Mechanical And Microstructure Properties Of Chilled Cast Iron Camshaft 6y5z29

Materials and Design 30 (2009) 927–938

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Mechanical and microstructure properties of chilled cast iron camshaft: Experimental and computer aided evaluation Levent Cenk Kumruog˘lu * Department of Metallurgical and Materials Engineering, Cumhuriyet University, 54187 Sivas, Turkey

a r t i c l e

i n f o

Article history: Received 29 May 2008 Accepted 2 July 2008 Available online 15 July 2008 Keywords: Casting Chilled cast iron Hardness Microstructure Foundry methods Computer simulation

a b s t r a c t In the present study, the mechanical and metallographic properties of camshafts produced from chilled cast iron were examined experimentally and numerically relating with the solidification, cooling rate and metal flow. First of all, with the help of the 3-dimensional drawing and design programs the whole casting methodology was planned. This planning was analyzed with casting simulation software. After that camshafts were produced experimentally at the foundry. Chill material was used in order to get rapid cooling. The phases, dendrite structure and dendrite arm spacing on the lobes of camshaft were examined with an optic microscope and XRD. The Brinell, Rockwell and Vickers hardness tests were carried out on surfaces of lobes and camshafts. On the rapid cooling areas, the ledeburitic phase and high hardness values, on the slow cooling surfaces, rosette like graphite in pearlitic and low ferrite phase and low hardness values were examined. On the chill areas 2–15 lm DAS (dendrite arm spacing) were measured. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, numerical simulations and computer aided modeling of casting solidification are receiving increasing attention because of its great quantity of potential in increasing productivity of the metal casting industry by reducing the time associated with the traditional experimental based design of casting [1]. There is an extensive and increasing effort in the numerical simulation of a wide range of processes involving solidification and heating–cooling, including a variety of casting processes [2]. In order to identify the conditions and optimum values, simulation of solidification process is done by running indigenously developed computer software for the casting process selected for investigation [3]. The program output provides the details on determination of the stress concentration level [4], porosity caused by gas entrapment, humidity of sand, micro segregation [5], time–temperature profile, heat transfer coefficient values, flow rate, thermal modulus, solidification process which plays a key role in the effective design of castings [6,7]. Further, the computer simulation of solidification of castings offers a basis for predicting the solidification simulation package depends to a large extent on the use of accurate thermo-physical data and boundary conditions by the solidification modeler [1,8]. The structural integrity of shaped castings is closely related to their temperature–time evolution during solidification. In the last two decades to treat heat transfer during solidification a number of analytical and numerical models were developed and the conse* Tel.: +90 2642955790; fax: +90 2642955608. E-mail address: [email protected] 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.07.008

quent simulation of freezing patterns in castings has provided many improvements in foundry technology and processes. Using of casting solidification simulation could do much to increase knowledge of the process; however, some uncertainties must be eliminated before such simulations can be widely accepted as realistic descriptions of the process. The heat transfer at the metal–mould interface is one of these uncertainties, and reliable experimental values of heat transfer coefficients are required for various metal–mould combinations and superheats, as existing data is sparse. The way the heat flows across the metal and mould surfaces directly affects the evolution of solidification, and plays a notable role in determining the freezing conditions within the metal mainly in foundry systems of high thermal diffusivity like chill castings. Gravity or high–low pressure die casting, continuous casting, and squeeze casting are some of the processes where product quality is more directly affected by the interfacial heat transfer conditions. Once information in this area is accurate, foundryman can effectively optimize the design of their chilling systems to produce sound castings [9]. During the solidification and freezing of cast alloys, using of chills plays an important role in promoting the directional solidification. One of the important factors that affects heat transfer from the solidifying casting to chills is the resistance offered by the casting/chill interface. The interface becomes significant when the metal and the chill have reasonable good rates of thermal conductance [1]. Bishop et al. has been studied that the effect of chills on the solidification characteristics of cast iron [10]. Importance of time on the solidification of castings has been showed by Berry [11]. Sun and Loper [12] showed that intensive super-cooling (both constitutional

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and thermal) in front of the chilling interface results in the development of cellular dendrite growth of the graphite into a form recognized as exploded graphite. The microstructures of gray iron and ductile irons are determined by cooling rate, composition, nucleation and growth conditions prevailing during solidification [13].

2. Chilled cast iron camshafts Gray cast iron is an important metal, offering versatility and low cost. Gray cast iron is used widely in the manufacturing of automotive parts since presents good wear resistance, pressure tightness,

Fig. 1. Intake (I) and Exhaust (E) lobes, cylinders, camshafts and their location in the casting branch.

Fig. 2. Foundry technology for producing camshafts.

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good machinability, even at hardness high enough to impart excellent wear resistance, and high vibration absorption. Also, gray cast iron could be chilled to improve its wear surface properties [14] maintaining the inside structural body easily machineable, and reducing the manufacturing costs [15]. Also mechanical properties especially good wear resistance and hard surface hardness could be achieved by starting from the same molten metal to which small amounts of some elements are added, or by a variation of the cooling rate during solidification or solid-state transformation [16]. In general, the cooling rate during casting is largely governed by the design and thermal nature of the casting procedure, one significant factor being the mould material. Metal–moulds generally offer higher chilling action on the solidifying mass due to their higher heat diffusivity. Therefore, the influence of higher cooling rate is normally responsible for the superior properties of chilled castings. The use of chills firstly favors the refinement of microstructure.

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Thus, there is a marked effect upon, grain size and microstructure over a wide range of cooling rates and, consequently, on the mechanical properties [17]. When manufacturing a camshaft using chilled cast iron, casting techniques ensures that the cast iron solidifies more quickly, increasing the hardness. In order to obtain a wear-resistant cam surface of cast camshafts, chills are used to increase the cooling rate and create a hard ledeburitic structure [18]. Fast cooling prevents graphite formation. Ledeburite, cementite and some martensitic phase can be seen in the chilled layer on the outer surface of the gray cast iron, as ledeburite and graphite flakes. Ipek et al. [14] investigated that chilled gray cast iron has typical ledeburite and Fe2C, Fe3C and FeC iron carbides on the surface, which are known as a white cast iron structure. The carbon in the outer region solidifies as iron carbide (cementite) and remains bound to the iron atoms, while otherwise it precipitates out of the iron structure as graphite;

Table 1 Chemical composition of examined camshaft (wt.%)

Chilled area Center of lobe

Fe

C

Si

Mn

Cr

P

S

Cu

Ni

Mo

Sn

Ti

Ceq

91.53 91.27

3.54 4.09

2.25 2.21

0.79 0.702

0.68 0.555

0.06 0.059

0.03 0.03

0.4 0.453

0.2 0.17

0.41 0.385

0.03 0.027

0.014 0.014

4.31 4.84

Ceq: carbon equivalent.

Fig. 3. Chills assembly on camshafts (a), and the original cam shafts branch (b).

Fig. 4. Physical properties of camshaft casting material calculated by computer analysis.

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also controlled chilling eliminates ferrite (too soft and susceptible to corrosion) but promote pearlitic structure [15,19]. Chilled cast iron belongs to a group of metals possessing high strength, hardness, toughness and wear resistance [19]. For a metallurgist, there is sufficient information available on the solidification mode and cell size of ordinary cast iron cast in sand moulds. However, there is a serious lack of possibility of future development of cast irons possessing information on the mode of solidification of chilled cast high strength and hardness [13]. Related with the undercooling of cast iron the mechanical properties such as hardness, toughness, strength increases. Chilled iron castings are produced by casting the molten metal against a metal chill, resulting in a surface virtually free from graphitic carbon. In the production of chilled iron, the composition is selected so that only the surfaces cast against the chill will be free from graphitic carbon. The more slowly cooled portions of the casting will be gray or mottled iron. The depth and hardness of the chilled portion can be controlled by adjusting the composition of the metal, the extent of inoculation, and the pouring temperature [20].

In this study, a camshaft, used in engine, was considered for the simulation. In order to analyze the cooling rate, solidification and liquid metal flow effect of the camshaft, the simulation software called NOVA Flow&castÒ were used. These programs enable the simulation of casting processes, including mould filling, thermal modulus, temperature, shrinkage and solidification. Particularly, the simulation of mould filling and solidification can provide a better insight to the process, since it is difficult to observe mould filling and solidification in a manufacturing operation. SOLIDWORKSÒ 2003 was used to create solid models of the branch of camshafts.

3. Experimental details Variable camshaft consisting of two cam lobes, one inside the other, in which the exhaust cams are connected firmly to the camshaft tube, and the intake cams are ed to the inside cam lobe by a connecting element. This achieves the functionality of two adjustable camshafts using just one camshaft. Fig. 1 shows Intake (I) and Exhaust (E) lobes, cylinders, camshafts and their location in the casting branch with feeder, riser, and sprue. 3.1. Experimental and computer aided evaluation Flow of liquid metal and mould filling phenomena is one of the most important parameter when producing camshafts and the other plenty of cast material by casting techniques. In order to obtain soundness casting; pouring speed, casting temperature, total casting time, feeder, sprue, ingate, pour basin, chills location, chills material and mould design are very crucial parameters by using trial and error methods the optimization of these parameters is so difficult therefore computer simulation has a great importance on optimizing.

Fig. 6. Thermocouples locations in camshaft’s lobes and bearing cylinders.

Fig. 5. Physical properties of silica sand and gray iron (chill) material obtained by computer analysis.

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These models were converted into STL (STereo Lithography) formats and imported into simulation software for mesh generation. The initial settings of parameters used in simulation were as follows. Dimension of sand mould box was 600 mm width, 450 mm length, 230 mm height. Minimal mould thickness was 25 mm. According to the finite element method, total number of cells 55877640 and total number of casting cells 2223868. The mould material was chosen as silica sand and initial mould temperature was 20 °C. The volume of casting 2744,926 cm3 and volume of chilled cast iron branch 883,458 cm3 were measured. Pouring temperature 1280 °C, liquidus temperature 1157 °C and solidus temperature 1045 °C were selected. 4. Results and discussion 4.1. Foundry technology for producing camshafts and simulation studies The melts utilized in the present study were produced by using a 500 kg induction melting furnace in Camshaft Company – Turkey.

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The foundry technology is illustrated in Fig. 2. AFS grain fines number of silica sand 50–55 is used as sand mould material. CaSi (to promote uniform structure) inoculants were used in crucible. The camshaft material is gray cast iron with a carbon equivalent of 4.31–4.32%. Chemical composition of camshafts is given in Table 1. The pouring temperature is approximately 1380–1400 °C. Inoculation is carried out in a ladle. All eight cams are chilled using grey cast iron chills, the chills location of each camshaft and the original casting equipment are shown in Fig. 3. The simulation software called JMat-ProÒ was used to obtain heat conduction, specific heat, density properties of chilled cast iron, gray cast iron mould (chill) material. These properties are shown in Fig. 4. In addition, the simulation software was used to obtain physical and thermal properties of both gray iron chill that is set in mould for rapid cooling and silica sand. The thermal and physical properties of these materials are shown in Fig. 5. To obtain meaningful results the thermocouples were located in camshaft lobes and cylinders. The detail of location is presented in Fig. 6. The numbers 1–8 were located as X shape in lobes and the numbers 9–11 were located as X shape in cylinders. CS1–CS4 are acronyms of camshaft. After the liquid metal was poured in the

Fig. 7. Simulated cooling curve by computer simulation software for camshaft 2 and camshaft 3.

Fig. 8. Simulated cooling curve by computer simulation software for camshaft 1 and camshaft 4.

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mould cavity, each thermocouple supplied temperature and cooling curve data. CS2–CS3 and CS1–CS4 cooling curves are given in Figs. 7 and 8, respectively, for each lobe. According to the simulation results, the cooling curves of CS2 and CS3 are same therefore they were shown in the same graphic. As shown in Fig. 7 the lobes 2, 3, 6, 7 which belong to CS2 and CS3 got cold faster than the lobes 1, 4, 5, 8. As the situation happened in CS2 and CS3, the cooling curves of CS1 and CS4 are the same, therefore they are shown in the same graphic too. As shown in Fig. 8 the lobes 2, 3, 6, 7 got cold faster

than the lobes 1, 4, 5, 8. Despite the fact that the cooling curves in CS1–CS4 and CS2–CS3 look the same; the cooling rate in CS1– CS4 was obtained a little faster than the cooling rate in CS2–CS3. In Fig. 9 the cooling curves were shown for bearings 9–11. Compared with the cooling curves in CS2–CS3 and CS1–CS4, the cooling rate in the bearings 9–11 was obtained more slowly. To characterize the chilled and non-chilled area, the CuCl2 test was carried out. The CuCl2 test gives macroscopic vision. Cu is deposited on the chilled cast iron and the Fe dissolves by forming Fe2+. Fe is spontaneously oxidized to Fe2+ by Cu2+. The Cu2+ is spon-

Fig. 9. Computer simulation of cooling curve for camshaft cylinders.

Fig. 10. Computer simulation of cooling curve of camshaft lobe.

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FeðsÞ þ CuCl2 ðaqÞ ! FeCl2 ðaqÞ þ CuðsÞ:

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ð1Þ

Electrons flow from anode to cathode because the cathode has a lower electrical potential energy than the anode. On the atomic level, a Cu2+ (aq) ion comes into with a Fe(s) atom on the surface of the lobe. The Cu(s) atoms supply better vision to have a macroscopic view on the chilled (ledeburitic) area and non-chilled area. In order to understand the cooling phenomena in the lobes clearly, 14 thermocouples were located in each lobe as shown in Fig. 10 and the computer simulation was carried out. The results of the computer solidification and cooling simulation showed that on the outer surface of the lobes solidification was occurred more rapidly. The transformed ledeburitic phase was examined on the outer surface of the lobes because of the rapid cooling rate. Flow of liquid metal is one of the most important parameters when producing camshafts by casting techniques. After the liquid metal poured into mould cavity, metal flows in sprue, runner, ingates, respectively, then fills mould cavity. The flow simulation results shown in Fig. 11 that the liquid metal firstly reaches the last feeder which is at the end of the runner then flows into bearing 3, lobe 8 and 7, respectively, then reaches bearing 2 and flows towards lobe 4 and 3. Because of firstly ing, chill materials of these lobes are heated by liquid metal. Heated chills supply slow cooling rate as compared with to the other chills. At the same time because of the lobe 8 locations near the feeder, the high thermal modulus feeder cools slowly and heats around. Lobe 1 has the same cooling conditions, due to the nearest location of high thermal modulus feeder. Total casting time was measured as 10 s. Flow of liquid and solidification simulation steps are illustrated in Fig. 11. The numbers 1–8 in Fig. 11 were presented as step by step of flow-solidification simulation. 4.2. Metallographic studies Fig. 11. Flow simulation of camshaft branch.

taneously reduced to Cu0 by Fe. The entire CuCl2 process is spontaneous.

After the camshafts were poured and solidified, all camshafts branches were separated from the mould sand. Then the sand on the surface of the camshaft was cleaned by grinding method. After that all the camshaft’ lobes were cut with a metallographic fine cutter disc. After cross sectional cutting of the specimen, grinding

Fig. 12. Microstructure pictures of lobe 1.

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Fig. 13. Microstructure pictures of lobe 4.

Fig. 14. Microstructure pictures of lobe 6.

was performed by using emery papers of grade 180–1200. Alumina polishing from 6 to 1 lm particle size followed. Specimen surfaces were etched with 2% Nital to reveal the microstructures. An optic microscope employed all lobes’ microstructures. For each lobe the microstructure photos were taken from the top area to the bottom area with the range of approximately 430 lm. 23 photos were taken for each sample. Eventhough all lobes were investigated, only lobe 1, 4 and 6 are illustrated in Figs. 12–14,

respectively. Because, lobe 1, 4 and 6 have different cooling curve and solidification time. The typical microstructure adjacent to the surface of the camshaft lobe 1 is shown in Fig. 12. At the top location of the lobe, the microstructure is consistent with transformed ledeburitic phase – it contains a greater percentage of cementite and less pearlitic than the gray cast iron microstructure in the camshaft core. This type of microstructure was likely achieved during solid-

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ification by the use of chill plates situated in the mould adjacent to the cam lobes. At the center of lobe, a cast iron microstructure is presented consisting of graphite surrounded with pearlite and low ferrite.

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The microstructure at average cooling rates is occurred as mottled iron and the carbon in this structure is as primary cementite and graphite. Very slow cooling of irons that contain large percentages of silicon and carbon is likely to produce considerable fer-

Fig. 15. Overall views of the cross-sections of lobes.

Table 2 Rockwell hardness values of camshaft lobes alone X- and Y-axis 1 Lobe

1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

2 Lobe

3 Lobe

4 Lobe

5 Lobe

6 Lobe

7 Lobe

8 Lobe

X-axis

Y-axis

X-axis

Y-axis

X-axis

Y-axis

X-axis

Y-axis

X-axis

Y-axis

X-axis

Y-axis

X-axis

Y-axis

X-axis

Y-axis

57 50 38 34 31 33 38 46 51

56 54 48 33 36 31 35 40 44 50

54 50 46.5o 36.5 39 31 42 51 53

54 52.5 48 42 39 39 40.5 48 54 54

53 50 42 39 36 35 40 48 56

52 49 46 43 39 26 35 35 47 52

49 41 35 32 35 27 40 45.5

54 54 49 45.5 38 35 34 35 40 50

52.5 48 44 38 37 30 39 45 54.5

54.5 52 49 47 39 37 34 39 50 54

50 43 44 36 36 34 43 46 50

54 54 48 46 41 36 36 40 45 56

55 50 46 40 36 38 41 44 54

55 54 47 47 38 35 37 42 47 51

51 43 40 40 40 35 37 40 53

54 49 45 43 39 40 34 41 51 54

All data in the table is Rockwell C.

Fig. 16. HRc Hardness test results along the X-axis.

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rite and pearlite throughout the matrix, together with coarse graphite flakes. As shown in Figs. 12–14 the ledeburitic structure was obtained at the top area and the bottom area. Especially it was examined that there was a great amount of ledeburitic phase depth at the top area. The mottled graphite was also examined in the center of the lobe. That kind of microstructure was approximately 30– 35 HRc hardness. On the other way ledeburitic structure hardness was between 45 HRc and 57 HRc. 4.3. Hardness testing The hardness tests methods, micro-hardness, Rockwell C and Brinell hardness, were used on the surface of the camshafts’ lobes. The hardness tests performed on cross-sectional surface are shown in Fig. 15.

Rockwell hardness testing was carried out on the all camshaft lobes surfaces from top (nose) area to the bottom area. The hardness tests results are summarized in Table 2. Also the table results are presented in Figs. 16 and 17 as graphic. Along the Y-axis 10 hardness tests were carried out for each lobe at the same time 9 hardness tests were carried out for each lobe along the X-axis. The hardness test was performed on each lobe surface from left to the right. The hardness tests numbers were illustrated in the lobe drawing in Figs. 16 and 17. All hardness tests results gave us an important knowledge. The computer simulation results and the hardness tests results had a great agreement together. As shown previously, cooling speed is a very important parameter. In addition to this, on the rapid cooling areas high hardness results, approximately 50–60 HRC, in the center of the lobe lower hardness results, approximately 30– 35 HRC, were obtained. After that micro-hardness tests were per-

Fig. 17. HRc Hardness test results along the Y-axis.

Fig. 18. Average values of micro-hardness test result of camshaft’ lobes.

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Fig. 19. XRD data obtained from the chilled zone of lobe.

formed for each lobe in order to be understood well. Micro-hardness tests as shown in Fig. 18 were carried out from top to the bottom. According to the micro-hardness tests results, various values from 350Hv0.3 (Hardness Vickers) to the 750Hv0.3 were obtained. It was examined that there was cementite phase in high hardness areas, graphite phase in lower hardness areas. In Fig. 18, the dark line symbolizes an average micro hardness values. The Fe3C and Fe phases were determined by XRD studies and the detailed results are given in Fig. 19. The phase composition investigation of the samples was carried out on the polished metal surface using RIGAKU diffractometer Cu Ka, radiation, 1.54050 Å). As can be seen in Fig. 19, the rapid cooling area consist Fe3C carbides in matrix. The carbides phases improve hardness, wear and abrasion resistance. The microstructure of the unalloyed iron generally consists of M3C carbides and pearlite. On the other hand, carbides and perlite do not supply sufficient properties for some engineering applications. For instance, toughness is as significant as hardness for camshafts. Ni, Cr, Mo and Co additions both increase the hardness and toughness. 5. Conclusion As the result of this work, the following conclusions are derived. The cooling rate of a casting is primarily a function of its section size. Increasing the cooling rate refines both hard transformed ledeburitic phase and fine pearlitic (without ferrite inclusions) matrix structure (on nose area of the lobes) therefore the hardness increases. Lobe 6, 7, 2 and 3 have rapid cooling rate therefore harder values are expected than the other lobes. Because of the strong cooling effect of chill, top of lobes are rapidly solidified under the high cooling rate therefore deeper chill zone occurs and the DAS(dendrite arm spacing) is between 2 lm

and 15 lm. In lower cooling areas the DAS rises but in the central areas of the lobes DAS doesn’t occur. Thermal characteristics such as cooling time, cooling flow rate, chill and feeder locations, have shown the most pronounced effect on microstructure. The casting simulation software is very useful for predicting chill effect and casting parameters. So trial and error period could be decreased. Acknowledgements The author gratefully acknowledge the technical s extended by Mr. Hakan Bog˘atur and ESTAS Company (Sivas). References [1] Gafur MA, Haque MN, Prabhu KN. Effect of chill thickness and superheat on casting/chill interfacial heat transfer during solidification of commercial pure aluminium. J Mater Process Technol 2003;133:257–65. [2] Spim JA, Garcia A. Numerical analysis of solidification of complex shaped bodies: coupling of mesh elements of different geometries. Mater Sci Eng 2000;A277:198–205. [3] Vijayaram TR, Sulaiman S, Hamouda AMS, Ahmad MHM. Numerical simulation of casting solidification in permanent metallic molds. J Mater Process Technol 2006;178:29–33. [4] Bayrakceken H, Ucun I, Tasgetiren S. Fracture analysis of a camshaft made from nodular cast iron. Eng Failure Anal 2006;13:1240–5. [5] Attar E, Homayonifar P, Babaei R, Asgari K, Davamil P. Modelling of air pressure effects in casting moulds. Modell Simul Mater Sci Eng 2005;13:903–17. [6] Kalpakjian S, Schmid SR. Manufacturing engineering and technology. 4th ed. International Edition Prentice-Hall; 2001. [7] Kim C, Kim CW. Numerical simulation of casting solidification in automotive applications. The minerals, metals and materials society. USA: TMS; 1991. [8] Su X. Computer aided optimization of an investment bi-metal casting process. PhD thesis, University of Cincinnati; 2001. [9] Santos CA, Quaresma JMV, Garcia A. Determination of transient interfacial heat transfer coefficients in chill mold castings. J Alloys Compd 2001;319: 174–86.

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