As-deposited Specimens Research Articles

Laser Directed Energy Deposition Additive Manufacturing of Al7075 alloy: Process Development, Microstructure, and Porosity

7xxx series aluminum alloys are priority materials adopted in aerospace because of their lightweight property and excellent mechanical performance. The combination of lightweight material and additive manufacturing techniques can significantly promote lightweight design in many industries. This study developed the optimal process for manufacturing Al7075 alloy by laser-aided directed energy deposition and revealed the material properties of the as-deposited Al7075 specimens. Firstly, single-track experiments were conducted by varying laser power, printing speed, and powder delivery rate in broad ranges. Results indicated that the printing speed and powder delivery rate impact the surface finish and shape of single-track beads considerably. Block specimens printed with the optimal parameters selected from the single-track experiments present crack-free quality and have a high relative density of up to 99.89%. Second phases with rich Cu, Mg, and Zn elements along the inter-dendritic regions and grain boundaries were observed from the microstructure inspection. The composition of zinc in the deposited samples is lower than usual, which leads to a relatively low microhardness.

Improved material properties of wire arc additively manufactured Al-Zn-mg-cu alloy through severe deformation interlayer friction stir processing and post-deposition heat treatment

In this study, Al-Zn-Mg-Cu alloys were prepared by wire arc additive manufacturing (WAAM) combined with interlayer friction stir processing (IFSP). To enhance the FSP region, an improved stirring pin was designed to broaden the stir zone effectively. Moreover, the effects of typical T6 and T73 heat treatments on the microstructure and mechanical properties of as-deposited (AS) specimen were meticulously investigated. The results indicated that heat treatments had minimal impact on grain size, dislocation density, and texture strength, but significantly altered the type, size, and distribution of precipitates. The differences in strength were primarily attributed to precipitation strengthening rather than grain boundary or dislocation strengthening. Following T6 heat treatment, the precipitates were predominantly η’, which were smaller in size and exhibited a significantly increased number density compared to AS specimen. Therefore, the average yield strength (YS) and ultimate tensile strength (UTS) increased by 39.51 % (500.21 MPa) and 15.84 % (584.26 MPa), respectively. In contrast, T73 treatment caused a substantial number of fine η’ to transform into coarser η, leading to a significant decrease in precipitate number density. Consequently, compared to T6 specimen, the average YS and UTS decreased by 47.19 % and 13.13 %, respectively.

Improved wear and corrosion resistance of additively manufactured SS316L by laser remelting process

Directed energy deposition is an efficient and flexible additive manufacturing technique that has broad application prospects due to its ability to fabricate intricate structures with heterogeneous properties. In this study, the laser-based directed energy deposition (L-DED) technique is employed to deposit an austenitic stainless steel 316L powder at a significantly high deposition rate. In recent developments, laser remelting (LR) has been explored as a superior alternative for enhancing the surface integrity of the components produced by L-DED. This study endeavours to examine the impacts of LR, emphasizing the improvement of wear and corrosion resistance. The grain refinement is achieved through laser remelting, which eventually leads to a 34 % improvement in micro–hardness over the as-deposited sample; as a result of increased micro–hardness, wear resistance improved by 66 % when compared to the as-deposited specimen. Furthermore, the corrosion resistance performance of the laser-remelted samples is compared with as-deposited samples by electrochemical impedance spectroscopy (EIS) in a solution of 3.5 % NaCl solution. The remelted sample showed improved corrosion resistance, which is attributed to the grain refinement due to rapid solidification in LR. This study demonstrates that laser remelting can be an effective way to improve the wear and corrosion resistance of SS316L, which enlarges its application in harsh environments.

Effects of Tempering on Microstructure and Properties of Additive Manufacturing Cu-Bearing AISI 431 Steel.

AISI 431 martensitic stainless steels (MSS) with 2.5 wt% Cu were fabricated via laser-directed energy deposition additive manufacturing followed by single-step tempering treatment. The influences of different tempering times at 600 °C on microstructure and mechanical properties of the as-deposited 431-2.5Cu MSS have been explored and analyzed. The as-deposited MSS specimen primarily consisted of lath martensite, austenite and M23C6 carbide. After the single-step tempering treatment at 600 °C, Cu-enriched (ԑ-Cu) nano-precipitates and reverse austenite can be formed and promoted by extending the tempering treatment. The microhardness, strength and elongation can be improved with increasing the tempering time up to 1.0 h, and subsequently reduced with the tempering time prolonging to 2.0 h. Compared to 431 MSS that requires a multiple-step heat treatment for excellent performance, the 431-2.5Cu MSS specimen presented superior tensile properties after single-step tempering at 600 °C for 1.0 h in the present work. The ultimate tensile strength (UTS), yield strength (YS) and elongation (EL) of one-hour tempered MSS were 1611 MPa, 1334 MPa and 16.3%, respectively. This study provides a quantitative theoretical reference and experimental basis for realizing short-process fabrication of the Cu-bearing MSS with high strength and ductility.

The Effect of Interlayer Delay on the Heat Accumulation, Microstructures, and Properties in Laser Hot Wire Directed Energy Deposition of Ti-6Al-4V Single-Wall.

Laser hot wire directed energy deposition (LHW-DED) is a layer-by-layer additive manufacturing technique that permits the fabrication of large-scale Ti-6Al-4V (Ti64) components with a high deposition rate and has gained traction in the aerospace sector in recent years. However, one of the major challenges in LHW-DED Ti64 is heat accumulation, which affects the part quality, microstructure, and properties of as-built specimens. These issues require a comprehensive understanding of the layerwise heat-accumulation-driven process-structure-property relationship in as-deposited samples. In this study, a systematic investigation was performed by fabricating three Ti-6Al-4V single-wall specimens with distinct interlayer delays, i.e., 0, 120, and 300 s. The real-time acquisition of high-fidelity thermal data and high-resolution melt pool images were utilized to demonstrate a direct correlation between layerwise heat accumulation and melt pool dimensions. The results revealed that the maximum heat buildup temperature of the topmost layer decreased from 660 °C to 263 °C with an increase to a 300 s interlayer delay, allowing for better control of the melt pool dimensions, which then resulted in improved part accuracy. Furthermore, the investigation of the location-specific composition, microstructure, and mechanical properties demonstrated that heat buildup resulted in the coarsening of microstructures and, consequently, the reduction of micro-hardness with increasing height. Extending the delay by 120 s resulted in a 5% improvement in the mechanical properties, including an increase in the yield strength from 817 MPa to 859 MPa and the ultimate tensile strength from 914 MPa to 959 MPa. Cooling rates estimated at 900 °C using a one-dimensional thermal model based on a numerical method allowed us to establish the process-structure-property relationship for the wall specimens. The study provides deeper insight into the effect of heat buildup in LHW-DED and serves as a guide for tailoring the properties of as-deposited specimens by regulating interlayer delay.

Effects of heat treatments on the microstructure and mechanical properties of Inconel 718 alloy fabricated by electron beam freeform fabrication

The present study investigated the effects of different heat treatments on the microstructure and mechanical properties of Inconel 718 alloy fabricated by electron beam freeform fabrication. The Laves phases, γʹ phases, and (Nb,Ti)C particles were precipitated in the as-deposited specimen. Solution at 960 °C promoted the partial dissolution of Laves phases and the precipitation of δ phases. Further increasing the solution temperature to 1060 °C dissolved the Laves phases effectively, facilitating the adequate precipitation and uniform distribution of γʹ and γʹʹ phases during the subsequent double aging heat treatment. The ultimate tensile strength (UTS) and elongation of the as-deposited specimen were 770 MPa and 12.2 % respectively. The solution heat treatment at 1060 °C increased the elongation to 38.1 % but decreased the UTS to 734 MPa. The microhardness and UTS of specimens that underwent double aging heat treatment were significantly increased, with the maximum average microhardness and UTS reaching 491 HV0.5 and 1285 MPa, respectively. The undissolved Laves phases and the precipitated δ phases at grain boundaries impaired the ductility. The transformation from single solution or double aging heat treatment to solution plus double aging heat treatment led to a shift from a ductile fracture mode to a mixed ductile and brittle fracture mode.

Effects of solution treatments on the microstructure and mechanical properties of Inconel 625 fabricated by electron beam freeform fabrication

The present study investigated the effects of solution treatments on the microstructure and mechanical properties of Inconel 625 fabricated by electron beam freeform fabrication. The results indicated that the microstructure of the as-deposited specimen exhibited typical dendritic characteristics. Meanwhile, the Laves ((Ni,Fe,Cr)2(Nb,Mo,Ti)) phases and needle-shaped δ phases precipitated. The deposition process led to the development of a preferred orientation feature along the building direction, and the typical {001}<100> plate texture was observed. Solution treatments at 900 °C and 1000 °C resulted in the precipitation of more δ phases, and the volume fraction was approximately 5.29 % and 7.96 %, respectively. The precipitated phases fully dissolved at solution temperatures of 1100 °C and 1200 °C, and equiaxed grains with a twin substructure were formed. The recrystallization weakened the preferred orientation, leading to a decrease in grain size and the formation of {111}<1–10> texture. The as-deposited specimen exhibited an ultimate tensile strength (UTS) of 682 MPa, a yield strength (YS) of 410 MPa, and an elongation of 56.4 %. Solution treatment had minimal impact on the UTS, and the YS was mainly contributed by the solid solution strengthening. However, the elongation was greatly improved at elevated solution temperatures, and the elongation was increased to 72.1 % at most. The primary mode of fracture was toughness fracture, which was attributed to the coalescence of microvoids. The solution treatment at 900 °C led to a fracture pattern that exhibited a combination of toughness and brittleness, and decreased the elongation to 48.5 %.

Comparative erosion performance of HVOF and LVOF sprayed NiCrSiBCFe-WC-Co coating

Thermal spray coatings enhance one or more surface properties of engineering components. In this work, NiCrSiBCFe-WC-Co composite coating powder is deposited over SA213-T12 boiler steel using the high velocity oxy fuel spraying (HVOF) and low velocity oxy fuel spraying (LVOF) thermal spray methods to improve erosion performance. XRD and scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDS) were used to examine the surfaces and constituents of as-deposited specimens, whereas SEM was used to investigate the eroded specimens. The erosion behaviours of developed coatings were investigated at 30° and 90° impact angles, 53 m/s and 400°C. XRD analysis of the HVOF coating showed an additional peaks of NiCo2O4, SiC and Ni2B with common peaks Ni, WC, W2C, Cr23C6 and C6(Cr,Co,Ni)23 to that of LVOF coating. HVOF-sprayed coating demonstrated higher microhardness and adhesion strength with lower porosity than LVOF-sprayed coating. The HVOF-sprayed NiCrSiBCFe-WC-Co coating demonstrated comparatively better erosion resistance than LVOF-sprayed NiCrSiBCFe-WC-Co coating, attributed to its superior properties. The mechanism of erosion, concerning the different characteristics of the coatings, is discussed.

Study of microstructure and mechanical properties and residual stresses of 24CrNiMo steel prepared by selective laser melting and laser melting deposition

Selective laser melting (SLM) and laser direct metal deposition (LMD) have shown promise in advanced manufacturing. However, limited research fabricating the same material using these methods, particularly regarding mechanical properties and residual stresses. In this study, 24CrNiMo blocks were prepared using SLM and LMD and subjected to stress relief annealing (SR). The microstructure, residual stresses, and mechanical properties were investigated using SEM, XRD, EBSD, mechanical testing, and μ-X360n residual stress analysis. The results showed that the microstructure of both blocks composed of α-Fe. However, due to different thermal histories, there were differences in grain size between the SLM and LMD specimens. SLM specimens exhibited higher dislocation density, resulting in increased tensile strength and yield strength, while the LMD samples had higher proportions of high-angle grain boundaries (HAGBs), providing enhanced plasticity. LMD-SR specimens exhibited stronger texture and higher content of HAGBs in the <111> direction, further improving plasticity. Both as-deposited blocks exhibited high residual stresses, ranging from 450 MPa to 580 MPa for SLM and 20 %–50 % of SLM values for LMD. After SR treatment, the strength of both specimens decreased due to a reduction in dislocation density, but the increased content of HAGBs improved plasticity. Additionally, the residual stresses in both specimens significantly decreased, transitioned from tensile stresses to compressive stresses, greatly enhancing their wear resistance. Compared to the as-deposited specimens, the SLM-SR and LMD-SR specimens exhibited a reduction in weight loss due to wear by 46.1 % and 75.2 %, respectively, indicating a significant improvement in wear resistance.

Effect of heat treatment on microstructure and wear properties of laser additively manufactured 316 L stainless steel/Co-Cr-W alloy part using directed energy deposition method

The directed energy deposition additive manufacturing was applied to fabricate hard Co-Cr-W layers on the 316 L stainless steel substrate. The effect of different heat treatment cycles was investigated. The primary microstructure of deposited layers contained a cobalt matrix with interdendritic eutectic carbides mainly M7C3, M23C6, and M6C. Most of the primary carbides were dissolved in the matrix after the solution treatment at 1250 °C for 1 h. The aging treatment promoted the precipitation of (W/Co)6C carbides. The hard carbides caused the third-body wear and increased the wear volume loss of specimens. The solution & double aging treatment provided the optimum combination of hardness and wear resistance due to the best distribution of W/Co carbide. The wear volume loss after heat treatment was decreased from about 6.6 mm3 to 2.8 mm3 compared to the as-deposited specimen. The proposed solution & double aging heat treatment improved wear resistance by 50%.

Effect of ECAP on fracture toughness and fatigue endurance of DED-processed Ti-6Al-4V investigated on miniaturized specimens

Additive manufacturing (AM) has revolutionised the production of complex components with custom mechanical properties. However, as-deposited AM-ed materials often exhibit inferior mechanical behaviour compared to conventionally manufactured counterparts because of defects generated during the deposition. In this study, the effects of equal channel angular pressing (ECAP) on fracture toughness and fatigue endurance of Ti-6Al-4V titanium alloy manufactured by direct energy deposition (DED) are investigated. The aim of this study is to compare the mechanical performance of the material in the as-deposited and ECAP-ed states, exploiting the potential benefits of ECAP in improving the properties of additively manufactured components. ECAP processing is used to induce severe plastic deformation (SPD) and create a unique microstructure in Ti-6Al-4V specimens. As-deposited and ECAP-ed specimens were evaluated using stress intensity factor (SIF) analysis, fatigue crack growth, and high cycle fatigue tests using miniaturized specimens. Fractographic analysis was performed using scanning electron microscopy (SEM) to examine the fracture surfaces and identify the underlying failure mechanisms. The results revealed that while ECAP led to a slight increase in porosity, it significantly reduced fracture toughness and had a negligible effect on the rate of propagation of fatigue cracks. In addition, the ECAP-ed specimens showed reduced fatigue endurance of the material.

Enhanced high-temperature mechanical properties of laser-arc hybrid additive manufacturing of Al-Zn-Mg-Cu alloy via microstructure control

Recently, rapid and cost-effective additive manufacturing solutions for lightweight aluminum alloys with excellent high-temperature mechanical properties have been increasingly in demand. In this study, we combined laser-arc hybrid additive manufacturing with solution and artificial aging treatments to achieve Al-Zn-Mg-Cu alloy with favorable high-temperature strength via microstructure control. Hydrogen pores became the major defect in the as-deposited and heat-treated specimens. The continuous distribution of eutectics with hard-brittle characteristics at the grain boundaries was destructed following heat treatment. High-density η′ precipitates were uniformly dispersed in the heat-treated Al-Zn-Mg-Cu alloy, whereas appeared coarsened and dissolved at 473 K, owing to the rapid diffusion of Zn and Mg. The average 0.2% yield strength (318 ± 16 MPa) and ultimate tensile strength (362 ± 20 MPa) at 473 K after heat treatment were enhanced by approximately 58% and 51%, respectively, compared to those of the as-deposited specimen. In addition, the η′ precipitates contributed to lattice distortions and strain fields, which prevented dislocation motion and increased slip deformation resistance at high temperatures. The as-deposited specimen exhibited intergranular fracture at 473 K, with cracks preferring to propagate along the aggregated eutectics. However, crack propagation proceeded in the sections with more pores in the heat-treated specimen. Our approach may provide a valid option for achieving aluminum alloys with excellent high-temperature mechanical properties.

Quantitative study on the correlation between microstructure and mechanical properties of additive friction stir deposited 6061-T6 Al-Mg-Si alloy

Additive friction stir deposition (AFSD) technology is attractive for its ability to create freeform and fully dense structures without melting and solidification. Hence, AFSD is an alternative to fusion-based additive manufacturing technology. However, the quantitative relationship between mechanical properties and microstructure has not been established yet. The purpose of the research is to establish a quantitative relationship between the microstructure and mechanical properties of AFSD-ed 6061-T6 aluminum alloy. In this study, 6061-T6 aluminum alloy feedstock was processed using AFSD technology and then subjected to post-deposited heat treatment (PDHT). The microstructure evolution and mechanical properties of the feedstock, as-deposited and PDHT-ed 6061-T6 aluminum alloy specimens, were investigated for a comparative study. The microstructure was analyzed using electron backscatter diffraction (EBSD), X-ray diffraction (XRD), and transmission electron microscopy (TEM). Hardness testing and tensile testing were conducted for the feedstock, as-deposited, and PDHT-ed specimens. The strengthening mechanisms were discussed. It was found that precipitation strengthening was the main strengthening mechanism for feedstock and PDHT-ed specimens, which contributed to 69% and 83% of the total yield strength, respectively. Whereas grain refinement strengthening was the main strengthening mechanism for the as-deposited specimen, which occupied 68% of the total yield strength. Finally, the relationship between microstructure and mechanical properties of additive friction stir-deposited 6061-T6 Al–Mg–Si alloy was quantified.

Specimen size effect on the strength of nickel-boron alloys

Nickel-boron alloys are prepared by an electrodeposition method. The as-deposited nickel-boron alloy is then annealed at 250 °C and 400 °C for 4 h. Their mechanical strengths and specimen size effects on the strength are evaluated by micro-compression test using pillar-type micro-specimens fabricated by focused ion beam system. In all as-deposited and annealed specimens, a smaller-is-stronger specimen size effect is confirmed. The compressive strength of the nickel-boron alloys reaches a maximum of 5.79 GPa with the smallest specimen size in this work.

Microstructure characteristics of a René N5 Ni-based single-crystal superalloy prepared by laser-directed energy deposition

This work focused on the microstructure and mechanical properties of nickel-based single crystal (SX) superalloy René N5 prepared by laser-directed energy deposition (L-DED) using a flat-top laser beam. A comparison was conducted with the conventional directional solidification technique. Results show that the crack-free single-crystal superalloys with a fraction of low-angle grain boundary (LAGB) higher than 98.5% were obtained by epitaxial growth under different laser powers. The flat fusion line using a flat-top laser beam can suppress the formation of stray grain and high-angle grain boundaries (LAGB) during the L-DED process. Microstructure observation shows the extremely refined dendrites in L-DED SX superalloys with primary dendrite arm spacing (PDAS) values around 20–30 µm, which is significantly lower than as-cast ones (∼350 µm). PDAS increases with the increasing laser power. Segregation of refractory elements of Re, W, and Ta increases with increasing laser power, and the SX specimens with laser power of 1100 W and 1300 W have lower elemental segregation compared with as-cast specimens. The SX specimens exhibit a uniform square γ′ phase at the dendrite core, with an average size of 79–101 nm and a volume fraction of 57–68%. Besides, the carbides with discontinuous and refined shapes show a uniform distribution in the L-DED specimens in comparison with as-cast specimens. The microhardness of as-deposited specimens increases with the increasing laser power, which is higher than that of as-cast ones. This work can provide potential guidance for the microstructure control in the laser additive manufacturing of Ni-based SX superalloys and a further improvement in high-temperature mechanical performance.

Effect of electroshocking treatment on the microstructure and mechanical properties of laser melting deposited near-β Ti-55531 thin-wall

The near-β Ti-55531 (Ti-5Al-5Mo-5V-3Cr-1Zr, wt%) alloy with thin-wall structures were manufactured by laser melting deposition (LMD), and followed by the post-processing of electroshocking treatment (EST). The microstructure evolution before and after EST were characterized via using the scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). The results showed that the microstructure of as-deposited specimen was mainly composed of short columnar grains and equiaxed grains, showing a unique structure of alternating short columnar grains-equiaxed grains-short columnar grains. The results of EBSD showed that EST could reduce the orientation concentration and weaken the texture of β phase. XRD results indicated that the α phase content slightly increased after EST. The micro-strain and lattice constant of β phase were changed after EST, which were probably ascribed to the precipitation of α, the generation of crystal defects on α/β interface, and the decrease of texture intensity. The results of mechanical properties indicated that EST could increase the microhardness, the yield strength and the tensile strength, which were due to the variation of microstructure. All results show that EST is an effective approach for optimizing the microstructure and improving the mechanical properties of LMD near-β titanium alloy.

Microstructures, heat treatments and mechanical properties of AerMet100 steel fabricated by hybrid directed energy deposition

As a kind of directed energy deposition (DED) method, wire and arc additive manufacturing (WAAM) is a solution for the short-process manufacturing of medium complex and integral parts. Ultrahigh-strength steel AerMet100 has been widely applied in large-scale aerospace structural parts like landing gear and the increasing demand of integrated design promotes the application of WAAM. The microstructure and mechanical properties of as-deposited AerMet100 steel are greatly affected by post-heat treatments, and thus researches on heat treatments are essential to the wider application of AerMet100 in the aerospace industry. In this paper, an AerMet100 steel sample was fabricated by WAAM with in situ microrolling (a hybrid DED method). Two heat treatment designs, based on standard and homogenization, were applied to the sample. The microstructures of the as-deposited and heat-treated specimens were examined with optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD), and the mechanical properties were examined using an electronic universal testing machine. The results indicate that austenitization is the main grain refinement mechanism of the as-deposited specimens, and a smaller layer height and an appropriately large heat input are beneficial for the refinement. Cellular segregation formed during solidification has severe heredity and strong effects on the microstructures of the nonhomogenized heat-treated group, causing lower mechanical properties and anisotropy. A special fracture mechanism related to segregation causes fracture morphology featuring parallel structures in both the non-homogenized X-direction tensile specimens and X-Y direction KIc specimens. Compared with the nonhomogenized heat-treated group, homogenization heat treatment basically eliminated cellular segregation and achieved a better combination of the tensile and KIc properties.

Decomposition behavior of yttria-stabilized zirconia and its effect on directed energy deposited Ti-based composite material

• The present study focuses on the rule of YSZ nanoparticles in the refined both α and β grains and increased strength of additively manufactured Ti-6Al-4 V (Ti64). • The YSZ nanoparticles decomposed during the deposition process and led to the formation of Y 2 O 3 and some excess oxygen in the Ti64 matrix. • The decrease in the sizes of the prior β grains could be attributed to the increasing amount of dissolved oxygen and yttrium, which promoted constitutional supercooling. • The reduction in the size of the α grains could be ascribed to a shift of the onset of the β → α+β transformation to a higher temperature and shorter time with increasing concentration of dissolved oxygen. • The contributions of the underlying strengthening mechanisms (α grain size, oxygen solid solution, Y 2 O 3 precipitate) for the as-deposited specimens were quantitatively determined. We report on the microstructure and the strengthening mechanisms of additively manufactured parts fabricated by directed energy deposition of Ti-6Al-4V (Ti64) powders blended with yttria-stabilized zirconia (YSZ) nanoparticles. These specimens showed refined microstructures as compared to bare as-deposited Ti64, where the α and columnar prior β grain sizes decreased with increasing YSZ content. The YSZ nanoparticles decomposed during the deposition process and led to the formation of yttrium oxide and some excess oxygen in the Ti64 matrix. The decrease in the sizes of the prior β grains could be attributed to the increasing amount of dissolved oxygen and yttrium, which promoted constitutional supercooling. Furthermore, the reduction in the size of the α grains could be ascribed to a shift of the onset of the β → α+β transformation to a higher temperature and shorter time with increasing concentration of dissolved oxygen. Finally, the contributions of the underlying strengthening mechanisms for the as-deposited specimens were quantitatively determined.

Microstructure and mechanical properties of unalloyed molybdenum fabricated via wire arc additive manufacturing

Molybdenum is an important high-temperature structural material but has poor processability. Additive manufactured unalloyed Mo is generally very small and the mechanical properties is seldomly studied. In this work, wire arc additive manufacturing was adopted, and crack-free molybdenum parts with high-density (99.0%) and characteristic size of 20 mm×20 mm×120 mm were successfully fabricated by short-track scanning. The microstructure and mechanical properties of samples in both the as-deposited and heat-treated states were studied and compared. Large columnar grains were observed, which were basically along the 〈001〉 direction. Heat treatment leads to grain coarsening, and the elimination of some sub-grain boundaries. Due to the weakened effect of grain and sub-grain boundary hardening, the mechanical properties of heat-treated specimens were worse than that of as-deposited specimens at room temperature. Both of them exhibit brittle fracture features. Under high temperature, the ductile fracture is observed, and the as-deposited specimen has similar strength and ductility, compared with the heat-treated specimens, suggesting a weak role of grain and sub-grain boundary at high temperature. A large number of fragments were observed at the fracture surface after high-temperature tests, which was MoO3 by energy dispersive spectroscopy test.

Investigation of short-term creep properties of a coarse-grained Inconel 718 fabricated by directed energy deposition compared to traditional Inconel 718

Short-term creep performance of a coarse-grained Inconel 718 alloy fabricated by Directed energy deposition (DED) technique has been investigated at 650 °C in comparison to a conventional wrought material. In order to study the effect of heat treatment and sample orientation on the creep behaviour, as-deposited specimens oriented parallel and perpendicular to the building direction have been treated by double aging (DA), solution annealing + double aging (SA) and homogenization + solution annealing + double aging (HSA) conditions. Microstructural changes along with their impacts on high temperature mechanical properties have been examined with respect to grain size, crystallographic orientation, precipitates, and dislocation substructure. Results show that, superior creep strength and creep ductility of DED-manufactured DA specimen can be achieved compared to wrought specimen. The process-induced dislocation substructure and the irregular columnar grains with the random network of grain boundaries primarily contribute to the superior creep behaviour. However, when the unique columnar grain features formed during DED process are replaced by recrystallized, equiaxed grain structure in the HSA case, the worst creep performance is observed due to the coarse grains with regular-shaped grain boundaries facilitating the crack propagation. In addition, anisotropy of creep performance with respect to sample orientation observed for the heat-treated DED specimens can be significantly alleviated by a higher temperature of solution treatment.