Until recently, the most important free-machining aluminium alloys were from the AA2000 and AA6000 series with additions of lead or combinations of lead, bismuth, and tin. Due to lead toxicity, international directives (ELV, RoHS, REACH) were adopted in the European Union, which from 1 July 2008 limit the use of lead in aluminium alloys to a maximum of 0,4 wt.%. In recent years, even stricter measures have been imposed, which is why manufacturers are striving to develop new free-machining aluminium alloys without the addition of lead. The aim of the master's thesis was a synthesis of AA6026 alloy with the addition of bismuth, as a substitute for toxic lead.
Within the experimental work of the master's thesis, we produced the alloy AA6026 with 1,1 wt.% of bismuth. We studied how the addition of bismuth at various stages of alloy manufacturing affects the composition, shape and distribution of bismuth phases in the alloy. In the first case, bismuth was added to the induction furnace, in the second in the holding electric resistant furnace, and in the third example few minutes prior to casting. Characterization of the microstructure was carried out by optical and scanning electron microscopy (SEM) and x-ray diffraction analysis (XRD). The temperature of the phase transitions during heating and cooling was followed by the differential scanning calorimetry (DSC) method.
On the basis of the obtained results, we developed a solidification model of the alloy with an emphasis on the formation of bismuth phases. It was found that bismuth was bound with magnesium in the Mg3Bi2 phase in all cast samples, which appeared in the microstructure in two forms: as the primary α-Mg3Bi2 phase in the form of angular and hexagonal particles, which was located inside the primary αAl crystals, as well as the rod-like eutectic which segregates on the grain boundaries together with Mg2Si or Al15Si2(FeMn)3 phases. We have confirmed that the addition of bismuth at different stages of alloy manufacturing affects the distribution and the fraction of bismuth phases. The smallest fraction of primary α-Mg3Bi2 particles were phases formed in the case of bismuth addition in the induction furnace.