Excellent thermal conductivity and lower density make Al–Si alloys a suitable alternative for cast iron in the fabrication of engine components. The increase in the maximum operation temperature and pressure of engines necessitates improving the thermomechanical fatigue performance of Al–Si alloys. This paper has two major parts focussing on the use of Al–Si based alloys in cylinder heads and engine blocks.In the first part, the structural stress–strain and material property requirements of cylinder heads are discussed. In addition, the physical and mechanical properties of different competing materials used in the manufacture of engine components are reviewed. The physical metallurgy, solidification sequence and thermal conductivity of Al–Si based alloys are reviewed. Also discussed is the effect of microstructural features on thermomechanical fatigue lifetime. This part also includes an overview of the strengthening mechanisms of cast Al–Si alloys, by dispersed phases and heat treatment.Demands ...

Application of cast Al–Si alloys in internal combustion engine components

Aluminium alloys have gained popularity for automotive applications, due to their high strength-to-weight ratio and thermal conductivity. In particular, cylinder heads are commonly produced from Al–Si–Cu alloys, given their age-hardenability and excellent castability. However, operating temperatures can induce thermal stresses in the heads, which can cause failure if the mechanical and thermal properties are insufficient. This research is a pioneering attempt at presenting several relationships between microstructure, mechanical properties, and thermal conductivity via the characterization of commercial 319 Al alloy cylinder heads. Optical microscopy, SEM-EDX, XRD, porosity measurements, tensile testing, Rockwell hardness testing, and thermal conductivity measurements using the transient plane source method were conducted at several critical head locations after thermal sand reclamation (TSR) and T7 heat treatments. The higher cooling rate at the bottom of the castings led to a finer dendritic structure, smaller intermetallic compounds, lower aspect ratio eutectic Si particles, and less porosity. Also, the T7 treatment promoted the formation of fine Al 2 Cu precipitates distributed throughout the matrix and the coarsening and spheroidization of eutectic Si particles. Improvements in yield strength, tensile strength, and hardness were attributed to both grain boundary and precipitation strengthening, whereas improvements in thermal conductivity were attributed to solute depletion during aging.

Factors influencing thermal conductivity and mechanical properties in 319 Al alloy cylinder heads

Failure analysis of a diesel engine cylinder head based on finite element method

Thermo-mechanical fatigue (TMF) failure is the major problem of the cylinder head subjected to combined variations in temperature and loading during operation. This study mainly focuses on the TMF property and the life prediction of vermicular graphite iron (VGI). The ferrite clusters can be easily found from the microstructure and fractography images. Compared with the TMF experimental data testing at 125–400 ℃ and 125–500 ℃, significant cyclic hardening occurs in the former and slight hardening does in the latter. Depending on the difference in the damage mechanism between TMF and iso-thermal low-cycle fatigue (LCF), based on the hysteresis energy, a life prediction method has been proposed at first. By the minimum amount of LCF and TMF tests, the present method can predict the TMF life rapidly, accurately and cheaply. And based on the difference in the fatigue crack propagation thresholds between pearlite and ferrite, the fracture mechanism of TMF was also discussed.

Thermo-mechanical fatigue property and life prediction of vermicular graphite iron

http://liu.diva-portal.org/smash/get/diva2:850966/FULLTEXT01

Thermo-mechanical and superimposed high-cycle fatigue interactions in compacted graphite iron

While the deformation and damage behavior of aluminum cylinder heads under complex thermal mechanical loading has been the subject of numerous studies in the past, cast iron cylinder heads have been in the focus of thermomechanical fatigue (TMF) only to a minor extent. In this paper, a feasible procedure is presented to set up material models and estimate service life of cast iron cylinder heads under variable thermomechanical loading conditions by the use of computer-aided engineering tools. In addition, the influence of thermal load and mechanical constraints on TMF life span is shown. A specimen model is used for parameter identification in material model setup and a cylinder head model is used for correlation with cracking phenomena. Investigation of different thermomechanical load influences is conducted on the cylinder head model. The principal strain and energy based fatigue criteria are used in assessment of TMF lifetime for the cast iron family and material specific evaluation procedures are pointed out. The results highlight the importance of exact definitions of the boundary conditions and underline the sensitivity of TMF lifespan of cast iron cylinder heads with respect to the defined boundary conditions. Considering this sensitivity, an approach conforming to the engine development requirements is proposed. It is shown that both the crack location and fatigue lifetime are predicted with high accuracy.

Stefan Trampert

Papers

Thermomechanical Fatigue Life Prediction of Cylinder Heads in Combustion Engines

Simulation von realen Fahrszenarien am Prüfstand

Effiziente Kalibrierung an Prüfständen