Selective Laser Melting (SLM) is one of the additive manufacturing techniques used to fabricate three-dimensional metal parts from a computer aided design. SLM is gaining more and more attention due ...to its ability to produce parts with intricate shape and added functionalities. Moreover, materials fabricated by SLM show an unique microstructure and in some cases a metastable cellular microstructure is observed. However, its formation during the SLM process is not fully understood, yet. Hence, the present manuscript attempts to explain the development of metastable cellular microstructures during the SLM solidification process by considering the Bernard Marangoni driven instability (BMI) and particle accumulated structure formation (PAS) mechanisms, where both thermodynamics and kinetics play a role. According to the considered theories it is proposed that the cellular microstructure forms only when the following conditions are satisfied: (1) A binary alloy system is preferred. If a multicomponent system is chosen, then only two phases should form in the respective alloy; (2) the solute should be immiscible with the solvent or have at least negligible solubility around the solidification temperature of the solvent phase; (3) the solute phase should possess a higher melting point than the solvent, and (4) the solute and the solvent should have a melting point difference of at least 673 K.
•Metastable cellular microstructure formation during the selective laser melting process.•Criteria for the formation of cellular microstructure is established.•Mechanism for the cellular microstructure formation during the selective laser melting process is proposed.
The tensile, fracture, and fatigue crack growth properties of 316L stainless steel (SS) produced using the selective laser melting (SLM) technique were evaluated and compared with those of ...conventionally manufactured (CM) austenitic SSs. For SLM, both single melt (SM) and checker board (CB) laser scanning strategies were employed, so as to examine the effect of scanning strategy on the mechanical properties. The experimental results show that the SLM alloys' yield strength is significantly higher than that of CM 316L SS, a result of the substantial refinement in the microstructure. In contrast, only a marginal improvement in the ultimate tensile strength and a marked reduction ductility, which are a result of the loss of work hardening ability, are attributed to the absence of stress induced martensitic transformation common in CM austenitic SSs. In spite of these, the fracture toughness, which ranges between 63 and 87MPam0.5, of the SLM alloys is good, which is a result of the mesostructure induced crack tortuousity. The SLM process was found to marginally reduce the threshold stress intensity factor range for fatigue crack growth initiation and enhance the Paris exponent within the steady state crack growth regime. Both tensile and toughness properties were found to be anisotropic in nature. SLM with CB scanning strategy improves both these properties. All these observations on the mechanical properties are rationalized by recourse to micro- and meso-structures seen these alloys.
The microstructure and room temperature mechanical properties of a18Ni (300) grade maraging steel (MS) fabricated using the selective laser melting (SLM) technique were studied, in both before- and ...after-aging conditions. Microstructural analysis reveals fine cellular structure in the as-SLM MS. Upon aging, nanoscale precipitation of intermetallic compounds occurs within the cells, which in turn, result in marked improvements in yield and ultimate tensile strengths, substantial reductions in ductility and fracture toughness, and a transition from dimple to quasi-cleavage fracture morphology. Overall, the mechanical performance, including the fatigue crack growth characteristics, of the SLM MS after aging is found to be similar to that of conventionally manufactured MS of the same grade. Importantly, the reduced ductility does not lead to a reduction in toughness, attributed to the stress induced martensitic transformation as a result of austenite presence in aged SLM MS. Although the SLM alloy possesses a mesostructure, which is a result of line-by-line laser scanning and layer-by-layer building of the components, no significant anisotropy in the mechanical behavior is observed, which is a result of strong metallurgical bonding between adjoining lines and layers. These results are discussed in terms of the meso- and micro-structural features.
•Mechanical properties of SLM maraging steel were studied comprehensively.•Properties of SLM and CM steels are similar, except ductility of aged-SLM.•Reduced ductility does not lead to reduction in toughness.•No significant anisotropy in the mechanical properties was observed.
Al-12Si specimens are produced by selective laser melting (SLM) from gas atomized powders. An extremely fine cellular structure is observed with residual free Si along the cellular boundaries. Room ...temperature tensile tests reveal a remarkable mechanical behavior: the samples show yield and tensile strengths of about 260. MPa and 380. MPa, respectively, along with fracture strain of ~3%. The effect of annealing on microstructure and related tensile properties is examined and the results demonstrate that the mechanical behavior of the Al-12Si SLM samples can be tuned within a wide range of strength and ductility through proper annealing treatment.
Among the additive manufacturing processes, selective laser melting has gained wide popularity for manufacturing austenitic stainless steel (316L) due to the inherent advantages offered. The ...synergetic effect of non-equilibrium microstructure and crystallographic texture on mechanical properties has shown that its properties can be tuned by controlling scanning strategies such as interlayer hatch rotation, hatch strategies, etc. Recent studies have explored the mechanical properties and related responses during tensile related deformation. Accordingly, the present article highlights the evolution mechanism for microstructure and crystallographic texture as a function of hatch style. This manufacturing strategy provides an additional tuning parameter to control the morphology of grains and their crystallographic texture. Additionally, the relationship between the hatch style variation and the compressive deformation has been discussed. It was observed that in cases where the initial texture was along compression direction has shown enhanced twin formation propensity, thereby, increased strain hardening response. The deformation asymmetry pertaining to tension and compression has also been discussed.
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•The paper provides a unique manufacturing strategy to control microstructure – crystallographic texture and resulting mechanical property.•Detailed explanation is given for texture formation as fibre or a combination of and fibre along build direction.•Crystallographic texture as a combination of and promotes both multiple slip and twinning and gives enhanced hardening.
An Al-Zn-Mg-Cu alloy with high zinc content was successfully produced by selective laser melting (SLM). A small amount of discontinuous η particles precipitates into the inter-dendritic areas, which ...is less than in the corresponding cast sample. After T6 heat treatment, almost all η particles dissolve in the Al-matrix. The hardness of the as-synthesized SLM material is 133 ± 6 HV0.05, similar to the homogenized cast sample, and it increases to 219 ± 4 HV0.05 after the T6 heat treatment. This value is higher than for the corresponding T6 cast sample, which indicates that SLM processing leads to a material with improved mechanical behavior.
•Fabrication of Al-Zn-Mg-Cu (7XXX series with high Zn content) by selective laser melting.•Al-Zn-Mg-Cu samples produced by SLM and Cast show different microstructures.•Improved hardness for annealed SLM sample compared to the annealed cast samples.
A group of Ti--xNb--7Fe (x=0, 1, 4, 6, 9, 11wt.%) alloys was designed and produced by cold crucible levitation melting process. The microstructural characteristic of the alloys with Nb addition and ...its effect on their mechanical properties as well as wear resistance were investigated. Microscopic and phase analysis results show that all the alloys, except for the Ti--11Nb--7Fe, exhibit orthorhombic alpha " and body-centred cubic beta phases, while Ti--11Nb--7Fe alloy consists of only beta phase. It is proposed that increasing the Nb content enhances beta phase stability and its proportion in the microstructure of the designed alloys. Depending on the proportion of beta and alpha " phases, Ti--xNb--7Fe alloys show varied hardness (3.57-5.92GPa) and compressive strength (1990-2093MPa). Additionally, they present wear rates in the range of 310-15-110-13 m3/m which correlates well with the changes in the corresponding microstructures and mechanical properties. Among the studied alloys, Ti--11Nb--7Fe with beta phase microstructure, presents the lowest elastic modulus (86GPa) and the highest compressive strain (41.5%) along with high compressive strength, hardness and wear resistance. Therefore, it is suggested that this beta -type Ti--11Nb--7Fe alloy is a promising candidate, more suitable than the commercially used CP--Ti and Ti--6Al--4V, for orthopedic applications.
•Effect of laser remelting on mechanical properties under cyclic loading was studied.•Effect of laser remelting on wear volume and friction coefficient was studied.•Effect of laser remelting and heat ...treatment on Charpy impact energy was studied.•Anisotropy in hardness, residual stress, Charpy impact energy and wear was studied.•Reciprocating sliding tests with ball above and below configuration were conducted.
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The reliability and quality of additively manufactured parts are questionable. The present study aimed to improve the surface quality and mechanical and tribological properties of a Ti6Al4V alloy manufactured using selective laser melting (SLM). The effects of the laser remelting on its surface topology, mechanical properties, and sliding wear in a ball-on-plate configuration with a ball either above or below the plate were investigated systematically. The influence of the laser remelting approach on the anisotropy of the mechanical and tribological properties was compared with that of the heat treatment approach. The surface quality and high cycle fatigue strength improved with an increase in the number of melting steps. The compressive and impact strengths also increased with an increase in the number of melting steps. The wear resistance in both configurations was higher in the remelted samples than in the samples after SLM. The results confirmed that laser remelting, as an affordable approach, could significantly improve the reliability of parts fabricated by the SLM process.
The present study investigates the wear properties of commercially pure titanium (CP-Ti) parts produced using selective laser melting (SLM) and casting. Scanning electron microscopy (SEM) ...investigations show that SLM-produced CP-Ti parts have martensitic (α΄) microstructure, whereas cast-produced CP-Ti samples exhibit plate-like (α) microstructure. SEM studies on the wear surfaces at moderate loads (15N) show shallow ploughing grooves at certain regions and some delamination cracks for both SLM and cast CP-Ti samples. On increasing the load to 30N, deeper ploughing grooves were observed in both samples along with delamination of material at certain regions. However, ploughing grooves were found to be very shallow in SLM samples compared with the cast parts. Although both SLM and cast CP-Ti exhibited similar wear mechanisms, SLM CP-Ti showed better wear resistance due to its martensitic microstructure, finer grain size and superior microhardness.
•Wear-properties of commercially pure-Ti prepared by SLM and casting processes.•Both samples show similar wear mechanisms.•SLM-processed sample show superior wear performance than cast specimens.•The superior wear properties are attributed to the martensitic microstructure.
Commercially pure titanium (CP-Ti) and Ti–TiB composite parts with three different porosity levels (i.e. 10%, 17% and 37%) were produced by selective laser melting (SLM). Scanning electron microscopy ...(SEM) investigations show that martensitic (α′) microstructure exists in SLM-processed CP-Ti parts, whilst SLM-processed Ti–TiB composites present needle-shape TiB particles distributed in α-Ti matrix. Mechanical properties of these porous samples decrease with porosity level increasing. The yield strength and elastic modulus of porous CP-Ti parts range 113–350MPa and 13–68GPa respectively, which are much lower than those for porous Ti–TiB counterparts (234–767MPa and 25–84GPa respectively) mainly due to the strengthening effect induced by TiB particles in Ti–TiB samples. Compression stress–strain curves of 37% porous CP-Ti parts show a typical three-stage behavior of ductile porous metals. Also, the elastic moduli of both 37% porous CP-Ti and Ti–TiB samples are similar to that of human bone. SEM investigations of the porous CP-Ti samples after compression testing show that no crack presents until 50% compressive strain and most of deformation is absorbed by porous areas. In contrast, μ-CT investigations indicate that all porous Ti–TiB samples fail at early stages of compression testing due to cracks resulting from insufficient ductility of struts of porous areas, because they are not able to accommodate high strains of the deformation at high strengths.
