The effect of vermicularity on the thermal conductivity of vermicular graphite cast iron

doi:10.1016/j.matdes.2015.12.169

Materials & Design

Volume 93, 5 March 2016, Pages 418-422

https://doi.org/10.1016/j.matdes.2015.12.169 Get rights and content

Highlights

•
Interfacial thermal contact resistance affects the thermal transfer of cast iron obviously.
•
Interfacial thermal contact resistance is about 0.1 W/(m²·K).
•
The thermal conductivity of cast iron can be predicted by finite element model with relatively high accuracy.

Abstract

As an ideal material for the cylinder head of high power density diesel engine, vermicular graphite cast iron (VGI) with higher thermal conductivity was received more and more attention recently. In this article, finite element (FE) software ANSYS was applied to investigate the changes of thermal conductivity of VGI with the vermicularity and the FE-models are carried out based on the real metallographic images of VGI. The results indicate that there is a big difference between the numerical simulation and the reality without the consideration of the thermal contact resistance of interface between matrix and graphite. The numerical simulation result is close to the reality when the contact thermal conductance is set at 0.1 W/(m²·K). In addition, the equivalent thermal conductivity along X direction k_equ,x rises from 34.437 W/m·K to 38.902 W/m·K and increases by 13% or so when the vermicularity is improved from 40% to 90% according to the numerical simulation.

Graphical abstract

Introduction

VGI has been gradually the primary material for cylinder heads of engine, especially high power density diesel engine because of its excellent mechanical and physical properties compared to the traditional gray cast iron or aluminum alloys [1], [2], [3], [4]. As one of the important parts of diesel engine, the cylinder head is a typical thin-walled complex casting. The cylinder head endures the cycle impact of high temperature and pressure under bad service conditions. The mechanical and thermal load becomes higher and higher with the increasing specific power of engine. Besides good mechanical behavior under high temperature, the thermal transport properties are of significant importance for materials of cylinder head since the ability to transfer away heat enhances the resistance to such factors as thermal fatigue and distortion [5], [6], [7], [8].

Thermal conductivity of graphite is significantly higher than that of the matrices in cast irons. Most material scientists think distribution and morphology of the graphite phase are the key influence on thermal transport. Nodular iron, where graphite is similarly spherical, exhibits lower conductivity than vermicular graphite, which in turn has a lower conductivity than gray iron with its flake graphite [9], [10]. Just the opposite of thermal conductivity, nodular iron possesses the best mechanical properties, VGI takes the second place and gray iron is the worst [11], [12], [13], [14]. Although the casting ability and damping properties of gray iron is excellent compared to vermicular and nodular graphite iron, the outstanding comprehensive properties of VGI gradually makes it to be the primary material for cylinder head of engine instead of gray iron. If the thermal conductivity of VGI could be improved further, its advantage would be greater compared to gray iron. And the mechanical properties of cast iron have been already studied in depth to some extent [11], [12], [13], [14], [15], [16]. Therefore the thermal conductivity of VGI has received more and more attention in recent years [8].

A feasible technical solution should be applied to evaluate the thermal transport ability of VGI by means of numerical simulation, for example, finite element methods [17], [18]. And then some constructive suggestions could be obtained to improve the thermal conductivity of cast iron at a lower price [19], [20], [21], [22], [23], [24]. The research in this area is relatively rare in the literature because cast iron is a kind of traditional material and numerical simulation is a new technical method. In this paper, we try to combine the traditional material with new simulation method. Consequently, VGI was taken as research object, finite element software ANSYS was applied to investigate the influence factors of the thermal conductivity. Besides the distribution and morphology of the graphite phase, interface between graphite inclusion and matrix also plays an important role in thermal transport. In addition, the effect of vermicularity on the thermal conductivity of VGI was also emphatically discussed. These studies can make it possible to predict thermal conductivity changes of VGI with relatively high accuracy using FE-simulation.

Section snippets

Finite element model

Fig. 1 shows the schematic diagram how to build FE model of VGI. In order to properly capture the conductivity of VGI, the FE-simulations are carried out based on the real image of microstructure (shown in Fig. 1a). The matrix of VGI is restricted as a pure ferrite phase, without the separate modeling of the pearlitic phase. Although this simplification has some little deviation from the reality, the key issue discussed in the present analysis is not affected basically. In that way, the FE

Analytical expression of the equivalent thermal conductivity

The equivalent thermal conductivity of VGI is calculated according to the steady heat conduction with the constant physical properties. The distribution of temperature t complies with the heat conduction equation as follows. $\frac{\partial^{2} t}{\partial x^{2}} + \frac{\partial^{2} t}{\partial y^{2}} + \frac{\partial^{2} t}{\partial z^{2}} = 0$

Three kinds of boundary conditions can be used to solve the Eq. (1). The first one defines the temperature t_w on the boundary and the simplest case is that t_w is constant. $t_{w} = const$

The second one defines the value of heat flux on the boundary condition

Interfacial contact thermal conductance between matrix and graphite

The objective of the following representative simulations using the proposed FE-based simulation strategy is to evaluate the effect of interfacial thermal contact resistance between matrix and graphite on the thermal transfer properties of VGI. Fig. 2 shows the distribution of temperature and heat flux at a steady heat transfer boundary condition without the consideration of thermal contact resistance of interface. The uniformity of them is obviously influenced by the graphite inclusions,

Conclusions

In this paper, FE method was used as the verification method of measured thermal conductivity and its prediction. The corresponding FE models were carried out according to the real metallographic images. The distribution of temperature and heat flux can be calculated by ANSYS software and k_equ,x can be deduced according to Fourier law. Some conclusions can be drawn as follows.

1.
The numerical simulation results of k_equ,x is clearly higher than the actual value and heat transfer is mainly carried

Acknowledgments

The authors would like to thank the financial support by the State Key Laboratory for Mechanical Behavior of Materials (20131303), Schoolmaster Fund of Xi'an Technological University (XAGDXJJ1309), the National Basic Science Development Foundation of China (2012CB619602 and 2012CB619606) and Shaanxi Education Foundation of China (04JK211 and 2012JC13).

References (28)

T. Rausch et al.
Application of quantitative image analysis of graphite structures for the fatigue strength estimation of cast iron materials
Prod. Eng.
(2010)
A.R. Ghaderi et al.
Effects of graphite morphologies on the tribological behavior of austempered cast iron
Wear
(2003)
I. Hervas et al.
Graphite \nodule morphology as an indicator of the local complex strain state in ductile cast iron
Mater. Des.
(2013)
C. Delprete et al.
Experimental characterization of a Si–Mo–Cr ductile cast iron
Mater. Des.
(2014)
Z.J. Ma et al.
Numerical simulation of densification during consolidation of titanium-matrix coated fibers
Compos. Part A
(2006)
G. Ljustina et al.
A FE based maching simulation methodology accounting for cast iron microstructure
Finite Elem. Anal. Des.
(2014)
N.K. Fukumasu et al.
Numerical analysis of the stresses developed during the sliding of a cylinder over compact graphite iron
Wear
(2005)
F.D. Carazo et al.
Effective properties of nodular cast-iron: a multi-scale computational approach
Comput. Mater. Sci.
(2014)
J.P. Papa et al.
Computer techniques towards the automatic characterization of graphite particles in metallographic images of industrial materials
Expert Syst. Appl.
(2013)
W.M. Mohammed et al.
Modeling the effect of the microstructure of compacted graphite iron on chip formation
Int. J. Mach. Tools Manuf.
(2011)

A. Velichko et al.

Estimation of the effective conductivities of complex cast iron microstructure using FIB-tomographic analysis

Acta Mater.

(2009)

J.C. Pina et al.

Thermo-mechanical analyses of heterogeneous materials with a strongly anisotropic phase: the case of cast iron

Int. J. Solids Struct.

(2015)

A. Velichko et al.

Unambiguous classification of complex microstructures by their three-dimensional parameters applied to graphite in cast iron

Acta Mater.

(2008)

C. Chuang et al.

3D quantitative analysis of graphite morphology in high strength cast iron by high-energy X-ray tomography

Scr. Mater.

(2015)

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View full text

The effect of vermicularity on the thermal conductivity of vermicular graphite cast iron

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Finite element model

Analytical expression of the equivalent thermal conductivity

Interfacial contact thermal conductance between matrix and graphite

Conclusions

Acknowledgments

Prod. Eng.

Wear

Mater. Des.

Mater. Des.

Compos. Part A

Finite Elem. Anal. Des.

Wear

Comput. Mater. Sci.

Expert Syst. Appl.

Int. J. Mach. Tools Manuf.

Acta Mater.

Int. J. Solids Struct.

Acta Mater.

Scr. Mater.