Rapidly Solidified Structures and Their Mechanical Properties in Aluminum Alloys

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Experiments are carried out on the effects of heat inputs on the micro-structural features of deposited metals in a commercially pure aluminum, Al-4.5% Mg and Al-4% Zn-2% Mg alloys and followed by the investigations of their effects on the mechanical properties such as tensile, bending, impact and hardness.Grain structures scarcely coarsen with increase in heat inputs, but the dendrite cell size increases markedly, showing a parabolical relationship with them if three-dimensional heat flow seems to be possible, while it is observed to show a linear relationship when two-dimensional heat flow seems to be obtained. These results are in good agreement with weld solidification theory involving the mass transport and heat conduction theories.Concentrations of solute elements at dendrite cell boundaries and their neighbors increase with heat inputs, while those in cell matrix decrease reversely, which means that solute concentrations in cell matrix decrease with coarsening dendrite cell.Mechanical properties of deposited metals in Al-4.5% Mg and Al-4% Zn-2% Mg alloys are markedly reduced with increase in dendrite cell size. In particular, the tensile strength can be represented by a linear relationship with the reciprocal square root of dendrite cell size as given from Petch’s equation.From another angle, however, the mechanical properties of these solidified metals seem to be mainly influenced by the distribution of second phases with eutectic compositions and the concentration of solute elements in the cell matrix. In addition, it is necessary to consider the influences of distributions of iron and silicon as impurities, which tend to form the network at boundaries. These effects of iron and silicon, however, are scarcely observed for a commercially pure aluminum, where they would rather act as strengthening elements.