Papers by Author: Dmitry G. Eskin

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Authors: D. Ruvalcaba, Dmitry G. Eskin, Laurens Katgerman
Abstract: In the present investigation, serial sectioning and 3D reconstructions are made on samples quenched at selected temperatures during unconstrained solidification in order to observe the evolution in morphology of coarse dendrites in 3D. The 3D microstructure reconstruction during the solidification of an Al−7 wt.% Cu alloy allowed the identification of a complex coarse morphology of dendrites. High-ordered branches present different morphologies at different temperatures and locations in the microstructure due to coarsening and coalescence. 3D visualization of complex dendritic structures is discussed in the present investigation.
1015
Authors: D. Ruvalcaba, Dmitry G. Eskin, Laurens Katgerman
Abstract: In the present research the possibility of studying the solidification of aluminum alloys by using the quenching technique is analyzed. Since the quenching technique does not provide reliable information (i.e. due to an overestimation of solid fraction) when measuring the solid fraction over 2D images from samples quenched at high temperature, the overestimation problem is investigated by analyzing 3D reconstructed microstructures from quenched samples. The 3D reconstructed microstructure may provide better understanding about the cause of overestimation of solid fraction when quenching at high temperatures. Consequently, the reconstruction of the microstructure that has existed before quenching may be possible after identifying and removing the solid phase that develops during quenching. In the present research, binary aluminum alloys are solidified and quenched at different temperatures, and then 3D reconstructed images are analyzed. The possibility of reconstructing the microstructure that develops during solidification before quenching is discussed.
1707
Authors: Mehdi Lalpoor, Dmitry G. Eskin, Hallvard Gustav Fjær, Andreas Ten Cate, Nick Ontijt, Laurens Katgerman
Abstract: Direct chill (DC) casting of high strength 7xxx series aluminum alloys is difficult mainly due to solidification cracking (hot cracks) and solid state cracking (cold cracks). Poor thermal properties along with extreme brittleness in the as-cast condition make DC-casting of such alloys a challenging process. Therefore, a criterion that can predict the catastrophic failure and cold cracking of the ingots would be highly beneficial to the aluminum industry. The already established criteria are dealing with the maximum principal stress component in the ingot and the plane strain fracture toughness (KIc) of the alloy under discussion. In this research work such a criterion was applied to a typical 7xxx series alloy which is highly prone to cold cracking. The mechanical properties, constitutive parameters, as well as the KIc values of the alloy were determined experimentally in the genuine as-cast condition and used as input data for the finite element package ALSIM5. Thermomechanical simulations were run for billets of various diameters and the state of residual thermal stresses was determined. Following the contour maps of the critical crack size gained from the model, the casting conditions were optimized to produce a crack-free billet.
1432
Authors: Aage Stangeland, Asbjørn Mo, Dmitry G. Eskin
Abstract: A constitutive equation for thermal strain in the mushy zone has recently been established [1]. The parameters in this constitutive relation are in the present study determined for the commercial alloys A356, AA2024, AA6061 and AA7075 in addition to an Al-4 wt% Cu alloy by combining experimentally measured contraction of a cast sample with thermomechanical simulations. The constitutive equation for thermal strain in the mushy zone reflects that there is no thermal strain in the solid part of the mushy zone at low solid fractions and that the thermal strain in the mushy zone approaches thermal contraction in fully solid as the solid fraction increases towards one. Experiments were performed at cooling rates in the range from 2 to 5.5 °C/s. The solid fractions when the tested alloys start to contract, gsth , are in the range from 0.63 to 0.94. Grain refinement increases gsth for all the tested alloys.
343
Authors: Nikolay A. Belov, V.V. Cheverikin, Dmitry G. Eskin, A.N. Turchin
Abstract: The formation of eutectics in Al–Zn–Mg–Ni and Al–Zn–Mg–Si systems is studied by means of metallography, DSC, EPMA, X-ray spectroscopy and thermodynamical calculations. Polythermal sections of the corresponding phase diagrams are constructed. The concentrations and temperatures of binary eutectic reactions L → (Al) + Al3Ni and L → (Al) + Mg2Si in quaternary alloys are determined. Nonequilibrium solidification in Al–7% Zn–3% Mg-based alloys ceases at approximately 480 °C. The alloys close by composition to binary eutectics have considerably improved casting properties as compared to the base Al–7% Zn–3% Mg composition. In particular, hot tearing susceptibility is much less in alloys with Al3Ni or Mg2Si. These results are corroborated by measurements of thermal contraction during solidification. The alloys containing binary eutectics exhibit much lower temperatures of contraction onset and less thermal strain is accumulated in the solidification range. Fine eutectic morphology enables fragmentation and spheroidization of intermetallic particles during annealing. The presence of Al3Ni and Mg2Si particles does not decrease the precipitation hardening effect associated with precipitation of the T′ (AlMgZn) phase. Improved casting properties and good mechanical properties of castings allow the application of Al–Zn–Mg alloys with binary eutectics formed by Al3Ni or Mg2Si as foundry alloys.
413
Authors: Dmitry G. Eskin, Laurens Katgerman
Abstract: Hot tearing is a significant problem upon direct-chill casting of high-strength aluminum alloys. The occurrence of hot cracks is related to the thermal contraction of the solid phase and to the lack of feeding by the liquid phase during solidification. It has been identified that structure features such as grain size and amount of nonequilibrium eutectics influence both phenomena involved in hot tearing. Experimental and computer-simulation results are presented for a range of model and commercial aluminum alloys. The results are obtained both during special small-scale experiments and during industrial-scale direct-chill casting. It is shown that grain refinement reduces hot tearing susceptibility of aluminum alloys through the related decrease of the temperature of thermal contraction onset and increased permeability of the mushy zone. The effects of process parameters on hot tearing are also discussed.
995
Authors: A.N. Turchin, Dmitry G. Eskin, Laurens Katgerman
Abstract: Effects of solidification range on macro- and microstructure of pure aluminium and binary Al–Cu alloys obtained under conditions of constant melt flow are studied experimentally. The solidification range of binary alloys was varied by changing the concentration of the alloying element. An electromagnetic pump with a specially designed melt-guiding system is used to organize controlled unidirectional melt flow along the solidification front. Temperature and melt flow velocity are controlled during the experiment. It is observed that the extent of solidification range changes the macro– and microstructure, affects width and deflection angle of columnar grains, and alters the dendrite arm spacing in the presence of melt flow. The melt flow itself is found to change the macro- and microstructure, e.g. the increase of melt flow velocity clearly decreases the dendrite arm spacing.
1789
Authors: T.V. Atamanenko, Dmitry G. Eskin, Laurens Katgerman
Abstract: It was shown on laboratory and industrial scale that ultrasonic melt treatment (UST) significantly refines structure of aluminium alloys and improves the quality of castings. However, despite considerable efforts which have been made over decades in the field of ultrasonic processing of aluminium melts, quite a few problems remain unclear. One of them is addressed in this project. The aim of the project is to understand which mechanism is responsible for cavitation-aided grain refinement. It is expected that the knowledge gained as a result of this work can be used in directchill, shape and die casting. The paper describes an experimental setup and first results on the correlation between parameters of UST, solidification conditions and degree of structure refinement. In separate experiments, a model Al-Cu alloy with different amount of solidification sites is solidified with and without UST. The final microstructure is analyzed.
987
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