FATIGUE CRACKS IN ALUMINUM CYLINDER HEADS FOR DIESEL ENGINES R. Fuoco and M. Moreira
Institute for Technological Research—IPT, São Paulo, Brazil Copyright © 2010 American Foundry Society Abstract
During the development of diesel engine cylinder heads a series of parts were produced using different casting conditions in order to evaluate the process capability and to establish the limits of the process parameters. The cylinder heads were gravity cast in a water-cooled permanent mold with sand cores using 357 alloy in the refined and modified condition. The parts were heat treated to the T7 condition. Cylinder heads with high contents of microporosity or with new oxide film inclusions failed during a thermo-cycle durability test. The article describes the failure analysis of some of these failed cylinder
Introduction
Alloys of the 319 Al-Si-Cu family in the as-cast or T6 heat treated condition are commonly used for cylinder heads of gas- oline engines. In the last few decades, with the great demand for weight reduction in the automotive industry, the application of aluminum alloys has also been extended to the cylinder heads of diesel engines for cars and small trucks. At the same time, regulation imposing lower emissions limits required diesel en- gines to work under higher pressures and temperatures.1-3
Some
recent diesel engine cylinder heads for small trucks work at around 150 bar of pressure and temperatures around 250C (482 F), promoting a high tendency for mechanical fatigue cracks near the water jacket areas and thermo-mechanical fatigue cracks in the combustion chamber.1
To counteract the tendency
for the fatigue problems, engineers specify aluminum alloys with improved elevated temperature performance, normally in the T6 or T7 heat-treated condition, and high grade castings.1-8
Aluminum Alloys for Diesel Engine Cylinder Heads
Several aluminum alloys have been specified for diesel engine cylinder heads. Most of them can be classified into three main categories:1-3, 7
1. Al-Si-Mg alloys (A356, 357); 2. Al-Si-Mg-Cu alloys (for example: A356+0.5%Cu); 3. Al-Si-Mg or Al-Si-Cu alloys with small additions of Ni, Mn, Zr and/or V to improve the creep resistance through intermetallics and dispersoid phases (for example: A356+0.4%Cu+0.3%Mn+0.1%Zn+0.4 %Fe+0.5%Ni, A356+1%Cu+0.15%Zr+0.15%Mn and 319+0.15%Mn+0.25%V+0.15%Zr).
International Journal of Metalcasting/Fall 10
heads that developed fatigue cracks. Clear evidence was found of fatigue crack nucleation at microporosity and new oxide film inclusions. A detailed analysis identified the causes of the failures and allowed the selection of process parameters for the production of the cylinder heads. New parts, produced with low hydrogen content and redesigned gating systems, complied with the specified test.
Keywords: cylinder heads, fatigue cracks, aluminum oxides, gating system
Alloys based on the A356 (families 1 and 2) present good combinations of strength and ductility, but their strength re- duces rapidly above 250C. On the other hand, alloys based on the 319 present higher yield and creep strength above 250C, but exhibit lower ductility. Additions of alloying ele- ments to form intermetallics and dispersoids (third family) increase the creep strength of the alloy of both families.1-3, 7
High Grade Cylinder Heads
The general conditions for production of high grade castings require some special features, as described in Table 1.
In the specific case of cylinder heads, the main process parameters that must be controlled in order to obtain high grade castings are described as follows:2-8
1. Alloy selection, as discussed earlier, is a function of the working pressure and temperature of the engine;
2. Selection of high purity alloy especially consid- ering low iron content (Fe
3. Low microporosity levels, normally reached by low hydrogen content (typically below 0.20 ml/100g Al), efficient grain refinement treatment (small grain size) and high solidification rate;
4. As-cast silicon particles with fibrous morphology (obtained by modification treatment) and rounded and isolated silicon particles formed by breaking and spheroidization of the fibrous particles during the solution treatment;
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