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Effects of slope plate variable and

reheating on semi-solid structure of

ductile cast iron*M. Nili-Ahmadabadi, F. Pahlevani and P. Babaghorbani

School of Metallurgy and Materials Engineering, University of Tehran, Tehran, Iran

S e m i - s o l i d p r o c e s s i n g i n v o l v e s a l l o y s w i t hnon-dendrite microstructure that contain spherical solid

particles in the liquid matrix [1,2]. The process was invented in1971 and developed industrially in 1996 when the automotiveindustry needs for, durable, safe and low density parts might befor the first time fulfilled with a pressure die cast techniques [3,4].Most of the initial research had been focused on alloys with lowmelting point; nonetheless recently several approaches have beenperformed with alloys of higher melting point [5-8].

In comparison with other traditional method of casting, diecasting results in better mechanical properties and less porositywhich could be attributed to lower level of volume reductionand also homogeneous filling of the mould [9]. Mechanicalproperties of these products could be equivalent to forged oneswhile more complicated shapes are attainable.

Dendritic structure along with microporosity is intrinsic tosolidification of ductile iron. Therefore, semi-solid process seemsto be a proper solution for improvement of ductile iron structureand mechanical properties. In addition, it seems that by usingsemi-solid process of ductile iron it may be easier to producenear net shape and thin section parts.

The semi-solid process is generally divided into three main

steps [10]: feedstock manufacturing, reheating and forming.Various methods, such as strain induced metal (SIM) activation,the isothermal treatment and casting using a slope plate are usedto produce feedstock for thixoforming [11-13].

Slope plate method is one of the new methods for applyingshear stress to produce semi-solid casting with globular structure.In this method, molten metal with a suitable superheat is castinto the mould after crossing the slope plate. Solid nucleationare formed because of contacting between the melt and slopeplate and also heat transferring. These nucleations are detachedfrom the surface as a result of applying shear stress and meltflow. Finally they are distributed into the melt. In the slope platemethod different parameters such as superheat, length of slopeplate, mould material, slope and inclined plate’s material canaffect the final microstructure.

To provide semi-solid state with an accurately controlled solidfraction and fine spherical particles uniformly dispersed in aliquid matrix prior to forming, reheating is carried out. Thedriving force is the reduction of the interfacial energy betweenthe solid and liquid phases and this process is a diffusioncontrolled process [14]. Temperature and holding time are twocrucial factors to obtain an optimum microstructure duringreheating [15-19]. Therefore, the reheating temperature controls thesolid fraction in the slag and fine uniform globular microstructureis achieved by minimizing the temperature difference betweenthe center and the surface parts of the work piece [20].

In this research the effect of reheating on the ingot of ductileiron prepared by slope plate was studied. The effect of different

Male, Professor. Research interests: solidification and solid state

phase transformation in metals and alloys, modeling of phase

transformation.

E-mail: nili@ut.ac.ir

Received: 2007-05-15; Accepted: 2007-10-20

*M. Nili-Ahmadabadi

Abstract: Semi-solid metal casting and forming are known as a promising process for a wide range of metal

alloys production. In spite of growing application of semi-solid processed light alloys, a few works have been reported

about semi-solid processing of iron and steel. In this research inclined plate was used to change dendritic structure

of iron to globular one. The effects of length and slope of plate on the casting structure were examined. The results

show that the process can effectively change the dendritic structure to globular. In the slope plate angle of 7.5° and

length of 560 mm with cooling rate of 67K�s-1 the optimum nodular graphite and solid globular particle were achieved.

The results also show that by using slope plate inoculant fading can be prevented more easily since the total time of

process is rather short.

In addition, the semi-solid ductile cast iron prepared by inclined plate method, was reheated to examine the effect

of reheating conditions on the microstructure and coarsening kinetics of the alloy. Solid fraction at different reheating

temperatures and holding time was obtained and based on these results the optimum reheating temperature range

was determined.

Keywords: ductile cast iron; semi-solid; reheating; thixoforming

CLC number: TG 143.5 Document Code: A Article ID: 1672-6421(2008)01-036-03

Research & DevelopmentFebruary 2008

37

variables during casting and reheating on the final microstructureis explained.

1 Experimental procedure

Cast iron with chemical composition shown in Table 1 wasprepared in a medium frequency induction furnace with 20 kgclay-graphite crucible. The sandwich method spheroidizationprocess was performed by addition of 2.5% Fe-Si-Mg and also0.4% Fe-Si as post inoculants.

A 600 mm�150 mm �10 mm copper plate was used to studythe effect of slope plate technique on the structure of semi-solidductile iron. The plate was coated with boron nitride to preventadhesion between the alloy and the plate. The melt at temperatureof about 1,260� was cast on the plate, arranged to provide fordifferent lengths and angles and was consequently quenched in waterand steel mould with 90 mm in diameter and 160 mm in height.

To reheat, samples were cut from alloys prepared by optimumprocessing condition (contact length and slope of inclined plate).The samples were heated in a resistance furnace at a controlledatmosphere. The range of reheating temperature was 1,140� to1,165� for 3 to 20 min holding time.

Nikon optical microscope equipped with Buehler Omnimetimage analyzer was used to study the volume fraction andmorphology of solid particles in the both processes.

2 Results

2.1 Semi-solid casting

By increasing the angle of the plate, the applied stress increaseswhile the duration of applied stress and the solid fractiondecreases, if the slope plate length remains constant. Therefore,at constant plate length the solid globularity should be a functionof slope plate angle. In the following, the effect of slope plateangle on the structure of semi-solid after 10 min reheating isdescribed, for a fixed length (L=560 mm).

Figure 1 shows that when plate angle is 20° the dendriticstructure has changed into a structure of mixed spheroidal andirregular shaped solid particles that are heterogeneouslydistributed. The results of this slope angle imply that appliedstress level was enough to change dendritic structure but theperiod of stress application was not enough to achieve ahomogeneously distributed globular structure. In the case of lowplate angle (5°), the distribution of solid particle was homogeneous,but the particles exhibited a range of irregular shapes (Fig. 2).

Figure 3 illustrates that when slope plate is 7.5°, solid particleshave uniform distribution and size, and are well globularized.Figure 4 shows the aspect ratio of solid particles vs. differentslope plate angles. As the figure shows aspect ratio decreases asslope plate angle decreases from 20° to 7.5° and it increases asthe angle decreases to 5°.

The results of slope plate process show that when slope platehas zero angle, aspect ratio of the solids is about 20 which meanssolid has a fully dendritic structure. However, even a small slopeangle (5°) sharply reduces solids aspect ratio. Figure 4 showsthat above and below 7.5° slope plate the solid particles did notwell globularized. It is supposed that at higher angle althoughshear rate is high, the duration of applied shear is not sufficientwhile in the lower angle the situation is reverse. Figures 3 and 4show that optimized combination of homogeneous distribution,uniform size and well globularized solid particles should beachieved around the slope plate angle of 7.5° due to appropriateapplied stress and period.

Table 1 Chemical composition of cast iron

C

3.60

Si

2.68

S

0.007

P

0.017

Mn

0.89

Ti

0.01

Mg

0.04

Fe

Balance

Fig. 1 Microstructure of specimen prepared by slope

plate at angle 20° . The arrow shows the solidparticles

Fig. 2 Microstructure of specimen prepared by slope

plate at angle 5°

Fig. 3 Microstructure of specimen prepared by slopeplate at angle 7.5° . The arrow shows the nodular

graphite embedded inside solid particle

Fig. 4 Aspect ratio of solid particles prepared by slope

plate at different slope angle

40µm

40µm

50µm

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2.2 Reheating

Figure 5 (a, b) shows the liquid fraction vs. temperature andholding times (10, 13 and 15 min) of ductile iron. Figure 5(a)shows that although liquid fraction increases as heatingtemperature increases, in the case of reheated specimen for 13min a slope reduction can be realized when liquid fractionchanges between 40%J50%. This slope reduction in this liquidfraction interval could be very useful for semi-solid forming.Figure 5(b) shows that different reheating temperature leads todifferent liquid fraction and in all cases the liquid fraction ofspecimens reach to almost a steady state after 15 min. Figure 6 illustrates the microstructures of reheated samplesof ductile iron for different conditions. These figures indicatethat with increasing of the holding time in the semi-solid state,the liquid fraction, solid globularity and grain size increase.According to Fig. 5(b) and Fig. 6(d), the liquid fraction ofspecimen heat treated for 13 min should be the maximum.

3 Conclusions

Application of slope plate process was found successful for thedevelopment of semi-solid ductile cast iron and the followingresults were found as well:

(1) The optimized duration and applied stress were achievedat angle of 7.50 and length of 560 mm.

Fig. 6 Microstructures of reheated ductile iron samples at1,160����� for different holding time

(2) Liquid volume fraction, solid globularity and size arefunction of heating time and temperature.

(3) The optimum reheating condition for forming was achievedwhen specimens are reheated for 13 min.

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Fig. 5 Liquid fraction vs. reheating temperate (a),holding time (b) for ductile iron

(a) 5 min (b) 7 min

(a) (b)

a b

c d

(c) 10 min (d) 13 min