CORRELATION BETWEEN THERMAL PARAMETERS, STRUCTURES, DENDRITIC SPACING AND CORROSION BEHAVIOUR OF ZN-AL ALLOYS WITH COLUMNAR TO EQUIAXED TRANSITION

ALICIA ESTHER ARES; LILIANA MABEL GASSA; SERGIO FABIAN GUEIJMAN; CARLOS ENRIQUE SCHVEZOV

Salt Lake City, Utah

Conferencia; The 15th INTERNATIONAL CONFERENCE ON CRYSTAL GROWTH; 2007

ICCG15

The columnar to equiaxed transition (CET) has been examined for many years and the significance of CET has been treated in several articles. Experimental observations in different alloy systems have shown that the position of the transition is dependent on parameters like cooling rate, velocity of the liquidus and solidus fronts, local solidification time, temperature gradients andrecalescence. The dendritic structure in alloys results in microsegregation of solute species which affects significantly the mechanical properties of the material. The main parameters characterizing the microstructure and the length range of microsegregation is the spacing which is classified as primary, secondary and tertiary. Properties like mechanical resistance and ductility are influenced by the dimensions and continuity of the primary branches, while the secondary and tertiary branches permit the isolation of interdendritic phases which can deteriorate the mechanical behavior of the material. Since the morphology and dimensions of the dendritic structure is related to the solidificationparameters mentioned above, for each type of alloy it is essential tocorrelate dimensions and solidification conditions in order tocontrol the structure.The objective of the present research consist onstudying the influence of solidification thermal parameters with thetype of structure (columnar, equiaxial or with the CET); and with thedendritic spacing (primary and secondary) in Zn-Al alloys (Zn-2%Al,Zn-4wt%Al, Zn-16wt%Al, Zn-27wt%Al and Zn-50%Al, weightpercent). Also, correlate the type of structure and dendritic spacingwith the corrosion resistance of these alloys. In order to obtain theCET, the alloys were solidified directionally upwards in anexperimental set up consisting of a heating unit, a temperaturecontrol system, a temperature data acquisition system, a samplemoving system and a heat extraction system, with a set ofthermocouples in the samples which permit to determine the timedependent profiles during growth. From these profiles and thelocation of thermocouples it was possible to calculate the coolingrates, growth velocities and the temperature gradients along thesample. The primary and secondary spacings were measured andcorrelated with the solidification parameters. The corrosion studywas realized by electrochemical impedance spectroscopy andpolarization curves. The CET zone and the equiaxed structurespresented a better corrosion resistance than the columnar zone. Thechoice of the alloys is due to their relative importance in the foundryindustry. Zn-Al (ZA) alloys combine high strength and hardness,good machinability with good bearing properties and wearresistance often superior to standard bronze alloys.