Selective Laser Melting Alloy Research Articles

The effect of strain rate on stress corrosion performance of Ti6Al4V alloy produced by additive manufacturing process

Ti6Al4V alloy is considered as a favourable material for additive manufacturing (AM) and in particular for selective laser melting (SLM), due to the inherent difficulties relating to casting and plastic forming of this alloy. However, the significantly increased solidification rate associated with the SLM process results in a modified micro-structure and inherent casting imperfections that can have a detrimental effect on the alloy stress corrosion performance. The main objective of this study was to evaluate the effect of strain rate on stress corrosion behavior of Ti6Al4V alloy produced by SLM, as compared to its wrought alloy (Grade 5) counterpart of the same chemical composition. Micro-structure and phase identification were examined by scanning electron microscopy and X-ray diffraction analysis. General corrosion resistance was evaluated in terms of open circuit potential, potentiodynamic polarization analysis and by impedance spectroscopy, while stress corrosion behavior was examined at various strain rates in a 3.5% NaCl solution at ambient temperature. The results obtained revealed that general corrosion resistance and stress corrosion endurance at slow strain rate, in terms of time to failure and ductility of the SLM alloy, were relatively reduced, as compared to the wrought alloy counterpart. This was mainly due to the formation of a strained martensitic matrix in the form of α' phase and the significant reduction of the β phase in the alloy produced by SLM process.

Enhanced strength of the selective laser melted Al-Mg-Sc-Zr alloy by cold rolling

To further refine the coarse columnar grains (CGs) in selective laser melted (SLMed) alloy, an Al-4.0Mg-0.7Sc-0.4Zr-0.5Mn alloy was fabricated by selective laser melting (SLM) and then subjected to cold rolling and annealing at 300 °C for 1 h. Microstructural details and mechanical properties were examined by SEM, EBSD, TEM, and tensile tests. The results show that the microstructure of the SLMed alloy can be further refined by cold rolling. A yield strength of 573 MPa is obtained with an average grain size of 2.4 ± 1.3 μm in the CGs areas, a reduction of approximately 30% compared with the as-fabricated alloy. The enhanced strength of the rolled alloy is derived from the refined grains and high density of Al3(Sc, Zr) phase. Additionally, the Al3(Sc, Zr) particles can enhance the microstructure stability.

Investigation of Laser Polishing of Four Selective Laser Melting Alloy Samples

Selective laser melting (SLM) is a layer by layer process of melting and solidifying of metal powders. The surface quality of the previous layer directly affects the uniformity of the next layer. If the surface roughness value of the previous layer is large, there is the possibility of not being able to complete the layering process such that the entire process has to be abandoned. At least, it may result in long term durability problem and the inhomogeneity, may even make the processed structure not be able to be predicted. In the present study, the ability of a fiber laser to in-situ polish the rough surfaces of four typical additive-manufactured alloys, namely, Ti6Al4V, AlSi10Mg, 316L and IN718 was demonstrated. The results revealed that the surface roughness of the as-received alloys could be reduced to about 3 μm through the application of the laser-polishing process, and the initial surfaces had roughness values of 8.80–16.64 μm. Meanwhile, for a given energy density, a higher laser power produced a laser-polishing effect that was often more obvious, with the surface roughness decreasing with an increase in the laser power. Further, the polishing strategy will be optimized by simulation in our following study.

Analysis of Structure and Mechanical Properties of Co–Cr–Mo Alloy Obtained by 3D Printing

Abstract—A comparative study was performed of microstructure and strength characteristics of the Co–Cr–Mo alloy obtained by the additive manufacturing (AM) method using selective laser melting (SLM) in the EOSINT M280 3D printer and by arc melting in a helium atmosphere. The occurrence of a martensitic transformation in SLM samples was detected. It was found by nanoindentation that the microhardness HIT of the SLM alloy is higher than that of the cast alloy, which is likely due to the high level of surface residual stresses in the SLM sample. It was found that the SLM alloy has a two-phase (fcc + hcp) structure after standard annealing, in contrast to the cast alloy, which retains a single-phase fcc structure after such annealing.

Structure Formation and Properties of Eutectic Silumin Obtained Using Selective Laser Melting

An Al–Si eutectic alloy was synthesized by the selective laser melting (SLM) method using a pilot wrought Al-Si powder (of Russian AKD-12 grade) as the starting material. On the basis of electron microscopy, the microstructure of the SLM alloy was certified with EDS mapping of elements (Al and Si). It has been established that the main structural component is an Al-based solid solution with a cellular-dendritic structure (with an average cell size of ~500 nm). The Si phase, which is a component of the eutectic, is located at the boundaries in the form of rounded crystals about ten nanometers in size. This paper presents the results of measuring the mechanical and nanomechanical properties, which show the competitiveness of the use of the pilot (experimental) powder.

Effect of Heat Treatment on Microstructure and Properties of FGH4096M Superalloy Processed by Selective Laser Melting

A self-designed nickel-based superalloy, designated as FGH4096M, was prepared by selective laser melting (SLM). Different heat treatments were performed to improve the mechanical properties of the SLM alloy through optimizing the microstructures and the features of γ′ precipitates. Compared with the as-deposited alloy, the columnar grains with dendritic structures and equiaxed structures were retained in the alloy after direct aging, but a large amount of tertiary γ′ phase precipitated, especially around the sub-grain boundaries, resulting in the highest tensile strength but the lowest elongation. During solid solution and aging treatment (SSA), the recovery and recrystallization occurring in the alloy facilitated the grains to be equiaxed with the increase of solution temperature. For lower solution temperature (below 1100 ℃), the secondary γ′ precipitates decreased with the increase of solution temperature, while the tertiary γ′ precipitates from the subsequent aging process gradually increased; for higher solution temperature over 1100 ℃, exceeding the complete dissolution temperature of the γ′ phase, only tertiary γ′ precipitates from the subsequent aging process were uniformly distributed in the alloy. After double solid solution (1170 ℃ + 1050 ℃) + aging heat treatment (DSSA), there were three sizes of γ′ precipitates in the alloy. In general, the SLM + SSA (1130 ℃) alloy obtained the best comprehensive properties, which could be related to the homogenized microstructures and the uniform and dense distribution of single sized tertiary γ′ precipitates in the alloy.

Effect of post-treatments on microstructure and mechanical properties of a novel nickel-based powder metallurgy superalloy processed by selective laser melting

A newly developed third generation nickel-based powder metallurgy (PM) superalloy, designated as FGH100L, was produced through Selective Laser Melting (SLM). Post-treatments, including Heat Treatment (HT), Hot Isostatic Pressing (HIP) and combined HIP + HT treatment, were carried out on the as-SLMed alloy to improve its mechanical properties by controlling the microstructure and/or reducing the defects in the alloy. Both HT and HIP treatments facilitated the recovery and recrystallization of the superalloy, especially when the solution temperature reached 1150 °C, thereby homogenizing the microstructure and refining the grains. During the post-treatment, the number density of γ′ precipitates increased obviously with the process varying from SLM, SLM + HIP, SLM + HT to SLM + HIP + HT, and both yield strength and ultimate strength increased accordingly, indicating that the precipitation strengthening played a major role in improving the alloy strength. During the HIP process, the size and quantity of the pores were greatly reduced, which contributed greatly to a much higher elongation of the SLM + HIP alloy than that of the SLM and SLM + HT alloys. The fracture surfaces of the SLM alloys after different post-treatments showed transgranular fracture features, while the intergranular fracture was the main fracture mode of the as-SLM alloy.

Prediction of Residual Stress Random Fields for Selective Laser Melted A357 Aluminum Alloy Subjected to Laser Shock Peening

AbstractResidual stress (RS) is a major processing issue for selective laser melting (SLM) of metal alloys. Postprocessing by way of heat treatment or hot isostatic pressing is usually required for acceptable mechanical properties. In this work, laser shock peening (LSP) treatment on both SLM and cast aluminum A357 alloys are compared with regard to the development of beneficial near-surface compressive RS. Experiments are conducted using high energy nanosecond pulsed laser, together with a fast photodetector connected to a high-resolution oscilloscope and high-speed camera to identify detailed temporal and spatial laser pulse profiles to improve numerical predictions. Constitutive modeling for SLM A357 alloy is performed using finite element simulation and data obtained from X-ray diffraction (XRD) measurements. Since XRD-RS measurements are accompanied with significant machine-reported error, an effective method is introduced to quantify the material constitutive model uncertainty in terms of a joint probability mass function. Conventionally, most constitutive behavior research for LSP involves deterministic material modeling. Predicted RS using deterministic approaches fail to reflect real-world variations in the materials, laser treatment, or RS measurements. A discretized Bayesian inference is used to quantify the rate-dependent plasticity material model parameters as a joint probability function. RS are then characterized as random fields, which provides far greater insight into the practical ability to attain desired residual stresses. Moreover, for identical LSP treatments, it is determined that the material models are significantly different for the SLM and the conventional cast A357 aluminum alloys, resulting in much lower magnitude of compressive RS in the SLM alloy.

Investigation of Localized Corrosion Phenomena in Selective Laser Melted 316L Stainless Steel

Selective laser melting (SLM) refers to the additive manufacturing of bulk material from powder on a layer by layer basis through melting via laser and rapid solidification. In recent years, additively manufactured metallic alloys produced by SLM have garnered attention due to opportunities in waste reduction, time and cost efficiency, and fabrication of geometrically complex structures. However, limited work has been performed to investigate the corrosion properties of these alloys. Understanding these mechanisms is crucial in assessing the attendant risks associated with the use of AM alloys in engineering design. Stainless steels such as 316L produced by SLM have presented advantages due to the corrosion resistant properties of these alloys. However, the non-equilibrium microstructure in these SLM materials, significantly different from that of the wrought counterpart, result in heterogeneities that may lead to localized corrosion such as melt pool boundary attack. A point of concern in stainless steels is intergranular corrosion due to the formation of secondary phases such as chromium carbides that deplete chromium from grain boundaries when the material is exposed to temperature ranges of 450 – 800 °C. While low-carbon steels such as 316L have great resistance to deleterious chromium carbide precipitation, this phenomenon has not been investigated in great detail in the case of selective laser melted 316L. However, it has been shown that precipitation of secondary phases is accelerated in SLM alloy systems such as Inconel 625 with respect to the wrought counterpart. Intergranular corrosion can ultimately lead to intergranular stress corrosion cracking, a form of brittle failure. This study investigates localized corrosion phenomena such as melt-pool boundary attack and intergranular corrosion and the associated microstructure-based mechanisms in the corrosion of these SLM 316L alloys. Galvanostatic etching, potentiodynamic scans through the double-loop potentiokinetic reactivation method, and mass-loss testing of wrought and SLM 316L are utilized to reveal the electrochemical behavior of these alloys. Additionally, focused ion beam and transmission electron microscopy are utilized to understand the microstructural origins of the observed corrosion phenomena.

Mechanical and thermal characterisation of AlSi10Mg SLM block support structures

Selective Laser Melting (SLM) is an Additive Manufacturing (AM) process based on the local fusion of powdered metal. SLM requires support structures that enable heat transfer and structural support during fabrication while being frangible to enable removal. Despite the criticality of support structures, very little quantitative data exists to characterise their behaviour. AlSi10Mg is an important SLM alloy which is particularly appropriate for laser applications due to its ease of processing. Block support structures are the most versatile of the commercially implemented support structure designs. This research presents an experimental study of the mechanical strength and numerical analysis of the thermal behaviour during SLM fabrication of ALSi10Mg block support structures. It was found that: support structure peel strength is significantly lower than normal strength due to fundamentally different failure mechanisms; support strength is influenced by height; and, reduced support spacing increases heat transfer and load bearing capability. Design equations were generated to enable prediction of the strength of a given support structure. This previously unavailable data allows AM designers to unambiguously specify support structures that are optimal for a specific design scenario. Furthermore, these insights potentially allow the redesign of block support structures such that response is independent of geometry.

Microstructural evolution and defect formation in a powder metallurgy nickel-based superalloy processed by selective laser melting

In this study, the selective laser melting (SLM) technology has been employed to manufacture a nickel-based superalloy which was conventionally prepared through powder metallurgy (PM) route. The microstructural features and defects were systematically investigated both prior to and after heat treatment and compared with the PM counterpart. Both solidification cracking and liquation cracking were observed in the SLM specimen in which the grain misorientation and low melting point (γ + γ’) eutectic played a vital role in their formation mechanism. Columnar grains oriented along building direction were ubiquitous, corresponding to strong <001> fiber texture. Solidification cell structures and melt pools are pervasive and no γ’ precipitates were detected at about 10 nm scale before heat treatment. After super-solvus solution and two-step aging treatments, high volume fraction γ’ precipitates emerged and their sizes and morphologies were comparable to those in PM alloy. <001> texture is relieved and columnar grains tend to become more equiaxed due to static recrystallization process and grain boundary migration events. Significant annealing twins formed in SLM alloy and are clarified as a consequence of recrystallization. Our results provide fundamental understandings for the SLM PM nickel-based superalloy both before and after heat treatment and demonstrate the potential to fabricate this group of alloys using SLM technology.

Failure of ceramic coatings on cast and selective laser melted Co-Cr dental alloys under tensile test: Experiment and finite element analysis

Present paper deals with investigation the failure of ceramic coatings on cast and selective laser melted (SLM) Co-Cr dental alloys by tensile test and finite element analysis (FEA). It is established that the failure mode of porcelain coatings is mixed/cohesive in SLM alloy and mixed/adhesive in cast. In both alloys, maximum shear and normal stresses are concentrated at coating free edge on the porcelain-metal interface. Stresses distribution indicates the possible sites of coating fracture initiation and the way it will start - adhesively in the cast alloy due to lower bond strength and cohesively for SLM alloy. Coating failure is cohesive along the sample's length, due to change of maximum stresses location from the coating free edge through its thickness to the surface. The FEA gave an adequate explanation of the failure mechanism of porcelain coatings on cast and SLM Co-Cr dental alloys upon tensile test.

Assessment of mechanical properties and fatigue performance of a selective laser melted nickel-base superalloy Inconel 718

In the present work a nickel-base superalloy Inconel 718 manufactured by selective laser melting (SLM) was investigated with focus on microstructure, orientation-dependent mechanical property and fatigue performance. Comparative material testing and characterization of the SLM and the forged Inconel 718 revealed significant differences in both microstructures and mechanical properties. The columnar grain structure of SLM alloy leads to the orientation-dependent mechanical properties, which matches the Hall-Petch relation. The inhomogeneous microstructures and slit shaped lack-of-fusion (LoF) defects from the SLM process result in worse fatigue performance and deteriorate the fatigue crack growth behavior.

Laser welding of a selective laser melted Ni-base superalloy: Microstructure and high temperature mechanical property

Inconel 625 specimens manufactured by selective laser melting (SLM) were welded using a laser welding process. The tensile strength in the welding zone was higher than that of the base material at 815 °C. Fractures were observed in the base alloy, but not in the welding zone. The welding zone was mainly composed of columnar grains that exhibited greater resistance to tensile fractures than the as-received SLM alloy with equiaxed grains. The failure was characterized by brittle fractures in the base material, whereas the welding zone featured enhanced plastic deformation ability. The present work established laser welding as a highly efficient method for joining SLM Inconel 625 and for achieving outstanding high temperature performances.

Refining the Microstructure of Fe‐Nd‐B by Selective Laser Melting

Lab scale additive manufacturing of Fe‐Nd‐B based permanent magnet material (Fe75‐Nd18‐B7) has been performed. For fabrication a special inert gas process chamber for laser powder bed fusion has been used. This is required because Fe‐Nd‐B powders are extremely sensitive to oxidation. Selective laser melting (SLM) of the powders allows to realize very fine microstructures with a directed crystal growth and finely dispersed Nd‐rich phase. It is shown, that this fine and textured microstructure leads to large coercivity in sintered Fe‐Nd‐B magnets. A prototype has been realized by milling the SLM alloy and sintering.

Formation mechanism and mechanical properties of the selective laser melting Ni/Co base alloy

This work presents a comprehensive study of the microstructure, formation mechanism and mechanical properties of the selective laser melting 18Ni300 alloy. The test results indicated that the density of this alloy was highest (99.34%) when laser power was 450 W; the elastic deformation occurred during the initial period of the loading, which accorded to Hooke Law; large quantity of the columnar grains can be produced near the fusion line, their growth directions were perpendicular to the fusion line; also, lots of the isotropics equiaxed grains were observed; with increasing of the loading, the plastic/necking deformations occurred; lots of the crystals' boundaries increased greatly the energy for the crack propagation, resulting in the enhancements of the strength/toughness of this selective laser melting alloy. This research may provide the essential theoretical and experimental basis to improve the quality of the laser additive manufacturing composites.

Enhanced biocompatibility of a Ni–Cr alloy prepared by selective laser melting: a preliminary in vitro study

Against a general consensus regarding the high toxicity and low biocompatibility of cast nickel–chromium (Ni–Cr) alloys, the in vitro biocompatibility of selective laser melting (SLM)-fabricated Ni–Cr alloys was investigated and compared with that of cast alloy. Using a single Ni–Cr alloy powder, two SLM alloys with high (SLM_a) and low (SML_b) porosity were prepared by adjusting the laser process parameters in addition to a cast alloy. The alloys were studied in terms of their microstructure, metal ion release, and cell response. All three alloys were composed of only the γ (face-centered cubic) phase. However, both SLM alloys showed more homogeneous dispersion of Ni, Cr, and molybdenum elements and finer grain formation than the cast alloy. Immersion test results indicated that both SLM alloys (in particular, SML_b) exhibited a significantly lower Ni ion release than the cast alloy (P<0.001). A water-soluble tetrazolium salt-8 assay also showed that the viability of L929 mouse fibroblasts was significantly higher for both SLM groups (in particular, SML_b) than for the cast alloy group (P<0.001). Thus, the combined results of this in vitro study suggest that the SLM-processed Ni–Cr alloys exhibited greatly enhanced biocompatibility in comparison with the cast alloy.

Characterization of Ni-Based Superalloy Built by Selective Laser Melting and Electron Beam Melting

Alloy718 specimens built by selective laser melting (SLM) and electron beam melting (EBM) were comparatively examined. The higher cooling rate resulted in a high dislocation density and interdendritic precipitates, which brought about inferior creep properties in SLM alloys. However, a low dislocation density, distributed δ precipitates and a strong 〈001〉 texture were observed in the EBM material. As a result, the creep life of the direct-aged EBM specimen approached 1100 hours and was comparable to that of conventional cast-and-wrought alloys at 650 °C.

The origins for tensile properties of selective laser melted aluminium alloy A357

In this work the tensile behaviour of selective laser melted (SLMed) aluminium alloy A357 in the as-fabricated and heat-treated states is explained using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and transmission electron backscatter diffraction (t-EBSD). The as-built sample has an ultrafine microstructure, with high residual stresses and non-equilibrium solid solute concentration of Si in the supersaturated Al matrix. Consequently, the tensile properties of the SLMed Al alloy A357 are comparable or better than traditional cast counterparts. The Al grains in the SLMed alloy consist of sub-micron sized Al cells, and both high angle and low angle boundaries are initially occupied by eutectic nano-sized Si particles, which are beneficial for strength but detrimental for ductility. With subsequent solution heat treatment, the Si particles on the low angle cell boundaries (LACBs) dissolve while those at the high angle grain boundaries (HAGBs) coarsen. Simultaneously internal stresses decrease, as does solute content in the matrix. The evolution of these microstructural features explains the improved tensile ductility (at its maximum >23%) and reduced tensile strength for the heat treated SLMed aluminium alloy A357 samples.

Improvement in the oxidation resistance of Nb-Si based alloy by selective laser melting

Nb-18Si-24Ti-2Cr-2Al-2Hf (at.%) alloy was fabricated by selective laser melting (SLM). Results showed that an ultra-fined microstructure was achieved by SLM. After oxidation at 1250°C for 50h, the weight gains of Nb-Si based alloys via SLM and vacuum induction melting (VIM) were 96.48mg/cm2 and 198.91mg/cm2, respectively. Compared with the VIM alloy, the SLM alloy exhibited a slower oxidation kinetic rate, featured by the formation of a more compact oxide scale without spallation. The improvement in the oxidation resistance was attributed to the microstructure refinement.