M. Barkhudarov, Flow Science Inc., Santa Fe, New Mexico; S. Mascetti, XC Engineering, Italy; R. Pirovano, XC Engineering, Italy Abstract High pressure die casting is one of the most complex processes in the foundry world due to the wide range of physical phenomena and process parameters that control the outcome. A particular challenge is achieving optimal conditions in the shot sleeve from which metal is injected into the die cavity. The speed of the plunger in a horizontal shot sleeve must be carefully controlled to avoid unnecessary entrainment of air in the metal and, at the same time, minimize heat losses in the sleeve. The paper presents a general solution for the flow of metal in a shot sleeve, based on the shallow water approximation of the interaction of the moving plunger and liquid metal. The derived analytical solution allows engineers to precisely control the behavior of metal in the shot sleeve during the slow-shot stage of the high pressure die casting process, minimizing the risk of air entrainment. Results are validated with three-dimensional numerical modeling of the process. Coupled with parametric optimization, the numerical model can improve the process conditions predicted by the analytical model. Introduction The speed of the plunger in a horizontal shot sleeve must be carefully controlled to avoid unnecessary entrainment of air in the metal and at the same time minimize heat losses in the sleeve. If the plunger moves too fast, large waves are created on the surface of the liquid metal that may overturn and entrain air into the metal, which will then be carried into the die cavity. A plunger moving too slow results in waves reflecting from the opposite end of the shot sleeve. The reflected waves prevent proper expulsion of air into the die cavity. In either case, the outcome is excessive porosity in the final casting. Moreover, a slow plunger increases also oxidation of the free surface of liquid metal, and the heat losses because of the long contact time with the mold walls. In this article two approaches are used to limit these effects: a general solution for the plunger speed as a function of time and a full-physics, three-dimensional CFD optimization. Mathematical model The dynamics of waves in a horizontal shot sleeve can be analyzed by drawing an analogy with flow in an open channel. A detailed analysis is possible by modeling the flow of metal in a rectangular shot sleeve of length L and height H (justified for initial fill fractions in the range of 40-60% [1]) using the shallow water approximation [3]. In this approximation the flow in the vertical direction is neglected in comparison with the horizontal velocity component. The flow is modeled in two dimensions, with the x axis directed along the direction of motion of the plunger, and the z axis pointing upwards. If viscous forces are omitted, then the flow has only one velocity component, u, along the length of the channel. The plunger speed in the positive x direction is given by dX/dt=X’(t), where X(t) defines the position of the plunger at time t>0. At the moving surface of the plunger, the velocity is defined as . As the plunger moves along the length of the channel it sends waves traveling forward along the metal surface. Each wave is associated with a small segment of the metal free surface and the column of metal directly below it (Fig. 1). The location, metal speed and depth in a wave that separates from the surface of the plunger at time t=tp are given by [3]: (1) Where According to Eq. (1), the metal speed, u, and depth, h, in each wave are constant and depend only on the time of the wave separation from the plunger, tp. They both increase with the speed of the plunger X’. Therefore, the first conclusion is that to maintain a monotonic slope of the metal surface in the direction away from the plunger, the latter must not decelerate. If this condition is not satisfied, then there will be waves sloped in both directions. When they reflect off the end of the sleeve and travel back towards the plunger, it creates unfavorable conditions for the evacuation of air from the sleeve and into the die cavity. Figure 1: Schematic representation of the flow in a shot sleeve and the coordinate system. Controlling the Waves Once a wave detaches from the plunger it travels at a constant speed given by (2) If the plunger accelerates, then each successive wave will move faster than the waves generated earlier. This will lead to a steepening of the surface slope as the waves travel further down the channel, and can potentialy result in overturning. If the speed of the plunger can be controlled as to limit the wave steepening during the slow shot stage, then the overturning can be avoided. Figure 2: The illustration for calculation of the slope of the metal’s free surface. Let us analyze the evolution … [Read more...]
A new frontier in the simulation of gas defects
Thanks to their high reliability, efficiency and precision, process simulation software is increasingly used on a daily basis: many of the defects commonly found in foundry parts are already successfully grasped by existing numerical models and can therefore be prevented. These technologies are rapidly evolving and, thanks to the increasing availability of computing power, you are now able to simulate very complex problems by dividing the domain with millions of cells. Despite this, some important physical aspects have dimensions that remain much smaller than the size of the grid cell: in the modeling of these phenomena are introduced the main approximations, not being able to simulate them directly. Among the numerical challenges that propose the "sub-grid" models - those that involve precisely physical phenomena with a characteristic length smaller than the cell size - one of the largest is to accurately and effectively simulate the smallest air bubbles that remain embedded within the metal. THE SOFTWARE FLOW-3D® CAST The aim is therefore to show the current solutions available for the simulation and analysis of air incorporation and to propose an innovative solution able to overcome the limits of traditional methods. This was done using FLOW-3D® CAST software, one of the most accurate software for modelling a wide range of foundry processes. Its peculiarities are the ability to interpret with great accuracy the geometries of the piece through the FAVOR algorithm, despite the use of a structured grid, and the absolute precision in modeling the movement of the fluid alloy during the filling phase, using the TruVOF algorithm. In addition, the software has numerous numerical models able to simulate all the physics and particularities characterizing the foundry processes, from the thermal cycle of preheating to the final extraction of the piece from the mould. Thanks to this it is possible to carry out simulations with a high precision in determining the position of the defects related to the filling phase. Furthermore, FLOW-3D® CAST has a partially open-source code that can be easily customised: allowing the creation of new numerical models or the improvement of existing ones, the software adapts perfectly to the purpose of this work. NUMERICAL MODELS TO SIMULATE AIR ENTRAPMENT Within FLOW-3D® CAST there are different approaches to simulate air incorporation, of increasing complexity. The simplest approximation is to completely ignore the influence of air on the metal, except for a condition of constant and uniform pressure imposed on the exposed surface of the liquid alloy. With this approximation, the metal behaves as if a perfect vacuum had been created inside the mold, or similarly as if the air could be evacuated instantly from anywhere in the mold. The most complete modeling, on the other hand, consists in simulating also the dynamics of the air, calculating its speed at each point. Although it is possible to consider the complete and coupled fluid dynamics of metal and air simultaneously, in most cases this is not necessary. Due to the relatively small influence of air on the dense and viscous surface of the metal, it is possible to significantly reduce the calculation time by concentrating most of the resources on resolving the motion of the alloy. On the other hand, completely neglecting the influence of air does not allow to obtain realistic solutions to the problem, both in terms of filling dynamics and with regard to the identification of defects related to the presence of air. To obtain the best compromise between simulation speed and precision of the result, considering all the physical phenomena that have a significant influence during the filling phase of a die-casting process, within FLOW-3D® CAST it was decided not to calculate the complete dynamics of the air but to approximate its main contributions with two additional numerical models: In fact, air can be incorporated in the fluid because it is completely surrounded by it, in the form of compressible bubbles (bubble model), or trapped as a quantity dispersed by the effect of turbulence (air entrainment model). Air Entrainment model The air entrainment model [1] was developed in the 1990s to simulate the effect of turbulence on the surface of a moving liquid. During filling, in fact, the high speeds at which the fluid is found are sufficient to disturb the surface to the point of incorporating air in the form of microscopic bubbles. Every single air particle is much smaller than the calculation cell: for this reason the air is represented as a diffused quantity in the metal. Despite the fact that the individual bubbles, being microscopic, are not able by themselves to create a significant defect in the piece, their presence, at different concentrations, can affect the movement of the metal itself. The presence of air in the alloy, in fact, causes the density and local viscosity of the fluid to vary significantly. … [Read more...]
Low Pressure Die Casting, early solidification issues
In Low Pressure Die Casting processes, the liquid metal is moved from a furnace through a rising tube, typically ceramic, into a metal mould. To do that, the furnace is incrementally pressurized to control the height of the metallic fluid and to impose a final high compression once the part is completely filled. In this process risers are not necessary, reducing the trimming costs, and it is possible to obtain parts with a very good surface finishing and with very low porosity. Anyway, the flow rate of the metal and the die temperature should be accurately calculated to find the best compromise between the velocity of the process and quality of the cast part. In fact, a too fast flow rate could lead to a highly turbulent flow and to an excessive entrainment of air; on the other hand, a too slow filling (in combination with a low temperature of the die) could provoke an early solidification, preventing the complete filling of the part. Using FLOW-3D® CAST it has been possible to reproduce through an accurate simulation, the real process, in which a complete filling is not obtained. The flow in the rising pipe is not included in the setup, imposing the flow rate directly at the entrance in the metal mould, with its real temperature. Figure 1 shows the fluid at the half of the filling phase, coloured with the solid fraction. From this picture the early solidification can already be noticed, underlining the excessive cooling rate. Figure 1 - Solidification front Figure 2, instead, shows the final shape of the aluminium, compared with a picture of the real part. The solidification of the metal front creates a blockage that forces the still liquid metal to flow around it to the upper part of the casting, where it also solidifies generating a big hole in the final shape. Figure 2 - Early solidification defects The video, finally, shows the complete dynamic of the flow, underlining the phase of the early solidification and showing in details how the metal slows down and stops due to the increasing solid fraction. https://youtu.be/x-VaIm05q6s Video of the dynamic of the filling and solidification … [Read more...]
Sleeve filling and slow shot phase analysis with FLOW-3D Cast
High pressure Die Casting is a complex field of foundry. The liquid hot metal is generally poured into a shot sleeve for few seconds, until the desired volume is reached. Then, after a short waiting time, the plunger pushes the metal into the die cavity. First a slow shot phase is performed to avoid air entrainment in the sleeve, then a final high speed phase that fill the casting part in a very short amount of time. One of the targets of any producer is to find the best compromise between a fast process, to increase the productivity and to reduce the heat losses, and a slow filing and shot necessary to minimize the air entrainment. FLOW-3D Cast, due to its capabilities, is one of the best software to analyse this process. It can combine easily moving objects, mass sources, heat transfer and solidification, everything in fast and accurate simulations. Several studies were already done to determine the best plunger velocity curve, also coupling FLOW-3D Cast to numerical optimization software. The aim of the present simulation, instead, is to focus on the sleeve filling, underlining the possibility to control also this phase and the defects that could arise from a not-optimal solution. https://www.youtube.com/watch?v=cGQUmH8EHZ0 In the video both fluid and walls are coloured by temperature, with two different colour scales. The heat transfer coefficients have been artificially increased to emphasize the temperature change. Thanks to this fact, it is possible to notice that some drops of metal flow on the beginning of the runner system, solidifying and influencing the casting phase until they are melted again. It is possible also to notice the big waves generated when the filling is finished, and how this waves contribute to entrain some big air bubbles that are pushed into the casting part, generating defects. … [Read more...]
Investigation of Mould Leakages in a Gravity Casting
This article was contributed by Gabriele Taricco of CM Taricco and Stefano Mascetti of XC Engineering. Mould design is a very complex undertaking that must consider not only fluid dynamics and metal solidification patterns, but problems that may arise from the mould itself and how it reacts to stresses from heat transfer. CM Taricco, a mould maker company based in Italy, recently encountered a problem of metal leakages at the bottom of one of their new moulds. The cause of the mould leakages was initially obscure and only appeared after a few process cycles. It was evident that the problem was critical, since it would compromise the production timeline and dramatically increase the costs to cast the part. Investigation of an idea The process itself was a gravity casting, with well-controlled pouring dynamics and overflow designs, so the problem could not come from the fluid dynamics part. The hypothesis of Gabriele Taricco (owner of CM Taricco) was that the metal leakages were resulting from a bad design of the thermal dissipation of the mould, causing a non-uniform distribution of temperature and hence large and unwanted deformations at the bottom of the mould, that were enforced cycle after cycle up to the opening of a critical area where metal could flows out. To verify this and to find a quick solution to the problem, a FLOW-3D simulation was run to exactly visualize what was happening to the mould as it was being heated. The analysis After a filling simulation, to ensure a good filling pattern, the focus of the simulation was redirected to a thermal die cycling analysis. The setup in this case is fast and straightforward requiring only 1 hour to reproduce 10 production cycles on a common desktop machine (i7 5930K, commercial value 1500 dollars). The result confirmed CM’s initial hypothesis: by looking at the temperature field, from various points of view and cross sections in a single image using FlowSight, it was clear that the temperature distribution of the mould would easily cause the expected deformations and metal leakages. Simulation of the mould’s temperature during the die cyclings Further analysis with the Fluid-Structure Interaction module Once the problem was identified and the technical staff could start designing an improved mould, CM Taricco wanted to have a final confirmation running a FEM analysis of stresses and deformations on the die. To perform this analysis, XC Engineering Srl helped CM in setting up and performing the calculation. The result of the analysis showed exactly what CM thought was happening: FLOW-3D was able to reproduce with extreme accuracy the same location and size of the real deformations found on the mould after few pouring cycles. This was good news for CM, and enforces an additional recommendation to use the FSI module at the design stage to predict the real die deformations based on the real casting conditions. Deformation of the mould during the die cyclings, simulated using the Fluid Structure Interaction model. Deformations are amplified x20. Read more... … [Read more...]
