Microstructure Of Selective Laser Melting Research Articles

The Effects of Heat Treatment on Microstructure and Mechanical Properties of Selective Laser Melting 6061 Aluminum Alloy.

Selective laser melting technology can be used for forming curved panels of 6061 aluminum alloy thermal shield devices for the International Thermonuclear Experimental Reactor (ITER), in order to make the formed parts with better performance. This study proposes different heat treatment processes, including annealed treatment at 300 °C for 2 h, solution treatment at 535 °C and then aging at 175 °C over 2 h, to control the mechanical behavior of the 6061 aluminum alloy samples prepared by selective laser melting (SLM). The mechanical properties such as ductility, tensile strength, and hardness of SLM 6061 aluminum alloy were investigated, and the microstructure of the samples was analyzed. The eutectic silicon skeleton shape disappeared after annealing treatment at 300 °C for 2 h. The tensile strength decreased by 22.86% (from 315 MPa to 243 MPa of the deposited state samples), and the elongation increased from 2.01% to 6.89%. Moreover, the hardness reduced from 120.07 HV0.2 to 89.6 HV0.2. After solution aging, the unique microstructure of SLM disappeared. Furthermore, the precipitation of massive Si particles on the α-Al matrix increased, and a trace amount of the Mg2Si(β) phase was generated. Compared with the deposited samples, the tensile strength decreased by 12.06%, while the hardness of specimens was 118.8 HV0.2. However, the elongation showed a remarkable increase of 297% (from 2.01% to 7.97%). Therefore, solution aging can critically improve the plasticity without losing significant tensile stress in the SLM 6061 aluminum alloy. This study proposes the use of SLM 6061 aluminum alloy for the thermal shields on the ITER and provides a reference for choosing a reasonable heat-treatment method for the optimal performance of the SLM 6061 aluminum alloy.

Microstructure and machinability evaluation in micro milling of selective laser melted Inconel 718 alloy

Selective laser melting (SLM) of Inconel 718 (IN718) alloy is common these days. The microstructure and surface integrity of fabricated components are highly depend on SLM parameters and post-processing processes. Micro milling is a promising solution to achieve good surface finish. However, the machinability of SLM fabricated IN718 is different than the wrought IN718 due to their significant differences in microstructure and properties, thus technical parameters applicable for wrought IN718 are unsuitable to SLM fabricated IN718. Therefore, the microstructure and machinability of SLM fabricated IN718 during micro milling processes were evaluated in this work, and comparisons with wrought IN718 were made. Micro milling experiments with varied spindle speed and feed rate were performed and comparisons between tool wear, surface roughness, residual stress and micro hardness were made. Further, the microstructure of SLM fabricated and wrought IN718 before and after machining were compared in terms of grain geometry and crystallographic texture. SLM fabricated IN718 shows slight grain coarsening and texture weaken behaviors, which is different with the obvious grain coarsening and slight texture strengthen phenomena of wrought IN718. The SLM fabricated component has lower compressive residual stress as compared with wrought alloy, and the tool wear, surface roughness and micro hardness values are higher while micro milling wrought IN718 alloy. The reasons leading to these differences were analyzed from the perspectives of technological characteristics and material properties. Observed phenomena and clarified mechanisms can be used as basis to determine suitable technological parameters of micro milling SLM fabricated IN718 alloy.

On enhancing surface wear resistance via rotating grains during selective laser melting

Abstract Selective laser melting (SLM) enables fast and reliable fabrication of metallic parts. Yet post-processing techniques are still required to achieve desirable surface properties, such as wear resistance, surface quality, before the components are ready for deployment in the field. Given the complexities associated with post-processing, alternative approaches are needed. To this end, tailoring process parameters during SLM could be an efficient way to increase wear resistance. However, the relationship between process parameters, microstructure, and wear mechanism must be clarified first. In this study, three scanning strategies are applied to tailor the microstructure of SLM fabricated 316 L stainless steel samples. They are: without remelting (zigzag), remelting with a 0° (zigzag-R0), and 90° (zigzag-R90) rotation and different hatch spacings. The effects of grain orientation on wear resistance are studied by performing scratch tests. It is shown that remelting with small hatch spacings results in the transition of crystallographic texture from the single fiber texture to cubic textures, causing the remelted samples to have high wear resistance through aligning the grains orientations to resist slip. The intensity of textures tends to increase with decreasing the hatch spacings. The maximum enhancement of wear resistance is found in the zigzag-R90 samples using a hatch spacing of 10 μm, leading to 90.8 % and 96.7 % reductions in scratch depth and wear rate, respectively. Such a method can be used in increasing surface wear resistance of SLM fabricated components.

Research on the limitations of laser energy density and microstructure characteristics of selective laser melted AlSi10Mg alloy

In this study, deficiencies in using the laser energy density (LED) to combine different process parameters to represent the heat input in selective laser melting (SLM) process are proposed. The change in melt pool shape could not always be clarified by the LED input, since the vary of LED input caused by altering hatch spacing cannot change the melt pool shape. In addition, the microstructure of SLM fabricated AlSi10Mg material was observed, and this paper gives a further elucidation on the formation of the hierarchical microstructures that discriminated by the morphology of Si phase. Microhardness is enhanced by the supersaturated α-Al matrix, and the hardness value of coarse zones and fine zones show different performance.

Study on the Measurement of Stress in the Surface of Selective Laser Melting Forming Parts Based on the Critical Refraction Longitudinal Wave

Measurement and control of stress in the metal forming layer is the basic problem of selective laser melting (SLM) forming parts. The critical refraction longitudinal (LCR) wave method to test stress in metallic materials has been extensively studied. However, when testing of stress in selective laser melting (SLM) forming parts using this method, some deep-seated regularities of this technology are still not clear. In order to reveal the mechanism of the LCR wave method to measure stress in SLM forming parts, specimens made of 316 L stainless steel were manufactured using meander, stripe, and chessboard scanning strategies. Static load tensile test were applied to SLM forming specimens, with the purpose to demonstrate the scanning strategy has important effect on the LCR wave method to test stress in SLM forming parts. The regularity of the LCR wave velocity on stress is obtained in this study. The anisotropic microstructure of SLM forming parts has an unneglectable effect on the LCR wave stress test. The essential principle of anisotropic microstructure effecting the LCR wave velocity in SLM forming parts were revealed in the experiments. The results of the experiment provide a basis for non-destructive and reliable test of stress in SLM forming parts and other inhomogeneous materials.

Effect of Ultrasonic Nanocrystal Surface Modification on the Microstructure and Martensitic Transformation of Selective Laser Melted Nitinol.

Nitinol has significant potential for biomedical and actuating-sensing devices, thanks to its functional properties. The use of selective laser melting (SLM) with Nitinol powder can promote novel applications aimed to produce 3D complex parts with integrated functional performances. As the final step of the production route, finishing processing needs to be investigated both for the optimization of the surface morphology and the limit alteration of the Nitinol functional properties. In this work, the effect of an advanced method of surface modification, ultrasonic nanocrystal surface modification (UNSM), on the martensitic transformation and microstructure of SLM built Ni50.8Ti49.2 (at.%) was investigated. Scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry indicated that the UNSM process can generate stress-induced martensite, at least partially suppressing the martensitic transformation. The microhardness profile indicates that the UNSM process can affect the mechanical properties of the SLMed Nitinol sample in a range of up to approximately 750 μm in depth from the upper surface, while electron backscatter diffraction analysis highlighted that the initial austenitic phase was modified within a depth below 200 μm from the UNSMed surface.

Additive manufacturing of CMSX-4 Ni-base superalloy by selective laser melting: Influence of processing parameters and heat treatment

Selective laser melting (SLM) provides an economic approach to manufacturing Ni-base superalloy components for high-pressure gas turbines as well as repairing damaged blade sections during operation. In this study, two advanced processing routes are combined: SLM, to fabricate small specimens of the nonweldable CMSX-4, and hot isostatic pressing (HIP) with a rapid cooling rate as post-processing to heal defects while the target γ/γ´ microstructure is developed. An initial parametric study is carried out to investigate the influence of the SLM process parameters on the microstructure and defects occurring during SLM. Special emphasis is placed on understanding and characterizing the as-built SLM microstructures by means of high-resolution characterization techniques. The post-processing heat treatment is then optimized with respect to segregation and the γ/γ´ microstructure.

Printability and Microstructure of Selective Laser Melting of WC/Co/Cr Powder.

The selective laser melting process is a growing technology for the manufacture of parts with very complex geometry. However, not all materials are suitable for this process, involving rapid localized melting and solidification. Tungsten has difficulties due to the high melting temperature. This study focuses on the possibility of processing a WC/Co/Cr composite powder using selective laser melting. Samples were fabricated and characterized in terms of density, defects, microstructure and hardness. Tests were conducted with hatch spacing of 120 μm and process speed of 40 mm/s. A constant laser power of 100 W and a powder layer thickness of 30 μm were used. A relative density of 97.53%, and therefore a low porosity, was obtained at an energy density of 12.5 J/mm2. Microscopic examination revealed the presence of small cracks and a very heterogeneous distribution of the grain size.

Ultra-high strength martensitic 420 stainless steel with high ductility

Martensitic 420 stainless steel was successfully fabricated by Selective laser melting (SLM) with >99% relative density and high mechanical strength of 1670 MPa, yield strength of 600 MPa and elongation of 3.5%. X-ray diffraction (XRD) and scanning electron microscopy disclosed that the microstructure of SLM 420 consisted of colonies of 0.5–1 μm sized cells and submicron martensitic needles with 11 wt. % austenite. Tempering of as-built SLM 420 stainless steel at 400 °C resulted in ultra-high strength material with high ductility. Ultimate tensile strength of 1800 MPa and yield strength of 1400 MPa were recorded with an elongation of 25%. Phase transformation analysis was carried out using Rietveld refinement of XRD data and electron backscattered diffraction (EBSD), which showed the transformation of martensite to austenite, and resulted in austenite content of 36 wt. % in tempered SLM 420 stainless steel. Transformation induced plasticity (TRIP), austenite formation and fine cellular substructure along with sub-micron martensite needles resulted in stainless steel with high tensile strength and ductility. The advanced mechanical properties were compared with conventionally made ultra-high-strength steels, and the microstructure-properties relationships were disclosed.

The Significance of Spatial Length Scales and Solute Segregation in Strengthening Rapid Solidification Microstructures of 316L Stainless Steel

Selective laser melting (SLM) is a novel manufacturing technique for producing complex parts, with unique mechanical properties that result from rapidly solidified structures. For SLM of austenitic 316L stainless steel, the reported mechanical properties have a large scatter, which reflects an incomplete understanding and control of the process-structure-properties linkage. This paper demonstrates how material micromechanical behavior --- including length scale dependent yield strength and hardening --- is linked to rapid solidification microstructure, and how this structure emerges from typical SLM process conditions. This linkage is produced by concurrently coupling phase field simulations with crystal plasticity modeling. In particular, the evolution of rapidly solidified SLM microstructures is described with a quantitative phase field model with imposed solute trapping kinetics. A range of process conditions are considered by varying the thermal gradient and pulling speed driving solidification. The phase field model can predict morphological transitions (dendritic-cellular-planar), microsegregation, and emergent microstructure length scales, as function of process conditions. Both 2D and 3D phase field simulations are conducted, yielding microstructure morphologies and cell spacing Vs. cooling rate data that are in good agreement with the available experiments in the literature. Micromechanical response of domains characterized by cellular subgrain solidification structures, generated from phase simulations, are analyzed with a Cosserat crystal plasticity model which includes calibrated length-scale and hardening effects, and a realistic description of solid solution strengthening. It is shown that length scale characteristics (cell spacing and grain size) and solute segregation greatly influence the overall hardening behavior, and also affect plastic localization and evolution of geometrically necessary dislocation (GND) type hardening. We conclude with a study of the micromechanical behavior of a polycrystalline structure, focusing on the effects of cell spacing and grain size, and the relative orientation between crystalline lattices and cells. Our results suggest that the material strength is more sensitive to the cell spacing (microstructural length scale) than to the magnitude of solute segregation. The concurrently coupled phase field-crystal plasticity modeling scheme is presented as a proof-of-concept for systematically investigating and discovering new compositions, processes and microstructures for additive manufacturing of metals.

Effect of Build Orientation on Mechanical Properties and Microstructure of Ti-6Al-4V Manufactured by Selective Laser Melting

Ti-6Al-4V with five build orientations relative to the substrate (0, 30, 45, 60, and 90 deg) prepared by selective laser melting (SLM) process was studied. Four distinct conditions were considered: as-built without any treatment, heat treatment below the β-transus, hot isostatic pressing, and heat treatment above the β-transus. The effect of build orientation on the mechanical properties and microstructure of SLM Ti-6Al-4V was comprehensively analyzed from the aspects of residual stress, pore distribution, texture, and microstructure. The results revealed that the residual stress, pore distribution, and texture were the main contributors to the differences in mechanical properties among samples with different build orientations. The tensile strength of samples in the as-built state reaches a maximum and minimum at 45 deg (1134 MPa) and 90 deg (1004 MPa), respectively. The elongation of SLM Ti-6Al-4V material reaches a maximum of 10 pct for the 0 deg sample because of its lower porosity and a minimum of 3.5 pct for the 45 deg sample due to its high porosity. The 0 deg sample in the as-built state exhibits the longest fatigue life because of its lower porosity and adequate strength, and the 90 deg sample exhibits the shortest fatigue life because of its lowest static strength caused by high residual stress.

The microstructure transformation of selective laser melted Ti-6Al-4V alloy

The microstructure transformation of Ti-6Al-4V was closely related to the complex thermal history in the selective laser melting (SLM) process. In this paper, the surface morphology and microstructure of SLM fabricated Ti-6Al-4V were characterized to explore the densification behavior and microstructure formation mechanism. The in-depth relationship between the bulk energy density, thermal cycle and microstructure transformation was explored. The results revealed that the low bulk energy density of 25 J/mm3 resulted in disordered solidification front and considerably terrible densification behavior. The microstructure characteristics and phase constitutions experienced a successive transformation as the bulk energy density increased: ultrafine Z-zigzag martensite α′- fine acicular martensite α′- coarse lamellar α. Moreover, the hierarchical martensite phase was differentiated into primary α′, secondary α′2, tertiary α′3, quartic α′4 and the dots or rods of α phase according to the morphology and transformation sequence. Furthermore, the hierarchical substructure formation and transformation mechanism based on the thermal cycle were proposed. Due to the present work, the control of phase constitution and martensitic morphology of fabricated Ti-6Al-4V can be realized by regulating the bulk energy density in SLM process.

Multiscale characterization of microstructures and mechanical properties of Inconel 718 fabricated by selective laser melting

A multiscale investigation of the microstructures and mechanical properties of Inconel 718 fabricated by selective laser melting (SLM) was performed on the as-SLM and heat-treated (HT) samples at various build locations. The microstructures were characterized by optical microscopy, high-resolution scanning electron microscopy, and electron backscatter diffraction over a wide range of length-scales. The as-SLM samples exhibited unique multi-scale microstructure features, including a few hundred-micrometer melt pools and columnar cube-textured grains, micrometer-sized dendritic and cellular sub-grain structures from chemical segregation and precipitates. Upon heat treatment, recrystallization occurred in the sample, replacing the unique SLM microstructures with homogenized grains, annealing twins, and nano-scale precipitates. Dry sliding wear tests, Vickers hardness tests, and force modulation microscopy in the atomic force microscopy were conducted to characterize the mechanical properties of the SLM parts. It was found that the wear resistance of the as-SLM sample was sensitive to the grain-level structures, exhibiting a relatively large spatial variation (∼11%) at different build locations. In contrast, the Vickers hardness barely changed (<1%) with locations. Heat treatment reduced the wear rate from 7.2 × 10−4 to 5.83 × 10−4 mm3/Nm and enhanced the hardness from 320 to 458 HV, resulting from the precipitation of nanoscale strengthening phases resolved from force modulation microscopy. Finally, the correlations of the mechanical properties with the hierarchical SLM microstructures were discussed, implying the importance of multi-scale conditions on tuning the SLM part performance.

Elaboration of oxide dispersion strengthened Fe-14Cr stainless steel by selective laser melting

This study presents the influence of the main selective laser melting (SLM) processing parameters on the densification behavior and microstructure evolution of oxide dispersion strengthened (ODS) Fe-14Cr stainless steel. Optimization of the process parameters allows to manufacture ODS stainless steel parts, which present high densities up to 98% and a fine dispersion of nanosized Y-Ti rich oxide particles. Laser power and scan speed are found to strongly influence the density and the microstructure of SLM builds. The ratio power over scan speed controls the width and the depth of the molten pool. Low energy densities (i.e. a low laser power or a high scan speed), inferior to 100 J mm−3, cause lack of fusion of the powder and induce the presence of numerous pores. Finer microstructure can be achieved in this condition since grains receive less energy to growth. The hatch distance does not affect the density in the range of testing values. A decrease in hatch distance causes re-fusion of previous neighbor track but does not re-melt the previous layer since the ratio power over speed is kept constant when varying the hatch distance. A slight coarsening of the microstructure is observed in this case. A large range of hatch distance can be used and especially large values, which decrease the time of production. This study can be a guideline to achieve materials with specific microstructure for high temperature applications.

Laves phases in selective laser melted TiCr1.78 alloys for hydrogen storage

Hydrogen is one of the most promising clean energy carriers within alternative fuels. The need for its absorption/release at reasonable p/T conditions within materials, like metallic Ti-Cr alloys, is primary for its use in renewable and sustainable energy applications.In this work TiCr1.78 compound was produced via selective laser melting (SLM) first time. Due to lack of commercially available powders of Ti-Cr compositions, two powders prototype precursors, premixed or prealloyed, were prepared and investigated. Correlation among powder type and microstructure of SLM built was studied. Prealloyed powders allowed Laves phases to be obtained in SLM parts while premixed did not fully react. Relative density of SLM products was strictly correlated to energy density.

Selective laser melting of an equiatomic CoCrFeMnNi high-entropy alloy: Processability, non-equilibrium microstructure and mechanical property

The selective laser melting (SLM) of an equiatomic CoCrFeMnNi high-entropy alloy (HEA) powder was studied, with emphasis on its non-equilibrium microstructural evolution and mechanical properties. The printed sample density increases gradually with increasing laser energy density and then decreases at an over-high laser energy density. The microstructure of SLM printed HEA exhibits a large number dislocation pile-ups and lattice distortion. Of particular intrigue are the existence of nanotwins and tetragonal σ phase in SLM printed HEA; such nanotwins and σ phase have never been observed in either cast or deformed HEA with true strain below 3.7%. The grain refinement by rapid solidification and σ phase synergistically improve the mechanical properties, as compared with conventionally solidified HEA. Hot isostatic pressing (HIP) was performed to improve the densification, yielding the increase of tensile strength from 601 MPa of SLM printed sample to 649 MPa after HIP.

Characterization of the microstructure of a selective laser melting processed Al-50Si alloy: Effect of heat treatments

Due to the high heating/cooling rates involved in selective laser melting, the samples fabricated by this process, typically present high tensile residual stresses. The goal of this work was thus to investigate the effects of heat treatments on the microstructure and residual stress of selective laser melting processed Al-50Si alloy. Heat treatments were performed under temperatures ranged from 300 to 600°C for 2h with subsequent water quenching. The microstructure and the distribution of the residual stress state of the samples were determined respectively by metallographic techniques and by Raman spectroscopy before and after heat treatments. In the as-fabricated condition, the crystal sizes and the residual stress present an inhomogeneous distribution. Heat treatment at 300°C allows a homogenization of the stress level without changing significantly the microstructure. While at higher temperatures, the average values of the residual stress decrease corresponding to an increase in the grain size.

Mechanical properties and microstructural characterization of selective laser melted 17-4 PH stainless steel

PurposeThe purpose of this paper is to understand the effect of four different factors: building orientation, heat treatment (solution annealing and aging), thermal history and process parameters on the mechanical properties and microstructural features of 17-4 precipitation hardening (PH) stainless steel (SS) parts produced using selective laser melting (SLM).Design/methodology/approachVarious sets of test samples were built on a ProX 100™ SLM system under argon environment. Characterization studies were conducted using mechanical tensile and compression test, microhardness test, optical microscopy, X-ray diffraction and scanning electron microscopy.FindingsResults indicate that building orientation has a direct effect on the mechanical properties of SLM parts, as vertically built samples exhibit lower yield and tensile strengths and elongation to failure. Post-SLM heat treatment proved to have positive effects on part strength and hardness, but it resulted in reduced ductility. Longer inter-layer time intervals between the melting of successive layers allow for higher austenite content because of lower cooling rates, thus decreasing material hardness. On the other hand, tensile properties such as elongation to failure, yield strength and tensile strength were not significantly affected by the change in inter-layer time intervals. Similar to other AM processes, SLM process parameters were shown to be instrumental in achieving desirable part properties. It is shown that without careful setting of process parameters, parts with defects (porosity and unmelted powder particles) can be produced.Originality/valueAlthough the manufacturing of 17-4 PH SS using SLM has been investigated in the literature, the paper provides the first comprehensive study on the effect of different factors on mechanical properties and microstructure of SLM 17-4 PH. Optimizing process parameters and using heat treatment are shown to improve the properties of the part.

Microstructures and Mechanical Properties of Ti6Al4V Parts Fabricated by Selective Laser Melting and Electron Beam Melting

This work compares two metal additive manufacturing processes, selective laser melting (SLM) and electron beam melting (EBM), based on microstructural and mechanical property evaluation of Ti6Al4V parts produced by these two processes. Tensile and fatigue bars conforming to ASTM standards were fabricated using Ti6Al4V ELI grade material. Microstructural evolution was studied using optical and scanning electron microscopy. Tensile and fatigue tests were carried out to understand mechanical properties and to correlate them with the corresponding microstructure. The results show differences in microstructural evolution between SLM and EBM processed Ti6Al4V and their influence on mechanical properties. The microstructure of SLM processed parts were composed of an α′ martensitic phase, whereas the EBM processed parts contain primarily α and a small amount of β phase. Consequently, there are differences in tensile and fatigue properties between SLM- and EBM-produced Ti6Al4V parts. The differences are related to the cooling rates experienced as a consequence of the processing conditions associated with SLM and EBM processes.