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As the material experiences non-uniform thermal expansion and contraction under different thermal behavior during laser melting deposition (LMD), the microstructure of the components will be affected, which also causes changes in mechanical properties. It is essential to understand the thermal behavior of the melt pool during LMD to improve the microstructure and joint quality. In this study, two 80-mm-thick Ti6Al4V plates are successfully joined using three different scanning speeds (10, 15, and 20 mm/s). The effects of thermal behavior on the grain morphology and dimension of deposition area, equiaxed crystal zone (EQZ), and the substrate at different scanning speeds are quantitatively analyzed. The results show that the fine equiaxed crystals are formed near the joint boundary due to the high-temperature gradient and cooling rate. The growth direction of fine columnar crystals continuously changes from the joint boundary to the center of the deposition area due to the change of heat-extraction direction. However, the epitaxial growth direction of the coarse columnar crystal is the same as the previous deposition layer at the center of the deposition area. Given the effect of high heat accumulation and low-temperature gradient during LMD, the dimension of columnar crystal is coarsen significantly at the center of the deposition area. [ABSTRACT FROM AUTHOR]

• 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]

• 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]

• 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]

Wire arc additive manufacturing (WAAM) and laser melting deposition (LMD) are two typical additive manufacturing technologies that fabricate large components and small complex structures. However, the composite manufacturing process of these two technologies is rarely reported. WAAM-LMD hybrid additive manufacturing is a feasible method to satisfy the dual requirements of production efficiency and structural accuracy in today's aerospace field. In this study, a 10-mm-thick wall structure was manufactured by this process. The microstructure in different areas was characterized using optical microscopy, scanning electron microscopy and electron backscattered diffraction to elucidate the formation mechanism of the interfacial microstructure. Meanwhile, the tensile properties were tested in different directions and regions of the component. The results indicate that the combined effect of WAAM and LMD areas lacks obvious defects. The average grain size of the bonding region is closer to the bottom of the LMD region and smaller than that of the WAAM region. Clear grain preference orientations and variant orientations were observed from the bottom to the top of the transition zone, which is affected by different thermal histories between the arc and the laser. In addition, equiaxed crystals and columnar crystals coexist in the remelting zone under the combined action of two factors: the epitaxial growth with columnar crystals and the high cooling rate of the molten pool. The properties of tensile specimens in different directions have few differences. However, the elongation obviously changes in the binding zone. Furthermore, the cleavage fracture gradually transforms into ductile fracture from the bottom to the top in the transition area. • The component is fabricated by WAAM-LMD hybrid additive manufacturing. • Equiaxed crystals and columnar crystals coexist in the combining zone. • The KAM value is higher and more uniform at the interface than in other regions. • The cleavage fracture gradually transforms into ductile fracture in transition area. [ABSTRACT FROM AUTHOR]

The microstructure under various conditions shows noticeable differences in the wire-laser directed energy deposition (DED) method using a coaxial head due to the intrinsic properties of aluminum alloy wire materials and the intricate heating process involved. The impacts of varied heat inputs and substrate preheating temperatures on solidification and microstructure evolution were explored in this study. A coupled model, including macroscopic finite element and microscopic cellular automata with interpolation, was employed to investigate the temperature field, thermal cycles, solidification parameters, and microstructure evolution under varied process parameters. This study revealed the transformation of four grain forms in the deposition layer, namely, lamellar grain, equiaxed grain, equiaxed fine grain, and columnar grain, through a comprehensive dynamic simulation of the microstructure. Quantitative research was conducted to determine the solidification region of equiaxed grains and the average length of columnar grain amid solidification under varied crystallization parameters. The findings demonstrated that under a lower heat input and preheating temperature, equiaxed fine grains are generated at the lower region of the deposition layer, while lamellar grains are formed above the equiaxed grains at the upper region of the deposition layer, which is similar to the morphology of a cast microstructure. The typical primary dendritic distance of columnar grain diminishes with the enlargement of the cooling rate and temperature gradient. The findings of this study offer a theoretical framework for regulating the microstructure of aluminum alloy deposition layers manufactured via wire-laser DED technology using a coaxial head. [Display omitted] • The effect of heat input on microstructure evolution was studied through CA-FE simulation and experimental comparison. • The transformation of four crystal forms was revealed through a comprehensive dynamic simulation of the microstructure. • The average length variation of columnar crystals was quantitatively analyzed under different crystallization parameters. [ABSTRACT FROM AUTHOR]