- Email: [email protected]
On the effect of SPD on recycled experimental aluminium alloys: Nanostructures, particle break-up and properties
Materials Science and Engineering A 410–411 (2005) 261–264
On the effect of SPD on recycled experimental aluminium alloys: Nanostructures, particle break-up and properties P. Szczygiel a , H.J. Roven a,∗ , O. Reiso b a
The Norwegian University of Science and Technology, Department of Materials Technology, N-7491 Trondheim, Norway b Hydro Aluminium AS, R&D Materials Technology, 6600 Sunndalsora, Norway Received in revised form 12 May 2005
Abstract As a part of on-going research on the effects of recycled alloy chemistries on alloy design, properties and applications, the present work focuses on SPD processing. ECAP is used to deform two different Al alloys with “recycled alloy chemistry” and a standard AlMgSi alloy. A comparison between deformation routes A and BC is performed to evaluate the effects of deformation path on particle break-up and particle distributions. Both alloy content, amount of accumulated strain and choice of deformation route influenced particle refinement. © 2005 Elsevier B.V. All rights reserved. Keywords: Aluminium; Recycling; Particles break-up; ECAP
1. Introduction Research communities worldwide have increased their efforts to explore challenges and opportunities in regard to recycling of light metals as production based on scrap is progressively increasing [1,2]. A technique which can be used for generic studies related to the effect of severe plastic deformation (SPD) on microstructure refinement in recycle type alloy chemistries is equal channel angular pressing (ECAP). It has been reported that SPD can lead to a refinement of both grain size and particle size [3,4], has an impact on solubility levels, e.g. [5,6] and mechanical properties, e.g. [7–9]. The present work characterizes the microstructure evolution including particle break-up characteristics during ECAP of a standard age-hardenable and two aluminium casting alloys.
were homogenized at 540 ◦ C/8 h. ECAP was performed at room temperature using 100 mm × 19.5 mm × 19.5 mm bars and an L-shaped split-die with Φ = 90◦ and Ψ = 20.6◦ [10]. This geometry leads to an imposed strain ∼1, e.g. [11]. Repeated pressing was conducted up to N = 4 passes using routes A and BC [12]. The initial microstructure was characterized by energy dispersive spectroscopy (EDS in the SEM), electron microprobe analyses (EMPA) and optical microscopy (OM). After ECAP standard metallographic investigations of the center y-planes [12] were conducted by means of OM and field emission scanning electron microscopy (FEG-SEM). Grain sizes were revealed from EBSD in the SEM. Particle image analyses were performed from OM pictures taken at 1000× magnification. Quantitative measurements were based on the mean area of particles (AlSi alloys) and the largest length of a rectangle encompassing each particle (AlMgSi). Each measured size distribution included ∼700–1000 particles.
2. Experimental Two experimental AlSi alloys having recycled-type chemistries with relatively high iron and silicon contents and a standard AlMgSi alloy were used in the present study (Table 1). Direct-chill (DC) castings obtained from Hydro aluminium AS ∗
Corresponding author. Tel.: +47 73594966; fax: +47 73550203. E-mail address: [email protected] (H.J. Roven).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.08.051
3. Results and discussion 3.1. Initial microstructure Before ECAP the Si-rich alloys had an ordinary cast and homogenized microstructure with an average grain size ∼100 m. Grain boundary areas were decorated with phases (Fig. 1), usually referred to as ‘constituents’, i.e. eutectic second-
P. Szczygiel et al. / Materials Science and Engineering A 410–411 (2005) 261–264
262
Fig. 1. Initial microstructure of AlSi7% alloy. Phase identification was based on coupled OM and EDS analysis.
Fig. 2. Example of particle break-up and re-distribution in AlSi7% after 4 passes, route A.
Table 1 Chemical composition of the alloys investigated (wt.%)
Comparing the mean area of individual particles (wp ) in the two Si-rich alloys after one pass, the reduction in wp was much more pronounced in the AlSi7 than in the AlSi3 alloy, i.e. reduction 38 and 10%, respectively (Fig. 3). However, the present results showed an increase of the wp parameter at 4 passes for alloy AlSi3, whereas it was almost constant for alloy AlSi7 (Fig. 3). This is un-expected and is probably due to the limited resolution of the OM technique. Particles tend to break-up even when the size is below the OM resolution limit, e.g. breakup of submicron precipitates during ECAP [8]. The total area of particles per investigated area supports this view since the measured total particle area was significantly reduced between 1 and 4 passes for the AlSi3 alloy (Fig. 3a) and even showed a reduction with increasing number of passes beyond the first pass for the AlSi7 alloy (Fig. 3b). It was also evident that the total area of particles per analyzed area was more reduced with route BC than with route A, i.e. as expected from earlier findings [12]. The refinement in the AlMgSi alloy expressed as reduction in the mean length of particles (lp ) showed a pronounced drop during the first pass following route A, i.e. ∼40% reduction. The refinement continued beyond one pass, but was much less pronounced. The observed size distributions in the AlSi-alloys (Fig. 4) and in the AlMgSi alloy supported the above results. However, for particles <2 m2 the area fraction was reduced, probably due to a continuation of refinement below the resolution limit of the OM.
Alloy
Al
Si
Mg
Fe
Mn
6082 AlSi3 AlSi7
Balance Balance Balance
0.98 2.76 6.81
0.64 0.61 0.62
0.19 0.41 0.64
0.51 – –
The two Si-rich alloys have typical recycled alloy chemistries.
phase particles that solidify around cells of aluminium during DC casting [13]. These particles are iron-containing phases as well as Mg2 Si precipitated during cooling from homogenizing temperature [14,15]. The particle average area size varied substantially but was in average 4.2 and 8 m2 in the AlSi3 and AlSi7, respectively (Fig. 3). However, some particles and dense particle clusters could extend to 25–30 m2 . The AlMgSi alloy also had a typical cast and homogenized microstructure before ECAP and the particle phases of the types [14,15] Al12 (Mn,Fe)3 Si2 and Mg2 Si were mostly distributed on grain boundaries. The average particle length was close to 5 m. 3.2. Particle break-up All types of particles broke-up as a result of the SPD and this was accompanied by a re-distribution giving a more homogeneous microstructure (Fig. 2).
Fig. 3. Measured mean area of individual particles (wp ) and particle area per area unit vs. number of passes (a) AlSi3 and (b) AlSi7.
P. Szczygiel et al. / Materials Science and Engineering A 410–411 (2005) 261–264
263
Fig. 4. Size distribution of silicon particles before (a and c) and after (b and d) pressing following route A for AlSi3 (a and b) and AlSi7 (c and d).
The break-up inducing change to finer particles was also more convincing in the AlSi7 than in the AlSi3 alloy, e.g. comparing Fig. 4c and d versus Fig. 4a and b. Particles larger than ∼7 m2 (AlSi7) and larger than ∼4–6 m2 (AlSi3) were clearly broken up, hence the populations of the respective smaller sizes were increasing. In the AlMgSi alloy the trend was quite similar, i.e. the particles having a mean length lp > 4 m showed a decreasing population after ECAP. 3.3. Grain size evolution It is known that large crystallographic rotations, extensive deformation banding, preference of low energy dislocation structures and a characteristic texture development during ECAP contribute to the formation of ultra-fine grains, e.g. [16]. Already during the first pass high angle grains 1–2 m in diameter were formed in the very proximity of larger particles and particle clusters, i.e. the formation of ultra-fine grains was stimulated by particles and particle clusters. After 4 passes an average grain size of ∼1 m was obtained and in accordance to earlier findings [17] route BC proved to be the more effective also in regard to grain refinement.
Fig. 5. Pressing force curves for route A and BC for alloys AlSi3 and AlSi7. Evident faster strengthening of the material with higher content of the particles and deformed by route BC .
The described microstructural changes increased the hardness of ECAP materials. The hardness increase was more rapid when applying route BC than following route A and more pronounced in the AlSi7 than in the AlSi3 alloy. This increase was consistent with earlier works, e.g. [3,4], and with the observed microstructural refinement and also in accordance to the development of the pressing force monitored during ECAP.
3.4. Pressing force 4. Conclusions The pressing force increased with (i) the number of passes, (ii) increasing Si content and (iii) applying route BC instead of route A (Fig. 5). A high operating pressing force seemed to induce a more extensive and effective break-up of particles, i.e. in general agreement to literature indications, e.g. [18]. The observed pressing force peak may be explained by a gradual build-up of intrinsic backpressure [6].
In spite of being a non-completed experimental study, one can suggest the following preliminary conclusions: • Constituent particles break-up as a result of the shearing processes taking place in ECAP. This phenomenon is more pronounced in route BC and more extensive in AlSi7
264
P. Szczygiel et al. / Materials Science and Engineering A 410–411 (2005) 261–264
than in AlSi3 but still clearly recognized in the AlMgSi alloy. • Large particles >4–6 m2 split up into larger populations of finer particles. Further investigations in the TEM are needed due to the limited resolution of the optical microscope. • Particles accelerate deformation induced grain refinement. Acknowledgements The authors wish to thank the Norwegian Research Council, Hydro Aluminium AS and Mr. Quentin Contrepois (INPG, Grenoble, France). References [1] G. Hoyle, Resour. Conserv. Recycl. 15 (1995) 181–191. [2] J. Blomberg, S. Hellmer, Proceedings of the Fourth ASM International Conference and Exhibition on the Recycling of Metals, Vienna, Austria, 1999, pp. 35–47. [3] P.J. Apps, J.R. Bowen, P.B. Prangnell, Acta Mater. 51 (2003) 2811–2822. [4] Y. Nishida, H. Arima, J.-C. Kim, T. Ando, Scripta Mater. 45 (2001) 261–266. [5] O.N. Senkov, F.H. Froes, V.V. Stoyarov, R.Z. Valiev, J. Liu, Nanostruct. Mater. 10 (5) (1998) 691–698.
[6] V.V. Stolyarov, R. Lapovok, I.G. Brodova, P.F. Thomson, Mater. Sci. Eng. A 357 (2003) 159–167. [7] M. Murayama, Z. Horita, K. Hono, Acta Mater. 49 (2001) 21–29. [8] C. Xu, M. Furukawa, Z. Horita, T.G. Langdon, Acta Mater. 53 (2005) 749–758. [9] C. Xu, M. Furukawa, Z. Horita, T.G. Langdon, Acta Mater. 51 (2003) 6139–6149. [10] Y. Iwahashi, J. Wang, Z. Horita, M. Nemoto, T.G. Langdon, Scripta Mater. 35 (1996) 143–146. [11] H.J. Roven, S. Dumoulin, J.C. Werenskiold, in: Y.T., Zhu, T.G., Langdon, R.Z., Valiev, S.L., Semiatin, D.H., Shin, T.C., Lowe, (Eds.), Ultrafine Grained Materials III, TMS, Philadelphia, USA, 2004, pp. 117– 124. [12] M. Furukawa, Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Mater. Sci. Eng. A 257 (1998) 328–332. [13] D.T.L. Alexander, A.L. Greer, Acta Mater. 50 (2002) 2571–2583. [14] A.L. Dons, J. Light Met. 1 (2001) 133–149. [15] A.L. Dons, E.K. Jensen, Y. Langsrud, E. Troemborg, S. Brusethaug, Metall. Mater. Trans. A 30 (1999) 2135–2146. [16] J.C. Werenskiold, PhD thesis no. 2004:129, Norwegian University of Science and Technology, 2004, pp. 1–262. [17] Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Acta Mater. 46 (1998) 3317–3331. [18] S.M. Roberts, J. Kusiak, P.J. Withers, S.J. Barnes, P.B. Prangnell, J. Mater. Proc. Techn. 60 (1996) 711–718.
On the effect of SPD on recycled experimental aluminium alloys: Nanostructures, particle break-up and properties P. Szczygiel a , H.J. Roven a,∗ , O. Reiso b a
The Norwegian University of Science and Technology, Department of Materials Technology, N-7491 Trondheim, Norway b Hydro Aluminium AS, R&D Materials Technology, 6600 Sunndalsora, Norway Received in revised form 12 May 2005
Abstract As a part of on-going research on the effects of recycled alloy chemistries on alloy design, properties and applications, the present work focuses on SPD processing. ECAP is used to deform two different Al alloys with “recycled alloy chemistry” and a standard AlMgSi alloy. A comparison between deformation routes A and BC is performed to evaluate the effects of deformation path on particle break-up and particle distributions. Both alloy content, amount of accumulated strain and choice of deformation route influenced particle refinement. © 2005 Elsevier B.V. All rights reserved. Keywords: Aluminium; Recycling; Particles break-up; ECAP
1. Introduction Research communities worldwide have increased their efforts to explore challenges and opportunities in regard to recycling of light metals as production based on scrap is progressively increasing [1,2]. A technique which can be used for generic studies related to the effect of severe plastic deformation (SPD) on microstructure refinement in recycle type alloy chemistries is equal channel angular pressing (ECAP). It has been reported that SPD can lead to a refinement of both grain size and particle size [3,4], has an impact on solubility levels, e.g. [5,6] and mechanical properties, e.g. [7–9]. The present work characterizes the microstructure evolution including particle break-up characteristics during ECAP of a standard age-hardenable and two aluminium casting alloys.
were homogenized at 540 ◦ C/8 h. ECAP was performed at room temperature using 100 mm × 19.5 mm × 19.5 mm bars and an L-shaped split-die with Φ = 90◦ and Ψ = 20.6◦ [10]. This geometry leads to an imposed strain ∼1, e.g. [11]. Repeated pressing was conducted up to N = 4 passes using routes A and BC [12]. The initial microstructure was characterized by energy dispersive spectroscopy (EDS in the SEM), electron microprobe analyses (EMPA) and optical microscopy (OM). After ECAP standard metallographic investigations of the center y-planes [12] were conducted by means of OM and field emission scanning electron microscopy (FEG-SEM). Grain sizes were revealed from EBSD in the SEM. Particle image analyses were performed from OM pictures taken at 1000× magnification. Quantitative measurements were based on the mean area of particles (AlSi alloys) and the largest length of a rectangle encompassing each particle (AlMgSi). Each measured size distribution included ∼700–1000 particles.
2. Experimental Two experimental AlSi alloys having recycled-type chemistries with relatively high iron and silicon contents and a standard AlMgSi alloy were used in the present study (Table 1). Direct-chill (DC) castings obtained from Hydro aluminium AS ∗
Corresponding author. Tel.: +47 73594966; fax: +47 73550203. E-mail address: [email protected] (H.J. Roven).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.08.051
3. Results and discussion 3.1. Initial microstructure Before ECAP the Si-rich alloys had an ordinary cast and homogenized microstructure with an average grain size ∼100 m. Grain boundary areas were decorated with phases (Fig. 1), usually referred to as ‘constituents’, i.e. eutectic second-
P. Szczygiel et al. / Materials Science and Engineering A 410–411 (2005) 261–264
262
Fig. 1. Initial microstructure of AlSi7% alloy. Phase identification was based on coupled OM and EDS analysis.
Fig. 2. Example of particle break-up and re-distribution in AlSi7% after 4 passes, route A.
Table 1 Chemical composition of the alloys investigated (wt.%)
Comparing the mean area of individual particles (wp ) in the two Si-rich alloys after one pass, the reduction in wp was much more pronounced in the AlSi7 than in the AlSi3 alloy, i.e. reduction 38 and 10%, respectively (Fig. 3). However, the present results showed an increase of the wp parameter at 4 passes for alloy AlSi3, whereas it was almost constant for alloy AlSi7 (Fig. 3). This is un-expected and is probably due to the limited resolution of the OM technique. Particles tend to break-up even when the size is below the OM resolution limit, e.g. breakup of submicron precipitates during ECAP [8]. The total area of particles per investigated area supports this view since the measured total particle area was significantly reduced between 1 and 4 passes for the AlSi3 alloy (Fig. 3a) and even showed a reduction with increasing number of passes beyond the first pass for the AlSi7 alloy (Fig. 3b). It was also evident that the total area of particles per analyzed area was more reduced with route BC than with route A, i.e. as expected from earlier findings [12]. The refinement in the AlMgSi alloy expressed as reduction in the mean length of particles (lp ) showed a pronounced drop during the first pass following route A, i.e. ∼40% reduction. The refinement continued beyond one pass, but was much less pronounced. The observed size distributions in the AlSi-alloys (Fig. 4) and in the AlMgSi alloy supported the above results. However, for particles <2 m2 the area fraction was reduced, probably due to a continuation of refinement below the resolution limit of the OM.
Alloy
Al
Si
Mg
Fe
Mn
6082 AlSi3 AlSi7
Balance Balance Balance
0.98 2.76 6.81
0.64 0.61 0.62
0.19 0.41 0.64
0.51 – –
The two Si-rich alloys have typical recycled alloy chemistries.
phase particles that solidify around cells of aluminium during DC casting [13]. These particles are iron-containing phases as well as Mg2 Si precipitated during cooling from homogenizing temperature [14,15]. The particle average area size varied substantially but was in average 4.2 and 8 m2 in the AlSi3 and AlSi7, respectively (Fig. 3). However, some particles and dense particle clusters could extend to 25–30 m2 . The AlMgSi alloy also had a typical cast and homogenized microstructure before ECAP and the particle phases of the types [14,15] Al12 (Mn,Fe)3 Si2 and Mg2 Si were mostly distributed on grain boundaries. The average particle length was close to 5 m. 3.2. Particle break-up All types of particles broke-up as a result of the SPD and this was accompanied by a re-distribution giving a more homogeneous microstructure (Fig. 2).
Fig. 3. Measured mean area of individual particles (wp ) and particle area per area unit vs. number of passes (a) AlSi3 and (b) AlSi7.
P. Szczygiel et al. / Materials Science and Engineering A 410–411 (2005) 261–264
263
Fig. 4. Size distribution of silicon particles before (a and c) and after (b and d) pressing following route A for AlSi3 (a and b) and AlSi7 (c and d).
The break-up inducing change to finer particles was also more convincing in the AlSi7 than in the AlSi3 alloy, e.g. comparing Fig. 4c and d versus Fig. 4a and b. Particles larger than ∼7 m2 (AlSi7) and larger than ∼4–6 m2 (AlSi3) were clearly broken up, hence the populations of the respective smaller sizes were increasing. In the AlMgSi alloy the trend was quite similar, i.e. the particles having a mean length lp > 4 m showed a decreasing population after ECAP. 3.3. Grain size evolution It is known that large crystallographic rotations, extensive deformation banding, preference of low energy dislocation structures and a characteristic texture development during ECAP contribute to the formation of ultra-fine grains, e.g. [16]. Already during the first pass high angle grains 1–2 m in diameter were formed in the very proximity of larger particles and particle clusters, i.e. the formation of ultra-fine grains was stimulated by particles and particle clusters. After 4 passes an average grain size of ∼1 m was obtained and in accordance to earlier findings [17] route BC proved to be the more effective also in regard to grain refinement.
Fig. 5. Pressing force curves for route A and BC for alloys AlSi3 and AlSi7. Evident faster strengthening of the material with higher content of the particles and deformed by route BC .
The described microstructural changes increased the hardness of ECAP materials. The hardness increase was more rapid when applying route BC than following route A and more pronounced in the AlSi7 than in the AlSi3 alloy. This increase was consistent with earlier works, e.g. [3,4], and with the observed microstructural refinement and also in accordance to the development of the pressing force monitored during ECAP.
3.4. Pressing force 4. Conclusions The pressing force increased with (i) the number of passes, (ii) increasing Si content and (iii) applying route BC instead of route A (Fig. 5). A high operating pressing force seemed to induce a more extensive and effective break-up of particles, i.e. in general agreement to literature indications, e.g. [18]. The observed pressing force peak may be explained by a gradual build-up of intrinsic backpressure [6].
In spite of being a non-completed experimental study, one can suggest the following preliminary conclusions: • Constituent particles break-up as a result of the shearing processes taking place in ECAP. This phenomenon is more pronounced in route BC and more extensive in AlSi7
264
P. Szczygiel et al. / Materials Science and Engineering A 410–411 (2005) 261–264
than in AlSi3 but still clearly recognized in the AlMgSi alloy. • Large particles >4–6 m2 split up into larger populations of finer particles. Further investigations in the TEM are needed due to the limited resolution of the optical microscope. • Particles accelerate deformation induced grain refinement. Acknowledgements The authors wish to thank the Norwegian Research Council, Hydro Aluminium AS and Mr. Quentin Contrepois (INPG, Grenoble, France). References [1] G. Hoyle, Resour. Conserv. Recycl. 15 (1995) 181–191. [2] J. Blomberg, S. Hellmer, Proceedings of the Fourth ASM International Conference and Exhibition on the Recycling of Metals, Vienna, Austria, 1999, pp. 35–47. [3] P.J. Apps, J.R. Bowen, P.B. Prangnell, Acta Mater. 51 (2003) 2811–2822. [4] Y. Nishida, H. Arima, J.-C. Kim, T. Ando, Scripta Mater. 45 (2001) 261–266. [5] O.N. Senkov, F.H. Froes, V.V. Stoyarov, R.Z. Valiev, J. Liu, Nanostruct. Mater. 10 (5) (1998) 691–698.
[6] V.V. Stolyarov, R. Lapovok, I.G. Brodova, P.F. Thomson, Mater. Sci. Eng. A 357 (2003) 159–167. [7] M. Murayama, Z. Horita, K. Hono, Acta Mater. 49 (2001) 21–29. [8] C. Xu, M. Furukawa, Z. Horita, T.G. Langdon, Acta Mater. 53 (2005) 749–758. [9] C. Xu, M. Furukawa, Z. Horita, T.G. Langdon, Acta Mater. 51 (2003) 6139–6149. [10] Y. Iwahashi, J. Wang, Z. Horita, M. Nemoto, T.G. Langdon, Scripta Mater. 35 (1996) 143–146. [11] H.J. Roven, S. Dumoulin, J.C. Werenskiold, in: Y.T., Zhu, T.G., Langdon, R.Z., Valiev, S.L., Semiatin, D.H., Shin, T.C., Lowe, (Eds.), Ultrafine Grained Materials III, TMS, Philadelphia, USA, 2004, pp. 117– 124. [12] M. Furukawa, Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Mater. Sci. Eng. A 257 (1998) 328–332. [13] D.T.L. Alexander, A.L. Greer, Acta Mater. 50 (2002) 2571–2583. [14] A.L. Dons, J. Light Met. 1 (2001) 133–149. [15] A.L. Dons, E.K. Jensen, Y. Langsrud, E. Troemborg, S. Brusethaug, Metall. Mater. Trans. A 30 (1999) 2135–2146. [16] J.C. Werenskiold, PhD thesis no. 2004:129, Norwegian University of Science and Technology, 2004, pp. 1–262. [17] Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Acta Mater. 46 (1998) 3317–3331. [18] S.M. Roberts, J. Kusiak, P.J. Withers, S.J. Barnes, P.B. Prangnell, J. Mater. Proc. Techn. 60 (1996) 711–718.