THE EFFECT OF STRUCTURE DEFECTS ON THE FATIGUE BEHAVIOUR OF AZ91 MAGNESIUM ALLOY PREPARED BY SAND CASTING AND SQUEEZE CASTING

摘要

The microstructure, tensile and fatigue properties of the magnesium alloy AZ91 prepared by two different casting methods, i.e. by conventional sand casting and squeeze casting, were studied. In addition, the morphology of fracture surface of fatigue specimens was examined. The mechanisms of crack initiation and growth were studied, too. Correlations between the occurrence of structure defects and the deterioration of alloy properties were looked for. The tensile and fatigue properties of the squeeze-cast samples were found to be better than those of the sand-cast samples. The former samples possess a finer grain structure and, in contrast to the latter, rarely contain tiny pores. The mean fatigue life for 50 percent probability to failure of the sand-cast alloy is of 3.3 x 10 cycles at maximum stress of 90 MPa and 3.9 x 10~4 cycles at maximum stress of 100 MPa. The squeeze-cast alloy exhibits fatigue life of 3.3 x 10~5 cycles at the higher load, i.e. 100 MPa. The number of fatigue cycles to fracture is affected appreciably by structure defects, i.e. by the presence of pores in the sand-cast material and of inclusions, oxide film and phase clusters in the material prepared by squeeze casting.