A Novel Characterization Technique to Determine Pore Susceptibility of Alloying Elements in Aluminum Alloys
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f the major problems associated with the casting of aluminum alloys is the formation of microporosity—a leading cause in the reduction of bulk mechanical properties,[1] particularly fatigue performance and toughness,[2–5] as well as loss of pressure tightness in cast parts. The main causes of pore formation during casting are insoluble gas evolution and inadequate feeding to compensate volumetric shrinkage during solidification. The defects that result from these causes are referred to as gas and shrinkage porosity, respectively. Depending on the shape and morphology of pores, they can be distinguished. Rounded pores generally are classified as gas pores, whereas irregular pores in the interdendritic spaces are classified as shrinkage pores. Hydrogen has a significantly lower solubility in solid aluminum compared with liquid aluminum. During solidification, hydrogen is rejected at the solid–liquid interfaces, and it builds up in concentration in the interdendritic spaces.[6] When the hydrogen levels in these liquid spaces reach a critical supersaturation, which is determined by the hydrogen solubility in the liquid at that temperature, hydrogen pores nucleate on preexisting bubble nucleation sites such as the folded G.S. VINOD KUMAR, formerly Research Fellow, Non-Ferrous Materials Technology Development Centre, Hyderabad, India, is now Research Fellow, Helmholtz Centre Berlin for Materials and Energy, Berlin, Germany. SURESH SUNDARRAJ, Staff Researcher, is with GM R&D, India Science Lab, ITPL, Bangalore, India. Contact e-mail: [email protected]. Manuscript submitted September 29, 2009. Article published online May 5, 2010. METALLURGICAL AND MATERIALS TRANSACTIONS B
double oxide film.[7,8] With further rejection and diffusion of hydrogen, these pores begin to grow as the solidification proceeds. The final pore volume and pore size distribution in the cast product depends on the amount of initially dissolved hydrogen content in the melt. Considerable work in the past has focused on quantifying the amount of dissolved hydrogen in the aluminum melt.[9–18] Among these techniques, the reduce pressure test (RPT), also known as the Straube–Pfeiffer technique has been accepted widely in the aluminum foundry community. The RPT uses a liquid aluminum sample that is poured into a pressure-tight chamber. The sample is left there to solidify under reduced pressure. Under this condition, the growth of porosity is virtually unrestricted, and the amount of porosity that is created by a given amount of hydrogen gas is much larger compared with what is obtained under actual casting conditions. Thus, it is possible to arrive at a correlation between the initial hydrogen content and the final pore size under partial pressure using RPT. However, to demarcate clearly the effect of pore susceptibility of individual elements present in a ternary or a multicomponent Al alloy, the conventional RPT technique needs to be modified to assess precisely the pore forming tendencies of the ternary elements (X) in Al-12.6Si-X alloys. We present a new approach
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