Bending in aluminium alloys AA 6111 and AA 5754 using the cantilever bend test

Abstract

Bendability is an important forming parameter in many applications, but particularly in automotive parts where the formed parts in structures can be quite complex, and where outer skins are joined to inner panels through the hemming process. In this paper the bend performance of two aluminium based automotive alloys, the heat treatable skin alloy, AA 6111 and the non-heat treatable structural alloy AA 5754, are assessed by the cantilever bend test. This test enables the loadndash;bend angle relationship to be monitored, and provides a bend surface that can be examined for different bend angles, since the bending pin does not contact the specimen surface in the local region of the bend. The results demonstrate that the cantilever bend test can differentiate between different bend performance, and the differences relate to the damage process involved in bending. In the heat treatable AA 6111 the bendability is dependent on the alloy temper, which controls the bend angle at which large surface cracks appear on the surface. This fracture process is a result of differences in the development of surface topography and surface damage with bending strain. The AA 5754 alloy has similar behaviour, but the performance is superior to the best of the AA 6111 tempers, and reflects a lower rate of surface topography development, and an absence of significant surface cracking over the bend angle range investigated.     

The paint–bake response of three Al–Mg–Zn alloys

The aging behaviors of three Al–Mg–Zn alloys have been investigated under conditions similar to the paint–bake cycle currently used in automotive manufacturing. The three alloys contain Mg in atomic concentrations from one to two times those of Zn. Natural aging at 25 °C after solutionizing is found to produce a linear increase in hardness with logarithmic time for times of up to 1 year. Hardnesses in naturally and artificially aged conditions are found to increase with Mg content. Artificial aging at 175 °C for 30 min, which simulates the automotive paint–bake cycle, produces increases in hardness of 15–36% over the solution-treated conditions. Peak hardness from artificial aging at 175 °C is produced in all alloys after approximately 8 h. Natural aging for 10 days prior to artificial aging at 175 °C does not produce significant changes in hardness compared with artificial aging alone. Natural aging for 1 year after simulated paint–bake aging increases hardnesses by 41–78% over those after simulated paint–bake aging alone. The precipitation strengthening mechanism in these alloys is consistent with η′ formation. Increases in hardness and strength with increasing Mg content are consistent with increased solid–solution strengthening, which is retained even after artificial aging.     

Development of mechanical properties during secondary aging in aluminium alloys

Abstract

Earlier work has shown that, if the artificial aging of aluminium alloys is interrupted by a dwell period at lower temperature, higher values of tensile properties and fracture toughness may be achieved than are possible with single stage T6 tempers. A second interrupted aging cycle has now been developed that involves underaging at the elevated temperature, quenching, and then allowing secondary precipitation to occur at, or just above, room temperature. Designated a T6I4 (I=interrupted) temper by the authors, this simpler aging cycle may reduce heat treatment costs. Tests on some 30 cast and wrought alloys have resulted in tensile properties close to those for a T6 temper, with higher values of fracture toughness being recorded for some cases. Such an aging treatment can be incorporated into a paint bake cycle to simplify the heat treatment of coated automotive components.     

基于人工神经网络的铜合金时效性能预测

通过对BP神经网络算法分析和收敛性的运用,对获得的时效实验数据进行训练,建立了Cu 0.30Cr 0.15Zr合金硬度和导电性与时效时间和时效温度的映射模型,从而可预测铜合金在一定时效条件下的硬度和导电性。结果表明,神经网络用于铜合金的时效性能预测是可行的。     

Metallurgical Observations on Fatigue Failure of a Bent Copper Tube

Deoxidized high phosphorus copper (C12200, DHP-Cu) is the principal construction material in hydraulic and HVAC installations due to its superior thermal/electrical conductivity, formability, corrosion resistance, and antibacterial properties. However, design and installation deficiencies or aggressive environmental conditions lead very often to unexpected failures due to abnormal operation or improper handling of material during construction. Copper water tubes experiencing leakage at the bend area, after short-time period in service, were disconnected from a hydraulic installation and brought for failure investigation. Optical and scanning electron microscopy, employed for microstructural and fractographic evaluation, were used as the principal analytical techniques in the context of the present investigation. Failure analysis findings suggest strongly that the leakage was caused by low-cycle fatigue failure (<10⁴ cycles), initiated from the tube outer surface and followed by brittle intergranular fracture. Final fracture occurred via the brittle intergranular failure process, when the remaining tube wall thickness reaches a critical size. The crack then becomes unable to sustain the operating load. Review of the design and installation conditions along with the special attention to material handling to avoid surface flaws (pits, scratches, gouges, etc.), which might be potential fatigue crack initiation sites due to increased stress concentration, are suggested as preventive actions to minimize the potential for further fatigue damage.     

Investigation on the effect of curvature on forming limit prediction for aluminium sheet alloys

The investigation of bendability of sheet metal alloys is of great importance for the evaluation of process robustness in production of hemmed sheet metal assemblies. Furthermore, material cracks due to bending can also occur at deep drawing operations. This contribution focuses on the experimental determination of material’s formability at deep drawing, the characterisation of bendability especially for hemming operations and the determination of formability under combined deep drawing and bending loads, respectively.     

Thermal history and tensile strength determination from fracture surface analysis

Aluminum alloys are used in various critical aerospace applications that require properties to be obtained from very specific heat treatments. This study shows that the fracture surface can indicate details of the thermal history and strength of the material. This is a novel technique to characterize the material from an analysis of the fracture surface features. This study evaluated the effect of varying the cooling rate and aging time on the fracture surface of an aluminum alloy.     

Comparative experimental study on the modification of microstructural and mechanical properties of friction stir welded and gas metal arc welded EN AW‐6061 O aluminum alloys by post‐weld heat treatment

In this paper, the effects of post‐weld heat treatment on modification of microstructures and mechanical properties of friction stir welded and gas metal arc welded AA6061‐O plates were compared with each other. Gas metal arc welding and friction stir welding were used as the applicable welding processes for AA6061‐O alloys. The applied post‐weld heat treatment consisted of solution heat treatment, followed by water quenching and finally artificial aging. The samples were classified as post‐weld heat treated and as‐welded joints. The microstructural evolution, tensile properties, hardness features and fracture surfaces of both as‐welded and post‐weld heat treated samples were reported. The results clearly showed that friction stir welding process demonstrated better and more consistent mechanical properties by comparison with the gas metal arc welding process. The weld region of as‐welded samples exhibited a higher hardness value of 80 HV0.1 compared to the base material. In addition, the feasibility of post‐weld heat treatment in order to enhance the mechanical properties and to obtain more homogeneous microstructure of 6061‐O aluminum alloys was evaluated.     

电液伺服阀弹簧管失效分析

统计分析了某电液伺服阀13个失效弹簧管的开裂情况(包括弹簧管的应用、开裂位置、裂纹扩展方向以及开裂方式的规律性)。采用外管分析、断口分析和金相组织分析等方法,研究了弹簧管断裂特征与机理。结果表明:弹簧管的开裂原因包括较大的疲劳应力、材质和加工因素。夹杂物在弹簧管冷拉成型过程中沿轴向分布,在周向打磨过程中沿夹杂物形成裂纹。     

Brittle fracture in rounded-tip V-shaped notches

Two failure criteria are proposed in this paper for brittle fracture in rounded-tip V-shaped notches under pure mode I loading. One of these criteria is developed based on the mean stress criterion and the other based on the point stress criterion which both are well known failure criteria for investigating brittle fracture in elements containing a sharp crack or a sharp V-notch. To verify the validity of the proposed criteria, first the experimental data reported by other authors from three-point bend (TPB) and four-point bend (FPB) tests on PMMA at −60 °C and Alumina–7% Zirconia ceramic are used. Additionally, some new fracture tests are also carried out on the rounded-tip V-notched semi-circular bend (RV-SCB) specimens made of PMMA for various notch opening angles and different notch tip radii. A very good agreement is shown to exist between the results of the mean stress criterion and the experimental data.     

Progressive failure monitoring and analysis in aluminium by in situ nondestructive evaluation

Damage initiation and progression in precipitate hardened alloys are typically linked to the failure of second phase particles that result from the precipitation process. These particles have been shown to be stress concentrators and crack starters as a result of both particle debonding and fracture. In this investigation, a precipitate hardened aluminium alloy (Al 2024‐T3) is loaded monotonically to investigate the role the particles have in the progressive failure process. The damage process was monitored continuously by combining the acoustic emission method either with in situ scanning electron microscopy or X‐ray microcomputed tomography to obtain both surface and volume microstructural information. Particles were observed to fracture only in the elastic regime of the material response, while void growth at locations predominantly near particles were found to be associated with progressive failure in the plastic region of the macroscopic response. Experimental findings were validated by fracture simulations at the scale of particle‐matrix interface.     

Effect of Mg and Sr-modification on the mechanical properties of 319-type aluminum cast alloys subjected to artificial aging

Aluminum-based 319-type cast alloys are commonly used in the automotive industry to manufacture cylinder heads and engine blocks. These applications require good mechanical properties and in order to achieve them through precipitation hardening, artificial aging treatments are applied to the products. The standard artificial aging treatment for alloy 319, as defined by the T6 heat treatment temper, consists in solution heat-treating the product for 8 h at 495 °C, water quenching at 60 °C, and then artificially aging at 155 °C for 2–5 h.

The present paper reports on aging heat treatments that were performed on four Al–Si–Cu–Mg 319-type alloys: 319 base alloy, Sr-modified 319 alloy, 319 alloy containing 0.4 wt% Mg, and the Sr-modified 319 + 0.4 wt% Mg alloy. This investigation was carried out in order to examine the effect of Sr-modification and additions of Mg on the microhardness, tensile strength and impact properties of 319-type alloys over a range of aging temperatures and times (150–240 °C, for periods of 2–8 h).

The results show that the best combination of properties is found in the Sr-modified alloy containing 0.4 wt% Mg (i.e. alloy 319 + Mg + Sr). Also, the optimum artificial aging temperature changes when Mg is present in the alloy.     

Effect of corrosion on the fatigue life and fracture mechanisms of 6101 aluminum alloy wires for car manufacturing applications

An innovative solution for the automotive industry is to replace the copper used for wiring harnesses with aluminum alloys, such as the aluminum–magnesium–silicon 6101 alloy. Wiring harnesses are composed of thin strand arms obtained by a wire drawing process. These strands are susceptible to exposure to a corrosive environment and fatigue solicitations simultaneously. The fatigue endurance of this alloy was studied using the stress-life approach for three metallurgical states representative of three cold-drawing steps. Fatigue tests performed in corrosive media tests highlighted a strong decrease of the 6101 alloy lifetime due to fatigue–corrosion interactions and a modification of failure modes.     

Nondisruptive material sampling and mechanical testing

The aging and inservice degradation of industrial equipment has underscored a need for efficient and reliable evaluation of the suitability of such equipment for continued service. The structural components of traditional energy production facilities, such as fossil- and nuclear-fueled electric power plants, are prime examples of aging equipment for which integrity during extended service is of major concern. The paper describes a recently developed nondisruptive miniature material sample removal and test approach that is being applied to a range of operating electric power plant components from turbine generators to pressure vessels, and to petrochemical plant reactor vessels for inservice integrity assessment. Thein situ removal of a thin wafer-like sample (less than 25 mm in diameter and 2.5 mm in thickness) from the component surface generally has no effect on component integrity. The miniature specimen small punch (disk bend) test has been developed to mechanically test the as-removed material, and is being used to estimate the material tensile behavior and fracture properties (ductile-to-brittle transition temperature and fracture toughness) required for a reliable component integrity assessment.     

Bend properties of sapphire fibers at elevated temperatures I: Bend survivability

The effect of temperature on the bend radius that a c-axis-oriented sapphire fiber can withstand was determined for fibers of various diameter. Bend stress rupture tests were performed for times of 1–100 h and temperatures of 300–1700 °C. Fibers would survive the bend test undeformed, would fracture or would deform. The bend survival radius was determined to be the radius above which no fibers fractured or deformed for a given time-temperature treatment. It was found that the ability of fibers to withstand curvature decreases substantially with time and increasing temperature and that fibers of smaller diameter (40–83 μm) withstood smaller bend radii than would be expected from just a difference in fiber diameter when compared with the bend results of the fibers of large diameter (144 μm). This was probably due to different flaw populations, causing high temperature bend failure for the tested sapphire fibers of different diameters.     

A probabilistic model for cleavage fracture with a length scale - Parameter estimation and predictions of growing crack experiments

A probabilistic model for the cumulative probability of failure by cleavage fracture was applied to experimental results where cleavage fracture was preceded by ductile crack growth. The model, introduced by Kroon and Faleskog [Kroon M, Faleskog J. A probabilistic model for cleavage fracture with a length scale - influence of material parameters and constraint. Int J Fract 2002;118:99-118], includes a non-local stress with an associated material related length scale, and it also includes a strain measure to account for the number of nucleated cleavage initiation sites. The experiments were performed on single edge cracked bend test specimens with three different crack lengths at the temperature 85 °C, which is in the upper transition region for the steel in question. The ductile rupture process is modelled using the cell model for nonlinear fracture mechanics. The original cleavage fracture model had to be modified in order to account for the substantial number of cleavage initiators being consumed by the ductile process. With this modification, the model was able to accurately capture the experimental failure probability distribution.     

Fracture analysis of dissimilar Al‐Al friction stir welded joints under tensile/shear loading

In this research, fracture of dissimilar friction stir welded (FSWed) joint made of Al 7075‐T6 and Al 6061‐T6 aluminum alloys is investigated in the cracked semi‐circular bend (CSCB) specimen under mixed mode I/II loading. Due to the elastic‐plastic behavior of the welded material and the existence of significant plastic deformations around the crack tip at the propagation instance, fracture prediction of the FSWed specimens needs some failure criteria in the context of the elastic‐plastic fracture mechanics which are very complicated and time‐consuming. For this purpose, the Equivalent Material Concept (EMC) is used herein by which the tensile behavior of the welded material is equated with that of a virtual brittle material. By combining EMC with the 2 brittle fracture criteria, namely the maximum tangential stress (MTS) and mean stress (MS) criteria, the load‐carrying capacity (LCC) of the FSWed CSCB specimens is predicted. Comparison of the experimental results and theoretical predictions from the 2 criteria showed that both criteria could accurately predict the LCC of the cracked specimens. Moreover, as the contribution of mode II loading increases, the size of the plastic region around the crack tip at failure increases, leading to increasing the LCC.     

泡沫金属三明治结构压印-粘接复合接头剥离性能分析

为研究三明治结构压印-粘接复合接头的抗剥离性能,选取AA5052铝合金板以及泡沫镍、泡沫铜以及泡沫铁镍进行压印-粘接复合连接,对接头进行拉伸剪切试验,采用扫描电镜对失效断口进行了观察,分析了接头的失效形式、失效载荷、能量吸收值以及失效机理。结果表明:夹层的存在会使接头中胶层的失效过程更加稳定,夹层对于压印-粘接复合接头的承载能力并无显著影响,但是会降低其失效过程中的能量吸收值。     

光氧老化对聚碳酸酯结构和性能的影响

研究了光氧老化后聚碳酸酯(PC)的结构和性能变化。力学性能测试结果表明,老化后,PC的冲击强度下降,拉伸强度、弯曲强度上升;扫描电镜断口形貌分析和差示扫描量热分析显示,光氧老化后PC材料的韧性有一定损失;傅立叶变换红外光谱分析显示PC老化后有酚类、酮类产物生成;PC的光氧老化可能是由于弗利斯重排反应产生自由基诱发PC分子链光氧降解反应所致。     

Relationship between base metal properties and friction stir welding process parameters

Friction stir welding (FSW) is a solid state welding process for joining aluminum alloys and has been employed in aerospace, rail, automotive and marine industries for joining aluminium, magnesium, zinc and copper alloys. In FSW, the base metal properties such as yield strength, ductility and hardness control the plastic flow of the material under the action of rotating non-consumable tool. The FSW process parameters such as tool rotational speed, welding speed, axial force, etc. play a major role in deciding the weld quality. In this investigation, an attempt has been made to establish relationship between the base material properties and FSW process parameters. FSW joints have been made using five different grades of aluminium alloys (AA1050, AA6061, AA2024, AA7039 and AA7075) using different combinations of process parameters. Macrostructural analysis has been done to check the weld quality (defective or defect free). Empirical relationships have been established between base metal properties and tool rotational speed and welding speed, respectively. The developed empirical relationships can be effectively used to predict the FSW process parameters to fabricate defect free welds.