Search

Individually fabrication forged parts and then joining them together through Laser Melting Deposition (LMD) is a viable way for manufacturing large components. For investigating the effect of the grain morphology of LMD joint, two 80mm-thick Ti6Al4V plates are successfully manufactured using three different scanning speeds (10, 15, and 20 mm/s). It is essential for understanding the thermal behaviour of melt pool during LMD to improve process quality. This study focuses on the energy density of heat source and the direction of heat flux, analyzing the effect of thermal behavior on the grain morphology and dimension of deposition area, equiaxed crystal zone (EQZ) and the substrate. The macrostructure is evaluated in the different thermal condition and scanning speeds. An extremely fine equiaxed crystal was observed near the joint boundary with a high temperature gradient and cooling rate. The curve epitaxial growth of fine columnar crystal rather than along straight lines is induced by the direction of heat conduction near the joint boundary. However, the orientation angle of epitaxial growth of the coarse columnar crystal is the same as previous deposition layer at the center of 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 deposition area.

The traditional welding processes are challenging to obtain an excellent jointed structure with a thickness of over 80 mm. Meanwhile, cyclic heating for large-scale manufacturing components worsens microstructure and mechanical properties after welding. Laser Melting Deposition (LMD) technology is put forward to break through the limit of large-scale jointing components widely used for manufacturing aerospace products. Therefore, the LMD experiment with coaxial powder feeding for joining a pair of X-groove Ti-6Al-4V thick plates (80 mm) with different thermal behaviors (including laser scanning speeds and cooling rates) is carried out in this paper. The morphology evolution of its microstructure at the equiaxed grain zone (EQZ) and the deposition area under different thermal behaviors are systematically investigated. Meanwhile, the relationship between microstructure evolution mechanism and tensile properties is further explored. The volume fraction and size of acicular α/α' phases at the EQZ or the deposition area keep a stable rising when the laser scanning speed increase from 10mm/s to 20mm/s. The α-Ti cluster phases are usually precipitate around the β grain boundary with a low scanning speed and cooling rate. By contrast, the orthogonal acicular martensitic α/α' phase can precipitate inside the β grain, forming a Basketweave structure with a high scanning speed and cooling rate. The Basketweave structure forming in β grains can coordinate the deformation and reduce the stacking effect of dislocations, which is beneficial for improving ductility. However, the α-Ti cluster phases precipitating around the β grain boundary usually cause local stress concentration at these locations, forming microcracks at this location and deteriorating the tensile properties.

The laser additive manufacturing technology with laser melting deposition as the core technology presents superior efficient connectivity of titanium alloy components with large thickness and complex structure in the aerospace field. The content of nanoparticles affects the microstructure and properties of TC4 large thickness titanium alloy joint when it is manufactured with laser additive manufacturing. A pair of TC4 titanium alloy test plates with a thickness of 80 mm were connected by laser additive manufacturing technology with the mixed powder as filler material which was produced by adding different content Y2O3 nanoparticles into TC4 powder. Optical microscopy, scanning electron microscopy, micro-hardness test and tensile test were conducted to study the effect of Y2O3 content on the microstructure and mechanical properties of the TC4 titanium alloy joint. The strengthening mechanism and agglomeration weakening mechanism of Y2O3 nanoparticles on microstructure and mechanical properties was investigated based on the experimental results. It was found that the addition of more Y2O3 nanoparticles resulted in partial powder agglomeration and degradation of tensile strength. The results indicated that the columnar grains in the middle of the deposition zone with Y2O3 nanoparticles were finer than those without Y2O3 nanoparticles. The joint fracture mode gradually changed from plastic fracture to quasi-cleavage fracture with the increase of Y2O3 content. The average hardness values of the joint with various Y2O3 contents are all higher than that of the TC4 titanium alloy substrate.

Additive manufacturing (AM) cannot meet the manufacturing requirements of specific large structures, so use the welding method to joint the small size parts of AM to solve the problem. In work, the laser weld 3mm thick samples cut from WAAM block under different laser powers and welding directions to explore the feasibility of technology. The properties evolution, and weldability of the laser-welded wire arc additive manufactured Al-Cu alloy joints are investigated. The results indicate that the wire arc additive manufactured Al-Cu alloy has excellent laser weldability without apparent detection. The microstructures of the laser-welded joints consist of columnar grains, dendrites and fine equiaxed grains. The 3500 W laser power joint has higher tensile strength and elongation than other laser powers, reaching 203.48 MPa, 4.13%. Compared with the different welding direction joints, “0°and 0°” has the higher tensile strength and elongation, reaching 180.77 MPa, 2.40%. The microstructures have a weak effect, but the anisotropy plays a significant role in the tensile properties. This study has demonstrated that it is possible to laser-welded WAAM samples, and provide solutions that AM technology difficultly produces large size parts.

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.

Fabricating an Inconel 718 superalloy using electromagnetic stirring assisted laser solid forming (EMS-LSF) is a novel method to modify its microstructure and mechanical properties by consuming the Nb element in the γ phase to alleviate interdendritic segregation. The precipitate of the δ phase at 950 °C after EMS-LSF can help to achieve the uniform diffusion of Nb, and can also improve its mechanical properties. The precipitation behavior of the δ phase in an EMS-LSF Inconel 718 superalloy with different heat treatment processes has been investigated. The results show that the morphology of the δ phase changes from rod-like to a long-needle shape and tends to grow from the inter dendrite to the core dendrite with electromagnetic field intensity increasing, which is accompanied by the “cutting” and “dissolution” of the Laves phase. Through precipitation kinetics analysis, the precipitation rate of the δ phase is seen to increase with the electromagnetic field intensity increasing. Under a combination of electromagnetic stirring and laser solid forming, the microhardness of the Inconel 718 samples increased slightly due to the fact that a higher content of Nb was distributed in the core dendrite resulting from the serious convection of liquid metal, which can strengthen the matrix.

Alloy sheets of type 2060 aluminum&ndash, lithium were welded by laser beam welding (LBW) filled with ER2319 Al-5.6Cu wire. Microstructural observations showed the uneven distribution of columnar grains, equiaxed grains and equiaxed dendrite grains in the weld. The &theta, &prime, (Al2Cu) phase and other phases precipitated in the weld. The &theta, (Al2Cu) phase centrally distributed at the grain boundaries. During the immersion corrosion, the pitting corrosion first occurred and then gradually expanded and transformed to intergranular corrosion and exfoliation corrosion. The electrochemical corrosion test showed a higher corrosion tendency of the base metal and heat-affected zone for the lower corrosion potential, but the corrosion current density of the weld was relatively larger. The segregation of Cu, Mg and other elements at the grain boundary aggravated the occurrence of intergranular corrosion.