5. “Zero” casting defects, such as shrinkage porosity, oxide film inclusions and others;
6. Small secondary dendritic arm spacing in the combustion chamber region (SDAS
7. T7 heat treatment condition that produces high mechanical properties, normally using a one or two step solution treatment at a high temper- ature, 540C (1004F) followed by a hot water quench and overaging around 200C (392F). The hot water quench is preferred to a cold water quench in order to reduce distortions and resid- ual stress level.
Aim of this Study
During the development of a new engine, some cylinder heads were produced in the selected alloy using different casting conditions. This was done in order to evaluate the process capability and to establish the limits of the process parameters. After some production trials and mechanical properties evaluation, the initially produced cylinder heads were evaluated in the thermo-cycle durability test. In these tests, some of the castings did not attain the specified re- quirements. The failed parts were examined in detail in order to identify the cause of the problem and to allow the adjust- ment of the process parameters.
The aim of this work is to present the failure analysis of two aluminum alloy diesel engine cylinder heads, which failed during the thermo-cycle durability test, as well as the im- provement of the process that led to successful castings.
Examples of Fatigue Cracks in Aluminum Alloy Cylinder Heads
The engines were tested using a thermo-cycle durability test (cycles alternating 16 minutes at maximum specified load at 90C [194F] and 16 minutes at zero load at 30C [86F]). Some castings failed at test times shorter than the specified 500 hours. The failed parts received a lon- gitudinal cut, parallel to the combustion chamber face. The exact locations of the cracks were detected using liquid penetrant. The cracks were opened in order to al- low the fractographic analysis using a Scanning Electron Microscope (SEM). To remove combustion products be- fore SEM analysis, the cracked surfaces were cleaned by brushing with a detergent and water solution, followed by etching using an acid solution for 6 minutes (70 ml ortho- phosphoric acid, 32 g chromic acid and 130 ml water) and ultrasonic stirring with acetone for 6 minutes. Samples for microstructural examination were polished and ana- lyzed with an optical microscope.
The process conditions used for the first production trial (“Cylinder Head A”) are described in Tables 2 and 3.
Failure Analysis of Cylinder Head A
A cylinder head that failed at 400 hours in the thermo- cycle durability test was examined in detail. The general features of Cylinder Head A after a longitudinal cut paral- lel to the combustion face is presented in Figure 1. This figure shows the position of the cracks in cylinders #3 and #4 revealed by using a liquid penetrant test. Both cracks initiated in the water jacket and propagated toward the combustion chamber.
Table 1. Process Parameters for the Production of High Grade Aluminum Alloy Castings. Adapted from References 6 and 7.
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International Journal of Metalcasting/Fall 10
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