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Materials Science and Engineering A 444 (2007) 99103 Semisolid microstructure of Mg 2 Si/Al composite by cooling slope cast and its evolution during Q.D. Qin, Y.G. Zhao , P.J. Street, accepted Abstract fabricated microstructure, , size that are shape K 1. lot minimizing and semisolid of ture needed is constituted of solid phase globules suspended in the liquid phase. The thixotropic effect of the semisolid alloys allows them to be handled as a massive solid and to attain fluid- like properties when sheared during shaping 2. Many different routes such strain tial Recently semisolid Kleiner allo gro flo of process allo spherical Suzuki thixoforming in of potential as automobile brake disc material because the inter- metallic compound of Mg 2 Si exhibits has high melting tem- perature, low density, high hardness, low thermal expansion coefficient and reasonably high elastic modulus 8. However, 0921-5093/$ doi: have been used to produce non-dendritic microstructure, as magnetohydrodynamic (MHD) stirring, spray forming, induced melt activated (SIMA)/recrystallisation and par- melting (RAP), liquidus/near-liquidus casting etc. 38. , Czerwinski 911 investigated the fabrication of Mg alloys components by injection molding process. et al. 12 studied the formation of semisolid MgAlZn y by extruded method. Wu et al. 13 constructed a model on wth morphology of semisolid metals, using solidification and w speed of the liquid as variations affecting the morphology crystals. Among all the techniques of SSP, the cooling slope Corresponding author. Tel.: +86 431 509 4481; fax: +86 431 509 5592. E-mail address: (Y.G. Zhao). the coarse reinforcement of the primary Mg 2 Si particles in the normal composite leads to poor properties. Therefore, the com- posite with coarse primary Mg 2 Si particles need to be modified to obtain adequate mechanical strength and ductility. It has been reported that rare earth elements such as Ce 16,Sr17 and its salts 18,19 have the power to modify the morphology of primary Mg 2 Si. A semisolid microstructure in the composite is expected to improve the mechanical properties. The semisolid of Mg 2 Si/Al composite has been produced via SIMA in pre- vious study 8. However, this technology is relative complex because of requiring cold extrusion and deformation. Less work has been carried out on semisolid Mg 2 Si/Al composite by the cooling slope cast and partial remelting process. In the present study, a semisolid of in situ Mg 2 Si/AlSiCu composite was prepared by the cooling slope cast and partial remelting process, see front matter 2006 Elsevier B.V. All rights reserved. 10.1016/j.msea.2006.08.074 Key Laboratory of Automobile Materials of Ministry of Education Jilin University, No. 142 Renmin Received 17 April 2006; An in situ Mg 2 Si/AlSiCu composite with semisolid structure was and effect of isothermal holding time on the morphology the morphology of primary Mg 2 Si and H9251-Al grains in the composite factor of H9251-Al grains increase with the isothermal holding time. 2006 Elsevier B.V. All rights reserved. eywords: Semisolid; Aluminum; Composites; Microstructure; Magnesium silicide Introduction It has been well known that semisolid processing (SSP) has a of significant advantages over conventional casting, such as the macrosegregation and solidification shrinkage reducing the forming temperature. The key that permits the alloys to shape is the absence of dendritic morphology the solid phase 1. The typical non-dendritic microstruc- partial remelting process Cong, W. Zhou, B. Xu and Department of Materials Science another reason related to the flow of the melt on the slope. The flowing melt cause partial fragments of the dendrites of the dendrites by vection. Fig. 3ad shows the evolution of the semisolid microstruc- of the composite by the cooling slope cast with the time of isothermal heat treatment of 30, 60, 180 and min, respectively. Fig. 3a shows that the as-cast coarse 2 Si dendrites are fragmented, changing to an irregular with slightly rounded tips, and the morphology of H9251-Al ve becomes globular with a mean size of 51H9262m. As the time increases to 60 min, the morphology of the Mg 2 Si in the composite becomes mainly elliptic shape and the of H9251-Al becomes more globular with a mean size 85H9262m seen in Fig. 3b. Furthermore, it also shows that some H9251-Al grains is not dissolved completely surviving in the as indicated by white arrows in Fig. 3b. Fig. 3c shows the cast (adopted from 8) and (b) the cooling slope cast. Q.D. Qin et al. / Materials Science and Engineering A 444 (2007) 99103 101 Fig. cast and microstructure 180 not increases grains” H9251 survi of as in particles. liquid during further grains” shape treatment the as ob of “smaller The solidification quenching may 3. Semisolid microstructures of the Mg 2 Si/Al composite by the cooling slope (d) 600 min. of the composite with a isothermal treatment for min. The morphologies of the Mg 2 Si and H9251-Al particles do change obviously, however, the mean size of H9251-Al particles to 111H9262m. It is of interest to note that some “smaller emerge on the surface of the large globular grains of -Al, as indicated by black arrows in Fig. 3c. The amount of the ved small solid particles increases, in comparison with that 60 min holding time. It seems that the liquid fraction increases well. Unfortunately, the liquid fraction could not be measured the present study, because of the survived of the small solid Poirier et al. 20 reported that the volume fraction of of AlCu alloy slightly decreased at the coarsening period semisolid isothermal treatment. The phenomenon needs study. Fig. 4a shows that the morphology of the “smaller is columnar and some survived solid phases are irregular as denoted by the white arrow in Fig. 4a. As the isothermal From the ticles of formation Fig. 4. Metallographs of the composites with the isothermal time of (a) with different isothermal holding time of (a) 30 min, (b) 60 min, (c) 180 min time increases up to 600 min, the morphologies of primary Mg 2 Si particles and H9251-Al grains are still globular, shown in Fig. 3d. The size of the H9251-Al grains increase viously with a mean size of 149H9262m. In addition, the amount the survived solid particles evidently decreases, and the grains” on the surface of large H9251-Al grains disappear. “smaller grains” emergence may be the consequence of of the liquid during handling of the samples before in water, and that emergence and disappearance be due to the difference of the handle time for quenching. Fig. 4b, it is clearly indicated that the morphologies of survived solid particles do not change obviously. To get better understanding of the evolution of the solid par- is of important, because it determines the final grain size the composite, and thus the mechanical properties 21. The of a semisolid structure by isothermal holding from a 180 min and (b) 600 min, showing the H9251-Al “smaller grains”. 102 Q.D. Qin et al. / Materials Science and Engineering A 444 (2007) 99103 Fig. con 8 is cast isothermal arms ellipsoidal and cles is joining coarsening the in can acterized F where respecti isothermal shape time time that that a slo solid v the of In microstructures of hard Fig. ing dissolv cess the 4. successfully remelting of ob increase mean morphology H9251 time Ackno neering F Refer 5. The relationship of the mean size of H9251-Al grains and the holding time. ventionally cast dendritic structure has been studied earlier . The transition of the solid phase from dendritic into spheral thought to be due to the liquid penetration, namely, the as- grain boundary is penetrated by liquid during the semisolid holding, causing the fragmentation of the dendrite and then, the fragmented arms change into spheroidal or grains. The relationship between the grain size of the H9251-Al particles holding time is shown in Fig. 5. The size of the H9251-Al parti- increases with the holding time. One coarsening mechanism the coalescence of the grains, namely, two grains encounter together and forming new bigger grain 22. Another mechanism is the Ostwald ripening 22,23, in which larger grains grow and the smaller grains remelt. Using the image analysis system, the number of the objects a selected area, and the perimeter and area of selected objects be measured 2. Normally, the shape of an object is char- by the shape factor F 0 defined as 2: 0 = 4A 0 P 2 0 (1) A 0 and P 0 represent the area and perimeter of the object, vely 2. The change of the shape factor during the treatment is shown in Fig. 6. It is indicated that the factor increases rapidly from 0.51 to 0.69 with the holding from 30 to 180 min and however, a much larger holding cannot result in a considerable change of F 0 , suggesting the F 0 seems to reach to a maximum value. It is reported the solid phase particles tend to become spherical, but, for longer holding time, the change of the shape of the particles ws down and even reverses in the case of the high values of volume fraction 21. Keeping in mind that the high solid olume fraction means also a high contiguity, this reversion from spherical shape can be attributed to the hard impingement the solid particles, leading to the local shape distortions 21. the present study, however, the solid volume fraction in the is lower relatively (0.6) according to the result the quantitative analysis, and consequently, the chance of the impingement is lower as well. With increase in the hold- 10 6. Relationship of the shaper factor of the H9251-Al grains ant the holding time. time, the higher curvature part of the solid particle will be ed, and leading to the increase of the F 0 . Finally, the pro- reaches to a dynamic equilibrium and the shape factor of grains will not change. Conclusion The semisolid structure of in situ Mg 2 Si/Al composite is produced by the cooling slope cast and partial process. The results show that: (a) the morphology primary Mg 2 Si phase is globular and/or elliptic not changing viously with increase in the isothermal holding time; (b) with in the isothermal holding time from 30 to 600 min, the size of H9251-Al grains increases from 50 to 150H9262m, and its becomes more globular; (c) the shape factor of the -Al solid particles rapidly from 0.51 to 0.69 with the holding from 30 to 60 min. wledgements This work is supported by The Project 985-Automotive Engi- of Jilin University and The Innovation and Invention oundation of Jilin University (2003CX029). ences 1 E. Tzimas, A. Zavaliangos, Mater. Sci. Eng. A 289 (2000) 217. 2 W.R. Loue, M. Suery, Mater. Sci. Eng. A 203 (1995) 1. 3 H.V. Atkinson, Prog. Mater. Sci. 50 (2005) 341. 4 M.P. Kenney, J.A. Courtois, R.D. Evans, G.M. Farrior, C.P. Kyonka, A.A. Koch, K.P. 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