Study On Structure Properties And Process Of Extruded Magnesium Alloy

This paper successfully designed double change channel angular pressing (DCCAP)and dural directional extrusion (DDE) combined the technology of equal channelangular extrusion (ECAE) and traditional extrusion, and through the argument of thefinite element simulation and actual extrusion factory, superior performance of extrudedAZ31magnesium alloy bar can be obtained, the extrusion temperature range from513Kto723K. We successfully carried out microstructure and properties for extruded AZ31with typical extrusion texture, the results show that grain refinement and texturevariation have an important effect on the mechanical properties of AZ31alloy. Thedeformation behaviors and the mechanism of grain refinement for DCCAP and DDEtechnology were analyzed via a series of research theory. Meanwhile, the elongatedgrains formation, and its influence on mechanical and corrosion properties were furtherstudied. In addition, the secondly cold forging deformation to extruded pure magnesiumpipe and subsequent static recrystallization (SRX) through annealing process were alsoanalyzed mainly from internal microstructure (such as twin, grain orientation et al) onthe cold deformation behavior and annealing restore, SRX nucleation and grain growth.According to the above research, we can draw some conclusions as follow:①The as-cast AZ31magnesium alloy specimens homogenized at723K for15hwere extruded at different extrusion temperature. The results show that DCCAP andDDE are very effective to refine grains, and their minmum sizes are about9and3μm,respectively. The refinement effect is better than ECAE and traditional extrusion. Thegrain size increases with the extrusion temperature elevated, while decreases with theincrease of extrusion ratio. Basal plane {0001} texture present some angular withextrusion direction (ED), but when the temperature up to573K, basal planes {0001}texture are nearly parallel with ED.②For DCCAP-ed AZ31alloys extruded at523K and673K, the yield strength are190MPa and178MPa, and the fracture strength are285MPa and276MPa, and thefracture strain are11%and14%, and strength factor K and strain hardening exponent nare516MPa,0.17and544MPa,0.20, respectively. This can be attributed to the co-effectof grain size and the extent of DRX; As for DDE-ed AZ31alloys with extrusion ratio4.5and extrusion temperature523K, compressive strength, compressive ratio,elongation and yield strength are up to414MPa,16.27%,14.75and200MPa, respectively. As the extrusion ratio up to10.1, the extrusion temperature at573K,compressive strength, compressive ratio, elongation and yield strength are increased to435MPa,19.5%,18.6%and232MPa. All of extruded AZ31alloys with the sameextrusion ratio, compressive strength, compressive ratio, elongation and yield strengthare presented a decrease tendency with the increase of temperature (≥573K).③The effective strain and strain ratio of DCCAP and DDE can get up to1.44,3.1, and1.93,4.1, meanwhile, effective stress are134MPa and142.96MPa, respectively.Thus, DCCAP and DDE will have excellent grain refinement effect via the theory ofdynamic recrystallization (DRX). The deformation region of DCCAP and DDE can bedivided into upsetting region, shearing region and extrusion ratio region. Themechanism of grain refinement is continuous dynamic recrystallization (CDRX).Simultaneously, coarse grains are likely to subdivide into fine grains due to higherplastic strain imposed.④For DDE-ed AZ31alloys extruded at573K and extrusion ratio of10.1, themicrostructure thermostbilization of transverse plane can be divided into three stages:473~548K, grains are gentle grow linearly;548~673K, grain growth is not changeobviously and placid trend; higher than673K, grains grow promptly. The activationenergies of grains growth for the three stages are80.1,18.1and97.8KJ/mol,respectively. Morever, the mechanical properties are in a good agreement withHall-Petch relationship.⑤Elongated grains formation can be attributed to flow stress on original coarsegrains disadvantage of basal slip and some original grains divided by twinning. The caxis of elongated grains are vertical to ED aroud <1120> direction. During thedeformation process at room temperature, the roatation extent of elongated grains andfine grains is different, resulting in accumulative stress at grain boundaries, thus triggercontraction twinning {1011}、{3032}、{1013}<3032> and double twinning{1011}-{1012}, finally, lead to fracture. There are many tangled dislocations and cellstructure at grain boundaries or in grains some local region, where are vulnerable tocorrosion pits. DDE can better improve the inner microstructure especially elongatedgrains, then, improve AZ31alloys the property to resist corrosion.⑥Twinning is an important deformation mechanism for cold forging on extrudedpure Magnesium, at the outset, extension twinning {1012}<1011> is the main kind oftwinning; With the increase of strain, c axis take place rotation, result in forming largeamount of contraction twinning {1013}<3032> and{1011}<1012>. Most of {0001}basal planes are keep a angle of45°with forging direction (FD) for6%samples,while, most of basal planes {0001} are pendicular to FD for the samples under strains of12%,16%and20%. The grains of14%cold forging samples can be refined from80μmto25μm via annealing SRX. The SRX nucleation sites are mainly at triple boundaryjunctions, contraction twinning and double twinning et al. SRX grains formed byextension twinning are keep almost the same orientation with its matrix, SRX grainsderived from triple boundary junctions are at different orientation from the matrix, butthis can not change the whole texture distribution. The growth of SRX and originalgrains inclined to FD. It can obviously define the stages of restore, SRX and graingrowth via the Hv curves. Increasing annealing temperature can contribute to SRXnucleation and growth, and lessen the duration of dynamic restore and SRX.