Microstructural Evolution of Electromagnetically Stirred Feedstock ...

Metallogr. Microstruct. Anal. (2013) 2:96–106 DOI 10.1007/s13632-013-0065-9

TECHNICAL ARTICLE

Microstructural Evolution of Electromagnetically Stirred Feedstock SSM Billets During Reheating Process S. Nafisi • R. Ghomashchi

Received: 6 January 2013 / Revised: 11 February 2013 / Accepted: 21 February 2013 / Published online: 8 March 2013 Springer Science+Business Media New York and ASM International 2013

Abstract Semi-solid metal (SSM) processing is an effective alternative to the classical manufacturing processes of casting and forging. The key issue in SSM is the production of suitable feedstock with guaranteed continuous supply. To produce good quality billets, the current work studied the thixocastability of electromagnetically stirred (EMS) Al–7Si alloy billets and compared them with conventionally cast billets to establish the effect of EMS in improving the quality of feedstock. In all cases, the intention was to study the microstructure of the billets just before feeding it into the high pressure die casting machines and therefore the billets were reheated to thixocasting temperature and isothermally held for 10 min before being quenched in water. The outcome of EMS application was the formation of refined and more globular SSM billet structures with less entrapped eutectic. The implication of pursuing such work is to guarantee continuous supply of feedstock to enable the establishment of manufacturing facilities, similar to mini mills for steel industry, where the feed stock is shipped to casting site to be shaped in high pressure die casting machines. Keywords

Stirring EMS Thixocasting Superheat

S. Nafisi (&) EVRAZ INC. NA, P.O. Box 1670, Regina, SK S4P 3C7, Canada e-mail: [email protected] R. Ghomashchi School of Mechanical Engineering, The University of Adelaide, Adelaide, SA 5005, Australia e-mail: [email protected]

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Introduction The concept of semi-solid metal (SSM) forming is over 40 years old. The original experiment leading to the invention of semi solid processing of metals was performed in early 1971, as part of a doctoral thesis at MIT, USA [1]. Since then, a large number of investigations have been carried out to both understand the process scientifically [e.g., 2–4] and find ways to implement the concept industrially [e.g., 5–10]. Basically, the process of SSM casting is carried out directly from the melt in one of two methods. In the rheocasting process, the slurry (liquid and solid mixtures) is prepared by stirring the superheated molten metal as it cools down to the mushy zone (the region between liquidus and solidus on phase diagram). In the thixocasting process, reheating of the alloy to temperatures above solidus and holding it isothermally within the mushy zone induces the desired structure [11]. One of the important issues in deciding whether to switch to one specific SSM fabrication route is the supply of feedstock. The term slurry-on-demand (SoD) describes the slurry-making operations to provide a constant supply of slurry for shaping operations. This may include the billets (slugs) produced via rheocasting, stored and used later by simple and short reheating to mushy zone. This in fact prescribes the SoD procedure based on a combination of rheocasting and thixocasting. Such an approach could be beneficial to establish manufacturing plants in regions with restricted environmental regulations while the feedstock can be produced in specialized sites and shipped to the manufacturing plants to fulfill the SoD requirements. This is similar to mini mills for steel industry where the feed stock is shipped to mills to be reheated and rolled to plates and sheets.

Metallogr. Microstruct. Anal. (2013) 2:96–106

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Fig. 1 (a) Casting facilities, (b) schematic details of the water-cooled copper mold [12]

The current research is therefore defined to study the effect of stirring feedstock to be thixocast later to fabricate engineering components. The conventional and electromagnetically stirred (EMS) thixocast billets are examined to highlight the beneficial effects of pre-thixocast stirring and furthermore to initiate the idea of a network of distinct sites specializing in feedstock production only and finished product fabrication only, respectively.

Experimental Procedure Binary Al–7% Si alloys (6.7–6.9% Si, 0.8–0.81% Fe) were prepared by melting 99.7% commercially pure aluminum in a SiC crucible in an electric resistance furnace. The addition of silicon and iron was carried out at 720 ± 5 C using pure silicon and Al–25% Fe master alloy. For achieving different cooling rates, two different molds were used. For the higher cooling rate, a copper mold with a water cooling jacket was used; for the lower cooling rate, a CO2 bonded silica sand mold was employed. Ingots were 76 mm in diameter and 300 mm long. The entire configuration was placed in an Electro-Magnetic Stirring machine, EMS (Fig. 1). The frequency was set to 50 Hz and the current was 100 and 30 A for copper and sand molds, respectively (the application of magnetic field was stopped when the alloy temperature in the EMS molds reached 400 C). The application of different currents was intended to ensure that the applied magnetic force to stir molten alloy is almost the same for both sand and copper molds. The higher conductivity of copper mold makes it difficult for the magnetic flux to penetrate the mold which results in decreasing the electromagnetic force responsible for stirring the molten aluminum alloy. This is not the case

for sand mold. The difference in applied current was to compensate such mold characteristics and therefore to create similar convective currents in the melt for the both molds. Experimental details are explained in further detail elsewhere [12]. Pouring temperature was changed between 630 and 690 C. The cooling rate in the copper and sand molds for the conventional ingot (with no stirring) was about 4.8 and 3.3 Cs-1, respectively (the cooling rates were calculated in the liquid state above the liquidus temperature). The cooling rate in the sand mold is relatively high at the beginning of the pouring which is due to the large volume of sand compared to the liquid metal. However, after the initial rapid heat dissipation, the bulk liquid temperature decreases slowly due to low heat diffusivity of sand mold. As a result, the sand could absorb significant amount of heat after filling the mold. The cooling rate is then reduced in the mushy zone. For the experiments with no stirring, the liquid was poured into the same molds and allowed to aircool. The samples with no stirring will be referred as conventional in the remaining of this article. For thixocasting trials (reheating to semi-solid region), samples were cut from the transverse sections (200 mm from the bottom of EMS billets), in areas between the billet center and wall, and were reheated in a single coil 5 kW induction furnace operating at 80 kHz. Samples were placed vertically on an insulator plate. Temperature variation during the tests was monitored by attaching thermocouples to both the billet center and the wall. The induction furnace was controlled by the central thermocouple and the wall thermocouple was used to establish if there is any transverse temperature gradient within the billet. The reheating cycle included 2–3 min of heating up to 583 ± 3 C and 10 min holding time at this temperature

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690, conventional

690, EMS

690, conventional, thixo

690, EMS, thixo

660, conventional

660, EMS


660, EMS, thixo

630, conventional

630, EMS


630, EMS, thixo

Fig. 2 Polarized light micrographs showing the effect of pouring temperature and stirring on the grain and globule size variations in the sand mold casting, conventional, EMS cast, and thixocast samples quenched after holding at 583 C for 10 min (scale bar is 800 microns)

before water quenching. (At the selected holding temperature of 583 ± 3 C, there is about 38–40% fraction solid according to ThermoCalc calculations.) For conventional and EMS cast samples, the metallographic specimens were cut transversely at 200 mm from the bottom of the billets, mounted in Bakelite, ground, and polished conventionally down to 0.05 lm colloidal silica. Thixocast samples were prepared simply by cutting a transverse section of the quenched samples. Image analysis was employed and the entire data were obtained from image processing of the resulting microstructure. Grain and globule sizes were measured using a linear intercept method on the anodized specimens. The outputs of the image system were classified according to the purpose of the study and for the following evaluations: •

Average circular diameter of primary a-Al particles based on the diameter of a circle having the same area as the measured object: sffiffiffiffiffiffiffiffiffiffiffiffiffiffi P ffi A Pi ; D ¼ 2 P ni

where Ai is the area of each a-Al particle and n is the number of particles measured. • Number density as the total number of particles per unit area (mm-2)

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•

•

Aspect ratio is given by the ratio of the longest over the shortest Feret diameter, i.e., length per width. Feret diameter is defined as the distance between two parallel tangents on each side of an object Sphericity is given by

Sphericity ¼

4pA ; P2

where A is the total area of primary a-Al particles and P represents peripheral length of eutectic-primary particles interface. The sphericity factor varies from 0 for objects having very elongated cross sections to a value of 1 for those having circular cross sections.

Results and Discussion Sand Mold The polarized light micrographs in Fig. 2 show the microstructural evolution due to variant pouring temperatures, stirring application, and reheating for thixocasting. In conventional casting, the morphology of primary a-Al particles becomes more dendritic with increasing the pouring temperature (Fig. 2). In other words, the


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Fig. 3 Grain/globule size measurements in: (a) conventional and (b) EMSstirred samples, sand mold thixocast

Fig. 4 Sintering effects of globules, EMS isothermally held after pouring at 660 C

morphology of a-Al phase changes by reducing the superheat to rosette, equiaxed, and finally globules with an active ripening process, i.e., remelting of thinner arms and growth of the thicker ones. The ripening process is more effective in the sand casting due to its lower cooling rate.

The sand mold with its low heat diffusivity reduces the heat extraction rate and thus establishes a shallow temperature gradient across the bulk liquid. Such low temperature gradient promotes nuclei survival with uniform distribution and their eventual freeze off as equiaxed grains [12]. From an SSM processing perspective, microstructural evolution is mainly due to the mechanical and/or thermomechanical fragmentation of dendrites and creation of multiple nucleation sites within the bulk liquid [11]. The application of electromagnetic stirring and the resulting forced convection of the bulk liquid is shown to have generated fragmentation of the dendrites accompanied by dendrite arms root remelting due to the thermal and solutal convections [13–15]. These fragmented particles are the most favored nucleation sites for the primary a-Al phase, as they are regarded as fresh nucleants having no disregistry with the solidifying material. During isothermal holding for the thixocasting operation, the eutectic is remelted while the primary a-Al phase is still in solid form. There is also a driving force toward reduction of interfacial area between liquid and primary a-Al particles associated with the interfacial energy and promoting a-Al particles globularization.

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Fig. 5 Image analysis results from various pouring temperatures/ application of EMS, sand mold thixocast

Fig. 6 Entrapped liquid measurement, sand mold thixocast

Conventional samples with high pouring temperatures preserved their coarse dendritic structure even after being held isothermally for 10 min at reheating temperature of

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Fig. 7 Entrapped liquid in billets poured at 690 C: (a) conventional, (b) EMS (numbers the globules)

583 ± 3 C, Fig. 2. The very coarse structure seems to form a solid network with full 3D interconnections and a relatively high proportion of intragranular liquid. Reducing the superheat results in the evolution of the a-Al particles to rosette/equiaxed as shown in Fig. 2. By EMS application and consequent fragmentation of primary dendrites, the structure is still capable of undergoing further globularization through reheating process and the liquid phase is mostly intergranular. Globule size is larger within the high superheat EMS samples but reduced by lowering the superheat. This is associated with higher number of favored nucleation sites, better thermal and solute convection, and more effective fragmentation of primary dendrites. There is a difference between globule and grain size. Globules are the primary particles which are detached from each other as clearly resolved by employing polarized light microscopy; however, there is the possibility that the neighboring individual particles are interconnected underneath of the polished surface. As a result, similar color of neighboring globules specifies a particular grain. By this method, grains could be differentiated from globules and


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690, conventional

690, EMS


690, EMS, thixo

660, conventional

660, EMS


660, EMS, thixo

630, conventional

630, EMS


630, EMS, thixo

Fig. 8 Polarized light micrographs showing the effect of pouring temperature (690, 660, 630 C) and stirring on the grain and globule size variations in the copper mold casting, conventional and EMS

cast, and thixocast samples quenched at 583 C for 10 min (scale bar is 800 microns)

Fig. 3 shows measured particles size of conventional and EMS reheated samples (particle size measurement by image analysis excluded all the entrapped eutectic areas). The globule size measurement in conventional cast samples would be erroneous due to the sectioning of dendritic branches. For conventional thixocast samples, the morphology of the primary a-Al particles is identical to conventional cast samples (Fig. 3a) although due to the reheating process, growth of primary particles is inevitable and grain size values are larger than the as-cast samples. In the EMS thixocast graph (Fig. 3b), both the grain and the globule sizes are presented. In contrast to conventional samples, there is no sudden reduction in grain size for EMS samples. In addition, the values of globule size have a considerable difference from grain size values. Examining the microstructure at higher magnifications, agglomeration of primary particles could be observed (an example is provided in Fig. 4). These agglomerations are formed due to the sintering processes being activated with prolonged holding time. Longer holding time and resultant agglomeration may encapsulate the eutectic mixture inside and form entrapped liquid area. Selected results of image processing are presented in Fig. 5. By lowering the pouring temperature from 660 to

630 C, average globule size reduced by *15%. Reduction in superheat promotes equiaxed particle formation with the added bonus of additional nucleation sites as confirmed by the primary a-Al number density which is increased abruptly at 630 C, Fig. 5a. The structure in the conventional cast sample is a function of pouring temperature and the greater the superheat, the more the structure will be dendritic. This concept is quite clear in the form of percentage of the primary a-Al particles having certain aspect ratio. By lowering the pouring temperature in conventional samples, the concentration of particles with an aspect ratio [2 decreases (Fig. 5b). The lowest amounts of primary particles with aspect ratio[2 were observed in EMS billets with a reasonable difference to that of the reheated conventionally cast samples. Decreasing the pouring temperature led to particles becoming more spherical and as a result, the percentage of particles with sphericity values greater than 0.8 increases. Comparing the conventional and EMS-treated samples, it is clear that the sphericity percentage is higher for stirred samples. In rheological studies, more spherical particles result in lower viscosity [16], which is expected to induce better flowability and filling of the die cavity during high pressure die casting.

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Fig. 9 Grain/globule size measurements in: (a) conventional and (b) EMSstirred samples, copper mold thixocast

Entrapped liquid has an impact on the viscosity of the semisolid slurries and less entrapped liquid results in better fluidity [2–4]. Conventionally cast samples have a tendency to trap the liquid which is normal in dendritic structure. Casting the billets with high superheat leads to a complex and massive dendritic structure which by reheating could encapsulate a portion of the residual liquid. Accordingly, by superheat reduction, the structure evolves to rosette/equiaxed with less probability of liquid entrapment. In the case of EMS samples, not only does the structure totally transform to globules but also it contains the least entrapped liquid, Fig. 6. It is observed that the majority of entrapped liquid in EMS samples initiates from liquid encapsulation by the surrounding globules, while for conventional samples, the main source is the liquid between secondary or tertiary dendrite arms. This notion is depicted in Fig. 7 for billets poured at 690 C. This may have some effect on the homogeneity of entrapped liquid where a more uniform composition is expected for the entrapped liquid for EMS– thixocast billets. Conventionally cast thixo billets have entrapped liquid that is more characteristics of the localized alloy concentration.

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Copper Mold Figure 8 shows the polarized light micrographs of the copper mold cast samples. The comparison between the two sets of micrographs in Figs. 2 and 8 clearly displays the effect of cooling rate. Thinner and more compacted dendrites are the result of the higher cooling rate in the copper mold in comparison to the sand mold samples. As for the influence of pouring temperature, similar trends to those of the sand cast are apparent for the copper mold as well, where reduction in pouring temperature encourages the formation of more equiaxed structure, see Fig. 8. Similar to the sand mold billets, the key point is the increased survival rate of nuclei with reducing pouring temperature and changes in the heat flow direction with increasing the number of stable solid particles in the melt. This means the nuclei could act as heat sinks and therefore the heat flow will no longer be directional. This results in a shorter solidification time for the copper mold which finally leads to the formation of finer dendrite size. The morphology of the primary a-Al phase changes to rosette and globular forms when EMS applied. When the micrographs of stirred samples are examined closely, it can


Fig. 10 Image analysis results from various pouring temperatures/ application of EMS, copper mold thixocast

be deduced that the resulting structure due to stirring is very much dependent on the pouring temperature, as it is the pouring temperature that controls the starting structure in the first place. Therefore, at higher pouring temperature, stirring has only caused mechanical disintegration of the dendrites and the resulting fragmented segments give the structure a rosette-type appearance. This is partially associated with the higher cooling rate of the copper mold to render fragmentation less effective due to shorter solidification time. At lower pouring temperatures, the morphology of the starting structure is not affected by stirring, as the a-Al particles are already less dendritic. The only effect of stirring in this case, as also active at higher temperatures—in addition to mechanical fragmentation, is to remove the concentration gradient within the boundary layer surrounding fragmented particles and thus slowing down the growth process. This is the main reason why the stirred billets are finer than the unstirred ones. In the case of thixocast specimens, conventional thixo structures reveal dendritic structures characterized by a

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continuous solid network and liquid pockets. However, thinner dendrite branches and smaller dendrite arm spacing are the main differences in comparison with sand mold cast results. Lowering the pouring temperature results in rounder and more isolated particles (Fig. 9) and with lesser entrapped liquid (Fig. 8). In contrast to conventional thixo specimens, the EMS-thixo (reheated) structures consist of almost globules with an average size of about 100 lm even with the higher superheats. The globules are well distributed within the eutectic network and nearly all the eutectic liquid pools are behaviour lar. Comparing the grain/globule size values for sand and copper molds billets may suggest that by stirring, the grain/ globule sizes approach a unique value regardless of the casting condition and therefore as a general conclusion, by EMS application the impact of the cooling rate and pouring temperature are less pronounced. The process of globularization and refinement of the primary a-Al particles are shown in Figs. 9 and 10. In EMS samples, the average globule size is reduced by lowering the pouring temperature. Interestingly, the globule size reduction rate is lower than the sand mold cast specimen which is associated with the role of the cooling rate as the key parameter. The increasing rate of number density is also higher than the sand cast billet which indicates a higher number of potent nucleants in copper EMS cast samples, Fig. 10a. The aspect ratio exhibits the same trend as sand cast samples, and the percentage of particles with aspect ratio [2 decreases with decreasing pour temperature in both conventional and EMS samples; however, the reduction rate is higher for conventional samples. In fact, during the reheating process, an initial higher cooling rate results in the billets having greater potential for microstructural evolution, i.e., larger interfacial area, and this is evident by comparing the results, Figs. 5, 10. By lowering the superheat, the percentage of particles with sphericity value 0.8 increases and the maximum value belong to EMS and lower super heated samples. According to Loue and Suery [17], during partial remelting, coarsening first proceeds predominantly through coalescence of dendrite arms. As the dendrite arms of the same grain have a perfectly matching crystallographic orientation, this results in a high quantity of intragranular liquid which depends on the cooling rate. In the case of EMS samples, the coalescence of short dendrite arms leads to an almost globular structure with a smaller amount of liquid pocket. The formation of spherical particles could be assisted by lowering the pouring temperature and thus having higher probability of equiaxed grains formation. Figure 11 shows the concept within the copper cast structures with high superheat value. Comparing the results with sand mold structures show that the percentage of liquid encapsulation in conventional

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Fig. 11 Copper mold samples, poured at 690 C: (a) as-cast, conventional, (b) EM stirred, (c) thixocast, sample ‘‘a’’, and (d) thixocast, sample ‘‘b’’

Fig. 12 Entrapped liquid measurement, copper mold thixocast

castings also depends on the cooling rate; it is higher for lower cooling rates. Interestingly, the stirred billets show less difference in the quantity of liquid pockets compared to the conventional billets, resulting in enhanced billet quality (Fig. 12).

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Fig. 13 Evolution of A/P ratio as a function of pouring temperature and stirring (filled triangle and filled circle for sand and copper mold cast samples, respectively)


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if one desires to differentiate between globule and grain terminologies as mentioned earlier in this article (further information could be acquired elsewhere [18]).

Conclusions

Fig. 14 Correlation between grain and globule size in EMS samples

Solidification time has an impact on partial remelting procedure. Basically, the driving force for the microstructural evolution within the mushy zone is the reduction of the interfacial area between liquid and solid; this value can be estimated by the area to perimeter ratio. In fact, as mentioned in the experimental procedure, this factor has an inverse relationship with the specific volume surface of the particles, Sv. In dendritic solidification, higher values of P (total solid–liquid interface length) resemble a structure with more dendrite branches. Solid–liquid interfacial length mainly depends on the solidification of the alloy. For instance, the higher cooling rate of copper mold samples leads to lower dendrite arm spacing and finer secondary and tertiary branches and therefore the interface boundary of the eutectic-primary particles increases. In conventional samples, copper cast samples have a shorter solidification time, which leads to a finer and more highly branched dendritic structure and therefore the A/P ratio is smaller. In the same condition, lowering the superheat results in an equiaxed structure, which, during isothermal holding, transforms to a globule/rosette structure. Similarly, higher cooling rate results in finer particles with smaller value of A/P (Fig. 13a). Electromagnetic stirred billets exhibited a similar trend. The higher cooling rate of the copper mold leads to smaller dendrite size, and with subsequent stirring, these dendrites and the dendrites that have detached from wall crystals are broken, fragmented, and distributed within the bulk liquid. A shorter solidification time also results in a limited growth with the eventual structure containing globules with smaller size compared to those of the sand mold billets (Fig. 13b). It is worth noting that the correlation between A/P and application of EMS is not significant for either sand or copper mold at higher casting temperatures, 660 and 690 C, but when temperature drops the effect of EMS on A/P ratio becomes more pronounced which is an indication of smaller particle size. There is a direct correlation between grain/globule sizes and macro/micro structural evolution. The relation is shown in Fig. 14 where the smaller the grain size, the smaller is the globule size. This concept becomes intricate

The thixocastabilily of EMS Al–7Si alloy billets was studied and compared with the conventionally cast billets to establish the effect of EMS in improving the quality of feedstock. The conventional, EM stirred, and thixocast conventional and EM-stirred billets were characterized quantitatively to confirm the qualitative observations. The concept of grain size and globule size is also discussed to provide guidelines for characterization of SSM structure. The effect of pre-thixocasting stirring has been shown to provide high quality feedstock materials for fabrication of engineering components based on SSMs processing. Prior stirring results in refined and more globular SSM billet structures, with less entrapped but more uniform eutectic. This is in contrast with the conventional thixo structures having dendritic structures characterized by continuous solid network and liquid pockets. In addition, the stirred billets showed less difference in the liquid pockets size comparing to the conventional ones. –

–

–

–

Electromagnetic stirring dramatically reduces the size of the primary a-Al particles, almost independent of the pouring temperature. In fact, following superheat, stirring seems to be the next most effective process parameter in size reduction of primary particles in Al–Si hypoeutectic alloy. Conventionally cast thixo-processed samples with high superheat retained their coarse dendritic structure regardless of the mold type, i.e., cooling rate. Reducing the superheat results in the formation of a large number of nuclei which survive and distribute well within the bulk liquid due to natural and pouring convections. In addition, the entrapped liquid in thixocast samples is also minimized with decreasing the superheat. In the EMS thixo-processed samples, the impact of superheat on the grain/globule sizes is small, i.e., there is no sudden variation in grain/globule size. Also the values of globule size have considerable difference with grain size which is an indication of two different concepts in optical microscopy. Comparing the conventional and EMS-treated samples, it is clear that the percentage of particles with greater sphericity is higher for stirred samples and for the copper mold. In fact, during the reheating process, the initial higher cooling rate results in the billets having a greater potential for structural evolution.

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Liquid entrapment is minimized by superheat reduction and evolution of dendrites to rosette/equiaxed primary a-Al morphologies. Thixo-processing of EMS billets also results in lower liquid entrapment. Solidification time has an impact on partial remelting procedure. Basically, the driving force for the microstructural evolution within the mushy zone is the reduction of the interfacial area between liquid and solid and it could be estimated by area to perimeter ratio. In conventional samples, copper cast samples have lower solidification time which leads to finer and highly branched dendritic structure and therefore A/P is smaller. In the same manner, lowering the superheat results in equiaxed structure which, during isothermal holding, transforms to a globule/rosette structure with lower A/P. Similarly, a higher cooling rate results in finer particles with a smaller value of A/P.

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Acknowledgment CANMET Materials Technology lab and Drs. M. Sahoo, D. Emadi, and M. Shehata are gratefully acknowledged for providing the EMS cast pieces. 13.

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