基于热加工图的7075铝合金热塑性变形工艺参数优化识别

在Gleeble-1500热模拟试验机上进行多组热压缩试验,得到7075铝合金在成形温度573～723K,应变速率0.01～10s-1下的真应力-应变数据,用此数据作为计算应变速率敏感指数（m值）、功率耗散因子（η值）、失稳判据（ξ（ε.）值）三重判据的基本模型。通过三重判据构建包含应变在内的7075铝合金热加工图,并观察试样变形后的微观组织来验证热加工图,最终判断该合金在试验范围内的最佳变形参数。结果表明7075铝合金热加工的安全区集中在高温低应变速率区,并随着应变的增加,η值逐渐增加;通过金相观察,稳定变形区,材料由于变形发生动态再结晶而使晶粒细化;不稳定变形区,裂纹伴随着流动位错带的产生而被发现,因此可以通过包含应变的热加工图所确定的最佳工艺参数来保证无缺陷的7075铝合金锻件。     

The room temperature mechanical properties of hot rolled 7075 aluminum alloy

The effect of equivalent rolling strain and temperature on the microstructural evolution and mechanical properties of 7075-T7351 aluminum alloy has been studied. This has been carried out using tensile testing method. The results indicated that the predominant restoration process during rolling at 250 and 350 °C is dynamic recovery, and the driving force is not high enough to trigger the dynamic recrystallization. However, the recrystallized grains are clearly evident in the microstructure of the specimens which were rolled at 450 °C. The tensile test results showed that the room temperature strength and ductility were decreased by increasing rolling temperature in the temperature range of 250–350 °C. The room temperature mechanical properties of the alloy were started to improve by increasing the temperature up to 450 °C. The observed trends were explained through considering the microstructural evolution upon hot rolling.     

An investigation to the hot deformation characteristics of AZ31 alloy through continuous cooling compression testing method

The design and control of thermomechanical processing (TMP) schedule are substantially facilitated by a thorough understanding of the dominant deformation mechanisms and phase transformations which occur in the alloy system under consideration. In this regards due to the effects of deformation history, the results of conventional methods such as dilatometry and thermal analysis are insufficient. In the present work, the hot deformation characteristics of AZ31 magnesium alloy has been studied through applying a series of continuous cooling compression (CCC) tests. The compression tests were conducted as the temperature was continuously reduced from 500 °C to 100 °C. The variations of true stress with true strain (or temperature) were extracted and the critical temperatures were determined. The derived true stress-true strain curves revealed five deviations at 440 ± 5 °C, 350 ± 5 °C, 300 ± 5 °C, 215 ± 5 °C and 150 ± 5 °C, which were properly addressed considering the related microstructural evolutions.     

Constitutive base analysis of a 7075 aluminum alloy during hot compression testing

In the field of deformation process modeling, the constitutive equations may properly represent the flow behavior of the materials. In fact, these valuable relationships are used as a calculation basis to simulate the materials flow responses. Accordingly, in the present study a hot working constitutive base analysis has been conducted on a 7075 aluminum alloy. This has been performed using the stress–strain data obtained from isothermal hot compression tests at constant strain rates of 0.004, 0.04 and 0.4 s⁻¹ and deformation temperatures of 450, 500, 520, 550 and 580 °C up to a 40% height reduction of the specimen. A set of constitutive equations for 7075 Al alloy have been proposed employing an exponent-type equation. The related material constants (i.e., A, n and α) as well as the activation energy Q for each temperature regime have been determined. The correlation of flow stress to strain rate and temperature can be deduced from the proposed equations. Furthermore, a change in deformation mechanism has been realized in the semi-solid temperature range. This has been related to the onset of lubricated flow mechanism during processing.     

Experimental investigation of microsegregation in low frequency electromagnetic casting 7075 aluminum alloy

7075 aluminum alloy ingots were prepared by the process of low frequency electromagnetic casting (low frequency electromagnetic casting = LFEC) and conventional direct chill (direct chill = DC) casting, respectively. The effects of low frequency electromagnetic field on microsegregation were investigated from eutectic analysis and electro probe microanalysis (EPMA). It was found that the amount of the eutectic, which was composed of α‐Al (aluminum phase), T phase and MgZn₂ was decreased markedly. In contrast, the solute concentration profiles which depend on solid fraction were increased to a certain extent with the presence of low frequency electromagnetic field. The effective distribution coefficient k_e was calculated, and the values of k_e for solutes in the LFEC sample were bigger than those in the DC sample. The LFEC process alleviated the microsegregation in 7075 aluminum alloy.     

Flow stress and ductility of AA7075-T6 aluminum alloy at low deformation temperatures

The present investigation has been conducted in order to develop a rational approach able to describe the changes in flow stress of AA7075-T6 aluminum alloy with deformation temperature and strain rate, when this material is deformed at temperatures in the range of 123-298 K at strain rates in the range of 4 × 10⁻⁴ to 5 × 10⁻² s⁻¹. The constitutive formulation that has been advanced to accomplish these objectives represents a simplified form of the mechanical threshold stress (flow stress at 0 K) model developed at Los Alamos National Laboratory (Los Alamos, New Mexico, USA). Thus, it is assumed that the current flow stress of the material arises from both athermal and thermal barriers to dislocation motion. In the present case, the effect of three thermal barriers has been considered: solid solution, precipitation hardening and work-hardening. The first two effects do not evolve during plastic deformation, whereas the last one is considered as an evolutionary component of the flow stress. Such an evolution is described by means of the hardening law earlier advanced by Estrin and Mecking (1984) [20]. The law is implemented in differential form and is integrated numerically in order to update the changes in strain rate that occur during tensile tests carried out both at constant and variable crosshead speed. The extrapolation of the hardening components from 0 K to finite temperatures is accomplished by means of the model earlier advanced by Kocks (1976) [19]. The results illustrate that the constitutive formulation developed in this way is able to describe quite accurately both the flow stress and work-hardening rate of the material, as well as temperature and strain rate history effects that are present when deformation conditions change in the course of plastic deformation. The evaluation of the ductility of the alloy indicates that the changes in this property are mainly determined by deformation temperature rather by strain rate. When deformation temperature decreases from 298 to 123 K, ductility also decreases from ∼35 to 24%. However, despite these relatively small variations, significant changes in the fracture morphology could be observed on the fracture surfaces of the examined specimens, with the predominance of a mixed ductile-brittle mechanism at lower temperatures.     

Mechanical and anisotropic behaviors of 7075 aluminum alloy sheets

Formability of 7075 aluminum alloy sheets was studied after annealing of 71% cold worked (CW) samples at different temperatures (270–450 °C). Uniaxial tensile test, deep drawing and Erichsen test were carried out at room temperature to evaluate formability parameters.     

Effect of aluminum on microstructural evolution during hot deformation of TX32 magnesium alloy

The effect of Al (0.4 and 1 wt%) addition on the hot deformation behavior of the Mg–3Sn–2Ca (TX32) alloy has been studied with the help of processing maps generated in the temperature and strain rate ranges of 300–500 °C and 0.0003–10 s^?1. The deformed specimens have been examined as regards changes in texture and microstructure using electron back scatter diffraction and transmission electron microscopy, respectively. The map for the TX32 base alloy exhibited two dynamic recrystallization (DRX) domains in the temperature and strain rate ranges: (1) 300–350 °C and 0.0003–0.001 s^?1, and (2) 390–500 °C and 0.005–0.6 s^?1. While 0.4 wt% Al addition to TX32 did not result in any significant change in the processing map, the map for the alloy with 1 wt% Al (TX32-1Al) exhibited four domains in the ranges: (1) 300–325 °C and 0.0003–0.001 s^?1, (2) 325–430 °C and 0.001–0.04 s^?1, (3) 430–500 °C and 0.01–0.5 s^?1, and (4) 430–500 °C and 0.0003–0.002 s^?1. In the first three domains, DRX has occurred, whereas in the fourth domain, grain boundary sliding takes place causing intercrystalline cracking in tension. In Domain 1 for all the alloys, DRX has occurred predominantly by basal slip and recovery by climb as confirmed by the resulting basal texture and tilt type sub-boundary structure. In Domain 2 of the base alloy and Domain 3 of the alloy with 1 wt% Al, second-order pyramidal slip dominates associated with cross-slip which randomizes the texture, and forms tangled dislocations and twist type sub-boundaries in the microstructure. The addition of 1 wt% Al causes solid solution strengthening and results in Domain 2 of the map of TX32-1Al alloy and in this domain basal+prismatic slip dominate.     

Effect of deformation temperature on microstructure evolution in aluminum alloy 2219 during hot ECAP

The effect of deformation temperature on microstructure evolution during equal channel angular pressing (ECAP) was studied in a coarse-grained aluminum alloy 2219 in a wide temperature interval from 250 to 475 °C. The structural changes taking place during ECAP up to strains of 12 are classified into the following three stages irrespective of deformation temperatures: i.e. (1) an incubation period for formation of the embryos of deformation bands (DBs) at low strains; (2) development of large-scale DBs followed by grain fragmentation at moderate strains; (3) rapid development of new grain at high strains. Microstructure development in stages 1 and 2 is hardly influenced by temperature, while that in stage 3 is most significantly affected at higher temperature. An increase in the pressing temperature leads to decreasing the volume fraction of new grains and increasing the average grain size in stage 3. This can be attributed to relaxation of strain compatibility between grains due to frequent operation of dynamic recovery and grain boundary sliding at higher temperature. The mechanism of grain refinement is discussed in detail.     

Fatigue properties of a modified 7075 aluminum alloy containing scandium

Fatigue properties such as the fatigue strength, fatigue notch sensitivity, and fatigue crack propagation rate, of a modified Al-7075 + Sc aluminum alloy were investigated in this study. The effects of solution treatment on the fatigue performance of this alloy were also investigated. The ultimate tensile strength of the as-extruded sample was 705.5 MPa. The ultimate tensile strength decreased by 12% after solution treatment. The fatigue limit σ_e of the as-extruded sample decreased from 201.2 to 154.4 MPa after solution treatment. The fatigue notch sensitivity for the as-extruded and solution-treated (ST) samples was 0.97 and 0.64, respectively. The crack growth rate in the as-extruded sample with fine precipitates was clearly lower than that of the ST sample that had coarse precipitates at R = 0.1 when ΔK < 15 MPa?m \sqrt m . However, the growth rates of both the samples were approximately the same when ΔK > 15 MPa?m \sqrt m . The higher yield strength of the as-extruded sample led to a lower crack growth rate when compared to the ST sample.     

LC4高强铝合金的慢应变速率拉伸试验

采用慢应变速率拉伸 (SSRT)技术测试了LC4铝合金在空气和质量分数为 3.5 %的NaCl溶液中的应力腐蚀断裂 (SCC)行为 .研究了应变速率对铝合金SCC行为的影响和氢在LC4高强铝合金应力腐蚀断裂过程中的作用 .试验结果表明 ,LC4合金具有SCC敏感性 ,在潮湿空气中发生应力腐蚀断裂 ,而在干燥空气中不发生应力腐蚀断裂 .对于长横取向的LC4铝合金试样 ,在应变速率为 1.331× 10 6s 1时 ,其SCC敏感性比应变速率为 6 .6 5 5× 10 6s 1时的敏感性大 .在潮湿空气和阳极极化条件下 ,铝合金的应力腐蚀断裂机理是以阳极溶解为主 ,氢几乎不起作用 .在预渗氢或阴极极化条件下 ,氢脆起主要作用 ,预渗氢时间延长可加速LC4合金的应力腐蚀断裂 .     

Microstructure-based multistage fatigue modeling of aluminum alloy 7075-T651

The multistage fatigue model for high cycle fatigue of a cast aluminum alloy developed by McDowell et al. is modified to consider the structure-property relations for cyclic damage and fatigue life of a high strength aluminum alloy 7075-T651 for aircraft structural applications. The multistage model was developed as a physically-based framework to evaluate sensitivity of fatigue response to various microstructural features to support materials process design and component-specific tailoring of fatigue resistant materials. In this work, the model is first generalized to evaluate both the high cycle fatigue (HCF) and low cycle fatigue (LCF) regimes for multiaxial loading conditions, with appropriate modifications introduced for wrought materials. The particular microstructural features of relevance to fatigue in aluminum alloy 7075-T651 include micron-scale Fe-rich intermetallic particles and rolling textures. The model specifically addresses the role of local constrained cyclic microplasticity at fractured inclusions in fatigue crack incubation and microstructurally small crack growth, including the effect of crystallographic orientation on crack tip displacement as the driving force. The model is able to predict lower and upper bounds of the fatigue life based on measured inclusion sizes.     

Microstructure evolution and mechanical properties of backward thixoextruded 7075 aluminum alloy

The present work deals with the microstructure evolution of 7075 aluminum alloy which has been backward thixoextruded in the temperature range of 550–600 °C using different ram diameters and ram displacement velocities. The recrystallization and partial melting (RAP) route has been used to obtain the semi-solid feedstocks for thixoforming. The results indicate that the back extruded microstructures mostly consist of semisolid grains which have been elongated along extrusion direction. The finest semisolid grain size has been obtained at lower deformation temperature and higher equivalent strain. This is attributed to the higher imposed shearing force on the liquid phase which could in turn fragment the grains. The current work also explores the room temperature mechanical properties of thixoformed work-pieces using shear punch testing method. It is found that the room temperature shear strength and ductility values have been substantially influenced by the deformation temperature, ram displacement velocity, and ram diameter.     

An investigation into the mechanical and tribological properties of plasma electrolytic oxidation and hard-anodized coatings on 6082 aluminum alloy

A ceramic coating on AA6082 aluminum alloy prepared by plasma electrolytic oxidation (PEO) has been studied and compared against a sulphuric acid hard-anodized coating on the same alloy. Surface morphology and microstructures of the coatings have been examined by scanning electron microscopy. X-ray diffraction is used to determine the phase composition of the coatings. The adhesion strength of the coatings has been evaluated using a scratch test method. The coating's mechanical properties such elastic modulus and hardness data have been generated using a dynamic ultra-microhardness tester. Sliding wear tests with different loading rates are performed on the coatings in order to assess their wear resistance. Test results show that the PEO treated samples exhibit significantly better mechanical properties compared to hard anodized samples. The elastic modulus and hardness of the PEO coating are 2-3 times greater than of the hard anodized coating and subsequently, an improved wear resistance of the PEO coating has been achieved. The mechanical properties of the coatings and their relations to their tribological performance are discussed.     

Hot compressive deformation behavior of 7075 Al alloy under elevated temperature

The hot compression tests were conducted with wide strain rates and forming temperature ranges to study the high-temperature deformation behavior of 7075 Al alloy. The material flow behavior and microstructural evolution during hot-forming process are discussed. Based on the measured stress–strain data, a new constitutive model is proposed, considering the coupled effects of strain, strain rate, and forming temperature on the material flow behavior of 7075 Al alloy. In the proposed model, the material constants are presented as functions of forming temperature. The proposed constitutive model gives good correlations with the experimental results, which confirms that the proposed model can give an accurate and precise estimate of flow stress for 7075 Al alloy.     

Effect of orientation on self-organization of shear bands in 7075 aluminum alloy

The spatial distribution of shear bands was investigated in the rolled 7075 aluminum alloy through the thick-walled cylinder (TWC) technique with 0°, 90° and 45° angles between the aluminum alloy cylinder axial direction and the rolling direction. Self-organization of multiple adiabatic shear bands was observed in different orientation specimens and investigated by using Schmid factor theories. The experimental results indicated obvious differences in the morphology and self-organization of shear bands for the specimens. At the initial stage, the spacing of the shear bands in the 0° specimen is smaller than in the other specimens. The nucleation of the shear bands in the 90° specimen is early. Due to the shielding effect, fast-developed shear bands block the development of the neighboring smaller shear bands in the 90° specimen. The spacing of the shear bands in the 45° specimen is much larger than in the other specimens under the similar effective strain. At the late stage, a large number of shear bands nucleate in the 0° specimen, and the spacing of the shear bands is small. The shear bands in the 90° specimen are well-developed with obvious shielding effect and the largest spacing. The 45° specimen has the maximum average nucleation rate of the shear bands. Owing to the close Schmid factors of the slip systems of the 45° specimen, the spacing of the shear bands in the 45° specimen is still larger than in the 0° specimen.     

Double aging and thermomechanical heat treatment of AA7075 aluminum alloy extrusions

The effects of double aging and thermomechanical double aging on enhancing mechanical properties and accelerating the kinetics of precipitation in aluminum alloy AA7075 were studied by means of hardness tests, tensile tests, and transmission electron microscopy. Using the appropriate heat treatment schedule, the time to peak aging was reduced by a factor of up to 36 without substantial decrease in hardness or tensile properties. The use of appropriate double aging and thermomechanical double aging showed that there can be a significant impact on energy savings and productivity resulting from the accelerated kinetics of precipitation.     

Corrosion-fatigue behaviour of 7075-T651 aluminum alloy subjected to periodic overloads

The corrosion-fatigue behaviour of 7075-T651 aluminum alloy subjected to periodic overloads was examined. This aluminum alloy is typically used in aerospace structural components such as the wing spars of aircraft. Axial fatigue specimens were subjected to a loading spectrum that consisted of a fully reversed periodic overload of near-yield magnitude followed by 200 smaller cycles at high R-ratio. The specimens were fatigue tested while they were fully immersed in an aerated and recirculated 3.5 wt% NaCl simulated seawater solution.The results for the corrosion-fatigue testing were compared to data obtained for the same overload spectrum applied in laboratory air. A damage analysis showed that the presence of the corrosive environment accelerated the damage accumulation rate to a greater extent than that observed in air, particularly at low stress ranges. This resulted in a reduction in the fatigue strength of the material when it was simultaneously subjected to overloads and a corrosive environment. It is believed that the reduced fatigue life was due primarily to corrosion pit formation and a combination of anodic dissolution at the crack tip and hydrogen embrittlement. For practical purposes, the endurance-limit of the material disappears under these conditions.