Impact of Aspect Ratio on the Mechanical Properties of Squeeze-Cast Aluminum Alloys

Optimizing AA6061 Performance: How Component Geometry Dictates Squeeze Casting Process Success

This technical summary is based on the academic paper "IMPACT ASPECT RATIO ON MECHANICAL PROPERTIES OF ALUMINUM ALLOY PRODUCED BY SQUEEZE CASTING PROCESS" by S.S. Mutar and N.S. Abtan, published in the International Journal on "Technical and Physical Problems of Engineering” (IJTPE) (2023).

Keywords

• Primary Keyword: Squeeze Casting Process
• Secondary Keywords: Aluminum Alloy AA6061, Mechanical Properties, Aspect Ratio, Hardness, Porosity, Wear Rate, Direct Squeeze Casting

Executive Summary

• The Challenge: Eliminating shrinkage porosity in aluminum castings to achieve superior mechanical properties required by demanding industries like automotive and aerospace.
• The Method: Applying a direct squeeze casting process at a constant pressure of 90 MPa to AA6061 aluminum alloy, while systematically varying the component's height-to-diameter (H/D) aspect ratio (2/3, 3/3, and 4/3).
• The Key Breakthrough: A lower aspect ratio (H/D = 2/3) dramatically improved all measured mechanical properties, resulting in the highest density, greatest hardness, and lowest wear rate.
• The Bottom Line: For squeeze-cast AA6061 components, the geometric aspect ratio is a critical design and process parameter that directly governs the final material quality and performance.

The Challenge: Why This Research Matters for HPDC Professionals

In high-performance applications, particularly in the automotive and aerospace sectors, components must exhibit excellent mechanical qualities. Traditional casting methods can be plagued by shrinkage porosity, a critical defect that compromises strength, density, and overall integrity. The squeeze casting process was developed to combat this by applying high pressure during solidification. However, even with this advanced method, achieving consistent, optimal properties requires a deep understanding of all process variables. This research addresses a crucial but often overlooked variable: the physical geometry of the cast component itself, specifically its aspect ratio (H/D). Understanding this relationship is key to moving from near-net shape to truly high-performance, reliable parts.

The Approach: Unpacking the Methodology

The study was designed to isolate the effect of the H/D aspect ratio on the mechanical properties of squeeze-cast AA6061 aluminum alloy.

Method 1: Material and Mold Preparation
- Material: AA6061 aluminum alloy, a widely used material known for its high strength-to-weight ratio, formability, and corrosion resistance. The chemical composition is detailed in Table 1 of the paper.
- Molds: The experiment utilized a direct squeeze casting setup with three cylindrical die molds made of medium carbon AISI 1040 steel. The molds had a fixed inner diameter of 30 mm but varied in height (25, 35, 45 mm) to produce castings with H/D ratios of 2/3, 3/3, and 4/3.

Method 2: Squeeze Casting Process
- Process Parameters: The alloy was melted to 800 °C and the mold assembly was preheated to 300 °C. After pouring the melt, a constant pressure of 90 MPa was applied within 7 seconds and maintained for 120 seconds during solidification.
- Data Collection: A K-type thermocouple and data logger were used to record the cooling curves for each H/D ratio, capturing one reading every two seconds.

Method 3: Mechanical Property Testing
- Density and Porosity: The Archimedes principle (ASTM D3800) was used to determine the real density of each specimen. Porosity was then calculated based on the difference between the measured density and the theoretical density of AA6061 (2.727 g/cm³).
- Hardness: A Vickers hardness tester was used to measure the hardness of the samples.
- Wear Rate: A Pin-on-disc wear tester was employed (Speed: 480 rpm, Load: 20 N, Time: 20 Sec) to determine the wear rate by measuring weight loss.

The Breakthrough: Key Findings & Data

The study revealed a clear and direct correlation between the H/D aspect ratio and the resulting mechanical properties of the squeeze-cast AA6061 samples. A lower aspect ratio consistently produced superior results.

Finding 1: Lower Aspect Ratio Drastically Reduces Porosity and Increases Density

The effectiveness of the applied pressure in eliminating porosity is highly dependent on the component's geometry. As the H/D ratio decreased, the density increased and porosity fell sharply.

• As shown in Figure 5, the sample with the lowest H/D ratio of 2/3 achieved a density of 2.724 g/cm³ and a porosity of just 0.08%. In contrast, the sample with the highest H/D ratio of 4/3 had a lower density of 2.71 g/cm³ and a significantly higher porosity of 0.66%. The density of the 2/3 ratio sample closely approached the theoretical maximum for the alloy.

Finding 2: Hardness and Wear Resistance Improve with Lower Aspect Ratio

The improved material integrity achieved at lower H/D ratios translated directly into better performance in hardness and wear testing. This is attributed to the faster cooling rate associated with the smaller volume, which results in a finer grain structure.

• As shown in Figure 6, the hardness for the 2/3 H/D ratio sample was the highest at 69 HV. This value decreased to 65 HV for the 3/3 ratio and fell to 59.5 HV for the 4/3 ratio.
• Similarly, Figure 7 shows that the 2/3 ratio sample exhibited the best wear resistance, with a wear rate of 27×10⁻⁸ g/cm. The wear rate increased to 33×10⁻⁸ g/cm for the 3/3 ratio and 38×10⁻⁸ g/cm for the 4/3 ratio.

Practical Implications for R&D and Operations

• For Process Engineers: This study suggests that for a given applied pressure, its effectiveness in reducing porosity is diminished as the casting height (and thus H/D ratio) increases. Optimizing the cooling rate and pressure transmission by favoring lower-profile component geometries can directly contribute to reducing scrap and improving part quality.
• For Quality Control Teams: The data in Figure 5 and Figure 6 of the paper illustrates the direct effect of H/D ratio on density and hardness. This provides a tangible benchmark for setting new quality inspection criteria, linking a geometric feature to key mechanical properties.
• For Design Engineers: The findings indicate that component aspect ratio is a critical factor influencing defect formation during squeeze casting solidification. Designing parts with a lower H/D ratio where possible is a valuable consideration in the early design phase to ensure optimal mechanical performance and manufacturability.

Paper Details

IMPACT ASPECT RATIO ON MECHANICAL PROPERTIES OF ALUMINUM ALLOY PRODUCED BY SQUEEZE CASTING PROCESS

1. Overview:

• Title: IMPACT ASPECT RATIO ON MECHANICAL PROPERTIES OF ALUMINUM ALLOY PRODUCED BY SQUEEZE CASTING PROCESS
• Author: S.S. Mutar, N.S. Abtan
• Year of publication: 2023
• Journal/academic society of publication: International Journal on "Technical and Physical Problems of Engineering” (IJTPE), Volume 15, Number 4, Issue 57
• Keywords: Aspect Ratio, 6061 Aluminum Alloy, Direct Squeeze Casting, Mechanical Properties.

2. Abstract:

Squeeze casting, a technique used to avoid the development of shrinkage porosity, involves applying external pressure to an alloy melt while it solidifies. The squeeze casting method was created with sectors like the auto and aerospace in mind since the squeeze cast components have better mechanical qualities. Series (6xxx) alloys are among the most important aluminum alloy chains containing silicon and magnesium as basic elements and are widely used in various engineering applications. These alloys have characteristics like high strength to weight, formability and moderate corrosion resistance. The effect of H/D ratios of 2/3, 3/3, 4/3 at applied pressure of 90 MPa on the mechanical properties (hardness - density - porosity - wear rate) of AA6061 aluminum ingot was studied after the examination process, the hardness, density decreased with the increasing of the H/D ratio, while the values of the porosity ratio and the wear rate increased at the applied Pressure values.

3. Introduction:

Direct squeeze casting is a technique where molten metal is solidified under pressure inside a closed mold, resulting in a fine-grain casting with mechanical properties similar to a wrought product. The method is easily mechanized for producing near-net to net shape components of high quality from aluminum, magnesium, and copper alloys. The high pressure ensures tight contact between the molten metal and the mold surface, increasing the heat transfer rate and leading to rapid solidification. This results in a fine secondary arm spacing, which is necessary for achieving adequate strength and ductility. Squeeze casting offers an alternative to conventional casting and forging, reducing material consumption and producing higher-strength parts for weight savings. This study investigates the effect of section thickness (via H/D ratio) on the density, porosity, hardness, and wear rate of AA6061 alloy squeeze cast at a pressure of 90 MPa.

4. Summary of the study:

Background of the research topic:

The study focuses on direct squeeze casting, a manufacturing process that applies pressure during the solidification of molten metal to eliminate defects like shrinkage porosity and improve mechanical properties. The material under investigation is AA6061, a common aluminum alloy used in engineering applications due to its favorable strength-to-weight ratio and corrosion resistance.

Status of previous research:

Previous studies have been conducted to improve the squeeze casting process by examining various process parameters for different materials and alloys. Research has established that squeeze cast parts have improved mechanical characteristics over conventionally cast parts.

Purpose of the study:

The purpose of this study was to investigate the effect of the height-to-diameter (H/D) aspect ratio on the mechanical properties (hardness, density, porosity, wear rate) of AA6061 aluminum alloy produced by the squeeze casting process under a constant applied pressure of 90 MPa.

Core study:

The core of the study involved casting cylindrical specimens of AA6061 alloy with three different H/D ratios (2/3, 3/3, 4/3) using a direct squeeze casting method. A constant pressure of 90 MPa was applied during solidification. The resulting specimens were then subjected to a series of tests to evaluate their density, porosity, Vickers hardness, and wear rate. The relationships between the H/D ratio and these mechanical properties were analyzed.

5. Research Methodology

Research Design:

An experimental research design was employed. The independent variable was the aspect ratio (H/D) of the cylindrical specimens, which was controlled by using molds of different heights with a fixed diameter. The dependent variables were the measured mechanical properties: density, porosity, hardness, and wear rate. The applied pressure (90 MPa), casting temperature (800 °C), and mold preheat temperature (300 °C) were held constant.

Data Collection and Analysis Methods:

• Cooling Rate: A K-type thermocouple connected to a data logger recorded temperature over time during solidification.
• Density & Porosity: Density was measured using the Archimedes method per ASTM D3800. Porosity was calculated using the formula based on theoretical and real density (ASTM C948).
• Hardness: A Vickers hardness test machine was used to measure the hardness of the samples.
• Wear Rate: A pin-on-disc wear tester was used to determine the wear rate based on mass loss over a set sliding distance.
• The collected data was plotted on graphs to analyze the relationships between the H/D ratio and each mechanical property.

Research Topics and Scope:

The research was focused on AA6061 aluminum alloy. The scope was limited to the direct squeeze casting process at a single pressure level (90 MPa). The investigation centered on how varying the H/D aspect ratio (2/3, 3/3, 4/3) impacts the final density, porosity, hardness, and wear rate of the cast components.

6. Key Results:

Key Results:

• The cooling rate decreased as the H/D ratio increased. For H/D ratios of 2/3, 3/3, and 4/3, the cooling rates were 20, 15.5, and 14 °C/Sec, respectively.
• Density decreased and porosity increased with an increasing H/D ratio. The density values were 2.724, 2.718, and 2.71 g/cm³ for H/D ratios of 2/3, 3/3, and 4/3, respectively. The corresponding porosity values were 0.08%, 0.32%, and 0.66%.
• Hardness decreased as the H/D ratio increased. The recorded Vickers hardness values were 69, 65, and 59.5 HV for H/D ratios of 2/3, 3/3, and 4/3, respectively.
• Wear rate increased as the H/D ratio increased. The wear rate values were 27×10⁻⁸, 33×10⁻⁸, and 38×10⁻⁸ g/cm for H/D ratios of 2/3, 3/3, and 4/3, respectively.
• The best mechanical properties (highest density, highest hardness, lowest porosity, lowest wear rate) were obtained at the lowest H/D ratio of 2/3.

Figure Name List:

• Figure 1. Used molds and presses
• Figure 2. Data logger thermometer
• Figure 3. Hardness tester
• Figure 4. Cooling curves of samples with aspect ratio H/D
• Figure 5. Porosity level and density at pressure (90Mpa)
• Figure 6. Hardness and aspect ratio H/D at pressure (90Mpa)
• Figure 7. Wear rate and aspect ratio H/D at pressure 90 MPa

7. Conclusion:

The study concluded that the H/D ratio has a significant impact on the mechanical properties of squeeze-cast AA6061. An increase in the H/D ratio resulted in a decrease in the cooling rate, a decrease in density, and an increase in porosity. Consequently, hardness and wear resistance also degraded with a higher H/D ratio. The reason is attributed to the reduced effectiveness of the applied pressure over a larger volume (greater height), which impairs the feeding mechanism between dendritic arms needed to eliminate shrinkage porosity. The optimal mechanical properties were achieved with the lowest H/D ratio of 2/3, which yielded a density value closest to the theoretical maximum for the alloy.

8. References:

• [1] P. Vijian, V. Arunachalam, “Optimization of Squeeze Casting Process Parameters Using Taguchi Analysis", The International Journal of Advanced Manufacturing Technology, Issue 83, Vol. 33, pp. 1122-1127, August 2007.
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• [3] K. Sukumaran, et al., “Studies on Squeeze Casting of Al 2124 Alloy and 2124-10% SiCp Metal Matrix Composite", Materials Science and Engineering, Issue 1, Vol. 490, pp. 235-241, 25 August 2008.
• [4] L. Yang, "The Effect of Solidification Time in Squeeze Casting of Aluminum and Zinc Alloys", Journal of Materials Processing Technology, Issue 1, Vol. 192, pp. 114-120, October 2007.
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• [6] N.S. Abtan, K.H. Ghlaim, L.A. Proff, “Comparisons of Applied Pressure Effect on Improving Density, Hardness, and Microstructure by both Squeeze Casting Process and Pressure Die Casting Process for 380-Al Alloy", The Iraqi Journal for Mechanical and Material Engineering, Issue 2, Vol. 12, pp. 334-343, 2012.
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Expert Q&A: Your Top Questions Answered

Q1: Why was a constant pressure of 90 MPa chosen for this study?

A1: The paper specifies that the experiment was conducted at an applied pressure of 90 MPa. The study's objective was not to evaluate different pressures but to isolate the effect of the H/D aspect ratio. By keeping the pressure constant, the researchers could ensure that any observed differences in mechanical properties were directly attributable to the geometric changes in the test samples.

Q2: How does the cooling rate, as shown in Figure 4, directly correlate with the observed hardness in Figure 6?

A2: The paper establishes a direct relationship. The H/D ratio of 2/3 had the fastest cooling rate (20 °C/Sec) and the highest hardness (69 HV). Conversely, the 4/3 ratio had the slowest cooling rate (14 °C/Sec) and the lowest hardness (59.5 HV). The authors explain that a faster cooling rate, facilitated by the smaller volume of the 2/3 sample and effective pressure, leads to a finer grain structure and reduced dendritic arm spacing, which in turn increases hardness.

Q3: What is the proposed mechanism for how applied pressure reduces porosity in this process?

A3: According to Section 3.2 of the paper, the applied pressure plays a major role by enhancing the feeding mechanism during solidification. The pressure forces the remaining liquid metal to penetrate the gaps between the developing dendritic arms. This action effectively fills the voids that would otherwise become shrinkage porosity, thus increasing the overall density and integrity of the casting.

Q4: The paper's conclusion mentions the "compressibility factor of the molten metal." Can you elaborate on its role?

A4: In Conclusion 2, the authors attribute the superior density of the 2/3 ratio sample to the effect of the compressibility of the molten metal being greater than the rate of cooling. This implies that in a smaller, lower-profile casting, the applied pressure is more efficient at compacting the solidifying metal and closing internal voids before the part solidifies completely. In taller castings (higher H/D ratio), the pressure's effect is diminished over the greater distance.

Q5: Why is there a direct link between lower porosity (Figure 5) and lower wear rate (Figure 7)?

A5: The paper links improved wear resistance to a smoother grain structure and uniform distribution of the eutectic phase, which are characteristics of a dense, low-porosity casting. Porosity acts as a stress concentration point and a source of weakness in the material's surface. A denser material with higher hardness, like the one achieved with the 2/3 H/D ratio, presents a more uniform and robust surface that is more resistant to material loss from friction, resulting in a lower wear rate.

Conclusion: Paving the Way for Higher Quality and Productivity

This research clearly demonstrates that in the Squeeze Casting Process, component geometry is not just a design feature but a critical process variable. The study on AA6061 alloy shows that a lower height-to-diameter aspect ratio allows applied pressure to work more effectively, leading to faster cooling, near-zero porosity, and significantly improved hardness and wear resistance. These findings provide a clear directive for engineers: to achieve the highest mechanical performance, component design and the casting process must be considered in tandem.

"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 ASPECT RATIO ON MECHANICAL PROPERTIES OF ALUMINUM ALLOY PRODUCED BY SQUEEZE CASTING PROCESS" by "S.S. Mutar and N.S. Abtan".

Source: The paper was published in the International Journal on "Technical and Physical Problems of Engineering” (IJTPE), Vol. 15, No. 4, Dec. 2023, Pages 89-93. A direct link is not provided in the source document.

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