Quenching Distortion of Aluminium Castings – Improvement by Gas Cooling

For quenching of age hardenable aluminium alloys today predominantly aqueous quenching media are used, which can lead due to the Leidenfrost phenomenon to a non‐uniform cooling of the parts and thus to distortion. Particularly at thin‐walled or complex shaped parts local plastic deformations can occur by the uneven thermal stresses. In relation to the conventional quenching procedures in aqueous media, gas quenching exhibits a number of technological, ecological, and economical advantages. The quenching intensity can be adjusted by the variable parameters gas pressure and gas velocity as well as the kind of gas and thus can be adapted to the requirements of the part. The distortion behaviour of serial production aluminium parts was researched after high‐pressure gas quenching with nitrogen 10 bar and after water quenching. Aluminium castings and forgings are considered as interesting applications of gas quenching, because of their near‐net shape before age hardening. Cost savings would be possible, because of reduced distortion and therefore less reworking.     

Comparing bond formation mechanism between similar and dissimilar aluminium alloy friction welds

Abstract

Friction welding of high strength aluminium alloys was considered in the current study. The mechanism of bond formation in dissimilar alloy welding was compared to similar alloy welding. The differences in flow properties between two different alloys of aluminium lead to uneven deformation behaviour while welding. Since flow behaviour directly affects bond formation and strength, it is important to understand its behaviour. Rods of aluminium alloys AA 2024 and AA 6061 were welded to themselves and to each other denoting similar and dissimilar weld systems. Burn-off length, which is one of the important parameters in friction welding was varied in steps to see how it affects the bond formation. The flash geometry of the welds was observed to interpret the flow differences. Reasons for variations in bond formation were discussed with the help of fracture surfaces.     

Investigation of fatigue cracks in aluminium alloys 2024 and 6013 in laboratory air and corrosive environment

Fatigue cracks have been generated in the commercial aluminium alloys AA2024 and AA6013 and analysed with a positron microprobe. This instrument provides laterally resolved positron annihilation measurements, which are sensitive to lattice defects like vacancies and dislocations. These commercial alloys have undergone a solution heat treatment and quenching prior to fatigue testing. Subsequently, they have been aged at room temperature and 190°C for AA2024 and AA6013, respectively. We performed the fatigue crack generation both in air and under the influence of a chemically aggressive environment (artificial seawater). Due to the corrosive environment hydrogen is probably produced at the fresh fractured surface in the vicinity of the crack tip. We discuss the possible implications of in-diffused hydrogen on the produced lattice defects, especially when there is a delayed migration of vacancies in the lattice, due to a reduced mobility.     

A Biaxial Fatigue Specimen for Uniaxial Loading

Abstract: The aim of this paper is to present a novel un‐notched fatigue test specimen in which a biaxial stress state is achieved using a uniaxial loading condition. This allows the problem of multi‐axial fatigue to be studied using relatively common one‐axis servo‐hydraulic testing machines. In addition the specimen presented here is very compact and can be made using a small volume of material (100 × 40 × 4.5 mm). For this specimen, the degree of biaxiality, defined by the parameter is equal to approximately 0.45. The specimen geometry was optimised using the Dang Van multi‐axial fatigue criterion. In addition to use as a fatigue specimen, it has been demonstrated that the biaxial specimen presented here is also suitable for biaxial tensile tests, to determine the rupture strength of a material in a biaxial stress state. Two different materials have been investigated: The first was wrought aluminium 2024‐O in the form of 5 mm sheets. The second was a cast aluminium‐silicon alloy AlSi7Cu0.5Mg0.3, commonly used in automotive and aeronautical applications. The fatigue strengths were determined at 2 × 10⁶ cycles and at various R‐ratios using a staircase procedure. For the aluminium 2024, it is shown that the biaxial stress state increases the maximum permissible first principal stress when compared to the uniaxial condition. However, in terms of the cast aluminium alloy, it has been demonstrated that this type of fatigue specimen is not suitable for materials containing casting defects, in particular micro‐shrinkage pores, because the volume of material, in which the stress state is biaxial, is not large enough.     

Synergistic effect of environmental media and stress on the fatigue fracture behaviour of aluminium alloys

The present study aims at explaining the synergistic effect of environmental media and stress/strain on fatigue lives of aluminium alloys. Rotating bending fatigue tests were carried out using four different aluminium alloys LY12‐CZ, 2024‐T4, 7475‐T7351 and 7075‐T651, at air state, 3.5% and 5.0% NaCl aqueous solutions. These results indicated that synergistic actions of the environmental media and cyclic loading accelerated the fatigue crack propagation of aluminium alloys. Furthermore, various influence factors (such as solution concentration, cyclic numbers, high (low) strength aluminium alloys etc.) of the fatigue life at synergistic actions of the environmental media and stress were quantificationally discussed in this paper.     

Acoustic quality factor of aluminium and selected aluminium alloys from 50 mK to 300 K

The amplitude-independent quality factors (Qs) of polycrystalline aluminium of 99.999 wt% purity, and the commercial aluminium alloys 2024, 6061 and 7075 have been measured in the temperature range 50 mK–300 K. Data were obtained at about 1 kHz from a torsional mode of machined monolithic mechanical resonators. The as-machined alloys exhibited maximum Qs of 1.5 to 9.5 million. The annealed alloys exhibited maximum Qs from 4.3 to 7.4 million. After several weeks at room temperature, substantially higher Qs were observed in all of the annealed materials, as compared to values obtained within a few days subsequent to the thermal treatment. Nevertheless, the maximum Qs were appreciably lower than those which have been measured in aluminium–magnesium alloys. Three loss peaks were observed in the aluminium, and a single peak in two of the alloys, even after annealing. The temperature dependence of the shear moduli was also obtained. This study is part of the continuing effort in this laboratory to evaluate the acoustic loss of high-Q materials at low temperatures, of particular interest for the construction of sensitive detectors of gravitational radiation.     

铝合金淬火使用聚合物淬火剂的应用研究

针对铝合金淬火使用聚合物淬火剂进行了实验研究。配制了聚合物体积分数为16%和33%的淬火水溶液,选择典型材料和零件进行了一系列对比实验。结果表明:采用33%和16%两种体积分数的AQ251聚合物淬火剂得到的拉伸性能、电导率和耐蚀性与水淬比较无明显差别,处于同一水平;在聚合物冷却介质中淬火可以显著减小零件变形。     

Controlling of Distortion by means of Quenching in adapted Jet Fields

Controlled temporal and/or spatial inhomogeneous (adapted) quenching within the heat treatment process creates the possibility of influencing the distortion of a workpiece. Suitable heat transfer conditions at the workpiece surfaces within the quenching process are realized by impressing and regulating of adjusted flexible flow fields on the basis of jet arrays inside liquid or gaseous media. For analysis of workpiece distortion in heat treatment, the locally asymmetric quenching process is analysed and modelwise described in the framework of the Collaborative Research Centre (SFB570) “Distortion Engineering” at the University of Bremen. Here the potential of measures for avoidance as well as reduction of distortion within the heat treatment process is appraised on the basis of simulation models. These model calculations are verified by experimental examinations on the asymmetric quenching of simple shaped parts like rings of SAE 52100 (100Cr6) and cylinders of AISI 5120 (20MnCr5) in the framework of flexible quenching conditions.     

Directional pressure quench casting of aluminium alloys

Abstract

A novel method of casting is described in which liquid metal in permeable moulds is subjected to high ambient pressure and quenching simultaneously. A small pressure quenching chamber was used and shapes of two different geometries cast into sand moulds. Two non-heat treatable alloys, LM6M and LM21M, comprised the raw material. The results show that increasing ambient pressure alone up to a value of 2 MPa (20 bar) can reduce porosity of castings, has a limited effect on tensile strength, and little effect on dendrite arm spacing. Casting under pressure with quenching increased tensile strength by up to 30% compared with conventional castings and virtually eliminated porosity. The dendrite arm spacing is also considerably reduced. Pressure quench casting is a potentially inexpensive means of improving the mechanical and microstructural properties and integrity of a wide range of aluminium alloys made by existing sand casting methods. For heat treatable alloys it offers the possibility of solution treating in the mould, thus eliminating one operation in a production process.

MST/3099     

Study on the accumulative fatigue damage rules under multiaxial two‐stage step spectra constructed by loadings with similar lives

In this paper, the influence on the multiaxial fatigue damage accumulation caused by loading path variation was studied. For 2024‐T4 aluminium alloy, the damage evolution during the entire life was first observed. On the basis of the observation, the stage I of fatigue damage evolution was further divided into two sub‐stages, and the dominant stress parameters of these two sub‐stages were proposed. Taking the dominant stress parameters into account, a phased accumulative fatigue damage model was proposed. Then, 12 multiaxial two‐stage step spectra constructed by loadings with approximately identical fatigue lives were carried out on 2024‐T4 aluminium alloy. The accumulative fatigue damage was calculated by the proposed model, and another five commonly used models and the calculated results were compared. According to the comparison, the newly proposed model had the most accurate results with the smallest scatter.     

Plastic CTOD as fatigue crack growth characterising parameter in 2024‐T3 and 7050‐T6 aluminium alloys using DIC

The plastic range of crack tip opening displacement (CTOD) has been used for the experimental characterisation of fatigue crack growth for 2024‐T3 and 7050‐T6 aluminium alloys using digital image correlation (DIC). Analysis of a complete loading cycle allowed resolving the CTOD into elastic and plastic components. Fatigue tests were conducted on compact tension specimens with a thickness of 1 mm and a width of 20 mm at stress ratios of 0.1, 0.3 and 0.5. The range of plastic CTOD could be related linearly to da/dN independent of stress ratio for both alloys. To facilitate accurate measurements of CTOD, a method was developed for correctly locating the crack tip and a sensitivity analysis was performed to explore the effect of measurement position behind the crack tip on the CTOD. The plastic range of CTOD was demonstrated to be a suitable alternate parameter to the stress intensity factor range for characterising fatigue crack propagation. A particularly innovative aspect of the work is that the paper describes a DIC‐based technique that the authors believe gives a reliable way to determine the appropriate position to measure CTOD.     

Variation of the cyclic strain-hardening exponent in advanced aluminium alloys

The cyclic strain-hardening exponents for five fatigue-resistant aluminium alloys were determined throughout the fatigue life to study the degree of cyclic stability of these alloys. Data were compared with results for 2024-T4 aluminum and for two high-pressure steels. The strain-hardening exponent increased logarithmically in all cases except 2024-T4, although the increase was small and did not exceed 33% over the fatigue life. 7475-T351 aluminium alloy was found to be entirely stable, and 7075-T7351 almost so. These were followed in order of rising sensitivity by 2014-T6, 7050-T73651, and 2124-T851 aluminium alloys, and 28NiCrMo7.4 and 30CrNiMo8 steels. 2024-T4 aluminum alloy demonstrated a strong decrease in strain-hardening exponent with fatigue life.     

Homogenization of hardness distribution and distortion in high pressure gas quenching

Quenching with gases rather than oil or other liquid media has the advantages of reducing the risks concerning health and environment, while simultaneously homogenizing the quenching results and minimizing distortion due to a wide range of possible process parameter variations and the pure convective heat transfer. In this contribution, a coupled solution for increasing homogenization of quenching results within high pressure gas quenching will be presented. In the first stage, an experimental test facility was set up for flow investigations and in the second stage a numerical simulation model was generated. The numerical and experimental results of the flow through the chamber were compared for several boundary conditions. Finally, after complete verification of the simulation, the model may be used to assist in parameter variation for optimization of homogeneous high pressure gas quenching.     

Mechanical characterisation and quality index of A356-type aluminium castings heat treated using fluidised bed quenching

Abstract

The current study aimed at investigating the effect of fluidised sand bed quenching on the mechanical performance and quality index of A356·2 aluminium cast alloys. Traditional water and conventional hot air quenching media were used to establish a relevant comparison with fluidised sand bed quenching. Quality charts were generated using two models of quality indices to support the selection of material conditions on the basis of the proposed quality indices. The use of a fluidised sand bed for the direct quenching aging treatment of A356·2 casting alloys yields greater UTS and YS values compared to conventional furnace quenched alloys. For the same aging conditions (170°C/4 h), the fluidised bed quenched aged 356 alloys show nearly the same or better strength values than those quenched in water and then aged in a CF or an FB. Based on the quality charts developed for alloys subjected to different quenching media, higher quality index values are obtained by water quenched T6-tempered A356 alloys. Using hot sand as a quenching medium at different temperatures, namely 170, 190 and 210°C, reduces both the strength and the quality with increase in quenching temperature for the alloys investigated. The regression models indicate that the eutectic silicon modification factor has the most significant effect on the quality results of the alloys investigated, for all heat treatment cycles, as compared to other metallurgical parameters.     

The effect of pitting corrosion on fatigue life

Fatigue testing of pre‐pitted 2024‐T3 aluminium alloy specimens is performed in laboratory air at 22 °C and 40% RH to characterize the effect of pitting corrosion on fatigue life. Specimens, pre‐corroded in a 0.5 M NaCl solution from 48 to 384 h, have fatigue lives that are reduced by more than one order of magnitude after 384 h pre‐corrosion as compared to those of uncorroded specimens. The reduction in fatigue life is interpreted in terms of the influence of the time of exposure to the corrosive environment or pit size. The crack‐nucleating pit sizes, ranging from 20 to 70 μm, are determined from post‐fracture examinations by scanning electron microscopy. Fatigue lives are estimated using a fracture mechanics approach and are shown to be in good agreement with the actual data. A probabilistic analysis shows that the distribution of fatigue life is strongly correlated to the distribution in nucleating pit size.     

Monotonic and low cycle fatigue behaviour of 2024‐T3 aluminium alloy between room temperature and 300 °C for designing VAWT components

In the present study, the results of the monotonic tension tests and low cycle fatigue tests performed on aluminium alloy EN AW‐2024‐T3 under various operating temperatures are presented in order to assess the fatigue behaviour of the aluminium alloy under evaluated temperatures. Monotonic tests were performed to determine the influence of temperature on mechanical properties of the material. The aim of cyclic tests was to acquire the parameters required for Manson–Coffin equation in order to plot strain–fatigue life curves. Moreover, stress–strain behaviour of the alloy and the cyclic hardening behaviour were evaluated using Ramberg–Osgood equation. Finally, P_SWT‐damage parameters for each temperature have been calculated for further investigation of the effects of the temperature on fatigue life using acquired data while taking the account of mean stress effect into calculations. Variations in the experimental data due to various test temperatures are presented for both monotonic and cyclic tests.     

Single-sided laser beam welding of a dissimilar AA2024–AA7050 T-joint

In the aircraft industry double-sided laser beam welding of skin–stringer joints is an approved method for producing defect-free welds. But due to limited accessibility – as for the welding of skin–clip joints – the applicability of this method is limited. Therefore single-sided laser beam welding of T-joints becomes necessary. This also implies a reduction of the manufacturing effort. However, the main obstacle for the use of single-sided welding of T-joints is the occurrence of weld defects. An additional complexity represents the combination of dissimilar and hard-to-weld aluminium alloys – like Al–Cu and Al–Zn alloys. These alloys offer a high strength-to-density ratio, but are also associated with distinct weldability problems especially for fusion welding techniques like laser beam welding. The present study demonstrates how to overcome the weldability problems during single-sided laser beam welding of a dissimilar T-joint made of AA2024 and AA7050. For this purpose a high-power fibre laser with a large beam diameter is used. Important welding parameters are identified and adjusted for achieving defect-free welds. The obtained joints are compared to double-sided welded joints made of typical aircraft aluminium alloys. In this regard single-sided welded joints showed the expected differing weld seam appearance, but comparable mechanical properties.     

Artgleiche und artverschiedene reibrührgeschweißte Verbindungen aus AA2124+25 % SiC und AA2024

Similar and dissimilar friction stir welded joints made from AA2124+25 % SiC and AA2024 An aluminium matrix composite (AMC) consisting of an AA2124 matrix reinforced by 25 vol.% SiC particles was used to produce similar AMC+AMC and dissimilar AMC+2024‐T3 joints by friction stir welding. When the particle reinforced composite was located on the retreating side, material mixing was less intense for dissimilar joints. Nevertheless, a higher strength has been determined for this arrangement due to a hook‐like interlocking of both materials. Tensile test and S‐N fatigue behaviour is shown to be compromised by alignment of the reinforcement particles perpendicular to loading direction already in the particle reinforced base material. Welding residual stresses were determined through the cut‐compliance method in terms of stress intensities acting at the crack tip. The underlying residual stress distribution in the un‐cracked structure was calculated by the weight function method. Longitudinal tensile residual stresses were found to be higher in the monolithic material as compared to the particle reinforced composite. This held true both for similar and within dissimilar joints. Growth behaviour of cracks crossing the joint line was described and correlated with residual stresses for similar joints.     

Characterization of the protective effect of aluminium surface treatments by d.c. and a.c. measurements

Steady-state and electrochemical impedance spectroscopy measurements have been made on anodic layers on 1050 and 2024T3 aluminium alloys prepared from solutions of phosphoric acid, boric acid and sodium tetraborate, before and after impregnation treatment with zinc. Corrosion characteristics of the anodic layers were dependent on the aluminium substrate and the electrolyte. Aluminium alloy composition was found to be the most important factor for corrosion resistance; alloying elements of 2024T3 alloy (like copper) had a harmful influence on this layer property. Steady-state measurements allowed the oxide layer behaviour to be studied in the anodic range by the determination of an anodic passivity domain. This domain was characterized by a weak aluminium oxidation through the oxide layer. The zinc impregnation treatment had a marked protective effect on each studied anodic layer. This treatment can be used as an alternative to hot sealing in water or chromic acid solution.     

腐蚀环境下2024-T3铝合金疲劳裂纹扩展和剩余强度实验研究

对2024-T3铝合金在5种典型实验室环境和3种组合环境下的疲劳裂纹扩展和剩余强度进行了实验研究.通过实验获得的裂纹扩展数据,对Paris公式进行条件拟合,得到各种环境下的裂纹扩展常数,并作了对比分析.结果表明,腐蚀环境的参与使2024-T3铝合金的疲劳裂纹扩展速率明显加快,不同腐蚀环境对疲劳裂纹扩展速率的影响程度不同,其影响的严重程度由重到轻依次为:油箱结构区、厨房与厕所、油箱积存水、盐水、潮湿空气、高空环境、干燥空气.实验数据还进一步表明,腐蚀介质对临界裂纹长度的影响很小,说明环境对剩余强度能力无直接影响.