Effect of Liquid Hot Isostatic Pressing on Structure and Mechanical Properties of a Sand-Cast A356.02 Alloy
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INTRODUCTION
IN the automotive industry, components manufactured from age-hardened A356 alloy are widely used due to the alloy’s excellent castability and intermediate strength.[1] However, A356 castings have insufficient reliability, which is attributed to the variability in mechanical properties, particularly elongation to failure and fatigue strength. This is caused by the fact that A356 castings usually contain inherent defects such as shrinkage and gas pores.[1–3] In addition, Sr-modified A356 alloy is highly susceptible to hydrogen pickup, which increases the gas porosity.[1,4] To diminish porosity, new casting techniques were recently developed.[3,5] However, currently, shrinkage and gas porosities are the main factors responsible for premature fracture in tension[1] and under fatigue.[2,3] Hence, the minimization or even elimination of these structural defects is vital to meet the demand for highly reliable low-weight automotive components produced from high-strength A356 alloy. It is well known that hydrogen and shrinkage types of porosity in aluminum castings can be easily removed by hot isostatic pressing (HIP).[2,6] The densification of A356 alloy significantly improves the alloy’s mechanical properties and reduces scattering.[5,6] As a result, the two-step technique consisting of conventional casting followed by HIP is considered an advanced process used to produce high-quality castings.[6] Initially, the isostatic pressure in HIP was induced by pressurizing a gas against the surface of an object.[6] The application of a high isostatic pressure at high temperature eliminates DAMIR TAGIROV, Research Associate, ELENA LASHINA, Research Engineer, and RUSTAM KAIBYSHEV, Doctor of Science, and Head, are with the Laboratory of Mechanical Properties of Nanoscale Materials and Superalloys, Belgorod State University, Pobeda 85, Belgorod 308015, Russia. Contact e-mail: ivanova_e@ bsu.edu.ru Manuscript submitted October 29, 2010. Article published online December 19, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
shrinkage porosity and hydrogen pinholes. This process occurs in two stages.[6–8] In the first stage, the closure of an isolated pore takes place under an isostatic pressure that is larger than the yield stress (YS) of a material at the HIP temperature. The pores shrink rapidly.[6,8] In the second stage, the bonding of the mutually opposing surfaces of the collapsed pores, which are pushed together as in a planar crack, occurs because atoms diffuse in both directions across the interface.[6,8] In other words, a kind of diffusion-controlled local bonding takes place, completely eliminating pore trails. HIP can be used to improve the casting process by allowing for densification.[6] The duration of 1 hour or longer is necessary to complete this process,[6] which is controlled by drift diffusion.[8] As a result, HIP cycle times are generally ~5 to 16 hours[6] and can be decreased to ~2 hours.[9] However, this technique is characterized by high added cost and is not suitable for the mass production of highvolume comp
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