Improving High Pressure Die Casting Designs

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Improving High Pressure Die Casting Designs

The content for this article was contributed by Mark Littler of Littler Diecast Corporation.

Littler Diecast Corporation, a producer of high pressure die castings, was recently able to redesign and die cast an electrical switch frame for an aerospace application. Formerly produced by a different manufacturer, there were defect problems in a high number of the castings and a new design was needed to achieve a lower scrap rate. Littler Diecast was able to demonstrate that they could pinpoint the defects through simulation without previous knowledge of the problems. This impressed the client enough to land them the job.

Identifying the Problem

The switch is cast from A380 aluminum and is approximately 1 ¼” x 1” x 1/2” in size. Littler Diecast found that porosity problems were plaguing the part in two locations: the plate and the chimney. This was confirmed by the customer. Holes were forming in each of the locations because of the way the part filled. The flow would enter through a single gate as shown in Figure 1, jet to the far side of the plate and then backfill, trapping air pockets that do not always close due to early solidification. The same problem was found in the chimney: fluid would jet to its furthest extent and then backfill, creating trapped air that could not vent through the parting line.

X-ray of original part, showing porosity problems
X-ray of original part, showing porosity problems
Original design with a single gate
Figure 1: Original design with a single gate. Plot colored by velocity magnitude.
Final design with three gates
Figure 2: Final design with three gates. Plot colored by velocity magnitude.

The Original Part Design

There were other problems with the original design of the part. There was a lot of die erosion around the slot for the lock washer and the sealing surfaces on the bottom of the plate. The overflows located at the corners of the part were not large enough to allow defects to flow out.

Using FLOW-3D, Littler Diecast was able to analyze the flow behavior and visually determine what was occurring. With such a small part, early solidification is a problem due to the rapid cooling in thin sections. If flow jets across the part and back, the fluid has more time to cool and create entrapped air. It is best to have the hottest liquid coming in last. With this in mind, Littler Diecast was able to test a number of ideas and achieved a design that minimized the potential for problems and maximized the process window.

The Final Part Design

After three major design changes, the part quality was vastly improved. First, the gating and runner was redesigned so the fluid entered through three gates in an entirely new direction. This, combined with the second design change of creating a larger overflow, meant that there was much less back flow in the plate allowing the hottest fluid to enter last. Third, the approach angle and locations of the gates were altered, which helped to prevent backflow in the chimney.

This new design also reduced the potential for die erosion in the new tool. Instead, fluid would jet onto a core pin used for the center hole in the chimney. The core pin is easily replaced, a much faster and less expensive repair than repairing the die steel. All of these design changes took place before any new die steel was cut eliminating the costly process of engineering changes if problems are discovered after the tooling has already been produced.

Physical Verification

After the trial run of the production tool, Littler Diecast was able to verify the design changes through short shots, x-rays and destructive tests. The short shots showed a balanced runner, and there was no porosity visible in the x-rays. Break testing showed a consistent crystalline grain structure with no voids, demonstrating that the failure was due to the strength of the material and not a casting defect.

Littler Diecast x-ray validation metalcasting
X-rays at different angles of a sample final part that was picked up from the shop floor.

Learn more about the versatility and power of modeling metal casting processes with FLOW-3D CAST.

Realizing Da Vinci’s Il Cavallo

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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.

Leonardo da vinci bronze horse casting
Leonardo da Vinci bronze horse casting
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

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.

Based on Da Vinci’s notes, IMSS built CAD models for the simulation of the casting process with FLOW-3D. “The dimensions of the runners, the external canals from furnace to runners and the bronze alloy were deducted from contextual sources, such as The Pirotechnia by Vannoccio Biringuccio (1540), Cellini’s Treatise on Sculpture (1541) and Lives of the Artists (1551) by Giorgio Vasari,” Bernardoni said. “From Vasari, we know that usually the bronze to cast statues was 10% tin.”

Simulating Leonardo’s Design

The simulations performed by XC Engineering demonstrated that molten bronze would fill the statue’s molds in a few minutes, and that all the metal would have weighed exactly the amount Da Vinci had calculated, according to Stefano Mascetti, who performed most of the FLOW-3D simulations at XC Engineering.

“I have studied Leonardo’s horse case for two years, exploring each possibility, experiment, machine and layer of his drawings,” Bernardoni said. “I had the sensation everything would go in the right way, and the results confirmed my sensation.”

Based on the FLOW-3D results, IMSS hopes to cast the horse in Milan, where Il Cavallo was meant to originally stand. “The result of the numerical simulation reinforced my decision in the end to do the real casting of the horse, following step by step Leonardo’s notes building his foundry,” Bernardoni said.

Learn more about the versatility and power of modeling metal casting processes with FLOW-3D CAST.

Integration of CFD Analysis into Die-Cast Process Design

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Integration of CFD Analysis into Die-Cast Process Design

This article was contributed by Alex Reikher, Ph.D., of Shiloh Industries

In today’s marketplace, organizations face increasing pressure from both old, well-established, and new, rapidly-growing economies. Globalization of the marketplace forces companies to be on the lookout for avenues to sustain their competitive advantage. Rapid developments in internet technology and free exchange of information, are some of the factors that reduce the period when companies can hold on to their competitive advantage. One of the ways organizations can maintain a leading position in the industry is to reduce time required to bring innovations to the marketplace. With the goal of compressing die-cast process development time, modeling with FLOW-3D has become an integral part of Shiloh Industries’ engineering department.

At Shiloh Industries, new projects start with conceptual development of the gate and runner system, approximate slow shot profile calculations, shot cylinder diameter, minimum ventilation area, and process pressure requirements. Flow analyses are performed in order to develop the best possible flow pattern and minimize air entrainment. After runner design is finalized, thermal analyses are run to help make the best decision on waterline placement.

For over seven years, we have been able to prove to our group the accuracy and reliability of the predicted results using FLOW-3D as a die casting process modeling tool. These results have had good correlations with the actual casting defects, temperature distribution and flow patterns.
Alex Reikher
Shiloh Industries

An attractive feature of FLOW-3D is the ability to run separate analyses for every stage of the development process. It allows for a short development time in choosing the right shot profile, gate design, and waterlines location. A fully coupled flow and thermal analysis need to be carried out only once to verify that all components work well together without adverse interactions. An introduction of a general moving object (GMO) model allows the setting of the best plunger velocity in the shot sleeve during the slow shot stage. In the project described here, the part design has drastically changed from its current production version.

The part geometry is shown in Figure 1. It poses challenges during filling and solidification to ensure the required casting quality. For example, high internal stresses may develop in the tall rib section during solidification and subsequent cooling, resulting in undesirable buckling forces.

In the initial stages of the design process, twenty-one runner configurations were suggested for evaluation. FLOW-3D was used to evaluate all variants. Figure 2 shows some of the runner designs considered.

Original geometry of the metalcasting part
Figure 1: The casting part
Examples of different runner systems
Figure 2: Three of the twenty-one runner systems modeled in FLOW-3D

The initial evaluation criterion of the runner systems was the flow pattern. After the first stage of the design process was completed, two conceptually different runner designs, shown in Figure 3, were accepted for further evaluation.

Runner systems evaluated for flow pattern
Figure 3: Runners selected for further evaluation based on the flow pattern

Solidification analyses were ran during the second stage of evaluation. Temperature distributions in the casting as well as in the die were analyzed. Figure 4 shows temperature distribution in the part at the end of solidification, using the final runner system design.

Temperature distribution at the end of solidification

Conclusion

For over seven years, we have been able to prove to our group the accuracy and reliability of the predicted results using FLOW-3D as a die casting process modeling tool. These results have had good correlations with the actual casting defects, temperature distribution and flow patterns.

We are using FLOW-3D not only as a die casting process simulation tool, but also as a general CFD modeling tool. If during process development, design changes need to be recommended to a customer, FLOW-3D allows us to quickly and reliably evaluate these changes and present to the customer not only the proposed changes, but also the effects these changes will have on the part performance.

Learn more about the versatility and power of modeling metal casting processes with FLOW-3D CAST.

Aluminum Integral Foam Molding Process

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Aluminum Integral Foam Molding Process

This application note was contributed by Johannes Hartmann and Vera Jüchter, Department of Materials Science, Chair of Metals Science and Technology, University of Erlangen-Nuremberg.

Aluminum foams show exceptional properties such as good damping and high energy absorption and mass specific flexural stiffness [1]. The stiffness makes it especially attractive for use in load-bearing and at the same time lightweight structures. In order to increase this weight-specific stiffness and for better load transmission, a compact skin is needed [2], as realized in Aluminum Foam Sandwiches (AFS).

At the Chair of Metals Science and Technology at the University of Erlangen-Nuremberg, the modified die casting process “Integral Foam Molding (IFM)” has been developed in order to produce aluminum foams with an integral solid skin, a foamed core and a gradual transition region in-between (see Fig. 1). This process was developed from foam injection molding of polymers and is therefore appropriate for cost-effective one-step mass production of complex foam parts with compact layer. A simulation technique, described in this note, has been adapted to model this process as an aid in selecting process parameters.

Cross section of an aluminum integral foam
Figure 1. Cross section of an aluminum integral foam with a compact skin, a transition region with decreasing relative density and smaller pores, as well as a foamed core.

Aluminum Integral Foam Molding Technology

A certain amount of blowing agent (magnesium hydride, MgH2) is therefore placed in the runner system and the shot chamber filled with aluminum melt (the schematic process cycle is depicted in Fig. 2; the process is described in detail in [3]). As the piston advances, the powder is entrained in a turbulent way into the mold. In case of the technology variant “High Pressure Integral Foam Molding (HP-IFM),” the part is completely filled at a high ambient pressure as known from the standard die casting process, guaranteeing an excellent surface quality. Starting from the tempered surfaces of the mold, the melt starts to solidify to an integral solid skin. After some milliseconds – the so-called delay time – the mold is opened over a core puller system and the volume locally increased (and pressure decreased) which initiates pore growth in the still semi-solid inner region due to the thermal decomposition and hydrogen release of the magnesium hydride particles. Every blowing agent particle represents a pore nucleus from which pore growth starts until it is stopped by the counter-pressure of neighboring pores expanding simultaneously. The forming cell walls are hereby stabilized by primarily solidified particles of the aluminum alloy, so-called endogenous stabilization [4].

High pressure integral foam molding
Figure 2. Schematic process cycle of “High Pressure Integral Foam Molding (HP-IFM)” of aluminum.

A prerequisite for a homogeneous foam morphology in the entire volume of the casting part is a good distribution of the particles at the moment of decomposition initiation. Furthermore, the temperature of the melt during blowing agent entrainment is on the one hand decisive for the decomposition kinetics of the magnesium hydride (see Fig. 3) and on the other hand determines the amount of solid phase during foaming. Insufficient solidified alpha-grains lead to non-stabilized struts between forming neighboring pores as drainage of the melt due to capillary forces takes place leading to pore coarsening. However, a very high amount of solid phase increases the rigidity of the matrix and leads to disrupted structures by hindering spheroidization of the developing pores [2].

Microcellular Aluminum Integral Foams: Approaching the Process Limits

Simulation of the integral foam molding process represents a powerful tool that not only helps to investigate the mold filling properties of a new part design but can also predict particle entrainment and determine the foam evolution conditions saving cost-intensive trial series. The goal of current research is to decrease pore size while keeping the porosity level constant. Computational fluid dynamics (CFD) simulation would help get as close as possible to the current process limits or to even push them further. Improvement in foam morphology would not only lead to more homogeneous structures with smaller scatter in the mechanical properties but would also allow production of thinner parts whose mechanical properties might then be determined by finite element. This objective can only be achieved by a high particle distribution density within the melt and at the same time a totally stabilized pore growth with a decrease in coalescence phenomena.

Schematic curves of decomposition of magnesium hydride
Figure 3. Schematic curves of decomposition of magnesium hydride as a function of the melt temperature, calculated by the Johnson-Mehl-Avrami approach [2]
Schematic curves of decomposition of magnesium hydride
Figure 3. Schematic curves of decomposition of magnesium hydride as a function of the melt temperature, calculated by the Johnson-Mehl-Avrami approach [2]

Adapting the Simulation Parameters to Practical Integral Foam Molding Experiments

In order to be able to use CFD simulation for the reliable prediction of particle behavior or temperature fields, different simulation parameters have to be determined by adjusting those to match real experiments. To this end, integral foam parts were produced with varying delay times between ca. 30 and 130 ms resulting in different dense skin thicknesses where foam formation was impossible due to a solid phase fraction exceeding a certain percentage1 at the moment of mold expansion and pore growth initiation. This leads to a characteristic so-called solidification curve with an axis intercept and slope depending on the die temperature and other chosen process parameters (see Fig. 4). Simulating the casting cycle for the alloy AlSi9Cu3(Fe) by varying the heat transfer coefficient (the value for fully liquid melt as well as for fully solidified melt), the experimental solidification curve can be fitted. In order to achieve this goal, it was necessary to extend the simulation to the dosing of the melt into the shot chamber to depict the real temperature distribution before the beginning of piston movement. The temperature was locally measured in the shot chamber by placed thermocouples and could be successfully depicted in good agreement with the real data within the simulation. The same can be referred to temperature measurements at the die surface during mold filling whose evolution over time correlates well with the simulation results.

In a second step, further parameters defining the melt flow behavior such as the surface tension or the coefficient of solidification drag are adjusted by comparing simulations with different settings to experimental studies where the piston is stopped before filling the mold (see Fig. 5). As soon as the flow of the melt within the simulation is consistent with the practical tests, the parameters are set.

Melt flow defining parameters in runner system
Figure 5. Adjustment of melt flow defining parameters such as the surface tension by comparisons of real experiments (left) to simulations (right)

After defining the cooling as well as the flow characteristics of the melt, the entrainment of the particles is simulated. In order to adjust the simulation for correct particle/fluid-interaction, the particle-defining parameter coefficient of particle drag is fitted by comparisons to x-rayed samples where substitute particles with a higher contrast in x-ray characterization to aluminum than magnesium hydride are entrained, e.g., copper or iron particles (see Fig. 6). The simulation results fit quite well with the experiments so that a reliable forecast of particle distribution as a function of process parameters can be deduced.

Parameters influencing particle melt interactions
Figure 6. Adjustment of parameters influencing particle/melt-interactions by comparisons of x-rayed samples left); produced by the entrainment of copper particles) to simulations (right)

Conclusion

Altogether it could be demonstrated that FLOW-3D can be an important instrument to investigate potential weak points in the fabrication of new integral foam parts before their actual production. In that way, a successful filling and blowing agent distribution without cold flow or dead zones can be assured. Furthermore, thanks to the correct depiction of temperature fields to be expected, the formation of compact skin and decomposition properties of magnesium hydride (and so the pore formation conditions) can be deduced. This offers the potential to define the process parameters to satisfy customer requests with regard to integral foam structures

1 Criterion is the solid phase fraction where the shear strength and therefore the resistance to pore evolution increases drastically.

References

[1] C. Körner, R. F. Singer, Adv. Eng. Mater. 20002 (4), pp. 159-165.
[2] C. Körner, in Integral Foam Molding of Light Metals – Technology, Foam Physics and Foam Simulation, Springer, Berlin, Heidelberg, Germany 2008.
[3] H. Wiehler, C. Körner, R. F. Singer, Adv. Eng. Mater. 200810 (3), pp. 171-178.
[4] J. Hartmann, A. Trepper, C. Körner, Adv. Eng. Mater. 201113 (11), pp. 1050-1055.

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