Study on typical hole defects in AZ91D magnesium alloy prepared by low pressure lost foam casting

The hole defects can easily occur in magnesium alloy castings that are prepared by low pressure lost foam casting(LP-LFC)process when the process parameters such as vacuum,pouring temperature and f illing velocity are not properly selected.In this study,the forming mechanism of the hole defects in AZ91D magnesium castings by LP-LFC process was investigated.The shape,location and surface appearance of the hole defects were observed using optical microscopy and scanning electron microscopy,and the chemical composition on the surface of the holes was analyzed using energy spectrometer.The result indicates that there are two types of hole defects,i.e.,the pyrolysis products related hole defects,including concentrative hole and blow hole defects,and slag related hole defects.The concentrative hole and the blow-hole defects were formed either by the liquidEPS degradation products entrapped in the molten metal under the condition that the pouring temperature is equal to or lower than 730℃,or by the hindered transport of EPS pyrolysis products.Some irregular shape hole defects were caused by slag or by coating slough entrapment when the pouring temperature is equal to 750℃and the f illing velocity is equal to or greater than 100 mm·s-1.To reduce or eliminate the hole defects,the vacuum and f illing velocity must be properly chosen to ensure that the metal front prof ile exhibits convex shape and in laminar current state,and the pouring temperature should be just high enough to ensure that the molten melt has adequate heat energy to complete the foam pyrolysis and to fully occupy the mould.For AZ91D magnesium castings in this study,the parameters should be 730℃pouring temperature,0.02-0.03 MPa vacuum and 80mm·s-1f illing velocity.