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Abstract
This thesis focuses on the microstructure evolution during different techniques of grain refinement through melt treatment, conventional and advanced thermomechanical treatment (stir friction processing). Two AZ31 alloys with and without Ce-mishmetal addition were cast under Ar/CO protective atmosphere at 750oC, then homogenized at 460oC for two hours. After that they 2 subjected to conventional thermomechanical treatement (hot forging followed by annealing). Firstly, hot forging operated at 350oC to 30% reduction, then, water quenching. Finally, the forged alloys were annealed at 250oC for 16 minutes. Part of commercial AZ31 was conducted to stir friction process (FSP) as a new thermomechanical treatment technique. Friction stir processing was conducted at rotational speed (885 rpm) and transverse speed (62.5 mm/min). On the other hand, finest structure is observed in AZ31-1%Ce (5æm) as a result of conventional thermomechanical treatment and its microstructure is characterized by incomplete fragmentation of Al Ce precipitates. While, the grain size obtained after friction stir processing is 8æm with few fine 4 precipitates within grains. The results of hardness indicates that the maximum hardness is obtained in both stir friction processed AZ31 (75 HV) and conventionally thermomechanical treated AZ31- 1%Ce (82 HV). At the same time, the maximum yield and ultimate tensile strengths are obtained in both stir friction processed AZ31 (160MPa and 260MPa, respectively) and thermomechanical treated AZ31-1%Ce (179 MPa and 255MPa, respectively). Also, higher elongation is recognized in AZ31 (16%) stir friction processed and AZ31-1%Ce (14%) thermomechanical treated