Papers by Author: A.N. Turchin

Abstract: The interaction between flow and progressing solidification front is of great importance, since it occurs in all casting processes. The present paper provides a better understanding of the flow phenomena and associated complex effects on solidification in a rectangular cavity under forced flow conditions, by means of experiments and computer simulations. It is shown that the cavity-driven flow with solidification is determined by several interacting features. The variation in bulk flow velocity and initial superheat dramatically changes the macro- and microstructure, promoting grain refinement, formation of peculiar grain and dendrite morphologies, etc. In particular, twinned feathery grains are found in the structure formed under certain heat and flow conditions during solidification. Some correlations between twinned feathery morphology, flow and solidification parameters are obtained. The effect of flow vortices on progressing solidification front and their effects on structure evolution are analyzed. Finally, the quantitative correlations between microstructure, solidification and flow parameters are established.

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.

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.