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Near-equiaxed β grain was achieved in the near-α Ti60 (Ti-5.7Al-4.0Sn-3.5Zr-0.4Mo-0.4Si-0.4Nb-1.0Ta-0.05C) titanium alloy via laser directed energy deposition (LDED). The microstructural evolution along the building direction and the room-temperature tensile properties along the horizontal and vertical directions (building direction) were systematically studied through SEM and OM. EBSD and XRD were utilized to accurately demonstrate the texture of the α and β phases. The results showed that the α phase presented a low texture intensity, which was ascribed to the weak textured β grain with near-equiaxed morphology, since there are Burgers orientation relationships during the β→α transition. In addition, numerical simulation, combined with the CET curve of Ti60 alloy considering the effect of multi-composition, was utilized to elucidate the formation mechanism of the near-equiaxed β grains. Furthermore, according to the solidification theory, we proposed that the solidification temperature range Δ T f was more accurate than the growth restriction factor Q in predicting the formation tendency of equiaxed β grain in different titanium alloys. Tensile results showed that the horizontal and vertical samples had similar strength, while the former exhibited larger elongation than the latter. The effect of the near-equiaxed β grain and the internal α phase on mechanical properties were revealed at last.

Additive manufacturing (AM) enables the processing of heterogeneous materials with customised architectures to improve strength-ductility synergy. Herein, layerwise-heterostructured high-strength steels were processed by AM of two steel powders following the designed architectures. The fabricated high-strength steels with hierarchical heterogeneous characteristics at the layer, melt-pool and grain scales, exhibit good strength-ductility combination, reaching a strength of 1.32 GPa together with an elongation of 7.5%. The increased strength attributes to hetero-deformation induced strengthening. In-situ deformation monitoring reveals many unique deformation bands in heterostructure materials, which delays necking and improves ductility. The findings demonstrate a novel potential approach to circumvent material property trade-offs. Additively manufactured heterogeneous materials with controllable architectures achieved a good strength-ductility combination; the underlying strengthening mechanism and unique deformation behaviour are revealed.

The 2024Al alloy and 3%TiB2 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, TiB2 reinforced 2024Al composite presents texture-less structure, consisting of the dendrite and fine equiaxed structures. Some TiB2 particles distribute within the Al matrix, others distribute along the grain boundaries and intertwine with the Al2Cu phase. The incorporation of TiB2 gives rise to significant grain refinement, which is one of the important reasons for the improvement of mechanical properties. Moreover, the TiB2 reinforced 2024Al sample exhibits 284 MPa high tensile strength, 163 MPa yield strength, 108.5HV microhardness, and 18.7% excellent elongation.

Long-striped Laves phases are considered to be crack sources in laser repaired Inconel 718 during stress rupture test. In order to obtain fine Laves phases, Nd:YAG pulse laser is used to fabricate Inconel 718 superalloy on a wrought Inconel 718 substrate in this study. The microstructures and stress rupture properties of pulse laser repaired Inconel 718 superalloy after DA treatment (720 °C × 8 h/furnace cooling at 50 °C/h to 620 °C × 8 h/air cooling) and 900STA treatment (900 °C × 20 min/air cooling+720 °C × 8 h/furnace cooling at 50 °C/h to 620 °C × 8 h/air cooling) are investigated by using SEM and TEM. The results shows that granular Laves phases can be obtained when pulse laser is used. The formation of granular Laves phase is related to high cooling rate (G·R) and low ratio of temperature gradient to growth rate (G/R). The critical values of G·R and G/R are 104 and 107, respectively. After 900STA treatment, δ phase precipitates around the granular Laves phases. But there is no δ phases existing in the DA samples. The surrounding δ phases can induce stress concentration, resulting in that the elongation of the 900STA samples is worse than that of the DA samples. However, the regions denuded in Nb element around the δ phases can delay the formation of the vacancies, which leads to that the stress rupture life of the 900STA samples is better than that of the DA samples.

The influence of solution heat treatment on microstructure and tensile property of Inconel 718 formed by high-deposition-rate laser metal deposition has been investigated in this paper. Two different heat treatment regimes were employed: 1100 °C × 0.5 h/WQ+720 °C × 8 h/FC to 620 °C × 8 h/AC (this type of samples is called type-S) and 1100 °C × 1 h/WQ+720 °C × 8 h/FC to 620 °C × 8 h/AC (this type of samples is called type-L). The results showed that niobium segregation and Laves phases were eliminated both in type-S and type-L samples. This means that heat treatment at 1100 °C for no more than 0.5 h is enough, if initial epitaxial columnar grains are needed and niobium segregation and Laves phases are tend to be removed. Twins only existed in type-L sample, which was the main reason why the elongation of type-L was superior to that of type-S. The fracture mechanism of these two samples was microvoids coalescence ductile rupture. The separation of the granular sub-micron particles and the γ matrix was the main nucleus of the micro-voids.

In this paper, a comparative study of high-cycle fatigue tests (T=650 °C, f=110 Hz, R=0.1, Kt=1) were carried out with wrought Inconel 718 and LAMed Inconel 718. The results show that the influences of the Laves phases on high-cycle fatigue properties are based on the applied stress amplitudes. At a low stress amplitude, most of the Laves phases held their original morphologies. The fatigue cracks stopped in front or detoured around them, which means that the unbroken Laves phases play an important role in hindering crack propagation. In this way, the high-cycle fatigue life of LAMed Inconel 718 was superior to that of wrought Inconel 718. However, at a high stress amplitude almost all of the Laves phases in the crack propagation region splintered into smaller fragments, parts of which separated from the austenite matrix. Microscopic holes or cracks were formed at the interface, which provided passages for the fatigue cracks to propagate. In this case, the Laves phases were harmful, leading to the degradation of fatigue performance in LAMed Inconel 718 compared with wrought Inconel 718.

Laser repairing technology, based on laser additive manufacturing technology, has been used in repairing damaged components for saving time and economic cost. Due to the existence of laser deposited zone, the microstructure and mechanical properties of laser repaired (LRed) components are different from original wrought parts. In this paper, the tensile deformation behavior of LRed Inconel 718 was compared with that of wrought Inconel 718 by using digital image correlation (DIC) method. The microstructure and the failure mechanism of LRed Inconel 718 were also investigated. The results showed that the LRed Inconel 718 sample could be divided into two parts: laser deposited zone (LDZ) and substrate zone (SZ). The laser deposited zone consisted of columnar dendrite growing epitaxially along the deposition direction, where the γ″ phases gathered around the interdendritic Laves phases. However, the microstructure of the substrate zone was equiaxed grain with the uniform distribution of δ and γ″ phases. It means that the microstructure of the LRed Inconel 718 was non-uniform. The deformation compatibility of the LRed Inconel 718 was inferior to that of the wrought Inconel 718 because of the existence of the Laves phases and the heterogeneous distribution of the γ″ phases. As a result, strain concentrated in the laser deposited zone resulting in that most plastic deformation was clustered in this region. The hard and brittle Laves phases slipped and broke up into smaller parts due to local stress concentration. Microscopic holes formed at the interface of the Laves phases and the γ matrix and combined together to cause the fracture of the LRed Inconel 718.

The CoCrNi medium entropy alloy with different amounts of oxides was fabricated by laser aided additive manufacturing (LAAM). The cryogenic tensile properties and microstructure evolution during tensile deformation were investigated. For the Sample B with higher oxide content (2.03 vol%), the yield strength (YS), ultimate tensile strength (UTS) and elongation (El) were all inferior to those of the Sample A with much fewer oxides (0.47 vol%). The lower YS of Sample B was mainly attributed to the lower initial dislocation density. The oxides contributed slightly to the increase in YS, while reducing the El significantly. The El of Sample B was comparable at 298 K and 143 K, owing to the compensation effect from twin boundaries (TBs). Higher YS and UTS were obtained at 143 K for both samples. With decreasing temperature from 298 K to 143 K, the YS and UTS of Sample A increased almost linearly, whereas the El decreased. Though a large amount of TBs were formed during the tensile deformation, they were unevenly distributed among the grains near the fractured location. Under higher stress at cryogenic temperature, the interaction of grain boundaries with massive TBs caused micro-voids to initiate more readily along the grain boundaries, resulting in premature failure.

Improvement in high temperature material properties of aerospace alloys is critical to extend the performances of aeroengines. Apart from new alloy development, there is also great potential in optimizing the microstructure of laser directed energy deposited (Laser-DEDed) parts. This work reports a novel method of handling the undesirable and large-sized Laves phase particles in Laser-DEDed alloys. Experimental results demonstrate improvement in high temperature mechanical properties of IN718, by fully leveraging on the fast dissolution behaviors of the Laves phase in Laser-DEDed microstructure. The obtained Laser-DEDed IN718 contains about 1.55 vol % Laves phase with the dimension of 0.4–0.6 μm and aspect ratio of approximately 1.4. In particular, the high-temperature stress rupture (HTSR) performance of Laser-DEDed IN718 samples tested at 650 °C and 725 MPa (~130 h) exhibit a ~148% improvement in HRST life compared with the forged material (~51 h). It has been ascertained that the small and granular Laves phases have positive effects on the high-temperature mechanical properties of IN718. They can induce dynamic recrystallization to lower localized stress concentration and enhance high temperature performance.

The 50% laser additive manufactured (50%LAMed) tensile samples of Inconel 718 superalloy, i.e., the laser-deposited zone and the substrate zone, occupied 50% volume fraction respectively along the tensile direction, have been fabricated by using laser additive manufacturing (LAM) technology. The inter-dendritic Laves phases were reserved because solution heat treatment could not be used in case of the deteriorating mechanical properties of a forging substrate. Meanwhile, most of the γ″ phases were precipitated in the inter-dendritic area, leading to local stress concentration around the Laves phases in the process of a tensile test. With tensile stress increasing, the degree of deformation and fracture of the Laves phases was closely related to the morphologies of themselves. For the long striped Laves phases, in order to deform with the austenite matrix, they slipped and broke up into small parts. For most of the granular Laves phases, they did not break up and held original morphologies in the process of the tensile test. The broken Laves phases were separated from the γ matrix, and micropores formed at the surface between them. The fracture mechanism was the microvoid coalescence ductile fracture, and the Laves phases were the main nucleuses for the formation of micropores. This study indicates that the mechanical properties of 50%LAMed Inconel 718 can be improved by controlling the morphologies of the Laves phases.

A comparative study on the metallurgical properties and material microstructures of Inconel 718 (IN718) deposited by High Deposition Rate Laser Metal Deposition (HDR-LMD) using Gas Atomization (GA) and Plasma-rotating Electrode Process (PREP) powders has been carried out. Initially, powders of same nominal particle size produced by different methods were selected and the chemical composition, porosity and morphology of which have been characterized. After that, parallel experiments have been designed and performed, and the metallurgical properties and material microstructures have been analyzed. It is found: compared to IN718 deposited with PREP powder, material formed by GA powder has higher porosity and higher dilution zone whereas finer dendrite structure, lower Nb element segregation and lower volume fraction of Laves phase. In order to figure out the reasons, the mechanisms of pores formation, the laser energy allocation, heat dissipation and solidification of the processes using different powders have been qualitative and quantitative analyzed. It concludes: the high porosity of GA IN718 is due to that more gas has been drawn into the process because of the characteristics of GA powder; the microstructures of GA IN718 is superior to that of PREP powder, showing finer dendrite structure, lower Nb segregation and lower Laves phase fraction, which is due to its higher cooling rate. Keywords: High deposition-rate LMD, Inconel 718 (IN718), Gas atomized (GA), Plasma Rotating Electrode Process (PREP), Metallurgical properties, Material microstructures

Laser solid forming (LSF) Ti5Al2Sn2Zr4Mo4Cr titanium alloy usually exhibits brittle fracture with a poor elongation when the tensile direction is perpendicular to the growth direction of columnar β grains. In the present study, the effect of powder feed rate on the columnar to equiaxed transition (CET) and grain refinement of Ti5Al2Sn2Zr4Mo4Cr titanium alloy during the LSF process was investigated. The results showed that increasing in powder feed rate could effectively promote CET during LSF process. However, keeping increasing in the powder feed rate did not further refine the β grains effectively, especially in the regions near the fusion lines. The pauses of the LSF process aimed to obtain equiaxed β grains also lead to the precipitation of very fine α laths inside the β grains since the heat accumulation was avoided. The microstructure containing equiaxed β grains and homogeneous α laths was obtained after heat treatment. The tensile test was performed both in the vertical (parallel to the build direction) and horizontal (perpendicular to the build direction) directions under the as-deposited and heat treatment conditions. The tensile properties, especially the ductility, were significantly improved compared to that in the previous studies. It is partly because the equiaxed β grains effectively prevented the crack propagation.

IN100 Ni-based superalloy fabricated by micro-laser aided additive manufacturing (micro-LAAM) was investigated in this study. After solution treatment and aging (STA) of the micro-LAAMed IN100 alloy, hierarchical γ′ phases were recognized and characterized, which contributed significantly to the high ultimate tensile strength (~1050 MPa) and acceptable ductility (5%) at 25 °C and 600 °C. The tensile fractured surfaces of the as-built and STAed IN100 were characterized by dimples and dimples/cleavages, respectively. The high cycle vibration fatigue (HCVF) behavior was preliminarily studied through simulating the service conditions of some cantilever structures in automobile and aerospace industries. Compared with the cast IN100, the micro-LAAMed IN100 superalloys (as-built or STAed) both exhibited inferior HCVF lives. The HCVF behavior was discussed and correlated with the microstructure characteristics, such as the preferred growth direction of the grains in micro-LAAMed IN100 and the massive interfaces existing in the final obtained material. In the present study, the hierarchical γ′ phases were beneficial to the static tensile property, whereas detrimental to the HCVF behavior of the final obtained IN100 to some extent.

In this paper, a powder feeding laser additive manufacturing technology has been used for fabricating the Inconel 718 super-alloy. Laves phases of different sizes and morphologies have been obtained by using three types of heat treatments. The influence of Laves phases on the room temperature tensile properties of laser additive manufactured Inconel 718 has been investigated. The results show that small and granular Laves phase can be gained after heat treatment at 1050 °C for 15 min (S-15 sample). When the holding time extends to 45 min, the morphologies of Laves phases basically remain unchanged while its volume fraction further decreases (S-45 sample). Nevertheless, irregular and long-striped Laves phases still exist in the samples only after direct aging heat treatment (DA sample). The room temperature tensile results reveal that the S-15 samples have better tensile strength and ductility than that of S-45 samples. Besides, the DA samples with irregular and long-striped Laves phases show the lowest tensile strength and ductility. Hence, a certain amount of small and granular Laves phases are presumably beneficial for the room temperature tensile properties of Inconel 718. Moreover, a model has been established to describe the fracture of the Laves phase. On the basis of the fracture model, the critical stress needed for the fracture of long-striped Laves phases is lower than that needed for the fracture of granular Laves phases. Therefore, the former generally suffered internal fracture while the latter often fail by interfacial decohesion. Through influencing the volume fraction, the size and the distribution of g" phase, the effect of the Laves phases on the room temperature tensile property is achieved. Furthermore, a yield strength model has been developed to reveal this influence in terms of numbers. The yield strength increments caused by grains, solution elements and g' phase are almost the same for the three kinds of samples. The differences of the yield strength are mainly caused by g" phase. In addition, in terms of ductility, granular Laves phases are more favorable than long-striped Laves phases.

A near &beta, titanium alloy, Ti5Al2Sn2Zr4Mo4Cr, was fabricated by directed energy deposition (DED) with different powder feed rates to investigate the formation of fully equiaxed &beta, grains. A two-dimensional numerical model was developed to investigate the thermal conditions of the molten pool. Experimental results showed that the formation of an epitaxial cellular structure at the bottom of the molten pool is almost unavoidable. An increase in the powder feed rate produces a moderate thermal condition and promotes the formation of equiaxed grains in a single cladding layer. However, it could not guarantee the formation of a fully equiaxed microstructure in a block sample. From a low to high powder feed rate, fully columnar, mixed equiaxed&ndash, columnar, and fully equiaxed microstructures were obtained. Grain morphology was also affected by the remolten process. Increasing the powder feed rate reduced the remolten depth and broke the continuity of the epitaxial cellular structure, leading to different grain morphologies.

The mechanical properties of Inconel 718 are closely related to the morphology and size of the Laves phase, which must be quantitatively controlled to change the effect of the Laves phase from deleterious to beneficial. In this study, post-heat treatment was used to regulate the morphology and size of the Laves phase in Inconel 718 fabricated using laser directed energy deposition, and the dissolution behavior of the Laves phase during solution heat treatment was investigated. The results indicated that the sharp corners and grooves of the Laves phase preferentially dissolved, causing the morphology of the Laves phase to change from a long-striped to granular shape during dissolution. The dissolution kinetics of the Laves phase were also investigated using the Johnson–Mehl–Avrami–Kolmogorov and Singh–Flemings models. The initial stage of dissolution was controlled by both the long-range diffusion of Nb and the interfacial reaction. However, with decreasing degree of Nb segregation, the interfacial reaction became dominant.

For the purpose of improving the productivity of laser metal deposition (LMD), the focus of current research is set on increasing the deposition rate for it, in order to develop high-deposition-rate LMD (HDR-LMD). Plasma rotating electrode process (PREP) and gas atomization (GA) are the most commonly used processes for producing LMD powders. In the presented work, the microstructures, the tensile properties, and the fracture mechanisms of Inconel 718 formed by HDR-LMD with PREP and GA powders have been examined. Initially, blocks were built up with different powders under identical experimental conditions. Afterwards, the samples built were heat treated, and then manufactured to tensile test specimens, after which they were tested in order to determine their tensile properties. Finally, microstructure and fracture analysis has been conducted. According to the results of the optical microscope observations for grain morphology, the EBSD analysis for grain orientations, and the analysis of twin boundary and grain boundary orientation, the GA sample has a higher recrystallization degree than the PREP sample. In addition, the intergranular δ phases are needle-like in the PREP sample, and some spherical δ phases have been observed in the GA sample. The tensile test shows that the quasistatic properties of the GA sample are slightly higher than those of the PREP sample. Based on the fracture analysis, different fracture mechanisms have been observed: in PREP sample, intergranular fracture has been identified; in GA sample, mixed fracture - both intergranular fracture and trans-granular fracture - has been observed. Furthermore, the pores found in the GA sample may be not definitely detrimental for the tensile properties. This hypothesis is supported by the following facts: the GA samples have significantly higher porosity, but meanwhile higher tensile properties; the dimples on the fracture surface that are around the pores develop well and fine; cracks formed in the pores did not extend to the outside of the pores.

In order to meet the requirements for rapid manufacturing of large-scale high-performance metal components, the unique advantages of high-deposition-rate laser directed energy deposition (HDR-LDED, deposition rate ≥ 1 kg/h) technology have been attracted great attention. HDR-LDED technology significantly improves the efficiency by simultaneously increasing the mass and energy input on basis of conventional laser directed energy deposition (C-LDED, deposition rate ≤ 0.3 kg/h), which dramatically changes the solidification condition and thermal cycling effect compared to C-LDED processes. Based on this, Inconel 718 bulk samples were fabricated with a deposition rate of 2.2 kg/h and a height of 75 mm. Through experimental observation combined with finite element simulation, the precipitation morphology, thermal cycling effect and tensile properties at room temperature of the block samples at heights of 6 mm (bottom region), 37 mm (middle region) and 69 mm (top region) from the substrate were investigated. The results show that both temperature interval and incubation time satisfy the precipitation conditions of the second phases because of the intense thermal cycling effect so that δ, γ" and γ' phase are precipitated in the bottom and middle region of the as-deposited sample during the HDR-LDED process. As a result, the micro-hardness and the yield strength of the bottom region (385 HV; 745.1 ± 5.2 MPa) are similar to those of the middle region (381 HV; 752.2 ± 12.1 MPa), respectively. And they are both higher than those of the top region (298 HV; 464.7 ± 44.2 MPa). The tensile fracture mechanism is shown in both fracture and debonding of the Laves phase. The inhomogeneous microstructures and corresponding mechanical property differences of Inconel 718 fabricated by HDR-LDED along the deposition direction suggest the necessity to conduct further research of the post heat treatment in the future.