Characterisation of Al-Cu and Al-Mg-Si Free-Cutting Alloys

Free cutting alloys of Al-Cu (AA2011 and AA2111B) in T6 temper and Al-Mg-Si system (AA6023 and AA6262) in tempers T6 and T8 were subjected to Charpy U - notch impact testing at the temperatures ranging from 20 degrees C to 350 degrees C. The microstructure of the materials was characterized by light metallography, fracture surfaces were observed using scanning electron microscope (SEM). The alloys showed a significant decrease in the impact energy KU at temperatures similar to 125 degrees C (AA2011, AA2111B), similar to 170 degrees C (AA6023), and similar to 250 degrees C (AA6262), respectively. This decrease of KU was caused by melting of disperse phases containing low-melting point metals (Pb, Sn, Bi), which was confirmed by differential scanning calorimetry. Additional annealing of the AA6262-T8 alloy for 2h at 400 degrees C followed by slow cooling led to the transformation of Pb + Bi particles accompanied by the shift of the melting temperature from 250 to 310 degrees C. Higher temperature solution annealing of the AA6023 alloy for 30 min at 540 degrees C (as a replacement of common 30 min at 520 degrees C) resulted in a partial transformation of Sn + Bi particles accompanied by melting point shift from similar to 170 to similar to 200 degrees C. Chemical composition of the corresponding phases was monitored by energy dispersive X-ray spectroscopy in SEM.