Zur thermischen Ermüdung der Magnesium‐Basislegierung AZ91

Thermal fatigue of magnesium‐base alloy AZ91 Thermal fatigue tests of the magnesium‐base alloy AZ91 were carried out under total strain control and out‐of‐phase‐loading conditions in a temperature range between ‐50°C and +190°C. Specimens produced by a vacuum die casting process were loaded under constant total strain and uniaxial homogeneous stress. To simulate the influence of different mean stresses, experiments were started at different temperature levels, e.g. the lower, mean or upper temperature of the thermal cycle. The thermal fatigue behavior is described by the resulting stress amplitudes, plastic strain amplitudes and mean stresses as a function of the number of thermal loading cycles. Depending on the maximum temperature and the number of loading cycles, cyclic softening as well as cyclic hardening behavior is observed. Due to the complex interaction of deformation, recovery and recrystallization processes and as a consequence of the individual temperature and deformation history, thermal fatigue processes of the material investigated cannot be assessed using results of isothermal experiments alone. The upper temperatures or the resp. temperature amplitudes determine the total fatigue lifetime.     

Einfluss von Randschichtzustand und Haltezeit auf die thermische Ermüdung der Magnesiumbasislegierung AZ31

In this paper thermal fatigue of magnesium base alloy AZ31 in the temperature range between +50°C and +290°C is investigated. Experiments were carried out under constant total deformation (out‐of‐phase loading) and the resulting stress amplitudes as well as the plastic strain amplitudes were recorded as a function of the number of thermal loading cycles. In particular the consequences of mechanical surface treatments (deep rolling) and of hold‐times were investigated. In both cases no particular influence compared with untreated specimens loaded without hold‐times was observed, which is due to the interaction of deformation and recrystallization processes during thermal fatigue.     

The modified temperature term on Johnson Cook model for AZ31 magnesium alloy

The quasi‐state and dynamic mechanism of AZ31 magnesium alloy at a strain rates range of 0.001 s^‐1–2500 s^‐1 under a temperature range of 20 °C–250 °C were researched by compression tests using the electronic universal testing machine and split Hopkinson pressure bar system. The true stress‐strain curves at different strain rates and evaluated temperatures were obtained. The result shows that the thermal soften effect of AZ31 magnesium alloy is significant. By modifying the temperature term of the original Johnson Cook model of AZ31 magnesium alloy, a modified Johnson Cook model of AZ31 magnesium alloy has been proposed to reveal thermal soften effect on the deformation behavior of AZ31 magnesium alloy more precisely. With the modified Johnson Cook model and fracture model, the finite element method simulation of AZ31 magnesium alloy hat shaped specimen under impacting was conducted. The numerical simulation result is consistent with the experimental result, which indicates that the modified Johnson Cook model and fracture model are greatly valid to predict the deformation and fracture behavior of the AZ31 magnesium alloy hat shaped specimen under impacting.     

Ermüdungslebensdauer der Magnesiumlegierung AZ91: Experiment und Modellierung

Fatigue Life of the Die‐Cast Magnesium Alloy AZ91: Experiments and Modelling The cyclic deformation behaviour of the die‐cast magnesium alloy AZ91HP was investigated under total strain control at constant total strain amplitudes between 1.4 × 10⁻³ to 2 × 10⁻² at room temperature and at 130°C. Microstructural investigations in combination with the determination of crack‐growth behaviour using the replica technique and measurements of changes of the stiffness (compliance) of the specimen during a fatigue experiment led to a detailed understanding of the evolution of damage and the main damage mechanisms. Based on these findings, a microstructurally based life‐prediction concept was formulated.     

An experimental study of creep‐ratchetting behavior of rolled AZ31B magnesium alloy at room temperature

To study the interaction effect of creep and ratchetting for rolled AZ31B magnesium alloy at room temperature, a series of stress‐controlled tests were designed. In the tests, four loading types with different mean stresses were considered, and dwell loading was applied to explore the creep effect on the ratchetting. The test results indicated that the sequence of ratchetting and creep loading is crucial for the strain evolution. The amount of twinning/detwinning increased as the mean stress decreased, leading to an exhaustion of nonbasal slip during ratchetting, and then suppressed the creep ductility. However, the creep sequence exerted little influence on the strain shift of ratchetting while large amount of twinning/detwinning was involved.     

Hot Cracking in AZ31 and AZ61 Magnesium Alloy

This paper examined the impact of the number of thermal cycles and augmented strain on hot cracking in AZ31 and AZ61 magnesium alloy. Statistical analyses were performed. Following observation using a scanning electron microscope (SEM), an energy dispersive spectrometer (EDS) was used for component analysis. Results showed that Al content in magnesium alloy has an effect on hot cracking susceptibility. In addition, the nonequilibrium solidification process produced segregation in Al content, causing higher liquid Mg-alloy rich Al content at grain boundaries, and resulting into liquefied grain boundaries of partially melted zone (PMZ). In summary, under multiple thermal cycles AZ61 produced serious liquation cracking. AZ61 has higher (6 wt%) Al content and produced much liquefied Mg17Al12 at grain boundaries under multiple thermal cycles. The liquefied Mg17Al12 were pulled apart and hot cracks formed at weld metal HAZ due to the augmented strain. Since AZ31 had half the Al content of AZ61, its hot-cracking susceptibility was lower than AZ61. In addition, AZ61 showed longer total crack length (TCL) in one thermal cycle compared to that in three thermal cycles. This phenomenon was possibly due to high-temperature gasification of Al during the welding process, which resulted in lower overall Al content. Consequently, shorter hot cracks exhibited in three thermal cycles. It was found the Al content of AZ31 and AZ61 can be used to assess the hot-cracking susceptibility.     

Lifetime behaviour of unidirectionally wire reinforced lightweight metal matrix composites

In the field of lightweight construction for transportation means, hybrid structures composed of high-strength and low-density materials exhibit a high application potential. The current work shows that a spring steel wire reinforcement content of 11.1 vol.% leads to a significant increase in lifetime of the magnesium alloy AZ31 under stress controlled fatigue loading. Based on the knowledge of the quasi-static behaviour of the single components and the fatigue behaviour of the single matrix material and on the understanding of mechanisms and mechanics of constituent deformation and damage evolution a new model is deduced from the quantitative strain response in order to predict the lifetime for different unidirectionally reinforced material systems.     

Cyclic hysteresis of AZ31B extrusion under load‐control tests using embedded sensor technology

The fatigue behaviour of AZ31B extrusion magnesium alloy under load‐control cyclic test conditions is estimated using a combination of simulation and experimental results. The strain measurement of this asymmetric material is found experimentally using a Fibre Bragg Grating (FBG) sensor during rotating bending tests. Then, to analyse applied stresses in the sample – particularly in the plastic deformation range – the Variable Material Property (VMP) method is employed. Using this simulation method, the hysteresis loops of two critical top and bottom elements of the sample's cross section under different bending moments are obtained. Finally, the strain of the sample during rotating bending, as measured by the embedded FBG sensor, is related to the stresses obtained from the modeling using a mapping function. The hystereses obtained from this combination of the modeling and experimental results are compared with the results of a companion strain‐control pull–push test in which the input strain history was that of measured by the FBG sensor. Observations verify that the stresses of the combined VMP‐FBG hysteresis loops have good compatibility with the stress responses obtained through the experiment. The hybrid model introduced in this work can be employed to capture cyclic hysteresis, and hence estimate the fatigue life, under load‐controlled rotating bending tests.     

Thermisch‐mechanisches Ermüdungsverhalten der partikelverstärkten Aluminiumknetlegierung EN AW‐6061‐T6 und die Entwicklung von Eigenspannungen im Matrixwerkstoff unter thermisch‐mechanischer Beanspruchung

Thermal mechanical fatigue behaviour of particle reinforced EN AW‐6061‐T6 and development of residual stresses in the matrix material by thermal mechanical loading The behaviour of non reinforced and 15 Vol.‐% α‐alumina particle reinforced wrought aluminium alloy EN AW‐6061‐T6 in thermal mechanical fatigue loading was investigated at different maximum temperatures. The tests were performed in strain controlled mode by means of an electro‐mechanical testing machine. Alternating load deformation and life cycle behaviour either materials were compared. It came out, that the reinforcement leads to an decreasing thermal mechanical fatigue life cycle while keeping constant the maximum temperature and mechanical loading. The two materials showed softening behaviour due to high maximum temperatures of 573 K to 673 K. However, there is an intense scatter of the number of cycles to failure of the non reinforced alloy aggravating the interpretation of the results. On the other hand the thermal mechanical life cycle increases in combination with increasing maximum temperatures. Simultaneously the part of plastic deformation in mechanical loading increases for both materials, while for a constant total strain range the effective maximum and minimum stresses are decreasing. Furthermore, the development of residual stresses in the matrix of the reinforced alloy by thermal mechanical fatigue loading was analysed. It was observed that only small absolute values of residual stresses will be obtained for these loads. Nevertheless, tendencies of mounting tensile residual stresses can be identified in the direction of thermal mechanical fatigue loading and subsequently reduction of the residual stresses.     

异步轧制AZ31镁合金板材的超塑性工艺及变形机制

经过异步轧制工艺获得AZ31镁合金薄板。在300~450℃范围内,分别通过5×10-3,1×10-3s-1和5×10-4s-1不同应变速率进行高温拉伸实验研究其超塑性变形行为,计算应变速率敏感指数m值、超塑性变形激活能Q及门槛应力σ0值。通过EBSD分析和扫描电镜观察拉伸断裂后的断口形貌,分析AZ31镁合金的超塑性变形机制。结果表明:AZ31镁合金的塑性变形能力随着变形温度的升高及应变速率的降低而增强。当拉伸温度为400℃、m=0.72、应变速率为5×10-4s-1时,AZ31具有良好的超塑性,伸长率最大为206%。温度为400℃时,异步轧制AZ31镁合金的超塑性变形是以晶格扩散控制的晶界滑移和基面滑移共同完成的。     

Effect of 1.5 wt% tin addition on the properties of AZ31 magnesium alloy

In this paper, 1.5 wt%Sn was added to the AZ31 magnesium alloy aiming at improving the mechanical properties by using a low cost alloying element. Both alloys were prepared in the cast/heat treated (HT), rolled at 350 °C, rolled/heat treated at 400 °C and extruded at 350 °C. The results indicate that with addition of tin an improvement was obtained in both tensile strength and ductility of the AZ31 alloy in the cast/heat treated and in the extruded conditions. The yield and ultimate tensile strengths reached 98 MPa and 224 MPa respectively with 14 % elongation in the cast/heat treated condition while in the extruded condition these values were 212 MPa and 286 MPa with 20 % elongation. The tensile strength was even higher after rolling reaching 315 MPa for AZ31 with tin addition; however, as the material temperature during the last passes has decreased to relatively low values, the % elongation decreased to 1 %. After heat treatment at 400 °C for 2 hours the % elongation was restored and reached 12 %; this was accompanied by a decrease in tensile strength which reached 276 MPa. The results are discussed in relation to the microstructure evolution including grain size, phase identification, and volume fraction of phases.     

AZ31 镁合金轧制-剪切-弯曲变形工艺数值模拟研究

目的研究温度和道次压下量的变化对AZ31镁合金轧制-剪切-弯曲变形工艺的影响规律。方法对AZ31镁合金轧制-剪切-弯曲变形过程进行数值模拟,探究变形过程中应力、应变分布规律。结果压下量越大,模具转角处累积的等效应变值越大;随着温度的升高,模具转角处等效应力逐渐减小,等效应变逐渐增大。结论试样在模具转角处发生了剧烈的塑性变形,研究结果为板材的制备提供了依据。     

Fatigue behaviour of welded joints from magnesium alloy (AZ31) according to the local strain concept

In the present study, the results of fatigue tests with the magnesium alloy AZ31 (ISO‐MgAl3Zn1) in the material states base metal, heat affected zone and weld metal obtained under strain control at room temperature within a range from 2·10² to 5·10⁶ cycles are presented. The fatigue behaviour was characterized by the Coffin–Manson–Basquin equations and the stress – strain behaviour by the Ramberg–Osgood equation. The data can be used to assess welded magnesium joints according to the local strain concept.     

Evaluation of a phenomenological elastic‐plastic approach for magnesium alloys under multiaxial loading conditions

Magnesium alloys are greatly appreciated due to their high strength to weight ratio, stiffness, and low density; however, they can exhibit complex types of cyclic plasticity like twinning, de‐twinning, or Bauschinger effect. Recent studies indicate that these types of cyclic plastic deformations cannot be fully characterized using the typical tools used in cyclic characterization of steels and aluminium alloys; thus, it is required new approaches to fully capture their cyclic deformation and plasticity. This study aims to propose and evaluate a phenomenological cyclic elastic‐plastic approach designed to capture the cyclic deformation of magnesium alloys under multiaxial loading conditions. Series of experimental tests were performed to characterize the cyclic mechanical behaviour of the magnesium alloy AZ31BF considering proportional loadings with different strain amplitude ratios and a nonproportional loading with a 45° phase shift. The experimental results were modulated using polynomial functions in order to implement a cyclic plasticity model for the AZ311BF based on the phenomenological approach proposed. Results show good correlations between experiments and estimates.     

Fatigue lifetime of AZ91 magnesium alloy subjected to cyclic thermal and mechanical loadings

In the present paper, thermo-mechanical fatigue (TMF) and low cycle fatigue (LCF) or isothermal fatigue (IF) lifetimes of a cast magnesium alloy (the AZ91 alloy) were studied. In addition to a heat treatment process (T6), several rare elements were added to the alloy to improve the material strength in the first step. Then, the cyclic behavior of the AZ91 was investigated. For this objective, strain-controlled tension–compression fatigue tests were carried out. The temperature varied between 50 and 200 °C in the out-of-phase (OP) TMF tests. The constraint factor which was defined as the ratio of the mechanical strain to the thermal strain, was set to 75%, 100% and 125%. For LCF tests, mechanical strain amplitudes of 0.20%, 0.25% and 0.30% were considered at constant temperatures of 25 and 200 °C. Experimental fatigue results showed that the cyclic hardening behavior occurred at the room temperature in the AZ91 alloy. At higher temperatures, this alloy had a brittle fracture. But also, it was not significantly clear that the cyclic hardening or the cyclic softening behavior would be occurred in the material. Then, the high temperature LCF lifetime was more than that at the room temperature. The OP-TMF lifetime was the least value in comparison to that of LCF tests. At the end of this article, two energy-based models were applied to predict the fatigue lifetime of this magnesium alloy.     

Structural durability of magnesium alloys and components

The fatigue behaviour of the magnesium die cast alloys AZ91, AE42 and AM50 was investigated at constant amplitude and in variable amplitude tests. The ambient conditions of these tests varied between laboratory air at room temperature, at 125 °C and a permanent influence of NaCl‐solution at room temperature. More than 40 test series were analysed in a generalized way to determine standardized slopes of S‐N curves and mean stress sensitivity. The behaviour of the three alloys was investigated also in strain‐controlled cyclic tests at normal and elevated temperature. Based on this substantial data set several variants of methods following both the nominal‐stress concept and the local‐strain approach were applied to determine guidelines to improve the reliability of lifetime estimation of components made of magnesium. The corrosion fatigue behaviour of these magnesium alloys was extensively investigated under rotating bending to clarify the damaging influence of the corrosive load component. Under simultaneous action of corrosion and cyclic mechanical loading several influencing factors have to be considered which attain special importance during the testing of magnesium alloys.     

Zum Wechselverformungsverhalten der Magnesiumbasislegierung AZ31

The cyclic deformation behaviour of Mg – base alloy AZ31 The cyclic deformation behaviour of Mg – base alloy AZ31 was investigated in stress controlled tension-, compression-tests. Experiments with zero mean stress (R = -1) as well as with tensile or compression mean stress (R = 0, R = - ∞ resp.) were carried out. Cyclic strain hardening and a pronounced anisotropy of strength during the first loading cycles was observed with higher yield strength in tension compared with compression. Consequently, in tests with zero mean stress cyclic creep and compressive mean strains occured.     

AZ91D室温环境棘轮及其低周疲劳行为

通过AZ91D室温环境应力控制下的低周疲劳试验,对铸造镁合金棘轮及其低周疲劳行为进行了研究,讨论了室温环境下材料的应力循环特性、棘轮行为、塑性应变范围、全应变范围等疲劳参量随载荷水平和加载历史的变化规律,同时基于平均应力修正对材料的应力-寿命曲线进行了讨论。研究结果表明:AZ91D在室温环境下的应力循环呈循环硬化,材料的棘轮行为和塑性应变范围、全应变范围等疲劳参量依赖于载荷水平和加载历史,另外考虑平均应力修正后的应力-寿命曲线预测效果有明显改观。     

Bonded aircraft repairs under variable amplitude fatigue loading and at low temperatures

Bonded repairs can replace mechanically fastened repairs for aircraft structures. Compared to mechanical fastening, adhesive bonding provides a more uniform and efficient load transfer into the patch, and can reduce the risk of high stress concentrations caused by additional fastener holes necessary for riveted repairs. Previous fatigue tests on bonded Glare (glass‐reinforced aluminium laminate) repairs were performed at room temperature and under constant amplitude fatigue loading. However, the realistic operating temperature of ?40 °C may degrade the material and will cause unfavourable thermal stresses. Bonded repair specimens were tested at ?40 °C and other specimens were tested at room temperature after subjecting them to temperature cycles. Also, tests were performed with a realistic C‐5A Galaxy fuselage fatigue spectrum at room temperature. The behaviour of Glare repair patches was compared with boron/epoxy ones with equal extensional stiffness. The thermal cycles before fatigue cycling did not degrade the repair. A constant temperature of ?40 °C during the mechanical fatigue load had a favourable effect on the fatigue crack growth rate. Glare repair patches showed lower crack growth rates than boron/epoxy repairs. Finite element analyses revealed that the higher crack growth rates for boron/epoxy repairs are caused by the higher thermal stresses induced by the curing of the adhesive. The fatigue crack growth rate under spectrum loading could be accurately predicted with stress intensity factors calculated by finite element modelling and cycle‐by‐cycle integration that neglected interaction effects of the different stress amplitudes, which is possible because stress intensities at the crack tip under the repair patch remain small. For an accurate prediction it was necessary to use an effective stress intensity factor that is a function of the stress ratio at the crack tip R_{crack tip} including the thermal stress under the bonded patch.     

对双峰织构类型AZ31镁合金板材力学性能各向异性的分析

目的 制备双峰织构类型的AZ31镁合金板,以改善板材微观组织和弱化基面织构,研究微观组织对力学性能各向异性的影响规律,以提高镁合金板材的成形性能。方法 通过弯曲限宽矫直技术对0°、30°和60°轧向切样的板材进行热加工以预制拉伸孪晶,获得双峰织构类型的AZ31镁合金板材,通过EBSD获取板材的微观组织。对RD、45°和TD方向的原始板材进行室温单向拉伸实验,获得板材的工程应力-应变曲线及力学性能参数,并计算r值（塑性应变比）与n值（应变硬化指数）。结果 弯曲限宽矫直技术可诱发大量拉伸孪晶形成ED偏转织构,将偏转织构与基面织构共存的板材称为双峰织构类型AZ31镁合金板材。拉伸孪晶的出现显著细化了晶粒,弱化了基面织构强度,使板材的屈服强度下降,极大提升了材料塑性。其中30°轧向切样的板材ND面塑性力学性能各向异性的改善效果最好,其r值最小、n值最大。结论 双峰织构类型能够弱化AZ31镁合金板材基面的织构强度,提高材料塑性。拉伸孪晶含量越高,板材的强度与塑性越好,力学性能各向异性的改善效果也越显著。