Study On NZ30K Magnesium Alloy Processed By Repeated Compression Severe Plastic Deformation

Mg-3.0Nd-0.2Zn-0.4Zr (wt.%, NZ30K) magnesium alloy was chosen as the researchmaterial in this article. The couple thermal mechanical numerical simulation of NZ30Kmagnesium alloy during repetitive upsetting (RU) and cyclic closed-die forging (CCDF)was carried out by finite element simulation software DEFRORM-3D. The influence ofprocessing parameters on material flow, temperature field, stress field and strain field wasstudied. The NZ30K magnesium alloy was subjected to repetitive upsetting and cyclicclosed-die forging at400°C for various passes. The influence of RU and CCDF onmicrostructure, mechanical properties and texture evolution was investigated.The numerical simulation of NZ30K magnesium alloy during repetitive upsetting(RU) for0,1,4and8passes was done by finite element simulation softwareDEFRORM-3D. The material flows laterally during RU under the pressure of the punchand gradually fills the lower cavity. The region of stress concentration is at the corner ofthe mould, where the plastic deformation is most intense there. With the increase of RUpasses, the cumulative equivalent strain increases gradually and reaches11.1~12.7after8passes. The distribution of equivalent strain becomes more homogenous with increasingRU passes. The peak load obtained from the software decreases with RU passes.NZ30K magnesium alloy was processed by repetitive upsetting (RU) at400°C for0,1,4and8passes, respectively. The influence of RU on the microstructure evolution andmechanical properties was investigated. Results show that with the increase of RU passes,the grains of the alloy are significantly refined and the microstructure homogeneity allover the sample is greatly enhanced. The alloy exhibits a homogenous equiaxedmicrostructure with an average grain size of4μm after RU for8passes. Both the strengthand ductility are notably improved with RU passes. The YS, UTS and elongation of the 8-passes alloy are208.2MPa,248.5MPa and30.1%, compared with the153.3MPa,214.0MPa and8.4%prior to RU, respectively.The simulation of NZ30K magnesium alloy during cyclic closed-die forging (CCDF)for0,1and4passes was carried out by DEFRORM-3D. NZ30K magnesium alloy wasalso processed by CCDF at400°C for0,1and4passes, respectively. The microstructureevolution and mechanical properties were investigated. The simulation result suggests thatthe material undergoes upsetting deformation and area of cross section is enlarged. Thedegree of deformation in the upper part of the material is larger than that of the lower part.The two ends in upper part of the material always have the largest equivalent strain duringdeformation. With increasing CCDF passes, the cumulative equivalent strain increasesgradually and reaches6.2~8.5after4passes. CCDF is effective in refining grains,improving mechanical properties and ductility of NZ30K alloy. After4passes, theaverage grain size of the alloy decreases from the initial90μm to5μm. The YS and UTSof4-passes alloy reach226.4MPa and259.4MPa, which increase47.7%and21.2%compared with the initial alloy. The ductility of the4-passes alloy is30.7%, whichincreases264.2%compared with that before CCDF.The tensile curve of4-passes RUed sample shows a clear yield phenomenon withapparent upper and lower yield points followed by an immediate increase in flow stress.On the tensile curve of the8-passes alloy, an apparent yielding plateau forms However, noyield phenomenon is observed in the0-pass and1-pass alloy. It appears that the yieldphenomenon during tensile test becomes more pronounced with grain refinement. Similaryield phenomenon was also observed on the tensile curves of the4-passes CCDFed alloy.Yield phenomenon during tensile tests is rarely found in magnesium alloys. It might relatewith mobile dislocations due to the activation of non-basal and basal slip systems.