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The effect of crystal orientation and heat treatment on microstructure and wear properties of Inconel 718 superalloy fabricated by laser directed energy deposition (LDED) were analyzed and compared with forged samples. Fully heat treated samples presented the lowest wear rate and better resistance to friction compared with other condition samples. The direct aging treated samples also had better wear resistance attributed to its microhardness and microstructure and were mainly abrasive wear with a large amount of wear debris and deeper scratches on the worn surface. As for the anisotropy of microstructure in LDED samples, the wear properties in different crystal orientations were different, and better wear performance was obtained in the section which is vertical to the deposition direction. • Anistropic microstructure leads to different wear resistance. • The solution of Laves phase and precipition of γ″ phase enhance the wear property. • Heat treatment can decrease the difference in wear property of differnt sections. [ABSTRACT FROM AUTHOR]

The coarse columnar grains and special interface in laser solid formed (LSFed) Inconel 718 superalloy workpieces seriously affect their mechanical properties. To improve the microstructure and mechanical properties of LSFed Inconel 718 superalloy, electromagnetic stirring (EMS) was introduced to alter the solidification process of the molten pool during LSF. The results show that EMS could not completely eliminate the epitaxially growing columnar grains, however, the strong convection of liquid metals can effectively influence the solid–liquid interface growing mode. The segregation of alloying elements on the front of solid–liquid interface is inhibited and the degree of constitutional supercooling decreases correspondingly. Comparing the microstructures of samples formed under different process parameters, the size and amount of the γ+Laves eutectic phases formed in interdendritic area decrease along with the increasing magnetic field intensity, resulting in more uniformly distributed alloying elements. The residual stress distribution is proved to be more uniform, which is beneficial to the grain refinement after recrystallilzaiton. Mechanical properties testing results show an improvement of 100 MPa in tensile strength and 22% in elongation was obtained after EMS was used. The high cycle fatigue properties at room temperature was also improved from 4.09 × 10 4 cycles to 8.21 × 10 4 cycles for the as-deposited samples, and from 5.45 × 10 4 cycles to 12.73 × 10 4 cycles for the heat treated samples respectively. [ABSTRACT FROM AUTHOR]

To improve the high temperature oxidation resistance of TA2 titanium alloy, a gradient Ni-Ti coating was laser cladded on the surface of the TA2 titanium alloy substrate, and the microstructure and oxidation behavior of the laser cladded coating were investigated experimentally. The gradient coating with a thickness of about 420–490 µm contains two different layers, e.g. a bright layer with coarse equiaxed grain and a dark layer with fine and columnar dendrites, and a transition layer with a thickness of about 10 µm exists between the substrate and the cladded coating. NiTi, NiTi 2 and Ni 3 Ti intermetallic compounds are the main constructive phases of the laser cladded coating. The appearance of these phases enhances the microhardness, and the dense structure of the coating improves its oxidation resistance. The solidification procedure of the gradient coating is analyzed and different kinds of solidification processes occur due to the heat dissipation during the laser cladding process. [ABSTRACT FROM AUTHOR]

Abstract: The microstructure variation of as-deposited Inconel 718 superalloy sample in deposition direction fabricated by laser solid forming (LSF) was investigated. It was found that the as-deposited microstructure was mainly consisted of predominant γ columnar dendrites directionally growing along the deposition direction. Primary dendrite arm space measurement revealed spaces of 11.5, 17.5 and 38.0μm at the bottom, the middle and the top section of the LSF sample respectively. The result was further studied by comparing with the Kurz–Giovanola–Trivedi (KGT) dendrite growth model, which was in a reasonable agreement with the experimental result. The various microstructures and micro-segregation situations at different sections of the LSF sample, which were induced by the thermal accumulation during LSF process, were also carefully discussed. [Copyright &y& Elsevier]

Abstract: The effect of overlap rate on the residual stress, recrystallization nucleation location and the distribution of recrystallized grains of LSFed Inconel 718 superalloy were investigated. It is found that the as-deposited microstructures with different overlap rates have the similar characteristics including that columnar grains grow along the deposition direction. The overlap area possesses higher residual stress compared with the inner-pass area, and the position of peak residual stress is closer to previous pass. The increase in the overlap rate will broaden the variation range of the residual stress. During heat treatment, the recrystallization nucleation first occurs in the overlap area with the lower overlap rate, and then expands to both overlap area and inner-pass area paralleling with the increasing of the overlap rate. Although the overlap rate is only one of basic factors in the Laser Solid Forming process, it plays an important role in controlling residual stress distribution, material microstructure and mechanical properties. The higher overlap rate adopted, the finer and the more evenly distributed grains could be obtained through recrystallization for LSFed Inconel 718 superalloy. [Copyright &y& Elsevier]

Abstract: Two kinds of laser scanning paths, i.e. single direction raster scanning (SDRS) and cross direction raster scanning (CDRS), were used to prepare Inconel 718 alloy parts by laser solid form (LSF) technology. The microstructures and mechanical properties of LSF Inconel 718 samples were investigated. It is shown that the as-deposited microstructure of SDRS sample is composed of columnar dendrites which grow epitaxially along the deposition direction; but in the CDRS sample, the continuous directional growth of columnar grains is inhibited and the orientation deviation of dendrites in two adjacent layers increases. The as-deposited grains of the CDRS sample are finer than those of the SDRS sample. After heat treatment, recrystallization occurs and grains of both samples are refined. However, the recrystallized grains in SDRS sample are not as uniform as that of the CDRS sample. Tensile testing at room temperature indicates that the ultimate tensile strength of these two samples is similar; however, the ductility of the CDRS sample is much better than that of the SDRS sample. Fracture surface examination presents a transgranular mode of crack propagation for the SDRS sample and a mixture of transgranular and intergranular modes of crack propagation for the CDRS sample. The inhomogeneity of grain size is considered to be the prime reason for the worse ductility of the SDRS sample. [Copyright &y& Elsevier]

Abstract: The microstructure and residual stress of laser rapid formed (LRFed) nickel-base superalloy Inconel 718 was investigated. The as-deposited microstructure of an LRFed Inconel 718 alloy is composed of columnar dendrites growing epitaxially along the deposition direction, and the columnar dendrites transformed to unevenly distributed equiaxed grains after annealing treatment at high temperature. Residual stress evaluation in microstructure scale by Vickers micro-indentation method indicates that the residual thermal stress is unevenly distributed in the LRFed sample, and it has a significant effect on the recrystallization during solution annealing treatment. The residual stress is introduced by rapid heating and cooling during laser rapid forming. There is an alternative distribution between high residual stress regions and low residual stress regions, within a single deposited layer, resulting in a similar distribution of recrystallized grain size. [ABSTRACT FROM AUTHOR]

Abstract: Aluminum matrix composites reinforced by different contents of multi-walled carbon nanotubes (MWCNTs) were fabricated by friction stir processing (FSP). The microstructure of nano-composites and the interface between aluminum matrix and MWCNTs were examined using optical microscopy (OM) and transmission electron microscopy (TEM). It was indicated that MWCNTs were well dispersed in the aluminum matrix throughout the FSP. Tensile tests and microhardness measurement showed that, with the increase of MWCNT content, the tensile strength and microhardness of MWCNTs/Al composites gradually increased, but on the contrary, the elongation decreased. The maximum ultimate tensile strength reached up to 190.2MPa when 6vol.% MWCNTs were added, and this value was two times more of that of aluminum matrix. Appearances and fracture surface micrographs of failed composite samples indicated that the composites become more and more brittle with the increase of the MWCNT content. [Copyright &y& Elsevier]

Abstract: The crystallization characteristics of Zr55Al10Ni5Cu30 bulk metallic glasses BMGs during pulsed laser surface melting (PLSM) were examined, and the crystallization behavior during Laser solid forming (LSF) of Zr55Al10Ni5Cu30 BMGs with the pre-laid powder method on the amorphous substrates was further investigated. It was found that the BMG could keep the amorphous state after PLSM with six pulses and crystallization began to occur in heat-affected zone (HAZ) after PLSM with twelve pulses. There was no crystallization occurred in the deposit with one and two layers during LSF, and the volume fraction of amorphous phase in the deposit with seven layers deposit was about 92.44%. The crystallization degree did not increase remarkably with the increasing of deposited layers. The crystallization mainly occurred in HAZ during PLSM and LSF. A physical model was proposed to describe laser solid forming of BMGs, which explained the formation mechanism of BMGs during laser solid forming. It is shown that the crystallization during the PLSM and LSF process was mainly caused by the accumulation of structural relaxation in the HAZ. The size of HAZ should be smaller than the thickness of single pulsed laser deposited layer during LSF of BMGs without crystallization. Based on the present model and experiment results, we can reckon that bulk metallic glasses could be achieved by LSF without size limitation. [Copyright &y& Elsevier]

Ni-Sn anomalous eutectic is obtained by single track laser cladding with the scanning velocity from 1 mm/s to 10 mm/s using the Ni-32.5 wt.%Sn eutectic powders. The microstructure of the cladding layer and the grain orientations of anomalous eutectic were investigated. It is found that the microstructure is transformed from primary α-Ni dendrites and the interdendritic (α-Ni + Ni 3 Sn) eutectic at the bottom of the cladding layer to α-Ni and β-Ni 3 Sn anomalous eutectic at the top of the cladding layer, whether for single layer or multilayer laser cladding. The EBSD maps and pole figures indicate that the spatially structure of α-Ni phase is discontinuous and the Ni 3 Sn phase is continuous in anomalous eutectic. The transformation from epitaxial growth columnar at bottom of cladding layer to free nucleation equiaxed at the top occurs, i.e., the columnar to equiaxed transition (CET) at the top of cladding layer during laser cladding processing leads to the generation of anomalous eutectic. [ABSTRACT FROM AUTHOR]

In this paper, creep-fatigue behaviors of the thin-walled tubular specimen with film holes were experimentally studied to address the influence of cyclic stress interruptions on creep strength and creep-fatigue life of a directionally solidified superalloy. The specimens with and without multiple holes were designed and manufactured to consider the effect of stress concentration produced by film cooling holes. The creep life of the specimen with holes has reduced significantly with the introduction of fatigue. An interesting result was found that the creep strength and life was enhanced on the stress concentrated specimens even if the creep loading was interruption by fatigue loadings, while creep fatigue life will be commonly shortened for notched specimen. Failure mechanisms related with cooling holes were analyzed on the basis of microscope observation. • Creep-fatigu tests of specimens with film-cooling holes were carried out. • Fatigue creep interaction is reflected in the formation of dimples. • High-temperature deformation and stress concentration is a key factor leading to damage. • The creep life of the specimen with holes has reduced with the introduction of fatigue. • The relationship between rafting type and stress state around film cooling hole was found. [ABSTRACT FROM AUTHOR]

The aim of this study was to elucidate cyclic hardening behavior and deformation mechanisms of the strain-hardened zones of friction-stir-welded (FSWed) dissimilar AA5083-to-AA2024 aluminum alloy joints (with a heterogeneous microstructure) after cyclic deformation via quaasi-in-situ electron backscatter diffraction (EBSD) technique and transmission electron microscopy (TEM) examinations. The extent of cyclic hardening was observed to decrease with increasing number of cycles at a constant total strain amplitude. Strain localization occurred mainly in the stir zone (SZ) and heat-affected zone (HAZ) on the AA5083 side, accompanied by an increasing density of dislocations during cyclic deformation identified via both EBSD and TEM. The strain hardening capacity of AA5083 SZ was significantly higher than that of AA5083 HAZ based on the density of geometrically-necessary dislocations (GNDs). The {111}-oriented grains exhibited a higher strain hardening capacity than other grains. Cracks initiated and propagated in the AA5083 HAZ along with the presence of shear bands. The evolution of GNDs played a predominant role in the AA5083 SZ, while the evolution of statistically-stored dislocations (SSDs) was dominant in the AA5083 HAZ during cyclic deformation. • In-situ EBSD was used to clarify the fatigue mechanisms of FSWed dissimilar Al. • The cyclic hardening behavior of FSWed dissimilar Al was quantified. • The strain hardening capacity of the joints are highly inhomogeneous. • The evolution of GNDs plays a predominant role in the AA5083 SZ. [ABSTRACT FROM AUTHOR]

• After high temperature heat treatment, a static recrystallization of the deposit occurred. • Element segregation altered the precipitation kinetics of γ′′/γ′ phases. • A judicious post-heat-treatment increased Δ K th and decreased FCGR of LDED IN718. • The Δ K th of HSA specimen increased by ~ 2.6 times vs. as-deposited specimen. In this study, Inconel 718 (IN718) specimens, manufactured by laser directed energy deposition (LDED), are subjected to three different heat-treatments to have different microstructural characteristics. Fatigue crack growth (FCG) testing is performed on both as-deposited and heat-treated specimens. The coarse Laves phases with uneven γ′′/γ′ precipitates severely decrease FCG resistance of LDED IN718. The soft precipitate-free zone around short-acicular δ-phase partly reduces FCG resistance of LDED IN718. The FCG threshold (Δ K th) of a judicious heat-treatment specimen increases by ~ 2.6 times vs. as-deposited specimen and reaches 10.8 MPa∙m1/2, and its Δ K th and FCG curve are comparable to those of wrought IN718. [ABSTRACT FROM AUTHOR]

• The effects of inclusions on fatigue life of FGH96 were investigated at 600 °C. • The scatter of inclusion-induced fatigue life is interpreted from fracture mechanics. • A unified maximum SIF calculation method is proposed for different types of crack. • Life is calculated by fracture mechanics for various inclusion sizes and positions. This paper studied the effect of inclusion factors on the fatigue life. of a P/M superalloy. Tests were conducted at 600 °C and fractographies were examined to identify the failure source. The degree of the detrimental effect on life was related to inclusion size, position and applied stress level. An explanation for these factors on fatigue life based on fracture mechanics was proposed. Inclusions were considered as crack and K max calculation was correlated with inclusion position and shape. The small crack growth behaviour was also considered and good life prediction was obtained by integrating the rate equation with initial defect size. [ABSTRACT FROM AUTHOR]