Unlock Higher Strength in ADC12 Castings: How Cooling Rate Control Replaces T6 Treatment

This technical summary is based on the academic paper "Effect of Cooling Rate on the Precipitation Characteristics of Cast Al-Si-Cu Alloy" by M. Okayasu, N. Sahara, and M. Touda, published in ARCHIVES of FOUNDRY ENGINEERING (2021).

Keywords

• Primary Keyword: Precipitation Hardening ADC12
• Secondary Keywords: Aluminum alloy, Casting, Solid solution, Aging, Solidification rate, Cooling rate, Mechanical properties

Executive Summary

• The Challenge: Traditional T6 heat treatment for strengthening cast aluminum alloys like ADC12 can cause high-temperature defects such as blisters.
• The Method: The study systematically varied the solidification cooling rate of ADC12 cast samples to determine its effect on subsequent strengthening via a low-temperature aging process.
• The Key Breakthrough: A solidification cooling rate exceeding 0.03 °C/s is sufficient to trap copper in a solid solution within the aluminum matrix, enabling significant precipitation hardening during a simple aging process without requiring a separate, high-temperature solution treatment.
• The Bottom Line: Precisely controlling the cooling rate during casting offers a viable pathway to enhance the mechanical properties of ADC12 parts, bypassing the risks associated with the high-temperature solutionizing step of a full T6 treatment.

The Challenge: Why This Research Matters for HPDC Professionals

In the automotive industry, the drive for vehicle weight reduction to improve fuel efficiency and reduce emissions has put a spotlight on high-strength aluminum alloys. While casting is an economical method for producing complex parts, cast Al-Si-Cu alloys like ADC12 typically lack the strength of their wrought counterparts.

The standard method to increase strength is a T6 heat treatment, which involves a high-temperature solid solution treatment followed by quenching and aging. However, for cast components, this high-temperature step (often around 500°C) can lead to critical defects like blistering, rendering the parts unusable. This limitation has hindered the widespread adoption of high-strength cast aluminum in structure-critical applications. This research tackles this exact problem by exploring whether the casting process itself can be manipulated to achieve the first step of the T6 process, eliminating the need for the problematic high-temperature treatment.

The Approach: Unpacking the Methodology

The researchers designed an experiment to isolate the effect of solidification cooling rate on the age-hardenability of commercial ADC12 (Al-10.4Si-1.7Cu-0.3Mg-0.9Fe) alloy.

Method 1: Controlled Solidification
To achieve a wide range of cooling rates, 95-g samples of molten ADC12 at 630°C were poured into molds with different thermal properties and cooled under different conditions:
- GC (Gravity Casting, Steel Mold): Rapid air cooling in a steel mold.
- AC (Air Cooling, Ceramic Mold): Slower air cooling in a ceramic mold.
- FAC (Furnace Air Cooling, Ceramic Mold): Even slower cooling with forced air in a furnace.
- FC (Furnace Cooling, Ceramic Mold): Very slow cooling by turning off the furnace.
This resulted in four distinct, measured cooling rates ranging from 1.17 °C/s down to 0.006 °C/s.

Method 2: Low-Temperature Aging (LTA)
Instead of a full T6 process, the "as-cast" samples from each cooling group were directly subjected to a low-temperature aging process. This was conducted at 175°C and 220°C for durations ranging from 1 to 96 hours to trigger precipitation hardening.

Method 3: Comprehensive Material Analysis
The mechanical and microstructural properties were thoroughly evaluated using:
- Micro-Vickers Hardness and Nanoindentation: To measure hardness changes in the overall material and specifically within the α-Al phase.
- Microscopy (Optical, EBSD, STEM): To observe the grain structure, internal strain, and, crucially, the presence of nano-scale precipitates responsible for hardening.

The Breakthrough: Key Findings & Data

The study delivered clear, data-driven evidence linking the solidification rate directly to the potential for strengthening the alloy.

Finding 1: A Critical Cooling Rate Unlocks Precipitation Hardening

The ability of the ADC12 alloy to harden during aging was entirely dependent on how quickly it was cooled during casting. As shown in Figure 5(a), samples cooled at rates above 0.03 °C/s (GC, AC, and FAC) showed a dramatic increase in hardness after aging at 175°C, reaching a peak of approximately 115 HV after 20 hours. In stark contrast, the slowest-cooled sample (FC, 0.006 °C/s) exhibited almost no change in hardness, indicating that the slow cooling failed to set the stage for precipitation.

Finding 2: Microscopic Proof of Hardening Mechanism

Scanning Transmission Electron Microscopy (STEM) analysis revealed the microscopic reason for the difference in hardness. As seen in Figure 7, the FAC sample (cooled at 0.030 °C/s) clearly shows the formation of fine θ' (CuAl₂) metastable precipitates after aging. These precipitates act as obstacles to dislocation movement, which is the fundamental mechanism of precipitation hardening. Conversely, the STEM image of the slow-cooled FC sample shows no such precipitates, explaining its inability to gain strength through aging.

Practical Implications for R&D and Operations

• For Process Engineers: This study suggests that adjusting die casting parameters to achieve solidification rates above 0.03 °C/s can enable significant strength gains through a simple, low-temperature aging process. This could involve optimizing die cooling channels, cycle times, and spray applications to replace the need for a separate, high-temperature solution treatment.
• For Quality Control Teams: The data in Figure 5 demonstrates a direct correlation between cooling rate (which influences SDAS) and the potential for age hardening. This relationship could inform new, non-destructive quality inspection criteria, where microstructural features are used to predict the final mechanical properties of a part after aging.
• For Design Engineers: The findings indicate that part geometry, particularly wall thickness, will critically influence the local cooling rate and thus the final mechanical properties. Thicker sections may not achieve the necessary cooling rate to enable precipitation hardening, a crucial consideration for ensuring uniform strength in the early design phase of a component.

Paper Details

Effect of Cooling Rate on the Precipitation Characteristics of Cast Al-Si-Cu Alloy

1. Overview:

• Title: Effect of Cooling Rate on the Precipitation Characteristics of Cast Al-Si-Cu Alloy
• Author: M. Okayasu, N. Sahara, M. Touda
• Year of publication: 2021
• Journal/academic society of publication: ARCHIVES of FOUNDRY ENGINEERING, Volume 21, Issue 4/2021
• Keywords: Aluminum alloy, Casting, Precipitation, Solid solution, Aging, Solidification rate

2. Abstract:

The influence of the cooling rate on the extent of precipitation hardening of cast aluminum alloy (ADC12) was investigated experimentally. This study explored the cooling rate of the solidification of Cu in the α-Al phase to improve the mechanical properties of ADC12 after an aging process (Cu based precipitation hardening). The solid solution of Cu occurred in the α-Al phases during the casting process at cooling rates exceeding 0.03 °C/s. This process was replaced with a solid solution process of T6 treatments. The extent of the solid solution varied depending on the cooling rate; with a higher cooling rate, a more extensive solid solution was formed. For the cast ADC12 alloy made at a high cooling rate, high precipitation hardening occurred after low-temperature heating (at 175 °C for 20 h), which improved the mechanical properties of the cast Al alloys. However, the low-temperature heating at the higher temperature for a longer time decreased the hardness due to over aging.

3. Introduction:

Due to the demand for vehicle weight reduction, age-hardenable aluminum alloys have gained significant attention. Casting technologies are advantageous for producing complex automotive parts, but the relatively poor material strength of cast Al alloys has limited their use in structure-critical applications. Mechanical properties can be improved by controlling the cooling rate to refine grain size (reduce SDAS) or by applying a T6 treatment (solid solution followed by aging) to create metastable precipitates like Guinier-Preston zones and θ' (CuAl₂). However, the high-temperature solid solution step of T6 treatment is often unsuitable for cast alloys as it can create defects like blisters. Previous research suggested that the solid solution process could be replaced by a rapid solidification process, but the specific cooling rate required was not clarified. This study thus aims to examine the effect of the solidification rate on the precipitation hardening of cast Al alloy.

4. Summary of the study:

Background of the research topic:

The research is situated within the context of the automotive industry's need for lightweight, high-strength materials to reduce vehicle weight and emissions. Cast aluminum alloys are economically favorable for complex parts, but their mechanical properties often fall short of requirements.

Status of previous research:

Previous studies have established that T6 heat treatment is an effective method for strengthening Al-Cu alloys through precipitation. However, it is also known that the high-temperature solutionizing stage of T6 can introduce defects in cast components. Some work has suggested that rapid solidification during casting could serve as an alternative to this solutionizing step, but the quantitative parameters for this process were not well-defined.

Purpose of the study:

The primary purpose of this study was to experimentally determine the influence of the solidification cooling rate on the precipitation hardening characteristics of cast ADC12 aluminum alloy. The goal was to identify a critical cooling rate that would allow for effective age hardening without the need for a conventional high-temperature solid solution treatment.

Core study:

The core of the study involved producing cast ADC12 samples under four different, controlled cooling rates. These as-cast samples were then subjected to a low-temperature aging (LTA) process. The resulting changes in mechanical properties (hardness) and microstructure (precipitate formation) were systematically analyzed to establish a direct relationship between the initial cooling rate and the material's response to aging.

5. Research Methodology

Research Design:

The study employed a comparative experimental design. A commercial ADC12 alloy was cast into four groups, each with a distinct solidification cooling rate. The mechanical properties and microstructure of these groups were analyzed both in the as-cast state and after undergoing a low-temperature aging process at two different temperatures (175°C and 220°C) for various time intervals (1-96 h).

Data Collection and Analysis Methods:

• Material Preparation: ADC12 alloy was melted and gravity cast into steel and ceramic molds under different cooling conditions (air cooling, furnace cooling) to achieve four distinct cooling rates, which were measured using a thermocouple.
• Mechanical Testing: Material hardness was evaluated using a micro-Vickers hardness tester and a nanoindentation system to assess both bulk properties and the properties of the α-Al phase specifically.
• Microstructural Analysis: The microstructure was examined using optical microscopy, electron backscattering diffraction (EBSD) to analyze crystal orientation and strain, and scanning transmission electron microscopy (STEM) to identify nano-scale precipitates.

Research Topics and Scope:

The research focused on a commercial cast aluminum alloy, ADC12 (Al-Si-Cu). The investigation was scoped to quantify the effect of solidification cooling rates, ranging from 0.006 °C/s to 1.17 °C/s, on the material's capacity for precipitation hardening.

6. Key Results:

Key Results:

• The four casting processes successfully produced samples with distinct cooling rates: 1.17 °C/s (GC), 0.052 °C/s (AC), 0.030 °C/s (FAC), and 0.006 °C/s (FC).
• A higher cooling rate resulted in a finer microstructure, as indicated by a smaller secondary dendrite arm spacing (SDAS), which ranged from 13.2 µm for the fastest cooling rate to 95.0 µm for the slowest.
• Samples cooled at rates of 0.030 °C/s or higher (FAC, AC, GC) showed significant age hardening at 175 °C, with hardness increasing from a baseline of ~80 HV to a peak of ~115 HV after 20 hours.
• The sample cooled at the slowest rate (FC, 0.006 °C/s) showed no significant increase in hardness after the same aging treatment.
• STEM analysis confirmed that the hardening in the FAC sample was due to the formation of θ' (CuAl₂) metastable precipitates. These precipitates were absent in the slow-cooled FC sample.
• Heating for longer durations (>20 h at 175°C) or at a higher temperature (220°C) led to over-aging, characterized by a decrease in hardness.

Figure Name List:

• Fig. 1. Schematic diagram of various solicUfication processes to obtain different cooling rate
• Fig. 2. Temperature profiles during the solidification of the ADC12 samples (GC, AC, FAC, and FC)
• Fig. 3. Micrographs of the cast ADC12 samples obtained using four different cooling rates (i. e., the GC, AC, FAC, and FC samples) and various heating processes
• Fig. 4. EBSD analysis (IPF and KAM maps) of the GC, AC, FAC, and FC samples of ADC12 before LT aging.
• Fig. 5. Vickers hardness as a function of aging time for the GC, AC, FAC, and FC ADC12 samples: (a)175°C and (b) 220°C
• Fig. 6. (a) Harclness values determined by nano-indentation and (b) curves of load vs. depth before and after the heating processes for the GC, AC, FAC, and FC ADC12 samples
• Fig. 7. STEM images for the FAC-LT, and FC-LT samples

7. Conclusion:

1) A solid solution of Cu element occurred in the α-Al matrix during the casting process at cooling rates exceeding 0.03 °C/s (FAC). This cooling process could be replaced with a solid solution of T6 to induce the precipitation hardening of CuAl₂. The extent of the solid solution varied depending on the cooling rate; with a higher cooling rate, a more extensive solid solution was formed.
2) For the cast ADC12 sample cooled at a cooling rate > 0.03 °C/s, precipitation hardening occurred after the aging process. Furthermore, high Vickers hardness values were obtained for the GC and AC samples after aging at 175°C for 20 h. With higher temperature and longer heating process, over-aging occurred, resulting in lower mechanical properties.

8. References:

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Expert Q&A: Your Top Questions Answered

Q1: Why were 175°C and 220°C chosen for the low-temperature aging (LTA) process?

A1: The paper notes that these temperatures were determined based on a previous study by the authors [12]. This prior work likely established that this temperature range is optimal for inducing the precipitation of copper-based phases in ADC12 alloy, allowing for an effective comparison of the hardening response among the different sample groups.

Q2: The paper states that the solid solution process was replaced by rapid solidification. What is the critical cooling rate required to achieve this effect in ADC12?

A2: The study identifies a critical cooling rate threshold of 0.03 °C/s. The results clearly show that samples cooled faster than this rate (FAC at 0.030 °C/s, AC at 0.052 °C/s, and GC at 1.17 °C/s) successfully formed a solid solution of copper that enabled subsequent hardening. The sample cooled slower than this rate (FC at 0.006 °C/s) did not, demonstrating this is the minimum rate required under these conditions.

Q3: What is the specific microstructural reason for the increased hardness in the rapidly cooled samples after aging?

A3: The increased hardness is a direct result of precipitation hardening. The STEM analysis shown in Figure 7 provides visual evidence of this. In the rapidly cooled FAC sample, the aging process caused the formation of fine, plate-like θ' (CuAl₂) metastable phases within the α-Al matrix. These nano-scale precipitates effectively impede the movement of dislocations, thereby strengthening the material and increasing its hardness.

Q4: Did the aging process cause any visible changes in the eutectic structure?

A4: No, it did not. According to the micrographs in Figure 3(b), there were no evident microstructural changes observed in the eutectic phases (the complex network of Si, Cu, and Fe-based phases between the α-Al dendrites) even after prolonged aging for 96 hours. This indicates that the significant changes in hardness were due to nano-scale phenomena within the α-Al grains, not large-scale changes to the eutectic structure.

Q5: The paper mentions over-aging. What were the conditions that led to a decrease in hardness?

A5: Over-aging, which results in a loss of hardness, was observed under two conditions. As shown in Figure 5(a), aging at 175°C for more than 20 hours caused the hardness to decrease from its peak value. Additionally, aging at the higher temperature of 220°C (Figure 5(b)) resulted in a lower peak hardness and a more rapid decline, indicating that the precipitates were coarsening and losing their strengthening effectiveness more quickly.

Q6: How does secondary dendrite arm spacing (SDAS) correlate with the cooling rate in this study?

A6: Table 1 shows a strong inverse correlation between cooling rate and SDAS. The highest cooling rate (GC, 1.17 °C/s) produced the finest microstructure with the smallest SDAS of 13.2 µm. As the cooling rate decreased, the SDAS increased proportionally, reaching 95.0 µm for the slowest cooling rate (FC, 0.006 °C/s). This confirms the well-established principle that faster solidification leads to a more refined dendritic structure.

Conclusion: Paving the Way for Higher Quality and Productivity

This research provides a clear and actionable framework for enhancing the strength of cast ADC12 aluminum alloy while circumventing the risks of traditional T6 heat treatment. The key takeaway is that by carefully controlling the solidification cooling rate to exceed 0.03 °C/s, manufacturers can enable significant Precipitation Hardening ADC12 with a simple, low-temperature aging process. This approach not only improves mechanical properties but also eliminates the potential for high-temperature blister defects, leading to higher yields and 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 "Effect of Cooling Rate on the Precipitation Characteristics of Cast Al-Si-Cu Alloy" by "M. Okayasu, N. Sahara, M. Touda".

Source: https://doi.org/10.24425/afe.2021.138679

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