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Green Sand Metalcasting Foundry News

Core and Mold Gas Evolution: Porosity In Castings

Posted by Hill and Griffith Company on Apr 7, 2020 10:00:36 AM

A global consortium of metalcasting companies, resin and coating suppliers, additive and sand suppliers, and researchers, aims to spread the word for CPR casting porosity reduction.

Abstract from the June 2007 Foundry Management & Technology article by Charles E. Bates and Robert Burch

According to a recent survey of 121 product engineers, design engineers, metallurgists, laboratory personnel, and others associated with the design, production, and use of castings, the metalcasting industry's greatest need for improvement lies in eliminating porosity in castings. By consensus, this applies to aluminum, gray iron, and ductile iron, and to all types of steel castings. The distribution of quality issues based on this broad survey is illustrated in Figure 1. Concerns over porosity rated twice as high as any other issue in the survey.

A consortium of companies around the world is being formed to address this concern, led by AlchemCast L.L.C. The consortium is known as the Foundry Casting Porosity Reduction (CPR) Consortium, and to assure the future success of castings as structural components in engineered products, a kick-off meeting for foundries and suppliers was held in Birmingham, AL, April 12-13, 2007.

Warut Sintapano Core Mold

Speakers and attendees came from Mexico, Japan, and from across the United States. Raymond Monroe of the Steel Founders Society summarized 30 years of research and development on porosity, beginning with his own groundbreaking research at the Southern Research Institute in the mid 1970s. Franco Capello, of Manufacturas Cifunsa S.A., an automotive iron foundry group in Saltillo, Mexico, outlined some of the issues associated with porosity in iron blocks and heads. George Goodrich of Professional Metallurgical Consultants in Buchanan, MI, presented microscopic techniques for determining the gas that produced specific porosity defects in iron castings. Dr. Hiroshi Onda from Nissan's Powertrain Group in Yokahama, Japan, outlined some porosity issues associated with production of aluminum heads. Pavan Chintalapati and John Griffin, both of the University of Alabama in Birmingham, discussed finding micro-porosity using ultrasonic techniques and the new ASTM E2224 digital x-ray standard.

The remainder of the presentations detailed techniques for measuring the volume, rate of gas evolution, and pressures inside cores. The focus was on porosity produced by excess gas pressures in molds and cores. Leonard Winardi, a Ph.D. candidate at UAB has developed three techniques for determining the gas pressure in molds. These include inserting a probe attached to a pressure transducer in cores; immersing samples in molten metal to various depths, depending on the calculated gas pressure in the mold or core; and observing whether or not bubbles are ejected, and observing gas evolution from cores during pouring in a real-time X-ray unit. Any or all of these three techniques may be used to determine peak pressures, at least in simple cores.

Winardi measured the volume and rate of gas evolution from a core produced in a commercial foundry, and calculated the pressure inside the core as a function of time (see Figure 2). The binder being used produced two gas evolution peaks which resulted in two gas pressure peaks. The peak calculated pressure was almost identical to the peak measured pressure. Further improvements in the shape of the pressure curves may be obtained when more accurate information on the composition of the gases produced is obtained.

Winardi's theory of gas bubble formation in castings is that if the gas pressure in the mold or core exceeds the metal head pressure, gas will bubble from the core or mold into the metal. A gas rate of evolution curve is illustrated in Figure 3. If the metal head pressure can be developed faster than the gas pressure, such as illustrated by the red line (a'), the release of gases into the mold cavity is suppressed, and gas is forced through the core prints. If the pouring rate is lower, as illustrated by the red line (a), the gas pressure exceeds the metal head pressure for a period, and during this time, gas bubbles through the metal to produce dross, oxide folds, and slag defects. The importance of the peak rate of gas evolution in relation to the pouring rate helps to explain why gas defects can sometimes be eliminated by gate changes to pour the casting faster. Sometimes, blows also can be eliminated by pouring hotter to allow the bubbles to blow through the metal. This does not eliminate dross formation, but it may allow the gas to exit the casting.

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