Weld Center Research Articles

Microstructure and mechanical properties of dissimilar pinless friction stir spot welded 2A12 aluminum alloy and TC4 titanium alloy joints

The microstructure and mechanical properties of dissimilar pinless friction stir spot welded joint of 2A12 aluminum alloy and TC4 titanium alloy were evaluated. The results show that the joint of Al/Ti dissimilar alloys can be successfully attained through pinless friction stir spot welding (FSSW). The joint can be divided into three zones (SZ, TMAZ and HAZ). The microstructure of joint in Al alloy side changes significantly but it basically has no change in Ti alloy side. At the same rotation speed, the maximum load of welded joints gradually rises with the increase in dwell time. At the same dwell time, the maximum load of the welded joint increases with the increase of the rotational speed. In addition, optimal parameters were obtained in this work, and they are rotation speed of 1500 r/min, plunge speed of 30 mm/min, plunge depth of 0.3 mm and dwell time of 15 s. The fracture mode of welded joints is interfacial shear fracture. The microhardness of the joint on the Al side distributes in a typical “W” type and is symmetry along the weld center, but the distribution of the microhardness on the Ti side has no obvious change.

Microstructural evolution and mechanical properties of a linear friction welded two-phase Ti-6.5Al-3.5Mo-1.5Zr-0.3Si titanium alloy joint

The microstructure, microtexture evolution and mechanical properties of a linear friction welded two-phase Ti-6.5Al-3.5Mo-1.5Zr-0.3Si titanium alloy joint were investigated. Results show clear microstructural differences at the joint center and edge. A martensitic and acicular α microstructure was found in the weld center zone (WCZ) at the joint center and a Widmanstatten microstructure can be identified in the WCZ at the joint edge. Further away from the WCZ, the microstructural differences smoothed out. The cross-weld crystallographic texture was assessed using electron back scattered diffraction. There are different α texture components in WCZ and thermomechanically-affected zone, but the β textures are similar in these two areas. The microstructural differences at the joint center and edge are the cause of the differences in microhardness distribution and tensile properties along the welding direction. Following post weld heat treatment, the inhomogeneity in both microstructure and mechanical properties were reduced.

Microstructure and mechanical properties of magnesium alloys joints with Ag-Cu interlayer by ultrasonic-induced transient liquid phase bonding in air for structure lightweight design

Magnesium alloys with Ag-Cu interlayer were joined by ultrasonic-induced transient liquid phase bonding. The influence of ultrasonic vibration time was studied. The mechanical properties of joints at room temperature and higher temperature were measured. With ultrasonic vibration time increasing, the ternary eutectic reaction between base metal and interlayer formed eutectic liquid phase. Mg element gradually diffused toward the center of weld, the liquid phase increased, and then the width of weld widen, substances such as solid solution α-Mg, AgMg3, CuMg2 were observed in weld. The weld was gradually closed with the extrusion of liquid phase and the diffusion of elements. At room temperature, the shear strength of joints could reach to ∼92 MPa, and the fracture partly occurred in base metal. The shear strength of nearly closed joints increased with temperature decreased from 300 °C to room temperature.

High-Speed Welding of Stainless Steel with Additional Compensatory Gas Jet Blow Molten Pool

To avoid humping bead defects in high-speed welding, this paper proposes the method of an additional and compensatory gas jet blow molten pool. A pulsed metal inert gas high-speed welding test platform was constructed for compensatory gas jet blow molten pool. A total of 304 stainless steel sheets were used as the welding workpieces under equal heat inputs. Two high-speed butt welding processes were conducted and compared, in which the workpieces were welded with and without compensatory gas jets at 154 cm/min and 167 cm/min, respectively. After high-speed welding with compensatory gas jet blow, the weld appearance was straight, uniform, and high-quality, with no humping bead or undercut defects. The macroscopic morphologies and microstructures of cross-sections of the weld at the toe, near the surface, the middle, and the bottom portion all showed the stirring effect of the gas jet on the molten pool and improved grain refinement degrees. Hardness was enhanced in the weld center and the heat-affected zone. At welding speeds of 154 cm/min and 167 cm/min, the fracture load capacities of the welds were increased by 24.9 and 10.4%, respectively.

On microstructure and property differences in a linear friction welded near-alpha titanium alloy joint

In this study, a near-alpha titanium alloy (Ti-6Al-2Zr-1Mo-1V) was welded with linear friction welding (LFW). The microstructural evolution and mechanical properties of the joint were evaluated. Results show that grains are remarkably refined in the weld center zone due to dynamic recrystallization because of the effect of thermo-mechanical coupling, while the fractions of deformation grains, sub-structure grains and recrystallized grains are significantly different from those of the base metal (BM). The room temperature tensile strength of the welded joint is not lower than that of the BM, because all the failure occurs in the BM. It is more interesting to find that the joint impact toughness is increased by 19.1% compared to the BM.

Investigation on the resistance to fatigue crack growth for weld metals with different Ti addition in near-threshold regime

The resistance to fatigue crack growth and its correlation with microstructure for weld metals with different trace Ti element addition was systematically investigated in this paper. It is found that there were some obvious differences in grain size, block ferrite and precipitations for Ti-free and Ti-containing weld metal. Fatigue crack growth tests were conducted in weld center under different stress ratios, the results indicate that weld metal without Ti addition performed better on fatigue resistance in near-threshold regime at each stress ratio. Transgranular growth with crack deflections and intergranular growth with bifurcations are the typical crack growth behavior in Ti-free and Ti-containing weld centers respectively. It is revealed that large size of prior austenite grains in Ti-free weld center lead to more crack deflection via analysis on the fatigue crack growth path. Moreover, the carbides scattered on ferrite boundaries also lead to crack deflection and reduce the possibility of intergranular fracture at near-threshold for Ti-free weld center. Compared to crack bifurcations, the crack deflections contributed more to crack closure inducted by contact surface roughness, and the fatigue growth resistance is enhanced for Ti-free weld center. The correlation between fatigue resistance performance and misorientation of ferrite boundaries in weld centers was further clarified by electron back-scattered diffraction (EBSD) characterization.

Mechanical property and characterization of TA1 titanium alloy sheets welded by vacuum electron beam welding

Vacuum electron beam welding (EBW) was used to weld the thick TA1 titanium alloy sheets. Test results indicated that the coarse serrated-shape α and needle-shape α′ grains were observed in weld zone (WZ); the heat affected zone (HAZ) near the weld center consisted of α and α′ gains; the tensile strength of the TA1 joint was 344 MPa, which was significantly higher than that of the base metal (BM); the fracture of the tensile samples with different penetration depths all occurred on BM, the tensile property of the upper-bottom joint was better than that of the middle joint; moreover, lots of dimples can be observed on the fatigue fracture surface, exhibiting the characterization of the ductile fracture.

Investigation of the oxide film on 308L weld surface produced during welding process

The oxide film formed during welding on the inner surface of 308L weld was investigated. The morphology, composition and structure of the oxide film were characterized by optical microscopy, scanning electron microscopy, laser Raman spectroscopy, X-ray diffraction, Auger electron spectroscopy and transmission electron microscopy. The results indicated that the oxidation degree of the weld metal was generally uniform from the weld center to the fusion line. Micro-scale cracks were found on the outer surface of the oxide film and nano-scale gaps scattered on the inner oxide film-weld interface. The oxide film exhibited a duplex structure including outer and inner layers. The outer layer, made of tightly bound particles, was mainly composed of Fe2O3 oxide particles with a diameter of 50–150 nm. And the inner layer, made of relative loosely bound particles, was mainly composed of Fe3O4 and FeCr2O4 oxide particles with a diameter of 10–50 nm. The corresponding oxidation behavior and mechanism of the oxide film were also discussed.

Experimental and numerical investigations of bonding interface behavior in stationary shoulder friction stir lap welding

Stationary shoulder friction stir lap welding (SSFSLW) was employed to weld 2024 aluminum alloy. A coupled Eulerian-Lagrangian (CEL) model was developed to investigate the lap interface behavior during SSFSLW. Numerical results of material movement and equivalent plastic strain were in good agreement with the experimental work. With increasing welding speed, the distances from the hook tip to the top surface of the upper workpiece on the retreating side (RS) and the advancing side (AS) increase, while the distance between two wave-shaped alclads decreases. A symmetric interface bending is observed on the AS and the RS during plunging, while the interface bending on the AS is bigger than that on the RS during welding. The peak temperature of the interface on the AS is higher than that on the RS. The equivalent plastic strain gradually increases as the distance to the weld center decreases, and its peak value is obtained near the bottom of the weld.

Microstructure and porosity characteristics of 5A06 aluminum alloy joints using laser-MIG hybrid welding

5A06 aluminum alloy plates with thickness of 6.9mm are welded by KUKA Robot with a fiber laser of IPG YLS-6000. The laser-metal inert gas (MIG) hybrid welding method is adopted to provide high overall productivity in this paper. To investigate the joints microstructure and porosity distribution, optical microscope observation, scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) detection are conducted. The pores of laser-MIG hybrid welding joint are mainly metallurgical pores which are caused by the dissolved gases and evaporation of Mg element. This metallurgical pores are densely populated in the upper area of weld joint. Besides, the micro-hardness along the weld center line and perpendicular to the weld center line are separately analyzed. The joint softening can be evidently observed near heat affected zone (HAZ) and in weld bead (WB), where the grain enlargement or pores problem exist. It claims that the formation of pores has a much greater influence on the joint softening than grain enlargement. In order to get a low-porosity weld joint, as well as owning a high level of micro-hardness, the porosity and micro-hardness in different laser-MIG hybrid weld joints are analyzed comprehensively. The welding speed of 3m/min is more suitable to be applied in laser-MIG hybrid welding for the 6.9mm 5A06 aluminum alloy.

Feature point extraction based on contour detection and corner detection for weld seam

In an arc welding robot system with vision sensor, the extraction precision of the weld seam position has an important effect on welding quality. However, due to disturbances such as camera resolution, scanning posture, reflections, etc., it is a great challenge to identify weld seam accurately. In this paper, weld seam is considered as a special curve which consists of feature points, based on that, an effective method is proposed to extract the feature points by using contour detection and corner detection. Experiment results show that feature points can be extracted successfully by using this method, so that the robot can weld the work part along the feature points detected. The limitation of this method is that the detected feature points are sometimes inconsistent with the weld center points, which may affect the welding quality.

X-ray phase contrast observation of solidification and hot crack propagation in laser spot welding of aluminum alloy.

The inner characteristics of solidification and crack propagation in laser spot welding of four representative aluminum alloys, A1050, A2024, A5083 and A6061, were firstly observed with the X-ray phase contrast method. Keyhole disappeared within 1 ms after the laser was shut down. The solidification process finished in 2 ms for A1050, 3 ms for A2024, 5 ms for A5083, and 3 ms for A6061, respectively. Longitudinal view area of the molten pool decreased as the thermal conductivity increased, while the average solidification rate increased with increase of the thermal conductivity. Hot crack was observed to propagate from the bottom to the upper surface in the center of spot weld of A2024, A5083, and A6061, which was also the first in situ observation of crack during the welding process. Both the SEM, EBSD and Micro-X-ray computed tomography (CT) results validated that there was a crack propagation in the spot weld, and the mechanism for this crack formation was discussed. This paper provides a better understanding of solidification and crack formation in laser manufacturing.

Hybrid Laser-Arc Welding of X90 Pipeline Steel: Effect of Laser Power on Microstructure and Mechanical Properties

In this work, hybrid laser-arc welding process was applied to X90 pipeline steel which has wide potential applications in the future pipeline project. The effect of different laser power (1.0, 1.5 and 2.5 kW) on microstructure and mechanical properties of weld joints was investigated. It has been found that a macroscopic morphology of “wine cup like” is observed in the weld joint with increasing laser power, where fusion zone (FZ) and heat-affected zone (HAZ) can be clearly identified. The FZ microstructure mainly includes massive ferrite, acicular ferrite (AF), and increased laser power resulting in a decrease in AF content. The HAZ consists of coarse-grained HAZ (CGHAZ), fine-grained HAZ (FGHAZ) and mixed-grained HAZ (MGHAZ). The hardness ranging from the weld center to base metal decreases and then increases, and the effect of laser power on hardness is not significant. The increased laser power leads to an evident decrease in the ultimate tensile strength and impact toughness of weld joint. The highest ultimate tensile strength and impact energy are 815 MPa, 239.1 J respectively at a laser power of 1.0 kW. A number of inclusions are observed at the bottom of dimples, which may be the (Ti,Mn)2O3 particles.

Melt flow and grain refining in ultrasonic vibration assisted laser welding process of AZ31B magnesium alloy

During the laser welding process of magnesium (Mg) alloy, it is easy to produce welding defects, which deteriorates the mechanical properties of welded joint. Since ultrasonic vibration can decrease weld defects considerably, the ultrasonic-assisted welding technology has become a hotspot in the current research field of welding and joining. In this study, the ultrasonic vibration assisted laser welding technology used for the joining of AZ31B Mg was carried out. Setting amplitude transformer and welding specimen in contact makes ultrasonic travel to the weld pool as well as promote melt flow and bubble escape by the cavitation, acoustic streaming and thermal effects. Owing to these effects, the weld porosity decreases significantly to less than 1%. Besides, in the crystallization process of the weld pool, ultrasonic vibration increases the number of nucleation particles and refine grain, which improve the mechanical properties of welded joint. As a result, the average grain area in weld center area decreases from 359.9 μm2 to 213.7 μm2. Also, compared with laser welding, the tension strength σb, and elongation Agt increase from 235.1 MPa to 274.1 MPa and 6.6% to 7.5%, respectively.

Microstructure Evolution and Mechanical Properties of Keyhole Repair Welds in AA 2219-T851 Using Refill Friction Stir Spot Welding

The search for a suitable friction-based keyhole repair technique that fulfills the requirements for high-quality repair welds has become an important research topic, especially for aerospace applications. In order to provide and analyze an universal keyhole repair method for lightweight metals, refill friction stir spot welding is applied to through-hole repairs in 3- and 6-mm-thick sheets of precipitate hardening AA 2219-T851. Keyholes with a diameter of 7.5 mm were repair welded achieving a defect-free microstructure. The correlation between the microstructural evolution imposed by the repair process and the resulting mechanical properties is shown. A comprehensive analysis of the precipitate evolution in peak-aged AA 2219 during RFSSW is presented. Thermal cycle measurements revealed high heating rates and peak temperatures of up to 520 °C in the weld center. The thermal cycle caused mainly dissolution and minor equilibrium phase formation in the stirred zone. In the HAZ, overaging of the strengthening precipitates dominates with minor dissolution and equilibrium phase formation only in the direct proximity of the SZ. Microstructural analysis revealed typical weld zone formation with inhomogeneous grain size distribution in the SZ. The resulting mechanical properties are dominated by an inhomogeneous hardness distribution with lowest hardness in the TMAZ at 5 mm from the center of the weld. During tensile loading main yielding and the final fracture occur in the area of lowest strength. Tensile testing showed yield strength of 40 to 46% and UTS of 28 to 25% below BM values in 3- and 6-mm-thick sheets, respectively. The sheet thickness and post-weld natural aging were found to influence the mechanical properties of the weld significantly.

Fabrication and Mechanical Properties of Tungsten Inert Gas Welding Ring Welded Joint of 7A05-T6/5A06-O Dissimilar Aluminum Alloy.

In this paper, Tungsten Inert Gas Welding (TIG) and single-sided welding and double-sided forming have been used to weld the 7A05-T6/5A06-O dissimilar aluminum alloy circular welded joint of a ring-stiffened closed cylindrical sandwich shell. Microstructural characterization and mechanical properties of welded joints were investigated by use of scanning electron microscopy (SEM), backscatter electron diffraction (EBSD) and transmission electron microscopy (TEM), respectively. Hardness distribution and tensile properties of the welded joints were examined. The results showed the failure of the welded joints produced in the fusion zone (FZ). The tensile strength and yield strength of the welded joints were, respectively, 78.87% and 97.24% of the 5A06-O base metal (BM), and the elongation reached 84.29% of 7A05-T6 base metal. Welding high heat input led to the coarse grain size in the fusion zone, and the long-term similar quenching effect can lead to the full dissolution of the strengthening zone of the fusion zone resulting in the reductions of strength and hardness. Around 7A05 heat-affected zone (HAZ), there is an obvious hardening zone and softening zone, the solid solution precipitates into the Rayleigh brilliant η′ (MgZn2) phase, which results in natural aging strengthening, thus obtaining high hardness. η′ (MgZn2) enhanced phase dissolved fully and the dislocation density decreased rapidly in the HAZ region resulted in a softening zone with a lower hardness at about (10–18) mm from the center of the weld center.

Study the correlation between Microhardness, Microstructure & In-situ thermal analysis of PCATIG weldedments of Al-SiC composite

Activated TIG welding has been performed on Al – 8% SiC composite 5mm plate with various fluxes such as Al2O3, MnO2, CaO, MgO, SiO2 & TiO2. Microstructure, Micro hardness and cooling rate are observed. Correlation between micro hardness, microstructure and cooling rate of Pulsed Current TIG welding and Pulsed Current Activated TIG welding on Al-SiC composite has been studied. Combining the pulsed current TIG and activated TIG welding (PC-A-TIG) on Al-SiC composite showed very fine superior weld microstructure on PCATIG – SiO2 & PCATIG – TiO2 which resulted in higher micro hardness values around 70 – 75 Hv. Micro hardness are observed in different locations of weld surface such as 1mm, 2mm & 3mm below the weld surface and it also observed along the weld zone to heat affected zone upto 12mm for the profiling the micro hardness value around the weldment. Microstructure observed in weld centre for observed on weld zone & heat affected zone. Minimum micro hardness value between 63 – 65 Hv found in PCATIG – MnO2, PCATIG – CaO & PCATIG – MgO due to intermediate micro structure between coarse and fine. PCATIG – Al2O3 weld zone & heat affected zone and heat affected zone of PCATIG – MnO2, PCATIG – CaO & PCATIG – MgO shows coarse microstructure. This resulted in reduced micro hardness around 55 – 60 Hv. Coarse micro structure in weld zone and heat affected zone have least cooling rate. Fine micro structure in weld zone is due to higher cooling rate. Heat affected zone directly depends on temperature gradient between the weld centre and weldment heat affected zone.

Experimental investigation of pug cutter embedded TIG welding of Ti-6Al-4V titanium alloy

Light-weight materials, such as titanium alloy, have been extensively investigated. This paper primarily focuses on microstructural, metallurgical, and mechanical properties of the pug cutter embedded tungsten inert gas (TIG) welding of Ti-6Al-4V titanium alloy. In this work, TIG welding was modified to control the speed and the arc length. Analysis revealed that the microstructure underwent severe changes at different zones: coarsened in the fusion zone (FZ) and enlarged in the heat-affected zone (HAZ). The microstructure was finer at the base metal than those in the FZ and the HAZ, and the Vickers hardness values decreased from weld center line to the base metal. Tensile test showed that the specimen with low current and high speed was broken below its ultimate tensile strength, whereas most of the samples were broken at the base metal region that had a similar strength to that of the parent metal. The elongation of the metal was primarily affected by the heat input. A high volume of heat increased the hardness and the brittleness, resulting in low ductility.

Gas tungsten arc welding of 7075 aluminum alloy: microstructure properties, impact strength, and weld defects

Precipitation-hardened 7075 (Al-Zn-Mg-Cu) aluminum alloys have low specific density and high strength. Because of these advantages, they are commonly used as construction materials in the aerospace industries. Even though they have such important and common areas of usage, their weldability is quite difficult. It is important to weld this alloy used in these industrial areas. In this study, 3 mm-thick 7075-T651 aluminum alloy materials were joined using different welding currents via gas tungsten arc welding (GTAW) method. The metallographic examinations were carried out to determine macrostructural and microstructural properties of the weld zones. Also, notch impact and hardness tests were performed to determine the mechanical properties of the welded samples. The results obtained as a result of the tests were evaluated and interpreted. The grain size of the weld center increased due to heat input occurring with increase of the welding current. Micro cracks (hot cracking) occurred in the roots of welding seams. The increase in the welding current affected the hardness distribution of the weld zone. The impact strength of the welded sample was negatively affected by the grain coarsening and micro cracks in the welding seam.

Underwater local dry cavity laser welding of 304 stainless steel

The relationship between the gas rates and the shielding condition of the double-layer gas curtain nozzle utilized in underwater laser beam welding (ULBW) was observed. The welding morphology, weld porosity and mechanical properties of butt joints produced in different process parameters were investigated. With the increase of the heat input, the welding porosity decreased firstly and then increased. Higher welding porosity degraded the mechanical properties significantly and the formation of the welding pores could be attributed to the water vapor generating on the bottom surface of the joint and the solidification rate of the molten pool. The high-quality butt joint of ULBW without welding pores was obtained under optimal process parameters, whose tensile strength, impact toughness and microhardness profile approached to these of the in-air welded joint. Compared with in-air joint, the lathy ferrite appears in the ULBW bead and some equiaxed crystals were replaced by the columnar dendrites in the weld center region owing to the cooling effect of water.