Development of a 3D Filling Model of Low-Pressure Die-Cast Aluminum Alloy Wheels
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eferred trend in the automotive industry is toward aluminum alloy wheels over steel wheels principally because of the greater design flexibility they offer.[1] However, despite their increased use, they remain one of the most difficult automotive castings to make as they must possess both good mechanical performance and good surface finish, a combination not required in other automotive aluminum castings such as cylinder heads and blocks.[2] The main castingrelated defects in automotive aluminum alloy wheels include porosity, entrained oxide films, and hot cracks.[2,3] The porosity may be large in scale (>1 mm) when related to liquid encapsulation during wheel solidification or gas entrainment during die filling,[3–9] or, small in scale (0
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The pressure is applied to the surface connected to the initialized layer of liquid metal at the inlet boundary; this insures that only liquid metal enters the domain. The pressure profile applied to this boundary is the relevant section of the production pressure-fill-curve, shown in Figure 4 or a variant under investigation. As
the vent to approximately correct for the change in geometry. When air flows from the die cavity (a large volume) into and through the vent (a smaller volume), there are two major sources of energy loss: one is related to the friction loss as the gas travels along the vent (the major loss); the other is related to the energy dissipation when secondary flows, such as vortices and recirculation, develop because of an abrupt geometry change (the minor loss).[29] The momentum source term describing these losses is presented in Eq. [8]. l q SM ¼ u KI u2 ½8 KF 2
Fig. 4—The inlet pressure curve used in the model which is limited to the die filling portion of the production pressure curve.
where SM is the user-defined momentum source (N/m3), l is the viscosity of air (Pa s), q is the air density (kg/m3), u is the air velocity through the vent (m/s). KF is the frictional coefficient of the vent (m2), and KI is the inertial coefficient of the vent (m1). Integration of Eq. [8] over the length of the vent yields the pressure drop—e.g., DP ¼
previously described, the portion of the industrial pressure-fill-curve related to the liquid metal being pushed up through the transition pipe and sprue has been removed, as has been the portion of the pressureholding stage (refer to Figure 2). 2. Outlets As previously described, gaps between the various die sections serve as one of the areas through which gas is vented from the die cavity during filling. Referring to Figure 3, two such areas identified industrially are the interface between the wheel ejection pins and the top die, located at the top of the hub, and the interface between the top die and the side die, located on the in-board rim flange. The gap at the ejection pins is effectively an annulus, with an inside diameter of 12.0 mm, width of 0.05 mm, and axial length of about 15 mm.[25] The gap at the interface between the top die and the side die is 1 mm wide and 20 mm long extending around the circumference of the in-board r
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