Re-aging Treatment Research Articles

Effect of multi-stage aging treatments on the precipitation and mechanical properties of Al-Zn-Mg alloys

The mechanical properties of Al-Zn-Mg alloys via multi-stage aging treatments were studied to explore the strength-toughness trade-off. The microstructures of aged alloys were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). The results indicated that large numbers of GP zones and some part of η′ phases were formed during the low-temperature (90 °C) aging at the third-stage aging, which consumed considerable solid solubility. This prevents fast growth and coarsening of precipitates at the fourth-stage aging (150 °C). Finally, the mixed precipitation state of mainly η′ phases and some η phases formed. The grain boundary phase grew insignificantly, which is beneficial to decrease the strength gap between intragranular and grain-boundary. The intragranular equilibrium η phase is beneficial to pin the movement of dislocation, which avoids fracture caused by accumulation of dislocations around the large second-phase and on the grain boundaries. Although the strength of materials decreased slightly, the impact toughness of materials significantly increased due to the positive effect of the intragranular equilibrium η phase. Compared with the conventional retrogression and re-aging (RRA) treatment (120 °C/24 h + 185 °C/105min + 120 °C/12 h), the designed four-stage ageing treatment (FSA) (120 °C/24 h + 185 °C/105min + 90 °C/12 h + 150 °C/12 h) resulted in the decrease of tensile strength and yield strength by only 3% and 4%, respectively, but contributed to the increases of elongation and impact toughness by 15% and 17%, respectively.

Effect of different aging treatment on high temperature properties of die-forged Al-5.87Zn-2.07Mg-2.42Cu alloy

High temperature tensile test over the temperature range from 100 °C to 250 °C of die forged Al-5.87Zn-2.07Mg-2.42Cu alloy were carried out and the effect of aging treatment on microstructures and mechanical properties of alloy were studied. The strength of alloys processed with peak aged (T6), over aged (T73) and retrogression and re-aging (RRA) treatment suffered from different extent of decreasing for the increase of the stretching temperature. Among them, the RRA alloy has the most obvious drop in tensile strength, which has dropped from 615 MPa to 268 MPa with temperature rising from room temperature to 250 °C. While T6 and T73 alloys drop by 338 MPa and 333 MPa, respectively. With the increase of test temperature, the coarsening rate of precipitates in RRA sample is faster than that of T6 and T73, resulting in more significant decrease in the high temperature strength. The T73 sample exhibits good stability of high temperature mechanical properties, due to the slowing coarsen rate of precipitates.

Effect of repetitious retrogression and re-aging treatment on the microstructure, strength and corrosion behavior of powder hot-extruded 7055 Al alloy

The precipitation evolution, mechanical properties and corrosion behavior of powder hot-extruded 7055 Al alloy treated by repetitious retrogression and re-aging (RRA) were investigated. The results suggest that the dominant intragranular precipitates after different numbers of RRA treatment change from GP zones +η' phase (RRA1) to η' phase (RRA2, RRA3) and then to η' phase + η phase (RRA4), and both strength and corrosion resistance deteriorate with the increase of RRA repetition times. Transmission electron microscope (TEM) observation indicates that the coarsening and transformation of intragranular precipitates during repetitious RRA treatment induce the decrease of strength, while the increases in the area fraction of grain boundary precipitates (GBPs) and the width of precipitate free zones (PFZs) are detrimental for the corrosion resistance of the alloy. Therefore, the RRA1 treatment can achieve the best comprehensive performance of the powder hot-extruded 7055 Al alloy with excellent strength and good corrosion resistance.

Influence of various heat treatment procedures on the corrosion behavior of Al–Zn–Mg–Cu alloys

In this study, we have investigated the effect of different heat treatment sequences on the microhardness and corrosion resistance of Al–Zn–Mg–Cu. Five different heat treatment sequences include water quenching, solution heat treatment (SHT), and re-aging (RA) treatment for different times (0.5 h, 0.7 h, and 0.9 h). The microstructural analysis reveals the formation of a higher amount of precipitates in RA0.7 h sample while the least amount of precipitates were noticed for SHT sample. SHT dissolves most of the precipitates present in the alloys as confirmed from the microstructural analysis. The maximum value of hardness and yield strength (YS) were estimated for RA0.7 h and SHT treated samples, respectively. The electrochemical analysis depicted the lowest corrosion rate for RA0.9 h followed by SHT sample whereas the highest corrosion rate was noticed for T6 treated sample. It is also noteworthy that the corrosion rate of RA0.7 h is higher than the RA0.9 h among re-aged samples. The importance of heat treatment cycle for optimizing the mechanical and electrochemical performance of Al–Zn–Mg–Cu alloys under harsh conditions are also discussed.

Effect of carbides on high-temperature aging embrittlement in 12%Cr martensitic heat-resistant steel

It is well-known that the 9–12%Cr martensitic heat-resistant steels own excellent mechanical properties; yet, in the case of aging embrittlement mechanism, little has been documented with carbides effect. In this study, to understand the aging embrittlement mechanism in steel 10Cr12Ni3Mo2VN, we focused on high-temperature short-term aging embrittlement by investigating the relationship between the heat treatment, microstructures, and aging embrittlement. It is mainly interested in the effect of carbides on the impact toughness of the steel sample after a heat treatment. The results show that the steel appears a reversible short-term aging embrittlement at temperatures within 500–600℃. It was found that carbides play an important role in the aging embrittlement, especially the M7C3-type carbides. Carbides formed at boundaries cause boundaries weakening and induce tempered martensite matrix to fracture in quasi-cleavage mode, which leads to the aging embrittlement. It was also found that the aging embrittlement can be eliminated by re-tempering at a higher temperature due to the transformation of carbides from M7C3-type to M23C6-type. Aging embrittlement arises again but becomes weak after a re-aging treatment. In addition, by refining prior austenite grains, increasing temper temperature and reducing carbon content, it can improve the resistance to aging embrittlement in the sample as refining grains increase the resistance to brittle fracture; what’s more, it could reduce the precipitation of carbides at boundaries during aging by increasing temper temperature and reducing carbon content.

Effect of retrogression treatments on microstructure, hardness and corrosion behaviors of aluminum alloy 7085

The microstructure, hardness and corrosion resistance of aluminum alloy 7085 after different retrogression and re-aging (RRA) treatments were investigated. The results showed that the properties of alloy 7085 are sensitive to the temperature and dwell time of the retrogression treatment. Desirable mechanical property and corrosion resistance can be obtained for the alloy pre-aged at 120 °C for 24 h, followed by a retrogression at 160 °C for 1.5 h and re-aging at 120 °C for 24 h. After the optimized RRA treatment, the hardness of alloy 7085 is improved by 10.2% as compared with the peak-aged one, which is mainly due to the distribution of dispersed rod-like η′ precipitates in the matrix; while the improved corrosion resistance is mainly attributed to the discrete and coarse η phase with higher Cu content, and narrow precipitate-free zone about 45–50 nm in width along the grain boundaries.

Effect of RRA heat treatment on corrosion and mechanical properties of AA7075

In this study, a retrogression process (R) has been applied to AA7075 at 180, 240, and 370°C for 15, 30, 90, and 120 min. After the retrogression, reaging treatment (RA) was reapplied with T6 conditions (120°C for 24 hr). The effect of different temperatures and time periods of retrogression and reaging temper on exfoliation corrosion (EXCO) of the AA7075 is examined. The EXCO solution was prepared in accordance with the ASTM G34‐01 standard and the samples were kept in the solution for 48 hr. Susceptibility of the alloy to EXCO was determined by weight loss detection, macro and microstructure studies, scanning electron microscope, transmission electron microscope, and energy dispersive X‐ray analyses. The mechanical properties of aged samples were determined by hardness, tensile tests, and fractography.

Influence of retrogression and re-ageing heat treatment on the fatigue crack growth behavior of 7010 aluminum alloy

Aluminum alloys are widely used in aircraft structural components where light weight, high strength and good corrosion resistance are the primary requirements. These alloys are generally used in peak-aged (T6) condition in which they are susceptible for stress corrosion cracking. In the recent years, retrogression and re-ageing (RRA) treatment on aluminum alloy is carried out to enhance their corrosion resistance maintaining the ultimate tensile strength. The aim of this work was to study the influence of RRA treatment on the fatigue crack growth rate (FCGR) behavior. The 7010 aluminum alloy was heat treated to two different conditions i.e., T6 and RRA. The microstructures of these alloys were characterized by using TEM. Standard compact tension (CT) specimens were prepared and FCGR tests were carried out by using a 100 kN servo-hydraulic test machine as per ASTM E647-15e1. The constant amplitude FCGR tests were carried out at a stress ratio, R = 0.5 using sine wave loading pattern at 10 Hz. Crack length was monitored by following compliance technique. Microstructural studies show that RRA treated alloy contain fine and densely populated precipitates in the matrix along with coarse and discontinuous precipitates in the grain boundary. The fatigue crack growth rate was observed to reduce along with an increase in the threshold stress intensity factor range (ΔKth) for RRA treated alloy compared to the T6 alloy. The mechanisms for reduction in fatigue crack growth rate of RRA treated alloy is attributed to the microstructural modifications. The increased resistance is expected to enhance the damage tolerance capability of the alloy.

Effects of brazing temperature and post weld heat treatment on 7075 alloy brazed joints

The microstructural evolution and mechanical properties of brazed joints were evaluated before and after the post weld heat treatment. The microstructure and shear strength of brazed joints were improved with the brazing temperature increased, and the highest shear strength of 42 MPa was obtained when brazed at 600 °C. The coarse grains were presented in the joints brazed at the higher brazing temperature, which deteriorated the shear strength dramatically. Post weld heat treatment which including solution treatment and retrogression and re-aging treatment were performed to improve the microstructure and mechanical properties of brazed joints. Through the solution treatment, the brittle intermetallic compounds were reduced significantly and the recrystallization occurred in brazed joints. The coarse lath grains were transformed to the fine grains. After the solution treatment, the retrogression and re-aging treatment were performed and the precipitates which continuously distributed along the grain boundaries were dissolved into the aluminum matrix gradually with the retrogression temperature increased. The fine precipitates were uniformly distributed within the grains and discontinuously distributed on the grain boundaries. When the retrogression temperature was 200 °C, the highest shear strength of 108 MPa was obtained which increased by 160% compared to the shear strength of the brazed joints without post weld heat treatment.

Microstructure and mechanical properties of 7075 alloy during laser heat treatment

Influence of retrogression and re-aging (RRA) and laser retrogression and re-aging (LRRA) treatment on the microstructure, fracture surface and mechanical properties of 7075 alloy has been investigated. The results show that LRRA (650 W) can increase the hardness, wear resistance and ultimate tensile strength (UTS) of 7075 alloy. The maximum hardness is 181.2 HBW and the maximum UTS is 552.2 MPa. Although the change of elongation is less, the elongation of LRRA samples is higher than that of RRA sample. The small size of shallow dimples can increase the strength of the 7075 alloy. The mechanical properties of RRA sample is lower than that of LRRA (650 W) sample. There are many fine and homogenously distributed precipitates in the matrix after LRRA (650 W) treatment. The size of MPts is about 20 nm. The strength of 7075 alloy can be increased by fine and homogenously distributed precipitates.

Intergranular corrosion resistance of Zn modified 5××× series Al alloy during retrogression and re-aging treatment

The effects of retrogression and re-aging treatment on the intergranular corrosion behavior of Al-5.0Mg-3.0Zn alloys are systematically studied. The alloy's strength increases when retrogression temperature increases from 390 °C to 420 °C. Samples retrogressed at 390 °C then re-aged are very susceptible to intergranular corrosion and those retrogressed at 405 °C and 420 °C express good corrosion resistance. Then the alloy's corrosion susceptibility rises with increasing retrogression temperature. Grain boundary precipitates are coarsened and distributed discontinuously with decreasing retrogression temperature. Higher retrogression temperature results in higher Mg and Zn content difference and intergranular corrosion susceptibility.

Fatigue and corrosion behavior of in-service AA7075 aircraft component after thermo-mechanical and retrogression and re-aging treatments

Fatigue crack growth and corrosion are the two important failure mechanisms for aircraft structural components and, therefore, various treatments have been developed to improve the fatigue and corrosion resistance of aircraft materials. In the present study, thermo-mechanical and retrogression and re-aging treatments were applied to AA7075T7352 specimens, which were extracted from a nearly 40 years in-service F-4 Phantom component. The in-service component was selected in order to observe the influence of thermo-mechanical and retrogression and re-aging treatments on the properties of a used aircraft material and it was expected that the service life of the material is extended in the maintenance stage. In the experimental work, electrical, mechanical, fatigue crack growth, and corrosion tests were carried out using the specimens with T7352 (as-received), thermo-mechanical and retrogression and re-aging conditions. Based on the results, fatigue crack growth resistance of the material benefited from the thermo-mechanical and retrogression and re-aging treatments; however, both treatments lowered the corrosion resistance of the material.

Corrosion performance of 7075 alloy under laser heat treatment

Microstructure, exfoliation corrosion (EXCO), intergranular corrosion (IGC) and potentidynamic polarization test of the 7075 aluminum alloy after retrogression and re-aging (RRA) treatment, and laser retrogression and re-aging (LRRA), respectively, were studied by using scanning electron microscope, and transmission electron microscope (TEM). The results show that after pre-aging, laser treatment (650 W, 2 mm s−1) and re-aging a lot of matrix precipitates of alloy were precipitated again. The semi-continuous grain boundary precipitates and the wider precipitate-free zones (PFZ) improve the corrosion resistance of the alloy. The corrosion properties of the alloy after LRRA (650 W, 2 mm s−1) treatment are better than that after RRA treatment.

Effect of Retrogression Heat Treatment Time on Microstructure and Mechanical Properties of AA7010

The effect of retrogression time during retrogression and re-aging (RRA) treatment of AA7010 is evaluated by performing tensile tests and characterizing the microchemistry of the grain boundary precipitates (GBPs) using transmission electron microscope coupled with the energy-dispersive spectroscopy. Retrogression time is evaluated so that the ultimate tensile strength of the RRA-treated sample is equal to that of the T6-treated sample and the grain boundary microstructure similar to that of the over-aged (T7451) condition. The investigation reveals that the sample retrogressed at 200 °C for 20 min has UTS of 586 MPa which is equivalent to that of the T6 sample and 11.5% higher than that of the T7451 condition. The fracture toughness of the RRA-treated sample was 41 MPa√m. Microstructure of the RRA-treated sample is similar to T7451, along the grain boundaries and in the grain interior similar to that of the T6-treated sample. Energy-dispersive spectroscopy confirmed the increment of Cu content on the GBP’s with increase in the retrogression time, which is expected to improve the stress corrosion cracking resistance of the alloy.

Effect of retrogression and re-ageing heat treatment on microstructure and microhardness of aluminium 7010 alloy

Aluminium alloy 7010 is subjected to retrogression and re-ageing (RRA) heat treatment to study the influence of microstructural changes on hardness. Retrogression is performed at 190 °C for different time intervals ranging from 10 to 60 minutes. Optimum time for retrogression treatment is estimated based on the retrogression time that result with equivalent mechanical properties as that of peak aged (T6) condition. Retrogression performed for 30 minutes resulted with micro hardness of 203 HV, which is equivalent to that obtained by following T6 treatment. Microstructural characterization done with the help of transmission electron microscope (TEM) indicates RRA treatment results with the coarsened and discontinuous precipitates along the grain boundary which is similar to over aged (T7) condition, where as fine and densely populated precipitates in the matrix similar to T6 condition. Coarse and discontinuous grain boundary precipitates (GBP’s) improves resistance to stress corrosion cracking. Fine and dense precipitates in the matrix ensures hardness equivalent to that of T6.

Mechanical properties and corrosion behavior of a new RRA-treated Al–Zn–Mg–Cu–Er–Zr alloy

The effect of various retrogression and re-aging (RRA) treatments on mechanical properties and corrosion behavior of a new type Al–Zn–Mg–Cu–Er–Zr alloy was investigated by microhardness testing, tensile testing, intergranular corrosion (IGC) and exfoliation corrosion (EXCO) testing. The results show that the RRA treatment can effectively improve the IGC and EXCO resistance with less strength sacrificing because the grain interior precipitates and the grain boundary precipitates are similar to that of T6 temper and T73 temper. Meanwhile, as the microhardness of the retrogressed alloy reaches the peak value during the retrogression at 170 °C, the corresponding RRA-treated alloy possesses the better strength, but the corrosion resistance is poor in comparison with that of the retrogression at 190 °C. The optimal combination of strength, IGC and EXCO resistance is obtained after retrogression at 180 °C for 60 min. Moreover, for different heat treatment tempers, the corresponding microstructural evolution of the precipitates including Al3(Er, Zr) particles was also discussed in detail.

Retrogression on corrosion behavior of spray formed Al-7075

Abstract

Influence of heat input/multiple passes and post weld heat treatment on strength/electrochemical characteristics of friction stir weld joint

ABSTRACTThe V-95 and D-19 precipitation hardened Russian aluminum alloys are widely used in the Russian aircraft industry and these alloys are not weldable by conventional fusion weld techniques. This paper intends to evaluate the effect of spindle and weld speed on joint strength characteristics of a single pass (SP) and double pass (DP) friction stir lap weld through a common heat index and to analyze the effect of retrogression and re-ageing treatment (RRA) on joint strength and corrosion characteristics. The strength characteristics were analyzed by welding and shear testing of specimens and corrosion susceptibility of joint through immersion in EXCO solution as per ASTM G34. The trials revealed that the joint strength of the welded alloy is inversely proportional to the heat index and the DP weld provided significantly higher strength than an SP weld. The heat affected zone of the joint was found most sensitized to corrosion. The RRA treatment was found to improve the strength of the joints welded with higher heat input while it slightly degraded the joint strength for low heat input welds. The corrosion characteristics of the welded joint is also significantly improved by the post weld RRA treatment.

Effect of friction stir lap weld and post weld heat treatment on corrosion behavior of dissimilar aluminum alloys

This study has been undertaken to evaluate the electrochemical behavior of friction stir lap welded V-95CHT1 and D19CHT plates and to study the effect of post weld heat treatment on the corrosion characteristics of the welded alloys. The corrosion characteristics were tested as per ASTM G-34 standard by immersion in EXCO solution and analyzing the corroded plates through optical imaging and scanning electron microscopic analysis. Cyclic polarization tests were also undertaken to validate the results of the immersion test. It was observed that friction stir lap weld had sensitized the V-95 plate with the heat-affected zone of the joint found most susceptible and weld nugget least susceptible to corrosion. Corrosion resistance of the welded plates was also found to be higher for those joints that were welded employing process parameters providing higher weld heat input. Post weld retrogression and re-aging heat treatment was also found to be beneficial in increasing the corrosion resistance of the alloy.

Effect of Stress-Aging Treatments on Precipitates of Pre-retrogressed Al-Zn-Mg-Cu Alloy

Abstract According to retrogression and re-aging treatment (RRA), a novel retrogression and stress-aging treatment (RSA) was put forward for Al-Zn-Mg-Cu alloys. The effect of stress-aging time and external stress on precipitates of pre-retrogressed Al-Zn-Mg-Cu alloy was investigated by transmission electron microscope (TEM). The observation shows that there is a large number of matrix precipitates (MPts) and slightly discontinuous grain boundary precipitates (GBPs) in the pre-retrogressed alloy. The effects of stress-aging time and external stress on precipitates of the retrogressed alloy are significant. With the increase of stress-aging time and external stress, the size of MPts increases and the density of MPts decreases. Meanwhile, the size of GBPs, the distance between GBPs and the width of precipitate free zone (PFZ) increase. Compared with the traditional retrogression and re-aging treatment, the retrogression and stress-aging treatments increase the size of MPts, narrows the width of PFZ and make the GBPs more discontinuous for the studied Al-Zn-Mg-Cu alloy.