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Cylinders made of tantalum (Ta) and tubes made of 304 stainless steel (304SS) were successfully joined by laser welding under different offsets. The experimental results indicated that satisfactory weld formations were obtained. The fusion zone (FZ) was found to consist of the Fe-Cr solution and the Fe2Ta phase. In addition, the intermetallic compounds (IMCs) layer was generated at Ta/FZ interface. When the laser beam offset was -0.2 mm (0.2 mm laser beam offset on the 304 stainless steel side), the interfacial IMC consisted of Fe2Ta. Thicker IMC with the compositions of Fe2Ta+FeTa was generated at offsets of 0 mm (center) and +0.2 mm (0.2 mm laser beam offset on the Ta side). Numerical simulation results showed that the interfacial peak temperature would be increased when the laser beam offset from 304 stainless steel side to Ta side, which led to the thicker IMC and new generation of FeTa. The microhardness in the FZ also fluctuated since the FZ was composed of hard Fe2Ta and soft Fe-Cr solution. Tensile test results indicated that the highest value of 308.3 MPa was obtained under the laser beam offset of -0.2 mm.

The oxide film could realize the formation of the chemical bonding between metal and plastic. In this study, surface pre-oxidation of AZ31B magnesium alloy was performed by two common methods to investigate the effect of oxide film on laser assisted joining of carbon fiber reinforced thermoplastic composites (CFRTP) and AZ31B. Oxide layer with porous structure was fabricated by micro-arc oxidation (MAO), in contrast to the dense and smooth oxide film produced by annealing. Both types of oxide film could effectively enhance the mechanical properties of the joint. Oxide layer could inhibit the decomposition of CFRTP and eliminate defects and relieve the residual stress because of the lower interfacial heat conduction. Porous structure of MAO coating further reduced the heat conduction. A new chemical bonding MgCO3 was identified at the joint with oxide. MAO coating could facilitate the formation of the new chemical bonding and enhance the mechanical interlocking, further strengthening the mechanical properties of the joint. The tensile-shear strengths of annealed and MAO joints were 8.8 MPa and 13.3 MPa respectively, reaching 1.60 and 2.41 times that of untreated joints (5.5 MPa). The MAO coating realized the comprehensive enhancement of AZ31B/CFRTP interface in terms of the interfacial heat transfer, mechanical interlocking and chemical bonding.

Here, in order to develop the metallic materials with excellent relationship between strength and toughness for application, the FeCoCrNi + FeCoCrNiAl-laminated high-entropy alloys (HEAs) had been fabricated by laser melting deposition (LMD) additive manufacturing technique. The process parameter databases for FeCoCrNi and FeCoCrNiAl HEAs were built by orthogonal experiments and statistics method, which helped to select the optimised process parameters for the two HEAs. Then, the differences in the forming process, phase structure, and grain morphology of FeCoCrNi and FeCoCrNiAl HEAs were systematically investigated during the fabrication of the FeCoCrNi + FeCoCrNiAl-laminated HEAs. The results show that variations in surface tension, wettability, and other physical properties between the two HEAs led to significant differences in their LMD processes. Al not only influenced the fabrication process of both HEAs, but also promoted the phase transition from FCC (FeCoCrNi) to BCC (FeCoCrNiAl). In addition, Al also acted as a strong limiting factor to inhibit the effect of supercooling on the grain morphology, which transformed from coarse columnar grains to fine equiaxed grains. The columnar grains, with a preferred orientation in the FeCoCrNi-deposited wall, promoted the suppression of the inhibitive effects of the phase structure and strong limiting factor, led to the growth of the columnar grains in the FeCoCrNiAl-deposited wall, and helped achieve long-distance continuous growth of columnar grains across the interface in the subsequently deposited wall. The methodology used in this study could be meaningful for the exploration of process parameters of LMD, and the discoveries of grain characteristic help to understand the microstructure transition in laminated heterogeneous HEAs fabricated by laser additive manufacturing.

Laser welded–brazed Al/steel lapped joint was successfully obtained under various shielding gas contents (pure Ar, CO2+Ar and pure CO2) with laser power of 2000W. Influence of shielding gas contents on weld formation, interfacial microstructure and tensile–shear strength was investigated. The addition of CO2 in the shielding gas would not only enhance the total fusion volume but also improve the wettability of molten filler. Interfacial microstructure observations showed that addition of CO2 in the shielding gas would thicken the interfacial IMC. Continuous layered η–Fe2(Al,Si)5 and serration shaped θ–Fe(Al,Si)3+τ5–Fe1.8Al7.2Si was newly formed. In addition, higher peak temperatures were produced with the increase of CO2 volumes, which was proven by numerical simulation results. Higher laser energy absorption for CO2, alternation of melt flow pattern and smaller thermal conductivity of CO2 were responsible for these changes. Highest average tensile–shear strength of 163MPa was obtained under the shielding gas content of 50 %Ar+50 %CO2 when laser power was 2000W.

Laser welding–brazing Al/steel with different Zn–Al filler metals was performed. Experiments and thermodynamics calculations were conducted to analyze microstructure evolution and elemental diffusion behavior, respectively. The interfacial intermetallic compounds (IMC) was composed of dominating layered η–Fe2Al5Zn0.4 phase and two different types of δ–FeZn10 phase. Scattered δ–FeZn10 phase among layered η–Fe2Al5Zn0.4 matrix was found in all joints while continuous δ–phase adjacent to steel substrate appeared in joint obtained with Zn–Al2 and Zn–Al15 fillers and disappeared in the case of Zn–Al22 filler. Thermodynamics calculation showed that Zn element preferentially diffused to the Fe–Al interface and steel substrate, and reacted with generated Fe–Al IMC and residual Fe elements, leading to the presence of Zn element in η–Fe2Al5Zn0.4 phase and the formation of δ–FeZn10 phase. The increasing Al addition in filler metals induced a more sufficient Fe–Al reaction, causing a thicker Fe–Al IMC layer and insufficient residual Fe elements at the interface. It would be harder for the Zn elements to diffuse through the η–Fe2Al5Zn0.4 layer with larger thickness. A higher Gibbs free energy of δ–FeZn10 phase compared with η–Fe2Al5Zn0.4 phase, insufficient Fe and Zn elements at the interface were all responsible for the disappearance of continuous δ–FeZn10phase. Joint with the highest tensile strength was produced with Zn–Al22 filler owing to the disappearance of continuous δ–FeZn10 phase and crack–inhibitation effect of scattered δ–FeZn10 phase among layered Fe2Al5Zn0.4 matrix.

A reliable numerical model is developed to predict the deformation of mismatch and normal butt-welded joints after laser and gas metal arc hybrid welding (Laser-GMAW). A smaller deflection is produced in the mismatch joint as a result of a more uniform weld profile along the thickness direction, while a larger shrinkage deformation is obtained as a result of the higher heat input compared to the normal joint. The transverse plastic strain is much larger than the longitudinal plastic strain for both joints. An interruption is observed at the bottom surface of the mismatch joint owing to the discontinuity of the weld profile. The calculated inherent deformation, based on the plastic strain, is much larger along the transverse direction than along the longitudinal direction and it is also the dominating welding deformation. The deformation of a pipe assembled with a longitudinal welding line is predicted from the calculated inherent deformation parameters. A smaller roundness (ΔR) and deflection (ΔZ) are produced in the mismatch joint due to its smaller inherent bending deformation compared to the normal joint.

6061-T6 aluminum alloy and DP590 steel were joined successfully by a laser welding–brazing process with pure Al, AlSi5, and AlSi12 filler metals. Interfacial microstructure of the joints with highest tensile strength (under laser powers of 2000 W, 2000 W and 2500 W for joints obtained with pure Al, AlSi5 and AlSi12 filler metals, respectively) were selected for comparison. The interface produced with the pure Al filler metal consisted of a thick layer of η-Fe2Al5 with scattered rod-shaped θ-FeAl3, with average thicknesses of 10.1 μm in the bottom region and 16.7 μm in the top region. When 5 wt.% Si was added to the filler metal, the interfacial intermetallic compound (IMC) components remained similar, whereas the interface thickness decreased to 3.8 μm in the bottom region and 7.5 μm in the top region. On further increasing the Si addition to 12 wt%, the interfacial IMC acquired 1.2 μm-thick τ5-Fe2Al8Si in the bottom region with 5.6-μm-thick θ-Fe(Al,Si)3 and τ5-Fe2Al8Si in the top region. The chemical potential of Si in the Fe-Al-Si ternary system was lower at the Fe-Al side comparing with weld seam and steel substrate, which led to a preferentially diffusion of Si to the Fe/Al interface and finally caused the aggregation of Si in the Fe/Al interface. Si addition improved the joint strength and reduced the required laser power for a suitable joint. The joint produced with the AlSi5 filler metal had the highest tensile strength and largest fracture displacement.

The keyhole is unstable with the presence of humps at the keyhole walls during single-spot laser welding (SSLW). The keyhole stability is improved and the presence of humps is reduced during dual-spot laser welding (DSLW). The vortices present at the rear keyhole wall during SSLW disappear during DSLW. There is an upward motion of the melt flow at the rear keyhole wall and the melt flow velocity is higher along the width of the molten pool during DSLW. A sandwich specimen is constructed to investigate the effect of keyhole and melt flow dynamics on the final shape of the weld seam. The results show that DSLW produces smoother, more regular weld beads due to higher stability of the keyhole and molten pool. DSLW produces a weld seam with fewer porosities and a more homogeneous nickel distribution due to the upward motion and absence of vortices in the melt flow. DSLW produces a wider molten pool due to the higher melt flow velocity along the width of the molten pool.

Laser welding-brazing 2-mm-thick 6061-T6 aluminum alloy to DP590 dual phase steel in butted configuration was performed with AlSi12 filler metal. The influence of laser power on weld appearance, interfacial microstructure and tensile strength was studied. Thermal cycle at brazing interface was calculated by numerical simulation in order to clarify the interfacial reaction mechanism. A sound weld appearance and cross section could be obtained under laser powers of 2200 W and 2500 W. When the laser power was 1800 W, the interfacial intermetallic compound (IMC) was continuous thin 2-μm-thick τ5 phase while no reaction layer was detected at the bottom of groove at the brazing interface. When the laser power was 2200 W, the interfacial IMC consisted of needle-shaped θ phase+serration-shaped τ5 phase with average thickness of 5.2 μm. With the further increase of laser power to 2500 W, the interfacial IMC was composed of planar η phase, needle-shaped θ phase and serration-shaped τ5 phase with average thickness of 12 μm. When the laser power increased to 3000 W, the interfacial IMC components were composed of thick planar η phase, θ phase and τ5 phase with average thickness of 30 μm. Highest tensile strength with 140 MPa was obtained when the laser power was 2200 W. The interfacial reaction mechanism under different laser powers and relationship between IMC components and tensile strength was clarified.

Defocused laser welding-brazing of 2-mm-thick 6061-T6 aluminum alloy and DP590 steel in butted configuration was performed with AlSi12 flux-cored filler metal. Three different groove shapes at steel side were made: square-shape, half Y-shape and half V-shape. The experimental results indicated that inhomogeneous intermetallics compounds (IMC) morphology along the steel appeared from top to bottom region. In top region, IMCs consisted of needle θ-Fe(Al,Si)3 + serrated τ5-Fe1.8Al7.2 for three joints and their average thickness was different(10.2-μm for square-shape, 10.1-μm for half Y-shape while 8.6-μm for half V-shape). In middle region, only 2.0-μm τ5-Fe1.8Al7.2 was detected in joint with square-shape groove while θ-Fe(Al,Si)3 + τ5-Fe1.8Al7.2 with the average thickness of 7.2-μm and 4.9-μm were observed in joints with half Y-shape and half V-shape grooves. In bottom region, insufficient metallurgical reaction occurred in joint with square-shape groove while τ5-Fe1.8Al7.2 with the average thickness of 1.2-μm and 1.1-μm were formed in joints with half Y-shape and half V-shape grooves, respectively. The smallest temperature gradient along the steel interface in thickness direction was noticed in joint with half V-shape groove, resulting in the least difference of IMC thickness values from top to bottom. The results of tensile test indicated that the highest tensile strength was achieved in joint with half V-shape groove because of its excellent spreading behavior of molten filler metal, largest bonding interface area and suitable interfacial IMC distribution along the steel interface.

Al/steel butt joints were successfully obtained using a dual–spot laser welding–brazing process with different R (R = EFe:EAl = 4:6, 5:5, and 6:4; EFe and EAl represented the energy ratios of the laser near steel and Al, respectively). The molten filler metals wet and spread well along the front and back surfaces of the steel when R was 5:5 and 6:4, whereas an undercut occurred when R was 4:6. The interfacial intermetallic compound (IMC) along the brazing interface consisted of only thin–layered τ5–Fe1.8Al7.2Si when R was 5:5 and 4:6. The IMC had a greater thickness when R was 5:5 owing to its higher peak temperatures than when R was 4:6. A mixed IMC (θ–Fe(Al,Si)3+τ5–Fe1.8Al7.2Si) was newly formed in the top region of the brazing interface when R was 6:4. TEM observations revealed that two types of grains existed in θ–Fe(Al,Si)3: normal and twinned. Numerical simulations for the thermal cycles along the brazing interface indicated that the IMC thickness was determined by the interfacial peak temperature, while the IMC homogeneity was determined by the temperature gradient. The results of an experiment involving the tensile properties indicated that the highest average joint strength (193 MPa) and largest average fracture displacement (0.64 mm) when R was 5:5, owing to its satisfactory weld formation and reasonable IMC distribution.

Laser welding under subatmospheric pressures exhibits significant potential in the field of large-scale structure connections. In this study, a set of disk laser welding experiments are performed on 35-mm-thick 5A06 aluminium alloy under various ambient pressures. The influence of ambient pressures on weld formation, plasma plume behaviours, microstructures, as well as the mechanical properties is investigated. The experimental results show that the penetration depth increased sharply when welded under the ambient pressure of 104 Pa, while the plasma plume and porosity defects were suppressed effectively. With further decrease in ambient pressures, there was little variation in weld characteristics. The microstructure characteristics of joints produced under subatmospheric pressures exhibited more uniformity with a wider heat-affected-zone, and the joints’ mechanical properties improved markedly compared with those produced under atmospheric pressure. To obtain a high weld quality, by considering the weld appearance, porosity defects as well as welding stability, the ambient pressures for the laser welding process should be limited to less than 102 Pa. Apart from these, with the decrease in ambient pressures, a smaller burning loss of the element Mg as well as greater low-angle grain boundaries and dislocations would be present in the weld seam. This contributes to improvements in the mechanical properties.