On determining lost core viability in high-pressure die casting using Computational Continuum Mechanics

1. Overview:

  • Title: On determining lost core viability in high-pressure die casting using Computational Continuum Mechanics
  • Author: Sebastian Kohlstädt
  • Year of Publication: 2019
  • Journal/Conference: KTH Royal Institute of Technology (Doctoral thesis)
  • Keywords:
    • high-pressure die casting
    • lost salt cores
    • computational continuum mechanics
    • two-phase compressible flow
    • OpenFOAM
    • CFD
    • fluid-structure interaction
    • volume-of-fluid method

2. Research Background:

  • Social/Academic Context:
    • High-pressure die casting is an important process in the automotive and related industries [16, 17, 88].
    • It is capable of producing near net-shape raw geometries [92].
    • It is an indispensable constituent of the global supply chain in manufacturing [63].
    • There is a limitation in design freedom regarding the integration of functionality into housings or brackets [26].
    • Integrating lost cores in high-pressure die casting components has not been possible for significant annual output in the automotive industry [59].
    • Lost cores provide the possibility to include hollow sections or undercuts inside the housing.
    • Applications include channels for oil-flow [44, 45, 60, 62] and heat transfer [67, 103], even with the advent of electric power-trains.
  • Limitations of Existing Research:
    • "As of spring 2019, no application in serial production of lost cores, i.e. cores that are destroyed during deforming, in high-pressure die casting is known."
    • "The reason for this is believed to be the absence of an engineering tool that can tell upfront whether a concept of casting and process combined will be viable."
  • Necessity of Research:
    • "This thesis aims to fill precisely that void by presenting, implementing and testing a CCM model inside the OpenFOAM toolbox in order to determine upfront whether a design of a housing will be manufacturable with lost cores."
    • Need for a tool to "determine a priori whether a housing concept with inlying geometries that so far only exists in Computer Aided Design (CAD) will have the desired cooling performance and will be manufacturable with an acceptable number of rejects."

3. Research Objectives and Research Questions:

  • Research Objectives:
    • To investigate whether Computational Continuum Mechanics (CCM) can serve as a valuable tool for the casting engineer.
    • To develop and test a CCM model within the OpenFOAM toolbox.
    • To determine upfront whether a housing design will be manufacturable with lost cores.
  • Core Research Questions:
    • Can Computational Continuum Mechanics (CCM) be used as a valuable tool for the casting engineer to determine lost core viability in high-pressure die casting?
    • Can a CCM model predict upfront whether a housing design will be manufacturable with lost cores?
  • Research Hypothesis:
    • CCM can serve as a valuable tool for casting engineers in determining lost core viability.
    • A CCM model implemented in OpenFOAM can predict the manufacturability of housing designs with lost cores.

4. Research Methodology

  • Research Design:
    • Development, implementation, and testing of a CCM model inside the OpenFOAM toolbox.
    • Experimental validation of the CCM simulations using a test mold.
  • Data Collection Methods:
    • Numerical simulations using the developed CCM model in OpenFOAM.
    • Experimental determination of the deformation of lost cores in hpdc using a manufactured test mold.
  • Analysis Methods:
    • Comparison of simulation results with experimental results.
    • Analysis of forces on lost cores using different turbulence models and mesh resolutions.
    • Evaluation of core deformation during die filling using Fluid-Structure Interaction (FSI) approach.
    • Assessment of heat transfer between melt and core using conjugate heat transfer model.
  • Research Scope:
    • Focus on determining lost core viability in high-pressure die casting.
    • Modeling two-phase flow of air and melt using the "volume-of-fluid-concept".
    • Turbulence modeling using the Reynolds-Averaged-Navier-Stokes (RANS) approach, mostly with the Menter SST k-w-model.
    • Assuming an isotropic linear elastic model for solid mechanics.
    • Experimental validation using a test mold and casting experiments.

5. Main Research Results:

  • Core Research Results:
    • - "It was proven that housings made with lost cores can improve the heat transfer capabilities of castings."
    • - "It was possible to produce castings with cores up to an impact velocity of 30 ms¯¹."
    • - "The impact velocity was found to be the most decisive parameter."
    • - "The slamming events at first impact of the melt were found to be not failure-critical if crack-free cores are used."
    • - "It was also found that the approach of evaluating only the peak force does not go far enough. Effects later in the process may have a more important impact due to larger force-time integrals."
    • - "Also, different from the original assumptions, the heat transferred from the melt to the core may not be neglected even though filling times are below 0.1 s."
    • - "Defining general numerical constraints for conditions under which salt cores are a viable technology is very difficult as geometry alterations play an important role too."
    • - "This underscores the power and usefulness of the presented model even further as the engineer is now capable of testing each setup individually."
    • - "The presented strategy in this thesis together with the developed CCM tools can therefore provide a powerful tool for the casting or CAD-engineer to decide case by case whether a concept for a casting will be producible or not."
    • - "All models have been tested and validated with high-pressure die casting experiments and are in line with previously published findings with deviations of 5-10 % at maximum."
  • Statistical/Qualitative Analysis Results:
    • - Mesh study (Figure 11) showed that "simulation requires a mesh as fine as a spacing of 0.3 mm in order to be above the lower limit of the slamming factor according to the von Karman model."
    • - Turbulence model selection (Figure 13) showed that "the selection of the turbulence model is of minor importance. Even with different mesh spacings, the result of the slamming factor stayed more or less the same. The variation was at maximum for all values plotted below 2%."
    • - Validation experiments (Figure 20, 21) revealed that "the core failure may also appear in the shape of deformation, not only cracking, as it was previously assumed." and "All cores see deformation. The deformation was in this experiment towards the other direction than the direction of the inflowing melt."
    • - FSI simulations (Figure 22, 23, 24) showed that "the model is capable of reproducing the bending of the core" and "the displacement of the core is at 1.1 mm, matching the eventual shape that was also observed in the casting tests (1.2 mm on average)."
    • - Heat transfer simulations (Figure 26, 27, 28, 29, 30) indicated that "While a very thin outer layer heats up immediately to the melt temperature, the temperature in the middle of the core will not rise before a time interval of 1 s has elapsed."
  • Data Interpretation:
    • CCM model is a useful tool for casting engineers to assess lost core viability.
    • Impact velocity is a critical parameter for lost core survival.
    • Core deformation is a significant failure mode, not just cracking.
    • Heat transfer effects on the core need to be considered for accurate predictions.
    • Finer mesh resolution is needed for accurate slamming force calculations.
    • Turbulence model selection has a minor impact on the slamming force.
  • Figure Name List:
    • Figure 1: The layout and components of a high-pressure die casting machine according to DIN 24480
    • Figure 2: Process steps during high-pressure die casting (HPDC)
    • Figure 3: Conceivable applications of lost core technology for creating inlying channels in cast housings in the automotive industry
    • Figure 4: Illustration of the volume-of-fluid method of distinguishing between the phases via an indicator function assigning a value between 0 and 1 to each cell
    • Figure 5: Solving process scheme of fsiFoam Solver
    • Figure 6: PISO algorithm before and after the adjustments
    • Figure 7: Mechanism of transferring the field data at the fluid-solid interface
    • Figure 8: The tool concept for validation of the simulations in high-pressure die casting experiments. There is always a pair of inserts for the moving side (ms) and the fixed side (fs); (a) is the setup for the straight ingate, (b) represents the tooling parts for the fork ingate
    • Figure 9: An engineering drawing of the salt core geometry of the ultimately selected shape: a rectangular cross-section with 10 mm in width and 3, 5 and 7 mm in thickness.
    • Figure 10: The geometry for investigating the slamming on a salt core in a channel; all dimensions in mm
    • Figure 11: Mesh study of the slamming factor in comparison with models by von Karman [118] and Wagner [121]
    • Figure 12: Comparison of the computed result with reference studies in previously published articles
    • Figure 13: Influence of the selected turbulence model on the computed result of the slamming factor
    • Figure 14: Different strategies for calculating the turbulence in the Navier-Stokes equations
    • Figure 15: The flow pattern on the 2D-mesh at three different time steps, illustrating the influence of the selected turbulence model on the morphology of the melt-air interface: (a) k-ɛ; (b) k-w-SST; (c) Spalart-Allmaras. Red areas characterise the melt, blue areas the air and the mixed colours (white/orange) regions are the interface between melt and air
    • Figure 16: Comparison of the effective mean pressure force in the x-direction when using different turbulence models; the vertical bars show the standard deviation
    • Figure 17: Comparison of the necessary computational time in minutes when doing the simulations with the particular turbulence model
    • Figure 18: Benchmarking of the presented OpenFOAM model with previously published data by Korti and Aboudi [71]; the figure shows the interface positions at various time steps
    • Figure 19: The fraction occupied by air after the melt-front has propagated into the ingate; the numerical values represent the different piston velocities in ms-1
    • Figure 20: The deformation of the salt core in a casting experiment; Uin = 30 ms-1
    • Figure 21: The deformation of a cracked salt core in a casting experiment; Uin = 50 ms-1
    • Figure 22: The deformation of the salt core after 0.016 s, as predicted by the CFD simulation; Uin = 30 ms-1
    • Figure 23: The filling pattern of the melt and core displacement at a fill fraction of 95 %; Uin = 30 ms-1
    • Figure 24: The displacement of the salt core centre in the x-direction over time
    • Figure 25: Sketch of the simplified simulation; note that the entire domain is filled with melt as this is a single-phase flow model
    • Figure 26: Result of the mesh independency study
    • Figure 27: Temperature in salt core at times t=(0.1s, 0.5s, 1s, 1.5s, 2s) from left to right
    • Figure 28: Spatial temperature distribution in x-direction through the middle of the core
    • Figure 29: Dimensionless temperature distribution through the salt core at time t=1 s
    • Figure 30: Dimensionless temperature distribution through the salt core at time t=2 s
    • Figure 31: A strategy proposal for designing castings with lost cores and the corresponding manufacturing process
    • Figure 32: The impossible triangle of computational continuum mechanics in high-pressure die casting

6. Conclusion and Discussion:

  • * Summary of Key Results:
    • - Computational Continuum Mechanics (CCM) is a powerful tool for supporting casting engineers in designing high-quality castings with lost cores.
    • - A segregated approach is more useful than a fully comprehensive "fire-and-forget" model for designing castings with lost cores.
    • - Key factors for lost core viability include ingate velocity, core diameter, and die cross-section at the melt's first impact site.
    • - Heat transfer to the core and core deformation are important factors.
    • - The developed CCM model in OpenFOAM is capable of predicting cooling performance and core deformation.
  • * Academic Significance of Research:
    • Demonstrates the applicability of CCM for designing high-pressure die casting processes with lost cores.
    • Provides a validated CCM model and a segregated strategy for practical application.
    • Contributes to the understanding of the complex physics involved in lost core die casting.
  • * Practical Implications:
    • Offers a practical strategy for casting engineers to design and optimize high-pressure die casting processes using lost cores.
    • Provides tools to predict lost core viability upfront, reducing trial-and-error in process design.
    • Helps in designing castings with complex internal geometries for improved functionality.
  • * Limitations of Research:
    • "a fully comprehensive model is still for future researchers to develop."
    • "Stability on industrial geometries remains a subject of concern as well as solution times."
    • "The still largest drawback of the model is its reversibility." (referring to the FSI model and core deformation)
    • "If the model was to yield proper results at 100 % fill fraction still, the physical effects of plastic deformation and heat transfer need to be included."

7. Future Follow-up Research:

  • Directions for Follow-up Research:
    • Improve CCM model stability and computational efficiency for industrial applications.
    • Expand the solid model to include plastic deformation and core fracture.
    • Incorporate heat transfer into the FSI model.
    • Advance production technology for salt cores.
    • Conduct experimental measurements of slamming force for model validation.
    • Apply the models to industrial geometries and validate with real-world applications.
  • Areas Needing Further Exploration:
    • Impact of more advanced turbulence treatment strategies.
    • Development of tailored solvers for improved efficiency.
    • Investigation of elasto-plastic material models for salt cores.
    • Activation of heat transfer models in specific zones for isothermal melt treatment.

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9. Copyright:

  • This material is based on the thesis of Sebastian Kohlstädt: On determining lost core viability in high-pressure die casting using Computational Continuum Mechanics.
  • Thesis Source: ISBN 978-91-7873-206-7

This material is a summary written based on the above thesis, and unauthorized use for commercial purposes is prohibited.

Copyright © 2025 CASTMAN. All rights reserved.

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