Microstructure and tensile properties of laser beam welded Ti–22Al–27Nb alloys

Ti–22Al–27Nb alloys were welded using the laser beam welding process. The microstructure characterization and the tensile properties of the laser beam welded joints were investigated. The experimental results showed that a well-quality joint could be obtained using laser beam welding method. The fusion zone of the welded joint was composed of B2 phase. The tensile strength of the joints at room temperature was basically comparable to that of the base metal and the tensile ductility of the joints achieved 56% of the base metal. The average tensile strength of the welded joints at 650 °C was tested to be about 733 MPa, with the elongation of 2.93%.     

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.     

Microstructure and mechanical properties of friction stir welded 7136–T76 aluminium alloy

Abstract

The microstructure of the weld was examined by light and electron microscopy (scanning and transmission). The various regions, i.e. thermomechanically affected zone, heat affected zone and unaffected base material, were studied in detail to better understand the microstructural evolution during friction stir welding and its impact on basic mechanical properties. The change in morphology of the strengthening phases reflected the relative temperature profile and the amount of deformation across the welded joint during the stir welding process. The centre of the weld was composed of fine grains and coarse particles identified mainly as MgZn₂. In the thermomechanically and heat affected zones, the grain size was not uniform, and the strengthening phases filled the grain interiors, while grain boundaries were surrounded by precipitation free zones. The size of the strengthening phase decreased towards the base material. The hardness profile of the friction stir weld displayed the lowest hardness on the retreating side. Tensile properties of the weld itself were superior to those for material containing weld.     

Nucleus geometry and mechanical properties of resistance spot welded coated–uncoated DP automotive steels

In this study, mechanical properties of resistance spot welding of DP450 and DP600, galvanized and ungalvanized automotive sheets have been investigated. The specimens have been joined by resistance spot welding at different weld currents and times. Welded specimens have been examined for their mechanical, macrostructure and microstructure properties. Depending on the weld current and time, effects of zinc coating on tensile properties, microhardness values as well as microstructure nugget geometry and nucleus size ratio have been investigated. X-ray diffraction analysis has been used to investigate the phase that formed at the joint interface. Result of the experiment show that nugget diameter, indentation depth and tensile load-bearing capacity are affected by weld parameters. Coating prevents full joining at low parameters. Microhardness increased in heat-affected zone and weld metal.     

Microstructure and mechanical properties of laser welded joints between 2198/2060 Al–Li alloys

Dissimilar Al–Li alloys (2198 and 2060) were laser welded without the addition of filler material. The effects of welding parameters on the formation of the welded joints, microstructure evolution, solute segregation, porosity and their relationships with the mechanical properties of the joint were investigated. It was found that reducing the weld heat input can effectively prevent grain coarsening, while decreasing porosity and reducing the tendency for hot cracking, thereby enhancing the properties of the joint. The dissolution of precipitates such as the T1 phase and θ′ phase in the base metal, and the variations in Cu and Mg content between grain interiors and boundaries, resulted from solute segregation during welding, leading to reductions in the strength of the weld.     

Microstructural characteristics and mechanical properties of friction stir welded joints of Ti–6Al–4V titanium alloy

The α + β titanium alloy, Ti–6Al–4V, was friction stir welded at a constant tool rotation speed of 400 rpm. Defect-free welds were successfully obtained with welding speeds ranging from 25 to 100 mm/min. The base material was mill annealed with an initial microstructure composed of elongated primary α and transformed β. A bimodal microstructure was developed in the stir zone during friction stir welding, while microstructure in the heat affected zone was almost not changed compared with that in the base material. An increase in welding speed increased the size of primary α in the stir zone. The weld exhibited lower hardness than the base material and the lowest hardness was found in the stir zone. Results of transverse tensile test indicated that all the joints had lower strength and elongation than the base material, and all the joints were fractured in the stir zone.     

Tensile properties of friction stir welded and friction stir welded-superplastically formed Ti–6Al–4V butt joints

A University and Industry collaborative research project was undertaken to evaluate the performance of as friction stir welded (FSW) and friction stir welded-superplastically formed Ti–6Al–4V alloy sheets. The purpose of this particular effort was to evaluate the tensile properties of friction stir welded and superplastically formed friction stir welded Ti–6Al–4V. Welds were produced out of both standard grain and fine grained titanium and tested in the as welded, stress relieved (SR) and superplastically formed (SPF) conditions. The preliminary results of the FSW and post FSW–SPF joint were found to be close to that of as received titanium with respect to strength, but elongations were decreased.     

Influence of welding parameters on microstructures and mechanical properties of electron beam welded aluminium–lithium plates

Abstract

The present work focused on the welding characteristics of electron beam welding (EBW) in 8090 Al–Li plates, evaluated in terms of strength or toughness degradation in post-weld impact and bending tests with loading rates of 10³, 10^-1, and 10^-4 s^-1. The influence of welding parameters, such as welding power, welding speed, and electron beam focus position, on the post-weld microstructures, porosity, and mechanical properties were examined. Although the joint efficiency for the maximum flexure strength (F_p ), or tensile strength, can be as high as 85–90%, the joint efficiency for the fracture absorption energy (E_t ) was only 20–40%, a level usually unsuitable for applications. It was found that changing the welding power and speed by a factor of 3 resulted in significant variation in E_t but only minor variation in F_p . Changing the welding focus position had little effect on post- weld mechanical properties. The abundant grain boundary precipitates in the welded specimens were thought to be the main cause of the degraded post-weld properties. Other microstructural factors included the δ′ precipitate and grain sizes. The volume fraction of porosity did not play any decisive role owing to the small size (<0·3 mm), low quantity (1–2%), and spherical shape of the EBW induced pores. Finally, given the same post-weld microstructures, the toughness degradation of the EBW specimens was worst under high rate impact loading.     

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.     

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.     

Fatigue crack growth behaviour of gas tungsten arc,electron beam and laser beam welded Ti–6Al–4V alloy

The present investigation is aimed to evaluate fatigue crack growth parameters of gas tungsten arc, electron beam and laser beam welded Ti–6Al–4V titanium alloy for assessing the remaining service lives of existing structure by fracture mechanics approach. Center cracked tensile specimens were tested using a 100 kN servo hydraulic controlled fatigue testing machine under constant amplitude uniaxial tensile load. Crack growth curves were plotted and crack growth parameters (exponent and intercept) were evaluated. Fatigue crack growth behavior of welds was correlated with mechanical properties and microstructural characteristics of welds. Of the three joints, the joint fabricated by laser beam welding exhibited higher fatigue crack growth resistance due to the presence of fine lamellar microstructure in the weld metal.     

Comparative study on microstructure and mechanical properties of laser welded–brazed Mg/mild steel and Mg/stainless steel joints

A laser welding–brazing (LWB) technology using Mg based filler has been developed for joining Mg alloy to mild steel and Mg alloy to stainless steel in a lap configuration. Microstructure and mechanical properties of laser welded–brazed lap joints in both cases were comparatively studied. The results indicated that no distinct reaction layer was observed at the interface of Mg/mild steel and subsequently the interface was confirmed as mechanical bonding, whereas an ultra thin reaction layer with a continuous and uniform morphology was evidenced at the Mg/stainless steel interface, which was indicative of metallurgical bonding. The newly formed interfacial layer was indexed as FeAl phase by transmission electron microscopy (TEM) combined with energy dispersive spectroscopy (EDS). The average tensile–shear strength of Mg/mild steel joint was only 142 N/mm with typical interfacial failure, while that of Mg/stainless steel joint could reach 270 N/mm, representing 82.4% joint efficiency relative to the Mg alloy base metal. The fracture location of Mg/stainless steel joint was at Mg fusion welding side, suggesting the interface was not weak point due to the formation of ultra thin interfacial layer. The role of alloying elements in base metal and bonding mechanism of the interfacial layer were discussed, respectively.     

Microstructure of copper–AISI type 304L electron beam welded alloy

Abstract

Heterogeneous butt welding of copper and AISI type 304L stainless steel was carried out using the electron beam process. Examination by scanning and transmission electron microscopy has indicated the possibility of obtaining joints free of cracks and porosity. Energy dispersive microanalysis of the weld bead cross-section has demonstrated the presence of non-equilibrium phases. The results show that the binary Cu-Fe equilibrium diagram is unable to predict the weld microstructure even at the moderate cooling and solidification rates expected under the present welding conditions. The feasibility of the Cu-304L electron beam welding process is therefore hindered by the problem of microstructural stability of the joint because of possible phase transitions during the service life of welded components.     

Defect features and mechanical properties of friction stir lap welded dissimilar AA2024–AA7075 aluminum alloy sheets

5 mm-Thick dissimilar AA2024-T3 and AA7075-T6 aluminum alloy sheets were friction stir lap welded in two joint combinations, i.e., (top) 2024/7075 (bottom) and 7075/2024. The influences of process conditions (welding speed and joint combination) on defects (hook and voids) features and mechanical properties of joints were investigated in detail. It was found that the hook deflects largely upwards into the stir zone (SZ) at lower welding speeds (50, 150 mm/min) in both combinations. The process conditions significantly affect the hook geometry which in return affects the lap shear strength. In all 2024/7075 joints, voids appear and the joints fracture from the tip of hook on AS along the SZ/TMAZ (thermomechanically affected zone) interface in lap shear test (tensile fracture mode). In 7075/2024 joints, the hook on RS horizontally extends a large distance into the bottom stir zone at higher welding speeds (225, 300 mm/min). The joints fracture in three modes: shear fracture along the lap interfaces, tensile fracture and the mix fracture of both. In both joint combinations, the lap shear strength generally increases with the increase of welding speed. 7075/2024 Joints show higher failure load than 2024/7075 joints at lower welding speeds while the opposite result appears at higher welding speeds.     

Consideration of mean-stress effects on fatigue life of welded magnesium joints by the application of the Smith–Watson–Topper and reference radius concepts

Three types of welded joints have been assessed with regard to their fatigue strength based on the mean-stress damage parameter model according to Smith, Watson, and Topper (P_SWT) and on the reference notch radius concept. These analyses were performed with three different stress ratios, R = −1, R = 0 and R = 0.5, under axial loading. For each stress level, the corresponding Neuber-Hyperbolas, Masing-loops and their maximum stress and maximum strain values were determined in order to calculate damage parameter (P_SWT) values. For a given weld geometry, this damage parameter is able to unify the fatigue results for different R-values within at a tight scatter band and therefore to consider the mean-stress effect. The unification of the results for different weld geometries is performed by applying the reference radii r_ref = 0.05 and r_ref = 1.00 mm as suggested by the IIW-Recommendations.     

Analytical investigation and numerical simulation of the stress–strain state in mechanically heterogeneous welded joints with a single-V butt weld

This paper aims to investigate the stress–strain state in mechanically heterogeneous welded joints with a single-V butt weld by an analytical model along with a numerical simulation. Analytical expressions for the stress–strain state in both the weld and the main material are proposed. In order to verify the proposed expressions, a numerical simulation of the stress–strain state in a mild welded joint with a single-V butt weld was carried out on the basis of the finite element method and the results were compared with the analytical solution obtained applying the proposed analytical model. EP-787 and JONI 13/45А steels were used for the weld while 15X2MFА steel was used for the main material in the analysed welded joints. A comparison of the analytical solution with the finite element analysis results showed a good agreement. The proposed equations could be used in design practice for calculations of the stress–strain state in welded joints.     

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.