Novel Approach to Thermal Processing Development for Precision Sand Casting Process (PSCP) of Aluminum Engine Blocks
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Copyright Ó 2020 American Foundry Society https://doi.org/10.1007/s40962-020-00526-y
Abstract The latest generation of high-performance engine blocks are produced with the precision sand casting process (PSCP). Their enhanced mechanical properties are provided by an integrated cast iron chill in the bulkhead section and thus result in large variations in secondary dendrite arm spacing (k2) throughout the whole casting, providing unique challenges in both heat treat (T7 temper) development and compliance to the CQI-9 protocol. This research examines the influence of chemistry, controlled solution/quench, residence time between quench and the start of artificial age and artificial age temperature, on the mechanical properties and cryogenic testing observations on PSCP engine blocks. The results are obtained from large population engine block castings (n = 204), heat treated within an industrial plant environment with strict
control over furnace temperature gradients. The results indicated that age temperature differences of as little as 4 °C (within CQI-9 for furnace temperature specification of ± 5 °C) can alter yield strength and elongation significantly, which in turn changes crack susceptibility (warranty issues). In addition to the above analysis, it was found that extending the time between the solution/quench operation and the start of artificial age results in a continual drop in yield strength due to the influence of natural aging.
Introduction
only) for cylinder block castings due to blister formation 1 that may be encountered with using a solution stage.
Most aluminum alloy castings require a thermal process stage to provide the requisite mechanical properties and dimension stability. Thermal processing also has a significant impact on machining characteristics, ensuring that casting sections which are to be machined are not too brittle (causing excessive tool wear), or too soft (tool metal 1 buildup and larger burrs). The T7 process (Aluminum Association Temper Designation for solution and over-age) is commonly used to achieve target properties and characteristics for gravity permanent mold (GPM), gravity semipermanent mold (GSPM) and precision sand casting process (PSCP), all three of which make up nearly 15–18% of the aluminum casting supply in terms of volume. Highpressure die casting (HPDC), representing nearly 80% of the aluminum casting industry, will generally use a T5 (age
International Journal of Metalcasting
Keywords: T7 heat treatment, natural aging, mechanical properties, aluminum alloys
The casting process that will be of focus in this manuscript will be the PSCP as it tends to have the widest range in 1 microstructures –8 which in turn will have a variation in property responses with a single heat treat regime. Many iterations of heat treat development may have to occur to ensure that both coarse and fine microstructure regions of the same casting not only meet target mechanical property thresholds but do not have any deleterious characteristics (tensile fracture and distortion being
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