Aluminum alloys are characterized by their low specific weight, low melting point, negligible gas solubility with the exception of hydrogen, excellent flowability, good machinability and good corrosion resistance. Superior quality castings are an essential requirement for critical structural components used in the automotive and aerospace industries. The production of such molded parts requires that the inclusions and the porosity in aluminum alloys be minimized (or even eliminated) in order to eliminate their harmful influence on the mechanical properties, so that these properties are then mainly controlled by the microstructure of the molded parts.
The presence of hard non-metallic inclusions in aluminum alloys can not only degrade the mechanical properties of the final product, but also create a number of other processing problems in die casting, including:
Reduction of the mechanical properties of the casting
Tensile strength and elongation are significantly reduced with increasing inclusion content. Clean metal, in general, is characterized by high tensile properties (yield strength, ultimate tensile strength, percent elongation).
Poor machinability and high tool wear
Some of the inclusions that can often be present in the casting are brittle and extremely hard ceramic particles. The hardness of certain inclusions can be measured on the Mohs scale against 10 for diamonds. When a molten alloy is cleaned by filtration before casting, the machinability improves quite significantly.
Loss of fluidity and nourishing properties
Inclusions in the molten metal produce a decrease in fluidity. The production of clean molten metal by ceramic foam filter filtration can dramatically increase flowability, up to 30% at normal operating temperatures.
Increased porosity of gases
The level of dissolved hydrogen in aluminum alloy melts is the primary factor responsible for the formation of gas porosity during solidification. The solubility limit of hydrogen in molten aluminum is large, but decreases dramatically as the metal solidifies, leading to the formation of gas porosity. The porosity of gases after solidification depends not only on the hydrogen content of the molten metal, but also on the cleanliness of the metal. The oxides act as pore nucleation sites and the hydrides break down on cooling to produce dissolved hydrogen.
Poor surface quality and lack of pressure tightness
Defects in aluminum products such as pinholes in thin sheets and razor streaks in lithographic plates are usually caused by inclusions. Inclusions, ranging in size from 10 to 20 m in effective diameter, can, at a concentration of a few parts per million, be harmful in rolled aluminum products.