J. L. LaGoy, D. S. J. Mashl, M. J. C. Hebeisen, Bodycote HIP, Andover, MA

Summary:

Oxide inclusions and porosity are common defects in aluminum alloy castings. Typically, porosity plays a greater role in limiting fatigue life in these components because of its larger size and/or higher concentration than oxides. It is well established that hot isostatic pressing (HIP) eliminates internal porosity thereby improving fatigue life by up to an order of magnitude [1-12]. With porosity removed by HIP, oxides become the predominant defect to limit fatigue life. In particular, research has shown that oxides in HIPed commercial-grade castings can affect fatigue properties to an extent that ranges from totally dominating the failure process to having minimal, if any, effect. This is supported by fractography and energy dispersive spectroscopy results [5-8].

Because of the dependence of fatigue life on oxide concentration [5-9, 12], it would be beneficial to identify a practical method for quantifying oxide inclusions to determine which aluminum castings will not benefit from the HIP process as expected. This study examines the use of destructive testing techniques, including chemical analysis and stereology, to quantify the concentration of oxides. Results indicate that while chemical analysis by solid-state infrared detection via inert gas fusion can measure the percentage of total oxygen successfully, a large number of tests are required because of the sporadic distribution of oxides within castings. Stereological techniques, such as oxide area fractions measured by optical and/or scanning electron microscopes, are evaluated for comparison. Statistical analysis is employed to determine the degree of correlation between various oxide measurement methods and fatigue life in order to determine the suitability of a specific testing technique to predict the potential benefit of HIP processing.