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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.

To figure out the evolution of the heterogeneous microstructure across the molten pool boundaries (MPBs) and its effects on tensile properties of selective laser melted (SLM) AlSi10Mg alloy, two of the most commonly used scanning strategies, i.e., bidirectional scanning within a layer and with a 90° and 67° rotation for the successive layers (zigzag-90 and zigzag-67) were adopted. Grain morphology, solidification microstructure, texture and room temperature tensile properties of the SLMed AlSi10Mg alloy under the two scanning strategies were studied. The coarse columnar and fine equiaxed grains were observed in a single solidified track under the two scanning strategies. The columnar grains dominated the bulk of the molten pool (MP) while the equiaxed grains mainly distributed along the MPBs. The equiaxed grains along MPBs break the continuous growth of the columnar grains and lead to a weak texture. A new mechanism based on heterogeneous nucleation and constitutional supercooling was proposed to explain the formation of heterogeneous microstructure across the MPBs. The tensile test was performed in the horizontal (perpendicular to the build direction) and the vertical (parallel to the build direction) directions fabricated under the two scanning strategies. The mechanical properties showed obvious fluctuation, and the yield strength of the horizontal direction is generally higher than that of the vertical direction. The distribution of pores and MPBs is the main factors to influence the tensile properties.

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.

The electrochemical corrosion behavior of laser aided additive manufactured (LAAMed) CoCrNi medium entropy alloys (MEAs) with different oxide inclusion level was investigated in 3.5 wt.% NaCl solution. Different from the 316 L stainless steel, no pitting appeared on the LAAMed CoCrNi within the test regime used in this study. The LAAMed sample with a higher oxide inclusion level demonstrated lower corrosion current density and higher charge transfer resistance, implying a more compact passive film. After holding in the solution at 0.2 VSCE for 30 mins, the passive films formed on the LAAMed CoCrNi MEAs have similar thickness and dominant constituent (i.e. Cr2O3 and Cr(OH)3). More oxides in the LAAMed CoCrNi MEA could facilitate the formation of a compact passive film, which leads to superior corrosion resistance.

Current heterostructured materials demonstrate the high potential to circumvent strength-ductility tradeoff but face challenges with unconfigurable hetero-zone distribution and mechanical anisotropy. This study explored laser aided additive manufacturing (LAAM) of non-laminar spatially heterostructured materials (SHM) with configurable architectures to combine superior properties from the constitutive AISI 420 stainless steel and C300 maraging steel to enhance the overall properties. It is proven that the hatch spacing (h) has a significant effect on the microstructural evolutions and mechanical properties of the SHMs as it affects the layer thickness and inter-dilution regions. The mechanical properties of the SHMs were evaluated at multi-scales. The sample with h of 1.5 mm possesses a high tensile strength of about 1.6 GPa along with a reasonable break elongation of 8.1%, showing a good strength-ductility combination. Micropillar compression tests were conducted to measure localised mechanical properties, which is critical to understand the strengthening mechanism. Additionally, the SHM substantiates a much higher strength than many lamellar and linear functionally graded materials reported in the literature. This can be explained by the rule-of-mixture effect and hetero-deformation induced strengthening (HDIS). Furthermore, in-situ deformation observation reveals multiple deformation bands in SHM, which delays necking and contributed to ductility in tandem with the transformation induced plasticity (TRIP) effect. The findings highlight a novel approach for the development of SHM with a tunable performance by using LAAM of multiple materials following the configurable architectures.

The nonuniform microstructure of the as-built Inconel 718 superalloy fabricated by high-deposition-rate laser directed energy deposition (HDR-LDED) brings about heterogeneous room temperature tensile properties along the building direction (BD). Thus, it is imperative to explore suitable post heat treatment processes for homogeneous and better performances of HDR-LDED built Inconel 718 superalloy. In this work, three kinds of heat treatment schedules were employed to investigate the microstructure evolution and room temperature tensile properties. The results showed that the microstructure uniformities along the BD after three heat treatment processes were all significantly improved. In addition, the three heat treated samples present different microstructural characteristics which were reflected by the morphology, size and distribution of the Laves, δ and γ" phases. However, all the samples exhibited similar microhardness, indicating that the difference in the characteristic parameters of the second phases was not enough to influence the microhardness. Furthermore, the heterogeneity in room temperature tensile property along the BD was improved when compared with the as-built sample. And the S-30 sample demonstrated the best combination of strength and ductility (yield strength: 1013 ± 13 MPa, ultimate tensile strength: 1192 ± 11 MPa, elongation: 16 ± 6%). The fragmentation and the debonding of the “Laves+δ” phase contributed to the fracture in all the samples. The initial dislocation density inside the bulk sample may become one of the factors that have to be considered in the future when optimizing the heat treatment processes in HDR-LDED process.

Aerospace is a key market driver for the advancement of additive manufacturing (AM) due to the huge demands in high-mix low-volume production of high-value parts, integrated complex part geometries and simplified fabrication workflow. Rapid and significant progress has been made in the laser additive manufacturing (LAM) of aeroengine materials, including the advanced high-strength steels, nickel-based superalloys and titanium-based alloys. Despite the extensive investigation of these three families of materials by the research community, there is a lack of comprehensive review on LAM of high strength steels, and existing gaps in published reviews on Ti-based alloys and Ni-based superalloys. Furthermore, although emerging materials such as high/medium entropy alloys and heterostructured materials exhibit promising mechanical performance, rigorous characterization, testing, qualification, and certification are still required before actual application in engine parts. Thus, it is still important and relevant to have a deep understanding on the relationship between process parameters – microstructures – mechanical properties in these widely used aeroengine materials, to drive the development of superior high-value alloys. This review aims to provide a critical and in-depth evaluation of laser powder bed fusion (LPBF) and laser directed energy deposition (LDED) technologies of the mentioned aeroengine materials. The review will summarize the material properties, performance envelops and outlines the research gaps of these aeroengine materials. Furthermore, perspectives on research opportunities, materials development, and new R&D approaches of LAM for the aeroengine materials are also highlighted.

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.

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.