Optimize Al-Si Alloy Performance: How Pouring Temperature in Semi-Solid Metal Casting Dictates Final Component Strength

This technical summary is based on the academic paper "SEMI SOLID METAL POURING TEMPERATURE EFFECTS ON MECHANICAL PROPERTIES OF AL-SI ALLOY" by NURUL HIDAYATI BINTI SALLEH, published as a thesis for Universiti Malaysia Pahang (JUNE 2012).

Keywords

• Primary Keyword: Semi-Solid Metal Casting
• Secondary Keywords: Al-Si Alloy, Pouring Temperature, Mechanical Properties, Microstructure, A356 Alloy, Sand Casting

Executive Summary

• The Challenge: To produce high-quality, strong, and reliable Al-Si alloy components by controlling process parameters to avoid defects and inconsistent mechanical properties.
• The Method: A356 Al-Si alloy was sand cast at three distinct semi-solid and liquid pouring temperatures (620°C, 640°C, and 695°C) to assess the impact on the final product.
• The Key Breakthrough: A lower pouring temperature of 620°C resulted in significantly improved mechanical properties, achieving a maximum strength of 124.34 N/mm² and a hardness of 62.3.
• The Bottom Line: For semi-solid Al-Si casting, a lower pouring temperature is not a limitation but a critical advantage, producing a finer, globular microstructure that directly enhances component strength and hardness.

The Challenge: Why This Research Matters for HPDC Professionals

In the world of high-performance components, particularly in the automotive and aerospace sectors, consistency is key. Semi-solid metal (SSM) casting offers a unique blend of casting and forging benefits, enabling the production of complex, near-net-shape parts with excellent accuracy. However, the process is highly sensitive to thermal conditions.

Engineers constantly face a critical trade-off: pouring at higher temperatures ensures good mold filling but risks defects like shrinkage and warping. Conversely, pouring at lower temperatures can lead to premature solidification and incomplete filling. This study directly addresses this challenge by investigating how varying the pouring temperature of A356 Al-Si alloy affects its fundamental mechanical properties—strength, hardness, and internal microstructure. The findings provide a clear, data-backed path to optimizing casting parameters for superior, reliable components.

The Approach: Unpacking the Methodology

This research employed a systematic and controlled experimental approach to isolate the effects of pouring temperature. The methodology provides a robust foundation for the study's conclusions.

Method 1: Material and Casting Process
The study utilized A356 Al-Si alloy, a widely used material in industrial casting. The components were produced via sand casting, a versatile method suitable for a wide range of geometries. Three distinct pouring temperatures were investigated: 620°C, 640°C, and 695°C, representing a range from the lower semi-solid state to a higher, more liquid state.

Method 2: Mechanical Property Testing
To quantify the performance of the cast samples, standardized mechanical tests were conducted. Tensile strength was measured using a Shimadzu Universal Testing Machine to determine yield strength, ultimate tensile strength (UTS), and elongation. Hardness was evaluated using a Matsuzawa Rockwell Hardness Testing machine.

Method 3: Microstructural Analysis
The internal structure of the alloy—a key determinant of its mechanical behavior—was examined. Cast samples were prepared and observed using a LEICA DME working microscope to analyze the morphology (shape and size) of the primary phase structures formed at each pouring temperature.

The Breakthrough: Key Findings & Data

The results clearly demonstrate a strong correlation between pouring temperature and the final mechanical properties of the A356 alloy.

Finding 1: Lower Pouring Temperature Creates a Superior Globular Microstructure

Microstructural analysis revealed a dramatic shift in the alloy's internal structure based on temperature. As shown in Figure 4.1 of the paper, the sample poured at the lowest temperature (620°C) exhibited a fine, globular (spherical) primary phase. In contrast, the sample poured at the highest temperature (695°C) developed a coarse, dendritic (tree-like) structure. The globular structure is known to enhance mechanical properties by reducing stress concentration points.

Finding 2: Strength and Hardness Peak at the Lowest Pouring Temperature

The mechanical testing data directly confirmed the benefits of the globular microstructure. The sample poured at 620°C achieved the highest ultimate tensile strength of 124.34 N/mm² and the highest Rockwell hardness value of 62.3. As the pouring temperature increased to 640°C and 695°C, both strength and hardness progressively decreased. This is because the faster cooling rate associated with a lower pouring temperature refines the microstructure, leading to a stronger, harder final product.

Practical Implications for R&D and Operations

• For Process Engineers: This study suggests that adjusting pouring temperatures downwards, into the semi-solid range, is a viable strategy for significantly improving the strength and hardness of A356 Al-Si components without compromising casting quality.
• For Quality Control Teams: The data in Figure 4.1 of the paper illustrates the effect of pouring temperature on microstructure. This visual evidence can be used as a new quality inspection criterion, where the presence of a globular structure serves as a reliable indicator of superior mechanical performance.
• For Design Engineers: The findings indicate that the inherent sensitivity of Al-Si alloys to thermal processing conditions is a critical consideration. This knowledge allows for better material property specification in the early design phase, ensuring the final component meets performance requirements.

Paper Details

SEMI SOLID METAL POURING TEMPERATURE EFFECTS ON MECHANICAL PROPERTIES OF AL-SI ALLOY

1. Overview:

• Title: SEMI SOLID METAL POURING TEMPERATURE EFFECTS ON MECHANICAL PROPERTIES OF AL-SI ALLOY
• Author: NURUL HIDAYATI BINTI SALLEH
• Year of publication: JUNE 2012
• Journal/academic society of publication: Thesis submitted in fulfilment of the requirements for the award of the degree of Bachelor of Mechanical Engineering with Manufacturing Engineering, Faculty of Mechanical Engineering, UNIVERSITI MALAYSIA PAHANG.
• Keywords: Semi-solid metal, Al-Si alloy, pouring temperature, mechanical properties, microstructure, sand casting, A356 alloy.

2. Abstract:

In this study, mechanical properties and morphology study of semi-solid metal Al-Si alloy casting was investigated. The sand castings were conducted at three different pouring temperatures. The pouring temperatures for the investigation were 620°C, 640°C and 695°C. The three different samples were tested for their properties such as strength, hardness and the microstructure. Tensile test was conducted using Shimadzu Universal Testing Machine. Hardness was measured by Matsuzawa Rockwell Hardness Testing machine. Microstructure of cast samples were observed using LEICA DME working microscope. The different pouring temperatures caused different cooling rates on the cast samples. The results and observation indicates that a lower temperature produced good quality castings with the maximum values of strength and hardness of 124.34 N/mm² and 62.3 respectively. From metallographic study, primary phase of cast sample at the lower pouring temperature was globular structure while dendrite structure occurs due to a higher pouring temperature. The lower pouring temperature provides a finer microstructure and high hardness samples due to faster cooling rate produced at lower pouring temperature.

3. Introduction:

Semi-solid metal (SSM) processing, discovered in the early 1970s, is a casting process that combines the advantages of liquid metal casting with solid metal forging. It is primarily used for complex products requiring near-net-shapes and high dimensional accuracy. The process operates at a temperature between the solidus and liquidus, where the metal exists in a slurry state (30% to 60% solid). To produce good quality castings, process parameters must be well understood. Pouring temperature is a critical parameter; lower temperatures can lead to incomplete mold filling, while higher temperatures can cause shrinkage and warping. This study investigates the effect of different pouring temperatures on the mechanical properties and microstructure of a semi-solid Al-Si alloy to determine optimal processing conditions.

4. Summary of the study:

Background of the research topic:

Semi-solid metal processing is an established manufacturing route for automotive, aerospace, and military components. Al-Si alloys are widely used in industry due to their favorable properties. The microstructure of these alloys, which can be hypoeutectic, eutectic, or hypereutectic, dictates their mechanical performance. The formation of this microstructure during solidification is highly dependent on process variables, including cooling rate, which is directly influenced by pouring temperature.

Status of previous research:

Previous studies have established the relationship between pouring temperature and microstructure morphology. Lashkari (2006) reported that higher pouring temperatures (675°C–695°C) in A356 alloy lead to dendritic structures with high viscosity, while lower temperatures (615°C) result in globular morphology with lower viscosity. Liu et al. (2006) also demonstrated that as pouring temperature decreases, the primary phase morphology in A356 alloy transitions from rosette-like to globular-like, with a corresponding decrease in grain size. These findings suggest that controlling pouring temperature is a feasible method for refining grain structure.

Purpose of the study:

The objective of this project is to investigate the effect of different pouring temperatures on the mechanical properties of A356 Al-Si alloy.

Core study:

The study involved conducting sand casting of A356 Al-Si alloy at three pouring temperatures: 620°C, 640°C, and 695°C. The resulting cast samples were then subjected to metallographic analysis to study the microstructure, tensile testing to determine strength and elongation, and hardness testing to measure Rockwell hardness. The results from the three temperature groups were then compared to establish a relationship between pouring temperature and the material's mechanical properties.

5. Research Methodology

Research Design:

The research was an experimental investigation designed to compare three groups of cast samples, with the independent variable being the pouring temperature (620°C, 640°C, 695°C). The dependent variables were the measured mechanical properties (tensile strength, yield strength, percent elongation, hardness) and the observed microstructure morphology.

Data Collection and Analysis Methods:

• Casting: An oil-fired crucible furnace was used for melting the A356 alloy. Sand molds were prepared using O.B.B Sand "E". Pouring was conducted manually after verifying the molten metal temperature with a thermocouple.
• Mechanical Testing: Tensile tests were performed on dog-bone-shaped samples using a Shimadzu Universal Testing Machine. Hardness was measured with a Matsuzawa Rockwell Hardness Testing Machine.
• Microstructural Analysis: Samples for analysis were sectioned, mounted, ground, and polished. The microstructure was then observed using a LEICA DME Working Microscope.

Research Topics and Scope:

The scope of the study included:
1. Conducting sand casting of semi-solid metal A356 alloy at three different pouring temperatures.
2. Performing a metallographic study of the cast samples.
3. Conducting tensile tests at room temperature.
4. Measuring the hardness properties of the cast samples.

6. Key Results:

Key Results:

• Microstructure: The morphology of the primary phase was significantly affected by the pouring temperature. At 620°C, a globular structure was observed. At 695°C, a dendritic structure was formed. The sample cast at 640°C showed a transitional structure.
• Mechanical Properties: The sample cast at the lowest temperature of 620°C exhibited the highest mechanical properties. The ultimate tensile strength (UTS) was maximized at this temperature.
• Hardness: The hardness of the samples showed a clear trend, with the highest Rockwell hardness value (62.3) recorded for the sample poured at 620°C. Hardness decreased as the pouring temperature increased.
• Strength: The maximum ultimate tensile strength of 124.34 N/mm² was achieved at the 620°C pouring temperature.

Figure Name List:

• Fgure No. 2.1 Hypeeutetic of Al-Si
• Fgure No. 2.2 The Al-Si binary phase diagrams
• Fgure No. 2.3 Morphologies of primary phase in A356 alloy obtained at different pouring temperatures: (a) 650; (b) 630; and (c) 615
• Fgure No. 2.4 Strain-time graph for the same a-Al morphology at different temperature.
• Fgure No. 2.5 % Elongation / % Reduction in diameter with pouring temperature
• Fgure No. 2.6 Variation of UTS with temperature
• Fgure No. 2.7 Variation of hardness with pouring temperature
• Fgure No. 3.1 O.B.B sand
• Fgure No. 3.2 (a) Molding box and (b) the pattern
• Fgure No. 3.3 The pattern was placed on the mold (b) the white flour was tabor on the pattern and the mold (c) the pattern was taken out from mold and (d) mold ready for pouring
• Fgure No. 3.4 Oil fired crucible furnace
• Fgure No. 3.5 Checking the temperature before pouring to the mold
• Fgure No. 3.6 Thermocouple for checking the molten metal temperature
• Fgure No. 3.7 Molten metal was pouring to the mold
• Fgure No. 3.8 Before fettling
• Fgure No. 3.9 Dog bone for tensile test samples
• Fgure No. 3.10 MSX200M Sectioning cut-off machine
• Fgure No. 3.11 Buhler SimpliMet 1000, Automatic Mounting Press
• Fgure No. 3.12 Buehler HandiMet 2, Roll Grinder
• Fgure No. 3.13 Metkon FORCIPOL 2V Grinder-Polisher
• Fgure No. 3.14 LEICA DME Working Microscope
• Fgure No. 3.15 (a) Shimadzu Universal Testing Machine (b) Tensile test stage
• Fgure No. 3.16 (a) Matsuzawa Rockwell Hardness Testing Machine (b) The indenter stage
• Fgure No. 4.1 The morphology of primary phase in A356 alloy obtained at pouring temperature (a) 620°C, (b) 640°C, and (c) 695°C
• Fgure No. 4.2 Graph yield stress versus pouring temperature
• Fgure No. 4.3 Graph UTS versus pouring temperature
• Fgure No. 4.4 Graph percent elongation versus pouring temperature
• Fgure No. 4.5 Graph of Rockwell hardness versus pouring temperature

7. Conclusion:

The study concludes that pouring temperature has a significant effect on the mechanical properties and microstructure of sand-cast A356 Al-Si alloy. A lower pouring temperature (620°C) produces castings with superior quality, characterized by maximum strength and hardness. This improvement is attributed to the formation of a fine, globular microstructure, which results from the faster cooling rate associated with lower pouring temperatures. Conversely, higher pouring temperatures lead to the formation of dendritic structures and a corresponding degradation in mechanical properties.

8. References:

Note: A complete, formatted reference list is not available in the provided document excerpt. The following citations were identified within the text.
- Flemings (1991)
- Grill (1982)
- Gupta et al. (1999)
- Haizhi (2002)
- Kalpakjian et al. (2006)
- Lancer (1981)
- Lashkari et al. (2006)
- Lashkari et al. (2007)
- Lee et al.
- Liu et al. (2006)
- Liu et al. (2009)
- Massalski et al. (1990)
- Miller et al.
- Mondolfo (1976)
- Nayak et al. (2011)
- Ndaliman et.al (2007)
- Ravi (2006)
- Smith et al. (2006)
- Stroganov et al. (1977)

Expert Q&A: Your Top Questions Answered

Q1: Why were the specific temperatures of 620°C, 640°C, and 695°C chosen for the investigation?

A1: These temperatures were selected to represent a meaningful range for the A356 alloy's solidification process. The lowest temperature, 620°C, is within the semi-solid (or "mushy") zone, ideal for promoting a globular structure. The highest temperature, 695°C, is well into the liquid phase, which typically results in dendritic growth upon cooling. The intermediate 640°C point allows for observation of the transition between these two states, providing a comprehensive view of the temperature's effect.

Q2: The study used A356 Al-Si alloy. What are the key characteristics of this alloy that make it suitable for this type of research?

A2: A356 is a hypoeutectic aluminum-silicon alloy known for its excellent castability, good corrosion resistance, and favorable mechanical properties after heat treatment. Its wide solidification range between liquidus and solidus temperatures makes it an ideal candidate for semi-solid metal processing. This characteristic allows for the formation of the slurry needed for SSM and makes the alloy sensitive to the cooling rate variations investigated in this study.

Q3: The paper attributes the improved properties at 620°C to a "faster cooling rate." How does a lower pouring temperature lead to faster cooling?

A3: This may seem counterintuitive, but it relates to the total heat that must be extracted by the mold. When metal is poured at a lower temperature (e.g., 620°C), it is already closer to its solidification point. The mold needs to remove less thermal energy to solidify the metal, so the solidification process itself occurs more rapidly. This faster solidification rate prevents the large, tree-like dendrites from growing and instead favors the nucleation of many small, fine, globular grains.

Q4: How exactly does a globular microstructure lead to higher strength and hardness compared to a dendritic one?

A4: The shape and distribution of the primary phase particles are critical. Dendritic structures are characterized by long, interlocking arms with sharp internal angles. These sharp features act as stress concentrators, providing easy initiation points for cracks to form and propagate under load. In contrast, a globular structure consists of smooth, rounded particles that distribute stress more evenly. This morphology inhibits crack initiation and makes it more difficult for cracks to travel through the material, resulting in higher overall strength, ductility, and hardness.

Q5: This study was conducted using sand casting. Would the results be applicable to other processes like high-pressure die casting (HPDC)?

A5: While the specific values for strength and hardness would differ, the underlying principle is highly applicable. HPDC inherently involves very high cooling rates, which naturally promote a fine microstructure. However, this study's findings reinforce the importance of precise thermal management even in HPDC. Injecting the metal slurry at the lowest possible temperature that still ensures complete die filling would likely yield an even finer, more optimized globular microstructure, further enhancing the mechanical properties of the final HPDC component.

Conclusion: Paving the Way for Higher Quality and Productivity

This research provides clear, actionable evidence that for Al-Si alloys, precise thermal control is paramount. The core challenge of balancing fluidity with solidification defects can be effectively managed by operating at lower pouring temperatures. The key breakthrough is the confirmation that a lower temperature in Semi-Solid Metal Casting is not a compromise but a direct path to superior mechanical properties, driven by the formation of a fine, globular microstructure. These insights empower R&D and operations teams to refine their processes, reduce variability, and produce stronger, more reliable components.

"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 "SEMI SOLID METAL POURING TEMPERATURE EFFECTS ON MECHANICAL PROPERTIES OF AL-SI ALLOY" by "NURUL HIDAYATI BINTI SALLEH".

Source: The paper is identified as a thesis from Universiti Malaysia Pahang, accessible via the CORE (core.ac.uk) repository provided by UMP Institutional Repository.

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