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Size evolution of the unmelted phase during injection molding of semisolid magnesium alloys
Scripta Materialia 48 (2003) 327–331 www.actamat-journals.com
Size evolution of the unmelted phase during injection molding of semisolid magnesium alloys F. Czerwinski
*
Index Development Engineering, Husky Injection Molding Systems Ltd., 560 Queen St, Bolton, Ont., Canada L7E 5S5 Received 30 August 2002; received in revised form 30 August 2002; accepted 17 September 2002
Abstract A size of the unmelted phase within injection molded Mg–Al–Zn alloy depends on the solid/liquid ratio and the smallest particles are generated at very low liquid contents. For high liquid fractions the reduction of the solid content is mainly accompanied by a reduction in the number of particles, while their size is only slightly decreased. Ó 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Semisolid processing; Injection molding; Thixomolding; Magnesium alloys
1. Introduction The phenomena of thixotropy are described as viscosity–time relationships that appear in some fluids after changes in shear rate [1]. One of numerous technologies, utilizing the benefits of thixotropy, is the semisolid processing of metallic alloys. The key factor controlling both the flow of a slurry and product properties is the equiaxed, non-dendritic microstructure that behaves thixotropically and can be formed into a net shape. As a result, the major effort of all semisolid methods is focused on the generation of globular structures. Although there are a number of distinct semisolid routes, for a majority of them the origin of globular morphologies takes place during primary solidification. This is directly seen in rheoprocess-
* Corresponding author. Tel.: +1-905-951-5000; fax: +1-905951-5365. E-mail address: [email protected] (F. Czerwinski).
ing, represented by rheocasting or rheomolding, where the globular forms are created during cooling of the molten alloy under mechanical [2], ultrasound [3] or magnetic agitations [4]. Thixoprocessing, such as thixocasting or thixoforming, requires only partial melting, but the globular structures are formed during the previous step and retained after billet solidification and subsequent reheating [5]. More complex conditions exist during the one-step process of semisolid injection molding [6] where feedstock particulates of a specific nature are subjected to the simultaneous influence of the heat supplied from an external source and shear force imposed by the injection screw. While the fine dendritic structure governs the thermal decomposition of the rapidly solidified feedstock, the plastic deformation plays a key role in mechanically comminuted chips [7]. There is a distinction made in the literature between dendritic and globular features in terms of their influence on the flow characteristics of the semisolid slurry, with well-documented advantages
1359-6462/03/$ - see front matter Ó 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 6 2 ( 0 2 ) 0 0 4 5 4 - 2
328
F. Czerwinski / Scripta Materialia 48 (2003) 327–331
of the latter [2]. It is further anticipated that the size of globular solid particles plays a significant role in the permeability of the solid skeleton, the ability to flow in narrow channels and the residual porosity. In spite of its engineering importance, a description of controlling the size of the primary solid during semisolid processing in general, and injection molding in particular, is still missing. The objective of this experiment was, therefore, to reveal the morphological changes of the unmelted phase within Mg–9%Al–1%Zn alloy, injection molded in the semisolid state, to provide the preliminary data for an extensive and fundamental study.
2. Experimental details The research material was a mechanically chipped magnesium alloy with a nominal composition of 8.5% Al, 0.75% Zn, 0.3% Mn, 0.01% Si, 0.01% Cu, 0.001% Ni, 0.001% Fe and an Mgbalance which corresponds to the AZ91D grade. A Husky TXM500-M70 prototype system with a clamp force of 500 tones and equipped with various molds, was used for injection molding experiments. The machine barrel had a diameter of 70 mm, a length of approximately 2 m and its temperature profile was maintained by a number of electric resistance heater bands, grouped into several independently controlled zones. The alloy was injected at a screw velocity in the range of 0.7–2.8 m/s. For parts with a weight of between 250 and 520 g, the typical cycle time was between 25 and 40 s, which corresponds to an average residency time of the alloy within the machine barrel of the order of 100–300 s. To examine the very initial stages of thermal decomposition, chipped feedstock was immersed into the semisolid slurry of the same alloy beyond the influence of the injection screw. To preserve the morphology and content of the unmelted phase which existed at high temperatures, the alloy was quenched by contact with a cold metal. Surface preparation consisted of grinding with progressively finer SiC paper, mechanical polishing with 1 lm diamond paste and colloidal alumina, followed by etching in a 1% solution of nitric acid in ethanol. Stereological analysis was
conducted using optical microscopy, equipped with an Omnimet Enterprise quantitative image analyzer. In order to avoid errors related to insufficient image contrast, stereological measurements were conducted in a semiautomatic way where the features were manually selected.
3. Results 3.1. General microstructure A general view of the microstructure of injection molded Mg–9%Al–1%Zn alloy is shown in Fig. 1a–d. The major difference between individual micrographs is expressed by the relative contribution of two constituents: the unmelted phase and matrix. The image depicted in Fig. 1a (22% liquid) is typical for semisolid extrusion molding, conducted at relatively low temperatures of the solidus–liquidus range to achieve ultra-high solid contents. The microstructures obtained at higher temperatures, corresponding to 40–95% liquid, are a characteristic of thixomolding. Examples for 49.8%, 87.1% and 95.2% liquid are shown in Fig. 1b–d, respectively. For all four solid contents, the particles were roughly globular with some differences in their internal structure, already detected under low magnifications of the optical microscope. The internal inhomogeneity of the primary solid is caused by chemical segregation, which is typical for magnesium alloys. During reheating, the regions of higher Al and Zn contents led to the solidstate precipitation of Mg17 Al12 intermetallic or, at higher temperatures, the formation of socalled ‘‘entrapped liquid’’ pools as a result of partial melting [8]. After solidification, the latter are represented by dark-contrast islands (Fig. 1b and c). The particles, accompanied by low liquid volume exhibit numerous, and fine precipitates. This is in contrast to particles surrounded by higher volumes of the former liquid which contain less precipitates with a generally larger size. 3.2. Size analysis of the unmelted phase Statistical analysis of the size of primary solid particles was conducted for a liquid content
F. Czerwinski / Scripta Materialia 48 (2003) 327–331
329
Fig. 1. Exemplary microstructures of Mg–9%Al–1%Zn alloy injection molded at various fractions of the liquid phase: (a) 22%; (b) 49.8%; (c) 87.1% and (d) 95.2%.
between 22% and 95.2%. Each data point in Fig. 2 was collected during a separate experiment, performed under individual settings of the injection molding system. For a comparison, a data point for 0% liquid is included as well. This represents
Solid Particle Diameter, um
80 Processing: TXM500-M70 System Alloy: Mg-9%Al-1%Zn
70 60 50 40 30 20 0
10
20
30
40
50
60
70
80
90 100
Liquid Fraction, %
Fig. 2. The solid particle size, plotted as a function of the liquid fraction within Mg–9%Al–1%Zn slurry.
the grain size of the fully recrystallized structure of the mechanically comminuted chip, directly before melting. The recrystallization took place when chips were immersed in the semisolid slurry of the same magnesium alloy. Microstructural details of the thermal decomposition of mechanically chipped Mg–9%Al–1%Zn alloy were described previously [7]. The plot in Fig. 2 reveals that the solid particle size at low liquid fractions, which represents the beginning of melting, is comparable to the recrystallized grain of the chipped feedstock. In increasing the liquid content, the size of particles increases rapidly, reaching the value of 70–80 lm for 60–70% of liquid. It is of interest to note that further increase in the processing temperature caused a rather small reduction in the particle size. As a result, the solid particles in almost complete liquid alloy (95% liquid) were comparable to those present in the slurry with a liquid content of 45%. Additional information, regarding the size evolution of solid particles within the slurry, is provided by the particle-size distribution. Exemplary
330
F. Czerwinski / Scripta Materialia 48 (2003) 327–331 35 Mg-9%Al-1%Zn
Frequency, %
30 25 Liquid fractions: 22% 49.80% 95.20% 0%
20 15 10 5 0 0
20
40
60
80
100
120
140
160
Solid Particle Diameter, um
Fig. 3. The histograms of solid particle diameters for selected liquid fractions within Mg–9%Al–1%Zn alloy. The data point for 0% liquid represents the recrystallized structure of the chipped feedstock, directly before melting.
histograms are shown in Fig. 3. The particles within the slurry with the lowest liquid content tested, i.e. 22%, exhibited a sharp maximum around 30 lm. The distribution profile was generally similar to that of the recrystallized grains within chips and the major difference was expressed by a 10% fraction of grains above 70 lm in the recrystallized structure. The slurry with a content of 50% liquid exhibited a broad scatter of particle sizes between 20 lm and 150 lm. As seen from the histogram for 95.2% liquid, particles with a size of 150 lm were still present at the end of melting. The lower average size of the particles within the alloy slurry having 95.2% liquid resulted from the higher fraction of smaller particles, generally of the order of 40 lm.
4. Discussion of results Although there are many features which characterize primary solid particles, the major attention of this experiment was focused on their size. The stereological examinations revealed that the relationship between the size of the unmelted phase and its volume fraction is complex. As described previously [7], the initial morphology of the semisolid slurry, which is formed during injection molding, is of an equiaxed nature. Such
a structure is created by recrystallization of the plastically deformed feedstock, and its further disintegration by grain boundary melting. As can be deduced from Fig. 1a, at ultra-high solid contents, the alloy represents a deformable, semicohesive granular solid, saturated with liquid. The solid grains are partially interconnected by unwetted grain boundaries and liquid fills the intergranular spaces. When subjected to external stress, resulting from the screw action or flow under high pressure into the mold cavity, the alloy will respond by disagglomeration of partially bonded grains. The macroscopic deformation of such structures is accommodated by: (i) elastic–plastic deformation at grain contacts, (ii) bulk grain deformation or fragmentation and (iii) grain rearrangement by sliding or rolling [9]. A comparison of the grain size for 0% and 22% liquid (Fig. 2), shows no evidence of substantial refining. Since the response of the alloy also includes structure agglomeration due to bond formation among particles caused by impingement and reaction [10], the external stress effect should be interpreted as a potential for prevention of particle coarsening. In addition to structure breakdown and agglomeration caused by external strain, the reduction of interfacial energy between the particles and liquid provides the driving force for morphological and dimensional changes [11]. Two potential contributors include coalescence and Ostwald ripening. Coalescence is usually defined as the nearly instantaneous formation of one large particle upon contact of two smaller ones [12]. Ostwald ripening is governed by the Gibbs–Thompson effect which alters the concentration at the particle-matrix interface, depending on the curvature of the interface, thus creating the concentration gradient for the diffusional transport of the material [12]. Both mechanisms are superimposed on alloyÕs melting. The preferential melting of some particles mainly due to their smaller size is a possibility. The contribution of the latter is, however, limited here due to a very short processing times. Particle coarsening, particularly intense between 20% and 50% of the liquid fraction, indicates that the effectiveness of the mechanism which preserved the particle size is diminished by increasing liquid content. It seems that the particle coarsening seen in Fig. 2 should be
F. Czerwinski / Scripta Materialia 48 (2003) 327–331
understood as a competition between the shear and coalescence effects. Shear forces between particles, which may be carried only by the solid phase [9], are reduced since they can easily separate within the surrounding liquid. At the same time, the interparticle distances are short, so particles can meet each other to coalesce. The plot of particle size in Fig. 2 indicates that there is a critical fraction of liquid at which the particle size starts to decline. It is surprising that the solid size does not significantly reduce during advanced stages of melting. This is so, despite the dramatic reduction in the solid content. The concept of competition outlined above, explains these changes. The coalescence is less active, because at higher liquid contents, the probability that particles will come in contact with each other, is reduced. On the other hand, the role of Ostwald ripening could be greater because of faster diffusion at high temperatures. Since the particle size affects the slurry flow mode and properties of the final component, its control is of engineering importance. The results of this study indicate that the key factor in this process is the initial particle size, inherited from the feedstock. Practically, the role of screw may have some contribution only at very high solid contents. While the liquid fraction is higher, the influence of the injection screw disappears.
5. Conclusions The size of the unmelted phase within Mg– 9%Al–1%Zn alloy depends on the liquid fraction within the semisolid slurry. The initially small particles, being of the same order as the grain of the recrystallized feedstock before melting, expe-
331
rienced significant coarsening in the range of the liquid fraction between 20% and 50%. Although further increase in the liquid content caused some particle refinement, changes were significantly lower than that expected from the reduction in a solid volume fraction. It is postulated that at very high solid fractions, the particle–particle interaction resulting from the external shear is the effective mechanism which preserves the fine structure inherited from the recrystallized feedstock. The effectiveness of this mechanism is reduced by the presence of higher volume fractions of the liquid alloy. At higher liquid fractions the mechanisms of coalescence, Ostwald ripening or selected melting control the size of solid particles.
References [1] Schalek E, Szegvari A. Koll Z 1923;32:318–26. [2] Spencer DB, Mehrabian R, Flemings MC. Metall Trans 1972;3A:1925. [3] Genma N, Soejima T, Fukumoto H, Ayata K. ISIJ Int 1988;29:1056. [4] Abramov VO, Abramov OV, Straumal BB, Gust W. Mater Des 1997;18:323. [5] Fehlbier M, Aguilar J, Sahm PR. Int J Cast Metals Res 2001;14:71. [6] Decker RF, Carnahan RP, Vining R, Eldener E. Proceedings 4th International Conference on Semisolid Processing of Alloys and Composites, Sheffield, 1996. p. 221. [7] Czerwinski F. Acta Mater 2002;50:3265. [8] Czerwinski F, Zielinska-Lipiec A, Pinet PJ, Overbeeke J. Acta Mater 2001;49:1225. [9] Tzimas E, Zavaliangos A. Acta Mater 1999;47:517. [10] Chen CP, Tsao CA. Acta Mater 1998;45:1955. [11] Cahn RW. Physical metallurgy. 4th ed. Amsterdam: North-Holland; 1966. [12] Wolfsdorf-Brenner TL, Voorhees PW, Sutliff J. Metall Mater Trans 1999;30:1955.
Size evolution of the unmelted phase during injection molding of semisolid magnesium alloys F. Czerwinski
*
Index Development Engineering, Husky Injection Molding Systems Ltd., 560 Queen St, Bolton, Ont., Canada L7E 5S5 Received 30 August 2002; received in revised form 30 August 2002; accepted 17 September 2002
Abstract A size of the unmelted phase within injection molded Mg–Al–Zn alloy depends on the solid/liquid ratio and the smallest particles are generated at very low liquid contents. For high liquid fractions the reduction of the solid content is mainly accompanied by a reduction in the number of particles, while their size is only slightly decreased. Ó 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Semisolid processing; Injection molding; Thixomolding; Magnesium alloys
1. Introduction The phenomena of thixotropy are described as viscosity–time relationships that appear in some fluids after changes in shear rate [1]. One of numerous technologies, utilizing the benefits of thixotropy, is the semisolid processing of metallic alloys. The key factor controlling both the flow of a slurry and product properties is the equiaxed, non-dendritic microstructure that behaves thixotropically and can be formed into a net shape. As a result, the major effort of all semisolid methods is focused on the generation of globular structures. Although there are a number of distinct semisolid routes, for a majority of them the origin of globular morphologies takes place during primary solidification. This is directly seen in rheoprocess-
* Corresponding author. Tel.: +1-905-951-5000; fax: +1-905951-5365. E-mail address: [email protected] (F. Czerwinski).
ing, represented by rheocasting or rheomolding, where the globular forms are created during cooling of the molten alloy under mechanical [2], ultrasound [3] or magnetic agitations [4]. Thixoprocessing, such as thixocasting or thixoforming, requires only partial melting, but the globular structures are formed during the previous step and retained after billet solidification and subsequent reheating [5]. More complex conditions exist during the one-step process of semisolid injection molding [6] where feedstock particulates of a specific nature are subjected to the simultaneous influence of the heat supplied from an external source and shear force imposed by the injection screw. While the fine dendritic structure governs the thermal decomposition of the rapidly solidified feedstock, the plastic deformation plays a key role in mechanically comminuted chips [7]. There is a distinction made in the literature between dendritic and globular features in terms of their influence on the flow characteristics of the semisolid slurry, with well-documented advantages
1359-6462/03/$ - see front matter Ó 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 6 2 ( 0 2 ) 0 0 4 5 4 - 2
328
F. Czerwinski / Scripta Materialia 48 (2003) 327–331
of the latter [2]. It is further anticipated that the size of globular solid particles plays a significant role in the permeability of the solid skeleton, the ability to flow in narrow channels and the residual porosity. In spite of its engineering importance, a description of controlling the size of the primary solid during semisolid processing in general, and injection molding in particular, is still missing. The objective of this experiment was, therefore, to reveal the morphological changes of the unmelted phase within Mg–9%Al–1%Zn alloy, injection molded in the semisolid state, to provide the preliminary data for an extensive and fundamental study.
2. Experimental details The research material was a mechanically chipped magnesium alloy with a nominal composition of 8.5% Al, 0.75% Zn, 0.3% Mn, 0.01% Si, 0.01% Cu, 0.001% Ni, 0.001% Fe and an Mgbalance which corresponds to the AZ91D grade. A Husky TXM500-M70 prototype system with a clamp force of 500 tones and equipped with various molds, was used for injection molding experiments. The machine barrel had a diameter of 70 mm, a length of approximately 2 m and its temperature profile was maintained by a number of electric resistance heater bands, grouped into several independently controlled zones. The alloy was injected at a screw velocity in the range of 0.7–2.8 m/s. For parts with a weight of between 250 and 520 g, the typical cycle time was between 25 and 40 s, which corresponds to an average residency time of the alloy within the machine barrel of the order of 100–300 s. To examine the very initial stages of thermal decomposition, chipped feedstock was immersed into the semisolid slurry of the same alloy beyond the influence of the injection screw. To preserve the morphology and content of the unmelted phase which existed at high temperatures, the alloy was quenched by contact with a cold metal. Surface preparation consisted of grinding with progressively finer SiC paper, mechanical polishing with 1 lm diamond paste and colloidal alumina, followed by etching in a 1% solution of nitric acid in ethanol. Stereological analysis was
conducted using optical microscopy, equipped with an Omnimet Enterprise quantitative image analyzer. In order to avoid errors related to insufficient image contrast, stereological measurements were conducted in a semiautomatic way where the features were manually selected.
3. Results 3.1. General microstructure A general view of the microstructure of injection molded Mg–9%Al–1%Zn alloy is shown in Fig. 1a–d. The major difference between individual micrographs is expressed by the relative contribution of two constituents: the unmelted phase and matrix. The image depicted in Fig. 1a (22% liquid) is typical for semisolid extrusion molding, conducted at relatively low temperatures of the solidus–liquidus range to achieve ultra-high solid contents. The microstructures obtained at higher temperatures, corresponding to 40–95% liquid, are a characteristic of thixomolding. Examples for 49.8%, 87.1% and 95.2% liquid are shown in Fig. 1b–d, respectively. For all four solid contents, the particles were roughly globular with some differences in their internal structure, already detected under low magnifications of the optical microscope. The internal inhomogeneity of the primary solid is caused by chemical segregation, which is typical for magnesium alloys. During reheating, the regions of higher Al and Zn contents led to the solidstate precipitation of Mg17 Al12 intermetallic or, at higher temperatures, the formation of socalled ‘‘entrapped liquid’’ pools as a result of partial melting [8]. After solidification, the latter are represented by dark-contrast islands (Fig. 1b and c). The particles, accompanied by low liquid volume exhibit numerous, and fine precipitates. This is in contrast to particles surrounded by higher volumes of the former liquid which contain less precipitates with a generally larger size. 3.2. Size analysis of the unmelted phase Statistical analysis of the size of primary solid particles was conducted for a liquid content
F. Czerwinski / Scripta Materialia 48 (2003) 327–331
329
Fig. 1. Exemplary microstructures of Mg–9%Al–1%Zn alloy injection molded at various fractions of the liquid phase: (a) 22%; (b) 49.8%; (c) 87.1% and (d) 95.2%.
between 22% and 95.2%. Each data point in Fig. 2 was collected during a separate experiment, performed under individual settings of the injection molding system. For a comparison, a data point for 0% liquid is included as well. This represents
Solid Particle Diameter, um
80 Processing: TXM500-M70 System Alloy: Mg-9%Al-1%Zn
70 60 50 40 30 20 0
10
20
30
40
50
60
70
80
90 100
Liquid Fraction, %
Fig. 2. The solid particle size, plotted as a function of the liquid fraction within Mg–9%Al–1%Zn slurry.
the grain size of the fully recrystallized structure of the mechanically comminuted chip, directly before melting. The recrystallization took place when chips were immersed in the semisolid slurry of the same magnesium alloy. Microstructural details of the thermal decomposition of mechanically chipped Mg–9%Al–1%Zn alloy were described previously [7]. The plot in Fig. 2 reveals that the solid particle size at low liquid fractions, which represents the beginning of melting, is comparable to the recrystallized grain of the chipped feedstock. In increasing the liquid content, the size of particles increases rapidly, reaching the value of 70–80 lm for 60–70% of liquid. It is of interest to note that further increase in the processing temperature caused a rather small reduction in the particle size. As a result, the solid particles in almost complete liquid alloy (95% liquid) were comparable to those present in the slurry with a liquid content of 45%. Additional information, regarding the size evolution of solid particles within the slurry, is provided by the particle-size distribution. Exemplary
330
F. Czerwinski / Scripta Materialia 48 (2003) 327–331 35 Mg-9%Al-1%Zn
Frequency, %
30 25 Liquid fractions: 22% 49.80% 95.20% 0%
20 15 10 5 0 0
20
40
60
80
100
120
140
160
Solid Particle Diameter, um
Fig. 3. The histograms of solid particle diameters for selected liquid fractions within Mg–9%Al–1%Zn alloy. The data point for 0% liquid represents the recrystallized structure of the chipped feedstock, directly before melting.
histograms are shown in Fig. 3. The particles within the slurry with the lowest liquid content tested, i.e. 22%, exhibited a sharp maximum around 30 lm. The distribution profile was generally similar to that of the recrystallized grains within chips and the major difference was expressed by a 10% fraction of grains above 70 lm in the recrystallized structure. The slurry with a content of 50% liquid exhibited a broad scatter of particle sizes between 20 lm and 150 lm. As seen from the histogram for 95.2% liquid, particles with a size of 150 lm were still present at the end of melting. The lower average size of the particles within the alloy slurry having 95.2% liquid resulted from the higher fraction of smaller particles, generally of the order of 40 lm.
4. Discussion of results Although there are many features which characterize primary solid particles, the major attention of this experiment was focused on their size. The stereological examinations revealed that the relationship between the size of the unmelted phase and its volume fraction is complex. As described previously [7], the initial morphology of the semisolid slurry, which is formed during injection molding, is of an equiaxed nature. Such
a structure is created by recrystallization of the plastically deformed feedstock, and its further disintegration by grain boundary melting. As can be deduced from Fig. 1a, at ultra-high solid contents, the alloy represents a deformable, semicohesive granular solid, saturated with liquid. The solid grains are partially interconnected by unwetted grain boundaries and liquid fills the intergranular spaces. When subjected to external stress, resulting from the screw action or flow under high pressure into the mold cavity, the alloy will respond by disagglomeration of partially bonded grains. The macroscopic deformation of such structures is accommodated by: (i) elastic–plastic deformation at grain contacts, (ii) bulk grain deformation or fragmentation and (iii) grain rearrangement by sliding or rolling [9]. A comparison of the grain size for 0% and 22% liquid (Fig. 2), shows no evidence of substantial refining. Since the response of the alloy also includes structure agglomeration due to bond formation among particles caused by impingement and reaction [10], the external stress effect should be interpreted as a potential for prevention of particle coarsening. In addition to structure breakdown and agglomeration caused by external strain, the reduction of interfacial energy between the particles and liquid provides the driving force for morphological and dimensional changes [11]. Two potential contributors include coalescence and Ostwald ripening. Coalescence is usually defined as the nearly instantaneous formation of one large particle upon contact of two smaller ones [12]. Ostwald ripening is governed by the Gibbs–Thompson effect which alters the concentration at the particle-matrix interface, depending on the curvature of the interface, thus creating the concentration gradient for the diffusional transport of the material [12]. Both mechanisms are superimposed on alloyÕs melting. The preferential melting of some particles mainly due to their smaller size is a possibility. The contribution of the latter is, however, limited here due to a very short processing times. Particle coarsening, particularly intense between 20% and 50% of the liquid fraction, indicates that the effectiveness of the mechanism which preserved the particle size is diminished by increasing liquid content. It seems that the particle coarsening seen in Fig. 2 should be
F. Czerwinski / Scripta Materialia 48 (2003) 327–331
understood as a competition between the shear and coalescence effects. Shear forces between particles, which may be carried only by the solid phase [9], are reduced since they can easily separate within the surrounding liquid. At the same time, the interparticle distances are short, so particles can meet each other to coalesce. The plot of particle size in Fig. 2 indicates that there is a critical fraction of liquid at which the particle size starts to decline. It is surprising that the solid size does not significantly reduce during advanced stages of melting. This is so, despite the dramatic reduction in the solid content. The concept of competition outlined above, explains these changes. The coalescence is less active, because at higher liquid contents, the probability that particles will come in contact with each other, is reduced. On the other hand, the role of Ostwald ripening could be greater because of faster diffusion at high temperatures. Since the particle size affects the slurry flow mode and properties of the final component, its control is of engineering importance. The results of this study indicate that the key factor in this process is the initial particle size, inherited from the feedstock. Practically, the role of screw may have some contribution only at very high solid contents. While the liquid fraction is higher, the influence of the injection screw disappears.
5. Conclusions The size of the unmelted phase within Mg– 9%Al–1%Zn alloy depends on the liquid fraction within the semisolid slurry. The initially small particles, being of the same order as the grain of the recrystallized feedstock before melting, expe-
331
rienced significant coarsening in the range of the liquid fraction between 20% and 50%. Although further increase in the liquid content caused some particle refinement, changes were significantly lower than that expected from the reduction in a solid volume fraction. It is postulated that at very high solid fractions, the particle–particle interaction resulting from the external shear is the effective mechanism which preserves the fine structure inherited from the recrystallized feedstock. The effectiveness of this mechanism is reduced by the presence of higher volume fractions of the liquid alloy. At higher liquid fractions the mechanisms of coalescence, Ostwald ripening or selected melting control the size of solid particles.
References [1] Schalek E, Szegvari A. Koll Z 1923;32:318–26. [2] Spencer DB, Mehrabian R, Flemings MC. Metall Trans 1972;3A:1925. [3] Genma N, Soejima T, Fukumoto H, Ayata K. ISIJ Int 1988;29:1056. [4] Abramov VO, Abramov OV, Straumal BB, Gust W. Mater Des 1997;18:323. [5] Fehlbier M, Aguilar J, Sahm PR. Int J Cast Metals Res 2001;14:71. [6] Decker RF, Carnahan RP, Vining R, Eldener E. Proceedings 4th International Conference on Semisolid Processing of Alloys and Composites, Sheffield, 1996. p. 221. [7] Czerwinski F. Acta Mater 2002;50:3265. [8] Czerwinski F, Zielinska-Lipiec A, Pinet PJ, Overbeeke J. Acta Mater 2001;49:1225. [9] Tzimas E, Zavaliangos A. Acta Mater 1999;47:517. [10] Chen CP, Tsao CA. Acta Mater 1998;45:1955. [11] Cahn RW. Physical metallurgy. 4th ed. Amsterdam: North-Holland; 1966. [12] Wolfsdorf-Brenner TL, Voorhees PW, Sutliff J. Metall Mater Trans 1999;30:1955.