| 1. |
- Zhu, Bai, et al.
(författare)
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A study of anodising behaviour of al-si components produced by rheocasting
- 2019
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Ingår i: Solid State Phenomena. - : Trans Tech Publications. - 1012-0394 .- 1662-9779. - 9783035713732 ; 285, s. 39-44
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Tidskriftsartikel (refereegranskat)abstract
- This paper aims to investigate the anodising behaviour of Al-Si components produced by rheocasting, to understand the effect of the surface liquid segregation (SLS) on the anodising response. The material investigated was EN AC 42000 Al-alloy with an addition of 150 ppm Sr. The component was rheocast and conventionally liquid cast for benchmarking. The RheoMetal™ process was used to prepare slurry and subsequently cast using a vertical pressure die casting machine. Prior to anodising, mechanical grinding was used as pre-treatment method for selected samples as comparison with components in the as-cast state. Anodising was performed on the components using a constant controlled voltage at 25 V, in 1 M H2 SO4, at room temperature. The duration of anodising was varied from 30 mins to 120 mins to examine the relationship between oxide layer thickness and the anodising time. The oxide layer was investigated and characterised. The results demonstrated that the presence of the SLS layer, which was enriched with alloying elements, had a significant influence on the anodising behaviour of the cast component. The oxide layer thickness of the components produced by rheocasting and fully liquid casting was measured and compared. The relations between the oxide layer thickness and anodising time, as well as the casting methods are presented and discussed in this paper..
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| 2. |
- Zhu, Baiwei, 1990-, et al.
(författare)
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A study of formation and growth of the anodised surface layer on Al-Si casting alloys based on different analytical techniques
- 2015
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Konferensbidrag (refereegranskat)abstract
- One of the major limitations in the application of anodizing of aluminum casting alloys is the non-uniform thickness of the oxide layer. Previous researches have studied the formation of the oxide film during anodizing in detail. These investigations have mainly been limited to aluminum and wrought aluminum alloys, and only a few papers have given some insights about the formation of the oxide layer on cast aluminum alloys. The majority of cast aluminum alloys contains relatively higher amounts of Si and other elements (e.g. Cu and Fe) than wrought alloys. This paper aims to investigate the mechanisms of formation and growth of the anodized surface layer on Al-Si casting alloys by applying different analytical techniques such as optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray computer tomography (X-ray CT) scanning. Three different Al alloys with various Si content (2.4%, 3.5% and 5.5%) were investigated. Si morphological modification by Sr addition, as well as gradient solidification was used to vary the microstructural coarseness in a controlled manner to study the influence of these parameters on the growth of the oxide layer. The microstructure features of the anodized layer and the interface between matrix and the oxide layer were studied, employing optical microscopy (figure 1), and SEM/EDS (figure 2). X-Ray CT-scanning (figure 3) was used to examine the high density phases (Fe and Cu bearing phases) and also to give a 3-D view of the anodized oxide surface. Furthermore, TEM was used to examine the general filming behavior of anodized layer and the local effects associated with Si particles in the matrix.It was found that: (i) during anodizing, the oxide front grew around the Si particles and tends to engulf them; (ii) the oxide front grew inwards but did expand in other directions in the eutectic areas; (iii) a scalloped interface between substrate and oxide indicated different anodizing rates in dendrites and eutectic phases. The oxide front was moving faster in the primary dendrite Al-phase than in the eutectic, and the growth rate was particularly slow in larger eutectic areas; (iv) different anodizing rates in dendrite and eutectic phases resulted in thinner layers in specimens with lower SDAS (secondary dendrite arm spacing); and (v) a more uniform layer thickness in Sr-modified specimens illustrates that oxide growth speed was faster in modified eutectic phase. However, diffusion of Si or long distance between Si particles after modification could even results in the adverse situation with a bad anodized surface appearance.
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| 3. |
- Zhu, Baiwei, 1990-, et al.
(författare)
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A study of formation and growth of the anodised surface layer on cast Al-Si alloys based on different analytical techniques
- 2016
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Ingår i: Materials & design. - : Elsevier. - 0264-1275 .- 1873-4197. ; 101, s. 254-262
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Tidskriftsartikel (refereegranskat)abstract
- This paper aims to investigate the mechanisms of formation and growth of the anodised surface layer on Al-Si castings by applying different analytical techniques such as optical microscopy, scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and X-ray computer tomography (X-ray CT) scanning. Three different Al alloys with various Si content (2.43%, 3.53% and 5.45%) were investigated. Si particle morphological modification by Sr addition, as well as directional solidification, was used to vary the microstructural coarseness in a controlled manner to study the influence of these parameters on the growth behaviour of the oxide layer. This study observed residual unanodised Al phases trapped beneath or between Si particles in the oxide layer. It was found, depending on the geometry and morphology of Si particles, that Al can be shielded by Si particles and prevented from oxidising.
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| 4. |
- Zhu, Baiwei, 1990-
(författare)
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Casting and anodising of Al alloys- Alloy design, manufacturing process and material properties
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Casting and semi-solid metal (SSM) casting are widely used to manufacture near-net-shape components of Al-Si alloys in the automotive and electronic industries. In such applications, casting components need to meet the combined requirements of good mechanical properties and corrosion and wear resistance. Hence, a good understanding of the relevant aspects such as material design, manufacturing and surface treatments have a significant impact on the final performance of the component. The objective of this thesis is to understand the interaction among manufacturing and surface treatments and how their combination impacts the microstructure and final properties. The results will accordingly highlight the potential for improving the mechanical and anodising properties of rheocast components.The influence of the most relevant alloying elements has been investigated in this study. It is found that Si and Fe have a significant influence on anodising. During anodising, Si particles are oxidised at a much lower rate than Al phase and embedded in the oxide layer. Due to the presence of Si particles and their morphology, residual metallic Al phase and cracks are introduced in the oxide layer. A reduced number of residual metallic Al phase, as well as defects, can be obtained by changing the Si particle morphology to disconnected fibrous by Sr modification. On the contrary, Fe-rich intermetallics could be partly dissolved during anodising, leaving vacancies or voids as defects in the oxide layer. So, it was proved that by modifying Si particles and removing Fe-rich intermetallics from the surface, the defects in the oxide layer are reduced, and better corrosion protection is achieved.The SSM process increases the microstructural inhomogeneity such as transverse macrosegregation and longitudinal macrosegregation in the cast component. The results show that the presence of surface liquid segregation (SLS) layer by transverse macrosegregation does not have a significant impact on the corrosion resistance and hardness of the oxide layer of as-cast surfaces compared to liquid casting. The longitudinal macrosegregation influences the corrosion protection provided by the anodised layer but does not affect the hardness of the rheocast component before or after anodising. In this study, it is also found that, during the casting of Al-Si alloys, the surface of the component can be enriched in Fe-rich intermetallics due to the SLS or interaction with the die material. Despite this affects only the very superficial thickness, it has a big impact on the corrosion resistance and hardness of the oxide layer.This study has revealed that the high value of the oxide layer thickness, as well as the hydrothermal sealing, is not a guarantee for improving the corrosion resistance of the oxide layer. An increase of the oxide layer thickness by increasing applied voltage or anodising time decreases both the corrosion resistance and hardness of the oxide layer. Moreover, the hydrothermal sealing after anodising significantly decreases the corrosion protection provided by the anodised layer in Al-Si alloys due to cracks formation.This study has observed that the casting defects such as oxide film, cold shots and the solute-rich layer which are related to the casting process dominate the fatigue behaviours of the SSM cast components. The fractographic examination indicates that the oxide film, cold shots and solute-rich layer act as crack initiation points during fatigue testing. Therefore, it was found that, in these conditions, the anodising does not have an evident impact on fatigue properties, despite the anodising process adds a brittle anodised layer on the surface.
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| 5. |
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| 6. |
- Zhu, Baiwei, 1990-, et al.
(författare)
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Hardness and corrosion behaviour of anodised Al-Si produced by rheocasting
- 2019
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Ingår i: Materials & design. - : Elsevier. - 0264-1275 .- 1873-4197. ; 173
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Tidskriftsartikel (refereegranskat)abstract
- The anodised layer of Al-Si alloys produced by rheocasting was studied and compared to anodised traditional liquid casting in this paper. The anodising was performed in 1.0 M H2SO4 solution at room temperature on the as-cast substrates, and anodising voltage and time were optimised as process parameters. This study focuses on understanding the effect of the surface liquid segregation (SLS) layer by rheocasting on the hardness and corrosion protection of the oxide layer. The hardness depends on the anodising parameters and varies along the oxide thickness. The corrosion protection given by the oxide layer was evaluated by electrochemical impedance spectroscopy (EIS) in 3 wt-% NaCl solution, and the results revealed that the longitudinal macrosegregation influences the corrosion protection, with the near-to-vent region showing lower corrosion protection due to a higher eutectic fraction. A comparison between liquid and rheocast samples indicated that the presence of SLS layer by the transverse macrosegregation does not have a significant impact on the corrosion resistance of the oxide layer. Moreover, it was found that an increase of the oxide layer thickness by longer anodising time or higher applied voltage decreases both the hardness and corrosion resistance of the oxide layer.
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| 7. |
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| 8. |
- Zhu, Baiwei, 1990-, et al.
(författare)
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Influence of Fe-rich intermetallics and their segregation on anodising properties of Al-Si-Mg rheocast alloys
- 2021
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Ingår i: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 422
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Tidskriftsartikel (refereegranskat)abstract
- During rheocasting of Al alloys, the component's surface can be enriched in Fe-rich intermetallics due to the surface liquid segregation or the interaction with the die material. Although this precipitates enrichment affects only the surface, it can have a big impact on the quality of the surface treatments or durability. In this paper, the effect of Fe-rich intermetallics on the hardness and corrosion resistance of the anodized layer of AlSiMg alloys was examined by using nanoindentation and electrochemical impedance spectroscopy (EIS) techniques. A sulfuric acid anodising process was performed on unground and mechanical ground surfaces of TX630 substrates produced by the rheocasting process. During anodising, Fe-rich intermetallics can be dissolved, leaving voids in the oxide during its growth. The anodised samples with two different oxide layer thickness (4 and 7 μm) were tested by EIS in 3 wt-% NaCl solution for 12 h. A comparison of the EIS results of unground and mechanically ground surfaces demonstrated that the mechanical grinding successfully removed the Fe enriched layer, improving corrosion protection of the anodised oxide. Moreover, the hardness measurements indicate that the oxide layer on the mechanically ground surface with low Fe content shows higher hardness than on the unground surface with high Fe content.
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| 9. |
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| 10. |
- Zhu, Baiwei, 1990-, et al.
(författare)
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Influence of Si and cooling rate on microstructure and mechanical properties of Al-Si-Mg cast alloys
- 2015
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Ingår i: Proceedings of the 7th Symposium of Aluminium Surface Science and Technology.
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Konferensbidrag (refereegranskat)abstract
- This paper aims to investigate the effect of Si-content and microstructure on mechanical properties of Al-Si-Mg alloys with the purpose of to develop high strength cast aluminum alloys that are anodizable. The Si-content was 2.5, 3.5 and 5.5 wt% Si and both Sr-modified and unmodified conditions were used. The samples were produced using the gradient solidification furnace to generate a well-controlled microstructure. The resulting secondary dendrite arm spacing was 10µm and 20µm. The microstructural features were evaluated by employing SEM/EDS, and optical microscopy. Furthermore, CT-scan technology was used to provide a 3-D view of high density phases in the microstructure. The mechanical properties of these alloys were studied by means of tensile and hardness testing where the latter were performed on the macro- and micro-level. The results demonstrate clearly how the Si growth in the microstructure is restricted by increased cooling rate and modification and its role in strength development in Al-Si alloys. Additionally, the CT-scan visualized the morphology of intermetallics as well as supported in identifying the oxide layer growth as a result of the anodizing process.
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