Friction stir welding of thick section Al‐Zn‐Mg‐Cu aluminum alloy

Friction stir butt welding of 25 mm thick AA7075–T651 plates has been investigated. Careful process parameter selection resulted in single pass, full‐penetration defect free welds. The weld nugget exhibits a significant grain refinement while facing the dissolution of strengthening precipitates. Microhardness survey gives a W‐shaped profile with lower hardness values recorded in the thermo‐mechanically‐affected zone. Tensile fractures occur, again, in the thermo‐mechanically‐affected zone, where minimum hardness occurred. The friction stir welds demonstrate an excellent root bend performance while falling behind base material in face bend test. The welds also displayed outstanding impact toughness compared to that of parent material. It is concluded that defect free single pass friction stir welds can be successfully made on 25 mm thick AA7075–T651 plates.     

铝合金板厚度对硼/环氧补片单面修复试件疲劳性能的影响

采用单向硼/环氧复合材料补片真空袋压工艺单面修复含中心裂纹不同厚度铝合金板,测试了修复试件的疲劳性能,从疲劳寿命、疲劳裂纹扩展速率和裂纹扩展纹线考察不同厚度铝合金板修复后疲劳性能的差异。结果表明：硼/环氧补片胶接修复后,铝合金板的疲劳寿命大幅度提高,且疲劳寿命提高幅度随铝合金板厚度增大而降低。厚度为1.76mm、5.20mm和10.20mm 3种铝合金板修复试件的疲劳寿命分别是未修复试件的22.30倍、12.84倍和8.40倍。厚度为1.76mm铝合金板修复试件在铝合金板完全断裂后还能继续承担疲劳载荷,而厚度为5.20mm和10.20mm 2种铝合金板修复试件在铝合金板断裂后完全破坏。裂纹扩展速率和归一化裂纹长度差均随铝合金板厚度增大而增大。     

Size effects in the fatigue behavior of welded steel tubular bridge joints

Tubular space trusses for bridge applications use thick‐walled tubes. The reduction in fatigue resistance due to geometrical size effects is thus an important issue. In order to carry out a thorough study, both fatigue tests on large‐scale specimens and advanced 3D crack propagation modelling were carried out at ICOM/EPFL. The study is limited to circular hollow sections (CHS) K‐joints. An alternate current potential drop (ACPD) system is used to measure crack depth on nodes of the tested truss specimens. The results obtained from the tests are given in the paper in terms of S‐N data, crack depth versus number of cycles and deduced crack propagation rates. The numerical model was developed using the dual boundary elements method (DBEM), software BEASY?, and was validated with fatigue tests data. The stress intensity factors (SIF) along the doubly curved crack front at different crack depths were obtained. With this model, a parametric study investigates the influence of geometry, size and load case on fatigue life. The results of both proportional and non‐proportional sizing effects on fatigue strength are presented. The paper shows that size effects (proportional and non‐proportional) can be expressed as a function of the non‐dimensional parameters and chord thickness.     

Effect of temperature on the fracture behaviour of heat‐treated Al–Cu–Li alloy laser welds under low‐cycle fatigue loading

The paper presents the results of the studies of the effect of temperature on the fracture behaviour of Al–Cu–Li alloy laser welds under low‐cycle fatigue loading. The mechanical properties and the microstructure of the welded joints without and after postweld heat treatment (PWHT) were investigated. The tensile strength and the low‐cycle fatigue resistance of the welded joints were studied at various test temperatures (20°C, 85°C and ? 60°C). It was been found that heating up to 85°C and cooling down to ?60°C reduced the maximum number of loading cycles of the welded joints after PWHT by 1.5–2.0 times compared with that at a test temperature of 20°C.     

Fatigue resistance of weld ends – Analysis of the notch stress using real geometry

Modern welded structures often contain weld start and end points which are the failure critical location. In particular, for special investigations into thin sheet structures, no approach for the determination of the fatigue life has been established thus far. In this research, the primary aim was to obtain the real geometry of weld ends with high precision using a three dimensional scanner to find a general approach using the notch stress concept. Going one step further, analyses have been performed regarding to unify the notch stress concept. The existing results of Olivier – who examined long welds with no start and end points – were re‐evaluated to unify the results of long regular welds with the local weld end under one scatter band.     

Determination of local characteristics for the application of the Weakest‐Link Model

The Weakest‐Link model is based on defects that are statistically distributed within the material with local stress. The failure at least at one defect causes the failure of the total structure. Based on this model, the so‐called statistic size effect can be evaluated in the case of cyclic loading and in the case of static loading the failure behaviour of ferritic steel within the brittle fracture range is highlighted. The application of the Weakest‐Link model requires the allocation of the local characteristics ‐ surface and / or volume ‐ to the discrete points of the stress. By using the method “SPIEL” which is independent from the FE code used, the allocation of couples of values ‐surface and stress and / or volume and stress ‐ by a suitable choice of unit load cases is possible. In consequence of the method “SPIEL” particularities are to be taken into consideration. In the present paper these particularities will be described exemplarily for the FE programs ABAQUS¹ and ANSYS².     

Effects of creep age forming time on microstructure and high cyclic fatigue of 7075 aluminum alloy

In this paper, the hardness, ultimate tensile strength, yield strength, elongation E₁₀₀, S−N curves, and fatigue performance of 7075 aluminum alloy were obtained after aged at 170 °C for different times (10 h, 15 h, and 20 h). Additionally, the microstructure and fatigue fracture of the alloy were observed. The investigation results show that as the forming time increased, the hardness, ultimate tensile strength, and yield strength decreased, the elongation first decreased, then increased, and the fatigue limit increased. As the forming time increased, the metastable phase gradually transformed into a steady phase and coarsened, the width of precipitate free zone increased, and the width of the fatigue strip decreased. After creep age forming for 20 h, the precipitate free zone was the widest, approximately 100 nm.     

Structure‐property investigations on a laser beam welded dissimilar joint of aluminium AA6056 and titanium Ti6Al4V for aeronautical applications. Part II: Resistance to fatigue crack propagation and fracture

Investigations were continued on the dissimilar laser beam welds of AA6056 and Ti6Al4V, fabricated by inserting Ti‐sheet into the profiled Al‐sheet and melting AA6056 alone. By using microstructure, hardness and strength as the criteria, sites exhibiting non‐uniform microstructure and localized plastic deformation due to strength mismatch were investigated in two orientations: ? crack parallel to the weld and ? crack perpendicular to the weld for fatigue crack propagation and fracture toughness at room temperature. Effect of temper of AA6056 on these properties was studied for two conditions; welding in T4 followed by post weld heat treatment T6, and welding in T6 and naturally aged for a defined period. The orientation “crack parallel to the weld” was investigated in 3 locations on the side of AA6056: the interface and the two changeovers on the Al‐side. Firstly, between the fusion zone and the heat affected zone (3 mm from the interface) and secondly, between (primary) heat affected zone and towards the base material (7 mm from the interface). Although brittle intermetallic TiAl₃ had been formed at the interface, uncontrolled separation or debonding at the interface was not observed. Insofar the bond quality of the weld was good. However, the ranking of interface was the lowest since fatigue crack propagation was relatively faster than that in the fusion zone and heat affected zone, and fracture toughness was low. Therefore, unstable fatigue crack propagation is observed when the crack propagates perpendicular to the weld from AA6056 towards Ti6Al4V. The results have shown that the dissimilar joints exhibit improved performance when laser beam welded in the T6 condition.     

Fatigue behavior of a 7075-T6 aluminum alloy coated with an electroless Ni–P deposit

An investigation has been carried out in order to study the fatigue and corrosion–fatigue behavior of a 7075-T6 aluminum alloy coated with an electroless Ni–P (EN) deposit, in the as-plated condition, of approximately 38–40 μm in thickness and a high P content, of approximately 18 wt%. The results obtained, show that the EN coating can give rise to a significant improvement in the fatigue and corrosion–fatigue performance of the substrate, depending on the testing conditions. When the coated system is tested in air, it is observed that the increase in fatigue properties decreases as the alternating stress applied to the material increases. At stresses of the order of 0.4 σ_0.2% the increase in fatigue life is more than about 100%. However, as the stress increases to values in the range of 0.7 σ_0.2%, no improvement in the fatigue performance of the system is observed and the behavior is similar to that of the uncoated substrate. Under corrosion–fatigue conditions, the fatigue life is observed to increase between approximately 60% and 70%, depending on the stress applied. It is shown that fatigue cracks are associated with nodular-like defects present on the surface of the coated samples. The deleterious effect of such defects seems to be more pronounced as the alternating stress applied to the material increases. A crude estimate of the yield strength of the EN coating from tensile measurements indicates that such a parameter is in the range of 3.8 GPa, in agreement with the computation of the absolute hardness of the deposit, of about 4 GPa, by means of Meyer’s law. It is also shown that the EN deposit has a very good adhesion to the substrate even when the system is subjected to tensile stresses greater than the yield strength. Such characteristics as well as the higher mechanical properties of the EN coating in comparison with the aluminum alloy substrate and the preservation of its integrity during fatigue testing contribute to the better fatigue performance of the coated system.     

A prediction method for endurance of carburized components based on fatigue of corresponding material states with different carbon contents

Carburizing generates different material properties and residual stresses in the carburized layer. A material mechanics based model for endurance prediction of carburized steel components has been developed, in which thin walled tubes are used under cyclic internal and monotonic external pressures allow the determination of the material properties of the carburized layers. An unexpected independence of endurance and mean stress sensitivity from carbon content and hardness has been found, contrary to existing scientific knowledge. Component like specimens were tested to validate the endurance prediction model. The deviations of the predictions from the experiment are in the range of ±8 percent.     

Influence of carbon nanotubes and dispersions of SiC on the physical and mechanical properties of pure copper and copper‐nickel alloy

This paper investigates the physical and mechanical properties of copper‐nickel alloy (at 50 wt.%–50 wt.%) and pure copper, mixed with various types of reinforcement materials such as carbon nanotubes (0.5 wt.%–2 wt.%) as nanoparticles, silicon carbide (1 wt.%–4 wt.%) as microparticles. The acquired composite specimens characteristics were estimated such as microstructure, density, electrical and thermal conductivity, hardness, and compression stress properties to determine the suitable reinforcement percentage that has the best physical and mechanical properties with different main matrix material whether copper‐nickel mechanical alloying or pure copper powder. The micron‐sized silicon carbide and nanosized carbon nanotubes were added to improve the mechanical and physical properties of the composite. The electrical and thermal conductivity of pure copper alloy enhanced compared with the copper‐nickel alloy matrix material. The hardness and compression yield stress of both pure copper and copper‐nickel composites have enhancement values and for copper‐nickel base composites hardness and compression yield stress have enhanced with the most positive enhancement values to examined an optimum percentage of reinforcing material.     

Magnesium addition in copper‐chromium alloys: The refinement of aging precipitates and improvement on mechanical properties

Herein, we demonstrate the synthesis of copper‐chromium and copper‐ chromium‐magnesium alloys by melting and casting process and explore the effect of the magnesium addition on mechanical and electrical properties of the alloys. This article focuses on the variation of the precipitation sequence and the decrease of strengthening phase sizes induced by the addition of trace magnesium element. The results show that magnesium element has little effect on the hardness of copper‐chromium alloy, but it significantly improves the hardness of the aging alloy and maintains high conductivity. The addition of magnesium element inhibits the growth and structural transformation of the precipitated phase. The refinement impact of magnesium addition on precipitated phase and change in alloy precipitation sequence may be the main reasons for the high hardness of copper‐chromium‐ magnesium alloy. In addition, the magnesium addition shows a significant refinement effect on small size precipitation phase, but it does not present the same refinement effect on large size precipitation phase. This attributes to the presence of a semi‐coherent interface between the matrix and the large size of precipitates, which provides the dislocation‐based diffusion channels for high‐rate chromium diffusion and promotes the precipitate growth.     

Effect of post-weld heat treatment on fracture toughness and fatigue crack growth behaviour of electron beam welds of a titanium (α+β) alloy

The effects of a post-weld heat treatment on the fracture toughness and fatigue crack growth behaviour of electron beam welds of an α + β titanium alloy, Ti–6.5Al–1.9Zr–0.25Si have been studied. Welds in the stress-relieved condition exhibited poor fracture toughness due to poor energy absorbing capacity of the thin α and α' phases. Post-weld heat treatment which resulted in the decomposition of α' to α + β and the coarsening of intragranular and intergranular α resulted in improved toughness. This improvement in the toughness is related to improved ductility leading to crack blunting, crack path deviation at the thick intragranular and intergranular α phase. Fatigue crack growth resistance of welds was superior to the base metal in the α + β heat-treated condition. The superior crack growth resistance of the welds is due to the acicular α microstructure which results in a tortuous crack path and possible crack closure arising from crack path tortuosity.     

Investigation on low cycle fatigue of nickel‐based single crystal turbine blade in different regions

Low cycle fatigue experiments of nickel‐based single crystal superalloy miniature specimens were carried out at 760 °C/1000 MPa and 980 °C/750 MPa. According to testing results, low cycle fatigue life is dependent on sampling position of turbine blade under same test conditions. Fracture surface morphology and longitudinal profile microstructure indicated that the fracture mechanism transformed from cleavage fracture to ductile fracture with the changing of medium temperature to high temperature due to the particle cutting at yield stress intensity. The scanning electron microscopy observation of original material demonstrated that the smaller precipitate size of samples have a shorter fatigue life. Meanwhile, the constitutive model considering size effect was built based on the crystal plastic theory. The finite element analysis demonstrated that the smaller precipitate size could dramatically reduce the plastic deformation suffering the same cycle loading.