Cold Wire Drawing Deformation And Recrystallization Behaviours Of AZ31 Mg Alloy

AZ31 magnesium alloy has been recognized as one of the most promising wrought Mg alloys due to the excellent mechanical properties and plastic deformation capability. However, it is also restricted when deformed at room temperature because of the hexagonal close packed (HCP) structure.In this paper, cold wire drawing processing was used to manufacture AZ31 Mg alloy wires. Severe plastic deformation of 75% was successfully carried out by multiple-pass wire drawing with small deformation per pass. Finally, AZ31 Mg alloy wires were drawn from 5.0 mm and 2.0 mm in diameter to 0.09 mm at room temperature with several-pass intermediate annealing treatments. On basis of the high deformation, a series of investigations were studied systematically such as the plasticity mechanism, the work hardening behaviours and the deformation texture evolution with deformation by means of optical microscopy (OM), transmission electron microscopy (TEM), X-ray diffraction (XRD), electron back scatter diffraction (EBSD), differential scanning caborimetry (DSC), microhardness and tentile tests. The primary conclusion was as follows:The severely plastic deformation mechanism was studied by microstructure evolution observation at cold drawn reductions of 10～75%. At reduction less than 30%, deformation twinning and dislocation slip were the main mechanism. At 30～40%, the number of twinned grains was dramatically decreased. The long thin twins {10-11} became the main type of deformation twins. Up to 50～60%, the interaction of deformation twinning and slipping dislocations was enhanced. As a result, plenty of fine grains were observed in the severely deformed zones with the loss of deformation bands and deformation twins. At high deformation of 70%, more and more refined grains were observed with angle grain boundary in the broken deformation strips. The microstructure evolution revealed the mechanism evolved to dislocation slip and refined deformation grains.Grain refinement was greatly dependent on the cold drawn reduction and annealing temperature. The critical cold deformation of AZ31 Mg alloy was 12%. When higher than 12%, the recrystallized grain size was refined with deformation increasing at temperatures lower than 300℃; at temperatures higher than 350℃, abnormal grain growth occurred with deformation increasing. The microhardness tests revealed that the lowest recrystallized temperature of cold drawn alloys was gradually decreased from 260～280℃at to 210～220℃with deformation decreasing from 60% to 10%. With the initial grain size decreasing, the lowest temperature was also decreasing. The schematic sketch of grain refinement versus cold drawn reduction and annealing temperature was achieved based on those experimental results.The mechanical properties greatly improved with cold deformation increasing and initial grain size decreasing. For example, the microhardness increased from 53.7 Hv to 91.3 Hv for the 60% cold drawn wire, and the yield strength (YS) and ultimate tensile strength (UTS) increased to 376Mpa and 477MPa. Grain refinement promoted the enhancement of mechanical properties. The microhardness even improved to 110Hv for the 70% drawn wires with an initial grain size of around 1μm. The YS and UTS improved to 430 MPa and 467 MPa with refined grain size of 1μm. All the cold drawn wires exhibited excellent comprehensive mechanical properties after annealing treatments, with the YS, UTS and elongation of more than 300MPa, 400MPa and 15%, respectively.The deformation texture evolution was the same dependent on the cold drawn deformation. At 30～40%, stable deformation texture was gradually formed with {0002}<10-10> parallel to the wire drawing direction. The intensity of deformation texture was strengthened with cold deformation. At 60～70%, the maximum intensity had little decrease due to the transformation of plastic mechanism and the formation of plenty of refined grains with large angle grain boundary. After annealing treatments, the texture system gradually turned to the typical recrystallization texture: {0002}<11-20> parallel to the wire drawing direction.