Experimental and simulation research on hollow AZ31 magnesium alloy three-channel joint by hot extrusion forming with sand mandrel

Magnesium alloy is one of the lightest metal structural materials.The weight is further reduced through the hollow structure.However,the hollow structure is easily damaged during processing.In order to maintain the hollow structure and to transfer the stresses during the high temperature deformation,the sand mandrel is proposed.In this paper,the hollow AZ31 magnesium alloy three-channel joint is studied by hot extrusion forming.Sand as one of solid granule medium is used to fill the hollow magnesium alloy.The extrusion temperatures are 230℃ and 300℃,respectively.The process parameters(die angle,temperature,bottom thickness,sidewall thickness,edge-to-middle ratio in bottom,bottom shape)of the hollow magnesium alloy are analyzed based on the results of experiments and the finite element method.The results are shown that the formability of the hollow magnesium alloy will be much better when the ratio of sidewall thickness to the bottom thickness is 1:1.5.Also when edge-to-middle ratio in bottom is about 1:1.5,a better forming product can be received.The best bottom shape in these experiments will be convex based on the forming results.The grain will be refined obviously after the extrusion.Also the microstructures will be shown as streamlines.And these lines will be well agreement with the mold in the corner.

Effect of grain refinement induced by wire and arc additive manufacture (WAAM) on the corrosion behaviors of AZ31 magnesium alloy in NaCl solution

Additive manufacturing(AM)of Mg alloys has become a promising strategy for producing complex structures,but the corrosion performance of AM Mg components remains unexploited.In this study,wire and arc additive manufacturing(WAAM)was employed to produce single AZ31 layer.The results revealed that the WAAM AZ31 was characterized by significant grain refinement with non-textured crystallographic orientation,similar phase composition and stabilized corrosion performance comparing to the cast AZ31.These varied corrosion behaviors were principally ascribed to the size of grain,where cast AZ31 and WAAM AZ31 were featured by micro galvanic corrosion and intergranular corrosion,respectively.

Edge crack damage analysis of AZ31 magnesium alloy hot-rolled plate improved by vertical roll pre-rolling

In the present study,through vertical roll pre-rolling of AZ31 magnesium alloy hot-rolled plate at room temperature,the effect of different vertical roll pre-rolling reduction on edge crack of the plate during flat rolling was systematically studied.The evolution of microstructure and texture in the edge and middle of the plate after vertical roll pre-rolling,heating and rolling was analyzed by using EBSD technology.The results show that during the vertical roll pre-rolling,{10–12}primary twins and{10–12}-{10–12}secondary twins dominated the edge deformation,while{10–12}primary twins dominated the middle deformation.With the increase of vertical roll pre-rolling reduction,the twin volume fraction of the edge and the middle increased,and the difference between them decreased gradually.After heating,the twin orientation caused by vertical roll pre-rolling was still maintained,and there was a significant difference in grain size between the edge and the middle.When the reduction rate of flat rolling was 30%,cracks have already appeared in the initial plate,while the vertical roll pre-rolled samples showed extremely high rolling performance.When the reduction rate of flat rolling was 50%,the 8PR sample still had no crack initiation.In addition,after flat rolling of 50%,the initial plate showed a strong basal texture,while the maximum pole density of the 8PR plate was small,and the texture distribution was very scattered,showing the characteristics of weak orientation.

Microstructure and mechanical properties of curved AZ31 magnesium alloy profiles produced by differential velocity sideways extrusion

Lightweight curved profiles are widely utilised in the transportation industry considering the increasing need for improving aerodynamic efficiency,aesthetics and cutting emissions.In this paper,curved AZ31 Mg alloy profiles were manufactured in one operation by a novel process,differential velocity sideways extrusion(DVSE),in which two opposed rams were used.Effects of extrusion temperature and velocity(strain rate) on curvature,microstructure,and mechanical properties of the formed profiles were examined.Profile curvature was found to be more readily controlled by the velocity ratio of the bottom ram v2to the top ram v1,whereas extrusion temperature(T=250,300,350℃)and extrusion velocity(v_(1)=0.1,1 mm/s) slightly affect curvature for a given velocity ratio.A homogeneous microstructure with equiaxed grains(~4.5 μm) resulted from dynamic recrystallisation(DRX),was observed after DVSE(v_(2)/v_(1)=1/2) at 300 ℃ and v_(1)=0.1 mm/s,where the initial billet had an average grain size of ~25 um.Increasing extrusion temperature leads to grain growth(~5 μm) at 350 ℃ and v_(1)=0.1 mm/s.DRX is incomplete at the relatively low temperature of 250℃(v_(1)=0.1 mm/s),and higher strain rate with v1=1mm/s(T=300℃),resulting in inhomogeneous bi-modal necklace pattern grains ranging in size around 1-25 μm for the former and 2-20μm for the latter.Grain refinement is attributed to DRX during the severe plastic deformation(SPD) arising in DVSE,and initiates at the prior boundaries of coarse grains in a necklace-like manner.Compared with the billet,micro-hardness and ultimate tensile strength of the profiles have been enhanced,which is compatible with grain refinement.Also,an obvious increase in tensile ductility was found.However,yield strength slightly decreases except for the complete DRXed case(300℃,v_(1)=0.1 mm/s),where a slightly higher value was found,indicating strengthening by grain refinement is greater than softening caused by texture modification.The initial billet had a strong basal texture wherein the {0002} basal plane is oriented parallel to the extrusion direction(’hard’ orientation),while DVSE results in the profiles having weak basal textures and the {0002} basal plane oriented ~5-10° to the extrusion direction(i.e.towards the orientation for easier slip).This significantly modified texture contributes to the softening of the profiles in the extrusion direction,in which tensile tests were performed,and the related elongation improvement.

Review on the effect of different processing techniques on the microstructure and mechanical behaviour of AZ31 Magnesium alloy

Magnesium(Mg)alloys despite being the ideal candidate for structural applications,owing to their high specific strength and low density,are not widely used due to lack of active slip systems at room temperature in their hexagonal close-packed crystal structure,eliciting poor ductility and formability.Amongst the various series of Mg alloys,the AZ and ZK series alloys have been standouts,as they inherit better room temperature strength and flow characteristics through their solute elements.Grain refinement,as well as eliminating casting defects through metal processing techniques are vital for the commercial viability of these alloys since they play a key role in controlling the mechanical behaviour.As such,this review highlights the effect of different Bulk-deformation and Severe Plastic Deformation techniques on the crystal orientation and the corresponding mechanical behaviours of the AZ31 alloy.However,every process parameter surrounding these techniques must be well thought of,as they require specially designed tools.With the advent of finite element analysis,these processes could be computationally realized for different parameters and optimized in an economically viable manner.Hence,this article also covers the developments made in finite element methods towards these techniques.

Effects of RE on Microstructures and Mechanical Properties of Hot-Extruded AZ31 Magnesium Alloy

Effects of rare earth (RE) additions on microstructure and mechanical properties of the wrought AZ31 magnesium alloy were investigated. The results show that, by adding 0.3%, 0.6% and 1.0% RE elements, the as-cast microstructure can be refined, and the as-cast alloys′ elongation and tensile strength can be improved. After extrusion, the alloy with 0.3% and 0.6% RE additions obtain a finer microstructure and the best mechanical properties, but the alloy with 1.0% RE addition has the coarse Al-RE compound particles in grain boundaries which decreased elongation and tensile properties. Usually, Rare earth (RE) elements were used to improve the creep properties of aluminium-containing magnesium pressure die cast alloys at elevated temperatures. In this paper, it is also found that the high temperature strength of extruded materials can be increased by RE elements additions.

Effect of asymmetric rolling process on the microstructure,mechanical properties and texture of AZ31 magnesium alloys sheets produced by twin roll casting technique

Symmetric rolling(SR)and asymmetric rolling(ASR)processes were carried out on 6 mm thick AZ31 magnesium alloy sheets that were produced by twin roll casting(TRC)technique.Before rolling processes,sheets were heat treated in order to obtain a homogenized microstructure.In this study,for the ASR process the rolling speed ratio between upper roller and lower was selected as 1.25.Both SR and ASR processes were utilized with 40%reduction per passes using 2 pass schedule for a total reduction ratio of 0.67.Symmetric and asymmetric rolled sheets were characterized using optical microscopy(OM),scanning electron microscopy(SEM)and transmission electron microscopy(TEM)techniques.Texture measurements were performed by using X-ray diffraction(XRD)technique and mechanical properties were investigated by tensile tests and also hardness measurements.

Evaluation of self-healing ability of Ce-V conversion coating on AZ31 magnesium alloy

This study investigated the influence of cerium nitrate in vanadate solutions on the properties of Ce–V conversion coatings on AZ31 magnesium alloys,and evaluated the self-healing behavior of the Ce–V conversion coating for AZ31 magnesium alloy.The results showed that the additions of cerium nitrate prevented pentavalent vanadium from reducing to tetravalent vanadium in the coatings during conversion reaction process.Adding appropriate cerium nitrate to vanadate solution led to a thicker coating with a more compact CeVO_(4) layer.The corrosion behavior of the Ce–V conversion coating was investigated by the electrochemical tests and the scratch immersion test in 3.5 wt.%NaCl solution.The self-healing ability of the coating was confirmed from all tests.The surface analysis revealed that the self-healing effect of the Ce–V conversion coating was only provided by the release and migration of vanadium compounds.

Improving the fatigue property of welded joints for AZ31 magnesium alloy by ultrasonic peening treatment

The fatigue property of AZ31 magnesium alloy and its TIG welded joints were investigated. The ultrasonic peening treatment （UPT） was used to improve the fatigue property of the TIG welded joints, which was treated at the weld toe by the UPT process. The test results show that the fatigue strength of the base metal of AZ31 magnesium alloys is 57.8 MPa, and those of the fillet joint and the transverse cross joint are respectively 20. 0 MPa and 17.2 MPa at 2 × 10^6 cycles. The fatigue strengths of two kinds of welded joints treated by the UPT are respectively 30. 3 MPa and 24. 7 MPa, which have been improved by 51.5% and 43.6%, respectively. The fatigue life of the fillet joint specimens is prolonged by about 2. 74 times and the fatigue life of the transverse cross joint specimens is prolonged by about 1.05 times when the stress range is at 40. 0 MPa.

Effects of Er on the microstructure and properties of AZ31 magnesium alloy prepared via the EMS process

The effects of small amounts of the rare-earth element erbium on the microstructure and the mechanical properties of AZ31 magnesium alloy via the electromagnetic stirring （EMS） process have been studied. It has been shown that AZ31-Er alloys are mainly composed of α-Mg solid solution and β-Mg17TA112 phases. When the Er content reaches 0.12 wt.%, the characteristic peaks of A12Er can be observed. The micro- structure is obviously refined and the tensile strength of the AZ31-based alloy at ambient temperature is significantly improved by contents of 0.03 wt.% Er, especially the elongation （8= 19%）. More addition of Er obviously decreases the tensile strength and elongation of the AZ31-based alloy because of the grain coarsening and the reduction of β-Mg17A112 phases.

Mathematical modeling of thermo-mechanical behavior of strip during twin roll casting of an AZ31 magnesium alloy

The effect of set-back distance on the thermo-mechanical behavior of the strip during twin roll casting(TRC)of an AZ31 magnesium alloy was modeled using finite element method(FEM).Model validation was done by comparing the predicted and measured exit strip surface temperature as well as the secondary dendrite arm spacing(SDAS)through the thickness of the sheet to those measured during experiments.Model results showed as the set-back distance increases,the strip exit temperature decreases and the solidification front moves toward the entry of the roll gap.The cast strip also experiences more plastic deformation and consequently,the normal stress on the strip surface and effective strain at the strip center-line increase.Moreover,higher separating forces were predicted for longer set-back distances.Model predictions showed that changing the set-back distance by varying the final thickness has a more significant effect on the temperature and stress-strain fields than altering the nozzle opening height.

Microstructural evolution characterization of friction stirring welded AZ31 magnesium alloy

A friction stirring welding to joint 5 mm rolled AZ31 magnesium had beendeveloped. The microstructures in various regions including the weld nugget, thermo-mechanicallyaffected zone (TMAZ) and heat affected zone (HAZ) were investigated and compared with unaffectedparent metal using optical microscopy. The results showed that the heat and mechanical process hadgreat effect on the microstructure evolution. In weld nugget, the heat was enough to producesufficient superplastic material flow and the mechanical effect was greatest, and the dynamicrecrystallization was completed thoroughly. In TMAZ, the mechanical effect was indirectly affectedby the welding tool and only some grains had undergone dynamic recrystallization. The variousregions were studied in detail to better understand the microstructure evolution during frictionstirring welding (FSW). The cross section near the 'key hole' showed clear onion rings because thematerial was stirred only by the rotation of the probe and materials rotated with the probe and didnot move along welding direction and in vertical direction, there was no material flow and the flowmovement can be regarded as two dimensional layer flow.

The Stamping Formability of AZ31 Magnesium Alloy Sheet Improved by Equal-channel Angular Pressing

Equal channel angular pressing （ECAP） processing and annealing were applied to the AZ31 magnesium alloy sheets to evaluate the potential improvement in the mechanical properties and formability. The ECAP experiment was conducted at 300 ℃ in a die having an included angle of 90o between two channels by the BCZ route with the sheets rotated by 90°about the normal axis of plate plane. The tensile tests and conical cup tests were conducted at various temperatures from 20 to 250 ℃. The experimental results indicated that improving the working temperatures could lead to the soft in the material and the enhancement of ductility. Comparatively, the ECAPed AZ31 alloy sheets showed the lower yield strength and smaller conical cup value （CCV） than the unECAPed counterpart in the room temperature. The difference in yield strength between them became small in the elevated temperature, but the ECAPed samples still had the smaller CCV value, implying the improved formability. The texture of the AZ31 alloy sheets could be modified by ECAP and the decrease in the yield strength and more uniform deformation realized in the material, so the formability of AZ31 alloy sheets was improved.

High speed cutting of AZ31 magnesium alloy

Using LBR-370 numerical control lathe,high speed cutting was applied to AZ31 magnesium alloy.The influence of cutting parameters on microstructure,surface roughness and machining hardening were investigated by using the methods of single factor and orthogonal experiment.The results show that the cutting parameters have an important effect on microstructure,surface roughness and machine hardening.The depth of stress layer,roughness and hardening present a declining tendency with the increase of the cutting speed and also increase with the augment of the cutting depth and feed rate.Moreover,we established a prediction model of the roughness,which has an important guidance on actual machining process of magnesium alloy.

Unveiling the mechanical response and accommodation mechanism of pre-rolled AZ31 magnesium alloy under high-speed impact loading

Split Hopkinson pressure bar(SHPB)tests were conducted on pre-rolled AZ31 magnesium alloy at 150–350℃ with strain rates of 2150s-1,3430s^(-1) and 4160s-1.The mechanical response,microstructural evolution and accommodation mechanism of the pre-rolled AZ31 magnesium alloy under high-speed impact loading were investigated.The twin and shear band are prevailing at low temperature,and the coexistence of twins and recrystallized grains is the dominant microstructure at medium temperature,while at high temperature,dynamic recrystallization(DRX)is almost complete.The increment of temperature reduces the critical condition difference between twinning and DRX,and the recrystallized temperature decreases with increasing strain rate.The mechanical response is related to the competition among the shear band strengthen,the twin strengthen and the fine grain strengthen and determined by the prevailing grain structure.The fine grain strengthen could compensate soften caused by the temperature increase and the reduction of twin and shear band.During high-speed deformation,different twin variants,introduced by pre-rolling,induce different deformation mechanism to accommodate plastic deformation and are in favor for non-basal slip.At low temperature,the high-speed deformation is achieved by twinning,dislocation slip and the following deformation shear band at different deformation stages.At high temperature,the high-speed deformation is realized by twinning and dislocation slip of early deformation stage,transition shear band of medium deformation stage and DRX of final deformation stage.

Research on deformation mechanism of AZ31 magnesium alloy sheet with non-basal texture during uniaxial tension at room temperature:A visco-plastic self-consistent analysis

The relationship between activities of involved deformation mechanisms and the evolution of microstructure and texture during uniaxial tension of AZ31 magnesium alloy with a rare non-basal texture has been thoroughly investigated in the present study by means of electron backscattered diffraction(EBSD) measurement and visco-plastic self-consistent(VPSC) modeling. These results show that except basal slip and prismatic slip, {10■2} extension twin(ET) also plays a significant role during plastic deformation. With the increasing tilted angle between loading direction and rolling direction(RD) of sheet, the activity of {10■2} ET possesses a decreasing tendency and its role in plastic deformation changes from the one mainly sustaining plastic strain to the one mainly accommodating local strain between individual grains. When {10■2} ET serves as a carrier of plastic strain, it mainly results in the formation of basal texture component(c-axis//ND, normal direction). By comparison, when the role of {10■2} ET is to accommodate local strain, it mainly brings about the formation of prismatic texture component(c-axis//TD, transverse direction). At large plastic deformation, the competition between basal slip and pyramidal<c+a> slip is responsible for the concentration of tilted basal poles towards ND within all deformed samples. The larger difference is between the activities of basal slip and pyramidal <c+a> slip, the smaller separation is between these two tilted basal poles. Besides,VPSC modeling overesttmates volume fractions of {10■2} ET in samples with angle of 0 to 30° between loading direction and RD of sheet because interactions between twin variants are not included in VPSC modeling procedure at the present form. In addition, as compatible deformation between individual grains cannot be considered in VPSC modeling, the predicted volume fractions of {10■2} ET in samples with angle of 45 to 90° between loading direction and RD of sheet are smaller than the correspondingly measured results.

Effect of welding speed on the material flow patterns in friction stir welding of AZ31 magnesium alloy

The clear zigzag-line pattern on transverse cross sections can be used to explain the formation mechanism of the weld nugget when friction stir welded AZ31 magnesium alloy without any other insert material is used as mark. It provides a simple and useful method to research the joining mechanism of friction stir welding. The rotation speed is kept at 1000 r/min and the welding speed changes from 120 mm/min to 600 mm/min. The macrostructure on the transverse cross section was divided into several parts by faying surface. The results show that the shape and formation procedure of the weld nugget change with the welding speed. There are two main material flows in the weld nugget： one is from the advancing side and the other is from the retreating side. A simple model on the weld nugget formation of FSW is presented in this article.

Improved Mechanical Properties in AZ31 Magnesium Alloys Induced by Impurity Reduction

Mechanical properties and microstructures of AZ31 magnesium alloys containing different impurity levels but having the same alloying element content, were investigated at ambient temperature. These AZ31 alloys were produced by semi-continuous casting, wherein the content of impurity was varied systematically. Microstructure observation shows that finer grains are existent in the alloy with lower impurity level. Tensile testing reveals that a reduction of impurity content results in a noticeable increase of the strength and elongation in the alloys in the cast, homogenized and extruded states. As the impurity content decreases from 0.0462wt% to 0.0163wt%, the ultimate tensile strength is evidently enhanced by 62 MPa and the elongation is nearly doubled in the homogenized specimen. The observed property improvement was discussed in terms of the microstructure variation with impurity reduction.

Microstructure evolution and corrosion resistance of AZ31 magnesium alloy tube by stagger spinning

This study fabricates an AZ31 magnesium alloy tube by spinning technology-power stagger forward spinning.The microstructure evolution of the tube is investigated by combining electron backscatter diffraction and transmission electron microscopy analysis,and the corrosion resistance is measured by an electrochemical corrosion test.Results show that the grains are obviously more uniform and finer along the wall thickness’s direction of the AZ31 alloy tube after the third spinning pass.The number of twins ascends first and then descends,while the varying trend of low-angle grain boundaries(LAGBs)is opposite to that of the twins as the spinning pass increases.With the increase of the total spinning deformation,the deformation texture initially increases and the c-axis of the{0001}crystal plane gradually rotates to the axial direction of the tube;the deformation texture then decreases and the orientation of grains becomes more random.The main mechanism of grain refinement is dynamic recrystallization by the twin-induced way and bowing out of the nucleation at grain boundaries during the first and second pass.However,the dominant mechanism of the refined grain is the high-temperature dynamic recovery in the third pass,and the microstructure mainly consists of substructured grains.After the spinning deformation,the corrosion resistance of the AZ31 alloy tube decreases due to the combined effect of twins and high density-dislocations.

Effect of driver roll rotational speed on hot ring rolling of AZ31 magnesium alloy

Based on the ABAQUS/Explicit code,A 3D elastic-plastic and coupled thermo-mechanical FE model of radial ring rolling of AZ31 Magnesium alloy has been proposed to analyze the influence of rotational speed of driver roll to study the inhomogeneity distribution of strain and temperature,fishtail coefficient,rolling force parameters.The results show that:(1)when the rotational speed of driver roll n increases,the strain distribution of the rolled ring becomes less homogeneous,and the temperature distribution more homogeneous yet,and leading to an optimal n value;(2)the fishtail coefficient firstly decreases,then increases with the increase of n;(3)the rolling force,contact area and rolling moment gradually descend with the increase of n.