A Novel Process in Semi-Solid Metal Casting

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J. Mater. Sci. Technol., 2012, 28(10), 946950.

A Novel Process in Semi-Solid Metal Casting

Bijan Abbasi-Khazaei and Saeid GhaderiDepartment of Material Eng., Faculty of Eng., Razi University, Kermanshah, Iran

[Manuscript received December 7, 2011, in revised form April 21, 2012]

In this research a new process for semi-solid casting of ductile iron based on the high nucleation rate combined

with locally mechanical stirring is presented. In this process at first fully liquid ductile iron was poured onthe peripheral surface of a wheel rotating against pouring direction. At this stage, the solid crystals nucleated

at the chilling surface were pushed to the melt by a heat resistance steel cutter and finally the semi-solid

slurry was generated. Reheating treatment was done on the samples to achieve more efficiency of semi-solid

casting process. The effects of the travelling distance of solid particles during casting, the reheating time and

temperature were examined. The results showed that the process effectively changes the dendrite structure to

globular one.

KEY WORDS: Semi-solid; Casting; Ductile cast iron

1. Introduction

The process of semi-solid casting offers a num-ber of advantages such as improved mechanical prop-erties, good surface finish, and low segregation andso on. The key to the process is to obtain semi-solid slurry free of dendrite, with the solid beingpresent as non-agglomerated, fine and spherical parti-cles, and with minimum entrapped liquid in the solid.The semi-solid slurries are obtainable by a numberof methods with and without liquid agitation such as

single slug production method, low superheat casting,low superheat pouring with a shear field, swirled en-thalpy equilibration device process, cooling slop, twinroll casting equipped with a cooling slope, and partialmelting.

As demonstrated by Hirt and Koop[1], usingthe single slug production method, a magnetic fieldcaused stirring of the molten mass. The growing den-drite structure was destroyed by shear forces gener-ated by the flow and the slurries were created.

In the low superheat casting process, a slightlysuperheated melt is poured directly into the die; the

seed of the crystals are generated at the die surface

Corresponding author. Assist. Prof., Ph.D.; Tel.: +98831 8369655; Fax: +98 831 4283263; E-mail address: [email protected] (B. Abbasi-Khazaei).

and the casting is carried out before the crystal seedscould be re-melted[2]. By this method shallow tem-perature gradient removes directional heat extractionfrom the melt and prevents the formation of dendritesand the semi-solid slurry is created.

The process of low superheat pouring with a shearfield, used solidification conditions to control nucle-ation, nuclei survival and grain growth by means oflow superheat pouring, vigorous mixing and rapidcooling during the initial stage of solidification, com-bined with a much slower cooling thereafter.

The swirled enthalpy equilibration device process,involves two main steps, at first the heat is extractedto achieve the desired liquid-solid mixture and thenexcessive liquid is drained to produce a self-supportingsemi-solid slug that is formed under pressure[3]. Theprincipal is based on achieving rapid thermal equilib-rium between the metallic container and the bulk ofthe metal by proper process parameter selection suchas pouring temperature, eccentric mechanical stirringand drainage of a portion of eutectic liquid.

For the cooling slope method, the slurries aremade by the simple process of pouring the slightly su-

perheated melt down a cooling slope with subsequentsolidification in a die[4]. Granular crystals nucleateand grow on the slope and are washed away from thesurface by fluid motion. The melt, containing a large


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number of nuclei crystals, solidifies in the die, andresults in a fine globular microstructure.

For the twin roll casting method, at first a moltenmetal is poured on the cooling slope, the melt becomesa semisolid, flowing into a preheated nozzle and beingdragged by a twin roll caster machine. The method

was used for strip casting of aluminum by Haga[5] andmagnesium alloys by Watari et al.[6]. Slow castingspeed, using a lubricant in order to prevent the stick-ing of the strip to the roll, and coating of the cool-ing sloop can be considered as disadvantages of thismethod.

In partial melting process, the solid alloy isheated so that recrystallization takes place before thesemi-solid temperature range is reached; at temper-atures above solidus line, liquid wetting at newlyformed grain boundaries leads to a spheroidizedmicrostructure[7].

Most of these processes make use of the high nu-cleation rate associated with low-temperature castingand chill cooling, but destroying dendrite by shearforces and other principles are also used. Althoughthe potential of semi-solid processing is already well-known, the industrial production of semi-solid mate-rial with high melting point such as steels and ironshas some limitations because of technological prob-lems, mostly due to the higher process temperature.The problem is more complicated for ductile iron be-cause of inoculants fading at more casting time.

There are a few researches about semisolid casting

of ductile irons[810]

. Nili-Ahmadabadiet al.

recentlyused cooling slope method to produce slurry of ductileiron[8]. The optimum graphite nodularity and solidparticle globularity were obtained at sloped plate an-gle of 7.5 and length of 560 mm with a cooling rateof 67 Ks1 without inoculant fading. It seems thatat the mentioned plate angle, the low casting speedand subsequently low productivity of this method inmass production could be questioned.

In the present study a new process based on thehigh nucleation rate combined with locally mechani-cal stirring is presented for preparing semisolid slurryof high melting point material such as ductile iron.The method is easy and fast, with no coating of thesurface due to rapid flow of heat without any stickingof the solidified metals to the surface.

2. Experimental

2.1 Melt preparation

Ductile cast iron with the chemical compositionshown in Table 1 was treated in a crucible furnace.Magnesium was added by using the sandwich methodwith 2%FeSiMg as inoculant and then 75% FeSiwas used as post inoculant.

2.2 Semi-solid casting

Fig. 1 shows a schematic illustration of the exper-

Table 1 Chemical composition of ductile iron (wt%)

C Si Mn Mg S P Fe

3.1 2.2 0.98 0.05 0.005 0.015 Bal.

Fig. 1 Schematic representation of the semi-solid process

imental apparatus, and Table 2 shows the experimen-tal conditions and specifications of the wheel. Moltenmetal at 1300 C was cast on the top surface of therotating wheel in opposite direction of melt pouring.A large number of solidified nucleis were created atthe surface and some of them were washed away from

the surface by fluid motion. This stage is similar tothe cooling sloop method. A cutter made by heat-resisting tools steel at the top of the wheel was usedfor locally mechanical stirring and diattaching of theremained solidified nuclies; this stage is similar to themethods based on mechanical stirring of the moltenmetals, and finally the melt became slurry and flowedinto the die. It should be mentioned that the wheelwas air cooled with no additional instruments for cool-ing. In order to investigate the effect of apparatus pa-rameters, the cutter was positioned at the four trav-eling distances of 4, 5, 7 and 8 cm (length of OA inFig. 1).

2.3 Reheating temperature and time

For the reheating, samples were cut from castsprepared using the optimum processing condition ob-tained at the traveling distance of 8 cm. The sampleswere reheated in a resistance furnace with a controlledatmosphere at the temperatures of 1150, 1165, 1175,1185 and 1197 C for 5 to 20 min holding time andfollowed by air cooling.

An optical microscope equipped with an image an-alyzer was used to study the volume fraction and

morphology of the solid particles. Aspect ratio ofthe grains was measured as dmax/dmin, where dmaxand dmin are maximum and minimum diameter of thegrains.


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Table 2 Experimental conditions

Wheel Mould Molten metal Cutter Traveling distanceof solid particles

(length ofOA in Fig. 1)

Diameter: 22 cm; Material: sand; Temperature: 1300 C; Material: heat 4, 5, 7 and 8 cmWidth: 10 cm; Temp.: 25 C; Weight: 60 kg; resisting tools steel

Material: brass 7030, Dimension: Material: ductile ironself cooling, non-coating; 200 mm100 mm55 mm

Temperature just aftercasting: 60 C;

Speed: 75 rev./min

Fig. 2 Dendritic structure of ductile iron by normal cast-ing

3. Results and Discussion

3.1 Semi-solid casting

Fig. 2 shows the microstructure of the normal

casted ductile iron, which is mainly dendritic. Mi-crostructures of semi-solid casted ductile iron withdifferent traveling distance of solidified particles areshown in Figs. 3. As can be seen the structure ofthe grains in all cases are globular and their size and

globularity change as the traveling distance increases.The variation of the count, size, and the aspect ratioof the grains are shown in Fig. 4. It is shown thatas the traveling distance rises, the grain size and theaspect ratio decrease because of more heat transferand more primary solid nucleation due to more stir-ring time which resulted in more grain count. Resultsshow that, at the traveling distances of 4, 5, 7 and8 cm, the hardness of the casts are 18.5, 21, 24 and26 HRC, respectively. The increase of the hardnessto respect of traveling distance can be attributed tomore fraction of pearlite.

3.2 Reheating

For optimizing the time and temperature of re-

Fig. 3 Optical micrographs of semi-solid ductile iron at traveled distance: (a) 4 cm, (b) 5 cm, (c) 7 cm and (d) 8 cm


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Fig. 4 Variation of size (a), count (b) and aspect ratio (c) of the grains

Fig. 5 Effect of reheating at of 1165 C for different time: (a) 5 min, (b) 10 min, (c) 15 min and (d) 20 min

heating, a series of samples obtained at the travel-ing distance of 8 cm were reheated at temperatures of1150, 1165, 1175, 1185 and 1197 C for 5, 10, 15 and

20 min, respectively.Fig. 5 shows the microstructure of reheated semi-

solid cast ductile iron for different holding time. Asit can be seen, at the holding time of 5 min, the liq-uid is formed only at the corner of the grain bound-aries because of their low melting points. At thereheating time of 10 min, the most grain bound-aries become liquid but the primary solid nod-ules remain unchanged. At the holding time of15 min, the shape of the primary solids graduallychanges to spherical and the rounded islands are cre-ated. With increasing holding time to 20 min andmore, liquid fraction and solid globularity changeslightly. The liquid fractions are graphically cal-culated and its variation vs holding time is shownin Fig. 6. As it can be seen, the liquid frac-

Fig. 6 Calculated liquid fraction vs reheating time at1165 C


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950 B. Abbasi-Khazaei et al.: J. Mater. Sci. Technol., 2012, 28(10), 946950.

Fig. 7 Effect of reheating temperatures on the microstructures: (a) 1150 C, (b) 1165 C, (c) 1175 C, (d) 1185 C,(e) 1197 C and (f) aspect ratio of the primary solid at the holding time of 15 min

tion rises as holding time increases and approximatelyreaches a steady state after 15 min. Fig. 5 shows that,the time of 15 min can be selected as optimum re-heating time for all holding temperatures, when, therounded islands are surrounded by the minimum ofstabilized liquid fraction. Some dark laths can be seenin Fig. 5(c) which are martensite and bainite (markedwith M and B).

The effect of reheating temperatures on the mi-crostructures at the optimum reheating time is shownin Fig. 7. The temperature of 1150 C is not suffi-cient to obtain desired semi-solid mixture because ofinsufficient liquid fraction and non spherical shape ofprimary solids. At the other hand at the tempera-tures of 1175, 1185 and 1197 C, the liquid fraction insemi-solid mixture notably rises which is undesirabledue to further solidification with dendritic structure.In addition, at the temperature more than 1185 C,the graphite nodules and the solid grains agglomerateand reduce because of more stability. So it can be

concluded that the optimum reheating temperaturein this wrok is 1165 C.

4. Conclusion

In this research a new process for semi-solid cast-ing of alloys especially for high melting point materi-als such as steel and iron is presented. The proposedprocess is based simultaneously on the mixed mech-anism of breaking dendrites and nucleation and the

growth of granular crystals. The process is applied inthe semi-solid casting of ductile iron. The optimizedduration was achieved at traveling distance of 8 cm.The optimum reheating condition was attained at thetemperature of 1165 C for 15 min.

REFERENCES

[1 ] G. Hirt and R. Koop: Thixoforming, Semi-solidMetal Processing, KGaA, Weinheim, Wiley-Vch Ver-lag GmbH & Co., 2009, 15.

[2 ] T. Haga and P. Kapranos: J. Mater. Process. Tech-nol., 2002, 130131, 594.

[3 ] D. Doutre, G. Hay, P. Wales and J.P. Gabathuler: inProc. of Light Metal Conference, COM, Vancouver,Canada, 2003, 293.

[4 ] T. Motegi, N. Ogawa, K. Kondo, C. Liu and S.Aoyama: in Proc. ofthe ICAA-6, Toyohashi, Japan,1998, 297.

[5 ] T. Haga: J. Mater. Process. Technol., 2002, 130131, 558.

[6 ] H. Watari, K. Davey, T. Rasgado, T. Haga and S.Izawa: J. Mater. Process. Technol., 2004, 155156,1662

[7 ] M. Margarido and M.H. Robert: J. Mater. Proc.Technol., 2003, 133, 149.

[8 ] M. Nili-Ahmadabadi, F. Pahlavani and P. Babaghor-bani: J. Tsinghua Sci. Technol., 2008, 13, 147.

[9 ] B. Hidarian, M. Nili-Ahmadabadi and M. Moradi: J.Trans. Nonferrous Met. Soc. China, 2010, 20, 798.

[10] M.H. Robert and R. Cristofolini: J. Achievements inMater. Manufact. Eng., 2008, 28, 115.

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A Novel Process in Semi-Solid Metal Casting