Re: What causes Porosity of Die-Castings and How to Improve Die-Casting
Hi Johhny
Zinc alloys are not my area of familiarity. However, there are some general indicators I can give.
When the metal has been polished, (<1 micron surface finish) what shape are the porosity voids when you look at them under a microscope? If they are round, they are most probably gas. You may be able to fix this by degassing the molten metal with nitrogen or argon before you die cast. Don't overdo the degassing, some metals can react with or absorb nitrogen.
Of course, you could have the problem that you are pulling the metal into the die so quickly that you get turbulence and the gas voids are actually air that has been pulled into the metal as you are casting.
If the voids are irregular and follow grain boundaries, they may be shrinkage voids. This has at least two causes. One is that you are not "feeding" the casting fast enough. In other words, the metal is not getting into the die cavity fast enough. Another cause is that the metal is cooling too fast when it hits the die wall. In this case the metal may cool so fast that shrinkage voids are unavoidable.
Does the porosity appear at the start of the day, and get less as the dies are used more? If so, the temperature of the die may be having an effect.
Strange but true, even the time of year can have an effect. During the summer, the average humidity of the air is greater, so you can get hydrogen pickup having a greater effect during the summer.
Sorry if this is a bit general, but solving metallurgical problems is not something that can be done in a forum. All I can do is give general pointers to places to look at.
NC
