Texture Effect On Tension/Compresion Respond Via Combined-Extrusions For Wrought Magnesium Alloy Rods

The strong basal texture in extruded wrought magnesium can often lead to severe tension/compression yield asymmetry along deformation direction,which can limit the application for magnesium alloys in sophisticated working environment such as alternative loading or multi-direction loading.Therefore,based on previous cold deformation researches,this paper investigated influence of upsetting and spiral mold on microstructure and mechanical properties.The microstructure evolutions in secondary extrusion,die upsetting and extrusion,cyclic extrusion and compression were further investigated.The finite element modelling and viscoplastic selfconsistent(VPSC)model were combined to quantitively analyze the influence principle of strain path onto deformation mechanism and texture evolution in the combined extrusion process and specify the basal texture weaken principle by strain path variation.Semi Sachs flow stress prediction model was established to predict the influence of texture types on room temperature quasi-static uniaxial tensioncompression behavior and yield asymmetry.This paper provided theoretical model and data support for the technique realization of basal texture weakening modification and the improvement of load-bearing properties for magnesium alloys.Die upsetting technique for magnesium alloys could reach the effect of weakening of strong basal texture along extrusion direction(ED)and promote recrystallization refinement by twinning division.The spiral mold could only control the texture along circular direction and comparatively higher deformation heat could cause microstructure to be coarsen.Comparing the effect of extrusion ratio and extrusion temperature,it turned out that low temperature and small extrusion ratio in die upsetting and extrusion could attain high tensile and compressive strength in extruded rods with refined microstructure and weak texture.Based on these,aiming for high strength ZK61 magnesium alloy rods,three combined extrusion techniques were adopted to fabricate three typical texture states.Setting ED as reference,secondary extrusion formed strong {0002}//ED basal texture;inclined texture by cyclic extrusion and compression oriented ?45° away from ED was obtained;the inclined texture reduced the asymmetry but also decreased yield values of both tension and compression.The dispersed texture by die upsetting and extrusion almost eliminated the asymmetry and attained high strength of ?300MPa for tension and compression.Although the inclined texture could improve yield asymmetry but accompanied by the value of both yield strengths declining.The microstructure evolution was further investigated for the three combined extrusion techniques.For secondary extrusion,the extruded fiber texture of{0002}//ED maintained unchanged.For die upsetting and extrusion,when compressed,the texture was changed from{0002}//ED to {0002}?ED,and then extruded to {0002} plane deflected 47°?78° away from ED.For cyclic extrusion and compression,with the strain path changing twice from extrusion to compression,{0002} moved into ED when extruded and away from ED when compressed.The angle between {0002} and ED fluctuated its way into 45°.Viscoplastic self-consistent model(VPSC)provided quantitative analysis of the influence of strain path on deformation mode activation for the aforementioned microstructure evolution.The VPSC model enlightened the change in deformation modes and predicted the texture evolvement with accuracy.For secondary extrusion,basal slip and prismatic slip took turns to be dominant.For die upsetting and extrusion,the tensile-compressive change of the strain path could activate the tensile twinning when compressed.For cyclic extension and compression,under the constant change of tension and compression basal slips remained dominant for most of deformation process.Semianalytical Sachs model predicting flow stress and evaluating slip/twinning activity was used to understand asymmetry improvement.In general,weaker scattered basal texture led to less twinning activity,gentler strain hardening slope and low tension-compression asymmetry.For inclined texture,the asymmetry improvement was achieved by decreasing yield stresses in both tension and compression with large slip Schmid factors;for dispersed texture,the suppressed twinning activity in compression elevated compressive yield strength,thus improving the asymmetry.Yield strength prediction equation was further conducted from the Semi Sachs model and simplified the ambient temperature microstructure-property prediction for magnesium alloys.