Effect of pre-induced twinning on microstructure and tensile ductility in GW92K magnesium alloy during multi-direction forging at decreasing temperature

Effect of pre-induced twinning on the microstructure evolution and mechanical properties of extruded Mg-9.26Gd-2.08Y-0.36Zr(GW92K)alloy have been investigated during multi-direction forging with large strains at decreasing temperature from 400℃ to 300℃.The results showed that,whether there pre-induced twinning existing in the initial microstructure by pre-deformation or not,a mixed microstructure of residual coarse grains and notably refined grains formed under both conditions,combing with some residual coarse grains with less deformation inside grains and lots of dispersed nano-precipitates distributed along refined grain boundaries.However,a significant improvement with the tensile ductility was obtained by the pre-induced twinning in the former alloy.It was suggested that,the pre-induced twinning largely contributed to the grain refinement and lead to an increase in the ratio of fine grain structure which would be responsible for the better properties.Furthermore,during subsequent forging deformation in the pre-deformed sample,the grain refinement mechanism by gradual grain orientation rotation around the surface section in residual coarse-grains was a little different from that by the macro-shear deformation in the as-extruded condition.

A two-step superplastic forging forming of semi-continuously cast AZ70 magnesium alloy

A two-step technology combined forging with superplastic forming has been developed to enhance the forgeability of semi-continuously cast AZ70 magnesium alloy and realize the application of the as-cast magnesium alloy in large deformation bullet shell.In the first step,fine-grained microstructure preforms that are suitable for superplastic forming were obtained by reasonably designing the size of the initial blanks with the specific height-to-diameter ratio,upsetting the blanks and subsequent annealing.In the second step,the heat treated preforms were forged into the end products at the superplastic conditions.The end products exhibit high quality surface and satisfied microstructure.Consequently,this forming technology that not only avoids complicating the material preparation but also utilizes higher strain rate superplastic provides a near net-shaped novel method on magnesium forging forming technology using as-cast billet.

Forgeability of AZ31B magnesium alloy in warm forging

To understand the forming behaviour of magnesium alloys, the upsetting test of Mg alloys at evaluated temperature was conducted. It is found that Mg alloys are brittle at temperatures below 200 ℃. When temperature is higher than 400 ℃, Mg alloy is not suitable for forging because of heavy oxidation. Mg alloy shows an excellent workability at temperatures from 250 to 400 ℃. Because in warm forging the flow stress decreases with increasing strain, which is so called work softening, no appropriate material formulation is available for Mg alloys. For the evaluation of flow stress of Mg alloys in warm forging processes, a formula is derived by analyzing the stress data measured at various temperatures in upsetting test. It is demonstrated that the formula fits the flow stress obtained from experiment.

Fracture behaviors of AZ80 magnesium alloy during multiple forging processes

The initiation, propagation and the accompanied dislocation structures of the cracks in AZ80 magnesium alloy during multiple forging processes were investigated. The results show that the cracks firstly initiate at the Mg/Mg17Al12 interface under the hoop tensile stress on equatorial free surface. On further deformation, the cracks in the Mg17Al12 particles tend to propagate along the grain boundaries(GBs) in a zigzag pattern and link with adjacent cracks in other Mg17Al12 particles to form one whole crack, leading to the fracture surface. Low deformation temperature and too many forging passes during the deformation will promote the nucleation of interfacial microcrack inside the specimens due to the strong plastic strain incompatibility and the high internal stresses near the GBs. The loading axis rotating during the process can change the stress field at the tip of cracks, leading to the change of the crack propagating path and assisting in inhabiting microcracking development.

Grain size and texture changes of magnesium alloy AZ31 during multi-directional forging

Grain size and texture changes of magnesium alloy AZ31 were studied in multidirectional forging(MDF) under decreasing temperature conditions.MDF was carried out up to large cumulative strains of 4.8 with changing the loading direction during decrease in temperature from pass to pass.MDF can accelerate the uniform development of fine-grained structures and increase the plastic workability at low temperatures.As a result,the MDFed alloy shows excellent higher strength as well as moderate ductility at room temperature even at the grain size below 1μm.Superplastic flow takes place at 423 K and depends on the anisotropy of MDFed samples.The mechanisms of strain-induced fine-grained structure development and of the plastic deformation were discussed in detail.

Ductility improvement methods for commercial AZ31B magnesium alloy in cold forging

In order to realize cold forging of magnesium alloys in practical application,some methods for ductility improvement of a commercial wrought AZ31B magnesium alloy(Mg-3%Al-1%Zn,mass fraction) at room temperature were suggested.The effects of heat treatment before forging and hydrostatic pressure during forging on the ductility were investigated in cold upsetting and cup forging.High-temperature annealing was effective to reduce the degree of the texture anisotropy of the specimen,and it was found that the forging limit of the annealed specimen was improved in cold forging.On the other hand,cold cup forging of the annealed specimen was carried out with applying counter pressure.By applying counter pressures of 100-200 MPa during forging,the critical punch stroke for forging limit of the specimen without crack was improved by 25%in punch stroke.

Semisolid die forging process,microstructures and properties of AZ31 magnesium alloy mobile telephone shells

A semisolid slurry of AZ31 magnesium alloy was prepared by vibrating wavelike sloping plate process,and the semisolid die forging process,microstructures,and properties of the magnesium alloy mobile telephone shell were investigated.The semisolid forging process was performed on a YA32-315 four-column universal hydraulic press.The microstructures were observed by optical microscopy,the hardness was analyzed with a model 450SVD Vickers hardometer,the mechanical properties was measured with a CMT5105 tensile test machine,and the fractograph of elongated specimens was observed by scanning electron microscopy （SEM）.The results reveal that with the increase of die forging force,the microstructures of the product become fine and dense.A lower preheating temperature and a longer dwell time are favorable to the formation of fine and dense microstructures.The optimum process conditions of preparing mobile telephone shells with excellent surface quality and microstructures are a die forging force of 2000 kN,a die preheating temperature of 250℃,and a dwell time of 240 s.After solution treatment at 430℃ and aging at 220℃ for 8 h,the Vickers hardness is 61.7 and the ultimate tensile strength of the product is 193MPa.Tensile fractographs show the mixing mechanisms of quasi-cleavage fracture and ductile fracture.

High cycle fatigue behavior of the forged Mg-7Gd-5Y-1Nd-0.5Zr alloy

This paper investigated the high cycle fatigue behavior of a forged Mg-7Gd-5Y-1Nd-0.5Zr alloy with different stress concentration factor(Kt),under different stress ratio(R),and along different loading direction.The smooth specimen(Kt=1),under R=0.1 and along longitude direction,shows a high fatigue strength of 162 MPa at 107 cycles.The fatigue behavior of the forged Mg-7Gd-5Y-1Nd-0.5Zr alloy exhibits a high sensitive to the notch.Moreover,change of stress ratio from 0.1 to−1 may also result in a bad fatigue property.The flux inclusions were elongated along longitude direction and/or transverse direction during the forging process of the Mg-7Gd-5Y-1Nd-0.5Zr alloy.The interface between the flux inclusion and the matrix may debond and serve as the crack initiation site during the fatigue loading process,leading to the deterioration of the fatigue property along thickness direction and a high anisotropic fatigue behavior between longitude direction and thickness direction.

Gradient microstructure and superior strength-ductility synergy of AZ61 magnesium alloy bars processed by radial forging with different deformation temperatures

Gradient microstructure modification is a cost-efficient strategy for high strength without compromising ductility,which is urgently needed in the fundamental science of engineering materials.In this study,heterogeneous structures of AZ61 alloy bars with anisotropic gradients(with different grain size distributions from the surface to the center)were observed to exhibit strong strength-ductility synergies under different deformation tem peratures.The results reveal that the grain refinement process under mediumlow temperature deformation conditions(≤350℃)consists of four transition stages along the radial direction,i.e.,twin activations and deformation band formations,dislocation cells and pile-ups,ultrafine sub-grains,and randomly orientated quasi-micron grains.Different deformation temperatures have a great influence on twin activations and deformation band formations,and the high temperature can easily provoke the initiation of non-basal slip.The deformation bands were determined as a primary nucleation site due to their highly unstable dislocation hindrance ability.Analysis in combination with the Radial forging(RF)deformation process,the differences of dynamic precipitates can be attributed to microstructural difference and solubility limit of Al at different tem peratures.By summarizing the tensile test results,the sample forged at 350℃exhibited the best strength-ductility synergy,exhibiting the highest elongation(EL)of 23.2%with a 251 MPa yield strength(YS)and 394 MPa ultimate tensile strength(UTS)in center region,and combined with the highest strength value of 256 MPa YS and 420 MPa UTS in the center region,while the EL was slightly degraded to 19.8%.

Microstructural evolution of Mg-7Al-2Sn Mg alloy during multi-directional impact forging

Multi-directional impact forging(MDIF)was applied to a Mg-7Al-2Sn(wt.%)Mg alloy to investigate its effect on the microstructural evolution.MDIF process exhibited high grain refinement efficiency.After MDIF 200 passes,the grain size drastically decreased to 20µm from the initial coarse grains of~500µm due to dynamic recrystallization(DRX).Meanwhile,original grain boundaries remained during MDIF and large numbers of fine sphericalβ-Mg_(17)Al_(12) particles dynamically precipitated along the original grain boundaries with high Al concentration,acting as effective pinning obstacles for the suppression of DRXed grain growth.Besides,micro-cracks nucleated during MDIF and propagated along the interface between the remained globular or cubic Al-Mn particles and Mg matrix.

Microstructure and mechanical properties of AZ61 Mg alloy prepared by multi directional forging

AZ61Mg alloy was multi directionally forged(MDFed) during decreasing temperature condition from 643 K to 483 K at a true strain rate of 3×10-3 s-1 up to cumulative strain of∑△ε=4.0 at maximum.A pass strain of△ε=0.8 was employed.While average grain size decreased gradually with increasing cumulative strain,the evolution of fine-grained structure strongly depended on the MDF temperature.Under the condition where the temperature was higher than the most adequate one,grain coarsening partially took place during MDF.In contrast,at lower temperature,inhomogeneous microstructure composed of the initial coarse and newly appeared fine grains was evolved.After straining over∑△ε=3.2(i.e.,over 4 passes of MDF) ,equiaxed ultrafine grains(UFGs) having average size of about and lower than 1μm were uniformly evolved.While the MDFed alloy to∑△ε=4.0 possessed relatively high hardness of HV 99,and it accepted further about 20%cold rolling almost without cracking.Because of the superior formability of the UFGed AZ61Mg alloy,the hardness was further easily raised to HV 120 by following cold rolling.

Dynamic precipitation during multi-axial forging of an Mg-7Gd-5Y-1Nd-0.5Zr alloy

The dynamic precipitation behavior during multi-axial forging in an Mg-7Gd-5Y-1Nd-0.5Zr alloy has been investigated and compared with that in static precipitation treatment. The results indicated that dynamic precipitation does occur during multi-axial forging. The dynamic precipitate can be deduced as β phase with face-centered cubic crystal structure (a = 2.22 nm). Most of the β precipitates locate at the dynamic recrystallization grain boundaries. The morphology and orientation relationship is different from that of the β phase formed in the static precipitation treated alloys, although the crystal structure is the same. The precipitation temperature of β phase during MAF is higher than that in the static precipitation treatment.

Forming of magnesium alloy by underwater shock wave

Magnesium alloys are best known for their light weight and high strength-to-weight ratio.In the forging process of magnesium alloys,hot working of has been often carried out.However,the strength of the material is reduced because the material temperature reaches the recrystallization temperature during hot working.Therefore,the cold method working is required to maintain or increase the strength of a material.In this study,we have designed experimental equipment to examine whether the forming of magnesium alloy is possible in the cold working by utilizing underwater shock wave,and the hardness of the material changes by receiving the underwater shock wave.Experimental results revealed increase in coefficient of extension and hardness of materials after receiving a shock wave.

钛含量对锻压态机械壳体镁合金性能的影响

为了研究钛(Ti)含量对锻压态机械壳体Mg-6Al-4Sn-Ti镁合金耐磨损性能和耐腐蚀性能的影响,优化Mg-6Al-4Sn-Ti镁合金中的钛含量,制备了锻压态Mg-6Al-4Sn-xTi(x=0,0.1,0.2,0.3,0.4,0.5(质量分数,%))镁合金试样,并进行了合金显微组织、耐磨损性能和耐腐蚀性能的测试与对比分析。结果表明:合金元素Ti有助于细化锻压态Mg-6Al-4Sn镁合金晶粒、提高合金耐磨损性能和耐腐蚀性能。随着合金元素Ti含量从0逐渐增加到0.5%,合金晶粒先细化后粗化,磨损体积先减小后增大,腐蚀电位先正移后负移,耐磨损性能和耐腐蚀性能均先提高后下降。与不添加Ti的锻压态Mg-6Al-4Sn镁合金相比,添加0.3%Ti的锻压态Mg-6Al-4Sn-Ti镁合金磨损体积减小幅度达21%、腐蚀电位正移幅度达13%。锻压中机械壳体Mg-6Al-4Sn-xTi镁合金的合金元素Ti含量优选为0.3%。

Enhancing strength and ductility in back extruded WE71 magnesium alloy cylindrical parts by introduction of multi-direction forging process

Magnesium cylindrical parts have relatively poor mechanical properties and distinct anisotropy of microstructure,which hinder their application as structural components.To improve the performance of WE71 cylindrical parts,multi-direction forging(MDF)was introduced before back extrusion,and the microstructure and mechanical properties were investigated.Results of microstructure show that the grain size in the outer of the cylindrical bottom is refined from 30.1 to 27.7μm,the micro structure is more uniform and the dislocation density is higher.The bimodal grain structure is formed in the outer of the cylindrical wall,which is ascribed to the formation of MgsRE phases along grain boundaries.These phases result in the Zener pinning effect on grain boundaries and the reduction of DRX volume fraction.The texture type of the cylindrical bottom is<0001>‖ED and the cylindrical wall is<1010>‖ED,and the maximum pole intensity is 1.986 and 1.664,respectively.Results of the tensile test at room temperature show that combined improved strength and ductility of the cylindrical part is attained after introducing the MDF process.The ultimate tensile strength(UTS),yield strength(YS)and elongation are279 MPa,185 MPa and 12%at the bottom and 299 MPa,212 MPa and 20%at the wall.

生物医用镁合金Mg-Zn-Zr的组织性能和腐蚀降解行为研究

选取组织性能和力学性能较好的锻造态Mg-Zn-Zr可降解生物医用镁合金作为研究对象,通过金相组织观察、SEM扫描、XRD物相分析及拉伸试验研究了锻造态Mg-Zn-Zr镁合金的宏观和微观组织以及力学性能,并进一步确定了锻造态Mg-Zn-Zr合金的第二相组成.通过静态浸泡腐蚀失重实验研究纯镁和锻造态Mg-Zn-Zr镁合金在(37±0.5)℃恒温SBF模拟体液中的腐蚀降解速率;采用SEM扫描+EDS能谱分析技术手段对锻造态Mg-Zn-Zr镁合金腐蚀后的试样表面及腐蚀产物形貌进行分析,研究锻造态Mg-Zn-Zr镁合金的腐蚀类型、腐蚀程度及腐蚀机理.结果显示,与纯镁相比,锻造态Mg-5.45Zn-0.38Zr镁合金具有优良的力学性能.随着Zn、Zr的加入,锻造态镁合金的显微组织细化,从而使锻造态Mg-5.45 Zn-0.38Zr合金的力学性能得到很大的提高.锻造态Mg-5.45Zn-0.38Zr静态浸泡腐蚀实验结果表明,与纯镁相比,添加Zn、Zr元素可以提高镁合金耐腐蚀性.用失重法测得的锻造态Mg-5.45Zn-0.38Zr合金在(37±0.5)℃恒温SBF模拟体液浸泡腐蚀240 h后的平均腐蚀速率为0.93 mm/a.锻造态Mg-5.45Zn-0.38Zr合金的耐腐蚀性能优于纯镁.

New extrusion process of Mg alloy automobile wheels

The recent research and development of forged magnesium road wheel were reviewed.Methods of flow-forming,spin forging of manufacturing a forged magnesium alloy wheel were introduced.A new extrusion method was investigated especially. Extrusion from hollow billet was proposed in order to enhance the strength of spoke portion and reduce the maximum forming load. By means of the developed technique,the one-piece Mg wheels were produced successfully by extrusion from AZ80+alloy.At the same time,the existing problems on the research and development of forged magnesium road wheel were analyzed.The impact testing,radial fatigue testing and bending fatigue testing results show that AZ80+wheel can meet application requirement in automobile industry.

锻造态Mg-Zn-Zr/-Y镁合金体外生物相容性研究

研究了锻造态Mg-Zn-Zr和Mg-Zn-Zr-Y镁合金在模拟体液中的pH值变化规律;按照ISO 10993的标准要求,分别制备了锻造态Mg-Zn-Zr/-Y合金不同浓度(100%、50%、10%)脐静脉血管内皮细胞(HUVEC)浸提液,通过体外溶血率测试了锻造态Mg-Zn-Zr/-Y镁合金的溶血率;通过CCK-8法体外细胞毒性试验及细胞形貌考察评价了锻造态Mg-Zn-Zr/-Y镁合金对HUVEC的细胞毒性。结果表明:锻造态Mg-Zn-Zr/-Y在SBF模拟体液中的腐蚀速度均高于纯镁的腐蚀速度;锻造态Mg-Zn-Zr镁合金溶血率为0.6%,锻造态Mg-Zn-Zr-Y镁合金溶血率为0.5%,不会导致严重溶血;锻造态Mg-Zn-Zr/-Y镁合金材料的浸提液浓度为10%时,对细胞的毒性较小,几乎不影响细胞的增殖。

Ca微合金化对Mg-8Gd-1Er-1Zn-0.5Zr-xCa合金显微组织和力学性能的影响

本文研究了Ca微合金化对铸态及锻造态Mg-8Gd-1Er-1Zn-0.5Zr-xCa(x=0.1,0.3,0.5,质量分数,%)合金的显微组织和力学性能的影响规律。结果表明:随合金中Ca含量的增加,合金第二相转变过程为:非连续的鱼骨状Mg_(3)Gd相→粗化的Mg_(3)Gd相+Mg_(5)Gd相+Mg_(2)Ca相→少量Mg_(3)Gd相+Mg_(5)Gd相+Mg_(2)Ca相+块状LPSO相,合金晶粒尺寸减小,第二相体积分数增加,力学性能提高。添加0.5%Ca的合金抗拉强度、屈服强度和伸长率最高,分别为241.0 MPa、149.2 MPa和5.7%。固溶处理后,晶界处形成LPSO相,且其体积分数随合金中Ca含量的增加而增加,促进了合金在后续锻造过程中发生动态再结晶,基面织构强度随之增加,基面Schmid因子随之降低。当合金中Ca含量为0.5%时,合金锻后力学性能最佳,其抗拉强度、屈服强度和伸长率分别达到344.6 MPa,274.5 MPa和8.1%。分析认为基面织构增强和晶界强化是锻造态合金力学性能提高的主要原因。

径锻AZ61镁合金退火后晶粒长大及组织演变规律

目的通过研究径锻AZ61镁合金棒材在后续退火处理时的晶粒长大和组织均匀性演变行为,构建等温条件下径锻AZ61镁合金晶粒长大模型,总结径锻镁合金退火后组织均匀性演变规律,为此棒材后续热处理工艺的制定提供理论借鉴和指导。方法基于系统的等温正交退火试验,量化统计各退火条件下样品组织的平均晶粒尺寸和晶粒尺寸分布,分析径锻AZ61镁合金棒材的晶粒长大行为和组织均匀性演变规律。结果退火后,径锻AZ61镁合金组织中晶粒趋于长大。在250~300℃温度区间内,晶粒长大速度较为缓慢。随着退火温度的升高,晶粒长大速率随之上升,尤其是退火温度升高到450℃以后,晶粒长大速率显著增大。当退火温度一定时,退火起始30 min内,晶粒快速长大。在之后的长时间退火过程中,晶粒长大速度逐渐放缓。同时,随着退火的进行,锻后样品中存在的超细晶条带状组织逐渐消失,样品组织均匀性显著提高。结论通过研究晶粒长大现象构建的等温条件下径锻AZ61镁合金晶粒长大模型与试验结果拟合良好。结合组织均匀性演变规律,径锻AZ61镁合金退火后的组织状态能够被很好地预测。