MOLTEN CLEANLINESS ASSESSMENT IN A LOW-PRESSURE FOUNDRY

Until a couple of years ago, the most progressive aluminum foundries making automotive castings had relied on the off-line, batch sampling techniques such as PoDFA (Porous Disc Filtration Analysis), and Prefil (Pressure Filtration Technique) for their molten cleanliness assessments. Nevertheless, such techniques have been used sporadically for process auditing rather than as a part of the daily process. Recent advancements in the ultrasonic inclusion detection system (MV 20/20 system) for on-line assessment of molten aluminum cleanliness have proven to be encouraging for the technology to be used more frequently. A central point of this article is to illustrate that even with present day advancements, and/or the limitations of the inclusion detection and measurement techniques, it is feasible to measure quantitatively the cleanliness of molten aluminum in a foundry, as well as to use such data to learn about the process, and to be able to make improvements. During the study, the molten cleanliness assessments were designed to be led by the results of the ultrasonic inclusion detection system. The Prefil results associated to the ultrasonic detection results are the ones that corresponded to the same time in which Prefil and ultrasonic assessments were performed simultaneously. Although PoDFA results did not necessarily correspond to the same timing as the Prefil and ultrasonic assessments, they still provided analogous outcomes of the molten aluminum cleanliness trends. In a very simple and positive comparison between the three mentioned technologies, the main differences between the ultrasonic inclusion detection system with PoDFA and Prefil are: While the ultrasonic inclusion detection system can analyze continuously, Prefil and PoDFA can only evaluate one sample at a given point in time. The ultrasonic inclusion detection system provides instantaneous and continuous measurements of inclusion size distributions but does not provide information about what type of inclusions are present. Although each Prefil’s filtration curve provides immediate information on the melt cleanliness, the accuracy of the curves’ interpretation must be based on the metallographic analysis of the samples. PoDFA does not provide a filtration curve. The analysis is only based on the metallographic analysis of the samples. PoDFA and Prefil metallographic results provide information about the existing types of inclusions but the results, if expedited, would take more than 2 weeks to be available, while timing also depends on the number of tests solicited. As in typical aluminum wheel foundries, the aluminum alloy in this study is A356.2. While molten aluminum is refined with a master alloy of Al-Ti-B in the form of rods, modification takes place with an Al-Sr master alloy. The molten aluminum is delivered to the casting machines’ holding furnaces via transfer ladles of 2,200 lbs capacity (1,000 kg). The transfer ladles are filled with molten aluminum from gas-fired furnaces located in a central melting area. Among the many different melting furnaces available, with different melting rates and holding capacities, five particular furnaces were selected for the molten cleanliness assessments. Those furnaces were identified as melting furnace 1, melting furnace 2, stack melter, and chip melting furnaces 1 and 2. Sourcing from morden casting.