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COMPARISON BETWEEN WEDGE TEST CASTINGS AND COMPONENT ENGINE BLOCK CASTING PROPERTIES


R. Mackay


Nemak of Canada Corporation, Windsor, Ontario, Canada J. Sokolowski


University of Windsor, Windsor, Ontario, Canada Copyright © 2010 American Foundry Society Abstract


A significant portion of the literature on new and/ or modified alloy compositions that could serve as a replacement for the traditional 300 series aluminum alloys used for engine components are performed on test castings, which at times have limited complexity, lending themselves to advantageous liquid feeding conditions in the semi-solid state. More specifically the limited complexity of the test casting design may lead to conditions where progressive solidification provides enhanced soundness and limited segregation, which may not repeat in the component casting, made of the same alloy with the same melt treatment strategy.


Introduction


Aluminum alloys containing silicon and copper (Aluminium Association Designation 300 Series alloys) represent near- ly 80% of all aluminum cast by weight.1-4


One of the most


important factors that controls the service life of aluminum castings is the degree of soundness.5


Castings whose service


conditions have tensile and compression cycles may result in fatigue fracture, such as those found in aluminum cast engine blocks. Lowering porosity in cast components (both area fraction percentage and in the largest pores) has been known to extend service life.6-16


A considerable portion of metallurgical research has es- tablished the melt conditions that lead to lowering po- rosity in the cast component (made from the 319 and 356 alloys) both of which have traditionally been used for engine blocks. Dispersed porosity in thick cast sections can be reduced with melt treatments such as degassing and furnace filtering, particularly when changing the specific section geometry and/or utilization of chills to accelerate the solidification rate to reduce porosity is not an option. Turbulent mould filling conditions (including the filtered melt in the poorly designed gating system) can cause bi-films leading to pore formations in the cast structure.17-19


International Journal of Metalcasting/Fall 10


The research contained in this paper will investigate both a directionally chilled wedge test casting and a component engine block (CEB) poured with four development test alloys having the same melt treatment, with the aim to establish similarities and differences encountered in the test results of both casting designs used. The authors will make a recommendation regarding the best approach for successful alloy conversion strategies for production castings.


Keywords: wedge casting, engine block components, porosity, α-Al dendrites, Al-Si eutectic, interpretative thermal analysis, fraction solid calculation, mechanical characteristics


Boileau investigated the structure of the 319 alloy in a wedge casting and a component engine block casting and found that progressive solidification conditions not only minimized segregation but also provided smaller mushy zone sizes that reduced the size of the pores found on the tensile and fa- tigue fracture surfaces. However, in the engine blocks cast, pores seen on the fracture surfaces of the tensile and fatigue samples were significantly larger because progressive solidi- fication conditions were not typically encountered.


The question the authors faced was what limitations exist using a wedge casting design for optimizing alloy composi- tions with the aim of improving fatigue life. Previous work by Mackay et al.,20


set out to utilize the method of interpreta-


tive thermal analysis to assess feeding kinetics of alloys hav- ing different Si and Cu concentrations. The alloys previously studied for feeding capacity will be further investigated in a wedge casting and in an engine block casting.


Experimental Methodology Liquid Alloy Treatment


Figure 1 shows the process flow and its conditions for en- gine block and wedge castings as well as sample prepara- tion for the presented research. The wedge casting (Stage 1)


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