Post-processing Heat Treatment Research Articles

CT scan, EBSD and nanoindentation analysis of 3D-printed parts with post-process heat-treatment

Heat treatment is vital for improving the characteristics of Laser Powder Bed Fusion (LPBF) components. The technique has the potential to change the microstructure of the material as well as its mechanical properties, such as yield strength, hardness, and ultimate tensile strength. To avoid undesirable impacts on the microstructure, temperature, heating, and cooling rates must be precisely controlled. Several parts were printed using LPBF from Steel 316L powder and went through post-process heating. The CT scan analysis revealed that heating the 3D printed parts for 40 min at 900 °C and 950 °C increased the porosity level across the parts although the porosity then decreased after 950 °C. From 850 °C to 1050 °C, EBSD analysis resulted in inverted pole figure maps demonstrating a relative increase in grain size. ImageJ software was used to determine the actual grain size and phase, revealing a grain size growth. Furthermore, as heat treatment temperatures increased, the ferrite phase enlarged. The cellular structure and high temperatures had a major impact on mechanical characteristics. Hardness test findings revealed a decreased mechanical characteristic as heat treatment temperature rose represented by increased porosity population and grain size. To increase the mechanical properties of these materials, an effective strategy is to achieve an even distribution of micro grains while limiting the porosity population.

Multi-physics Approach to Predict Fatigue Behavior of High Strength Aluminum Alloy Repaired via Additive Friction Stir Deposition

A smooth particle hydrodynamic (SPH) simulation of an additive friction stir deposition (AFSD) repair was used to inform a multi-physics approach to predict the fatigue life of a high strength aluminum alloy. The AFSD process is a solid-state layer-by-layer additive manufacturing approach in which a hollow tool containing feedstock is used to deposit material. While an understanding of the evolving microstructures is necessary to predict material performance, the elevated temperatures and strain rates associated with severe plastic deformation processes (SPDP) make accurate collection of experimental data within AFSD difficult. Without the ability to experimentally determine material history within the AFSD process, an SPH model was employed to predict the thermomechanical history. The SPH simulation of an AFSD repair was used to inform several microstructural models to predict material history during and after processing with AFSD and a post-processing heat treatment. These microstructure models are then used to inform a mechanistic microstructure and performance model to predict the fatigue life of an AFSD repair in AA7075.

Dynamic tensile deformation behavior of AISI 316L stainless steel fabricated by laser-beam directed energy deposition

The dynamic deformation behavior of metallic materials is the key to crash-safe design in many applications, e.g., in the automotive sector. However, in the field of additive manufacturing, dynamic deformation properties have only been scarcely studied. Based on the intrinsic interrelationships of process parameters, post-process heat treatments and the resulting microstructure, the present study seeks to provide a holistic overview of the dynamic tensile deformation properties of AISI 316L stainless steel, fabricated by laser-beam directed energy deposition. Utilizing in situ digital image correlation, the local deformation behavior at varying strain rates from 100 s−1 to 1000 s−1 was unveiled. In combination with pre- and post-mortem microstructure analysis using electron backscatter diffraction and scanning transmission electron microscopy, the microstructural effects on the active deformation mechanisms were studied. The results show that twinning contributes to the overall deformation in the heat-treated condition owing to the weaker crystallographic texture. On the contrary, twinning is less prominent in the non-heat-treated condition due to its pronounced <001> texture. As a result of the differences in terms of active deformation mechanisms, superior fracture elongations are observed in heat-treated condition, which are rationalized by delayed necking due to a more homogeneous strain distribution along the gauge section. Independently from the sample condition, fractography analysis revealed a ductile mode of failure being characterized by equiaxed dimples on the fracture surfaces. Consequently, it can be concluded that AISI 316L stainless steel is excellently suited for dynamic tensile loading, although heat-treatment-depended differences have to be taken into account.

Influence of post-processing heat-treatment on the mechanical performance of AISI 410L stainless steel manufactured by the L-DED process

Additively manufactured martensitic stainless steel components can combine complex geometry with superior mechanical and corrosion performance. In this work, the mechanical performance of AISI 410L processed by laser directed energy deposition (L-DED) additive manufacturing using previously optimized parameters is assessed. Microstructure, hardness, tensile strength, and Charpy impact toughness are evaluated in the as-built and heat-treated conditions. Four heat-treatment routes are investigated: (I) austenitization and water quenched, and austenitization, water quenched, and tempered at (II) 300, (III) 450, and (IV) 600 °C, followed by air cooling. The results show that, for tempering temperatures up to 450 °C, the hardness, yield strength, and ultimate tensile strength show an increasing trend when compared with the as-built condition and reference commercial standard (annealed AISI 410) because of microstructure refinement induced by recrystallization. Tempering at 600 °C, on the other hand, enhances the ductility of the specimens, accounting for an increased deformation until fracture and superior Charpy impact toughness. In summary, this work demonstrates that, for all tested conditions, the tensile strength of the AISI 410L additively manufactured by L-DED outperforms that of the standardized commercial AISI 410 martensitic stainless steel, and that post-processing heat-treatments can be used to further enhance toughness and ductility, making it even more competitive.

Diamond-Like Carbon (DLC) and AlCrN films onto Ti-6Al-4V substrates by Laser-Powder Bed Fusion (L-PBF): Effect of substrate heat treatment and surface finish

This paper focuses on the performance of thin-film coatings onto additively manufactured Ti-6Al-4V. Specifically, because metal parts obtained by laser-powder bed fusion (L-PBF) often require post-process heat treatments and surface finishing to meet end-user specifications, we studied how the resulting changes to mechanical strength and surface roughness affect the performance of films deposited by physical or plasma-enhanced chemical vapor deposition (PVD, PE-CVD). L-PBF Ti-6Al-4V substrates were heat-treated either below or above the β-transus and finished by grinding with different grit sizes, and then were coated with PVD AlCrN or a PE-CVD DLC-based film.Scratch adhesion on harder surfaces treated below the β-transus was higher with both coating types, whilst the substrate finishing had a negligible effect. Conversely, in ball-on-disc sliding wear tests, substrate roughness had a dominant effect: rough surfaces always resulted in earlier cracking and delamination of the coatings. Substrate hardness had a minor effect only with the AlCrN film. Moreover, the DLC-based films, because of the low-friction conditions they establish through a graphitized tribofilm as well as their higher H/E ratio, survived severe contact conditions better than the stiffer AlCrN. The results were interpreted in the light of the plasticity indices of the coated systems and their tribochemical interactions.

Thermo-Mechanical Treatment for Reducing the Wear Rate of CuCrZr Tool Electrodes during Electro-Discharge Machining.

The research presented in this paper focused on optimising the process of unconventional plastic forming by hydrostatic extrusion (HE) with post-processing heat treatment of a copper alloy (CuCrZr) for electro-discharge machining (EDM) applications. The treatment was carried out in such a way as to obtain a material with an improved microstructure, characterised by a significant increase in hardness and strength while maintaining a high electrical conductivity, thus achieving the main goal of reducing electrode wear in the EDM process. As part of the research, a material with an ultrafine-grained structure was obtained with an average grain size of d2 = 320 nm and a much higher strength of UTS = 645 MPa compared to the material in the initial state (UTS = 413 MPa). The post-processing treatment (ageing) allowed us to obtain a material with a high electrical conductivity after the HE process, at 78% IACS. The electrodes made of CuCrZr subjected to HE had a reduced electrical discharge wear in relation to electrodes made of the initial material. The best results were obtained for electrodes made of the material subjected to a five-stage HE process combined with ageing at 480 °C for 1 h. The electrical discharge wear in these electrodes was reduced by more than 50% compared to electrodes made of non-deformed copper.

Improving the Impact Resistance through Annealing in PLA 3D Printed Parts

This study conducts an experimental exploration and thorough analysis of the influence of annealing on the impact resistance of PLA 3D-printed components. The investigation extends its scope to encompass the influence of printing parameters, specifically layer thickness and infill percentage. The research highlights that the impact resistance of annealed 3D printed PLA components is predominantly influenced by the infill percentage, with the highest impact energy observed at a full 100% infill. It is noticeable that the application of annealing post-processing heat treatment results in a remarkable, up to threefold, increase of the impact energy highlighting its potential efficacy as a viable technique for enhancing the mechanical integrity of PLA 3D printed products. Consequently, this study establishes annealing as a promising methodology, particularly for PLA 3D printing applications that encounter significant mechanical loads.

Part Deflection Measurements of AM-Bench IN718 3D Build Artifacts.

One of the primary barriers for adoption of additive manufacturing (AM) had been the uncertainty in the performance of AM parts due to residual stresses/strains. The rapid melting and solidification which occurs during AM processes result in high residual stresses/strains that produce significant part distortion. While efforts to mitigate residual stresses, such as post-process heat treatment, can reduce these effects, they nullify the benefits of the as-built component microstructure. Therefore, the ability to predict as-built component residual stresses and component deflection is crucial. AM-Bench seeks to provide modelers with high-fidelity data in well-characterized AM components to aid in model development and calibration. The measurements reported here are part of the 3D builds of nickel-based superalloy IN718 test objects for the CHAL-AMB2022-01-PD modeling challenges. The part deflection measurements were performed using a coordinate measurement machine after the part was partially separated from the build plate.

Effects of Paint Baking Heat Treatments on Mechanical Properties and Microstructure of Resistance Spot-Welded A5022-O and A6014-T4 Alloys

This study presents information on the behavior of paint baking (PB) after resistance spot welding of the 5- and 6xxx series aluminum alloys. The weld parameters are optimized, and the weld specimens are baked three times for 20 min at 180 °C to simulate the heat treatments required for paint baking. The mechanical properties of the samples were characterized by using the lap shear test, micro/nanoindentation hardness, and fatigue test. As the mechanical properties of the weld are affected by the characteristics of the heat-affected zone and those of the fusion zone, the microstructure of the cross-sections was also analyzed through optical and electron microscopy. The investigation of the 6xxx series welds showed that the post-processing heat treatment decreased both the strength and the toughness of the weld, which resulted from the reduced hardness of the microstructure. Additionally, the lap shear test indicated that the failure mode for the 6xxx series changed from nugget failure to partial nugget failure after the paint baking process. However, the mechanical properties of the 5xxx welds were not affected as much as the 6xxx series during baking heat treatment. The fatigue test for the 6xxx series showed a different tendency from the lap shear test. Its fatigue properties improved due to an increased elastic modulus after the heat treatment.

Photovoltaic performance of magnetron sputtered antimony selenide thin film solar cells buffered by cadmium sulfide and cadmium sulfide /zinc sulfide

Antimony selenide (Sb2Se3)-based thin-film solar cells have recently attracted worldwide attention as an abundant, low-cost, and efficient photovoltaic technology. The highest efficiencies recorded for Sb2Se3 solar cells have been obtained using cadmium sulfide (CdS) as a buffer layer. The Cd-included hybrid buffer layers could be one option to increase device efficiency through more effective usage of light. Therefore, in this work, the effect of single CdS and hybrid CdS/zinc sulfide (ZnS) buffer layers on the photovoltaic performance of Sb2Se3 thin-film solar cells has been investigated in detail. Sb2Se3 thin films have been deposited on molybdenum (Mo)-coated soda-lime glass (SLG) substrates by radio frequency magnetron sputtering technique followed by a post-heat treatment process. The morphological, and structural properties of Sb2Se3 thin films have been investigated by X-Ray Diffraction and Scanning Electron Microscopy. To compare the device performances of single CdS and hybrid CdS/ZnS buffered Sb2Se3 thin-film solar cells, SLG/Mo/Sb2Se3/CdS/ZnS/indium tin oxide (ITO) and SLG/Mo/Sb2Se3/CdS/ITO structures have been fabricated. The findings of this study have revealed a reduction in solar cells’ performance from η=3.93% for CdS buffer to η=0.13% for CdS/ZnS hybrid buffer. The change in the solar cell performance using the CdS/ZnS hybrid buffer has been discussed in detail.

Reduction of ASZ size and enhancement of mechanical properties of TLP bonded IN718 joint using post bond heat treatment process

Reduction of ASZ size and enhancement of mechanical properties of TLP bonded IN718 joint using post bond heat treatment process

On the transformation temperatures of Ti-6Al-4V: Effect of oxygen pick-up during Laser Powder Bed Fusion

Ti-6Al-4V (Ti64) parts manufactured by Laser Powder Bed Fusion (L-PBF) exhibit a fine martensitic microstructure and significant residual stresses that are detrimental for their mechanical performance. Consequently, post-processing heat treatments are necessary to relieve these internal stresses and achieve a microstructure adequate for the mechanical requirements. Given the variations in powder compositions and processing parameters, there is a large scattering of physical properties of both as-built and heat-treated parts. In this context, this study quantifies the link between interstitial contents (specifically oxygen and nitrogen) and the transition temperatures of as-built parts, using as-atomized pre-alloyed powders for reference. The martensite decomposition temperature and beta transus temperature are assessed using high-temperature X-Ray diffraction and differential scanning calorimetry for both the powder and as-built parts. It was found that the martensite that composes the as-built microstructure can be decomposed into a stable α+β structure at temperatures exceeding 490 °C. Oxygen and nitrogen pick-up occurs during the L-PBF process, leading to an elevated beta transus temperature in as-built parts relative to the original powder. This insight provides a guide for optimizing post-processing heat treatments for Ti64 as-built components.

A Review of the Mechanical Properties of 17-4PH Stainless Steel Produced by Bound Powder Extrusion

17-4PH Stainless Steel is a mechanically high-performing alloy that is widely used across chemical and mechanical processing industries. The alloy is conventionally fabricated by cast methods, but emerging additive manufacturing techniques are presently offering an economic, efficient, and environmentally friendly alternative. Bound Powder Extrusion (BPE) is a relatively new additive manufacturing technique that is used to fabricate three-dimensional, free-form components. Investigation into the mechanical properties and behavior of 17-4PH stainless steel fabricated by BPE is vital to understanding whether this technique proposes a competitive substitute to the cast alloy within industry. Published literature has investigated the as-fabricated mechanical properties, microstructure, porosity, and post-processing heat treatment of the BPE alloy, with limited comparison evident among the papers. This paper, therefore, aims to review published findings on the mechanical properties of 17-4PH stainless steel produced by additive manufacturing techniques, with a key focus on BPE. It is important to highlight that this review study focuses on the MetalXTM 3D printer, manufactured by Markforged. This printer is among the widely utilized BPE 3D printers available in the market. The key results, together with the impact of post-heat treatments, were discussed and compared to provide a more comprehensive picture of the patterns that this alloy presents in terms of its microstructure and mechanical properties. This enables the manufacture of components relative to desired material performance, improving overall functionality. A comparison of yield strength, ultimate tensile strength (UTS), Young’s modulus, ductility, and hardness was made relative to microstructure, porosity, and density of published literature for the as-fabricated and post-heat-treated states, identifying areas for further research.

In-situ Selective Laser Heat Treatment for microstructural control of additively manufactured Ti-6Al-4V

As-built Laser Powder Bed Fusion (LPBF) Ti-6Al-4V typically exhibits a fully acicular α′-martensite microstructure, and requires post-process heat treatment in order to decompose the martensite and achieve sufficient ductility. In the present study, we demonstrate a simple concept based on in-situ Selective Laser Heat Treatment (SLHT) that can effectively alter the microstructure and activate the decomposition of the α′-martensite into a lamellar α+β microstructure within a short time scale (~30s). SLHT consists of multiple rescanning of the printed part, with low energy density, triggering solid-state phase transformations. Operando X-ray diffraction has been performed on cuboid and thin wall geometries, and was augmented by thermal finite element simulations. Upon SLHT, a gradual formation of the β phase as well as an α/α′peak narrowing trend have been evidenced through X-ray diffraction, as an indication of the diffusional nature of α′-martensite decomposition. Moreover, through fine tuning of the process parameters at the final stage of SLHT, a controlled temperature evolution during cooling was achieved, leading to preservation of the β phase, a product of the decomposition, down to room temperature. Complementary microstructural characterizations via EBSD, SEM, and TEM confirm the presence of a lamellar α+β microstructure after SLHT. Our results evidence, for the first time, the fast kinetics of α′-martensite decomposition under in-situ SLHT. The approach is meant to be implemented at selected locations during the LPBF process, avoiding time-consuming post processing steps, and leading to composite-like, architected microstructures.

The effects of pore size and heat treatment on compression and corrosion behaviors of Ti–6Al–4V sheet-based gyroid implants fabricated by laser powder-bed fusion process

The mechanical performance and corrosion behavior are crucial factors for additively manufactured Ti–6Al–4V porous implants, which are influenced by implant design, fabrication method, and post-processing. In the present study, effects of geometrical parameters and heat treatment on the compression and corrosion behaviors of Ti–6Al–4V sheet-based gyroid implants fabricated by laser powder-bed fusion (LPBF) process are investigated. The results showed that an increase in the pore size (ranging from 600 to 1200 μm) did not alter the microstructure, while decreased the Young's modulus and strength of the implants. The post-processing heat treatment resulted in the α' → α + β phase transformation which influenced mechanical properties. The mechanical properties of all scaffolds were found to be comparable to those of human bone. The heat-treated implants showed superior corrosion resistance than the as-built ones. Furthermore, the corrosion resistance of both as-built and heat-treated implants improved with increasing the pore size. The LPBFed Ti–6Al–4V implants with gyroid-sheet structure can be a promising candidate for biomedical implants from both compressive and corrosion behaviors points of view.

On the Influence of Volumetric Energy Density and Inter-Layer Time on the Material Properties of Case-Hardening Steels

Case-hardening steels are gaining increasing interest in the field of laser powder bed fusion (PBF-LB/M) due to their excellent weldability. In combination with post-process carburization heat treatment, the surface properties can be improved to generate high-strength products. When manufacturing larger products by means of PBF-LB/M, the in situ heat accumulation and the altered cooling rates affect the resulting material properties. Therefore, the fabrication of larger products requires an understanding on the influencing factors that affect the material properties. This work investigates the effect of different volumetric energy densities (VED) on the resulting microstructural and mechanical properties. It is found that the hardness decreases continuously along the build direction. The gradient depends on the applied energy and is stronger for higher energy inputs due to heat accumulation and lowered cooling rates. Furthermore, countering strategies are investigated to avoid process-specific hardness reduction along the build direction. This includes a reduced number of parts within the build job as well as a modified inter-layer time (ILT) between consecutive layers of the specimen. Applying a moderate inter-layer time helps to counter process-specific overheating, which is indicated by an almost homogeneous material hardness and melt pool size along the build direction.

Heat treatments of 17-4 PH SS processed by SLM to improve its strength and biocompatibility in biomedical applications

This research investigates the impact of heat treatment on the microstructure, hardness, and corrosion resistance of 17-4 PH SS (stainless steel) processed by Selective Laser Manufacturing (SLM) in Phosphate Buffer Solution (PBS) and compared to its commercial wrought counterparts. The SLM process produced a segregated microstructure with significant variations in composition and phases. Post-process heat treatment resulted in a uniform and reproducible microstructure in the SLM samples with a significant improvement in corrosion resistance in the solubilised samples and a remarkable hardening in the solubilised and aged samples. Additionally, heat-treated SLM samples showed no relevant release of metallic elements to PBS electrolyte after 75 days of immersion, indicating its potential use as a biomaterial. The study concludes that the manufacturing process used to produce SS have a significant impact on its properties, moreover, post-build treatment improve microstructure providing uniformity which positively impact in the corrosion resistance. The electrochemical results also suggest that, after homogenization, the additively produced 17-4 PH SS shows a better behaviour in biological environment than the wrought 17-4 PH SS.

In situ laser generation of NiOX nanoparticles embedded in graphene flakes for ambient-processed hole-transport-layer-free perovskite solar cells

Carbon-based hole-transport-layer-free perovskite solar cells (HTL-free C–PSCs) have gained tremendous attention due to their low cost, ease of fabrication, low-temperature processability, and excellent long-term stability. However, HTL-free C–PSCs suffer from poor interfacial contact at the carbon/perovskite and limited hole extraction ability, thereby limiting the device's performance. Herein, an in situ one-step synthesis strategy is presented to simultaneously generate laser-induced graphene flakes (LIG) embedded with the uniformly distributed fine NiOX nanoparticles (LIG@NiOX) as the electrode for HTL-free C–PSCs. Due to the desired morphology of the LIG flakes, it enables the formation of a compact LIG@NiOX electrode without a post-heat treatment or hot-pressing process. As a result, the fully ambient-processed HTL-free C–PSCs prepared under a high relative humidity of around 50–70% based on the LIG@NiOX achieve a power conversion efficiency (PCE) of up to 14.46%, compared to a PCE of 10.36% for the PSCs based on the commercial graphite/carbon black. This is due to a remarkable improvement in the physical contact at the carbon/perovskite interface using LIG@NiOX. Moreover, the PSCs based on LIG@NiOX retained 94% of their initial PCEs after 185 days of storage in ambient air, compared to those based on the Spiro-OMeTAD/Au that only retained 78% of their initial PCEs after 84 days of storage under the same ambient condition. The laser process opens a new avenue for simultaneous forming LIG embedded with the in situ formed metal oxide nanoparticles for various applications.

Effects of aging heat treatment on microstructure and corrosion behavior of friction surfacing treated Al–Zn–Mg–Cu matrix composite

The effect of artificial aging heat treatment on the microstructure and corrosion behavior of Al–Zn–Mg–Cu matrix composite was studied. Nickel-aluminide reinforcement was fabricated in-situ during the stir casting process of Al–Zn–Mg–Cu alloy. After friction surfacing of the consumable rod of an as-cast composite on AA1050 substrate and post-processing artificial aging heat treatment at 125 °C for 18 h, significant grain growth was observed in the nickel-free sample. Moreover, the presence of nickel-aluminide reinforcement in the microstructure of composite resulted in the formation of bimodal microstructure. The presence of nickel-rich particles and the strengthening mechanism through precipitates could help increase the hardness and strength in the nickel-containing sample. After artificial aging heat treatment, the shear strength and hardness of composite containing nickel-aluminide increased by 24% and 13%, respectively, more than those of the aluminum matrix. Before aging heat treatment, the pitting corrosion resistance was significantly improved by the presence of nickel-containing particles. After aging heat treatment due to the adsorption of more significant amount of soluble copper by nickel-rich particles in the nickel-containing sample, corrosion resistance decreased by 39% compared to that of the nickel-free sample.

Gamma Prime Characterization in Additively Manufactured Haynes 282 after One-Step and Two-Step Post-Process Heat Treatments.

Gamma Prime Characterization in Additively Manufactured Haynes 282 after One-Step and Two-Step Post-Process Heat Treatments.