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Two-Phase Fluid Dynamics in High Pressure Die Casting

High pressure die casting (HPDC) is a novel manufacturing method with capability of mass production with higher accuracy. In this process molten metal is injected into the die at high velocity (30-100 m/s) leading to a less physically understood filling behavior. Due to the turbulent nature of the flow, liquid jet may undergo severe instability, breakup and even atomization each of which could culminate in one or more typical casting defects.

Porosity is one of the challenging defects in the final product and may be affected by jet instability and atomization during injection phase. In case of atomization a large number of droplets with high velocity impinge the colder confining walls of the casting mold and might solidify consecutively. Different time scales of the impingement of the droplets and their
solidification may result in heterogeneous structures near the surface of final product.

Accordingly, an appropriate numerical investigation of this process should include liquid jet instability and breakup as well as porosity formation near the surface.

Therefore a numerical framework using an Eulerian-Lagrangian approach is established to simulate the liquid metal jet breakup and droplet formation during the injection phase. First an analytical model for flow instability leading to turbulent breakup is considered to realize the droplet formation near the liquid jet interface, and then a novel idea is used to capture the droplet formation as Lagrangian droplets coupled with numerical simulations of continuum phase by volume of fluid approach (VOF) to model the global spreading and primary breakup of the turbulent liquid jet. This approach helps to reduce computational costs by using the sub-grid model instead of resolving small liquid structures in finer grid resolutions. Afterwards, an analytical model for droplet impact on mold walls and solidification is being studied and implemented in the numerical framework. This approach enables the prediction of porosity formation near the surface of the final product.

In Figure 1, the numerical simulation of primary breakup of turbulent water jet is presented against experimental study performed in our laboratory.

Fig. 1: Numerical simulation of turbulent water jet (Re=69000) using the Eulerian-Lagrangian approach (left) and experiments (right) for two consecutive times.

Figure 2 shows the experimental setup and Figure 3 presents the schematic of analytical model used for droplet-wall interaction with the solidification effects.

Fig. 2: Experimental study on primary breakup of water jet using high speed camera and back illumination

Fig. 3: Schematic of droplet-wall impact sub-model for porosity prediction.

(Mahdi Saeedipour, Supervision: Simon Schneiderbauer)