Low-cycle fatigue properties of an ultrafine-grained magnesium alloy processed by equal-channel angular pressing

Magnesium alloy Mg-3%Al-1%Zn (AZ31) billets prepared from equal channel angular pressing (ECAP) were utilized in low-cycle fatigue tests in order to investigate their fatigue life. Fully reversed strain-controlled tension-compression fatigue tests were conducted at the frequency of 1 Hz in ambient air. The microstructures were examined by optical microscopy (OM) and scanning electron microscopy (SEM). The hysteresis loops of the ECAP processed and conventionally extruded samples display obviously different shapes in the total strain amplitude range from 0.2% to 0.6%. Accordingly, the low cycle fatigue lives of ECAP processed samples are found to be longer than those of extruded samples, which can be attributed to the different in the hysteresis energy incorporating tensile strain energy.     

Damage prediction of 7025 aluminum alloy during equal-channel angular pressing

Equal-channel angular pressing （ECAP） is a prominent technique that imposes severe plastic deformation into materials to en- hance their mechanical properties. In this research, experimental and numerical approaches were utilized to investigate the mechanical prop- erties, strain behavior, and damage prediction of ECAPed 7025 aluminum alloy in various conditions, such as die channel angle, outer comer angle, and friction coefficient. Experimental results indicate that, after the first pass, the yield strength, ultimate tensile strength, and hardness magnitude are improved by approximately 95%, 28%, and 48.5%, respectively, compared with the annealed state, mainly due to grain re- finement during the deformation. Finite element analysis shows that the influence of die channel angle is more important than that of outer comer angle or friction coefficient on both the strain behavior and the damage prediction. Also, surface cracks are the main cause of damage during the ECAP process for every die channel angle except for 90°; however, the cracks initiated from the neighborhood of the central re- gions are the possible cause of damage in the ECAPed sample with the die channel angle of 90°.     

Superplastic Deformation Behavior of Hot-rolled AZ31 Magnesium Alloy Sheet at Elevated Temperatures

1IntroductionAs a typical wrought magnesiumalloy,AZ31alloyhas a wide prospect for applications inthe fields of auto-mobiles,electronic appliances and aeronautic facili-ties[1,2].However,due to the hexagonal close-packed(HCP)structure of magnesium,the ductility of AZ31al-loy at roomtemperature is rather poor,which greatly re-stricts its applications in structural fields[3-5].Owing tothe activation of non-basal slip system[6],the ductility ofMg alloycan be significantlyimproved at elevatedtem…     

AZ31镁合金板冷拉深变形特点

在室温条件下,对最常用的镁合金AZ31板材在经过不同的退火热处理后进行冷拉深试验研究中,借助有限元数值模拟技术对其拉深变形过程进行分析,探索其冷拉深变形特点及规律,并合理解释在拉深过程中的载荷特点、破裂形态、极限拉深比、各向异性现象、厚度分布规律以及退火工艺条件、模具结构及尺寸对它们的影响规律.     

Constitutive analysis of AZ31 magnesium alloy plate

The plastic deformation simulation of AZ31 magnesium alloy at different elevated temperatures (from 473 to 723 K) was performed on Gleeble-1500 thermal mechanical simulator at the strain rates of 0.01, 0.1, 1, 5 and 10 s-1 and the maximum deformation degree of 80%. The relationship between the flow stress and deformation temperature as well as strain rate was analyzed. The materials parameters and the apparent activation energy were calculated. The constitutive relationship was established with a Zener-Holl...     

Hot Deformation Kinetics of Magnesium Alloy AZ31

1 IntroductionManyirreplaceable characteristics andthe abundanceof magnesiumalloyin nature have drawn a world wide at-tentionin 21st century.In contrast with the cast magne-siumalloy,wrought magnesiumalloyis much betterinitscomprehensive mechanical properties ,so a wide applica-tionis found in many fields like transportation,aeronau-tics ,electricity and communication.The flowstress of a material depends upontwo majorfactors ,namely,its own characteristics and its deforma-tionterms .The basic …     

Microstructure and mechanical properties of ultrafine grained Mg15Al alloy processed by equal-channel angular pressing

An as-cast magnesium alloy with high Al content Mg15Al was subjected to equal-channel angular pressing (ECAP) through a die with an angle of ϕ=90° at 553 K following route B_c. It is found that the network β-Mg₁₇Al₁₂ phases in the as-cast Mg15Al alloy are broken into small blocks and dispersed uniformly with increasing numbers of pressing passes. Moreover, many nano-sized Mg₁₇Al₁₂ particles precipitate in the ultra-fine α-Mg matrix. The grains are obviously refined. However, the grain structure is inhomogeneous in different areas of the alloy. The average size of the primary phase α-Mg is reduced to about 1 μm while grains of around 0.1–0.2 μm are obtained in some two-phase areas. With additional ECAP passes (up to 8), coarsening of the grains occurs by dynamic recovery. Room temperature tensile tests show that the mechanical properties of Mg15Al alloys are markedly improved after 4 ECAP passes. The ultimate tensile strength and elongation to failure increase from 150 MPa to 269.3 MPa and from 0.05% to 7.4%, respectively. Compared with that after 4 passes, the elongation to failure of the alloy increases but the strength of the alloy slightly decreases after 8 ECAP passes. Fracture morphology of the ECAP-processed alloy exhibits dimple-like fracture characteristics while the as-cast alloy shows quasi-cleavage fractures.     

AZ31B变形镁合金板材的TIG焊接

采用TIG焊对5 mm厚AZ31B挤压镁合金板材进行了焊接试验。采用万能拉伸试验机、金相显微镜、扫描电子显微镜和显微硬度仪等分析测试手段对焊接接头的组织、力学性能以及断口形貌等进行了分析。探讨了焊接电流对焊接接头的组织及力学性能的影响,揭示了不同焊接电流下焊接接头的断裂机制。结果表明,采用TIG焊焊接5 mm厚AZ31B镁合金板时,开X型坡口,采用双面焊接双面成型工艺,在130~145 A焊接电流及合适焊接速度条件下,能得到表面成型良好的焊接接头。当正反面焊接电流均为145 A时,AZ31B镁合金板焊接接头的抗拉强度达到最大值248.6 MPa,约为母材强度的84.0%。AZ31B镁合金板焊缝区显微硬度比母材区稍有所下降,热影响区显微硬度下降比较严重。当焊接电流为145 A时,AZ31B镁合金板焊接拉伸断口有大量韧窝,属韧性断裂。     

ECAP处理后ZK40的组织及超塑性

研究了镁合金ZK40经等通道挤压1道次和4道次后的超塑性和微观组织.在523 K进行的拉伸实验表明，随着挤压道次的增加，ZK40的延伸率也得到了提高.实验通过对ZK40作不同条件下的拉伸测试，得到了超塑性表现最好的温度和应变速率.经4道次等通道挤压后的ZK40在523K的温度和1×10^-4/s的应变速率下其断裂延伸率达到了660%.微观组织的分析表明，ZK40在等通道处理过程中，晶粒随着挤压道次的增加不断细化，而且晶粒的等轴性和均匀性得到显著改善.     

MgCO_3对AZ31镁合金晶粒的细化

研究了MgCO3对AZ31镁合金凝固组织的影响。结果表明:随着MgCO3的增加,合金中的β-Mg17Al12相由晶内分布变为晶间分布。在AZ31中添加0.6%的MgCO3,于760℃时保温10min,细化效果最佳。α-Mg晶粒的尺寸由基体合金的570μm降至100μm,降低幅度约为82.5%,抗拉强度提高了57.9%,延伸率约提高了11.1%。通过能谱分析,结合能计算及自由能计算证实,细化机理是MgCO3反应后生成的部分Al4C3质点作为异质核心细化晶粒。多余的Al4C3质点钉扎晶界阻碍晶粒长大。Al元素随固/液界面前沿被快速推至晶界,生成沿晶界生长的β-Mg17Al12相,起到进一步固定晶界的作用。合金元素的分布均有改变。     

镁合金AZ31阻尼性能的实验研究

采用脉冲响应衰减法研究镁合金AZ31的减振降噪效果.由脉冲激励激发镁合金薄板自由振动,测得镁合金薄板表面的辐射声压,将声压信号进行带通滤波,分离出薄板各个简正振动对应的辐射声压,或分析带宽内几个简正振动产生的复合声压,将该声压信号曲线进行多项式拟合,由拟合曲线的斜率得出镁合金的阻尼损耗因子,并与钢的阻尼性能进行比较.结果表明,镁合金的阻尼损耗因子随频率的增大逐渐减小,在660、1090、1230和1760Hz处存在阻尼峰值,是减振性能最佳的频率.在中低频段,镁合金的阻尼性能约为钢的2、3倍.镁合金构件在使用中必将体现出良好的抗冲击和减振降噪性能.     

As-extruded AZ31B magnesium alloy fatigue crack propagation behavior

The fatigue crack propagation rate of as-extruded AZ31B magnesium alloy was studied. Compact tension [C(T)] of the notch direction parallel (T-L), vertical (L-T), and inclined at 45o to the extrusion direction was investigated. The experimental results indicate that the crack propagation direction is parallel to the extrusion direction for T-L and L-T specimens, whereas the specimen inclined at 45o has an angular deflection of 9° to 11° toward the extrusion direction. The T-L specimen has the fastest fatigue crack propagation rate, and the L-T specimen has the slowest rate, the fatigue crack propagation rate of the specimen inclined at 45o is between the two directions. The crack tip propagates by both transgranular and intergranular fractures. Fatigue fractures consist of cleavage plane or quasi-cleavage and are brittle fractures. The fatigue striation occurs for specimens inclined at 45o and its size is 3-15 μm.     

Microstructure and mechanical properties of ZE10 magnesium alloy prepared by equal channel angular pressing

ZE10 magnesium alloy was subjected to equal-channel angular pressing(ECAP) up to 12 passes in a die with an angle of 120° between the two channels at 250-300°C.An inhomogeneous microstructure of bimodal grains including fine grains of 1-2 μm as well as coarse grains of about 20 μm was obtained after the initial 1-4 ECAP passes.The grain size became increasingly homogeneous with further ECAP processing and the grains were significantly refined to 1-2 μm after 8 passes and further refined to 0.5-1 μm after 12...     

Thermal-stress simulation of direct-chill casting of AZ31 magnesium alloy billets

Two-dimensional (2D) transient coupled finite element model was developed to compute the temperature and stress field in cast billets, so as to predict the defects of the I-type billets made from AZ31 magnesium alloy and find the causes and solutions for surface cracks and shrinkages during direct-chill (DC) casting process. Method of equivalent specific heat was used in the heat conduction equation. The boundary and initial conditions used for primary and secondary cooling were elucidated on the basis of the heat transfer during the solidification of the billet. The temperature and the thermal-stress fields were simulated with the thermal-structural coupled module of ANSYS software. The influences of casting parameters on the distributions of temperature and stress were studied, which helped optimize the parameters (at pouring temperature of 680 °C, casting speed of 2 mm/s, heat-transfer coefficient of the second cooling equals to 5 000 W/m²·°C⁻¹). The simulation results of thermal stress and strain fields reveal the formation mechanism of some casting defects, which is favourable for optimizing the casting parameters and obtain high quality billets. Some measures of controlling processes were taken to prevent the defects for direct-chill casting billets. Funded by the 973 National Grand Theoretical Research Program(No. 2007CB613700), the National Sci&Tech Support Program(No. 2007BAG06B04), National Natural Science Foundation of China (No. 50725413), and the Natural Science Foundation of Chongqing(No. CST, 2007bb4413)     

Microstructure evolution of AZ31 Mg alloy during change channel angular extrusion

Microstructure evolution of AZ31 Mg alloy during change-channel angular extrusion (CCAE) was investigated. The grains of AZ31 Mg alloy were refined significantly from 500 mm to 15 mm after CCAE deformed at 523 K. Dislocations were induced at the initial stage of extrusion and they rearranged themselves to form dislocation boundaries and sub-grain boundaries during deformation. When the specimen through the horizontal change channel with the strain increased, the sub-boundaries evolved to high angle grain boundaries (HAGB). The process of grain refinement can be described as continuous dynamic recovery and recrystallization (CDRR).     

焊接电流对AZ31镁合金接头的影响

为了研究AZ31镁合金的焊接性,对AZ31镁合金板进行交流钨极氩弧（TIG）焊.试验采用x 射线衍射仪、扫描电子显微镜、光学金相显微镜对试样的焊缝显微组织进行分析,并对不同焊接电流下的试样进行抗拉强度和硬度测试.研究发现：随焊接电流的增加,焊缝成形变差,晶粒逐渐粗化,同时易产生气孔和裂纹等缺陷,使接头性能降低;焊缝区由基体α Mg和附集于晶界的β Al12 Mg17两相组成.结果表明：焊接电流对AZ31镁合金接头的熔池形状及焊接质量有显著的影响.     

汽车内板用冷热冲压薄板成形性对比

因生产工艺不同,薄板坯连铸连轧所生产的0.8～3.0mm的热轧产品与传统冲压用冷轧产品存在着一定的差异。从材料的化学成分、力学性能、扩孔性能等方面与冷轧冲压薄板进行对比研究。研究结果表明,两种材料化学成分相近,热轧SPHE薄板因屈服强度过高,r值过小,致使成形性低于冷轧产品。与汽车用冲压材料要求对比结果表明,SPHE板满足汽车用板冲压要求,可在汽车内板采用热轧材料进行＂以热代冷＂冲压生产。     

不同温度下AZ31镁合金绝热剪切变形局域化

为了了解镁合金在不同温度高应变率载荷作用下发生变形局域化的特点,进而揭示镁合金在高速冲击载荷作用下发生绝热剪切的特殊规律,采用分离式Hopkinson压杆对挤压态AZ31镁合金进行了常温及高温的高速冲击压缩试验,而后对不同温度冲击后的试样通过光学显微镜、扫描电镜和透射电镜进行变形机制的分析.结果表明:常温下试样在受到剪切力后,在剪切区的裂纹周围产生了大量的孪晶;高温下试样的剪切区内产生了明显的绝热剪切带,并且在剪切带周围发现了大量平行的孪晶.在TEM下观察到剪切带内为等轴晶晶粒,在剪切区内发生了动态再结晶过程.     

Kinetics of recrystallization for twin-roll casting AZ31 magnesium alloy during homogenization

The kinetics of recrystallization for twin-roll casting AZ31 magnesium alloy with different thicknesses during homogenization was analyzed.It is shown that fine grains are first formed at the boundaries of deformed bands in the twin-roll casting slab.The recrystallized grains with no strain are gradually substituted for the deformed microstructure of twin-roll casting AZ31 magnesium alloy.The incubation temperature and time for the recrystallization of a twin-roll casting AZ31 magnesium alloy strip with a thickness of 3 mm are lower and shorter than those of the 6-mm thick strip,respectively.The 3-mm thick twin-roll casting magnesium alloy has finer grains than the 6-mm thick strip.The activation energies of recrystallization for twin-roll casting AZ31 magnesium alloy slabs with the thickness of 3 and 6 mm are 88 and 69 kJ/mol,respectively.The kinetics curves of recrystallization for twin-roll casting AZ31 magnesium alloy were obtained.