Specific Character of Material Flow in Near-Surface Layer during Friction Stir Processing of AZ31 Magnesium Alloy

Microstructure and texture evolution in the near-surface layer during friction stir processing (FSP) of AZ31 magnesium alloy was studied. Material flow was found to be a very complex process consisting of several stages. The material in front of the friction stir tool was first deformed by the rotating shoulder. Then, approaching the tool, it experienced a secondary deformation caused by the rotating pin, and finally, behind the tool, it again underwent a tertiary deformation induced by the shoulder. The texture evolution was shown to dictate the grain structure development.     

Influence of Texture on Mechanical Behavior of Friction-Stir-Processed Magnesium Alloy

Friction stir processing (FSP) improves the mechanical properties of metallic materials. In this study, a magnesium alloy AZ31B was friction stir processed by using single and multiple pass. The friction-stir-processed magnesium alloy exhibits higher tensile strength and ductility in the transverse direction (TD) compared to the longitudinal direction (LD). Both single pass and multiple (two) pass friction-stir-processed material show similar anisotropy in tensile properties, but the multiple pass friction-stir-processed material shows fine-grained microstructure with higher tensile strength and ductility. The tensile anisotropy in the friction-stir-processed AZ31B originated from the textured microstructure that evolved during FSP.     

Machining Characteristics and Corrosion Behavior of Grain Refined AZ91 Mg Alloy Produced by Friction Stir Processing: Role of Tool Pin Profile

In the present study, influence of friction stir processing (FSP) tool pin profile on the microstructure evolution, corrosion and machining characteristics of the AZ91 magnesium alloy was investigated. Three different pin profiles namely simple taper, threaded taper and square taper were selected and FSP was carried out at 1400 rpm and 25 mm/min tool travel speed. Microstructural observations indicated grain refinement from a starting grain size of 166.5–7.9, 22.1 and 4.08 µm for FSPed samples processed by simple taper, threaded taper and square taper pins, respectively. In all the FSPed samples, decreased amount of secondary phase (Mg₁₇Al₁₂) was observed compared with that of the unprocessed sample. From the X-ray diffraction analysis, it was observed that the square taper pin tool had induced higher texture effect compared with the other two FSP tools. From the electrochemical studies, the corrosion resistance of the sample processed with square taper pin tool was observed to be more in comparison to that of the other samples; which could be attributed to the texture effect and decreased fraction of secondary phase. Machining behavior assessed by conducting drilling experiments showed a significant influence of grain refinement on the cutting forces.     

Facilitating Basal Slip to Increase Deformation Ability in Mg-Mn-Ce Alloy by Textural Reconstruction Using Friction Stir Processing

The widespread application of wrought magnesium alloys is hampered by their insufficient formability at room temperature. The tensile ductility of a newly developed Mg-Mn-Ce alloy has been dramatically improved by friction stir processing (FSP). The microstructure of the stir zone was characterized mainly by elongated fine grains which were highly separated by low-angle grain boundaries because of the high contribution of continuous dynamic recrystallization. A new {0002} distribution with high basal plane tilt angles which facilitated 〈a〉 basal slip when the specimens were pulled along the FSP direction was obtained. Both the enhanced basal slip and crystallographic orientation evolution of Mg crystals increased the strain hardening exponent of the FSP specimen, and hence improved its tensile ductility. A material flow model, developed based on the local textural evolution, could reasonably explain the phenomenon that the FSP specimen exhibited warping and a high normal anisotropy ratio during tensile test.     

Experimental and Numerical Study of Electromagnetic Forming of AZ31B Magnesium Alloy Sheet

Wrought magnesium alloys are interesting materials for automotive and aeronautical industries due to their low density in comparison to steel and aluminium alloys, making them ideal candidates when designing a lower weight vehicle. However, due to their hexagonal close‐packed (hcp) crystal structure, magnesium alloys exhibit low formability at room temperature. For that reason, in this study a high velocity forming process, electromagnetic forming (EMF), was used to study the formability of AZ31B magnesium alloy sheet at high strain rates. In the first stage of this work, specimens of AZ31B magnesium alloy sheet have been characterised by uniaxial tensile tests at quasi‐static and dynamic strain rates at room temperature. The influence of the strain rate is outlined and the parameters of Johnson‐Cook constitutive material model were fit to experimental results. In the second stage, sheets of AZ31B magnesium alloy have been biaxially deformed by electromagnetic forming process using different coil and die configurations. Deformation values measured from electromagnetically formed parts are compared to the ones achieved by conventional forming technologies. Finally, numerical study using an alternative method for computing the electromagnetic fields in the EMF process simulation, a combination of Finite Element Method (FEM) for conductor parts and Boundary Element Method (BEM) for insulators, is shown.     

Bonding Interface Formation between Mg Alloy and Steel by Liquid-phase Bonding using the Ag Interlayer

Liquid-phase bonding between a Mg alloy (AZ31) and low-carbon steel was attempted at 773 K (500 °C) using Ag as an interlayer that forms a eutectic melt with the Mg alloy at this temperature. On the AZ31 side, eutectic melting and subsequent isothermal solidification were observed, and it was confirmed that the solidification of the eutectic liquid was promoted by the diffusion of Ag into the AZ31 base metal. On the steel side, Al was transported from AZ31 during the eutectic melting and isothermal solidification. This transported Al was enriched at the steel surface and reacted with steel to form a uniform, thin Fe-Al intermetallic compound layer. After the isothermal solidification, strong bonding was achieved via the thin intermetallic compound layer between AZ31 and steel, and no Ag remained at the bonding interface. The strength of the joint was found to be higher than the yield strength of AZ31.     

挤压态AZ31B镁合金在超塑变形过程中的微观组织及力学性能研究

通过正向温挤压获得了细晶微观组织的AZ31B镁合金。研究了在310～460℃范围内,应变速率1×10-3～1×100/s下的超塑性流变行为。结果表明,在415℃、1×10-3/s的条件下AZ31B镁合金具有良好的超塑性,最大延伸率可达380%。应变速率敏感指数达到0.47。通过光学显微镜和扫描电镜（SEM）分别观察了AZ31B镁合金在超塑变形过程中的微观组织演变和断口形貌。晶界滑移机制为AZ31B超塑变形的主要机制。     

Mechanics of Intermittent Plasticity Punctuated by Fracture During Shear Deformation of Mg Alloys at Near-Ambient Temperatures

Microstructure evolution of basal-textured Mg alloy AZ31B (Mg: Al: Zn; 96: 3: 1 wt pct) during simple shear deformation at near-ambient temperatures was studied by plane-strain machining. Using Schmid factor calculations in conjunction with quantitative electron microscopy, it was found that plastic deformation in AZ31B in the primary deformation zone of machining commences by extension twinning followed by basal slip. Characteristics of twinning in individual grains were described by correlating the direction of twinning with the principal stress state. The implications of these deformation mechanics for the microstructure inherited by the freshly generated surfaces in shear-based material removal processes are examined. These include the identification of extensive surface texture reorientation at machined surfaces via extension twins, limits on surface integrities wrought by fracture events that punctuate plastic deformation, and their relationship to the cutting tool geometry.     

镁合金的超塑性与损伤定量分析

研究了轧制AZ31B镁合金板材的超塑性与空洞损伤，对拉伸试样在超塑性变形各阶段轴剖面的空洞进行了观察，通过对空洞演化的分析建立了空洞体积分数与变形程度的定量关系。研究结果表明：AZ31B镁合金板材在一定的变形条件下具有良好的超塑性；变形伴随着空洞的形核、长大，继而发生空洞的连接，导致材料断裂；空洞体积分数随着变形程度的增加呈指数规律变化。     

Thermochemical Analysis of Phases Formed at the Interface of a Mg alloy-Ni-plated Steel Joint during Laser Brazing

The thermodynamic stability of precipitated phases at the steel-Ni-Mg alloy interface during laser brazing of Ni-plated steel to AZ31B magnesium sheet using AZ92 magnesium alloy filler wire has been evaluated using FactSage thermochemical software. Assuming local chemical equilibrium at the interface, the chemical activity–temperature–composition relationships of intermetallic compounds that might form in the steel-Ni interlayer-AZ92 magnesium alloy system in the temperature range of 873 K to 1373 K (600 °C to 1100 °C) were estimated using the Equilib module of FactSage. The results provided better understanding of the phases that might form at the interface of the dissimilar metal joints during the laser brazing process. The addition of a Ni interlayer between the steel and the Mg brazing alloy was predicted to result in the formation of the AlNi, Mg₂Ni, and Al₃Ni₂ intermetallic compounds at the interface, depending on the local maximum temperature. This was confirmed experimentally by laser brazing of Ni electro-plated steel to AZ31B-H24 magnesium alloy using AZ92 magnesium alloy filler wire. As predicted, the formation of just AlNi and Mg₂Ni from a monotectic and eutectic reaction, respectively, was observed near the interface.     

Modeling the Stress-Strain Behavior and Hot Tearing during Direct Chill Casting of an AZ31 Magnesium Billet

Quantitative knowledge of the thermal mechanical history experienced during direct chill (DC) casting aids the scientific understanding of the process especially in terms of defect formation such as hot tearing. In this work, a thermomechanical finite element (FE) model has been developed to simulate the DC casting of magnesium alloy AZ31 billets. The mathematical model simulates the evolution of temperature, stress, and strain within the billet during an industrial DC casting process. These quantities were then used to calculate the evolution in pressure, and hence hot tearing tendency, within the semisolid regime via the Rappaz–Drezet–Gremaud (RDG) criterion. The temperature predictions were validated against experimental thermocouple data measured during a plant trial at an industrial magnesium DC casting facility. In addition, the residual elastic strains predicted by the model were compared to residual strain measurements made at the Canadian Neutron Beam Centre (CNBC) using a magnesium billet produced during the industrial casting trial. The validated model was then used to quantitatively assess the impact of casting speed on the hot tearing tendency in AZ31 billets.     

稀土Nd对AZ 31镁合金铸态组织和力学性能的影响

利用气氛电阻炉制备了AZ 31-xNd合金(x=0.05%,0.1%,0.2%,0.4%,0.6%),采用光学显微镜(OM)、扫描电子显微镜(SEM)和电子能谱分析仪(EDS)对不同Nd含量的实验合金进行了显微组织观察和分析,结果发现,Nd在AZ 31-xNd合金中形成了Al_3Nd和Mg_(12)Nd相,这些含Nd相导致AZ 31镁合金在凝固过程中的晶粒细化,从而提高了AZ 31镁合金的铸态室温力学性能,随着Nd含量的增加,合金的铸态室温抗拉强度极限和延伸率均先升高后降低.     

Developing an Empirical Relationship to Predict Corrosion Rate of AZ31B Magnesium Alloy under Sodium Chloride Environment

Magnesium alloys have gained considerable interest as a structural material for automotive and aerospace applications due to their low-density, high specific strength and good castability. As a consequence, these light alloys have a promising future. The limitation of low corrosion resistance restricts their practical applications. Corrosion behaviour of the AZ31B magnesium alloy was evaluated by conducting immersion corrosion test in NaCl solution at different chloride ion concentrations, pH value and immersion time. An attempt was also made to develop an empirical relationship to predict the corrosion rate of AZ31B magnesium alloy. Three factors, five level, central composite rotatable design matrix was used to minimize the number of experimental conditions. Response surface methodology was used to develop the relationship. The developed relationship can be effectively used to predict the corrosion rate of AZ31B magnesium alloy at 95 % confidence level.     

Process Parametric Dependency of Axial Downward Force and Macro- and Microstructural Morphologies in Ultrasonically Assisted Friction Stir Welding of Al/Mg Alloys

Metallurgical and Materials Transactions A - An elementary analysis is proposed to quantify the effects of ultrasonic vibrations during friction stir welding (FSW) of AA6061-T6 to AZ31B Mg alloy....     

AZ31-0.84%Sb合金的机械性能与变形特征

The mechanical properties and deformation features of AZ31-0.84% Sb alloy have been studied by means of the measurement of the properties and morphology observation. Results show that UTS of AZ31-0. 84% Sb alloy at room temperature is 297 MPa, a higher value of UTS is still maintained up to 189 MPa as temperature elevated to 200℃. One of the main reasons for enhancing UTS of the alloy is attributed to the high volume fraction of the precipitates dispersed in the matrix, including Mg3Sb2 phase, which effectively hindered the movement of dislocations during the elevated temperature deformation. The deformation mechanisms of AZ31-0. 84% Sb alloy are the twins and dislocations activated on basal and non-basal planes, a c dislocations may be activated on the basal and nonbasalplanes in twins regions, and some of the thinner twins may shear through the dense dislocations within the thicker twins.     

A New Constitutive Strain-Dependent Garofalo Equation to Describe the High-Temperature Processing of Materials—Application to the AZ31 Magnesium Alloy

A new strain-dependent equation derived from that of Garofalo is developed in this work. This equation describes mathematically the deformation behavior of materials as a function of strain, strain rate, and temperature and is valid over a wide range of strain with good statistical accuracy. An explicit expression s( e) = f( e,T,[(e)\dot] ) \sigma \left( \varepsilon \right) = f\left( {\varepsilon ,T,\dot{\varepsilon }} \right) is introduced that reproduces stress-strain curves. Statistical tools for determining the validity of the equation have been applied. Predictions from this expression were compared with torsion data obtained in AZ31 magnesium alloy that was deformed at various temperatures and strain rates. Analysis of the strain dependence of the Garofalo parameters allowed us to establish a steady state at strains of about 0.6. It also allows drawing conclusions about the microstructural changes that occur during deformation of the alloy. In addition, the characteristic points in the evolution with strain of the parameters of the equation are related to the most significant values that characterize the microstructural processes occurring during deformation of the AZ31 alloy. The observed decrease of Q and n as a function of strain is attributed first to a softening process due to dynamic recrystallization and grain size refinement and finally to flow localization. The predicted values obtained with the new constitutive equation and the experimental values for the AZ31 alloy are in good agreement with an average relative error of about 6.5 pct.     

Tensile behavior of friction-stir-welded AZ31-H24 Mg alloy

In the present study, tensile behavior of friction-stir-welded AZ31 (Mg-3.6Al-1Zn-0.6Mn in wt pct)-H24 Mg alloy was investigated. It was found that the tensile property, particularly tensile elongation, of AZ31-H24 alloy was significantly degraded with friction stir welding (FSW). The tensile fracture always occurred at the boundary between the thermomechanically affected zone (TMAZ) and the stir zone (SZ) on the advancing side. The fractographic examination on the tensile-fractured AZ31-H24 alloy specimen showed a mixed mode of cleavage and dimpled rupture. The AES analysis suggested that the significant reduction in tensile elongation of friction-stir-welded AZ31-H24 Mg alloy was attributable to the entrapped oxides along the boundary between the TMAZ and SZ.     

The Effect of Concurrent Straining on Phase Transformations in NiAl Bronze During the Friction Stir Processing Thermomechanical Cycle

Equivalent strains up to a value of ≈2.7 were determined by evaluation of the shape changes of the phases in a duplex α(fcc)/β(bcc) microstructure formed ahead of the pin tool extraction site during the friction stir processing (FSP) thermomechanical cycle in a cast NiAl bronze alloy. Correlation of the local strains with volume fractions of the various microstructure constituents in this alloy shows that the concurrent straining of FSP results in acceleration of the α + β → β reaction in the thermomechanically affected zone (TMAZ) ahead of the pin extraction site. The resulting volume fraction of β (as determined by the volume fraction of its transformation products formed during post-FSP cooling) corresponds closely to the volume fraction expected for the peak stir zone temperature measured separately by means of thermocouples embedded within the tool pin profile along the tool path. The stir zone (SZ) in this material exhibits near-equilibrium microstructures despite brief dwells near the peak temperature (T _peak ≈ 0.95T _melt), reflecting large local strains and strain rates associated with this process.     

Grain Refinement Mechanisms in Gradient Nanostructured AZ31B Mg Alloy Prepared via Rotary Swaging

Metallurgical and Materials Transactions A - Gradient nanostructured AZ31B Mg alloy rods were prepared via cold rotary swaging in this study. The swaged sample exhibited the best...