CHAPTER 2

MELTING AND HANDLING

Diran Apelian

INTRODUCTION

Melting the metal and handling liquid metal are two of the most critical components in the overall metal casting operation. The manner in which the metal is melted, the way the metal is transferred into the casting cavity, and the whole liquid metal handling process have a significant impact on productivity, on the cost of operations, and certainly on the quality of the resultant cast component. In this case the old adage "garbage in -- garbage out" is certainly applicable.

Processing the metal in its molten state is the activity wherein the most gains can be achieved. Molten metal processing is an opportunity for refining and quality enhancement. For example, processes such as alloying, degassing, filtration, fluxing, and grain refinement and modification in aluminum are usually carried out in the liquid metal prior to casting. The mass transfer rates and the kinetics are such that these reactions are carried out much more effectively in the melt.

In the United States there is extensive awareness and recognition of the importance of molten metal processing throughout the foundry industry. It is well understood that the highest quality can be obtained in the molten state. The level of hydrogen can be reduced and liquid and solid inclusions can be removed and controlled, thus improving the quality of the overall casting. There are active developments in refining technologies in North America; in particular, degassing processes have been developed and have evolved during the last decade whereby we have several corporations commercially supplying degassing equipment. The practice of degassing the melt prior to casting is carried out as a norm throughout the industry. Filtration and the removal of inclusions is another molten metal processing technology that has evolved over the last decade or so in the United States, and today we find that most foundries and metal casting operations filter the metal to remove inclusions prior to casting. In addition, there is active development of measurement devices and equipment to monitor the quality of molten metal. The concept, of course, is that to control quality one needs to measure it. To establish context, we find that in the United States there are developments and commercial instrumentation to measure hydrogen content of the melt; inclusions present in the melt are monitored through LIMCA and other tests such as the PODFA and LAIS. The LIMCA is based on a coulter counter technique whereas the PODFA and the LAIS are based on a metallographic technique.

These technologies are not laboratory curiosities, but rather are used through the metal casting industry to assure that the melt being cast is of the highest quality. During the last decade, we have also witnessed the development and commercial utilization of thermal analysis equipment (i.e., AludeltaTM sold by MPT products in Exton, Pa., which has enabled aluminum foundries to monitor the level of modification and grain refinement).

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Published: March 1997; WTEC Hyper-Librarian