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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]

Abstract Wire feed speeds of 3.5, 4.5, and 5.5 m/min and offset positions of 1 and 2 were employed for this study with an ERCuSi-A weld wire. The microstructures of the joints, which include a Cu/Ti interface layer consisting of Ti 2 Cu, TiCu, and AlCu 2 Ti, a Cu-matrix seam consisting of Cu and petal-shaped Fe-Si-Ti intermetallics, and a Cu/Fe interface layer consisting of α-Fe and Cu, were studied. The formation enthalpy calculated from the Miedema model can explained the microstructure evolution mechanism. The interface thickness and ultimate tensile strength were found to increase with wire feed speed. The highest tensile strength of the joint was 294 MPa, fracturing at the Cu/Ti interface. Offsetting the welding torch to the TC4 side increased the amount and size of the Fe-Si-Ti intermetallics, degrading the tensile strength. Four fracture modes were proposed to differentiate the crack propagations in the joints, which were determined by the interfacial bonding strength and the Fe-Si-Ti intermetallics in the weld seam. [ABSTRACT FROM AUTHOR]

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]

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]

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 instability of the keyhole and bubble being captured by solidification interface are the main factors that affect the formation of porosity in laser welding of both aluminum alloy and steel. In laser welding of aluminum alloy, the more violent fluid flow behind the keyhole, the larger depth to width ratio can cause the keyhole more unstable, and susceptive to collapse that can be described as three necessary steps. The much larger solidification rate and longer escaping distance make the bubble easily captured by the solidification interface, and easily combined. All of these reasons contribute to the formation of larger number and size of porosity in laser welding of aluminum alloy. The numerical and experimental results are in good agreement. [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]

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]

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]

Shrimp is a very important source of protein which is patronized by almost half of the world's population, and hence a very important specie in aquaculture. There is the need for increase in shrimp production worldwide to meet consumer demands. However, shrimp production is hampered by high cost of commercial feeds. Increase in prices of fish oil and fish meal has led to calls for their substitute. This calls for substitute has resulted in researchers studying the nutritional requirement of shrimp. The rationale for this article is to review the literature available on recent studies toward the development of practical diets for shrimps focusing on the nutrients required by different species qualitatively as well as quantitatively. This review highlights on nutrient requirements with respect to growth and feed utilization. Digestibility of nutrients used in shrimp diets is also accounted for in this article. [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]

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 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]

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]

The pre-welding condition of the base metal, the welding heat input, and the state of the interlayer in multi-layer welding are examined to identify means of controlling the hot cracking in Inconel-718 cast sheets during tungsten inert gas (TIG) welding. A finite element method (FEM) incorporating the cast microstructure and alloy chemical composition was used to calculate the welding stresses and predict crack initiation. The crater crack, weld-seam center longitudinal solidification crack, and the weld toe compound crack mixed with the transversal solidification crack and the heat-affected zone (HAZ) liquation crack are discussed. A pre-welding homogenization heat treatment, lower welding heat input, and post-welding crater grinding of each individual welding seam can decrease its hot cracking susceptibility. [ABSTRACT FROM AUTHOR]

• Fe 2 Al 5 is the main factor influencing the Fe-Al IMC interface thickness. • Fe 2 Al 5 without Si has obvious (0 0 1) preferential orientation tendency. • Established Fe 2 Al 5 thickness prediction model with calculation error under 10 %. • Si addition is more effective in controlling Fe 2 Al 5 growth compared to energy. Further development of the car lightweight in automotive industry significantly depends on the reliability of the Al/steel dissimilar joint, since more and more Al alloy parts are used in the car manufacturing. It has been proved that the Al/steel dissimilar joint strength can be effectively increased when the intermetallic compound (IMC) growth, especially the Fe 2 Al 5 layer thickness, is controlled at certain level. Because the Fe 2 Al 5 is the dominant phase of the entire IMC layer in the Al/steel dissimilar joint, and cracks usually initiate at this location when damage is conducted. Therefore, for the automotive industry assembly line, the prediction model monitoring the Fe 2 Al 5 layer thickness of each Al/steel weld seam is desired to ensure the dissimilar joint quality and reliability. In the present research, a numerical dynamic growth model based on element diffusion laws is established to calculate the IMC layer thickness of Fe 2 Al 5 phase during the variable polarity cold metal transfer (VP-CMT) welding of aluminum to steel in the function of welding heat input and location. To build up effective connection between the welding heat input and the actual temperature history of Al/steel interface, a finite element analysis (FEA) model is customized for the present VP-CMT process. The Fe 2 Al 5 layer thickness calculation results are further validated using electron back-scattered diffraction (EBSD) characterization under scanning electron microscopy (SEM), confirming a 2.3 % deviation between the model prediction and actual layer thickness. [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]

Wrought, as-cast, and homogenized sheets were welded using the tungsten inert gas (TIG) process. The precipitates in both the base metal and the coarse-grained heat-affected zone (CGHAZ) for each joint were analyzed by optical microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). Statistical data regarding the precipitates were also obtained from metallographs. With the alloy element contents determined by EDS and the phase diagram obtained using ThermoCalc, the diffusion of Nb and the dissolution of carbides and the Laves phase were analyzed. The wrought base metal contained massive M 23 C 6 particles and MC blocks. As the heat input increased, the amount of carbides in the CGHAZ decreased. No Laves phase was discovered in the base metal or CGHAZ. In the as-cast or homogenized base metal, MC blocks and the Laves phase were distributed in the interdendritic segregated region. As the heat input increased, the MC and Laves content did not change significantly. The prewelding heat treatment in the homogenization process reduced the amount of precipitates in the base metal. The CGHAZ of the joint of the homogenized metal lost more dendritic structure than did the CGHAZ of the joint of the as-cast metal. [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]

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]

Abstract: 5052 aluminum alloy sheets and galvanized mild steel sheets were joined in lap configuration by alternate-current double pulse gas metal arc welding with pure Al, Al–5Si, Al–12Si and Al–4.5Mg (wt%) filler wires. The effect of alloying elements on the microstructure of intermetallic compounds (IMC) layers formed between weld seam and steel, and tensile strength of the resultant joints were investigated. The thickness of IMC layer in all samples varied along the cross-section of the joint, the intermediate part of the IMC layer was thicker than the head and root parts. The diffusion of Si into Fe2Al5 sub-layer could restrain the growth of Fe2Al5 sub-layer and IMC layer, and joint's mechanical property improved with the increasing Si content in Fe2Al5 phase. Due to the high hot crack sensitivity of Al–4.5Mg alloy, cracks generated at the root of joint made with Al–4.5Mg filler, resulting in poor mechanical property. [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]

Abstract: 5052 Al alloy sheets and galvanized mild steel sheets were joined by gas metal arc welding with three different current modes, including direct-current pulse gas metal arc welding (DPG), alternate-current pulse gas metal arc welding (APG) and alternate-current double pulse gas metal arc welding (ADG). The effect of current mode on size, distribution and volume fraction of pores generated in Fe–Al dissimilar materials lap joint was quantitatively studied. EDS result showed that pores in Fe–Al joint were mainly caused by trapped zinc metal vapor from galvanized steel. Volume fraction of pores in joints made by APG and ADG processes was larger than that in joint resulted from DPG process. Moreover, pores in joints made by APG and ADG processes had smaller diameter, and tended to distribute in the middle of the weld seam. On the contrary, pores with large diameter were inclined to distribute close to upper weld surface of the joint resulted from DPG process. These results are attributed to the difference of arc stirring force and linear heat input in these three processes caused by different current modes. [Copyright &y& Elsevier]

Abstract: This paper studies Al–steel lap joints resulted from direct-current pulsed gas metal arc welding (DPG) and alternate-current double-pulse gas metal arc welding (ADG). Base metals being welded are 1mm thick 5052 Al alloy sheet and 1mm thick galvanized mild steel sheet with filler metal being 1.2mm 4047. Effect of the different welding processes on joint properties is studied through characterization of the steel/Al interphases and mechanical strength. It is found that the thickness of intermetallic compound layer composed of Fe2Al5, FeAl3and Fe3Al, and the diffusion of Fe element from steel to weld seam are dependent on the amount of welding heat input. Lap shear samples of Al–Fe joints resulted from the ADG process fractured at Al base metal or weld seam with average ultimate tensile strength (UTS) of 201MPa. On the other hand, all lap shear samples of the joints resulted from DPG process fractured along the interphase between the Fe2Al5 sub-layer and the steel sheet with average UTS of 115MPa. ADG process offers higher joint strength than that offered by DPG process due to thinner intermetallic compounds layer caused by the reduced heat input, illustrating that cyclic polarity switching and a secondary low frequency current pulse can effectively reduce the heat input in joining Fe–Al dissimilar materials, which means the ADG process is much more feasible for making robust Fe–Al lap joints. [Copyright &y& Elsevier]

This study evaluated the effect of dietary rapeseed meal (RM) and hydrolyzable tannin on grass carp (Ctenopharyngodon idella) and determined the possible role of tannin on health when RM was added to the diet. Eight diets were formulated. Four were semipurified-diets with 0, 0.75, 1.25, and 1.75% hydrolyzable tannin (T0, T1, T2, and T3), and the other four were practical diets containing 0, 30, 50, and 70% RM (R0, R30, R50, and R70), which had similar tannin contents as semipurified-diets. After the 56 d feeding trial, the antioxidative enzymes and relative biochemical indexes showed a similar tendency in practical and semipurified groups. In hepatopancreas, superoxide dismutase (SOD) and catalase (CAT) activities increased with RM and tannin levels, respectively, while glutathione (GSH) content and glutathione peroxidase (GPx) activity increased. Malondialdehyde (MDA) content increased and decreased in T3 and R70, respectively. In the intestine, MDA content and SOD activity increased with RM and tannin levels, while GSH content and GPx activity decreased. The expression levels of interleukin 8 (IL-8) and interleukin 10 (IL-10) were upregulated with RM and tannin levels, and the Kelch-like ECH-associated protein 1 (Keap1) expression was upregulated in T3, whereas it was downregulated in R50. This study demonstrated that 50% of RM and 0.75% of tannin induced oxidative stress, injured hepatic antioxidant ability, and resulted in intestinal inflammation in grass carp. Therefore, the tannin in rapeseed meal cannot be neglected in aquatic feeding. [ABSTRACT FROM AUTHOR]

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]

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]

• 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 this paper, the microstructure optimization and mechanical property of joints are studied when the Cu, CuSi3, and CuNi10 wires are used to Cold Metal Transfer arc-braze TC4 to 304L. Based on the microstructure analysis and thermodynamic calculation, the existing state of Ti and Fe atoms are transferred from high brittleness Ti-Fe to less brittle Ti-Fe-Si intermetallics when the welding wire is changed from Cu to CuSi3 due to the lower Gibbs free energy of Ti-Fe-Si intermetallics. Furthermore, the (Fe, Ni) solution and Ti-Ni intermetallics with the lowest hardness are formed when CuNi10 wire is used. Thus, three Ti-Fe suppression mechanisms are concluded: suppressed by forming intermetallics and diffusion inhibition, suppressed by forming intermetallics, and suppressed by forming intermetallics and solution. Based on the microstructure optimization, the max hardness of intermetallics layer decreases from 769 to 654 Hv and the max tensile strength increases from 186.4 to 319.4 MPa. However, all cracks of the joints with different wires will initiate from the bottom of the seam and propagate in the TC4/seam transition zone due to the high Schmid factor. Besides, the crack propagation rate of more than 200 m/s can be in-situ measured by high-speed camera with 200000 fps. • Si addition can transfer Ti-Fe into Ti-Fe-Si intermetallics. • Ni addition can transfer Ti-Fe into Ti-Ni intermetallics. • The crack initiation and propagation are in-situ observed by High speed camera. [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]

• The initial and complete cleaning thresholds of 2 μm oxide layer were 12.7 J/cm2 and 25.5 J/cm2, respectively. • Thermal stress coupling effect led to the separation of substrate and oxide layer. • Phase explosion and impact effect led to the splattering and removal of oxide. • Energy density affects plasma characteristic by changing plasma temperature and electron density. Through comparing the cleaning effect of aluminum alloy at different energy densities, the model of materials thermodynamics was established, illuminating the cleaning mechanism and plasma behavior. The research showed laser cleaning could remove oxide layer completely. The initial cleaning threshold was 12.7 J/cm2 and complete cleaning threshold was 25.5 J/cm2. At low energy density, phase explosion caused by laser ablation was the main cleaning mechanism. At high energy density, besides phase explosion, impact effect induced by evaporation pressure caused the splattering and removal of oxide layer. Plasma lifespan had positive correlation with energy density. Its lifespan in 25.5 J/cm2 and 51.0 J/cm2 were about 6 μs and above 10 μs, respectively. The model of materials thermodynamics showed that at low energy density, substrate evaporation pushed out molten oxide layer and formed pulse craters. At high energy density, transient energy absorption caused thermal stress coupling effect and separated substrate with oxide layer. The impact effect induced by evaporation pressure resulted in the removal of oxide layer. The vapor of Al absorbed laser energy and formed plasma. The high energy density enhanced the laser-plasma coupling. It caused high temperature and intensive electron density. The time of plasma cooling and atomization became longer. [ABSTRACT FROM AUTHOR]

Magnetic intensities from 0 to 18.0 mT and CuNi10 wire were employed in Cold Metal Transfer (CMT) joining of Ti6Al4V to 304 L stainless steel. The axial external magnetic field (EMF) could improve the wetting behavior and modify the microstructure of arc-brazed joints. A rotated bell-shaped arc was formed owing to the Lorentz force acting during the welding process, resulting in short arc length, wide arc width, and low temperature of the melting pool. The liquid metal in the melting pool exhibited an incomplete circular motion on the surface owing to the pulsed plasma shear stress and Lorentz force caused by the radial current and axial magnetic field. With increasing magnetic intensity, the inner layer containing β + π + Ф (NiTi + NiTi 2 +CuTi) and π + ψ + Ф (NiTi 2 + CuTi 2 + CuTi) and outer layer containing γ + Δ ((Cu, Ni) + TiNiCu) became thinner and more uniform because of the intense flow and short reaction time at high temperature. The microstructure on the 304 L side of the joints changed from columnar dendrites to coarsened grains. With increasing magnetic intensity, the tensile strength of the joints increased from 270.7 to 387.8 MPa. All the samples fractured at the Ti6Al4V/seam interfacial layer of the joints. [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]

A three-dimensional numerical model that coupled the arc, keyhole and weld pool was developed to investigate the keyhole behavior and energy transfer of an aluminum alloy in a variable polarity plasma arc welding process. The convective pattern in the weld pool was measured using an advanced x-ray transmission system. The elemental distribution of the bead was determined using an electron-probe microanalyzer. The coupled mechanisms of the keyhole, energy transfer, and compositional change were revealed. With a keyhole, both the arc energy loss and energy input increase, while the total energy transported from the arc to the weld pool decreases a bit, so the arc energy efficiency decreases. The heat flux at the keyhole is low and decentralized, resulting in low keyhole temperature; thus, the loss of elements with low boiling points due to evaporation can be ignored. Elements can be sufficiently mixed and distributed uniformly throughout the bead. [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]

• 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]

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]

Carbon-steel filler wire was used to solve the coating problem in laser welded blanks of Al-Si-coated 22MnB5 without a decoating procedure. The effect of the filler wire on the fusion-zone microstructure and mechanical properties was investigated. The Al content was diluted by the filler wire. The δ-ferrite phase was also reduced after the reduction in Al and homogenization. The joint strength was ∼1210 MPa, and the elongation was 1.1% without filler wire. When filler wire was used, the joint strength increased to ∼1550 MPa, and the elongation was greater than 3%. The fracture surface of the joint with the filler wire was ductile. The effects of welding speed and feeding speed were also studied. A reduction in the Al content at the higher feeding speed or the use of a slower welding speed was found to be most effective because either approach enabled more filler material to be added to lower the ratio of the Al-Si coating in the fusion zone. In the absence of the filler wire, the δ-ferrite phases were large and long, connecting to a bulk zone. When the filler wire was used, the δ-ferrite phases were broken up and became narrow and small with a uniform distribution in the martensite matrix. [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]

Due to low density and good high temperature performance, titanium aluminide has been continuously attractive for aerospace and automotive industries. In recent years, additive manufacturing of titanium aluminide has become a popular research area. The present work has first time fabricated commercial graded TiAl-4822 alloy using an innovative twin-wire directed energy deposition-arc (TW-DED-arc) process, which uses a unique twin-wire feeding method to produce the target alloy in-situ. According to experimental results, TW-DED-arc fabricated TiAl-4822 alloy possesses direct full density, homogeneous chemical composition, uniformed microstructure, and stable mechanical property. Also, the formation mechanism of layered microstructure consisting of alternative columnar and equiaxed grain regions has been investigated in detail. Even under as-fabricated condition, TW-DED-arc fabricated TiAl-4822 performs competitive tensile properties compared to the same alloy produced by casting and other additive manufacturing methods. Generally, successful TW-DED-arc buildup of TiAl-4822 provides a high-efficiency and low-cost route for the further development of titanium aluminide production. [ABSTRACT FROM AUTHOR]

Mechanical and electrical properties of joints between stranded Ag-plated Cu conductors and Ag-plated Kovar interconnectors are crucial for the reliability and energy efficiency of space solar arrays. Parallel gap resistance welding (PGRW) is a conventional approach for fabricating the welds. However, the PGRW involves two critical issues, namely underheated wire-interconnector interfaces and overheated wires, which narrowed the weld lobe and resulted in unsatisfying joint properties. In this study, an indirect resistance spot welding (IRSW) with a redesigned electrode system was developed to address the issues. By the IRSW, 80% enlargement in the weld lobe and 18% reduction in joint resistance were achieved. The two processes regarding interfacial microstructure and thermal behaviors were comparatively investigated to understand the fundamental mechanism behind the advancements. Compared with the PGRW, the IRSW elevated the temperature history at the wire-interconnector interface, which reinforced the interfacial bonding by transforming the sold-state diffusion bonding to brazing. The IRSW decreased the temperature of the upper wires while uniformizing the temperature among the multi-layered wires (e.g. the peak temperature gap between the upper and low wires was narrowed from 118℃ to 15℃), which inhibited the formation of high-resistivity Ag-Cu solid solution. The process also reduced the temperature at the heat-affected zone of wires, which suppressed the softening. The key to achieving high-quality micro welds between Ag-plated Cu conductors and Ag-plated Kovar interconnectors was to obtain brazing-based interface bonding while suppressing alloy behavior through uniformized electric and thermal fields. [Display omitted] • Indirect resistance spot welding (IRSW) with redesigned electrodes was developed. • IRSW reduced thermal damage at the electrode-wire interface and heat-affected zone. • IRSW facilitated change of interface bonding from solid state diffusion to brazing. • IRSW significantly expanded the welding window and reduced the joint resistance. [ABSTRACT FROM AUTHOR]

TiAl alloy becomes a promising high-temperature structural material due to excellent mechanical properties at elevated temperature. However, the inherent brittleness makes it difficult to be processed by traditional technologies. Therefore, an innovative twin-wire plasma arc additive manufacturing (TW-PAAM) process is developed to fabricate TiAl alloy. Substrate heating is indispensable during additively manufactured TiAl alloy, which can alleviate its crack tendency. In this paper, crack-free TiAl alloy samples are fabricated using TW-PAAM, and the effect of substrate temperature (560, 620 and 680 °C) on as-deposited TiAl alloy is investigated in detail. With the increase in substrate temperature, microstructural lamellar spacing and colony size exhibit the tendency of increase. Also, α2 phase content, recrystallization degree and high Schmid factor frequency present the decrease tendency. These variations of microstructure characteristics further lead to the decrease in both microhardness and tensile properties of the deposit. These findings provide a valuable reference for optimizing microstructures and mechanical properties of additively manufactured TiAl alloys. [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]