Influence of Cold Work on the Microstructural Evolution and Hardness During Aging of AA6061 Alloy

It is well known that the kinetics of precipitation is altered with the degree of cold working and this study aims to bring out the effect of cold working on the aging response of the alloy AA6061 along with the studies of microstructural evolution. The as-received samples were initially solutionized at 530 °C and subsequently aged at 160 °C for varying time from 2 to 28 h at intervals of 2 h. A peak hardness of 124 HV was observed at 15 h beyond which the hardness decreased due to overaging. The solutionized samples were also subjected to various degrees of cold working and the increase in hardness by virtue of strain hardening was observed. The samples that were cold worked to various levels (10, 20, 30% etc.) were subsequently aged at 160 °C for 4 h. A peak hardness of 134 HV was obtained for the sample cold worked to 75% and subsequently aged for 4 h and this was attributed to the increase in the dislocation density that improved the kinetics of precipitation. The evolution of microstructure for various samples during the course of aging was observed using transmission electron microscopy and the changes in the properties were correlated to the obtained microstructure.     

焊后时效对6061-T6铝合金搅拌摩擦焊接头的影响

对4组6061-T6铝合金搅拌摩擦焊接头进行焊后人工时效处理,采用OM,SEM等分析了时效处理工艺对接头的组织和力学性能的影响规律和机制。研究表明:焊缝截面形貌呈现典型的3个区,时效处理后,焊缝区晶粒形貌无显著变化,晶界较自然时效清晰,晶内析出相也明显增多;接头强度和显微硬度随人工时效处理时间的延长而提高,提高幅度随保温时间逐渐减小;时效处理为8 h时接头力学性能优良,接头抗拉强度均达到265 MPa以上,约为母材的93.0%;焊核区硬度达到90 HV,约为母材的90%。时效处理中,焊接过程中固溶在基体中的第二相粒子析出,形成弥散强化效应,大幅提高接头的强度和硬度。随着时效时间的延长,析出粒子逐渐较少,强化效应逐渐减弱;接头最薄弱的区均位于后退侧的热机械影响区,其最低硬度值约为母材的75%。拉伸断口均与拉伸方向成约45°角,断口平整,呈典型的切断断口形貌,与自然时效接头的断口形貌相比,时效处理后接头的断口韧窝大而浅,塑性稍有降低。     

Modeling of Strain Hardening in the Aluminum Alloy AA6061

In this paper, the evolution of work-hardening and dynamic recovery rates vs the flow stress increase (σ ? σ _y ) in Al-Mg-Si alloys is presented. The experimental data have been extracted from stress–strain curves. All curves show an initial very rapid decrease in slope of the σ–ε curve, which is associated with the elastic–plastic transition. After the elastic–plastic transition, there are typically two distinctive behaviors. For underaged alloys, there is an approximately linear decrease of work-hardening rate as (σ ? σ _y ) increases. However, for overaged alloys after elastic–plastic transition, there is a plateau in the work-hardening rate followed by an almost linear decrease. The maximum work-hardening and dynamic recovery rates are found to be dependent on the aging state. In order to investigate these phenomena, a model has been employed to simulate the work-hardening behavior of Al-Mg-Si alloys. The model is based on a modified version of Kocks–Mecking–Estrin (KME) model, in which there are three main components: (1) hardening due to forest dislocations, grain boundaries, and sub-grains; (2) hardening due to the precipitates; and (3) dynamic recovery. The modeling results are discussed and compared with the experimental data.     

Buckling of Multiwalled Carbon Nanotubes Using Timoshenko Beam Theory

A Timoshenko beam model is presented in this paper for the buckling of axially loaded multiwalled carbon nanotubes surrounded by an elastic medium. Unlike the Euler beam model, the Timoshenko beam model allows for the effect of transverse shear deformation which becomes significant for carbon nanotubes with small length-to-diameter ratios. These stocky tubes are normally encountered in applications such as nanoprobes or nanotweezers. The proposed model treats each of the nested and concentric nanotubes as individual Timoshenko beams interacting with adjacent nanotubes in the presence of van der Waals forces. In particular, the buckling of double-walled carbon nanotubes modeled as a pair of double Timoshenko beams is studied closely and an explicit expression for the critical axial stress is derived. The study clearly demonstrates a significant reduction in the buckling loads of the tubes with small length-to-diameter ratios when shear deformation is taken into consideration.     

Wear Behavior of the Lead-Free Tin Bronze Matrix Composite Reinforced by Carbon Nanotubes

Copper-coated carbon nanotubes were prepared by the electroless plating route. The structure and component of copper/carbon tubes were characterized using a transmission electron microscope and energy dispersive spectrometer. The results show that the surface of the carbon tubes was covered by the copper particles. Copper/carbon tubes were used as the substitute of part of tin and all of lead in the tin bronze matrix, and the tribological properties of carbon nanotube-reinforced Cu-4 wt pct Sn-6 wt pct Zn composites were studied. The effects of the carbon nanotube volume fraction and sliding distance in unlubricated ball-on-disc wear test were investigated. The 3 vol pct carbon nanotube-reinforced Cu-4 wt pct Sn-6 wt pct Zn composite shows the Vickers hardness of 126.9, which is approximately 1.6 times higher than that of Cu-6 wt pct Sn-6 wt pct Zn-3 wt pct Pb tin bronze. The wear rate and average friction coefficients of 3 vol pct carbon nanotube-reinforced Cu-4 wt pct Sn-6 wt pct Zn composite were lower than those of the Cu-6 wt pct Sn-6 wt pct Zn-3 wt pct Pb tin bronze, respectively.     

7075合金的双级时效 节能减排新工艺

分析了7075合金的成分与状态,并对等温时效及双级时效进行了研究,得出采用双级时效处理7075合金,节省能源,减少污染,提高生产率。     

碳纳米管/聚合物纳米复合材料研究进展

碳纳米管/聚合物复合材料是近几年来的一个研究热点。碳纳米管具有独特的力学、电学性能,而聚合物则易于加工、适用性强,将两者的优势结合起来可以制备出性能优异、用途广泛的新材料。本文从碳纳米管/聚合物制备方法、性能及应用等方面进行了综述,并对存在的一些问题进行了讨论。     

Effect of Multiwalled Carbon Nanotubes on the Thermal Conductivity and Porosity Characteristics of Blast Furnace Carbon Refractories

Different amounts of carbon nanotubes (CNTs) (0–5 mass pct) containing carbon refractory specimens for a blast furnace were prepared and coked for 3 hours at 1473 K (1200 °C) and 1673 K (1400 °C). The thermal conductivity and porosity characteristics of the coked specimens were evaluated using the flash diffusivity technique and mercury porosimetry, respectively. It was found that CNTs acted as carbon source, and most of them were consumed during coking. With the increase of CNT content, the aggregation of CNTs became more severe, the amount of SiC whiskers formed increased and their aspect ratio became larger, and the SiC whiskers tended to be distributed nonhomogeneously. The thermal conductivity of a 4 mass pct CNT containing a carbon specimen was highest because of the contributions of SiC and residual CNTs. The porosity characteristics of a 0.5 mass pct CNT containing a carbon specimen was best because of the uniform filling of SiC whiskers. The excessive addition of CNTs degraded the porosity characteristics because of the severe aggregation of CNTs.     

Studies on the Work-Hardening Behavior of AA2219 under Different Aging Treatments

Uniaxial tensile tests were performed to examine the influence of the precipitation state on the yield strength and work-hardening behavior of AA2219 for different aging treatments. The microstructural observations in four aging treatments (viz. natural aging, underaging, peak aging, and overaging) were made through transmission electron microscopy (TEM) to understand the type of phase or intermediate stages of the phase present (Guinier–Preston (GP) zones, θ″, θ′, and θ). To characterize the work-hardening behavior, the analysis of the experimental results has focused on two parameters, viz. the initial work-hardening rate Θ_max (≡dσ/dε) and the slope (dΘ/dσ) of the Θ-σ plot, which is related to the rate of dynamic recovery. The initial work-hardening rate (Θ_max) first drops as aging proceeds and then increases significantly upon overaging. The large increase in Θ_max is also associated with a concomitant increase in the slope (dΘ/dσ) of the Θ-σ curve. The material constants in the differential equation for the dislocation density are evaluated and flow stress vs plastic strain curves are generated using the flow stress contributions from the solid-solution, dislocation, and precipitation hardening. The model predictions are found to be in excellent agreement with the experimental data for a range of precipitation states from underaged (UA) to overaged (OA) conditions. Curves of flow stress due to dislocation hardening with the plastic strain were also generated in the presence of shearable and nonshearable precipitates.     

硝酸铈对6061铝合金磷化过程的影响

通过SEM,XRD,FTIR和电化学的方法,研究了硝酸铈对6061铝合金磷化过程的影响,并对铝合金磷化动力学行为进行了探讨.结果表明,硝酸铈的加入使得在铝合金表面上所形成的磷化膜结晶细化,成膜速度加快,磷化完成时间缩短.磷化膜组成为Zn3(PO4)2*4H2O和Zn2Fe(PO4)2*4H2O,硝酸铈并没有构成磷化膜的组成成分,硝酸铈在铝合金磷化过程中起到了形核与促进磷化的作用.     

On Plastic Deformation Behavior of Cryorolled AA8090 Alloy

AA8090 alloy was rolled up to 50 and 75 % reductions at both liquid nitrogen (LNR) and room temperatures (RTR). Both hardness and tensile behavior were evaluated on rolled samples. Optical microscope, TEM and EBSD were used for detailed microstructural examination of rolled samples. Williamson-Hall peak broadening analysis on X-ray diffraction data was made to evaluate crystallite size, lattice strain and dislocation density. An enhanced tensile strength was evidenced in LNR samples when compared to RTR samples without sacrificing ductility, which was ascribed to the higher density of dislocations in LNR samples than RTR samples. A large number of dislocation tangled regions along with ultrafine grain structure were evidenced through TEM and EBSD. Significant fraction of special boundaries in combination with increased fraction of texture components like S, Brass, Cu and Goss would be another reason for enhanced properties in LNR conditions than that of RTR. These components were observed to be strengthened with increased rolling reduction. Work hardening behavior clearly evidenced the variation in amount of work hardening and recovery phenomenon. It showed large variation in recovery in the case of 50 % reduction than that of 75 % reduction, which was attributed to significantly higher density of dislocations in 75 % rolled samples in RTR and LNR.     

Shear Behavior of AA6061 Aluminum in the Semisolid State Under Isothermal and Nonisothermal Conditions

The shear behavior of a 6061 aluminum alloy was studied in the semisolid state at large solid fractions. The tests were carried out either at constant temperature after partial solidification (i.e., isothermal shear tests) or during solidification at low cooling rate (i.e., nonisothermal shear tests). In isothermal conditions, results show that (1) the mechanical behavior depends on the volume fraction of the solid phase present in the sample at the temperature of the test, (2) there is a critical solid fraction corresponding to the coalescence of the solid grains beyond which shear stress increases very sharply with increasing solid fraction, and (3) the mushy alloy exhibits viscoplastic behavior with a strain-rate-sensitivity parameter close to about 0.17. In nonisothermal conditions, results show that stress increases continuously with decreasing temperature whatever the strain rate. However, at high strain rate, it was observed that cracks developed when the solid fraction approaches 1, leading to a slower stress increase compared to that observed at low strain rate. Finally, modeling of this behavior is carried out by considering a cohesion parameter of the solid phase, which depends on solid fraction and strain rate.     

Effect of Dispersoid Size and Volume Fraction on Aging Behavior and Mechanical Properties of TiO2-Dispersed AA7075 Alloy Composites

TiO₂-dispersed AA7075 alloy composites were produced by mechanical milling followed by hot uniaxial compaction and sintering. The effects of volume fraction and dispersoid size on precipitation kinetics, densification, and hardness of the composites were studied in detail. While the sinterability of the composites decreases with increasing volume fraction of the particulate reinforcement (dispersoid), the same increases with decreasing particle size of the reinforcement. Microstructural analysis using X-ray diffraction and scanning electron microscopy shows an improvement in the distribution of reinforcement with decreasing particle size. The hardness of the composites increases with increasing volume fraction and decreasing TiO₂ particle size. Further, the reinforced composites do not show age hardenability unlike unreinforced AA7075 alloy. Microstructural analysis reveals the formation of MgTiO₃ and ZnO near the TiO₂-AA7075 interface, which suppresses the formation of Guinier-Preston (GP) zone resulting in no age hardenability of the composites.     

Influence of the Tool Pin and Shoulder on Microstructure and Natural Aging Kinetics in a Friction-Stir-Processed 6061–T6 Aluminum Alloy

The influence of the stirring pin and pressing tool shoulder on the microstructural softening during friction-stir processing (FSP) and subsequent natural aging behavior was investigated for a 6061-T6 aluminum alloy. The evolution of hardness profiles in various characteristic regions of the FSP plates was investigated as a function of time from 4 to 5760 hours after the FSP through the thickness of the plates and correlated to the microstructure and residual strain profiles measured by a neutron-diffraction technique. The results show that the microstructural softening and the natural aging observed in the dynamic recrystallized zone and thermomechanically affected zone are mainly caused by the frictional heating from the tool shoulder, resulting in dissolution and reprecipitation of strengthening precipitates. On the other hand, the softening in the heat-affected zone is due to the dissolution/growth of the precipitates and is not followed by the natural aging under the current processing condition. The kinetics of the natural aging behavior is also discussed.     

碳纳米管对镁锌合金组织及性能的影响

采用钟罩浸块铸造法制备了不同碳纳米管含量的镁锌合金铸锭,经热挤压后制备出不同碳纳米管含量变形镁锌合金,研究碳纳米管作为增强材料加入到不含铝的镁锌二元合金中,对其组织和性能产生的影响。利用光学显微镜、扫描电镜及拉伸强度实验机对其显微组织形态及力学性能进行了分析。结果表明,碳纳米管的加入能够显著细化合金铸锭的晶粒起到了细化树枝晶二次枝晶臂间距和转变晶体生长方式的作用,且经过热挤压变形后组织变得更加均匀细小,具有细晶组织;碳纳米管对热挤压镁锌合金的力学性能有增强作用,提高了镁锌合金的抗拉强度、屈服强度、延伸率,当加入量为1.0%时,分别达到最大值219.740,215.969 MPa,27%,其中以延伸率的提高最为显著,抗拉强度和屈服强度值趋于稳定,可得碳纳米管加入到镁锌合金中,起到了较好的复合效果,对晶粒和晶界起到细化和强化的作用,使得该材料具有较高塑形的同时没有降低其强度;合金的断裂特征为典型韧性断裂,有较深的圆形韧窝和撕裂棱组成,碳纳米管与基体合金结合紧密,在拉伸断口处的白色须状物质即为表面被基体合金包覆的碳纳米管。     

Effect of Aging Treatment on Fatigue Behavior of an Al-Cu-Mg-Ag Alloy

An investigation of the fatigue properties of an Al-Cu-Mg-Ag alloy with two different heat treatments—peak aged (T6), and peak aged interrupted (T6I4)—has been conducted. While the strength levels resulting from the two heat treatments were similar, the main difference between the microstructures was that the peak aged interrupted material contained a higher volume fraction of the θ′ precipitates. This study specifically focused on the effects of these treatments on the fatigue lifetime distribution, and the role of crack initiation vs the small crack growth behavior. Several total fatigue lifetime tests were completed at room temperature and at a given stress level to characterize the distribution in fatigue lifetimes. Fatigue results indicate that there is almost no difference in the mean lifetime for either heat treatment, but there is a significant difference in the minimum lifetimes, where the peak aged condition exhibited a higher propensity for life-limiting failure mechanisms. The small crack growth behavior of the two aging treatments was studied both at room temperature and elevated temperature by means of a standard acetate replication method. The small crack growth rates at both temperatures were largely unaffected by the different aging treatments. Based on the given number of tests, results suggest that the life-limiting fatigue failures of the two aging treatments are primarily governed by different crack initiation mechanisms due to the differences seen in the density of θ′ precipitates.     

合金元素对无取向电工钢磁时效的影响

研究了碳、硅、铝合金元素的含量变化对无取向电工钢磁时效的影响。实验结果表明:碳含量是影响无取向电工钢磁时效的主要因素,硅、铝合金元素由于其影响碳元素的分布对电工钢磁时效也有重要影响;当w(C)小于0.003%时,无取向电工钢无明显的磁时效现象,当w(C)大于0.003%时,无取向电工钢容易产生磁时效现象,且磁时效随着w(Si+Al)的增加而降低,当w(Si+Al)大于2.5%时,无取向电工钢磁时效消失。     

Mechanical Behavior of AA6061 Aluminum in the Semisolid State Obtained by Partial Melting and Partial Solidification

The tensile properties of a 6061 aluminum alloy have been studied in the semisolid state at large solid fractions. The tests have been carried out either after a partial melting treatment or after partial solidification. Results show the following: (1) the mechanical behavior depends on the liquid-phase distribution and, therefore, on the way the semisolid state has been achieved (melting or solidification); (2) there is a critical solid fraction range where the semisolid alloy is relatively brittle; and (3) the mushy alloy exhibits viscoplastic behavior with the occurrence of micro-superplasticity at low strain rate. Modeling of this behavior is carried out by considering either the area fraction of grain boundaries wetted by the liquid or a cohesion parameter of the solid phase, which depends on solid fraction and thermal treatment.