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Effects of processing parameters on microstructure of thixoformed ZA27 alloy
Materials & Design Materials and Design 28 (2007) 1279–1287 www.elsevier.com/locate/matdes
Effects of processing parameters on microstructure of thixoformed ZA27 alloy T.J. Chen *, Y. Hao, Y.D. Li State Key Laboratory of Advanced Non-ferrous Materials, Lanzhou University of Technology, 287# Langongping Road, Lanzhou 730050, PR China Received 24 January 2005; accepted 15 December 2005 Available online 13 March 2006
Abstract In this paper, the effects of processing parameters on microstructures of thixoformed ZA27 alloy cylindrical rods were investigated. The results indicated that the microstructure of the formed rod was basically uniform except that of the section near the inner gate. But the detailed microstructure observation or quantitative examination showed that the primary particle fraction slightly decreased along the axial direction but it relatively more obviously decreased along the radial direction. The primary particle size increased and the primary particle fraction decreased with the increase of reheating duration. Both the size and fraction of primary particles decreased with elevating the reheating temperature and increased with increasing the die temperature. The secondary solidification began with the direct growth of the secondary primary phase on the surface of the primary a 0 particles and completed by forming lamellar eutectics. The secondarily solidified structure was significantly affected by die temperature while the other two parameters, reheating temperature and reheating time had no obvious effect. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Non-ferrous metals and alloys; Forming; Microstructure
1. Introduction Thixoforming is a novel metal processing technology which combines the elements of both casting and forging. It offers significant advantages, such as reducing macrosegregation and porosity and lowering forming efforts [1]. It generally comprises three steps, production of non-dendritic ingot, reheating and forming. In recent years, much effort has been made to simplify this three-step process to two steps, or even to one-step [2–6]. Some investigations integrated the former two steps, the non-dendritic material production and reheating, into one step through reheating traditionally casting alloy ingot with fine dendrites [2,3]. The starting ingot with fine dendrites can be produced by adding refiner prior to pouring, or by chilling upon solidification. This technology for production of non-dendritic ingot (or slurry) was termed semi-solid thermal transfor*
Corresponding author. Tel.: +86 931 2806921; fax: +86 931 2755806. E-mail address: [email protected] (T.J. Chen).
0261-3069/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2005.12.010
mation (SSTT) [6]. The primary particles in the resulting semi-solid microstructure are finer, more spherical and more uniform than those produced by the magnetohydrodynamic stirring, the most common method in the threestep process in practice [1,3]. The others developed a single integrated machine to produce magnesium alloy components. But so far, this technology, named thixomoulding, is limited to relatively low solid fraction and magnesium alloys for thin wall components due to relatively low pressure capability of the machine [5,7]. Therefore, thixoforming with SSTT is useful in metal processing method and has much potential for future use. It is well known that the performance of a final component is determined by its microstructure, and that the microstructure is determined by processing technology. Thus, for the thixoforming, it must be firstly verified the effects of processing parameters on its microstructure in order to understand the effect of microstructure on mechanical properties of an alloy. However, previous studies have been mainly focused on microstructural evolution
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during partial remelting and subsequent isothermal holding [2,3,5,6,8–10]. Only a few investigations have involved microstructures of thixoformed alloys [4,7,11]. Therefore, in the present work, the effects of processing parameters, such as reheating temperature, reheating duration and die temperature on microstructure of a Zr modified ZA27 alloy thixoformed with SSTT were studied. 2. Experimental procedure The nominal compositions of ZA27 alloy used for this work were 26–28%Al, 1.7–2.0%Cu, 0.2%Zr, 0.02– 0.04%Mg (in wt.), with a balance of Zn. The role of Zr was to achieve a microstructure with fine dendrites through modification [3]. A quantity of the alloy was melted, then degassed by using C2Cl6. Pouring followed at 550 °C into permanent mould with ambient temperature to form rods 190 mm long and 45 mm in diameter. Some small ingots, 50 mm long, were cut from these rods, as the starting ingots for thixoforming. The small ingots were heated in an electric resistance furnace at various temperatures of 465 °C, 470 °C, 473 °C, 475 °C, 480 °C and 485 °C, respectively, for different periods ranging from 65 min to120 min, and then transported to a die to be pressed using a 40 ton pressure. The temperature fluctuation was kept within ±1 °C by a temperature controller
D80. Three die temperatures were employed, 250 °C, 300 °C and 350 °C. The die for this procedure is shown in Fig. 1. The thixoformed product is schematically shown in Fig. 2, which also clearly reflects the dimensions of the die cavity. The rod part with 165 mm long was the desired formed output product. The thixoformed products were sawed into two parts along their central lines, and one was divided into nine small samples according to the dimensions shown in Fig. 2. Starting from the left, the small samples were numbered 1–9. The cross-sections of these small samples were ground and polished. Metallographic observations were carried out using a back-scattered electron imaging system in an S-520 scanning electron microscope (SEM) and an optical microscope Mef-3 (OM). Quantitative examinations on the microstructures were done using an automatic quantitative metallography analytical system in OM. 3. Results and discussion 3.1. Effects of processing parameters on microstructures of thixoformed rods 3.1.1. Effect of reheating duration Fig. 3 shows some typical micrographs of the different sections in the rods thixoformed at reheating temperature
Fig. 1. Photos of die used in this work.
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Fig. 2. Schematic for thixoformed parts.
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Fig. 3. OM micrographs of the different positions in the rods thixoformed at the reheating temperature of 470 °C heated for different durations (die temperature: 300 °C): (a) 70 min, (b) 90 min and (c) 120 min.
of 470 °C heated for different times. It can be seen that except the part near the inner gate in the section No. 1, the microstructure along the axial direction was basically uniform. The primary particle size slightly increased while the particle fraction decreased with prolonging the reheating duration. Simultaneously, the segregation of former liquid phase appeared, and its size and amount increased as the time increased. After heated for 120 min, the longest duration applied in this work, large size segregation could be observed near the inner gate (Fig. 3(c)). The result of quantitative image examination shown in Fig. 4 also supports the phenomena mentioned above. It must be noted that the primary particle fraction was the average value of the eight sections except the No. 1 section. In each section, two images (edge and center) were analyzed. It can be seen that the particle fraction decreased obviously during reheating from 70 min to 75 min, and
then decreased slowly as the time further increased (Fig. 4(a)). The particle size also decreased during reheating from 70 min to 75 min, but then increased (Fig. 4(b)). In fact, the primary particle coarsening from Ostwald ripening and coalescence and the size reducing from the partial melting of the particles were in a competitive condition before the solid/liquid equilibrium state was attained during reheating [8]. Temperature changes in the center of the ingot with the reheating time indicated that the semi-solid system was up to its thermal equilibrium state between the solid and liquid phases till the reheating duration was over 170 min (Fig. 5). But the longest reheating duration used in this work was 120 min. So this solid– liquid equilibrium state was not achieved. The obvious decrease in primary particle fraction at reheating from 70 min to 75 min indicated that the partial melting of the particles was extensive, which resulted in an obvious
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(a)
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Fig. 5. Temperature changes in the ingots during reheating at 470 °C. Showing reheating process was divided into five stages: I was a rapid heating stage; II a solid phase transformation stage, in which transformations (a + g + e ! b) occurred at about 285 °C; III another heating stage; IV partial remelting stage (started from 385 °C), and V isothermal holding stage.
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Heating time/min Fig. 4. Variations of primary particle (a) fraction and (b) size in the thixoformed rods with reheating duration (reheating temperature: 470 °C, die temperature: 300 °C).
reduction in size of the particles. So the particle reducing at this stage was superior to the particle coarsening. But after this stage, the particle reducing was inferior to the particle coarsening, so the resulting particle size increased with the time although the particle fraction decreased. Due to the characteristics of the resistance furnace heating method, the surface of the ingot was first heated and the heat then gradually transferred toward the center. Thus, the microstructural evolution near the surface was faster than that in the inner part. In addition, the ingot’s gravity was equivalent to a pressure being exerted on its top surface. Under this pressure, the liquid phase in the inner part would flow toward the radial edge while the primary particles would move towards each other in axial direction (the axial line of the cylindrical ingot was perpendicular to ground during reheating) [12,13]. Simultaneously, the Al-rich primary particles moved toward the top surface and the zinc-rich liquid phase moved toward the bottom due to the difference in densities between the primary particles and the liquid phase. All of these resulted in the ununiform microstructure of the semi-solid ingot: the size and fraction of the primary particles in the inner section or near the top surface were generally larger than those near the edge or the bottom, respectively. Fig. 6
shows that both the size and fraction in the center part was obviously larger than those in the bottom edge. This ununiform microstructure of the semi-solid ingot must result in the inhomogeneous microstructure of the thixoformed rod. It must be indicated that this microstructure inhomogeneity of the semi-solid ingot would be also aggravated by coarsening during reheating, e.g., the relatively larger particles in the inner part of the ingot might partially attributed to the relatively more intensive Ostwald ripening and coalescence because of the shorter diffusion distance and higher frequency of primary particle contact or meeting each other resulted from the relatively higher solid fraction [14,15]. Furthermore, the segregation degree of the heated ingot microstructure would increase with increasing the heating duration, thus the segregation of the resulting thixoformed microstructure would also gradually become serious. In addition, the pressure exerted on the top surface of the semi-solid ingot by the pressure head of the pressure machine during the subsequent thixoforming, similar to the pressure from the gravity, also caused the segregation [12,13]. It is remarkable that the speed of the pressure head was quite slow, about 0.2 m/s, which could aggravate the segregation [16]. It can be found that the reheating time or the time for the ingot to achieve the thermal equilibrium state was very long. Except the heat transfer characteristics of resistance furnace, the other reason why the time was so long is that some phase transformations occurring during reheating need much heat. It can be expected that the main transformation is the partial melting of solid metal. Fig. 5 indicates that another endothermic reaction (a + g + e ! b) also occurred. Furthermore, the speed of heat conduction decreased with the increase and homogenization of the temperature in the ingot because the temperature gradient
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Fig. 6. OM micrographs of: (a) the inner part and (b) the bottom edge of ingot reheated for 90 min at 470 and then water-quenched.
between the furnace chamber and the ingot or within the ingot decreased. Especially, the speed was very slow at the final reheating stage (Fig. 5). 3.1.2. Effect of reheating temperature Fig. 7 gives the variations of the fraction and size of the primary particles in the rods with reheating temperature. It can be seen that both the primary particle fraction and size slightly decreased as the temperature elevated. The result of
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micrograph observation revealed that the size of liquid pools entrapped within the primary particles increased with elevating the temperature (comparing Fig. 8(a) and (b)). As the reheating temperature elevated, the amount of the liquid in the microstructure of the heated ingot prior to forming must increase at a given heating time, and thus the degree of particle reducing due to partial remelting would also increase. In addition, during reheating at high temperature the time for soaking at low semi-solid temperature interval relatively decreased due to rapid heating speed from the large temperature gradient between the ingot and the furnace chamber. This could also decrease the particles size due to relatively short time for coarsening, especially for the coalescence at high solid fraction state. Furthermore, the coarsening of the liquid pools within the primary particles, controlled by diffusion, would also be accelerated with the increase of the temperature, resulting in the increase in size and decrease in number (Fig. 8). In order to evaluate the thixoformed microstructures along the radical direction, the microstructure in the edge of the rod formed at reheating temperature of 475 °C heated for 75 min was observed and shown in Fig. 9. Simultaneously, the micrographs along the center line were also presented for comparison. It can be seen that the primary particle fraction at the edge was lower than that at the center. The largest difference in the primary particle fraction was in the part near the inner gate. The quantitative examination given by Fig. 10 clearly shows the difference. It also indicates that the microstructural uniformity along the edge was inferior to that along the central line. No obvious difference in particle size between the edge and center was observed. From discussion in Section 3.1.1, it can be known that the microstructure of the semi-solid ingot prior to forming was relatively inhomogeneous, especially the primary particle fraction near the edge was lower than that in the inner section. And this segregation could become more serious driven by pressure exerted by pressure machine head during forming. During forming, the slurry with relatively lower primary particle fraction at the ingot’s bottom center was firstly pressed into the cavity because the slurry at this site was the nearest to the inner gate. Just like this, the slurry always near the inner gate was continuously pressed
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Fig. 8. OM micrographs of sections No. 7 of thixoformed rods at reheating temperature of: (a) 470 °C and (b) 480 °C.
Fig. 9. OM micrographs of (a) the edge and (b) the center of the rod formed at reheating temperature of 475 °C heated for 90 min (die temperature: 300 °C).
into the cavity till it was filled up. It can be expected that the slurry with relatively higher primary particle fraction near the inner part of the ingot would be pressed into the cavity as the forming process proceeded. That is to say that the primary particle fraction of the slurry pressed in increased with the proceeding of forming. Furthermore, the mould-filling sequence of the slurry pressed in was from the edge of the cylindrical cavity to the inner part and from the bottom to the top of the cavity [17] (as indicated by Fig. 1, the cavity with large diameter was towards the top during forming). Based on this analysis, it can be easily explained why the primary particle fraction of the edge was lower that of the center. It also can be deduced that the particle fraction in the top part would be higher than that
near the bottom, which can not only be seen by comparing the micrographs shown in Figs. 3(a), (b) or 9(a), but also can be quantitatively demonstrated by the curve corresponding to the center shown in Fig. 10. However, the changes expressed by Fig. 3(c) seemly do not obey this rule. This might be caused by the change of the ingot shape during handling from the reheating furnace to the die, e.g. the ingot might collapse during handling, resulting in the change of the mould-filling sequence of the slurry in different positions of the ingot. 3.1.3. Effect of die temperature Fig. 11 shows the variations of the fraction and size of the primary particles with the die temperature. It can be
T.J. Chen et al. / Materials and Design 28 (2007) 1279–1287
Primary particle fraction/%
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Fig. 10. Variations of primary particle fraction along the longitudinal direction of the rod formed at reheating temperature of 475 °C heated for 90 min (die temperature: 300 °C).
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seen that both the fraction and size increased as the die temperature elevated. It is well known that the higher the die temperature, the slower the solidification and thus the longer the time for the liquid retaining. Because the secondary primary phase (the solidification of the liquid phase
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after the semi-solid slurry being pressed into the cavity and the resulting primary phase was named secondary solidification and secondary primary phase, respectively), and the former primary particles had similar crystal structure and both of them belonged to Al-rich a 0 phase, the secondary primary phase first directly developed from the surface of the primary particles without re-nucleation. It was just for this reason that there was longer time for the secondary primary phase growing from the surface of the primary particles when the die temperature was relatively higher, and thus the amount of the secondary primary phase attached to the primary particles was also more, resulting in larger primary particles and higher primary particle fraction. This can be clearly seen by comparing Fig. 12(a) and (b). 3.2. Secondary solidification process Undoubtedly, the microstructure formed from secondary solidification has significant effects on the performance of the final thixoformed components, so it is of particular importance to study the secondary solidification process. As described in Section 3.1.3, the secondary solidification began with the direct growth of secondary primary a 0 phase from the surface of the primary a 0 particles without re-nucleation, generally resulting in a toe-shape structure or a continuous a 0 phase layer surrounding the primary particle (Fig. 13(a)). Subsequently, a peritectic transformation, a + L ! b occurred, leading the freshly developed secondary primary a 0 phase becoming into a Zn-rich b phase (in the SEM micrographs the primary a 0 phase was in black and b was in gray) [18]. Of course, the surfaces of the former primary particles might partially participate in this reaction. Finally, the solidification completed by forming a well-defined b + g lamellar eutectics through eutectic reaction of L ! b + g. Fig. 13(b) shows the microstructures solidified from the former liquid pools entrapped within the primary particles. It can be seen that their solidification was similar to that of the liquid between the primary particles. The difference was that the toe-shape b phase structure was always substituted by a continuous b circle. Results indicated that the processing parameters, reheating temperature and reheating time had no obvious effect on the secondarily solidified microstructure, but the die temperature had significant effect. Fig. 14 shows the micrographs of section No. 5 of the rods formed at die temperatures of 250 °C and 350 °C. It can be seen that the amount of eutectics decreased with elevating the die temperature, and the b layer around the primary particles thickened till the primary particles were connected by b phase at temperature of 350 °C. The cause has been described in Section 3.1.3. For the ZA27 alloy, the eutectics result from non-equilibrium solidification and its amount decreases with decreasing the solidification rate [18]. Similarly, it is just for this reason that the amount of the eutectics solidified from the liquid pools also decreased and only
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Fig. 12. OM micrographs of sections No. 7 of the rods formed at die temperatures of: (a) 250 °C and (b) 350 °C (reheating temperature: 470, reheating duration: 90 min).
Fig. 13. SEM micrographs of the section No. 5 of the rod formed at reheating temperature of 480 °C heated for 90 min (die temperature: 300 °C).
Fig. 14. SEM micrographs of products formed at die temperature of: (a) 250 °C, (b) 350 °C (reheating temperature: 470 °C, heating time: 90 min).
in the structure solidified from the large size liquid pools the eutectics could be seen at 350 °C. Furthermore, because the primary particles had longer time to participate in the
peritectic reaction and this reaction was also rapider at higher temperature, thus they almost completely became into gray b phase at 350 °C.
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4. Conclusions (1) Except that of a short section near the inner gate, the microstructure of the thixoformed rods were basically uniform. However, the detailed microstructure observation or quantitative examination showed that the primary particle fraction slightly decreased along the axial direction while it relatively more obviously decreased along the radical direction. All of these changes in particle fraction were attributed to the microstructural characteristics of the semi-solid ingot prior to forming and the subsequent mould-filling process. (2) In the microstructures of the formed rods, the primary particle size increased and the primary particle fraction decreased with the increase of heating duration. Both these two parameters decreased with elevating the reheating temperature and increased as the die temperature increased. (3) The secondary solidification began with the direct growth of the secondary primary a 0 phase towards the liquid on the surfaces of the primary a 0 particles. Subsequently, the freshly developed secondary primary a 0 phase transformed into b phase through a peritectic reaction. Finally, the solidification completed by formation of a welldefined b + g lamellar structure through a eutectic reaction. (4) The secondarily solidified structure was significantly affected by die temperature while the other two parameters, reheating temperature and reheating duration had no obvious effect.
Acknowledgements The authors gratefully acknowledge the Development Program for Outstanding Young Teachers in Lanzhou university of Technology for their financial support, and also thank the Opening Foundation of State Key Laboratory of Advanced Non-ferrous Materials.
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References [1] Kirkwood DH. Semisolid metal processing. Int Mater Rev 1994;39:173–89. [2] Tzimas E, Zavaliangos A. Evolution of near-equiaxed microstructure in the semisolid state. Mater Sci Eng 2000;A289:228–40. [3] Tijun Chen, Yuan Hao, Ying Ma, Song Lu, Guangji Xu. Structural evolution of ZA27 alloy during semi-solid isothermal heat treatment. Trans Nonferr Met Soc China 2001;11:98–102. [4] Czerwinski F, Zielinska-Lipec A, Pinet PJ, Overbeeke J. Correlation of microstructure and tensile properties of a thixomoulded AZ91D magnesium alloy. Acta Mater 2001;49:1225–35. [5] Czerwinski F. On the generation of thixotropic structures during melting of Mg-9%Al-1%Zn alloy. Acta Mater 2002;50:3265–81. [6] Bergsma SC, Tolle MC, Kassner ME, Li X, Evangelista E. Semi-solid thermal transformations of Al-Si alloys and the resulting mechanical properties. Mater Sci Eng 2002;A237:24–34. [7] Fan Z. Semisolid metal processing. Int Mater Rev 2002;47:49–85. [8] Loue WR, Suery M. Microstructural evolution during partial remelting of Al–Si7Mg alloys. Mater Sci Eng 1995;A203:1–13. [9] Zoqui EJ, Shehata MT, Paes M, Kao V, Es-Sadiqi E. Morphological evolution of SSM A356 during partial remelting. Mater Sci Eng 2002;A325:38–53. [10] Chen TJ, Hao Y, Sun J. Microstructural evolution of previously deformed ZA27 alloy during partial remelting. Mater Sci Eng 2002;A337:3–81. [11] Yu Y, Kim S, Lee Y, Lee J. Phenomenological observations on mechanical and corrosion properties of thixoformed 357 alloys: a comparison with permanent mould cast 357 alloys. Metall Mater Trans 2002;33A:1399–412. [12] Chen CP, Tsao C-YA. Semi-solid deformation of non-dendritic structures–phenomenological behavior. Acta Mater 1997;45:1955–68. [13] Tzimas E, Zavaliangos A. Mechanical behavior of alloys with equiaxed microstructure in the semisolid state at high solid content. Acta Mater 1999;47:517–28. [14] Seyhan I, Ratke L, Bender W, Voorhees PW. Ostwald ripening of solid–liquid Pb–Sn dispersions. Metall Mater Trans 1996;27A:2470–8. [15] Snyder VA, Alkemper J, Voorhees PW. The development of spatial correlations during Ostwald ripening: a test of theory. Acta Mater 2000;48:2689–701. [16] Zavaliangos A. Modeling of the mechanical behavior of semisolid metallic alloys at very high volume fractions of solid. Int J Mech Sci 1998;40:1029–41. [17] Chen TJ, Hao Y, Li YD. Mould-feeding process during thixoforming ZA27 alloy cylindrical rod. Met Mater Int [in press]. [18] Durman M, Murphy S. An electron metallographic study of pressure die-cast commercial zinc–aluminum-based alloy ZA27. J Mater Sci 1997;32:1603–11.
Effects of processing parameters on microstructure of thixoformed ZA27 alloy T.J. Chen *, Y. Hao, Y.D. Li State Key Laboratory of Advanced Non-ferrous Materials, Lanzhou University of Technology, 287# Langongping Road, Lanzhou 730050, PR China Received 24 January 2005; accepted 15 December 2005 Available online 13 March 2006
Abstract In this paper, the effects of processing parameters on microstructures of thixoformed ZA27 alloy cylindrical rods were investigated. The results indicated that the microstructure of the formed rod was basically uniform except that of the section near the inner gate. But the detailed microstructure observation or quantitative examination showed that the primary particle fraction slightly decreased along the axial direction but it relatively more obviously decreased along the radial direction. The primary particle size increased and the primary particle fraction decreased with the increase of reheating duration. Both the size and fraction of primary particles decreased with elevating the reheating temperature and increased with increasing the die temperature. The secondary solidification began with the direct growth of the secondary primary phase on the surface of the primary a 0 particles and completed by forming lamellar eutectics. The secondarily solidified structure was significantly affected by die temperature while the other two parameters, reheating temperature and reheating time had no obvious effect. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Non-ferrous metals and alloys; Forming; Microstructure
1. Introduction Thixoforming is a novel metal processing technology which combines the elements of both casting and forging. It offers significant advantages, such as reducing macrosegregation and porosity and lowering forming efforts [1]. It generally comprises three steps, production of non-dendritic ingot, reheating and forming. In recent years, much effort has been made to simplify this three-step process to two steps, or even to one-step [2–6]. Some investigations integrated the former two steps, the non-dendritic material production and reheating, into one step through reheating traditionally casting alloy ingot with fine dendrites [2,3]. The starting ingot with fine dendrites can be produced by adding refiner prior to pouring, or by chilling upon solidification. This technology for production of non-dendritic ingot (or slurry) was termed semi-solid thermal transfor*
Corresponding author. Tel.: +86 931 2806921; fax: +86 931 2755806. E-mail address: [email protected] (T.J. Chen).
0261-3069/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2005.12.010
mation (SSTT) [6]. The primary particles in the resulting semi-solid microstructure are finer, more spherical and more uniform than those produced by the magnetohydrodynamic stirring, the most common method in the threestep process in practice [1,3]. The others developed a single integrated machine to produce magnesium alloy components. But so far, this technology, named thixomoulding, is limited to relatively low solid fraction and magnesium alloys for thin wall components due to relatively low pressure capability of the machine [5,7]. Therefore, thixoforming with SSTT is useful in metal processing method and has much potential for future use. It is well known that the performance of a final component is determined by its microstructure, and that the microstructure is determined by processing technology. Thus, for the thixoforming, it must be firstly verified the effects of processing parameters on its microstructure in order to understand the effect of microstructure on mechanical properties of an alloy. However, previous studies have been mainly focused on microstructural evolution
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during partial remelting and subsequent isothermal holding [2,3,5,6,8–10]. Only a few investigations have involved microstructures of thixoformed alloys [4,7,11]. Therefore, in the present work, the effects of processing parameters, such as reheating temperature, reheating duration and die temperature on microstructure of a Zr modified ZA27 alloy thixoformed with SSTT were studied. 2. Experimental procedure The nominal compositions of ZA27 alloy used for this work were 26–28%Al, 1.7–2.0%Cu, 0.2%Zr, 0.02– 0.04%Mg (in wt.), with a balance of Zn. The role of Zr was to achieve a microstructure with fine dendrites through modification [3]. A quantity of the alloy was melted, then degassed by using C2Cl6. Pouring followed at 550 °C into permanent mould with ambient temperature to form rods 190 mm long and 45 mm in diameter. Some small ingots, 50 mm long, were cut from these rods, as the starting ingots for thixoforming. The small ingots were heated in an electric resistance furnace at various temperatures of 465 °C, 470 °C, 473 °C, 475 °C, 480 °C and 485 °C, respectively, for different periods ranging from 65 min to120 min, and then transported to a die to be pressed using a 40 ton pressure. The temperature fluctuation was kept within ±1 °C by a temperature controller
D80. Three die temperatures were employed, 250 °C, 300 °C and 350 °C. The die for this procedure is shown in Fig. 1. The thixoformed product is schematically shown in Fig. 2, which also clearly reflects the dimensions of the die cavity. The rod part with 165 mm long was the desired formed output product. The thixoformed products were sawed into two parts along their central lines, and one was divided into nine small samples according to the dimensions shown in Fig. 2. Starting from the left, the small samples were numbered 1–9. The cross-sections of these small samples were ground and polished. Metallographic observations were carried out using a back-scattered electron imaging system in an S-520 scanning electron microscope (SEM) and an optical microscope Mef-3 (OM). Quantitative examinations on the microstructures were done using an automatic quantitative metallography analytical system in OM. 3. Results and discussion 3.1. Effects of processing parameters on microstructures of thixoformed rods 3.1.1. Effect of reheating duration Fig. 3 shows some typical micrographs of the different sections in the rods thixoformed at reheating temperature
Fig. 1. Photos of die used in this work.
195 165 20
20
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20
20
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20
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Fig. 2. Schematic for thixoformed parts.
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Fig. 3. OM micrographs of the different positions in the rods thixoformed at the reheating temperature of 470 °C heated for different durations (die temperature: 300 °C): (a) 70 min, (b) 90 min and (c) 120 min.
of 470 °C heated for different times. It can be seen that except the part near the inner gate in the section No. 1, the microstructure along the axial direction was basically uniform. The primary particle size slightly increased while the particle fraction decreased with prolonging the reheating duration. Simultaneously, the segregation of former liquid phase appeared, and its size and amount increased as the time increased. After heated for 120 min, the longest duration applied in this work, large size segregation could be observed near the inner gate (Fig. 3(c)). The result of quantitative image examination shown in Fig. 4 also supports the phenomena mentioned above. It must be noted that the primary particle fraction was the average value of the eight sections except the No. 1 section. In each section, two images (edge and center) were analyzed. It can be seen that the particle fraction decreased obviously during reheating from 70 min to 75 min, and
then decreased slowly as the time further increased (Fig. 4(a)). The particle size also decreased during reheating from 70 min to 75 min, but then increased (Fig. 4(b)). In fact, the primary particle coarsening from Ostwald ripening and coalescence and the size reducing from the partial melting of the particles were in a competitive condition before the solid/liquid equilibrium state was attained during reheating [8]. Temperature changes in the center of the ingot with the reheating time indicated that the semi-solid system was up to its thermal equilibrium state between the solid and liquid phases till the reheating duration was over 170 min (Fig. 5). But the longest reheating duration used in this work was 120 min. So this solid– liquid equilibrium state was not achieved. The obvious decrease in primary particle fraction at reheating from 70 min to 75 min indicated that the partial melting of the particles was extensive, which resulted in an obvious
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(a)
500
70 60
400
50
Temperature/
Primary particle fraction/%
80
40 30
300
200
20 10
100
0 70
80
90
100
110
120 0
Heating time/min
0
80
Primary particle size/um
50
100
150
200
250
Heating time/min
(b)
Fig. 5. Temperature changes in the ingots during reheating at 470 °C. Showing reheating process was divided into five stages: I was a rapid heating stage; II a solid phase transformation stage, in which transformations (a + g + e ! b) occurred at about 285 °C; III another heating stage; IV partial remelting stage (started from 385 °C), and V isothermal holding stage.
70 60 50 40 30 20 10 0 70
80
90
100
110
120
Heating time/min Fig. 4. Variations of primary particle (a) fraction and (b) size in the thixoformed rods with reheating duration (reheating temperature: 470 °C, die temperature: 300 °C).
reduction in size of the particles. So the particle reducing at this stage was superior to the particle coarsening. But after this stage, the particle reducing was inferior to the particle coarsening, so the resulting particle size increased with the time although the particle fraction decreased. Due to the characteristics of the resistance furnace heating method, the surface of the ingot was first heated and the heat then gradually transferred toward the center. Thus, the microstructural evolution near the surface was faster than that in the inner part. In addition, the ingot’s gravity was equivalent to a pressure being exerted on its top surface. Under this pressure, the liquid phase in the inner part would flow toward the radial edge while the primary particles would move towards each other in axial direction (the axial line of the cylindrical ingot was perpendicular to ground during reheating) [12,13]. Simultaneously, the Al-rich primary particles moved toward the top surface and the zinc-rich liquid phase moved toward the bottom due to the difference in densities between the primary particles and the liquid phase. All of these resulted in the ununiform microstructure of the semi-solid ingot: the size and fraction of the primary particles in the inner section or near the top surface were generally larger than those near the edge or the bottom, respectively. Fig. 6
shows that both the size and fraction in the center part was obviously larger than those in the bottom edge. This ununiform microstructure of the semi-solid ingot must result in the inhomogeneous microstructure of the thixoformed rod. It must be indicated that this microstructure inhomogeneity of the semi-solid ingot would be also aggravated by coarsening during reheating, e.g., the relatively larger particles in the inner part of the ingot might partially attributed to the relatively more intensive Ostwald ripening and coalescence because of the shorter diffusion distance and higher frequency of primary particle contact or meeting each other resulted from the relatively higher solid fraction [14,15]. Furthermore, the segregation degree of the heated ingot microstructure would increase with increasing the heating duration, thus the segregation of the resulting thixoformed microstructure would also gradually become serious. In addition, the pressure exerted on the top surface of the semi-solid ingot by the pressure head of the pressure machine during the subsequent thixoforming, similar to the pressure from the gravity, also caused the segregation [12,13]. It is remarkable that the speed of the pressure head was quite slow, about 0.2 m/s, which could aggravate the segregation [16]. It can be found that the reheating time or the time for the ingot to achieve the thermal equilibrium state was very long. Except the heat transfer characteristics of resistance furnace, the other reason why the time was so long is that some phase transformations occurring during reheating need much heat. It can be expected that the main transformation is the partial melting of solid metal. Fig. 5 indicates that another endothermic reaction (a + g + e ! b) also occurred. Furthermore, the speed of heat conduction decreased with the increase and homogenization of the temperature in the ingot because the temperature gradient
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Fig. 6. OM micrographs of: (a) the inner part and (b) the bottom edge of ingot reheated for 90 min at 470 and then water-quenched.
between the furnace chamber and the ingot or within the ingot decreased. Especially, the speed was very slow at the final reheating stage (Fig. 5). 3.1.2. Effect of reheating temperature Fig. 7 gives the variations of the fraction and size of the primary particles in the rods with reheating temperature. It can be seen that both the primary particle fraction and size slightly decreased as the temperature elevated. The result of
70
(a)
Solid fraction/%
60 50 40 30 20 10 0 470
475
480
Reheating temperature/ (b) 70
Particle size/um
60 50 40 30 20 10 0
470
475
480
Reheating temperature/ Fig. 7. Variations of primary particle: (a) fraction and (b) size in the thixoformed rods with reheating temperature (reheating time: 90 min, die temperature: 300 °C).
micrograph observation revealed that the size of liquid pools entrapped within the primary particles increased with elevating the temperature (comparing Fig. 8(a) and (b)). As the reheating temperature elevated, the amount of the liquid in the microstructure of the heated ingot prior to forming must increase at a given heating time, and thus the degree of particle reducing due to partial remelting would also increase. In addition, during reheating at high temperature the time for soaking at low semi-solid temperature interval relatively decreased due to rapid heating speed from the large temperature gradient between the ingot and the furnace chamber. This could also decrease the particles size due to relatively short time for coarsening, especially for the coalescence at high solid fraction state. Furthermore, the coarsening of the liquid pools within the primary particles, controlled by diffusion, would also be accelerated with the increase of the temperature, resulting in the increase in size and decrease in number (Fig. 8). In order to evaluate the thixoformed microstructures along the radical direction, the microstructure in the edge of the rod formed at reheating temperature of 475 °C heated for 75 min was observed and shown in Fig. 9. Simultaneously, the micrographs along the center line were also presented for comparison. It can be seen that the primary particle fraction at the edge was lower than that at the center. The largest difference in the primary particle fraction was in the part near the inner gate. The quantitative examination given by Fig. 10 clearly shows the difference. It also indicates that the microstructural uniformity along the edge was inferior to that along the central line. No obvious difference in particle size between the edge and center was observed. From discussion in Section 3.1.1, it can be known that the microstructure of the semi-solid ingot prior to forming was relatively inhomogeneous, especially the primary particle fraction near the edge was lower than that in the inner section. And this segregation could become more serious driven by pressure exerted by pressure machine head during forming. During forming, the slurry with relatively lower primary particle fraction at the ingot’s bottom center was firstly pressed into the cavity because the slurry at this site was the nearest to the inner gate. Just like this, the slurry always near the inner gate was continuously pressed
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Fig. 8. OM micrographs of sections No. 7 of thixoformed rods at reheating temperature of: (a) 470 °C and (b) 480 °C.
Fig. 9. OM micrographs of (a) the edge and (b) the center of the rod formed at reheating temperature of 475 °C heated for 90 min (die temperature: 300 °C).
into the cavity till it was filled up. It can be expected that the slurry with relatively higher primary particle fraction near the inner part of the ingot would be pressed into the cavity as the forming process proceeded. That is to say that the primary particle fraction of the slurry pressed in increased with the proceeding of forming. Furthermore, the mould-filling sequence of the slurry pressed in was from the edge of the cylindrical cavity to the inner part and from the bottom to the top of the cavity [17] (as indicated by Fig. 1, the cavity with large diameter was towards the top during forming). Based on this analysis, it can be easily explained why the primary particle fraction of the edge was lower that of the center. It also can be deduced that the particle fraction in the top part would be higher than that
near the bottom, which can not only be seen by comparing the micrographs shown in Figs. 3(a), (b) or 9(a), but also can be quantitatively demonstrated by the curve corresponding to the center shown in Fig. 10. However, the changes expressed by Fig. 3(c) seemly do not obey this rule. This might be caused by the change of the ingot shape during handling from the reheating furnace to the die, e.g. the ingot might collapse during handling, resulting in the change of the mould-filling sequence of the slurry in different positions of the ingot. 3.1.3. Effect of die temperature Fig. 11 shows the variations of the fraction and size of the primary particles with the die temperature. It can be
T.J. Chen et al. / Materials and Design 28 (2007) 1279–1287
Primary particle fraction/%
80 70 60 50 40
Centre Edge
30 20 10 0 40
60
80
100
120
140
160
180
200
Distance/mm
Fig. 10. Variations of primary particle fraction along the longitudinal direction of the rod formed at reheating temperature of 475 °C heated for 90 min (die temperature: 300 °C).
80
(a)
Particle fraction/%
70 60 50 40 30 20 10 0 240
260
280
300
320
340
360
Die temperature/ (b)
Particle size/um
80
60
40
20
0 250
300
350
Die temperature/ Fig. 11. Variations of primary particle: (a) fraction and (b) size in the thixoformed rods with die temperature (reheating temperature: 470 °C, reheating time: 90 min).
seen that both the fraction and size increased as the die temperature elevated. It is well known that the higher the die temperature, the slower the solidification and thus the longer the time for the liquid retaining. Because the secondary primary phase (the solidification of the liquid phase
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after the semi-solid slurry being pressed into the cavity and the resulting primary phase was named secondary solidification and secondary primary phase, respectively), and the former primary particles had similar crystal structure and both of them belonged to Al-rich a 0 phase, the secondary primary phase first directly developed from the surface of the primary particles without re-nucleation. It was just for this reason that there was longer time for the secondary primary phase growing from the surface of the primary particles when the die temperature was relatively higher, and thus the amount of the secondary primary phase attached to the primary particles was also more, resulting in larger primary particles and higher primary particle fraction. This can be clearly seen by comparing Fig. 12(a) and (b). 3.2. Secondary solidification process Undoubtedly, the microstructure formed from secondary solidification has significant effects on the performance of the final thixoformed components, so it is of particular importance to study the secondary solidification process. As described in Section 3.1.3, the secondary solidification began with the direct growth of secondary primary a 0 phase from the surface of the primary a 0 particles without re-nucleation, generally resulting in a toe-shape structure or a continuous a 0 phase layer surrounding the primary particle (Fig. 13(a)). Subsequently, a peritectic transformation, a + L ! b occurred, leading the freshly developed secondary primary a 0 phase becoming into a Zn-rich b phase (in the SEM micrographs the primary a 0 phase was in black and b was in gray) [18]. Of course, the surfaces of the former primary particles might partially participate in this reaction. Finally, the solidification completed by forming a well-defined b + g lamellar eutectics through eutectic reaction of L ! b + g. Fig. 13(b) shows the microstructures solidified from the former liquid pools entrapped within the primary particles. It can be seen that their solidification was similar to that of the liquid between the primary particles. The difference was that the toe-shape b phase structure was always substituted by a continuous b circle. Results indicated that the processing parameters, reheating temperature and reheating time had no obvious effect on the secondarily solidified microstructure, but the die temperature had significant effect. Fig. 14 shows the micrographs of section No. 5 of the rods formed at die temperatures of 250 °C and 350 °C. It can be seen that the amount of eutectics decreased with elevating the die temperature, and the b layer around the primary particles thickened till the primary particles were connected by b phase at temperature of 350 °C. The cause has been described in Section 3.1.3. For the ZA27 alloy, the eutectics result from non-equilibrium solidification and its amount decreases with decreasing the solidification rate [18]. Similarly, it is just for this reason that the amount of the eutectics solidified from the liquid pools also decreased and only
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Fig. 12. OM micrographs of sections No. 7 of the rods formed at die temperatures of: (a) 250 °C and (b) 350 °C (reheating temperature: 470, reheating duration: 90 min).
Fig. 13. SEM micrographs of the section No. 5 of the rod formed at reheating temperature of 480 °C heated for 90 min (die temperature: 300 °C).
Fig. 14. SEM micrographs of products formed at die temperature of: (a) 250 °C, (b) 350 °C (reheating temperature: 470 °C, heating time: 90 min).
in the structure solidified from the large size liquid pools the eutectics could be seen at 350 °C. Furthermore, because the primary particles had longer time to participate in the
peritectic reaction and this reaction was also rapider at higher temperature, thus they almost completely became into gray b phase at 350 °C.
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4. Conclusions (1) Except that of a short section near the inner gate, the microstructure of the thixoformed rods were basically uniform. However, the detailed microstructure observation or quantitative examination showed that the primary particle fraction slightly decreased along the axial direction while it relatively more obviously decreased along the radical direction. All of these changes in particle fraction were attributed to the microstructural characteristics of the semi-solid ingot prior to forming and the subsequent mould-filling process. (2) In the microstructures of the formed rods, the primary particle size increased and the primary particle fraction decreased with the increase of heating duration. Both these two parameters decreased with elevating the reheating temperature and increased as the die temperature increased. (3) The secondary solidification began with the direct growth of the secondary primary a 0 phase towards the liquid on the surfaces of the primary a 0 particles. Subsequently, the freshly developed secondary primary a 0 phase transformed into b phase through a peritectic reaction. Finally, the solidification completed by formation of a welldefined b + g lamellar structure through a eutectic reaction. (4) The secondarily solidified structure was significantly affected by die temperature while the other two parameters, reheating temperature and reheating duration had no obvious effect.
Acknowledgements The authors gratefully acknowledge the Development Program for Outstanding Young Teachers in Lanzhou university of Technology for their financial support, and also thank the Opening Foundation of State Key Laboratory of Advanced Non-ferrous Materials.
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