Below is a list of the top 10 frequently asked questions our inventors receive about the Mercalloy family of alloys. Don’t see your question on the list? Email Dr. Ray directly and he’ll respond with an answer.
In the most general sense, what is the simplest definition of a structural aluminum die casting alloy?
A structural aluminum die casting alloy is a die casting alloy that exhibits higher ductility than a permanent mold alloy of similar composition.
How can a structural aluminum die casting alloy exhibit higher ductility than a permanent mold alloy of similar composition? I thought castings made by the permanent mold casting process had the highest mechanical properties of all conventional casting processes.
The cooling rate associated with the high-pressure die casting process, which uses pressure to push metal against the uncoated dies, is much higher than for permanent mold casting, which doesn’t use pressure but does use insulating coatings on the metal dies. The microstructure is finer for the die casting process than permanent mold casting and the mechanical properties, including the ductility, are also higher for die casting.
Why do conventional high-pressure die casting alloys have inferior mechanical properties to both permanent mold castings and sand castings?
Conventional die casting alloys require approximately 1 percent to 1.3 percent iron content to avoid die soldering. However, this level of iron destroys mechanical properties, particularly the ductility, because iron needles form in the microstructure during the solidification phase and clog the interdendritic passageways. This hinders feeding and contributes to porosity issues. In the solidified state, the iron needles produce stress risers that further decrease mechanical properties below the porosity, itself.
What is the main chemistry difference between a conventional die casting alloy and a structural aluminum die casting alloy?
The main difference is the iron level in the alloy. Conventional die casting alloys contain 1 percent to 1.3 percent iron content, which decreases mechanical properties, while structural aluminum die casting alloys contain a lower level of iron comparable to most permanent mold alloys (less than 0.40 percent iron) and in most sand castings (less than 0.25 percent iron).
Since the iron levels in structural aluminum die casting alloys are low, what additional elements do structural aluminum die casting alloys need to provide die soldering resistance?
There are currently two classes of structural aluminum die casting alloys. The first group, which includes Silafont-36 and Aural-2, has been around for approximately 20 years and relies on 0.5 percent to 0.8 percent of manganese content to resist die soldering. The second, more new group is the Mercalloy family of alloys which has been used almost exclusively by Mercury Marine for marine applications that require high-corrosion resistance in a saltwater environment and high ductility in high-strain loading conditions. The Aluminum Association registered Mercalloy alloys rely on strontium at a 0.05 percent to 0.07 percent level to create a thin strontium oxide at the molten metal surface to resist die soldering. Mercury Marine has 100-percent adopted the Mercalloy family for its operations.
Do Mercalloy alloys have advantages over the previous generation of structural aluminum alloys?
Yes. Structural aluminum alloys that rely on manganese for die soldering resistance contain 10 types of micron-sized intermetallic manganese compounds due to the solubility of manganese in aluminum alloys that contain iron. The Mercalloy family of alloys, which rely on strontium for die soldering resistance, do not contain intermetallic compounds and can outperform the older generation in high-tension testing that are more typical of a crash.
What distinguishes the composition of a structural aluminum die casting alloy from a sand casting alloy, if both alloys have an iron specification of 0.25 percent maximum iron?
Manganese and silicon are the distinguishing elements. Conventional and sand casting alloys allow for no more than 0.5 percent manganese content because intermetallic compounds of manganese form and decrease the mechanical properties. The previous generation of structural die casting alloys relied on manganese levels above 0.5 percent for die soldering resistance. Structural die casting alloys that rely on strontium (Mercalloys) for die soldering resistance contain less than 0.35 percent manganese, but the real difference between this next generation of structural die cast alloys and conventional or sand casting is the silicon content. Structural die casting alloys require 9 percent to 11 percent silicon for fluidity, but conventional casting and sand casting require much less.
Why are sand casting alloys, such as Alloy 356 and Alloy 357, not recommended to be modified with 0.03 percent strontium for fear of increasing the porosity level, but the structural aluminum die casting alloy 367 can contain strontium levels as high as 0.07 percent?
In the high pressure die casting process, there is pressure intensification while the die casting alloy is solidifying that completely surprises hydrogen porosity formation. Thus, high levels of strontium are preferred for die soldering resistance because strontium has no effect on porosity formation in the die casting process.
Since strontium is a reactive metal, what are the effects on strontium if the alloy is in a holding furnace for a long period of time?
Strontium will be lost at the same rate as magnesium is lost in aluminum-silicon-magnesium alloys. Strontium can be added back into the melt in 50 gram packets of 90 percent strontium and 10 percent aluminum.
Will I have problems holding chemistry if I recycle runners and biscuits in my holding furnace?
It is much easier to hold chemistry for Mercalloy alloys 367, 368 and 362 than for other alloys, such as Silafont-36 or Aural-2, which are made from prime aluminum.
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