Strength-ductility balance of AZ31 magnesium alloy via accumulated extrusion bonding combined with two-stage artificial cooling Author links open overlay panel

AZ31 Mg alloy with heterogeneous bimodal grain structure(smaller grain size of 5-20µm and coarser grain size of 100-200µm)was subjected to accumulated extrusion bonding(AEB)at 250℃combined with two-stage artificial cooling in this work,viz.local water cooling and artificial cooling.The microstructure developed consecutively as a result of discontinuous dynamic recrystallization(DDRX)for the AEBed samples.{10-12}tensile twinning also played an important role for the AEB with local water cooling at the initial extrusion stage in the container.Local water cooling could further reduce the DRXed grain size to~2.1µm comparing that without water cooling.And the grain growth rate was reduced by artificial cooling out of extrusion die.Under the combination of two-stage cooling,the fine DRXed grains at sizing band were almost retained with average grain size of~2.3µm after the sheet out of extrusion die,and the unDRXed grains with high residual dislocation density accumulation were also reserved.The tensile tests results indicated that a good strength-ductility balance with a high ultimate tensile strength(319 MPa vs.412 MPa)and fracture elongation(19.9%vs.30.3%)were obtained.The strength enhancement was mainly owing to the grain refinement and local residual plastic strain reserved by the artificial cooling.The excellent ductility originated from fine DRXed microstructure and ED-tilt double peak texture.

Improved strength and ductility of AZ31B Mg alloy sheets processed by accumulated extrusion bonding with artificial cooling

In this study,accumulated extrusion bonding(AEB)process with application of artificial water cooling was successfully performed to fabricate fined-grains AZ31B Mg alloy sheets at 150℃,200℃ and 250℃.The resultant microstructure and mechanical properties are systematically investigated.It reveals that the processing temperature has an important effect on the microstructural evolution during extrusion.During AEB process at 150℃ and 200℃,{10-12}tensile twinning was activated at early stage of extrusion,and subsequently continuous dynamic recrystallization(CDRX)occurred and dominated the further deformation.However,for the sample extruded at 250℃,hardly any twins can be observed,and new fined dynamic recrystallized grains were found along grain boundaries.Artificial cooling was utilized to reduce the rate of grain growth out of the extrusion die,resulting the grains significantly refined from 11μm to 2.5μm.Local high dislocation density region was also observed in the microstructure of sample processed at 150℃ in artificial cooling condition,and the degree decreased with the processing temperature increase.The results summarized from tensile tests indicated that due to the grain refinement the strength and ductility was significantly enhanced(YS of 186 MPa vs.145 MPa,UTS of 391 MPa vs.336 MPa and FE of 31.5%vs.24.5%compared with the as-received sample).Subsequently,annealing treatment at different temperatures was applied to eliminate the high dislocation density.The sample annealed at 200℃ exhibited the best comprehensive mechanical property with YS of 179 MPa,UTS of 390 MPa and FE of 33.0%.As the annealing temperature increasing,the dislocation density was reduced by static recrystallization(SRX)and grain growth,leading to a decreased strength and ductility.

Variation in Deformation Behaviors Along the Transverse Direction During the Warm Rolling of a 1480-mm-Wide AZ31B Plate

A decrease in the weight of aerospace vehicles,large ships,weapons,and high-speed trains will increase the demand for wide-width magnesium alloy plates and their composite parts to replace steel and plastic.An investigation was conducted to study the variation in deformation behaviors along the transverse direction during the warm rolling of a 1480-mm-wide AZ31B plate.A uniaxial thermal compression test with a 59%reduction was performed at different positions on a 13.7-mm-thick rolled plate along the width direction at a temperature of 220℃ and 270℃ and strain rate of 15 s^(−1).At the same time,the 13.7-mm-thick plate was rolled in a single pass to 5.6 mm on a mill with a 1725-mm-wide roll to confirm the thermal deformation behavior and the dynamic recrystallization(DRX).The results show that the main texture type does not change and the grain size does not have a clear deflection when the magnesium alloy plate reaches a certain value under rolling accumulative reduction.The grain size of a 13.7-mm-thick plate increases with a decrease in the distance to the center layer in the thickness direction.In the width direction,the edge(R6)first decreases and then increases toward the symmetric plane(R1).The critical stress required for dynamic recrystallization in the transition zone R3 of the rolled plate width is minimum,and the average grain size is minimum owing to the relatively complete recrystallization.