Influence of secondary cooling strategies on thermal gradients in the direct chill casting of magnesium alloys
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Influence of secondary cooling strategies on thermal gradients in the direct chill casting of magnesium alloys P. V. Sai Divya1 · P. K. Penumakala1 · A. K. Nallathambi2 Received: 25 February 2020 / Accepted: 9 September 2020 © Akadémiai Kiadó, Budapest, Hungary 2020
Abstract Direct chill (DC) casting is the widely used technology for the production of ingots or billets of nonferrous alloys. Nonuniform thermal gradients generated during the process play a predominant role in generation of cracks within the billet. In the present work, a temperature-based finite element technique is used to estimate thermal gradients developed in different regions of billet during DC casting of magnesium alloys. Shrinkage-dependent realistic heat transfer coefficient and temperature-dependent nonlinear boiling curves are used as boundary conditions in the mould and secondary cooling zones, respectively. The simulated temperature profiles during DC casting of AZ31 alloy are validated with real-time plant measurements. Secondary cooling strategies such as pulsed water flow and water removal with wiper are widely used in industry for reducing thermal gradients in DC casting. In this work, the influence of these two techniques on the reduction of thermal gradients is investigated for the DC casting of wrought AZ31 and cast AZ91 alloys. Usage of wiper reduces thermal gradients by 30% in case of AZ31 alloy casting. Surface reheating is observed with the use of wiper in AZ91 alloy. The use of pulsed water is found to reduce thermal gradients at the bottom of the sump in AZ31 casting, whereas no thermal gradient reduction is observed in AZ91 casting for the investigated conditions. Keywords Direct chill casting · Magnesium · Secondary cooling · Wiper · Pulsed flow List of symbols k Thermal conductivity (W m−1 K−1) 𝜃 Temperature (°C) 𝜌 Density (kg m−3) Cp Specific heat capacity (J kg−1 K−1) Δh Latent heat (J kg−1) fpc Phase fraction g(θ) Phase change function 𝜃li Liquidus temperature (°C) 𝜃so Solidus temperature (°C) K Thermal conductance matrix C Capacitance matrix L Latent heat vector F Force vector 𝜃s Surface temperature (°C) * P. K. Penumakala [email protected] 1
Department of Mechanical Engineering, Birla Institute of Technology and Science-Pilani, Hyderabad Campus, Hyderabad, India
Department of Mechanical Engineering, National Institute of Technology, Tiruchirappalli, India
2
𝜃w Cooling water temperature (°C) 𝜃coh Coherency temperature (°C) Rw Thermal resistance due to convective cooling of mould wall with water (m2 K W−1) Rm Thermal resistance through the mould wall (m2 K W−1) Rint Interfacial thermal resistance (m2 K W−1) Lm Mould wall thickness (m) Lg Air gap thickness (m) km Thermal conductivity of the mould wall (W m−1 K−1) kair Thermal conductivity of air (W m−1 K−1) htot Total heat transfer coefficient from the molten metal to water in the mould (W m−2 K−1) hw Cooling water heat transfer coefficient (W m−2 K−1) hint Interface heat transfer coefficient (W m−2 K−1) Xo Solidified shell
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