Turbulent breakup of non-metallic inclusions and equiaxed crystals during solidification of a hypoeutectic Al-Si alloy

Ewan Lordan,Kun Dou,Yijie Zhang,Chrysoula Tzileroglou,Alain Jacot,Paul Blake,Zhongyun Fan

doi:10.1016/j.mtla.2021.101114

Abstract

The breakup of agglomerates and bodies suspended in turbulent flows are important phenomena that influence many aspects of modern solidification processing. It is often assumed that breakup operates in high-pressure die casting, wherein molten metal is transported at high speed through a narrow orifice system. To test this assumption, X-ray tomography and electron backscatter diffraction mapping are used to characterise pores, inclusions, and primary α-Al grains in die-cast samples produced with different flow field intensities. Numerical simulations are performed in ProCAST (ESI Group) to quantify the three-dimensional flow fields and to relate the derived quantities to breakage. Increasing the dissipation rate of turbulent kinetic energy is shown to induce a refinement of both non-metallic inclusions and primary α-Al1 grains nucleated in the shot chamber, a phenomenon which is ascribed to breakage. Several breakup mechanisms are discussed, with emphasis on the role of fluid turbulence.

Highlights

The breakup of bodies and aggregates suspended in a turbulent flow are important phenomena that influence many aspects of commercial casting processes
We demonstrate that higher flow field intensities promote breakage, enhancing the tensile properties of the casting
The increase in tensile strength and 0.2 % proof strength is more modest in comparison, with average values of 270±19 MPa and 313±9 MPa, and 124±6 MPa and 137±7 MPa obtained for the traditional runner system (TRS) and lean runner system (LRS) specimens, respectively

Summary

Introduction

The breakup of bodies and aggregates suspended in a turbulent flow are important phenomena that influence many aspects of commercial casting processes. Breakup may play a prominent role in die-casting processes where defect-forming suspensions, such as gas bubbles [3,4] and oxide films [5], are readily transported by the bulk-liquid flow. Extrinsic melt treatments are ubiquitously used in foundries to control liquid metal quality prior to solidification processing These treatments typically utilise an external field to impart a force on the liquid, either directly by agitation [7,8] or indirectly through ultrasonic cavitation [9]. Some uncontrolled solidification takes place producing a multi-phase mixture consisting of up to 30 vol.% solid [11] This multiphase fluid—which carries an array of defect-forming suspensions—is transported at high speeds through a narrow orifice system where it is subject to shear rates in the order of 104~105 s-1 [12]. This invites an obvious question: can fluid flow be manipulated in such a way as to promote breakage during the transportation of liquid metals?

Results

Discussion

Conclusion