Melt Quality Assessment

Abstract

It is well known that the reaction of liquid aluminum with the moisture in the environment results in two products: aluminum oxide and hydrogen gas that dissolves in aluminum. Both of these products are considered to be detrimental to the properties of aluminum alloys. Therefore, test equipment has been developed to check the levels of these defects in the melt. Many of these involve expensive and consumable tools. In addition, an experienced personnel may be required to interpret the results. Nonetheless, aluminum oxide is harmless as long as it remains on the surface. The problem begins when this oxide is entrained into the liquid aluminum such as turbulence during transfer or mold filling in a non-optimized design. This can only happen by folding of the oxide. During this action, rough surface of the oxides comes in contact to form no bonds. These defects are known as bifilms that have certain characteristics. First, they act as cracks in the cast parts since they are oxides. It is important to note that aluminum oxide has thin amorphous oxide (known as young oxides) and thick crystalline oxide (γ-Al2O3) that may be formed in a casting operation. Second, almost zero force is required to open these bifilms due to the unbonded folded oxide skins. Thus, these defects can easily form porosity by unravelling during solidification shrinkage. On the other hand, the formation of porosity by hydrogen is practically impossible. Theoretically, hydrogen has high solubility in the liquid but it has significantly low solubility in solid aluminum. Thus, it is suspected that hydrogen is rejected from the solidification front to form hydrogen gas and porosity. However, the hydrogen atom has the smallest atomic radii and high diffusibility. Therefore, segregation of hydrogen in front of the growing solid is difficult. In addition, the energy required for hydrogen atoms to segregate and form hydrogen gas molecule is around 30,000 atm. Under these conditions, porosity formation by hydrogen is not likely to be achieved. Hydrogen probably stays in a supersaturated state or diffuses homogeneously through the cast part. The effect of hydrogen can only be seen when it can diffuse into the unbonded gap between the bifilms to open them up to aid the unravelling of bifilms to form porosity. This phenomenon can be easily detected by a very simple test called reduced pressure test. When a sample is solidified under vacuum, the bifilms start to open up. Since all porosity is formed by bifilms, the cross section of the sample solidified under vacuum can be analyzed by means of image analysis software. The sum of maximum length of pores can be measured as an indication of melt quality. Since bifilms are the most detrimental defects, this value is called “bifilm index” given in millimetres, which makes this test the only test that can quantify aluminum melt quality in such detail including both the effects of bifilms and hydrogen together. Several Al-Si alloys were used at various conditions: degassing with lance, ceramic diffusers, and graphite rotary has been compared. Gravity sand casting, die casting, and low-pressure die casting methods were evaluated. The effect of grain refiners and modifiers was studied. And the evolution of the bifilm index has been presented.