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Hot extrusion tests for the purpose of recycling of cutting chips were carried out for AZ31B and AZ91D magnesium alloys. The tensile properties at room temperature and the surface quality of the extrusions were evaluated. Excellent mechanical properties at room temperature were obtained for an extrusion temperature of 423 K for AZ31B and for 573 K for AZ91D alloy, respectively. The tensile strength and yield strength of AZ31B cutting chip extrusion were higher than those of the extrusion from AZ31B cast billet, probably due to a grain refinement effect. However, the peculiar and wavy surface roughness almost perpendicular to the extrusion direction was formed in the extrusions from cast billet as well as in the extrusions from cutting chips of both alloys. On the extrusion surface of AZ31B alloy cutting chips, a mixed texture of basal and non-basal planes was formed. Moreover, a strong basal plane texture was observed in the layer approximately 0.4 mm from the surface. Discontinuous metal flow due to the difficulty of basal plane slip along the die exit corner with a sharp rectangular edge appeared to form the peculiar and wavy surface roughness.

In order to recycle machined chips, hot extrusion tests were performed on magnesium alloys AZ31B and AZ91D. Two types of extrusion dies were used-one with an exit corner having a sharp rectangular edge N-die and the other with an exit corner having a round edge R-die ; the radius of the round edge was 1.5 mm. In the extrusion tests using N-die, peculiar and wavy surface roughness that was almost perpendicular to the extrusion direction was formed in the extrusions obtained from machined chips of both the alloys. This roughness appears to be due to discontinuous metal flow caused by the basal plane slip along the exit corner of N-die. Therefore, the exit corner of R-die was essentially effective in improving the surface roughness of the extrusions because it led to smooth and continuous metal flow. The surface roughness tended to decrease with an increase in the extrusion ratio R from 19 to 100. Grain boundary sliding in the extrusion surface layer at the die exit appears to be a possible mechanism of another type of surface roughness, particularly in the extrusion tests performed with lower extrusion ratios, i.e., at lower strain rates.

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