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Fatigue performance is always an important factor affecting the application of aluminum alloys. The service life of the 7B04 aluminum alloy tends to reduce under continuous alternating loads. Therefore, a new method is urgently needed to improve fatigue performance. Laser shock peening (LSP) is a widely proposed method to enhance fatigue performance. It is found that LSP can prolong the fatigue life of 7B04 by improving the surface stress state. During the strengthening process, the residual stress is mainly attributed to the change in microstructure, which the statistical results of grain size can reflect. The microhardness of the treated 7B04 is 22.7% higher than that of the untreated sample. In addition, there is a significant residual compressive stress from the specimen surface to its interior of about 1500 µm after the process of laser shock peening. The fatigue life is extended to 93%, and the ultimate fracture changes macroscopically. The fatigue performance of 7B04 is greatly improved by the LSP treatment. The strengthening mechanism of LSP is established to reveal the relationship between microstructure and stress state to improve the fatigue performance of metal parts of any shape. [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]

Specific energy is a key process parameter during laser cladding of high entropy alloy (HEA); however, the effect of specific energy on the microstructure, hardness, and wear resistance of HEA coating has not been completely understood in the literature. This paper aims at revealing the influence of specific energy on the microstructure and properties of laser cladded FeCoCrNi high entropy alloy on the Ti6Al4V substrate, and further obtains feasible process parameters for preparation of HEA coating. Results indicate that there are significant differences in the microstructure and properties of the coatings under different specific energy. The increase of specific energy plays a positive role in coarsening the microstructure, promoting the diffusion of Ti from the substrate to HEA coating, and subsequently affects the hardness of samples. The HEA coating is mainly composed of the face-centered cubic phase and body-centered cubic phase, precipitating a small amount of Fe-Cr phase and Laves phase. Metallurgical bonding is obtained between the base metal and the coatings of which the bonding region is mainly composed of columnar crystal and shrinkage cavities. The microhardness of the HEA coating reaches 1098 HV, which is about 200% higher than that of the TC4 substrate, and the wear resistance is significantly improved by the HEA coating. [ABSTRACT FROM AUTHOR]

The uneven microstructure and element distribution between the inner-layer zone (INZ) and the inter-layer zone (ITZ) resulting from the thermal cycling effect during the directed energy deposition process with the electric arc energy source (DED-Arc) can lead to inferior mechanical properties. In order to address this problem, the ultrasonic treatment of the 2319 Al-Cu alloy during the DED-Arc process is investigated to control the multi-regional microstructure quantitatively. Therefore, the effect of ultrasonic vibration during the DED-Arc process on grain morphology, distribution of precipitates and mechanical properties in different regions are explored in detail. The formation mechanism of differential microstructure is elucidated by using an ultrasonic-heat-flow coupling simulation model for the DED-Arc process. The results indicate that ultrasonic vibration refines the average grain size, leading to 16% and 40% reductions at the molten pool center (MPC) and boundary (MPB), respectively. However, the grains in the semi-melting zone (SMZ) experience coarsening due to the reduced temperature gradient after ultrasonic treatment. The enhanced melt flow rate is considered responsible for both the homogenization of Cu and the fragmentation of the θ-Al 2 Cu phase. The higher subgrain boundary density in the ITZ leads to an average microhardness of approximately 60 HV, which is lower than that found in the INZ (72.3 HV). Meanwhile, ultrasonic treatment increases the average strength and elongation by 38.1% and 17.5%, respectively, mainly through precipitation strengthening, which results in a strength increment of 91.6 MPa. The slip directions tend to be consistent after ultrasonic vibration, which is conducive to crystal sliding and displays excellent ductility. [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]

Regionalization of microstructure regulation and mechanical property adjustment during the thermal cycle of the deposition process plays a vital role in the grain morphology and orientation of aluminum alloy components manufactured using wire arc additive manufacturing (WAAM) technology. However, the microscopic mechanism of the mechanical property difference caused by uneven microstructure in different regions remains unclear for the WAAM 2319 alloy. The geometric properties of grains are described quantitatively in different regions for the WAAM 2319 alloy through EBSD data processing and analysis. The effect of grain orientation on grain boundary, including subgrain boundaries and the boundary of coincidence site lattice (CSL boundary), are explored with EBSD data. The grain boundary and dislocations are characterized by transmission electron microscopy (TEM). The distribution of geometrically necessary dislocation (GND) in the WAAM block at different regions is studied through EBSD analysis. The high Schmidt factor or Taylor factor (M) percentage was quantitatively calculated in different regions. Then the relationship between grain morphology and plastic deformation ability is established through these factors. The results indicate that the average grain size is small and located in the top and middle of the WAAM sample with a low critical Taylor factor (M c). The Taylor factor in most grains easily exceeds M c (M c < M), which causes most dislocations to pass through grain boundaries quickly, and plastic deformation easily occurs in these regions. There are various CSL boundaries except Σ3 boundaries existing in the middle of the sample, which can maintain the high mobility of the grain boundaries. Therefore, more slip transmission in the middle of the WAAM sample can provide the driving force for grain boundary migration. Most grains with a high subgrain boundary density (above 2.5) and the geometrically necessary dislocation (GND) density (5 × 1014 m−2) can hinder the movement of dislocations and improve the strength of materials at the sidewall of the sample. The Schmidt factor of columnar crystals is generally low (below 0.25), and the sliding system does not start rapidly in columnar crystals, especially at the bottom of the WAAM sample. • Regionalization of grain geometric properties after the WAAM process is fitted. • The influence mechanism of grain morphology on plastic deformation is revealed. • The relationship between grain misorientation and slip transmission is built. • The mechanisms of slip transmission around the grain boundary are explored. [ABSTRACT FROM AUTHOR]

The microstructure affects the performance of the formed component during the wire arc additive manufacturing (WAAM) process of aluminum alloy. The present research utilized a finite element model to investigate the temperature field, thermal cycle curve, temperature gradient and cooling rate curve of the deposition layer at different positions in the WAAM process. The microstructure distribution and grain size variation of the deposition layer at different positions were examined according to the experimental results. The microstructure evolution of the deposition layer in the WAAM was simulated using a macro-micro coupled grain growth model. The variation law of solid phase and liquid phase distribution with solidification time steps of molten pool under varying temperature gradients is analyzed. The results indicated that the average length and width of the grains continuously decrease from right to left in the top zone of the deposition layer. When the temperature gradient increases to a certain extent, both the liquid phase area difference and the solid phase area ratio difference within the same time step gradually decrease with the development of the temperature gradient. This research supplies a reference for optimizing the microstructure of WAAM aluminum alloy by adjusting the heat input and solidification parameters. • The variation of solid and liquid phase distribution with time steps under different temperature gradients is analyzed. • The average length and width of the grains continuously decrease from right to left in the top zone. • The average length of the grains first increases and then decreases from the top to the bottom of the deposition layer. • The liquid phase area difference within the same time step decrease with the development of the temperature gradient. [ABSTRACT FROM AUTHOR]

Laser additive manufacturing (LAM) with directed energy deposition (DED) as the predominant foundation is an efficient and flexible joining technology that makes the fabrication of components with large thicknesses and complex structures possible. Heat accumulation phenomena caused by successive layer deposition strongly affect the microstructure and properties of components created with LAM. This study presents the conditions for component joining of constant process parameters and laser power gradient reduction process parameters to compare and investigate the influence of heat accumulation on the uniformity of mechanical properties and microstructure. The effects of the thermal cycle curve in different zones during LAM on the microstructure and mechanical properties of the top, middle and bottom of the deposition zone (DZ) under various parameters were investigated through finite element simulation and experiment. Results indicated that the cooling speed at the top of the DZ is slow and the heat accumulation effect is high. The aspect ratio of the α phase at the top of the deposition zone is high and high heat accumulation deteriorates mechanical properties. The heat accumulation effect is weakened when the gradient process parameters are used for joining. The content and size of the α phase at different joint heights are further homogenized. The tensile strength difference at various positions decreases and is higher than the tensile strength of the joint under constant parameters. This study promotes the LAM technology and provides a control reference for heat accumulation and microstructure regulation of large-thickness Ti6Al4V titanium alloy joints fabricated by LAM. [Display omitted] • The effect of heat accumulation on the microstructure and mechanical properties of the TC4 titanium alloy joint was investigated. • The distribution characteristics of temperature field and the change rule of thermal cycle curve in different zones during the laser additive manufacturing process were explored. • The differences of microstructure and mechanical properties at the top, middle and bottom of the addition zone under gradient parameters and constant parameters were comparatively evaluated. [ABSTRACT FROM AUTHOR]

In the laser welding process using a focused Gaussian beam, high thermal gradient and molten pool instability usually lead to coarse columnar grain, poor surface quality and degradation of mechanical properties. In this paper, a beam shaping strategy that coaxially combines fiber laser and diode laser is applied in laser welding of austenitic stainless steel. Effects of the diode laser power on both the microstructures and mechanical properties are systematically studied by experiments and simulations. Results show that a fiber-diode hybrid heat source could effectively avoid unstable states of the keyhole, and has significant advantages in controlling spatter and surface morphology. Equiaxed grains tends to form on both sides of the junctions in synergistic and dominant regions due to the reduction in the thermal gradient produced by the hybrid beam. EBSD results show that the addition of diode beam weakens the formation of columnar grains along the easy slip texture <100> due to the disturbance in the liquid metal by a transverse distribution of the diode laser. Compared with the Gaussian heat source, the joint welded by the fiber-diode hybrid laser welding provides higher strength and elongation, with typical fracture morphologies of deeper and larger dimples. The optimum mechanical properties of the joint can be obtained by 2000 W fiber laser power and 1000 W diode laser power, where cracks occur in the base metal. [ABSTRACT FROM AUTHOR]