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Laser cleaning has been widely used in manufacturing industries due to its lower pollution and higher cleaning precision and efficiency compared with traditional ways. However, laser cleaning would inevitably induce processing roughness on the workpiece surface and only a few researches have been conducted to this field. In this work, the influence of process parameters on surface roughness was explored in detail. The results showed that process parameters changed surface roughness by changing energy density and overlap ratio. And surface roughness was showed linear positive correlation with energy density. At same energy density, surface roughness will first increase and then decrease with increasing overlap ratios. The maximum roughness was in the range of 50 to 66.7%. It could be explained that after laser irradiation, the sample surface absorbed pulse energy and formed craters. The rim and craters made the sample surface coarse. Due to the depth and diameter of craters were dependent on energy density, the roughness increased with increasing energy densities. The adjacent craters will also change the height of rim and depth of crater with different overlap ratios. It led to different surface roughness. [ABSTRACT FROM AUTHOR]

The keyhole behaviors were observed directly by laser welding experiment with quartz glass. Based on Fresnel absorption of laser beam and multi-reflection combined with volume of fluid (VOF) method, a three-dimensional mathematical model was established to study the role of welding speed on keyhole behaviors and keyhole-induced porosity formation in laser welding of aluminum alloy. The keyhole behaviors and weld pool fluid flow were discussed, and the result shows that, although the welding speed varies, the mechanism of keyhole collapse was similar. However, the keyhole stability at higher welding speed was improved due to reduced weld depth fluctuations, spatter number, keyhole depth to width ratio, and keyhole collapse frequency. Furthermore, the improved keyhole stability impeded the keyhole collapse, and the lower weld depth and solidification rate facilitated bubble escaping from the weld pool easily, which are the main factors for suppression of porosity at higher welding speed. [ABSTRACT FROM AUTHOR]

Hybrid laser-MIG (metal inert gas) welding has not been extensively applied in welding of aluminium alloy with thickness more than 10 mm because of the existence of weld defects and low joint strength. In this study, an original hybrid laser-MIG welding plus MIG welding process was developed. Double-sided hybrid laser-MIG welding method was designed and applied to the welding of 30-mm-thick Al 5083 alloy. The process was then optimized by overlaying a MIG welding upon the weld. Analysis on the weld revealed that the overlaid MIG welding played multiple roles, including making up the bead formations, eliminating the keyhole-induced porosities as well as refining grains. Along the longitudinal direction of the weld, the grains became finer from two fringes to the middle, and the mechanical properties improved accordingly. Compared with MIG welding, the hybrid laser-MIG welding plus MIG welding process showed overwhelming superiorities over the MIG welding, which will be promising in industrial applications. [ABSTRACT FROM AUTHOR]

Multi-pass gas metal arc welding was carried out for Al 5083 alloy plate with thickness 30 mm. During multi-pass welding of thick plate, the former welding pass experienced multiple thermal cycles that came from subsequent welding passes. This work focused on the effect of thermal cycle on metallurgical characteristics and mechanical properties. Different from what had long been recognized that welding thermal cycle was harmful, the results indicated that multiple thermal cycles caused the partial transition from columnar grains to equiaxed grains, which finally contributed to the refinement of microstructure. Increased precipitated particles resulted in the improvement of strengthening effect. These beneficial effects of microstructure and precipitated phase led to the increase of micro-hardness and the improvements of tensile strength and ductility. As the progress of welding process, overall properties of the welded joint improved. Conclusion could be made that appropriate thermal cycles were beneficial during multi-pass welding of Al 5083 alloy. [ABSTRACT FROM AUTHOR]

Thick plates of 5083 aluminum alloy are welded with GMAW and GMAW-TW processes; the influence of weld pool convection on porosity formation, microstructure, and mechanical property of the welding joints is investigated. The results showed that due to the appearance of outward flow and 'push-pull' patterns, and intense mixing and stirring effects in GMAW-TW process, a sound weld seam with less porosity can be obtained. The less probability of bubble formation, the shorter escaping distance of the bubble, the easy splitting of bubble, the higher bubble escape velocity, and lower solidification rate are responsible for porosity suppression. Even though the grain size in GMAW-TW process is larger than that in GMAW process, the ultimate tensile strength of the joints is larger, which is caused by less porosity formation, larger number, and uniform distribution of second-phase particles in the welds. Compared with GMAW process, the GMAW-TW process possesses the advantages of higher efficiency, less weld defects, and higher mechanical property in the welding of 5083 aluminum alloy thick plates. [ABSTRACT FROM AUTHOR]

CuSi CMT brazing is an advanced welding technology widely used in automotive industry. But dark deposits defect was found on the weld seam surface. SEM and EDS analyses were operated in this paper and this surface was found to contain three areas: area A with silicates thin film and Cu, area B with MnO·SiO silicates, and area C with CuO. And thus the formation mechanism of surface deposits, which was mainly attributed to the generation and uneven distribution of large amount of dark CuO, was proposed. Based on the mechanism of the deposit formation, four methods were processed to suppress the surface dark deposits. Ar + 2%O would prevent the generation of dark deposits because of enhanced cathode crushing effect. Besides, delay gas protection time, keep torch travel angle positive, and maintain a low heat input would be benefit for suppressing dark deposits on the weld seam surface. [ABSTRACT FROM AUTHOR]

A three-dimensional numerical model was established to improve the accuracy of simulation with stationary three-wire GMAW method. High-speed photography was utilized to detect the details of welding process. The effective radius for arcs was calculated by Abel inversion method. Besides, due to arc interference, transverse velocities of droplets were added by a theoretical calculation method. The simulated longitudinal cross-sectional welded shape indicated good corresponding to the optical result. Moreover, the flow pattern of simulated results accorded well with the reality. According to the simulation, there existed upward mobility in the leading and trailing pools which can be attributed to the droplet impact. This mobility can cause the accumulation of molten metal at the side near middle pool. Therefore, it will accelerate the process to form common weld pool. [ABSTRACT FROM AUTHOR]

In order to improve the understanding on arc interference behavior and verify the welding operating point change during twin wire MIG welding, the synchronous acquisition system has been established to acquire instantaneous information of electric signals, arc profile, and droplet transfer including dynamic arc length variation. It showed that the current and voltage signals changed significantly after trailing arc (T-arc) was added to the leading arc (L-arc) in a stable welding process. Moreover, the arc agitation of L-arc increased with increasing the current of T-arc but decreased with increasing the inter-wire distance. The results also showed that the stable spray transfer of L-arc was destroyed and deflected due to the effect of T-arc. With respect to a single wire welding, the arc length of L-arc increased and the welding operating point changed during the twin wire welding. [ABSTRACT FROM AUTHOR]

A numerical model of a three-dimensional transient weld pool was conducted to analyze the hump formation mechanism in high-speed single-wire gas metal arc welding (GMAW) and the hump suppression in triple-wire GMAW. The effect of heat convection, radiation, and vaporization on temperature field and the impact of arc pressure, surface tension, Lorentz force, buoyancy force, and droplet impinging force on the fluid flow field were considered during simulations. In good agreement with the experimental beads and images recorded by high-speed camera, the simulated results are shown: during the hump formation in single-wire GMAW, the liquid channel was formed, elongated, pinched, and solidified due to the strong backward molten metal flow and the normal surface tension. However, in the same line energy, humps were suppressed in triple-wire GMAW. The 'pull-push' flow in the triple-wire GMAW weld pool could dramatically slow down the backward trend, which reduced the solidification rate. Consequently, the liquid channel disappears and the humps could not be formed any more. [ABSTRACT FROM AUTHOR]

A three-dimensional numerical model is established to study the temperature and fluid flow fields in the twin-wire gas metal arc welding (GMAW) process. The high-speed photography system is used to capture the images of the weld pool during the welding. Based on simulation and experimental results, the weld pool formation, convection, and stability in twin-wire GMAW process are investigated. Both 'push-pull' and outward flow patterns exist in the twin-wire GMAW weld pool, which can contribute to decreasing the height of the bulge and increasing the width of the pool. The convection in the weld pool can proceed adequately, the arc force between the leading and trailing arcs is relatively balanced, surface tension normal force is uniform along the liquid channel, and the liquid channel is capillary stable, all of those contribute to the stability of the weld bead. The simulation results are in good agreement with those in the experiment. [ABSTRACT FROM AUTHOR]

In order to better understand the arc interference behavior and welding stability in twin wire GMA welding, the synchronous acquisition system has been established to acquire instantaneous information of electric signals, arc profile, and droplet transfer including dynamic arc length variation. The results showed that after Trailing arc (T-arc) was added to the Leading arc (L-arc) in a stable welding process, the current of L-arc decreased and the voltage kept constant. The stable globular transfer of L-arc was destroyed and deflected, turned into spray transfer due to the effect of T-arc. Compared to the single wire welding, the arc length of L-arc increased and the welding operating point changed in twin wire welding. [ABSTRACT FROM AUTHOR]

The plasma behavior and metal transfer in CO laser + GMAW-P hybrid welding have been investigated. A 650 nm laser, in conjunction with the shadow graph technique, is used to observe the metal transfer process. The effect of the mutual distance and laser power on the metal transfer has been discussed. The results indicate that the laser-induced plasma plume have a significant impact to the arc shape, resistance, electrode melting, droplet formation, detachment, and impingement onto the workpiece. The laser-induced plasma changes the conductive path and forces affecting on the droplet. High laser power and short distance between laser beam and arc (DLA) reduce the pulse base time (PB) of the voltage of phase, increase the droplet detachment time (PD) and the pulse current time (PP) of the voltage of phase, and it also lead to an upward and inward force near the bottom of the droplet. As a consequence, the droplet formation time is increased, and eventually an off-axis droplet phenomenon is deduced. The vapor jet force induced by the the keyhole plasma acts on the droplet as a retention force; this force decreases when the DLA becomes larger and increases when the laser power becomes higher. The observation may help in understanding the weld characteristics with respect to variation in mutual distance and laser power which may be beneficial in using the main process parameters to produce desired weld quality. [ABSTRACT FROM AUTHOR]

In the process of directed energy deposition-arc (DED-arc), the geometrical shape of weld bead in arc-striking (AS) and arc-extinguishing (AE) areas are usually inconsistent with that in the middle section, which directly affects the forming accuracy of component thus needs to be optimized. In this paper, high-speed photography is used to observe the molten pool behavior of nickel aluminum bronze (NAB) bead deposition, and the causes of abnormal bead shape in the AS and AE areas are analyzed. On that basis, an optimization method for AS and AE is put forward. In the AS area, the welding current and travel speed are increased. And after the arc strikes, a hovering time is used during which the torch hovers at the AS point. In the AE area, the welding current is gradually reduced while the torch keeps hovering at the AE point. Then, the optimization effect of AS parameters on the bead shape are explained through factorial experiments, and the optimization effects of AE parameters are explained through orthogonal experiments. At the same time, the optimal parameter combination of AS and AE is obtained. With the proposed method and the optimal parameters, the height and width deviation of the deposited single bead are controlled within 5%. After adopting the optimization method, the effective height ratio of the DED-arc–manufactured NAB thin wall is increased from 67.5% to 91.5% under the unidirectional deposition path, which is also increased from 88.9% to 98.3% under the bidirectional deposition path. [ABSTRACT FROM AUTHOR]

The directed energy deposition-arc (DED-arc) process is gaining popularity for cost-effective production of large parts, especially in the marine industry. However, precise shaping control, especially for parts with sharp corners, has been challenging. Nickel aluminum bronze (NAB) alloy, widely used in the marine field, is an expensive material, making it crucial to improve its forming accuracy. To tackle this challenge, the Angle-Driven Parameter Control Model (APCM) has been introduced. This model is designed to enhance corner paths in the DED-arc process for NAB alloy. By adjusting the travel speed, the APCM aims to reduce height errors at corners. To validate the effectiveness of the APCM, single bead depositions and a multi-layer thin-wall component with seven different angles (10°, 20°, 30°, 45°, 60°, 75°, and 90°) were tested. Results indicate that the APCM has a more significant optimization effect at smaller angles. For angles above 60°, relatively small height deviations can be achieved without parameter control. For the 15-layer thin-wall component, the APCM control reduced the height deviation at the 10° angle from 7.72 to 2.09 mm. Therefore, the proposed APCM is ideal for corner paths in the DED-arc process of NAB alloy. [ABSTRACT FROM AUTHOR]

In recent years, the significant residual stress in the wire-arc additively manufactured components has been continuously one of the most concerning issues for the further development of the wire-arc additive manufacturing (WAAM) technology. Although many pre/post processing methods have been proved capable of modifying the residual stress, few works have been conducted to reduce the residual stress using appropriate path planning. In the present research, a WAAM path planning strategy is developed combining both zigzag and contour-offset methods, which aims to obtain a single WAAM deposition buildup layer with both optimal outline accuracy and residual stress status. The path planning and residual stress measurement processes have been carefully designed to acquire reasonable results. According to the residual stress results measured using X-ray diffraction method, the arc deposits finished with more contour-offset paths contain smaller tensile residual stresses, which is due to the hooping function of the outer contour deposit path to the inner deposit. Therefore, it can be concluded that the offsetting of the external contours before zigzag filling the internal dimensions is beneficial to both the residual stress distribution and shape outline accuracy of the WAAM buildup layer. [ABSTRACT FROM AUTHOR]

The anisotropy of wire-arc additively deposited nickel-aluminum bronze alloy is studied by microstructure observation and room temperature mechanical property testing. The applied heat treatments have effectively modified the as-fabricated {0 1 1} <2 1 1>, {1 1 1} <1 1 0> texture into {1 1 1}. According to the microstructures, the single tempering has resulted in lowest anisotropy while heat treatment with prior homogenization and quenching process induces even intensified annealing textures. However, as shown by the tensile tests, the quenching process is necessary for favorable mechanical properties in the wire-arc additive manufacturing fabricated nickel-aluminum bronze component; therefore, a balance between homogenization annealing and κ-phase precipitation is indicated. Meanwhile, the function of κ-phase precipitates on grain rotation and growth during post-production heat treatment is explained in detail. [ABSTRACT FROM AUTHOR]

Twin-wire plasma arc additive manufacturing (TW-PAAM) is an innovative process for titanium aluminide manufacture which is low in equipment cost and high in efficiency. However, as an in situ alloying process, the composition homogeneity is typically a source of concern. The present work investigates this subject by observing the element input mode at various welding wire positions with a high-speed camera system and particle tracking method. It is found that when the welding wires are in a high position, the droplet enters the molten pool from the tip of the titanium wire, and the alloy element input interval is long, resulting in the formation of bands with high aluminium content at the edge of the deposition layer. When the welding wires are close to the workpiece surface, the droplet transfer mode switches to a mixture of dual wire-bridge transfer and Ti wire-bridge transfer. The diameter and transfer interval of the droplets decrease, as well as the heterogeneity of the deposition layer. The conclusion establishes a correlation between droplet transfer mode and composition homogeneity, which is beneficial for optimizing the TW-PAAM titanium aluminide fabrication process. [ABSTRACT FROM AUTHOR]

As a complex thermo-physical process, the plasma arc welding (PAW) is easy to be unstable due to external interferences. Weld quality monitoring is important for intelligent robot PAW welding. Due to the different instability mechanisms, it is difficult to obtain high adaptivity and accuracy with features extracted in a single time window. In this paper, a novel feature extraction method based on sliding multiscale windows is proposed to improve model accuracy and calculation speed. A group of windows with different time widths are established to extract multiscale information. Windows slide throughout welding process and are synchronized on the timeline for feature correlation. The welding current and arc voltage are processed to extract features inside windows, including signal denoising by discrete wavelet transform (DWT) and dimension reduction by primary components analysis (PCA). Based on the feature vectors extracted from multiscale-windows, support vector machine (SVM) with radial basis function (RBF) kernel is used. The best window width is determined automatically by model training. The proposed method is used to predict weld quality for PAW in the field of shipbuilding. The results show that the model with multiscale feature extraction is helpful to improve prediction precision and recall ratio. [ABSTRACT FROM AUTHOR]

A novel detection method of penetration status was presented for a high-power fiber laser welding. The metallic vapor and molten pool was recorded by a high-speed camera during welding process. The radiation intensity of metallic vapor, as well as the morphology of molten pool end, was calculated by image processing algorithm as image features. Four image features, the radiation intensity of metallic vapor (RIMV), the area of molten pool end (AMPE), the rear angle of molten pool end (RAMPE), and the aspect ratio of molten pool end (ARMPE), were extracted. The mean value, relative range, variation coefficient, and frequency ratio were computed for the four features to obtain the 16 characteristic parameters. Aiming at penetration status, the characteristic parameters were reorganized to form two complex indicators by the principal component analysis. Experimental results showed that the detection method was potential for online detection on the penetration status in a high-power laser welding process. [ABSTRACT FROM AUTHOR]

It is essential to acquire the electron temperature distribution of plasma plume to determine the laser energy loss caused by laser-induced plasma. In this paper, a set of synchronous high-speed cameras were positioned in different angles to obtain plasma images during CO laser welding process shielded by He-Ar mixed gas. The absorption coefficient and refractive index of incident laser was calculated based on the 3D distribution of laser-induced plasma. The results indicated that the temperature field of plasma core region was nonuniform while the temperature distribution was relatively smooth and uniform for plasma periphery portion. It was found that the shielding gas affected the 3D distribution of plasma, as well as the energy interaction between laser and base material. Laser energy loss caused by plasma increased with argon percentage in the shielding gas. Relatively, laser energy loss caused by refraction is slightly higher than that caused by absorption. [ABSTRACT FROM AUTHOR]