Alloy Joints Research Articles

Investigation of the microstructure and mechanical properties of Mg–Al–Zn alloy joints formed by different laser welding processes

Abstract The microstructure and mechanical properties of Mg–Al–Zn alloy joints by using autogenous laser beam welding (LBW) and laser-MIG hybrid welding, respectively, are investigated. The results show that the weld formation of the hybrid welding joint is relatively good, and there are mainly α-Mg matrix phases and β-Mg17Al12 strengthening phases in the weld metal. The microstructure in the fusion zone (FZ) of the two joints is different. The LBW joint is composed of columnar crystal and equiaxed dendrite. The hybrid welding joint consists of fine equiaxed grains, and the grain size in the laser zone is larger than that in the arc zone. The microhardness in FZ is higher due to the precipitation of the β-Mg17Al12 phase in this region. Under the optimized welding procedure, the strength coefficient of the two joints is >90%. There are many dimples on the tensile fracture surface of the hybrid welding joint, which is characterized by the pattern of the ductile fracture.

Microstructure and mechanical properties of tungsten inert gas welded dissimilar aluminum alloys joint subjected to normal multi-pass friction stir process

This work investigated the effect of multi-pass friction stir processing on the tungsten inert gas welded joints of AA6082/AA8011. The friction stir processing approach is broadly used in the marine, automotive, and aerospace engineering sectors. The flow of plasticized material seems to be significant in the mixed flow region, which causes an erratic intercalation pattern. By using a stirring tool action, deformation was induced in situ extrusions of dissimilar alloys. The plasticized material was moved layer by layer to generate a lamellae structure. Theparent materials AA6082 and AA8011 were seen to be penetrating and vigorously mixing in this zone. After the fourth pass friction stir processing, the tungsten inert gas weldment's ultimate tensile strength and % strain dramatically increased. The defects that had been present before friction stir processing were eliminated when the coarse grain structure of the tungsten inert gas weldments was changed into a refined grain structure from 18.09 to 8.92 µm. The microstructure study shows the linear relationship between friction stir processing passes and grain size. The increases in friction stir processing pass resulted in a decrease in grain size. The maximum ultimate tensile strength (89.60) was observed in tungsten inert gas + 4 pass friction stir processing compared to the other joints.

Forming process prediction of a self-piercing riveted joint in carbon fibre reinforced composites and aluminium alloy based on deep learning

As a mechanical internal locking structure, the forming process of a self-piercing riveted (SPR) joint is currently mainly investigated through riveting interruption experiments and finite element simulation. To solve the problems of complex simulation modelling processes, high experimental costs, and low efficiency, an SPR process prediction method based on deep learning was proposed to predict the deformation process and damage of SPR joints in carbon fibre-reinforced composites and aluminium alloys. The original images of the model dataset were obtained based on the simulation results, and image segmentation technology was used to classify the cross-section and damage morphology of the joint. A deep learning model based on a convolutional neural network and conditional generation antagonism model architecture was established, and the section shape and damage morphology of the joint were predicted by inputting the percentage of punch displacement. The k-fold cross validation method was used for model training, and the untrained data were used as the test set to verify the predictive ability of the model by comparing the forming section parameters of the SPR joint. The results show that the deep learning model used can accurately predict the deformation state and damage evolution of riveted materials at different joining stages. The average prediction accuracies of the height of the riveted head, residual thickness, and rivet spread are 95.80 %, 95.68 %, and 92.40 %, respectively.

A hybrid shoulder to achieve a significant improvement in tensile strength and fatigue performance of friction stir welded joints for Al–Mg–Si alloy

In this paper, a hybrid shoulder friction stir welding (HS-FSW) process was developed combining the advantages of rotational shoulder friction stir welding (RS-FSW) and non-rotational shoulder friction stir welding (NRS-FSW). Both static and fatigue performance of the HS-FSW Al–Mg–Si alloy joints were examined and compared with those of the RS-FSW and NRS-FSW joints. The ultimate tensile strengths of the joints welded by HS-FSW are increased by 17.3 % and 6.7 %, compared with those of RS and NRS FSW, respectively. Besides, fatigue strengths of HS-FSW joints grow by respective 25.4 % and 1.8 %. For a better understanding of the significant improvements, the fatigue crack propagation (FCP) test and residual stress measurement were carried out. FCP test indicates that the advancing side (AS) is the most sensitive zone for FCP. The residual stress measurements suggest that the HS-FSW joint has the lowest distribution amplitude of residual stress compared with that of the RS-FSW and NRS-FSW joints. Based on above evolution, an assessment is firstly conducted, which reveals that the axial force and the lower heat input to mitigate the amplitude and the peak value of tensile stress during welding, jointly contribute to the superior imprisonment of the tensile strength and fatigue strength of the HS-FSW joints.

Correlation between Microstructure and Bending of FSW and TIG Welded Mg-Rich Aluminium Alloy Joints

This paper reports on the mechanical properties in relation to the microstructure of the Mg-rich aluminium alloy joints fabricated through friction stir welding (FSW) and tungsten inert gas (TIG) welding techniques. The microstructure, tensile, and bending tests were conducted on friction stir, and TIG welded joints. Most coarse grains (27.81mm) dominated the joint produced using the TIG welding technique, while the refined grains (11.26 mm) mostly dominated the joint made through the FSW technique. The ultimate tensile strength (UTS) of the joint fabricated using the FSW technique was higher (379 MPa) compared to the TIG-welded joint (260 MPa). However, the inverse behaviour was observed when looking at the tensile elongation of the very same joints. The bending results correlated with UTS results, and this phenomenon was attributed to the microstructural grain size.

Influence of post weld aging treatment on microstructural features and fatigue behaviour of unnotched and notched specimens of friction stir welded AA6061-T651 aluminium alloy joints under completely reversed stress cycle

A comparative investigation on rotating bending fatigue behaviour between unnotched and notched specimens of friction stir-welded (FSW) AA6061-T651aluminium alloy joints has been conducted under completely reversed cyclic loading (R = −1). Post-weld aging treatment was done for 8 h at 200°C on the welded joints. After post-weld aging treatment, the fatigue strength was significantly improved, while the grain size remained unchanged. Microstructural evolution before and after post-weld aging treatment was studied using orientation imaging microscopy (OIM) and transmission electron microscopy (TEM). It was found that post-weld aging treatment decreased dislocation density and facilitated precipitation hardening through the reprecipitation of fine β″(Mg2Si) particles. Fine particles of post-weld aged specimens of FSW joints were sheared by dislocations generated during cyclic loading, resulting in difficult crack initiation and tortuous crack propagation. Fractographic results confirmed that Post-weld aged specimens of FSW joints have better fatigue resistance than as-welded specimens, owing to the presence of significant precipitates along the grain boundary.

Effect of Tool Travel Rate on Microstructure Evolution and Mechanical Properties of Dissimilar Friction Stir Welded Joints of AA7075 and AA6061 Aluminium Alloys

Effect of Tool Travel Rate on Microstructure Evolution and Mechanical Properties of Dissimilar Friction Stir Welded Joints of AA7075 and AA6061 Aluminium Alloys

Influence of friction stir welding process parameters and statistical behaviour of the novel interlock aluminum alloys joint with SiCp reinforcement

Influence of friction stir welding process parameters and statistical behaviour of the novel interlock aluminum alloys joint with SiCp reinforcement

Crater array generated on Al alloy surface by nanosecond laser: Surface characteristics and bonding strength

The strength of aluminium (Al) alloy adhesive joints is critical for aerospace and automotive applications. This work uses an infrared nanosecond laser to create a crater array (CA) and thereby modify the substrate bonding area and enhance the strength of 7075-T6 Al alloy joints bonded by epoxy resin adhesives. We study how the parametrisation (crater overlap ratio β and pulse energy density Ф) of the CA affects the surface texture features (surface roughness Sa and fractional increase Sdr in surface area), wettability, chemical properties, and shear strength τ. The mechanism by which CA affects τ is revealed by the surface characteristics and failure modes. Sandblasted specimens were used as a reference. The results show that, for laser treatment, Sdr, Sa, and τ are closely related to β but not to Ф. When β = 30 % or 60 %, the CA surface is superhydrophilic. Regardless of β and Ф, the substrate absorbs carbon-containing pollutants from the air, so that the contact angle of the CA surface gradually increases over time until, 15 days after the laser treatment, the CA surface becomes hydrophobic. Zn, Cu, and Fe are uniformly distributed over the alloy surface, whereas C, O, Mg, and Al are unevenly distributed. The C content decreases with increasing β but is not correlated with Φ. The O content increases with the increasing β and Φ. When β = 30 %, the area of cohesive failure in the substrate bonding area is maximal. When β ≤ 0, the untreated area outside the craters is susceptible to adhesive failure. When β ≥ 60 %, the substrate bonding area is covered by a thin layer formed by molten residue, which is susceptible to substrate failure. Laser treatment is more effective than sandblasting in modifying the texture, wettability, and chemical properties of the bonding area, which can further improve the bonding strength. The maximum strength of the bonded joint obtained by laser treatment is 26.89 MPa at Φ = 21.6 J/cm2 and β = 30 %, which is 16.2 % stronger than sandblasting.

Carbon–Carbon Composite Metallic Alloy Joints and Corresponding Nanoscale Interfaces, a Short Review: Challenges, Strategies, and Prospects

Brazing of carbon–carbon (C/C) composites with metallic materials currently faces a series of difficulties, such as the poor wettability of metallic materials on the surface, the nanoscale interface bonding of C/C composites and metallic materials, thermal stress problems for these different materials, etc. Especially, the practical problems, including the low joint strength and insufficient reliability, still limit the large-scale practical application of brazing technology for C/C composites and metal materials. Herein, in order to guide the fabrication of high-quality joints, we present a brief discussion on the latest research progress in the joints of C/C composites and metallic materials, including current challenges, solution methods, mechanisms, and future prospects. More importantly, the nanoscale interface in the carbon–carbon composites and metallic alloys is paid very little attention, which has been critically discussed for the first time. Then, we further outline the possible solutions in joint problems of C/C composites and metallic materials, proposing feasible strategies to control the reaction in the brazing process, such as surface treatments, the addition of reinforcing phases, a transition layer sandwiched between the base material and the intermediate layer, etc. These strategies are being envisioned for the first time and further contribute to promoting the converged applications of C/C composites and metallic materials.

Multi-response Optimization of Friction Stir Welded Reinforced Joints of Dissimilar Aluminum Alloys

Multi-response Optimization of Friction Stir Welded Reinforced Joints of Dissimilar Aluminum Alloys

Interface microstructure and mechanical response of similar and dissimilar joints of (γ + α2) titanium aluminide and α-titanium alloy produced by diffusion bonding

Titanium aluminides are one of the potential materials for aerospace applications because of their high specific strength and good resistance to creep and oxidation at elevated temperatures. However, it is quite challenging to obtain similar and dissimilar joints with these materials by fusion welding due to their high crack sensitivity, limited ductility, and low thermal conductivity. In this study, joints of (γ + α2)-titanium aluminide (TiAl) to itself and with α-Ti alloy were produced by solid-state diffusion bonding performed under vacuum in the temperature range of 750–950 °C and pressure of 10–20 MPa for 10–30 min. The resultant interface microchemistry and microstructures were examined in-detail by scanning electron microscope in conjunction with energy dispersive spectroscopy and electron backscattered diffraction. Instrumented indentation and shear testing were employed to examine hardness variation across the interface and to evaluate the bond strength of the joints, respectively. The TiAl/TiAl joint possessed a single thin interface layer of α2 (Ti3Al), whereas two interphase layers comprised, viz. α + α2 (layer 1) and α2 (layer 2) were formed at interface of the α-Ti/TiAl joint. The minimum bonding temperatures of 900 °C and 800 °C were identified for producing metallurgically sound joints of TiAl/TiAl and α-Ti/TiAl, respectively, under an applied pressure of 10 MPa for 30 min. The bond strength of ∼400 MPa, which is ∼90% shear strength of TiAl base alloy, was achieved for TiAl/TiAl joints produced at 900 °C and above. The α-Ti/TiAl joint produced at 950 °C showed a maximum bond strength of ∼504 MPa. The intimate contact of surfaces through the complete collapse of their asperities and formation of phases, similar to that present in TiAl base alloy, at the interface led to enhanced joint strengths. The fracture surface of the shear tested specimens displayed a transition in failure mode from brittle to transgranular failure of the interface along with TiAl base alloy with an increase in bonding temperature.

Effect of notch location on fatigue crack growth behavior for inhomogeneous material domains in friction stir welded magnesium alloy

The fatigue crack growth behavior of friction stir welded joints of AZ31 Mg alloy is investigated for three different notch locations. The extended finite element method based simulation is developed to capture the effect of stir zone on fatigue crack growth behavior. The notch locations with asymmetric material domain experience an asymmetric stress field, which leads to crack path deviation from the orthogonal direction to the applied force. This leads to a longer crack path. The near-tip stress field from the simulation enhances the understanding of unusual crack path. The experimental results including crack path trajectories from experiments compare well against the numerical predictions.

Studying the STA effects on aluminium alloy joints by friction welding

This article examines the properties of joints fabricated by friction welding (FW) using AA6061-T6 and AA7075 as examples of both similar and dissimilar metals. It also looks at how heat treatment after welding affects the metallurgical characteristics of the joint. The heat treatment method under investigation combines solutionizing heat treatment with synthetic ageing (STA). The tensile characteristics, microhardness, and microstructure of the as-welded and STA-treated welded specimens are assessed in the paper using mechanical testing equipment and metallurgical characterization methods. The findings demonstrate that the joint characteristics are enhanced by precipitate nucleation occurring in the joint interface of the FW similar and dissimilar joint under STA treatment. On the other hand, the STA influence on the tensile strength (TS) of joints made with AA7075 was greater than that of similar joints made with AA6061-T6, as well as dissimilar joints made with both AA6061-T6 and AA7075. Additionally, after STA treatment, the similar joints showed a uniform distribution of hardness. However, in the dissimilar joint after STA treatment, the hardness dropped suddenly on the AA6061-T6 side, specifically from the partially deformed zone (PDZ) towards the heat affected zone (HAZ).

Investigations into the Microstructure and Texture Evolution of Inertia-Friction-Welded Dissimilar Titanium Alloys

The welded joint of a dissimilar titanium alloy was obtained via inertial friction welding technology. The characteristics of the bonding interface and the microstructure of the welded joint were investigated via optical microscopy, scanning electron microscopy and electron backscattered diffraction. The results show that fine, equiaxed grains and interdiffusion bands of the elements Mo and Sn were formed in the weld zone under the high temperature and plastic deformation of the inertial friction welding. The weld zone and thermo-mechanically affected zone formed ⟨1¯21¯0⟩ α texture and ⟨111⟩ β texture, respectively.

Component test of nickel-based alloys and their welded joints under long-term creep loading

The aim of the paper is to investigate experimentally the creep behavior of improved materials and welded joints at component scale under near-to-service operating conditions, which are promising for the application in highly efficient and flexible future power plants.It focusses on the creep behavior under multiaxial loading of similar and dissimilar welded joints of high temperature resistant nickel-based alloys (A740H, A617B and HR6W) in the temperature range between 650 °C and 750 °C by means of thick-walled-component tests. In this case, the temperature, the external axial load and the internal pressure remain constant during testing.Numerical simulations are used for the estimation of the component axial load in order to induce a meaningful damage evolution without causing a total component failure. The metallographic investigation on the welds A740H/A740H and A617B/A740H indicated no test-related damage. Small cracks in circumferential direction were found on the welds HR6W/HR6W and HR6W/A617B after having reaching 2% local creep strain. The damage mechanism could be classified as a typical creep damage with carbides formation in the grain boundaries and thus grain boundary separations occur, which ultimately lead to the crack formation.In general, the FEM calculation underestimates the axial creep strain on the investigated welds, especially the initial creep strain evolution is not accurately predicted. However, the long-term creep strain evolution could be well predicted by FEM. In this case, the material parameters for the creep law should be further optimized in order to minimize the underestimation.

Strength calculation and microstructure characterization of HAZ softening area in 6082-T6 aluminum alloy CMT welded joints

Degradation of mechanical properties in the heat-affected zone (HAZ) is a common issue when welding 6082-T6 aluminum alloy. Even for cold metal transfer (CMT) welded joints with little heat input, it is always challenging to avoid this problem. In this study, we examined the effects of various heat inputs on the softening of CMT welded 6082-T6 aluminum alloy joints as well as the mechanism underlying the softening of HAZ. The hardness of the welded joint at the HAZ softening area directly decreases as the heat input increases. The softening of HAZ is the most severe under the impact of thermal cycling, with a softening zone width of up to 5.5 mm and a minimum hardness of about 61 HV, reaching 58.65% of the base material (BM). According to the strength model established in the study, the most significant factors affecting the strength of welded joints were the weakening of precipitation strengthening and solid solution strengthening due to welding thermal cycling. These factors accounted for 51.51% and 30.26% of the total strength, respectively. Compared with BM, the contribution of precipitation strengthening and solid solution strengthening in HAZ softening area decreased by 85 MPa and 23 MPa, respectively. Where the weakening of precipitation strengthening is mainly due to the β" and β' phase transitions induced by welding thermal cycling. The decrease in solid solution strengthening is then mainly influenced by the change in the content of Mg and Si elements in the matrix. The revelation of softening mechanism provides theoretical support for how to control the loss of mechanical properties of welded joints of Al-Mg-Si aluminum alloy.

Mechanical properties and microstructure of aluminum alloy 7075-T6 spot-welding joint

In this study, an orthogonal experiment was conducted to investigate the effects of various process parameters on the mechanical properties of resistance spot welding joints in AA7075-T6 aluminum alloy. A numerical simulation was also performed to determine the optimal parameter combination. The mechanical properties, macro-morphology, and microstructure of the joints were characterized and analyzed through a series of tests including tensile testing, super depth-of-field testing, metallographic observation, and microhardness testing. The results show that the welding parameters have a significant effect on the mechanical properties of the joints with the following order of influence: welding time > welding current > electrode pressure. The combination of 60 ms welding time, 17 kA welding current, and 0.22 MPa electrode pressure yielded the best comprehensive mechanical properties with an average tensile load of 2364N and an energy absorption value of 6.8 J. The numerical simulation results are consistent with the nugget size of this parameter combination. During spot welding, spatter defect occurs, shrinkage cavity exists at nugget interface, strengthened T-phase and finer equiaxed grain structure precipitate in nugget center. After welding process and microstructure change, the hardness of the nugget zone is lower than that of the base material while the hardness of the nugget center is higher than that of the edge. The failure mode of the joint is interface fracture.

A novel process for improving fatigue properties of TIG welded AZ31B mg alloy joints by laser bionic treatment

It is crucial to obtain better fatigue life for the safe application of materials, and nature provides a new idea for effectively improving fatigue property. A ‘soft-hard’ alternating structure imitated biological morphology was fabricated on the surface of welded joints by laser bionic treatment (LBT) for improving the fatigue property of TIG welded AZ31B Mg alloy joints. After LBT, the grains in the weld zone were refined with the average grain size about 5.55 μm and decreased by 76.1%, resulting in an increase of hardness of 12.0% to about 60.12 HV0.1, and formed a ‘soft-hard’ structure on the surface of welded joints. By adjusting the distribution angle of laser bionic stripe, the high cycle fatigue strength of welded joints was increased by 129.4% at most. The fracture morphology of welded joints was changed by LBT. The weld zone of bionic welded joints was deformed by twinning and slip, while the laser-treated zone was only deformed by slip. The mechanisms of strengthening fatigue properties were discussed based on the microstructure and structural characteristics from EBSD analyze.

Local microstructure and strengthening mechanisms of double-sided friction stir welded Al-Mg-Mn-Er alloy joint

Local microstructure and strengthening mechanisms of double-sided friction stir welded Al-Mg-Mn-Er alloy joint were investigated to reveal its softening mechanism. The results showed that a fine equiaxed grain structure (5.61 μm) was formed in the nugget zone (NZ), while the thermo-mechanically affected zone (TMAZ, 45.32 μm) and heat-affected zone (HAZ, 100.73 μm) maintained a fibrous grain structure. The fraction of low angle grain boundaries decreased from 75.6% of the base metal (BM) to 15.6% of the NZ. The annealing effect resulted in obvious reduction in dislocation density from 1.8×1014 m−2 of the BM to 4.5×1012 m−2 of the NZ. The average diameter size and volume fraction of Al3(Er, Zr) precipitates of the NZ, TMAZ and HAZ were close to those of the BM (13.7 nm and 0.13%). The NZ and TMAZ exhibited the lowest yield strength of about 201 MPa while the BM had the highest yield strength of about 295 MPa. The loss of the dislocation strengthening and substructure strengthening was the main reason for the decrease of yield strength from the BM to the NZ.