•Design method of sheet based functionally graded Gyroid structure based on the mathematical expression of triply periodic minimal surfaces.•Sheet based functionally graded Gyroid exhibited failure ...mechanism of layer-by-layer buckling deformation.•Superior energy absorption capability of sheet based functionally graded Gyroid structure.•The Johnson-Cook models were implemented to simulate the deformation response of the lattice structures.
The triply periodic minimal surfaces (TPMS) have caught a lot of attention to many applications recently such as biomaterials, lightweight components with high strength and functionally graded material (FGM). In this study, the designing methods of the network based functionally graded Gyroid (N-FGG) and sheet based functionally graded Gyroid (S-FGG) structures were presented. The specimens of N-FGG and S-FGG based structures were fabricated by selective laser melting (SLM) with Ti-6Al-4V powder, then followed by quasi-static compression tests to measure the mechanical properties. The S-FGG based structure showed higher elastic modulus, yield strength and more stable stress fluctuation than N-FGG based structure with the same range of volume fraction gradient. The dominated deformation behaviors of both graded lattice structures were layer-by-layer. However, the S-FGG based structure showed more of buckling failure while the N-FGG based structure exhibited more of brittle fracture. Furthermore, the finite element analysis (FEA) with the Johnson-Cook plastic and damage models was implemented to simulate the plastic deformation and the failure behavior of the lattice materials at the post-yield stages. The simulated results illustrated that the compressive stress concentrated in the middle area of struts which connected the two adjacent layers of N-FGG based structures, while the stress in S-FGG based structures was distributed much uniformly in the middle connection region. S-FGG based structure also showed higher ultimate stress with the increase of compressive strain. Finally, the energy absorption capability of lattice structures was investigated, and the results indicated that the S-FGG based structure showed more total energy absorption per unit volume and higher energy efficiency, which means good prospects especially in the applications of relatively high allowable stress.
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Commercially pure titanium (CP–Ti) gyroid scaffolds with interconnected pores and high porosities in the range of 68–73% and three different unit cell sizes of 2, 2.5, and 3 mm were manufactured by ...selective laser melting (SLM) for bone implant applications. The microstructure and mechanical properties of the scaffolds with different unit cell sizes and sample orientations were evaluated. The microstructure of as-built struts was dominated by massive martensite and the average microhardness of the struts was 2.27 GPa, which is ∼50% higher than that of dense cast CP–Ti. The elastic modulus and yield strength of the as-built scaffolds ranged from 1465 to 2676 MPa and from 44.9 to 56.5 MPa, respectively, values which are close to the elastic modulus of trabecular bone and presumably strong enough to bear the physiological loading of implants. The as-built scaffolds exhibited excellent ductility up to 50% strain and no sign of fracture up to 20–30% strain under compression. The dominant compressive response of the scaffolds was observed by formation of a plastic hinge which led to rotation of the struts about the plastic hinges followed by development of local shear bands in struts in the long plateau region. These SLM-manufactured gyroid CP–Ti scaffolds with significantly enhanced hardness and compressive strength exhibited an elastic modulus close to that of trabecular bone and offer a promising improvement on CP–Ti scaffolds for bone implant applications.
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High-power laser melting deposition provides an efficient solution for the fabrication of large-sized titanium alloy components. In this study, Ti6Al4V blocks with well-formed structures were ...prepared using a 7 kW laser power, and their internal defect distribution, microstructure along the deposition direction, tensile properties, and fatigue performance were investigated. The results showed that the density of the as-deposited Ti6Al4V blocks could reach 99.94%, and the main internal defects were cavities, primarily spherical and dispersed, with a maximum diameter of 326.7 μm. Among them, 65.53% of cavities had a diameter smaller than 100 μm, and 92.76% had a diameter smaller than 200 μm. The microstructure at the top region of the specimen consisted of a needle-shaped α phase, the middle region had a mixture of needle-shaped and lamellar α-phase, and the bottom region showed a lamellar α-phase structure. With an increase in deposition height, the aspect ratio of α-phase increased, and the average width decreased from 3.22 μm to 0.88 μm. The internal structure was mainly composed of a basket-weave structure, with a small amount of Widmanstätten structure present at grain boundaries. Hardness and tensile properties exhibited significant non-uniformity along the deposition height. The tensile strength and yield strength at the top region were approximately 50 MPa higher than those in the middle and bottom regions, while the elongation was about 2% lower. The top region's tensile fracture surface displayed a sawtooth pattern, whereas the middle and bottom regions exhibited fractures along the 45° direction of the principal stress. The length of α-bundles and the interface density within the bundles were crucial factors affecting crack propagation. In the top region, α bundles were the longest, and the interface density was the highest. During crack propagation, when the crack extended perpendicular to the α bundles, it encountered high resistance, and when it extended non-perpendicular, it rapidly propagated along the bundle boundaries, exhibiting high strength and low plasticity. The fatigue performance of the Ti6Al4V specimens fabricated by high-power laser melting deposition showed significant dispersion, with cracks originating from subsurface lack of fusion defects. At different stages of fatigue crack propagation, the role of α phase laths varied: during the fatigue source region, cracks primarily propagated along the boundaries of α laths, displaying significant cleavage fracture characteristics. In the fatigue propagation region, crack propagation velocity was mainly influenced by the direction of the laths, and in the fatigue final rupture region, ductile fracture was predominant. Α laths parallel to the tensile direction exhibited the best plasticity, promoting the formation of large dimples and high tear ridges.
High-performance grade 300 maraging steels were fabricated by selective laser melting (SLM) and different heat treatments were applied for improving their mechanical properties. The microstructural ...evolutions, nanoprecipitation behaviors and mechanical properties of the as-fabricated and heat-treated SLM parts were carefully characterized and analysed. The evolutions of the massive submicron sized cellular and elongated acicular microstructures are illustrated and theoretically explained. Nanoprecipitates triggered by intrinsic heat treatment and amorphous phases in as-fabricated specimens are observed by TEM. High-resolution TEM (HRTEM) images of the age hardened specimens clearly exhibit massive nanosized needle-shaped nanoprecipitates Ni3X (X=Ti, Al, Mo) and 50–60nm sized spherical core-shell structural nanoparticles embedded in amorphous matrix. XRD analyses reveal austenite reversion and probable phase transformations during heat treatments. The hardness and tensile strength of the as-fabricated and age-treated SLM specimens absolutely meet the standard wrought requirements. Furthermore, the lost ductility after aging can be compensated by preposed solution treatments. Relationships between massive nanoprecipitates and dramatically improved mechanical performances of age hardened specimens are elaborately analysed and perfectly explained by Orowan mechanism. This study demonstrates that high-performance grade 300 maraging steels, which is comparable to the standard wrought levels, can be produced by SLM additive manufacturing.
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•Evolutions of the typical SLMed microstructures are illustrated and theoretically explained.•Precipitation behavior and phase transformation of SLMed maraging steel are characterized by TEM and XRD.•Significant improvement of strength after solution and aging treatment was evaluated and explained.•Relationships between massive nanoprecipitates and improved mechanical performances are elucidated.
In this study, the GH4251 high-temperature superalloy was fabricated using selective laser melting (SLM) and hot isostatic pressing (HIP) post-treatment. The hot corrosion behavior of the SLMed and ...SLM-HIPed GH4251 was systematically investigated and compared with the as-forged one in a mixed salt of 75 % Na2SO4 and 25 % NaCl at 900 °C. The results indicated that three GH4251 superalloys rank in order of superiority for high-temperature corrosion resistance as follows: SLMed > SLM-HIPed > as-forged. The hot corrosion resistance of the alloy is closely related to its microstructures. A high density and a high proportion of small-angle grain boundaries effectively prevent substrate corrosion from molten salts. High dislocation densities and fine grains can increase the elemental diffusion channels and promote the formation of dense protective oxide films, thereby enhancing the hot corrosion resistance. The SLMed superalloy has high densities, high dislocation densities, a high proportion of small-angle grain boundaries, and an appropriate grain size. This is the reason why the SLMed alloy exhibits the most superior hot corrosion resistance. In addition, the hot corrosion mechanisms of the GH4251 superalloy were also discussed in depth.
•The superalloys rank in order of superiority for high-temperature corrosion resistance as follows: SLMed > SLM-HIPed > as-forged.•High dislocation densities and fine grains can promote the formation of protective oxide films, thus improving the hot resistance.•The high hot corrosion resistance of SLMed alloy is attributed to its high densities, dislocation densities, low-angle grain boundaries content and finer grains.
Selective laser melting is a promising additive manufacturing technology for manufacturing porous metallic bone scaffolds. Bone repair requires scaffolds that meet various mechanical and biological ...requirements. This paper addresses this challenge by comprehensively studying the performance of porous scaffolds. The main novelty is exploring scaffolds with different porosities, verifying various aspects of their performance and revealing the effect of their permeability on cell growth. This study evaluates the manufacturability, mechanical behaviour, permeability and biocompatibility of gyroid scaffolds. In simulations, mechanical behaviour and permeability exhibited up to 56% and 73% accuracy, respectively, compared to the experimental data. The compression and permeability experiments showed that the elastic modulus and the permeability of the scaffolds were both in the range of human bones. The morphological experiment showed that manufacturing accuracy increased with greater designed porosity, while the in vitro experiments revealed that permeability played the main role in cell proliferation. The significance of this work is improving the understanding of the effect of design parameters on the mechanical properties, permeability and cell growth of the scaffolds, which will enable the design of porous bone scaffolds with better bone-repair effects.
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•Proposing a design method to improve the accuracy of manufactured porosity of high-porosity scaffolds.•Comprehensively studying the manufacturability, mechanical and mass transport properties, and biocompatibility of gyroid scaffolds.•The simulation of the mechanical and mass-transport properties of scaffolds both showed predictability.•Greater pore size gave bone scaffolds higher permeability, which promotes bone repair•The factors possibly affecting the prediction accuracy of the mechanical and mass-transport properties of lattice scaffolds were summarised.
The tensile properties, mode I fracture toughness (KIc), fatigue crack growth behavior, and unnotched fatigue strength of additively manufactured Ti-6Al-4V (Ti64) alloy using selective laser melting ...(SLM) technique were investigated. Four different combinations of layer thickness (t) - scan rotation between successive layers (ϕ), which resulted in mesostructures that range from through-thickness columnar prior β grains with square cross-sections, whose side lengths equal to the scan spacing, to near-equiaxed mesostructures in both build and transverse directions, were explored. Possible anisotropy in mechanical properties was investigated by conducting tests on samples whose loading axis is either parallel or perpendicular to the build directions. In all cases, the microstructure consisted of fine α/α′ lath structure, where α′ is the metastable martensitic Ti phase that is acicular in shape, within the prior β grains. Experimental results show that the process parameter combinations of t = 60 μm and ϕ = 67° results in an alloy that exhibits high yield strength (>1100 MPa) and ductility (>12%) simultaneously, KIc of 58 MPa m , and unnotched fatigue strength, which is similar to that of the same alloy but manufactured using conventional techniques. The anisotropy in properties, overall, was found to be not substantial, even in the case where columnar growth of prior β grains occurs in the build direction. The values of the Paris exponents for steady state fatigue crack growth (FCG) are much lower than those reported for conventionally manufactured Ti64, suggesting higher FCG resistance in SLM Ti64. Analysis of the effective microstructural length scale that controls the near-threshold FCG rate suggests that it is the colony size that dominates this behavior. Overall, the results of this study indicate directions for process parameter optimization that would lead to SLM Ti64 that is not only has high strength, but also is damage tolerant.
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A bimodal globularized microstructure in contrast to martensitic laths is known to impart high strength and toughness in Ti-6Al-4V. Heat treatment for the phase transformation of the laths to the ...globularized microstructure must be preceded by plastic deformation. This work reports an innovative strategy to obtain the bimodal microstructure consisting of globular α in additively manufactured Ti-6Al-4V alloy by heat treatment alone. The heat treatment schedule involves repeated thermal cycling close to but below the β transus temperature to form globular α eliminating the need for plastic deformation prior to heat treatment. A new mechanism of globularization other than known in literature is proposed to explain the formation of globular α. The inherent dislocation sub-structure of the martensitic laths initiates globularization by thermal grooving and boundary splitting but is unable to completely globularize the microstructure. Mechanisms such as cylinderization and edge spheroidization also do not lead to globularization. The purposefully designed thermal cycling causes oscillations in the volume fractions of α and β phases that in synergism with the slow cooling segments of the cycle globularize the α phase by epitaxial growth. The bimodal microstructure thus produced led to a significant improvement in the ductility by 80% and the toughness by 66%, which are desirable for structural applications. Furthermore, beneficial compressive stresses were generated in the alloy because of cyclic heat treatment. It is envisaged that the exceptional combination of mechanical properties observed here will lead to the fabrication of SLM printed Ti-6Al-4V parts that could leverage the advantages of additive manufacturing with material properties that are comparable to those obtained by conventional fabrication routes.
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Any variation in the processing parameters of selective laser melting fabrication could impact the performance of the final product. This study is concentrated on the effects of laser power and ...scanning speed alteration on the microstructure, transformation temperatures, texture, and shape memory response of Ni50.8Ti49.2. In this regard, multiple samples were systematically fabricated to demonstrate that careful selection of process parameters can lead to fabrication of parts with distinctive features and behaviors. The samples processed with low laser power showed significantly higher strain recovery and lower mechanical hysteresis compared to those processed with high laser power. It was demonstrated that the samples fabricated with same energy level, using a combination of different processing parameters each displayed unique responses. The sample fabricated with a laser power of 100 W and scanning speed of 125 mm/s exhibited almost perfect superelasticity with a recovery ratio of 96% and strain recovery of 5.77% in the first cycle. The corresponding stabilized superelastic response demonstrated full strain recovery of 5.5% after 10 cycles.
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Functional graded cellular materials (FGCMs) have attracted increasing attentions for their improved properties when compared to uniform cellular structures. In this work, graded Gyroid cellular ...structures (GCSs) with varying gradient directions were designed and manufactured via selective laser melting (SLM). As a reference, uniform structures were also manufactured. The surface morphology and mechanical response of these structures under compressive loads were investigated. Results indicate high manufacturability and repeatability of GCSs manufactured by SLM. Optimized density distribution gives these structures novel deformation and mechanical properties. GCSs with density gradient perpendicular to the loading direction exhibit deformation behaviours similar to uniform ones, while GCSs with the gradient parallel to the loading direction exhibit layer-by-layer deformation and collapse behaviour. A novel phenomenon of sub-layer collapses is found in GCSs with gradient parallel to the loading direction. Furthermore, mathematical models were developed to predict and customize the mechanical properties of graded cellular structures by optimizing the relative density of each layer. These significant findings illustrate that graded cellular structures have high application prospect in various industries, particularly given the fact that additive manufacturing has been an enabler of cellular structure fabrication.
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•Continuous graded Gyroid cellular structures (GCSs) were fabricated by SLM.•Novel deformation and mechanical properties were gained compared to uniform cellular.•The effect of gradient direction was investigated for GCSs.•Novel sub-layer collapses are founded in GCSs with gradient along building direction.•Mathematical models were developed to calculate Young's modulus and strength of GCSs.
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