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On the formation of spheroidal microstructures in a semi-solid Al–Si alloy by thermomechanical processing
Scripta Materialia 57 (2007) 1165–1168 www.elsevier.com/locate/scriptamat
On the formation of spheroidal microstructures in a semi-solid Al–Si alloy by thermomechanical processing E.A. Vieira,1 A.M. Kliauga and M. Ferrante* Federal University of Sa˜o Carlos, Materials Engineering Department, 13565-905 Sa˜o Carlos, Brazil Received 12 July 2007; accepted 17 July 2007 Available online 19 September 2007
Spheroidal microstructures suitable for thixoforming, can be formed by a variety of techniques. From optical microscopy and electron backscattered diffraction observations, the mechanism of formation of such microstructure in an Al–Si alloy conditioned by the so-called deformation–recrystallization technique is described as an internal partition of the deformed dendrites. During heating and partial remelting, low angle boundaries turn into high angle boundaries, whose penetration by the liquid is energetically favorable, thus leading to a dispersion of spheroidal Al-a particles. 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Semi-solid processing; Al–Si; Thermomechanical processing; Recovery; EBSD
In order to reduce fuel consumption and tailpipe emission, by 2020 the average passenger car is expected to be 17% lighter [1], mostly by increasing the proportion of Al and Mg alloys. Since replacement of steel by light alloys in the powertrain appears to be approaching saturation, the body shell and suspension must be considered, but for the latter application cast Al alloys do not possess enough strength and fatigue resistance. However, a relatively new technology, denominated thixocasting or semi-solid state (SSS) casting, is capable of ‘‘upgrading’’ the mechanical properties of cast products to the level of their forged counterparts. The main requirement for the success of SSS forming is the production of a non-dendritic, spheroidal microstructure, from which suitable viscosity and adequate flow behavior depends. There are four main techniques for raw material preparation, hereafter named microstructural conditioning: (i) mechanical stirring; (ii) electromagnetic stirring (magnetohydrodynamics, MHD); (iii) new rheocasting; and (iv) a thermomechanical treatment (TMT), involving plastic deformation and recrystallization. The first three conditioning modes have been satisfactorily described in the literature, from both
* Corresponding author. Tel.: +55 16 3351-8256; fax: +55 16 33615404; e-mail: [email protected] 1 Present address: CEFET (Federal Centre of Technological Education), Vito´ria (ES), Brazil.
a descriptive and a scientific point of view [2,3], but less attention has been given to the TMT process, which attains very favorable microstructural parameters by employing a very simple technology [4]. In the present work, electron backscatter diffraction (EBSD) and optical microscopy were employed to study the mechanism of spheroidal microstructure production by the TMT process, contrasting it with the MHD conditioning mode. The TMT-conditioned group of samples was produced by remelting a commercial A356 ingot. After degassing, the melt was Sr-modified, grain refined by Ti–B and poured into a steel mould with a 145 · 35 mm2 section and 250 mm height. The MHD group of samples was cut from a 100 mm diameter commercial ingot supplied by FORMCAST. The chemical composition of each group is given in Table 1 in wt.%, and it can be seen that differences are negligible. The TMT ingot was homogenized at 540 C for 24 h and water cooled. It was reheated to 300 C and deformed by rolling to 30% thickness reduction. The MHD material was already conditioned. In all experiments the SSS was attained by induction heating to 580 C samples taken from the ingot and from the deformed plate, a temperature which, as calculated by the Scheil equation, corresponds to a solid volume fraction (fs) equal to 0.5. Soaking times at that temperature varied between 0 and 10 min, and samples were water quenched.
1359-6462/$ - see front matter 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.07.046
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Table 1. Chemical composition of raw materials (wt.%)
TMT MHD
Si
Mg
Cu
Fe
7.13 6.60
0.40 0.34
0.01 –
0.12 0.15
Both optical microscopy and EBSD measurements were employed to characterize microstructure, general mesotexture and orientation of pairs of contiguous Ala particles. Grain boundaries were classified as low angle boundaries (LAB, <15) and high angle boundaries (HAB, >15). Figure 1 shows optical images of the initial microstructure of the TMT and MHD samples before partial remelting. It can be seen that the MHD ingot microstructure is constituted by small rosette-like dendrites. Although there are small differences between the ingot centre and its periphery, the former being coarser than the latter (the present micrograph shows the ingot centre), dendrites rapidly coalesce and the microstructure ends up by being quite uniform after reaching the SSS and being soaked at the chosen temperature. As for the as-rolled TTM sample, Figure 1b shows dendrites approximately 50 lm thick, making an angle of 24 with the rolling direction. The interdendritic distance before rolling was close to 67 lm, a figure compatible with the thickness reduction. By partial melting, small Al-a particles are produced, which rapidly coarsen and spheroidize. The formation of non-dendritic microstructures by mechanical stirring of semi-solid alloys has been studied by Doherty et al. [3], and it can be assumed that, if correct, their model would also apply to magnetically stirred semi-solid alloys. According to the model, dendrites would bend and fragment under the influence of shear forces. Bending is accomplished by dislocation generation, followed by their climb and alignment forming grain boundaries. When the misorientation (a) across these boundaries exceeds 20, it can be shown that the grain boundary energy is more then twice the solid– liquid energy. Therefore, fragmentation along the boundary will be energetically favorable and the dendrite debris disperse into the liquid where they growth and spheroidize. In contrast, TMT microstructural conditioning was assumed to be based simply on deformation–recrystallization processes, followed by liquid formation and penetration along the grain boundaries.
Figure 2 shows EBSD images of TMT and MHD samples in the as-received condition, that is, rolled plate and MHD ingot, and after 0 and 10 min in the SSS. In the MHD sample it is apparent that the rosettes exhibit a single orientation within, suggesting that the bending– fracturing mechanism acts on selected dendrite arms. This should leave groups of dendrites, or rosettes, dispersed in the liquid instead of individual debris as predicted [3]. For the TMT sample the pattern is completely different. In the first EBSD image it is apparent that within the dendrites a number of orientations exist; this indicates internal fragmentation, certainly due to the plastic deformation during rolling. The second and third micrographs show the sample as quenched from the semi-solid state. Penetration of the subgrains by the interdendritic liquid is complete and the isolated particles assume random orientation; their competitive growth is clearly shown by the third image. The frequency distribution of grain boundary angles is shown in Figure 3. It is important to take into account that for the as-received samples (solid state condition) measurements were carried out in the conventional way averaging a large area, while for the remaining samples (0, 3 and 10 min in the SSS) the misorientation concerns only pairs of contiguous particles. Analyzing the MHD material it can be seen that in the as-received condition approximately 60% of the boundaries are LAB and that, once in the SSS, their proportion increases to 70%, with the great majority consisting of boundaries with a < 5. This behavior supports the Doherty et al. model since dendrite separation when a > 20 is consistent with the observed increase of LAB proportion. After 3 min the material is well into the SSS and is composed by discrete Al-a particles (fs = 0.5) plus the liquid phase; hereafter random motion of the particles decreases the proportion of LAB. As for the TMT sample, the evolution of grain boundaries angles frequency is different in the sense that LAB are continuously replaced by HAB, except for the 3 min SSS sample (see the explanation below). It must be pointed out that according to the Mackenzie distribution, the probability of finding boundary angles smaller than 10 in a non-deformed cubic structure is 4% [5]. Figure 4 shows EBSD images of MHD and TMT individual dendrites in the as-received and deformed conditions, together with the respective internal misorientation, both from one extremity of the dendrite to the
Figure 1. Microstructures of A356 alloys produced by: (a) electromagnetic stirring and (b) thermomechanical treatment.
E. A. Vieira et al. / Scripta Materialia 57 (2007) 1165–1168
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Figure 2. EBSD images showing dendrites/grains orientation. First row: TMT samples; second row: MHD samples. First column: as-received samples; second column: partially remelted samples, soaked for 0 min at 580 C; third column: idem, soaked for 10 min at 580 C. The standard stereographic triangle (color key) is included. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Figure 3. Misorientation angle distribution. First row: MHD samples; second row: TMT samples. First column: as-received samples; second column: partially remelted samples soaked for 0 min at 580 C; third column: idem, soaked for 3 min; fourth column: idem, soaked for 10 min.
other (point to origin) and point to point. It must be recalled that the former material has already experienced the local melting–fragmentation process described in Ref. [3]. The following comments are relevant: (i) in practical terms, within any MHD dendrite a single crystallographic orientation exists; and (ii) the internal partitioning of the TMT dendrite is very clearly shown; the point-to-point misorientation measurements show the existence of at least 35 boundaries or subboundaries, all of them exhibiting a < 20; of these, only five or six have a > 10. The accumulated misorientation within
this dendrite is 55, which must be contrasted with the 6 or 7 of the MHD. Figure 5 is an optical micrograph of a TMT sample reheated to 570 C, thus very close to the A356 melting temperature, which is 557 C, and rapidly quenched. The time to reach the said temperature was 3 min and the soaking time was 0 min. It is apparent that the deformed dendrites are partitioned by what appears to be subgrain boundaries. Approximate measurements taken on this sample show that the number of internal intercepts met by a test-line of the same length as that of Figure 4b, that is, 260 lm, is close to five.
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Figure 4. Accumulated and point-to-point internal misorientation for: (a) MHD dendrite, as-received; and (b) TMT dendrite in the as-deformed condition. The standard stereographic triangle (color key) is included. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Figure 5. Optical micrograph of a TMT sample. After deformation by rolling, the material was heated to 570 C and immediately quenched.
Hence, the sequence of events leading to the formation of spheroidal microstructures in TMT-conditioned A356 alloys can be described as follows: (i) plastic deformation, giving a high number of LAB by internal partition of the dendrites; (ii) while heating to the SSS, the LAB are replaced by HAB, and their proportion decreases; (iii) after 3 min in the SSS, high energy boundaries begin to disintegrate, as predicted in Ref. [3], thus proportionally more LAB are left, hence their proportion increases; (iv) similarly to the MHD sample, 10 min in the SSS is sufficient to randomize the particles’ orientation, but still without reaching the Mackenzie distribution. The presence of two peaks in the orientation distribution of both the TMT and MHD samples characterizes a microstructure composed by isolated and bonded Al-a particles. Mechanism (i) above is the accepted model for deformation microstructures described by Hasen and coworkers [6]. In the sequence, the main events are dislocation climb and rearrangement, meaning that in TMTconditioned Al–Si alloys, the spheroidal microstructure
is a product of dislocation recovery rather than grain recrystallization. Semi-solid spheroidal microstructures can be produced by a number of techniques, including TMT and MHD. These two processes show some similarity in the sense that the plastic deformation of dendrites (by stirring in the semi-solid state for MHD and by rolling in the solid state for TMT) is followed by dislocation rearrangement when the alloy is heated to the SSS. Optical microscopy and EBSD observations of the TMT material showed that plastic deformation caused flattening and internal partition of the solidification dendrites. The evolution of the internal boundaries’ misorientation suggests a mechanism by which dislocation migration turns LAB into HAB, which are liable to be penetrated by the liquid phase, thus forming spheroidal Al-a particles. This mechanism is very similar to that proposed by Doherty et al. for the MHD process, although there are differences between the two techniques. [1] Mercer und HypoVereinsbank (Ed.) Automobiltechnologie (2001) 2010. [2] J.L. Jorstad, 8th Int. Conf. on Semi-solid Processing of Alloys and Composites, September 21–23, 2004, Limassol (Cyprus). [3] R.D. Doherty, H.I. Lee, E.A. Feest, Mat. Sci. Eng. 65 (1984) 181. [4] M.P. Kenney, J.A. Courtois, R.D. Evans, G.M. Farrior, C.P. Kyonka, A.A. Koch, K.P. Young, in Metals Handbook. Metals Park, OH, ASM International, 9th Ed., vol. 15 (1988) 327. [5] J.K. Mackenzie, Biometrika, vol. 45 (1958), 229. Quoted in V. Randle, O. Engler, Texture Analysis, Gordon & Breach Science Publishers (2000), p. 225. [6] B. Bay, N. Hansen, D.A. Hughes, D. Kuhlmann-Wilsdorf, Acta Metall. Mater. 40 (1992) 205.
On the formation of spheroidal microstructures in a semi-solid Al–Si alloy by thermomechanical processing E.A. Vieira,1 A.M. Kliauga and M. Ferrante* Federal University of Sa˜o Carlos, Materials Engineering Department, 13565-905 Sa˜o Carlos, Brazil Received 12 July 2007; accepted 17 July 2007 Available online 19 September 2007
Spheroidal microstructures suitable for thixoforming, can be formed by a variety of techniques. From optical microscopy and electron backscattered diffraction observations, the mechanism of formation of such microstructure in an Al–Si alloy conditioned by the so-called deformation–recrystallization technique is described as an internal partition of the deformed dendrites. During heating and partial remelting, low angle boundaries turn into high angle boundaries, whose penetration by the liquid is energetically favorable, thus leading to a dispersion of spheroidal Al-a particles. 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Semi-solid processing; Al–Si; Thermomechanical processing; Recovery; EBSD
In order to reduce fuel consumption and tailpipe emission, by 2020 the average passenger car is expected to be 17% lighter [1], mostly by increasing the proportion of Al and Mg alloys. Since replacement of steel by light alloys in the powertrain appears to be approaching saturation, the body shell and suspension must be considered, but for the latter application cast Al alloys do not possess enough strength and fatigue resistance. However, a relatively new technology, denominated thixocasting or semi-solid state (SSS) casting, is capable of ‘‘upgrading’’ the mechanical properties of cast products to the level of their forged counterparts. The main requirement for the success of SSS forming is the production of a non-dendritic, spheroidal microstructure, from which suitable viscosity and adequate flow behavior depends. There are four main techniques for raw material preparation, hereafter named microstructural conditioning: (i) mechanical stirring; (ii) electromagnetic stirring (magnetohydrodynamics, MHD); (iii) new rheocasting; and (iv) a thermomechanical treatment (TMT), involving plastic deformation and recrystallization. The first three conditioning modes have been satisfactorily described in the literature, from both
* Corresponding author. Tel.: +55 16 3351-8256; fax: +55 16 33615404; e-mail: [email protected] 1 Present address: CEFET (Federal Centre of Technological Education), Vito´ria (ES), Brazil.
a descriptive and a scientific point of view [2,3], but less attention has been given to the TMT process, which attains very favorable microstructural parameters by employing a very simple technology [4]. In the present work, electron backscatter diffraction (EBSD) and optical microscopy were employed to study the mechanism of spheroidal microstructure production by the TMT process, contrasting it with the MHD conditioning mode. The TMT-conditioned group of samples was produced by remelting a commercial A356 ingot. After degassing, the melt was Sr-modified, grain refined by Ti–B and poured into a steel mould with a 145 · 35 mm2 section and 250 mm height. The MHD group of samples was cut from a 100 mm diameter commercial ingot supplied by FORMCAST. The chemical composition of each group is given in Table 1 in wt.%, and it can be seen that differences are negligible. The TMT ingot was homogenized at 540 C for 24 h and water cooled. It was reheated to 300 C and deformed by rolling to 30% thickness reduction. The MHD material was already conditioned. In all experiments the SSS was attained by induction heating to 580 C samples taken from the ingot and from the deformed plate, a temperature which, as calculated by the Scheil equation, corresponds to a solid volume fraction (fs) equal to 0.5. Soaking times at that temperature varied between 0 and 10 min, and samples were water quenched.
1359-6462/$ - see front matter 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.07.046
1166
E. A. Vieira et al. / Scripta Materialia 57 (2007) 1165–1168
Table 1. Chemical composition of raw materials (wt.%)
TMT MHD
Si
Mg
Cu
Fe
7.13 6.60
0.40 0.34
0.01 –
0.12 0.15
Both optical microscopy and EBSD measurements were employed to characterize microstructure, general mesotexture and orientation of pairs of contiguous Ala particles. Grain boundaries were classified as low angle boundaries (LAB, <15) and high angle boundaries (HAB, >15). Figure 1 shows optical images of the initial microstructure of the TMT and MHD samples before partial remelting. It can be seen that the MHD ingot microstructure is constituted by small rosette-like dendrites. Although there are small differences between the ingot centre and its periphery, the former being coarser than the latter (the present micrograph shows the ingot centre), dendrites rapidly coalesce and the microstructure ends up by being quite uniform after reaching the SSS and being soaked at the chosen temperature. As for the as-rolled TTM sample, Figure 1b shows dendrites approximately 50 lm thick, making an angle of 24 with the rolling direction. The interdendritic distance before rolling was close to 67 lm, a figure compatible with the thickness reduction. By partial melting, small Al-a particles are produced, which rapidly coarsen and spheroidize. The formation of non-dendritic microstructures by mechanical stirring of semi-solid alloys has been studied by Doherty et al. [3], and it can be assumed that, if correct, their model would also apply to magnetically stirred semi-solid alloys. According to the model, dendrites would bend and fragment under the influence of shear forces. Bending is accomplished by dislocation generation, followed by their climb and alignment forming grain boundaries. When the misorientation (a) across these boundaries exceeds 20, it can be shown that the grain boundary energy is more then twice the solid– liquid energy. Therefore, fragmentation along the boundary will be energetically favorable and the dendrite debris disperse into the liquid where they growth and spheroidize. In contrast, TMT microstructural conditioning was assumed to be based simply on deformation–recrystallization processes, followed by liquid formation and penetration along the grain boundaries.
Figure 2 shows EBSD images of TMT and MHD samples in the as-received condition, that is, rolled plate and MHD ingot, and after 0 and 10 min in the SSS. In the MHD sample it is apparent that the rosettes exhibit a single orientation within, suggesting that the bending– fracturing mechanism acts on selected dendrite arms. This should leave groups of dendrites, or rosettes, dispersed in the liquid instead of individual debris as predicted [3]. For the TMT sample the pattern is completely different. In the first EBSD image it is apparent that within the dendrites a number of orientations exist; this indicates internal fragmentation, certainly due to the plastic deformation during rolling. The second and third micrographs show the sample as quenched from the semi-solid state. Penetration of the subgrains by the interdendritic liquid is complete and the isolated particles assume random orientation; their competitive growth is clearly shown by the third image. The frequency distribution of grain boundary angles is shown in Figure 3. It is important to take into account that for the as-received samples (solid state condition) measurements were carried out in the conventional way averaging a large area, while for the remaining samples (0, 3 and 10 min in the SSS) the misorientation concerns only pairs of contiguous particles. Analyzing the MHD material it can be seen that in the as-received condition approximately 60% of the boundaries are LAB and that, once in the SSS, their proportion increases to 70%, with the great majority consisting of boundaries with a < 5. This behavior supports the Doherty et al. model since dendrite separation when a > 20 is consistent with the observed increase of LAB proportion. After 3 min the material is well into the SSS and is composed by discrete Al-a particles (fs = 0.5) plus the liquid phase; hereafter random motion of the particles decreases the proportion of LAB. As for the TMT sample, the evolution of grain boundaries angles frequency is different in the sense that LAB are continuously replaced by HAB, except for the 3 min SSS sample (see the explanation below). It must be pointed out that according to the Mackenzie distribution, the probability of finding boundary angles smaller than 10 in a non-deformed cubic structure is 4% [5]. Figure 4 shows EBSD images of MHD and TMT individual dendrites in the as-received and deformed conditions, together with the respective internal misorientation, both from one extremity of the dendrite to the
Figure 1. Microstructures of A356 alloys produced by: (a) electromagnetic stirring and (b) thermomechanical treatment.
E. A. Vieira et al. / Scripta Materialia 57 (2007) 1165–1168
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Figure 2. EBSD images showing dendrites/grains orientation. First row: TMT samples; second row: MHD samples. First column: as-received samples; second column: partially remelted samples, soaked for 0 min at 580 C; third column: idem, soaked for 10 min at 580 C. The standard stereographic triangle (color key) is included. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Figure 3. Misorientation angle distribution. First row: MHD samples; second row: TMT samples. First column: as-received samples; second column: partially remelted samples soaked for 0 min at 580 C; third column: idem, soaked for 3 min; fourth column: idem, soaked for 10 min.
other (point to origin) and point to point. It must be recalled that the former material has already experienced the local melting–fragmentation process described in Ref. [3]. The following comments are relevant: (i) in practical terms, within any MHD dendrite a single crystallographic orientation exists; and (ii) the internal partitioning of the TMT dendrite is very clearly shown; the point-to-point misorientation measurements show the existence of at least 35 boundaries or subboundaries, all of them exhibiting a < 20; of these, only five or six have a > 10. The accumulated misorientation within
this dendrite is 55, which must be contrasted with the 6 or 7 of the MHD. Figure 5 is an optical micrograph of a TMT sample reheated to 570 C, thus very close to the A356 melting temperature, which is 557 C, and rapidly quenched. The time to reach the said temperature was 3 min and the soaking time was 0 min. It is apparent that the deformed dendrites are partitioned by what appears to be subgrain boundaries. Approximate measurements taken on this sample show that the number of internal intercepts met by a test-line of the same length as that of Figure 4b, that is, 260 lm, is close to five.
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Figure 4. Accumulated and point-to-point internal misorientation for: (a) MHD dendrite, as-received; and (b) TMT dendrite in the as-deformed condition. The standard stereographic triangle (color key) is included. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Figure 5. Optical micrograph of a TMT sample. After deformation by rolling, the material was heated to 570 C and immediately quenched.
Hence, the sequence of events leading to the formation of spheroidal microstructures in TMT-conditioned A356 alloys can be described as follows: (i) plastic deformation, giving a high number of LAB by internal partition of the dendrites; (ii) while heating to the SSS, the LAB are replaced by HAB, and their proportion decreases; (iii) after 3 min in the SSS, high energy boundaries begin to disintegrate, as predicted in Ref. [3], thus proportionally more LAB are left, hence their proportion increases; (iv) similarly to the MHD sample, 10 min in the SSS is sufficient to randomize the particles’ orientation, but still without reaching the Mackenzie distribution. The presence of two peaks in the orientation distribution of both the TMT and MHD samples characterizes a microstructure composed by isolated and bonded Al-a particles. Mechanism (i) above is the accepted model for deformation microstructures described by Hasen and coworkers [6]. In the sequence, the main events are dislocation climb and rearrangement, meaning that in TMTconditioned Al–Si alloys, the spheroidal microstructure
is a product of dislocation recovery rather than grain recrystallization. Semi-solid spheroidal microstructures can be produced by a number of techniques, including TMT and MHD. These two processes show some similarity in the sense that the plastic deformation of dendrites (by stirring in the semi-solid state for MHD and by rolling in the solid state for TMT) is followed by dislocation rearrangement when the alloy is heated to the SSS. Optical microscopy and EBSD observations of the TMT material showed that plastic deformation caused flattening and internal partition of the solidification dendrites. The evolution of the internal boundaries’ misorientation suggests a mechanism by which dislocation migration turns LAB into HAB, which are liable to be penetrated by the liquid phase, thus forming spheroidal Al-a particles. This mechanism is very similar to that proposed by Doherty et al. for the MHD process, although there are differences between the two techniques. [1] Mercer und HypoVereinsbank (Ed.) Automobiltechnologie (2001) 2010. [2] J.L. Jorstad, 8th Int. Conf. on Semi-solid Processing of Alloys and Composites, September 21–23, 2004, Limassol (Cyprus). [3] R.D. Doherty, H.I. Lee, E.A. Feest, Mat. Sci. Eng. 65 (1984) 181. [4] M.P. Kenney, J.A. Courtois, R.D. Evans, G.M. Farrior, C.P. Kyonka, A.A. Koch, K.P. Young, in Metals Handbook. Metals Park, OH, ASM International, 9th Ed., vol. 15 (1988) 327. [5] J.K. Mackenzie, Biometrika, vol. 45 (1958), 229. Quoted in V. Randle, O. Engler, Texture Analysis, Gordon & Breach Science Publishers (2000), p. 225. [6] B. Bay, N. Hansen, D.A. Hughes, D. Kuhlmann-Wilsdorf, Acta Metall. Mater. 40 (1992) 205.