Selective Laser Melting Steel Research Articles

Microstructural and electrochemical behaviour of severely surface-deformed 316L steel manufactured by conventional and selective laser melting routes

This study thoroughly examines the influence of conventional and selective laser melting (SLM) routes and surface mechanical attrition treatment (SMAT) on the microstructural and electrochemical properties of 316L steel. Compared to wrought specimens, the SLM specimens exhibit significantly smaller grains (∼41 vs. ∼83 µm) and higher dislocation density (∼7.2 × 1013 vs. ∼3.7 × 1012 m−2). Both specimens show nearly doubled surface hardness after SMAT, with the SLM surface displaying a ∼30 nm grain size and minimal α’ phase. The microstructure significantly influences passivation and corrosion behaviour. The SLM specimens exhibit superior electrochemical characteristics to wrought counterparts in SMATed (0.00299 mmpy) and non-SMATed (0.00771 mmpy) conditions. SMAT effectively eliminates surface porosity, enhancing the passivation and corrosion resistance of SLM steel.

Numerical and Experimental Analysis of Strength Loss of 1.2709 Maraging Steel Produced by Selective Laser Melting (SLM) under Thermo-Mechanical Fatigue Conditions.

The result of the development of additive manufacturing (AM) methods is the increasing use of the selective laser melting (SLM) method as a technique for producing tooling for injection moulds and die casting pressure moulds from maraging steel powders. The mould components are subjected to varying thermo-mechanical loads during these operations. This paper presents a numerical model that is used to predict the fatigue life of a material that is loaded with a time-varying temperature field according to the classic and modified Coffin test. Using a computational model, the temperature changes in the resistance-heated specimen and the stress and strain fields that are caused by this phenomenon were determined. Using three different multiaxial fatigue criteria, the fatigue life of SLM steel was determined. Numerical calculations were verified using experimental thermal fatigue tests on 1.2709 SLM steel that was aged at 490 °C as well as via metallographic tests. The numerical model was used to predict the durability of the same steel aged at 540 °C. The effect of specimen clamping conditions on the fatigue life of SLM steel was determined numerically. The value of the decrease in strength of SLM steel as a result of the increasing number of cycles of temperature changes was determined experimentally; a great influence of ageing temperature on fatigue life was found. Changes in the structure of steel occurring during cyclic changes in temperature are presented.

Tribological, Corrosion, and Mechanical Properties of Selective Laser Melted Steel

In additive manufacturing (AM), selective laser melting (SLM) is a relatively novel technique that utilizes thermal energy via laser beams to melt and solidify metallic powders into three-dimensional components. Compared to traditional manufacturing techniques, SLM is advantageous because it is more time-efficient, cost-effective, and allows for the fabrication of components with superior mechanical, tribological, and corrosion performances. However, much of the existing literature highlights the influence of SLM on softer materials such as aluminum or magnesium due to their thermal expansion coefficients rather than on materials such as steel. This review aims to encapsulate the existing literature on SLM steel and understand the factors that allow for its fabrication and the underlying mechanisms that dictate its mechanical, tribological, and corrosion performance. By understanding the trends of laser energy density (LED), scanning patterns, and building directions for these properties, a comprehensive understanding of SLM steel can be achieved. Additionally, through this understanding, the future directions of this research and suggestions will be provided to continue progressing the field in an impactful direction.

Fatigue fracture morphology of AISI H13 steel obtained by additive manufacturing

The paper focuses on researching the effect of fatigue loading on metallic structure, lifetime, and fracture surface topographies in AISI H13 steel specimens obtained by selective laser melting (SLM). The topography of the fracture surfaces was measured over their entire area, according to the entire total area method, with an optical three-dimensional surface measurement system. The fatigue results of the SLM 3D printed steel specimens were compared with those reported for conventionally manufactured 13H steel. The investigation also considers the roughness of the specimens’ side surface. Moreover, the fractographic evaluation conducted using scanning electron microscopy confirms that the predominant fracture mechanism is transgranular fracture. Microtomography done after mechanical loading also showed the influence of the stress level on the porosity distribution. Both fractographic and Micro-CT investigations confirm that higher stresses result in coarser and much more uniform porosity observed in fractured samples. These comprehensive quantitative and qualitative fracture analyses are beneficial to predict the failure conditions of SLM steel parts, especially in the case of fatigue damage. From the quantitative analysis of the H13 SLM-manufactured fracture surface topography, it was possible to conclude that the larger the loadings acting on the specimen, the rougher the fracture surface because the ductile fracture mode dominates. It has also been proven that the porosity degree changes along the length of the sample for the most stressed specimens.

Adhesion strength of Tii_xCx – DLC multilayer nanocomposite thin films coated by ion-plasma deposition on martensitic stainless steel produced by selective laser melting followed by plasma-nitriding and burnishing

[Ti0.2C0.8/a-C]40 multilayer thin films composed of forty pairs of TiC and pure carbon layers were formed on a selective laser melted (SLM) martensitic stainless steel by means of ion-plasma deposition process. SLM steel was pre-treated by one of the two following schemes: (1) oil quenching from 1040°C followed by heating to 480°C for 4 hours and air cooling (HT), finish milling (FM); (2) HT, FM, ion-plasma nitriding followed by burnishing. Mechanical failure mode and critical load Lc for damaging the coatings were determined using linear scratch tests performed at linearly-increased normal force. Indentation by conical diamond tip were carried out in order to asses an elastic recovery and energy dissipation coefficient defined as the ratio of plastic to total deformation energy. The scratch test results showed that the post-processing of the substrate strongly influenced the failure mode of the coating and increased the critical load from 320 mN to 920 mN. Indentation revealed that nitriding and burnishing before coating deposition increase the elastic recovery of the [Ti0.2C0.8/a-C]40 coating-substrate system from 24% to 68%. The energy dissipation coefficient drops from 79% to 45%.

Microstructure and Tribological Response of Selective Laser Melted AISI 316L Stainless Steel: The Role of Severe Surface Deformation

This study investigates the effect of surface mechanical attrition treatment (SMAT) (a severe surface-deformation process) on microstructure and tribological behavior of AISI 316L steel samples manufactured using the selective laser melting (SLM) technique. The specimens are built in different directions (0°, 45°, and 90°). The microstructure of annealed SLM samples shows the non-uniform distribution and random orientation of grains. It contains high-angle grain boundaries and a high density of dislocations. The average grain size is about 63, 51, and 41 μm for 0°, 45°, and 90° build direction, respectively. SMAT is beneficial for SLM steel to reduce surface roughness (by ~ 87%) and eliminate internal porosity. The deformed layer of SLM steel shows a highly dense network of slip bands, distortion of grains, and hardness gradient (up to the depth of about 600 μm). Increase in surface hardness due to SMAT is maximum (~ 54%) for the sample having 90° build direction. Typical observation of deformation-induced martensite is absent for the SMAT-processed SLM steel. Under the higher load (especially, 20 N), the tribological response of sample manufactured in the 90° direction is superior amongst the non-treated samples. Severe surface deformation enhances the wear resistance and reduces the COF of SLM steel.

A Comparison Study of the Austenite Grain Growth and Its Transformation Behavior during Uniform Continuous Cooling of a Wrought and Selective Laser Melting 4340 Steels

Abstract This study compares austenite grain growth and its continuous cooling transformation (CCT) behavior between selective laser melting (SLM) 4340 and conventional wrought 4340 steels. Standard dilatometry tests were used to determine the austenite decomposition behavior at different cooling rates. The analysis of the data from the controlled cooling paths was used to generate CCT diagrams for the two steels investigated in this study. Advanced microstructural characterization techniques, Scanning Electron Microscopy and Orientation Imaging Microscopy, were employed to support the CCT diagrams. Electron Backscatter Diffraction (EBSD) and a special EBSD-Image Quality characterization technique were used to assess the percentage of the microstructural components observed. Three important observations were made: (1) the hardenability of the wrought 4340 steel was higher than the SLM 4340 steel; (2) the presence of granular bainite was observed after slow cooling conditions in both steels; (3) the SLM steel exhibited finer austenite grain size distribution than the wrought steel in the temperature range studied. MTEX, a MatLab program, was used to reconstruct the prior austenite grains. The SLM steel exhibited finer austenite grain size distribution than the wrought steel in the temperature range studied.

In-situ neutron diffraction study of wrought and selective laser melted maraging stainless steels

Bulk phase transformations and dislocation density were monitored in a maraging stainless steel on a bulk level using in-situ neutron diffraction up to 1340 °C, i.e. 30 °C below the melting temperature. Three materials with different initial microstructure and/or different composition were studied: wrought, as-built SLM-ed (selective laser melted) and re-austenitized SLM-ed. In contrast to the wrought martensitic steel, the as-built SLM-ed steel was essentially ferritic. However, re-austenitized SLM-ed steel recovered the usual martensitic microstructure. A δ-ferritic domain above 1200 °C was confirmed for all the materials studied. However, the amount of δ-ferrite at the melting point strongly depends upon steel composition. Additionally, it was shown that the initial microstructure of the steel (ferritic or martensitic) has little to no influence on re-austenization. Dislocation densities were estimated from diffraction peak broadening. The ferritic as-built SLM material contains a high dislocation density (~4 × 1014 m−2), which is however far less than in the martensitic materials (~5 × 1015 m−2). Dislocations start to annihilate from 550 °C / 600 °C in all the materials studied, but a measurable dislocation density of ~1013 m−2 is still observed at 950 °C / 1000 °C.

Tribo-mechanical properties and adhesion behavior of DLC coatings sputtered onto 36NiCrMo16 produced by selective laser melting

The combination of selective laser melted nickel-chromium-molybdenum alloyed steel and diamond-like carbon (DLC) coatings is an aspiring approach to produce light-weight components with improved tribological properties. In order to assure the tribo-mechanical properties of the DLC coatings, good adhesion on the additively manufactured substrates under high loads is essential. 36NiCrMo16, produced by Selective Laser Melting (SLM) and wrought material techniques, served as substrate material for hydrogen-free a-C and hydrogenated a-C:H coatings with a chromium carbide interlayer. The structural and mechanical properties of 36NiCrMo16 were examined in order to consider their effects on the tribo-mechanical properties and adhesion of the DLC coatings deposited by magnetron sputtering. In x-ray diffraction analyses, retained austenite was identified for the 36NiCrMo16 substrate processed by SLM. This increased austenite content in the SLM built steel is attributed to the pre-heating temperature of 200 °C during fabrication. Additionally, a relative density of 99.98% was detected for the SLM manufactured tempering steel by using an x-ray microscope. In contrast to the conventional 36NiCrMo16, the SLM substrates exhibited a higher surface roughness, which is ascribed to the different phase composition and microstructure. In nanoindentation tests, the a-C and a-C:H coatings deposited onto 36NiCrMo16 exhibited a hardness of ~22 and ~19–20 GPa, respectively. In general, the DLC coatings revealed a good adhesion to the conventional and SLM 36NiCrMo16 in Rockwell C indentation tests. Local, limited spalling failures were identified for a-C and a-C:H on the SLM substrate, while no delamination was observed for the coatings on conventional steel. The differences in microstructure, such as the retained austenite and higher surface roughness, do appear to affect the adhesion of the DLC coatings. In tribometer tests against 100Cr6 counterparts, the DLC coatings significantly reduced the friction and increased the wear resistance of 36NiCrMo16. Therefore, the DLC coatings are promising to enhance the tribo-mechanical properties of SLM 36NiCrMo16 substrates, but it is crucial to consider the surface integrity of SLM steel on the adhesion behavior of DLC coatings.

Correlation between heat treatment process parameters, phase composition, texture, and mechanical properties of 12H18N10T stainless steel processed by selective laser melting

The phase composition, texturing degree, oriented stresses and strains of the crystal lattice, the physical and mechanical properties of austenitic steel 12H18N10T (AISI 321) manufactured by selective laser melting (SLM) before and after heat treatment with various temperatures and protective gas composition have been investigated. SLM stainless steel has better mechanical properties, given that the phase volume content is as follows: γ-Fe ∼ 65%, α-Fe ∼20%, NiTi ∼ 5%; the texturing degree equals to T111∼0.70 and the dominant crystal-lattice orientation is (111). Such a phase composition is formed in the steel after annealing at 620 °C in argon. As the temperature rises during the stabilizing annealing process up to 825 °C, the phase transformation γ→α takes place in SLM steel with a change in the phase concentration γ-Fe and α-Fe in the range from 40 to 50%. In the high-temperature intermetallic phase NiTi, dissolution at 825 °C and a multiple rise of the FeO concentration lead to a change in the crystal lattice parameter of the γ-Fe and α-Fe phases and a decrease in the mechanical properties of SLM steel. This study showed that the improvement of mechanical properties was due to the separation of high-temperature intermetallic phase from austenite and an increase in the concentration of iron carbide Fe3C in SLM steel.

Particles control in selective laser melting in-situ oxide dispersion strengthened method

Stainless steel selective laser melting (SLM) can be considered as a new possible approach for in-situ formation of oxide dispersion strengthened (ODS) steels because of the dispersion of amorphous oxide nano-particles due to the trace amounts of laser chamber oxygen and in-situ internal oxidation of reactive elements. In this paper it is demonstrated that the particle sizes and distributions can be adjusted by choosing different chamber oxygen level through controlling the quantity of initial reactive cores and the available reactive solutes of each core.

Homogenization of selective laser melting cellular material for impact performance simulation

Purpose – The purpose of this paper is to present, the global behaviour of sandwich structures comprising cellular cores is predicted by finite element (FE) analysis. Two modelling approaches are investigated, providing different levels of accuracy; in both approaches, the sandwich structure is idealised as a layered stack with the skin modelled using shell elements; while the core is either modelled with fine detail using beam micro-elements representing the cell struts, or is modelled by three-dimensional solid elements after an appropriate core homogenisation. Design/methodology/approach – The applied homogenisation methodology, as well as the all important modelling issues are presented in detail. Experimental tests performed using a mass-drop testing machine are used for the successful validation of the simulation models. Findings – It was concluded that the core microscale models having detailed FE modelling of the core unit cells geometry with fine scale beam elements are suitable for the analysis of the core failure modes and the prediction of the basic core stiffness and strength properties. It was demonstrated that the homogenised core model provides significant advantages with respect to computing time and cost, although they require additional calculations in order to define the homogenised stress-strain curves. Research limitations/implications – Special microscale material tests are required for the determination of appropriate materials parameters of the core models, as steel selective laser melting (SLM) microstrut properties differ from the constitutive steel material ones, due to the core manufacturing SLM technique. Stress interactions were not taken into account in the homogenisation, as the applied core material model supports the introduction of independent stress-strain curves; however, the predicted load deflection results appeared to be very close to those obtained from the detailed core micromodels. Originality/value – The paper is original. The dynamic behaviour of conventional sandwich structures comprising conventional honeycomb type cores has been extensively studied, using simple mass-spring models, energy based models, as well as FE models. However, the response of sandwich panels with innovative SLM cellular cores has been limited. In the present paper, novel modelling approaches for the simulation of the structural response of sandwich panels having innovative open lattice cellular cores produced by SLM are investigated.

Constitutive and failure behaviour in selective laser melted stainless steel for microlattice structures

The emerging selective laser melting (SLM) technology makes possible the manufacturing of metallic microlattice structures with better tailorability of properties. This work investigated the constitutive formulation of the parent material and the failure mechanism in the SLM stainless steel microlattice structure. The constitutive behaviour of SLM stainless steel was quantitatively formulated using the Johnson–Cook hardening model. A finite element model incorporating the constitutive formula was developed and experimentally validated to predict the localised stress evolution in an SLM stainless steel microlattice structure subjected to uniaxial compression. The predicted stresses were then linked to the fracture process in the SLM steel observed by scanning electron microscope. It was found that the tensile and compressive stress state is localised in the strut members of the microlattice, and determines the macroscopic cracking mode. The tensile opening and shear cracking dominate the tension and compression zones, respectively. However, the microscopic examination on the fracture surfaces reveals the formation of substantial slip bands in both the tension and compression zones, implying that the ductile fracture in the SLM stainless steel is transgranular.

Sliding Wear Characteristics and Corrosion Behaviour of Selective Laser Melted 316L Stainless Steel

Stainless steel is one of the most popular materials used for selective laser melting (SLM) processing to produce nearly fully dense components from 3D CAD models. The tribological and corrosion properties of stainless steel components are important in many engineering applications. In this work, the wear behaviour of SLM 316L stainless steel was investigated under dry sliding conditions, and the corrosion properties were measured electrochemically in a chloride containing solution. The results show that as compared to the standard bulk 316L steel, the SLM 316L steel exhibits deteriorated dry sliding wear resistance. The wear rate of SLM steel is dependent on the vol.% porosity in the steel and by obtaining full density it is possible achieve wear resistance similar to that of the standard bulk 316L steel. In the tested chloride containing solution, the general corrosion behaviour of the SLM steel is similar to that of the standard bulk 316L steel, but the SLM steel suffers from a reduced breakdown potential and is more susceptible to pitting corrosion. Efforts have been made to correlate the obtained results with porosity in the SLM steel.

Laser-arc hybrid welding of wrought to selective laser molten stainless steel

Selective laser melting (SLM) is a successful tool-free powder additive technology. The success of this manufacturing process results from the possibility to create complex shape parts, with intrinsic engineered features and good mechanical properties. Joining SLM steel to similar or dissimilar steel can overcome some limitations of the product design like small dimension, undercut profile, and residual stress concentration. In this way, the range of applications of the SLM process can be broadened. In this paper, the hybrid laser welding of selective laser molten stainless steel was investigated. A high-power fiber laser was coupled to an electric arc and austenitic stainless steel wrought and SLM parts were welded together. The power and speed parameters were investigated. The joints were analyzed in terms of weld bead profile, microstructure, microhardness, and tensile test. The efficiency of the welding process was evaluated through the line energy input versus the weld molten area.