Evaluation of detrimental effect on the ductility caused by the inhomogeneous skin and casting defects in a high pressure die cast recycled secondary alloy

This introduction paper is based on the paper "[Evaluation of detrimental effect on the ductility caused by the inhomogeneous skin and casting defects in a high pressure die cast recycled secondary alloy]" published by "[Materials Characterization]".

Fig. 1. (a) Diagram of HPDC configuration showing different parts, (b) Top view of an actual cast part, and (c) Side view of an actual cast part showing the steps with wall thickness of 1-, 2-, 4-, 6-, 10- and 15-mm. (Adapted with permission from Dalai et al. [19]).
Fig. 1. (a) Diagram of HPDC configuration showing different parts, (b) Top view of an actual cast part, and (c) Side view of an actual cast part showing the steps with wall thickness of 1-, 2-, 4-, 6-, 10- and 15-mm. (Adapted with permission from Dalai et al. [19]).

1. Overview:

  • Title: Evaluation of detrimental effect on the ductility caused by the inhomogeneous skin and casting defects in a high pressure die cast recycled secondary alloy
  • Author: Biswajit Dalai, Simon Jonsson, Manel da Silva, Fredrik Forsberg, Liang Yu, Jörgen Kajberg
  • Year of publication: 2025
  • Journal/academic society of publication: Materials Characterization
  • Keywords: Secondary alloy, AlSi10MnMg(Fe) alloy, High pressure die casting, Ductility, Inhomogeneous skin, Porosity, Cold flake

2. Abstract:

The usage of recycled alloys in the high pressure die casting (HPDC) applications for automobiles is gaining rapid interest. Even though the skin microstructure, which is typically induced on the casting surface during the HPDC process, is believed to improve the properties of the HPDC castings, it may not always form continuously throughout on the casting surface, and thereby can influence the mechanical properties. Thus, the current study evaluated and compared the effects of inhomogeneously formed surface skin with that of other defects on the ductility exhibited by the HPDC castings of a recycled secondary AlSi10MnMg(Fe) alloy. The formation of inhomogeneous skin in the current study was attributed to a phenomenon related to the “waves and lakes” type of defects created by the HPDC process. Such skin structure limited the ductility of the HPDC castings, irrespective of the tested strain rates in the current case, by undergoing abrupt fracture due to its poor bonding with the adjoining matrix resulting from the aforementioned inhomogeneity. Even if the investigated AlSi10MnMg (Fe) alloy contained an abundance of porosity, cold flakes and intermetallics, which are usually considered the driving factors behind the fracture of HPDC processed alloys, the effect from the inhomogeneous skin layer dominated all other factors in the current case. The order of detrimental effect on the ductility of HPDC processed AlSi10MnMg(Fe) alloy followed a sequence of inhomogeneous skin, cold flakes and pores, with the inhomogeneity in skin turning out to be the most harmful one.

3. Introduction:

The automotive industry's focus on cost-effectiveness and safety has led to increased use of high pressure die casting (HPDC) for structural parts due to its geometrical accuracy, reduced cycle time, and ability to produce thin-walled castings. Material development has progressed towards primary AlSi10MnMg alloys with controlled Fe and Mn content to suppress brittle β-Al5FeSi (β-Fe) compounds and promote less harmful α-intermetallic compounds (α-Fe), improving mechanical properties [5-7, 9]. Recently, sustainability concerns have driven interest in recycled secondary AlSi10MnMg(Fe) alloys. These typically have higher Fe content due to scrap recycling [4, 7], posing a risk of β-Fe formation. Research aims to optimize secondary alloy compositions, particularly the Mn:Fe ratio, to achieve properties comparable to primary alloys [11, 14-17]. A "skin" layer, a fine-grained α-Al phase formed by rapid solidification at the die surface [20], is often associated with HPDC. While generally considered beneficial, literature suggests this skin can be inhomogeneous [21-23], a factor potentially overlooked in previous studies [20, 24-27]. This raises questions about the influence of inhomogeneous skin compared to other common HPDC defects like porosity (gas and shrinkage pores) [26-29] and cold flakes [32-34], which are known to limit ductility. Existing literature details the negative impact of these defects, but studies evaluating and comparing their relative influence, especially concerning inhomogeneous skin formation mechanisms and effects in secondary alloys, particularly under high strain rates relevant to crash scenarios, are limited.

4. Summary of the study:

Background of the research topic:

High pressure die casting (HPDC) is increasingly used for automotive structural components. There is a growing trend towards using recycled secondary Al-Si-Mn-Mg alloys (like AlSi10MnMg(Fe)) for sustainability, replacing primary alloys. However, secondary alloys often have higher Fe content, potentially forming detrimental phases, and their properties can be affected by casting defects, including porosity, cold flakes, and the surface skin layer. The formation and effect of an inhomogeneous skin layer are not fully understood compared to other defects.

Status of previous research:

Previous research established the benefits of primary AlSi10MnMg alloys with controlled Fe/Mn [6, 7, 9]. Studies on secondary AlSi10MnMg(Fe) alloys explored optimizing Mn:Fe ratios to avoid brittle β-Fe phases [11, 14-17]. The detrimental effects of porosity [26, 27, 29-31] and cold flakes [32-34] on HPDC alloy ductility are known. Some studies noted skin layer inhomogeneity [21-23], and the authors' prior work [19] indicated that an inhomogeneous skin limited the ductility of the studied secondary alloy. However, comparative studies ranking the detrimental effects of these different features (inhomogeneous skin, porosity, cold flakes) are lacking, as is a detailed understanding of the inhomogeneous skin formation mechanism and its impact, especially under varied strain rates.

Purpose of the study:

The aim of the current work is to investigate the evolution and effect of microstructure and casting defects on the ductility and fracture behavior of an HPDC processed recycled secondary AlSi10MnMg(Fe) alloy with varied casting thickness (2 mm to 10 mm). The study initially focused on porosity as a potentially dominant factor but expanded to evaluate and compare the influence of inhomogeneous skin, cold flakes, and pores. Additionally, the effect of strain rate (0.001 s⁻¹ to 10 s⁻¹) on the tensile properties and fracture behavior of the secondary alloy was examined.

Core study:

The study involved producing HPDC step castings of a recycled secondary AlSi10MnMg(Fe) alloy with varying wall thicknesses (1 mm to 15 mm). The microstructure of as-cast samples (2, 4, 6, 10 mm sections) was characterized using optical microscopy (OM) to identify phases (α-Al, Al-Si eutectic, α-Fe intermetallics) and features like the skin layer and casting defects (cold flakes, cold shots). Porosity in 2, 6, and 10 mm thick sections was quantified using X-ray microtomography (XMT). Uniaxial tensile tests were performed on specimens machined from these sections under different strain rates (0.001, 0.1, 10 s⁻¹) to evaluate mechanical properties (stress-strain behavior, ductility). Fracture surfaces and lateral surfaces of deformed specimens were analyzed using scanning electron microscopy (SEM) and OM, combined with high-speed camera imaging during tests, to identify crack initiation sites and fracture mechanisms. The study correlated the observed mechanical behavior and fracture modes with the specific microstructural features and defects present, ultimately ranking the detrimental effect of inhomogeneous skin, cold flakes, and pores on the alloy's ductility.

5. Research Methodology

Research Design:

The study employed an experimental approach. HPDC castings with varying step thicknesses were produced from a recycled secondary AlSi10MnMg(Fe) alloy. Material characterization techniques (OM, XMT, SEM) were used to analyze the microstructure, porosity, and defects. Mechanical properties were evaluated through uniaxial tensile testing under quasi-static and dynamic strain rates. Fracture analysis was conducted to correlate microstructural features and defects with mechanical behavior and failure modes. The research compared results across different casting thicknesses and strain rates.

Data Collection and Analysis Methods:

  • Material: Recycled secondary AlSi10MnMg(Fe) alloy (composition in Table 1).
  • Casting: HPDC process using a Buhler cold chamber machine (details in Sec 2.2, Fig. 1), producing step castings (1-15 mm thickness). Vacuum assist (VDS) was used.
  • Microstructure Analysis: OM on polished cross-sections (unetched) from 2, 4, 6, 10 mm steps (locations in Fig. 2).
  • Porosity Analysis: XMT scans on tensile specimens from 2, 6, 10 mm steps using Zeiss Xradia 620 Versa (details in Sec 2.4, Table 2). Data reconstructed and analyzed using Zeiss software and Dragonfly Pro for visualization, pore segmentation (Otsu thresholding), number density, volume fraction, and size distribution.
  • Mechanical Testing: Uniaxial tensile tests on specimens (geometry Fig. 3) using Instron 1272 (0.001, 0.1 s⁻¹) and Instron VHS (10 s⁻¹) machines (details in Sec 2.5, Table 3). High-speed camera (Phantom V2512) used for crack initiation observation at 0.001 s⁻¹.
  • Fracture Analysis: SEM (FEI Magellan 400) observation of fracture surfaces. OM observation of polished lateral surfaces adjacent to the fracture.

Research Topics and Scope:

  • Alloy System: Recycled secondary AlSi10MnMg(Fe) HPDC alloy.
  • Variables: Casting wall thickness (2, 4, 6, 10 mm analyzed), strain rate (0.001, 0.1, 10 s⁻¹).
  • Characterization: Microstructure evolution (α-Al grain size, α-Fe intermetallic types/morphology, Al-Si eutectic), skin layer formation (presence, thickness, homogeneity), casting defects (porosity distribution/size/type, cold flakes, cold shots).
  • Properties: Tensile stress-strain behavior, ultimate tensile strength (UTS), total elongation (TE) / ductility.
  • Analysis: Correlation between microstructure/defects and tensile properties/fracture behavior. Identification of crack initiation sites and failure mechanisms. Comparison of the detrimental effects of inhomogeneous skin, cold flakes, and porosity on ductility.

6. Key Results:

Key Results:

  • The studied secondary AlSi10MnMg(Fe) alloy (Mn:Fe ratio 2.1) successfully avoided the formation of brittle β-Fe phases, instead forming α-Fe intermetallics (Fig. 4).
  • A fine-grained skin layer was formed only on the surfaces of the 2 mm (30-150 µm thick) and 4 mm (20-90 µm thick) castings, but not on the 6 mm and 10 mm castings (Fig. 5).
  • Where present, the skin layer was significantly inhomogeneous and discontinuous, sometimes appearing to migrate into the casting thickness before terminating abruptly (Fig. 6). This formation was linked to complex melt flow and localized cooling rate variations during HPDC, resembling "waves and lakes" defect formation (Fig. 21).
  • Common casting defects like cold flakes and cold shots were observed, increasing in size and frequency with casting thickness (Figs. 7, 8).
  • Porosity (gas and shrinkage pores) was present in all castings. The amount (number density and volume fraction) was similar in 2 mm and 6 mm castings (approx. 21-22 pores/mm³, 0.1% vol fraction) but increased significantly in the 10 mm casting (52 pores/mm³, 0.8% vol fraction), primarily due to larger shrinkage pores (Figs. 13, 14, 15).
  • Tensile tests showed large variations in ductility (TE ranging from 1.6% to 6.8% for 2mm samples), irrespective of strain rate (Fig. 9). Flow stress slightly increased with strain rate.
  • Ductility varied with thickness (tested at 0.001 s⁻¹): 1.5% for 2 mm, 2.9% for 6 mm, and 3.6% for 10 mm (Fig. 16). All samples failed before significant necking.
  • Fracture analysis revealed that the inhomogeneous skin layer acted as the primary crack initiation site in many samples (B, C, E, F, H, I) due to poor bonding with the underlying matrix, leading to delamination and abrupt fracture at low elongations (Figs. 10, 11, 12, 18, 22).
  • Cold flakes caused abrupt fracture when their flat boundary (weak interface) was oriented at approximately 30° or more to the loading axis (Figs. 17b, 19). Their effect was negligible when oriented parallel to the load (Fig. 10i).
  • Large shrinkage pores were identified as the fracture initiation site in the 10 mm thick sample (Fig. 17c).
  • Cracking of α-Fe intermetallics and Al-Si eutectics occurred during deformation in all samples but did not appear to be the primary factor controlling the large variations in overall ductility (Fig. 20).
  • The order of detrimental effect on the ductility of this HPDC secondary alloy was determined to be: Inhomogeneous skin (most harmful) > Cold flakes (effect dependent on orientation) > Porosity (Table 4).
Fig. 2. Graphical representation of an as-cast part of 2 mm step thickness, with green boxes indicating the locations used for analysis using optical microscopy. The pink arrows indicate the two casting surfaces on which the skin layer is supposedly formed. (Adapted with permission from Dalai et al. [19]). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 20. SEM images on the fractured surfaces of tensile tested samples displaying: (a) matrix of the fractured surface with yellow and blue arrows indicating α-Al phase and Al–Si eutectics, respectively, (b) Al–Si eutectics, (c) fractured Si particle within Al solid solution (adapted with permission from Dalai et al. [19]), (d) cracked Chinese script α-Fe compound, (e) cracked (α-Fe)I and (α-Fe)II compounds, and (f) uncracked (α-Fe)I and (α-Fe)II compounds. The red arrows in (b – e) indicate cracks. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 15. (a) Normalized distribution of pores size, in terms of equivalent diameter, for different casting thickness. The green, blue and red arrows indicate the largest equivalent diameter of pores found in 2-, 6- and 10-mm thick castings, respectively. (b) is the magnified profile of the porosity distribution indicated by black dashed rectangle in (a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 15. (a) Normalized distribution of pores size, in terms of equivalent diameter, for different casting thickness. The green, blue and red arrows indicate the largest equivalent diameter of pores found in 2-, 6- and 10-mm thick castings, respectively. (b) is the magnified profile of the porosity distribution indicated by black dashed rectangle in (a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 10. Fracture analysis of HPDC tensile specimens tested with 0.001 s − 1 strain rate: (a – c) Sample A, (d – f) Sample B, and (g – i) Sample C. (a), (d) and (g) are the images captured by the high-speed optical camera during the tensile tests, with yellow arrows indicating the crack initiation. (b), (e) and (h) are the SEM images taken on the fracture surface of the deformed samples, with yellow dashed boxes indicating their corresponding crack initiation area, and blue arrows in (h) indicating a fringe of delamination. (c), (f) and (i) are the OM images taken on the lateral surface adjacent to the corresponding crack initiation area in the deformed samples, with orange arrow in each image indicating the loading direction. The yellow arrow and the purple dashed line in (i) indicate the crack initiation side and the flat cold flake boundary, respectively. The red dashed lines in (f) and (i) indicate the skin layer. (adapted with permission from Dalai et al. [19]). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. (a) Engineering stress vs engineering strain curves obtained from the uniaxial tensile tests of 2 mm thick HPDC AlSi10MnMg(Fe) specimens deformed with 0.001, 0.1 and 10 s − 1 strain rates, and (b) Corresponding total elongation (TE) undergone by each specimen.
Fig. 8. Cold flakes observed in the HPDC castings having step thickness of (a) 2 mm, and (b) 10 mm. The red dashed line in each image indicates the flat edge of the cold flake particle. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. Cold flakes observed in the HPDC castings having step thickness of (a) 2 mm, and (b) 10 mm. The red dashed line in each image indicates the flat edge of the cold flake particle. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. OM images showing the casting defects produced in the as-cast AlSi10MnMg(Fe) alloy: (a) Cold flake with conspicuous flat edge indicated by red arrows, and (b) Cold shot. (Adapted with permission from Dalai et al. [19]). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. OM images showing the casting defects produced in the as-cast AlSi10MnMg(Fe) alloy: (a) Cold flake with conspicuous flat edge indicated by red arrows, and (b) Cold shot. (Adapted with permission from Dalai et al. [19]). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. OM image of a 2 mm thick as-cast step part showing an inhomogeneous skin layer, where the skin appears to be migrating through the thickness of the casting and ending abruptly, as indicated by the red dashed lines. (Adapted with permission from Dalai et al. [19]). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. OM images showing the presence or absence of skin at the casting surface of as-cast AlSi10MnMg(Fe) alloy having wall thickness of: (a) 2 mm, (b) 4 mm, (c) 6 mm, and (d) 10 mm.
Fig. 4. OM image showing characteristic features formed in the as-cast AlSi10MnMg(Fe) alloy. (Reprinted with permission from Dalai et al. [19]).
Fig. 4. OM image showing characteristic features formed in the as-cast AlSi10MnMg(Fe) alloy. (Reprinted with permission from Dalai et al. [19]).
Fig. 3. (a) Schematic diagram showing the dimensions of specimen used for uniaxial tensile tests and X-ray microtomography, and (b) The actual tensile specimen of 2 mm wall thickness used for the tests. (Adapted with permission from Dalai et al. [19]).
Fig. 3. (a) Schematic diagram showing the dimensions of specimen used for uniaxial tensile tests and X-ray microtomography, and (b) The actual tensile specimen of 2 mm wall thickness used for the tests. (Adapted with permission from Dalai et al. [19]).

Figure Name List:

  • Fig. 1. (a) Diagram of HPDC configuration showing different parts, (b) Top view of an actual cast part, and (c) Side view of an actual cast part showing the steps with wall thickness of 1-, 2-, 4-, 6-, 10- and 15-mm. (Adapted with permission from Dalai et al. [19]).
  • Fig. 2. Graphical representation of an as-cast part of 2 mm step thickness, with green boxes indicating the locations used for analysis using optical microscopy. The pink arrows indicate the two casting surfaces on which the skin layer is supposedly formed. (Adapted with permission from Dalai et al. [19]). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
  • Fig. 3. (a) Schematic diagram showing the dimensions of specimen used for uniaxial tensile tests and X-ray microtomography, and (b) The actual tensile specimen of 2 mm wall thickness used for the tests. (Adapted with permission from Dalai et al. [19]).
  • Fig. 4. OM image showing characteristic features formed in the as-cast AlSi10MnMg(Fe) alloy. (Reprinted with permission from Dalai et al. [19]).
  • Fig. 5. OM images showing the presence or absence of skin at the casting surface of as-cast AlSi10MnMg(Fe) alloy having wall thickness of: (a) 2 mm, (b) 4 mm, (c) 6 mm, and (d) 10 mm.
  • Fig. 6. OM image of a 2 mm thick as-cast step part showing an inhomogeneous skin layer, where the skin appears to be migrating through the thickness of the casting and ending abruptly, as indicated by the red dashed lines. (Adapted with permission from Dalai et al. [19]). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
  • Fig. 7. OM images showing the casting defects produced in the as-cast AlSi10MnMg(Fe) alloy: (a) Cold flake with conspicuous flat edge indicated by red arrows, and (b) Cold shot. (Adapted with permission from Dalai et al. [19]). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
  • Fig. 8. Cold flakes observed in the HPDC castings having step thickness of (a) 2 mm, and (b) 10 mm. The red dashed line in each image indicates the flat edge of the cold flake particle. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
  • Fig. 9. (a) Engineering stress vs engineering strain curves obtained from the uniaxial tensile tests of 2 mm thick HPDC AlSi10MnMg(Fe) specimens deformed with 0.001, 0.1 and 10 s⁻¹ strain rates, and (b) Corresponding total elongation (TE) undergone by each specimen.
  • Fig. 10. Fracture analysis of HPDC tensile specimens tested with 0.001 s⁻¹ strain rate: (a - c) Sample A, (d – f) Sample B, and (g - i) Sample C. (a), (d) and (g) are the images captured by the high-speed optical camera during the tensile tests, with yellow arrows indicating the crack initiation. (b), (e) and (h) are the SEM images taken on the fracture surface of the deformed samples, with yellow dashed boxes indicating their corresponding crack initiation area, and blue arrows in (h) indicating a fringe of delamination. (c), (f) and (i) are the OM images taken on the lateral surface adjacent to the corresponding crack initiation area in the deformed samples, with orange arrow in each image indicating the loading direction. The yellow arrow and the purple dashed line in (i) indicate the crack initiation side and the flat cold flake boundary, respectively. The red dashed lines in (f) and (i) indicate the skin layer. (adapted with permission from Dalai et al. [19]). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
  • Fig. 11. Fracture analysis of HPDC tensile specimens tested with 0.1 s⁻¹ strain rate: (a, b) Sample D, (c, d) Sample E, and (e, f) Sample F. (a), (c) and (e) are the SEM images taken on the fracture surface of the deformed samples, with yellow dashed box in each case indicating the corresponding crack initiation area, and blue arrows in (c) and (e) indicating fringe of delamination. (b), (d) and (f) are the OM images taken on the lateral surface adjacent to the corresponding crack initiation area in the deformed samples, with orange arrow and red dashed lines respectively indicating the loading direction and skin layer in each case. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
  • Fig. 12. Fracture analysis of HPDC tensile specimens tested with 10 s⁻¹ strain rate: (a, b) Sample G, (c, d) Sample H, and (e, f) Sample I. (a), (c) and (e) are the SEM images taken on the fracture surface of the deformed samples, with yellow dashed box in (c) and (e) indicating the corresponding crack initiation area. (b), (d) and (f) are the OM images taken on the lateral surface adjacent to the respective crack initiation area (for H and I) or a random area (for G), with orange arrow and red dashed lines respectively indicating the loading direction and skin layer in each case. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
  • Fig. 13. XMT images showing porosity distribution in the HPDC samples having wall thickness of (a) 2 mm, (b) 6 mm, and (c) 10 mm. The legend in each image indicates the volume of pores in mm³.
  • Fig. 14. Number density and fraction of pores induced in the HPDC castings of different wall thickness.
  • Fig. 15. (a) Normalized distribution of pores size, in terms of equivalent diameter, for different casting thickness. The green, blue and red arrows indicate the largest equivalent diameter of pores found in 2-, 6- and 10-mm thick castings, respectively. (b) is the magnified profile of the porosity distribution indicated by black dashed rectangle in (a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
  • Fig. 16. Engineering stress vs engineering strain curves obtained from the uniaxial tensile tests of specimens having different wall thickness.
  • Fig. 17. SEM images showing the fracture surface of tensile samples having wall thickness of (a) 2 mm, (b) 6 mm, and (c) 10 mm. The yellow dashed rectangle in (a) indicates brittle feature, with blue arrows indicating a fringe of delamination. The red dashed portion in (b) displays a very flat fracture. The yellow arrows in (c) indicate the dendritic arms of α-Al surrounding the shrinkage pore. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
  • Fig. 18. (a) OM image showing the lateral surface of the two fractured parts belonging to the tensile sample having 2 mm wall thickness, with the yellow arrow approximating the location of brittle feature displayed in the SEM image of Fig. 17 (a). (b) and (c) are respectively the magnified images of the areas enclosed by pink and green rectangles in (a). The yellow and white arrows in (c) indicate the crack initiation by delamination of skin and the resultant final fracture, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
  • Fig. 19. (a) OM image showing the lateral surface of the two fractured parts belonging to the tensile sample having 6 mm wall thickness, with the red arrow indicating the location of flat surface shown in the SEM image of Fig. 17 (b). (b) is the magnified image of the area enclosed by red box in (a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
  • Fig. 20. SEM images on the fractured surfaces of tensile tested samples displaying: (a) matrix of the fractured surface with yellow and blue arrows indicating α-Al phase and Al-Si eutectics, respectively, (b) Al-Si eutectics, (c) fractured Si particle within Al solid solution (adapted with permission from Dalai et al. [19]), (d) cracked Chinese script α-Fe compound, (e) cracked (α-Fe)₁ and (α-Fe)Ⅱ compounds, and (f) uncracked (α-Fe)₁ and (α-Fe)Ⅱ compounds. The red arrows in (b - e) indicate cracks. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
  • Fig. 21. Still images from the simulation of die filling stage with increasing time from left to right. (Reprinted with permission from Dalai et al. [19]).
  • Fig. 22. Type of skin pattern observed close to the crack initiation area in each deformed specimen of HPDC processed AlSi10MnMg(Fe) alloy tested at different strain rates, summarized in relation to their corresponding total elongation.

7. Conclusion:

The following conclusions can be drawn from the research:
(i) The HPDC process imparted fine-grained skin layer only on the surface of 2- (width of skin = 30–150 µm) and 4-mm (width of skin = 20–90 µm) thick castings and not in the 6- and 10-mm thick ones. Moreover, this layer was significantly discontinuous, meaning that it was not formed throughout on the casting surfaces.
(ii) The absence as well as the inhomogeneous nature of the surface skin was attributed to the complex melt flow pattern inside the die cavity which results in varied heat transfer and cooling rate. The inhomogeneous skin layer acted as a crack initiation site owing to its weak bonding with the adjacent matrix, subsequently leading to abrupt fracture.
(iii) When tested with different strain rates, on one hand, the overall flow strength of the HPDC processed secondary alloy arguably increased with increase in the strain rate. On the other hand, there was no relation between the ductility exhibited by the material and the strain rate.
(iv) The large variation in ductility (between 1.6 and 6.8 %) demonstrated by the secondary alloy at varied strain rates could be correlated to the skin formation in the HPDC castings. When the castings possessed no skin or a continuous skin on its surface, it could withstand larger TE. Whereas if it contained an inhomogeneous skin on the surface, the material failed abruptly at different lower elongations.
(v) The negative effect of the cold flake was insignificant when its weak flat surface boundary was nearly parallel to the loading direction. Whereas it became detrimental when the boundary was oriented at approximately 30° from the loading direction.
(vi) The abundance of pores in the 10 mm thick casting restricted its ductility to 3.6 %. However, even though the 2- and 6-mm thick castings contained lower amount of pores, their ductility was even more restricted (to 1.5 and 2.9 %, respectively) because of the abrupt fracture triggered respectively by inhomogeneous skin and cold flake.
(vii) The microstructural features were ranked according to their effect on the ductility exhibited by the HPDC processed secondary alloy, which follows a sequence of inhomogeneous skin, cold flakes and pores, with the inhomogeneity in skin formation turning out to be the most harmful one.

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  • This material is a paper by "Biswajit Dalai et al.". Based on "Evaluation of detrimental effect on the ductility caused by the inhomogeneous skin and casting defects in a high pressure die cast recycled secondary alloy".
  • Source of the paper: https://doi.org/10.1016/j.matchar.2025.114775

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