Effect of combinative addition of mischmetal and titanium on the microstructure and mechanical properties of hypoeutectic Al-Si alloys used for brazing and/or welding consumables

Effects of mischmetal (RE) and/or Ti modifier on the microstructure including alpha-Al dendrites, eutectic Si phases and other secondary phases of Al-Si brazing and/or welding alloys were investigated by differential scanning calorimetry (DSC), optical microscopy (OM), scanning electron microscopy (SEM). The DSC results showed that an addition of RE decreased the eutectic temperature and caused supercooling, promoting the nucleation of eutectic Si crystals. In addition, the maximum temperature of the first endothermic peak varied with the different RE contents, which had a good correlation with the microstructural modification of the eutectic Si phase. The alpha-Al dendrites were well refined by increasing the cooling rate or adding 0.08 wt.% of Ti. When 0.05 wt.% RE was added to the Al-5Si-0.08Ti alloy, the morphology of eutectic Si phase was transformed from coarse platelet to fine fibers and the mechanical properties of the resulting welding rod were well improved. Whereas, when excess RE was added, a large number of beta-Fe phases appeared and the aspect ratios of beta-Fe phases increased. The morphologies and chemical components of two kinds of RE-containing intermetallic compounds (IMCs) were also discussed.