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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.

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.