1-s2.0-s1044580306000866-main

57 (2006) 371–385
Materials Characterization
Combined grain refining and modification of conventional andrheo-cast A356 Al–Si alloy

Shahrooz Nafisi 1, Reza Ghomashchi ⁎,2

Advanced Materials and Processing Research Group, University of Quebec at Chicoutimi, Chicoutimi, Quebec, Canada G7H 2B1

Received 6 June 2005; accepted 8 March 2006

Abstract

This paper describes a comprehensive study on the combined addition of Ti–B grain refiner and Sr modifier elements to A356Al–Si alloy. Using different qualitative and quantitative techniques in conventional and semi-solid metal castings, it is shown that,while the refiner and modifier elements affect respectively the nucleation and eutectic reactions, the combined addition not onlyreplicates both individual element effects but also gives the added bonus of better globularity in the semi-solid metal process. Anew innovative concept is introduced for fluidity measurement by using the magnitude of remaining liquid in the form of drainage,which is increased by combined treatment.© 2006 Elsevier Inc. All rights reserved.

Keywords: Semi-Solid Metal; Rheocasting; SEED Process; Grain refiner; Modifier; Thermal analysis

1. Introduction

Grain refining and modification of Al–Si alloys offersubstantial benefits in casting processes. Finer grainsensure better mechanical properties, improved machin-ability, better feeding, while with modification thesilicon morphology changes from flake to fibrous,resulting in improved properties, especially ductility [1–4]. Thus it is reasonable, and has become the norm forthe last two decades, to use treatments that combinegrain refining and modification, in order to takeadvantages of both methods [5]. However, as reported

⁎ Corresponding author.E-mail address: [email protected]

(R. Ghomashchi).URL: http://www.ampr-institute.com/.

1 Research fellow, Facility for Electron Microscopy Research,McGill University, Montreal, Canada.2 Former NSERC-ALCAN-UQAC Professor and Chair holder;

Director, Advanced Materials and Processing Research Institute.

1044-5803/$ - see front matter © 2006 Elsevier Inc. All rights reserved.doi:10.1016/j.matchar.2006.03.016

by the authors [6,7], each treatment has its owncharacteristics and, with combination, considerablechanges may occur in the percentages, shapes andsizes of both primary α-Al and eutectic Si.

During the last two decades, there has been intensiveresearch into finding new and improved casting routesto compensate for some of the drawbacks of conven-tional casting routes. Such efforts eventually led to theintroduction of semi-solid metal (SSM) processing.SSM processing was realized as a side issue in a PhDstudy [8] and has since attracted more attention, in boththe scientific and the technological domains, due to themany potential technical, technological and economicadvantages.

In conventional diecasting, melt treatment, i.e.,refining and modification, is not very common. Thiscan be explained by the nature of the process, whichinvolves high cooling rates to deliver finer microstruc-tures. For SSM die casting, however, the process isdivided into a two-stage solidification, wherein a slowly

mailto:[email protected]

http://dx.doi.org/10.1016/j.matchar.2006.03.016

http://www.ampr-institute.com/

Fig. 1. Schematic of the SEED process.

372 S. Nafisi, R. Ghomashchi / Materials Characterization 57 (2006) 371–385

cooled mush (liquid and solid mixture) with a mainlyglobular structure is injected into a die assembly capableof imposing rapid cooling of the mush, i.e., rheocasting.An alternative is also recognized in which the slowlycooled and solidified mush is reheated to temperaturesabove the solidus and then injected into a die cavity toshape, i.e., thixocasting.

One of the main aims of SSM processing, withspecific emphasis on Al alloys, is to obtain a primaryphase of globular structure, in order to improve thethixotropic behavior and the mechanical properties ofthe alloy. As a result, most of the studies have focusedon globule refinement [9–12] and there has been littlepublished on the combined effects [13]. The purpose ofthe present paper is to report on the findings of thecombined effects of Ti–B and Sr-based additions to anAl–7Si–0.35Mg alloy using a recently patented SSMprocessing route involving the “swirled enthalpyequilibration device” (SEED) [14].

2. Experimental procedure

The purpose of this research was to examine theeffects of the combined addition of grain refiner andmodifier in conventional casting and in SSM processingof Al–Si alloys. The semi-solid slurry for the SSMprocessing was prepared using the SEED technology(Fig. 1).

Table 1Chemical analysis of A356 melts (wt.%)

Si Mg Fe Mn Cu

Base alloy 6.68 0.4 0.07 0.003 0.001Treated alloy 6.68 0.39 0.08 0.003 0.001

Grain refining and modification were carried outon Al–7Si–0.35 Mg alloy by adding titanium, boronand strontium in the forms of Al–5Ti–1B andAl–10Sr master alloys respectively. The additionswere made between 680 and 700 °C, after whichthe degassed melts rested for 20–30 minutes beforesampling, then stirred for two minutes immediatelyprior to sampling. The chemical compositionsbefore and after the combined treatment are givenin Table 1.

Graphite cups of 25 mm inner diameter and 5 mmwall thickness were used for conventional casting.These were immersed into the melt for approximately1 min before sampling to ensure uniform temperaturedistribution across the sample at the beginning ofsolidification. Each cup with ∼50 g of alloy wastransferred to the cooling station and two K-typethermocouples were quickly immersed into the melt,near the center and the mold wall, with thermo-couples tips located 10 mm from the bottom of thecups. Temperature readings were collected using ahigh-speed high resolution National Instrument datalogger system with a sampling rate of 10 readingsper second. For these series of experiments, thecooling rate was between 1.5 and 2 °C s−1 abovethe liquidus temperature. The analysis of thermaldata was carried out based on analysis of the Al–Sisolidification temperature [15,16] and the following

Ti B P Sr Al

0.0058 0.0001 0.0003 max. 0.000 bal.0.058 0.0098 0.0003 max. 0.014 bal.

373S. Nafisi, R. Ghomashchi / Materials Characterization 57 (2006) 371–385

critical points (temperature, T, and time, t) wereidentified:

– TnucAl: start of primary α-Al dendrites nucleation– TminAl: newly nucleated crystals have grown to suchan extent that the latent heat liberated balances outthe heat extracted from the sample

– TgAl: recalescence of steady state growth tempera-ture due to release of latent heat of primary α-Aldendrites

– ΔTRec: temperature difference between unsteady(TminAl) and steady (TgAl) states growth tempera-tures of primary α-Al particles

– tRec: time difference between TminAl and TgAl, thetimes associated with TminAl and TgAl (in theliterature, it was called the liquidus undercoolingtime, [16,18])

– Tnuceut: start of eutectic nucleation– Tmineut and Tmaxeut: minimum and maximum ofeutectic temperatures.

For SSM processing, 2 kg of the alloy were poured at645 °C into a SEED cylindrical refractory-coated steelmold of diameter 75 mm and length 200 mm (Fig. 1).The mold was initially swirled at ∼2.5 Hz for a certainduration to form approximately 0.2–0.3 fraction solid.After a short resting period, the bottom enclosure of themold was opened to drain the remaining liquid for aspecific period of time. The overall casting proceduredid not exceed ∼90 s. After draining, the billet wasquenched into cold water. The quenching temperaturewas 596±2 °C. In all experiments, a 0.8 mm diameterK-type thermocouple was inserted in the mold center ata distance 80 mm from the bottom of the mold to collectthermal data. The cooling rate during solidification wasabout ∼5 °C s−1 for the SEED billets.

Transverse samples were prepared by sectioning thecylindrical billets near the thermocouple tips (that is,10 mm and 80 mm from the bottom of the conventionaland the SEED billets respectively). For a completedescription of the microstructure, it was necessary tocharacterize and quantify such microstructural para-meters as the primary α-Al particle size, grain size,number density, sphericity3, and area to perimeter ratio,A/P, which is proportional to the inverse of the surfacearea per unit volume4, Sv. For this purpose, a Clemex

3 Sphercity ¼ 4pAP2

, where A is total area of primary particles and Pis perimeter of liquid–solid interface. The closer the sphericity to 1,the higher is the globularity of the particle.4 Sv¼ 4P

pA, where Sv is specific volume surface of the particles (an

estimation of 3D).

image analysis system was used and a routine written todifferentiate effectively the contrast variations betweenthe eutectic and the primary α-Al particles, and a macro-etching technique was employed for better distinction. Inorder to obtain a representative structure, all the datawere obtained from image processing of the resultantmicrostructure between the center and the wall surface ofthe billet. A total of 85 randomly selected fields with thetotal area of 255 mm2 were scanned (at a magnificationof 50×) for the α-Al particle measurements, while 50fields with a total area of 1.5 mm2 (magnification 500×)were examined in order to evaluate the siliconmorphology (conventional cast specimens only).

Because the silicon morphology changes due to theaddition of Sr in the SEED process were beyond theresolution limit of the light optical microscope of theimage analysis system, a Hitachi S4700 scanningelectron microscope (SEM) was used to characterizethe structure using deep-etched (10% HF) specimens.

3. Results and discussion

3.1. Conventional casting

3.1.1. Thermal analysisFig. 2 shows the nucleation and growth segments of

typical cooling curves for primary α-Al formation andthe eutectic reaction of both untreated and treated alloys.As is evident, the nucleation and growth temperatureshave shifted up for the primary α-Al, while the eutecticreaction has been suppressed to lower temperatures as aresult of the grain refiner and modifier co-addition.

From the solidification point of view, Fig. 2(A), asthe temperature falls at a given cooling rate below thenucleation temperature, TnucAl, the nucleation rate risesexponentially to such an extent that it compensates heatextraction and eventually overrides it after the minimumtemperature, TminAl. In other words, the evolution oflatent heat retards the cooling rate up to the minimumand thereafter the melt is reheated to TgAl due mainly tothe growth of primary nuclei. It is quite important tonote that by the time recalescence is complete, all thenucleation opportunities have ceased and the final graindensity is predictable.

The effects of separate individual additions of refinerand modifier to the A356 Al–Si alloy have beenreported previously by the authors [6,7]. Individualadditions of the grain refiner raised the primary α-Alnucleation temperature by as much as 4–5 °C, whilesole additions of the Sr-based modifier depressedeutectic temperature by almost 7–8 °C. In otherwords, if both additives performed in the same manner

Fig. 2. Cooling curves before and after additions. (A) Beginning of solidification; (B) Eutectic reaction zone.


in the combined case, the solidification range (TnucAl–Tnuceut) should increase by 11–13 °C. This, in fact, iswhat was observed in the present case for the combinedaddition (see Fig. 3). Widening the solidification rangemay not be regarded important for conventional castingor even be a drawback due to increasing the incident ofporosity, but it is quite crucial for SSM processes.Furthermore, development of a wider freezing rangeslows down directional solidification, which is again apoint in favor for SSM processing.

As was mentioned previously, the small addition ofTi and B shifted the curves up and recalescencedecreased. In other words, the addition of grain refinershas catalyzed the nucleation of primary α-Al sooner andwith more nuclei in the melt. Furthermore, bothnucleation and growth temperatures of the α-Al

particles increased but the temperature rise was greaterfor TnucAl, as can be seen clearly in Fig. 3(A). Thismeans that there are more nuclei with a lower chance ofgrowth at any one time during nucleation and earlygrowth of the primary phase.

Fig. 3(C) and (D) show the changes in Tnuceut,Tmaxeut, eutectic undercooling (Δθ=Tmaxeut−Tmineut),and the solidification range (ΔTα) due to the combinedtreatment. As has been shown in previous publications[6,7], the changes in eutectic temperature parameters arethe same for the individual and the combined treatments,which may confirm the independent functions of thegrain refiner and the modifier during solidification. Inother words, the strong affinity of B for Ti to form TiB2

nucleants impedes any reaction between Sr and B to formSrB6 compounds and thus deactivate Sr as a modifier.

Fig. 3. (A, B) Variation of nucleation and growth temperatures of α-Al particles due to the grain refiner effect. (C, D) Variation of nucleation, maximum, and recalescence of eutectic andΔTα due to themodifier effect.

375S.

Nafisi,

R.Ghom

ashchi/Materials

Characterization

57(2006)

371–385


It is evident from Fig. 3(C) and (D) that the eutecticnucleation temperature and eutectic maximum temper-ature decreased with the addition of Sr, while at the sametime, the eutectic undercooling, Δθ, increased by∼2 °C. In addition, the depression of the eutectictemperature gave rise to a larger α-Al solidificationrange and consequently more α-Al formation. Theinteresting point here is that the increase inΔTα is a littlemore here when comparison is made with the Sraddition alone since the addition of the refiner broughtabout an increase in the α-Al particle nucleationtemperature and in the solidification range.

3.1.2. MicrostructureThe light optical micrographs in Fig. 4 illustrate the

resulting microstructural changes due to combinedtreatment. The primary α-Al phase has a fully columnar(dendritic) and transforms to an equiaxed morphology

Fig. 4. Light optical micrographs showing the effect of the combined treatmenTi, 98 ppm B, 140 ppm Sr.

with the combined addition. Such an effect is believed tobe solely due to the grain refiner segment of thecombined treatment. The eutectic mixture is alsoaffected by this treatment, with the flake silicon presentin the untreated specimen (Fig. 4(A) and (B)) transform-ing fully to a fibrous structure (Fig. 4(C) and (D)), theeffect being due to the Sr content.

The correlation between the average grain sizemeasurement, the α-Al nucleation temperature and theliquidus under-cooling time is demonstrated in Fig. 5.With the simultaneous addition of Ti, B and Sr, the grainsize was reduced more efficiently than when the grainrefiner was added separately, the final grain size being∼520 μm (a 56% reduction as against 33% by therefiner by itself). It seems that combined effect givesbetter refinement than the addition of individual masteralloys. Such a conclusion may be reached if therecalescence values for the combined (−0.5 to −0.1)

t. (A, B) Untreated base alloy; (C, D) alloy after the addition of 580 ppm

Fig. 7. SEMmicrographs of selected samples, deep-etched in 10% HF.(A) Base alloy without any additions; (B) alloy with the addition of580 ppm Ti, 98 ppm B, 140 ppm Sr.

Fig. 6. Polarized light micrographs showing the effect of Ti, Baddition: (A) base alloy without any additions; (B) alloy with theaddition of 580 ppm Ti, 98 ppm B and140 ppm Sr.

Fig. 5. Relationship between grain size and the parameters TnucAl and (TgAL–TminAl).


Fig. 8. Microstructural parameters of eutectic Si. (A) Si %; (B) number density of Si particles; (C) circular diameter and area/perimeter ratio; (D) percentage of particles having aspect ratio greater than 2.

378S.

Nafisi,

R.Ghom

ashchi/Materials

Characterization

57(2006)

371–385


and the sole treatment (−1.0 to −0.5) are compared. It iswell established that the smaller the magnitude ofrecalescence, the more effective is the grain refiner.

Interestingly, by calculation of the liquidus under-cooling time, tRec, it was observed that this timedecreased by refiner addition. A small value of tRecsignifies that the grains do not have a long growthperiod. In other words, longer values of tRec are relatedto more growth opportunity and consequently moredendrite formation. This is in line with the micro-graphs shown in Fig. 6 and with previous findings onthe effects of refiner additions to the Al–Si alloys[17,18].

The refiner addition gave rise to an increase in the α-Al nucleation temperature due to the presence of morepotent and effective nucleants in the bulk liquid. Whenthere is a high density of nucleants in the system, i.e.,multiplication or copious nucleation, the mean free path

Fig. 9. Light optical micrographs showing the effect of simultaneous melt trewith the addition of 580 ppm Ti, 98 ppm B, 140 ppm Sr.

between the nuclei becomes small and thus grain growthis restricted. Furthermore the heat flow becomesmultidirectional due to the high density of nucleants.Such solidification conditions should eventually lead tothe formation of somewhat spherical primary phaseparticles, Fig. 6.

As mentioned in the previous section, the Sr contentof the combined treatment modifies the morphology ofeutectic Si. This can be seen in the two SEMmicrographs in Fig. 7, where the change of the flakystructure of the Si to a fibrous one and a seaweed-likestructure is visible.

3.1.3. Image analysisFig. 8 shows the data obtained from image proces-

sing of the microstructure. The percentage of Si in themicrostructure increases with the addition of themodifier Sr, as expected based on the thermal analysis

atment in the SEED process. (A, B) Untreated base alloy; (C, D) alloy

Fig. 10. Variation of silicon morphology due to Sr addition: (A)without Sr, (B) 140ppm Sr.

Fig. 11. (A) Polarized light micrograph and (B) schematic represen-tation of dendrites transformation to cramped dendrites due to stirring.


described in the previous section. The addition of Srdepresses the eutectic point and shifts it to the right. Asimple lever rule calculation on the Al–Si phasediagram confirms this conclusion.

The transformation of Si flakes to a fibrousmorphology is also visible through the number density,the area-to-perimeter ratio, and the aspect ratio, asshown in Fig. 8. With the addition of the modifier, thenumber density increased while the average circulardiameter, the area/perimeter ratio, and the aspect ratiodecreased.

It should be noted that a correlation always existsamong the above-mentioned parameters to confirm themorphological changes of flake to fibrous growth of theeutectic Si. The rapid rise of the number density of the Siparticles together with the reduction in the eutectic Sisize in the form of the circular diameter and the loweraspect ratio verify a greater number of Si branchesintersecting the 2D polished surface. This is character-

istic of highly branched structures, e.g., fibrousmorphology.

3.2. SEED process

3.2.1. MicrostructureThe light optical micrographs in Fig. 9 show the

effect of the combined treatment. Grain refiningefficiency is obvious through the size reduction of theglobules and the presence of more primary particles.The Sr modifier effect, the morphological change of theSi particles from lamellar to fibrous, can be seen in thehigher magnification images of Fig. 10.

It may be argued that there is not much differencebetween the morphology of the α-Al particles of theuntreated alloy and that of the treated alloy. For theuntreated alloy it has to be mentioned that, in mostcases, the α-Al particles have a complex morphologyand therefore two-dimensional characterizations are notgood enough to comprehend the complete character-istics of the particles. In other words, what is supposedto be one globule may have stretched or interconnected

Fig. 13. SEM micrographs of selected SEED samples, deep etched in10% HF. (A) Untreated base alloy; (B) alloy after treatment with580 ppm Ti, 98 ppm B, 140 ppm Sr.

Fig. 12. Polarized light micrographs of SEED quenched billets. (A)Untreated base alloy, (B) alloy after treatment with 580 ppm Ti,98 ppm B and 140 ppm Sr.


to other globules. This concept is illustrated in Fig. 11,a magnified polarized light micrograph of an actualpolished sample together with a model proposed forthis evolution. By stirring and forced flow, i.e.,convection, pre-formed dendrites are compelled eitherto break up by fragmentation [20] or by a root re-melting mechanism [21], or, in another case, may beplastically bent, the final shape being a “crampeddendrite”. Fig. 11 confirms that the individual globulescould indeed be part of one single cramped dendriteand only appeared as globules due to metallographicsectioning. In order to determine whether the globulesare independent grains or segments of larger crampedα-Al phase, polarized light microscopy must be used.The polarized light micrographs in Fig. 12 confirm theeffectiveness of the combined treatment in generatingbetter globularization of the primary α-Al. As isevident, the number of globules in the α-Al particlescolony is higher in the untreated specimen. The

optimum or, indeed, the desired structure is that onegrain in the macrostructure has the same size of oneglobule.

The effect of the combined treatment in renderingbetter globularity of the primary α-Al particles couldalso be explained in terms of the modifier and itseffect on the morphological changes in the Si. This isdue to the growth nature of the fibrous structure of Siin the eutectic mixture, which encompasses theprimary α-Al phase without being a continuation ofit. Therefore, the globules are indeed true globuleswhen Sr is added. As was shown by Dahle et al. [19],in unmodified alloy the eutectic grows from theprimary phase, while for Sr-modified alloy eutecticgrains nucleate and grow separately from the primarydendrites.

The Si morphological changes due to the combinedtreatment are illustrated by the SEM micrographs in

Fig. 14. Microstructural parameters of the SEED samples. (A) Primary α-Al percentage; (B) circular diameter and number density of the α-A rticles; (C) area/perimeter ratio and aspect ratio; (D)sphericity.

382S.

Nafisi,

R.Ghom

ashchi/Materials

Characterization

57(2006)

371–385

l pa


Fig. 13. The flaky structure of the Si has changed to afibrous morphology having seaweed-like attribute.Furthermore, in contrast to the conventional castspecimens (see Fig. 7), extra refinement of thestructure is attributed partly to the quenching of theSSM slurries from the mushy zone and partly to thefluid flow and thermal uniformity within the SSMmold. This means that branching of the Si fibers is nothindered and it can branch off in any direction sincethere is no directional heat flow in the mold (for moredetails, see [7]).

3.2.2. Image analysisAs discussed in the previous sections, the

microstructure is quite fine in the SSM billets andoptical quantitative metallography was not viable forthe eutectic Si. Therefore, the results as presented inFig. 14 are concentrated solely on the primary α-Alphase.

The primary α-Al percentage appears to haveincreased which is due to the higher number of effectivenuclei from the refining side plus the increase in the α-Al solidification range arising from the Sr, Ti, and Badditions (the combined treatment increased the α-Alsolidification range by 11–13 °C).

It was shown previously that with modificationalone the number density of the primary α-Al particlesremained unchanged while the quantity increased andhence the average circular diameter rose. This is not thecase for the combined treatment in this situation, as canbe seen in Fig. 14(B), where the average circulardiameter decreased slightly but the change in numberdensity is more pronounced. This is attributed to the

Fig. 15. Drainage of liquid du

grain refining effect in shifting the nucleation temper-ature to higher values and causing the formation ofmore nuclei per unit volume. The greater number ofnuclei compensates for the enlargement of the solidi-fication range due to the combined treatment. Further-more, the average area/perimeter ratio is approximatelyconstant while the percentage of α-Al particles withaspect ratio >2% decreases. Such a finding, coupledwith the increasing sphericity values as shown in Fig.14(D), is an indication of improving globularity of thestructure.

As was mentioned in the experimental proceduresection, drainage is part of the SEED process to providea self-standing billet, but it is the belief of the authorsthat it can also be an indication of the fluidity of thealloy. Here it has to be noted that this may becontradictory to the classical meaning of fluidity,which is normally defined as the distance to which ametal will run before solidification. The drainagepercentage increased with the combined treatment, ascan be seen in Fig. 15. Such a finding may lookcontradictory to the fact that, by increasing the primaryα-Al percentages, the drainage should decrease due tothe agglomeration and blockage of the mold bottomorifice.

Drainage augmentation could be explained fromdifferent aspects. According to the literature onconventional casting [22], the dendrite coherencypoint, DCP, has a direct relationship with the fluidityconcept, where in a postponed DCP improves fluidity,i.e., smooth channels without interlocking pathways forliquid flow. On the other hand, from the modificationpoint of view, Sr postpones the DCP, meaning that, in

ring the SEED process.


spite of the increasing α-Al percentage, more flow existsin the slug. This is supported by the better sphericitynumbers where more globular particles are formed withthe combined treatment. That is to say, there aresmoother inter-particle channels in which liquid canflow. These are in line with the increased fluidity of themodified and refined alloys [23–25].

4. Conclusions

A comprehensive study has been reported on thecombined addition of grain refiner and modifierelements to A356 Al–Si alloy. Using different qualita-tive and quantitative techniques in conventional andsemisolid castings, it has been shown that, while therefiner and modifier elements affect, respectively, thenucleation and eutectic reactions, combined addition notonly replicates both effects but also gives the addedbonus of better globularity in the SSM process. A newinnovative concept is introduced for fluidity measure-ment by using the magnitude of remaining liquid in theform of drainage, which is increased by combinedtreatment.

Acknowledgments

The work reported here is part of an NSERC-ALCAN-UQAC industrial research chair, grant No.IRCPJ268528-01, on the “Solidification and Metallur-gy of Al alloys”. The financial support provided byNSERC and ALCAN International Limited is grate-fully acknowledged. Special thanks are due toProfessor H. Vali, Director of Electron Microscopy,and Mrs. L. Mongeon of the Facility for ElectronMicroscopy Research at McGill University for theSEM analysis.

References

[1] McCartney DG. Grain refining of aluminum and its alloys usinginoculants. Int Mater Rev 1989;34(5):247–60.

[2] Closset B, Gruzleski JE. Structure and properties of hypoeutecticAl–Si–Mg alloys modified with pure strontium. Met Trans A1982;13A:945–51.

[3] Argo D, Gruzleski JE. The fluidity of sodium and strontiummodified sand cast aluminum-silicon foundry alloys. Cast Met J1989;2:109–12.

[4] Gruzleski JE, Closset BM. The treatment of liquid aluminum-silicon alloys. Schaumburg, IL: American Foundrymen'sSociety; 1990.

[5] Bondhus E, Sagstad T. Strobloy, the new combined grain refinerand modifier for hypoeutectic Al Si foundry alloys. In: PetersonRD, editor. Light Metals 2000. Warrendale, PA: TMS Publica-tions; 2000. p. 845–9.

[6] Nafisi S, Ghomashchi R. The influence of grain refining on themicrostructural evolution of A356 alloy: classical view inconventional and semi-solid metal casting. J Mater Proc Tech2006;174:371–83.

[7] Nafisi S, Ghomashchi R. Effects of modification duringconventional and semi-solid metal processing of A356 Al–Sialloy. J Mater Sci Eng A 2006;415:273–85.

[8] Spencer DB. PhD Thesis, Massachusetts Institute of Technology,Cambridge, MA; March 1971.

[9] Wan G, Witulski T, Hirt G. Thixoforming of Al alloys usingmodified chemical grain refinement for billet production. Proc.Intl. Conf. on Aluminum Alloys: New Process Technologies”,Ravenna, Italy, June 1993. Milan, Italy: Associazione Italiana diMetallurgia; 1993. p. 129–41.

[10] Ferrante M, Freitas E, Bonilha M, Sinka V. Rheologicalproperties and microstructural evolution of semi-solid aluminiumalloys inoculated with mischmetal and with titanium. In: BhasinAK, Moore JJ, Young KP, Midson S, editors. Proc. 5th Intl.Conf. on Semi-Solid Processing of Alloys and Composites,Golden, CO, June 1998. Golden, CO: Colorado School of Mines;1998. p. 35–42.

[11] Brusethaug S, Voje J. Manufacturing of feedstock for semi-solidprocessing by chemical grain refinement. In: Chiarmetta GL,Rosso M, editors. Proc. 6th Intl. Conf. on Semi-Solid Processingof Alloys and Composites, Turin, Italy, September 2000. Brescia,Italy: Edimet Spa; 2000. p. 451–6.

[12] Pan QY, Arsenault M, Apelian D, Makhlouf MM. SSMprocessing of AlB2 grain refined Al–Si alloys. AFS Trans2004;112:4–53.

[13] Grimmig T, Aguilar J, Fehlbier M, Bührig-Polaczek A.Optimization of the rheocasting process under consideration ofthe main influence parameters on the microstructure. In: ApelianD, Alexandrou A, editors. Proc. 8th Intl. Conf. on Semi-SolidProcessing of Alloys and Composites, Limassol, Cyprus,September 2004.

[14] Doutre D, Hay G, Wales P. Semi-solid concentration processingof metallic alloys. United States Patent No. 6428636; 2002.August 6.

[15] Backerud L, Chai G, Tamminen J. Solidification characteristicsof aluminum alloys: Volume 2 Foundry alloys. Des Plaines, IL:American Foundry Society Inc.; 1990.

[16] Tuttle BL. Definitions in thermal analysis. Modern casting; 1985.p. 39–41. November.

[17] Charbonnier J. Microprocessor-assisted thermal analysis testingof aluminum alloy structures. AFS Trans 1984;92:907–22.

[18] Gloria D. Control of grain refinement of Al–Si alloys bythermal analysis. PhD Thesis, Department of Mining andMetallurgical Engineering, McGill University, Montreal,Canada; 1999. June.

[19] Dahle AK, Nogita K, Zindel JW, McDonald SD, Hogan LM.Eutectic nucleation and growth in hypoeutectic Al–Si alloys atdifferent strontium levels. Met Trans A 2001;32A:949–60.

[20] Flemings MC. Solidification processing. New York, NY:McGraw-Hill; 1974.

[21] Hellawell A. Grain evolution in conventional and rheo-castings.In: Kirkwood DH, Kapranos P, editors. Proc. 4th Intl. Conf. onSemi-Solid Processing of Alloys and Composites, Sheffield,UK, June 1996. Sheffield, UK: University of Sheffield; 1996.p. 60–5.

[22] Chai G, Rolland T, Arnberg L, Backerud L. Studies of dendritecoherency in solidifying aluminum alloy melts by rheologicalmeasurements. In: Brown SB, Flemings MC, editors. Proc. 2nd


Intl. Conf. on Semi-Solid Processing of Alloys and Compo-sites, Cambridge, MA, June 1992. Warrendale, PA: TMS; 1992.p. 193–201.

[23] Alsem WHM, Van Wiggen PC, Vader M. The combined effect ofgrain refinement and modification in aluminum alloys. In: CutshallEuel R, editor. Light Metals 1992. Warrendale, PA: TMSPublications; 1992. p. 821–9.

[24] Kwon YD, Lee ZH. The effect of grain refining and oxideinclusion on the fluidity of Al–4.5 Cu–0.6 Mn and A356 alloys.Mater Sci Eng A 2003;360:372–6.

[25] Dahle AK, Tondel PA, Paradies CJ, Arnberg L. Effect of grainrefinement on the fluidity of two commercial Al–Si foundryalloys. Met Trans A 1996;27A:2305–13.

1-s2.0-s1044580306000866-main

Documents

semisolid metal process

combined grain

combined treatment

semisolid metal castings

advanced materials

processing research

processing research

improved casting routesto