Scanning Laser Epitaxy Research Articles

On the Spatial Variation of the Microstructure and Microhardness Properties of Nickel-Based Superalloy René 142 Fabricated via Scanning Laser Epitaxy (SLE)

Abstract This article seeks to investigate and characterize the spatial variation of the microstructure and microhardness properties of René 142, a high-γ′ nickel-based superalloy, fabricated through single-pass depositions using scanning laser epitaxy (SLE), a laser powder bed fusion (LPBF)–based additive manufacturing (AM) process. Various advanced material characterization techniques such as optical microscopy, scanning electron microscopy, and Vickers microhardness measurements are employed to thoroughly characterize the SLE-fabricated René 142 specimens. Whereas optical microscopy analyses reveal that the primary dendritic arm spacing (PDAS) remains relatively constant, scanning electron microscopy investigations demonstrate that the size of the γ′ precipitates increase as the scan progresses. Microhardness measurements indicate that the hardness values remain unaffected along the scan direction. Through this study, it is demonstrated that the microstructure and the microhardness properties of the AM-fabricated René 142 deposits are strong functions of the spatial location.

Epitaxial deposition of nickel-based superalloy René 142 through scanning laser epitaxy (SLE)

Single-pass depositions of columnar René 142 on investment cast single-crystal (SX) René N5 substrates having [100] and [001] primary dendrite growth directions were obtained through scanning laser epitaxy (SLE), a laser powder bed fusion (LPBF)-based additive manufacturing (AM) process. The microstructure and the microhardness properties of the René 142 deposits were investigated through high-resolution optical microscopy (HR-OM), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), x-ray diffraction (XRD), electron backscatter diffraction (EBSD), and micro-hardness measurements. HR-OM investigations revealed the capability of SLE in depositing more than 1000 μm of columnar René 142 in a single pass. SEM investigations demonstrated that the primary γ/γ′ precipitates in the deposit region were 90% finer in size compared to the substrate. XRD investigation revealed the presence of a strong [200] peak and EBSD analysis confirmed SX René 142 growth. Microhardness measurements showed an increase in the hardness values by ∼10% in the deposit region compared to the cast substrate. The results showed that the SLE process has tremendous potential in producing epitaxial deposits of nickel-based superalloys and, therefore, the findings reported in this work can pave ways to fabricate components with dissimilar-chemistry high-γ′ nickel-based superalloys using an LPBF-based AM process.

Additive Manufacturing of Nickel-Base Superalloy IN100 Through Scanning Laser Epitaxy

Scanning laser epitaxy (SLE) is a laser powder bed fusion (LPBF)-based additive manufacturing process that uses a high-power laser to consolidate metal powders facilitating the fabrication of three-dimensional objects. In the present study, SLE is used to produce samples of IN100, a high-γ′ non-weldable nickel-base superalloy on similar chemistry substrates. A thorough analysis is performed using various advanced material characterization techniques such as high-resolution optical microscopy, scanning electron microscopy, energy dispersive x-ray spectroscopy, and Vickers microhardness measurements to characterize and compare the quality of the SLE-fabricated IN100 deposits with the investment cast IN100 substrates. The results show that the IN100 deposits have a finer γ/γ′ microstructure, weaker elemental segregation, and higher microhardness compared with the substrate. Through this study, it is demonstrated that the SLE process has tremendous potential in the repair and manufacture of gas turbine hot-section components.

Microstructures and Microhardness Properties of CMSX-4® Additively Fabricated Through Scanning Laser Epitaxy (SLE)

Epitaxial CMSX-4® deposition is achieved on CMSX-4® substrates through the scanning laser epitaxy (SLE) process. A thorough analysis is performed using various advanced material characterization techniques, namely high-resolution optical microscopy, scanning electron microscopy, energy-dispersive x-ray spectroscopy, x-ray diffraction, and Vickers microhardness measurements, to characterize and compare the quality of the SLE-fabricated CMSX-4® deposits to the CMSX-4® substrates. The results show that the CMSX-4® deposits have smaller primary dendritic arm spacing, finer γ/γ′ size, weaker elemental segregation, and higher microhardness compared to the investment cast CMSX-4® substrates. The results presented here demonstrate that CMSX-4® is an attractive material for laser-based AM processing and, therefore, can be used in the fabrication of gas turbine hot-section components through AM processing.

Microstructure of nickel-base superalloy MAR-M247 additively manufactured through scanning laser epitaxy (SLE)

Abstract Scanning laser epitaxy (SLE) is a laser powder bed fusion (LPBF) based additive manufacturing (AM) process developed for the repair and manufacture of gas turbine hot-section components made of nickel-base superalloys. In the present study, the single-pass fabrication of more than 1500 μm thick deposits of MAR-M247, a non-weldable superalloy atop similar chemistry substrates using a high-power laser beam is demonstrated. Metallurgical continuity is achieved across the entire deposit-substrate interface and the samples show little or no warpage across a broad range of processing parameters. In order to relieve the residual stresses and enable precipitation of the strengthening phases, the SLE-deposited MAR-M247 samples are subjected to a commercially available heat treatment process. Microstructures of the as-deposited and the heat-treated MAR-M247 samples are investigated using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and Vickers microhardness measurements. The crack-free deposits obtained here for MAR-M247 represent one of the first few successes reported for a non-weldable alloy of its kind. The results demonstrate that the SLE process has significant potential for the AM-based repair of existing and fabrication of entirely new gas turbine hot-section components utilizing feedstocks of high γ′ content nickel-base superalloy powders. The results also establish that MAR-M247 is an attractive material for the LPBF-based AM processes.

Additive Manufacturing of Nickel‐Base Superalloy René N5 through Scanning Laser Epitaxy (SLE) − Material Processing, Microstructures, and Microhardness Properties

Nickel‐base superalloy René N5 is deposited on cast René N5 substrates with [100] and [001] crystallographic orientations through scanning laser epitaxy (SLE) applied to gas‐atomized pre‐alloyed René N5 powder. Single‐pass fabrication of crack‐free deposits exceeding 1000 micron is achieved. High‐resolution optical microscopy reveals that the deposits have 10 times finer primary dendritic arm spacing compared to the substrate. SEM reveals the presence of finer microstructure in the deposit region compared to the substrate region. XRD and EBSD investigations show that the substrate crystallographic orientation does not affect the unidirectional crystal growth in the deposit region. Vickers microhardness values are higher in the deposits compared to the substrate.

A Study on the Microstructural Characterization of René 142 Deposited Atop René 125 Processed through Scanning Laser Epitaxy

Advancements in the design, optimization and manufacture of turbine engine hot-section components during the past few decades have contributed enormously to the improvement in power-ratings and efficiency levels of gas turbine engines. Nickel-base superalloys are extensively used to produce the hot-section components as this class of alloys offer improved creep strength and higher fatigue resistance compared to other alloys due to the presence of precipitate-strengthening γ' phases i.e. Ni3[Ti, Al, Ta etc.] in the normally face centered cubic (FCC) structure of the solidified nickel. Although this second phase is the main reason for the improvement in properties, it also results in increased processing difficulty as these alloys are prone to crack formation. In this work, we demonstrate powder-bed additive manufacturing of René 142 onto René 125 substrates through scanning laser epitaxy (SLE). René 142 is a high strength, nickel-base directionally solidified (DS) alloy that has high rupture strength, excellent resistance to grain boundary cracking, and superior high-velocity oxidation resistance. Successful deposition of René 142 on René 125 provides an avenue to repair legacy hot-section components by depositing superior quality alloys at the damage locations. The microstructure of the deposited René 142 is observed to follow the polycrystalline or EQ morphology of the underlying René 125 substrate. The SLE processed René 142 exhibits dense and crack-free deposits, and microstructure refinement compared to the underlying cast René 125 substrate. This work is sponsored by the Office of Naval Research through grants N00014-11-1-0670 and N00014-14-1-0658.

Additive Manufacturing of Single-Crystal Superalloy CMSX-4 Through Scanning Laser Epitaxy: Computational Modeling, Experimental Process Development, and Process Parameter Optimization

This paper focuses on additive manufacturing (AM) of single-crystal (SX) nickel-based superalloy CMSX-4 through scanning laser epitaxy (SLE). SLE, a powder bed fusion-based AM process was explored for the purpose of producing crack-free, dense deposits of CMSX-4 on top of similar chemistry investment-cast substrates. Optical microscopy and scanning electron microscopy (SEM) investigations revealed the presence of dendritic microstructures that consisted of fine γ′ precipitates within the γ matrix in the deposit region. Computational fluid dynamics (CFD)-based process modeling, statistical design of experiments (DoE), and microstructural characterization techniques were combined to produce metallurgically bonded single-crystal deposits of more than 500 μm height in a single pass along the entire length of the substrate. A customized quantitative metallography based image analysis technique was employed for automatic extraction of various deposit quality metrics from the digital cross-sectional micrographs. The processing parameters were varied, and optimal processing windows were identified to obtain good quality deposits. The results reported here represent one of the few successes obtained in producing single-crystal epitaxial deposits through a powder bed fusion-based metal AM process and thus demonstrate the potential of SLE to repair and manufacture single-crystal hot section components of gas turbine systems from nickel-based superalloy powders.

Additive Manufacturing and Characterization of René 80 Superalloy Processed Through Scanning Laser Epitaxy for Turbine Engine Hot‐Section Component Repair

This article demonstrates single‐pass fabrication of thick, crack‐free deposits of René 80, a high gamma‐prime “non‐weldable” superalloy, atop like‐chemistry substrates through scanning laser epitaxy (SLE), a metal powder bed‐based laser additive manufacturing (AM) technique. Microstructural investigations and processing zone identification are presented. Optical microscopy and scanning electron microscopy reveal the segregated dendritic microstructure with the presence of finer carbides and primary γ′ particles. Transmission electron microscopy reveals a bimodal distribution of finer secondary γ′ particles in the deposit region. Microindentation hardness measurements show an increase in the hardness value by 25% in the deposit region compared to the cast substrate due to microstructural refinement. In addition to the achievement of metallurgical continuity across the interface and improved hardness in the deposit, the crack‐free deposits obtained here for René 80 represent one of the few successes reported for a non‐weldable alloy of its kind. The results presented here illustrate the significant potential of SLE for the AM‐based repair of existing and AM‐based construction of entirely new gas turbine hot‐section components utilizing powders of high γ′ nickel‐based superalloys that have long been known to have high tendency for cracking under the processing conditions associated with traditional welding, joining, and cladding approaches.

Additive Manufacturing of IN100 Superalloy Through Scanning Laser Epitaxy for Turbine Engine Hot-Section Component Repair: Process Development, Modeling, Microstructural Characterization, and Process Control

This article describes additive manufacturing (AM) of IN100, a high gamma-prime nickel-based superalloy, through scanning laser epitaxy (SLE), aimed at the creation of thick deposits onto like-chemistry substrates for enabling repair of turbine engine hot-section components. SLE is a metal powder bed-based laser AM technology developed for nickel-base superalloys with equiaxed, directionally solidified, and single-crystal microstructural morphologies. Here, we combine process modeling, statistical design-of-experiments (DoE), and microstructural characterization to demonstrate fully metallurgically bonded, crack-free and dense deposits exceeding 1000 μm of SLE-processed IN100 powder onto IN100 cast substrates produced in a single pass. A combined thermal-fluid flow-solidification model of the SLE process compliments DoE-based process development. A customized quantitative metallography technique analyzes digital cross-sectional micrographs and extracts various microstructural parameters, enabling process model validation and process parameter optimization. Microindentation measurements show an increase in the hardness by 10 pct in the deposit region compared to the cast substrate due to microstructural refinement. The results illustrate one of the very few successes reported for the crack-free deposition of IN100, a notoriously “non-weldable” hot-section alloy, thus establishing the potential of SLE as an AM method suitable for hot-section component repair and for future new-make components in high gamma-prime containing crack-prone nickel-based superalloys.

Erratum to: A Coupled Thermal, Fluid Flow, and Solidification Model for the Processing of Single-Crystal Alloy CMSX-4 Through Scanning Laser Epitaxy for Turbine Engine Hot-Section Component Repair (Part I)

Scanning laser epitaxy (SLE) is a new laser-based additive manufacturing technology under development at the Georgia Institute of Technology. SLE is aimed at the creation of equiaxed, directionally solidified, and single-crystal deposits of nickel-based superalloys through the melting of alloy powders onto superalloy substrates using a fast scanning Nd:YAG laser beam. The fast galvanometer control movement of the laser (0.2 to 2 m/s) and high-resolution raster scanning (20 to 200 µm line spacing) enables superior thermal control over the solidification process and allows the production of porosity-free, crack-free deposits of more than 1000 µm thickness. Here, we present a combined thermal and fluid flow model of the SLE process applied to alloy CMSX-4 with temperature-dependent thermo-physical properties. With the scanning beam described as a moving line source, the instantaneous melt pool assumes a convex hull shape with distinct leading edge and trailing edge characteristics. Temperature gradients at the leading and trailing edges are of order 2 × 105 and 104 K/m, respectively. Detailed flow analysis provides insights on the flow characteristics of the powder incorporating into the melt pool, showing velocities of order 1 × 10–4 m/s. The Marangoni effect drives this velocity from 10 to 15 times higher depending on the operating parameters. Prediction of the solidification microstructure is based on conditions at the trailing edge of the melt pool. Time tracking of solidification history is incorporated into the model to couple the microstructure prediction model to the thermal-fluid flow model, and to predict the probability of the columnar-to-equiaxed transition. Qualitative agreement is obtained between simulation and experimental result.

A Microstructure Evolution Model for the Processing of Single-Crystal Alloy CMSX-4 Through Scanning Laser Epitaxy for Turbine Engine Hot-Section Component Repair (Part II)

Part I [Metall. Mater. Trans. B, 2014, DOI: 10.1007/s11663-014-0117-9 ] presented a comprehensive thermal, fluid flow, and solidification model that can predict the temperature distribution and flow characteristics for the processing of CMSX-4 alloy powder through scanning laser epitaxy (SLE). SLE is an additive manufacturing technology aimed at the creation of equiaxed, directionally solidified and single-crystal (SX) deposits of nickel-based superalloys using a fast-scanning laser beam. Part II here further explores the Marangoni convection-based model to predict the solidification microstructure as a function of the conditions at the trailing edge of the melt pool formed during the SLE process. Empirical values for several microstructural characteristics such as the primary dendrite arm spacing (PDAS), the columnar-to-equiaxed transition (CET) criterion and the oriented-to-misoriented transition (OMT) criterion are obtained. Optical microscopy provides visual information on the various microstructural characteristics of the deposited material such as melt depth, CET location, OMT location, PDAS, etc. A quantitative and consistent investigation of this complex set of characteristics is both challenging and unprecedented. A customized image-analysis technique based on active contouring is developed to automatically extract these data from experimental micrographs. Quantitative metallography verifies that even for the raster scan pattern in SLE and the corresponding line heat source assumption, the PDAS follows the growth relation w ~G −0.5 V −0.25 (w = PDAS, G = temperature gradient and V = solidification velocity) developed for marginal stability under constrained growth. Models for the CET and OMT are experimentally validated, thereby providing powerful predictive capabilities for controlling the microstructure of SX alloys processed through SLE.