Effect Of Overaging Research Articles

The electrochemical corrosion performance of aluminum alloys grade 6082-T6 weld repair

This research investigated the corrosion behavior of standard current metal inert gas weld repair for 6082-T6 aluminum alloy using ER5356 filler metal. The new and repaired (NW and RW) welds were studied. The welds comprised the weld metal (WM), the heat affected zone (HAZ) (solid solution and softened zones), and the base metal (BM). The study focused on investigating electrochemical corrosion using polarization and electrochemical impedance spectroscopy (EIS) methods in 3.5% NaCl solutions, especially in HAZ, including metallurgical and mechanical examinations. The BM containing an α-Al matrix with Al(Fe,Mn)Si and Mg2Si phases exhibited the maximum hardness (70–104 HV0.1). The WM hardness decreased (67–76 HV0.1) with the α-Al, β-Mg3Al2, and Mg2Si phases. Despite having comparable phases to BM, HAZs showed lower hardness (Solid HAZ: 70–82 HV0.1) due to more intermetallic phases. The RW’s softened HAZ revealed the minimum hardness (52–63 HV0.1) compared to that of the NW (55–70 HV0.1). Besides, the tensile strength of the RW (179.7 MPa) was also lower than that of the NW (174.4 MPa) because of the reheating effect. The electrochemical corrosion results indicated that the BM exhibited the maximum corrosion resistance (the lowest corrosion current density (icorr), the highest corrosion potential (Ecorr), and the charge transfer resistance (Rct)), followed by the HAZ and the WM, respectively. The softened HAZ demonstrated better corrosion resistance than the solid solution HAZ. Conversely, the over-aging effect reduced the softened zone’s pitting corrosion resistance (Ep) compared to the solid solution zone. The RW exhibited inferior corrosion resistance compared to the NW due to increased intermetallic phases, which was consistent with the mechanical results. However, the RW’s softened HAZ corrosion characteristics were inconsistent with its mechanical properties; its hardness and tensile strength were the lowest, but its corrosion resistance was not. Pitting corrosion was observed on the weld surfaces using the SEM.

Effects of Static Strain Aging on Mechanical Performance of Ductile Cast Iron

EN-GJS-400-15U nodular cast iron intended to be used as load-bearing element in long-term geological disposal canisters containing spent nuclear fuel in Finland and Sweden was studied for static strain aging (SSA). Tensile test specimens manufactured from the nodular cast iron were pre-strained to 1%, 2% and 3% nominal plastic strains. The pre-strained specimens were aged at different temperatures ranging from room temperature to 400 °C for varying times. The aged specimens were tested with conventional tensile testing using constant cross-head speed of 0.016 mm/s. Additionally, four specimens were studied with digital image correlation (DIC) during the tensile testing to obtain full-field strain measurements. SSA resulted in elevated pronounced yield point in all the conditions, while the as-received material showed continuous yielding behavior. SSA reduced the elongation to fracture. DIC tests showed more localized yielding behavior in the SSA specimens. Over-aging effect was observed at 400 °C where increasing pre-strain did not increase the yield stress more. For 1-day aging time, the highest yield stress increment was found after aging at 200°C. The yield stress of the material was almost identical after aging in 100°C and 200°C.

Mitigation effects of over-aging (T73) induced intergranular corrosion on stress corrosion cracking of AA7075 aluminum alloy and behaviors of η phase grain boundary precipitates during the intergranular corrosion formation

The stress corrosion cracking (SCC) behaviors of over-aged and peak-aged AA7075 aluminum alloys in a 5 wt% NaCl solution were compared. Even though the over-aging process induced intergranular corrosion (IGC), it decreased the SCC susceptibility of the material. Grain boundaries (GBs) with misorientation angles less than 30° and the grain matrix were considered factors that reduced the IGC propagation rate, thereby preventing the development of SCC in the over-aged material. η phase grain boundary precipitates (GBPs) underwent de-alloying, forming Cu-enriched particles during the IGC formation. However, the cathodic properties of these particles are not the key factors determining whether the IGC will occur or not.

Influence of reverted austenite on fatigue life of 18 % nickel maraging steel

AbstractMaraging steels containing 18 % nickel are based on a low‐carbon iron‐nickel‐cobalt‐molybdenum system. These steels demonstrate a remarkable combination of high strength and high toughness. To investigate the effect of overaging and resulting austenite reversion on the fatigue behavior of C250 maraging steel, specimens in the solution annealed state were overaged at 510 °C for various intervals, resulting in volume fractions of reverted austenite ranging from 2.6 % to 11.4 %. The staircase method was used to calculate the fatigue limit. S−N curves were generated by testing a minimum of four stress levels and at least three samples at each stress level. Basquin's equation was used to fit the experimental stress‐life data, and the Basquin exponent and fatigue strength coefficient were calculated. The presence of 2.6 % reverted austenite in the microstructure improved the fatigue limit and fatigue ratio, without significantly reducing tensile strength. The results can be interpreted in terms of reverted austenite having a crack blunting effect on the propagating cracks. A small amount of reverted austenite in the range of 2 %–3 % vol has a beneficial effect on the fatigue life.

Dependency of mechanical properties on artificial aging conditions for EN AW 6082 aluminum alloy

Aluminum 6xxx series alloys attract attention owing to their decent strength, formability and corrosion characteristics. Mechanical properties of these alloys can be altered via precipitation hardening by applying an aging treatment known as artificial aging (AA). This investigation was focused on the behavior of mechanical properties of EN AW 6082 alloy for various aging conditions. Mechanical property response for different heat treatment procedures were analyzed together with DSC results and TEM micrographs. Solution heat treatment (SHT) temperature was found to affect the obtained yield strengths after AA considerably and higher SHT temperatures lead to higher yield strength for same artificial aging condition. It was also shown that yield strength values obtained after 480 min—180 °C artificial aging can be surpassed in shorter times at higher artificial aging temperatures. Aging times as short as 60 min may be employed using temperatures above 200 °C to obtain higher strength than 480 min—180 °C aging. Linear relationships between Vickers hardness and mechanical strengths were suggested for the complete data of the study. Negative effects of over-aging on ductility were presented and β′′ precipitates which are responsible for hardening during aging were presented using TEM micrographs. Additionally, high cycle fatigue tests were conducted for AA procedures 480 min—180 °C and 240 min—200 °C and similar fatigue strengths were reported.

Effect of over-aging on the microstructure, mechanical properties and crashing performance of thin-walled Al–Mg–Si–Cu alloy profiles

The thin-walled Al–Mg–Si–Cu alloy (AA6008) crash-box profiles were treated by over-aging at 215 °C for 60 min, 120 min and 300 min, and then their corresponding microstructure, mechanical properties, and crashing performance were investigated. The results show that with the increase of aging time, the number density of precipitation is decreased significantly, the size of precipitate is coarsened, and the precipitate type is transformed from β″ to β′, resulting in a reduction of microhardness and strength. The crashed morphology of the profile is improved showing a more regular folding deformation and less crack, but the corresponding values of energy absorption and mean crash force are decreased owing to the decrease of the strength. The medium overaged treatment, i.e., AA 215 °C-120 min sample, can obtain a balanced combination of excellent mechanical properties (yield strength > 280 MPa) and crashing performance. The influence of precipitation on the mechanical properties and crash deformation is also discussed based on experimental results.

Processing of friction stir welded AA6061-T6 joints reinforced with nanoparticles

The solid-state joining of AA6061-T6 - 0.2% Al2O3 (15 nm) and TiO2 (15 nm) nanoparticles via friction stir welding was done. The microstructure, metallurgical, and mechanical properties of weldments were examined and compared with FSW weld Without Nanoparticle (WNP) weldment and base material. Results reveal that the WNP weld welded at many FSW conditions, while in the case of weldments With Al2O3 Nanoparticle (WAL) and With TiO2 Nanoparticle (WTI)), the working window was narrower. Such welds were only feasible at high heat inputs concerning high rotational speed. Microstructural and metallurgical studies reveal the expansion of the Weld Nugget Zone (WNZ) on the retreating side, as a result of high heat input and microhardness for WAL weldment than WTI weldment. Whereas the overaging effect caused due to frictional heat leads to a decrease in microhardness of WTI compared to WAL weldment. The grain refinement in WNP weldment with appreciable microhardness is governed by Dynamic Recrystallization (DRX). The tensile tests showed high tensile properties of WAL and WTI weldments than WNP weldments.

Microstructural Analysis of Friction Stir Butt Welded Al-Mg-Sc-Zr Alloy Heavy Gauge Sheets

Friction stir welding (FSW) on a heavy gauge sheet of a hereditary fine-grained Al-Mg-Sc-Zr alloy was carried out to study the specifics of plasticized metal flow and microstructural evolution in different sections and zones of the joint. It was found that the stir zone (SZ) macrostructure may contain either a single or many nugget zones depending on the metal sheet thickness and the seam length. The effect of grain kinking in a thermomechanically affected zone (TMAZ) under pressure from the stir zone metal was discovered. The stir zone metal was fine-grained but had a microhardness lower than that of the base metal, which may be explained by the overaging effect of FSW on the Al3Sc precipitates. The tensile strength of the joint was almost equal to that of the base metal (BM). The grain size distributions were obtained in different sections below the sheet surface and away from the exit hole, which allowed us to suggest the specific adhesion-assisted layer-by layer metal transfer mechanism in FSW.

Effect of post-weld heat treatment on the mechanical behavior and dislocation density of friction stir welded Al6061

Al6061 is a heat-treatable aluminum alloy and an extremely versatile material used in settings where medium to high strength is required. To produce good joints using this alloy, a particular solid-state welding process called friction stir welding (FSW) is preferable instead of the conventional welding methods. In this study, the effects of T6 post-weld heat treatment (PWHT) on the mechanical properties, dislocation density, and microstructure of a friction stir welded (FSWed) Al6061 aluminum alloy were investigated. Results indicated that FSW had degraded the mechanical properties of Al6061. The welding efficiency decreased to 65% compared with the tensile strength of the base metal. After FSW, the samples demonstrated a serrated flow behavior known as the Portevin–Le Chatelier (PLC) effect. This effect was attributed to the nonuniform distribution of hard Mg2Si particles, coarsening and/or solutionizing of the strengthening elements in the stir and thermomechanically affected zones, and an over-aging effect in the heat-affected zone of the FSWed sample. However, the T6-PWHT performed on the FSWed sample diminished the PLC effect and concurrently improved the mechanical properties back to its original state. The PWHT also promoted precipitation hardening through a better distribution of Mg2Si particles that prevented grain growth and increased the dislocation density due to the applied strain, which subsequently improved the mechanical properties of the FSWed Al6061 alloy.

A Mechanical Evaluation of an Overaged Aluminum Metal-Matrix-Composite (2009 Al/SiC/15p MMC)

Abstract The effect of overaging on the mechanical properties of an aluminum matrix composite with silicon carbide particles (2009 Al/SiC/15p MMC) was studied. Test specimens were fabricated from a rolled billet and then aged in oil or air over a range of times and temperatures prior to mechanical testing. It was found that overaging led to a decrease in material strength (and hardness). However, it had no appreciable effect on the impact resistance of the material. The samples aged in an oil bath had a similar degradation of mechanical properties to those aged in air under the same conditions. Furthermore, it was also found that the hardness of the material continually decreases as the aging temperature is incrementally increased up to 290 °C.

Effects of Platform Pre-Heating and Thermal-Treatment Strategies on Properties of AlSi10Mg Alloy Processed by Selective Laser Melting

The AlSi10Mg alloy was processed by selective laser melting using both hot- and cold-build platforms. The investigation was aimed at defining suitable platform pre-heating and post-process thermal treatment strategies, taking into consideration the peculiar microstructures generated. Microstructural analyses, differential scanning calorimetry, and high-resolution diffraction from synchrotron radiation, showed that in the cold platform as-built condition, the amount of supersaturated Si was higher than in hot platform samples. The best hardness and tensile performance were achieved upon direct aging from cold platform-printed alloys. The hot platform strategy led to a loss in the aging response, since the long processing times spent at high temperature induced a substantial overaging effect, already in the as-built samples. Finally, the standard T6 temper consisting of post-process solution annealing followed by artificial aging, resulted in higher ductility but lower mechanical strength.

Effect of Overaging on Fatigue Crack Propagation and Stress Corrosion Cracking Behaviors of an Al-Zn-Mg-Cu Alloy Thick Plate

The fatigue property as well as stress corrosion cracking (SCC) resistance of an Al-Zn-Mg-Cu alloy thick plate in peak-aged and overaged tempers (T7351 and T7651) is systematically investigated by fatigue crack propagation (FCP) test and slow strain rate test (SSRT). Microstructural characterization is examined by transmission electron microscopy and scanning electron microscopy. Results reveal that the T7351 alloy has lower strength but higher electrical conductivity as compared to T7651 alloy. The FCP resistance of T7351 alloy is superior to that of the T7651 alloy due to the coarser precipitates in the highly overaged alloy in which the strain localization is reduced by promoting homogeneous slip. In addition, the SSRT test suggests a higher SCC resistance in T7351 alloy. The enhanced SCC resistance is found to depend on grain boundary precipitate characteristics and crack propagation resistance of the alloys.

Effect of Overaging on the Cyclic Deformation Behavior of an AA6061 Aluminum Alloy

The present work encompasses the effect of overaging on the strain-controlled low-cycle fatigue (LCF) behavior of an extruded AA6061 aluminum alloy at varying strain amplitudes. During the T7 aging treatment, the size of precipitates increased from 60 nm under T6 conditions to 220 nm after aging for 48 h at 200 °C, leading to a decrease in the monotonic tensile strength. During the LCF tests, nearly symmetrical hysteresis loops can be observed in the mid-life cycle under all test conditions, whereas the first-cycle hysteresis loops were moderately inflected under long-aging conditions. With increasing aging time, the cyclic peak stresses decreased and the plastic strain increased. Nearly ideal Masing behavior was exhibited under T6 conditions, while it was lost under T7 overaging conditions. The cyclic stress responses were similar under all tested conditions, involving stabilization at low strain amplitudes and softening at high strain amplitudes, with initial hardening for the first few cycles. Compared to the T6 condition, the fatigue life increased with increasing T7 aging time. Various LCF parameters were estimated based on the Coffin-Manson and Basquin relationships and on the LCF experimental results. The relationship between the fatigue life, strength, and microstructure of the investigated AA6061 aluminum alloy under various aging conditions was discussed.

Effects of overaging on microstructure and tensile properties of the 2055 Al-Cu-Li-Ag alloy

The lightweight, unconventional 2055 Al-Cu-Li-Ag alloy exhibits an excellent specific strength in the T83 state, but no literature reports the effects of overaging on this alloy. In the present work, the suitability of the alloy for lightweight components operating at high temperature is evaluated. Thermal exposure in the range 215–305°C was investigated, highlighting its consequences on both microstructure and mechanical properties. In the most severe overaging state (24h at 305°C), the typical T1 precipitates (Al2CuLi) are dissolved, leading to the formation and coarsening of ϑ’ and Ω phases. In all overaging conditions, the alloy performance was superior or at least comparable to that of another third generation Al-Li alloy, AA2099, which is characterised by a slightly lower density and encouraging mechanical properties for high temperature applications. Compared to AA2099, the AA2055 alloy provides a higher specific strength (the basic requirement for mass savings) both in the T83 and in the most severe overaging state (24h at 305°C). This work highlights that AA2055 is a promising candidate for lightweight components operating up to 305°C, and it lays the basis for high temperature tests of the alloy.

EN AW-4032 T6 Piston Alloy After High-Temperature Exposure: Residual Strength and Microstructural Features

This study aims to evaluate the effects of prolonged thermal exposure on both microstructural evolution and mechanical properties of the EN AW-4032 T6 piston alloy. For the purpose, the experimental activities have been carried out on samples machined from forged and heat-treated automotive pistons. The effects of overaging have been investigated in the temperature range of 140-290 °C, firstly by evaluating the time-temperature-hardness curves and then by carrying out room-temperature tensile tests on overaged samples. The material softening was substantial and extremely rapid when the soaking temperature exceeded 250 °C. During overaging, both the tensile strength and the residual hardness considerably decreased, and a relationship between these parameters has been established. The alloy behavior in the plastic field has been modeled according to the Hollomon’s equation, showing that both the strain hardening exponent and the strength coefficient are a function of the residual hardness. The results were finally related to the corresponding microstructural changes: OM and FEG-SEM metallographic and fractographic analyses on overaged samples gave evidence of coarsened precipitates along the grain boundaries.

Effect of Weld Characteristic on Mechanical Strength of Laser-Arc Hybrid-Welded Al-Mg-Si-Mn Aluminum Alloy

Laser-arc hybrid welding (LAHW) was employed to improve the tensile properties of the joints of 8-mm-thick Al-Mg-Si-Mn alloy (AA6082) using Al-5Mg filler wire. The weld microstructures were examined by scanning electron microscope, electron backscattered diffraction, and transmission electron microscopy in detail. The LAHW joints with pore-free and high-tensile performances were obtained. The strength enhancement of the fusion zone and heat-affected zone in the LAHW joint was mainly attributed to the grain refinement strengthening and the precipitation strengthening, respectively. The microstructure characteristics were related to the effects of laser-arc interaction on the energy transfer within the molten pool. The arc caused the majority of laser energy to dissipate out of the keyhole, and then it reduced the heat input. The lower heat input refined the grain size, weakened the overaging effect, and thus improved the tensile strength.

Influence of chemical composition of Ti/TiC/a-C:H coatings deposited on 7075 aluminum alloy on their selected mechanical properties

This paper presents the results of research on the deposition of gradient antiwear Ti/TiC/a-C:H coatings on Al–Zn alloys (7075 series) using a hybrid method Radio Frequency Plasma Assisted Chemical Vapor Deposition (RF PACVD). In order to eliminate the over-aging effect of the aluminum substrate it was subjected to T6I6 treatment. The alloy has been strengthened by precipitation and a first aging process, at a temperature of 813K for 6h and at 430K for 1h, respectively. Synthesis of the gradient coatings on 7075 aluminum alloy was performed simultaneously during the second stage of the aging process at 390K for 6.5h. It has been proved that the change in the substrate bias potential in the range of 150–600V (constant value for each process) influences the mechanical properties of the resulting coatings (hardness in the range 10–19GPa and adhesion to the substrate from 16 up to 33mN).It has been proved in this case that during the substrate aging process it is possible to control the contribution of disordered carbon. Its value (calculated as D/G area ratio) in the range of 0.75 allows obtaining coatings with a coefficient of friction of about 0.05 (Al2O3 or Si3N4 counterparts, 5N load) and wear rates of about 4.4∗10−5mm3/Nm, which due to their chemical structure behaves as solid lubricant in the friction pair.

The Evolution of Defects in Deformed Cu-Ni-Si Alloys during Isochronal Annealing Studied by Positron Annihilation

The effect of isochronal annealing on the deformation-induced defects in pure Cu and Cu-Ni-Si alloys is studied by positron annihilation spectroscopy. For the cold-rolled Cu, annealing up to 900°C causes a gradual recovery of the deformation-induced defects and monotonous decrease of the hardness. This indicates that its hardening is mainly related with defects such as dislocations. However, for the hot-rolled and quenched Cu-Ni-Si alloy, although there is a partial recovery of defects after annealing below 500°C, formation of additional defects is observed after annealing above 500°C. The hardness of Cu-Ni-Si alloy has a maximum value after annealing at 500°C, which suggests that the hardening of Cu-Ni-Si alloy is not due to defects, but primarily due to the precipitation formed during annealing. Further annealing of the Cu-Ni-Si alloy above 500°C results in over-aging effect and the precipitates lose coherence with the host matrix, which leads to positron trapping by vacancy clusters in the incoherent interface region.

Aging Behaviors of Ti-Al-Mo-Fe Titanium Alloy

Ageing behaviors and microstructural characterization by aging condition of Ti-Al-Mo-Fe alloy was investigated. Due to the formation of ω phase, it occurs a drastic change in mechanical properties of β alloys. There is large increasing in hardness and yield strength, accompanied with serious ductility reduction. However, it has been proved that proper control of ω phase volume fraction can bring to improve strength with a reasonable ductility. In this work new beta titanium alloy was designed and developed in Ti-Al-Mo-Fe alloy to investigate the hardening behavior of ω and α precipitation during aging. The results showed that a small amount of athermal ω was observed in the β matrix during water quenching from above the β transus temperature. Isothermal ω formation was also found during aged at temperatures ranging from 573 K to 773 K although it has a limited time of stability at 773 K. The hardening due to isothermal ω precipitation exhibits no over-aging effect as long as ω phase exists inside the matrix. The hardness of this alloy is very sensitive to size and volume fraction of ω phase and its existence, which depends on aging temperature, time and alloy compositions.

Effect of over-aging on the microstructural evolution in an Al–Cu–Mg–Ag alloy during ECAP at 300 °C

This report explores over-aging as it relates to microstructural evolution during equal channel angular pressing (ECAP) at 300°C using a novel Al–Cu–Mg–Ag alloy doped with zirconium and scandium. Material characterization was conducted using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray analysis in which microstructural evolution in a quenched alloy (QA) material was compared to that in an over-aged alloy (OA) material. It was shown that over-aging facilitates grain refinement where, at ɛ∼12, a fraction of the high angle boundaries (HABs) and average misorientation angles are ∼0.72 and 30°, respectively. In contrast, a partially recrystallized structure evolves in the QA sample even after ɛ∼12. Intense plastic straining affects precipitation. In the QA, ECAP leads to the precipitation in Ω-phase with a plate-like shape, initially. Further strain leads to a shearing offset associated with a localized dislocation glide followed by dissolution of Ω-phase and precipitation of the θ-phase (Al2Cu) and S-phase (Al2CuMg), both of which share an equaxed shape. In the OA, the dissolution of the Sc enriched θ-phase consisting of relatively coarse particles is followed by precipitation of nanometer-sized scale dispersoids of a Sc depleted θ-phase on deformation-induced boundaries. In addition, Ag and Mg-rich particles being β′(MgAg) and U(AlMgAg) phases precipitate during ECAP, in both states of the Al–Cu–Mg–Ag alloy explored herein.