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Multistage stress-strain curve correction based on an instantaneous friction factor was studied for axisymmetric uniaxial hot compression of 7150 aluminum alloy. Experimental friction factors were calculated based on continuous isothermal axisymmetric uniaxial compression tests at various deformation parameters. Then, an instantaneous friction factor equation was fitted by mathematic analysis. After verification by comparing single-pass flow stress correction with traditional average friction factor correction, the instantaneous friction factor equation was applied to correct multistage stress-strain curves. The corrected results were reasonable and validated by multistage relative softening calculations. This research provides a broad potential for implementing axisymmetric uniaxial compression in multistage physical simulations and friction optimization in finite element analysis. [ABSTRACT FROM AUTHOR]

The precipitation kinetics and its effect on hot working behavior of an Al-Zn-Mg-Cu alloy were studied under non-isothermal conditions and over a wide range of temperatures. The characterization techniques employed included in situ electrical resistivity measurements, differential scanning calorimetry, hardness testing and microstructure examination by scanning and transmission electron microscopy. The results showed that the precipitation kinetics during non-isothermal treatments were highly dependent on the thermal paths. Slow cooling processes led to coarse and sparse particle distribution. When heating to high temperatures, very fine precipitates with high density and homogeneous distribution were present at low temperature. Such fine precipitates showed unstable characters and dissolved quickly at higher temperature. Quantification of precipitate evolution during non-isothermal processing was obtained from in situ electrical resistivity measurements. The effects of heating paths on dynamic and static flow behaviors were also demonstrated to be significant. The degree of initial supersaturation controlled the nucleation rates and led to strong effect on dynamic and static precipitation, and on the flow stress softening. [ABSTRACT FROM AUTHOR]

In situ electrical resistivity monitoring technique was employed to continuously evaluate the precipitate evolution of an Al–Zn–Mg–Cu-based commercial alloy during typical artificial aging treatments. The effects of artificial aging on the precipitates stability during non-isothermal heat treatments were also explored. Conventional hardness test, transmission electron microscopy and differential scanning calorimetry were also adopted to verify the electrical resistivity results. The results indicated that both the precipitation process and its timely rate could be followed by the monitored electrical resistivity during artificial aging treatments. The electrical resistivity results gave overall information on continuous precipitation and dissolution processes, especially under high heating rates. Samples artificial aging heat treated at 120 °C for 24 h followed by aging at 150 °C for 24 h presented more stable state and coarser precipitates than the samples only artificial aging heat treated at 120 °C for 24 h or triple artificial aging heat treated at 120 °C/24 h + 195 °C/15 min + 120 °/24 h. While the incoherent η precipitates in the samples artificial aging heat treated at 120 °C for 24 h followed by aging at 150 °C for 24 h were more easiness to coarsening and dissolve during non-isothermal heat treatments as well. [ABSTRACT FROM AUTHOR]

Previous studies have demonstrated that the static softening kinetics of 7150 aluminum alloy showed typical sigmoidal behavior at 400 °C and softening plateaus at 300 °C (F.L. Jiang, et al., Mater. Sci. Eng. A, vol. 552, 2012, pp. 269–275). In present work, the static softening mechanisms, the microstructural evolution during post-deformation holding was studied by optical microscopy, scanning electron microscope, electron back-scattered diffraction and transmission electron microscopy. It was demonstrated that recrystallization is essentially absent during post-deformation holding, and that static recovery was the main contribution to static softening. Strain induced precipitation and coarsening caused softening plateaus at 300 °C. In order to better understand the static softening mechanism, physically-based modeling, which integrated recovery and multicomponent particle coarsening modeling, was employed to rationalize the experimental results. [ABSTRACT FROM AUTHOR]

Isothermal and non-isothermal multistage hot compression tests were carried out on 7150 aluminum alloy at temperature range of 325–425 °C and strain rate range of 0.01−1 s −1 . During isothermal multistage hot compression, the flow stress decreased with increasing temperature, accumulative strain as well as decreasing strain rate and pass strain. However, the interval time had a complex influence on flow stress, which ascended flow stress values with increasing interval time at 425 °C. The isothermal and non-isothermal constitutive characteristics were studied based on hyperbolic-sine law, indicating complex influences of various processing parameters on material constants. Non-isothermal multistage flow stresses were found to be easier than normal continuous single pass method to manipulate for constitutive equations calculation with better linear fitting reference and comparable activation energy. Precipitation at low temperature and its dissolution at high temperature were found to affect flow behavior and constitutive characteristics remarkably based on microstructural observation of deformed sample. [ABSTRACT FROM AUTHOR]

Abstract: The hot deformation characters of two Al–Mn–Mg–RE alloys (without and with Ti addition, respectively) were studied by hot compression tests, and the deformed microstructures were observed by optical microscopy and transmission electron microscopy, respectively. The results show that the flow stress, hot deformation activation energy and stress exponent were higher with the addition of Ti during hot deformation. Less indistinct subgrain boundaries were also obtained with addition of Ti. The main enriched rare earth constituent particles could make positive purifying effects on impurity elements of Fe and Si in the Al–Mn–Mg–RE alloy. While the addition of Ti not only promoted the formation of a more stable Ti2 Al20 (Ce x , La1−x ) phase, but also resulted in more refined and dispersive constituent particles, which could improve the hot workability of the alloy. [Copyright &y& Elsevier]

Abstract: Isothermal continuous and interrupted hot compression tests have been carried out on Gleeble-1500 system at temperatures of 300°C and 400°C, strain rates of 0.01s−1 and 0.1s−1, respectively. The dynamic and static softening kinetics of 7150 aluminum alloy during multistage hot deformation were investigated by means of flow curves and relative softening rates. The results show that the flow curve exhibits a peak and some work softening during hot deformation of 7150 aluminum alloy. The flow stress increased with increasing strain rate and decreasing temperature while the flow softening decreased. The static softening increased with increasing strain rate, deformation temperature and delay time. The static softening curves appeared as a typical sigmoidal shape of the type at temperature of 400°C, while plateaus were observed when temperature decreased to 300°C. The appearance of static softening at 400°C exceeding 100% was observed. An empirical static recrystallization model based on Johnson–Mehl–Avrami–Kolmogorov model with the recrystallization activation energy of 219.17kJ/mol for 7150 aluminum alloy was derived. [Copyright &y& Elsevier]

Strong metals are greatly desired for lightweight and energy-efficient industrial design. The strengthening of metals is traditionally accomplished by the additive contributions of various obstacle families (e.g., solid solutions, particles, and grain boundaries) and dislocation self-interactions that impede dislocation motion. In the present work, unlike a traditional additive understanding, a distinctive nonadditive strengthening mixture rule for obstacles and dislocations were validated based on experimental and modeling analyses in numerous cold-worked steels and aluminum alloys. Concretely, numerous well-annealed body-centered cubic steels and face-centered cubic aluminum alloys were prepared, in which the hierarchical strength levels of solid solutions, grain boundaries, and/or particles were estimated. The above specimens were then cold rolled to various strain levels. Dislocation densities were quantified by utilizing X-ray diffraction line-profile-analysis, and the dislocation density was found to increase faster with an increasing strain level when a high strength was presented in well-annealed specimens. When plotting the yield stresses as a function of the square roots of the dislocation densities in massive distorted samples based on the Taylor hardening law, it is interesting to note that an approximate single linearity was obtained. Individual dislocation strengthening was found to respond to the total strength in the deformed specimens, which indicated that the full nonadditive strengthening mixture rule was employed between the obstacle families and the dislocation contributions. The mechanisms of the observed nonadditive strengthening were also discussed by implementing additional experiments and transmission electron microscopy observations. Then, the modified constitutive models based on both one and two internal parameters' Knocks-Mecking models were developed respectively, which excellently captured the effects of the various obstacle families on dislocation storage processes. The developed models also rationalized the observed nonadditive strengthening mixture rule. Image 1 • The dislocation densities accumulated faster in the cold-worked steels and Al alloys with higher primary strength. • Approximate single linearity was obtained by plotting the yield stresses as a function of square roots of the dislocation densities. • The full nonadditive strengthening mixture rule between the contributions of various obstacle families and dislocation junctions were observed. • The modified constitutive models based on both one and two internal parameters' Knocks-Mecking models were developed respectively. • The observed nonadditive strengthening mixture rule was captured well by the developed models. [ABSTRACT FROM AUTHOR]

In order to evaluate the role of external Si addition in the intermetallic formation of 8009 aluminum alloy, the mixed powders of AlSi20 alloy and rapidly solidified 8009 aluminum alloy were extruded and annealed under various annealing conditions. The employed characterization techniques included scanning electron microscope, energy dispersive spectroscopy, X-ray diffraction and transmission electron microscope. The results showed that the formation of Al 4.5 FeSi phase was enhanced while the contents of Al 12 (Fe, V) 3 Si dispersion in 8009 aluminum alloy matrix and Si phase were decreased in the annealed AlSi20/8009 aluminum alloy with increasing annealing temperature (480 °C–540 °C) and annealing time (15 min–60 min at 540 °C). A novel nano-sized phase containing 5.58–6.98Al, 64.14–66.89Si, 25.59–26.20V and 1.91–2.65Fe (at. %) was observed at annealing temperature of 540 °C, which occupied a slice stacking morphology and had many sharp edges and corners. By comparing with the annealed 8009 aluminum alloys, the external Si addition was found to be responsible for the formation of Al 4.5 FeSi and the nano-sized phases in the annealed AlSi20/8009 aluminum alloys. The intermetallic formation mechanism and the role of external Si addition were also discussed based on experimental observations. • The mixed powders of AlSi20 and 8009Al were extruded and annealed. • The effects of annealing temperature and time were investigated. • A novel nano-sized phase was observed at annealing temperature of 540 °C. • The mechanism of intermetallic formation were discussed. [ABSTRACT FROM AUTHOR]

Continuous severe plastic deformation (SPD) processes, which overcome the limitations of economic efficiency and sample size for classical SPD approaches, are promising industry technologies for manufacturing aluminum alloy products with refined microstructures and enhanced properties. In the present work, the repetitive continuous extrusion forming (R-Conform) process combined with auxiliary thermal treatments was implemented in Al-Zn-Mg-Cu alloys, which had poor workability. By combining microstructural observation, mechanical testing and finite element simulation, the influence of auxiliary heat treatments on the mechanisms of grain refinement and strengthening for Al-Zn-Mg-Cu alloys processed by R-Conform were systematically investigated. The results indicated that the grains of Al-Zn-Mg-Cu alloys were significantly refined by a continuous dynamic recrystallization (CDRX) during the R-Conform process with special thermomechanical conditions. The increased supersaturated solid solution concentration and constituent precipitate fraction caused by prehomogenization were found to accelerate CDRX for grain refinement through the pinning effect on dislocation motion. Due to the simultaneous effect of shear deformation and online solution, the R-Conformed alloys contained massive lattice distortions and high concentrations of solute atoms. As a result, the processed Al-Zn-Mg-Cu alloys combined with prehomogenization and direct aging treatments possibly possess a good combination of strength and ductility due to the integrated functions of fine grains, massive lattice distortion, and evenly distributed nanosized precipitates. [Display omitted] • R-Conform process with auxiliary thermal treatments was implemented in an Al-Zn-Mg-Cu alloy. • The influence of auxiliary heat treatments on grain refinement and strengthening were systematically investigated. • The processed alloys contained massive lattice distortions and solid solution atoms due to shear deformation and on-line solution. • The optimal process route for fabricating Al-Zn-Mg-Cu alloys with good strength and ductility was obtained. [ABSTRACT FROM AUTHOR]