Directed energy deposition (DED) is a major additive manufacturing (AM) process, which employs high energy beams as the heat source to melt and deposit metal powder in a layer-by-layer fashion such that complex components can be manufactured. In this study, a magnetic-field-assisted DED method is applied to control the microstructure and element distribution in the deposited materials. For this purpose, to control the microstructure of DED-built 316L stainless steel, a horizontal magnetic field is introduced during the DED process at different levels of magnetic field intensities (i.e., 0T, 1.0T and 1.8T). Scanning electron microscopy (SEM) and energy dispersive X-Ray spectroscopy (EDS) are used to characterize the microstructure of components obtained with different magnetic field strengths. The results show that the microstructure of deposited materials is significantly affected by the external magnetic field. Also, the result of interdendritic microsegregation pattern presents a transformation from continuous morphology to discrete morphology because of the applied magnetic field. Along with the increasing horizontal magnetic field intensity, nickel and chromium content are changed significantly in austenite and ferrite.

This study investigates the effects of laser shock peening (LSP) on residual stress, near surface modification, and hardness of Inconel 718 (IN718) specimens manufactured by selective laser melting (SLM) technique. Optical microscope and electron backscattered diffraction (EBSD) is used to characterize the microstructures of both heat-treated and as-built specimens. A nanoindentation test is performed to determine the properties such as the hardness of as-built and heat-treated specimens. Afterward, the hardness along the distance from the LSP treated surface is also defined. To investigate the effect of LSP energy on the mechanical properties of specimens, two levels of LSP energy, e.g., low energy LSP (6.37 GW/cm 2 ) and high energy LSP (8.60 GW/cm 2 ), are carried out on selected samples. With the increase in laser energy density, it is found that both compressive residual stress and hardness increase after LSP treatment. The as-built specimens after high energy LSP treatment show the compressive residual stress of −875 MPa, and the surface hardness increases from 468 HV to 853 HV.

3D printing (additive manufacturing) has become a popular method to create three-dimensional objects due to its high efficiency and is easy to operate. 3D printing of continuous fiber reinforced polymers has been a challenge. The fused deposition modeling (FDM) processes for this purpose were proposed and made possible only several years ago. The 3D printed continuous fiber reinforced polymers are able to improve the mechanical properties by leaps and bounds. In this paper, we aim to investigate the possibility of further improve the mechanical properties of 3D printed continuous fiber reinforced polymers by adding nano fillers to the polymer matrix. In experiment, the Kevlar fiber is chosen to be the continuous fiber material, and nylon 6 (PA 6) is chosen to be the polymer matrix material. Carbon nanotubes (CNTs) and graphene nano platelets (GNPs) nanoparticles are first mixed with nylon 6 pellets to make nanocomposites. The nanocomposites are then extruded into filaments for 3D printing. During the 3D printing process, both Kevlar filament and nanocomposite filament are fed through the printing nozzle and deposited on the platform. Tensile specimens are made from pure PA 6 and four types of nanocomposites, namely, 0.1wt% CNT/PA 6, 1wt% CNT/PA 6, 0.1wt% GNP/PA 6, 1wt% GNP/PA 6. By incorporating four layers of Kevlar fiber, which leads to the weight percentage of about 9% for Kevlar fiber in materials, fiber composite tensile specimens are made from Kevlar/PA 6 composite and four fiber reinforced nanocomposites, namely, Kevlar/0.1%CNT/PA 6, Kevlar/1%CNT/PA 6, Kevlar/0.1%GNP/PA 6, and Kevlar/1%GNP/PA 6. The tensile tests reveal that CNTs filled PA 6 nanocomposites show less significant improvements in mechanical properties as compared to the GNP filled PA 6. With only 0.1wt% of GNP, the tensile modulus improves by 101%, and with 1wt% of GNP, the modulus improves by 153%. The results also indicate that although Kevlar fibers dominate the main mechanical properties of the printed composite materials, the existence of GNP nano fillers also provide noticeable contribution to the enhancement of tensile strengths and moduli, while the effect of CNTs is much less pronounced.

Graphene is an ideal reinforcement material for metal matrix composites (MMC) owing to its high strength, high ductility, light weight, as well as good bonding with metal matrix. Additive manufacturing such as selective laser melting (SLM) brings the advantages of low material waste, high flexibility, and short production lead cycle. In this study, graphene nano-platelets (GNPs) reinforced Inconel 718 composites are fabricated by SLM technique and processed under various post heat treatment schemes. It is found that fabrication of GNPs reinforced MMC using SLM technique is a viable approach. The obtained composite possesses dense microstructure and enhanced tensile strength. Post heat treatments at two levels of solution temperature (980 and 1220°C) for 1 hour followed by two-step aging are carried out. The experiment results indicate that addition of GNPs into Inconel 718 matrix results in significant strength improvement. At as-built condition, the ultimate tensile strengths are 997 and 1447 MPa, respectively at 0 and 4.4vol.% GNP content. Moreover, under as-built and solution treated condition, high content of GNPs results in overall higher UTS value and the strengthening effect is most significant at as-built condition. Meanwhile, γ′ and γ″ precipitation hardening is suppressed in the GNPs reinforced composite under aged condition due to the formation of MC carbide and depletion of Nb. Incorporating GNPs in Inconel 718 effectively inhibits the grain growth during post heat treatment.

Graphene possesses many outstanding properties, such as high strengths, light weight, making it an ideal reinforcement for metal matrix composite (MMCs). Meanwhile, fabricating MMCs through laser assisted additive manufacturing (LAAM) has attracted much attention in recent years due to the advantages of low waste, high precision, short production lead time, and high flexibility. In this study, graphene reinforced aluminum alloy AlSi10Mg is fabricated using selective laser melting. Composite powder is prepared using high-energy ball milling. Room temperature tensile tests are conducted to evaluate the tensile properties. Scanning electron microscopy (SEM) observations are conducted to investigate the microstructure and fracture surface of obtain composite. It is found that adding GNPs significantly increases porosity and therefore deteriorates material tensile performance. The relationship between porosity and material strength are numerically investigated. Taking into consideration the strength reduction caused by large porosity, the strengthening effect of GNPs turns out to be significant, which reaches 60.2 MPa.

Selective laser melting (SLM) is an additive manufacturing process that uses laser beam to melt metal powders and allow the melt to solidify in a layerwise way. SLM has drawn much attention from industry and academia in recent years. Improving the mechanical properties and performance of components fabricated by SLM has been a focused research area. Adding hard second phase particles into metal matrix has been proven an effective measure to strengthen metal material by SLM. In this research, we adopt nano sized TiC particles to reinforce pure iron matrix using the SLM process. The reinforced TiC/iron composite with 0.5 wt.% TiC is successfully fabricated. Tensile tests and fatigue tests are carried out to demonstrate the strengthening effect, and fatigue fracture surfaces are characterized by SEM to understand the fatigue failure mechanism. The obtained experimental data are compared with an existing composite fatigue life prediction model. The results indicate that nano TiC is effective in improving the tensile performance of pure iron, where the ultimate tensile strength (UTS) and yield strength (YS) increase by 17% and 6.3% respectively. TiC nano particles improve the fatigue life principally at lower cycle fatigue regime, while the beneficial effect at high cycle fatigue regime is not significant, mainly due to the large porosity introduced in SLM process. In addition, it is discovered that traditional Ding’s model does not accurately predict the fatigue life of nano TiC/iron composite, and thus more accurate fatigue modeling work is called for.

Laser assisted additive manufacturing (LAAM) is regarded as a complementary manufacturing method to traditional manufacturing technologies. Meantime, improving the mechanical performance of components fabricated by LAAM is an important research focus in recent years and it has drawn significant attention from both industrial and research aspects. In the present study, in order to obtain high-performance metal components by LAAM, nano-TiC particles are used to reinforce Inconel 718 and the mixed raw powder is processed by selective laser melting (SLM) technique. To investigate the effect of TiC amount on the property and performance of the composite, samples with four levels of nano-TiC addition (0, 0.4, 0.8 and 1.6 wt.%) are prepared, all other manufacturing parameters are set fixed. Furthermore, standard solid solution treatment at 980 °C for 1 hour is carried out to investigate its effect on the final properties. SEM observations are performed to analyze the microstructure of the composites. In addition, to understand the reinforcing mechanism of nano particles in LAAM-produced metal composites at both as-built and heat treated state, we consider four main strengthening mechanisms, (a) load-bearing effect, (b) enhanced dislocation density due to the residual plastic strain caused by the difference in the coefficients of thermal expansion (CTE) between the matrix and reinforcing particles, (c) Orowan strengthening effect, and (d) Hall-Petch strengthening. The effect of TiC nano particle amount on each of the four strengthening mechanisms is investigated separately and the results show that within the investigated range, the increase of reinforcement content leads to higher tensile strength. With 1.6 wt.% reinforcement, the ultimate tensile strength increases by 15%. At as-built condition, the composites have the maximum yield strength (YS) and ultimate tensile strength (UTS), while for solution treated samples, the tensile strengths are overall lower due to microstructure coarsening. Through quantitative investigation, it is found that both as-built and solution treated conditions, the load-effect strengthening effect is very small as compared with other contributors. Thermal mismatch strengthening effect is most significant at any volume fraction under as-built condition, mainly due to high SLM temperature. However, for solution-treated condition, CTE mismatch strengthening is weakened because solution treatment significantly equilibrates the thermal strain in the composite, and diminishes most strain-induced dislocations. However, Hall-Petch strengthening becomes dominating as large amount of nanoparticles effectively inhibit the grain coarsening during solution treatment.

Graphene nanoplatelets (GNPs) have many outstanding properties, such as high mechanical strengths, light weight, and high electric conductivity. These unique properties make it an ideal filler material for various composites. On the other hand, the development MMNCs (metal matrix nanocomposites) through additive manufacturing (AM) processes has become a major innovation in the field of advanced structural materials, owing to shorter production lead time, less material waste, high production flexibility. It is of great innovativeness to have the attractive features combined to produce GNPs reinforced MMNCs using AM techniques. In addition, metal components produced by laser assisted additive manufacturing (LAAM) methods usually have inferior mechanical properties, as compared to the counterparts by the traditional metal forming processes. To achieve optimized mechanical properties, the obtained MMNCs are subjected to various post treatment routines and the effect of post heat treatment on material properties is investigated. In this study, pure Inconel 718 and GNPs reinforced IN718 with 1.1 vol.% and 4.4 vol.% filler material are fabricated by selective laser melting (SLM). Room temperature tensile tests are conducted to evaluate the tensile properties. Scanning electron microscopy (SEM) observations are conducted to analyze the microstructure of materials and to understand the reinforcing mechanism. It is found that fabrication of GNPs reinforced MMC using SLM is a viable approach. The obtained composites possess dense microstructure and enhanced tensile strength. The strengthening effect and mechanisms involved in the composites are discussed. Solution treatments at three levels of temperature (940, 980, and 1020°C) for 1 hour period are carried out to evaluate the effect of the heat treatment on the material microstructure and therefore the resulted mechanical properties of the composite material. The results of samples with and without heat treatment are also compared. The experiments results indicate that that addition of GNPs into Inconel 718 results in significant strength improvement. Moreover, at any volume content of reinforcement, higher solution treatment leads to lower strength, mainly due to coarsened microstructure. The addition of GNPs effectively inhibits the grain growth during the post heat process and the average grain size is significantly refined compared to unreinforced samples. Moreover, through the investigation of various strengthening mechanisms, it is found that Orowan strengthening effect is small and can be neglected for both as-built and heat treated conditions. Load transfer effect is the dominating strengthening effect among all contributors and solution treatment significantly reduces thermal mismatch strengthening.

Fabricating metal matrix composites (MMCs) through laser assisted additive manufacturing (LAAM) has attracted much attention in recent years. This is because the traditional metal components produced by LAAM are usually inferior to the counterparts produced by conventional manufacturing processes, reflected by porosity, lower density, and thus poorer mechanical properties and service performance. Adding reinforcements to metal matrix in LAAM process can alleviate the challenge. Also, for components produced by LAAM processes, post treatment is often required to further strengthen the material, reduce residual stress, or clean off surface for dimensional accuracy. However, research regarding how post treatment affects the microstructure and mechanical properties of LAAM-produced MMCs is still very rare in literature. In this study, a nano-TiC reinforced Inconel 718 composite is prepared using selective laser melting (SLM) technique. Various post heat treatment processes have been adopted to investigate their effect on final product properties. The motivation is that Inconel 718 is a Ni-based superalloy, whose full potential is explored in heat treatment after manufacturing processes. A composite with 0.5 wt.% nano-TiC addition is prepared. Three levels of solution temperatures at 940, 980, and 1020 °C (for 1 hr) or one level of annealing temperature at 1100 °C (for 1 hr) are adopted for the treatment, combined with the standard two-step aging (720 °C, 8 h, furnace cooling + 620 °C, 8 h, air cooling) on both the MMC and unreinforced Inconel 718 materials. Scanning electron microscopy (SEM) observation is conducted to analyze the microstructure of the composite and understand the reinforcing mechanism. Tensile tests are conducted to evaluate the tensile properties. It is discovered that compared with the pure Inconel 718 by SLM, the Inconel 718-TiC MMC exhibits improved ultimate tensile strength for both as-built and solution/annealing treated conditions. Microscopy observation shows that the dendritic structures of Inconel 718 is remarkably refined by the TiC particles in as-built samples, and grain coarsening is largely inhibited by the TiC particles for solution/annealing treated samples. For both reinforced and unreinforced Inconel 718, dissolving of Laves phase and precipitation of δ phase is observed, but annealing at 1100 °C is not favorable for the formation of δ phase. Aging treatment significantly increases the UTS values for both type of material. Moreover, the strengthening effect of added nano particles becomes less significant in the aged condition, due to the precipitation of γ′ and γ″. Future work includes the process parameter optimization, and further evaluation of other mechanical properties.

Graphene nanoplatelets (GNPs) have many outstanding properties, such as high mechanical strengths, light weight, and high electric conductivity. These unique properties make it an ideal reinforcement used for metal matrix composite (MMCs). In the past few years, many studies have been performed to incorporate GNPs into metal matrix and investigate the properties of obtained metal matrix composites. Meanwhile, fabrication of MMCs through laser assisted additive manufacturing (LAAM) has attracted much attention in recent years due to the advantages of low waste, high precision, short production lead time, and high workpiece complexity capability. In this study, the two attractive features are combined to produce GNPs reinforced MMC using selective laser melting (SLM) process, one of the LAAM processes. The target metal matrix material is Inconel 718, a nickel-based Ni-Cr-Fe austenitic superalloy that possesses excellent workability and mechanical performance and has wide applications in industries. Inconel 718 holds outstanding mechanical performance, corrosion, oxidation, and wear performance over a wide range of temperatures, making it an attractive superalloy used for high temperature service components. In the experiment, pure Inconel 718 and GNPs reinforced Inconel 718 composites with two levels of GNPs content (i.e., 0.25 and 1 wt.%) are obtained by SLM. Note that the SLM process, a novel powder mixture procedure is adopted to ensure the even dispersion of GNPs in the Inconel 718 powders. Room temperature tensile tests are conducted to evaluate the tensile properties. Scanning electron microscopy (SEM) observations are conducted to analyze the fracture surface of materials and to understand the reinforcing mechanism. It is found that fabrication of GNPs reinforced MMC using SLM is a viable approach. The obtained composite possesses dense microstructure and significantly enhanced tensile strength. The ultimate tensile strengths (UTS) are 997.8, 1296.3 and 1511.6 MPa, and the Young’s moduli are 475, 536, and 675 GPa, for 0 wt.% (pure Inconel 718), 0.25 wt.%, and 1 wt.% GNP additions, respectively. The bonding between GNPs and matrix material appears to be strong, and GNPs are well retained during the SLM process. The strengthening effect and mechanisms involved in the composites are discussed. Load transfer, thermal expansion coefficient mismatch, and dislocation hindering are believed to be the three main reinforcing mechanisms involved. It should be noted that more work needs to be conducted in the future to obtain more comprehensive information regarding other static and dynamic properties, and the high temperature performances of the GNP reinforced MMCs produced by SLM. Process parameter optimization should also be investigated.

Laser assisted additive manufacturing (LAAM) is regarded as a complementary method to traditional manufacturing processes and it has drawn significant attention from both industry and academia. Improving the performance of components fabricated by LAAM has been an important research task in recent years due to the inevitable negative effects introduced by in the LAAM process, such as porosity, inhomogeneity, and tensile residual stress. In this study, to obtain high-performance metal components by LAAM, nano-TiC particles are adopted to reinforce Inconel 718 and the mixed powders of TiC and Inconel 718 are processed by selective laser melting (SLM). To investigate the effect of TiC concentration on the property and performance of the metal matrix composite, samples with three levels of nano-TiC addition (0, 0.25, 0.5 wt.%) are prepared, in which the SLM process parameters are kept the same for all cases. Three-point bending tests are performed to determine the optimum nano TiC addition. It is found that the material of 0.5 wt.% TiC addition results in the highest bending modulus. Compared with pure Inconel 718, the composite with 0.5 wt.% nano-TiC shows 38% increase in flexural modulus. In addition, to better understand the reinforcing mechanism of nano TiC particles, the SLM processed samples are characterized using a self-designed micro/nano three-point bending tester that is incorporated with an atomic force microscope (AFM) to in situ observe the nanoparticles movement on the sample surface under loading. The sample surface is scanned by AFM at 0%, 2%, 4% and 6% strain condition during the test, and the migration of individual nano particles at the sample surface is tracked at each strain condition. The AFM observations show that the dispersion of TiC nanoparticles is overall random and uniform in the Inconel 718 matrix, and localized agglomeration of TiC particles exists. The interfaces between nanoparticles and the matrix are generally continuous and free of any deleterious micro cracks, indicating a favorable metallurgical bonding feature. The surface morphologies obtained by AFM at different strain conditions are processed by digital image correlation technique to obtain the strain fields during the deformation process. The results show that the agglomerated TiC particles serve as defects in the material, leading to relative sliding of particles with respect to the matrix. This is believed to be the main factor that limits the performance of the composites made by SLM.

Selective laser melting (SLM) is an important additive manufacturing process. It applies focused laser energy to quickly melt and solidify material powders, and a controlled layered operation can result in a free form build that is often out of reach for machining processes. As such, it has attracted much attention in recent years. However, metal components produced by this process often have inferior mechanical properties, as compared with the counterparts by the traditional manufacturing processes. To strengthen the metal components by SLM, adding reinforcement particles and applying post treatment are regarded as the two effective ways. Although adding reinforcement particles to create metal matrix composites has been studied by researchers in literature, much fewer has been done to use post treatment processes to further improve the properties and performance of the metal matrix composites from SLM. In this study, a nano-TiC reinforced Inconel 718 composite is prepared using SLM technique. The material has 0.5 wt.% nano-TiC addition. Solid solution treatments at three levels of temperature (940, 980, 1020 °C) are carried out to evaluate the effect of the heat treatment methods on the microstructure and resulted mechanical properties of the composite material. The results of samples with and without heat treatment are also compared. SEM observations are carried out to analyze the microstructure of the composite and understand the reinforcing mechanism. Tensile tests are conducted to evaluate the mechanical properties of the formed composites. It is discovered that compared with the pure Inconel 718 by SLM, the Inconel 718-TiC composite exhibits improved ultimate tensile strength. Microscopy observation of as-built samples indicates that the dendritic structures of Inconel 718 is remarkably refined by the TiC particles. Suspected laves phase particles are observed in as-built Inconel-TiC composite, and they partially transform to large amount of needle-like δ phase during the solid solution treatment.

The objective of this paper is to experimentally investigate the machinability of nano-TiC reinforced Nickel based super alloy Inconel 718 fabricated by direct metal selective laser melting (SLM). Four 10×10×3 square test coupons were fabricated with different amount of nano-TiC: (1) pure Inconel 718, (2) Inconel 718+0.25% TiC, (3) Inconel 718 + 0.5% TiC, of 508 microns. The machinability of the four materials were examined in terms of cutting forces, tool wear and chip morphology. Three level of federates (1.0, 1.5 and 2.0 um/flute) and three level of spindle speeds (12,000, 15,000 and 18,000 rpm) were selected and a 3 2 full factorial experiment was performed on each test coupon. Full immersion slotting was selected with a fixed axial depth of cut at 20 microns. The SEM images of the tools reveal that the dominant wear mechanisms were abrasive wear at the tool tip and flank face. The adhesion and build up edge were also common. The wear rate increases with the addition of nano-TiC. The loss of the AlTiC coating will result in accelerated wear, which was observed for machining of nano-TiC reinforced Inconel 718, but not on pure Inconel 718. The edge chipping and abrasive wear at the tool tip reduced the effective cutting diameter, enlarged the edge radius, and caused the increase of the cutting force. For all the materials tested, the cutting chips had serrated edge on the free surface and much smoother surface on the other side, which suggests that a cyclic chip formation of alternating high shear strain followed by low shear strain. This is in agreement with the chip formation mechanism for the Inconel 718 fabricated with conventional method rather than DMLS. The serration is more severe with the and (4) Inconel 718 + 1% TiC. Tensile tests were performed on all four material and the material strength increases with the increase of the TiC content up to 0.5% then plateaued. The elongation drop significantly with the inclusion of TiC in Inconel substrate. Micro-endmilling experiments were conducted using AlTiN coated WC micro-mill with nominal diameter addition of nano-TiC. The cutting forces were collected with a Kistler 3-axial load cell 9017B. The cutting forces increases with the increase spindle speed (hence surface speed) within the range examined, but the effect of feedrate is not statistically significant. The cutting forces were much higher for TiC reinforced Inconel and the magnitude of the cutting forces increases with the increase of the weight percentage of TiC contents.

Metal components produced by additive manufacturing processes usually have inferior properties and performances as compared with the counterparts by the traditional forming and machining processes. To close the gap, the metal matrix can be strengthened by adding reinforcement particles in additive manufacturing processes. This research presents the fabrication of nano-TiC reinforced Inconel 718 composites using selective laser melting (SLM). Tensile and wear performance tests are conducted to evaluate the mechanical properties of the formed composites. It is discovered that the composites exhibit improved mechanical properties in terms of ultimate tensile strength and yield stress. Compared with the pure Inconel 718 specimens by SLM, the ultimate tensile strength and yield stress of the reinforced Inconel 718 increase by 207 MPa and 204 MPa, respectively, with 0.5 wt.% addition of nano-TiC particle. Smaller increases are observed with 0.25 wt.% and 1.0 wt.% nano-TiC additions. On the other hand, the addition of nano-TiC particles decreases the ductility of Inconel 718. To investigate the strengthening mechanism of nano reinforcement particles in SLM, the microstructures with different levels of nano-TiC particles are observed. The results indicate that the microstructure of Inconel 718 is remarkably refined by the TiC particles, and the reinforcement particle significantly impede the growth of columnar grain in the solidification process.

Shot peening is a well-known surface finishing process that is often applied to enhance the residual stress distribution in the component surfaces. In this study, we carry out a molecular dynamics (MD) simulation study to investigate the single particle shot peening process on the (100) silicon surface at nano scale. The MD simulation enables in-situ observation and analysis of complex material deformation mechanisms in the presences of ultra-high strain rate, such as stress distribution, dislocation propagation, and particle/workpiece interface interaction. Three levels of shot speed (i.e., 200, 500, and 800 m/s) and three levels of impinging angle (i.e., 30°, 60°, and 90°) are configured in the simulation. It is found that higher shot speeds result in deeper depths of residual indentation, deeper penetration depths of residual stress below surface, and higher magnitude of maximum compressive stress. The residual stress distributions underneath the shot-peened surfaces exhibit strong anisotropic manner. Residual stress only concentrates in the area along the impact direction; and the decrease of impact angle results in shallower penetration of residual stress.