Identifying the Causes and Determining the Solutions
Excerpt from the Die Casting Engineer January 2008 article by Dr. David V. Neff
Defective castings, scrap... Yikes! — The scourge of the high pressure die caster trying to make a good living in a highly competitive marketplace! What's a body to do? Well, for one thing, he can try to avoid making defective castings in the first place! But just what is a "defective" casting? For this article, we wish to consider a defective casting as one which has a defect, right? But there are many ways that a casting can be "defective." For our purposes, we shall limit this discussion to those defective castings where the cause can be attributed to "inclusion" defects — out of the many other defects and their causes that possibly could exist. Inclusions in castings are foreign bodies that exist in the finished casting and are detrimental to the casting's properties and functionality. Where do they come from? If they are present, how do we get rid of them? More importantly, how can we avoid the "inclusion of the inclusion" into the casting in the first place? So, let's start at the beginning.
Die Casting Process Basics
"In the beginning" (at least the beginning of the modern high pressure die casting era), there was aluminum — but there also was, and is oxygen present! Aluminum readily oxidizes at extremely low vapor pressures and concentration of oxygen present. Consequently, since we melt and process our aluminum alloys on planet Earth and not Mars, there is always sufficient oxygen present to create aluminum oxides within the melt. Crystallographically, there are many different forms of aluminum oxide. Other oxides can also be present in aluminum die casting alloys as well. Of course, at the melt surface the oxide layer can build up and create "dross." In large melting furnaces, particularly reverberatory configurations, the dross layer that develops can grow to significant thicknesses, retarding heat transfer, making skimming and dross removal much more laborious and increasing the chances for dross entrainment and carryover into a transfer ladle or casting furnace. But, in addition to the oxides which can be generated during the melting process itself, one must also consider the charge materials. Specification ingot, remelt sows, alloying agents and foundry returns — trim and gating, flash, machining chips, etc. — will also always contain oxide skins. Then, there is the potential, and often eventual, erosion of the refractory vessel linings themselves, which can add inclusions into the melt. This is especially true even with "best practice" preventive maintenance and cleaning of the furnace vessel, as scraping the refractory and skimming the resultant debris can never be quite complete with total removal.
The use of furnace fluxes to aid in cleaning or melt treatment can also leave behind particulate matter, resulting in entrained inclusions when not sufficiently removed from the melt. Flux inclusions start out as liquid, but as they pick up oxides, they become pasty. Flux inclusions often do not separate readily to the melt surface where they may be skimmed; hence, these can be carried over and ultimately into a casting. Subsequent reactions with moisture in the air can result in a corroded surface or a "leaker" casting in operation.
One rather insidious form of refractory erosion results from corundum formation in the melting furnace. Corundum is an extremely dense, hard form of aluminum oxide which forms at high temperatures and can be fairly common with die casting alloys. Excessively high melting temperatures, high oxygen potential with furnaces operating under negative pressure and high free-silica refractories enhance corundum formation. Once formed, corundum adhesions on the refractory vessel walls can only be removed by mechanical means, and there is always the potential for not being able to skim or remove all such debris from a cleaning campaign. These can become inclusions in the finished casting. One other key source of inclusions pertinent to high pressure die casting alloys is the formation of "sludge." There are two kinds of sludge we can define — "heavy metal" sludge (no relation to heavy metal music) and "metallurgical" sludge. Heavy metal sludge develops when the heavier common alloying elements in aluminum die casting alloys — iron, manganese, copper, zinc — fall out of solution and segregate to the bottom of the furnace. This can often occur with metal held for an extended period or when larger melts are not sufficiently circulated. The second kind of sludge, "metallurgical sludge," actually forms in-situ in melting and in holding furnaces with the right combination of temperature and composition.
This metallurgical sludge is defined as an aluminum-silicon inter-metallic precipitate complex, which also contains iron, manganese and chromium. Metallurgical sludge formation is dependent only on temperature and composition and not on time. There is a "sludge factor," usually Fe+2Mn+3Cr, which should be less than or equal to 1.8. This determines the minimum melt holding temperature, which must be maintained, often about 1220°F, in order to avoid sludge formation. If the sludge factor based on actual composition within the furnace itself is greater than 1.8, a higher holding temperature must be maintained to avoid sludge formation.
