2002 Mech Stir Properties

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  Materials Science and Engineering A326 (2002) 370–381 Mechanical stir casting of aluminium alloys from the mushy state:process, microstructure and mechanical properties D. Brabazon, D.J. Browne *, A.J. Carr Department of Mechanical Engineering  ,  Uni   ersity College Dublin ,  Belfield  ,  Dublin  4  ,  Ireland  Received 2 April 2001; received in revised form 29 August 2001 Abstract A comprehensive study was carried out to establish the effects of controlled stirring during solidification on the microstructureand mechanical properties of aluminium alloys, in comparison to conventionally gravity chill cast material. A novel devicecomprising a grooved reaction bonded silicon nitride rod rotating in a tube-like crucible was used to process aluminium alloys inthe mushy state. The stir casting device was specially designed to also enable rheometric study of the alloys in this condition. Afactorial design of experiments was used to determine the effect of the process variables shear rate (    ), shear time ( t s ), and volumefraction solid during shear (  f  s ) on microstructure and both static and dynamic mechanical properties of the stir cast alloy.Investigation of the microstructure consisted of computer-aided image analysis of the primary phase morphology. A moreglobular primary phase was achieved at low values of   f  s , but this was not the optimum morphology for mechanical properties.In all cases, improved mechanical properties and reduced porosity were obtained in the stir cast condition in comparison withconventional casting and in comparison with previous work on stir casting. Comparison with alloy commercially rheocast viaelectromagnetic stirring, however, showed that the latter had superior mechanical properties. It is proposed that the mechanicalstir casting process be considered as an alternative to gravity die casting in cases where very simple and thick walled shapes arerequired. © 2002 Elsevier Science B.V. All rights reserved. Keywords :   Stir casting; Aluminium alloys; Image analysis; Microstructure; Mechanical properties; / locate / msea 1. Introduction In conventional casting processes, liquid metal ispoured into a mould and solidifies as heat is extractedvia the mould walls. The morphology of the growingsolid–liquid interface is typically dendritic. The naturalprogression of filling followed by solidification oftenleads to internal structural defects, such as entrainedoxide or shrinkage porosity, which combine to yield acasting of relatively poor mechanical properties.Research at the MIT in the 1970s into the rheologyof alloys in the mushy state, as reported in [1], generallyinvolved use of a rotational viscometer which acted tofragment the dendritic solid morphology in a time-de-pendent fashion, revealing the thixotropic nature of metallic materials in this state. This work inspired threedecades of subsequent research into the processing andproperties of alloys in this so-called semi-solid state, theresults of which are reported at the biennial interna-tional conference on the subject [2–4]. It is beyond thescope of this paper to present a comprehensive reviewof the field, but it should be noted that the semi-solidprocessing (SSP) of alloys is utilised in a number of manufacturing routes today for high quality aluminiumand magnesium castings, with reduced levels of castingdefects.Over the years, a number of devices have been con-structed to either investigate the rheological behaviourof semi-solid metals, or to produce billets with a non-dendritic microstructure. Rheological characterisationhas typically been carried out by an active mechanicalshearing method, normally using augers or impellersmounted on a central rotating shaft (e.g. [5–8]). Elec-tromagnetic stirring (EMS), on the other hand, due inpart to its high production rate, has become the mainmethod of producing SSP billet commercially. EMS * Corresponding author. Tel:  + 353-1-716-1901; fax:  + 353-1-283-0534. E  - mail address : (D.J. Browne).0921-5093 / 02 / $ - see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0921-5093(01)01832-9  D .  Brabazon et al  .  /   Materials Science and Engineering A 326 (2002) 370  –  381  371 also avoids contact of molten metal with stirrers, and insome cases the crucible, and may be easier to imple-ment for high temperature alloys [9  –  12].A more recently developed method of SSP has beentermed liquidus casting or the New RheoCasting pro-cess (NRC) [13  –  15]. This involves pouring the alloywith a low superheat into a chilling environment tonucleate many small grains. Once held for a shortperiod of time at semi-solid forming temperature thesegrains ripen and develop a non-dendritic morphology.NRC is beginning to be used commercially for  ‘ slurryon demand ’  SSP whereby the production of non-den-dritic alloy and subsequent forming via die casting areintegrated into one operation.For the commercial production of industrial compo-nents via the SSP route, the  fi nal step is often that of high pressure die casting of mushy alloy with non-den-dritic or globular morphology; a process known as thixocasting  . And the process by which the requisitestarting globular structure is attained has becomeknown as  rheocasting  .The thixocasting step produces near net shape prod-ucts, and adds signi fi cant value to the alloy. For thesereasons there have been many studies of the effects of thixocasting process variables on microstructure andproperties of the product. There have also been studiescarried out on the mechanical properties of rheocastmaterials, but these have not been comprehensive. Tothe knowledge of the authors, there has been no de-tailed quantitative study on the effects of rheocastingvariables on the microstructure and both static anddynamic mechanical properties of the rheocast material.This was an oversight, because it is possible that suf  fi -cient improvements in quality and mechanical proper-ties of alloys could be produced via rheocasting suchthat, in certain cases, the additional cost of thixocastingwould not be justi fi ed. For example, in cases where thinwalls and  fi ne detail are not a feature of a part,rheocasting could be considered as an alternative togravity die casting. For this reason, in addition toscienti fi c interest, such a study was undertaken by theauthors.It was decided that use of mechanical stirring was themost direct and cost effective way of altering shear rate,and because of this, in addition to enabling rheometry,a mechanical rheocaster was designed and built. Thedesign brief was to produce materials with a range of microstructures, from fully dendritic to fully globular.At this point a note on terminology is appropriate. Themost common SSP route today is rheocasting to pro-duce a solid billet, and later reheat of this billet fol-lowed by thixocasting. However, in cases where there isno intermediate billet solidi fi cation stage, the process of die casting a rheocast slurry has also been referred to asrheocasting [1]. As the current work is not directlyconcerned with the production of feedstock for thixo-casting, and also to avoid confusion with the latterde fi nition of rheocasting,  stir casting   [16] has beenadopted to describe the current process. The principalinterest here is the difference between conventionalgravity die casting and the casting of sheared semi-solidalloy into a metal mould.The distinguishing features of the new mechanicalstir caster were to include:1. a capability for semi-continuous stir casting of alu-minium alloys;2. top feeding with liquid, and semi-solid poured in acontinuous laminar stream from the bottom;3. a well-de fi ned shear zone in which process parame-ters could be closely controlled;4. shear zone design to avoid porosity-inducing vortexformation;5. dual purpose i.e. to act also as a rheometer;6. use of unique rotor and crucible materials to enablecontinuous clean operation.Some of these features are shared with other devices,but this stir caster is unique in its design and in that ithas  all   of these attributes. The authors have establishedthat at least 30 mechanical stirring systems have beenconstructed over the past three decades, and it is practi-cal to cite only some examples here. The devices havebeen used either for rheological [5  –  8] or stircasting[16  –  18] investigations, but rarely for both. Most previ-ous systems are also much smaller than the one pre-sented here.This study involved a Taguchi designed test pro-gramme to reveal microstructural features and deter-mine mechanical properties, including toughness andfatigue performance, of stir cast materials, in compari-son to conventionally cast material. In this way, theproperties of the processed alloys could be related tothe microstructure, and conclusions drawn regardingnot only optimum microstructures, but also optimumprocessing conditions. Image analysis techniques wereused to supply quantitative data on the microstructure.This follows the previous work on microstructuralcharacterisation of EMS [19] and mechanically [20]rheocast alloys. 2. Experimental 2  . 1 .  Stir caster design A mechanical stir caster / rheometer as illustrated inFig. 1, was designed and built to produce the variouscast morphologies. The semi-solid alloy was sheared ina heated tubular zone between a grooved rotor and acrucible. An independent in-line torque meter was posi-tioned between the stirring rotor and the drive motor toenable rheological measurements. The caster furnacewas heated by means of four resistance heating ele-  D .  Brabazon et al  .  /   Materials Science and Engineering A 326 (2002) 370  –  381 372 ments. One element around the wide reservoir at thetop of the crucible and three along the lower narrowsection were used to control the temperature in thesemi-solid range of the alloy. This con fi guration en-abled a maximum temperature of 850  ° C and controlof the temperature gradient within the narrow sectionof the crucible, where the shearing occurred. A lineardrive provided lift to the rotor, enabling evacuation of the stir caster after the desired period of shear. Duringshear, with the rotor in the lower position, the devicealso acted as a rheometer.The rotor and crucible (Fig. 2) were both, uniquely,of Reaction Bonded Silicon Nitride (RSBN), whichenabled these two parts to be easily lapped togetherduring operation of the stir caster. RBSN has goodthermal shock resistance, good high temperaturestrength, does not contaminate the melt, and has a lowcoef  fi cient of thermal expansion and moment of inertia.An additional external immersion heating elementwas needed in the reservoir to provide suf  fi cient moltenalloy there for an adequate metallostatic head for stircasting at higher fractions solid. A batch casting trolley,which also held a plug against the crucible outlet, wasused to carry the chill moulds into which the stir castmaterial poured. Control of stirring speed, stirring time,stirrer height, and the temperature pro fi le of the fur-nace, was implemented on a PC by means of   LABVIEW control software, and data input and output controlboards. The software also displayed and logged thestirring speed, height of the stirrer, temperatures in thefurnace, and the torque experienced by the stirrer, on areal time basis. Apparent viscosity, shear rate, andshear stress were also calculated and logged againsttime by the program. Detailed design, construction andoperation of this stir caster have been previously de-scribed [21]. 2  . 2  .  Operation of the stir caster When setting up the stir caster before an experimentthe rotor was  fi rst lowered into the crucible, Fig. 1. Itsheight was accurately adjusted to form a partial seal atthe exit such that it was held concentrically duringstirring. Only a partial sealing of the outlet was allowedto ensure that torque pick-up from the rotor-crucibleinteraction was negligible. An external plug attached tothe batch casting trolley provided a full seal at the exit.After the caster set-up, metal melted in an inductionfurnace was transferred to a resistance holding furnacewhere it was stabilised at a temperature 20  ° C abovethe liquidus temperature. The melt was then pouredinto the stir caster furnace which had been preheated to570  ° C for A356 and to 595  ° C for Al  –  4%Si. Once thetemperature of the semi-solid melt ( T  ss ) was stabilised,giving the desired  f  s , via the element controllers, rota-tion of the stirrer was started. After shearing the alloyat the speci fi ed shear rate and for the speci fi ed length of time, the rotor was raised, the plug on the batch castingtrolley was released and the alloy allowed to  fl ow intoa 35 mm diameter cylindrical steel mould, of height90 mm.Conventional gravity chill castings, poured from20  ° C above the liquidus, were also made in thesemoulds, for comparison purposes.The resultant bars were examined radiographically.Quality indicator wire showed that a resolution of about 0.1 mm could be obtained from the procedure. 2  . 3  .  Thermal analysis The slope of the  T  ss  –   f  s  curves close to the eutectictemperature affects  f  s  control. With too small an abso-lute slope here, accurate  f  s  control becomes dif  fi cult[17]. The upper limit of   f  s  at which stir casting ispossible depends on the stir casting device. For exam-ple, local solidi fi cation may occur due to a lack of accurate temperature control, or insuf  fi cient motortorque may be available to stir the more solid structure. Fig. 1. Schematic of stir casting device.Fig. 2. RBSN ceramic crucible reservoir and rotor.  D .  Brabazon et al  .  /   Materials Science and Engineering A 326 (2002) 370  –  381  373Table 1Chemical composition of A356 and Al  –  4%Si alloys (in wt.%)Si Cu Mg Fe Mn Ti Ni Zn Pb Sn Al7.14A356 0.1 0.4 0.31 0.12 0.14 0.013 0.056 0.07 0.007 Bal0.004 0.01 0.173 0.005 0.007 0.0054.02 0.013Al  –  4%Si 0.004 0.006 BalTable 2Stir casting parameters used for producing test bars     (s − 1 )  f  s Material  T  s  (  ° C)Material type  t s  (s)  –  1  –  Al  –  4Si chill cast  – –  54.93 0.36Al  –  4Si stir cast 6302 60112.843 0.36Al  –  4Si stir cast 630 60  – –  A356 chill cast  –  4  –  54.93 0.35 601A356 stir cast 60112.84 0.3A356 stir cast 6016 607 54.93A356 stir cast 0.3 601 300112.84 0.3A356 stir cast 6018 30054.93 0.259 605A356 stir cast 60112.84 0.25A356 stir cast 60510 6054.93 0.25 605 30011 A356 stir cast112.84 0.25A356 stir cast 60512 300  – – – –  13 EMS rheocast Higher fractions solid may be stir cast by using arelatively large liquid metal head, to provide a pressureon the semi-solid material during stir casting, and / or bykeeping the caster exit well insulated to avoid localsolidi fi cation. The  T  ss  –   f  s  relationship and coherencypoints for the alloys under investigation, A356 andAl  –  4%Si, were determined using thermal analysis fol-lowing the methodology of Ba ¨ ckerud et al. [22]. Suchthermal analysis of A356 has been carried out previ-ously [22,23] but at faster cooling rates. A slow coolingrate of 0.06  ° C s − 1 (the slope of the cooling curve aftersolidi fi cation) was used in this work in order to matchthe stabilised temperatures in the experiments. Thechemical compositions of the alloys used are shown inTable 1. 2  . 4  .  Design of experiments Process parameters used for the stir casting experi-ments may be seen in Table 2. Those listed for A356follow a Taguchi factorial design [24] with three factors(    ,  f  s ,  t s ) and two levels (2 3 ). Conventional chill castspecimens (materials 1 and 4), poured from 20  ° Cabove the liquidus temperatures, were tested and theresults compared with those obtained for the stir cast-ings. An upper fraction solid of 0.3 was used for A356to ensure  fl uid castings and a lower fraction solid of 0.25 was used to ensure that the alloy was above thecoherency point (the fraction solid at which equiaxeddendritic grains start to impinge upon one anotherunder normal solidi fi cation conditions). The levels of shear rate were chosen with the lower value about half of the upper one. Previous work (e.g. [8,25]) has shownthat particle size diminishes early on during shear dueto morphological disintegration, but begins to increaseat extended shear times due to primary phase coarsen-ing and coalescence. In order to avoid the latter effect,shear times were restricted to 5 min. Material 13 iscommercial EMS rheocast A356 alloy from a Europeansupplier. 2  . 5  .  Metallography and image analysis Samples cut from the stir cast bars were prepared formetallographic examination. A  fi nal hand polish wasperformed on Selvyt cloth with  ‘ Brasso ’  metal polish[26]. This  fi nal polishing stage also served to etch theAl  –  4%Si microstructure. A356 was etched with Keller ’ sreagent.Particle size, distribution, and shape were investi-gated by image analysis techniques. Analysis was per-formed on a PC using the  IMAGETOOL  program(developed at the University of Texas Health ScienceCentre, San Antonio). The primary phase particles andagglomerates that do not connect with neighbouringprimary phase were analysed as isolated particles [27].The equivalent average diameter ( D ) of the isolatedparticles was calculated from their average area ( A )according to Eq. (1): D =  4 A   1 / 2 (1)

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