Subsequent Artificial Aging Research Articles

The Influence of Composition on the Clustering and Precipitation Behavior of Al-Mg-Si-Cu Alloys

The natural aging (NA) and artificial aging (AA) behavior of Al-Mg-Si-Cu alloys with different Mg/Si ratios and Cu additions were systematically investigated by means of hardness test, atom probe tomography, transmission electron microscopy, and Monte Carlo simulation. The Si-rich low-Cu alloys displayed higher hardness compared to the Mg-rich equivalents because Si atoms play a dominant role in clustering of solute atoms during both natural and artificial aging. In the high-Cu alloys, Cu did not obviously change the cluster distribution during NA, but significantly refines the clusters and precipitates due to the strong interaction of Cu atoms with Mg atoms during AA. In contrast to the low-Cu alloys, the Mg-rich high-Cu alloys exhibit higher hardness in the early and over-aged stages of artificial aging, with similar or slightly higher hardness in the peak aging condition compared to their Si-rich counterparts. Three types of precipitates (β″, Q′, and L) are favored in the high-Cu alloys. The Mg-rich high-Cu alloy has more L phase, while the Si-rich variant is abundant in Q′ phase. The negative effect of NA on subsequent AA behavior is less dependent on Mg/Si ratio in the high-Cu alloys due to a synergistic action of the residual Si and Cu atoms, but is closely related to Mg/Si ratio in low-Cu alloys.

Mechanical properties enhanced by deformation-modified precipitation of θ′-phase approximants in an Al-Cu alloy

The influence of large pre-deformation on the age hardening behaviors, tensile properties and precipitate characteristics in an Al-Cu alloy during artificial ageing was investigated using mechanical property measurements and electron microscopy. Room-temperature rolling of Al-Cu alloy with a thickness reduction of 80% plus a subsequent artificial ageing can result in a strength increase of 35% without sacrificing ductility compared with an alloy that is peak–aged using a conventional heat treatment process (T6). A large quantity of dislocations that accumulated during cold-rolling was retained in the alloy after post-ageing at 150°C. Although dense dislocation walls and dislocation tangles that exist in cold-rolled Al-Cu alloys were expected to cause micro-structural heterogeneity, the plate-shaped precipitates on the {001}Al planes exhibited an exceptionally uniform distribution. Atomic-scale angular dark-field scanning transmission electron microscopy (ADF-STEM) revealed these precipitates are θ′–approximants with a crystal structure similar with the well–known tetragonal θ′ phase but with the body-centered position not fully occupied by Cu. In addition, the precipitate sizes were considerably refined as compared to the θ′ precipitates that were formed in the T6 peak-aged alloys. The microstructure–property relationships in the alloy are discussed to explain the observations. Our results suggest the deformation-modified precipitation may play a vital role in achieving better strength-ductility synergy.

Processing of high strength EN AW 6082 forgings without a solution heat treatment

Light weight EN AW 6082 alloy forgings are used extensively in automotive suspension components. The conventional production cycle involves a solution heat treatment that is responsible not only for coarse surface grains, potentially degrading for fatigue properties, but also for microstructural features that reduce the impact toughness. The separate solution treatment may be omitted if hot forging operation is fine tuned to solutionize sufficient Mg and Si during forging to offer adequate age hardening during the subsequent artificial ageing cycle. Uniform structures with a predominantly fine fibrous core thus obtained, ensure better fatigue and impact properties and a longer service life. The proposed process not only improves the quality of the automotive suspension components but also offers significant cost savings.

Effect of aging treatment on mechanical properties and fracture behavior of friction stir processed Mg–Y–Nd alloy

In this study, precipitation behavior of Mg–Y–Nd cast alloy during friction stir processing (FSP), and the effect of subsequent artificial aging on mechanical properties and fracture behavior of the FSP alloy were investigated. It is found that the coarse α-Mg grains and large second phases are greatly refined after FSP. Moreover, due to the heat input during processing and the natural cooling, β′ and β1 precipitates are also observed in the FSP alloy. The FSP specimens were subjected to subsequent artificial aging treatment, and the peak hardness is obtained at 150 °C for 54 h and 180 °C for 30 h. Strengths of the peak–aged specimens are further increased, which is attributed to the large quantity of β″ and β1 precipitates, respectively. Meanwhile, elongations of the peak-aged specimens are both decreased. Due to the comprehensive effects of banded structures and fine grains, failure mechanisms of FSP and peak-aged specimens are all mixed ductile–brittle fracture mode. However, compared to the FSP specimens, different fracture paths are exhibited in peak–aged specimens.

The different dependencies of the mechanical properties and microstructures on hot extrusion and artificial aging processing in case of the alloys Al 7108 and Al 7175

In view of optimizing the strength and the ductility of the almost copper-free alloy 7108 and the copper-containing alloy 7175 the influences of extrusion processes with different parameters and subsequent artificial peak aging procedures have been studied. The work was divided into three steps. In the first step the extrusion ratios, the product speeds and the billet temperatures have been varied and their influence on the extrudability of the alloys 7108 and 7175 were examined. In the second step the resulting strands were analysed in the water quenched condition to investigate the correlation of the morphology and the texture to the mechanical properties. In the last step the strands were subjected to peak aging, to analyse the influence of the extrusion parameters on the precipitation behaviour and the mechanical properties. For the analyses of the morphology and the texture a combination of optical microscopy, X-ray diffraction and electron backscatter diffraction has been used. The particles and the precipitates were analysed by scanning and transmission electron microscopy. In addition the mechanical properties were measured by compression and tensile tests. The investigations show that the almost Cu-free alloy 7108 is extrudable within a wide parameter field and that the mechanical properties of this alloy can be varied and optimized by the parameters of artificial aging processes. So the combination of the extrusion process and the artificial aging results in an elongation at fracture up to 17.4±0.2% and in strength values up to 426±1MPa. In case of the Cu-containing alloy 7175 the extrusion process is limited to lower product speeds, because the temperature increment, which is introduced through the plastic deformation, is higher in case of this alloy. Therefore, eutectic phases form and cause hot cracking of the profiles. The investigation shows that the mechanical properties of the Cu-containing alloy 7175 are mainly influenced by the extrusion process. It was shown that the extrusion process using high billet temperatures leads to a ductility up to 14.5±0.5% and to strength values up to 636±4MPa.

Effect of natural ageing and pre-straining on the hardening behaviour and microstructural response during artificial ageing of an Al–Mg–Si–Cu alloy

Al–0.42Mg–0.5Si–1.0Cu alloy was prepared by hot extrusion, followed by quenching and ageing treatment. The keeping and pre-straining immediately after quenching were performed before ageing, to investigate natural ageing and pre-straining effects on the age-hardening and microstructural response of the alloy, hardness measurement, TEM, and HRTEM characterisation were used. The results indicated that, room-temperature keeping led to a loss of peak ageing hardness for subsequent artificially aged alloy. The peak ageing hardness decreased and the peak ageing time increased with increasing keeping time. The formation of clusters/GP zones induced by natural ageing decreased the densities of β″ and Q′ phases, which contributed to the decrease of peak-ageing strengthening. Pre-straining before ageing significantly increased the peak ageing hardness and shortened the peak ageing time of the alloy. Furthermore, it significantly mitigated the effects of natural ageing on the hardening behaviour of the alloy during subsequent artificial ageing by restraining the formation of clusters/GP zones and accelerating the nucleation of β″ and Q′ phases in the early stage of subsequent artificial ageing. In the over-ageing stage, dislocations induced by pre-straining facilitated the coarsening of precipitates and consequently led to an apparent decrease of hardness.

Effect of High-Speed Impact Compression on Natural Aging and Subsequent Artificial Aging of a 6061 Aluminum Alloy

Effect of High-Speed Impact Compression on Natural Aging and Subsequent Artificial Aging of a 6061 Aluminum Alloy

Influence of the grain boundary chemistry on the intergranular corrosion mechanisms of a high‐strength Al‐Mg‐Si alloy

The influence of solution annealing (T4) and artificial aging (T6) on the corrosion properties of a technical Al‐Mg‐Si alloy was investigated. In the T4 condition, the material was not susceptible to intergranular corrosion; however, pitting was identified to occur. Subsequent artificial aging of the solution annealed and quenched alloys introduced susceptibility to intergranular corrosion. This phenomenon was attributed to two microgalvanic coupling situations: one that occurred between the noble Cu‐phases located on the grain boundary and the anodic adjacent area and one between a Mg‐ and Si‐depleted zone along the grain boundaries and the adjacent area. Pitting corrosion was proposed to occur as a result of the absence of these microgalvanic coupling situations, and therefore with the absence of a distinct, preexisting corrosion path in the solution annealed condition. Copyright © 2015 John Wiley & Sons, Ltd.

Effect of natural ageing on microstructure and mechanical properties of Mg–10Gd–2Y–0.8Zr Alloy

The effect of natural aging on the microstructure and mechanical properties of Mg–10wt%Gd–2wt%Y–0.5wt%Zn–0.5wt%Zr alloy (GW102) was investigated via optical microscopy (OM), field emission gun scanning electron microscopy (FEGSEM), high-resolution transmission electron microscopy (HRTEM), and differential scanning calorimetry (DSC). The results indicated that the natural placement had a significant effect on the microstructure, mechanical properties, and subsequent artificial aging behaviors of the extruded GW102 alloy. The α(Mg) isometric crystal of the as-extruded alloy was composed entirely of deformation structures, formed by several types of cross-slip. However, the deformation structure within α(Mg) was significantly reduced by performing a 3-year natural aging treatment after extrusion. Many strain-free zones were observed near the grain boundary. Moreover, the β″ phase precipitated within the α(Mg) crystalline grains. The alloy was subjected to a 3-year natural aging treatment, followed by artificial aging at 200°C; the resulting hardness and tensile strength were relatively high during the early stage, but decreased significantly at peak aging and the time required to reach peak aging was shortened. In addition, an aging peak platform (44–132h) was observed. Moreover, the formation of β″ and the reduction of the deformation zone at natural aging process promoted the subsequent artificial aging process.

Enhanced Strengthening in Ultrafine‐Grained Al–Mg–Si Alloys Produced via ECAP with Parallel Channels

This paper investigates a dynamic aging effect induced by severe plastic deformation (SPD) and subsequent artificial aging on structure and mechanical properties of aluminum Al–Mg–Si alloys. Application of optimal processing parameters of SPD via equal‐channel angular pressing with parallel channels and subsequent artificial aging ensures simultaneous enhancement of strength and ductility in the processed alloys. Ultrafine‐grained microstructures with strengthening metastable β′ and β″ precipitates generated during SPD processing and subsequent artificial aging provide enhanced strength due to significant grain boundary hardening and precipitation hardening.

The Influence of Prolonged Natural Aging on the Subsequent Artificial Aging Response of the AA6111 Automotive Alloy

The influence of prolonged ambient temperature storage on the subsequent precipitation behavior of the AA6111-T4P automotive alloy during artificial aging has been investigated using hardness testing and high-resolution transmission electron microscopy. The results indicate that growth of atomic co-clusters and GP zones during ambient storage suppress the onset of Q′ and Q precipitation, and retard the formation of elemental silicon particles. However, the overall hardening response of the stored AA6111 T4P alloy to subsequent peak-aging and over-aging treatments is not altered by the significant reduction in the volume fraction of Q′ and Q precipitates due to their lower strengthening effects compared with GP zones and the Q″ phase precipitates.

The influence of Mg/Si ratio on the negative natural aging effect in Al–Mg–Si–Cu alloys

The effects of natural aging (NA) on subsequent artificial aging (AA) at 180°C in Al–Mg–Si–Cu alloys with varied Mg/Si ratios (0.5, 1 and 2) were systematically studied by Vickers micro-hardness measurements, differential scanning calorimetry and transmission electron microscopy (TEM). The alloy with large Mg/Si ratio possesses a significant negative NA effect on the maximum hardness achieved during AA preceded by an extended NA, while the alloy with small Mg/Si ratio shows a negligible negative NA effect. Though few lath-like Qʺ/L precipitates exist, needle-like βʺ precipitates are the primary hardening precipitates in all the peak-aged alloys. The negative NA effect is demonstrated to be determined by precipitate coarsening, which is manifested microscopically as the broader precipitate length distributions (PLD) and shift of PLD toward larger length range, in AA with the prolonging of NA. Our results suggest the nature of NA clusters is quite different in Al–Mg–Si–Cu alloy varying in Mg/Si ratio. Only a small fraction of NA clusters in alloy with large Mg/Si ratio are stable and could induce preferential growth of precipitates to be considerably coarsened during AA. A large fraction of stable NA clusters in alloy with low Mg/Si ratio lead to synchronous growth of βʺ precipitates, thus restricting the preferential growth.

Microstructural impact on intergranular corrosion and the mechanical properties of industrial drawn 6056 aluminum wires

Abstract The influence of individual manufacturing steps during industrial wire drawing processes on the mechanical and corrosion properties of the 6056 aluminum alloy was investigated. These steps demonstrated an essential influence on the microstructure, and thus, the susceptibility to intergranular corrosion (IGC). No clear correlation between IGC susceptibility and hardness was observed. Although the highest resistance against intergranular attack was determined for those alloys in the solution annealed condition, pitting corrosion was identified to occur. Subsequent artificial aging of the solution annealed and quenched wires reintroduced IGC susceptibility; this phenomenon was attributed to the occurrence of galvanic coupling between the noble Cu-phases, located on the grain boundary, and the anodic grain boundary area.

Formation and reversion of clusters during natural aging and subsequent artificial aging in an Al–Mg–Si alloy

The continuous changes in number density, size distribution and chemical composition of clusters during natural aging (NA), and subsequent artificial aging (AA) at 443K, in an Al–0.62Mg–0.93Si (mass%) alloy have been evaluated using atom probe tomography. The data show almost no change in the average size and Mg/Si ratio (at% ratio) of clusters during NA. The highest hardness and most-dense clusters, following NA, and the largest drop in hardness, during AA for 1.2ks, are observed in the material with the longest NA period. In contrast with the published literature, which suggests that only small clusters revert into solution, the number density of clusters decreased, with no dependence on the cluster size, during AA for 1.2ks, resulting in a slight decrease in the average radius of clusters. Things to be emphasized are that the number density of clusters with the Mg/Si ratio below 0.4 stays pretty much the same during AA, whereas other clusters decrease during the first 1.2ks and then increase again during the prolonged AA for up to 3.6ks. This work is the first to reveal that the typical Si-rich clusters can neither be dissolved nor grow further, although the size is small. In other words, it is believed that the typical Si-rich clusters can lead to the retardation of the hardness increase during AA called as negative effect of NA.

High-speed extrusion of heat-treatable Mg–Al–Ca–Mn dilute alloy

A newly developed dilute magnesium alloy, Mg–0.27Al–0.13Ca–0.21Mn (at.%), shows extraordinary high-speed extrudability of a die-exit speed of 60m/min. Subsequent artificial aging at 200°C (T5) enhanced the proof stress from 170MPa to 207MPa due to the precipitation of Guinier Preston (G.P.) zones and weakened basal texture while keeping good ductility of 12.5%. High-speed extrusion reduces the processing cost, so the dilute Mg–Al–Ca–Mn alloy could be an industrially viable low-cost medium strength structural material.

Effects of pre-aging treatment on subsequent artificial aging characteristics of Al-3.95Cu-(1.32Mg)-0.52Mn-0.11Zr alloys

The effects of pre-aging treatments on subsequent artificial aging response were investigated by means of transmission electron microscopy observations and hardness test in age hardened Al-3.95Cu-(1.32Mg)-0.52Mn-0.11Zr alloys. In Al-3.95Cu-0.52Mn-0.11Zr alloy, when the pre-aging temperature is 25 °C, the pre-aging treatment has no evident effect on the peak hardness of subsequent artificial aging, while a positive effect (increase of peak hardness) appears when pre-aging temperature is 50 °C. However, in Al-3.95Cu-1.32Mg-0.52Mn-0.11Zr alloy, it is found that whether the pre-aging temperature is 25 °C or 50 °C, the peak artificial aging hardness is lower than that of T6 treated alloy, that is to say, pre-aging treatment has a negative effect on the peak hardness of subsequent artificial aging in the alloys.

6061アルミニウム合金の自然時効およびその後の人工時効に及ぼす高速衝撃圧縮の影響

6061アルミニウム合金の自然時効およびその後の人工時効に及ぼす高速衝撃圧縮の影響

Effect of natural aging on quench-induced inhomogeneity of microstructure and hardness in high strength 7055 aluminum alloy

The effect of natural aging on quench-induced inhomogeneity of microstructure and hardness in high strength 7055 aluminum alloy was investigated by means of end quenching technique, transmission electron microscopy and differential scanning calorimetry thermal analysis. The hardness inhomogeneity in the end-quenched specimens after artificial aging decreases with the increase of natural aging time prior to artificial aging. The quench-induced differences in the amount and size of η′ phase are large in the end-quenched specimen after artificial aging at 120°C for 24h, leading to high hardness inhomogeneity. Natural aging for a long time results in a larger amount of stable GPI zones in the slowly-quenched sample, and thus decreases such differences in the end-quenched specimens after subsequent artificial aging, leading to lower hardness inhomogeneity. The hardness inhomogeneity can be reduced from 14% to be 4% by natural aging for 17,280h prior to artificial aging.

The Influence of Quenching and Aging on Fracture Toughness of the Al-Zn-Mg-Cu Alloy 7449

Abstract This investigation characterises the fracture toughness of the very high strength Aluminium alloy 7449. This Al-Zn-Mg-Cu alloy is heat treatable and relies on rapid quenching from the solution heat treatment temperature to promote subsequent artificial aging. While the influence of quench paths on strength is well understood and can be predicted using techniques like quench factor analysis, the influence of quench rate on fracture toughness is more challenging. The rate of quenching can influence the fracture toughness through complex precipitation reactions occurring during cooling. The precipitate locations dictate the magnitude of the detrimental effect on the fracture toughness. In this investigation, the fracture toughness of 7449 in two product forms was measured using compact tension specimens cut from forged blocks and rolled plate. These plane strain (KIC) results were also augmented with Charpy impact tests. Various quench conditions were investigated, including water at three different temperatures and poly oxyethylene glycol (PAG) in two concentrations. The influence of standard and novel aging procedures including retrogression and reaging was also determined. The combinations of strength and toughness have been related to the prevailing microstructural condition. Fracture toughness magnitudes were found to vary most significantly with rapidity of cooling from the solution treatment temperature, with the subsequent aging treatments having a much smaller effect.

Tensile Properties and Fusion Zone Hardening for GMAW and MIEA Welds of a 7075-T651 Aluminum Alloy

Tensile and hardness values for 7075-T651 aluminum alloy in the as welded and post weld heat treated conditions (solubilization and artificial aging-T6), obtained using GMAW and modified indirect electric arc (MIEA) welding processes are presented. Results showed that the base material along rolling direction exhibited a tensile strength of around 600 MPa and elongation of 11%. For the as welded condition, tensile strength was 260 MPa and elongation percent of 3%. This behavior was attributed to brittleness induced by the microstructural characteristics of the welded alloys, as well as high porosity. Hardness profiles along the welds were obtained and different welded zones were identified. A soft zone (~100 HV0.1) in the heat affected zone for GMAW and MIEA was observed, the minimum hardness corresponding to weld metal (~85 and ~96 HV0.1 for GMAW and MIEA, respectively). The high dilution between filler and base metal during welding in MIEA allows to the Zn and Cu to flow from the base metal into the weld metal, inducing hardening by solution and subsequent artificial aging. In this regard, the hardness of the weld metal for MIEA increases by 56%, while the tensile strength reaches a value close to 400 MPa. For GMAW, non-favorable hardening effect was observed for the weld metal after solution and artificial aging.