As-built Microstructure Research Articles

Multi-scale microstuctural investigation of a new Al-Mn-Ni-Cu-Zr aluminium alloy processed by laser powder bed fusion

Traditional high strength aluminum alloys such as the 2xxx or 7xxx series are prone to cracking when processed by additive manufacturing. Designing new aluminum alloys that can be processed crack-free by Laser Powder Bed Fusion (L-PBF) while exhibiting comparable or enhanced mechanical properties is a major target, which may be reached by including in the alloy design strategy specific features of this processing route such as the very high cooling rates. Here, we study a novel Al-4Mn-3Ni-2Cu-2Zr alloy processed by L-PBF, which shows some specific features in comparison to other Al-alloys developed for additive manufacturing. We establish the relationships between the processing conditions and the specific features of the microstructure inherited from L-PBF based on a multi-scale microstructural characterization approach from the melt pool scale up to the nanoscale using X-ray diffraction and electron microscopy with a special focus on Automated Crystal Orientation Mapping (ACOM) in transmission. At the melt pool scale, three regions have been identified: FEZ (Fine Equiaxed Zone), CZ (Columnar Zone) and CEZ (Coarse Equiaxed Zone) giving a hierarchical architecture to the microstructure. Each region has been thoroughly characterized by coupling ACOM and chemical mapping. Five different intermetallic phases have been identified in the as-built microstructure: Al3Zr, Al3Ni2, Al9Ni2, Al60 Mn 11Ni 14, and Al2Cu. The spatial distribution of these intermetallic phases has been found to vary within a given molten pool. The solidification sequence and the various mechanisms involved in the formation of this peculiar microstructure are discussed in the light of our multi-scale microstructural observations along with solidification thermodynamic calculations.

Propagation of Input Uncertainties in Numerical Simulations of Laser Powder Bed Fusion

Laser powder bed fusion has the potential of redefining state-of-the-art processing and production methods, but defect formation and inconsistent build quality have limited the implementation of this process. Numerical models are widely used to study this process and predict the formation of these defects. Presently, the uncertainties of model input parameters and thermophysical properties used by these numerical simulations have not been investigated. In the present study, the uncertainty in these input parameters and material properties are quantified for laser powder bed fusion, with and without a simulated powder bed, to determine their influence on the predictive accuracy of an experimentally validated numerical model. Accounting for all possible sources of uncertainty quickly becomes computationally expensive on account of the curse of dimensionality. Uncertainty in laser absorption, solid, and liquid specific heat of the metal were found to have the largest effect on model prediction reliability with or without the use of a powder bed. Results also illustrate that accounting for these three uncertain parameters still captures the majority of model prediction uncertainty. Furthermore, the methodology of this study may be used to understand the uncertainty in as-built microstructure through propagation to microstructure prediction models, or applied under processing conditions where high Péclet numbers are observed and the thermal convection and fluid flow within the molten pool are substantial.

Influence of solidification structure on austenite to martensite transformation in additively manufactured hot-work tool steels

The microstructure of a hot-work tool steel additively manufactured using laser powder-bed fusion (L-PBF), and its response to post heat treatment, is studied in detail by microstructure characterization and computational thermodynamics and kinetics. The high solidification and cooling rates during the L-PBF process lead to suppression of δ-ferrite and instead solidification of an austenite phase directly containing a cellular substructure where the alloying elements have segregated to the inter-cellular regions and where solidification carbides have formed in the cell junctions. The austenite is then partly decomposed into martensite at lower temperatures. The micro-segregation can be predicted by reducing the complex solidification behavior to a diffusion problem in one dimension enabling detailed comparisons with the measured segregation profiles quantified at a nanometer scale. Martensite start temperature (Ms) calculations along the spatially varying composition show that the Ms temperature decreases in the inter-cellular regions where austenite is observed. The network of austenite in the as-built microstructure can be understood from the combined influence of the composition dependence of the Ms temperature in relation to the build plate temperature and the mechanical stabilization of the small-sized austenite regions. This work demonstrates the power of computational tools based on computational thermodynamics and kinetics for designing tool steels for additive manufacturing by predictions of the steel's response to the L-PBF process and post heat treatments.

Effect of Heat Treatment on Microstructure Evolution of X38CrMoV5-1 Hot-Work Tool Steel Produced by L-PBF

The X38CrMoV5-1 hot-work tool steel produced by laser powder bed fusion was investigated to assess the effect of quenching and tempering and direct tempering on the as-built microstructure. After the printing process, the material microstructure appeared to be characterized by a fine cellular network consisting of γ-Fe cell boundaries and α′-Fe cores. Scheil–Gulliver curves, X-ray diffraction patterns, and transmission electron microscopy images suggested a transformation of the inner core zone from δ-Fe to α′-Fe through γ-Fe. Air quenching promoted the transition of the solidification structure into a fully martensitic microstructure. Both as-built and quenched samples revealed the presence of manganese oxides and vanadium carbonitrides forming core-shell structures. After tempering, starting from as-built and from quenched condition, a dispersion of nano-sized V and Cr-rich second phases was formed in the microstructure, achieving hardness values comparable to those obtained by the same alloy produced by conventional methods. The specimen tempered directly after the laser powder bed fusion process showed a hardness peak shifted towards higher temperatures compared to the conventionally tempered sample.

Understanding the Effects of CoAl2O4 Inoculant Additions on Microstructure in Additively Manufactured Inconel 718 Processed Via Selective Laser Melting

The effect of varying amounts of CoAl2O4 inoculant ranging from 0 to 2 wt pct on the microstructure evolution of Inconel 718(IN718) fabricated by selective laser melting (SLM) was evaluated. Characterization of the as-built microstructure revealed that addition of CoAl2O4 resulted in a modest degree of grain refinement with a slight increase in microstructural anisotropy. Increasing the total CoAl2O4 content beyond 0.2 wt pct resulted in severe agglomeration of the non-metallic particles and the formation of slag inclusions measuring up to 100 μm in size present in the as-built microstructure. In addition to large agglomerates, the inoculant was chemically reduced to form a fine dispersion of submicron-sized Al2O3 particles throughout the IN718 matrix. The fine dispersion of oxides significantly hindered grain recrystallization during the post-fabrication heat treatment due to a Zener pinning effect. The findings from this study indicate in order to effectively utilize CoAl2O4 as a grain refining inoculant for additive manufacturing, the process parameters need to be optimized to avoid agglomeration of the non-metallic particles and other process-related defects.

Tempering behavior of a direct laser deposited hot work tool steel: Influence of quenching on secondary hardening and microstructure

Direct Laser Deposition (DLD) opens new ways for the fabrication of tools with intricate designs as well as for dies and molds repairing. However, non-equilibrium, and highly dynamic characteristics of the DLD process, generally result in non-equilibrium microstructures, inhomogeneities across the build height, and unwanted phase transformations. This might lead to non-uniform and highly scattered mechanical properties. This highlights the need for a proper thermal post-processing aimed at achieving application-specific mechanical properties. AISI H13, which is commonly used in hot-forming applications (e.g., die casting tooling, extrusion dies), shows high potential in the rapid manufacturing of the tools by additive manufacturing (AM). In this study, the influence of the as-built microstructure on direct tempering response as well as tempering after austenitization and quenching of DLD H13 tool steel is evaluated. The factors governing hardness and tempering behavior are discussed in detail with the aid of microstructural analysis and isochronal tempering studies. As-built microstructure comprised martensite, inter-cellular/inter-dendritic retained austenite (RA), and solidification carbides. Decomposition of retained austenite by direct tempering of the as-built samples led to a stronger secondary hardening peak and a shift of this peak to higher temperature. Austenitized and quenched samples contained ≤2 vol% retained austenite. Increasing the austenitization temperature and time led to the dissolution of a larger vol% of solidification carbides, recovery of the micro-segregation, recrystallization and grain growth. In quenched and tempered parts, hardness increased by increasing austenitization temperature from 1020 °C to 1060 °C as a result of higher supersaturation of quenched martensite leading to larger vol% of secondary carbide precipitation. Within the whole technically significant tempering range, highest hardness was achieved by direct tempering of the as-built material.

Selective laser melting of AlCu-TiB2 alloy using pulsed wave laser emission mode: processability, microstructure and mechanical properties

The present work explores the processability of an advanced aluminum alloy, namely AlCu-TiB2, by selective laser melting (SLM), and the correlation between microstructure and mechanical properties of as built specimens. The principal process parameters of the laser emission in pulsed wave (PW) mode, namely laser power and exposure time, were varied to achieve full density parts starting from prealloyed powders. Detailed analysis of defects in the highest relative density conditions was performed thzrough X-ray computed tomography. It was found that the highest relative density, up to 99.5%, was achieved at a laser energy density of 82 J/mm3. Microstructure of as-built specimens was analyzed through scanning electron microscopy, coupled with EDX and EBSD analyses, and transmission electron microscopy. The as-built microstructure was considerably uniform, and characterized by fine equiaxed grains, decorated with micrometric and nanometric second phases and an even distribution of reinforcing TiB2 particles; no epitaxial grain growth was observed. Finally, tensile behaviour of the as-built specimens indicates a yield and ultimate tensile stress of 325 MPa and 395 MPa, respectively, combined with an elongation of 13%. The low work hardening rate and jerky flow characterizing plastic deformation are analyzed and discussed.

Effect of input powder attributes on optimized processing and as-built tensile properties in laser powder bed fusion of AlSi10Mg alloy

A systematic material characterization approach is presented to demonstrate the critical impact of input powder attributes on the optimized parameters and the as-built properties in the Laser Powder Bed Fusion (LPBF) of AlSi10Mg alloy. Two batches of powders (conventional and specialty) with widely different attributes, including particles’ morphology, size distribution, internal oxidation, surface chemistry and roughness, were separately investigated for the optimized LPBF parameters. It is showed that the specialty powder with a spherical morphology and a larger mean particle size exhibits a higher flowability, as well as a higher apparent density. The latter, together with a smoother surface topography in the specialty powder particles (thus a lower laser absorptivity), necessitates a higher volumetric energy density (VED) requirement for a complete fusion during the LPBF. The as-built microstructures which resulted from both powders show a similar evolution of total porosity content, melt-pool boundaries and solidification structure, both qualitatively and quantitatively, which is consistent with their comparable YS values. The specialty powder, however, yields a higher UTS and %El in the as-built state, which is believed to be due to a scarce presence of O-rich particles (which otherwise preside primarily within the intercellular regions, as is the case for the conventional powder). The emergence of such O-rich particles in the as-built microstructure could be traced back to the powder source (i.e., in the form of internal O-rich phases and a thick oxide surface layer in the conventional powder). In other words, an O-lean powder translates into an O-lean printed part. One-time recycling of the specialty powder is shown to have a negligible effect on its morphological characteristics, and thus on the optimized set of parameters, consistently yielding as-built YS values similar to those of the original powder. The as-built UTS and %El values resulting from the recycled powder, however, are notably lower than those from the original powder, which are suggested to be due to the presence of contaminants and/or oxide inclusions resulting from the LPBF processing itself and/or during the recycling procedure (e.g., sieving). We believe the correlations established here between the input powder attributes and the LPBF performance of AlSi10Mg alloy can be applicable to many metallic systems.

On the variety and formation sequence of second-phase particles in nickel-based superalloys fabricated by laser powder bed fusion

We examine by transmission electron microscopy the precipitation state in the as-built microstructure of a hot-cracking sensitive nickel-based superalloy fabricated by laser powder bed fusion. Most observations are carried out on carbon extraction replica to isolate the precipitate signals from those of the matrix. Chemical maps measured by energy dispersive X-ray spectrometry are compared to phase and orientation maps obtained on the same precipitates by automated crystal orientation mapping. The chemical analyses are completed by electron energy-loss spectrometry measurements made on thin foils. The large fields of view investigated allow for achieving a representative picture of the second-phase particles in the as-built microstructure. A large variety of nano-particles is found: (Ti,Nb)(C,N) carbonitrides, (δ) Al2O3 alumina, (Cr,Mo)3B2 and (Cr,Mo)5B3 borides, and the intermetallic phase Ni7Zr2. The precipitates themselves are also complex in terms of their inner structure: carbonitrides can present a core-shell structure of composition and are frequently twinned, alumina particles serve as nucleation sites for carbonitride aggregates, and several crystallographic forms of borides may coexist next to each other. These observations provide the necessary information to retrace the solidification path of the microstructure during fabrication and to discuss the precipitation mechanisms under the extreme processing conditions associated with laser powder bed fusion.

Atom probe tomography study of κ-phases in additively manufactured nickel aluminum bronze in as-built and heat-treated conditions

The distribution, morphology, and chemical composition of various κ-phases present in the wire-arc additive manufactured (WAAM) nickel aluminum bronze (NAB) of nominal composition Cu-9Al-4Ni-4Fe-1Mn (wt%) was investigated in the as-built and three different heat-treated conditions (350 °C for 2 h, 550 °C for 4 h, and 675 °C for 6 h). The precipitation of intermetallic κ-phases plays a crucial role in determining the mechanical and corrosion properties. The microstructural changes were analyzed using scanning electron microscopy (SEM), transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS), and complemented with atom probe tomography (APT) at the nano-scale. The as-built microstructure consists of copper-rich α, κII (globular Fe3Al) and κIII (lamellar NiAl) phases in the interdendritic regions, and nano-scale Fe-rich κIV particles (5–10 nm) in the Cu-matrix. Heat-treatment at 350 °C for 2 h (HT-1) has not produced any significant microstructural changes. When heat-treated at 550 °C for 4 h (HT-2), a new-phase needle-like κv (NiAl based) was formed, which differs from other κ-phases in morphology. After HT-2, globular κII was coarsened, lamellar κIII was partially spheroidized, and κIV precipitation in the matrix was reduced. Under conditions of 675 °C for 6 h (HT-3), globular κII and needle-like κv were coarsened, lamellar κIII was completely spheroidized, and the amount of κIV was significantly reduced.

Micro-cracking, microstructure and mechanical properties of Hastelloy-X alloy printed by laser powder bed fusion: As-built, annealed and hot-isostatic pressed

This study analyses literature data to identify optimised print parameters and assesses the consolidation, microstructure, and mechanical properties of Hastelloy-X printed by laser powder bed fusion. Effects of post annealing and hot-isostatic pressing (HIP) on the microstructure and mechanical properties are also revealed. The susceptibility to the solidification cracking and the as-built microstructure such as precipitation and chemical segregation were predicted by the calculation of thermodynamics phase diagrams. The distribution of solidification cracks throughout the builds was quantified for the as-built, annealed and HIP conditions. The assessment reveals the variation of crack density towards the bottom, top and free surface of solid builds. This distribution of cracks is found to be associate with the thermal gradient and thermal conductivity which were estimated by analytical thermal calculations. While the annealing and HIP both can alter the as-printed microstructure thanks to recovery and recrystallisation, the micro-cracks and pores were only successfully removed by the HIP. In addition to the removal, recrystallisation and precipitation in the HIP (stronger than in annealing), resulting in optimal mechanical properties including a substantial increase in elongation from 13% to 20%, significant improvement of ultimate tensile stress from 965 MPa to 1045 MPa with moderately high yield stress thanks to precipitation.

Microstructure and mechanical properties of Haynes 282 superalloy produced by laser powder bed fusion

Ni-base superalloys are essential materials for high-temperature applications in the energy and aerospace sectors. Significant benefits in design, function, and manufacture of high-temperature components may be realized from additive manufacturing (AM) of these materials. However, because of cracking issues during AM fabrication, only a handful of materials have been tried and qualified. This article provides an initial evaluation of the processability and properties of Haynes 282 by laser-powder bed fusion (LPBF), which is a relatively new Ni-base superalloy with properties superior to those of many legacy wrought superalloys. The results demonstrated that crack-free Haynes 282 can be manufactured by means of LPBF with full density. The mechanical properties at ambient temperature exceeded the properties of the reference material in the as-built and heat-treated conditions, albeit with significant anisotropy. Mechanical properties at 800 °C indicated that the yield strength of heat-treated Haynes 282 by LPBF was comparable to that of the reference material, however, ductility was significantly reduced. Promising stress rupture performance also indicates that Haynes 282 is an ideal candidate for adoption in additive manufacturing, especially if heat treatments can be re-designed for the additively manufactured as-built microstructure.

Effect of solutionizing temperature on the microstructural evolution during double aging of powder bed fusion-additive manufactured IN718 alloy

Quantitative microstructural analysis was carried out to determine the size and volume fraction of strengthening phases: gamma double prime (γ″) and gamma prime (γ′); in additively manufactured (AM) Inconel 718 (IN718) with an emphasis on the utilization of Nb for γ″ formation during aging treatment. Powder bed fusion AM (PBF-AM) technique was used to generate coupons using indigenously synthesized IN718 alloy powder. Microstructural studies using transmission electron microscope (TEM) and small angle x-ray scattering (SAXS) were carried out on powder bed fusion-additive manufactured (PBF-AM) coupons in as-built condition (as processed) and in three different heat-treated conditions: solutionizing at three different temperatures (980, 1030 and 1080 °C) followed by double aging treatment. As-built microstructure consists of dendritic structure accompanied with Nb segregation along the interdendritic regions resulting in the formation of a Nb rich interdendritic phases dispersed within the interdendritic Nb-rich γ matrix. The inhomogeneous Nb distribution resulted in preferential precipitation of coarse δ during solutionization at 980 °C and further coarse γ″during direct aging and aging subsequent to solutionizing at 1030 °C. As-cast structure and related elemental segregation got modified during solutionization at 1080 °C and resulted homogeneous γ″ precipitation. Though the average size of γ″precipitates remained same in all aged samples, volume fraction increased with increasing solutionization temperature. SAXS studies were carried out on PBF-AM as well as conventionally produced wrought samples with the similar heat treatment conditions and confirmed that PBF-AM built IN718 aged samples solutionized at 1030 °C and 1080 °C resulted in γ″ precipitates with average precipitate size and volume fraction similar to that of the wrought-IN718 sample solutionized at 1080 °C.

Nanometer-scale precipitations in a selective electron beam melted nickel-based superalloy

The nanometer-scale (nano-scale) microstructural evolution in an additively manufactured Re-free Ni-based superalloy, with single crystal compositions, is investigated through field emission scanning electron microscopy, transmission electron microscopy, and atom probe tomography. We find that nano-scale primary γ′ precipitation occurs in the fine as-built microstructure, leading to an exceptional microhardness of 480.0 ± 6.7 HV at room temperature. Presence of ultra-fine γ′ precipitates, ~ 20 nm in size, is observed in the bottom few layers of the as-built samples, which is hitherto undocumented and contrary to the widespread consensus regarding hierarchical γ′ phase evolution in additively manufactured Ni-based superalloys. Moreover, considerable precipitation of tantalum-rich C14 Laves phase at the grain boundaries and interdendritic regions in the as-built samples emphasizes the need for additive manufacturing specific alloy design.

Cracking mechanism and its sensitivity to processing conditions during laser powder bed fusion of a structural aluminum alloy

In this paper, an analysis of the hot cracking susceptibility as a function of processing parameters is presented for the structural aluminum alloy 6061 (Al-0.8Si-1.2Mg wt%) processed by laser powder bed fusion (L-PBF). The hot cracking mechanism is identified as solidification cracking on the basis of experimental observations in as-built microstructures. In agreement with previous works, cracks are found to occur at high angle grain boundaries and are preferentially located at the center of the melt pools. Using the Rappaz-Drezet-Gremaud model combined with Rosenthal simulations, we show that the location of hot cracks corresponds to the regions of highest mechanical solicitation during solidification. Hot cracking sensitivity maps are then developed to predict in a simple manner the variations of hot cracking susceptibility as a function of the first order process parameters, namely the laser power and scanning speed and as well for the preheating conditions. The predicted trends are qualitatively in agreement with the experimental observation. The results allow the impact of processing conditions on reducing hot cracking to be discussed.

Directed energy deposition additive manufacturing of a Sc/Zr-modified Al–Mg alloy: Effect of thermal history on microstructural evolution and mechanical properties

Recently, laser additive manufacturing (LAM) of Al–Mg–Sc–Zr alloys has been considered an important method for developing a new generation of high-performance Al alloys. However, one of the key differences between LAM and conventional casting/wrought processes is that it has a unique combination of rapid solidification and in-situ solid-state transformation induced by repeated thermal cycling during deposition. In addition, owing to its near-net shape characteristics, subsequent plastic deformation processing is typically not an option to improve the as-built microstructure. To gain insight into the effect of the complex thermal history during LAM on the microstructural evolution and mechanical properties of the novel alloy systems, an Al–Mg–Sc–Zr alloy was processed via laser-directed energy deposition (L-DED) using a substrate with air cooling (AC) or water cooling (WC). The results show that the solidification microstructure was closely related to the dynamic solidification conditions in the molten pool, which promoted the transition from an equiaxed grain structure at low cooling rate (AC) to a heterogeneous grain structure at high cooling rate (WC). Furthermore, in-situ precipitation occurred during the repeated high-temperature thermal cycling in the AC sample, while it was effectively suppressed in the WC sample by lowering the peak temperature during thermal cycling. After aging, the yield strength of the WC sample was enhanced to ~2 times that of the AC sample, while the uniform elongation was still comparable to that of the AC sample. To achieve good mechanical properties for L-DED-processed Al–Mg–Sc–Zr alloys, it is crucial to conduct integrated control of the thermal history during both rapid solidification and in-situ precipitation.

On the microstructure and solidification behavior of new generation additively manufactured Al-Cu-Mg-Ag-Ti-B alloys

Abstract Laser powder bed fusion (LPBF) was used to horizontally print A205 ( Al — Cu — Mg — Ag — Ti — B ) metal powder. The samples have been cut in various directions to analyze their phases, grain morphology, size, crystallographic orientation, and distribution of elements via X-ray diffraction (XRD), electron backscatter diffraction (EBSD), and X-ray energy dispersive spectroscopy (XEDS) in a transmission electron microscope (TEM). To monitor deformation behavior and strain distribution, several uniaxial tensile tests accompanied by digital image correlation (DIC) technique were carried out. The results showed that the as-built microstructure fully contains fine equiaxed grains with no strong preferential crystallographic orientation. The XEDS elemental maps confirmed the solute trapping phenomenon taking place during the LPBF. The stress-strain curves obtained by the uniaxial tensile testing revealed a quasi-isotropic behavior in terms of building direction; moreover, the DIC strain contour maps confirmed yield-point phenomenon and Luders bands propagation during uniaxial tensile testing. Finally, a time/temperature interdependence grain growth model was successfully applied based on no diffusion in solid and partial mixing in liquid through the entire melt pool.

Effect of post heat-treatment on the microstructure and high-temperature oxidation behavior of precipitation hardened IN738LC superalloy fabricated by selective laser melting

The present study investigated the effect of as-built and post heat-treated microstructures of IN738LC alloy fabricated via selective laser melting process on high temperature oxidation behavior. The as-built microstructure showed fine cell and columnar structure due to high cooling rate. Ti element segregation was observed in inter-cell/inter-columnar area. After post heat-treatment, the initially-observed cell structure disappeared, instead bimodal Ni3(Al, Ti) particles formed. High temperature (1273 K and 1373 K) oxidation test results showed parabolic oxidation curves regardless of temperature and initial microstructure. The as-built IN738LC fabricated via the selective laser melting process displayed oxidation resistance similar to or slightly better than that of IN738LC fabricated via wrought or cast process. Heat-treated SLM IN738LC, although had similar oxidation weight-gain values to those of the SLM as-built material at 1273 K, showed relatively better oxidation resistance at 1373 K. Bimodal Ni3(Al, Ti) precipitate formed in the post heat treatment changed the local chemical composition, thereby led to changes in alumina former/chromia former location and fraction on the alloy surface. It was concluded that in heat-treated IN738LC increased alumina former fraction was found, and this resulted in excellent oxidation resistance and relatively low weight-gain.

Microstructural and Tensile Properties Anisotropy of Selective Laser Melting Manufactured IN 625.

The present study was focused on the assessment of microstructural anisotropy of IN 625 manufactured by selective laser melting (SLM) and its influence on the material’s room temperature tensile properties. Microstructural anisotropy was assessed based on computational and experimental investigations. Tensile specimens were manufactured using four building orientations (along Z, X, Y-axis, and tilted at 45° in the XZ plane) and three different scanning strategies (90°, 67°, and 45°). The simulation of microstructure development in specimens built along the Z-axis, applying all three scanning strategies, showed that the as-built microstructure is strongly textured and is influenced by the scanning strategy. The 45° scanning strategy induced the highest microstructural texture from all scanning strategies used. The monotonic tensile test results highlighted that the material exhibits significant anisotropic properties, depending on both the specimen orientation and the scanning strategy. Regardless of the scanning strategy used, the lowest mechanical performances of IN 625, in terms of strength values, were recorded for specimens built in the vertical position, as compared with all the other orientations.

Effect of heat treatment on the microstructure and elevated temperature tensile properties of Rene 41 alloy produced by laser powder bed fusion

The microstructure and elevated temperature mechanical properties of a precipitation hardenable Nickel based superalloy, Rene 41, fabricated by laser powder bed fusion followed by two different heat treatment regimes were studied. The as-built (AB) microstructure consists of γ columnar grains aligned in <100> direction. No γ′ precipitates were observed in the AB condition. Following to a sub-solvus solutionizing and aging heat treatment, the AB grain morphology was maintained. The development of fine γ′ precipitates within the grains along with the discrete carbide particles on the grain boundaries occurred. Heat treatment above the solvus temperature of γ’ resulted in the formation of a random equiaxed grain morphology. The γ′ and carbide precipitation was also observed in this heat treatment regime, but their distribution and morphology were different. Uniaxial tensile tests were conducted at 760 °C. Average yield strength values for AB, sub-solvus and super-solvus heat treated alloys were 879, 824 and 855 MPa respectively. The three tested conditions showed similar strength values that are comparable with a wrought alloy at the testing temperature. However, the elongation and deformation behaviors were different for each condition. Sub-solvus heat treatment lead to the highest elongation at break with 22% whereas super-solvus heat treatment resulted in the highest work hardening rate during deformation.