Influence of Casting Distance on Microstructure of Simultaneously Mixed Controlled Diffusion Solidified Al-Si Alloys

This introductory paper is the research content of the paper ["Influence of Casting Distance on Microstructure of Simultaneously Mixed Controlled Diffusion Solidified Al-Si Alloys"] published by [特种铸造及有色合金/Special Casting & Nonferrous Alloys].

1. Overview:

• Title: Influence of Casting Distance on Microstructure of Simultaneously Mixed Controlled Diffusion Solidified Al-Si Alloys / 浇注距离对同时混合受控扩散凝固Al-Si合金组织影响
• Author: LI Shaosi, CHEN Tijun / 李绍斯, 陈体军
• Publication Year: Feb. 2025
• Publishing Journal/Academic Society: Special Casting & Nonferrous Alloys / 特种铸造及有色合金
• Keywords: Controlled Diffusion Solidification (CDS), Al-Si Alloy, Casting Distance, Nucleation Rate, Primary α-Al / 受控扩散凝固(CDS); Al-Si合金;浇注距离;形核率;初生a-Al

2. Abstracts / Introduction

The target alloy Al-8Si was obtained by controlled diffusion (CDS) using pure Al as precursor alloy 1 and Al-12Si as precursor alloy 2 at a certain mass ratio and temperature, and the flow field, temperature field, solute field and nucleation rate of the melt, as well as the experimental grain size and shape of the primary α-Al were analyzed by combination of numerical simulation and experimental verification. The results reveal that the casting distance affects the mixing effect of both master alloys, and when the casting distance is excessive or insufficient, the solute field and temperature field of the mixed melt are heterogeneous with lower nucleation rate, and the primary α-Al presents coarse and irregular grains. When the casting distance is 80 mm, the mixing effect is desirable, and temperature field and solute field are the most uniform with the highest nucleation rate, where the average grain size and shape factor of the primary α-Al are 57.6 µm and 1.55, respectively.

以纯A1为母合金1, Al-12Si为母合金2, 在一定的质量比和温度下, 采用受控扩散凝固(CDS)的方法得到目标合金Al-8Si。采用数值模拟和试验验证相结合的方法对混合熔体的流场、温度场、溶质场和形核率以及试验所得铸件的初生α-Al相尺寸、形状进行分析。结果表明, 浇注距离对两种母合金混合效果产生影响, 在浇注距离过小或过大时, 混合熔体的溶质场和温度场都不均匀, 形核率较低, 试验所得铸件的初生α-Al晶粒粗大且不规则。浇注距离为80 mm时, 混合效果最好, 温度场、溶质场最均匀且形核率最高, 初生α-Al相的晶粒尺寸和形状因子分别为57.6 µm和1.55。

3. Research Background:

Background of the Research Topic:

Aluminum alloys are widely used due to their high strength-to-weight ratio, low cost, abundant resources, good thermal conductivity, corrosion resistance, and ease of processing. In traditional casting, the large crystallization temperature range of aluminum alloys leads to coarse dendritic structures. Dendritic solidification can cause macro-segregation, porosity, and micro-cracks, severely impacting mechanical properties. Controlled Diffusion Solidification (CDS) is an effective method to obtain excellent casting properties by achieving non-dendritic (globular) structures.

铝合金具有较高的强质比，被归为轻金属，并且其具有成本低廉、资源丰富、导热性能好、耐腐蚀、容易加工等特点,应用前景广阔。在传统铸造中, 由于铝合金凝固过程的结晶温度区间大, 导致微观组织为粗大的树枝晶。研究发现, 铸件中枝晶状组织凝固方式除了会产生大量缩松、缩孔外, 还会导致成分不均匀, 内部形成大量气孔和显微裂纹, 从而严重影响铸件的力学性能。

Status of Existing Research:

Existing methods to obtain non-dendritic structures include adding grain refiners, electromagnetic vibration, and mechanical stirring during casting. However, these methods have limitations such as introducing impurities, high cost, and complex processes. While traditional CDS techniques offer advantages, they can still produce abnormal grains and uneven grain size.

研究者通过在浇注过程中添加晶粒细化剂、电磁振动、机械搅拌等方法来获得非枝晶浆料, 其中, 添加晶粒细化剂可能会引入杂质并且晶粒细化剂成本较高, 电磁振动和机械搅拌均需要外加其他设备, 成本高且工序复杂, 在工业应用上存在一定的局限性。传统的受控扩散凝固技术存在诸多限制, 即使完全满足要求的合金, 仍然会产生异常晶粒, 晶粒尺寸粗大且不匀整。

Necessity of the Research:

There is limited research on the effect of casting distance in CDS processes. To optimize CDS parameters and improve the mechanical properties and formability of Al-Si alloys, this study investigates the influence of casting distance on the controlled diffusion mixing effect, focusing on grain nucleation, growth, morphology, and formation mechanisms. This aims to provide a reference for the application of controlled diffusion solidification technology in non-ferrous alloys.

目前, 国内对CDS浇注距离的研究报道很少, 为了进一步完善同时混合CDS工艺参数, 提高Al-Si合金的力学性能和成形性能, 本研究将针对母合金不同浇注距离对受控扩散混合效果的影响进行分析, 并对晶粒的形核、生长方式和形貌等形成机制进行研究, 以期为受控扩散凝固技术在有色合金中的应用提供参考。

4. Research Purpose and Research Questions:

Research Purpose:

The purpose of this research is to investigate the influence of casting distance on the mixing effect of simultaneously mixed CDS for Al-Si alloys. This includes analyzing the temperature field, solute field, flow field, and nucleation rate during solidification, and correlating these with the microstructure, grain size, and shape of primary α-Al.

本研究将针对母合金不同浇注距离对受控扩散混合效果的影响进行分析, 并对晶粒的形核、生长方式和形貌等形成机制进行研究。

Key Research:

The key research questions are:

1. How does casting distance affect the temperature field, solute field, and flow field of the mixed melt in simultaneously mixed CDS?
2. How does casting distance influence the nucleation rate and solidification process in CDS?
3. What is the optimal casting distance to achieve a uniform microstructure and refined grain size in CDS Al-Si alloys?
4. How does casting distance affect the grain size and shape of primary α-Al phase in CDS Al-Si alloys?

本研究将针对母合金不同浇注距离对受控扩散混合效果的影响进行分析, 并对晶粒的形核、生长方式和形貌等形成机制进行研究, 以期为受控扩散凝固技术在有色合金中的应用提供参考。

5. Research Methodology

Research Design:

The research employed a combination of numerical simulation and experimental verification. Fluent software was used to simulate the CDS process of pure Al (alloy 1) and Al-12Si (alloy 2) to obtain Al-8Si alloy. Simulations varied casting distances (60mm, 80mm, 100mm) to analyze temperature, solute, and flow fields. Experiments were conducted with casting distances of 60, 70, 80, 90, and 100mm to validate simulation results and analyze microstructure.

利用 Fluent 软件对纯A1(合金1)与Al-12Si(合金2)混合得到目标合金Al-8Si的过程进行模拟, CDS流程示意图见图1。模拟过程中涉及的连续性方程(质量守恒方程)、Navier-Stokes方程(动量守恒方程)和能量守恒方程以及其他相关方程和热物性参数与前期研究相同。将试验的浇注距离分为5组, 分别为60、70、80、90和100mm并依次进行验证, 试验中坩埚的尺寸参数和CDS的工艺参数与模拟中的参数完全一致。

Data Collection Method:

Numerical simulation data included temperature field, solute field, flow field, and nucleation rate. Experimental data included microstructure images obtained by optical microscopy (OM) and grain size and shape factor measurements using Image-Pro-Plus 6.0 software.

利用 Fluent 数值模拟观察中间坩埚内混合熔体的流场、温度场、溶质场的分布以及预测随后凝固时的形核率, 结合试验验证以得到最佳混合参数。采用Axio Scope Al光学显微镜(OM)观察铸件横截面中心附近的显微组织。利用 Image-Pro-Plus 6.0软件计算初生α-Al的平均晶粒尺寸和形状因子。

Analysis Method:

Simulation results were analyzed to determine the effect of casting distance on melt mixing characteristics and nucleation rate. Experimental microstructures were analyzed to assess grain morphology and size. Grain size (D) and shape factor (F) were calculated using equations: D=2√(A₁/π), F = P² / (4πA₁).

利用 Fluent 数值模拟观察中间坩埚内混合熔体的流场、温度场、溶质场的分布以及预测随后凝固时的形核率。利用 Image-Pro-Plus 6.0软件计算初生α-Al的平均晶粒尺寸和形状因子, 评定判据如下: D=2√A₁/π, F = P² / (4πA₁),其中D为晶粒平均直径;A为晶粒截面积;P为晶粒边界周长;F为晶粒形状因子。

Research Subjects and Scope:

The research focused on Al-Si alloys, specifically targeting Al-8Si alloy produced by CDS of pure Al and Al-12Si. The casting distance was the primary variable, with distances of 60mm, 70mm, 80mm, 90mm, and 100mm investigated.

以纯A1(合金1)与Al-12Si(合金2)混合得到目标合金Al-8Si的过程进行模拟,CDS流程示意图见图1。将试验的浇注距离分为5组, 分别为60、70、80、90和100mm并依次进行验证。

6. Main Research Results:

Key Research Results:

• Casting distance of 80mm provides optimal mixing: Simulation results showed that a casting distance of 80mm resulted in the most uniform temperature and solute fields, and the highest nucleation rate.
• Microstructure refinement at 80mm: Experimentally, the microstructure at 80mm casting distance exhibited the finest and most equiaxed primary α-Al grains.
• Grain size and shape factor: At 80mm casting distance, the average grain size was 57.6 µm and the shape factor was 1.55, indicating refined and more globular grains compared to other distances.
• Influence of casting distance on fields: Too short or too long casting distances led to heterogeneous temperature and solute fields, lower nucleation rates, and coarser, irregular grains.

结果表明,浇注距离对两种母合金混合效果产生影响,在浇注距离过小或过大时,混合熔体的溶质场和温度场都不均匀,形核率较低,试验所得铸件的初生α-Al晶粒粗大且不规则。浇注距离为80mm时,混合效果最好,温度场、溶质场最均匀且形核率最高,初生α-Al相的晶粒尺寸和形状因子分别为57.6 µm和1.55。浇注距离为80mm时,形核的区域最多,其中深色区域为形核区域,灰色区域为液相区域(见图5b)。浇注距离为80mm时,晶粒的尺寸最细小、圆整,其平均晶粒尺寸为57.6 µm和1.55。

Analysis of presented data:

• Temperature and Solute Fields (Fig. 2 & 3): At 80mm, temperature and solute fields are most uniform. At 60mm and 100mm, heterogeneity is observed, indicating poor mixing.
• Flow Fields (Fig. 4): At 80mm, melt flow is evenly distributed, promoting mixing. At 60mm and 100mm, flow is biased, leading to segregation.
• Nucleation Rate (Fig. 5): Nucleation area is maximized at 80mm, correlating with uniform fields and flow.
• Microstructure (Fig. 6): Grain size is minimized and shape factor is closest to 1 (more globular) at 80mm, confirming optimal microstructure. Grain size and shape factor at 80mm are significantly better than at 60mm and 100mm.

图2为不同浇注距离的混合熔体半高截面处的温度场, 图3 不同浇注距离的混合熔体半高截面处的溶质场, 图4 不同浇注距离的混合熔体半高截面上的流场, 图5 不同浇注距离对应的凝固区域, 图6 不同浇注距离下CDS初生a-A1的微观形貌及对应的晶粒尺寸和形状因子. 浇注距离为80mm时,温度场、溶质场最均匀且形核率最高,初生α-Al相的晶粒尺寸最细小、圆整。

Figure Name List:

• Fig.1 Schematic diagram of CDS process / 图1 CDS流程示意图
• Fig.2 Temperature fields in the cross section at half height of mixtures with different casting distances / 图2 不同浇注距离的混合熔体半高截面处的温度场
• Fig.3 Concentration fields in the cross section at half height of mixtures with different casting distances / 图3 不同浇注距离的混合熔体半高截面处的溶质场
• Fig.4 Flow fields in the cross section at half height of the mixtures with different casting distances / 图4 不同浇注距离的混合熔体半高截面上的流场
• Fig.5 Solidified regions corresponding to different casting distances / 图5 不同浇注距离对应的凝固区域
• Fig.6 Micromorphologies of CDS primary α-Al with different casting distances as well as the corresponding grain size and shape factor / 图6 不同浇注距离下CDS初生a-A1的微观形貌及对应的晶粒尺寸和形状因子

7. Conclusion:

Summary of Key Findings:

The study demonstrated that casting distance significantly affects the microstructure of simultaneously mixed CDS Al-Si alloys. A casting distance of 80mm was identified as optimal, resulting in uniform temperature and solute fields, high nucleation rate, and refined, globular primary α-Al grains. Deviations from this distance, either shorter or longer, led to heterogeneous fields, lower nucleation rates, and coarser, irregular grains.

从同时混合受控扩散凝固模拟结果可得, 当浇注距离为80mm时, 其温度场和溶质场最均匀, 此时微溶池的数量最多且尺寸最小, 说明此时混合效果最好。从形核率可以看出, 形核区域在浇注距离为80mm时最大, 即形核率最高。观察不同参数下的铸件微观组织形貌, 当浇注距离为80mm时, 晶粒尺寸最细小、匀整。

Academic Significance of the Study:

This research provides valuable insights into the optimization of CDS parameters, specifically casting distance, for Al-Si alloys. It elucidates the relationship between casting distance, melt mixing, solidification behavior, and microstructure evolution in simultaneously mixed CDS. The findings contribute to a deeper understanding of the CDS process and its potential for producing high-performance aluminum alloy castings.

本研究还发现, 在最佳的浇注距离下, 初生a-Al进一步细小圆整, 从而得到理想的半固态非枝晶浆料和铸锭达到提高铸件性能的目的。可见, 本研究提出的同时混合CDS可以克服传统CDS的缺点。

Practical Implications:

The findings suggest that controlling casting distance is crucial in simultaneously mixed CDS to achieve desired microstructure and properties in Al-Si alloys. A casting distance of 80mm can be used as a reference for producing Al-8Si alloys with refined microstructures using this CDS method. This can lead to improved mechanical properties and formability of Al-Si alloy castings, benefiting industries utilizing these materials.

浇注距离为80mm时, 混合效果最好, 温度场、溶质场最均匀且形核率最高, 初生α-Al相的晶粒尺寸和形状因子分别为57.6 µm和1.55。这意味着所提出的同时混合CDS由于良好的共混效果而避免了传统CDS的这一缺点。

Limitations of the Study and Areas for Future Research:

The study focused on a specific Al-Si alloy composition (Al-8Si) and a limited range of casting distances. Future research could explore the effect of casting distance on other Al-Si alloy compositions and a wider range of casting parameters, including pouring temperature and mixing speed. Further investigation into the solidification mechanisms and microstructure evolution at different casting distances would also be beneficial.

研究对象为Al-Si合金。未来研究可以探索浇注距离对其他铝硅合金成分和更广泛的浇注参数范围的影响，包括浇注温度和混合速度。进一步研究不同浇注距离下的凝固机制和微观组织演变也将是有益的。

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9. Copyright:

• This material is "[LI Shaosi, CHEN Tijun]"'s paper: Based on "[Influence of Casting Distance on Microstructure of Simultaneously Mixed Controlled Diffusion Solidified Al-Si Alloys]".
• Paper Source: DOI: 10.15980/j.tzzz.T20240050

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