Mechanical properties of thixoformed hypoeutectic gray cast iron

Journal of Materials Processing Technology 226 (2015) 146–156

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Mechanical properties of thixoformed hypoeutectic gray cast iron Nadal R.L. a , Roca A.S. a , Fals H.D.C. a , Zoqui E.J. b,∗ a b

Departamento de Manufactura y Materiales, Facultad de Ingeniería Mecánica, Universidad de Oriente – UO, Santiago de Cuba 90400, Cuba Materials and Manufacturing Department, Faculty of Mechanical Engineering, University of Campinas – UNICAMP Campinas, SP 13083-860, Brazil

a r t i c l e

i n f o

Article history: Received 26 December 2013 Received in revised form 22 June 2015 Accepted 18 July 2015 Available online 23 July 2015 Keywords: Ferrous alloys Thixoforming Semisolid processing Mechanical properties Microstructure Hypoeutectic gray cast iron

a b s t r a c t The effects of thixoforming process variables on the final mechanical properties of a specially designed hypoeutectic cast iron were studied. An Fe-2.6 wt%C-1.5 wt%Si alloy was prepared using conventional sand casting molds and thixoformed in an eccentric press. Different temperatures and heat-treatment holding times were tested. After being heated to the semisolid state at 1160 ◦ C and 1180 ◦ C and held at these temperatures for 0, 30, 60 and 90 s, the samples were subjected to the thixoforming process. The holding time in the semisolid range simulates the industrial heating process, which is time controlled rather than temperature controlled. The results indicate that the forces applied during thixoforming are closely related to the liquid fraction of the hypoeutectic gray cast iron. A comparative analysis of the behavior of the tensile properties, Vickers microhardness and porosity before and after thixoforming was carried out. The stress-strain curves were similar for the different liquid fractions studied: at 1160 ◦ C the average values were YE = 282 MPa, UTS = 289 MPa, E = 0.21% and Vickers microhardness = 274HV, while at 1180 ◦ C the corresponding figures were YE = 272 MPa, UTS = 277 MPa, E = 0.2% and Vickers microhardness = 257HV. The small variations in tensile properties can be attributed to the final graphite morphology of the thixoformed alloy. The results also show that after thixoforming, the porosity of the Fe-2.6 wt%C1.5 wt%Si alloy decreased for all experimental conditions and had a maximum value of 5%. There was no significant difference in hardness between the thixoformed material and the as-cast alloy. © 2015 Elsevier B.V. All rights reserved.

1. Introduction In both their groundbreaking papers, “Rheocasting” and “Rheocasting Processes”, Dr. Merton Flemings et al. (1976a,b) showed that semisolid processing (SSM) using iron-based alloys is an attractive, innovative technology with various possibilities for use in a range of processes and production systems. Twenty years later Yoshida et al. (1996) presented the first paper explaining the behavior of cast iron in the semisolid state. However, it was only in 2003 that a more complete paper on this topic was presented by Tsuchiya et al. (2003), who demonstrated the potential of this technology and focused on the production of raw material for use in SSM. Ramadan et al. (2006) evaluated the effect of SSM on the mechanical properties of hypoeutectic gray cast iron. Recently, a series of papers focused on chromium-enriched cast iron, an example being the paper by Wiengmoon et al. (2008) that evaluated semisolid-processed 27 wt%Cr cast iron and described the relation-

∗ Corresponding author. E-mail addresses: [email protected] (R.L. Nadal), [email protected] (A.S. Roca), [email protected] (H.D.C. Fals), [email protected] (E.J. Zoqui). http://dx.doi.org/10.1016/j.jmatprotec.2015.07.015 0924-0136/© 2015 Elsevier B.V. All rights reserved.

ship between microstructure and hardness. Another example is the paper by Meng et al. (2013) that evaluated the effect of predeformation on thixoforming of Cr–V–Mo steels. There are few research papers on this topic that focus exclusively on SSM of cast iron or investigate the effect of thixoforming on the microstructure of hypoeutectic gray cast iron. The thixoforming of a hypoeutectic Fe-2.6 wt%C-1.5 wt%Si alloy was initially investigated by Roca et al. (2012). Here, the same alloy was heated to the semisolid state and then thixoforged. Roca et al. (2012) showed that the microstructure was stable in the semisolid range and that there were no significant changes in stress and/or corresponding viscosity with soaking time. Zoqui et al. (2013) showed that heating up to the semisolid state followed by thixoforming changes the material’s graphite morphology from type A to B (or E) but does not significantly affect the interdendritic arm spacing (IAS) between graphite lamellae. Note that the IAS corresponds to the globule size (GLS) of the semisolid material. While the paper by Roca et al. (2012) investigated the microstructure before forming and focused on the solid-to-liquid transition and its consequences for viscosity, the present paper focuses on the forming procedure and the resulting microstructure and properties. It aims to present a complete description of the microstructure and mechanical properties of thixoformed speci-

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Table 1 Chemical composition of the hypoeutectic cast iron (in wt%). Alloying element c

Gray cast iron Design parameters Fe-2.6 wt%C-1.5 wt%Si cast ironc a b c

Ca

Si

Mn

P

S

Othersb

2.7 2.502.70 2.61

1.3 1.401.60 1.54

0.5 ≤0.50 0.32

0.05 ≤0.10 0.05

0.03 ≤0.05 0.008

0.2 ≤0.30 0.024

Note: Carbon equivalent – 3.12 wt% CE. Note: Residual elements such as Ni, Cr, Cu and Mo. Note: Ordinary gray cast iron as specified in the MatWeb website (http://www.matweb.com).

mens, thereby contributing to the development of thixoforming processes for iron alloys. 2. Experimental procedure The methodology used here to assess the thixoformability of an alloy is the same as that described earlier by Zoqui et al. (2008), Zoqui and Torres (2010) and Zoqui and Naldi (2011). The methodology adopted in the Foundry and Thixoforming Laboratory (FTL) at the Faculty of Mechanical Engineering, Unicamp, to evaluate the thixoformability of a specific alloy is based on six basic steps: (a) thermodynamic analysis of the solid-to-liquid transition; (b) manufacture of the alloy and characterization of its initial microstructure; (c) evaluation of microstructural stability in the semisolid state; (d) determination of the combined effect of the liquid fraction and the morphology of the remaining solid on the alloy’s rheological behavior; (e) investigation of the thixoforming procedure itself; and (f) determination of the final microstructure and final mechanical characteristics. In many cases a seventh step (post-thixoforming finishing, such as machining and/or specific heat treatments) is added. As mentioned before, the Fe-2.6 wt%C-1.5 wt%Si alloy, whose chemical composition is shown in Table 1, was designed so as to obtain a material with good mechanical properties similar to those of an ordinary gray cast iron with a closely related chemical composition in the MatWeb database. As can be seen, the chemical composition of the alloy, which was characterized using an IMB model 007 spectrophotometer, was within the intended design range. As the first three steps in the evaluation of the thixoformability of an alloy identified above were discussed by Roca et al. (2012) and step (e) was briefly discussed by Zoqui et al. (2013), the present work seeks to discuss steps (e) and (f) in depth, thus closing the cycle, and to further analyze the thixoforming procedure and the resulting material. Previous papers have already shown that the solidus and liquidus as well as the SSM target temperatures are best determined by differential scanning calorimetry (DSC). Here, a Netzsch STA 409C thermogravimetric analyzer was used (at heating rates of 10, 15 and 20 ◦ C/min), and it was found that the amount of solid expected during the solid-to-liquid transition is proportional to the endothermic liquefaction process, i.e., the liquid fraction (fL ) can be calculated from the partial integral of the area under the endothermic curve. In the present case, using the fL vs. T curve, a new way of performing the thermodynamic evaluation is introduced: the stability of the semisolid transition as a function of the working (or target) temperature is determined by differentiating the fL vs. T curve to give the sensitivity of liquid fraction (dfL /dT in ◦ C−1 ), which shows the change in the amount of solid (or liquid) fraction per degree Celsius. The concept of dfL /dT at a given temperature was first introduced by Liu et al. (2005), who recommended that the dfL /dT at an fL of 40% should be lower than 0.030 ◦ C−1 . Fig. 1. shows (a) the complete transition from solid to liquid for the Fe-2.6 wt%C-1.5 wt%Si alloy at heating rates of 10, 15 and 25 ◦ C/min and (b) the corresponding sensitivity of liquid fraction, (dfL /dT), as a function of temperature. Both figures show the target temperatures chosen: 1160 and 1180 ◦ C. These values were chosen

so that solid fractions of approximately 40/50% and 20/30% could be obtained, i.e., so that thixoforming could be evaluated at a high and low solid content. Note that these temperatures are intended to exceed the eutectic melting point. The increasing heating rate reduces the accuracy with which the equipment can predict the solid-to-liquid transition in terms of fL and dfL /dT vs. T. Furthermore, it can be observed that the increasing heating rate reduces the temperature at which eutectic melting starts. It should be noted that industrial processes use heating rates close to 100 ◦ C/min. Fig. 1(b) clearly shows the instability of eutectic melting: at a low heating rate, there is a peak in the range 1106/1156 ◦ C, which indicates eutectic melting. After the peak, the solid-to-liquid transition occurs smoothly, and above 1156 ◦ C dfL /dT is below 0.015 ◦ C−1 . Clearly the use of the dfL /dT vs. T curve could be of great assistance in identifying temperatures corresponding to a more controllable range, which are essential when SSM technology is used in industry. It is important to note that the temperatures chosen (1160 and 1180 ◦ C) are expected to result in solid fractions of approximately

Fig. 1. Transition of Fe-2.6 wt%C-1.5 wt%Si alloy from solid to liquid, according to experimental DSC technique in (a) and the corresponding Sensitivity of Liquid Fraction as a function of the temperature in (b). Adapted from Roca et al. (2012).

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Fig. 2. Experimental setup for the thixoforging in the eccentric press: (a) general setup; (b) thixoforged sample view and (c) tensile test sample (dimension in mm).

40/50% and 20/30%, respectively. However, the solid-to-liquid transformation is a thermokinetic (non-equilibrium) rather than thermodynamic (equilibrium) phenomenon, since the heating rate of 100 ◦ C/min is much higher than the usual heating rate used for equilibrium analysis (close to 0.5 ◦ C/min). Hence, the actual amount of solid present during the forming process is probably greater than expected. Soaking time at temperatures in the semisolid range can promote liquid formation, since long periods at high temperatures result in the actual solid fraction in the billet approaching that expected for equilibrium. Moreover, the temperature can be expected to decrease slightly when the billet is transferred from the heating system to the compression device, further increasing the solid content. 2.1. Thixoforming procedure The experiments were performed using the setup shown in Fig. 2. The samples (142 mm high × 27.5 mm diameter) were initially heated from room temperature to the semisolid state at 1160 and 1180 ◦ C in approximately 11 min at a heating rate of 110 ◦ C/min in an 8 kHz 25 kW Norax induction furnace. The samples were kept at these temperatures for 0, 30, 60 and 90 s, after which they were transferred to the compression device, an instrumented SuperVictor 25 ton eccentric press, and thixoforged in an H13 open die at 170/180 ◦ C in one pass and then air cooled. The 0, 30, 60 and 90 s holding times were chosen because they are similar to those used in industrial systems, in which the process is time-dependent rather than temperature-dependent. An eccentric press was used for the thixoforming procedure because the FTL focuses on the development of low-cost materials and processing. Eccentric presses have the advantage that they are simple and easy to operate; however, final solidification after forming occurs without a load, which is a disadvantage, as solidification under continuous load prevents porosity, avoids shrinkage phenomena and results in better mechanical properties (Zoqui et al., 2008). The press was instrumented to allow the forces during the thixoforming process to be monitored. A 1-S40/25T S-type load cell with a sensitivity of 3 mV/V, precision of 0.05% and maximum load of 25 ton-force (245 kN) was fitted to the plunger of the press

together with a Micro-Epsilon LVDT VIP-200-ZA-2-SR7-I wear-free, inductive displacement sensor (linearity ± 0.2%). Both signals were captured by a National Instruments USB 6210 Data Acquisition System. Special Labview® software was developed to acquire and record force, time and position data (in N, ms and m, respectively) at a rate of 5000 readings per second. 2.2. Microstructure characterization After thixoforming, the microstructures were characterized to allow the relationship between the microstructure of the thixoformed samples and their mechanical properties to be established. The thixoformed samples were sectioned transversely and longitudinally in the area with the greatest deformation (i.e., the center of the samples), inset in Bakelite, sanded with 220, 320, 400, 600, 1200 and 1500 grit sandpaper and polished to a mirror finish with 6 ␮m and then 1 ␮m diamond paste. The polished samples were then etched using 2% Nital (HNO3 in ethanol at 2 vol%). The etched samples were rinsed under running water for about 30 s and then dried. The metallographic analysis was performed in a Leica DMIL optical microscope. IAS, i.e., primary particle size and average graphite size, was measured with software designed in MATLAB® for this purpose using the Heyn intercept method in accordance with the ASTM E112standard. The software also measured the amount of pearlite, ferrite and graphite. The primary particles/globules were counted in different fields on each micrograph using a minimum of 36 images from different sections of each sample. Fig. 3 shows the microstructure with and without chemical etching of the hypoeutectic cast iron specially designed for these experiments. As mentioned in the paper by Roca et al. (2012), when produced in a sand mold this alloy had the expected dendritic structure of pearlite–ferrite phases surrounded by graphite lamellae typical of gray cast iron. The graphite (Fig. 3a) does not exhibit a dendritic morphology. When a sand mold is used, the primary phase is surrounded by graphite flakes in the areas presumed to be the boundaries of the primary and secondary dendritic arms, characterizing type A graphite, or inter-dendritic segregation without a preferred orientation. This is the most desirable type of graphite in a gray cast iron, as it ensures the best possible mechanical properties.

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Fig. 3. Microstructures of the Fe-2.6 wt%C-1.5 wt%Si showing a) the initial A type graphite morphology, and b) the perlite-ferrite morphology.

Again, it is important to remember that, as described in Roca et al. (2012), “the raw material microstructure should be mapped in order to understand the material’s semisolid behavior. During the heating stage that precedes the forming operation liquefaction will begin (usually called secondary phase in thixoforming operations), and will continue as the primary ˛ phase is consumed with increasing temperature. A fine and well distributed graphite at the inter-dendritic boundaries will lead to a semisolid structure with the resulting liquid phase wetting the solid phase homogeneously, facilitating its detachment and forming the desired solid globules immersed in a liquid phase”. Fig. 3b illustrates the etched microstructure, which contains almost 90% pearlite with a few small areas of free ˛ phase. Although the pearlite is not homogeneous, in some areas the eutectoid exhibits a very fine distribution of ˛ + Fe3 C and in some restricted areas free coarse pearlite with an average size of less than 10 ␮m and Fe3 C lamellae. As porosity is one of the problems that can arise during thixoforming in an eccentric press, this property was characterized to determine the influence of liquid fraction and holding times on the development of porosity in the thixoformed samples. The gravimetric method, which takes into account the chemical composition and Archimedes’ principle, was used to determine the porosity volume.

3.1. Thixoforming procedure Fig. 4 illustrates the semisolid behavior during thixoforming. It shows the applied force vs. the displacement of Fe-2.6 wt%C1.5 wt%Si cast iron thixoformed in the semisolid state at (a) 1160 ◦ C and (b) 1180 ◦ C for holding times of 0, 30, 60 and 90 s. As expected, thixoforming with a smaller liquid fraction (i.e., at a temperature of 1160 ◦ C) requires a larger force than thixoforming at 1180 ◦ C. However, for both conditions, as the holding time increases, the liquid penetrates the structure and is distributed homogeneously in it, soaking the remaining solid particles, which become more spherical. Both phenomena (liquid penetration and globularization) reduce the forces required to thixoform the sample. The

2.3. Characterization of mechanical properties The tensile test specimens were machined parallel to the direction of compression to produce cylindrical tension-test specimens with a gauge length of 30 mm and diameter of 5 mm, as shown in Fig. 2c. Tensile tests were carried out in accordance with ASTM E8 M using a 10 ton MTS 810 universal test machine at a strain rate of 2 mm/min. Elongation was measured using a testing machine equipped with an extensometer. Three specimens were examined for each test condition, and the results were based on the average of the results for the three specimens for each test condition. The fracture surfaces of the specimens after the tensile tests were examined using a ZEISS EVO/MA15 scanning electron microscope (SEM). The Vickers microhardness of the thixoformed and heated samples (Roca et al., 2012) was measured on a Buehler 6300 machine under a 200 gf load applied for an indentation time of 15 s. The mean values are based on the results for ten different areas. 3. Results and discussion The results and discussion are presented in the same order as the experimental procedure. First, the results for the thixoforming procedure (in fact a thixoforging operation in a mechanical eccentric press) are presented, followed by characterization of the microstructure and the mechanical properties of the formed specimen.

Fig. 4. Force vs. displacement curves of the thixoformed Fe-2.6 wt%C-1.5 wt%Si at (a) 1160oC and (b) at 1180 ◦ C, for 0, 30, 60 and 90 s holding times.

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ior was observed in compression tests in a previous paper by Roca et al., 2012; where at 80% strain and 100 mm/s compression velocity, maximum stress was 35 and 22 MPa at 1160 ◦ C and 1180 ◦ C, respectively. In fact, the stress vs. strain curve reported by Roca et al. (Figs. 8a and c in their paper) is similar to the thixoforming force vs. displacement curve here (Fig. 4a and b in this paper). It is important to note that the compression velocity of the eccentric press reaches 250 mm/s and, as highlighted by Flemings (1991), the stress and consequent viscosity of semisolid materials decrease as the shear rate increases. Thus both sets of data are consistent. 3.2. Microstructure characterization

Fig. 5. Maximum thixoforming forces achieved for the Fe-2.6 wt%C-1.5 wt%Si at 1160 ◦ C and 1180 ◦ C, for holding times of 0, 30, 60 and 90 s.

maximum force used was 178 KN for a temperature of 1160 ◦ C without soaking time, which corresponds to 29.7 MPa if the projected area for the thixoformed part (40 mm × 150 mm) is taken into account. For longer soaking periods or when the temperature was increased to 1180 ◦ C, the highest force used was 110 KN, or approximately 18.3 MPa, which is low compared with most hot-die forging operations with ferrous alloys, where the force required can reach 60 to 400 MPa depending on the iron alloys being used and the temperature (Yeon et al., 2005; Jorge and Balancin, 2005). It should also be noted that at 1180 ◦ C (Fig. 4b), the behavior of the force vs. displacement curve was more consistent regardless of the holding time tested, suggesting that a temperature of 1180 ◦ C may be more appropriate for industrial thixoforming. In fact, for commercial purposes the stability of the process is more important than the real or final forces applied, as the presses used far exceed the maximum load required for thixoforming operations. Notice that the final, abrupt vertical curve at 15 mm displacement in Fig. 4 indicates contact between the upper and lower dies, when the force can reach up to 25 ton-force (245 kN). Fig. 5 illustrates the maximum thixoforming forces just before the upper and lower sections of the die come into contact as a function of temperature (and therefore liquid fraction) and holding time. The results in this figure indicate that the forces applied during the thixoforming process are more dependent on temperature and thus the volume of the liquid fraction. At the higher temperature the force required increases by approximately 30%, from an average of 145 kN at 1160 ◦ C to an average of 100 kN at 1180 ◦ C. These results suggest that at 1180 ◦ C the material fills the die more consistently. Again, at the lower temperature of 1160 ◦ C the results are widely dispersed and the force varies from 118 kN to 177 KN for soaking times of 90 s and 60 s, respectively, a difference of almost 60 kN. These results indicates that at 1160 ◦ C thixoforming is highly unstable and should be avoided. This instability can be explained by the fact that this temperature is too close to the final eutectic melting point (Fig. 1a and b) and that even small changes in the processing temperature, such as the temperature drop due to radiation heat loss when the semisolid material is transferred from the heating system to the eccentric press prior to the thixoforming, could lower the temperature of the cylindrical sample, or regions of the sample, and thus lead to a greater solid fraction. In comparison, at 1180 ◦ C the forces vary little, from 93 kN at 0 s to 117 kN at 60 s (a difference of only 24 kN), indicating more homogeneous and controllable semisolid behavior, as mentioned before. Both sets of data, the maximum thixoforming load (Fig. 5) and the semisolid transition (Fig. 1a and b), appear to indicate a more stable and controllable process at 1180 ◦ C. The same behav-

After thixoforming, the microstructure was characterized and tensile and hardness tests were carried out to compare the properties of the microstructure in terms of the type, quantity, size and dispersion of graphite and the amount of pearlite and ferrite and to determine the effect of these on the stress-strain curve and hardness of the thixoformed Fe-2.6 wt%C-1.5 wt%Si cast iron. Figs. 6 and 7 show the unetched and etched final microstructure of the thixoformed samples. The unetched microstructure on the left of each figure shows the graphite morphology, which is highly important in cast iron structures. Indeed, it is well known that the strength and strain behavior of cast iron is closely related to the type and shape of the graphite in it. The etched microstructure shows the distribution of pearlite and ferrite. After being heated to the semisolid state and then thixoformed, the Fe-2.6 wt%C-1.5 wt%Si cast iron exhibited a significant change in morphology in terms of the type of graphite, which changed from type A to type E (interdendritic flake graphite, a common type of graphite in hypoeutectic cast iron). It is in fact a modified type E as instead of the classical interdendritic structure it has a structure that is typical of the globules found in the most common thixoformed structures, i.e., there is no evidence of the dendritic skeleton observed in classical type E graphite. Additionally, the graphite flakes surrounding the globules are very fine compared with the conventional coarse graphite usually observed for type E structures. As reported in previous papers (Roca et al., 2012; Zoqui et al., 2013), although the amount of liquid phase formed even at the lower temperature is sufficient to promote some degree of globularization of the primary phase, which in this case is austenite, the forming process using an eccentric press also favored this globularization. It is possible that the amount of liquid formed was not enough to complete wetting of the boundary of the primary austenite phase, so that the quasi-globular structure was deformed under the high stress imposed, leading to a deformed austenite grain that recrystallized into a new globular-shaped grain because of the high temperature of the SSM process. This new recrystallized austenite becomes the round pearlite grain observed at room temperature. Furthermore, the random distribution pattern of the type A graphite that is clearly visible in the as-cast structure (Fig. 2a) is also clearly visible in the type E graphite in the thixoformed structure even at short holding times (0 s). For these short holding times, the graphite is arranged in small clusters, characterizing type E graphite, but with no preferred orientation. The thixoformed structure was also observed to exhibit good morphological stability since there were no significant changes in the size or shape of the white phase area (formed of pearlite or pearlite + ˛) or in the graphite structure. The graphite is well distributed around the edge of the globule for all conditions tested. The material usually remains in the semisolid state for at least 30 s before forming. This results in a situation where the austenite particles tend to deagglomerate, reducing the force needed to deform the material. However, some agglomeration of primary austenite particles is expected after thixoforming, leading to the

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Fig. 6. Microstructures of the thixoformed Fe-2.6 wt%C-1.5 wt%Si at 1160 ◦ C after different heat treatments and holding times: left un-etched and right etched.

formation of structures such as those shown in Fig. 6 (30 s) and Fig. 7 (90 s). Unlike the findings reported in an earlier paper (Zoqui et al., 2013), the results of the present study suggest that long holding times do not favor the agglomeration of austenite even at lower temperatures, so that an interparticle graphite skeleton is not formed. In some cases (60 and 90 s holding times) the graphite appears to form an almost interconnected structure for both temperatures tested. At longer holding times (30, 60 and 90 s), the presence of a liquid phase and diffusion mechanisms appear to favor wetting of the primary phase, resulting in a more marked globular shape, particularly for holding times of 60 to 90 s at 1160 ◦ C

and 30 s to 90 s at 1180 ◦ C. In other words, at higher temperatures globularization is faster, as expected. Again, recrystallization of the deformed austenite grain in the semisolid state also favored globularization. Table 2 shows the results of the metallographic analysis of samples thixoformed at 1160 ◦ C and 1180 ◦ C. In the simple compression test in the earlier paper by these authors (Zoqui et al., 2013) the graphite type changed from A to B, instead of the type E graphite found in the present study. This difference can be attributed to the sample size and the cooling method used. In the compression test the sample was 30 mm diameter × 30 mm high (with a weight of approximately 160 g) and was

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Fig. 7. Microstructures of the thixoformed Fe-2.6 wt%C-1.5 wt%Si at 1180 ◦ C after different heat treatments and holding times: left un-etched and right etched.

compressed to 73 mm diameter × 5 mm high. The greater heat loss and consequent fast cooling results in a rapid semisolid-to-solid transition where the solid already formed remains in place while the remaining carbon-rich liquid solidifies within the remaining space between globules (or interdendritic branches), favoring the formation of type E graphite. In the present study, the higher mass of the thixoformed cylindrical samples (27.5 mm diameter × 142 mm high, weight 640 g) resulted in slow cooling with sufficient time for the transition from the semisolid to the solid state to be completed, producing the characteristic type E graphite at the globule boundary.

The same comparison was made in the paper by Roca et al., 2012; who showed the microstructural evolution prior to deformation in compression tests and thixoforging for the same material at the same temperatures but with 0, 30, 90 and 120 s holding times. In their paper the objective was to characterize the microstructure prior to forming to evaluate its effect on the semisolid behavior. Without any deformation, the graphite changed from type A to type E for short soaking times and then to type B for soaking times of 30 s or longer. Without deformation and with rapid (air) cooling, carbon segregates into the liquid, favoring type B graphite.

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Table 2 Quantitative metallographic analysis of thixoformed Fe-2.6 wt%C-1.5 wt% Sia cast iron. Condition

Time (s)

As-cast ◦

Predominant graphite type

Microstructure phasea

Interdendritic arm spacing (␮m)

Graphite length (␮m)

87.2

59.1 ± 63.2

57.5 ± 56.2

G (%)

˛ (%)

P (%)

A

7.5 ± 1.1

5.3 ± 2.7

Thixoformedat 1160 C

0 30 60 90

E E E E

10.8 ± 2.2 7.6 ± 4.1 9.6 ± 2.6 7.9 ± 2.1

4.3 ± 2.5 8.0 ± 4.6 9.3 ± 4.3 11.1 ± 5.9

84.9 84.4 81.1 81.0

35.5 ± 10.0 50.5 ± 21.2 37.2 ± 11.6 40.0 ± 17.0

31.6 ± 12.6 31.9 ± 11.7 34.1 ± 9.7 21.1 ± 5.1

Thixoformedat 1180 ◦ C

0 30 60 90

E E E E

10.1 ± 2.8 5.9 ± 2.0 9.0 ± 2.2 9.4 ± 2.1

5.1 ± 2.1 7.7 ± 5.3 7.4 ± 3.6 7.5 ± 4.3

84.8 86.4 83.6 83.1

42.1 ± 14.8 49.9 ± 22.0 33.9 ± 10.3 42.1 ± 13.9

30.9 ± 10.1 17.5 ± 5.9 23.5 ± 7.6 34.5 ± 13.0

a

Note: G, graphite; ˛, ferrite; P, pearlite.

Fig. 8 shows the IAS (Fig. 8a) and average graphite particle size (Fig. 8b) of the Fe-2.6 wt%C-1.5 wt%Si cast iron as a function of heat-treatment holding time prior to thixoforming. No significant variation in IAS can be observed, indicating that there is no significant austenite grain growth during the semisolid phase. However, the average graphite size decreases during forming, probably because of the solidification effect, leading to improved mechanical properties. The same stability can be observed in the phase proportions. In Figs. 6 and 7, the amount of pearlite and ferrite appears to be sta-

ble, and after thixoforming the proportions of the graphite, ferrite and pearlite phases in the hypoeutectic cast iron listed in Table 2 were similar to those reported in previous papers (Roca et al., 2012; Zoqui et al., 2013). Again, these phase proportions seem to be much more dependent on the final cooling rate than on the semisolid processing itself, as shown in Fig. 9. In short, the different thixoforming temperatures and holding times tested here did not lead to any significant morphological changes in the alloy used, and, as expected, the final graphite and ferrite content was about 9% and 7%, respectively. Note that the stable phase proportions in the final product regardless of temperature and holding time favor SSM as a whole as a window of opportunity in terms of processing time and temperature is established that allows the billet to be heated to the desired temperature without adversely affecting the final microstructure and thus potentially rendering this type of processing impractical. 3.3. Characterization of mechanical properties After thixoforming, microhardness tests were carried out to compare the stress-strain response of the thixoforged Fe-2.6 wt%C1.5 wt%Si cast iron with the response of the original as-cast alloy. Fig. 10 shows the stress–strain curves of thixoformed samples for 0, 30, 60 and 90 s holding times at 1160 ◦ C (Fig. 10a) and 1180 ◦ C (Fig. 10b). It should be noted that the raw material used had a significantly higher yield strength (YE = 263 MPa) and ultimate tensile strength (UTS = 325 MPa) than traditional cast iron with a similar carbon content (YE = 65 to 172 MPa and UTS = 118–448 MPa) (source: www.matweb.com). These features are important for

Graphite Ferrite 20

15

10

0

30

60

o

o

1160 C

1180 C

o

o

1160 C

1180 C

o

o

1160 C

1180 C

o

0 As-Cast -30

o

1160 C

5 1180 C

Proportional Phase Area (%)

25

90

Heat Treatment Holding Time (s) Fig. 8. Morphological stability of the (a) interdendritic arm spacing, and (b) the average graphite particle size of the Fe-2.6 wt%C-1.5 wt%Si.

Fig. 9. Morphological stability of the graphite and ferrite proportion phase for the Fe-2.6 wt%C-1.5 wt%Si thixoforged at 1160 ◦ C and 1180 ◦ C.

154

a)

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350 300

Stress (MPa)

250 200 150

As-Cast Thixoformed after: 0s 30s 60s 90s o Soaking time at 1160 C

100 50 0 0.0

0.2

0.4

0.6

0.8

1.0

Strain (%)

b)

350

Stress (MPa)

300 250 200 150

As-Cast Thixoformed after: 0s 30s 60s 90s o Soaking time at 1180 C

100 50 0 0.0

0.2

0.4

0.6

0.8

1.0

Strain (%) Fig. 10. Stress–strain curves of the thixoformed Fe-2.6 wt%C-1.5 wt%Si after 0, 30, 60 and 90 s holding times: (a) at 1160 ◦ C, (b) at 1180 ◦ C.

industrial applications, both from a commercial point of view and from the point of view of efficiency. As mentioned earlier, heating to the semisolid state is imperative for thixoforming operations, which require that the specimen be kept in this state for a certain holding time. Since the heating process is time-controlled, short soaking times at temperatures corresponding to the semisolid state are expected in the thixoforming process, potentially leading to grain growth or morphological changes that would affect the mechanical properties. Compared with the as-cast material, the material thixoformed at 1160 ◦ C had a higher YS (obtained at 0.2% strain) and lower UTS and elongation (E). For samples with a strain of less than 2%, the YE is considered to be the UTS. The only exception was for a temperature of 1160 ◦ C and holding time of 90 s, for which the stress was lower and the strain higher, probably because of the greater amount of ferrite present (11%). Samples thixoformed at 1180 ◦ C had YE values similar to those of the as-cast material, with the same, significant decrease in UTS and E as samples processed at 1160 ◦ C. Table 3 shows the mechanical properties of the thixoformed Fe-2.6 wt%C-1.5 wt%Si cast iron in terms of YE, UTS, E and Vickers microhardness. Interestingly, while YE and Vickers microhardness increased, UTS and E, the most important mechanical properties of cast materials, decreased, indicating that thixoforming adversely affects the final mechanical properties. Nevertheless, the values for these parameters are still within the typical range of values for Fe-2.6 wt%C-1.5 wt%Si cast iron. A similar phenomenon occurs for A356 and A357 aluminum

Fig. 11. Iso-hardness map evolution for the Fe-2.6 wt%C-1.5 wt%Si heated and thixoformed at the semisolid state at 1160 ◦ C and 1180 ◦ C.

alloys, which exhibit mechanical properties inferior to or close to those of die cast parts (Zoqui et al., 2008; NADCA, 2006). As with aluminum alloys, further heat treatment may improve the mechanical properties of the thixoformed Fe-2.6 wt%C-1.5 wt%Si cast iron used here by increasing the YS, UTS and E. It should be noted that the thixoforming process used here was thixoforging in an eccentric press, in which deformation takes place quickly and the subsequent final solidification occurs freely without any load. Material thixoformed in this way probably suffers shrinkage, which is common in casting procedures, leading to small cracks within the structure. These cracks can be prevented by thixoforming in die cast machines. The combined use of dies specially adapted for use in the thixoforming of iron-based materials and heat treatment operations can be expected to improve the final mechanical properties of thixoformed specimens. Although thixoforming at 1160 ◦ C resulted in a higher YE (304 MPa), thixoforming at 1180 ◦ C appears to be the best option as the resulting mechanical properties varied little with soaking time. When a temperature of 1160 ◦ C was used, the average values of the mechanical properties measured were YE = 282 ± 22 MPa, UTS = 289 ± 14 MPa, E = 0.21 ± 0.12% and Vickers microhardness = 274 ± 24HV, while for a temperature of 1180 ◦ C the corresponding figures were YE = 272 ± 11 MPa, UTS = 277 ± 8 MPa, E = 0.2 ± 0.06 and Vickers microhardness = 257 ± 16HV. In other words, thixoforming at 1180 ◦ C yielded a more homogeneous result regardless of the soaking time used. This stability of the final mechanical properties is highly desirable in industrial operations and leads to improved machinability as the elongation remains practically unchanged with only a 7% decrease in microhardness. The thixoformed material has properties similar to those of ASTM 40 gray cast iron, and once again, these properties can be significantly improved by heat treatment. According to Table 3, there were no significant changes in hardness values between the Fe-2.6 wt%C-1.5 wt%Si cast iron in the as-cast form and after thixoforming. The highest hardness values were obtained for the lowest liquid fractions (1160 ◦ C). For a temperature of 1160 ◦ C, the hardness of the thixoformed material for 0 s holding time was similar to that of the as-cast alloy. At 30 s holding time, the highest hardness values were achieved for both solid fraction values, with a slight decrease at 1180 ◦ C. The increase in hardness at this holding time may be associated with smaller values of mean graphite particle size (see Table 2 and Fig. 8), indicating that agglomeration and deagglomeration is occurring. At 60 s holding time the hardness starts to decrease because of the formation of ferrite with a type E graphite morphology. Fig. 11 shows iso-hardness colour-coded contour maps of Vickers microhardness values for samples heated to 1160 ◦ C and 1180 ◦ C as reported in a

R.L. Nadal et al. / Journal of Materials Processing Technology 226 (2015) 146–156

155

Table 3 Mechanical properties of thixoformed Fe-2.6 wt%C-1.5 wt% Si cast iron. Condition

Yield strength (MPa)

Ultimate tensile strength (MPa)

Elongation (%)

Vickers microhardness (HV)

As-cast

Time (s)

263

325

0.78

250

Typicala

65–172

118–448

0.2– 0.7

161–321

◦

Thixoformedat 1160 C

0 30 60 90

304 276b 294b 255

306 276 294 279

0.23 0.11 0.13 0.37

254 257 305 281

Thixoformed at 1180 ◦ C

0 30 60 90

264 270b 267 288b

275 270 274 288

0.26 0.12 0.24 0.18

273 264 257 236

a b

Note: Typical mechanical properties taken from www.matweb.com. Note: For elongation below 0.2% the yield strength is considered to be equivalent to the ultimate tensile strength.

previous paper by Roca et al. (2012) and for samples thixoformed in the semisolid state for holding times of 0, 30, 60 and 90 s at 1160 ◦ C and 1180 ◦ C. It can be clearly seen that the highest hardness value was obtained for the material thixoformed at 1160 ◦ C with a 30-second holding time. Fig. 11 also shows that as-cast and thixoformed Fe-2.6 wt%C-1.5 wt%Si cast iron had higher hardness values than traditional gray cast iron with a similar composition, for which hardness values are usually in the region of 180–190 HV. Finally, Fig. 12 shows the results of the porosity tests for thixoformed samples and for as-cast Fe-2.6 wt%C-1.5 wt%Si cast iron. It was initially expected that the porosity would increase with hold-

ing time as gases such as oxygen and nitrogen were released in the sample. However, this did not occur, and the porosity actually decreased slightly, as shown in Fig. 12a. Statistically, the thixoformed materials showed no significant changes in porosity compared with the as-cast material. While both temperatures led to a small reduction in porosity, the smaller variation with holding time observed at 1180 ◦ C suggests again that this could be the best processing temperature for this material. Holding time was the factor that had the greatest influence on the porosity of the thixoformed part. The slight reduction in porosity at the higher temperature can also be attributed to the cooling time. At lower temperatures there is less cooling time, and the gases in the cavities of the original cast iron that formed during the thixoforming process remained trapped inside the material. Increasing the liquid fraction (i.e., using a temperature of 1180 ◦ C) results in increased cooling time so that the liquid fraction occupies the cavities more completely and releases gas more easily. Fig. 12b shows a pore in the thixoformed alloy. Thixoforming offers faster, more reliable processing than conventional casting and can be used to produce parts without producing significant porosity, a phenomenon that can adversely affect the performance of parts. Despite the poorer mechanical behavior in terms of UTS and E observed here, further heat treatment can be expected to improve the mechanical properties of thixoformed parts. 4. Conclusions The mechanical behavior of thixoformed Fe-2.6 wt%C-1.5 wt%Si cast iron specifically designed for this study was investigated and led to the following conclusions:

Fig. 12. Porosity of the Fe-2.6 wt%C-1.5 wt%Si thixoformed at the semisolid state at 1160 ◦ C and 1180 ◦ C, for holding times of 0, 30, 60 and 90 s in a) and b) example of the typical porosity found for these parts (condition: 1180 ◦ C after 60 s soaking time).

a) The Fe-2.6 wt%C-1.5 wt%Si cast iron designed for this study showed considerable potential as a thixoformable material. The microstructure exhibited good stability without significant changes in the size or shape of the primary ␣ phase or the graphite structure. The percentage of graphite ranged from 7 to 10%, while the ferrite content varied from 5 to 10%. The ferrite had a predominantly pearlite structure. Graphite particle size was 20–30 ␮m, while IAS, which represents the globule size, ranged from 30 to 45 ␮m, a very small variation. b) An increase in liquid fraction reduced the forces required for thixoforming; however, these were not significantly affected by holding time. The maximum stress required was 35 and 22 MPa at 1160 ◦ C and 1180 ◦ C, respectively. c) The ultimate tensile strength and elongation of the thixoformed alloy were relatively high compared with the values for traditional gray cast iron and were closely related to the type and shape of the graphite. At 1160 ◦ C, average YE

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was 282 ± 22 MPa, average UTS 289 ± 14 MPa and average E 0.21 ± 0.12%, while at 1180 ◦ C the corresponding figures were 272 ± 11 MPa, 277 ± 8 MPa and 0.2 ± 0.06%. d) Hardness values did not differ significantly from those of the as-cast alloy but were significantly greater than those of the traditional cast iron chosen for reference purposes. Vickers microhardness was 274 ± 24HV after thixoforming at 1160 ◦ C and 257 ± 16HV after thixoforming at 1180 ◦ C. e) The porosity of the thixoformed material decreased with increasing holding time but was not significantly affected by changes in the liquid fraction. Porosity was below 5% for all conditions tested. Acknowledgements The authors would like to thank José Maria Marquiori of Venturoso, Valentini & Cia Ltd., for producing the raw material used in this study. We are also indebted to CAPES (The Federal Agency for the Support and Assessment of Postgraduate Education, Brazil – Project CAPES/MES-Cuba no. 095/2010) and CNPq (The National Council for Scientific and Technological Development, Brazil, Project PQ no. 306896/2013-3) for providing technical and financial support. References Flemings, M.C., Riek, R.G., Young, K.P., 1976a. Rheocasting. Mater. Sci. Eng. 25, 103–117. Flemings, M.C., Riek, R.G., Young, K.P., 1976b. Rheocasting processes. AFS Int. Cast Met. J. 1 (3), 11–22. Flemings, M.C., 1991. Behavior of metal alloys in the semisolid state. Metall. Trans. A, 957–981, 22A. Jorge Jr., A.M., Balancin, O., 2005. Prediction of steel flow stresses under hot working conditions. Mater. Res. 8 (3), 309–315, http://dx.doi.org/10.1590/ S1516-14392005000300015

Liu, D., Atkinson, H.V., Jones, H., 2005. Thermodynamic prediction of thixoformability in alloys based on the Al–Si–Cu and Al–Si–Cu–Mg systems. Acta Mater. 53, 3807–3819. MatWeb’s database of material properties, 2015. http://www.matweb.com Meng, Y., Sugiyama, S., Soltanpour, M., Yanagimoto, J., 2013. Effects of predeformation and semi-solid processing on microstructure and mechanical properties of Cr–V–Mo steel. J. Mater. Process. Technol. 213 (3), 426–433. NADCA, 2006. Product Specification Standards for Die Castings Produced by Semi-Solid and Squeeze Casting Process, NADCA (North American Die Casting Association) Standards, 4th Ed. Publication N. 403, Wheeling, Il, USA, pp. 218. Ramadan, M., Takita, M., Nomura, H., 2006. Effect of semisolid processing on solidification microstructure and mechanical properties of gray cast iron. Mater. Sci. Eng. A 417, 166–173. Roca, A.S., Fals, H.D.C., Pedron, J.A., Zoqui, E.J., 2012. Thixoformability of hypoeutectic gray cast iron. J. Mater. Process. Technol. 212, 1225–1235. Tsuchiya, M., Ueno, H., Takagi, I., 2003. Research of semisolid casting of iron. JSAE Rev. 24 (2), 205–214. Wiengmoon, A., Chairuangsri, T., Poolthong, N., Pearce, J.T.H., 2008. Electron microscopy and hardness study of a semisolid processed 27 wt%Cr cast iron. Mater. Sci. Eng. A 480, 333–341. Yeon, Y.-C., Kin, S.-I.I., Byon, S.-M., Lee, Y., 2005. Prediction of flow stress and microstructural evolution during hot forging of three microalloyed medium carbon steels. Mater. Sci. Forum 169, 475–479, http://dx.doi.org/10.4028/ www.scientific.net/MSF.475-479.169 Yoshida, C., Kitamura, K., Ando, Y., Hironaka, K., 1996. Microstructure and properties of cast iron by semisolid die casting process. Nippon Imono Kyokai 68 (2), 141–147. Zoqui, E.J., Lourenc¸ato, L.A., Benati, D.M., 2008. Thixoforming of aluminium-silicon alloys in a mechanical eccentric press. Diffus. Defect Data Pt. B Solid State Phenom. 141–143, 517–522. Zoqui, E.J., Roca, A.S., Fals, H.D.C., 2013. Microstructure of thixoformable hypoeutectic cast iron. Diffus. Defect Data Pt. B Solid State Phenom. 192–193, 219–224. Zoqui, E.J., Naldi, M.A., 2011. Evaluation of the thixoformability of the A332 alloy (Al-9.5 wt%Si-2.5 wt%Cu). J. Mater. Sci. 46 (23), 7558–7566. Zoqui, E.J., Torres, L.V., 2010. Evaluation of the thixoformability of AA7004 and AA7075 alloys. Mater. Res. 13 (3), 305–318.