Precipitation-hardened Alloys Research Articles

Effect of ambient conditions in friction surfacing

AbstractFriction surfacing (FS) is a solid-state deposition process in which layers are deposited on a substrate surface by frictional heat and severe plastic deformation of a consumable stud material below its melting temperature. Bonding occurs due to accelerated diffusion. The deposition of several layers on top of each other is referred to as multi-layer FS (MLFS), a promising candidate for additive manufacturing (AM) as it offers advantages over fusion-based AM. In this study, the MLFS process for the precipitation-hardenable alloy AA2024 is investigated regarding the influence of environmental process conditions, i.e., preheating of the substrate like other AM processes as well as underwater and room temperature experiments. The influence of ambient conditions on the process behavior, the layer geometries, the microstructure, and the mechanical properties is shown. Preheating the substrate leads to an overall higher process temperature (424.1 °C), resulting in thinner and wider layers, larger grains, an overaged microstructure, and a smooth hardness transition in the MLFS stacks from top (140 HV0.1) to bottom (95 HV0.1). The lower the process temperatures, e.g., for underwater FS (326.5 °C), the thicker and less wide the layers and the smaller the grains. The hardness shows a periodic pattern at the layer interface, which is more pronounced at lower process temperatures, i.e., the hardness values range from 100 HV0.1 to 150 HV0.1.

Laser powder bed fusion of high-strength crack-free Al7075 alloy with the in-situ formation of TiB2/Al3Ti-reinforced phases and nucleation agents

The existence of solidification cracks caused by columnar grains in precipitation-hardened aluminium alloys limit the applicability of Al7075 components manufactured via laser powder bed fusion (LPBF) additive manufacturing. A novel approach was developed to co-incorporate submicron-sized B and micron-grade Ti6Al4V to eliminate hot cracks and to effectively transform coarse columnar grains into fine equiaxed grains, thus improving the mechanical performance of LPBF-fabricated modified Al7075 material. The grain refinement was mainly attributable to the heterogeneous nucleation promoted by the combination of in-situ-formed L12-Al3Ti and TiB2 nano-sized phases. After an optimised T6 heat treatment, excellent comprehensive mechanical properties were achieved, with a tensile strength of 460 MPa and an elongation of 13 %. This research provides an efficient and cost-effective path for addressing crack-sensitive metallic materials used for LPBF additive manufacturing processes.

High strain rate tensile deformation of similar and dissimilar AA6082 and AA7075 friction-stir-welded blanks

Friction-stir-welding process has become an established solid-state technique for joining of dissimilar lightweight materials over the past decade by overcoming fundamental welding challenges such as solidification cracking, phase segregation and surface oxidation. Despite these unique capabilities, the resulting microstructure feature in the weld zone consisting of fine grains with a high dislocation density, challenges further heat treatment and forming processes. Hence, a high solution annealing temperature results in degradation of the adjusted microstructure and in a lower to reduced mechanical properties in case of dissimilar joints of precipitation-hardenable aluminum alloys. Subsequent hot forming therefore involves a great effort and requires further heat treatment steps. Thus, the present study investigates the effect of a recently introduced novel thermo-mechanical forming process on local deformation behavior of similar as well as dissimilar joints processed by friction-stir-welding technique of thin-walled AA6082 and AA7075 blanks. To avoid the complete elimination of the adjusted microstructure, the welding process is performed after a thermo-mechanical process consisting of solution annealing, die cooling and peak aging. Uniaxial tensile tests are then carried out at high strain rates ranging from ε˙ = 40 s-1 to ε˙ = 400 s-1. The results show an increase in yield and ultimate tensile strength as well as in total elongation after failure with increasing strain rates. As the strain rate increases, the flow stress of similar weld of AA7075 is higher than those of AA6082 and the dissimilar weld of AA6082 and AA7075. Local deformation measurements reveal higher strain localization in the welded zone for similar welds, which leads to micro-crack initiation and failure due to strain accumulation. Microstructure analysis shows very fine equiaxed grain structure in the nugget zone and homogenous distribution of precipitates after friction stir welding process, which explain the plastic deformation behavior.

Effect of over-ageing on the tensile and torsional fatigue properties of the AlSi7Cu0.5 Mg0.3 precipitation-hardened cast aluminium alloy

The objective of this work is to evaluate the influence of over-ageing on the high cycle fatigue behaviour of the AlSi7Cu0.5Mg0.3 cast aluminium alloy. It is generally accepted that over-ageing has a large influence on the behaviour of these materials, however its effect on the fatigue strength is less understood. This work provides an experimental investigation of the effect of over-ageing on high cycle fatigue strength in both tensile and torsional loading conditions. It is shown that the torsional fatigue strength is more sensitive to over-ageing when compared to the tensile fatigue strength and that the fatigue defect sensitivity is negligible for torsional loads, in contrast to tensile loading conditions. The experimental results indicate that this is due to a change in the crack initiation mechanism, going from initiation in the matrix for torsion loads to initiation from casting defects for tensile loads. An original two-steps approach is proposed to rationalise the effect of over-ageing on the fatigue strength, with particular emphasis on decoupling the respective contributions from defects and the mechanical properties. This approach is used to build normalised Kitagawa-Takahashi diagrams that highlights the reduced defect sensitivity under tensile loads for the most severe over-ageing condition.

High-throughput strategy to design high entropy alloys with an FCC matrix, L12 precipitates, and optimized yield stress

This work proposes a methodology for designing high-strength precipitation-hardened high entropy alloys (HEAs) with an FCC matrix and L12 precipitates. High-throughput solidification calculations were conducted using the CALPHAD method, evaluating 11,235 alloys in the Cr-Co-Ni-Al-Ti system under specific boundary conditions. The acquired information was used to filter the alloys, focusing on alloys exhibiting an FCC+L12 phase field at 750 °C, a solidification interval narrower than 100 °C, and a solvus temperature under 1100 °C. The filtered alloys were analyzed to estimate their solid solution and precipitation hardening contributions to yield strength, with antiphase boundary energy (APB) assessed using two models. Three alloys were selected for testing the proposed strategy, including one with the highest yield stress and others for comparison. These alloys were produced, processed, and characterized using DSC, synchrotron XRD, SEM, and TEM. The results showed that the desired microstructure was achieved, with the alloys consisting of an FCC matrix and a high-volume fraction of L12 precipitates. Tensile tests at room temperature, 650 °C, 750 °C, and 850 °C demonstrated that the proposed model predicts well the yield strength trends, demonstrating the potential of the proposed approach for accelerating the discovery and development of novel HEAs with tailored properties.

Effect of forming parameters on the corrosion performance of retrogression- and warm-formed AA7075 alloy sheets

ABSTRACT Retrogression forming (RF) and warm forming (WF) are used to remedy formability limitations of high-strength precipitation-hardened AA7075 alloy sheets. RF is a thermomechanical treatment combining retrogression heat treatment and the forming of initially peak-aged T6 temper. WF is a thermomechanical treatment of initially pre-aged temper. The present work examines the corrosion performance of retrogression- and warm-formed AA7075 alloy sheets by determining the optimal pre-aging temperature (80°C or 100°C), forming temperature (room temperature, 150–200°C), strain rate (1, 0.1, or 0.01 s−1), and heating rate (1.1 or 20°C·s−1) for conventional slow and novel fast-forming technologies. Several alternating- and direct-current electrochemical tests were administered, including electrochemical impedance spectroscopy, potentiodynamic polarization, and linear polarization resistance. Results showed that the corrosion performance (i.e., passive film and corrosion-resistant properties) of formed AA7075 sheets was directly proportional to pre-aging and WF temperatures, inversely proportional to the RF temperature, and mildly affected by strain/forming rate and heating rate during forming.

Prediction of martensitic transformation and residual stress field on 17–4 PH stainless steel fabricated by laser powder bed fusion

The 17–4 precipitation-hardened (PH) alloy can be printed into diverse microstructures. Existing reports on the alloy have focused on the effect of Creq/Nieq ratios and the processing parameters of laser powder bed fusion (LPBF) on the final microstructures of the δ-ferrite and α’-martensite phases in the alloy. However, the effect of microstructural variations on the residual stress field of LPBFed 17–4 PH stainless steel has not yet been investigated. This study bridges the gap between what is known about the microstructure and residual stresses by proposing an alloy-equivalent phase prediction diagram for the additive manufacturing of the 17–4 PH alloy by varying the Creq/Nieq ratio and volumetric energy density to control residual stresses via microstructure manipulation. Dilatometry and Satoh tests were performed to elucidate the mechanism of the offset effect in martensite transformation. To control the microstructures, samples were fabricated with various Creq/Nieq ratios (2.05, 2.15, 2.30, and 2.49) and laser powers (130, 150, 170, and 190 W). The 17–4 PH alloy exhibited a low Ms temperature and approximately net zero residual stresses in the Satoh test. The study confirmed that martensitic transformation, which occurs more readily at lower Creq/Nieq ratios and higher laser powers, effectively offsets tensile residual stresses during the LPBF process.

Flexures for Kibble balances: minimizing the effects of anelastic relaxation

We studied the anelastic aftereffect of a flexure being used in a Kibble balance, where the flexure is subjected to a large excursion in velocity mode after which a high-precision force comparison is performed. We investigated the effect of a constant and a sinusoidal excursion on the force comparison. We explored theoretically and experimentally a simple erasing procedure, i.e. bending the flexure in the opposite direction for a given amplitude and time. We found that the erasing procedure reduced the time-dependent force by about 30%. The investigation was performed with an analytical model and verified experimentally with our new Kibble balance at the National Institute of Standards and Technology employing flexures made from precipitation-hardened Copper Beryllium alloy C17200. Our experimental determination of the modulus defect of the flexure yields 1.2×10−4 . This result is about a factor of two higher than previously reported from experiments. We additionally found a static shift of the flexure’s internal equilibrium after a change in the stress and strain state. These static shifts, although measurable, are small and deemed uncritical for our Kibble balance application at present. During this investigation, we discovered magic flexures that promise to have very little anelastic relaxation. In these magic flexures, the mechanism causing anelastic relaxation is compensated for by properly shaping and loading a flexure with a non-constant cross-section in the region of bending.

Diffusion Nitride Surface Layers on Aluminum Substrates Produced by Hybrid Method Using Gas Nitriding

While gas nitriding of steel is currently used in industry, nitriding of aluminum alloys remains an open challenge. The main obstacle is aluminum’s high susceptibility to passivation. The oxide film provides an effective barrier to nitrogen diffusion. Attempts to overcome this problem have mainly focused on glow discharge nitriding using cathode sputtering of an oxide layer. The produced AlN layers exhibit no diffusion zone and show limited performance properties. In this work, the effect of hybrid treatment aimed at producing diffusion layers of nitrides other than AlN on aluminum alloys was investigated on the model system of iron nitride–aluminum substrate. Hybrid treatment combines an electrochemical process involving the removal of the aluminum oxide layer from the substrate, its subsequent iron plating, and a further gas nitriding in high-purity ammonia. The obtained results prove that the hybrid treatment allows the production, at 530 °C/10 h, of diffusion layers of Fe3N iron nitrides on aluminum substrates with a nitrogen diffusion zone range in aluminum of ca. 12 µm. In alloys containing magnesium, its unfavorable effect on the nitrogen diffusion and the functional properties of the layers was observed. An interesting direction for further research is hybrid treatment of precipitation-hardened alloys without magnesium.

Influence of the process time on a self-piercing riveting process with tumbling kinematic

The increasing significance of ecological responsibility, stricter political regulations and economic objectives are driving innovation in research fields such as lightweight construction. One of the most important popular methods is the use of multi-material systems. Due to the different geometric and mechanical properties of the various materials used, resource efficient applications and utilizations are possible. Great challenges arise for the joining processes to realize these multi-material systems, since conventional joining processes reach their limits. In the field of mechanical joining processes, there are continuously new approaches, such as superimposing the punch in a self-piercing riveting process with a tumbling kinematic, to increase the number of adaptable process parameters and enhance the process control. Through various preliminary tests, a good understanding of the process has been developed, which allows to directly control the geometric joint parameters by configuring the tumbling strategy. A major challenge, particularly with regard to future industrial applications, is the process time, which is comparatively high due to the tumbling kinematics. In the investigations, a reduction of approximately 90% of the process time is targeted by adapting the joining and tumbling strategy. Therefore, the correlation of the traverse velocity and the tumbling velocity are examined in a gradual series of experiments. To represent realistic applications, the experiments are carried out with a dual-phase steel and a precipitation-hardening aluminum alloy. For identifying the influence of the process parameters on the joining process, a constant rivet–die combination is applied. Further, the examination of force–displacement curves is conducted. Moreover, the determination of geometric joint parameters is reliant upon macrographs to assess the influence of the joining time on the geometric joint formation. The test results show that a significant increase in joining speed with a resulting reduction in process time is feasible. Although the joining properties are affected, reliable joining is possible. In particular, the shaft thickness of the rivet is influenced by the varying proportion of the tumbling process in the joining operation and increases with higher joining speeds.

Asymmetric and anisotropic quench sensitivity of ZA81M magnesium alloy

The quench sensitivity of the ZA81M magnesium alloy is studied by combining experiment and crystal plasticity finite element simulation. The quench sensitivity is high in tension and low in compression making the single-valued quench sensitivity calculated from hardness insufficient. So, there is different loss rate of strength, along different test directions of the alloy, due to slow quenching, i.e., the quench sensitivity is asymmetric and anisotropic. The coarsening of the precipitate and widening of the precipitate-free zone are the origin of the quench sensitivity. Crystal plasticity finite element simulation is proposed to calculate the quench sensitivity. The phenomenological constitutive model is used to explicitly consider the strengthening effect of precipitates on different slip and twinning systems. So, the precipitate characteristics of the water-quenched and air-cooled alloy are easily captured. The anisotropy and asymmetry of the quench sensitivity in two perpendicular sample planes are determined by simulation. Such a method can be useful for determining the quench sensitivity of precipitation-hardening alloys with low crystal symmetry and with precipitates having different strengthening efficiencies on different slip or twinning systems.

Enhanced strength-ductility combination in the aluminum-gold system by heterogeneous distribution of nanoparticles via ultra-severe plastic deformation and reactive interdiffusion

Ultrafine-grained aluminum alloys are of interest due to their high strength-to-weight ratio, but they usually suffer from poor uniform ductility. In this study, an Al-Au alloy with a good combination of strength and ductility is produced by the heterogeneous distribution of Al2Au nanoparticles in an aluminum matrix. To generate such heterogeneity, the alloy is synthesized by ultra-severe plastic deformation of aluminum and gold powders via the high-pressure torsion (HPT) method. Reactive interdiffusion occurs during the process leading to the heterogeneous formation of intermetallic particles and a good strength-ductility synergy (200 MPa yield stress and 15% uniform elongation). Nanoparticles gradually distribute within the matrix and once a uniform nanoparticle distribution is achieved, the alloy shows no further increase in strength, but it completely loses its ductility. It is concluded that not only the presence of nanoparticles but more importantly the heterogeneity of their distribution can positively influence the strength-ductility combination in ultrafine-grained aluminum alloys. The findings of this study suggest that future studies on heterogeneous precipitation hardening can be a solution to achieve ductile precipitation-hardened alloys.

An additively manufactured precipitation hardening medium entropy alloy with excellent strength-ductility synergy over a wide temperature range

Modern engineering has long been in demand for high-performance additive manufactured materials for harsh working conditions. The idea of high entropy alloy (HEA), medium entropy alloy (MEA), and multi-principal-element alloy (MPEA) provides a new way for alloy design. In this work, we develop a Co42Cr20Ni30Ti4Al4 quinary MEA which exhibits a superiority of mechanical properties over a wide temperature ranging from 77 to 873 K via selective laser melting (SLM) and post-heat treatment. The present MEA achieves an excellent ultimate tensile strength (UTS) of 1586 MPa with a total elongation (TE) of 22.7 % at 298 K, a UTS of 1944 MPa with a TE of 22.6 % at 77 K, and a UTS of 1147 MPa with a TE of 9.1 % at 873 K. The excellent mechanical properties stem from the microstructures composed of partially refined grains and heterogeneously precipitated L12 phase due to the concurrence of recrystallization and precipitation. The grain boundary hardening, precipitation hardening, and dislocation hardening contribute to the high YS at 298 and 77 K. Interactions of nano-spaced stacking faults (SFs) including SFs networks, Lomer–Cottrell locks (L–C locks), and anti-phase boundaries (APBs) induced by the shearing of L12 phase are responsible for the high strain hardening rate and plasticity at 77 K. Our work provides a new insight for the incorporation of precipitation hardening and additive manufacturing technology, paving the avenue for the development of high-performance structural materials.

Ultrasound-assisted material characterization: An innovative method for the non-destructive in-situ detection of microstructural changes of high-strength aluminum alloys

The use of aluminum alloys in conjunction with thermal-assisted forming techniques permits the production of high-strength and geometrically complicated components without failure. By using innovative, temperature-dependent production techniques, it is possible to manufacture components with tailored properties. So far, however, it is not possible to characterize the microstructural behavior under thermal and mechanical load in real-time. Using a novel characterization method, which is based on the principle of laser ultrasonic testing, it has become possible to record contact-free and non-destructive microstructural changes in-situ during thermo-mechanical treatment. This allows the interaction-free detection of thermal as well as mechanical effects during thermal-supported forming operations. In this research work, the temperature-dependent behavior during the quenching and aging process of the precipitation-hardenable aluminum alloy AA6082 for a tailor-quenched forming process is recorded in real-time using laser ultrasonic techniques. First, the effects of different tool temperatures on quenching rates and material properties were investigated using conventional characterization methods, including temperature measurement, hardness and tensile tests. It was shown that a die temperature of 325 °C leads to the lowest material hardness and strength. Subsequently, the results were compared with the novel, non-destructive in-situ testing method and validated using microstructural analysis. It was proven, that microstructural changes can be identified during varying quench and aging operations. Thus, the influence of different process parameters on the microstructural and mechanical development of aluminum components can be detected in-situ for the first time.

Dual nanoprecipitation and nanoscale chemical heterogeneity in a secondary hardening steel for ultrahigh strength and large uniform elongation

Nanoprecipitates and nanoscale retained austenite (RA) with suitable stability play crucial roles in determining the yield strength (YS) and ductility of ultrahigh strength steels (UHSSs). However, owing to the kinetics incompatibility between nanoprecipitation and austenite reversion, it is highly challenging to simultaneously introduce high-density nanoprecipitates and optimized RA in UHSSs. In this work, through the combination of austenite reversion treatment (ART) and subsequent flash austenitizing (FA), nanoscale chemical heterogeneity was successfully introduced into a low-cost UHSS prior to the aging process. This chemical heterogeneity involved the enrichment of Mn and Ni in the austenite phase. The resulting UHSS exhibited dual-nanoprecipitation of Ni(Al,Mn) and (Mo,Cr)2C and nanoscale austenite stabilized via Mn and Ni enrichment. The hard martensitic matrix strengthened by high-density dual-nanoprecipitates constrains the plastic deformation of soft RA with a relatively low fraction of ∼15 %, and the presence of relatively stable nanoscale RA with adequate Mn and Ni enrichment leads to a marginal loss in YS but keeps a persistent transformation-induced plasticity (TRIP) effect. As a result, the newly-developed UHSS exhibits an ultrahigh YS of ∼ 1.7 GPa, an ultimate tensile strength (UTS) of ∼1.8 GPa, a large uniform elongation (UE) of ∼8.5 %, and a total elongation (TE) of ∼13 %. The strategy of presetting chemical heterogeneity to introduce proper metastable phases before aging can be extended to other UHSSs and precipitation-hardened alloys.

High-Throughput Simulations of Phase Precipitation in Additively Manufactured Al Alloy

Additive manufacturing (AM) provides numerous advantages compared to conventional manufacturing methods, such as high design freedom and low material waste. Among the available materials, precipitation-hardenable aluminum alloys are highly attractive for AM due to their high specific strength and low density. Precise control of the processing conditions during AM and post heat treatment (HT) is required to tailor the final mechanical properties. Consequently, many variables, such as the chemical composition and process and HT parameters, must be considered to design suitable alloys for AM. Experimental investigations are, however, limited in variation of these variables. Therefore, computational alloy design approaches allowing for a faster evaluation of many possible variations must be developed. This work presents a high-throughput approach to determine the precipitation kinetics and thermodynamic properties based on the CALculation of PHAse Diagrams (CALPHAD) method. The developed approach is successfully validated for an Al-Mg-Si-Ti-Fe alloy and is applied to screen 243 combinations of chemical compositions and HT parameters. The results confirm the microstructural stability of the Al-Mg-Si-Ti-Fe system to small composition variations.

High conductive copper alloys for additive manufacturing

For applications where high thermal and/or electrical conductivity combined with reasonably high strength is required, copper alloys may be used. Although many different alloys were already developed in the past, additive manufacturing like laser powder bed fusion (PBF-LB/M) opens up new possibilities for alloy development, mainly driven by the very high cooling rates. This allows for the usage of precipitation-hardened alloys with compositions exceeding the maximum solubility. The present work focuses on the investigation of a well-known CuCr1Zr alloy as well as CuZr alloys with 1 and 2 wt.% Zr. For a fast, resource-efficient screening and demonstration of feasibility, the investigated alloys were not printed from powder. Instead, solid sheets were partially re-melted in a PBF-LB/M machine to obtain a microstructure similar to the printed state. This rapid-solidification microstructure is investigated, and precipitates with a size 50 nm or even smaller are found. After subsequent aging heat treatments, the hardness of the alloys exceeds the maximum hardness achievable with conventional manufacturing methods (excluding work hardening). The investigations in this work revealed the great hardening potential of these alloys for usage in the PBF-LB/M process.

Ultra-fine grained structure and high-content precipitates enable ultrastrong yet strain-hardenable medium-entropy alloy

In this work, the mechanical response as a function of ageing time at 700 °C for a cold-rolled complex alloy with a chemical composition of Co40Ni33Cr15Al5Ti7 (at%) was explored. The alloy exhibited an ultrafine-scaled recrystallized microstructure combined with high-content coarse L12-type ordered precipitates after a long-term aging for 100 h at 700 °C. The resultant alloy demonstrated an extra strain-hardening mechanical response with yield strength of 2.1 GPa and tensile elongation of ∼12 % at room temperature. By transmission electron microscopy observations, it was affirmed that the occurrence of profuse stacking faults as well as deformation induced twins during the critical plastic straining stage was mainly responsible for the observed extra strain-hardened behavior. The long-term aging process at low temperature provides a promising strategy for a precipitation-hardening complex alloy with severe deformation to obtain the superb combination of yield stress and ductility.

Effect of Mo-doping on the stability of D022 superlattice and mechanical properties in the Ni2.1CoCrFeNb0.2 high entropy alloy

High entropy alloys (HEAs) with D022 superlattice have shown excellent yield strength. However, long-time annealing leads to the decrease of strength due to the coarsening of D022 superlattice. Therefore, stabilizing the D022 phase of HEAs is of critical importance. Here, we report that a minor addition of Mo significantly enhances the phase stability of D022 phase and mechanical performance of Ni2.1CoCrFeNb0.2 HEAs. Experimental results show that the Mo-doping slows down the coarsening of D022 phase significantly by decreasing the diffusivity and reducing the lattice misfit between precipitates and matrix. Due to the enhanced coarsening resistance of D022 phase, the Mo-doped Ni2.1CoCrFeNb0.2 HEA shows good strength and ductility after long-term aging. These insights provide some guidelines for the design of precipitation-hardened alloys for high-temperature application.

Atomic-scale investigation of precipitate phases in QE22 Mg alloy

Precipitation-hardenable commercial Mg alloy QE22 (Mg-2.5Ag-2.0Nd-0.7Zr, wt.%) has excellent mechanical properties, but precipitates in this alloy have not been well understood. In this work, precipitate phases γ'', γ, and δ formed during the isothermal ageing process at 150, 200, 250, and 300 °C have been characterized using atomic-resolution high-angle annular dark-field scanning transmission electron microscopy and atomic-scale energy-dispersive X-ray spectroscopy. The morphology, crystal structure, and orientation relationship of these precipitate phases have been determined. Domain boundaries usually exist in a single γ particle, which can be characterized by a separation vector of [11¯01]α. The δ phase forms in situ from its precursor γ phase, consequently leading to the formation of three different variants within a single δ particle. The nucleation of the δ phase is strongly related to the domain boundaries of the γ phase. The formation of the γ phase may be promoted by its precursor γ'' phase. The similarities in atomic structures of the γ'', γ, and δ phases are described and discussed, indicating that transformations between these precipitate phases can be accomplished through the diffusion of added alloying elements.