| 1. |
- Bjurenstedt, Anton, 1979-
(författare)
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On the influence of imperfections on microstructure and properties of recycled Al-Si casting alloys
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- There are great energy savings to be made by recycling aluminium; as little as 5% of the energy needed for primary aluminium production may be required. Striving to produce high quality aluminium castings requires knowledge of microstructural imperfections, which is extra important when casting recycled aluminium that generally contains higher levels of imperfections compared to primary aluminium. Imperfections include amongst others Si, Fe, and Mn as well as oxides. Si is needed for castability, but it may also initiate fracture. There are different types of Fe-rich intermetallics influencing properties of castings, generally in a negative direction. Oxides constitute cracks and they are elusive because they are difficult to quantify.This thesis aims to increase knowledge about imperfections in recycled aluminium castings originating from alloying elements and the melt. Experiments were performed in advanced laboratory equipment, including X-radiographic imaging during solidification and in-situ tensile testing in a scanning electron microscope. Experiments were also performed at industrial foundry facilities.The experiments showed that the nucleation temperature of primary α-Fe intermetallics increased with higher Fe, Mn, and Cr contents. Primary α-Fe are strongly suggested to nucleate on oxides and to grow in four basic morphologies. Lower nucleation frequency of α-Fe promoted faster growth and hopper crystals while higher nucleation frequency promoted slower growth rates and massive crystals. Results also showed that a decrease in the size of the eutectic Si and plate-like β-Fe intermetallics improved tensile properties, foremost the elongation to fracture. In β-Fe containing alloys the transversely oriented intermetallics initiated macrocracks that are potential fracture initiation sites. In alloys with primary α-Fe foremost clusters of intermetallics promoted macrocracks. In fatigue testing, a transition from β-Fe to α-Fe shifted the initiation sites from oxides and pores to the α-Fe, resulting in a decrease of fatigue strength. Oxides in Al-Si alloys continue to be elusive; no correlations between efforts to quantify the oxides and tensile properties could be observed.
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| 2. |
- Bogdanoff, Toni
(författare)
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The effect of microstructural features, defects and surface quality on the fatigue performance in Al-Si-Mg Cast alloys
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Global warming is driving industry to manufacture lighter components to reduce carbon dioxide (CO2) emissions. Promising candidates for achieving this are aluminium-silicon (Al-Si) cast alloys, which offer a high weight-to-strength ratio, excellent corrosion resistance, and good castability. However, understanding variations in the mechanical properties of these alloys is crucial to producing high-performance parts for critical applications. Defects and oxides are the primary reasons cast components in fatigue applications are rejected, as they negatively impact mechanical properties.A comprehensive understanding of the correlation between fatigue performance and parameters such as the α-aluminium matrix, Al-Si eutectic, surface roughness, porosities, hydrogen content, oxides, and intermetallic phases in Al-Si castings has not been reached.The research presented in this thesis used state-of-the-art experimental techniques to investigate the mechanical properties and crack-initiation and propagation behaviour of Al-Si-Mg cast alloy under cyclic loading. In-situ cyclic testing was conducted using scanning electron microscopy (SEM) combined with electron back-scattered diffraction (EBSD), digital image correlation (DIC), and focused ion beam (FIB) milling. These techniques enabled a comprehensive study of parameters affecting fatigue performance, including hydrogen content, surface roughness, oxides, and intermetallic phases. More specifically, we investigated the effect of melt quality, copper (Cu) content, oxide bifilms, surface quality, and porosity.The increased Cu concentration in heat-treated Al-Si alloys increased the amount of intermetallic phases, which affected the cracking behaviour. Furthermore, oxide bifilms were detected at crack-initiation sites, even in regions far away from the highly strained areas. Si- and Iron (Fe)-rich intermetallics were observed to have precipitated on these bifilms. Due to their very small size, these oxides are generally not detected by non-destructive inspections, but affect mechanical properties because they appear to open at relatively low tensile stresses. Finally, Al-Si alloy casting skins showed an interesting effect in terms of improving fatigue performance, highlighting the negative effect of surface polishing for such alloys.
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| 3. |
- Bogdanoff, Toni
(författare)
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The influence of microstructure on the crack initiation and propagation in Al-Si casting alloys
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- For reducing the CO2 footprint in many industrial fields, the goal is to produce lighter components. The aluminium-silicon (Al-Si) cast alloys are promising candidates to fulfill these goals with a high weight-to-strength ratio, good corrosion properties, excellent castability, and recyclable material. However, the variations within these components need to be understood to produce high-performance components for critical applications. The main reason for the rejection in these applications is defects and microstructural features that reduce the mechanical properties. The addition of copper (Cu) is one way of increasing the mechanical properties in Al-Si alloys and is commonly used in the automotive industry. Casting defects harm the mechanical properties, and these defects can be reduced by improving the melt quality, the correct design of the component, and the gating system.The study aims to investigate the static mechanical properties and the crack initiation and propagation under cyclic loading in an Al-7Si-Mg cast alloy with state-of-the-art experiments. The main focuses were on the effect of the HIP process and the role of Cu addition. In-situ cyclic testing using a scanning electron microscope coupled with electron back-scattered diffraction, digital image correlation, focused ion beam (FIB) slicing, and computed tomography scanning was used to evaluate the complex interaction between the crack path and the microstructural features.The amount of Cu retained in the α-Al matrix in as-cast and heat-treated conditions significantly influenced the static mechanical properties by increasing yield strength and ultimate tensile strength with a decrease in elongation. The three-nearest-neighbor distance of eutectic Si and Cu-rich particles and crack tortuosity were new tools to describe the crack propagation in the alloys, showing that a reduced distance between the Cu-rich phases is detrimental for the mechanical properties. Three dimensional tomography using a FIB revealed that the alloy with 3.2 wt.% Cu had a significantly increased quantity of cracked Si particles and intermetallic phases ahead of the crack tip than the Cu-free alloy. The effect of Cu and HIP process in this work shows the complex interaction between the microstructural features and the mechanical properties, and this needs to be considered to produce high-performance components.
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| 4. |
- Zhang, Qing
(författare)
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Mechanical properties of semi-solid Al castings : Role of stirring
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Aluminum castings have been widely used in the automotive industry to reduce the vehicle's weight. However, the existence of casting defects significantly limits its application. The most common and detrimental defects in aluminum castings are porosity and oxides. The formation of the pores comes from the solute hydrogen and volumetric reduction during the solidification process, resulting in the gas and shrinkage pores, respectively. The oxides can be introduced by either the operation during the process or the originally existing oxides in the melt. To reduce these defects, optimizing the casting process and controlling the melt quality is essential.In this work, the Semisolid Metal (SSM) process was used as it can significantly reduce the formation of shrinkage pores. The main focuses were on the influence of stirring intensity on the formation of casting defects and, thus, the resultant mechanical properties. In addition, to control the original melt quality, particularly the amount of oxides, efforts were made to develop proper methods for the melt quality assessment.The results show that the slurry-making process, mainly through stirring intensity, can affect the casting defects significantly. On the one hand, the increasing stirring intensity can distribute the primary Al particles more homogeneously, reducing the pores in terms of size and number by increasing the permeability during the solidification process. On the other hand, the increasing stirring intensity can affect the size of oxides differently, depending on the composition, for instance, the Mg content.For the alloys with sufficient Mg, the oxides would be MgAl2O4, which are small films with numerous cracks. Under intensive stirring, the oxides can break down into small oxide particles. As a result, intensive stirring can improve ductility by reducing the formation of big pores. However, the oxides would mainly be Al2O3 films for alloys with low Mg content. In this case, the current stirring intensity is insufficient to break the oxide films. Instead, the increased stirring has introduced more oxide films into the melt. Consequently, in the casting with intensive stirring, the increasing oxide films dominated the ductility rather than the reduced porosity.The SSM castings exhibit better bending fatigue properties than the casting using the traditional high-pressure die casting (HPDC) process. This improvement is mainly due to the formation of the harder surface liquid segregation (SLS) layer on the SSM casting surface. Furthermore, compared with the standard SSM process, the castings using intensive stirring (hereinafter referred to as the modified SSM process) show similar but more reliable fatigue properties. This reliable fatigue property can be attributed to eliminating the big internal pores through intensive stirring, which results in local stress concentration and significantly reduces fatigue performance. Besides, due to the gradient stress distribution in the bending loading, the surface defects play a significant role in the fatigue properties. With the increase of the specimens’ thickness, the failure mechanisms changed.The shrinkage pores in the reduced pressure test (RPT) test play a significant role in the accuracy of melt quality assessment. A good correlation between the bifilm index (BI)/ density index (DI) and hydrogen content is observed for the RPT samples without significant shrinkage pores. In addition, the correlation between the BI and elongation is also strongly affected by the clusters of shrinkage pores due to the conflict between the definition of the BI and the influence of clusters of shrinkage pores on the ductility. Based on this, we proposed an optimized BI where the clusters of shrinkage pores were treated as single pores, increasing the reliability of the correlation between the BI and elongation.
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| 5. |
- Santos, Jorge
(författare)
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Semi-solid Al-7Si-Mg castings – Microstructure and mechanical properties
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The use of lightweight parts by the vehicle industry produces immediate benefits on the reduction of emissions because less energy is consumed during the production, service, and recycle stages of a product life cycle. Therefore, the development of processes that allow high design freedom for topology optimisation and materials with high specific strength is a great need. Semi-solid Al-7Si-Mg castings provide great potential for weight reduction, particularly in critical applications where materials such as steel and cast iron are typically used. However, critical applications have higher requirements in mechanical and fatigue properties compared to conventional aluminium castings applications. Therefore, the control of microstructure and defect formation in all steps of the semi-solidcasting process is essential to produce lightweight, reliable castings for future demands.In semi-solid aluminium casting, a slurry consisting of primary α-Al crystals dispersed in the liquid is injected into the die-cavity. In this study, the slurry preparation involved the immersion of a cylinder (so-called EEM) while rotating into a superheated alloy. This investigation showed that the α-Al crystals in the slurry are a combination of equiaxed α-Al crystals that nucleate in the thermal undercooled liquid surrounding the EEM, crystal fragments from the columnar dendrites solidified on the EEM surface and undissolved crystals from the original EEM. The addition of grain refiners has no significant effect on the size and shape of the α-Al crystals in the slurry. The dissolution of the EEM duringslurry preparation was studied using a new tag-and-trace method of α-Al crystals. Whenthe EEM disintegrates into large α-Al crystal agglomerates during slurry preparation can result in detrimental effects on the fatigue properties of SSM castings.Alloy composition, cooling rate, strontium modification, and heat treatment affect the type, size, and shape of the intermetallic phases formed in the Al-7Si-Mg castings. This study showed that high cooling rates and strontium modification are beneficial for the formation of smaller and less detrimental iron-rich intermetallic phases to mechanical and fatigue properties.The precipitation hardening response of the SSM Al-7Si-Mg castings strongly affects mechanical and fatigue properties. The results in this study showed that the 0.2% offset yield strength increases linearly with the increase of the magnesium concentration in the interior of the α-Al crystals formed during slurry preparation of SSM Al-7Si-Mg castings in the T5 and T6 conditions. Macrosegregation regions surrounded by an oxide layer were preferential sites for fatigue crack initiation in the SSM Al-7Si-Mg castings tested in this study.
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| 6. |
- Siafakas, Dimitrios, 1982-
(författare)
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On particles and slags in steel casting
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Hadfield steel is widely accepted as one of the most important steel alloys utilized in industrial applications where high impact strength and wear resistance is required. Like in most metallic alloys used for component casting, the mechanical properties of Hadfield steel are directly connected with the microstructure of the material. It has been reported that Hadfield steel components with fine microstructure can present up to 30% increased strength and reduced risk of porosity formation during solidification when compared with their coarser microstructure counterparts.In the light-metal alloy and cast-iron industry, one of the most widely used methods for achieving refinement of the microstructure of the material is known as inoculation. As the name implies, inoculation is the practice ofadding selected compounds or alloying elements in a metal melt that have the ability to promote rapid grain nucleation during solidification. Even though it has been proved that inoculation is one of the most efficient methods for the refinement of a wide variety of metallic alloys, it has not yet gained adequate acceptance in the steel casting industry because researchers have not yet been able to identify proper inoculants for steel.The efficiency of the microstructural refinement when inoculating is influenced by several factors like the type of inoculant used and the processing conditions during melting, deoxidation, casting and heat treatment. Following proper deoxidation methods and application of tailored oxidic slags during melting could significantly promote the precipitation of desired inclusions that can act as potent nucleation sites for grains or as grain growth inhibitors.In any case, efficient inoculation is influenced by the complex interaction between the inoculant, the oxide slag, and the melt. The way this interaction happens is in many ways dictated by the chemical and thermophysical properties of the substances involved. Therefore, obtaining accurate values of basic thermophysical properties like viscosity and interfacial tension by improving current and utilizing novel measurement methods could significantly help in the effort of identifying and efficiently utilizing potent inoculants for austenitic steels.Considering the above, this work has a dual objective. The primary aim is to investigate if any of the by-products of deoxidation of Hadfield steel that remain in the material after solidification can act as potent inoculants by examining their qualitative and quantitative characteristics and their influence on the as-cast microstructure of the steel. The secondary aim is to acquire accurate values for oxide slag viscosity and slag-iron interfacial tension at high temperatures using different measurement methods and investigate how thermophysical properties are influenced by thermal and compositional conditions. This type of research is important because not only it can help to identify which substances are potent inoculants for austenitic steels but also pave the way for developing new or improving conventional deoxidation and inoculation processes with the ultimate goal of improving the cast component’s mechanical properties.The work is divided into 3 different stages. The first stage is dedicated to high-temperature oxide slag viscosity measurements. The viscosity of oxide slags with varying composition is measured in a wide temperature range utilizing the rotational bob and aerodynamic levitation methods. The systematic error is defined, and the compositional and thermodynamic dependence of viscosity is explained. In the second stage, the precipitation of particles in aluminum and titanium deoxidized Hadfield steel is investigated. The characteristics of particles, including type, size, morphology, composition, population, and sequence of precipitation are identified. The results are then compared against thermodynamic equilibrium calculations, a particle growth mathematical model is developed and the precipitation mechanism of each type of particle is described. Finally, in the third stage, the as-cast grain size of samples produced with varying deoxidation procedures is measured and the relationship between particle characteristics and grain size is determined. The particles are ranked according to their refining potency and compared to a ranking based on their disregistry with austenite.
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| 7. |
- Zhang, Qing
(författare)
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Mechanical Properties of Semi-Solid Al Castings : Role of Stirring
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Semi-solid metal (SSM) casting has been widely used in automotive industries to reduce the weight. In RheoMetalTM process which is one of the variations of SSM, the slurry fabrication can be finished within 30 second and can the slurry making process can be integrated into a high pressure die casting (HPDC) route without significant adjustments, making the process a promising alternative for industrial application. However, the application of SSM is still limited due to the semi-solid deformation-induced casting defects, such as macrosegregation and large pores. Due to the short stirring duration, the inefficient stirring is the leading cause of defects formation. Another critical issue in the RheoMetalTM process is the oxidation during the stirring process, which results in the increase of oxides in the castings, reducing the mechanical properties.This study aims to investigate the ductility and the fatigue performance of SSM castings. The main focuses were on the role of the stirring and oxides. The quenched slurry was analyzed to evaluate the effect of the stirring on the particle distribution in the slurry, and its effect on the formation of pores and segregation was discussed. To investigate the oxidation during the slurry making process, two alloys with different Mg content were cast. Scanning electron microscopy (JEOL JSM-7001F SEM) equipped with Energy-dispersive X-ray spectroscopy (EDS) was used to identify the oxides on the fracture surface. Tensile test and fatigue test accompanied with direct current potential drop (DCPD) were performed to investigate the ductility and fatigue performance, respectively.The results suggest that the intensive stirring can avoid the formation of the large clusters, making the particle distribution homogeneous in the slurry. The Mg contents determined the types of the oxides formed in the slurry making process. For alloys with sufficient Mg, the oxides would be MgAl2O4, which exist as small films with numerous cracks, while a large oxides film will be formed in the case of low Mg content.The combined influence of porosity and oxides was concluded. In the 42000 alloy, because of the low Mg content, the ductility was dominated by the large oxide films. In contrast, in the Magsimal 59 alloy, the presence of small oxides (less than 0.2 mm in majority) leads to the influence of oxides on the elongation negligible. However, a good correlation was obtained between the largest pores and ductility.The fatigue test shows that the surface liquid segregation (SLS) determined the fatigue strength under cyclic bend loading, due to its higher hardness. The effect of the inner pores on the fatigue performance was negligible, as the maximum stress was applied on the surface.
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| 8. |
- Dini, Hoda, 1984-
(författare)
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As-cast AZ91D magnesium alloy properties : Effects of microstructure and temperature
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Today, there is an essential need for lightweight, energy-efficient, environmentally benign engineering systems, and this is the driving force behind the development of a wide range of structural and functional materials for energy generation, energy storage, propulsion, and transportation. These challenges have motivated the use of magnesium alloys for lightweight structural systems. Magnesium has a density of 1.74 g/cm3, which is almost 30% less than that of aluminium, one quarter of steel, and almost identicalto polymers. The ease of recycling magnesium alloys as compared to polymers makes them environmentally attractive, but their poor mechanical performance is the primary reason for the limited adoption of these alloys for structural applications.The Mg-Al-Zn alloy AZ91D exhibits an excellent combination of strength, die-castability, and corrosion resistance. However, its mechanical performance with regard to creep strength, for example, at evaluated temperatures is poor. Moreover, very little is known about the correlation between its mechanical properties and microstructural features. This thesis aims to provide new knowledge regarding the role played by microstructure in the mechanical performance of the magnesium alloy. The properties/performance of the material in relation to process parameters became of great interest during the investigation.An exhaustive characterisation of the grain size, secondary dendrite arm spacing (SDAS) distribution, and fraction of Mg17Al12 was performed using optical and electron backscatter diffraction (EBSD). These microstructural parameters were correlated to the offset yield point (Rp0.2), fracture strength, and elongation to failure of the material. It was proposed that the intermetallic phase, Mg17Al12, plays an important role in determining the mechanical and physical properties of the alloy in a temperature range of room temperature to 190°C by forming a rigid network of intermetallic. The presence of this network was confirmed by studying the thermal expansion behaviour of samples of the alloy containing different amounts of Mg17Al12.A physically based constitutive model with a wide validity range was successfully adapted to describe the flow stress behaviour of AZ91D with various microstructures. The temperature-dependent variables of the model correlated quite well with the underlying physics of the material. The model was validated through comparison with dislocation densities obtained using EBSD.The influence of high-pressure die-cast parameters on the distortion and residual stress of the cast components was studied, as were distortion and residual stress in components after shot peening and painting. Interestingly, it was found that intensification pressure has a major effect on distortion and residual stresses, and that the temperature of the fixed half of the die had a slight influence on the component's distortion and residual stress.
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| 9. |
- Kasvayee, Keivan Amiri, 1986-
(författare)
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On the deformation behavior and cracking of ductile iron; effect of microstructure
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focuses on the effect of microstructural variation on the mechanical properties and deformation behavior of ductile iron. To research and determine these effects, two grades of ductile iron, (i) GJS-500-7 and (ii) high silicon GJS-500-14, were cast in a geometry containing several plates with different section thicknesses in order to produce microstructural variation. Microstructural investigations as well as tensile and hardness tests were performed on the casting plates. The results revealed higher ferrite fraction, graphite particle count, and yield strength in the high silicon GJS-500-14 grade compared to the GJS-500-7 grade.To study the relationship between the microstructural variation and tensile behavior on macroscale, tensile stress-strain response was characterized using the Ludwigson equation. The obtained tensile properties were modeled, based on the microstructural characteristics, using multiple linear regression and analysis of variance (ANOVA). The models showed that silicon content, graphite particle count, ferrite fraction, and fraction of porosity are the major contributing factors that influence tensile behavior. The models were entered into a casting process simulation software, and the simulated microstructure and tensile properties were validated using the experimental data. This enabled the opportunity to predict tensile properties of cast components with similar microstructural characteristics.To investigate deformation behavior on micro-scale, a method was developed to quantitatively measure strain in the microstructure, utilizing the digital image correlation (DIC) technique together with in-situ tensile testing. In this method, a pit-etching procedure was developed to generate a random speckle pattern, enabling DIC strain measurement to be conducted in the matrix and the area between the graphite particles. The method was validated by benchmarking the measured yield strength with the material’s standard yield strength.The microstructural deformation behavior under tensile loading was characterized. During elastic deformation, strain mapping revealed a heterogeneous strain distribution in the microstructure, as well as shear bands that formed between graphite particles. The crack was initiated at the stress ranges in which a kink occurred in the tensile curve, indicating the dissipation of energy during both plastic deformation and crack initiation. A large amount of strain localization was measured at the onset of the micro-cracks on the strain maps. The micro-cracks were initiated at local strain levels higher than 2%, suggesting a threshold level of strain required for micro-crack initiation.A continuum Finite Element (FE) model containing a physical length scale was developed to predict strain on the microstructure of ductile iron. The material parameters for this model were calculated by optimization, utilizing the Ramberg-Osgood equation. The predicted strain maps were compared to the strain maps measured by DIC, both qualitatively and quantitatively. To a large extent, the strain maps were in agreement, resulting in the validation of the model on micro-scale.In order to perform a micro-scale characterization of dynamic deformation behavior, local strain distribution on the microstructure was studied by performing in-situ cyclic tests using a scanning electron microscope (SEM). A novel method, based on the focused ion beam (FIB) milling, was developed to generate a speckle pattern on the microstructure of the ferritic ductile iron (GJS-500-14 grade) to enable quantitative DIC strain measurement to be performed. The results showed that the maximum strain concentration occurred in the vicinity of the micro-cracks, particularly ahead of the micro-crack tip.
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| 10. |
- Santos, Jorge
(författare)
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Al-7Si-Mg semi-solid castings – microstructure and mechanical properties
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The vehicles industry is facing increasing demands for fuel efficiency and cost reduction due to environmental legislation, sustainability and customer demands. Therefore, there is a great need to develop and produce lightweight components by using materials and processes that offer higher specific strength and/or design optimization. Semi‐solid aluminium casting offers design freedom and castings with lower shrinkage and gas entrapment defects compared to high pressure die castings. The lack of understanding of microstructure and defect formation, and design data, for semi‐solid castings is a barrier for foundries and designers in the vehicles industry to use semi‐solid castings.In this study, the effect of two grain refiners on slurry formation and surface segregation of semi‐solid Al‐7Si‐0.3Mg castings produced by the Rheometal™ process was evaluated. The influence of grain refinement on primary α‐Al grain size, shape factor and solid fraction was analysed in addition to the solute content of the surface segregation layer.The influence of magnesium on the formation of intermetallic phases during solidification and the heat treatment response of Al‐7Si‐Mg semi‐solid castings was investigated. The magnesium content was varied from 0.3 to 0.6wt.% and the semi-solid castings were analysed in the T5 and T6 conditions. Energy dispersive spectroscopy was used to identify the intermetallic phases formed during solidification. Tensile testing was performed and the results were correlated to the magnesium and silicon concentration measured in the interior of the α‐Al globules formed during slurry preparation.The results suggest that the addition of grain refiner decreases the solid fraction obtained in the Rheometal™ process. However, no significant effect was observed on the α‐Al grain size and shape factor.A good correlation was obtained between the magnesium concentration in the interior of the α‐Al globules formed during slurry preparation and the offset yield strength for all alloys. The low magnesium solubility in α‐Al at temperatures in the solidification range of the Al‐7Si‐Mg alloys is suggested to be the reason for the low hardening response for the T5 heat treatment compared to the T6 condition.
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