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• The effect on Ti-6Al-4V alloy grain evolution and mechanical properties of the LMD-WAAM process and laser powers have been studied. • The texture intensity of the α phase gradually increased from RZ to the top-LMD zone and from the WAAM zone to HAZ along the deposition direction. • The tensile fracture of LMD-WAAM samples (1000 W and 1200 W) occurs in the Interface zone, exhibiting apparent ductile fracture characteristics with a small amount of cleavage fracture and pore defects. • In the 1500 W LMD-WAAM sample, the tensile strength of the LMD-WAAM sample (906 MPa) is higher than LMD (897 MPa) but lower than the WAAM sample (920 MPa). Laser melting deposition (LMD) and Wire arc additive manufacturing (WAAM) are representative metal additive manufacturing (AM) technologies. To explore the feasibility of using the LMD-WAAM hybrid process to fabricate large and complex components, a preliminary investigation of Ti6Al4V was conducted. In this study, a WAAMed sample was used as a substrate for the LMD process at different laser powers. The effects of laser power on the grain evolution and mechanical properties of the Ti6Al4V alloy were studied. The results indicated that different thermal histories formed a graded microstructure from the LMD to the WAAM zones. The Electron backscatter diffraction texture intensity of the α phase gradually increased from the remelting zone (RZ) to the top-LMD zone and from the WAAM zone to the heat-affected zone. In the LMD zone, the length of the acicular α decreased and the lamellar α content increased with increasing laser power. Ductile fracture characteristics were apparent with few cleavage fractures and pore defects, which led to different fracture locations. Further, the microhardness of the interface zone exhibited a peak because the microstructure of the RZ was fine, leading to a higher microhardness. [ABSTRACT FROM AUTHOR]

• WC/Ti-6Al-4 V metal matrix composite (MMC) coating was synthesized on Ti-6Al-4 V alloy substrate by laser cladding technology. • The WC particles distribution, microstructure formation and evolution mechanism, and the relationship between microstructure and microhardness of the MMC coating were investigated. • The new phases TiC, W 2 C, W, and the [W, Ti]C 1-x solid solution were observed in MMC coating, and the WC particles were surrounded with a thickness of 3 um reaction layer. • The microhardness of the MMC coating was enhanced by 73.5%, with the peak of 633.4 HV. As an advanced repairing and strengthening technology, laser cladding technology has enormous potential in the surface modification of titanium alloys. This study applied laser cladding technology to synthesize WC/Ti-6Al-4 V metal matrix composite (MMC) coating on a Ti-6Al-4 V alloy substrate. The WC particles distribution, microstructure formation and evolution mechanism, microhardness of the MMC coating were investigated in detail. A prominent metallurgical bond was formed between the substrate and the MMC coating, and the uniform distribution of WC particles and reasonable transformation of microhardness from the substrate to the MMC coating were obtained. The phase composition of the MMC coating mainly contained TiC, Ti, WC, W 2 C, W, and the [W, Ti]C 1-x solid solution and the WC particles were surrounded with a thickness of 3 um reaction layer. The results indicated that the higher heat input is beneficial to the uniform distribution of WC particles. Meanwhile, the microhardness of the MMC coating was enhanced by 73.5%, with the peak of 633.4 HV. Furthermore, the microhardness of the MMC coating decreases with the increase of heat input, which was related to the severe dissolution behavior of the WC. [ABSTRACT FROM AUTHOR]

• Y 2 O 3 was added into the WC reinforced composite coating to improve the surface properties of Invar alloy. • The effect of Y 2 O 3 on average area, proportion and shape of equiaxed crystals and microstructure evolution were revealed. • The W and Y elements tend to converge at crystals boundaries. All phases in the cladding layer were distributed in γ-(Fe,Ni) matrix and formed a netty structure. As an essential material of aviation composite forming mold, Invar alloys will produce defects such as wear and crack during long-term service. In this paper, the rare earth oxide Y 2 O 3 was added into the WC reinforced composite coating to improve the surface properties of Invar alloy. The microstructural evolution, element distribution and microhardness of composite coating were investigated by optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and Vickers hardness tests. The mean area and proportion of regular cellular crystals were quantitatively measured. The effect of Y 2 O 3 on the distribution of the W element in the coating is revealed. Results indicate that the addition of Y 2 O 3 significantly increases the thickness of the coating and improves the melting efficiency. Furthermore, cellular dendritic crystals tend to transform into regular cellular crystals with higher Y 2 O 3 amounts. The proportion of regular cellular crystals reaches up to 30.0% when the mass fraction of Y 2 O 3 is 5%. The W and Y elements tend to converge at crystals boundaries. Meanwhile, the microhardness gradient of the cladding layer (CL) decreases with Y 2 O 3. [ABSTRACT FROM AUTHOR]

• The flow behavior of invar alloy molten pool was indirectly verified and studied based on the Laser melting deposition experiment with nano WC particles as tracer particles. • A significant Marangoni flow is occurred with the highest velocity in the middle of molten pool but lower in the surface and bottom of molten pool. • The element mapping of the sample deposited with mechanically alloyed powder shows an obvious element accumulation of WC in the bottom of the deposition layer. Continuous-wave laser melting deposition (LMD) offers several advantages, such as higher cooling rate, finer microstructures and improved mechanical properties over traditional processing technology. The fluid flow in the molten pool affects the solute distribution and the microstructure morphology when invar alloy mould is repaired with laser melting deposition. However, the effects of laser on the molten pool motion, thermal field and microstructure of invar alloy are not distinct. In this work, a three-dimensional model utilizing the volume of fluid method was developed to simulate the transient molten pool motion, heat transfer and fluid flow for the laser melting deposition process. The interaction among powder, laser and molten pool in LMD process was comprehensively considered in the model. The laser melting deposition with Nano-WC powder particles as a dispersoid was investigated. The simulated molten pool geometry was compared with experimental results. Moreover, temperature variation, fluid flow and their influence on the microstructure were analyzed. It is concluded that a significant Marangoni flow is observed with the highest velocity in the middle of the molten pool but lower in the surface and bottom of the molten pool. The lower velocity of fluid flow results in an element accumulation in the bottom of the molten pool. This study provides a referenced value for revealing the flow mechanism of molten pool in the LMD process of invar alloy. [ABSTRACT FROM AUTHOR]

• A novel laser mirror welding method which can reduce deformation is proposed. • The microstructure of laser mirror welding shows an asymmetrical distribution under the action of gravity. • Under the dual action of laser heat input and gravity, the effective bearing area of the weld near the upper fusion line is the smallest, which is prone to fracture. For thicker weldments, penetration requires higher welding heat input. The significant difference in temperature between the front and back of the weldment will inevitably produce more significant welding distortion. The symmetrical double laser heat sources act on the flat plate butt joint structure during the laser mirror welding process, which can realize the deformation-free and high-efficiency welding of thick plates. Due to the particular welding method of laser mirror welding, there are complex interactions of double keyholes and double-sided joint pools during the welding process. However, there is no relevant research on laser mirror welding of aluminum alloys. Therefore, experimental analysis on laser mirror welding of the 2219 aluminum alloy is performed in this paper. The relationship between welding process parameters and weld bead formation is explored by quantitative research about the macro morphology of the weld seam. Then, the typical microstructure distribution is studied by the microstructure of laser mirror welded joints. Finally, the tensile test experiment is performed to observe the microscopic morphology of the tensile fracture to analyze the fracture mode of the joint. This article discussed the intrinsic relationship between the stability of the molten pool and the tensile properties. The study found that when the molten pool is formed, the stability of the keyhole is the worst, and the tensile performance is poor. [ABSTRACT FROM AUTHOR]

Laser induced damage of Au-coated gratings (ACG) and metal multilayer dielectric gratings (MMDG) for pulse compression were measured using 800 ± 35 nm femto-laser with pulse width of 30.2 fs. The -1st order diffraction efficiency of the ACG is over 90% in wavelength range from 700 to 1000 nm. The MMDG has a 148 nm bandwidth (750–897 nm) with -1st order diffraction efficiency greater than 90%. The laser damage experiment on grating samples was performed in air for single-shot damage. The single-shot damage threshold of the ACG and MMDG was determined to be 0.32 ± 0.02 J/cm 2 and 0.31 ± 0.02 J/cm 2 , respectively. The damage morphologies of the ACG revealed that the damage was attributed to the pinholes at the base of the grating pillars and the weak adhesion between metal layer and photoresist gratings layer. The damage feature combined with near field distribution of MMDG indicated that the damage was due to the nonlinear ionization process of the valence electrons in HfO 2 film. According to analysis results, the laser damage resistance of the ACG can be enhanced through avoiding the appearance of pinholes and increasing adhesion between metal layer and photoresist layer. And for the MMDG, good performance of HfO 2 film, low near field enhancement and single HfO 2 grating structures may increase its laser damage resistance. [ABSTRACT FROM AUTHOR]

• The laser additive manufacturing method was adopted to join 80 mm thick Ti-6Al-4V Titanium alloy. • Irregular-shaped pores with a size of 1–10 μm are distributed in phase ɑ and phase β in the joint area without nanoparticles due to the larger dendrite spacing. • Nanoparticles can significantly reduce porosity defects in the weld seam, but excessive content can cause agglomeration of nanoparticles and Metallurgical pores。 • Nanoparticles can refine the microstructure of the weld zone, lessen the volume of the liquid region between dendrites in the mushy zone. Laser additive manufacturing is a method, through which parts are joined by laser melting deposition (LMD) layer by layer as opposed to the conventional welding processes. It presents superior advantages in joining large-size structural parts in aerospace due to its flexibility in technology and equipment. Owing to the large size of the welded joint and the complex welding process, the defect is easily produced in joining large structural parts. Given its detrimental influence on mechanical properties, defects are a major problem for the connection of large thickness components. This study aims to explore the formation mechanism of defects in joining 80 mm thick Ti6Al4V titanium alloy using laser additive manufacturing. The microstructure and element distribution detection of the welded joint is analyzed to verify the defect formation mechanism. It is indicated that defects during laser melting deposition (LMD) of large thickness Ti6Al4V titanium alloy can be classified into two types, porosity and lack of fusion (LOF). It is revealed that the irregular-shaped pores with a size of 1–5 μm can be observed in the joint area without nanoparticles due to larger dendrite spacing. Additionally, nanoparticles can reduce the porosity, but excessive content of nanoparticles will lead to metallurgical porosity, which decreases the performance of the weld joint. The LOF defects are caused by insufficient energy, which mostly appears between the layers or near the fusion line and has sharp edges. [ABSTRACT FROM AUTHOR]

• The laser mirror welding experiment of 2219 aluminum alloy was carried out. • The porosity morphology and distribution in laser mirror welding were discussed. • A numerical model was proposed to simulate keyhole and molten flow behavior effectively. • Evolution mechanism of process porosities in laser mirror welding was analyzed. Laser mirror welding, which inputs the symmetrical energy on the sides of the butt joint structure, has been employed to solve the problem of large angular distortion in single laser welding for medium-thick weldments. However, porosity defects is becoming a challenge for laser mirror welding due to dual laser beams and transverse welding direction. In this study, experimental analysis and numerical simulation modeling are used to reveal the characteristics and evolution mechanism of porosity in laser mirror welding of 2219 aluminum alloy. Compared with metallurgical porosity related to hydrogen, the size of process porosity is larger, the shape is more irregular, and the inner wall is rougher. The fluctuation of the coupled keyhole is the root cause of the formation of process porosity, manifesting as high-speed fluids on the wall. It is investigated that large process porosities mainly occur near the upper fusion line, which can be divided into two types: α region in the center and β region near the weld surface. The bubbles, which are formed owing to the collapse of the central keyhole in α region and the detachment of keyhole hump in β region individually, follow with upward melt flow by buoyancy. They are captured before crystallization resulting in the concentrated distribution of porosities in these regions. [ABSTRACT FROM AUTHOR]