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Highlights • Two mechanisms are responsible for keyhole-induced bubble formation. • The pressure balance equation is proposed to analyze the breaking of molten bridge by laser beam. • Large bubble and small bubble have high coalescence efficiency. Abstract Laser welding experiment with the aid of glass, and numerical simulation are carried out to study the keyhole behavior and keyhole-induced bubble formation. Two mechanisms are responsible for keyhole-induced bubble formation. The first mechanism is the strong fluid flow inside the weld pool, and the capillary instability of the whole keyhole, causing the collapse between rear keyhole wall and front keyhole wall. This mechanism contributes to most of keyhole-induced bubble formation. The second mechanism is the instability of the rear keyhole wall caused by the increase absorption of laser energy reflected by the bulge at front keyhole wall. The breaking of molten bridge is analyzed based on static pressure balance. The molten bridge with large curvature and low temperature is difficult to be broken. Bubble coalescence can be clearly observed at the bottom of the weld pool. Large bubble and small bubble have high coalescence efficiency. The bubbles at keyhole bottom have more time to escape without broken by laser beam, and the bottom part of rear keyhole wall is more easily depressed, so bubbles are easily formed at the keyhole bottom. [ABSTRACT FROM AUTHOR]

The droplet escape condition in laser welding is established in this paper. A three-dimensional numerical model is developed to study the weld pool convection and spatter formation at full penetration during the fiber laser welding of 5083 aluminum alloy. It is found that when laser power is 9 kW, the bottom of the keyhole is dynamically opened and closed. When the bottom of the keyhole is closed, the molten metal at the bottom of the back keyhole wall flows upwards along the fusion line. When the bottom of the keyhole is opened, few spatters can be seen around the keyhole at the top surface, two flow patterns exists in the rear part of the keyhole: a portion of molten metal flows upwards along the fusion line, other portion of molten metal flows to the bottom of the keyhole, which promote the spatter formation at the bottom of the keyhole rear wall. [ABSTRACT FROM AUTHOR]

The droplet escape condition is established, a three-dimensional numerical model is built to investigate the behavior of the weld pool in fiber laser welding of 5083 aluminum alloy, the shear stress caused by the metallic vapor flow that has great influence on spatter formation is considered. A high speed photography system is used to capture the transient images of the weld pool, the spatter formation is discussed. The numerical and experimental results show that the spatters generate mainly around the keyhole. Two factors are responsible for the spatter formation: the surface tension of the molten metal around the keyhole is low. The recoil pressure caused by evaporation and the shear stress caused by metallic vapor flow accelerate the upward moving melt around the keyhole. Increasing the welding speed and placing the nozzle at the trailing position can decrease the upward momentum of the molten metal around the keyhole, and stabilize the keyhole, which are help to suppress the formation of spatter. [ABSTRACT FROM AUTHOR]

Three-dimensional numerical models are established to investigate the convection in normal and high speed GMAW processes. A high speed photography system is used to capture the transient images of the weld pool. Based on the simulation and experimental results, the differences of weld pool formation, convection and stability are researched. The humping formation mechanism in high speed GMAW process, and humping inhibition mechanism in the twin wire GMAW process are also discussed. The results show that in normal speed GMAW process, a clockwise circulation and a backward fluid flow pattern exist in the weld pool behind the arc. While in high speed GMAW process, three main factors are responsible for the humping formation: the high momentum of the backward fluid flow, the large variation of the capillary pressure of the liquid channel in the welding direction, and the capillary instability. The first two factors impede the backfilling of molten metal, and make the liquid channel susceptible to premature solidification, the final factor makes the weld pool unstable and susceptible to collapse. In the twin wire GMAW process, these factors are suppressed, in order to obtain sound weld bead, the trailing wire current should not be larger than the leading wire current. [ABSTRACT FROM AUTHOR]

In this research, the thicknesses of Fe 2 Al 5 sub-layer ( l Fe2Al5 ) in bead welds were roughly calculated according to the formula l = Kt 1/2 . The values of constant K and residence time t were obtained through recording welding pools temperature. Fe 2 Al 5 sub-layers were also observed and their thicknesses were measured by SEM and EDS analysis. The calculated results were in close agreement with measured results, demonstrating the accuracy of these calculated Fe 2 Al 5 sub-layer thicknesses. [ABSTRACT FROM AUTHOR]

Three different spectroscopic methods were used to calculate the temperature and composition distribution of argon–helium arc plasma—the Fowler–Milne method, the two-line intensity correlation method and the Boltzmann plot method. Experimental errors, including random errors and systematic errors, were analyzed in detail to comparing the accuracy of different methods. Due to the large differences of physical characteristics between argon and helium, there were limited reports on the measurement of temperature and composition distribution in argon–helium arc plasma. To this end, The Fowler–Milne method and the Boltzmann plot method were modified in this paper. Three spectroscopic methods were compared with other׳s simulation result and showed good agreement with each other, except the Boltzmann plot method which had partly distinction. Through comparison and analysis of error bar in those methods, it was found that both the Fowler–Milne method and the two-line intensity correlation method had less error than the Boltzmann plot method, while the Fowler–Milne method, which is irrelevant to atomic transition probabilities and experimental apparatus calibration, had the minimum error. [ABSTRACT FROM AUTHOR]

Abstract: During laser–arc hybrid welding, the welding direction exerts direct effects on the plasma properties, the transient behavior of the droplet, the weld pool behavior, and the temperature field. Ultimately, it will affect the welding process and the weld quality. However, the behavior of the CO2 laser+GMAW-P hybrid welding process has not been systematically studied. In this paper, the current–voltage characteristics of different welding processes were analyzed and compared. The dynamics of the droplet transfer, the plasma behavior, and the weld pool behavior were observed by using two high-speed camera systems. Moreover, an optical emission spectroscopy was applied to analyze the plasma temperature and the electron number density. The results indicated that the electrical resistance of the arc plasma reduced in the laser leading mode. For the same pulse duration, the metal transfer mode was the spray type with the laser leading arrangement. The temperature and electron density distribution showed bimodal behavior in the case of arc leading mode, while this phenomenon does not exist in the caser of laser leading mode. The double elliptic-planar distribution which conventional simulation process used was not applicable in the laser leading mode. [Copyright &y& Elsevier]

Abstract: The metal transfer in CO2 Laser+GMAW-P hybrid welding by using argon–helium mixtures was investigated and the effect of the laser on the mental transfer is discussed. A 650nm laser, in conjunction with the shadow graph technique, is used to observe the metal transfer process. In order to analyze the heat input to the droplet and the droplet internal current line distribution. An optical emission spectroscopy system was employed to estimate default parameter and optimized plasma temperature, electron number densities distribution. The results indicate that the CO2 plasma plume have a significant impact to the electrode melting, droplet formation, detachment, impingement onto the workpiece and weld morphology. Since the current distribution direction flow changes to the keyhole, to obtain a metal transfer mode of one droplet per pulse, the welding parameters should be adjusted to a higher pulse time (TP) and a lower voltage. [Copyright &y& Elsevier]

Fe 3 Al based alloys have been continuously attractive to the fossil energy industry due to the excellent high temperature oxidation resistance and strength, light weight and low materials cost. In recent years, to overcome the fabrication difficulty of Fe 3 Al based alloys and reduce the room temperature induced high manufacturing cost, the twin wire-arc additive manufacturing (WAAM) technique is invented. During the post-production heat treatment aiming to modify the as-fabricated microstructures, the Al concentration induced precipitates are reported in the previous research, which can effectively refine the initial large grains and weaken the WAAM induced microstructural anisotropy. Therefore, the further characterization on the precipitate is meaningful for the development of the innovative WAAM in the Fe 3 Al fabrication. In the present research, to further characterize such Al concentration induced precipitates and find the functioning mechanism on the material properties, the precipitates are modified using various heat treatments and subsequently characterized in detail using microscopies and room temperature tensile test. According to the results, the Al concentration induced precipitates are characterized as the long-range ordered D0 3 structured Fe 3 Al superlattice. And the generation of such superlattice precipitates in the Fe 3 Al matrix performs dislocation pinning effect thus improves the alloy strength. However, the strengthening by D0 3 superlattice precipitates cannot compensate the tensile property anisotropy, thus the grain refinement is still the key to break down the large columnar grains and further improve the mechanical properties. • Fe 3 Al D0 3 superlattice precipitates are confirmed in WAAM produced Fe 30Al alloy. • D0 3 superlattice precipitates strengthen the alloy by dislocation pinning. • Heat treatments show little effect on the property anisotropy of WAAM made Fe 30Al. • Fracture morphology variation of Fe 30Al alloy is found by the heat treatment. [ABSTRACT FROM AUTHOR]

The further strengthening of the Al/steel dissimilar joint is significantly meaningful for car lightweight development in automotive industry. It has been experimentally proved that the use of Si containing aluminum alloy filler material such as ER4043 grade welding wire can effectively improve the joint strength by reducing the Fe Al intermetallic interface thickness. However, the specific functioning mechanism of Si has not been clarified explicitly thus controversy still exists. In the present research, an Al/steel dissimilar joint is generated using the variable polarity cold metal transfer (VP-CMT) technique. To address the suppression function of Si on the Fe Al intermetallic interface growth clearly, the focused ion beam (FIB) is conducted to extract the testing sample right at the interface, which is subsequently investigated under TEM. According to the obtained results, the intermetallics formed at the Fe Al interface are generally Fe 4 Al 13 phase on the aluminum side and Fe 2 Al 5 phase adjacent to the steel side. Also, rod-shaped Fe 5 Si 3 precipitates are confirmed in the Fe 2 Al 5 matrix near the steel side, rather than Si solid solutions or ternary Fe-Al-Si phases stated in the previous researches. As indicated by the dislocation distribution in the Fe 2 Al 5 matrix, the generation of Fe 5 Si 3 precipitates considerably suppresses the brittle Fe 2 Al 5 fracture. Such phenomenon clarifies the Si element functioning mechanism of improving the Al/steel dissimilar joint quality. • Focused ion beam is conducted to extract the Fe Al interface from the joint. • Existence state of Si in Fe Al interface is confirmed to be Fe 5 Si 3 precipitate. • Fe 5 Si 3 is the key to suppress intermetallic interface growth of Al/steel joint. • Fe 5 Si 3 can suppress the Fe 2 Al 5 matrix fracture by precipitate strengthening. [ABSTRACT FROM AUTHOR]

• Metal evaporation flux across Knudsen layer is discussed in laser keyhole welding; • A high-gas pressure state is indicated at the keyhole bottom; • Evaporation flux decreases with the keyhole depth due to the high-gas pressure; • The decrease of evaporation flux suppresses the element loss in the fusion zone. This study aims to calculate the metal evaporation flux across the Knudsen layer inside the keyhole in laser welding of Al–Mg alloys. The pressure balance at the keyhole wall was analysed and used as the boundary condition in the calculation. Based on the extracted keyhole shape parameters from high-speed images, the surface temperature and gas pressure inside the keyhole were calculated, and the evaporation fluxes for both Al and Mg were acquired. The results show that the front wall of the keyhole absorbs more laser energy than the rear wall. The gas pressure inside the keyhole increases and the evaporation flux decreases with increasing keyhole depth. In addition, the evaporation flux of Mg is much higher than that of the base element Al, which can result in the loss of Mg inside the melt pool. The distribution of the Mg evaporation flux inside the keyhole agrees with the measured Mg loss in the welded section of the Al–Mg alloy. Thus, the calculation method of the metal evaporation flux inside the keyhole can be applied in future simulations of element loss in laser keyhole welding. [ABSTRACT FROM AUTHOR]

In the present research an Fe–Fe 3 Ni functionally graded material has been fabricated using an innovative wire-arc additive manufacturing process. Considering the residual bcc -α-Fe in the bulk material, a homogenization heat treatment is conducted to the as-fabricated alloy to transform the residual bcc lattice into fcc structure. During the entire heat treatment, the phase evolution process in Fe–Fe 3 Ni functionally graded material is in-situ characterized using the high-intensity neutron diffraction instrument WOMBAT, therefore the temperature profile is set linear to accurately extract phase volume fractions and thermal lattice strains in the specific regions. According to the results, the as-fabricated functionally graded material contains both fcc -Fe 3 Ni and bcc -α-Fe, while after heat treatment the bcc -α-Fe is dissolved into the Fe 3 Ni matrix and leads to lower hardness in the alloy. Also, the detected phase evolutions are mainly performed by the low Ni content section of the Fe–Fe 3 Ni functionally graded material. In addition, compared to the bcc lattice in the as-fabricated functionally graded material, the dissolved Fe more considerably restraint the Fe 3 Ni lattice deformation thus a much lower thermal expansion coefficient is measured from cooling than the heating process. • Neutrons are conducted in-situ to the phase evolutions in Fe–Fe 3 Ni FGM. • bcc -α-Fe phase in the as-fabricated Fe–Fe 3 Ni FGM are dissolved by the heat treatment. • Responses of bcc-α-Fe and fcc-Fe 3 Ni phase lattice strains to temperature are calculated. [ABSTRACT FROM AUTHOR]

To suppress the amount of brittle Ti Fe intermetallics in Ti-6Al-4V/304L joints, this study adopts the variable polarity cold metal transfer arc-brazing method using ERCuSi-A wires. The effects of the thermal distribution strategy on the welding process, penetration ratio, microstructure, and tensile strength are investigated by varying the electrode positive/electrode negative (EP/EN) ratio. With the ratio decreasing, more energy is conducted to the wire, thereby decreasing the high-temperature holding time. When the ratio decreases from 8:1 to 1:8, the thickness of the Ti-6Al-4V/seam transition zone containing a Ti Cu interlayer and Ti 5 Si 3 intermetallic layer decreases from 118 to 81 μm. Along with the decrease in the interlayer thickness, the joint tensile strength initially increases from 216.28 to 261.69 MPa and then decreases to 211.29 MPa. A high EP/EN ratio results in a thick Ti-6Al-4V/seam transition zone that includes a Ti 5 Si 3 layer. A low EP/EN ratio results in a thin Ti-6Al-4V/seam transition zone that includes Ti 5 Si 3 distributed in the grain boundary. Both conditions reduce the tensile strength of the joints. The highest tensile strength is obtained when the ratio is 8:8, under which a thin transition zone and concentrated Ti 5 Si 3 intermetallic layer can be generated. Unlabelled Image • More electrode negative cycle leads to less thermal applied on the base metal. • Ti Fe intermetallics are suppressed due to extremely low base metal dilution ratio. • The formation of a purified Ti Cu layer is owing to the consumption of Ti 5 Si 3. [ABSTRACT FROM AUTHOR]

• A novel fully coupled plasma arc-keyhole-weld pool model is developed for the KPAW process in the quasi-steady state. • The coupled interaction between the forces and energy propagation is revealed. • The energy efficiency in the KPAW process is calculated and analyzed. The distribution of the energy in the keyhole plasma arc welding process has significant effect on the final weld quality. In this study, a novel fully coupled plasma arc-keyhole-weld pool model is built to study the energy propagation in the keyhole plasma arc welding process. The numerical results show that: the conduction energy flux, electric energy flux and total energy flux from the arc to the top weld pool are larger than these from the arc to the bottom weld pool. Under the influence of the arc pressure and plasma shear stress, two dominant convective eddies exist inside the weld pool, and the molten metal flows backward at the top and bottom surfaces, so the energy is not accumulated on the top surface, but transported to the rear weld pool together with the molten metal. The conduction energy from the arc to the anode is much higher than the thermionic heating. The penetrated keyhole formation can't increase the energy absorption, and 11.5% of the arc energy are lost by the efflux arc. The calculated energy efficiency for heating the anode in this study is only 60.7%. [ABSTRACT FROM AUTHOR]

In recent years, the twin wire arc additive manufacturing technique has been proved feasible of fabricating titanium aluminides with low cost. To address the concerns on the low thermal efficiency induced by the previous gas tungsten arc welding-based process, in the present research the twin-wire plasma arc additive manufacturing system is developed using the compressed plasma arc power. Subsequently, the newly developed system is used to fabricate the binary Ti–45Al titanium aluminide to verify the feasibility. And the analysis on microstructure and mechanical properties is performed afterwards. Generally, the titanium aluminide with target composition and low texture level has been successfully fabricated using the compressed plasma arc system. Compared to other titanium aluminides fabricated using the previous system, the layer band microstructure is considerably weakened. However, the intergranular cracks in the as-fabricated deposition indicate that higher interpass temperature is certainly required for the new technique. The improvement of the microstructural homogeneity achieved by the new system provides valuable reference for weakening the microstructural and mechanical anisotropies in the wire arc additively manufactured titanium aluminides. • The Ti–45 A l alloy is successfully fabricated by TW-PAAM system. • The microstructure evolution process of Ti–45 A l alloy is detailedly analyzed. • The reasons for intergranular cracks generation are investigated. • The weak anisotropy of mechanical properties are found and discussed. [ABSTRACT FROM AUTHOR]

The microstructure of TiAl alloy is extremely sensitive to the thermal history. During additive manufacturing, multiple thermal cycles and partial remelting of the TiAl deposition layers take place. Therefore, the heat input would exert significant influences on microstructural and mechanical characteristics of additively manufactured TiAl alloys. In this study, TiAl alloys were prepared successfully by twin-wire directed energy deposition-arc (TW-DED-arc). Systematic studies were conducted to regulate the microstructure characteristic and the mechanical property of the as-deposited TiAl alloys via the line energy. The results indicate strong effects of the line energy on the microstructure and performance of the TiAl alloys, which show alternative distribution of layer-band like microstructure with the dendritic grains and the fully lamellar colonies along the deposition direction. With the enhancement in the line energy, the layer-band like feature gradually weakens, and the fully lamellar colonies increase. In addition, the discontinuous coarsen phenomenon and the discontinuous dynamic recrystallization occurred, and the anneal twin was also formed. On the other hand, tensile fractures were observed in the dendritic grains region, and the maximum tensile strength was achieved with the medium line energy. However, the elongation and microhardness are proportional to the line energy. With the elevation in the line energy, the elongation and microhardness gradually increases and decreases, respectively. The microstructure formation mechanism of as-fabricated TiAl alloy is analyzed systematically, and the fracture mechanics was also discussed in detail. The findings of this research contribute not only to the methodology for fabrication of TiAl alloys with the novel TW-DED-arc process, but also to the deep understanding on the regulation of the microstructural and mechanical characteristics of the additively manufactured TiAl alloys. • The TiAl alloys are fabricated successfully by twin-wire directed energy deposition-arc. • The microstructure evolution mechanism is analyzed systematically for as-deposited TiAl alloy. • The relationships are established in detail among microstructure, properties and line energy for as-deposited TiAl alloy. • The microstructure and mechanical properties are regulated successfully by the line energy. • The tensile fracture mechanism is also revealed clearly for as-deposited TiAl alloy. [ABSTRACT FROM AUTHOR]

[Display omitted] • Bonding mechanism at Ag-Cu interfaces shifted from solid-state diffusion to brazing with increasing welding voltage. • Brazing-based bonding enhanced interfaces by eliminating micro gaps and building nano-scale interlocking structures. • Excessive Ag-Cu solid solution formation at interfaces led to degradation in electrical conductivity of joints. • High-strength and high-conductivity joints were obtained by controlling interfacial melting and alloying. In space flexible solar arrays, stranded Ag-plated Cu conductors and Ag interconnectors are preferred materials for energy transmission. Parallel gap resistance welding is a solderless micro-joining process that favors the reliability of joints between stranded Ag-plated Cu conductors and Ag interconnectors. It was found the interfacial microstructure presented diversity and complexity while mechanical and electrical properties showed unusual nonlinear variations with welding voltage. This study aims to clarify the interfacial microstructure evolution and its effects on the joint properties while elucidating the microstructure formation mechanisms. A shift in the bonding mechanism at interfaces from solid-state diffusion to brazing was found as welding voltage reached a critical value (1.5 V). It eliminated micro gaps and built nano-scale interlocking structures, resulting in substantial improvement in mechanical properties. Increasing the welding voltage from 1.2 V to 1.4 V improved electrical conductivity due to the enlarged bonding area at interfaces. However, high welding voltage (1.6 V) led to degradation in the electrical conductivity of joints due to excessive Ag-Cu solid solution formed at interfaces. The key to fabricating high-strength and high-conductivity joints lies in achieving appropriate interfacial melting while reducing alloying by controlling peak temperature and shortening the duration above the Ag-Cu eutectic point. [ABSTRACT FROM AUTHOR]

Conventional fabricating methods of TA2–304 SS laminated metal composite (LMC) (e.g., hot rolling, hot stamping, and explosive welding) have difficulty in removing brittle Ti Fe intermetallic compounds (IMCs) and are not suitable for the preparation of the small size structure components or transition joints. Additive manufacturing of LMCs has the potential to achieve the desired microstructure, properties, and structure. In this study, TA2–304 SS LMC, which has been extensively employed in pipe flanges of nuclear plants and bows of sonar dome, was well fabricated by utilizing directed energy deposition-arc (DED-arc) and copper‑nickel alloy transition interlayers, and the formation of brittle Ti Fe IMCs was suppressed. The results of this study indicated that the standard deviation of the height of the surface morphology reconstructed by adopting Python VTK technique was evaluated as 0.67. Furthermore, copper‑nickel transition interlayers possessed the capability of isolating Ti from Fe atoms through the formation of laminar-distributed microstructure. Specifically, the Interlayer1 was comprised of (Fe, Cr) grains with different shapes and small-size (Cu) equiaxial grains, Interlayer2 covered large-size (Cu, Ni) dendritic grains and intergranular (Cu), and the TA2 layer comprised α-Ti and Ti 2 Cu. The result also showed that thermal cycle-induced recovery and recrystallization and diffusion-induced elemental interaction accounted for the different microstructures. Moreover, the interface between the TA2 and Interlayer2 layers contained TiCu 4 + (Cu), Ti 3 Cu 4 + TiCu 4 , Ti 2 Cu + TiCu, and TiCu + Ti 2 Cu eutectoid microstructures, which resulted in the highest hardness of 450Hv as well as crack initiation and extension. Furthermore, due to the uniform-distribution of microstructure in the absence of brittle IMCs, the average shear strength was 234.0 MPa, which reached the strength of the conventional method and conformed to the standard requirements. Multiple groups of galvanic reactions between different transition materials can result in reduced corrosion resistance, and the oxidation-reduction reactions can lead to the formation of massive oxides. • TA2–304 SS LMC can be made by using DED-arc and Cu-based deposition interlayer. • Ti Fe IMCs can be removed by using chemical alloying and physical isolation. • Massive equiaxial grains will be formed due to the deposition thermal cycle. • The compress-shear strength conforms to the standard requirements. [ABSTRACT FROM AUTHOR]

The relationship between the microstructures and cryogenic mechanical properties of the dissimilar laser welded joint between ultra-low carbon 5% Ni steel (SA645) and 304L stainless steel (AISI304L) was investigated to identify the crucial microstructural characteristics determining the low-temperature toughness of the joint. The microstructural constituents of the weld metal were changed by varying the laser beam offset towards AISI304L side. The low-temperature (−120 °C) impact energy of the weld without beam offset which consisted of full lath martensite was only 10.22 J. High-density tangled dislocations in lath martensite resulted in the embrittlement of the weld metal. With increase in the offset of laser beam, the cryogenic toughness of the weld with a dual microstructure of lath martensite and retained austenite (RA) was enhanced more than seven times (72.16 J). Areas with relatively low strain/stress caused by the strain-induced martensite transformation of RA were observed near the crack propagation path, which deflected crack propagation path and increased the resistance of crack propagation. However, the weld metal with a triplex microstructure of martensite, RA and δ-ferrite exhibited a lower cryogenic impact toughness (37.15 J) despite the high volume fraction of RA when the beam offset was high. The thermal stability of RA decreased significantly owing to the weakening of restrain effect of surrounding martensite. The improvement effect of RA on cryogenic toughness was attenuated due to the decreased thermal stability of RA and the occurrence of δ-ferrite. Research results indicate that the volume fraction and stability of RA and the existence of δ-ferrite with increasing the laser beam offset were confirmed as the pivotal microstructural factors determining the cryogenic impact toughness. • The cryogenic impact toughness of the weld was enhanced by the strain-induced martensite transformation of RA. • The restrain effect of surrounding martensite played a dominant role in the thermal stability of RA. • The content and thermal stability of RA were the pivotal microstructural factors determining cryogenic toughness of the weld. • The existence of δ-ferrite was also crucial factor affecting cryogenic toughness of the weld when beam offset was high. [ABSTRACT FROM AUTHOR]