Abstract
The present paper describes the grain size refinement of AZ31 alloys by means of the HDDR process. Hydrogenation was carried out at 250∼450°C under a hydrogen pressure of 7 MPa for 24∼36 h. Upon hydrogenation, the disproportionation reaction occurred, leading to the formation of MgH2, Mg0.42Al0.58, and Al phases. In the subsequent desorption process, the three phases were recombined, and the AZ31 alloy was recovered. As a result of the HDDR process, the grain size of the AZ31 alloy powders was reduced from 50∼300 μm to about 100 nm in the case of the heat treatment at 350°C under a hydrogen pressure of 7 MPa for 24 h. The grain size tended to be increased with increasing process temperatures. In addition, the HDDR process was also applied to AZ31 alloy plates. By means of the HDDR process at 450°C, the grain size was reduced to less than 500 nm, while the layer thickness being treated was limited to the range of about 20 μm from the surface after 36 h at 7 MPa pressure of H2.
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