Evaluation of 3D printed 316L stainless steel microstructure and mechanical property by laser ultrasonics

Selective laser melting (SLM) is one of the metal additive manufactured technologies with the highest forming precision, which prepares metal components through melting powders layer by layer with a high-energy laser beam. The 316L stainless steel is widely used due to its excellent formability and corrosion resistance. However, its low hardness limits its further application. Therefore, researchers are committed to improving the hardness of stainless steel by adding reinforcement to stainless steel matrix to fabricate composites. Traditional reinforcement comprises rigid ceramic particles, such as carbides and oxides, while the research on high entropy alloys as reinforcement is limited. In this study, characterisation by appropriate methods, inductively coupled plasma, microscopy and nanointendation assay, showed that we successfully prepared the FeCoNiAlTi high entropy alloy (HEA)-reinforced 316L stainless steel composites using SLM. When the reinforcement ratio is 2 wt.%, the composite samples show higher density. The SLM-fabricated 316L stainless steel displays columnar grains and it varies to equiaxed grains in composites reinforced with 2 wt.% FeCoNiAlTi HEA. The grain size decreases drastically, and the percentage of the low angle grain boundary in the composite is much higher than in the 316L stainless steel matrix. The nanohardness of the composite reinforced with 2 wt.% FeCoNiAlTi HEA is twice as high as the 316L stainless steel matrix. This work demonstrates the feasibility of using a high-entropy alloy as potential reinforcement in stainless steel systems.

On the effect of rapid annealing on the microstructure and mechanical behavior of additively manufactured stainless steel by Laser Powder Bed Fusion

Mechanical properties and corrosion behavior of selective laser melted 316L stainless steel after different heat treatment processes

In this study, a thorough investigation of the microstructures and tensile properties of 316L stainless steel fabricated via laser powder bed fusion (L-PBF) was done. 316L stainless steel specimens with two different thicknesses of 1.5 mm and 4.0 mm fabricated under similar conditions were utilized. Microstructural characterization was performed using optical microscopy (OM) and scanning electron microscopy (SEM) equipped with electron backscatter diffraction (EBSD). Melt pools and cellular structures were observed using OM, whereas EBSD was utilized to obtain the grain size, grain boundary characteristics, and crystallographic texture. The 1.5 mm thick sample demonstrated a yield strength (YS) of 538.42 MPa, ultimate tensile strength (UTS) of 606.47 MPa, and elongation to failure of 69.88%, whereas the 4.0 mm thick sample had a YS of 551.21 MPa, UTS of 619.58 MPa, and elongation to failure of 73.66%. These results demonstrated a slight decrease in mechanical properties with decreasing thickness, with a 2.4% reduction in YS, 2.1% reduction in UTS, and 5.8% reduction in elongation to failure. In addition to other microstructural features, the cellular structures were observed to be the major contributors to the high mechanical properties. Using the inverse pole figure (IPF) maps, both thicknesses depicted a crystallographic texture of {001} <101> in their as-built state. However, when subjected to tensile loads, texture transitions to {111} <001> and {111} <011> were observed for the 1.5 mm and 4.0 mm samples, respectively. Additionally, EBSD analysis revealed the pre-existence of high-density dislocation networks and a high fraction of low-angle grain boundaries. Interestingly, twinning was observed, suggesting that the plastic deformation occurred through dislocation gliding and deformation twinning.

Additively manufactured (AM) 316L and 17-4PH stainless steel parts, concretely made by laser powder bed fusion (L-PBF), are characterized and micro-mechanical properties of those steels are analyzed. This study also explored and extended to proton irradiation and small-scale mechanical testing of those materials, to investigate how irradiation affects microstructural evolution and thus mechanical properties at the surface level, which could be detrimental in the long term in nuclear applications. In-depth anisotropy analysis of L-PBF 316L stainless steel parts with the variations of volumetric energy density, a combined study of nanoindentation with EBSD (electron backscatter diffraction) mapping is shown to be an alternative methodology for enriching qualification protocols. Each grain with a different crystallographic orientation was mapped successfully by proper indentation properties. &lt;122&gt; and &lt;111&gt; oriented grains displayed higher than average indentation modulus and hardness whereas, &lt;001&gt;, &lt;101&gt;, and &lt;210&gt; oriented grains were found to be weaker in terms of indentation properties. Based on an extensive nanoindentation study, L-PBF 17-4 PH stainless steels are found to be very sensitive to high load rates and irradiation further escalates that sensitivity, especially after a 0.25 s-1 strain rate. 3D porosity measurement via X-ray microscope ensures L-PBF stainless steel parts are of more than 99.7% density and could be promising for many industrial applications. High percentages of increment of nanohardness, maximum theoretical shear strength, and yield strength were observed due to proton irradiation of 5 um damage depth on the surface of 17-4 PH steel parts. Small-scale mechanical testing of irradiated AM nuclear stainless steels such as 17-4 PH was carried out and investigated by micro-compression of FIB fabricated pillars of different sizes of diameter. Irradiated 17-4 PH materials have never been investigated by this kind of testing procedure to asses the stress-strain characteristics of micro-scale volumes and to explore the structure-property relationship. Both as-built and irradiated AM 17-4 PH micropillars exhibited step-ups in the early stage of load-displacement curves with a varying number of slip bands intermittently formed throughout the pillar volume while compressed by the uniaxial load. As for the radiation-damaged zone, micropillars displayed lesser slip bands compared to as-built parts as irradiation damage creates an obstacle to dislocations movement and hence hardening. It requires higher loads to initiate plastic deformation as dislocation must overcome irradiation-induced obstacles for the slip to occur and localization of strain without increasing the load for a certain amount of time during the test. Proton irradiation effects on the compressive mechanical properties of AM 17-4 PH stainless steel parts depending on the volumetric energy density (VED) used during the parts' fabrication process. On as-built parts, compressive yield strength varied from 107.27 MPa to 150.70 MPa and it was in the range of 133.43 MPa to 244.57 MPa under irradiated conditions. All 2 μm pillars were fabricated as their height falls within the radiation damage depth of 5 μm. It was expected to generate the highest yield strength and tensile strength due to the radiation hardening effect as discussed earlier. Yield and tensile strength were found to be the highest as expected as of 244.57 MPa and 375.08 MPa in irradiated 17-4 PH sample 1 (VED = 54.76 J/mm3). Samples with lower VED exhibited better micro-mechanical compressive responses than higher VED AM 17-4 PH parts in both as-built and irradiated conditions.

Equiatomic CoCrFeMnNi high entropy alloy (HEA) powder was processed by laser powder bed fusion (LPBF) additive manufacturing (AM). The properties of the spherical pre-alloyed CoCrFeMnNi powder were characterized and its processability using LPBF AM was systematically investigated through the volumetric energy density (VED) based on the surface roughness, defects (micro-cracks and porosity) and densification. After optimization, LPBF processing at a VED of 104 J/mm3 achieved highly dense and crack-free vertical and horizontal test specimens with a porosity fraction lower than 0.01% and micro-pores having a mean size of, respectively, 25.9 μm and 13.4 μm, as determined from X-ray micro-computed tomography (μCT) inspection. Scanning electron microscope (SEM) analysis of the as-built (AB) CoCrFeMnNi processed at a VED of 104 J/mm3 showed a heterogeneous solidification microstructure, consisting of columnar grains with a cellular subgrain structure, and electron backscattered diffraction (EBSD) revealed a crystallographic texture mainly along the < 100 > direction. Post treatment with hot isostatic pressing (HIP) was effective in closing the remnant micro-pores in the bulk volume of the AB CoCrFeMnNi. Also, the cellular sub-grain structure in the AB CoCrFeMnNi completely disappeared after HIP and the resulting microstructure consisted of recrystallized equiaxed grains with annealing twins. The room temperature tensile response was anisotropic for AB CoCrFeMnNi with horizontally built specimens exhibiting higher strength and fracture strains (global and local) compared to vertically built ones; HIP reduced the anisotropy in the tensile properties and led to similar tensile strength with elongation values that were ~ 50% higher than in the AB condition. The HIPed CoCrFeMnNi also displayed higher Charpy impact toughness and absorbed energy at both room and liquid nitrogen temperatures compared to the AB material. Examination of the fracture surfaces after tensile and Charpy impact testing revealed ductile features with characteristic dimpled appearance and pointed to the important role of the remnant micro-pores on failure in the AB CoCrFeMnNi. Tribological assessments pointed to the superior low-stress abrasion resistance of AB and HIPed CoCrFeMnNi compared to 316L stainless steel (SS), which was included in this study to reinforce the analysis. SEM observations revealed that scratching and micro-fracture are the dominant wear mechanisms for the CoCrFeMnNi HEA, whereas ploughing and cutting parallel to the abrasive flow direction are the dominant mechanisms for 316L SS. To the authors’ knowledge, this study is the first to evaluate and report the low-stress abrasion resistance of any high entropy alloy. To understand the corrosion behavior, polarization curves of AB and HIPed CoCrFeMnNi were measured in 3.5 wt% NaCl and 1N H2SO4 solutions, and the results were compared to those of 316L SS. The findings indicate that AB and HIPed CoCrFeMnNi outperform 316L SS in a chloride-containing environment, but not in an acid-containing environment. Additionally, observations of hydrogen permeability revealed that AB CoCrFeMnNi permeates a lower volume of hydrogen atoms (by ~ 5 times) compared to 316L SS, despite its higher (by nearly 3 times) diffusion coefficient. Electrochemical hydrogen permeation data showed that the concentration of atomic hydrogen in the sub-surface of AB and HIPed CoCrFeMnNi was, respectively, about 32 and 26 times lower than in 316L SS. This study provides important material–structure–property data and indicates a promising outlook for LPBF of the CoCrFeMnNi HEA with high-performance.

The thermal stability of dislocation cellular structures in three additively manufactured (AM) austenitic stainless steels (SSs), 316L SS, 304L SS, and Al modified 316L SS (316L(Al)), were studied. Minor alloying elements, Mo and Al, were found affecting the stability of the cellular structures in AM austenitic SS, resulting in a stability ranking of AM 316L SS > AM 304L SS > AM 316L(Al) SS. As a result, their abilities towards recrystallization also differed. Owing to the high stacking fault energy (SFE) due to Al addition, AM 316L(Al) SS had the least stable subgrain cellular structure and exhibited the lowest recovery temperature. Although 316L SS possessed slightly higher SFE than 304L SS, the pinning effect due to Mo segregation at the cellular walls in AM 316L SS significantly enhanced its thermal stability. While the low-SFE AM 316L SS and AM 304L SS recovered their cellular structures via the equiaxed cell growth, the dislocation cellular walls in high-SFE AM 316L(Al) SS continuously vanished along a preferred direction. The fast recovery of cellular structures led to recrystallization retardation. The Hall–Petch model was found incapable of correlating cell size to strength because of the continuous weakening of cellular walls during heat treatment.

Achieving high strength 316L stainless steel by laser directed energy deposition-ultrasonic rolling hybrid process

Investigation of surface quality, microstructure, deformation mechanism, and fatigue performance of additively manufactured 304L stainless steel using grinding

Additive layer manufacturing is a blanket term for a wide range of processes operating on the same underlying principle. 3D geometry is created by depositing material, layer by layer to create a final 3D geometry. Selective laser melting (SLM) is a branch of additive layer manufacturing, using a laser to fuse a powder bed of metal into each layer. This thesis investigates the SLM process and its application to nickel based superalloy materials, IN625 and IN718. IN625 and IN718 are similar nickel-based superalloys developed for use in aerospace gas turbine engines. In their conventionally manufactured form, these materials have excellent high temperature mechanical properties which make them idea for use in the hot section of gas turbine engines. The aim of this thesis was to investigate how these materials interact with the SLM process and how the material produced can be optimised to improve the range of applications it can be used for. A gap in knowledge regarding a detailed understanding of how the powders morphological and rheological properties influence its ability to be processed by SLM was identified and investigated. A wide range of characterisation methods were implemented with certain important properties being identified to assess a powders processability, namely the particle size distribution and how a significant content of fine particles below 10 μm in size can be detrimental to processability. There is also a lack of a standard powder characterisation methodology specifically for SLM applications. This is addressed with certain methods and measurements being suggested as most promising for wider SLM application. Avalanche flow testing is found to be most applicable to the critical recoating process in SLM and most able to differentiate suitable and unsuitable SLM powders. Following characterisation of the raw material feedstock powder, this thesis also investigates the influence of processing parameters on the microstructure of the material produced by the SLM process. Significant microstructural changes were observed as a result of process parameter changes. This was identified to potentially enable for in-situ modification of material microstructure to suit a manufactured material to its end application. Of the process parameters investigated, laser scan speed was most interesting, suggesting that a faster laser scan speed was able to create a similar microstructure to a much slower one. This was attributed to the reheating effect of the laser beam returning quickly to the adjacent scan line. The validity of this explanation was investigated using a simple, computational thermal model. The result is a new understanding of laser scan speed SLM and its nonlinear relationship with material temperature and microstructure evolution. Finally, post process heat treatments of SLM manufactured IN718 material were investigated. This investigation was in response to a gap in current knowledge regarding heat treatments designed specifically for SLM material. SLM IN718 has been found to have reduced high temperature mechanical properties, specifically stress rupture, which limits its application in demanding environments. In this thesis a range of post process homogenisation heat treatments were investigated, with treatments between 1030 °C and 1060 °C being found to produce material with characteristics consistent with material with excellent stress rupture properties. This novel heat treatment route could provide a method for SLM IM718, and the increased design and geometric freedoms, to be applied in more demanding applications. An evolution of the grain structure in the material was also observed and measured during high temperature homogenisation treatments. This was investigated in the final chapter, and a novel mechanism is suggested for the process of grain coarsening observed. Previously published literature explains similar evolutions as recrystallisation however this did not fit the observations during this thesis. The evolution of grain structure was observed using a process of quasi in-situ electron back scatter diffraction, and a mechanism of grain boundary length reduction, followed by grain growth, is suggested to better fit the observations. It was determined that grains are preferentially selected for growth based on their proximity to a ‘path of least resistance’ of lower angle grain boundaries. The results of this work should benefit industrial users of SLM in the fabrication of Nickel-Based Superalloy material for aerospace applications. The conclusions on powder characterisation offer an insight into available methods to better control and characterise powder feedstock materials for consistent production. Aerospace users especially may find the work regarding post process heat treatments designed specifically for SLM material, to recover lost stress rupture performance, useful in enabling the use of SLM materials, and the design freedom that brings with it, in mor demanding environments than are currently possible.

Optical methods of laser ultrasonic testing technology in the industrial and engineering applications: A review

Surface mechanical attrition treatment of additively manufactured 316L stainless steel yields gradient nanostructure with superior strength and ductility

The positioning and quantitative testing of drilling-induced delamination in aircraft composite structures is not only one of the main difficulties in the field of non-destructive testing(NDT), but also a security problem need to be solved in aviation industry. Laser ultrasonic testing technique is a possible way to solve the problem. An experiment study concerning the verification of the feasibility of detecting drilling-induced delamination with laser ultrasonic technique is carried out. A composite laminate with drilling holes is made as specimen, the broadband characteristics of ultrasonic waves in composites generated by pulse laser are studied, and the wave signals with good signal-to-noise ratio(SNR), high sensitivity and resolution are extracted. The effects of the interface of drilling-induced delamination on the propagation characteristics of laser ultrasonic waves are studied, and the characterization methods of drilling-induced delamination with laser ultrasonics are acquired. Based on the pulse echo and transmission method, the laser ultrasonic C scan testing of the composite laminate with drilling holes is accomplished, and the morphologies, dimensions and positions of drilling-induced delamination are obtained. The research results show that laser ultrasonic testing is an effective method to detect the drilling-induced delamination in aircraft composite structures.

High precision detection of artificial defects in additively manufactured Ti6Al4V alloy via laser ultrasonic testing

Corrosion behavior of different building planes of selective laser melting 316L stainless steel in 0.1 M HCl solution