Development of Heat Treatments for Components Produced by Low Pressure Die Casting in Aluminum Alloys

Unlocking Peak Performance in Cast Aluminum: A New Look at Heat Treatment for AlSi7Mg0.3 Alloys

This technical summary is based on the academic paper "Development of Heat Treatments for Components Produced by Low Pressure Die Casting in Aluminum Alloys" by Ricardo Mil-Homens Jorge, submitted for the Master in Mechanical Engineering at the University of Porto (July 2025). It has been analyzed and summarized for technical experts by CASTMAN.

Figure 2 - Counter-pressure casting system [8].

Keywords

Primary Keyword: Aluminum Alloy Heat Treatment
Secondary Keywords: Low Pressure Die Casting, LPDC, AlSi7Mg0.3, A356 Alloy, T6 Heat Treatment, Porosity Reduction, Mechanical Properties

Executive Summary

The Challenge: To maximize the mechanical properties of high-performance aluminum alloy components (AlSi7Mg0.3) produced via Low Pressure Die Casting (LPDC) through optimized heat treatments.
The Method: An experimental study involving rotary degassing to reduce melt porosity, followed by a systematic evaluation of T6 heat treatment parameters, specifically varying artificial aging times and temperatures on cast steering knuckles.
The Key Breakthrough: Shorter, higher-temperature aging treatments (specifically 180°C for 4 hours) produced superior ultimate tensile and yield strength compared to traditional longer, lower-temperature cycles.
The Bottom Line: Optimizing heat treatment parameters is critical for unlocking the full performance potential of cast aluminum components, enabling the production of parts with mechanical properties that reach the upper limits defined by industry standards for gravity die casting.

The Challenge: Why This Research Matters for HPDC Professionals

In the automotive, aerospace, and other high-performance sectors, the demand for lightweight, high-strength, and reliable components is relentless. Low Pressure Die Casting (LPDC) is a key process for producing high-quality aluminum parts, but the final mechanical properties are critically dependent on subsequent treatments. While aluminum alloys like AlSi7Mg0.3 (A356) are known for their excellent response to T6 heat treatment, the specific parameters of that treatment—solution time, quenching, and aging temperature/duration—can be the difference between a standard part and a superior one.

This research was necessary to systematically study the effect of these variations, moving beyond conventional wisdom to find the optimal combination that maximizes strength and performance. For engineers in both LPDC and the related field of HPDC, understanding these nuances is crucial for pushing the boundaries of component quality, reducing cycle times, and achieving a competitive edge.

The Approach: Unpacking the Methodology

This study was conducted in a rigorous, multi-stage process to ensure the integrity of the findings.

Melt Treatment & Quality Control: The research began by addressing a fundamental challenge in aluminum casting: melt quality. An AlSi7Mg0.3 (A356) alloy melt was treated using a rotary degassing system with argon gas to reduce dissolved hydrogen and minimize porosity. The effectiveness of this treatment was evaluated at various time intervals (5 to 25 minutes) using the Reduced Pressure Test (RPT), with porosity quantified through Density Index calculations and image analysis.
Casting Procedure: After establishing an optimal 25-minute degassing treatment, steering knuckles were produced using an industrial Low Pressure Die Casting (LPDC) system. The process utilized a defined pressure curve to ensure controlled, non-turbulent mold filling and solidification.
Systematic Heat Treatment: The cast components were subjected to a T6 heat treatment. The solution heat treatment was held constant for all samples (4 hours at 540°C followed by a rapid quench). The key variable was the artificial aging stage, where parts were treated at four different temperatures (150°C, 160°C, 170°C, and 180°C) for durations ranging from 2 to 12 hours.
Mechanical Characterization: The effects of the treatments were measured through extensive mechanical testing. Brinell hardness tests were performed at each aging interval to map the hardness evolution and identify peak hardness. Subsequently, tensile tests were conducted on specimens treated to their peak hardness conditions to determine ultimate tensile strength (UTS), yield stress, and total elongation.

The Breakthrough: Key Findings & Data

The experimental work yielded two significant findings that offer valuable insights for aluminum casting professionals.

Finding 1: Quantifiable Improvement in Melt Quality Through Degassing

The study confirmed the effectiveness of rotary degassing in significantly reducing porosity. The Density Index (D.I.), a measure of potential porosity, showed a clear correlation with treatment time.

As shown in Figure 49, the D.I. dropped from an initial 4.68% to 0.58% after 25 minutes of degassing, well below the industry-accepted threshold of 1%. This was corroborated by direct porosity analysis. Figure 50 illustrates that the pore area fraction in RPT samples was reduced from a high of 18.6% in the untreated state to just 0.229% after a 25-minute treatment, demonstrating a dramatic improvement in melt cleanliness.

Finding 2: Shorter, High-Temperature Aging Delivers Superior Strength

Contrary to the common belief that longer aging times are always necessary, the research revealed that higher temperatures with shorter durations yielded the best mechanical properties. While all T6 treatments significantly improved strength over the as-cast state, the 180°C aging cycle proved most effective.

According to the data in Figure 65 and Table 19, the treatment of 4 hours at 180°C produced the highest average ultimate tensile strength (334 MPa) and yield stress (277 MPa). This surpassed the results from the longer, lower-temperature cycle of 12 hours at 150°C (311 MPa UTS, 219 MPa Yield Stress). This outcome suggests an opportunity to not only improve component strength but also to significantly reduce production cycle times and energy consumption.

Practical Implications for R&D and Operations

For Process Engineers: This study suggests that for AlSi7Mg0.3, adjusting artificial aging cycles towards shorter durations at higher temperatures (e.g., 4 hours at 180°C) may contribute to increased throughput and reduced energy costs while achieving superior mechanical strength.
For Quality Control Teams: The data in Table 19 of the paper illustrates the mechanical property benchmarks achievable with optimized T6 treatments on LPDC components. These values (up to 334 MPa UTS and 277 MPa Yield Stress) could inform new, more ambitious quality inspection criteria.
For Design Engineers: The findings indicate that components produced via LPDC and subjected to an optimized heat treatment can achieve mechanical properties reaching the upper bounds specified for gravity die casting. This affirms the process's viability for producing high-quality, high-performance structural components where strength-to-weight ratio is critical.

Paper Details

Development of Heat Treatments for Components Produced by Low Pressure Die Casting in Aluminum Alloys

1. Overview:

Title: Development of Heat Treatments for Components Produced by Low Pressure Die Casting in Aluminum Alloys
Author: Ricardo Mil-Homens Jorge
Year of publication: July 2025
Journal/academic society of publication: Master Dissertation, Faculty of Engineering of the University of Porto (FEUP)
Keywords: Low-pressure die casting; Heat treatment; Aluminum alloys; AlSi7Mg0.3

2. Abstract:

This dissertation is the result of an internship in a business environment and was carried out at the Advanced Manufacturing Technologies Unit (UTAF) of INEGI (Institute of Science and Innovation in Mechanical Engineering and Engineering Industry). The main objective of this work was to develop heat treatments for application to aluminum parts produced by low-pressure die casting, specifically for the AlSi7Mg0.3 alloy. This work began with a survey of low-pressure die casting processes in general, followed by a deeper dive into aluminum alloys and their treatments, including melt treatments, casting defects and heat treatments. After this review of the current literature, an experimental phase followed, in which the rotary degassing treatment was first studied, with the aim of obtaining raw castings with no porosity, analyzing the effect of various treatment times. This analysis proceeded by obtaining samples subjected to a Reduced Pressure Test and subsequent quantitative and qualitative porosity analysis. After this initial phase of testing and identification of optimum parameters, the castings were subjected to a set of solubilization and ageing heat treatments, and their mechanical characteristics were analyzed and compared in both states. The results obtained made it possible to identify optimum treatment times and temperatures, obtaining much higher elongation and tensile strength values when compared to other studies, and reaching the upper limits of the EN 1706 standard for die casting, affirming low-pressure casting technology as a viable alternative process for producing high-quality, high-performance components, highlighting the importance of defining suitable parameters for heat treatments.

3. Introduction:

Low Pressure Casting is an important process known for producing high-quality, efficient components. Its advantages come from precise control of material flow and solidification. This process, allied with the versatility of aluminum alloys and the multitude of treatments they can be subjected to, has the potential to produce the highest quality castings spanning a multitude of applications. The main objective of this dissertation is to study the effect of the variation of heat treatment parameters on the mechanical properties of aluminum parts produced through the process of Low-Pressure Die Casting, by performing a comparison of microstructures and mechanical properties between as-cast parts and parts in different stages of heat treatment.

4. Summary of the study:

Background of the research topic:

The study is situated within the field of advanced manufacturing, focusing on enhancing the properties of aluminum components. With increasing demands from industries like automotive for lightweight and high-performance parts, optimizing processes like Low Pressure Die Casting (LPDC) and subsequent heat treatments is critical. The research specifically targets the AlSi7Mg0.3 alloy (A356), which is widely used for quality parts due to its excellent response to precipitation hardening.

Status of previous research:

The paper reviews existing literature on LPDC, aluminum alloy metallurgy, melt treatments (degassing, grain refinement), casting defects (porosity, bifilms), and heat treatments (solution treatment, aging). It notes that while T6 treatment is common, studies have shown varied results depending on parameters. Some research indicated prolonged solution times could negatively impact aging response, while others showed that combined refinement, modification, and heat treatment could dramatically improve properties like impact toughness. The paper aims to build on this by systematically evaluating different aging strategies for LPDC components.

Purpose of the study:

The primary goal was to develop and identify optimal heat treatment parameters for AlSi7Mg0.3 components produced by LPDC to maximize their final mechanical properties. This involved two main phases: first, optimizing the melt degassing process to produce sound, low-porosity castings, and second, systematically testing various artificial aging time-temperature combinations to find the one that yields the best balance of tensile strength, yield strength, and elongation.

Core study:

The core of the study is an experimental investigation. First, a rotary degassing process was evaluated using a Reduced Pressure Test (RPT) to find the optimal treatment time for minimizing porosity. Second, steering knuckles were cast using LPDC with the optimized melt. Third, these castings were subjected to a standard solution heat treatment (540°C for 4 hours) and then artificially aged at four different temperatures (150°C, 160°C, 170°C, 180°C) for times up to 12 hours. Finally, the mechanical properties (hardness, tensile strength, yield strength, elongation) of the treated samples were measured and compared to identify the most effective heat treatment cycle.

5. Research Methodology

Research Design:

The research followed a two-part experimental design. The first part focused on process optimization, using varying degassing times to determine the ideal conditions for producing low-porosity melts. The second part was a parametric study on heat treatment, where the solution treatment was fixed, and the artificial aging temperature and time were systematically varied to map their effect on mechanical properties.

Data Collection and Analysis Methods:

Data was collected through physical testing. Melt quality was assessed using a Reduced Pressure Test (RPT), with density measurements used to calculate the Density Index and image analysis software (ImageJ) used to quantify pore area fraction and size distribution. Mechanical properties were determined using a semi-automatic Brinell hardness tester and an Instron universal testing system for tensile tests, following ASTM B557M standards. Microstructural analysis was performed using optical microscopy and SEM-EDS.

Research Topics and Scope:

The scope was focused on the AlSi7Mg0.3 (A356) aluminum alloy cast via the Low Pressure Die Casting (LPDC) process. The research investigated two key treatments: rotary degassing of the molten alloy and T6 precipitation hardening (solution treatment and artificial aging). The evaluation was based on the resulting porosity levels and mechanical properties, specifically hardness, ultimate tensile strength, yield strength, and elongation.

6. Key Results:

Key Results:

Rotary degassing for 25 minutes effectively reduced the Density Index to 0.58% and the pore area fraction to 0.229%, indicating a significant improvement in melt quality.
Solution heat treatment increased the hardness of the as-cast parts due to solid solution strengthening.
A plateau in hardness was observed during aging at 170°C and 180°C, where hardness did not decrease after reaching its peak within the 12-hour test window. The cause for this was not identified.
The optimal mechanical properties were achieved with a shorter, higher-temperature aging cycle: 4 hours at 180°C.
This optimal treatment resulted in an average ultimate tensile strength of 334 MPa, a yield strength of 277 MPa, and an elongation of 9.0%. These values reach the upper limits of the EN 1706 standard for gravity die casting.

Figure Name List:

Figure 1 - Typical LPC system configuration [2].
Figure 2 - Counter-pressure casting system [8].
Figure 3 - Backscattered electron images of iron intermetallic particles. Letters D and E identify coarse and fine β-iron. Adapted from [34].
Figure 4 - Pitting corrosion near iron-rich intermetallics. Adapted from [36].
Figure 5 - Projection of Al-Si-Fe ternary system, showing the solidification path of alloys in the Fecrit range, and three alloys with 0.8% Fe. Adapted from [38].
Figure 6 - Typical rotary degassing unit. Adapted from [46].
Figure 7 - Effect of impeller rotation speed and gas flow rate on AISi5Cu3Mg casting alloy tensile strength. Adapted from [47].
Figure 8 - Degassing mechanism of UTS treatment. a) Diffusion of hydrogen into cavitation bubbles; b) Transportation currents. Adapted from [48].
Figure 9 - Comparison of gas volume in aluminum after three different degassing treatments. Adapted from [48].
Figure 10 - a) Fully unmodified eutectic structure in hypoeutectic aluminum alloy, 800x. b) Fibrous silicon eutectic after modification, 800x. Adapted from [29].
Figure 11 - Sketch of a surface entrainment event in aluminum alloy. Oxide film is represented in solid white. Adapted from [78].
Figure 12 - Entrainment-free filling of mold cavity. Adapted from [78].
Figure 13 - Temperature dependence of hydrogen solubility in aluminum at 1 atm pressure. Adapted from [29].
Figure 14 - Relation between pore number density Npore and oxide inclusion density NA(i). Adapted from [93].
Figure 15 - Relation between pore volume percentage fpore with oxide inclusion density NA(i). Adapted from [93].
Figure 16 - Eutectic silicon particle morphology evolution during solution treatment. a) As-cast. b) After 1h. c) After 8h. Adapted from [102].
Figure 17 - Effect of solution treatment time on ageing response of A356 alloy castings. Adapted from [101].
Figure 18 - Effect of solution HT time on elongation and UTS of A356 alloy. Adapted from [102].
Figure 19 - Effect of various solute additions on yield strength of high-purity binary aluminum alloys. Adapted from [104].
Figure 20 - Typical ageing response of alloy system to different ageing temperatures T3>T2>T1. Adapted from [108].
Figure 21 - Representation of a GP zone and its effect on the surrounding matrix planes. Dotted lines indicate the regions affected by the coherency strains. Adapted from [104].
Figure 22 - Wide PFZ observed in Al-4Zn-3Mg alloys. Adapted from [104].
Figure 23 - Hardening mechanisms associated with different particle sizes. a) Looping of dislocation line around precipitate particle (Orowan looping). b) Particle shearing. Adapted from [104, 105].
Figure 24 - Comparison of the effect of combined grain refinement (G), modification (M) and T6 heat treatment (HT) on the impact toughness of an A356 alloy casting. Adapted from [110].
Figure 25 - Melt comparison before (left) and after slag cleaning (right).
Figure 26 - Reduced Pressure Test setup.
Figure 27 - FDU-2091 Mini-Degasser unit.
Figure 28 - AMETREX SPECTROMAXx arc/spark optical emission spectrometer.
Figure 29 - Density measurement procedure for submerged weight (left) and dry weight (right).
Figure 30 - Photograph of sample 1V after preparation for porosity analysis.
Figure 31 – First area selected for analysis in sample 1V.
Figure 32 - Second area selected for analysis in Sample 1V.
Figure 33 - Cropped section from sample 1V before (left) and after pre-processing (right).
Figure 34 - Cropped section from sample 1V after thresholding and binarization.
Figure 35 - Results of particle identification on sample 1V.
Figure 36 - LPDC system used in the production of castings.
Figure 37 - Cast steering knuckle model.
Figure 38 - Pressure curve used for the casting operation [116].
Figure 39 - Solution treatment control piece.
Figure 40 - Automatic heat treatment oven.
Figure 41 - Casting layout in homogenizing oven basket.
Figure 42 – Forced convection oven used for artificial ageing.
Figure 43 - Casting layout for artificial ageing.
Figure 44 - DuraVision® 20 G5 semi-automatic hardness testing machine.
Figure 45 - ONA VAD35® wire Electrical Discharge Machine.
Figure 46 - Cut piece from casting 38, before slicing into test specimens.
Figure 47 - Instron 5900R® universal testing system.
Figure 48 - Close-up view of instrumented specimen before testing.
Figure 49 - Evolution of Density Index with increasing degassing treatment time.
Figure 50 - Pore area fraction comparison between treated parts with increasing treatment times.
Figure 51 - Comparison of porosity distribution between samples 10V and 12V.
Figure 52 - Effect of degassing treatment duration on average pore diameter.
Figure 53 - Effect of degassing treatment duration on minimum pore diameter.
Figure 54 - Effect of increasing degassing treatment times on the standard deviation of pore area.
Figure 55 - Effect of increasing degassing treatment times on the standard deviation of pore diameter.
Figure 56 - Effect of solution heat treatment on part hardness.
Figure 57 - Hardness comparison between all tested temperatures.
Figure 58 - Individual hardness curves for all testing temperatures.
Figure 59 - Microstructure of untreated sample, 100x magnification.
Figure 60 - Microstructure of solution heat-treated sample, 100x magnification.
Figure 61 - Microstructure of sample after ageing treatment, 100x magnification.
Figure 62 - Points Z1-Z6 of sample 1, observed through SEM-EDS, 5000x magnification.
Figure 63 - Point Z7 of sample 1, observed through SEM-EDS, 25000x magnification.
Figure 64 - Points Z1-Z3 of sample 2, observed through SEM-EDS, 10000x magnification.
Figure 65 - Average ultimate and yield stress values for different heat treatments.
Figure 66 - Average total strain values for different heat treatments.

Figure 4 - Pitting corrosion near iron-rich intermetallics. Adapted from [36].

Figure 5 - Projection of Al-Si-Fe ternary system, showing the solidification path of alloys in the Fecrit range, and three alloys with 0.8% Fe. Adapted from [38].

7. Conclusion:

The aim of this dissertation was to develop heat treatments for LPDC-produced aluminum parts, in order to maximize the final mechanical properties. The degassing process using a rotary degasser proved to be an adequate method of reducing the overall hydrogen content, achieving a pore area fraction of 0.229 % and a Density Index of 0.58 %. However, its effectiveness plateaued, suggesting it is not fully effective at removing bifilm defects. When comparing different ageing treatment configurations, it was found that shorter, higher temperature treatments produce better mechanical properties. A combination of solution heat treatment at 540°C for 4 hours, and artificial ageing at 180°C for 4 hours produced the best results, yielding an ultimate tensile strength of 333.70 MPa, yield strength of 276.90 MPa, and an elongation of 9.05 %, reaching the upper bounds defined by the standard for gravity die casting.

8. References:

[List the references exactly as cited in the paper, Do not translate, Do not omit parts of sentences.]

Expert Q&A: Your Top Questions Answered

Q1: Why was the solution heat treatment fixed at 540°C for 4 hours instead of being a variable in the study?

A1: The paper states this decision was made "due to the considerable time expenditure necessary to complete a full T6 treatment." By fixing the solution treatment parameters, the researchers could isolate the effects of the artificial aging stage, which was the primary focus. This approach allowed for a more controlled and detailed investigation of how different aging times and temperatures impact the final mechanical properties of the AlSi7Mg0.3 alloy.

Q2: The hardness results in Figure 57 showed a plateau at 170°C and 180°C, where hardness didn't decrease after reaching its peak. What is the explanation for this lack of over-aging?

A2: This was an interesting and unexpected observation in the study. The researchers noted this plateau and conducted further microstructural analysis using optical microscopy and SEM-EDS to find a cause. However, as stated in the paper, "Even after this analysis, no explanation was found for the plateau." This highlights an area for future research, as it deviates from the typical aging response curve where hardness is expected to decrease after prolonged time at higher temperatures.

Q3: How significant was the rotary degassing treatment on the final quality of the cast material?

A3: The degassing treatment was highly significant. The data from the Reduced Pressure Test (RPT) provides a clear picture. As seen in Figure 50, the pore area fraction in an untreated sample was 18.609%. After just 15 minutes of degassing, this was reduced to 0.544%, and after 25 minutes, it was down to 0.229%. This demonstrates a nearly complete elimination of gas porosity, which is a critical first step for producing sound castings capable of achieving high mechanical properties after heat treatment.

Q4: The study concludes that shorter, higher-temperature aging is better. How much of a difference did it actually make in the tensile tests?

A4: The difference was substantial. Comparing the two extremes in Table 19, the 12-hour treatment at 150°C resulted in a yield stress of 219 MPa and a UTS of 311 MPa. In contrast, the 4-hour treatment at 180°C achieved a yield stress of 277 MPa and a UTS of 334 MPa. This represents a 26% increase in yield strength and a 7% increase in ultimate strength, all while reducing the aging time by 67%.

Q5: The paper focuses on Low Pressure Die Casting (LPDC). How relevant are these findings for High Pressure Die Casting (HPDC)?

A5: While the manufacturing process is different, the findings on the heat treatment response of the AlSi7Mg0.3 alloy are fundamentally about the material's metallurgy. HPDC components typically have a finer microstructure due to much faster cooling rates, which can affect the kinetics of precipitation. However, the principles of how Mg₂Si precipitates form and grow during aging are the same. This research provides a valuable data point for HPDC engineers, suggesting that exploring shorter, higher-temperature aging profiles for T6-treatable alloys could be a fruitful area for process optimization and improvement.

Conclusion: Paving the Way for Higher Quality and Productivity

This comprehensive study effectively demonstrates that the full potential of cast aluminum components is unlocked not just by the casting process itself, but by a deep, data-driven optimization of post-casting treatments. The core problem of maximizing mechanical properties was addressed by first ensuring a high-quality, low-porosity melt and then systematically testing aging parameters. The key breakthrough—that shorter, higher-temperature aging cycles can yield superior strength—challenges conventional practices and opens a path to greater efficiency.

By implementing an optimized Aluminum Alloy Heat Treatment, manufacturers can produce stronger, more reliable components while simultaneously reducing cycle times and energy costs. 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 "Development of Heat Treatments for Components Produced by Low Pressure Die Casting in Aluminum Alloys" by "Ricardo Mil-Homens Jorge".
Source: Master Dissertation, University of Porto, July 2025.

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