Original Article Understanding the occurrence of the surface turbulence in a nonpressurized bottom gating system: Numerical simulation of the melt flow pattern Ali Kheirabi 1 , Amir Baghani 2 , Ahmad Bahmani 3 , Morteza Tamizifar 1 , Parviz Davami 4 , Mohsen Ostad Shabani 1 and Ali Mazahery 5 Abstract Surface turbulence during the filling of the mold triggers the entrainment of oxide films, which appears to be detrimental to the soundness of the final casting. Nonpressurized and bottom-gating systems have been employed in order to avoid such casting defects by reducing the surface velocity of the liquid metal. However, recent studies have shown that the melt front velocity in the mold entrance may exceed the critical value in the nonpressurized and bottom-gating systems. Therefore, a study was conducted on numerical simulation melt flow pattern in nonpressurized and bottom-gating systems. It was noted that the liquid metal enters the gate and then mold cavity with a higher velocity by formation of dead zones and vortex flows in runner’s end. Therefore, the current designs based on conventional gating system ratio seem to be not optimized and unable to avoid the surface turbulence. Numerical results were in complete agreement with experimental observations. Understanding the reasons for occurrence of the surface turbulence in nonpressurized and bottom-gating systems provides information on the required steps to improve the design of the gating systems and minimize the entrainment of oxide films during the filling of the mold. Keywords Computational fluid dynamics, nonpressurized gating system, gravity casting, surface turbulence Date received: 3 April 2014; accepted: 20 November 2015 Introduction Casting defects have deleterious influences on the mechanical properties of the cast metal. 1 Filling of the mold is associated with formation of a number casting defects including bubble entrapment, oxide folding, and biofilms. 1–5 Bifilms are formed as a result of surface turbulence during the filling of the mold and are notorious for their effects on the reduc- tion of tensile strength and deterioration of fatigue properties. 6–8 Surface turbulent defects have been reported even in the cast aluminum wheels, which are made by low pressure die casting (LPDC) for automotive industry. 9–12 Bifilms act as initiation sites for the formation of other defects, such as hydrogen pores 13,14 and Fe-rich phases. 15 If the hydrogen content of the aluminum melt increases, an entrained bifilm acts as a site for the growth of hydrogen driven gas porosity. 14,16 In previous years, critical velocity was defined as an appropriate criterion to quantify the surface turbu- lence behavior. Campbell and Runyoro 17 measured the critical velocity of liquid aluminum (about 0.5 m/s). Increasing the melt front velocity causes sur- face turbulence and triggers severe rates of oxides 1 Iran University of Industries and Mines (IUIM), Tehran, Iran 2 Department of Mechanical and Industrial Engineering, University of Iowa, Iowa City, Iowa State, USA 3 Seoul National University, Seoul, South Korea 4 Department of Material Science and Engineering, Sharif University, Tehran, Iran 5 Department of Mechanical Engineering, McMaster University, Ontario, Canada Corresponding authors: Amir Baghani, University of Iowa, 3131 Seamans Center for the Engineering Arts and Sciences, Iowa City, IA 52242, USA. Email: amir-baghani@uiowa.edu Ahmad Bahmani, School of Materials Science and Technology, Seoul National Univesrsity, Seoul 08826, Korea. Email: ahmadbahmani@snu.ac.kr Proc IMechE Part L: J Materials: Design and Applications 0(0) 1–12 ! IMechE 2015 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/1464420715621930 pil.sagepub.com at Gazi University on December 25, 2015 pil.sagepub.com Downloaded from