Impact Strength of AE-type Alloys High Pressure Die Castings

Unlocking Higher Impact Strength in HPDC: The Critical Role of RE/Al Ratio in AE-Type Magnesium Alloys

This technical summary is based on the academic paper "Impact Strength of AE-type Alloys High Pressure Die Castings" by K. Braszczyńska-Malik and M.A. Malik, published in ARCHIVES of FOUNDRY ENGINEERING (2020).

Keywords

• Primary Keyword: AE-type Magnesium Alloys
• Secondary Keywords: High pressure die casting, impact strength, rare earth elements, magnesium alloys, Al₁₁RE₃ phase, RE/Al ratio, microstructure

Executive Summary

• The Challenge: AE-type magnesium alloys are promising for high-performance HPDC components, but optimizing their toughness requires a deep understanding of how chemical composition affects impact strength.
• The Method: Researchers prepared three AE-type alloys with a constant 5 wt% aluminum and varying rare earth (RE) element content (1, 3, and 5 wt%) and tested their impact strength and microstructure after fabrication via cold chamber HPDC.
• The Key Breakthrough: The study revealed a significant inverse relationship: a lower ratio of rare earth elements to aluminum (RE/Al) in the alloy's chemical composition results in a higher impact strength.
• The Bottom Line: For HPDC applications requiring superior toughness and impact resistance, carefully controlling the RE/Al ratio to be lower is a critical factor in maximizing the performance of AE-type magnesium alloys.

The Challenge: Why This Research Matters for HPDC Professionals

In the relentless drive for lightweighting in the automotive, aerospace, and electronics industries, magnesium alloys are highly desirable due to their low density. High Pressure Die Casting (HPDC) is the ideal method for producing complex, thin-walled components from these alloys. However, standard magnesium-aluminum (Mg-Al) alloys, like those in the AM and AZ series, suffer from poor mechanical properties at elevated temperatures (above 393 K). This is primarily due to the presence of the γ (Mg₁₇Al₁₂) phase, which leads to deformation by grain boundary sliding.

To overcome this, AE-type magnesium alloys, which incorporate rare earth (RE) elements, were developed. The RE elements suppress the formation of the undesirable γ phase by forming more thermally stable Al-RE intermetallic compounds. While this improves high-temperature performance, the precise effect of the RE content on other critical properties, like impact strength, has remained an area for deeper investigation. This research directly addresses that gap, providing crucial data for engineers selecting materials for demanding applications.

The Approach: Unpacking the Methodology

The researchers conducted a controlled experiment to isolate the effect of rare earth element concentration on impact strength.

• Materials: Three experimental AE-type magnesium alloys (designated AME501, AME503, and AME505) were produced. All were based on the commercial AM50 alloy, with a constant aluminum mass fraction of 5 wt%. The variable was the weight fraction of rare earth elements, which were added as a cerium-rich mischmetal at levels of approximately 1, 3, and 5 wt%.
• Casting Process: Casts were fabricated using a typical cold chamber high pressure die casting machine with a 3.8 MN locking force.
• Testing and Analysis:
  • Impact Strength: Un-notched specimens (6 mm x 6 mm x 55 mm) were tested using a Charpy V hammer with an impact energy of 150 J at room temperature.
  • Microstructural Analysis: The microstructure of the alloys was examined using a scanning electron microscope (SEM) to identify the phases present and their morphology.

This robust methodology ensures that the observed differences in impact strength can be directly correlated with the changes in the alloy's chemical composition and resulting microstructure.

The Breakthrough: Key Findings & Data

The study yielded clear, actionable data on the relationship between alloy chemistry and mechanical performance.

Finding 1: Lower RE/Al Ratio Directly Increases Impact Strength

The most significant finding is the inverse correlation between the RE content and the material's toughness. As the ratio of rare earth elements to aluminum increased, the impact strength decreased significantly.

As shown in Table 2, the results were definitive:
- AME501 (lowest RE content, ~1 wt%): Exhibited the highest impact energy of 13 J and an impact strength of 36.1 J/cm².
- AME503 (~3 wt% RE): Showed a notable drop to 9 J and 25.0 J/cm².
- AME505 (highest RE content, ~5 wt%): Had the lowest impact energy of 8 J and an impact strength of 22.2 J/cm².

This demonstrates that for applications where impact resistance is the primary concern, minimizing the RE content while maintaining enough to suppress the γ phase is the optimal strategy.

Finding 2: Microstructure Governs the Trade-off Between Strength and Toughness

The microstructural analysis revealed why this trend occurs. The primary phases in the alloys were an α-Mg solid solution and Al-RE intermetallic compounds, mainly Al₁₁RE₃. The study found that the volume fraction of the brittle Al₁₁RE₃ phase increased as the RE/Al ratio rose.

Interestingly, the paper notes this behavior is contrary to the effect on tensile strength, as described in a previous work [14]. In tensile tests, the Al₁₁RE₃ compound caused dislocation blocking, leading to higher tensile strength in the AME505 alloy. This highlights a critical engineering trade-off:
- High RE Content: Increases the volume of Al₁₁RE₃, which improves tensile strength but provides more sites for crack initiation under impact, thus lowering toughness.
- Low RE Content: Results in a finer intermetallic phase distribution, which is more advantageous for blocking crack growth under impact, leading to higher toughness.

Practical Implications for R&D and Operations

• For Process Engineers: This study underscores the sensitivity of AE-type magnesium alloys to their composition. While process parameters were not the focus, it implies that maintaining strict control over melt chemistry is paramount to achieving the desired mechanical properties in the final cast component.
• For Quality Control Teams: The data in Table 2 provides a clear correlation between the RE/Al ratio and impact strength. This can inform material specifications and incoming quality inspection criteria, ensuring the correct alloy grade is used for components that demand high toughness.
• For Design Engineers: The findings are crucial for material selection. For components subjected to impact loading, selecting an AE-type alloy with a lower RE/Al ratio (like AME501) is the superior choice. This trade-off between impact strength (favored by low RE) and elevated-temperature tensile strength (favored by high RE) is a critical consideration in the early design phase.

Paper Details

Impact Strength of AE-type Alloys High Pressure Die Castings

1. Overview:

• Title: Impact Strength of AE-type Alloys High Pressure Die Castings
• Author: K. Braszczyńska-Malik, M.A. Malik
• Year of publication: 2020
• Journal/academic society of publication: ARCHIVES of FOUNDRY ENGINEERING
• Keywords: AE-type magnesium alloy, Aluminum, Rare earth elements, High pressure die casting, Impact strength

2. Abstract:

The results of the Charpy impact test of AE-type magnesium alloys produced by the high pressure die casting method are presented. Three alloys with different weight fractions of rare earth elements (RE; e.g. 1, 3 and 5 wt%) and the same mass fraction of aluminium (5 wt%) were prepared. The casts were fabricated using a typical cold chamber high pressure die casting machine with a 3.8 MN locking force. Microstructural analyses were performed by means of a scanning electron microscope (SEM). The impact strength (IS) was determined using a Charpy V hammer with an impact energy equal to 150 J. The microstructure of the experimental alloys consisted of an α-Mg solid solution and Al₁₁RE₃, Al₁₀Ce₂Mn₇ and Al₂RE intermetallic compounds. The obtained results show the significant influence of the rare earth elements to aluminium ratio on the impact strength of the investigated materials. Lower the RE/Al ratio in the chemical composition of the alloy results in a higher impact strength of the material.

3. Introduction:

The high pressure die casting (HPDC) method, which ensures the production of thin-walled components of complicated shapes with dimensional precision, is very attractive in such applications as the automobile, aerospace and electronic industries. In recent decades this technology has especially been developed for aluminum alloys [1-3]. Then the intensively development of magnesium alloys caused this method to be widely used to produce elements from these alloys. Magnesium alloys are currently very desirable due to their low density and unique combination of residual properties [3-5]. High pressure die casting in both cold and hot chamber die casting machines is used to cast magnesium (like aluminum) alloys. It should be noted that magnesium alloys offer very good casting properties like castability and flow characteristics but require protective atmospheres as well as injection parameters and die designs different than those for aluminum alloys. From among the many different magnesium alloys, especially those from the AM or AZ series (based on the Mg-Al system) are widely used due to their low prices and high casting properties. On the other hand, a negative feature of those alloys is their low properties at elevated temperature (higher than 393 K) [6-9]. This behavior is caused by the presence of a γ phase (with the stoichiometric composition of Mg₁₇Al₁₂ at 43.95 wt% Al) in the microstructure of Mg-Al type alloys, which contributes to deformation by grain boundary sliding (especially during creep). For these reasons, many different types of alloys have been developed in which the formation of thermally stable phases along the grain boundaries could improve the properties at elevated temperatures, like with rare earth elements (RE) [8-11]. In Mg-Al-RE type alloys the γ phase is suppressed through the formation of Al-RE-type intermetallic compounds. Especially the Al₁₁RE₃ phase has an advantageous influence on the mechanical properties of the final components.

4. Summary of the study:

Background of the research topic:

HPDC is a key manufacturing process for lightweight magnesium alloy components. However, conventional Mg-Al alloys (AM, AZ series) exhibit poor mechanical properties at temperatures above 393 K due to the presence of the Mg₁₇Al₁₂ phase.

Status of previous research:

AE-type alloys (Mg-Al-RE) were developed to address this issue. The addition of rare earth elements suppresses the Mg₁₇Al₁₂ phase and forms more stable Al-RE intermetallic compounds, improving elevated-temperature properties. Previous studies by the authors have examined the microstructure and tensile properties of these alloys.

Purpose of the study:

To investigate the influence of the rare earth element mass fraction on the impact strength of HPDC AE-series magnesium alloys containing a constant 5 wt% of aluminum.

Core study:

Three alloys (AME501, AME503, AME505) with ~1, 3, and 5 wt% RE, respectively, were produced via cold chamber HPDC. Their impact strength was determined using a Charpy V hammer, and their microstructures were analyzed with SEM to correlate chemical composition with mechanical performance.

5. Research Methodology

Research Design:

The study used a comparative experimental design, where the primary variable was the weight percentage of rare earth elements in a Mg-5Al alloy. Three distinct alloy compositions were created and tested under identical HPDC and impact testing conditions.

Data Collection and Analysis Methods:

• Material Preparation: Alloys were produced from AM50 commercial magnesium alloy with additions of cerium-rich mischmetal.
• Casting: A cold chamber HPDC machine with a 3.8 MN locking force was used.
• Mechanical Testing: Impact strength (IS) was determined using a Charpy V hammer (150 J impact energy) on un-notched specimens (6x6x55 mm) at room temperature, according to relevant ASTM standards.
• Microstructural Analysis: Samples were prepared using standard metallographic techniques, etched, and analyzed with a JOEL JSM-6610LV scanning electron microscope (SEM).

Research Topics and Scope:

The research is focused on AE-type magnesium alloys (Mg-5Al-xRE) produced by HPDC. The scope is limited to the analysis of room-temperature impact strength and the corresponding microstructure as a function of the rare earth element content (1, 3, and 5 wt%).

6. Key Results:

Key Results:

• A lower RE/Al ratio in the chemical composition of the alloy results in a higher impact strength.
• The AME501 alloy (lowest RE content) had the highest impact energy (13 J) and impact strength (36.1 J/cm²).
• The AME505 alloy (highest RE content) had the lowest impact energy (8 J) and impact strength (22.2 J/cm²).
• The microstructure of the alloys consisted of an α-Mg solid solution and intermetallic compounds including Al₁₁RE₃, Al₁₀Ce₂Mn₇, and Al₂RE.
• The volume fraction of the Al₁₁RE₃ phase increases with the RE/Al ratio, which contributes to the decrease in impact strength.
• The undesirable Mg₁₇Al₁₂ phase was not present in any of the investigated alloys.
• Fracture surfaces showed typical brittle or quasi-cleavage fracture for magnesium, but with evidence of some ductility (dimples) and secondary cracks on intermetallic phases.

Figure Name List:

• Fig. 1. Macrograph of HPDC AME series alloy impact test specimens with dimensions in mm.
• Fig. 2. Microstructure of AME501 (a), AME503 (b) and AME505 (c) magnesium alloy casts obtained using cold chamber die casting machine; scanning electron microscopy
• Fig. 3. Macrographs of samples after impact test
• Fig. 4. SEM micrographs of fracture surface of HPDC AME501 alloy (after impact test)
• Fig. 5. SEM micrographs of fracture surface of HPDC AME503 alloy (after impact test)
• Fig. 6. SEM micrographs of fracture surface of HPDC AME505 alloy (after impact test)

7. Conclusion:

High pressure die cast technology allowed final components to be obtained from the AE series magnesium alloy, which is characterized by the α(Mg) solid solution and Al-RE-type intermetallic compounds. The low RE/Al ratio in the chemical composition of the alloy contributed to a high level of impact strength. Increasing the RE/Al ratio contributes to a decrease in the plasticity of the alloy and results in a lower impact strength of the material.

8. References:

• [1] El-Mahallawy, N.A., Taha, M.A., Pokora, E. & Klein, F. (1998). On the influence of the process variables on the thermal conditions and properties of high pressure die-cast magnesium alloys. Journal of Materials Process Technology. 73, 125-138.
• [2] Gutman, E.M., Unigovski, Y., Levkovich, M., Koren, Z., Aghion, E. & Dangur, M. (1997). Influence of technological parameters of permanent mold casting and die casting on creep and strength of Mg alloy AZ91D. Materials Science and Engineering, A234-236, 880-883.
• [3] Lee, S.G., Patel, G.R., Gokhale, A.M., Sareeranganathan, A. & Horstemeyer, M.F. (2006). Quantitative fractographic analysis of variability in the tensile ductility of high-pressure die-cast AE44 Mg-alloy. Materials Science and Engineering. A 427, 255-262.
• [4] Guangyin, Y., Yangshan, S. & Wenjiang, D. (2001). Effects of bismuth and antimony additions on the microstructure and mechanical properties of AZ91 magnesium alloy. Materials Science and Engineering A, 308, 38-44.
• [5] Asl, K.M., Tari, A. & Khomamizadeh, F. (2009). The effect of different content of Al, RE and Si element on the microstructure, mechanical and creep properties of Mg-Al alloys. Materials Science and Engineering. A, 523, 1-6.
• [6] Zhang, J., Wang, J., Qiu, X., Zhang, D., Tian, Z., Niu, X., Tang, D. & Meng, J. (2008). Effect of Nd on the microstructure, mechanical properties and corrosion behavior of die-cast Mg-4Al-based alloy. Journal of Alloys and Compounds. 464, 556-564.
• [7] Wang, J., Liao, R., Wang, L., Wu, Y., Cao, Z. & Wang, L. (2009). Investigations of the properties of Mg-5Al-0.3Mn-xCe (x = 0-3, wt.%) alloys. Journal of Alloys and Compounds. 477, 341-345.
• [8] Zhang, J., Leng, Z., Zhang, M., Meng, J. & Wu, R. (2011). Effect of Ce on the microstructure, mechanical properties and corrosion behavior of high-pressure die-cast Mg-4Al-based alloy. Journal of Alloys and Compounds. 509, 1069-1078.
• [9] Lu, Y., Wang, Q., Zeng, X., Ding, W., Zhai, Ch. & Zhu, Y. (2000). Effects of rare earths on the microstructure, properties and fracture behaviour of Mg-Al alloys. Materials Science and Engineering. A278, 66-76.
• [10] Wang, Y., Guan, S., Zeng & X., Ding, (2006). Effect of RE on the microstructure and mechanical properties of Mg-8Zn-4Al magnesium alloy. Materials Science and Engineering. A416, 109-118.
• [11] Zhang, J., Liu, S. Leng, Z., Zhang, M., Meng, J. & Wu, R. (2011). Microstructure and mechanical properties of heat resistant HPDC Mg-4Al-based alloys containing cheap misch metal. Materials Science and Engineering. A528, 2670-2677.
• [12] Żydek, A., Kamieniak, J. & Braszczyńska-Malik K.N. (2010). The effect of rare earth elements on the microstructure of as-cast AM50 alloy. Archives of Foundry Engineering. 10(spec.1), 147-150.
• [13] Braszczyńska-Malik, K.N. & Grzybowska, A. (2016). Influence of phase composition on microstructure and properties of Mg-5Al-0.4Mn-xRE (x = 0, 3 and 5 wt.%) alloys. Materials Characterization. 115, 14-22.
• [14] Braszczyńska-Malik, K.N. (2017). Effect of high-pressure die casting on structure and properties of Mg-5Al-0.4Mn-xRE (x = 1, 3 and 5 wt.%) experimental alloys. Journal of Alloys and Compounds. 694, 841-84.

Expert Q&A: Your Top Questions Answered

Q1: Why were un-notched specimens used for the Charpy impact test?

A1: The paper specifies in Figure 1 that the specimens were "without a notch" and had dimensions of 6 mm x 6 mm x 55 mm. While the paper doesn't state the reason for this choice, it notes the tests were carried out according to "relevant ASTM standards." Using an un-notched specimen allows the test to measure the intrinsic impact energy absorption of the bulk material itself, rather than focusing on its sensitivity to stress concentrations at a notch root.

Q2: The paper mentions the Al₁₁RE₃ phase is detrimental to impact strength but can be beneficial for tensile strength. Can you elaborate on this?

A2: The paper states that the increasing volume fraction of the Al₁₁RE₃ phase with higher RE content "contributed to a decrease in the impact strength." However, it contrasts this with findings from a cited previous work [14], where the same Al₁₁RE₃ intermetallic compound "caused dislocation blocking during tensile strength tests," leading to higher ultimate tensile strength. This suggests the phase acts as a strengthening agent under quasi-static tension but as a brittle phase and potential crack initiation site under high-strain-rate impact loading.

Q3: What specific rare earth elements were used in the study?

A3: The rare earth elements were added in the form of a cerium-rich mischmetal. According to the footnote in Table 1, the composition of this mischmetal was specifically: 54.8 wt% Ce, 23.8 wt% La, 16 wt% Nd, and 5.4 wt% Pr, with minor amounts of Fe and Mg.

Q4: Did the microstructure contain the undesirable Mg₁₇Al₁₂ phase that is typical of standard Mg-Al alloys?

A4: No. The paper explicitly states that "intermetallic compounds like Mg₁₇Al₁₂ (characteristic for the Mg-Al system) ... were not present in the investigated alloy." This is a key result, confirming that the addition of rare earth elements successfully suppressed the formation of this phase, which is known to be detrimental to properties at elevated temperatures.

Q5: What does the fracture surface analysis reveal about the failure mechanism?

A5: The SEM micrographs in Figures 4-6 show fracture surfaces with "brittle trough cleavage or quasi-cleavage," a mode typical for the hexagonal close-packed crystal structure of magnesium. However, the surfaces were not entirely brittle; they also showed evidence of some ductility, including "visible extension and the presence of small dimples." The presence of secondary cracks, especially on the intermetallic phases, indicates these phases act as points of weakness under impact.

Conclusion: Paving the Way for Higher Quality and Productivity

This research provides a clear and valuable guideline for engineers working with AE-type Magnesium Alloys in High Pressure Die Casting. The core takeaway is that a delicate balance must be struck in the alloy's formulation. To maximize toughness and impact resistance, a lower RE/Al ratio is definitively better, as it limits the volume fraction of brittle Al-RE intermetallic phases. This insight allows for more precise material selection and specification, directly contributing to the reliability and performance of lightweight components in critical applications.

At CASTMAN, we are committed to applying the latest industry research to help our customers achieve higher productivity and quality. If the challenges discussed in this paper align with your operational goals, contact our engineering team to explore how these principles can be implemented in your components.

Copyright Information

• This content is a summary and analysis based on the paper "Impact Strength of AE-type Alloys High Pressure Die Castings" by "K. Braszczyńska-Malik, M.A. Malik".
• Source: https://doi.org/10.24425/afe.2020.133321

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