Strain-controlled fatigue properties of dissimilar welded joints between Ti–6Al–4V and Ti17 alloys

The aim of this study was to evaluate the microstructure, hardness and cyclic deformation behavior of electron beam welded dissimilar joints of Ti–6Al–4V and Ti17 (Ti–5Al–4Mo–4Cr–2Sn–2Zr) titanium alloys. The welding resulted in a significant microstructural change across the joint, with hexagonal close-packed (hcp) martensite α′ and orthorhombic martensite α″ in the fusion zone (FZ), α′ in the heat-affected zone (HAZ) of Ti–6Al–4V side, and coarse β in the HAZ of Ti17 side. A characteristic asymmetrical hardness profile across the dissimilar joint was observed with the highest hardness in the FZ and a lower hardness on the Ti–6Al–4V side than on the Ti17 side, where a soft zone was observed. The dissimilar joint exhibited a lower Young′s modulus and higher cyclic strain hardening exponent than both Ti–6Al–4V and Ti17 base metals (BMs), and had the monotonic and cyclic yield strengths lying in-between those of two BMs with higher values for Ti17 alloy. Both BMs and joint showed essentially symmetrical hysteresis loops and equivalent fatigue life, and exhibited cyclic stabilization at lower strain amplitudes up to 0.6%, while cyclic softening occurred after initial cyclic stabilization at higher strain amplitudes. The initial cyclic stabilization was shortened with increasing strain amplitude. In the Ti–6Al–4V BM fatigued at a high strain amplitude of 1.2%, a short initial cyclic hardening emerged, corresponding to the presence of twinning and its resistance to the dislocation movement. Fatigue failure of the dissimilar joint occurred in the HAZ of Ti17 side where the soft zone was present, with crack initiation from the specimen surface or near-surface defect and crack propagation characterized by typical fatigue striations.     

Microstructures and properties of titanium–copper lap welded joints by cold metal transfer technology

Cold metal transfer (CMT) welding has been successfully used to weld dissimilar metals widely. However, a few investigations were carried out on the lap welding of commercially pure titanium TA2 to pure copper T2 with ERCuNiAl copper wire by CMT technique. In this paper, the affected mechanism of lapped location between the two metals on the microstructure and tensile shear strength of joints was revealed. The results indicated that satisfactory lapped joints between commercially pure titanium TA2 and pure copper T2 could be achieved by CMT welding method. A layer of intermetallic compounds (IMCs), i.e. Ti₂Cu, TiCu and AlCu₂Ti presented in titanium-weld interface, and the weld metal was composed of α-Cu solid solution and Ti–Cu–Al–Ni–Fe multi-phase. The two joints had almost same tensile shear strength, 192.5–197.5 N/mm, and fractured in the heat affected zone (HAZ) of Cu with plastic fracture mode during tensile shear tests.     

Effects of zinc coating on magnesium alloy–steel joints produced by cold metal transfer method

Magnesium alloys are lap-joined to galvanised and bare steel sheets by a cold metal transfer method. The weld appearance, cross-section, tensile strength and fracture behaviour of these joints are characterised by scanning electron microscopy, tensile tests and energy-dispersive spectroscopy. The joints were found to have good weld appearance and satisfactory tensile strength. The spreadability and wettability of the Mg alloy–galvanised steel joint are superior to those of the Mg alloy–bare steel joint, but the tensile strength is lower. In particular, the presence of Zn on the galvanised steel sheet improves wettability but decreases tensile strength. Aluminium has a high affinity for Fe, and the thinner layer of Fe–Al improves the mechanical properties of the Mg alloy–bare steel joint.     

Effect of temperature on high-cycle fatigue and very high cycle fatigue behaviours of a low-strength Cr–Ni–Mo–V steel welded joint

Axially push–pull fatigue tests of a low-strength Cr–Ni–Mo–V steel welded joint were conducted up to very high cycle fatigue regime at room temperature and 370 °C. The S–N curve at room temperature shows a duplex shape, while the S–N curve at 370 °C is continuously decreasing with lower fatigue strength. The welds at 370 °C undergoes dynamic strain ageing and has an enhanced load–defects interaction, leading to equal distribution of failures among different parts of the welds. The Z parameter model, with micro-defect location incorporated, having sound physical representation, is life-controlling of the welds at high temperature.     

Microstructure and mechanical properties of a new Al–Zn–Mg–Cu alloy joints welded by laser beam

A new type of Al–Zn–Mg–Cu alloy sheets with T6 temper were welded by laser beam welding (LBW). Microstructure characteristics and mechanical properties of the joints were evaluated. Results show that grains in the heat affected zone (HAZ) exhibit an elongated shape which is almost same as the base metal (BM). A non-dendritic equiaxed grain zone (EQZ) appears along the fusion line in the fusion zone (FZ), and grains here do not appear to nucleate epitaxially from the HAZ substrate. The FZ is mainly made up of dendritic equiaxed grains whose boundaries are decorated with continuous particles, identified as the T (AlZnMgCu) phase. Obvious softening occurs in FZ and HAZ, which mainly due to the changes of nanometric precipitates. The precipitates in BM are mainly η′, while plenty of GPI zones exist in FZ and HAZ adjacent to FZ, in the HAZ farther away from FZ, η phase appears. The minimum microhardness of the joint is always obtained in FZ at different times after welding. The ultimate tensile strength of the joint is 471.1 MPa which is 69.7% of that of the BM. Samples of the tensile tests always fracture at the FZ.     

Effects of package geometry on thermal fatigue life and microstructure of Sn–Ag–Cu solder joint

Abstract

Soldering experiments of chip scale package devices were carried out by means of diode laser soldering system with Sn–Ag–Cu solders. In addition, pull tests and a scanning electron microscope were used to analyse the effect of processing parameters on mechanical strength of solder joints. Viscoplastic finite element simulation was utilised to predict solder joint reliability for different package geometry under accelerated temperature cycling conditions. The results indicate that under the conditions of laser continuous scanning mode as well as the fixed soldering time, an optimal power and package geometry exists, while the optimal mechanical properties of microjoints are gained.     

Analysis of the S–N curves of welded joints enhanced by ultrasonic peening treatment

Contrast fatigue tests were carried out on T-shape tubular joints of 20 steel in three conditions: as welded, treated by ultrasonic peening treatment (UPT) before loading and UPT under loading. Results are: (1) Dispersity of test results measured by nominal stress is much larger than that measured by hot spot stress. After UPT before loading, fatigue strength of 20 steel tubular joints measured by hot spot stress increases by 67% and fatigue life is prolonged by 22–45 times. (2) Under low stress ratio R, UPT before loading can improve the fatigue performance of welded tubular joints significantly. (3) Under high stress ratio R, UPT under loading (static load) is recommended to improve the fatigue performance of welded tubular joints. UPT under loading not only enhances the fatigue properties at low stress level, but also at high stress level. (4) The general rule of S–N curves of welded joints treated by UPT is commonly effected by external load (static load) and self release of residual stress.     

Effect of rotation speed on microstructure and mechanical properties of Ti–6Al–4V friction stir welded joints

The effect of tool rotation speed on microstructure and mechanical properties of friction stir welded joints was investigated for Ti–6Al–4V titanium alloy. Joints were produced by employing rotation speeds ranging from 400 to 600 rpm at a constant welding speed of 75 mm/min. It was found that rotation speed had a significant impact on microstructure and mechanical properties of the joints. A bimodal microstructure or a full lamellar microstructure could be developed in the weld zone depending on the rotation speeds used, while the microstructure in the heat affected zone was almost not influenced by rotation speed. The hardness in the weld zone was lower than that in the base material, and decreased with increasing rotation speed. Results of transverse tensile test indicated that all the joints exhibited lower tensile strength than the base material and the tensile strength of the joints decreased with increasing rotation speed.     

Effects of tool wear on friction stir welded joints of Ti–6Al–4V alloy

Tool wear behaviour on microstructure and mechanical properties of friction stir welded zones of Ti–6Al–4V alloy was evaluated. SEM examination, EDS analysis and X-ray diffraction results indicated that severe wear of the tool is indicated by the presence of WC-Co particles in the stir zone at rotational speed of 630?rev?min^?1 and travel speed of 8?mm?min^?1. Micro-hardness, tensile tests and fractographical examinations also reflected that these particles make the material more brittle and reduce the mechanical strength by 40%. However at travel speed of 22?mm?min^?1, tool wear is less, hardness distribution is more uniform and enhanced ductility and strength is achieved.     

Influence of specimen type and reinforcement on measured tension–tension fatigue life of unidirectional GFRP laminates

It is well known that standardised tension–tension fatigue test specimens of unidirectional (UD) glass-fibre-reinforced plastics (GFRP) laminates tend to fail at end tabs. The true fatigue life is then underestimated. The first objective of this study was to find for UD GFRP laminates a test specimen that fails in the gauge section. The second objective was to compare fatigue performance of two laminates, one having a newly developed UD powder-bound fabric as a reinforcement and the other having a quasi-UD stitched non-crimp fabric as a reinforcement. In the first phase, a rectangular specimen in accordance with the ISO 527-5 standard and two slightly different dog-bone shaped specimens were evaluated by means of finite element modelling. Subsequent comparative fatigue tests were performed for the laminates with the three specimen types. The results showed that the test specimen type has a significant effect on the failure mode and measured fatigue life of the laminates. A significantly higher fatigue life was measured for the laminate with the powder-bound fabric reinforcement when compared to the laminate with the stitched reinforcement.     

The effects of electron beam irradiation on the thermal properties,fatigue life and natural weathering of styrene butadiene rubber/recycled acrylonitrile–butadiene rubber blends

The effects of electron beam (EB) irradiation on the thermal properties, fatigue life and natural weathering of styrene butadiene rubber/recycled acrylonitrile–butadiene rubber (SBR/NBRr) blends were investigated. The SBR/NBRr blends were prepared at 95/5, 85/15, 75/25, 65/35, or 50/50 blend ratios with and without the presence of a 3 part per hundred rubber (phr) of polyfunctional monomer, trimethylolpropane triacrylate (TMPTA). Results indicate that the crystallisation temperature (T_c) observed in polymeric blends is due to the alignment of polymer chains forming a semi-crystalline phase. Addition of TMPTA helps to align polymer chains through crosslinking. More crosslinking occurred between polymer blends with the help of TMPTA, upon irradiation. The improvement in fatigue life can also be associated with the stabilisation of SBR/NBRr blends upon irradiation and irradiation-induced crosslinking, which was accomplished with relatively low radiation-induced oxidative degradation in the presence of TMPTA. The tensile properties of both blends decreased over the periods of environmental exposure due to the effect of polymer degradation. After 6 months, the irradiated SBR/NBRr blends could not retain better retention [mainly with 25, 35 or 50 phr of recycled acrylonitrile–butadiene rubber (NBRr) particles] due to the samples becoming brittle over the long period of outdoor exposure.     

The new IIW recommendations for fatigue assessment of welded joints and components – A comprehensive code recently updated

The recommendations of the International Institute of Welding (IIW) on fatigue of welded components and structures and on the effect of weld imperfections in respect to fatigue have been published firstly in 1996. It was published in English, German, Japanese and French. A comprehensive code was established, which covered all current methods of verification, as e.g. component testing, nominal stress, structural stress, notch stress method as well as fracture mechanics assessment procedures. Detailed guidance for assessment of weld imperfections is also given. The safety philosophy covers the different strategies, which are used in various fields of application and gives a specified choice for the designer. The update of the recommendations was finalized in 2006. The main areas of update are the structural hot-spot stress concept, which allows now for an economic and coarser meshing in finite element analysis, the extension of the effective notch stress concept to welded aluminium structures and the numerical assessment of post weld treatments for improving the fatigue properties. It is expected that the new update will exert the same impact on design and codes as the old one.     

Effect of abnormal grain growth on tensile strength of Al–Cu–Mg alloy friction stir welded joints

An Al-4.5%Cu-1.5%Mg aluminum alloy with a T4 temper was friction stir welded, and the effect of the abnormal grain growth on the tensile strength of joints was investigated. Abnormal grain growth usually happens during post weld heat treatment. It is found that the tensile strength and elongation of the heat-treated joint will increase significantly if this phenomenon completely happens in stir zone. On the other hand stable grains in the stir zone have no effect on the mechanical properties of heat-treated joint.     

The effect of cutting speed and heat treatment on the fatigue life of Grade 5 and Grade 23 Ti–6Al–4V alloys

High speed machining is a necessary manufacturing method for ensuring productivity and profitability. However, research has demonstrated that the high speed machining process impairs the surface characteristics of materials such as Ti–6Al–4V including surface roughness and subsurface microstructural damage. Therefore, there is concern that high speed machining detrimentally influences the fatigue properties of Ti–6Al–4V components. This paper investigates the effect of cutting speed on the surface integrity and fatigue properties of Ti–6Al–4V (ASTM Grade 5) and Ti–6Al–4V ELI (ASTM Grade 23) alloys in the beta annealed and mill annealed heat treated conditions. It was found that the surface roughness and fatigue properties are not significantly influenced by cutting speed, however, the microstructure substantially influences the properties.     

Effect of aluminium on the microstructure and mechanical properties of as-cast magnesium–manganese alloys

In this paper, the effect of aluminium on microstructure and mechanical properties of as-cast magnesium–manganese alloy has been investigated by means of X-ray diffraction, optical microscopy and scanning electron microscopy. The results reveal that various Al–Mn intermetallic compounds form with an increase of Al content. As a result, microstructure of AM11 alloy has been effectively refined due to the formation of Al₈Mn₅ phase along the grain boundary, while Al addition is explained as the main reason on refining the microstructure of AM91 alloy due to its higher grain growth restriction factor value of ～4.32. The tensile yield strength (TYS) has been improved steadily from 27.4 to 122.9?MPa with increasing Al content, because of the combined effects of grain boundary strengthening, solid solution strengthening and precipitation hardening behaviours.     

Effect of the rolling speed on microstructural and mechanical properties of aluminum–magnesium alloys prepared by twin roll casting

In the present investigation the microstructures and mechanical properties of Al–Mg alloys prepared by twin roll casting (TRC), varying rolling speed, (i.e., 3, 4, and 5 rpm) were studied. Optical microstructures of the all the samples show dendritic morphology. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometer (EDS) studies show segregation of alloying elements at the interdendritic regions. Transmission electron microscopy (TEM) observation confirms the presence of intermetallic phase (Mg₅Al₈) at the grain boundaries. It was observed that the secondary dendritic arm spacing (SDAS) increases (marginally) with rolling speed. Also the microhardness and tensile strength decreases with increase in rolling speed.     

Investigation the effect of tightening torque on the fatigue strength of double lap simple bolted and hybrid (bolted–bonded) joints using volumetric method

In this research, the effect of the tightening torque on the fatigue strength of 2024-T3 double lap simple bolted and hybrid (bolted–bonded) joints have been investigated experimentally by conducting fatigue tests and numerically by implementing finite element analysis. To do so, three sets of specimens were prepared and each of them subjected to tightening torque of 1, 2.5 and 5 Nm and then fatigue tests were carried out under different cyclic longitudinal load levels. In the numerical method, the effect of the tightening torque on the fatigue strength of the considered joints has been studied by means of volumetric method. To obtain stress distribution around the notch (bolt hole) which is required for the volumetric method, nonlinear finite element simulations were carried out. In order to estimate the fatigue life, the available smooth S–N curve of Al2024-T3 and the fatigue notch factors obtained from the volumetric method were used. The estimated fatigue life was compared with the available experimental test results. The investigation shows that there is a good agreement between the life predicted by the volumetric method and the experimental results for different specimens with a various amount of tightening torques. The results obtained from the experimental analysis showed that the hybrid joints have a better fatigue strength compared to the simple bolted joints. In addition, the volumetric method and experimental results revealed that the fatigue life of both kinds of the joints were improved by increasing the clamping force resulting from the torque tightening due to compressive stresses which appeared around the bolt hole.     

Study on the mechanical performance and application of the composite cement–asphalt mixture

The rigidity of matrix mixture asphalt (MMA) is increased by the combination with cement mortar, and the performance of asphalt pavement has been better changed in the stabilities of high temperature rut and low temperature cracking and water damage. The research problems in this project mainly focus on the combinations among cement mortar and MMA and mechanical performance of composite cement–asphalt mixture (CCAM), etc. In the project, the CCAM is produced by pouring cement mortar into MMA, and the mechanical performance of CCAM has been researched. The research contents include the production technology and mechanical performance and construction technology of CCAM. Through the test data of CCAM in the mechanical performance, and compared with the performance of AC-16 MMA, the paper has analysed the mechanical performance and application of CCAM in high temperature rut and low temperature cracking and water damage, the components design and production technology and construction technology of CCAM have been put forward. The rigidity of MMA has been improved and the mechanical performance of CCAM has also met the design standards, it is suggested that CCAM can be applied as a new composite pavement materials.     

Effect of precipitate free zones on low cycle fatigue life of Al–Zn–Mg alloy

Abstract

The effect of precipitate free zones (PFZs) on the low cycle fatigue (LCF) life of an Al–4·70 wt-%Zn-2·80 wt-%Mg alloy at room temprature was investigated. The alloy used in the present study was uniquely prepared to have different widths of PFZ, whlle keeping the distribution and size of predominant precipitates in the matrix or on grain boundaries almost constant. As the width of the PFZ increased from 0·073 to 0·3 μm,the LCF life was observed to increase to some extent. However, the tensile properties such as yield strength, tensile strength, and elongation remained approximately unchanged. Studies using transmission electron mlcroscopy show that the extent of planar slip decreases with increasing PFZ width throughout cyclic deformation. Thls result suggests that the increase in LCF life is probably due to relaxation of the local stress in the soft PFZ regions.

MST/2093