defect detective D
The Case of the Tiny Holes American Foundry Society Technical Department
The Investigation THE CULPRIT
Metal: 300 series aluminum alloy. Casting Process: Permanent mold. Defect: Porosity on casting surface. Details: Many small pinholes be- came visible on the casting surface after the part was machined.
Clue 1: T e holes appear across the surface of the casting and not localized in a particular area or section. Clue 2: In general, the holes are round in shape and fairly
consistent in size, with the appearance of salt and pepper. Clue 3: T e holes were not visible on the cast surface prior to machining.
The Conclusion T e defect is caused by a problem in the melting process. T e cause of the
holes (porosity) is most likely excess hydrogen gas entrapped in the molten metal. In the molten state, aluminum can absorb hydrogen, which becomes entrapped and forms small bubbles. Hydrogen gas bubbles are generally round and evenly dispersed throughout the metal. Once the casting surface is broken through abrasive grinding or machining, the holes become visible.
The Solution T e source of the hydrogen is moisture, usually from the
Editor’s Note: T is column is the fi rst in a se- ries that will examine metal casting defects.
atmosphere in the form of humidity and from moisture on tools or the scrap metal that was charged in the furnace. To remedy the problem, your metalcaster must locate and minimize the source(s) of the hy- drogen and remove any hydrogen from the melt through a degassing process prior to pouring the casting.
48 | METAL CASTING DESIGN & PURCHASING | Jan/Feb 2012
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