Key Takeaways & Executive Findings
- •• Increasing mold wall thickness from 2 mm to 5 mm reduces filling speed from 25.41 m/s to 11.07 m/s, leading to wider and less distinct defect bands. • The ratio of defect band width to grain size (w/d) increases significantly to 24.47 for 5 mm thickness, compared to 7-18 for 2-4 mm, indicating a critical thickness effect. • Externally solidified crystals (ESCs) diffuse from center to defect band as wall thickness increases, maintaining high shear strength during filling and influencing defect band morphology. • The 5 mm-thick casting exhibits high porosity (5.25%), highlighting the detrimental effect of thick sections on casting quality.
Abstract
In order to investigate the effect of die wall thickness on morphologies of defect band, a stepped mold with a wall thickness of 5 mm, 4 mm, 3 mm, 2 mm, and 1 mm was designed to carry out high pressure die casting experiments with AlSi10MgMn alloy. For castings with wall thickness of 2-4 mm, the ratio of the mean defect band width (w) and mean grain size (d) in the defect band (w/d) ranges 7-18, while it increases to 24.47 for the 5 mm-thick casting. This difference is related with the filling speed and the distribution of externally solidified crystals (ESCs). The mold flow analysis indicates that the filling speed decreases from 25.41 m·s-1 to 11.07 m·s-1 when wall thickness increases from 2 mm to 5 mm. Due to the decreasing filling speed along the wall thickness, ESCs gradually diffuse from the center to the defect band, which keep the shear strength in the defect band at a high-level during filling. Meanwhile, the shear strength generated during the filling also decreases as the shear rate drops. Finally, the defect bands in the 5 mm-thick region become widen and indistinct, and the porosity is as high as 5.25%.
1. Introduction
The ultra-large integrated high pressure die casting (HPDC) technology equipped with large and complex molds is currently the first choice for achieving efficient near-net forming of complex lightweight aluminum alloy structural parts. It has good forming ability, high production efficiency, and good economic index, which is suitable for mass manufacturing of large-scale parts [1, 2]. HPDC contains different process parameters and the ultra-large integrated die inevitably contains different wall thicknesses. The microstructure of die castings must be influenced by uneven wall thickness and the relative study should have been and being conducted.
Defect band, as one of the representative features in HPDC castings, is often observed in outer contour of castings. Hou et al. [3] reported that macrosegregation and porosities exist in defect band, which is detrimental to the mechanical properties of castings. Dahle and StJohn [4] reported that the formation of defect band was closely related to the rheological and solidification behavior in the mushy zone. Li et al. [5] proposed that the shear stress developed during both filling and subsequent solidification processes was particularly important for the formation of defect band. Furthermore, Gourlay et al. [6] found that the formation of defect band was independent of ESCs, which had been confirmed by the experiments of Rodrigo and Ahuja [7]. But the presence and distribution of ESCs might influence mush rheology during filling and feeding to indirectly affect defect band formation. For example, Niu et al. [8] reported that ESCs could impede flow through the gates and thus influence filling and feeding. Gourlay et al. [9] found that the position of the defect band would move toward the die wall with increasing die temperature and external fraction solid fsESC. Jiao et al. [10] found that owing to the constraining relationship among the process parameters, complete avoidance of the ESCs in the microstructure of die castings seemed impossible. While, Laukli et al. [11] and Dinnis et al. [12] reported that the location and morphology of the defect band were influenced by the Si content, and the defect band width could be well reduced by decreasing Si content in Al-Si alloy. Yu et al. [13] systematically studied the influence of die-casting process on defect band width and proposed that the defect band width was strongly proportional with the size and quantities of ESCs. Jiao et al. [2] reported that increasing slow shot speed can make the defect band more obvious and closer to the center. While, there is still lack of the systematic research about the effect of wall thickness on the microstructure about HPDC castings, especially on the defect band.
Based on the above discussion, in order to study the effect of wall thickness on defect bands, the stepped castings were produced by a stepped mold with the wall thicknesses of 1 mm, 2 mm, 3 mm, 4 mm, and 5 mm. By microstructural characterization and mold flow analysis, the influence of ESCs distribution and the shear stress generated by the filling speed on defect band formation was elucidated. In addition, 3D characterization was conducted to analyze the porosity distribution in castings.
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Zhen-yu Sun, Wen-bo Yu, Jun-jie Li, Wei-chen Zheng, Guang-rui Wang, Jian-ru Fang, Shou-mei Xiong (2026). Influence of mold wall thickness on morphologies of defect band in high-pressure die casting technology. SinoTechIntel Verified Research. https://doi.org/10.1007/s41230-026-4124-6
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Frequently Asked Questions
What is the effect of mold wall thickness on defect band morphology in high-pressure die casting?
Increasing wall thickness from 2 mm to 5 mm reduces filling speed, causing defect bands to become wider and less distinct, with a higher w/d ratio and increased porosity.
How does filling speed change with wall thickness in HPDC?
Filling speed decreases from 25.41 m/s at 2 mm wall thickness to 11.07 m/s at 5 mm, as shown by mold flow analysis.
What role do externally solidified crystals (ESCs) play in defect band formation?
ESCs diffuse from the center to the defect band as wall thickness increases, maintaining high shear strength during filling, which influences defect band morphology.
What is the significance of the w/d ratio in this study?
The w/d ratio (defect band width to grain size) ranges from 7-18 for 2-4 mm wall thickness but increases to 24.47 for 5 mm, indicating a critical thickness effect on defect band severity.
What are the practical implications of this research for die casting industry?
The findings highlight the need to control wall thickness variations to minimize defect bands and porosity, improving the quality and mechanical properties of large aluminum alloy castings.
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