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In order to mitigate both residual stress and distortion induced by large thermal gradient in laser solid forming (LSF) process, the in-situ measurement first was used to monitor the thermal and distortion evolutions of the substrate during LSF and to calibrate the finite element model. Using the validated model, the effect of different scanning paths on the evolution of thermo-mechanical fields in LSF was analysed. The results show that good agreement is obtained between the numerical results and the experimental measurements. The maximum thermal gradient and the maximum tensile stress occur in the deposition of the first layer, and with the increasing of the layers, the temperature gradient is reduced gradually. The largest distortion is resulted from the long edge unidirectional scanning mode while the minimum residual stress and distortion is produced by short edge reciprocating scanning. The checkerboard scanning method can effectively reduce distortion of the substrate while hardly reduce the residual stress. In addition, the transverse bending of the substrate can notably curb the longitudinal bending of the substrate. Stress relaxation induced by the phase transformation has a significant effect on both residual stress and distortion of LSF part.

Titanium matrix composites (TMCs) with the addition of in situ ceramic reinforcing particulates have attracted extensive interest. Herein, additive‐manufactured Ti6Al4V/Al–5Ti–0.3C–0.2B alloys are fabricated from mixed powders by laser solid forming. The contents of Al–5Ti–0.3C–0.2B with 1.5, 2.5, and 5.0 wt% are considered. The results indicate the apparent microstructure transition from columnar grain to equiaxed grain with increasing content of Al–5Ti–0.3C–0.2B. The in situ reaction between the Ti6Al4V and Al–5Ti–0.3C–0.2B produces TiB, TiC, and Ti3AlC reinforcements. The increase of Al–5Ti–0.3C–0.2B content will refine grains and reduce α′ martensite phase, which is associated with the formation of TiB at the grain boundaries. As the content of Al–5Ti–0.3C–0.2B increases, the ultimate tensile strength of Ti6Al4V–x wt% (Al–5Ti–0.3C–0.2B) alloy increases from 993.8 MPa to 1233.6 MPa. The strength increases because of an increase of the solid phases TiB and Ti3AlC, which is ascribed to the presence of precipitation and grain refinement strengthening mechanisms. Meanwhile, the elongation of the alloy increases from 1.1% to 4.9% because the Ti3AlC phase is conducive to reducing thermal transfer and interfacial stress. These findings provide insights into the selection of reinforcement materials for optimizing mechanical properties of TMCs alloys. [ABSTRACT FROM AUTHOR]

The nearly rapid solidification and high-temperature thermal cycles associated with laser directed energy deposition of titanium alloys using laser solid forming (LSF) lead to the formation of coarse prior β columnar grains and continuous coarse-grain-boundary α (αGB) phases, which result in anisotropic properties and poor ductility. The influence mechanisms of trace B (up to 0.3 wt. %) on the solidification, solid state transformation, and tensile properties of LSFed Ti-6Al-4V after solution and aging treatment were studied using thermodynamic calculations. With an increase in B content, the size of prior β grains decreased owing to the growth-restricting effect it caused during the solidification and the Zener pinning caused by TiB during reheat cycles and heat treatments. The continuous coarse αGB phases were almost eliminated owing to the discontinuous distribution of TiB between the dendrites, and the length and aspect ratio of intragranular α phases were gradually reduced. Moreover, both ultimate tensile strength and yield strength increased, fracture elongation decreased, strength anisotropy decreased, and elongation anisotropy initially increased and then decreased significantly. The findings in this study will promote the development of new Ti alloys befitting additive manufacturing technology. [ABSTRACT FROM AUTHOR]

Surface improvement of laser solid forming Inconel 718 by electrochemical machining was investigated. Microstructure observation shows that there exists a fiber texture and the ⟨001⟩ direction is parallel to the build direction. Furthermore, the alloying elements of Nb, Mo, and Ti are rich in interdendritic regions, and γ/Laves eutectic phase and γ/carbides eutectic phase are located in the interdendritic regions. Besides, the surface morphologies are wavy due to the overlapping of the scanning tracks and the stacking effect between deposited layers. Electrochemical results show that the top surface requires less time to obtain a smooth surface profile than the side surface due to their original surface profiles difference and the less volume of the bulges on the top surface. The numerical simulation indicates that the uneven surface profiles become smooth owing to the fact that the surface peaks dissolve faster than the surface depressions. These results provide an insight into improving the surface quality of the as-deposited Inconel 718. [ABSTRACT FROM AUTHOR]

Laser solid forming (LSF) is a newly advanced additive manufacturing technique which offers a huge potential in efficiency and cost savings for fabricating the components of aerospace titanium alloys with high performance. In this research, the macrostructure/microstructure and electrochemical dissolution features of the Ti-6Al-4V alloy produced by LSF and forging were investigated. The LSFed Ti-6Al-4V that included as-deposited and annealing heat treatment (HT) conditions contains three periodically distributed microstructural morphologies: fine microstructure zone (FMZ), coarse microstructure zone (CMZ), and layer band. All zones of the different microstructural morphology in an entire deposit are constituents of α-laths and a small amount remained in the β phase. An alternating structure of coarse and fine microstructures was observed along the deposition direction in the deposit. Layer bands are generally spaced above the CMZ. The CMZ is a constituent of a slightly coarser basket weave structure than that of the FMZ. However, the layer bands present the microstructure of colony α. Additionally, the α-laths are slightly coarsened by HT, and this simultaneously results in a more homogeneous microstructure compared with the as-deposited microstructure. Moreover, the forged Ti-6Al-4V possesses the most homogeneous microstructure and the largest size of the α phase. Simultaneously, a variety of dissolved morphology features were observed in the three types of samples after potentiodynamic polarization tests. The order of the surface macrosmooth level of these samples is forged > HT > as-deposited. For LSFed Ti-6Al-4V, the FMZs and the layer bands present a faster electrochemical dissolution rate than that of the CMZs. Consequently, the uneven surface is revealed by the distinct dissolution rate of different microstructural zones at the same LSFed sample. The forged Ti-6Al-4V alloys present a uniformly dissolved behavior, which is attributed to the homogeneity of the microstructure. The elimination of the layer bands and the homogenization of the microstructure of the LSFed Ti-6Al-4V are of great significance for improving the surface quality during electrochemical machining. [ABSTRACT FROM AUTHOR]

Abstract The microstructure evolution and strengthening mechanism of a γ′-strengthening superalloy prepared by induction-assisted laser solid forming (LSF) were clarified by an investigation of the LSFed IN-738LC alloy with different solution treatments (at 1070 °C, 1120 °C and 1160 °C). The results show that both the as-deposited and heat-treated deposits were dominated by the columnar grains with a width of 100–270 μm. The equiaxed grains mainly originated from the columnar-to-equiaxed transition (CET) during LSF while a small portion of the equiaxed grains originated from recrystallization during the heat treatment, due to the relatively low temperature gradient in the induction-assisted LSF. Fine grain strengthening was still working in the columnar-grains dominated LSFed IN-738LC deposits. However, γ′ strengthening was found to play a major role in the strengthening mechanism by comparing the microstructure and properties of different heat-treated deposits. The γ′ dissolution and coarsening kinetics were calculated and the interaction mechanism between the dislocation and γ′ phase in LSFed IN-738LC was clarified. The deformation of the coarse (d > 200 nm) near cubic γ′ was found to be beneficial to the increase of ultimate tensile strength (UTS) and elongation (EL), while a brittle continuous phase formed at the grain boundary (GB) which was rich in Ni, Cr, Al and Ti was found to be detrimental to the deposit's plasticity. The γ′ phase deformation and desquamating, carbide breaking and GB phase breaking are the predominant fracture failure mechanisms. On the basis of relatively high strength, the IN-738LC deposits with a lower yield strength (YS)/UTS ratio than the cast and SLMed ones were obtained when solution-treated at 1070 °C. Graphical abstract Image 1 Highlights • The microstructure evolution was quantitatively characterized. • γ′ strengthening plays a major role in the strengthening mechanism. • A modified relational map between strength and γ′ size was established. • The deformation of the coarse (d > 200 nm) near cubic γ′ was beneficial to ultimate tensile strength and elongation. [ABSTRACT FROM AUTHOR]

Microstructure, electrochemical impedance, and Tafel and potentiodynamic polarizations were characterized to investigate the electrochemical anodic dissolution behavior of Ti-6Al-4V alloy produced by the laser solid forming (LSF) additive manufacturing process with a specific focus on the distinction on different planes. Electrochemical measurements show that the anodic dissolution characteristic of LSFed Ti-6Al-4V reveals anisotropic behavior on different planes. The horizontal-plane (XOY plane) is more resistant to corrosion than the vertical-plane (XOZ plane) in 15 wt% NaCl solution. Additionally, the vertical-plane shows a lower initial machining potential for the process of electrochemical machining compared to the horizontal-plane. The microstructure of Ti-6Al-4V alloy deposit is composed of dominant α-laths and small amounts of β phase, and its horizontal-plane has higher content of the β phase, lower content of the α phase, slightly finer α-laths, and more uneven α-lath width distribution compared to the vertical-plane. These differences in the microstructural characteristics produce the distinctions observed in the electrochemical anodic dissolution behavior of LSFed Ti-6Al-4V alloy on the vertical- and horizontal-planes. [ABSTRACT FROM AUTHOR]

Improvements to the surface of high-power laser solid formed Inconel 718 by use of electrochemical methods were systematically investigated. Formation features (surface microstructure characteristics and surface morphology) and the corresponding electrochemical anodic behavior (electrochemical dissolution behavior and surface levelling mechanism) were analysed. The electrochemical results show that the surface energy difference between the horizontal section and vertical section causes no difference to their transpassive current densities in 10 wt.% NaNO 3 solution up to 2.5 V, owing to the existence of a passive film. Analysis of micro-morphologies indicates that the high current density leads to a smooth micro-morphology due to the higher detachment rates of the interdendritic phases and surface products. Numerical simulations suggest that surface peaks dissolve faster than depressions during the electrochemical levelling process owing to the higher local current density on the surface peaks. Importantly, a detailed microstructure selection map for as-deposited Inconel 718 and an anodic current-density-potential map were developed, and the electrochemical levelling mechanism of its wavy surfaces was proposed. This work has the potential to integrate understanding of laser solid forming-process conditions, formation features, and electrochemical anodic behavior. [ABSTRACT FROM AUTHOR]

The cracking mechanism, especially the initiation and propagation process of cracking, in the overlapping zone (OZ) of laser solid forming (LSF) of IN-738LC alloy was investigated. The results showed that both the solidification cracks and liquation cracks can be found in the OZ of the LSFed IN-738LC specimen. The liquation cracking can be ascribe to the local melting of γ-γ′ eutectic at grain boundaries and could further become the initiation site of solidification cracks in the OZ of LSFed IN-738LC. According to the EBSD analysis and the Rappaz's theoretical model, the cracks in OZ always propagated along the high-angle grain boundaries due to their relatively wider solidification temperature range compared with the lower angle grain boundaries. The thermal stress and residual stress (cracking driving force) of the LSFed IN-738LC deposit was estimated by Vickers micro-indentation method combing a series of microhardness and room temperature tensile testing. It was shown that the residual stress in OZ was obviously higher than that in the inner-track zones due to the different thermal history of the two zones, while the microhardness of the two zones were about at the same level. Preheating was found to be effective in suppressing cracking by reducing the residual stress in LSFed IN-738LC deposit and the crack free deposit was obtained with preheating at 700 °C. [ABSTRACT FROM AUTHOR]

K465 nickel-based superalloy is widely used in the manufacturing of hot section components of aeroengines such as turbine blades and guide blades due to its excellent high temperature strength and superior corrosion resistance. In the present work, K465 superalloy deposits are fabricated on 316 L stainless steel substrate by laser solid forming technique. The microstructure and tensile properties of the as-deposited samples are investigated. The results show that the as-deposited K465 samples mainly consist of columnar dendrites which grow epitaxially from the substrate and the dendrite are fine with the average primary dendrite arm spacing of 18 µm, only a few fine equiaxed grains appear at the top of the deposit. The γ′ particles in the deposit seem uniform in spherical shape and are much finer than that in the as-cast K465 samples. The average hardness of the as-deposited samples is measured as 468 HV, which is higher than that of the as-cast K465 sample. The tensile strength and the yield strength of the as-deposited samples are also high and measured to be 1205.4 MPa and 917.4 MPa, while their elongation is up to 8.5%. [ABSTRACT FROM AUTHOR]

The influence of austenitizing treatment on the microstructure and mechanical properties of laser solid formed (LSFed) 300M steel is investigated within the austenitizing temperature range of 870-1050 °C. It is found that the dendritic segregation in 300M steel deposits (formed in the laser solid forming) gradually reduces with the increase of the austenitizing temperature, and even completely disappears when the austenitizing temperature reaches 1050 °C. Besides, the austenite grains and martensite packets in the LSFed 300M steel deposits changes little when the austenitizing temperature rises from 870 to 950 °C, but significantly coarsenes at the austenitizing temperature above 950 °C. The tensile test shows that the tensile strength of the samples reaches 2325 MPa after austenitizing heat treatment at 870 °C and, then, decreases slightly with the increase of the austenitizing temperature. For yields strength, it decreases slightly when the austenitizing temperature reaches 870-950 °C, while sharply decreases when the austenitizing temperature is above 950 °C. Moreover, a little effect of austenitizing temperature on the elongation and percentage reduction of area of the LSFed 300M steel is exhibited. [ABSTRACT FROM AUTHOR]

To investigate the mechanical properties of the Ti-6Al-4V alloy fabricated by laser solid forming technology, both static and dynamic shear tests were conducted on hat-shaped specimens by a servohydraulic testing machine and an enhanced split Hopkinson pressure bar system, over a temperature range of 173-573 K. The microstructure of both the original and deformed specimens was characterized by optical microscopy and scanning electron microscopy. The results show that: (1) the anisotropy of shear properties is not significant regardless of the visible stratification and the prior- β grains that grow epitaxially along the depositing direction; (2) the ultimate shear strength of this material is lower than that of those Ti-6Al-4V alloys fabricated by forging and extrusion; (3) the adiabatic shear bands of approximately 25.6-36.4 μm in width can develop at all selected temperatures during the dynamic shear deformation; and (4) the observed microstructure and measured microhardness indicate that the grains become refined in adiabatic shear band. Estimation of the temperature rise shows that the temperature in shear band exceeds the recrystallization temperature. The process of rotational dynamic recrystallization is considered to be the cause of the grain refinement in shear band. [ABSTRACT FROM AUTHOR]

In this research, the effect of microstructure on the fatigue crack growth rate (FCGR) of laser solid formed (LSFed) 300M steel was investigated. The results showed that the as-deposited microstructure of LSFed 300M steel consisted of martensite and coarse bainite. After heat treatment, the microstructure became homogeneous and was primarily composed of tempered martensite, bainite and a small amount of retained austenite. The FCGR of LSFed 300M steel gradually increases with increasing stress ratio (for a given stress intensity factor range and stress ratios between 0.1 and 0.5). The refined austenite grain of heat-treated LSFed 300M steel reduced the fatigue crack growth threshold (FCGT) and the FCGR. The fractography of as-deposited LSFed 300M steel was different from that of heat-treated LSFed 300M steel. [ABSTRACT FROM AUTHOR]

The microstructure and microhardness of Ni-base superalloy IN-738LC fabricated by laser solid forming (LSF) with the preheating was investigated. The results showed that with the increase of the preheating temperature (T 0 ) (25 °C, 800 °C, 900 °C and 1050 °C), the total crack length decreased significantly, the segregation of Ti and Al worsened, and the volume fraction of equiaxed grain increased, as well as the size of blocky carbides, γ-γ′ eutectic and γ′ particles. The high-angle grain boundaries, at which the continuous liquid film can be easily formed, were found to be susceptible to cracking. Both the concentration of Ti and Al and the increase of dendrite spacing contribute to the growth of carbides and γ-γ′ eutectic in the interdendrite while the size of the γ′ phases was largely affected by preheating. It was interesting to find that γ′ phase exhibited a bimodal distribution in the bottom of the deposit with the preheating at 1050 °C. The average microhardness of the deposits without preheating and with preheating at 800 °C and 900 °C is about at the same level, while the deposit with preheating at 1050 °C has the lowest average value. The crack-free deposit can be obtained when T 0 was up to 1050 °C, and its room temperature tensile properties are superior to cast IN-738LC alloy. [ABSTRACT FROM AUTHOR]

The microstructure and mechanical properties of laser solid formed (LSFed) 300M steel with tempering treatment was investigated. The microstructure of laser solid formed 300M steel with tempering treatment mainly consisted of the tempered martensite, bainite and a small amount of retained austenite. When the tempering temperature increased from 250 °C to 350 °C, the size of martensite lath and martensite blocks changed little. There are two kinds of tempered martensite. With increase of the tempering temperature, ε-carbide in the general tempered martensite transformed to cementite. In another kind of tempered martensite, ε-carbide were found to be precipitated along specific growth directions, with at least two variants for carbide precipitation. The hardness presented an incomplete linear correlation with the tensile strength and the yield strength for LSFed 300M steel. The hardness in LSFed 300M steel had not shown a significant change with increase of the tempering temperature. The tensile strength and yield strength increased first till the tempering temperature reaches 290 ° C–310 ° C then decreased with further increasing the tempering temperature. Especially, the strength dramatically decreased when the tempering temperature was above 310 ° C. However, the elongation and percentage reduction of area changed little with increase of the tempering temperature. The tensile fracture of LSFed 300M steel with tempering treatment presented a ductile fracture pattern. [ABSTRACT FROM AUTHOR]

A series of AISI 420 stainless steel samples were deposited under different oxygen partial pressures. The morphology, composition, size distribution and number density of the oxide inclusion were examined. Based on the thermodynamic calculation, the transformation process of the oxide inclusion was analyzed. Under air, the oxide inclusion comprised the angular spinel and allotriomorphic or globular compound oxide of spinel and silicate with large size. With decreasing the environmental oxygen content, the oxide inclusion became the globular oxide of mainly silicate with finer and more uniform size. The decrease of the O content in the sample mainly resulted in the decrease of the number density of the oxide particle but not the size. It is found that the oxide inclusion characteristics are determined mainly by the precipitation and solidification of the slag before and after the solidification of the liquid metal respectively. The size distribution of the oxide particle is dominated by the nucleation behavior when the O content is relatively low, while by the collision–coalescence behavior when the O content is high enough. The thermodynamic calculation can predict the oxide inclusion transformation reasonably for laser solid forming AISI 420 stainless steel. [ABSTRACT FROM AUTHOR]

Microstructure and mechanical properties of laser solid formed 300M steel were investigated. The as-deposited microstructure presented the tempered martensite, the martensite with a small amount of bainite, and the mixture of martensite and bainite from the bottom to the top of the deposits. After heat treatment, the microstructure was refined and uniform, and consisted of the mixture of tempered martensite, bainite and retained austenite. The hardness only showed a little increase from the bottom to the middle–upper of the as-deposited 300M steel, there was a significant increase in the hardness at the top of as-deposited 300M steel. The hardness in the heat treated deposits tended to be uniform and higher than that in the as-deposited 300M steel. Tensile strength and yield strength of the as-deposited 300M steel were much lower than the wrought standard. Through heat treatment, the tensile properties of laser solid formed 300M steel were improved significantly and reached the forgings standard. [ABSTRACT FROM AUTHOR]

Highlights: [•] Influence of atmosphere on deposition characteristics of LSF was studied. [•] The variation trend of width and height with processing parameters is not altered. [•] The atmospheres influence the width and height of a single track clad evidently. [•] Shape of the bulk sample is the best in nitrogen, then in air, the worst in argon. [•] Effect of atmosphere on effective energy and fluidity of melt serves the cause. [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: Present work applied a kind of new deposition pattern, fractal deposition pattern, in the LSF process. The influences of different depositing patterns (fractal, offset and raster) on the parts’ deformation, interior quality and mechanical properties are also investigated. The finite element method (FEM) simulation results show that the offsetout (the offset from the outside to the inside) and fractal deposition patterns generate the temperature distribution with the circular and eccentric circle symmetry at the end of deposition process, respectively. There are smaller temperature gradients using these two kinds of deposition patterns than those by the raster and offsetin (the offset from the inside to the outside) deposition patterns. It is also found that the magnitude of the transient temperature gradient has a critical effect on part distortion during LSF process. The fractal deposition pattern generates the smallest parts’ deformation, followed by offsetout. The parts interior quality investigation with fractal and offsetout deposition patterns shows that the overlap should be increased to 50% to avoid pore defects. Moreover, we find that adopting arc path instead of square path at the corner locations and “semi-spot vibration” strategy for the offset and fractal deposition patterns respectively can produce better LSFed parts’ quality. [Copyright &y& Elsevier]

Abstract: This paper studies the issue that the molten pool width gradually increases under some conditions during laser solid forming (LSF), which can decrease the shape and dimension accuracy of LSFed component to a large extent. By using the statics analysis method and calculating the interfacial tensions at the solid–liquid–gas triple point of molten pool, the proposed two-dimensional (2D) cross-sectional model of single deposition layer illustrates qualitatively that the deposition width would increase with the increasing pool temperature at a certain powder feeding rate, which we called the pool spread behavior here. Meanwhile, by calculating the maximum equilibrium contact angle for keeping solid–liquid–gas triple point balance, it is found that the molten pool is solidified during non-equilibrium state. Furthermore, in order to control the pool temperature and decrease pool spread amount, the optimal match of pool energy and mass inputs is determined for obtaining an optimum balance between the energy input and deposition efficiencies. [Copyright &y& Elsevier]

Abstract: Crack characterization in Rene88DT superalloy prepared by laser solid forming (LSF) was investigated; the effect of hot isostatic pressing (HIP) on crack healing, microstructure and mechanical properties was also analyzed. It was found that cracks in LSFed Rene88DT mainly nucleate and propagate in the overlap zone of laser-deposited passes, and can be attributed to the liquation cracks. The size and amount of the cracks in LSFed Rene88DT are significantly affected by the overlap of as-deposited passes. Through HIP processing, crack healing occurred, which leads to a substantial improvement of the mechanical properties of LSFed Rene88DT superalloy. However, the insufficient overlap will result in very large cracks in the deposit, which makes the diffusion bonding during HIP not completely eliminate the local segregation of Ti and Nb in the crack healing region. [Copyright &y& Elsevier]

The influence of post-heat-treatment on the resulting microstructure and room-temperature fracture toughness (K I C ) of Inconel 718 fabricated by laser solid forming (LSF), a kind of laser based directed-energy-deposition additive manufacturing technologies, is investigated. Detailed microstructure characterization was performed on as-fabricated and heat-treated samples using direct aging (DA), solution treatment plus aging (STA), homogenization plus STA (HSTA), respectively. The results indicate that as-fabricated sample mainly consists of γ columnar dendrites and a small quantity of (γ+Laves) eutectics in the interdendritic areas. After DA post-heat-treatment, the nonuniform γ′′/γ′ precipitates exist around Laves phases. After STA post-heat-treatment, the short-acicular δ-phases precipitate around/in the Laves phases, the micro-segregation reduces and the distribution of γ′′/γ′ precipitates in the dendrite arm is almost homogeneous. After HSTA post-heat-treatment, Laves phase mostly disappears, micro-segregation completely removes and the γ′′/γ′ precipitates (~30 nm) are distributed in bimodal recrystallized grains. The results of K I C testing indicate that as-fabricated sample possesses the lowest K I C mainly due to its lowest elastic modulus and yield strength. However, the K I C of DA sample (~89.8 MPa m) is close to that of as-fabricated sample, mainly due to its low ductility and strain-hardening exponent. But, the K I C of STA and HSTA samples increases by ~55.8% and 90.8% compared with as-fabricated sample respectively, mainly because of the better performance in strength–plasticity of HSTA sample (~1114 MPa, ~26.1%). Particularly, the K I C of HSTA sample with bimodal grains is ~60 MPa m higher than the lower limit of wrought IN718 (AMS 5662) and reaches ~164.1 MPa m. There exists significant difference in the K I C fractographs of as-fabricated and heat-treated samples. Overall, this research illustrates that an appropriate post-heat-treatment possesses obvious toughening effect on the LSF Inconel 718 alloy. [ABSTRACT FROM AUTHOR]

Laser solid forming (LSF) is a promising additive manufacturing technology. In the LSF process, deformation behaviors dictate the accuracy of the produced parts. In this study, by using a laser displacement detector based on laser triangulation principle, an accurate and effective real-time detection method was established to monitor the real-time deformation behavior of the key position during the LSF of a thin-wall structure. The results confirmed that increasing thin-wall length results in increasing final deformation of the edge. The displacement fluctuation range and value in the middle of thin wall are both smaller than that of the positions near the end, while the entire displacement changing direction in the middle is opposite to that of the end positions. When the deposition process is paused, the deformation of the thin wall during the cooling stage will deviate the position of the deposited thin wall, resulting in the dislocation between the subsequent deposited part and that before the pause, which affect the dimensional accuracy of the thin wall structure. This non-contact real-time detection method also confirmed the ability to monitor the initiation of cracking during the LSF process, and a potential to be used for the on-line feedback control of deformation of detected key position of deposited structure. [ABSTRACT FROM AUTHOR]

• The Cl- ions in the solution promote the passive dissolution reaction process. • The dissolved surface in the chloride-containing solution is significantly flatter. • The transpassive dissolution rate and current efficiency are enhanced by Cl- ions. The electrolyte solution significantly affects electrochemical machining. This study investigates the electrochemical dissolution behavior of a laser solid formed Hastelloy X nickel-based superalloy in 15 wt% NaNO 3 , 7.5 wt% NaNO 3 + 7.5 wt% NaCl, and 15 wt% NaCl solutions. The Cl- ions in the electrolyte solution accelerates the passive dissolution rate because the electrochemical reactions at the metal/film and film/solution interfaces are promoted. During the transpassive dissolution process, the Cl- ions in the electrolyte solution weaken the hindering effect of the transpassive film on the dissolution process, resulting in a faster transpassive dissolution rate and a higher current efficiency. Furthermore, the flatness of the dissolved surface in the chloride-containing solution is significantly higher than that in the chloride-free solution. Two models are proposed to illustrate the passive and transpassive dissolution behaviors. This study provides theoretical guidance for the selection of electrolyte solutions for the electrochemical machining of laser solid formed nickel-based superalloys. [ABSTRACT FROM AUTHOR]

For damage tolerance (DT) titanium alloy, the fatigue crack growth resistance (FCGR) is a critical properties requirement for engineering applications. However, the Ti-6Al-4V-DT parts fabricated by laser solid forming (LSF) suffer from low FCGR, because of predominant basket-wave microstructure. Here, we have explored a novel LSF fabrication design to produce full colony microstructure, via in-situ controlled growth. The creation of such microstructures leads to superior FCGR, which markedly exceed conventional additive manufactured and mill-annealed samples. The present works provide a significant guidance for LSF-fabricated titanium alloy with high DT properties. Unlabelled Image [ABSTRACT FROM AUTHOR]

Abstract The 2024Al alloy and 3%TiB 2 reinforced 2024Al composite were fabricated using laser solid forming (LSF). Different from the LSF processed 2024Al with a large columnar grain and apparent preferential growth orientation, TiB 2 reinforced 2024Al composite presents texture-less structure, consisting of the dendrite and fine equiaxed structures. Some TiB 2 particles distribute within the Al matrix, others distribute along the grain boundaries and intertwine with the Al 2 Cu phase. The incorporation of TiB 2 gives rise to significant grain refinement, which is one of the important reasons for the improvement of mechanical properties. Moreover, the TiB 2 reinforced 2024Al sample exhibits 284 MPa high tensile strength, 163 MPa yield strength, 108.5HV microhardness, and 18.7% excellent elongation. [ABSTRACT FROM AUTHOR]

The initial processing defects, i.e. voids, lack-of-fusion pores and inclusions, are the inevitable and unique microstructural features in the Ti-6Al-4 V (Ti64) alloy manufactured by laser solid forming (LSF). This study was to gain insight into the influences of these processing defects on the tensile thermomechanical response of the material under various strain rates of 0.001/s, 500/s, 1000/s and 3000/s, and under a wide range of temperatures from 298 K to 1273 K. The drastic effects of processing defects on the flow stress were founded to be susceptible to tensile loading and increase with the increasing strain rate. The SEM analysis was carried out to further understand the effect of initial processing defects on the dynamic fracture behavior. These processing defects, such as voids and lack-of-fusion pores, are the most vulnerable position to release impact energy instantaneously and then evolve into the initiation site and propagation path for dynamic tensile crack. The tensile crack under high strain rate and high temperature propagates through the columnar prior-β grain boundary α colony. It was analyzed that the typical tension-compression asymmetry in strength is positively associated with the strain rate in the Ti64 alloy by LSF. Such asymmetry is due to the differences in the deformation mechanism of processing defects and in the activated slip system under tensile and compressive loading. Additionally, the anisotropy of the mechanical property was discussed by examining the positive strain rate effects in two orientations (i.e., perpendicular and parallel to the crystal growth direction). This anisotropy is caused by different mechanical behaviors of lack-of-fusion defects and β grain boundary α colony in different orientations. [ABSTRACT FROM AUTHOR]

The limited ductility of laser solid formed (LSFed) titanium alloy is a critical issue, which hinders their potential engineering applications. Here, we reported a largely improved ductility (25.1% total elongation) of LSFed extra low interstitial Ti-6Al-4V titanium alloy with a comparable tensile strength (> 860 MPa, international standards) using triple heat treatment. Superior ductility was mainly attributed to the gradual globularization of the α laths during subcritical annealing. The secondary α lamellar obtained by the solution treatment and aging was responsible for the tensile strength. The present findings provide significant guidance for fabricating LSFed titanium alloy having high ductility and good strength. [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]

A series of Ti-2Al- y Mo ( y = 2,5,7,9,12) alloy samples were produced from blended elemental Ti, Al and Mo powders using the laser solid forming technology. The influence of the β stabilizer Mo on the morphology of the grains, the size and the distribution of the α laths, the Vickers hardness and the tensile properties of Ti-2Al- y Mo alloys were explored under designated laser processing parameters. It was found that the microstructure in Ti-2Al-5Mo, Ti-2Al-7Mo, Ti-2Al-9Mo, and Ti-2Al-12Mo deposited layers were composed of irregular columnar grains that grow epitaxially, with some small equiaxed grains in the upper region, while that of the Ti-2Al-2Mo alloy was composed of large columnar grains. The grain size decreased, while the thickness of equiaxed grains layer increased with increasing Mo content. This indicates a tendency towards columnar-to-equiaxed transition with increasing Mo. The dominant β grains form a basketweave microstructure composed of primary α laths, secondary α laths and retained β phase. It was also observed that with increasing Mo content, both the size of primary α laths and secondary α laths decreased significantly, while the number of secondary α laths increased sharply, and the Ti-2Al-12Mo is also constituted of α phase and β phase. The Young's modulus of Ti-2Al-7Mo, Ti-2Al-9Mo, and Ti-2Al-12Mo were basically the same within experimental error. Tensile testing of the specimens showed that Ti-2Al-7Mo exhibits a good combination of strength and elongation to failure. As a result, this material can be considered suitable as a viable feedstock for laser additive manufacturing. [ABSTRACT FROM AUTHOR]

Laser solid forming (LSF) is a newly developed additive manufacturing which offers a less material waste and reduction in lead-time for fabricating aerospace titanium alloys components. In this paper, two types of block with different build dimension (section geometries) were fabricated by LSF with same processing parameters. The corresponding microstructure, texture, and tensile properties were investigated systematically. The results show that the samples exhibits similar columnar β grains morphology and 〈100〉 fiber texture, but very different α variant characterizations (morphology and texture) due to the different thermal history they experienced respectively. The fine basket-weave microstructure with weak texture can be obtained under the fast cooling conditions, while the colony microstructure shows a strong transformation texture as a result of variant selection in the relative slow cooling rate. The α characterizations depend strongly on the competition growth mechanism between the α WGB (grin boundary Widmanstatten structure) and α I (intragranular α nuclei) during cooling process. The presence of α GB (grain boundary α layers) enhances the nucleation of certain variants in β→α phase transformation. Tensile results reveal that fine basket-weave microstructure has relative high strength and ductility with dimple fracture mode. The colony microstructure shows a feature of dominant brittle fracture appearance and results in low tensile ductility. [ABSTRACT FROM AUTHOR]

Abstract: By using a moving disc heat source model, an analytical model was developed to describe laser solid forming (LSF) process with the feedback of the surface temperature of the molten pool, which can be used to estimate the geometric characterizations (width and height) of the clad layer rapidly. An on-line temperature measurement system was established and some single-pass cladding experiments were conducted while the molten pool temperature was monitored. It was found that the estimated geometric characterizations agreed well with the experimental results. In addition, the power consumed by conduction, convection, radiation, evaporation and absorption during LSF were also estimated by the model. It was shown that the majority of the total absorbed power was conducted to the substrate. The effective model can not only be used to optimize the processing parameters but also potentially applied to the real-time feedback control. [Copyright &y& Elsevier]

Abstract: Laser solid forming (LSF) is a promising manufacturing technology. Thermal behavior is very significant for the research of microstructure, performance and geometric dimension of the fabricated part. In this research, a two-dimensional transient analytical model was developed on a moving square heat source with a uniform heat intensity distribution, and applied to estimate the temperature distribution and deposition thickness of the LSF thin-wall structures. The effects of two ends of the thin-wall structure and the temperature decline after closing the laser beam were investigated. The deposition thickness with different process parameters was also calculated and agreed well with the data measured by a CCD camera system under the practical process parameters despite some differences. Finally, a unique strategy (adjusting the dwell time of laser beam at both ends) was proposed to improve the dimensional accuracy at two ends of the thin-wall sample, and the experimental results demonstrated the validity of the strategy proposed. [Copyright &y& Elsevier]

Abstract: Laser solid forming (LSF) from blended elemental powders is a powerful tool for the synthesis of novel materials. Accurate composition control is critical for the application of this technique. It is found that the chemical composition of as-deposited sample can be controlled the same as the premixed elemental powders by keeping the identity of the divergence angles of the elemental powder streams. A mathematical model was established to describe the powder delivery process during LSF from blended elemental powders. Based on the consistency condition for divergence angles of different elemental powder streams, the match condition among the elemental powder characteristics (including particle size and density) can be obtained, which ensures the consistency in composition between the laser deposits and the premixed elemental powders. LSF experiments were carried out using a blend of Ti, Al, and V powders and the composition analysis was performed on as-deposited samples. The experimental results demonstrate the validity of the developed mathematical model. [Copyright &y& Elsevier]