Improving Quality in Mega-Casted Products: Identification of contributors to geometrical variation

1. Overview:

  • Title: Improving Quality in Mega-Casted Products: Identification of contributors to geometrical variation
  • Authors: Paul Adam, David Hermez
  • Year of Publication: 2024
  • Journal/Conference: Chalmers University of Technology, Master's thesis in Production Engineering
  • Keywords: HPDC, Megacasting, Geometrical variation, Distortions

2. Research Background:

The increasing global demand for environmentally friendly automotive manufacturing necessitates lightweighting techniques to address the weight increase associated with electric vehicle (EV) batteries. Volvo Cars is investing in large-scale casting (Mega-casting) using High-Pressure Die Casting (HPDC) to simplify assembly, improve efficiency, and enhance product quality. However, mega-casting presents challenges related to geometrical variation. While existing research focuses on minor defects like surface imperfections or porosity, this study concentrates on larger defects impacting dimensional accuracy.

3. Research Objectives and Questions:

  • Research Objective: To identify the contributors to geometrical variation in the HPDC process, specifically within mega-casting, and propose solutions for improved product quality.
  • Key Research Questions:
    • What factors contribute to geometrical variation in mega-casted products/parts?
    • What are the most critical contributing factors?
  • Research Hypothesis: The hypothesis was that design is the root cause of geometrical variation, with variations in thickness leading to residual stresses that affect the accuracy of the final shape.

4. Research Methodology:

  • Research Design: A mixed-methods approach combining qualitative and quantitative research methods was employed.
  • Data Collection Methods: A literature review was conducted. Eleven professionals in the HPDC field were interviewed. Production tests were performed at a foundry.
  • Analysis Methods: Qualitative analysis of expert interviews utilized Affinity Mapping. Quantitative analysis included correlation studies of measurement and process data, and analysis of Volvo Mega-casting production data to assess geometrical variation.
  • Subjects and Scope: The study focused on the HPDC process, particularly mega-casting, excluding surface defects, porosity, and machining.

5. Main Research Findings:

  • Key Findings: Design, particularly variations in wall thickness inducing residual stresses, was identified as the primary cause of geometrical variation. A method was found to successfully reduce geometrical variation in some instances; however, this led to secondary issues such as sticking to the trim-press. A Design of Experiments (DoE) was suggested for future studies to systematically optimize influential parameters.
  • Statistical/Qualitative Analysis Results: Expert interviews and data analysis revealed that gating system, die design, die temperature, melt temperature, alloy composition, vacuum-assisted casting, soldering during ejection, quenching (including media, rate, and orientation), and trimming all impact geometrical variation. The influence of vacuum-assisted casting on geometrical variation remained inconclusive.
  • Data Interpretation: Analysis of Volvo Mega-casting data highlighted specific areas with significant geometrical variation.
  • Figure List and Description: Numerous figures and tables visually presented simulation results, expert insights, and data analysis results.
Figure 2.6: Teslas’ and Volvos’ mega-castings of the rear floor
Figure 2.6: Teslas’ and Volvos’ mega-castings of the rear floor

6. Conclusion and Discussion:

  • Summary of Key Findings: Design flaws, especially thickness variations leading to residual stresses, were the root cause of geometrical variation. Various process parameters interact in complex ways to influence this variation.
  • Academic Significance: The study provides novel insights and knowledge for improving the quality of mega-castings.
  • Practical Implications: The findings underscore the need for design optimization considering geometrical variation from the outset, emphasizing the importance of optimizing the quenching process and improving the trimming process.
  • Limitations: The study's scope was limited by time constraints and resource limitations. Simulation analysis was partially hindered, and in-depth statistical analysis was restricted. Human factors and post-casting processes were excluded.

7. Future Research:

  • Future Research Directions: A Design of Experiments (DoE) should be conducted to identify optimal combinations of process parameters to minimize geometrical variation. Further simulation studies are needed to analyze the influence of design and process parameters on geometrical variation.
  • Areas Requiring Further Exploration: Research should focus on the trimming process, the impact of human factors, and the integration with subsequent processes.

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This summary is based on the master's thesis "Improving Quality in Mega-Casted Products: Identification of contributors to geometrical variation" by Paul Adam and David Hermez.

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