Rheological Behavior of Al-7Si-0.3Mg Alloy at Mushy State
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SEMI-SOLID manufacturing (SSM) is a relatively new metal/alloy processing technique in which metallic alloys are cast in the form of semi-solid slurry in the temperature range between liquidus and solidus. Introducing forced convection in the melt during solidification is one of the popular ways of preparing the slurry. This slurry has fragmented solid particles immersed in liquid, which offers less resistance to flow even at a high solid fraction. SSM slurries contain approximately 40 to 50 pct solid fraction and behave like solid when kept at rest and flow like liquid when sheared. This shear thinning phenomenon is known as thixotropic behavior, implying that the semi-solid slurry viscosity is high at lower stresses and progressively decreases with the stress value. Viscoplastic nature of the semi-solid slurry depends on the amount of solid fraction it contains and determines the feasibility of shaping it in the form of components using high pressure die casting or forging machines. Hence, it is evident that the rheological behavior of the slurry is key to the success of the semisolid processing technique. Characterization of viscoplastic nature of the semisolid slurry is generally performed by rheological tests using viscosity as the main physical parameter. Rheo-
PROSENJIT DAS, Scientist, is with the NNMT Group, CSIRCentral Mechanical Engineering Research Institute, Durgapur, 713209, India and also with the Department of Mechanical Engineering, Indian Institute of Science, Bangalore, 560012, India. SUDIP K. SAMANTA, Scientist, is with the NNMT Group, CSIR-Central Mechanical Engineering Research Institute, Durgapur, 713209, India. PRADIP DUTTA, Professor, is with the Department of Mechanical Engineering, Indian Institute of Science, Bangalore, 560012, India. Contact e-mail:[email protected] Manuscript submitted November 29, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
logical behavior of materials is generally expressed in terms of stress–strain–time relationships with respect to a wide range of process parameters such as temperature, strain rate, and fraction solid. During most rheological investigations, mushy state of matter is deformed under the application of shear upon which thin slices of matter glide over each other. Viscosity and deformation behavior of matter at mushy state establishes the correlation between its rheological and mechanical behavior. The changes occurring during a rheological investigation are reversible if the slurry is of viscoelastic nature; but in case of semi-solid alloy slurry, the changes are generally irreversible. A brief review on the activities of rheological investigation of semi-solid slurries is given below. Several researchers have worked on rheology of semisolid metal/alloys since the invention of semi-solid processing in the early 1970s. Some extensive review articles are also available in the published literature on the topic.[1–6] The present paper deals with the rheology of Al-7Si-0.3Mg alloy, which has emerged as the potential candidate for sem
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