Semisolid microstructure of Mg2Si Al composite by cooling slope cast and its evolution during partial remelting process.pdf

Materials Science and Engineering A 444 2007 99 103 Semisolid microstructure of Mg2Si Al composite by cooling slope cast and its evolution during partial remelting process Q D Qin Y G Zhao P J Cong W Zhou B Xu Key Laboratory of Automobile Materials of Ministry of Education and Department of Materials Science accepted 15 August 2006 Abstract An in situ Mg2Si Al Si Cu composite with semisolid structure was fabricated by cooling slope cast and partial remelting process The as cast microstructure and effect of isothermal holding time on the morphology size and shape factor of the grains were investigated The results show that the morphology of primary Mg2Si and Al grains in the composite are globular and or elliptic after partial remelting process The size and shape factor of Al grains increase with the isothermal holding time 2006 Elsevier B V All rights reserved Keywords Semisolid Aluminum Composites Microstructure Magnesium silicide 1 Introduction It has been well known that semisolid processing SSP has a lot of signifi cant advantages over conventional casting such as minimizing the macrosegregation and solidifi cation shrinkage and reducing the forming temperature The key that permits the semisolidalloystoshapeistheabsenceofdendriticmorphology of the solid phase 1 The typical non dendritic microstruc 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 fl uid like properties when sheared during shaping 2 Many different routes have been used to produce non dendritic microstructure such as magnetohydrodynamic MHD stirring spray forming strain induced melt activated SIMA recrystallisation and par tial melting RAP liquidus near liquidus casting etc 3 8 Recently Czerwinski 9 11 investigated the fabrication of semisolid Mg alloys components by injection molding process Kleineretal 12 studiedtheformationofsemisolidMg Al Zn alloybyextrudedmethod Wuetal 13 constructedamodelon growthmorphologyofsemisolidmetals usingsolidifi cationand fl ow speed of the liquid as variations affecting the morphology of 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 zhaoyg Y G Zhao process is a simple route The primary phase in the semisolid alloy by the cooling slope cast has been reported to become spherical after remelted in the semisolid state 14 Haga and Suzuki 14 15 investigated the producing process of ingots for thixoforming of Al 6Si alloys by cooling slope casting HypereutecticAl SialloyswithhighMgcontentisinfactan in situ aluminum matrix composites containing a large amount of hard particles of Mg2Si and the Mg2Si Al composite has a potential as automobile brake disc material because the inter metallic compound of Mg2Si exhibits has high melting tem perature low density high hardness low thermal expansion coeffi cient and reasonably high elastic modulus 8 However the coarse reinforcement of the primary Mg2Si particles in the normal composite leads to poor properties Therefore the com posite with coarse primary Mg2 Si particles need to be modifi ed toobtainadequatemechanicalstrengthandductility Ithasbeen reported that rare earth elements such as Ce 16 Sr 17 and its salts 18 19 have the power to modify the morphology of primary Mg2Si A semisolid microstructure in the composite is expected to improve the mechanical properties The semisolid of Mg2Si 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 Mg2Si Al composite by the cooling slope cast and partial remelting process In the present study a semisolid of in situ Mg2Si Al Si Cu composite was preparedbythecoolingslopecastandpartialremeltingprocess 0921 5093 see front matter 2006 Elsevier B V All rights reserved doi 10 1016 j msea 2006 08 074 100Q D Qin et al Materials Science and Engineering A 444 2007 99 103 Table 1 Chemical compositions of the Mg2Si Al composite wt MaterialsAlMgSiCuCrZnNiFe Al Si Mg CuBal 13 27611 2813 523 0 005 0 02399 7 purity and magnesium ingot 98 0 purity were used to prepare the experimental alloy About 520g of Al Si master alloy melt was molten in a graphite crucible in an electric resistance furnace About 100g of magnesium and 26g of Cu preheated at 300 C were added into the Al Si melt at 680 700 C After holding 15min the melt were poured into a steelmoldviaaaluminumcoolingslope preheatedat300 C to produce the in situ Mg2Si Al composite ingots and the chem ical compositions are listed in Table 1 The schematic of the casting process is shown in Fig 1 adopted from 15 Sub sequently the ingot was cut into a series of cubic samples of 12mm 12mm 12mm The partial remelting process was performedinaverticaltubefurnace andthesampleswereheated up to 560 C and held at the temperature for 30 60 180 and 600min respectively and then were quenched in cold water Metallographic specimens were polished through standard procedure and the microstructure in them examined using an optical microscopy A 0 5 hydrofl uoric acid HF aqueous solution was used as the etchant of polishing samples The grain size and area of the primary solid phase were analyzed sta tistically by a quantitative analysis system Omnimet Imaging Systems Buehler USA 3 Results and discussion According to the composition of the alloy and the previous studies 8 16 the as cast microstructure of the composite con sists of Mg2Si Al and eutectic Si phases Fig 2a and b shows the typical as cast microstructure of in situ Mg2Si Al compos ite by the normal cast and cooling slope cast respectively The microstructure of the composite reveals that the morphology of primary Mg2Si as cast in the composite by the normal cast was dendritic as indicated with a arrow in Fig 2a with a size of 200 m andthe Alwasdendriticaswell However afterthe composite solid with the cooling slope the morphology of Al phase in the composite by the cooling slop cast changes from dendritictospheralwithadiameterof 10 m andtheprimary Mg2 Si crystals become fi ne obviously as seen in Fig 2b One reason for it is due to the increase in the nucleation substrates in the melt after casting with the cooling slope another reason is related to the fl ow of the melt on the slope The fl owing melt will cause partial fragments of the dendrites of the dendrites by convection Fig 3a d shows the evolution of the semisolid microstruc tures of the composite by the cooling slope cast with the holding time of isothermal heat treatment of 30 60 180 and 600min respectively Fig 3a shows that the as cast coarse Mg2Si dendrites are fragmented changing to an irregular shape with slightly rounded tips and the morphology of Al have becomes globular with a mean size of 51 m As the holding time increases to 60min the morphology of the Mg2Si particlesinthecompositebecomesmainlyellipticshapeandthe morphology of Al becomes more globular with a mean size of 85 mseeninFig 3b Furthermore italsoshowsthatsome smaller Al grains is not dissolved completely surviving in the liquid as indicated by white arrows in Fig 3b Fig 3c shows the Fig 2 As cast microstructures of Mg2Si Al composites by a the normal cast adopted from 8 and b the cooling slope cast Q D Qin et al Materials Science and Engineering A 444 2007 99 103101 Fig 3 Semisolid microstructures of the Mg2Si Al composite by the cooling slope cast with different isothermal holding time of a 30min b 60min c 180min and d 600min microstructure of the composite with a isothermal treatment for 180min The morphologies of the Mg2Si and Al particles do not change obviously however the mean size of Al particles increasesto 111 m Itisofinteresttonotethatsome smaller grains emerge on the surface of the large globular grains of Al as indicated by black arrows in Fig 3c The amount of the survived small solid particles increases in comparison with that of60minholdingtime Itseemsthattheliquidfractionincreases aswell Unfortunately theliquidfractioncouldnotbemeasured in the present study because of the survived of the small solid particles Poirier et al 20 reported that the volume fraction of liquidofAl Cualloyslightlydecreasedatthecoarseningperiod during semisolid isothermal treatment The phenomenon needs further study Fig 4a shows that the morphology of the smaller grains iscolumnarandsomesurvivedsolidphasesareirregular shapeasdenotedbythewhitearrowinFig 4a Astheisothermal treatment time increases up to 600min the morphologies of the primary Mg2Si particles and Al grains are still globular as shown in Fig 3d The size of the Al grains increase obviously with a mean size of 149 m In addition the amount of the survived solid particles evidently decreases and the smaller grains on the surface of large Al grains disappear The smaller grains emergence may be the consequence of solidifi cationoftheliquidduringhandlingofthesamplesbefore quenching in water and that emergence and disappearance may be due to the difference of the handle time for quenching From Fig 4b it is clearly indicated that the morphologies of the survived solid particles do not change obviously To get better understanding of the evolution of the solid par ticles is of important because it determines the fi nal grain size of the composite and thus the mechanical properties 21 The formation of a semisolid structure by isothermal holding from a Fig 4 Metallographs of the composites with the isothermal time of a 180min and b 600min showing the Al smaller grains 102Q D Qin et al Materials Science and Engineering A 444 2007 99 103 Fig 5 The relationship of the mean size of Al grains and the holding time conventionally cast dendritic structure has been studied earlier 8 The transition of the solid phase from dendritic into spheral is thought to be due to the liquid penetration namely the as cast grain boundary is penetrated by liquid during the semisolid isothermal holding causing the fragmentation of the dendrite arms and then the fragmented arms change into spheroidal or ellipsoidal grains The relationship between the grain size of the Al particles and holding time is shown in Fig 5 The size of the Al parti clesincreaseswiththeholdingtime Onecoarseningmechanism is the coalescence of the grains namely two grains encounter joining together and forming new bigger grain 22 Another coarseningmechanismistheOstwaldripening 22 23 inwhich the larger grains grow and the smaller grains remelt Using the image analysis system the number of the objects in a selected area and the perimeter and area of selected objects can be measured 2 Normally the shape of an object is char acterized by the shape factor F0 defi ned as 2 F0 4 A0 P2 0 1 where A0and P0represent the area and perimeter of the object respectively 2 The change of the shape factor during the isothermal treatment is shown in Fig 6 It is indicated that the shapefactorincreasesrapidlyfrom0 51to0 69withtheholding time from 30 to 180min and however a much larger holding time cannot result in a considerable change of F0 suggesting that the F0seems to reach to a maximum value It is reported that the solid phase particles tend to become spherical but for a longer holding time the change of the shape of the particles slows down and even reverses in the case of the high values of solid volume fraction 21 Keeping in mind that the high solid volumefractionmeansalsoahighcontiguity thisreversionfrom the spherical shape can be attributed to the hard impingement of the solid particles leading to the local shape distortions 21 In the present study however the solid volume fraction in the microstructuresislowerrelatively 0 6 accordingtotheresult of the quantitative analysis and consequently the chance of the hard impingement is lower as well With increase in the hold Fig 6 Relationship of the shaper factor of the Al grains ant the holding time ing time the higher curvature part of the solid particle will be dissolved and leading to the increase of the F0 Finally the pro cess reaches to a dynamic equilibrium and the shape factor of the grains will not change 4 Conclusion The semisolid structure of in situ Mg2Si Al composite is successfully produced by the cooling slope cast and partial remelting process The results show that a the morphology of primary Mg2Si phase is globular and or elliptic not changing obviously with increase in the isothermal holding time b with increase in the isothermal holding time from 30 to 600min the mean size of Al grains increases from 50 to 150 m and its morphology becomes more globular c the shape factor of the Al solid particles rapidly from 0 51 to 0 69 with the holding time from 30 to 60min Acknowledgements ThisworkissupportedbyTheProject985 AutomotiveEngi neering of Jilin University and The Innovation and Invention Foundation of Jilin University 2003CX029 References 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 50 200