Post-fabrication Heat Treatment Research Articles

Additively manufactured low-gradient interfacial heterostructured medium-entropy alloy multilayers with superior strength and ductility synergy

Metallic additive manufacturing (AM) offers near net-shape fabrication but often results in insufficient strength and/or ductility due to voids or cracks, coarse initial crystalline microstructure, insufficient volume fraction of precipitates and yet post-AM strengthening/toughening methods non-destructive to shape are generally lacking. Here, we report a laser-directed energy deposited (L-DED) medium entropy alloy (MEA) with a heterostructure (HS) comprising alternate layers of solid-solution and intermetallic-dispersed MEA that possess ultimate tensile strength of 1132.8 MPa and elongation of 50.6 %, corresponding to strength-ductility synergy higher than other medium- or high-entropy alloys reported. The multilayers were produced from a dual source of CoCrNi MEA powder and a powder mixture of the same MEA, Al and Ti with stoichiometry (CoCrNi)86Al7Ti7, and the remarkable strength-ductility synergy is achieved only after post-L-DED heat treatment, which causes Al and Ti to diffuse across the layers to form a low-gradient HS. The significant back stress due to the HS contributes to the high strength, while the high ductility results from a high strain-hardening rate suppressing necking. This study demonstrates the concept of using AM not just as a near net-shape fabrication method but also a unique tool to produce low-gradient HSs with superb mechanical properties, via the use of multiple powder sources combined with suitable shape-preserving post-fabrication heat treatment.

Microstructure manipulation via machining and heat treatments in hybrid manufacturing of 316L stainless steel

Hybrid manufacturing combines additive and subtractive (machining) processes in a single platform to reduce the total time to fabricate a component in its final form. Here we show that if sufficient plastic deformation can be imparted to the material during machining, recrystallization can be triggered during post-fabrication heat treatment. This can enable highly localized microstructure control. We show that stainless steel 316L, when machined without coolant accumulates significant plastic strain compared to when machined with coolant that results in faster recrystallization kinetics during heat treatment. These effects are limited to the surface while the bulk microstructure remains unaffected.

Influence of Heat Treatment Parameters on the Microstructure of 17-4 PH Single Tracks Fabricated by Direct Energy Deposition

Post-fabrication heat treatment (PFHT) is one of the most applied strategies for achieving the desired microstructure and mechanical resistance on additive manufactured components because of the non-equilibrium microstructural state of the material in the as-built condition. In particular, during PFHT, 17-4 PH martensitic stainless steel is mainly strengthened by the precipitation of Cu-rich nanometric particles and Nb carbides into the metal matrix. In this work, the influence of specifically designed PFHTs on the microstructural and mechanical properties of 17-4 PH single tracks fabricated via direct energy deposition was studied. Different solubilization and aging times, as well as a direct aging strategy, were considered. Optical microscopy, X-ray diffractometry, and transmission electron microscopy were used to investigate the microstructure evolution induced by the PFHTs. Moreover, Vickers microhardness measurements were performed to evaluate the increase in mechanical strength. In all cases, the heat-treated single tracks showed a mean microhardness higher than that of the depositions in the as-built condition. In the single tracks subjected to solution treatment, followed by aging for about 100 h, the presence of both Cu-rich precipitates and Nb carbides was assessed; conversely, when directly aged from the as-built condition, only Nb carbides were detected. In the latter case, the carbides were finer and closer to each other than those in the single tracks aged after the solution treatment.

Influence of heat treatment on the tensile properties of Ti–15Mo additively manufactured by laser metal deposition

A bidirectional powder deposition strategy was employed to additively manufacture Ti–15Mo wt% using laser metal deposition. The as-built alloy was subsequently subject to post-fabrication heat treatment. Microstructural characterisation was conducted in the heat-treated state and also after uniaxial tensile deformation. X-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy were employed in the heat-treated state. Electron backscatter diffraction was used in investigating the deformed microstructure. Columnar β grain refinement was achieved by fragmentation from a combined contribution from precipitated phases and deformation induced products. The three distinct microstructural zones, namely the fusion, remelted and heat affected zones, observed in each deposited layer of the as-built microstructure were retained after sub-β-solvus heat treatment but completely erased in the super-β-solvus microstructure. Accommodation of plastic deformation in β matrix was by a combination of slip and primary α′′ martensite which formed preferentially at grain boundaries. Elastic modulus decreased from 86.85 ± 0.45 GPa in the as-built alloy to 72.8 ± 0.65 GPa after heat treatment. Ultimate tensile strength of 1168± 1.12 MPa from the heat-treated sample represents only a marginal increase from that of the as-built sample of 1099± 2.3 MPa. This was accompanied by a small decrease in total elongation.

Ageing response and strengthening mechanisms in a new Al-Mn-Ni-Cu-Zr alloy designed for laser powder bed fusion

Aluminum alloys designed for laser powder bed fusion (L-PBF) often show a bimodal grain structure and a strong out-of-equilibrium character with heterogeneities developing at scales ranging from the melt pool, i.e. several hundred microns, down to sub-nanometer. When subjected to post-fabrication heat treatments, microstructural evolutions arise at all scales. Herein are established the relationships between microstructure and mechanical properties at room temperature of a novel Al-4Mn-3Ni-2Cu-1Zr alloy, designed for L-PBF and subjected to direct ageing. On the basis of a multiscale microstructural study using scanning-electron microscopy (SEM), automated orientation mapping in the transmission-electron microscope (TEM-ACOM), atom probe tomography (APT), and synchrotron small-angle X-ray scattering (SAXS), we discuss and weigh the role of multiple strengthening mechanisms to the high strength of the material. In the stress-relieved conditions (300 °C/4 h), the yield strength is about 320 MPa and solid solution strengthening accounts for nearly two third of the yield strength (∼200 MPa) thanks to a very high content of Mn retained in solid solution (> 1.5 at.%) and, to a lesser extent, grain boundary strengthening. After ageing at 400 °C/1 h, the yield strength reaches 410 MPa. The additional contribution is brought by precipitation strengthening by L12-ordered Al3Zr, and to a lesser extent, Mn-rich precipitates. The composite effect due to the large fraction of relatively fine (< 1 μm) intermetallic particles (∼20%) is highlighted and cannot be neglected. This work provides guidelines to further optimize the mechanical properties and thermal stability of Al-alloys designed for L-PBF.

Effect of Post-Fabrication Heat Treatments on the Microstructure of WC-12Co Direct Energy Depositions

Laser-Directed Energy Deposition (L-DED) is an additive manufacturing technique that has lately been employed to deposit coatings of cemented carbides, such as WC-Co. During deposition, complex microstructural phenomena usually occur, strongly affecting the microstructural and mechanical behavior of the coatings. Post-fabrication heat treatments (PFHTs) may be applied to homogenize and strengthen the microstructure; nevertheless, to the best of the authors’ knowledge, just a few papers deepened the effect of these treatments on cemented carbides fabricated by additive manufacturing. This work evaluates the influence of four PFHTs on the microstructural evolution and hardness of L-DED WC-12Co. For each treatment, different combinations of solubilization time and temperature (between 30 and 180 min and from 400 °C to 700 °C, respectively) were adopted. The microstructure was investigated by optical and scanning electron microscopy equipped with energy-dispersive spectroscopy, whereas the mechanical properties were determined by Vickers hardness measurements. Based on the results, high microstructural heterogeneity in terms of WC particles, η-phase structures, and Co distribution was observed in the sample in the as-built condition. Some cracking defects were also observed in the samples, irrespective of the heat treatment conditions. Finally, a finer microstructure and a lower amount of brittle ternary η-phase, together with an increase in hardness (1030 ± 95 HV10), were found for the highest dwelling times (180 min) and for solubilization temperatures in the range of 500–600 °C.

Investigation of fatigue behavior of laser powder bed fusion Ti-6Al-4V: Roles of heat treatment and microstructure

High cycle fatigue (HCF) performance of additive manufactured (AM) titanium products is of great significance for its structural and functional applications. However, tying fatigue performance to the complex microstructures in AM titanium alloys is challenging. The work here carried out a thorough investigation into the influence of microstructures (particularly the grain boundary α-phase (GB-α) with varied morphologies) on the fatigue performance of laser powder bed fusion (LPBF) Ti–6Al–4V (Ti-64). Results showed that the improvement in the high-cycle fatigue life of LPBF Ti-64 could be achieved by the formation of low aspect ratio α lath and discontinuous GB-α via optimized post-fabrication heat treatment. Discontinuous GB-α could fully accommodate the deformation, improving the fatigue crack propagation resistance. Moreover, α lath with a low aspect ratio could lead to less strain accumulation on the interface between adjacent α lath, and thereby inhibit the crack initiation at these interfaces. This study enhances the understanding of how LPBF-induced complex microstructures influence fatigue behavior, and provides a pathway for the improvement of fatigue performance of additive manufactured titanium alloy.

Local characterization of stainless steel 17-4PH produced by material extrusion additive manufacturing: Influence of the post-treatment

This study focuses on the influence of porosities present in the stainless steel 17-4 PH made by material extrusion additive manufacturing (MEAM) process. A comparison is made at the sub-micron scale between an as-fabricated sample and a H900 post-fabrication heat treatment sample. To achieve such observations, gold nanoparticles (NPs) are used as markers in order to capture the evolution of the material behavior during an in-situ mechanical tensile test under a scanning electron microscope. The obtained strain maps allow to evidence strain concentrations and their evolutions regarding the microstructure and the applied post-treatment. The heat-treated sample reveals a change in size and number of porosities. The displacements of NPs during the test highlight the quantitative evolution of the crack initiations and the porosities growth, exposing a more homogenized strain evolution for the H900 sample compared to the as-fabricated one.

Reprint of: Bi-metal structures fabricated by extrusion-based sintering-assisted additive manufacturing

The manufacturing of bi-metal structure has received much interest because it provides compensated properties for a single metal alloy, meeting the harsh requirements for components used in key industries like aerospace, defense, energy, etc. Additive manufacturing (AM) has been extensively applied in building multi-material structures because of its ability to vary material type and composition in a layer-by-layer mode. However, fusion-based AM technologies usually induce interfacial cracks and delamination resulted from the large mismatch of coefficient of thermal expansion (CTE) between dissimilar materials, while the solid-state AM methods generate a large number of pores at the interface, requiring post-fabrication heat treatment. In this study, we aim to build high-quality 17–4 PH stainless steel and nickel alloy bi-metal structure with material extrusion AM method, followed by debinding and sintering processes. The microstructure and mechanical properties of the bi-metal structure were thoroughly investigated. It was revealed that small pores were distributed in the whole part, and no brittle intermetallic phase but slightly larger pores were formed at the interface. The material transition zone was relatively small in thickness, exhibiting low bonding strength and low hardness but a ductile deformation behavior. The completion of this study provides a pioneering analysis of bi-metal structures built by extrusion-based sintering-assisted AM, and shows a great promise for further adoption of this technology in a variety of industrial applications.

Bi-metal structures fabricated by extrusion-based sintering-assisted additive manufacturing

The manufacturing of bi-metal structure has received much interest because it provides compensated properties for a single metal alloy, meeting the harsh requirements for components used in key industries like aerospace, defense, energy, etc. Additive manufacturing (AM) has been extensively applied in building multi-material structures because of its ability to vary material type and composition in a layer-by-layer mode. However, fusion-based AM technologies usually induce interfacial cracks and delamination resulted from the large mismatch of coefficient of thermal expansion (CTE) between dissimilar materials, while the solid-state AM methods generate a large number of pores at the interface, requiring post-fabrication heat treatment. In this study, we aim to build high-quality 17–4 PH stainless steel and nickel alloy bi-metal structure with material extrusion AM method, followed by debinding and sintering processes. The microstructure and mechanical properties of the bi-metal structure were thoroughly investigated. It was revealed that small pores were distributed in the whole part, and no brittle intermetallic phase but slightly larger pores were formed at the interface. The material transition zone was relatively small in thickness, exhibiting low bonding strength and low hardness but a ductile deformation behavior. The completion of this study provides a pioneering analysis of bi-metal structures built by extrusion-based sintering-assisted AM, and shows a great promise for further adoption of this technology in a variety of industrial applications.

Effect of Successive Heat Treatment on the Performance of Superconducting Radio Frequency Niobium Cavities

One of the primary sources of radio frequency residual losses leading to lower quality factor is trapped residual magnetic field during the cooldown of superconducting radio frequency cavities. It has been reported that non-uniform recrystallization of niobium cavities after the post fabrication heat treatment leads to higher flux trapping during the cooldown, and hence the lower quality factor. Here, we fabricated two 1.3 GHz single cell cavities from high purity fine grain niobium and processed with successive heat treatments in the range 800-1000 °C to measure the flux expulsion and flux trapping sensitivity. The result indicates that although flux expulsion improves with increased heat treatments, there is a noticeable difference between the flux trapping sensitivity depending on the cavity. Evaluation of microstructure maybe crucial to understand the impact of flux trapping sensitivity on cavity performance.

Processing, microstructures and mechanical response of a β-metastable Ti-14Mo alloy fabricated by Electron Beam Powder Bed Fusion

Defect-tolerant materials are good candidates to be produced by additive manufacturing as parts often suffer from the presence of various kinds of defects: internal (pores) and especially surface defects (roughness). A β-metastable binary Ti-14Mo alloy exhibiting a TWIP effect with a composition adapted to take into account constraints of powder bed additive manufacturing processes was fabricated by electron powder bed fusion (E-PBF). The processing window was identified and dense samples with a relative density > 99.9% measured by X-ray computed tomography were fabricated. Microstructures were examined using metallography, X-ray diffraction, and electron microscopy. The mechanical properties were determined using monotonous and cyclic loading-unloading tensile testing. The as-built microstructure shows a two-phase α + β microstructure with a gradient in size of the α phase along the building direction. Post-fabrication heat treatment is therefore required to achieve a β-metastable microstructure. The peculiarities and similarities with other β-metastable binary Ti-Mo alloys fabricated by the more traditional cast and wrought processing route are highlighted. The possibility to tailor the mechanical response by low-temperature ageing via precipitation of the ωiso phase is also investigated. Such Ti-alloy could be used to improve the energy absorption capacity of architected materials for which the surface-to-volume ratio is very high and thus defect tolerance is of utmost importance.

Evidence of in-situ Cu clustering as a function of laser power during laser powder bed fusion of 17–4 PH stainless steel

The use of intrinsic heat treatments to control the microstructural evolution during additive manufacturing could eliminate the need for costly post-build processing. Using atom probe microscopy and cluster search algorithms, this study investigates the degree of Cu clustering and precipitation in 17–4 precipitate hardening (PH) stainless steel fabricated by laser powder bed fusion (LPBF). It was found that LPBF samples exhibit a greater than random degree of Cu clustering, irrespective of the laser power during fabrication. It is further shown that using a higher laser power (161.5 W rather than 127.5 W) led to a higher number density of Cu clusters, Cu precipitates, and higher hardness due to the greater heat input. The observations of Cu-rich clusters and precipitates within as-printed LPBF samples and its laser power dependence are novel and show potential for inducing desired strengthening phases directly during LPBF, mitigating the need for post-fabrication heat treatments.

Lightweight Design and Additive Manufacturing of a Fatigue-Critical Automotive Component

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;Metal additive manufacturing is finding growing applicability in motorsports and high performance car sectors. Laser-Powder Bed Fusion (L-PBF) is the most developed AM technology for lightweight aluminum alloys producing near-net-shape components of complex geometry that achieve outstanding lightweight targets. A key issue in the widespread industrial acceptability of L-PBF is the structural integrity of these lightweight components when subjected to dynamic loading conditions because it requires in-depth knowledge of the fatigue behavior of L-PBF aluminum under the combined effect of stress gradients, residual stresses, surface condition and process-induced internal defects. This contribution overviews the presentation and application of an integrated design workflow that includes the following phases: i) a topological optimization phase to achieve lightweight targets, ii) a design for additive manufacturing phase supported by process simulation to optimize part production, iii) actual part fabrication in an industrial-grade L-PBF system using AlSi10Mg alloy powder followed by post fabrication heat treatment iv) production and fatigue testing of witness specimens providing relevant design data; v) fatigue testing of the parts to determine actual performance and vi) final assessment of the design know-how developed for fatigue-critical additively manufactured metal components.&lt;/div&gt;&lt;/div&gt;

Editorial: Dental Ceramics: From Science and Technology to Clinical Application

Editorial: Dental Ceramics: From Science and Technology to Clinical Application

Texture evolution during processing and post-processing of maraging steel fabricated by laser powder bed fusion

In this study, the evolution of solidification texture during LPBF of Ti-free grade 300 maraging steel, and its effect on texture development during subsequent post-fabrication heat treatments was characterized using Electron Backscatter Diffraction (EBSD). It was found that in the as-fabricated state, no texture was observed in the room temperature martensitic phase. However, the reconstructed parent austenite phase displayed a Cube texture with a minor fraction of Rotated Goss texture. During subsequent aging treatments involving two different routes, namely direct aging of the as-fabricated samples, and conventional solution treatment + aging of the as-fabricated samples, significant changes in the texture components of parent austenite were observed, whereas no changes in texture were observed in the room temperature martensitic phase. During direct aging, it was found that with an increase in the aging temperature up to 520 °C, the texture components of the parent austenite changed from Cube/Rotated Goss to Brass, whereas during the conventional solution treatment and aging cycle, interestingly a change in texture component to rotated copper was observed. The transitions in texture components have been discussed using the concepts of recrystallization and twinning in austenite during annealing and/or aging, and strain energy release maximization (SERM) theory. Furthermore, the importance of these preferred orientations on the mechanical properties was quantified using transformation potential diagrams.

Laser Powder Bed Fusion of Defect-Free NiTi Shape Memory Alloy Parts with Superior Tensile Superelasticity

Laser powder bed fusion is a promising additive manufacturing technique for the fabrication of NiTi shape memory alloy parts with complex geometries that are otherwise difficult to fabricate through traditional processing methods. The technique is particularly attractive for the biomedical applications of NiTi shape memory alloys, such as stents, implants, and dental and surgical devices, where primarily the superelastic effect is exploited. However, few additively manufactured NiTi parts have been reported to exhibit superelasticity under tension in the as-printed condition, without a post-fabrication heat treatment, due to either persistent porosity formation or brittleness from oxidation during printing, or both. In this study, NiTi parts were fabricated using laser powder bed fusion and consistently exhibited room temperature tensile superelasticity up to 6% in the as-printed condition, almost twice the maximum reported value in the literature. This was achieved by eliminating porosity and cracks through the use of optimized processing parameters, carefully tailoring the evaporation of Ni from a Ni-rich NiTi powder feedstock, and controlling the printing chamber oxygen content. Crystallographic texture analysis demonstrated that the as-printed NiTi parts had a strong preferential texture for superelasticity, a factor that needs to be carefully considered when complex shaped parts are to be subjected to combined loadings. Transmission electron microscopy investigations revealed the presence of nano-sized oxide particles and Ni-rich precipitates in the as-printed parts, which play a role in the improved superelasticity by suppressing inelastic accommodation mechanisms for martensitic transformation.

Microstructural-micromechanical correlation in an Al–Cu–Mg–Ag–TiB2 (A205) alloy: additively manufactured and cast

In this paper, the processing-structure-property correlation of a high-strength titanium-modified aluminum-copper alloy, i.e., Al–Cu–Mg–Ag–Ti–B (A205), in the laser powder-bed fusion (LPBF) and casting conditions are studied. The as-fabricated and heat-treated materials were considered for this study. As A205 is a heat-treatable aluminum alloy, the effects of different post-fabrication heat-treatment processes, (i) solution heat-treatment (SHT: 520°C for 2 h) and (ii) over-aging and stabilizing treatment (T7), were also assessed. To test the stated relationship in LPBF and cast conditions depth-sensing indentation testing and extensive structural analyses were performed. The morphology and distribution of precipitates and nanoparticles in the microstructure were analyzed using optical, scanning electron, and transmission electron microscopy. The average nanoindentation hardness at as-fabricated, SHT, and T7 conditions are 1.48 ± 0.04, 1.50 ± 0.03, and 1.56 ± 0.06 GPa for the LPBF samples and 1.34 ± 0.04, 1.52 ± 0.10, and 1.72 ± 0.09 GPa for the cast samples. Detailed microstructural analyses confirmed the presence of precipitates and reinforced particles and revealed their morphology. The fine equiaxed grains with the average grain size of 1.1 ± 0.4 μm in the as-built condition highly enhanced the mechanical properties compared to the cast counter material, where the average grain size was measured as 46.4 ± 10.5 μm. Heat treatment played a more effective role on the cast samples and the strength of the cast material exceeded the LPBF material considerably after T7 treatment. This study offers a new compositional window for the additive manufactured lightweight high-strength material categories for various applications including automotive and aerospace industries. Besides, our findings demonstrate the significance of controlling the precipitates through the post-fabrication heat treatment processes.

Notched fatigue of additive manufactured metals under axial and multiaxial loadings, part II: Data correlations and life estimations

Fatigue performance of additive manufactured (AM) metals is highly sensitive to the existence of defects. This becomes more challenging when mechanically induced stress concentrations due to notches or complexities in the geometries are also present. Multiaxial stress states are additional important consideration in designing parts for critical load carrying applications. In this work, notched uniaxial and multiaxial fatigue and cracking behavior of two commonly used AM metals (Ti-6Al-4V and 17–4 PH stainless steel) with different post fabrication heat treatments and surface roughness conditions were studied. In part I of these two-part paper series, AM notch test results were compared to AM unnotched as well as to the notched and unnotched wrought counterparts. In this second part, both crack initiation (i.e., critical plane-based method) and crack growth (i.e., fracture mechanics-based method) approaches are used for uniaxial and multiaxial data correlations and life estimations. Both approaches are shown to result in satisfactory estimations of the experimental results.

Hydrogen-assisted failure in Inconel 718 fabricated by laser powder bed fusion: The role of solidification substructure in the embrittlement

The influence of hydrogen on the mechanical behavior of Inconel 718 fabricated by laser powder-bed-fusion was investigated through a series of tensile experiments. Samples subjected to two different post-fabrication heat treatments, viz. direct aging (DA) and homogenization plus aging (HA), were tested. Detailed microstructural characterization showed that a solidification substructure including a high density of dislocations and precipitates prevails in the DA sample while the HA sample is free from such a substructure. The DA sample exhibited a comparatively higher strength, but a lower resistance to hydrogen embrittlement. By recourse to a statistical analysis of the hydrogen-assisted cracks, the severe hydrogen embrittlement of the DA sample was proven to be due to the significant portion of hydrogen-assisted intergranular cracks that occurred without the aid of slip localization. These results are discussed in terms of the changes in microstructure upon heat treatments, and their influences on the hydrogen trapping sites.