment) could be a useful tool to improve the mechanical requirements. Acknowledgement This work was financed by the Norwegian Research Council. References Dons, A.L., 2002. Alstruc – a model for solidification and homogenization of industrial aluminum alloys. PhD thesis, Norwegian University of Science and Technology. Espedal, A., Gjestland, H., Ryum, N., McQueen, H.J., 1989. Hot deformation of Al–Mg–Si alloys. Scandinavian Journal of Metallurgy 18, 131–136. Gupta, A.K., Lloyd, D.J., Court, S.A., 2001. Precipitation hardening in Al–Mg–Si alloys with and without excess Si. Materials Science and Engineering A316, 11–17. Humphreys, H.J., Hatherli, M., 2004. Recrystallisation and Related Annealing Phenomena, second ed. Elsevier, Oxford, pp. 293–304. Langerweger, J., 1988. How casting billet methods can affect the quality of 6063 extrusion billets. In: Proceedings of the 4th International Aluminum Extrusion Technology Seminar, vol. 2, Washington, USA, pp. 381–384. Lefstad, M., 1994. Metallurgical speed limitations during the extrusion of AlMgSialloys. PhD thesis, Norwegian University of Science and Technology. Lefstad, M., Reiso, O., 1996. Metallurgical speed limitations during the extrusion of AlMgSi-alloys. In: Proceedings of the 6th International Aluminum Extrusion Technology Seminar, vol. 2, Chicago, USA, pp. 11–21. Mondolfo, L.F., 1967. Metallography of Aluminium Alloys. J. Wiley & Sons, Inc./Chapman & Hall, Limited, New York/London, pp. 245–247. Reiso, O., 1988. The effect of billet preheating practice on extrudability of AlMgSi alloys. In: Proceedings of the 4th International Aluminum Extrusion Technology Seminar, Washington, USA, pp. 287–295. Reiso, O., 2004. Extrusion of AlMgSi alloys. In: Nie, J.F., Morton, A.J., Muddle, B.C. (Eds.), Proceedings of the 9th International Conference on Aluminum Alloys. Brisbane, Australia, pp. 32–46.