Based on the forming principle of laser rapid melting and layered manufacturing, the selective laser melting (SLM) process is suitable for the preparation of permeable metal materials with a certain ...porosity. One of the important applications of this special structure is the development of permeable steel. In this study, the permeable steel with micron-sized controllable pores was prepared by gradient printing using SLM process. The formation and connectivity mechanism of pore structure were analyzed by adjusting the interlayer arrangement of scanning melt channels, and the influence of process parameters on the pore characteristics, permeability and mechanical properties was systematically investigated. The results revealed that the porosity and pore size of permeable steel were effectively improved by controlling the hatch distance and scanning strategy, with the results ranging from 7.81 % to 17.32 % and 57 μm to 121 μm, respectively. Based on the gas permeation test, it was verified that the permeability of permeable steel showed a clear upward trend with increasing porosity, and the results ranged from 2.17 × 10−12 m2 to 4.26 × 10−12 m2, indicating that the pore structure provides a strong basis for gas flow. Moreover, the mechanical properties of permeable steel decreased significantly with the increase of porosity and pore size, and the compressive strength and tensile strength ranged from 1061 MPa to 528 MPa and 1018 MPa to 609 MPa, respectively. The mechanical behavior and deformation mechanism were closely related to the gradient evolution of the pore structure. Overall, the gradient printed permeable steel using SLM process has good permeability and mechanical properties.
•Permeable steel with controllable pores was gradient printed by selective laser melting process.•Gradient design of pore structure was carried out by adjusting the arrangement of scanning melt channels.•The formation and connectivity mechanism of pore structure were analyzed.•The pore characteristics, permeability and mechanical properties of permeable steel were studied.
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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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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In this work, selective laser melting (SLM) technology was used to manufacture the Inconel 718 alloy from low to high power and the effects of laser power on formability, microstructural evolution ...and tensile performance were studied. It was found that the fully dense forming process window becomes narrower as the laser power increases. Moreover, as the laser power increases from 500 W to 2000 W, the build rate is increased from 6 mm3/s to 14.3 mm3/s. In comparison, the coarser γ-dendrite, the more precipitation of Laves phase and the finer columnar grain can be obtained under a higher laser power. Besides, the texture in the Inconel 718 samples can be reduced at a higher laser power. The tensile strength exhibits an obvious decreasing trend as the laser power increases. At the lowest laser power of 500 W, tensile strength reaches the highest values with the ultimate tensile strength of 1122 MPa and yield strength of 828 MPa, because of the higher relative density, the finer γ-dendrite and the less Laves phase. But the increase of the laser power benefits the improvement of elongation. When the laser power further increases to 1500 W, the elongation reaches its highest value of 33.45% due to the finer columnar grains.
•The build rate increases from 6 mm3/s to 14.3 mm3/s as the laser power increases.•Higher laser power leads to coarser γ-dendrite/Laves phase and finer columnar grain.•The number of strengthening phase remains unchanged as the laser power increases.•l At 500 W, UTS and YS reach 1122 MPa and 828 MPa, respectively. At 1500 W, EL reaches 33.45%.
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.
Grain boundary engineering (GBE) is a thermomechanical processing strategy to enhance the physical and mechanical properties of polycrystalline metals by purposely incorporating special types of ...grain boundaries—such as twin boundaries (TB)—in the microstructure. Because of the multiple strain-annealing cycles involved, conventional GBE is not directly applicable to near-net-shape parts, such as those produced via additive manufacturing (AM) technology. In this study, we explore a different GBE processing route that leverages TB multiplication during recrystallization of austenitic 316L stainless steel produced via selective laser melting (SLM). We find that recrystallization requires a minimum level of mechanical deformation, which scales with the laser scanning speed employed during SLM. We ascribe this relationship to the cell size and the amount of solute segregating at cell boundaries during rapid solidification, which are inversely and directly proportional to the laser scanning speed, respectively. The coarser the cell structure and the more uniform the chemical composition, the easier the nucleation and growth of recrystallized grains. Our results provide the groundwork for devising AM-compatible GBE strategies to produce high-performance parts with complex geometry.
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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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•Microstructure is analysed on different planes of SLM 316L stainless steel.•Scanning strategy and building direction affects microstructural characteristics.•Sample fabricated with ...67.5°-rotation scanning shows the lowest corrosion resistance.•The corrosion resistance of the planes perpendicular to building direction is higher.•The boundary of the molten pool is more susceptible to corrosion.
In-depth understanding of corrosion behaviour is a key aspect regarding the application of additively manufactured parts. In this study, 316L stainless steel was manufactured under different scanning strategies using selective laser melting (SLM). Microstructure characterization and electrochemical tests in NaCl aqueous solution (3.5 wt%), including open circuit potential (OCP), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS), were conducted to study the influence of scanning strategies on the corrosion behaviour. The microstructure and corrosion on different planes were characterized to reveal the influence of building direction. EBSD analysis shows that the scanning strategy affects the continuity of grain growth through adjacent layers and the growth of grains inside the melt track. Electrochemical tests indicate a clear difference in corrosion resistance perpendicular and parallel to building direction and with different scanning strategies. Pitting corrosion is the main form of corrosion in SLM 316L stainless steel and preferentially initiates on molten pool boundaries.
Al-12Si samples were fabricated by selective laser melting. Different processing parameters, including four different hatch styles, as well as contour and base plate heating, were employed to ...evaluate their impact on the room temperature tensile properties. The samples with different hatch styles show similar crystallite sizes and lattice parameters of Al and comparable amount of free residual Si, but different levels of texture. The yield strength (YS) varies between 235 MPa and 290 MPa, the ultimate tensile strength (UTS) between 385 MPa and 460 MPa and the tensile ductility ranges between 2.8% and 4.5%. Similarly, the samples produced with and without contour show differences in their tensile properties. Base plate heating was employed with three different temperatures and the samples display improved plasticity with increasing base plate temperature (BP 473 K, 573 K and 673 K samples show a ductility of 3.5%, ∼3% and 9.5%, respectively). The results indicate that the room temperature tensile properties can be tuned (between YS: 115 MPa–290 MPa, UTS: 220 MPa–460 MPa and ductility: 2.8%–9.5%) in-situ during the selective laser melting process giving an opportunity to define the mechanical properties of the SLM parts to suit their service requirements.
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