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- König, Mathias, 1980-
(author)
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Microstructure Formation and Mechanical Properties in Compacted Graphite Iron
- 2009
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Licentiate thesis (other academic/artistic)abstract
- Compacted graphite iron is rapidly becoming an attractive alternative for engine applications in the automotive industry. The improved process control now available that allows CGI components to be cast in a reproducible and reliable way has been the driving force for this development. To be able to optimise the properties in a CGI component it is crucial to have extensive knowledge of the factors influencing microstructure formation in the material. To understand microstructure formation and how the microstructure influences the mechanical properties a series casting trials were performed. A hemispherical sampling cup was used enabling temperature measurements as well as evaluation of the microstructure and a geometry consisting of cylinders of different diameters was used to obtain tensile test bars. A range of cooling rates and chemical compositions were studied in the casting experiments, resulting in substantial differences in both graphite morphology and matrix structure, suggesting that mechanical properties will vary accordingly. Nineteen trials were cast investigating the influence of varying: nodularity treatment level, Cu-content, Si-content, Sn-content and varying content of carbide promoting elements (Cr, Mn and Mo). It was found that the majority of the investigated alloying elements (Cu, Mg, Si and Sn) affect the graphite morphology. Similar to other graphitic cast irons Cu, Sn and carbide promoting elements promote pearlite formation in CGI, while Si promotes ferrite formation. It was also found that higher nodularity favours a pearlitic matrix structure. Mechanical properties are generally raised by increasing nodularity and increasing pearlite content, but a contribution from solution hardening of ferrite was found at high Si-contents. The influence of carbides on mechanical properties was negligible, for the investigated alloying contents and cooling conditions. From the microstructure investigation it was found that CGI is prone to develop a ferritic matrix. It was also clear that segregation of pearlite and ferrite promoting elements influenced the ferrite content. A deterministic model was developed to describe the ferrite growth in CGI. The growth rate of the ferrite border was assumed to be determined by an interface reaction at the ferrite-graphite interface, and that specifically the amount of carbon atoms that can be added to the graphite played a dominant role.
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- König, Mathias, 1980-
(author)
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Microstructure Formation During Solidification and Solid State Transformation in Compacted Graphite Iron
- 2011
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Doctoral thesis (other academic/artistic)abstract
- Compacted graphite iron (CGI) is rapidly becoming an attractive alternative material for engine components in the automotive industry, replacing lamellar graphite iron (LGI) in applications where high mechanical strength is desired. However, the gain in mechanical strength comes with a cost; thermal conductivity, process control and machining are three areas that are more challenging for CGI. This generates a need for research regarding various aspects concerning CGI. In this thesis the microstructure formation during solidification and solid state transformation will be the focus of interest. The phase transformations relevant for microstructure formation of importance to properties in CGI were studied. Experiments were performed in an industrial foundry giving this research direct relevance to regular production of CGI castings. Solidification of the grey (graphite/austenite) eutectic will be discussed, focusing on some relevant aspects influencing the graphite morphology of CGI. The formation of graphite nodules has been investigated by studying colour-etched microstructures. In a material containing mainly CGI cells it was found that nodules form either early during solidification as a consequence of high undercooling or late in the solidification sequence due to a combination of high undercooling and segregation of nodularising elements. Solidification of the white (cementite/austenite) eutectic was studied using chill wedges and the influence of some alloying elements on the amount of carbides was examined. To further enhance the understanding of carbide formation in CGI a commercial casting simulation software was used to correlate real castings to simulations. It was found that the alloying elements investigated influence the carbide formation in a similar way as in other graphitic cast irons and that high nodularity CGI is more prone to chill formation than low nodularity CGI. The solid state transformation was studied and a deterministic model was developed. The model divides a eutectic cell into layers, in order to take into account segregation of alloying elements, which was observed to be influential for the ferrite growth. Moreover, the effect of alloying elements on mechanical properties (tensile properties and hardness) was evaluated. Properties were correlated to microstructural features originating from both solidification and solid state transformations. The trends found generally confirmed previous results regarding properties in graphitic cast irons.
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| 8. |
- König, Mathias, 1980-, et al.
(author)
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Modeling of Ferrite Growth in Compacted Graphite Iron
- 2009
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In: Modeling of Casting, Welding and Advanced Solidification Processes - XII. - Warrendale, Pennsylvania, USA : The Minerals, Metals & Materials Society (TMS). - 9780873397421 ; , s. 505-512
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Conference paper (other academic/artistic)abstract
- In the present work the ferrite growth in Compacted Graphite Iron (CGI) has been modeled. The relative amounts of ferrite and pearlite are of vital importance for the mechanical properties of cast iron. Thus, the possibility to predict the matrix microstructure in CGI by means of simulation would be an indispensable tool for foundries and design engineers. The increasing use of CGI, especially for automotive applications, further creates a need for this research. This study shows that graphite in CGI grows in cells similar to eutectic grey iron cells. In cast irons the ferrite nucleates on the graphite and thereafter grows around the graphite particles until the formation of pearlite interrupts further ferrite formation. The complexity of the graphite particles in CGI imply that the ferrite will grow in different stages. A microstructure study of CGI reveals that ferrite nucleates on compacted graphite particles where low amounts of pearlite stabilizing elements can be found. Growth of ferrite then proceeds until it impinges with ferrite nucleated on adjacent graphite particles. Further ferrite growth takes place radially away from the center of the CGI cell until pearlite nucleation interrupts its growth. At some point during this process ferrite nucleates on graphite nodules in the last to freeze areas of the microstructure giving a contribution to the ferrite content. In the present work growth models of ferrite in nodular cast iron are adapted to CGI.
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| 9. |
- Payandeh, Mostafa
(author)
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Rheocasting of aluminium alloys : Process and components characteristics
- 2016
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Doctoral thesis (other academic/artistic)abstract
- Semi-Solid Metal (SSM) casting is a promising technology offering an opportunity to manufacture net-shape, complex geometry metal components in a single operation. However, the absence of foundry guidelines and limited design data for SSM casting makes it challenging to predict the performance of both process and components. The objective of this research was to develop and offer new solutions to material processing-related issues in the electronics industry. By investigating the opportunities afforded by the recently developed RheoMetalTM rheocasting process, a better understanding of the critical factors needed for an effective manufacturing process and optimised component characteristics was achieved.A study of the evolution of microstructure at different stages of the RheoMetalTM process demonstrated the influence of multistage solidification on the microstructural characteristics of the rheocast components. The microstructure of a slurry consists of the solute-lean and coarse globular α-Al particles with a uniform distribution of alloying elements, suspended in the liquid matrix. Additional solute-rich α-Al particles were identified as being a consequence of discrete nucleation events taking place after the initial slurry production. In the final components, macrosegregation was observed in the form of variations in the ratio of solute-lean coarse globular α-Al particles and solute-rich fine α-Al particles in both longitudinal and transverse directions.The relation between microstructural characteristics and material properties was established by determination of the local properties of a rheocast component. The fracture of a rheocast telecom component was strongly affected by microstructural inhomogeneity. In particular, macrosegregation in the form of liquid surface segregation bands and sub-surface pore bands strongly affected the fracture behaviour. Thermal conductivity measurements revealed that regions of the component with a high amount of solute-lean globular α-Al particles showed higher thermal conductivity. The effect of the local variation in thermal conductivity on the thermal performance of a large rheocast heatsink was evaluated by simulation. The results clearly show the importance of considering material inhomogeneity when creating a robust component design.
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| 10. |
- Payandeh, Mostafa, et al.
(author)
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Solidification and re-melting phenomena during the slurry preparation stage using the RheoMetalTM process
- 2016
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Reports (other academic/artistic)abstract
- The melting sequence of the Enthalpy Exchange Material (EEM) and formation of slurry in the RheoMetalTM process was investigated. The EEM was extracted, together with a portion of the slurry at different times before complete melting, and quenched. The EEM initially increased in size due to melt freezing onto its surface, forming a freeze-on layer. The initial growth of this layer was followed by a constant diameter of the EEM and thereafter subsequent melting. Microstructural characterization of the size and morphology of different phases in the EEM and the freeze-on layer was made. Dendritic equiaxed grains and eutectic regions containing Si particles and Cu-bearing particles were observed in the as-cast EEM. The freeze-on layer consisted of dendritic aluminum slightly tilted by about 30° toward the upstream direction, caused by the rotation of the EEM. Energy Dispersion Spectroscopy analysis showed that the freeze-on layer had a composition corresponding to a higher melting point than the EEM.Microstructural investigation of the EEM showed that the temperature rapidly increased to 495 ºC, causing incipient melting of Al2Cu and Al5Mg8Si6Cu2 phases in grain boundary regions. Following the incipient melting, the temperature in the EEM increased further and binary Al-Si eutectic started to melt to form a region of a fully developed coherent mushy state. Experimental results and a thermal model indicated that as the dendrites spheroidized and the interface at the EEM/freeze-on layer reached a mushy state with 25% solid fraction, coherency was lost and disintegration of the freeze-on layer took place. Subsequently, in the absence of the shielding effect from the freeze-on Layer, the EEM disintegrates at a higher solid fraction, estimated to be 50%. The fast and complex slurry generation in the RheoMetalTM process is a hybrid process with both rheocasting and thixocasting elements in the process.
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