Evaluation of the Microstructure and Properties of As-Cast Magnesium Alloys with 9% Al and 9% Zn Additions

This paper summary is based on the article Evaluation of the Microstructure and Properties of As-Cast Magnesium Alloys with 9% Al and 9% Zn Additions presented in Materials, MDPI.

1. Overview:

Title: Evaluation of the Microstructure and Properties of As-Cast Magnesium Alloys with 9% Al and 9% Zn Additions
Authors: Lechosław Tuz, Vít Novák, and František Tatíček
Publication Year: 2025
Publishing Journal: Materials, MDPI
Keywords: magnesium alloys; mechanical properties; microstructure; weldability; forming; elevated temperature; thermal conductivity

Figure 1. Sample for the tensile test (a) and the heating equipment (b), LabTest Model 5.100SP1, used for the tests.
Figure 1. Sample for the tensile test (a) and the heating equipment (b), LabTest Model 5.100SP1, used for the tests.

2. Research Background:

The ongoing demand for reduced energy consumption necessitates lighter vehicle designs, driving the re-evaluation of magnesium alloys as primary construction materials, particularly with the rise of electric vehicles. Magnesium alloys offer favorable mechanical properties, natural degradation, and are increasingly considered for both massive and thin-walled components, even in elevated temperature applications within the automotive and aviation sectors. However, a significant challenge in utilizing magnesium alloys, especially in massive castings produced via methods like sand casting or high-pressure die casting, is inherent porosity. This porosity negatively impacts mechanical and plastic properties and restricts the effectiveness of heat treatments. Furthermore, the presence of low-melting point structural components and phases within these alloys poses additional complexities. Therefore, understanding and mitigating these limitations is crucial for expanding the application of magnesium alloys.

3. Research Purpose and Research Questions:

This research aims to evaluate the microstructure and material properties of as-cast magnesium alloys with specific alloying additions relevant to industrial applications. The study focuses on comparing the effects of aluminum and zinc as primary alloying elements on the resulting alloy characteristics.

Key Research Questions:

  • How do 9% Aluminum (Al) and 9% Zinc (Zn) additions, respectively, influence the microstructure of as-cast magnesium alloys?
  • What are the effects of these alloying additions on the mechanical properties (tensile strength, hardness, plasticity) of as-cast magnesium alloys at both ambient and elevated temperatures?
  • How does the thermal conductivity of these magnesium alloys vary with temperature and alloying composition?

Research Hypotheses:
The study posits that high concentrations of aluminum or zinc in magnesium alloys will promote the formation of low-melting point phases, potentially increasing susceptibility to hot cracking during processing. Furthermore, the research investigates the temperature-dependent property changes in these alloys to understand their behavior under service conditions and during thermal processes like welding.

4. Research Methodology

Research Design: This study employed a comparative experimental design, examining two distinct magnesium alloys cast in sand molds: Mg-Al-Zn (approximately 9% Al, 0.5% Zn, balance Mg) and Mg-Zn-Al (approximately 9% Zn, 1% Al, balance Mg). Both alloys were tested in the as-cast state without subsequent heat treatment.

Data Collection Method: A comprehensive suite of characterization techniques was utilized:

  • Microscopy: Light Microscopy (LM) and Scanning Electron Microscopy (SEM) were used to observe the microstructure of etched samples. Energy Dispersive Spectroscopy (SEM-EDS) was employed for phase identification and elemental mapping.
  • Thermal Conductivity Measurement: The thermal conductivity coefficient was measured at temperatures ranging from 25 °C to 200 °C, adhering to ASTM E1461 standards.
  • Tensile Testing: Static tensile tests were conducted at ambient temperature and elevated temperatures (120 °C, 150 °C, 180 °C, and 240 °C). Samples were heated using contact heating with inductive elements. Elongation was measured using a laser extensometer.
  • Hardness Testing: Vickers hardness measurements (HV10) were performed to assess the hardness of both alloys.
  • Fractography: Fracture surfaces of tensile tested samples were examined using SEM to analyze fracture mechanisms.

Analysis Method: The collected data was analyzed using qualitative and quantitative methods. Microstructural features and phase compositions were identified through microscopy and EDS. Statistical analysis was applied to mechanical and thermal property data. Fractographic analysis provided insights into the fracture behavior of the alloys.

Research Subjects and Scope: The study focused on two specific magnesium alloy compositions, Mg-Al-Zn and Mg-Zn-Al, prepared by gravity casting into sand molds. The scope was limited to the as-cast condition and the evaluation of microstructure, thermal conductivity, and tensile properties up to 240 °C.

5. Main Research Results:

Key Research Results:

  • Microstructure: Both alloys exhibited a dendritic microstructure typical of cast materials. Interdendritic regions contained low-melting eutectic phases, intermetallic compounds, and Laves phases. The Mg-Al-Zn alloy showed local porosity in interdendritic areas (Figures 2 and 3). In the Mg-Al-Zn alloy, the γ-Mg17Al12 phase was prevalent in grain boundaries, while both γ-Mg17Al12 and β-MgZn2 phases were likely present in the Mg-Zn-Al alloy (Figures 4 and 5).
  • Thermal Conductivity: The thermal conductivity coefficient remained relatively constant with increasing temperature up to 200 °C for both alloys. The Mg-Zn-Al alloy demonstrated a significantly higher thermal conductivity (approximately 77 W/mK) compared to the Mg-Al-Zn alloy (approximately 49 W/mK) (Figure 8, Table 4).
  • Tensile Properties: Tensile strength (Rm) increased with temperature initially for both alloys, reaching peak values at elevated temperatures. For the Mg-Zn-Al alloy, the maximum tensile strength of 121 MPa was observed at 150 °C, while the Mg-Al-Zn alloy reached a maximum of 122 MPa at 180 °C (Figure 9, Table 5). Elongation remained low for both alloys in the as-cast state.
  • Hardness: The Mg-Al-Zn alloy exhibited a significantly higher Vickers hardness (83 HV10) compared to the Mg-Zn-Al alloy (46 HV10) (Table 6).

Statistical/Qualitative Analysis Results: EDS analysis confirmed the presence of alloying elements in interdendritic regions and identified the primary phases. Tensile test data showed a temperature-dependent trend in strength and elongation. Thermal conductivity measurements provided quantitative values for heat transfer characteristics. Fractographic analysis indicated brittle intergranular fracture in both alloys (Figures 10 and 11).

Data Interpretation: The results indicate that alloying elements, particularly Al and Zn, segregate to interdendritic regions during solidification, forming low-melting point phases. Zinc addition significantly enhances thermal conductivity, likely due to the formation of Laves phases. Elevated temperatures initially improve tensile strength, potentially due to increased atomic mobility, but can reduce plasticity. The presence of porosity in the Mg-Al-Zn alloy and the formation of brittle zinc-rich phases in the Mg-Zn-Al alloy influence their mechanical behavior and potential weldability.

Figure Name List:

  • Figure 1. Sample for the tensile test (a) and the heating equipment (b), LabTest Model 5.100SP1, used for the tests.
  • Figure 2. Macrostructure of the alloy in the cross-section: (a) Mg-Al-Zn, (b) Mg-Zn-Al.
  • Figure 3. Microstructure near porosity in the Mg-Al-Zn alloy.
  • Figure 4. Microstructure of the Mg-Al-Zn alloy. (a-d) light microscopy, (e,f) SEM images.
  • Figure 5. Microstructure of the Mg-Zn-Al alloy. (a-d) light microscopy, (e,f) SEM images.
  • Figure 6. Map of the element distribution in the Mg-Al-Zn alloy.
  • Figure 7. Map of the element distribution in the Mg-Zn-Al alloy.
  • Figure 8. Change in the thermal conductivity coefficient in the temperature function.
  • Figure 9. Change in mechanical properties in the function temperature of the (a) Mg-Al-Zn and (b) Mg-Zn-Al alloys.
  • Figure 10. Fracture morphology of samples 92 (a,b) and 96 (c,d).
  • Figure 11. Fracture morphology of samples L2 (a,b) and L6 (c,d).
Figure 2. Macrostructure of the alloy in the cross-section: (a) Mg-Al-Zn, (b) Mg-Zn
Figure 2. Macrostructure of the alloy in the cross-section: (a) Mg-Al-Zn, (b) Mg-Zn
Figure 3. Microstructure near porosity in the Mg-Al-Zn alloy. Figure 4. Microstructure of the Mg-Al-Zn alloy. (a–d) light microscopy, (e,f) SEM images. F
Figure 3. Microstructure near porosity in the Mg-Al-Zn alloy.
Figure 4. Microstructure of the Mg-Al-Zn alloy. (a–d) light microscopy, (e,f) SEM images.
Figure 4. Microstructure of the Mg-Al-Zn alloy. (a–d) light microscopy, (e,f) SEM images.
Figure 5. Microstructure of the Mg-Zn-Al alloy. (a–d) light microscopy, (e,f) SEM images
Figure 5. Microstructure of the Mg-Zn-Al alloy. (a–d) light microscopy, (e,f) SEM images

6. Conclusion and Discussion:

Summary of Main Results: This study demonstrates that both as-cast Mg-Al-Zn and Mg-Zn-Al alloys exhibit dendritic microstructures with alloying elements concentrated in interdendritic regions. The Mg-Zn-Al alloy, with 9% zinc, shows superior thermal conductivity but lower hardness compared to the Mg-Al-Zn alloy (9% aluminum). Both alloys exhibit peak tensile strengths at elevated temperatures, but are susceptible to hot cracking due to the presence of low-melting point phases.

Academic Significance of the Research: This research provides valuable insights into the comparative effects of aluminum and zinc alloying additions on the microstructure and properties of as-cast magnesium alloys. It contributes to a deeper understanding of phase formation, thermal behavior, and mechanical response in these industrially relevant alloy systems.

Practical Implications: The findings have practical implications for material selection and processing in die casting and welding of magnesium alloys. The higher thermal conductivity of the Mg-Zn-Al alloy may be advantageous in applications requiring efficient heat dissipation. However, the susceptibility to hot cracking in both alloys highlights the need for careful control of casting and welding parameters, including preheating, to ensure component integrity, particularly in large castings for automotive and aviation industries. Preheating in the range of peak tensile strength (150-180°C) is suggested to mitigate cracking during welding.

Limitations of the Research: The study is limited to two specific alloy compositions in the as-cast state. Weldability tests were not conducted, and further research is needed to optimize welding parameters and explore the effects of heat treatment on these alloys.

7. Future Follow-up Research:

Directions for Follow-up Research: Future research should focus on:

  • Detailed analysis of precipitation behavior within the magnesium matrix and phase transformations under heat treatment and artificial aging conditions.
  • Comprehensive weldability testing of both alloys to determine optimal welding parameters and strategies to prevent hot cracking.

Areas Requiring Further Exploration:

  • Investigating the optimization of preheating temperatures and cooling rates during casting and welding to minimize hot cracking susceptibility.
  • Further exploration of the influence of minor alloying elements and impurity levels on the microstructure, properties, and weldability of these magnesium alloys.

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9. Copyright:

*This material is Lechosław Tuz, Vít Novák, and František Tatíček's paper: Based on Evaluation of the Microstructure and Properties of As-Cast Magnesium Alloys with 9% Al and 9% Zn Additions.
*Paper Source: https://doi.org/10.3390/ma18010010

This material was summarized based on the above paper, and unauthorized use for commercial purposes is prohibited.
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