Measurement and interrelation of length scale of dendritic microstructures, tensile properties, and machinability of Al-9 wt% Si-(1 wt% Bi) alloys

Al-Si alloys are increasingly being employed in the aerospace industry, satellite bearings, and inertial navigation systems. Hypoeutectic compositions are more attractive because of the low material cost and excellent castability. The addition of alloying elements to these alloys is generally used to modify the morphology of Si particles with a view to meeting mechanical strength requirements. Considering manufacturing requirements, to efficiently predict the quality of the machining process, it is essential to analyze the machinability of the material to be used, which depends on several factors, including microstructural features. In the present study, the effects of solidification cooling rate and Bi addition to Al-Si alloys on microstructural features, tensile properties, and rate of heat generation during necking processes are analyzed in samples of directionally solidified Al-9 wt% Si-(1.0 wt% Bi) alloy castings. The microstructure is composed of α-Al dendrites characterized by measured primary, secondary, and tertiary dendritic arm spacings, with a eutectic mixture of α-Al and Si-(Bi) particles. The addition of Bi is shown to attenuate the sharp tips of Si crystals. The dendritic length scale of the Al-rich matrix is correlated with the solidification cooling rate through experimental equations, and the evolution of the ultimate tensile strength with the primary dendritic spacing is experimentally represented by a single Hall-Petch-type equation for both examined alloys. Considering a same primary dendritic arm spacing, the addition of 1 wt% Bi to the Al-9 wt% Si alloy is shown to have the following effects: similar ultimate tensile strength, the elongation to fracture is improved, higher machinability based on significant decrease in the heating rate.