Aging Heat Treatment Research Articles

Role of Cr in Mn-rich precipitates for Al–Mn–Cr–Zr-based alloys tailored for additive manufacturing

Novel alloy concepts enabled via additive manufacturing processes have opened up the possibility of tailoring properties beyond the scope of conventional casting and powder metallurgy processes. The authors have previously presented a novel Al–Mn–Cr–Zr-based alloy system containing three times the equilibrium amounts of Mn and Zr. The alloys were produced via a powder bed fusion-laser beam (PBF-LB) process taking advantage of rapid cooling and solidification characteristics of the process. This supersaturation can then be leveraged to provide high precipitation hardening via direct ageing heat treatments. The hardening is enabled with Zr-rich and Mn-rich precipitates. Literature study confirms that Mn-rich precipitates have a notable solubility of Cr, for example, the Al12Mn precipitate. This study aims to clarify the effect of Cr solubility in the thermodynamics and kinetics simulation and compare the precipitation simulations with samples subject to >1000 h isothermal heat treatment, thus creating an equilibrium-like state. The results show that Cr addition to the precipitates stabilizes the Al12Mn precipitate while slowing the precipitation kinetics thus producing a favourable hardening response. Such observations could be insightful while designing such alloys and optimising heat treatments of the current or even a future alloy system.

Direct artificial aging of the PBF-LB AlSi10Mg alloy designed to enhance the trade-off between strength and residual stress relief

The AlSi10Mg alloy is one of the most studied alloys processed by the Powder Bed Fusion-Laser Beam (PBF-LB). Many already published works focus on post-process heat treatments to reduce residual stress or improve mechanical strength. Instead, the present study aims to identify direct artificial aging (AA) heat treatment able to optimize both aspects, thus enhancing the trade-off between strengthening and residual stress relief for the PBF-LB AlSi10Mg alloy produced using a no-heated platform. Higher temperatures (190–240 °C) than those typically used in AA heat treatment were selected based on thermal analysis to benefit both residual stress relief and precipitation of reinforcing phases from the supersaturated solid solution of the metastable as-built alloy. The effects of AA heat treatment on mechanical properties (i.e. hardness) and residual stress were evaluated by plotting aging curves and by XRD and Raman analyses and demonstrated that different trade-offs between strengthening and stress relief can be achieved by tuning heat treatment parameters (temperature and time). In particular, the exposure at the lowest temperature (190 °C) induced a partial decrease in residual stress and a slight increase in hardness. By increasing heat treatment temperature and soaking time, the relief was more significant, whilst the decrease in hardness was rather limited. The results are supported by the microstructural changes observed on the samples due to the different heat treatment conditions applied and show the feasibility of designing an AA heat treatment for the PBF-LB AlSi10Mg alloy capable of satisfying the mechanical response required by the final application.

Tailoring precipitate distribution in 2024 aluminum alloy for improving strength and corrosion resistance

For the traditional peak-aged (PA) AA2024 alloy, the formation of large S-phase precipitates within the grains, wide precipitate-free zones (PFZs) near the grain boundaries (GBs), and continuous distribution of grain boundary precipitates (GBPs) can be observed. As a result, the PA alloy exhibits relatively high strength but poor corrosion resistance. However, with the application of cyclic plasticity treatment, high-density 1–2 nm clusters form within the matrix, and no PFZs form near GBs. In this study, this treatment yields the optimal balance between strength–elongation characteristics and corrosion resistance. By combining cyclic plasticity and ageing heat treatment with different heating rates, the nanoscale clusters play a crucial role as heterogeneous nucleation sites, resulting in the formation of finer and higher number density of S precipitates within the matrix. Additionally, the presence of these clusters reduces the formation of GBPs and minimizes the width of PFZs. Consequently, compared to the traditional PA sample, this approach achieves a significantly higher yield strength (increased by 46 %) and ultimate tensile strength (increased by 18 %), along with superior corrosion resistance. Although the influence of ageing heat treatment with different rates on mechanical properties is not significant, it notably affects the formation of GBPs and corrosion resistance. Specifically, a slower heating rate leads to an increase in the spacing between adjacent GBPs, resulting in improved corrosion resistance. In summary, cyclic strengthening, as a novel method for alloy strengthening, when combined with ageing heat treatment, modulates the distribution of S precipitates within the matrix and GBs. This optimization maximizes the effects of precipitation strengthening and breaks the inverse relationship between strength and corrosion resistance.

Effect of Aging Heat Treatment in an Al-4008 Produced by Liquid Metal Printing

In today's world, additive manufacturing (AM) is one of the most popular technologies and has the potential to revolutionize the manufacturing industry. As one of the most recent advances in this industry, liquid metal printing has a growing value in the engineering field. This study aims to evaluate the effect of two heat treatment conditions in an Al-4008 alloy produced by this technique in the microstructure and mechanical properties. It was concluded that the heat treatment (HT) enhances the Si particle coalescence and Fe-rich intermetallic compound precipitation, increasing the sample hardness significantly (50%). Density analysis showed a slight porosity decrease with HT. Tensile tests indicated heat-treated, same-directionally pulled samples exhibited brittleness compared to as-printed ones, while HT increased both yield strength (245 MPa) and ultimate tensile strength (294 MPa).

Phase Composition and Phase Transformation of Additively Manufactured Nickel Alloy 718 AM Bench Artifacts

Additive manufacturing (AM) technologies offer unprecedented design flexibility but are limited by a lack of understanding of the material microstructure formed under their extreme and transient processing conditions and its subsequent transformation during post-build processing. As part of the 2022 AM Bench Challenge, sponsored by the National Institute of Standards and Technology, this study focuses on the phase composition and phase evolution of AM nickel alloy 718, a nickel-based superalloy, to provide benchmark data essential for the validation of computational models for microstructural predictions. We employed high-energy synchrotron X-ray diffraction, in situ synchrotron X-ray scattering, as well as high-resolution transmission electron microscopy for our analyses. The study uncovers critical aspects of the microstructure in its as-built state, its transformation during homogenization, and its phase evolution during subsequent aging heat treatment. Specifically, we identified secondary phases, monitored the dissolution and coarsening of microstructural elements, and observed the formation and stability of γ’ and γ” phases. The results provide the rigorous benchmark data required to understand the atomic and microstructural transformations of AM nickel alloy 718, thereby enhancing the reliability and applicability of AM models for predicting phase evolution and mechanical properties.

A novel approach to suppression of reverted austenite in aerospace grade thick maraging steel weldments using the turbo-TIG welding process

This research examines aerospace grade-maraging steel weldments produced by the novel Turbo-TIG welding technique. The vital challenge in welding maraging steel is the development of reverted austenite in the weld region and HAZ. This reverted austenite degrades the mechanical characteristics of weldments. MDN 250 maraging steel with a 12 mm thick plate was welded at a high speed of 300 mm/min with consistent quality using the Turbo-TIG welding technique. The objective of the present work is to suppress the reverted austenite in the 12 mm thick plate of MDN 250 using the Turbo-TIG welding technique. Three different postweld heat treatments were applied, namely, (i) aging heat treatment (AT), (ii) solutionizing + aging (ST), and (iii) homogenization + solutionizing + aging (HT). The microstructure and X-ray diffraction analysis of the weld zone under AT and ST conditions demonstrate the existence of reverted austenite in the grain boundary regions. The SEM/EDS analysis of the weld zone of welded, AT, and ST conditions indicated Ni, Mo, and Ti microsegregation from the center matrix to the grain boundary regions. In contrast, neither microsegregation nor austenite reversion occurred in the HT condition. The tensile test demonstrates a maximum strength of 1797 MPa (UTS) under HT conditions. Likewise, the microhardness of the HT conditions exhibits a higher hardness of 552 HV in the fusion zone. In light of the metallurgical and mechanical characteristics of the weld, the HT heat treatments provide better weld performance in the case of Turbo-TIG welding of maraging steel grade MDN 250.

Multiscale microstructure containing nanometer-scale precipitations and stacking faults yields a high-strength Al-5Cu alloy by electron beam freeform fabrication

Electron beam freeform fabrication (EBF3) additive manufacturing technique has been successfully employed to print the non-weldable Al-5Cu thin-walled parts through an optimized set of process parameters. Post-heat treatment is conducted to engineer the desirable microstructure containing multiple nanostructures, i.e., the nanoscale θ" and θ' precipitates, stacking faulted zones and Al-Cu-Cd clusters, leading to a high-strength Al-5Cu alloy with good ductility. The grain formation mechanisms are revealed via a combinatorial simulation and experimentation. It is shown that EBF3 process can produce three different types of grain microstructures: fully equiaxed, fully columnar, and the mixture. In the layerwise melt pool solidification process, wide partially melted zone is observed to favor the formation of equiaxed grains due to its inheritance characteristic. After solution and aging heat treatment, Al-Cu-Cd nanoclusters (∼ 2 nm in Dia.) are generated, which facilitates the formation of nanoscale stacking faulted zones (∼11 nm). Meanwhile, as heterogeneous nucleation sites, Al-Cu-Cd nanoclusters effectively promote θ'-Al2Cu phase (∼ 19 - 114 nm) precipitation to enable the coexistence with fine θ"-Al3Cu precipitates (∼ 7 - 37 nm). The multiscale microstructure, especially incorporating the newly identified nanoscale stacking faulted zones, activates multiple strengthening mechanisms. Consequently, its tensile properties at room temperature reach an ultimate tensile stress of ∼ 496.5 MPa, yield strength of ∼ 435.7 MPa, and elongation of ∼ 9.6%, which are superior to other as-cast and additively manufactured Al-Cu alloys. This work offers a promising processing route to directly fabricate high-strength near-net-shape Al-Cu alloy parts for the aerospace and automotive industries.

Promoted aging precipitation behavior of TC4/GZ151K magnesium matrix composites

The precipitation behavior of TC4/GZ151K composite (Ti–6Al–4V reinforced Mg-15Gd-1Zn-0.4Zr) after solution and aging heat treatment (T6-treated) has been investigated. Quantitative measurements of precipitates revealed that nano-β′ phase in T6-treated TC4/GZ151K composite was denser and thinner than that in T6-treated GZ151K matrix, generating higher ultimate tensile strength of composite than that of matrix (410 MPa vs. 373 MPa). it was disclosed by geometric phase analysis that there were more geometrically necessary dislocations in solution-treated composite than that in the matrix, which was caused by different coefficient of thermal expansion between TC4 and Mg. High-density dislocations are beneficial to precipitate fine and dense β′ phase in the composite while low-density dislocations result in coarse and loose β′ phase in the matrix after aging process. This work confirmed the accelerating effect of TC4 particles on Mg-matrix's aging precipitation, which was of great significance to regulate the precipitation behavior of Mg alloy and produce high strength-ductile Mg materials.

Post-processing ageing treatment of friction surfaced Al-16Si alloy: experimental and theoretical studies

In this study, we investigated the impact of ageing heat treatment following friction surfacing of a solid solution-treated Al-16Si alloy on coating geometry, microstructure, and wear resistance. We employed a simulation using the smoothed-particle hydrodynamics (SPH) method alongside experimental analysis. The simulation results revealed that as the rotational speed increased from 600 to 1000 rpm, the coating efficiency decreased significantly from 31.2 to 13.1%. When comparing the coating to the substrate and the consumable rod, it was seen that after ageing heat treatment, the wear rate of the coating made at a rotational speed of 600 rpm dropped by a lot, by 70% compared to the substrate and by 53% compared to the consumable rod.

Effect of Heat Treatment Temperature on Microstructure, Tensile Properties and δ-Precipitate Phase in Ni-based Superalloy

Here, we investigated the influence of δ-precipitate (orthorhombic D0a Ni<sub>3</sub>Nb-ordered phase) on the room- and high-temperature tensile properties in wrought nickel-based Inconel 625 superalloys subjected to solution and aging heat treatment. Typically, solution heat-treatment temperatures in these alloys affect the solid-state precipitation of δ-phase, which governs high-temperature tensile properties. While precipitation of fine D0<sub>a</sub> δ-phase is known to have beneficial effects on the mechanical properties owing to the retardation of grain coarsening, Widmanstätten δ precipitation plays a deleterious influence on the fracture toughness, tensile ductility, and fatigue resistance. Therefore, to enhance the mechanical properties of this alloy series, it is key to generate a high number density of fine D0<sub>a</sub> δ precipitate by adjusting solid solution treatment temperatures. In this study, solution heat treatments were conducted above and below δ-phase solvus temperatures. By applying solution heat treatment at 900°C and 970°C, this alloy was confirmed to have a Widmanstätten δ phase and is composed similarly to the annealed microstructure. This Widmanstätten δ precipitate was densely distributed at both intergranular and intragranular grains. On the other hand, when solution treatment was applied at 1040 and 1100°C, more coarse particles (approximately 30 μm) with a significant reduction of Widmanstätten type δ phase were obtained. We found that grain size and Widmanstätten δ-phases have an important role in the high-temperature tensile properties of Inconel 625 superalloy series.

Novel Ni–Co-based superalloys with high thermal stability and specific yield stress discovered by directed energy deposition

We report on the rapid alloy screening of Ni–Co–Ti–Al–Mo superalloys with high thermal stability and specific yield stress by means of directed energy deposition. A laser directed energy deposition, a specific type of additive manufacturing, was employed using multiple powder feeders and elemental powders. Fifty superalloys of different compositions were deposited and the heat-treated microstructure and γ' solvus temperature were examined. The 43Ni–38Co–9Ti–6Al–4Mo superalloy (atomic percent composition) exhibited a uniform γ/γ' microstructure and a γ' solvus temperature of 1202°C. The beneficial properties of the superalloy were also found in the cast superalloy of identical composition. The cast superalloy exhibited a thermally stable γ/γ' microstructure with cuboidal γ' precipitates even after long-term aging heat treatments at 800°C, 900°C, and 1000°C up to 500 h. The γ'-coarsening mechanism was evaluated based on the Lifshitz–Slyozov–Wagner model and the trans-interface diffusion-controlled model. A transition between these two mechanisms was observed with an increase in aging temperature. Atom probe tomography analyses revealed that the sluggish interface diffusion of Co and corresponding reduction of the interfacial energy induced the transition of the γ'-coarsening mechanism. Moreover, the cast superalloy showed an enhanced specific yield stress attributed to its exceptionally low alloy density of 7.61 g/cm3.

Microstructure and mechanical properties of a newly developed Ti-6Al-2Mo-2Sn-2Zr-2Cr-2V alloy fabricated by laser directed energy deposition

With the rapid development of aviation industry, the study of novel titanium (Ti) alloy with superior mechanical properties is of great significance. In this paper, a newly developed Ti-6Al-2Mo-2Sn-2Zr-2Cr-2V alloy was fabricated by laser directed energy deposition (L-DED), which exhibits coarse β columnar grains containing the basket-weave structure knitted with α phases and β phases. After solution and aging heat-treatment, the fine secondary α phase precipitates, which leads to the increase in microhardness. The as-deposited sample exhibits the tensile strength of 1025 MPa and the elongation of 6.0%. With solution temperature increasing, the strength increases but the plasticity first increases and then decreases. The fracture mechanism shows a mixed of ductile and brittle fracture. The average impact toughness of as-deposited sample in building direction is 40.9 J cm−2 higher than that (23.4 J cm−2) in direction perpendicular to building direction. After solution and aging heat-treatment, the impact toughness improves significantly.

Ultrasonic fatigue tests on maraging 300 steel: Under solution annealed, after aging heat treatment and under pre-corrosion attack

Ultrasonic fatigue tests were carried out under continuous cycling on the maraging 300 steel for the following conditions: (A) solution annealed (as received from supplier), (B) after aging heat treatment of 490 °C for 6 h, (C) after pre-corrosion attack, and (D) specimens loaded at 293 MPa at room temperature without failure until 1.0E+10 cycles. The ultrasonic fatigue strength of the four modalities were compared and discussed in regard the crack initiation inclusion, the heat treatment and the testing conditions. Crack initiation and propagation under this fatigue testing modality was analyzed; revealing that ultrasonic fatigue strength is related to internal TiN-inclusions and its parameters of shape and orientation, in regard the uniaxial applied load. Numerical simulations were carried out to investigate the stress concentration of an ellipsoidal void of 150 mm (longer radius), and a TiN ellipsoidal inclusion of same dimensions. In addition, SEM (Scanning Electron Microscope) analysis was carried out on the fracture surfaces to determine the crack initiation and propagation zones.

Effect of thermomechanical training on recovery stress of Fe-based shape memory alloy

This research explores the impact of thermomechanical treatment on the recovery stress induced by the shape memory effect in an Fe-based shape memory alloy with the composition of Fe-17Mn-5Si-10Cr-4Ni-1(V,C) wt.%. Fe-based shape memory alloys have gained considerable attention for their potential applications in vibration control, energy dissipation, and structural strengthening or re-centering within the construction industry. To evaluate the recovery stress behaviour of this Fe-based shape memory alloy, specimens underwent a two-step aging heat treatment process, involving initial aging at 600 °C for 20 hours, followed by subsequent aging at 680 °C for 8 hours. Then heat-treated samples subjected to a cyclic thermomechanical treatment, involving a combination of deformation up to 2% and heating at 200 °C for 30 min in each cycle. The microstructure and mechanical properties were assessed using techniques such as scanning electron microscopy, as well as tensile tests. The results revealed that the thermomechanical treatment had a notable impact on the microstructure and mechanical properties of the Fe-based shape memory alloy. It resulted in an enhancement of the shape memory effect and recovery stress. These improvements were associated with an increase in the martensitic phase fraction. Additionally, the presence of VC precipitates in the treated samples contributed to the enhancement of the shape memory effect.

Microstructure and thermal properties of dissimilar M300-CuCr1Zr alloys by multi-material laser-based powder bed fusion

AbstractMulti-material laser-based powder bed fusion (PBF-LB) allows manufacturing of parts with 3-dimensional gradient and additional functionality in a single step. This research focuses on the combination of thermally-conductive CuCr1Zr with hard M300 tool steel. Two interface configurations of M300 on CuCr1Zr and CuCr1Zr on M300 were investigated. Ultra-fine grains form at the interface due to the low mutual solubility of Cu and steel. The material mixing zone size is dependent on the configurations and tunable in the range of 0.1–0.3 mm by introducing a separate set of parameters for the interface layers. Microcracks and pores mainly occur in the transition zone. Regardless of these defects, the thermal diffusivity of bimetallic parts with 50vol% of CuCr1Zr significantly increases by 70%–150% compared to pure M300. The thermal diffusivity of CuCr1Zr and the hardness of M300 steel can be enhanced simultaneously by applying the aging heat treatment.

Influence of aging heat treatment temperature on the mechanical properties of maraging steel 300 for application in ballistic armour

RESUMO Os aços maraging são aços especiais de ultra-alta resistência e alta tenacidade, mas, apesar de suas propriedades notáveis, suas aplicações ainda são limitadas devido ao seu alto custo de produção. O objetivo deste trabalho consiste em avaliar a relação entre a formação de precipitados e o teor de austenita revertida, correlacionando com a dureza e as alterações nas propriedades mecânicas relativas à tração do aço maraging 300 sob diferentes condições de tratamentos térmicos, para servir como parâmetro de análise no seu emprego em blindagens balísticas. Amostras de aço maraging 300 foram tratadas em 29 diferentes condições de envelhecimento e o comportamento de endurecimento por precipitação foi analisado empregando diferentes técnicas, como: ferritoscopia, ensaio de dureza nas situações como recebido e envelhecido, e ensaio de tração nas temperaturas de 440, 480 e 570 °C, a fim de verificar a influência da fase austenítica nas propriedades mecânicas. A influência do superenvelhecimento foi observada na amostra de 570 °C, resultando numa diminuição de aproximadamente 31% na resistência mecânica e um aumento em torno de 40% na ductilidade, em comparação com a amostra envelhecida a 440 °C, indicando uma melhora na tenacidade do material.

Effect of filler materials on the tensile properties and fracture toughness of laser beam welded AA2198 joints under different ageing conditions

The influence of different filler materials on the microstructure, tensile mechanical properties, and fracture toughness of laser beam welded AA2198 alloy as well as the effect of different artificial ageing heat treatments were investigated in this contribution. The welded joints were produced when exploited either, Al-Si (AA4047) or Al-Cu (AA2319) filler wires. It was shown that the Al-Si filler wire gave higher hardness values in the fusion zone when compared to the Al-Cu filler wire. The post heat treatment of the welded specimens increased by approximately +100 % the yield stress and by +20 % the ultimate tensile strength with increasing ageing time, in a similar way to the non-welded material. Elongation at fracture decreased in an inverse proportional manner to yield stress. Artificial ageing before welding gave improved elongation at fracture for the over-aged condition only. The quality index concept showed that the artificial ageing before the welding did not succeed in giving a higher quality of the welded joints, for both filler materials investigated. The opposite was shown on the post heat treatment, where the peak-aged condition increased substantially the ‘quality’ of the welded joints with both filler materials. The critical stress intensity factor was increased by +25 % for the under-aged condition for the post-welded condition and both investigated filler wires as a result of the balance between medium values in strength and ductility, respectively.

Effect of heat treatment on microstructure and wear properties of laser additively manufactured 316 L stainless steel/Co-Cr-W alloy part using directed energy deposition method

The directed energy deposition additive manufacturing was applied to fabricate hard Co-Cr-W layers on the 316 L stainless steel substrate. The effect of different heat treatment cycles was investigated. The primary microstructure of deposited layers contained a cobalt matrix with interdendritic eutectic carbides mainly M7C3, M23C6, and M6C. Most of the primary carbides were dissolved in the matrix after the solution treatment at 1250 °C for 1 h. The aging treatment promoted the precipitation of (W/Co)6C carbides. The hard carbides caused the third-body wear and increased the wear volume loss of specimens. The solution & double aging treatment provided the optimum combination of hardness and wear resistance due to the best distribution of W/Co carbide. The wear volume loss after heat treatment was decreased from about 6.6 mm3 to 2.8 mm3 compared to the as-deposited specimen. The proposed solution & double aging heat treatment improved wear resistance by 50%.

Change in Mechanical Properties of Laser Powder Bed Fused AlSi7Mg Alloy during Long-Term Exposure at Warm Operating Temperatures.

Al-Si-Mg alloys are most commonly used to produce parts by laser powder bed fusion for several industrial applications. A lot of papers have already focused on the effects induced by conventional heat treatments on the microstructure and mechanical properties of AlSi10Mg alloys, rather than on AlSi7Mg. Nobody has investigated thermal stability during long-term direct and artificial aging heat treatments of AlSi7Mg. This study investigates the changes in mechanical properties induced by long-term exposure (512 h) at 150 and 175 °C (the operating temperature of AlSi7Mg) after (i) the laser powder bed fusion process performed on a pre-heated build platform (150 °C), and (ii) heat treatments to the solution at 505 °C per 0.5 and 4 h. Thermal stability was evaluated through both Vickers microhardness measurements to obtain the aging profiles, and tensile tests to evaluate the mechanical properties in specific conditions. An optical microscope was used to investigate the microstructure. It was found that aging at 175 °C confers the same effects induced by a secondary aging heat treatment on as-built samples and, simultaneously, the worst effects on the solution heat treated AlSi7Mg alloy after long-term exposure. The AlSi7Mg DA at both 150 °C and 175 °C showed the same Vickers microhardness (~95 HV0.5), UTS (~300 MPa), and YS (~200 MPa) values for the longest exposure times because the fine and cellular α-Al matrix confers higher stiffness and strength despite the over-aged conditions. On the other hand, the coarsening effects that affected the precipitates during aging at 175 °C, as well as the formation of the precipitate-free zones along the grain boundaries, justified the highest detrimental effects induced on the SHTed samples.

Mechanism of improving corrosion resistance in heat affected zone of duplex stainless steel by tantalum addition

The effect of tantalum addition on corrosion resistance in HAZ of duplex stainless steel was investigated by pitting corrosion test and material characterizations (SEM, TEM, APT). The Ta addition improved corrosion resistance of HAZ simulating welding at low heat input and rapid cooling rate since Ta segregated in the Cr nitrides and suppressed formation of Cr-depleted zone. The Ta addition also improved corrosion resistance in HAZ subjected to isothermal aging heat treatment assuming welding at high heat input and low cooling rate because Ta promotes the formation of χ phase and delays the formation of σ phase and Cr-depleted zone.