A Simple Procedure to Determine Incoming Quality of Aluminum Alloy Ingots and Its Application to A356 Alloy Ingots
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Copyright 2020 American Foundry Society https://doi.org/10.1007/s40962-020-00414-5
Abstract A step-by-step procedure to determine the quality of ingots has been developed. This procedure is built on the reduced pressure test and is designed to guide the foundrymen to decide (1) whether the lot of ingots should be accepted and/ or (2) what remedial liquid metal processing should be done to bring the melt to the desired levels. A case study on two aluminum alloy A356 ingots is also presented. The results show wide variability in the initial quality of the
ingots. Moreover, traditional metallographic techniques to characterize the quality from the slice of the reduced pressure test sample have been shown to provide similar results to those obtained from computerized tomography.
Introduction
However, it has been recently shown in an industrial wheel production that sample parts from different suppliers provide different results.9 Hence, incoming inspection of ingots can provide useful information about not only the consistency of ingot quality but also what additional melt processing may be necessary.
To produce high-quality castings based on Campbell’s1–3 10 rules, it is imperative to start with a high-quality melt. It is common in the casting industry to have liquid metal go through several processes, such as rotary degassing and fluxing to attain a certain level of metal quality before pouring. These processes have the potential of providing high-quality melts, although it has been reported4–6 that such processes have a limit on quality improvement. Conversely, they may result in the degradation of quality4,7 if the initial metal quality is high. Therefore, the initial melt quality needs to be determined to decide whether additional processes are indeed necessary and, if so, to design such processes. To the authors’ knowledge, this approach has not been reported in the literature. Inspection of incoming raw material is common practice in most manufacturing operations, since the development of statistical techniques for acceptance sampling by Shewhart.8 Usually a sample is taken for evaluation, and subsequently, a decision to accept or reject the batch of raw material is made, based on the results from the evaluation of the sample. In the casting industry, incoming inspection is seldom practiced. Instead, there may be a requirement that only certified ingot be used to produce castings.
International Journal of Metalcasting
Keywords: computed tomography, reduced pressure test, aluminum, casting, quality, defects, bifilm, pores
The reduced pressure test (RPT) has been in use for several decades to determine liquid metal quality. For this test, either a metal cup is dipped into the liquid metal, or a small sample is poured into a special mold. In both instances, liquid metal is damaged due to the entrainment of surface oxides. It is well known10–17 that inclusions such as oxide films affect the results of RPT, especially when high levels of hydrogen are dissolved in liquid aluminum. This is because oxide films
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