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Additive manufacturing is an advanced forming method to fabricate complex components, while cracks appear in the deposited nickel-based single crystal superalloy samples undesirably. Though fabrication of crack-free superalloy were reported, effective solutions to fabricate crack-free nickel-based single crystal superalloy samples are still uncovered, thus the additive manufacturing in single crystal superalloy is still an extremely difficult work. In this work, we fabricated nickel-based single crystal superalloy samples through laser powder deposition, and analyzed the causes of crack during deposition processing. Experimental results demonstrate that the as-deposited thin-wall samples presented single crystal structure and its crystal orientation is well aligned with the [001] orientation of substrate, while some cracks appeared in the as-deposited block samples. Large misorientation angle in the local regions played significant roles in the cracks initiation and the further cracking propagation. Solidification cracks are attributed to the shrinkage cavities and the high-melting point interdendritic carbides, while liquation cracks are attributed to the low melting point compounds. These findings give a guide for causes analysis on cracks in the nickel-based single crystal superalloy fabricated by laser powder deposition additive manufacturing, which has bright prospects in single crystal superalloy manufacturing and repair. Keywords: Superalloy, Single crystal, Crack, Laser powder deposition, Additive manufacturing

As an advanced material forming method, additive manufacturing (AM) has been rapidly developed in many industry fields. Due to the non-weldability of nickel-based single crystal (SX) superalloys and complex solidification and stress conditions of AM process, hot cracks can easily form in depositions. Previous research focused on the additive manufacturing of single-track thin-walled SX samples, while effective solutions to fabricate crack-free multi-track SX block samples are still limited. Directed energy deposition (DED) technique was used to fabricate multi-track block samples in this work. The effects of the overlapping ratio, scanning velocity, and laser power on the microstructure and cracking susceptibility were analyzed. The experimental results demonstrated that improper process parameters play significant roles in the formation of high-angle grain boundaries and large internal stress, both of which will increase the cracking susceptibility. Crack-free SX multi-track block was successfully fabricated after adjusting the process parameters. These findings provide a guide to reduce the cracking susceptibility by controlling the process parameters, which aids in fabrication of large-scale crack-free SX blocks through DED technique.

In the present research, the microstructure and phase composition of high-Nb TiAl powder, produced by plasma rotating electrode process (PREP) have been investigated in details. PREPed powder with a mean particle size larger than 150 μm demonstrates a dendrite structure, and that smaller than 75 μm indicates a martensitic structure. PREPed powder is mainly composed of a hexagonal a2 phase, while in fine powder and coarse powders minor β phase and γ phase are detected, respectively, which is related to solidification path and cooling rate of molten droplets in PREP process. Phase composition and microstructure of the high-Nb TiAl powder have intrinsic effects on the resultant microstructure of hot isostatic pressed (HIPed) billets. After HIP processing, most martensitic and dendritic particles transformed into near γ structure, while few coarse dendritic powder was prone to introduce defects, such as residual primary particle boundaries (PPBs) or coarse lamellae structure, which were harmful to the mechanical properties of HIPed billets. The residual primary particle boundaries (PPBs) or coarse lamellae structure obviously decreased the ultimate tensile strength at room and elevated temperature. Keywords: Microstructure, High–Nb TiAl, Powder, Densification, Mechanical properties

In this work, Cu-based microcomposites were fabricated by powder metallurgy of gas-atomized Cu-15wt% Fe alloy powder, followed by drawing to various strains. The microstructure evolution and the resultant mechanical and electrical properties were investigated by scanning electron microscopy, electron backscatter diffraction, and tensile testing, among other techniques. The results indicated that during drawing, the Fe phase evolved into nanoscale filaments along the longitudinal direction. With increasing drawing strain, both the Cu grains and the Fe filaments were refined gradually, giving rise to a mean thickness of approximately 25nm for the Fe filaments at a total strain of 4.0. The iron oxides particles formed due to contamination of oxygen maintained unchanged during drawing process. In addition, it was found that an intermediate aging during the drawing can significantly enhance the electrical conductivity of the microcomposites without sacrificing their strength. The present process involving an initial powder with a homogenous distribution of Cu and fine Fe phases makes it possible to obtain microcomposites with an ideal combination of electrical and mechanical properties at lower drawing strain, compared to those produced by conventional casting methods. Keywords: Cu-Fe microcomposite, In situ technique, Hot consolidation, Electrical conductivity, Strength, Microstructure evolution

In the present research, the influence of Co addition on the microstructure and hot workability of IN713C alloy was investigated in detail by scanning electron microscope (SEM), transition electron microscope (TEM) and electron backscattering diffraction (EBSD) etc. Substitution of Ni by Co leads to lower driving force for formation of γ′ phase. In addition, with the increase of Co fraction, the γ′ phase precipitates turned finer and more cuboidal shape. As a result, the hot workability of IN713C alloy was improved significantly by Co addition. This can be ascribed to the decrease in activation energy, misfit between γ/γ′ phase lattice, the stacking fault energy as well as the fraction of γ′ phase at elevated temperature after adding Co. Keywords: Cobalt addition, Hot workability, DRX, Superalloy, SFE, Misfit

The precipitation behavior of a novel cobalt-based superalloy subjected to prior plastic deformations was investigated in details by using field emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM), etc. The precipitation of a μ-phase, which appeared at temperatures of 1073–1373 K, was confirmed, while MC-type carbide was observed at the temperatures >1173 K. The prior plastic deformations promoted the precipitation of the μ-phase during subsequent aging. Further, the precipitation of the μ-phase at temperatures lower than 1173 K was confirmed to occur predominantly along the {111} slip bands; this was the dominant deformation mechanism of this alloy during plastic deformation. In contrast, there was no evidence that the precipitation of the MC-type carbide was affected by the prior plastic deformations in the case of the specimens exposed at 1273 and 1373 K. Keywords: μ-Phase, Prior plastic deformations, TCP phase, Cobalt-based superalloy, Suzuki segregation

We created high-aspect-ratio sub-micron-sized lamellar porous structure by simple anodic selective dissolution of lamellar Ti-Al alloy in NaCl aqueous solution. The geometries of lamellar structures, which determined those of lamellar porous structure, were controlled by hot forging and subsequent ageing treatment before dissolution process. Although the changes in microstructure affected the selective dissolution behaviors, γ-TiAl phase could be dissolved selectively. As a result, the average sizes of unidirectional units of lamellar pores could be controlled in the range from to 25 μm to 500 μm. The pitches of the lamellar pores were controlled in the wide range from below 100 nm to over 1 μm. Keywords: Aluminum, Titanium, SEM, XPS, Anodic dissolution

TiC-dispersion-strengthened Cu alloys were prepared by a two-step ball-milling (BM) process followed by sparks plasma sintering (SPS), heat treatment and hot rolling in sequence. The two-step BM process is composed of a pre-ball-milling (pre-BM) on both Ti and graphite powders as well as a subsequent homogenizing by BM together with Cu powder. Microstructure evolution analysis was performed to evaluate the effects of BM conditions on the electrical and mechanical properties of Cu-based alloys. The X-ray results revealed that titanium carbide (TiC) formed from Ti and C under high impact energy BM. Moreover, the formation of TiC during the SPS and heat treatment processes was found to more beneficial in enhancing the mechanical properties of alloy. The residual Ti in Cu matrix was found to be the predominant factor lowering the electrical conductivity of Cu–Ti–C alloys.