CIM Bulletin, Vol. 97, No. 1082, 2004
S.G. Shabestari, P. Mokarian and S. Saeidinia
One of the major problems associated with the casting of aluminum alloys is the formation of gas porosity. Aluminum and its alloys are very susceptible to hydrogen absorption. Since solubility is much lower in solid state than liquid state, hydrogen atoms leave their position during solidification, and, by combining together, form hydrogen molecules. These micrometer scale cavities, called microporosity, cause the deterioration of mechanical properties, particularly strength, ductility, fracture toughness, fatigue resistance, pressure tightness, and corrosion resistance.
To avoid these problems and produce good quality castings, it is necessary to remove hydrogen from the melt. Several methods are available for degassing aluminum melt. One of them is a rotary degassing system using inert gas, e.g. Ar or N2. Various thermodynamic and kinetic factors govern the removal of hydrogen from molten aluminum. A large bubble size of inert gas results in very poor gas removal efficiency. For a given flow rate, large bubbles are far from each other and the hydrogen atoms have a long diffusion path to reach the bubble surface. Therefore, they contain a small fraction of hydrogen. A large number of small bubbles will be close together and hydrogen atoms will have only a short distance to travel in the melt to arrive at a bubble surface. Small bubbles will enhance gas removal efficiency, because of a higher surface-to-volume ratio and a longer residence time as their velocity is decreased.
Gas flow rate, rotor speed, temperature, and time are important parameters in the rotary degassing process. The experiments were conducted to investigate the influence of various process parameters in rotary degassing of about 350 kg of AS5U3G aluminum casting alloy. The melting unit was a crucible furnace with the capacity of 400 kg aluminum melts. After melting, a grain refiner of Al-5Ti-1B was added to the melt. The final chemical composition was controlled with a spectrophotometer (ARL spectrometer). The temperature was controlled and melt was poured in a ladle. Then degassing was done with a rotary degassing system with argon inert gas. To avoid the elemental loss, Al-Mg (hardener) and Al-Sr (modifier) were added at the last two minutes of the degassing operation. Degassing was done in a different set of argon gas flow rate (12 to 21 L/min.), rotor speed (350 to 500 rpm), degassing time (6 to 12 min.), and melt temperature (775°C to 780°C).
The efficiency of the degassing process was evaluated with a comparison of reduced pressure test (RPT) samples before and after degassing. Tensile test samples were prepared according to the ASTM B108 standard and tested in an Instron 4486 tensile machine. Inclusions and oxides of the reduced pressure test (RPT) samples were studied through Philips XL40 scanning electron microscopy (SEM). Also, the distribution of porosity and inclusion was investigated by means of SEM in the fracture surface of tensile samples.
The results show that by increasing the rotor speed from 350 to 450 rpm, the degassing efficiency is increased. Also, degassing efficiency increases with increasing gas flow rate up to 18 L/min. The degassing efficiency decreases as the melt temperature increased from 755°C to 780°C. This is because hydrogen absorption increases with temperature. The best degassing efficiency is obtained in eight minutes with a rotor speed of 400 rpm and a gas flow rate of 18 L/min. at 760°C. Tensile strength and elongation are increased considerably by the degassing process. Microstructural evaluation revealed that inclusions and iron intermetallics could act as nucleation sites for the formation of porosity. SEM shows small size and well-distributed microporosity in degassed samples. By optimizing the process parameters, RPT sample density is increased up to 2.73 g/cm3, which guaranties the production of high-quality aluminum castings.