Browsing by Keyword "Phase transformation"
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Item The initial stage of liquid phase sintering for an Al-14Si-2.5Cu-0.5Mg (wt%) P/M alloy(2010-06) Arribas, I.; Martín, J. M.; Castro, F.; TRAZABILIDAD CIRCULARThe present paper focuses on the initial stage of the liquid phase sintering (LPS) of a commercially available P/M Al-Si alloy, with nominal composition Al-14Si-2.5Cu-0.5Mg (in wt%). The microstructural examination of the as-received powder showed that it is constituted by pure Al particles and master alloy particles with composition Al-28Si-5Cu-1Mg (in wt%). Its compressibility is lower than for the conventional elemental P/M aluminium premixes, but it is still better than for prealloyed Al powders and other P/M powders. Thermogravimetry experiments showed that the elimination of the organic lubricant takes place between 275 and 490°C during heating to the sintering temperature. The phase transformations leading to the formation of the liquid phase were studied by differential scanning calorimetry (DSC). The dimensional changes associated with the generation of the liquid were measured by dilatometry. Samples quenched into water from different temperatures (between 450 and 575°C) and times (between 0 and 30min) were studied to reveal the microstructural evolution of the alloy. The first liquid is formed inside the master alloy particles at around 505°C. This liquid spreads across the compact, enhancing the chemical homogenization of the material. The alloying elements diffuse from the liquid inside the originally pure Al particles, reducing their melting temperature. This alloying process is almost concluded at around 535°C. When the temperature is increased the liquid starts to be formed also in the originally pure Al particles. The melting of the FCC Al-rich phase finishes around 575-590°C. The full melting of the alloy occurs at about 635-645°C. After analyzing the different possible causes, it is concluded that the main swelling mechanism is the volume change associated with the melting of a fraction of the material when the temperature is increased. The phases detected by X-ray diffraction (XRD) in the as-received powder and in the sintered compact are FCC Al-rich solid solution, Si, θ-phase (CuAl2), and Q-phase (Cu2Mg8Si6Al5).Item Laser hardening prediction tool based on a solid state transformations numerical model(2010) Martínez, S.; Ukar, E.; Lamikiz, A.; Liebana, F.; COMPOSITEThis paper presents a tool to predict hardening layer in selective laser hardening processes where laser beam heats the part locally while the bulk acts as a heat sink. The tool to predict accurately the temperature field in the workpiece is a numerical model that combines a three dimensional transient numerical solution for heating where is possible to introduce different laser sources. The thermal field was modeled using a kinetic model based on Johnson-Mehl-Avrami equation. Considering this equation, an experimental adjustment of transformation parameters was carried out to get the heating transformation diagrams (CHT). With the temperature field and CHT diagrams the model predicts the percentage of base material converted into austenite. These two parameters are used as first step to estimate the depth of hardened layer in the part. The model has been adjusted and validated with experimental data for DIN 1.2379, cold work tool steel typically used in mold and die making industry. This steel presents solid state diffusive transformations at relative low temperature. These transformations must be considered in order to get good accuracy of temperature field prediction during heating phase. For model validation, surface temperature measured by pyrometry, thermal field as well as the hardened layer obtained from metallographic study, were compared with the model data showing a good adjustment.Item Thermal model with phase change for process parameter determination in laser surface processing(2010) Ukar, E.; Lamikiz, A.; De Lacalle, L. N.López; Martinez, S.; Liébana, F.; Tabernero; COMPOSITEDespite the fast evolution of lasers in recent years, one of the main drawbacks for the implementation of surface laser treatment processes in industry is the previous experimentation necessary to get optimum process parameters. In order to reduce the necessary experimentation, the work presented focuses on the development of a numerical model for two different laser sources, including metallurgical transformations during the heating process. The model was experimentally validated for DIN 1.2379 tool steel, alloy typically used in die and mold making industry. Surface temperature obtained by pyrometry and the thermal field obtained from a metallographic study were compared with model data obtaining a good agreement.Item Vc-precipitation kinetics studied by small-angle neutron scattering in nano-steels(Trans Tech Publications Ltd, 2018) Ioannidou, Chrysoula; Arechabaleta, Zaloa; Rijkenberg, Arjan; Dalgliesh, Robert M.; van Well, Ad A.; Offerman, S. Erik; Shabadi, R.; Ionescu, Mihail; Jeandin, M.; Richard, C.; Chandra, Tara; Tecnalia Research & InnovationNano-steels are used in automotive applications to accomplish resource-efficiency while providing high-tech properties. Quantitative data and further understanding on the precipitation kinetics in Nano-steels can contribute to fulfil this goal. Small-Angle Neutron Scattering measurements are performed on a Fe-C-Mn-V steel, previously heat-treated in a dilatometer at 650o C for several holding times from seconds to 10 hours. The evolution of the precipitate volume fraction, size distribution and number density is calculated by fitting the experimental Small-Angle Neutron Scattering curves. The effect of phase transformation on precipitation kinetics is also discussed. Complementary Transmission Electron Microscopy, Scanning Electron Microscopy and Inductively Coupled Plasma Optical Emission Spectroscopy measurements are performed to support the Small-Angle Neutron Scattering data analysis.