Effects of humidity and contact material on fretting fatigue behavior of an extruded AZ61 magnesium alloy

Fretting fatigue tests of the extruded AZ61 magnesium alloy with the same contact material under low and high humidity were carried out to investigate basic fretting fatigue characteristics and effect of humidity on fretting fatigue behavior. Influence of contact material was also studied by using JIS S45C carbon steel contact material. Degradation of fatigue strength due to fretting was much more significant than that due to corrosion under high humidity condition. Therefore, no effect of humidity on fretting fatigue strength was found. Reduction rate of fatigue strength due to fretting for the magnesium alloy was between those of aluminum alloys and titanium alloys. Tangential force coefficient of the magnesium alloy was rather low compared to other materials such as steels, aluminum alloys and titanium alloys. Fretting fatigue strength with the S45C contact material was inferior compared to that with the same contact material. This is mainly due to higher tangential force in AZ61/S45C contact. Fretting fatigue cracks at the edge of fretting contact region were observed to nucleate in the very early stage of fatigue life, similar to other structural materials.     

Effects of Mn content and texture on fatigue properties of as-cast and extruded AZ61 magnesium alloys

Fatigue behavior of as-cast and extruded AZ61 magnesium alloys in ambient air (20 °C–55%RH) was investigated. It was found that size and distribution of cast defect influenced tensile and fatigue performance of the as-cast alloy. Fatigue limit of the as-cast alloy was significantly low compared to the extruded alloy. The casting defects served as stress concentration sites for fatigue crack nucleation. Fatigue tests were also carried out on a high Mn content alloy. All of the specimens failed from an inclusion near the specimen surface. Fatigue limit of Mg alloy with high Mn content was lower compared to that of the low Mn content alloy. Further, investigation on the effect of texture on fatigue and fatigue crack growth behavior of the extruded AZ61 magnesium alloy plate was carried out. The results showed that fatigue strength in the longitudinal direction to the extruded direction was higher compared to those in the transverse and 45° directions. Significant effect of specimen orientation on fatigue crack growth behavior for both short and long cracks was found near the threshold region. However, regardless of specimen orientation, the da/dN–ΔK_eff curves for all three kinds of specimens were in a narrow band. It is suggested that the difference in the fatigue life among the specimen orientations will be mainly due to the difference in the crack closure behavior. A transition of fracture mechanism was found for a long crack. Slip fracture mechanism was dominant above the transition point, whereas below the transition point, slip fracture mechanism was associated with cleavage fracture.     

AZ31B镁合金TIG焊焊接接头的疲劳性能

对8 mm厚AZ31B镁合金板及其三种TIG焊焊接接头的静载拉伸性能和疲劳性能进行试验研究。试验结果表明AZ31B镁合金母材的静载抗拉强度为245.50 MPa,TIG焊对接接头、纵向角接接头和非承载十字接头的静载抗拉强度分别为193.55 MPa、229.89 MPa、227.39 MPa。脉动循环(r = 0)疲劳试验表明,在2×106循环次数下,AZ31B镁合金母材的疲劳强度为57.81 MPa,为其静载抗拉强度的23.5％。相同循环次数下,AZ31B镁合金TIG焊对接接头、纵向角接接头和非承载十字接头的疲劳强度为24.60 MPa、20.14 MPa、17.25 MPa,分别为母材疲劳强度的42.6 %、34.8 %和29.8 %。按照国际焊接学会的规范,发现镁合金焊接接头的疲劳级别FAT仅为相应铝合金接头疲劳级别FAT的一半。由此看来,疲劳性能是影响镁合金在承受动态载荷结构中应用的主要因素之一。     

镁合金低周疲劳寿命预测模型探讨

通过铸造镁合金AZ91D和变形镁合金AZ31B室温环境下应力控制的低周疲劳试验,采用Basquin模型、SWT模型、应变能-寿命模型等模型进行了镁合金低周疲劳的寿命预测。在此基础上,基于连续介质损伤力学的不可逆热力学理论,将镁合金的低周疲劳损伤视为一个不可逆的耗散过程,用熵来反映系统的耗散过程,并以每一次循环的平均应变增量来反映平均应力对材料的影响,提出了一种新的镁合金低周疲劳寿命预测模型。用该模型进行了镁合金的低周疲劳寿命预测,预测结果与实测结果符合较好,同时相比上述其他模型,该模型具有较好的预测效果。     

Studies on the influence of process parameters on the AISI 316L resistance spot-welded specimens

Energy saving and improving product performance are long-term concerns in extrusion process. Therefore, this paper proposes a novel extrusion process called alternate extrusion (AE). The proposed process uses split punches alternately instead of the overall structure to apply the downward load, but receives an unexpected load-saving and grain refinement effect. Experimental and finite element method (FEM) methods were used to investigate the effects of different extrusion ratios on microstructure and mechanical properties. Results indicate that load value is significantly reduced, grain size is considerably refined, and tensile strength and elongation of material are improved after AE processing. The fractography shows that the fracture mode of AZ31 magnesium alloy changes from brittle to ductile. Although the actual extrusion ratio decreases in AE process, additional shear forces produced by different punch-alternating loads at the interface improve microstructure and mechanical properties. Therefore, AE can achieve grain refinement and load saving and improve strength and plasticity of magnesium alloys.     

Effect of high power diode laser surface melting on wear resistance of magnesium alloys

In the present work, an attempt has been made to improve the surface hardness and wear properties of magnesium alloys AZ31 and AZ61 through solid solution hardening and refinement of microstructures using a 1.5 kW high power diode laser (HPDL) as a heat generating source. One millimetre thick uniform overlapping melt tracks were produced on Mg alloy samples. Laser-melted samples were subjected to a two-body abrasive wear test using a modified pin-on-disc set up. The hardness and wear resistance of laser-melted samples were found far better than the as-received Mg alloy substrates. Scanning electron microscopy was used to examine microstructure of the laser-melted layers and the worn surface morphology. Fine microstructures were observed with an average grain size of less than 5 μm.     

Fretting wear behavior of AZ91D and AM60B magnesium alloys

The fretting wear behavior of the AZ91D and AM60B magnesium alloys are investigated using a reciprocating fretting wear machine under dry conditions with different numbers of cycles, different normal loads, slip amplitudes and frequencies. The worn surfaces and wear debris were examined using scanning electron microscopy and optical microscopy in order to understand the predominant wear mechanisms of two magnesium alloys. The results indicate that the AZ91D alloy displays a lower friction coefficient and lower wear quantity than the AM60B alloy. The AZ91D shows a higher capability than AM60B in resisting crack nucleation and propagation. Both AZ91D and AM60B show similar friction and wear characteristics. The wear quantity increases with increasing normal load, but decreases with increasing frequency. The friction coefficient also decreases as the normal load is increased. Fretting frequency had little effect on the friction coefficient. In a long term, the fatigue wear and abrasive wear were the predominant wear mechanisms for AM60B and delamination wear, adhesive wear and abrasive wear for AZ91D.     

Fatigue crack growth behavior in ultrafine grained low carbon steel

Ultrafine grained (UFG) low carbon (0.15 wt.% C) steel produced by equal channel angular pressing (ECAP) was tested for investigating the effect of load ratio on the fatigue crack growth rate. Fatigue crack growth resistance and threshold of UFG steel were lower than that of as-received coarse grained steel. It was attributed to the less tortuous crack path. The UFG steel exhibited slightly higher crack growth rates and a lower ΔK_th with an increase of R ratio. The R ratio effect on crack growth rates and ΔK_th was basically indistinguishable at lower load ratio (R>0.3), compared to other alloys, which indicates that contribution of the crack closure vanishes. The crack growth rate curve for UFG steel exhibited a longer linear extension to the lower growth rate regime than that for the coarse grained as-received steel.     

变形镁合金AZ31的研究进展

综述了国内外AZ31镁合金的研究进展.分别介绍了AZ31镁合金组织、力学性能及变形行为研究现状,讨论了合金元素对AZ31镁合金的影响,并对变形镁合金AZ31耐蚀性的相关研究进行了总结.最后对AZ31镁合金的发展前景进行了分析.     

AZ31B镁合金断裂应变与应力三轴度的关系研究

对AZ31B镁合金光滑圆棒和缺口圆棒进行了系列准静态拉伸试验,采用ABAQUS对各试样拉伸过程进行了模拟分析。拟合得到了Johnson-Cook断裂失效模型的部分材料常数,建立了AZ31B镁合金断裂应变与应力三轴度的关系模型。将建立的失效模型输入到ABAQUS中进行仿真模拟,模拟结果与试验结果基本一致,验证了断裂失效模型的正确性。     

Comparison of the machinability and wear properties of magnesium alloys

AZ and AS series magnesium alloys were used in this study and had different contents (i.e., 0 to 9 wt.% Al). The effect of zinc (Zn) and silicon (Si) on wear resistance and machinability was analyzed in AZ and AS series magnesium alloys. Zn amount in AZ series (1 %) and Si amount in AS series (1 %) were kept at a fixed rate. The effect of the changes in Al amount on hardness, wear resistance, and machinability in AZ and AS series magnesium alloys was comparatively analyzed. A higher increase was observed in the wear resistance of alloys in AS series magnesium alloys due to the rise in Al amount compared with AZ series. Intermetallic phases found in the microstructure of alloys (β-Mg₁₇Al₁₂ and Mg₂Si) were established to have an impact on the wear resistance and machinability of alloys.     

镍铝青铜焊丝进行镁/镀锌钢板点焊的焊接性分析*

以镍铝青铜焊丝为填充材料,对AZ31B镁合金/镀锌钢板进行冷金属过渡点塞焊试验研究,分析镁板上孔直径对镁钢点塞焊接头力学性能的影响.并通过分析焊接接头微观组织及其元素分布状况来研究其连接机理。研究结果表明：使用镍铝青铜焊丝能够得到焊缝美观的镁/镀锌钢异种金属的连接接头。镁/钢板焊接接头的焊缝主要由α-Cu和CuAl₂组成,熔合区由镁的固溶体α-Mg以及Al₂Cu₃Mg₂和Mg₂Cu混合的金属间化合物组成。镁板上孔的直径对镁钢接头性能有很大的影响。随着镁板孔径的增大,镁钢点塞焊接头的最大抗拉载荷先增大后减小,且当镁板孔径为5 mm时接头的最大抗拉载荷达到最大为3.4 kN。焊缝金属和镁母材的连接处即熔合区存在大量脆性金属间化合物,使得镁/钢接头整体力学性能较差。     

Deformation behavior and microstructure evolution of wrought magnesium alloys

There are many researches on the deformation behavior of wrought magnesium alloys, such as AZ31, AZ80, AZ91, and ZK60 magnesium alloys at different temperatures and strain rates, but few of them focuses on the deformation behavior of AZ41M and ZK60M alloys, especially under the twin-roll casting (TRC) state. Meanwhile, the existing researches only focus on the grain refinement law of the magnesium alloys under deformation conditions, the deformation mechanism has not been revealed yet. The hot compression behavior of AZ41M and ZK60M magnesium alloys under the temperature and strain rate ranges of 250-400 ℃ and 0.001-1 s-1 are studied by thermal simulation methods using Gleeble 1500 machine and virtual simulation using finite element analysis software. Simulation results show that sine hyperbolic law is the most suitable flow stress model for wider deformation conditions. The most reasonable selected deformation conditions of ZK60M alloy is 350 oC/0.1 s-1 for TRC and 350 oC/1 s-1 for conventional casting (CC), while AZ41M alloy is 300 oC/0.01 s-1 for TRC and 350 oC/0.1 s-1 for CC. Deformation behavior and dynamic recrystallization (DRX) mechanism of them are analyzed at the same deformation conditions. The microstructures of AZ41M and ZK60M alloys are observed at different deformed conditions by optical microscopy (OM) and electron back scatter diffraction (EBSD) and it reveals the flow behavior and deformation mechanism of them. Working harden and work soften contribute to the activation of basal, non-basal slip systems which promote DRX. The proposed research reveals the deformation behavior and mechanism of the AZ41M and ZK 60M magnesium alloys and concludes their optimized deformation parameters and processes and provides a theory basis for their manufacturing and application.     

Fretting fatigue in 2XXX series aerospace aluminium alloys

This research investigated the effects of microstructural characteristics on the fretting response in 2XXX series aerospace aluminium alloys. Fretting fatigue tests were conducted to determine the influence of slip character, alloy purity, grain structure and yield strength on fretting crack nucleation and growth. Crack length measurements and micrographs of the specimens indicated there was no significant difference in the fretting response of these alloys based on their microstructural characteristics. Results also showed that fretting caused cracks to nucleate in the first 1–5% of total life which resulted in much shorter fatigue lives. Additionally, fretting normalized the nucleation time in all alloys, eliminating the differences in intrinsic fatigue nucleation resistance. This resulted in the alloys with the highest stress-life (S–N) fatigue properties exhibiting a greater reduction in fatigue strength under fretting conditions. The total fretting fatigue life appeared to be primarily determined by the fatigue crack propagation resistance of the alloys.     

基于相对误差平方和的神经网络预测镁合金多轴疲劳寿命

提出一种以相对误差平方和(Surm squared relative error,SSRE)作为误差性能函数的反向传播(Back propagation,BP)神经网络算法(SSRE-BP),针对3种不同镁合金AZ31B、ZK60和AZ61A在单轴拉压、纯扭、45°比例和90°圆形非比例等4种不同加载路径下的疲劳寿命进行预测。并与以均方误差(Mean squared error,MSE)作为误差性能函数的传统BP神经网络(MSE-BP)以及基于临界平面法的SWT疲劳损伤模型预测的结果进行比较。结果表明,在3种镁合金材料总共138组疲劳数据中,神经网络只有一组预测值在3倍偏差界限外,而用SWT预测结果分别有16组、13组、10组数据在3倍偏差界限外。两种BP神经网络能够较好地预测镁合金不同加载路径下的疲劳寿命,相比于SWT疲劳模型预测的寿命在精度上有较大幅度的提升。其中,SSRE-BP算法的精度略高于传统的MSE-BP算法。     

基于红外热成像的镁合金疲劳裂纹扩展的研究

采用红外热成像技术监测疲劳裂纹扩展过程中试件表面温度的变化情况,对AZ31B镁合金板材室温下的疲劳裂纹扩展特征进行研究。分析疲劳裂纹尖端温升值与裂纹长度的对应关系,试件表面温度分布差异与裂纹扩展趋势的关系,探索镁合金材料疲劳裂纹扩展的规律。试验结果表明,疲劳裂纹扩展过程中,镁合金表面温度变化经过一个升温、降温的过程,在稳定扩展阶段,温度变化不大,在快速扩展阶段,温度呈明显上升趋势。三组试件最高温升值分别为A试件10.89℃、B试件15.19℃、C试件12.37℃。裂纹尖端及其附近组织观察发现,裂纹尖端发生转向,裂纹总体为穿晶断裂,并伴随少量沿晶断裂,在裂纹附近区域有少量塑性变形。疲劳试件表面的最高温度区域与材料的疲劳损伤机制相关,该区域对应材料的应力集中区,是疲劳微裂纹形成与扩展的部位,温度变化与试件的最终断面相吻合。     

Friction stir welding of AZ61A magnesium alloy

This paper deals with the development of an empirical relationship to predict tensile strength of friction stir welded AZ61A magnesium alloy. The process parameters such as tool rotational speed, welding speed, axial force and tool pin profile play a major role in deciding the tensile strength. The response surface method (RSM) was used to develop the empirical relationship. The four-factor, five-level central composite design was used to minimize the number of experimental conditions. The developed empirical relationship can be effectively used to predict tensile strength of friction stir welded AZ61A magnesium alloy joints at 95 % confidence level.     

超声振动对轻合金塑性压缩变形过程的影响

将超声振动叠加到AZ31镁合金、6061和7075铝合金的镦粗压缩变形过程,以研究高频振动对轻合金塑性压缩变形时的流动特性、断裂失效方式以及压缩表面状态的影响。实验发现,叠加振动后,"体积"和"表面"两种效应对3种材料的压缩变形过程都存在不同程度的作用。具体表现为材料的屈服强度、流动应力出现降低,且降低幅度与材料特性相关,其中6061对振动效应更为敏感;随着振幅的增加,AZ31材料体积效应中的硬化机制逐渐占据主导;此外,振动有助于提高压缩表面的质量。     

红外热像法预测镁合金的疲劳性能

对AZ31B镁合金板材在室温下的高周疲劳性能进行了研究,用红外成像仪测量了疲劳试验过程中合金表面的温度变化;根据疲劳试验过程中温度-应力关系及试样表面温度分布差异,确定疲劳断裂位置和疲劳极限。结果表明:根据合金表面温度变化可以预测疲劳断裂位置;以此法确定的AZ31B镁合金的疲劳极限为107.5MPa,与常规疲劳试验测得的疲劳极限吻合较好;加载初期镁合金升温到一定温度后又下降至室温左右,在断裂前温度快速升高。     

Effects of temperature and driver sheet for magnesium alloy sheet in magnetic pulse forming

Thermal effect from warm temperature is always used to improve the formability of AZ31 magnesium alloy sheet. However, it is seldom employed to deform AZ31 sheet in magnetic pulse forming process, due to increasing resistivity and decreasing effect of high strain rate. In this study, Al driver sheets without heating were used to strength effect of high strain rate and drive AZ31 sheet with warm temperature to deform. Method of numerical simulation was used to analyze magnetic pulse forming of AZ31 sheet with driver sheet and temperature. Magnetic flux density and magnetic force with and without Al driver sheet (thickness of 1, 1.5, and 2 mm) and different temperature (25, 100, 150, 200, and 250°C) were investigated. Deformation processes and velocity with Al driver sheet and different temperature were analyzed. The results indicate that it is better for formability of AZ31 sheet to adopt 1-mm Al driver sheet at higher discharge energy and warm temperature.