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Abstract: In order to improve the wear resistance of the γ-TiAl intermetallic alloy, microstructure, room- and high-temperature (600°C) wear behaviors of laser clad γ/Cr7C3/TiC composite coatings with different constitution of NiCr–Cr3C2 precursor-mixed powders have been investigated by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive spectrometer (EDS), block-on-ring (room-temperature) and pin-on-disk (high-temperature) wear tests. The responding wear mechanisms are discussed in detail. Results show that microstructures of the laser clad composite coatings have non-equilibrium solidified microstructures consisting of primary hard Cr7C3 and TiC carbides and the inter-primary γ/Cr7C3 eutectic matrix, about three to five times higher average microhardness compared with the TiAl alloy substrate. Higher wear resistance than the original TiAl alloy is achieved in the clad composite coatings under dry sliding wear conditions, which is closely related to the formation of non-equilibrium solidified reinforced Cr7C3 and TiC carbides and the positive contribution of the relatively ductile and tough γ/Cr7C3 eutectics matrix and their stability under high-temperature exposure. [Copyright &y& Elsevier]

Abstract: In order to improve the tribology and high-temperature oxidation properties of the Ti–48Al–2Cr–2Nb intermetallic alloy simultaneously, mixed NiCr–Cr3C2 precursor powders had been investigated for laser cladding treatment to modify wear and high-temperature oxidation resistance of the material. The alloy samples were pre-placed with NiCr–80, 50 and 20%Cr3C2 (wt.%), respectively, and laser treated at the same parameters, i.e., laser output power 2.8kW, beam scanning speed 2.0mm/s, beam dimension 1mm×18mm. The treated samples underwent tests of microhardness, wear and high-temperature oxidation. The results showed that laser cladding with different constitution of mixed precursor NiCr–Cr3C2 powders improved surface hardness in all cases. Laser cladding with NiCr–50%Cr3C2 resulted in the best modification of tribology and high-temperature oxidation behavior. X-ray diffraction (XRD), optical microscope (OM), scanning electron microscopy (SEM) and energy-dispersive spectrometer (EDS) analyses indicated that the formation of reinforced Cr7C3, TiC and both continuous and dense Al2O3, Cr2O3 oxide scales were supposed to be responsible for the modification of the relevant properties. As a result, the present work had laid beneficial surface engineering foundation for TiAl alloy applied as future light weight and high-temperature structural candidate materials. [Copyright &y& Elsevier]

Abstract: Mixed NiCr–Si precursor powders on γ-TiAl intermetallic alloy (Ti–48Al–2Cr–2Nb) have been investigated for laser cladding treatment to modify the tribology and high-temperature oxidation properties of the material simultaneously. The alloy samples are pre-placed with NiCr–50% and NiCr–40%Si (wt.%), respectively, and laser treated at the same parameters, i.e., laser output power 2.8 kW, beam scanning speed 2.0 mm/s, beam dimension 1×18 mm. The treated samples underwent tests of microhardness, full dry sliding wear and high-temperature oxidation. The results showed that laser cladding with different constitution of mixed precursor NiCr–Si powders improve the surface hardness in all cases. Laser cladding with NiCr–40% Si precursor mixed powders results in the better modification of tribology and high-temperature oxidation behavior. However, with NiCr–50%Si precursor powders shows decreased wear resistance compared to the original TiAl alloy due to the existence of a large amount of hard and brittle Ti5Si3 and TiSi compounds. Optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive spectrometer (EDS) analyses indicated that the formation of the reinforced primary Ti5Si3, γ/TiSi eutectics and the refinement of the microstructure and both the continuous and dense Al2O3, SiO2 hybrid oxide scales are supposed to be responsible for the modification of the relevant properties. Laser cladding with NiCr–Si mixed powders is anticipated to be a promising wear- and oxidation-resisting surface modification technique for TiAl intermetallic alloy. [Copyright &y& Elsevier]

The change of layer thickness can effectively influence the solidification condition and thermal history of the Al-5Si-1Cu-Mg aluminum alloy fabricated by laser additive manufacturing, and thus affect the microstructures and mechanical properties. Samples with ultrafine grain structures and excellent mechanical properties were successfully fabricated, and the microstructure and mechanical property differences related to the variations of layer thickness were systematically investigated. Results indicate that when higher layer thickness is employed, columnar grain structure is established with {100} Al texture due to high temperature gradient, while decreasing the layer thickness promotes the formation of equiaxed grains. Lower layer thickness sample with finer near-equiaxed grain structure has improved elongations. However, the strength is unexpectedly poorer than the higher layer thickness sample with near-columnar grain structure. The comparably better plasticity but poorer strength is attributed to the relatively coarser microstructure within grains, i.e. larger primary dendrite arm spacing with Si phases distributing more dispersed, introduced by more heat effect of subsequent deposition when applying lower layer thickness. For near-columnar sample, the cracked Si phases with different spacings perpendicular to loading direction would result in different microcrack coalescence behaviors and thus cause plasticity anisotropy. Whereas for near-equiaxed sample, due to the tailored distribution of Si phases, the direction-dependent elongation discrepancy is reduced. • High-property Al-5Si-1Cu-Mg samples were produced by laser additive manufacture. • The microstructure and tensile property related to layer thickness are examined. • Near-columnar and near-equiaxed grain structures are formed for different samples. • Near-equiaxed sample exhibits better plasticity but slightly lower strengths. • Plasticity anisotropy is lessen in near-equiaxed one and the mechanism is studied. [ABSTRACT FROM AUTHOR]

Abstract The thin-walled Al-5Si-1Cu-Mg aluminum alloy plate was fabricated by laser melting deposition additive manufacture technique followed by heat treatments. The microstructure and mechanical properties of the alloy are investigated considering the effects of solution/solution and artificial aging. Results show that due to the high cooling rate, a fine microstructure is formed with distinct grain structures and eutectic Si network embedded in the Al matrix, which gives rise to significantly better tensile properties (yield strength: 99 MPa, ultimate tensile strength: 223 MPa, elongation: 12.1%) as compared to as-cast Al-5Si-1Cu-Mg counterpart. After solution treatment, a superior ductility of approximately 22.4% can be achieved due to the increased mean distance and globularization of Si phases, and the appearance of nano-metric β〞and C phases during subsequent aging treatment gives a remarkable comprehensive mechanical behavior: The specimens show yield strength of 316 MPa, ultimate tensile strength of 416 MPa along with fracture strain of 15.2%. The combined effect of laser melting deposition and post-process heat treatments can yield parts with excellent mechanical properties, promoting the material for a wider range of applications. [ABSTRACT FROM AUTHOR]

Good hydrogen-environmentally embrittlement (HEE) resistance is necessary for safe service of critical main load-bearing ultra-high strength steel (UHSS) components of advanced aircrafts in marine atmosphere environment. Controlling isothermal tempering time is one important heat treatment method to optimize HEE resistance of UHSS by designing microstructure. Influence of isothermal tempering time at 482 °C on microstructures and HEE susceptibilities of laser additively manufactured (LAM) AerMet100 steel was investigated by several material characterization methods and slow strain rate tensile (SSRT) tests in both of air and 3.5 wt% NaCl aqueous solution. Results mainly indicate that microstructure evolution behaviors of LAM AerMet100 steel successively include martensite and retained austenite decomposition, M 2 C carbide formation and coarsening, and film-like reverted austenite formation and thickening. Short-time tempering specimens of LAM AerMet100 steel has the highest strength-loss-index in all of the specimens due to the lowest austenite amount. The lower HEE susceptibility of LAM AerMet100 steel in long-time tempering condition is mainly corresponding to M 2 C carbide coarsening and film-like reverted austenite thickening. Furthermore, difference of hydrogen charging method strongly influences macro-fracture behaviors of the SSRT specimens, but micro-cracking modes of the steel in different conditions are predominantly martensite packet/plate boundary cracking. Under open-circuit potential condition, HEE cracking zone is prone to be origin from pitting corrosion sites, and the SSRT specimen in long-time tempering condition have a good ductility; in contrast, under cathodic polarization condition, HEE cracking zone is around the circumferential zones, and the tensile specimens are brittle fracture. These findings are important when considering achieving improved HEE resistance for LAM AerMet100 steel tempered at 482 °C of a longer isothermal tempering time at the strength level of interest. [ABSTRACT FROM AUTHOR]

Ultrahigh-strength AerMet100 steel plate was fabricated by laser additive manufacturing process (LAM) followed by subsequent heat treatments. Microstructures and tensile mechanical properties of the steel were examined using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and universal mechanical testing machine. The results indicate that post-LAM heat treatments greatly change alloy carbide characteristics, decrease austenite content and prompt the formation of fine equiaxed prior-austenite grains for LAM AerMet100 steel. Compared to fine short rod-like Nb-rich MC carbides and needle-like M 3 C carbides in as-deposited specimens, alloy carbides in tempered specimens have the large changes, which include fine spherical Nb-rich MC carbides, fine rod-like M 2 C carbides, large-size spherical Mo-rich M 6 C carbides and Cr-rich M 23 C 6 carbides. And the fine rod-like M 2 C carbides are dispersive distribution and coherency with the martix martensite, resulting in strong coherent strain strengthening. After proper post homogeneity heat treatment processes, tensile mechanical properties of LAM AerMet100 steel have the comprehensive improvement, which are comparable to those of the forged ones. In comparison with low ductility of as-deposited specimen, the higher ductility of tempered specimen is mainly related to its extremely strong strain-strengthening capability achieved by dispersive precipitation of M 2 C carbides. With the increase of homogenization, the further improved ductility of tempered specimen mainly ascribe to the decreased amount and size of large-size alloy carbides (especially Mo-rich M 6 C carbide). [ABSTRACT FROM AUTHOR]

In order to evaluate the effects of microstructure characteristics on fatigue crack growth (FCG) resistance of laser additive manufactured (LAM) AerMet100 steel, microstructures and FCG behaviors (in Paris region) of as-deposited specimen and three types of tempered martensite specimens were examined. Results indicate as-deposited specimens of LAM AerMet100 steel have apparent texture characteristics of epitaxy unidirectional growth prior-austenite columnar grains and grain-interior inter-dendritic blocky retained austenite with [001] crystallographic orientation. And poor boundary cracking resistance of these texture characteristics along deposition direction mainly contributes to the FCG rate anisotropy of as-deposited specimens. After post-LAM heat treatments, the FCG resistance of all heat-treated specimens apparently improves with the fracture mode of transgranular cracking. With the increase of yield strength, the value of Paris coefficient C of the steel increases, but the value of Paris exponent m decreases. Compared to the poor dislocation slip resistance of bainite plates in as-deposited specimens, the improved dislocation slip resistance of martensite plates is mainly related to the strong dislocation pinning effect of fine dispersive rod-like coherent M 2 C carbides, resulting in the stronger FCG resistance of the heat-treated specimens. In the Paris region of low ΔK (< ~ 20 MPa m 1/2 ), fatigue cracks mainly propagate along the bainite (or martensite) plate interfaces, and the FCG rate of the steel can be effectively decreased by containing higher contents of thick film-like retained/reverted austenite; with the increase of Δ K , besides propagating along the soft inter-plate film-like austenite, fatigue cracks can also directly pass through the harder bainite (or martensite) plates with the striations and secondary cracks observed on fracture surfaces; in the Paris region of high Δ K (> ~ 70 MPa m 1/2 ), higher contents of retained/reverted austenite inversely accelerate the FCG rate of heat-treated LAM AerMet100 steel. In contrast, grain refinement has the little influence on the FCG rate (in most of Paris region) of the heat-treated specimens. [ABSTRACT FROM AUTHOR]

The control of solidification microstructure is critical to successful laser processing of single-crystal (SX) nickel-base superalloys and a practical tool for the microstructural control is processing–microstructure maps. However, the maps presented in literature do not consider the effects of powder feeding during laser additive manufacturing (LAM) of SX superalloys. This paper therefore presents a simple and feasible strategy to deal with the effects of powder feeding and to extend the combined numerical model used to calculate processing–microstructure maps. A characteristic ratio of epitaxial SX growth was defined to quantitatively compare the final solidification microstructure. Resulting processing–microstructure maps can estimate the influence of most processing variables, especially powder feeding rate, on the extent of epitaxial SX growth and the position of columnar-to-equiaxed transition. Using the processing parameters selected according to these processing–microstructure maps, a multi-layer SX deposit with fine dendrites was successfully fabricated by LAM. This successful SX LAM indicates that these new processing–microstructure maps involving powder feeding are reliable and useful because they can determine proper processing windows for LAM of SX superalloys and further advance the understanding of the processing–microstructure relationship in powder-feeding LAM process. [ABSTRACT FROM AUTHOR]

The near-β titanium alloy Ti-5Al-5Mo-5V-1Cr-1Fe (Ti-55511) fabricated by laser additive manufacturing was further annealed by multiple low temperature aging heat treatments. Microstructures and fracture surfaces were observed by optical and electron microscopes. Brittle tensile fracture behaviors were analyzed and cleavage fracture and quasi-cleavage fracture were distinguished. Results show that athermal ω phases in Ti-55511 have negligible strengthening effect because of their very small sizes and low content. With the aid of age hardening heat treatment, nano-scaled α phases precipitate, leading to higher microhardness. But the tensile properties of Ti-55511 did not effectively improve because brittle fracture happens due to the precipitation of nano-scaled α phases, which have similar sizes with brittle ω phases. The brittle fracture behavior is more relevant to α phase sizes instead of their volume fractions. With aging temperature increasing, the sizes of nano-scaled α phases increase and the alloy brittleness slightly decreases. [ABSTRACT FROM AUTHOR]

The near-β titanium alloy Ti-5Al-5Mo-5V-1Cr-1Fe (Ti-55511) was treated by laser surface remelting (LSR) with a series of holding times at 500 °C in order to study the growth behavior of ω-assisted α phase and its effect on mechanical properties. Transmission electron microscope and hardness test was used to characterize different phases and surface property of different samples. Results show that only a small quantity of athermal ω phases precipitate from β matrix at the remelted zone and there are incomplete collapsed {111} β lattice planes at the β/ω boundaries because Ti-55511 has low electron/atom ratio and high Al contents. So the athermal ω phase precipitated during LSR process has little effect on increasing surface hardness of Ti-55511. ω-Assisted nano-scaled α lamellas can be formed by the connection of some initial α phases at the early stage of low temperature isothermal heat treatment. And all the ω phase dissolved after 500°C/10min. Further increasing of holding times will cause volume fractions increase of the nano-scaled α lamellas, resulting in gradually improving hardness of the alloy at the remelted zone. [ABSTRACT FROM AUTHOR]

Gas tungsten arc welding was used to repair the laser additive manufactured Ti-5Al-5Mo-5V-1Cr-1Fe (Ti-55511) alloy with a subsequent triplex annealing treatment. The tensile properties of heat treated specimens containing of different proportions of weld zone were designed to evaluate the influence of weld zone on tensile properties of the alloy. Microstructures, microhardness and tensile tests were performed to study the mechanical properties and fracture behaviors of the specimens. Results show that dissolved oxygen in the weld zone has a strong influence on increasing the number of α phase nucleation sites that can lead to different α p morphologies in the base metal and weld zone. These different α p can lead to distinct microstructures after triplex annealing treatment but with similar α volume fractions. Besides, plasticity deterioration of the repaired tensile specimens is mainly attributed to the formation of columnar grain boundary α phases in the weld zone which are considered to be the earliest nucleation sites of microcracks and confirmed by in situ tensile test. With the increase of WZ proportions in the cross section of tensile specimens, the plasticity of the alloy gradually decreases. [ABSTRACT FROM AUTHOR]

Experimental process optimization is essential to obtain reliable processing conditions prior to performing actual laser additive manufacturing (LAM) of single-crystal (SX) nickel-base superalloys. The influence of processing parameters on deposited productivity and epitaxial SX growth in powder-feeding LAM process was systematically investigated by the orthogonal experiment (OE) method, a statistical experiment design method. This method can rapidly and economically estimate the effect of each processing parameter by a small number of experiments. Resulting relationship between the processing variables and each of deposited productivity and microstructure formation contributes to the selection of detailed processing conditions to balance the factors crucial to successful SX LAM, which means that appropriate adjustment of the processing parameters during actual SX LAM is easy to be performed. On the basis of the analyses of the OE results, a combination of relatively high power, low scanning velocity and moderate powder feeding rate is beneficial to both deposited productivity and epitaxial SX growth during powder-feeding LAM, and allows the preparation of good multilayer SX deposits with fine dendrites. [ABSTRACT FROM AUTHOR]

A novel α+β titanium alloy with multi-alloying addition and low-cost is designed from Ti-6Al-4V. In order to maintain the same [Al] eq and [Mo] eq , the Mo and Zr were added into the alloy instead of V, and the nominal composition of this novel alloy is determined as Ti-6Al-2V-1.5Mo-0.5Zr-0.3Si with a spot of Si added. In this study, a plate of the novel alloy was manufactured by laser additive manufacturing (LAM). The macrostructure, microstructure, compositions, microhardness and room temperature tensile properties of as-deposited alloy were investigated. The results show that the macrostructure of the novel alloy contains large columnar grains and small columnar grains. Compared to the reference alloy Ti-6Al-4V manufactured with the same process, the novel alloy had much finer basket-weave microstructure. This novel alloy, solid-solute with alloying elements Mo, Zr and Si, has higher microhardness and better combination of strength and plasticity compared to Ti-6Al-4V. [ABSTRACT FROM AUTHOR]

Primary dendritic arm spacing (PDAS) is an important microstructure feature in the nickel-base single-crystal (SX) superalloys by laser rapid directional solidification (LRDS). A combined numerical model was developed in this paper to investigate the influence of laser processing parameters on the PDAS. This model consists of (1) the theoretical PDAS models which relate PDAS to the solidification conditions and (2) the heat-flux calculations of laser processing which provide the solidification conditions as a function of the processing parameters. It is therefore able to immediately relate the PDAS to the processing parameters and obtain corresponding processing–microstructure maps. To verify the predicted accuracy, the PDAS values calculated by different theoretical models were compared with those produced under different processing conditions. Results show that the PDAS firstly decreases and then increases at a lower laser power whereas it decreases with increasing scanning velocity at a higher laser power, leading to nose-shape contour lines. The predicted accuracy depends on appropriate selections of material thermo-physical properties. These processing–microstructure maps can accurately capture the trends of the PDAS variation with the processing parameters and contribute to the control and optimization of dendritic microstructure while determining relevant processing windows for controlled SX laser processing. [ABSTRACT FROM AUTHOR]

The microstructure and mechanical properties of the Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy produced by laser melting deposition have been investigated. The columnar grains of the Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy due to the layer-by-layer epitaxial growth from the underlying template grains during laser melting deposition, which is roughly parallel to the deposition direction, had been found by optical microscopy; strong 10 1 ¯ 2 α crystallographic texture has been confirmed by X-ray diffraction and electron backscattered diffraction. Depending on the last heating history, the microstructure of the alloy undergoes a continued change opposite to the deposition direction: martensitic α′ (transformed β); coarsened primary α laths with transformed β; coarsened primary α laths; and the fine basketweave microstructure with a few coarsened α laths or annealed martensitic α′. The average Vickers micro-hardness value of the alloy is 364 HV. The alloy parallel to the deposition direction presents lower tensile strength (925 MPa) and better elongation (18.8%), compared with 1025 MPa and 8.2% of that vertical to the deposition direction, which has been discussed using its microstructure characterization. [ABSTRACT FROM AUTHOR]

Abstract: The compositional variation and the microstructural evolution in laser additive manufactured Ti/Ti–6Al–2Zr–1Mo–1V graded structural material were investigated. An anomalous compositional variation and its good correlation with hardness were found. Although, during the forming process, the ratio of Ti–6Al–2Zr–1Mo–1V in feeded powders abruptly changes from 0% to 100%, all the elements exhibit a gradual variation, which is preferred to a graded material because such gradient change can decrease the discontinuity of its microstructures and properties and can remarkably reduce the costs due to the simplified manufacturing process and the rational distribution of materials. [Copyright &y& Elsevier]

Abstract: Pulsed thermography (PT) as a non-contact non-destructive evaluation (NDE) technique was employed to examine as-sprayed air plasma spray (APS) thermal barrier coatings (TBCs) for quality evaluation. Thickness and microstructural characteristics play a vital role in determining the quality. In this paper, PT logarithmic peak second-derivative method was adopted to measure the thickness of top coat. Time dependent thermal images were used to characterize the microstructure which was confirmed by scanning electron microscope (SEM). The results showed that there was relationship between the temperature distribution of the surface and microstructure change in TBCs. Temperature distribution in thermal images and measurement results of thickness were in fairly good agreement with the microstructure change. It can be concluded that it was possible to employ these NDE methods as quality evaluation for as-sprayed TBCs. [Copyright &y& Elsevier]