Re-aging Treatment Research Articles

Study of a Formability Process Chain for a Copper-Free Al-Zn-Mg-Alloy by a Retrogression and Re-Aging Treatment (RRA)

The material for the experiment was a copper-free Al-Zn-Mg-alloy. Because of natural aging after solution treating these materials are not storable at room temperature in T4 temper. Therefore 7xxx series Al alloys are mainly delivered in an artificially aged T6 temper. This state is not proper for a cold formability process, a thermal formability processes like hot working has to be use. In this paper a process chain based on so-called retrogression and re-aging (RRA) treatment was investigated to allow cold forming after a short time treatment. To achieve this tensile test specimens were tested under various retrogression temperatures (220 - 360 °C) and at times (1 - 7 min) to evaluate the flow curve. The properties like strain hardening exponent n and strength coefficient C of the Ludwik-Hollomon equation, a common approximation of the flow curve, are illustrated and the influence of the process parameter is discussed. A re-aging treatment at 130 °C for 16 h after retrogression of non-deformed and deformed material is also examined.

Microstructure and Stress Corrosion Cracking of Spray Forming 7075 Alloy after Retrogression and Re-ageing Treatment

A typical age-strengthening 7075 aluminum alloy with high tensile properties can be spray-formed through T6 treatment,but its stress corrosion cracking(SCC) property appears very poor after this treatment. On the other hand,the SCC property may be improved by T7 treatment but the tensile strength of the alloy then be seriously harmed. Therefore,the effects of four kinds of retrogression and re-ageing(RRA) treatments on the microstructure and mechanical properties of a spray forming 7075 alloy are investigated by using mechanical,electric and transmission electron microscope observation measures in this research. It is obtained that the tensile strength and electric conductivity of the alloy reaches up to 748 MPa and 22.8 MS/m respectively under the condition of retrogressed at 200 ℃ for 10 min,and re-aged at 120 ℃ for 24 h. During retrogression treatment,GP zone and η′ phase are dissolved in matrix,η′ phase is then re-precipitated at 200 ℃ retrogression treatment over 10 min,which leads to the slight increase of tensile strength of the alloy. Because of the precipitation of shaped-η′ phases during retrogression and the corresponding RRA treatment a lots of η′ phases re-precipitate and η phases at grain boundary may improve the SCC resistance of the alloy.

Study of Ageing Treatment on Spray Forming Al-Zn-Mg-Cu Alloy

The Al-Zn-Mg-Cu (7A04) alloys were prepared by using the method of spraying forming and the hot extrusion. The changes of microstructure and mechanical properties of the spray deposited Al-Zn-Mg-Cu alloy underwent ageing, regression and reageing treatment (RRA) was studied by scanning electron microscopy, X-ray diffraction and other methods. The results show that the tensile strength of the alloy aged at 120°C for 24 hours can reach the peak value (760MPa), but the elongation is only 4.8 percent. The toughness of the alloy regression treated at 160°C for 2 hours can increase to 9.0 percent, but its tensile strength decreases greatly (680MPa). The reaging treatment at 120°C can make the tensile strength up to 740MPa, while the elongation which is 8.2 percent still keeps a higher level.

Reprecipitation behavior in Al–Cu binary alloy after severe plastic deformation-induced dissolution of θ′ particles

The microstructure and Vickers hardness behavior of Al–4 wt.% Cu alloy bearing θ′ precipitates, subjected to different pass of multi-axial compression (MAC) and re-aging treatments, has been studied. θ′ precipitates are almost completely dissolved after 12 passes (ɛ = 4.8) of MAC, and a non-equilibrium supersaturated solid solution is obtained. When this solid solution is re-aged at 100 °C, a slight increase of the hardness occurs due to the formation of GP zones. Precipitation of equilibrium θ phase is observed along the grain boundaries during re-aging. That is completely different from that of the coarse grained materials. The discrepancy for this unique re-aging behavior is attributed to extremely increasing of nucleation sites for θ phase at non-equilibrium grain boundaries (GB), as well as a large number of dislocations and vacancies acting as diffusion channel for Cu atoms.

Enhanced Fatigue Crack Propagation Resistance in an Al-Zn-Mg-Cu Alloy by Retrogression and Reaging Treatment

The microstructures and fatigue crack propagation (FCP) behavior of an Al-Zn-Mg-Cu alloy in T761 and retrogression and reaging (RRA) conditions were characterized by employing differential scanning calorimetry, optical microscopy, scanning electron microscopy, transmission electron microscopy, and electron backscatter diffraction. The results suggested that coarse η′ precipitates were present in T761-treated sample, while fine dispersed η′ precipitates and GP zones were uniformly distributed in RRA-treated ones. Besides, the width of precipitate-free zones (PFZs) in T761-treated sample was found to be much greater than that in RRA-treated ones. Compared with T761-treated sample, the enhanced FCP resistance of RRA-treated sample was attributed to the shearable particles in matrix and narrow PFZs.

The effect of pre-ageing temperature and retrogression heating rate on the strength and corrosion behaviour of AA7150

The effects of processing condition variations in the retrogression and re-ageing (RRA) treatment of AA7150 have been investigated. The results reveal that the corrosion resistance and strength of RRA-treated samples are sensitive to the pre-ageing temperature and retrogression heating rate. With an intermediate pre-ageing temperature of 60 °C and a slow retrogression heating rate of 5 °C/min, RRA-treated samples can be tailored towards a good combination of both corrosion resistance and strength, owing to a smaller potential difference between grain-boundary precipitates and the matrix, and a larger volume fraction of strengthening precipitates in the matrix.

Microstructure and mechanical properties of Al-Zn-Cu-Mg-Sc-Zr alloy after retrogression and re-aging treatments

The mechanical properties and stress corrosion cracking (SCC) resistance of an Al-Zn-Cu-Mg-Sc-Zr alloy under different aging conditions were investigated. The dependence of microstructure and mechanical properties on aging parameters was evaluated by tensile test, hardness test and conductivity measurement. The results show that for the alloys with retrogression and re-aging treatment (RRA), the conductivity increases with the retrogression time and temperature, while the tensile strength decreases. The transmission electron microscopy (TEM) results show that the precipitates η(MgZn2) at grain boundary aggregate apparently with retrogression time and the precipitates inside the matrix exhibit the similar distribution to T6 temper, which comprises fine GP zones, large η′(MgZn2) and η(MgZn2) phases. According to the mechanical properties and microstructure observations, the optimal RRA regime is recommended to be 120 °C, 24 h + 180 °C, 30 min + 120 °C, 24 h. The strength level of the alloy after the optimum RRA treatment is similar to that in T6 condition and the SCC resistance is improved obviously in contrast to T6 condition.

Effect of Stepped Aging Treatment on Microstructure and Properties of AA7085 Aluminum Alloy

The effect of stepped aging treatment including two-stepped retrogression aging and retrogression reaging treatment on the mechanical properties, electrical conductivity and the microstructure of AA7085 has been investigated. Electron microscopy observations were used to analyze the microstructures and tensile fracture surfaces of AA7085 processed via various treatment schedules. Besides, X-ray diffractometer and differential scanning calorimeter were used to explore the thermodynamic factors of heat treatment. Through the investigation of the effect of the retrogression time on the properties and microstructure of AA7085, the optimized retrogression time was confirmed. The results of comparing retrogression aging and retrogression reaging treatment showed that through RRA treatment, higher conductivity and fracture toughness were gained. Through the optimized RRA treatment based on appropriate retrogression time, the tensile strength, elongation, fracture toughness and conductivity of AA7085 were raised to 660MPa, 12%,36.6MPa•m1/2and 38.1%IACS.

Retrogression and re-aging treatment on short transverse tensile properties of 7010 aluminium alloy extrusions

The influence of retrogression and re-aging treatment (RRA) on short transverse tensile properties of 7010 aluminium alloy extrusions was studied. The short transverse ductility of extrusions, which was much lower in the T6 condition, was improved to the optimum level after retrogression and re-aging treatment. It is found that short transverse ductility is influenced by the nature of precipitate particles located along the grain boundary. It is observed that coarsening of the grain boundary precipitates and its copper enrichment that occurs during RRA are found to be the factors responsible for improvement in stress corrosion cracking (SCC) resistance. The optimum retrogression and re-aging schedule is established that gives rise to the best combination of strength, ductility and SCC resistance.

Influence of repetitious-RRA treatment on the strength and SCC resistance of Al–Zn–Mg–Cu alloy

Influence of repetitious-retrogression and reaging (repetitious-RRA) treatment on the strength and SCC resistance of Al–Zn–Mg–Cu alloy was investigated by the slow strain rate technique (SSRT) and electrical conductivity test, using TEM, EDS and SEM. Additionally, the strength of alloy was characterized by Vickers hardness. The results show that within 3 times RRA treatments, SCC resistance increases with the increase of time, while maintaining strength as compared to RRA treatment, TEM observation along with EDS analysis indicates that within 3 times, matrix precipitates (MPs) in repetitious-RRA treatment are similar to those by RRA treatment and grain boundary precipitates (GBPs) become more discrete and coarser with the time. However, RRA treatment reaches 4 times, the SCC resistance gets worse because MPs are also coarse. Besides, the Cu content of GBPs increases with the time, inversely for Zn content, which is another reason for improving the SCC resistance.

Influence of retrogression and re-aging treatment on corrosion behaviour of an Al–Zn–Mg–Cu alloy

Influence of retrogression and re-aging treatment on the microstructure, strength, exfoliation corrosion, inter-granular corrosion and stress corrosion cracking of an Al–Zn–Mg–Cu alloy has been investigated by means of optical microscope (OM), transmission electron microscope (TEM) and electrochemical impedance spectroscopy (EIS). The results show that retrogression and re-aging treatment can increase the size and the distribution discontinuity of the grain boundary precipitates, and lead to the increase of the corrosion resistance without the loss of strength and ductility. In addition, the analysis of electrochemical impedance spectroscopy shows that retrogression and re-aging treatment can enhance the resistance to exfoliation corrosion.

The Effects of Retrogression and Reaging on Aluminum Alloy 2099 (C458)

A retrogression and reaging (RRA) treatment was performed on 2195 Al-Li Alloy. The exposure times were from 5 to 60 min, and the temperatures were from 200 to 250 °C. Samples that were exposed to a salt spray test had overall similar mechanical properties as compared to those that were not exposed. The percent elongation, however, was significantly deteriorated due to the salt spray exposure. The mechanical properties of the 2195 samples were compared to those of 2099 samples exposed to similar treatments in an earlier study.

Small angle X-ray scattering study of the strengthen mechanism of Al–Mg–Li alloy during retrogression and reaging treatment

The δ′ precipitation behavior during retrogression and reaging (RRA) heat treatment in 1420 alloy has been investigated by small angle X-ray scattering (SAXS) method. SAXS results indicate that the transition interfacial layer exists between δ′ phase and matrix in earlier aging and reaging stages. The diffuse phase boundary gradually disappears with the increase of aging and reaging time. The specific inner surface ( S p) rapidly decreases during aging to retrogression. The sample of retrogression 5 min has a sharply defined phase boundary between δ ′ phase and matrix. The gyration radius of the particle decreases. Some larger and smaller δ ′ particles coexist in the sample. RRA treatment results in an increase in the volume fraction of δ ′ phase particles.

Microstructure and properties of an Al-Zn-Mg-Cu alloy pre-stretched plate under various ageing conditions

The microstructures after various ageing treatments and their relation to the strength, fracture toughness, and corrosion behavior of an Al-Zn-Mg-Cu alloy pre-stretched plate were investigated. The results show that retrogression and reaging (RRA) treatment led to a combination of high strength and stress corrosion cracking (SCC) resistance of the alloy. The TEM microstructure of the RRA-treated alloy is a distribution of very fine precipitates in the aluminum matrix grains, similar to that obtained under T6 condition, and the distribution of coarse η MgZn 2 precipitates on the grain boundaries similar to that obtained by T7 temper. SEM observations revealed that most of the intergranular fracture characteristics were present on the fracture surface of both the T6 and RRA-treated specimens. On the contrary, the fractographs of the T7 treated specimens mainly consisted of dimple-type ductile transgranular fracture with minor intergranular cracking.

Improved Resistance to Stress-Corrosion-Cracking Failures via Optimized Retrogression and Reaging of 7075-T6 Aluminum Sheets

The effect of retrogression and reaging (RRA) treatment on the hardness and stress corrosion cracking (SCC) susceptibility of 7075 aluminum alloy sheets was systematically investigated. The material in its as-received T6-temper state was retrogressed at 170 °C, 220 °C, 240 °C, and 380 °C for different durations between 15 seconds and 60 minutes. Subsequent reaging was performed at the T6 aging condition (120 °C for 24 hours). RRA-treated alloys exhibited a better combination of hardness and SCC resistance than the T6-temper state when retrogressed between 170 °C and 240 °C for short durations. However, a retrogression temperature of 380 °C resulted in considerable reduction in both hardness and SCC resistance. Among retrogression temperatures employed, 170 °C provided an optimum combination of hardness and SCC resistance, corresponding to 20 pct improvement in hardness and 85 pct improvement in SCC resistance. In this article, improvements in hardness and SCC resistance of 7075 alloy upon RRA treatment are discussed.

Redistribution and re-precipitation of solute atom during retrogression and reaging of Al-Zn-Mg-Cu alloys

The redistribution and re-precipitation of solute atom during retrogression and reaging of three different Al-Zn-Mg-Cu aluminum alloys were investigated. The results of hardness and tensile strength test indicate that after pre-aging at 100 °C or 120 °C and retrogressing at 200 °C for various time and re-aging treatment, the hardness and strength of the alloys are all larger than those under pre-aging condition, some of them even exceed the value under peak aging(T6) condition. TEM observation shows that the PFZ formed during retrogressing in short time becomes narrow and even disappears after re-aging treatment. However, the PFZ formed during retrogressing for a long time does not narrow after re-aging treatment. It is suggested that the redistribution and re-precipitation of solute atom during re-aging treatment result in the narrowing and even disappearance of the PFZ formed during retrogression, which reinforces the grain-boundaries and presents the value of tensile strength exceeding peak-aging strength in the RRA condition, while the precipitates in the matrix of the alloys still keep or even exhibit a more dispersed distribution, and a greater effect of precipitation strengthening is obtained.

Effect of heat treatment on the microstructure and mechanical properties of the spray-deposited Al–10.8Zn–2.8Mg–1.9Cu alloy

In this study, effect of various aging tempers (T6, T73 and RRA treatment) on the microstructure and mechanical properties of the spray-deposited Al–10.8Zn–2.8Mg–1.9Cu alloy was studied using high-resolution electron microscopy, selected area diffraction, and tensile tests. The results indicate that the two types of GP zones, GPI and GPII, are major precipitates for the alloy under T6 condition. No clear precipitation free zone was observed, and the grain boundary precipitates were continuous. Under two-step aging condition, the GP zones and η′ are major precipitates for the alloy, the discontinuous grain boundary precipitates are favorable to SCC resistance in over-aged condition, which reduces its strength 58 MPa (about 7%) compared to the peak-aged condition. After retrogression and re-aging treatment, the grain boundary precipitates are discontinuous, which is closed to that resulting from T73 temper. RRA treatment decreased ultimate tensile strength 25 MPa (about 3%) in values compared with the alloy at T6 condition.

Ageing behavior of an Al-Zn-Mg-Cu alloy pre-stretched thick plate

The ageing behavior of a pre-stretched thick plate of Al-Zn-Mg-Cu alloy was systemically studied including one-step ageing, two-step ageing, and retrogression and reageing treatment (RRA). One-step ageing of the alloy resulted in peak ultimate tensile strengths of 595 and 575 MPa after 22 and 6 h at 120 and 135°C, respectively. The strengthening phase in peak aged (T6 temper) alloy contained GP zones and the η' phase predominantly. After two-step ageing, the electrical conductivity was increased markedly, but the pre-stretched thick plate sacrificed a great loss of strength. RRA treatment provided a method for maintaining the strength close to that obtained by T6 temper and for obtaining the high electrical conductivity close to that obtained by T7 temper; the ultimate tensile strength and electrical conductivity were 583 MPa and 21.0 MS/m, respectively. TEM analysis of T7 and RRA specimens revealed two types of precipitates that contributed to age strengthening i. e. the η' and η phases.

Effects of RRA Treatments on Microstructures and Properties of a New High-strength Aluminum-Lithium Alloy-2A97

A new high strength 2A97 Al-Cu-Li-X alloy was subjected to triple-aging of retrogression and re-aging treatments (RRA). Transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and tensile tests were used to investigate the effects of RRA treatment on the microstructures and properties. DSC test reveals the reversion temperature range of the strengthening δ' (Al3Li) phase. The results show that the microstructure consists of δ' (Al3Li) phase, T1 (Al2CuLi) phase and θ″/θ'(Al2Cu) phase for 2A97 alloy treated by a triple-aging of a retrogression and re-aging treatment in the following order: (1) at 165 °C× 30 min, (2) at 220 °C or 240 ° × 15 min, (3) at 165 °C× 24 h. The plastic deformation, incorporated into the treatment after secondary high temperature aging, promotes the T1 precipitation during final re-aging. The tensile properties of the alloy treated by the retrogression and re-aging treatment reach the peak level of alloy single-aged at 165 °C in T6 temper.

Hydrogen embrittlement studies of aged and retrogressed-reaged Al–Zn–Mg alloys

The hydrogen embrittlement (HE) of Al–4.35Zn–1.4Mg–0.059Zr alloys in different artificial aging tempers and after retrogression and reaging (RRA) treatments has been investigated by tensile testing hydrogen precharged specimens. The influence of RRA and hydrogen charging on the dislocation structure was studied by TEM. The under-aged temper was the most susceptible while the over-aged temper was the most resistant to HE. The RRA treatment improved the HE resistance of all the tempers. This has been attributed to the reduction in dislocation density upon retrogression and reaging. Flat fractography features near the surface of the hydrogen charged specimen have been correlated to the depth of hydrogen penetration. The hydrogen dislocation interaction and hydride cracking mechanism of HE have been addressed.