Numerical simulation of the effects of a Phase Change Material (PCM) on solidification path of gravity sand cast Al-Cu alloy

Extending Columnar Growth: How Phase Change Materials Are Revolutionizing Solidification Control in Al-Cu Castings

This technical summary is based on the academic paper "Numerical simulation of the effects of a Phase Change Material (PCM) on solidification path of gravity sand cast Al-Cu alloy" by Z. Noohi, B. Niroumand, and G. Timelli, published in La Metallurgia Italiana (2021).

Keywords

• Primary Keyword: Phase Change Material Casting
• Secondary Keywords: Solidification Control, Macrostructure Control, Columnar to Equiaxed Transition (CET), Al-Cu Alloy Casting, Gravity Sand Casting, Casting Simulation, Thermal Management in Casting

Executive Summary

• The Challenge: Controlling the grain structure during solidification is a persistent challenge in achieving desired mechanical properties in metal castings.
• The Method: Researchers incorporated a Phase Change Material (PCM)—pure zinc—into a chiller system for gravity sand casting of an Al-Cu alloy.
• The Key Breakthrough: The PCM absorbed latent heat during its phase change, altering the thermal gradient and delaying the Columnar to Equiaxed Transition (CET), resulting in a larger columnar grain zone.
• The Bottom Line: Using PCMs is a viable and innovative method for precisely controlling casting macrostructure, offering a new way to engineer component properties.

The Challenge: Why This Research Matters for HPDC Professionals

The final solidification structure of a cast component has a significant effect on its mechanical and physical properties. For decades, engineers have relied on methods like controlling cooling rates with chillers, inoculation, and directional solidification to manage the macrostructure. However, these traditional methods have limitations. Metal or graphite chillers, for example, absorb a specific amount of heat before becoming saturated, limiting their effectiveness over the entire solidification process. This research was driven by the need for a new, more dynamic method to control the solidification path and, consequently, the final grain structure of high-performance aluminium casting alloys.

The Approach: Unpacking the Methodology

The study employed a comparative approach, using both experimental casting and numerical simulation to evaluate the effect of a Phase Change Material on the solidification of an Al-Cu alloy.

Method 1: Experimental Casting
An Al-4.5wt.%Cu-0.2wt.%Fe alloy was melted and poured at 750 °C into two different sodium silicate bonded silica sand moulds.
- The Chill Sample: This setup used a conventional solid low carbon steel chiller (30×30×13 mm³) placed at the end of the casting cavity to promote directional solidification.
- The PCM Sample: This setup replaced the solid chiller with a 2 mm thick steel container (30×30×25 mm³) filled with commercially pure zinc, which acted as the PCM. The dimensions were chosen so that the initial cooling power was equivalent to the solid chiller.
K-type thermocouples were placed at 10, 35, and 60 mm from the chill surface in the casting, as well as within the chiller and PCM, to record real-time temperature-time (T-t) curves.

Method 2: Numerical Simulation
To complement the experimental work and gain deeper insight into thermal dynamics, the researchers used ProCast 2018 software. A tetrahedral mesh was created for the casting, chiller, PCM, container, and runner system to simulate heat transfer and solidification. The simulation results were then validated against the experimental thermocouple data.

The Breakthrough: Key Findings & Data

The incorporation of a zinc PCM into the chiller assembly produced distinct and measurable changes to the solidification process and final macrostructure.

Finding 1: Delayed Columnar to Equiaxed Transition (CET) and an Extended Columnar Zone

The most significant finding was the PCM's ability to alter the grain structure. The PCM sample exhibited a larger columnar grain zone compared to the standard Chill sample.
- As shown in Figure 3, the length of the columnar region was measured to be approximately 38 mm for the Chill sample and 41 mm for the PCM sample. This demonstrates that the PCM's latent heat absorption sustained the thermal gradient necessary for columnar growth for a longer period, delaying the transition to equiaxed grains.

Finding 2: Increased Cooling Rates During Solidification

While both setups were designed for similar initial chilling effects, the PCM created a more dynamic cooling environment once it began to melt.
- The PCM temperature reached its melting point (419.5 °C) after 11 seconds. The subsequent absorption of latent heat enhanced heat extraction from the casting.
- Simulation data confirmed this effect. At a distance of 10 mm from the chiller, the cooling rate in the mushy zone was 4.6 °C/s for the Chill sample but significantly higher at 7.7 °C/s for the PCM sample. This increased cooling rate after the initial phase is responsible for extending the directional solidification.

Practical Implications for R&D and Operations

• For Process Engineers: This study suggests that incorporating a PCM like zinc into a chiller assembly can be used to extend the columnar grain region. This provides a new tool to tailor macrostructures for components that may benefit from enhanced directional strength or other specific properties.
• For Quality Control Teams: The data in Figure 3 of the paper visually demonstrates the distinct macrostructures achieved. This could inform new visual inspection criteria for components where a specific columnar-to-equiaxed grain ratio is a critical quality attribute.
• For Design Engineers: The findings indicate that the thermal management system (chiller design) has a profound impact on the final grain structure. This suggests that integrating PCM chillers could be a valuable design consideration for performance-critical parts where controlling the solidification path is paramount.

Paper Details

Numerical simulation of the effects of a Phase Change Material (PCM) on solidification path of gravity sand cast Al-Cu alloy

1. Overview:

• Title: Numerical simulation of the effects of a Phase Change Material (PCM) on solidification path of gravity sand cast Al-Cu alloy
• Author: Z. Noohi, B. Niroumand, G. Timelli
• Year of publication: 2021
• Journal/academic society of publication: La Metallurgia Italiana - International Journal of the Italian Association for Metallurgy
• Keywords: MACROSTRUCTURE CONTROL, GRAVITY SAND CASTING, PHASE CHANGE MATERIALS (PCMS), COLUMNAR TO EQUIAXED TRANSITION (CET)

2. Abstract:

Solidification structure has a significant effect on mechanical and physical properties of metallic materials and its control is a main direction in the research for the improvement of materials performance. Various methods, such as control of cooling rate, inoculation, imposing vibration and pressure, are traditionally used to control the solidification structure during casting and solidification processes. In this paper, the preliminary results of a new method for controlling the solidification structure during casting using Phase Change Materials (PCMs) are presented. The evolution of the solidification structure of a directionally chilled Al-Cu alloy poured in a silica sand mould with and without the use of pure zinc as a PCM was examined using experimental and simulation methods. It was shown that the PCM temperature could reach about 510 °C during the solidification of the aluminium alloy, therefore, absorbing its melting latent heat from the solidifying aluminium alloy melt which affects its local solidification cooling rate. Therefore, the solidification structure of the sample cast in the PCM fitted mould differed from that of the sample without PCM. While macrostructures of both samples showed the transition from columnar to equiaxed grains, the columnar zone in the PCM sample was larger than in the sample without PCM. In other words, columnar to equiaxed transition (CET) for the sample without PCM occurred sooner than that for the sample with PCM. In addition, the average size of the equiaxed grains at the Chill sample is smaller than the PCM sample.

3. Introduction:

Aluminium casting alloys are widely used in automotive, sport, and aerospace industries due to properties such as high thermal and electrical conductivity, castability, weldability, light weight, and corrosion resistance. It is established that processing parameters like cooling rate, electromagnetic fields, and directional solidification affect the casting macro- and microstructure. The use of metal or graphite chillers is a common method to control the final structure and minimize shrinkage defects. The effectiveness of a chiller depends on its material type and dimensions, which dictate its heat capacity and diffusivity. This paper proposes a new method for controlling the solidification macrostructure by incorporating a Phase Change Material (PCM) into a metal chiller. PCMs absorb or release latent heat at a nearly constant temperature during phase transition and are commonly used in applications like buildings and solar systems. This study investigates the effect of a zinc PCM on the cooling and solidification macrostructure of a directionally solidified Al-Cu alloy.

4. Summary of the study:

Background of the research topic:

The control of solidification structure is a primary area of research for improving the performance of metallic materials, as this structure significantly influences mechanical and physical properties.

Status of previous research:

Traditional methods for controlling solidification structure include managing cooling rates, inoculation, applying vibration or pressure, and using chillers made of materials like copper, iron, or graphite. Phase Change Materials (PCMs) are known for their ability to store and release thermal energy at a constant temperature, but their application in controlling casting solidification is novel.

Purpose of the study:

The study aimed to present and evaluate a new method for controlling the solidification structure of a cast Al-Cu alloy by using a Phase Change Material (pure zinc) integrated into a chiller system during gravity sand casting.

Core study:

The research involved a comparative analysis of two scenarios: one using a standard solid steel chiller ('Chill sample') and another using a steel container filled with pure zinc as a PCM ('PCM sample'). The solidification of an Al-4.5wt.%Cu-0.2wt.%Fe alloy was examined using both experimental methods (temperature measurement, macrostructure analysis) and numerical simulation (ProCast 2018) to understand the effect of the PCM on the cooling rate, solidification path, and the Columnar to Equiaxed Transition (CET).

5. Research Methodology

Research Design:

The study was based on a direct comparison between two casting configurations. The first was a control group using a conventional solid steel chiller. The second was the experimental group, which used a PCM (pure zinc) encapsulated in a steel container of dimensions designed to provide an equivalent initial cooling power.

Data Collection and Analysis Methods:

• Temperature Measurement: K-type thermocouples were placed at specified locations within the casting, chiller, and PCM to record temperature-time (T-t) profiles during pouring and solidification using a data acquisition system.
• Macrostructure Analysis: Longitudinal sections of the cast samples were prepared by grinding and etching with Keller's etchant. The macrostructures were then examined to determine the length of the columnar and equiaxed zones. ImageJ software was used for measurements.
• Numerical Simulation: ProCast 2018 software was used to perform a numerical simulation of the casting process. Heat transfer coefficients and mass flow were defined to model the thermal behavior and validate the experimental T-t curves.

Research Topics and Scope:

The research focused on a directionally solidified Al-4.5wt.%Cu-0.2wt.%Fe alloy cast in a sodium silicate bonded silica sand mould. The study's scope was to investigate the influence of a pure zinc PCM on the solidification cooling rate and the resulting macrostructure, specifically the Columnar to Equiaxed Transition (CET).

6. Key Results:

Key Results:

• The PCM (pure zinc) reached its melting temperature of 419.5 °C and absorbed latent heat from the solidifying Al-Cu alloy, with its temperature ultimately reaching about 510 °C.
• The columnar zone in the PCM sample was larger (approx. 41 mm) than in the Chill sample (approx. 38 mm), indicating that the Columnar to Equiaxed Transition (CET) was delayed in the presence of the PCM.
• The average size of the equiaxed grains was smaller in the Chill sample (1.8 ± 1.3 mm) compared to the PCM sample (2.2 ± 1.8 mm).
• The ratio of the thermal gradient (G) to the interface velocity (R) at the time of CET formation was calculated to be about 14.9 °C·s/cm² for the Chill sample and 0.05 °C·s/cm² for the PCM sample, confirming that CET formation was easier in the Chill sample.
• Numerical simulations showed good agreement with experimental T-t curves. The simulations revealed that while initial cooling at the casting-chiller interface was similar for both samples (~28.6 °C/s), the cooling rate further into the casting (at x=10 mm) was higher for the PCM sample (7.7 °C/s) than the Chill sample (4.6 °C/s) after the PCM began to melt.

Figure Name List:

• Fig.1 - Schematics of the casting moulds: (a) Chill sample and (b) PCM sample.
• Fig.2 - (a) Experimental T-t curves for both PCM and Chill samples and (b) Location of three thermocouples at 10, 35, and 60 mm from the chiller surface in both PCM and Chill samples, one thermocouple in the chiller (Chill sample) and two thermocouples, i.e. Zn1 and Zn2, in the PCM (PCM sample).
• Fig.3 - Macrostructure of (a) Chill and (b) PCM samples.
• Fig.4 - Experimental and numerical cooling curves for (a) Chill and (b) PCM samples.

7. Conclusion:

This paper introduced a novel method to control the solidification macrostructure of an Al-Cu alloy using a Phase Change Material (PCM). Experimental and simulation results demonstrated that incorporating a zinc PCM into a metal chiller affects the cooling and solidification conditions. The study observed a Columnar to Equiaxed Transition (CET) in both samples, but it occurred later in the sample with the zinc PCM. This effect is attributed to the absorption of the PCM's latent heat of melting during the early stages of solidification, as well as the different thermophysical properties of the PCM compared to the solid chiller. The results indicate that the proposed method can be used as an innovative cooling system to control the solidification macrostructure of castings.

8. References:

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Expert Q&A: Your Top Questions Answered

Q1: Why was pure zinc chosen as the Phase Change Material for this study?

A1: The paper specifies pure zinc was used as the PCM. Its melting point of 419.5 °C is within a suitable thermal range to interact with the solidifying Al-Cu alloy, which has a liquidus temperature above 644 °C. As shown in Figure 2, the PCM temperature reached over 500 °C, confirming it fully melted and absorbed its latent heat, which was the primary mechanism being studied.

Q2: What was the purpose of the 2 mm thick steel container for the PCM?

A2: The steel container served to encapsulate the pure zinc. The paper notes that this design influenced the initial heat transfer. At the beginning of the pour, an air gap between the solid zinc PCM and the container wall prevented localized, intense heat transfer. This allowed the container wall to heat up more uniformly, resulting in columnar grains that grew uniformly perpendicular to the chill surface, unlike the Chill sample where growth initiated from a corner.

Q3: The paper mentions the G/R ratio. How did it differ between the two samples and why is it important?

A3: The G/R ratio, which compares the thermal gradient (G) at the solid-liquid interface to the interface velocity (R), is critical for predicting the Columnar to Equiaxed Transition (CET). The paper states that at the time of CET, the G/R ratio was about 14.9 °C·s/cm² for the Chill sample and only 0.05 °C·s/cm² for the PCM sample. A higher G/R value, as seen in the Chill sample, makes CET easier to occur. The PCM's ability to maintain a steeper thermal gradient for longer resulted in a lower G/R value and delayed the CET.

Q4: Did the initial chilling power differ between the two setups?

A4: No, the two setups were intentionally designed to have a similar chilling effect at the beginning of solidification. The paper states, "the dimensions of the chiller and PCM were correctly designed, i.e. they provide a similar chilling effect at the beginning." This is confirmed by simulation results, which show the cooling rate at the direct chiller/casting interface (x=0 mm) was about 28.6 °C/s for both samples. The differences in cooling rate emerged further into the casting as the PCM began to melt.

Q5: What caused the difference in grain growth orientation between the two samples shown in Figure 3?

A5: In the Chill sample, the molten metal first contacted the lower corner of the solid steel chiller, causing intense, localized heat transfer that initiated nucleation and growth from that point. In the PCM sample, the PCM was encapsulated in a steel container. The paper suggests an initial air gap prevented direct, intense contact, allowing the container wall to heat up more evenly. This led to a more uniform heat transfer front and resulted in columnar grains growing uniformly perpendicular to the entire chill face.

Conclusion: Paving the Way for Higher Quality and Productivity

The challenge of precisely controlling solidification macrostructure is central to producing high-quality castings. This research demonstrates a significant breakthrough by applying Phase Change Material Casting technology to an Al-Cu alloy. The key finding—that a zinc PCM can absorb latent heat to sustain directional solidification and extend the columnar grain zone—provides engineers with a powerful new tool. This innovative approach offers a more dynamic thermal management system than traditional static chillers.

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 "Numerical simulation of the effects of a Phase Change Material (PCM) on solidification path of gravity sand cast Al-Cu alloy" by "Z. Noohi, B. Niroumand, G. Timelli".

Source: The paper was published in La Metallurgia Italiana - November/December 2021, pages 25-30.

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