Dans les procédés de coulée sous basse pression, le métal liquide est déplacé à travers un tube ascendant, d'un four, généralement en céramique, dans un moule métallique. Pour cela, le four est pressurisé progressivement pour régler la hauteur du fluide métallique et imposer une compression finale élevée une fois la pièce complètement remplie. Dans ce processus, les masselottes de coulée ne sont pas nécessaires, ce qui permet de réduire les coûts de coupe et d'obtenir des pièces avec une excellente finition de surface et une très faible porosité. Dans tous les cas, le débit de métal et la température du moule doivent être soigneusement calculés pour trouver le meilleur compromis entre la vitesse du processus et la qualité de la pièce finale. En fait, un débit trop important pourrait entraîner un écoulement très turbulent et un emprisonnement d'air excessif ; en revanche, un remplissage trop lent (combiné à une température de moule trop basse) pourrait provoquer une solidification précoce, empêchant le remplissage complet de la pièce. En utilisant FLOW-3D® CAST , il a été possible de reproduire par une simulation précise, le processus réel, dans lequel un remplissage complet n'est pas obtenu. Le débit dans le tube ascendant n'est pas inclus dans la simulation, imposant le débit directement à l'entrée du moule métallique, avec sa température réelle. La figure 1 montre le fluide à la moitié de la phase de remplissage, coloré avec la fraction solide. Sur cette image, on peut déjà remarquer la solidification précoce, ce qui souligne une vitesse de refroidissement excessive. Figure 1 - Front de solidification La figure 2 montre plutôt la forme finale de l'aluminium, comparée à une image de la pièce réelle. La solidification du front métallique crée un blocage qui force le métal encore liquide à s'écouler autour de lui jusqu'à la partie supérieure de la pièce coulée, où il se solidifie et génère un grand trou dans la forme finale. Figure 2 - Défauts de solidification précoce La vidéo, enfin, montre la dynamique complète de la coulée, soulignant la phase de la solidification précoce et montrant en détail comment le métal ralentit et s'arrête en raison de la fraction solide croissante. https://youtu.be/x-VaIm05q6s Vidéo de la dynamique du remplissage et de la 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...]
Realizing Da Vinci’s Il Cavallo
In the late 15th century, upon the commission of Ludovico Sforza, the Duke of Milan, Leonardo Da Vinci spent 17 years devising a plan to cast a 24-ft. tall bronze horse—the largest equestrian statue in the world—in a single pour. The horse, dubbed Il Cavallo, was never completed. After a full-scale clay model and necessary molds were prepared, French troops invaded Milan, forcing Ludovico to use the bronze earmarked for the immense statue to build cannons instead. Tragically, during the conflict, the molds were lost and Gascon archers from the victorious French troops used the massive model for target practice, reducing it to a mound of clay. In the succeeding centuries, many said the horse would never have been successfully cast anyway. Engineering studies asserted that the casting was impossible because the amount of bronze used in the single pour would result in large pockets of gas and possibly, explosions in the melt. Il Cavallo may yet have a happy ending. Using Da Vinci’s extensive notes on the project, the Institute and Museum of the History of Science (IMSS), located in Florence, Italy, where the great master apprenticed, worked with Flow Science’s Italian representative, XC Engineering (Cantu), to study the feasibility of Da Vinci’s design. The results, publicized by the Discovery Channel and other media sites, proved once again the genius of Da Vinci. The most amazing result, apart from the fact that both casting systems developed by Leonardo—vertical and horizontal—could work, is that the pouring of more than 70 tons of bronze would take three to four minutes. This result seems more incredible if we think to another famous casting described by Benvenuto Cellini. A smaller statue, his famous Perseus, seemed to take many hours. – Andrea Bernardoni, Historian at IMSS Replicating the Past Using Da Vinci’s notes on the casting of Il Cavallo, collected in a 34-page handbook, the IMSS and XC Engineering were able to demonstrate that Il Cavallo, often referred to as “the horse that never was,” can be successfully cast as designed. “At that time, engineers and artists were not used to writing down technological notes,” Bernardoni said, “The notes are not a modern technological plan, but they were files written down to help him understand the best way to achieve his goal.” The only numerical information in Da Vinci’s notes was the height of the horse—24 ft. (7.2 m). But the notes also provided drawings of the molds, ovens and casting system, as well as the posture of the horse. Da Vinci also detailed his intention to cast the bronze in a single pour without any steel reinforcement, and to make the two weight-bearing legs solid bronze. The mixture of earth used to make the molds and the furnace-opening sequence to cast the statue vertically in an upside-down position also were described in the notes. Read more... … [Read more...]
