A tungsten inert gas (TIG)-cold metal transfer (CMT) hybrid welding process is proposed. Compared with the conventional TIG-metal inert gas/metal active gas (MIG/MAG) hybrid welding method, the characteristic of TIG-CMT welding process is that there is no interaction between the two arcs. The addition of TIG can improve the wettability of molten metal. The microstructure in hybrid joint is coarser than that in conventional CMT joint, and the microhardness in weld metal (WM) of hybrid joint is higher than that of conventional CMT joint. In TIG-CMT hybrid welding process, the TIG current plays an important role in changing contact angle and the dilution of weld bead. Fine equiaxed grains have been observed in partially melted zone (PMZ) and coarsened equiaxed grains exist in heat-affected zone (HAZ). The width of PMZ increases with the increase of heat input. More heat input can also lead to the coarser columnar dendrite. In all weld joints, the microhardness in HAZ remains at a low level, and the microhardness in PMZ always has a sharp decrease from WM to HAZ. The outcome of this work shows that the TIG-CMT hybrid process is suitable for the multi-passes welding of aluminum alloy.

The continuous development of duplex stainless steels (DSSs) is due to their excellent corrosion resistance in aggressive environments and their mechanical strength, which is usually twice of conventional austenitic stainless steels (ASSs). In this paper, a designed lean duplex stainless steel 2101, with the alloy design of reduced nickel content and increased additions of manganese and nitrogen, is studied by being partly compared with typical ASS 304L steels. The microstructure, mechanical properties, impact toughness, corrosion resistance and weldability of the designed DSS 2101 were conducted. The results demonstrated that both 2101 steel and its weldment show excellent mechanical properties, impact toughness and corrosion resistance, so DSS 2101 exhibits good comprehensive properties and can be used to replace 304L in numerous applications.

The cold metal transfer (CMT) with addition of pulses (CMT + P) process is a new CMT welding method. The CMT + P transfer mode is a combination of a projected transfer mode with one droplet per pulse and a short circuit transfer mode during the cold metal transfer period. The results indicate that the current and voltage waveforms of the CMT + P welding process were quite different from those of the traditional CMT process. The appearance of the high initial arcing current and the current pulse steps as well as the low current for the pulse time phase for the first CMT period indicate a stable, smooth CMT + P transfer process. The high pulse current of the CMT + P process results in a higher heat input compared to the CMT process. A greater penetration and contact angle of the weld bead can be obtained by increasing the pulse number.

By using scanning electron microscopy, energy-dispersive spectrometry, X-ray diffraction, strength and hardness measurements, the microstructure, precipitation, mechanical properties, and corrosion resistance have been investigated for two super ferritic stainless steels, 26Cr–3.5Mo–2Ni and 29Cr–3.5Mo–2Ni, with the aim to consider the effect of Cr content. The results showed that with the addition of Cr content, the recrystallization temperature was increased; the precipitation of Laves and Sigma (σ) phases was promoted and the mechanical properties of super ferritic stainless steel were modified. Furthermore, the pitting corrosion resistance and corrosion resistance to H2SO4 of the two super ferritic stainless steels were improved. In addition, suitable annealing processing is a key factor to maintain integrated performance by optimizing microstructure and removing detrimental precipitation phases.

•A newly designed 30Cr-4Mo-2Ni super ferritic stainless steel (FSS) is developed.•Annealed 30Cr super FSS exhibits better mechanical properties than ordinary FSS.•Precipitation behaviors of Laves phases are determined by the annealing temperature.•The developed 30Cr super FSS exhibits excellent uniform resistance in sulfuric acid.A newly designed super ferritic stainless steel (FSS), i.e. 30Cr-4Mo-2Ni, was developed for use in severe environments, such as sulfuric acid. The annealing temperature had a significant effect on the precipitation behaviors of the steel, and ultimately on its mechanical properties and corrosion resistance. The results showed that the designed steel exhibits higher strength, higher hardness, lower elongation and far superior corrosion resistance than ordinary FSS. Detrimental Laves phases are readily precipitated in this steel, so suitable annealing parameters is necessary to change the precipitation behavior and promote the dissolution of Laves phases, and then render this economically produced steel, corrosion resistant to sulfuric acid.

Variable polarity cold metal transfer (VPCMT) welding process shows a large potential to join Mg/Al dissimilar alloys in the future manufacturing industry. In this study, Mg alloy AZ31B and Al alloy 6061 were joined by VPCMT process with ER4043 as filler metal, and the effects of the ratio of electrode positive/negative CMT cycles (EP/EN ratio) on the microstructure-strength relationships of welded joints were investigated. The results showed that hard brittle Mg–Al intermetallic compounds (IMCs) layers were formed in the weld interfaces and consisted of three intermediate layers: Mg2Al3 layer, Mg17Al12 layer, and Mg17Al12+α-Mg solid solution eutectic layer (very thin). With decreasing EP/EN ratio from 4:1 to 1:4, the thicknesses of the Mg2Al3 layer and Mg17Al12 layer were reduced from 80 µm to 10 µm and from 105 µm to 80 µm, respectively. The tensile strength, which was fairly dependent on the thicknesses of these intermediate layers, increased significantly by over 100 percent. The fractures of the joints with 4:1 and 1:4 EP/EN ratio occurred primarily within the Mg2Al3 layer and the Mg17Al12 layer, respectively. As a result, the more EN-CMT-cycles, the lower energy input, the thinner IMCs layer, and the higher joint strength.

The characteristics of microstructure, mechanical property and corrosion behavior of Cr26Mo3.5 super stainless steel joints by pulse tungsten inert gas(P-TIG)welding and laser welding were investigated. The results indicate that the widths of the center equiaxed grain zone(EGZ)and the columnar grain zone(CGZ)increase with the increase of heat input in both welding processes. The precipitates of Nb and Ti carbides and nitrides are formed in the weld metal(WM)and the heat affected zone(HAZ). The joints by laser welding show better tensile and corrosion resistance properties than those by P-TIG welding due to the heat concentration and lower heat input. The tensile strength and elongation increase with the decrease of heat input, and the fracture mode of the joints turns into ductile-brittle mixed fracture from ductile fracture when the welding method turns into P-TIG welding from laser welding. Moreover, the corrosion resistance of all joints declines slightly with the increase of heat input. Hence, laser welding is more suitable for welding Cr26Mo3.5 super stainless steel in engineering applications.

The effects of sensitization heat treatment on the microstructure and electrochemical behavior of 21% Cr ferritic stainless steel weld joints with or without 309L austenite stainless steel as an interlayer were investigated. The joints were processed by pulsed gas tungsten arc welding. With the interlayer, grains in weld bead were refined, and almost fully ferrite. When the joints with the interlayer were maintained at 500 °C for 1 and 4 h, no microstructure changes occurred, whereas Widmanstatten austenite and needle-like austenite formed in the weld bead after sensitization at 815 °C for 1 h. In general, sensitization treatment worsens the corrosion resistance of welds, but the resistance of samples with the 4-h treatment at 500 °C recovered in part compared to those subjected to sensitization at 500 °C for 1 h. This could be due to Cr diffusion from the ferrite that heals the chromium-depletion zone along the grain boundary. However, an increase in temperature does not have the same effect. The corrosion morphology of samples in the weld bead is different from those in base metal after heat treatment at 500 °C for 1 h; in base metal, pitting corrosion occurs, whereas grain boundary corrosion occurs in the weld bead. Corrosion morphology is closely associated with precipitation and segregation along the grain boundary.

•Pulsed laser welding of TA15 titanium alloy to a new BTi-6431S titanium alloy in•butt form was performed.•Relationship between the laser beam offset and microstructure was studied.•Relationship between the laser beam offset and mechanical properties was studied.Laser beam welding was used to weld dissimilar joints in BTi-6431S/TA15 titanium alloys. The effect of laser beam offset on microstructural characterizations and mechanical properties of the joints were investigated. Microstructural evolution of the joints was characterized by optical microscopy (OM) and X-ray diffraction (XRD). Tensile testing was conducted at room temperature and at 550 °C. The results demonstrated that with the exception of some porosity, a good quality joint could be achieved. Martensite α’ and acicular α structures were present in the fusion zone (FZ). The amount of martensite α’ present with the −0.2 mm beam offset was less than that with the 0.2 mm beam offset. Acicular α and martensite α’ transformations occurred in the high temperature heat-affected zone (HT-HAZ) of both the BTi-6431S and TA15 alloys. In the low-temperature heat-affected zone (LT-HAZ), the BTi-6431S and TA15 alloy microstructures exhibited a mixture of secondary α, primary α, and prior β phases. The microhardness values in the FZ followed the order: −0.2 mm> 0 mm> 0.2 mm. Tensile testing at room temperature and at 550 °C resulted in fracture of the TA15 alloy base metal. The fracture morphology exhibited a ductile dimple feature.

The effects of Ni content and heat-treatment process on the toughness of a super ferritic stainless steel with 26 wt% Cr and 3.5 wt% Mo were investigated. It was found that with the increase of Ni content, the Charpy impact toughness improved remarkably, and transformed from cleavage brittle fracture to the most ductile fracture. There were no obvious differences between the high- and low-Ni contents on the microstructure and mechanical properties since the addition of Ni did not influence crystal structure, phase composition, and precipitation of ultra pure ferritic stainless steels. Meanwhile, the heat-treatment process was a key point to maintain a high level of toughness by optimizing structure and removing detrimental precipitation, i.e., chi phase.

A combined numerical model of thermal field and the primary dendrite arm spacing (PDAS) was proposed to correlate the process parameters and PDAS in laser welding of Cu and Al. The solidification parameters simulated by the finite volume method with commercial software ANASYS FLUENT were applied in the PDAS model to predict the dendrite arm spacing of fusion zone. Dendrite was also examined by the metallographic method to validate the model. Results indicate that the calculated PDAS agrees with metallographic measurements reasonably, especially the Hunt model. PDAS increases apparently with increasing laser power while decreases slightly with increasing welding speed. Increasing laser power increases the secondary dendrite and increasing welding speed increases the microporosity in dendrite.

The behaviors of YAG laser welding process of ferritic stainless steel with activating fluxes were investigated in this study. Some conventional oxides, halides and carbonates were applied in laser welding. The results showed that the effect of oxides on the penetration depth was more remarkable. Most activating fluxes improved the penetration more effectively at low power than that at high power. The uniform design was adopted to arrange the formula of multicomponent activating fluxes, showing that the optimal formula can make the penetration depth up to 2.23 times as large as that without flux, including 50% ZrO2, 12.09% CaCO3, 10.43% CaO and 27.48% MgO. Through the high-speed photographs of welding process, CaF2 can minimize the plasma volume but slightly improve the penetration capability.

Thin plates of 21% Cr ferritic stainless steel welded by pulsed gas tungsten arc welding at different pulse frequencies were investigated for the microstructure characteristics and hardness behavior. The welds contained columnar grains in the outer part and fine equiaxed grains in the central region due to the pulsed process.

This paper introduces the complexity and particularity of tube-sphere intersection weld (J-groove weld) and establishes the mathematical model of tube-sphere intersection trajectory. Based on the characteristics of J-groove welds, the computational process of welding gun orientation is first simplified. Then the kinematic algorithm of a welding robot is obtained according to screw theory and exponential product formula. Finally, Solidworks and Sim-Mechanics are employed to simulate the kinematics of the welding robot, which proves the feasibility of the kinematic algorithm.

Cold metal transfer (CMT) depositing processes in aluminium alloy were conducted using various characteristic parameters which represent for the control of current output and wire motion. The effects of characteristic parameters on the energy input characteristic, metal transfer behavior, weld geometry, and microstructure of deposited weld metal were investigated. There were some limitations for characteristic parameters to achieve stable CMT process. For stable CMT process, the three distinct energy input periods of CMT cycle were controlled by characteristic parameters and thereby affect the features of deposited weld metal. The speed of wire feed motion affected not only burn phase duration but also short-circuiting duration. With the increased speed of wire feed motion, the whole energy input first increased and then decreased; the weld width and contact angle decreased, which was different from the effects of other characteristic parameters. An increment in short-circuiting current resulted in an increase in dilution ratio and grain size of weld metal although whole energy input was essentially constant. In conclusion, adjusting characteristic parameters can control the energy input process coupled with metal transfer behavior to design and optimize the weld properties for each special CMT application, which is suitable for additive manufacturing of aluminium alloy.

In actual manufacturing process, many weldments have large dimensions and complex shapes, and they are usually assembled through a multi-pass welding process. The joints of the tube–sphere intersection (J-groove joints) are complex. This paper presents a complete solution in determining the welding paths based on a developed J-groove joint robot. Generating complex welding paths in terms of cubic B-spline curves is made easy using path control modules such as interpolation module and local modification module. The point inversion module using particle swarm optimization is introduced to address the partition of path, which is required of the welding process. Experimental results show that higher efficiency as well as better weld quality can be achieved, indicating a promising and practical use of the robot for welding applications, which is rarely available at present.Highlights► We establish and simplify the mathematical model for the tube–sphere intersection. ► Data points of intersection can be compressed by B-spline interpolation. ► The point inversion algorithm of intersection based on PSO has high efficiency. ► We modify the intersection by Lagrange multiplier for approaching the real one.

•Single and dual beam laser welding of Ti–22Al–25Nb/TA15 are performed.•The fusion zone only consists of B2 phase in the mode of single laser beam.•O phase is formed in the fusion zone under dual beam laser welding.•The tensile properties are improved under dual beam laser welding.Laser beam welding (LBW) was used to join Ti–22Al–25Nb/TA15 dissimilar titanium alloys. The microstructure and mechanical properties of the welded joints under single and dual beam welding were analyzed and compared. In the mode of single laser beam, the fusion zone only consisted of B2 phase because of existence of β-phase stabilizer and rapid cooling rate of LBW. However, O phase was formed in the fusion zone while applying dual-beam laser welding due to decrease of the cooling rate. The microhardness distribution of the welded joint in dual-beam welding mode was consistent with that in single mode, but the hardness of the weld under dual laser beam was higher than that of single laser beam. In room-temperature tensile tests, the fractures all occurred in the weld, but the morphology exhibited a quasi-cleavage feature in single mode while the morphology was dimple fracture in the mode of dual laser beam. The tensile strength and elongation were both increased under dual-beam laser welding compared with those under single-beam laser welding.