Finite element analysis of flow and heat transfer of molten metal during the slow shot of die castings.

摘要

The aim of this thesis is to investigate the flow and heat transfer of molten metal in the shot sleeve of a cold-chamber die casting machine during the slow shot. A coupled fluid flow and solidification model is developed to predict the plunger profile that will produce the desired wave dynamics to minimize air entrapment and the amount and distribution of solidified phases in the shot sleeve.; A three-dimensional (3-D) finite element method was proposed for the numerical simulation of the fluid flow and heat transfer. The analysis was conducted by solving a coupled problem governed by the conservation of mass, momentum and energy. Simo's version of Newmark algorithm was implemented to solve the momentum equation. The corresponding software was developed and verified using well known test problems for fluid flow and heat transfer. The computed predictions were compared with experimental results. The present method is based on a Lagrangian finite element formulation whereby the fluid is discretized into volume elements that deform due to the movement of the plunger. When the deformation of elements becomes large, remeshing is done, and the data is mapped from the badly-deformed mesh to a new improved mesh.; Since the critical slow shot velocity has an important influence upon the flow pattern and wave formation of molten metal, the effects of inner diameter of a shot sleeve, plunger acceleration, and initial fraction filled on the critical slow shot velocity are investigated. The results are valuable since data acquisition in the shot sleeve is difficult.; In the cylindrical shot sleeve plunger velocities greater than the critical slow shot velocity with initial fraction filled of 0.5 cause the wave front to roll over and break at the ceiling of the shot sleeve, and entrap air. With the initial fraction filled of 0.3 the wave will break at both sides of the sleeve before the wave reaches the ceiling. Plunger velocities slower than the critical velocity do not create a full height wave which leads to the air entrapment in front of the plunger.; The coupling between the flow and heat transfer of the molten metal gives a better picture of the fluid in a shot sleeve. On one hand, the velocity field attained by the fluid flow analysis advects the enthalpy for the heat transfer analysis. On the other hand, the heat transfer analysis solves the enthalpy, temperature and fraction solid fields which consequently determine the density and viscosity fields for the fluid flow analysis. The analysis for the cylindrical shot sleeve found that the solidification of molten metal does happen during the slow shot of die castings although the solidification only occurs near the plunger which may not have a big effect on the following fast shot process.