Grain Refinement and Solidification Velocity of Supercooled Melts

Using a high speed thermal imaging camera, solidification velocity of undercooling is accurately determined for a highly undercooled Ni70Cu30 binary concentrated single phase alloy. It is observed that the velocity of solidification is first continuously and smoothly increasing at the low and mediate undercoolings then increasing discontinuously to a maximum value at the high undercooling range. Thus, the solidifcation velocity-undercooling relation has an abrupt discontinuity, which is caused by nonequilibrium diffusion of solutes in the liquid side near the advancing solid liquid interface. Grain refinement of high undercooling is directly induced by recrystallization, which was in clear contrast with the remelting induced dendrite fragmentation in conventional alloys.

Effect of Compound Fields of Ultrasonic Vibration and Applied Pressure on the 3D Microstructure and Tensile Properties of Recycled Al-Cu-Mn-Fe-Si Alloys

The effect of compound fields of ultrasonic vibration and applied pressure (UV+AP) on three-dimensional (3D) microstructure and tensile properties of recycled Al-Cu-Mn-Fe-Si alloys was systematically studied using conventional two-dimensional (2D) microscopy, synchrotron X-ray tomography, and tensile test. The properties of UV+AP treated alloys with the pouring temperature of 740, 710 and 680 °C were compared when those alloys achieved after gravity casting. After UV+AP treatment, the alloy with pouring temperature of 710 °C show the smallest grain size. Also, the sizes of Fe-rich phases and Al2Cu are greatly reduced and their 3D morphologies are compacted. The mechanical properties of UV+AP treated alloys are relatively higher than those measured for gravity cast equivalents. This improvement can be explained by the synergistic effect of acoustic cavitation, acoustic streaming, and force-feeding, which resulted in the dendrite fragmentation, uniform solute distribution, and microstructural refinement. The Orowan strengthening and solution strengthening were identified as the main strengthening mechanisms.

The Role of Electric Current-Associated Free Energy and Forced Convection on Grain Refinement in Pure Aluminum under Electropulsing

An experimental study with respect to the effect of an alternating electropulsing on grain refinement in pure aluminum was reported. The macrostructural observation with the mold preheated to different temperature and embedded the metal mesh indicated that the change of electric current-associated free energy related with the position of crystal nuclei (ΔGem) and forced convection dominated the generation of fine equiaxed grains (FEG). Under electropulsing with 480 A, ΔGem induced the dissociation of crystal nuclei from the upper interface of the electrode and the melt, leading to the generation of FEG. For a larger current intensity, FEG originated from the dissociation of crystal nuclei on the side wall besides the upper interface due to ΔGem and the forced convection. Furthermore, the model coupling the dissociation of crystal nuclei and dendrite fragmentation due to the forced convection and the dissociation of crystal nuclei due to ΔGem was presented to explain the formation mechanism of FEG in pure aluminum under electropulsing.

Dendrite Fragmentation in Semisolid Casting: Could we Do this Better?

A summary is given of the history of our understanding of dendrite coarsening, including particularly fragmentation. Much is now understood about this process as it takes place in directional solidification of a quiescent melt. Much less is understood about it in the rapidly cooled, turbulent environment of semi-solid casting. The importance of dendrite fragmentation in semi-solid processing is that it is key to obtaining fine final grain size, grain spheroidicity and rapid production rate. I have chosen in this keynote paper to talk about the fundamentals of an important part of the semisolid casting process ... that of “dendrite fragmentation.” The paper is written with an eye to its possible practical usefulness to researchers in process innovation. If we understood the dendrite fragmentation mechanism better, could we achieve finer, more numerous, grains than we do now? Could fully non dendritic structures be obtained industrially in short processing times?

On the Formation of Micro-Shrinkage Porosities in Ductile Iron Cast Components

A combination of direct austempering after solidification (DAAS) treatment and electron backscatter diffraction (EBSD) method was used to study the formation of micro-shrinkage porosities in ductile iron. Analyzing the aus-ferritic microstructure revealed that most of micro-shrinkage porosities are formed at the retained austenite grain boundaries. There was no obvious correlation between the ferrite grains or graphite nodules and micro-shrinkage porosities. Due to the absolute pressure change at the (purely) shrinkage porosities, the dendrite fragmentation rate during the DAAS process would be altered locally, which caused a relatively finer parent-austenite grain structure near such porosities.

Dendrite fragmentation: an experiment-driven simulation

The processes leading to the fragmentation of secondary dendrite arms are studied using a three-dimensional Sn dendritic structure that was measured experimentally as an initial condition in a phase-field simulation. The phase-field model replicates the kinetics of the coarsening process seen experimentally. Consistent with the experiment, the simulations of the Sn-rich dendrite show that secondary dendrite arm coalescence is prevalent and that fragmentation is not. The lack of fragmentation is due to the non-axisymmetric morphology and comparatively small spacing of the dendrite arms. A model for the coalescence process is proposed, and, consistent with the model, the radius of the contact region following coalescence increases as t 1/3 . We find that small changes in the width and spacing of the dendrite arms can lead to a very different fragmentation-dominated coarsening process. Thus, the alloy system and growth conditions of the dendrite can have a major impact on the fragmentation process. This article is part of the theme issue ‘From atomistic interfaces to dendritic patterns’.