No abstract

Porous magnesium alloys, Mg 9Al 0.75Zn(AZ91D), Mg 3Al 1Zn(AZ31B), Mg 3Al 0.25Zn(AZ91D dilution with magnesium), were fabricated by unidirectional solidification under various pressures of hydrogen.Optical micrographic observation of cross sections of the porous magnesium alloys shows that cylindrical pores are aligned in the direction parallel to the solidification direction in the vicinity of bottom of the ingots, while spherical pores are evolved in the region apart from the bottom. The number of the cylindrical pores depends on the concentration of the alloying elements and the solidification velocity. The difference in pore morphology was discussed in the light of the equilibrium phase diagrams.

Porous magnesium and its alloys AZ31B and AZ91D have been fabricated by casting technique and continuous zone melting technique under hydrogen atmosphere ranging from 0.15 MPa to 2.52 MPa. The maximum porosity of porous magnesium in the present work, which was fabricated under hydrogen of 0.2 MPa, is around 53 %. It has been found that many cylindrical pores extending to the solidification direction are formed in magnesium. In the magnesium alloys fabricated by the casting technique, the pores tend to extend more frequently to the solidification direction at the lower concentration of alloying elements. In the case of the continuous zone melting technique, cylindrical pores grow in the transference direction even in alloys with higher concentration of alloying elements, AZ91D, by controlling temperature gradient to be steeper. The latter technique is superior for production of the lotustype porous magnesium alloys.