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Taking advantage of laser additive manufacturing, continuous SiC fiber reinforced titanium matrix composites (SiC f /TMCs) with low density and excellent mechanical properties can be used to hybrid-fabricate large components with complex shape for aerospace applications. However, due to the anisotropy in both microstructure and heat transfer ability, the high-density and rapid heat input from laser during hybrid manufacturing process will cause the non-uniform damage of fibers and various solidification microstructure in the molten pool. In order to understand the solidification behavior of molten pool in SiC f /Ti17 composites, the fixed-point laser surface remelting was used to bring in rapid and symmetrical heat input. With the decrease of temperature from center to edge of the molten pool, both fiber damage degree and the contents of Si and C elements in melt decreased. The solidification microstructure contains TiC x , Ti 5 Si 3 and β-Ti, and their morphology varies according to the melt composition. SiC fibers act as heterogeneous nucleation sites for first solidified phases and promote their preference growth along the radial direction of fibers resulting from the excellent heat transfer ability. The faceted Ti 5 Si 3 grains solidified through peritectic reaction L+TiC x →Ti 5 Si 3 exhibit strong <0001> texture. [Display omitted] • Solidification behavior of SiC f /Ti17 during LSR basically follows Ti-Si-C system. • Ti 5 Si 3 solidified through L+TiC x →Ti 5 Si 3 has strong <0001> texture. • SiC fibers act as both base for solidification and fast channel for heat transfer. • Competition of heat transfer between molten pool and fibers create unique texture. [ABSTRACT FROM AUTHOR]

• Cracking susceptibility of nickel-based superalloy was investigated. • Directional solidified substrates existed lower cracking susceptibility. • Crack-free sample was successfully prepared by continuous scanning strategy. • Liquid films within high angle grain boundaries could exist at lower temperatures. Although laser additive manufacturing is a promising technique for fabricating nickel-based single crystal superalloy components, reducing hot cracking during the deposition is still a challenge. In this study, the cracking susceptibility and microstructures with different substrates are investigated. Results show that samples exhibit much lower cracking susceptibility when using the directional solidified substrates than the cast polycrystalline substrates. Hot cracking is attributed to a stable liquid film dependent on the misorientation angle. Besides, liquation cracking first occurs in the substrate and could propagate by extension into solidification cracks along the high angle grain boundaries. Based on these findings, a crack-free nickel-based superalloy with a polycrystalline substrate is successfully prepared using a continuous scanning strategy. [ABSTRACT FROM AUTHOR]

Controlling the grain structure while avoiding the stray grain (SG) formation is the key to fabricate the nickel-based single crystal (SX) superalloy components during laser additive manufacturing. In this study, a new approach is developed to inhibit SG formation by the selection of optimizing the substrate crystallographic orientation. Besides, a numerical model of combined substrate orientation with the tendency of equiaxed grain formation during epitaxial growth processing is developed to predict the SG formation in the molten pool. Combining the results from both experiments and simulations, results indicate that (001)/[110] and (01 3) / [ 100 ] substrate orientation could suppress the formation of SGs effectively. [Display omitted] [ABSTRACT FROM AUTHOR]

Laser surface nitriding technique is successfully applied to improve the surface modification efficiency of titanium alloys. Moreover, adjustment of nitride layer thickness can be achieved by applying multi-pass laser surface nitriding. Mass transfer behavior of laser surface nitriding could be totally different from the steady-state gas nitriding processes because it involves rapid heating and cooling procedures for many times, which is systematically analyzed in this research by conducting different number of laser surface nitriding passes and in-situ laser quenching after laser surface nitriding experiment on commercial pure titanium TA2 alloy. Results indicate that the thickness of nitridation layer increases with the increase of nitriding passes. After three passes laser surface nitriding, the depth of nitridation region can reach to about 56 μm. During multi-pass laser surface nitriding process, the pre-formed TiN layer will be partially decomposed under the thermal impact of subsequent laser processing. The decomposed nitrogen will further diffuse deeply into the substrate along the nitrogen concentration gradient and combine with the nitrogen from the environment to continually offer the nitrogen source during the entire laser nitriding process with high processing efficiency. • Depth of the nitridation region reaches to about 56 μm after three passes laser surface nitriding. • In-situ laser quenching is conducted to investigate the impact of subsequent laser processing. • Mass transfer behavior of nitrogen during laser surface nitriding process is given. [ABSTRACT FROM AUTHOR]

The dilution effect of the substrate during laser cladding is inherent, and controlling the dilution degree paves the way for the microstructural design of coatings with excellent mechanical properties. In this study, low-carbon 15–5 precipitation hardening (PH) stainless steel was selected as the cladding material on the surface of U75V pearlitic railway steels to improve wear performance. By altering the processing parameters, the cracking susceptibility, microstructures, and wear resistance of coatings with different dilution rates were investigated thoroughly. The results indicated that with the increase of laser beam power, the dilution effect of the eutectoid steel substrate enhanced obviously, resulting in the composition deviation of coatings. Microhardness of the coatings increased with the dilution rate but the excessive dilution effect led to severe cracking defects. The optimized cladding coating with a dilution rate of about 48 % exhibited a microhardness of 490 HV, and the wear tests showed that the wear volume decreased by about 85 % when compared to that of the U75V substrate. Microstructural characterization revealed that the increased carbon content induced by dilution, resulting in martensite hardening and carbides precipitation, was responsible for these performance improvements. This work demonstrated the potential of coating strengthening by dilution control, which expands the option for surface modification materials for railways. • Laser cladding of 15-5PH stainless steel powders was performed on U75V pearlitic steel with different laser power. • The dilution effect determined by processing parameters on the microstructures and wear resistance has been investigated. • Laser cladding coating prepared under optimized parameters showed improved microhardness and wear performance. • The increase in carbon content caused by the dilution effect was responsible for coating strengthening. [ABSTRACT FROM AUTHOR]

• Rapid W/SiC reaction induced by laser surface remelting. • W 2 C has ellipsoid shape with a P-31m space group ordered structure. • W 5 Si 3 is melted by extremely high temperature. Compared to alloys, metal-matrix composites experience more complicated microstructure evolution and interfacial reactions under extreme thermal conditions during laser additive manufacturing, welding-repairing and laser surface engineering. In this study, a cubic-shaped SiC f /Ti17 composite is heated by laser surface remelting and the rapid W-SiC reaction within the core-section of SiC fiber located in the molten pool were investigated. As fiber location close to the center of laser spot, severe reaction between W-core and SiC was observed. By applying SEM, XRD, nanoindentation and TEM, the reaction products are confirmed to be W 2 C and W 5 Si 3. The W 2 C has ellipsoid shape with a P-31m space group ordered structure, while W 5 Si 3 solidify between W 2 C resulting from its lower melting point. Because of the extreme temperature that melt W 5 Si 3 , the reaction products exhibit different morphology from those produced by solid-state diffusion such as sintering. [ABSTRACT FROM AUTHOR]

Titanium alloys fabricated by laser directed energy deposition (LDED) are usually subjected to sub-critical annealing to improve mechanical properties. This work mainly investigated the texture evolution of LDEDed Ti–6Al–2Zr–1Mo–1V alloy during sub-critical annealing and its effect on the anisotropy of yield strength. Results indicated that <0001> of α phase in the as-deposited sample exhibited two types of circle-shaped texture, which were perpendicular to the deposition direction (VD) and 45° away from VD, respectively. The intensities of these two types of texture were approximately equal because the α texture in the as-deposited sample were transformed from the strong <001> β //VD fiber texture following Burgers orientation relationship with weak variant selection. During sub-critical annealing, the formation of primary α phase, which tended to be transformed from the {110} β parallel to VD, led to the intensity of circle-shaped texture perpendicular to VD greater than that 45° away from VD. The calculation results suggested that all basal and prismatic slip systems of some α variants transformed from the {110} β parallel to VD were in hardly-activated orientation with Schmid factor (m) equal to zero for loading along the horizontal direction (HD), the slip systems of some α variants transformed from the {110} β 45° to VD were in hardly-activated orientation with m = 0 for loading along 45° direction (OD), while the systems of all α variants were in easily-activated orientation with high Schmid factors for loading along VD. After sub-critical annealing, the content of α phase in hardly-activated orientation increased for loading along HD but decreased for loading along OD, resulting in a smaller difference of yield strength between OD and VD or HD. • LDEDed Ti–6Al–2Zr–1Mo–1V alloy exhibited two types of circle-shaped <0001> α texture. • The relative content of the two types of texture was the key factor to affect the anisotropic yield strength. • The activation for slip systems of all possible α variants was analyzed. • Sub-critical annealing reduced the anisotropy of yield strength. [ABSTRACT FROM AUTHOR]

Laser surface nitriding technique has been successfully applied to greatly improve the nitriding efficiency of titanium alloy with marginally changing the surface roughness of substrate. However, due to the ultra-short action time, it is hard to understand the evolution mechanism of the microstructure during nitriding process, which is exactly the foundation to better control the quality of the nitride layer. In order to avoid the interference of alloying elements in titanium alloy on diffusivity of nitrogen, a commercial pure titanium TA2 alloy was used as the substrate in this research. Combined with finite element modeling (FEM) technology and microstructure characterizations, evolution mechanism of the microstructures during laser nitriding were systematically analyzed. Results indicate that microstructures of TiN -, α-Ti(N) - and (α' + α) are formed successively from the surface to the center. In the laser heating stage, TiN phase and stabilized α-Ti(N) phase formed, while in the cooling stage, α' martensite formed prior to the surrounding α phase, result in continuous orientation transition in α grains. Due to the gradual decrease of nitrogen contents from the surface to the center, α' martensite laths coarsened with the directional growth characterizations. • Nitriding microstructures with 27 μm depth are formed on TA2 surface by rapid laser nitriding. • Nitriding microstructures consist of the compound layer and the beneath lath zone. • Microstructures of TiN - α-Ti(N) - (α' + α) are formed from the substrate surface into the alloy. • α' martensite laths gradually coarsened with the directional growth characterizations. [ABSTRACT FROM AUTHOR]

• A novel laser deposition method to inhibit stray grain formation was developed. • The mechanism of stray grain formation at the bottom of the molten pool was revealed. • Stray grains formed are mainly due to the increased possibility of the equiaxed grain transformation. Although laser surface remelting displays excellent potential for repairing the nickel-based single crystal (SX) superalloy components, controlling the grain structure while avoiding the stray grain (SG) formation is still a challenge in the industry. In order to inhibit SG formation for nickel superalloys, a novel rotated laser irradiation approach is introduced in this paper and the study of the origin of the SG formation in laser directed solidification process. Besides, a numerical model of combined alloy composition with the tendency of equiaxed grain formation during epitaxial growth processing was developed in this paper to reveal the mechanism of SG formation at the bottom of the molten pool. Results indicate that SGs formed at the bottom of the molten pool are mainly due to the remarkably increased possibility of the equiaxed grain transformation with the rough fusion interface. During the laser process, the number of SGs gradually decreases with the increase of the angle (θ) between the laser irradiation and gravity direction. For θ = 90°, well-aligned epitaxial growth of [001] dendrites without any SGs are observed in the pool with the smooth fusion interface. Furthermore, applying the new laser deposition technique based on this phenomenon during laser directional solidification will be an effective method to control the grain-oriented growth in industrial manufacture for the single crystal components. [ABSTRACT FROM AUTHOR]

Near α titanium alloys fabricated by laser directed energy deposition (LDED) technique always display strong anisotropies of mechanical properties even they only contain a small amount of strong-textured β phase. This work gives a detailed investigation on the effect of α texture on the anisotropy of yield strength of a laser directed energy deposited (LDEDed) Ti–6Al–2Zr–1Mo–1V (TA15) alloy by consideration of the solid phase transformation. Results indicate that the yield strength of LDEDed TA15 alloy along three directions, the building direction (VD), the horizontal direction (HD) and 45° direction (OD), shows dramatically differences of 899.7 MPa, 937.3 MPa and 1016.7 MPa. The α texture is transformed from strong β texture respected to Burgers orientation relationship with weak variant selection. The crystallographic orientation and spatial orientation of textured α phase are the key factors to dominate the yield strength of LDEDed TA15 alloy at different loading directions. Along the VD direction, strong intensity of texture is found in the easily-activated <10 1 ¯ 1> and <11 2 ¯ 0> orientation of α phase, resulting in the largest Schmid factor. However, strong intensity along the OD direction in the hardly-activated <0001> orientation for basal and prismatic slip system of α phase leads to the lowest Schmid factor. The spatial orientation of textured α phase also affects the effective slip distance, which leads to the difference of resolved shear stress in different loading directions. The coupled effect of Schimd factor and effective slip distance controlled by transformed α texture mainly results in the anisotropy of yield strength. • The α texture is transformed from two types of <001> β texture. • The textured α phase has specific crystallographic and spatial orientations. • The crystallographic orientation is the key factor to dominate the anisotropic yield strength. • The spatial orientation leads to the anisotropy of resolved shear stress. [ABSTRACT FROM AUTHOR]

To give a deep insight into the nitrogen transfer mechanism during laser solid-state surface modification on titanium alloy, laser nitridation was conducted on Ti-6.5Al-3.5Mo-1.5Zr-0.3Si titanium alloy (TC11) without melting the metal surface. Microstructure characterizations of nitride layer formed under solid state condition were detailedly explored and the formation process of TiN phase was carefully discussed based on the lattice transformation. Results indicate that the nitride layer forming within a few seconds is composed of TiN phase layer and α phase layer with an orientation relationship: (1 1 ¯ 1) TiN // (0001) α , [011] TiN // [ 2 11 ¯ 0 ] α. Formation of TiN phase experiences fast solid-state transformation from α phase. Phase transformation from α-Ti lattice to TiN lattice experiences orthorhombic lattice transformation due to the shear deformation caused by rapid interstitial solution of nitrogen. After repeated laser nitridation process upon three times, nitride layer was thickened with an improved hardness of 824 HV. Linearly reciprocating ball-on-plane wear tests indicate that wear resistance of laser nitridation samples could be enhanced by a factor of 13. • Nitride layer is fabricated on TC11 alloy by laser nitridation in solid state. • The nitride layer is only composed of TiN phase and α phase. • Transformation from α phase to TiN phase experiences orthorhombic lattice. • Wear resistance of laser nitride sample can be enhanced by a factor of 13. [ABSTRACT FROM AUTHOR]

In aero-engines, cyclic softening can cause premature fracture of components, which is regarded as a challenge for the security of titanium alloy parts in service. In this work, the cyclic deformation behavior of LDEDed and β-forging TC17 titanium alloys with different morphology of α phase was explored, cyclic softening mechanism was investigated by using transmission electron microscopy (TEM) observation of dislocation morphology. The experimental results demonstrate that cyclic softening occurs when the total strain range increases to 2.0 %. The LDEDed specimen has a smaller stress amplitude and a larger plastic strain during cyclic softening than wrought specimen. In the softened deformation microstructure of LDEDed specimen, the size of α p decreases with the increase of strain, but the size and shape of α p and α s in wrought specimen do not change much. The main dislocation forms that dominate the cyclic softening in the α phase and the β matrix are slip bands and dislocation tangles, respectively, and the slip band in the α phase cannot pass through the secondary phase in the β matrix for slip transfer. Under the same strain, the lower dislocation density in the LDEDed microstructure is due to the larger directional back stress produced by the forward loading, which causes the dislocation rearrangement and annihilation under the reverse loading. • Cyclic deformation behavior of LDEDed and forging TC17 is compared. • Critical strain of cyclic softening/hardening of TC17 titanium alloy is obtained. • Cyclic softening mechanism related to α phase morphology are clarified. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel Al-Cu-Li-Mg-Ag-Sc alloy was fabricated by laser direct energy deposition. • The fully equiaxed grain structure of as-built alloy can be obtained by adding 0.3 wt% Sc. • The precipitation behavior of primary Al 3 Sc particles was studied based on transient nucleation theory. • T 1 , Al 3 Sc, T B , and quasicrystalline T 2 phases serve as primary strengthening phases. In this study, a novel Al-Li alloy with minor Sc addition was fabricated by laser directed energy deposition (LDED), resulting in a fully equiaxed grain structure. The precipitation behavior of primary Al 3 Sc particles was studied based on transient nucleation theory, and the results showed that sufficient heterogeneous nuclei by primary Al 3 Sc particles could preferentially form at initial solidification stage and induce into equiaxed grains. Besides, a large number of acicular T 1 (Al 2 CuLi) phases and spherical Al 3 Sc phases, as well as rod-shaped T B (Al 7 Cu 4 Li) phases and quasicrystalline T 2 (Al 6 CuLi 3) phases, were distributed in as-built alloy that served as primary strengthening phases by thermal cycling. However, the specific nucleation behavior of α-Al grains on primary Al 3 Sc particles and subsequent growth behavior during rapid solidification need to be further investigated. [ABSTRACT FROM AUTHOR]

One goal of metal additive manufacturing is to fabricate high-performance components without post processing. However, the last several deposited layers of as-deposited parts do not experience sub-β transus thermal cycles, resulting in slight differences in microstructure and mechanical properties from other deposited layers. Finer α lath in titanium alloys generally gives increased strength and decreased ductility, while in this study, although laser direct energy deposited Ti–6Al–2Zr–1Mo–1V alloy without sub-β transus thermal cycles exhibited finer α laths, it exhibited higher strength (921.0 ± 1.0 MPa) and similar ductility (9.2 ± 2.1%) compared to that with sub-β transus thermal cycles (898.3 ± 4.0 MPa/8.8 ± 1.3%). The ideal ductility was mainly attributed to the enhanced stain hardening ability. This study indicated that the laser direct energy deposited parts had ideal mechanical properties in the as deposited state. • The last five deposited layers experienced no sub-β transus thermal cycles. • The layers without sub-β transus thermal cycles showed finer α lamellae compared with the other layers. • The layers without sub-β transus thermal cycles exhibited higher strength and comparable ductility. [ABSTRACT FROM AUTHOR]

• Hot cracking possibility of CoCrFeNiMn high-entropy alloy could be reduced by introducing element Ti. • CoCrFeNiMnTi 0.6 without cracks was prepared by laser melted technology. • BCC phase formation released the local residual stress concentration. Additive manufacturing is a promising technique for fabricating high-alloyed metallic components such as CoCrFeMnNi high-entropy alloy with very fine microstructure and reduced macro-segregation. However, hot cracking nucleation at the interdendritic regions always occurs during preparation, which can hardly be controlled. In this study, the hot cracking possibility could be reduced by introducing element Ti into CoCrFeMnNi high-entropy alloy with the improved alloy microhardness (182 ± 8.7 HV to 973 ± 43.2 HV). At the final solidification stage, eutectic interaction takes place in the interdendritic regions. Body-centered-cubic (BCC) and a few σ phases form, dividing the liquid film and releasing the local stress concentration. CoCrFeNiMnTi 0.6 sample without cracks is observed. [ABSTRACT FROM AUTHOR]

Highlights: [•] The alternately grain morphology of columnar and equiaxed grains is found. [•] Ultrafine basket-weave microstructure is found within the prior β grains. [•] Microstructure of columnar grains is more uniform than that of equiaxed grains. [•] The LMDed TC11 acts higher strength and lower ductility than forged sample. [Copyright &y& Elsevier]