7075-H18铝合金板材的淬火敏感性与等温转变行为EI北大核心CSCD

以2 mm厚的7075-H18高强度铝合金板材为研究对象,采用微观组织分析、显微硬度测试、动力学建模与计算相结合的方法,对板材的淬火敏感性、等温相变行为及转变动力学进行系统研究,构建"时间-温度-性能"关系图。结果表明:7075-H18板材具有较高的淬火敏感性。"时间-温度-性能"关系图的鼻尖温度约为350℃,孕育期仅为0.23 s,淬火敏感温度区间为271~404℃,转变量为0.5%的曲线对应的临界线性冷却速率为969.7℃/s,超过了冷模淬火所能达到的冷却速率。板材在等温淬火过程中主要形成粗大的η平衡相,等温时间越长,晶内与晶界的η相尺寸越大,晶界η相越趋于连续分布,且无沉淀析出带变宽。基于实验数据构建的等温转变动力学模型,对7075-H18板材的等温相变过程进行准确预测,板材在350℃等温过程中沉淀相的析出速率最大,等温相变类型为"形核-长大"型相变,相变过程为层片状沉淀相的长大、增厚及互相吞并。理论计算结果与透射电子显微镜下观测到的η相特征以及"时间-温度-性能"关系图相一致。     

6082铝合金的TTT曲线及其研究

为研究6082铝合金的淬火敏感性,采用分级淬火的方法测定了6082铝合金的时间-温度-转化率（TTT）曲线,利用XRD、TEM并结合Avrami方程研究了6082合金在等温过程中的组织变化.结果表明：6082铝合金TTT曲线的鼻尖温度约为350℃,淬火敏感温度区间为300～420℃;6082铝合金过饱和固溶体在350℃...     

Hot deformation of AA6082-T4 aluminum alloy

High-temperature tensile deformation of 6082-T4 Al alloy was conducted in the range of 623–773 K at various strain rates in the range of 5 × 10⁻⁵ to 2 × 10⁻² s⁻¹. Stress dependence of the strain rate revealed a stress exponent, n of 7 throughout the ranges of temperatures and strain rates tested. This stress exponent is higher than what is usually observed in Al–Mg alloys under similar experimental conditions, which implies the presence of threshold stress. This behavior results from dislocation interaction with second phase particles (Mg₂Si). The experimental threshold stress values were calculated, based on the finding that creep rate is viscous glide controlled, based on creep tests conducted on binary Al–1Mg at 673 K, that gave n a value of 3. The threshold stress (σ _o) values were seen to decrease exponentially with temperature. The apparent activation energy for 6082-T4 was calculated to be about 245 kJ mol⁻¹, which is higher than the activation energy for self-diffusion in Al (Q _d = 143 kJ mol⁻¹) and for the diffusion of Mg in Al (115–130 kJ mol⁻¹). By incorporating the threshold stress in the analysis, the true activation energy was calculated to be about 107 kJ mol⁻¹. Analysis of strain rate dependence in terms of the effective stress (σ − σ _o) using normalized parameters, revealed a single type of deformation behavior. A plot of normalized strain rate () versus normalized effective stress (σ − σ _o)/G, on a double logarithmic scale, gave an n value of 3. Ehab A. El-Danaf—on leave from the Department of Mechanical Design and Production, College of Engineering, Cairo University, Egypt.     

On the extraordinary low quench sensitivity of an AlZnMg alloy

The scope of this work was to investigate the quench sensitivity of a high-purity wrought aluminum alloy Al6Zn0.75 Mg (in this work called 7003_pure). This is compared to a similar alloy with the additions of Fe, Si, and Zr at a sum less than 0.3 at.% (in this work called 7003_Fe,Si,Zr). Differential scanning calorimetry (DSC) was used for an in situ analysis of quench induced precipitation in a wide range of cooling rates varying between 0.0003 and 3 K/s. In 7003_pure, three main precipitation reactions were observed during cooling, a medium temperature reaction with a distinct double peak between 325 and 175 °C and a very low temperature reaction starting at about 100 °C. An additional high temperature reaction related to the precipitation of Mg₂Si starting at 425 °C has been observed for 7003_Fe,Si,Zr. In terms of hardness after natural as well as artificial aging, alloy 7003_pure shows a very low quench sensitivity. Hardness values on the saturation level of about 120 HV1 are seen down to cooling rates of 0.003 K/s. The as-quenched hardness (5 min of natural aging) shows a maximum at a cooling rate of 0.003 K/s, while slower and faster cooling results in a lower hardness. In terms of hardness after aging, 0.003 K/s could be defined as the technological critical cooling rate, which is much higher for 7003_Fe,Si,Zr (0.3–1 K/s). The physical critical cooling rates for the suppression of any precipitation during cooling were found to be about 10 K/s for both variants.

     

Evolution of microstructure and mechanical characteristics of friction stir welded ultrafine-grained aluminum alloy 6082

Bulk production of ultrafine-grained material is in great demand presently. Ultrafine-grained material can be synthesized using accumulative roll bonding, which is a prominent severe plastic deformation technique to develop such materials in bulk. There are further challenges in the fabrication of ultrafine-grained material. Friction stir welding is a potential technique to join the ultrafine-grained material while maintaining its mechanical and microstructural characteristics stability as no fusion is required. The present research work demonstrates the microstructural and mechanical characteristics of various welding zone after friction stir welding of ultrafine-grained aluminum alloy 6082. The microstructural features were examined using optical microscopy and the electron back-scattered diffraction technique. The variation in mechanical characteristics was observed using tensile and microhardness tests. The fractography of tensile specimens was studied to identify the mode of failure. The present study demonstrates the viability of friction stir welding to join ultrafine-grained aluminum alloy 6082 developed by accumulative roll bonding. The ultrafine grain size of 0.52 μm was achieved after four accumulative roll bonding cycles. The microhardness of accumulative roll bonding processed samples and the tensile strength of the weld joint were increased about two times and 1.6 times respectively compared to the annealed sample.     

Nanomechanical properties of friction stir welded AA6082-T6 aluminum alloy

Lightweight alloys are of major concern, due to their functionality and applications in transport and industry applications. Friction stir welding (FSW) is a solid-state welding process for joining aluminum and other metallic alloys and has been employed in aerospace, rail, automotive and marine industries. Compared to the conventional welding techniques, FSW produces joints which do not exhibit defects caused by melting. The objective of the present study is to investigate the surface hardness (H) and elastic modulus (E) in friction stir welded aluminum alloy AA6082-T6. The findings of the present study reveal that the welding process softens the material, since the weld nugget is the region where the most deformations are recorded (dynamic recrystallization, production of an extremely fine, equiaxial structure), confirmed by optical microscopy and reduced nanomechanical properties in the welding zone. A yield-type pop-in occurs upon low loading and represents the start of phase transformation, which is monitored through a gradual slope change of the load-displacement curve. Significant pile-up is recorded during nanoindentation of the alloy through SPM imaging.     

Mechanical properties and microstructure evolution in an aluminum 6082 alloy processed by high-pressure torsion

A commercial aluminum 6082 alloy was used to investigate the effect of the initial condition on subsequent processing by high-pressure torsion (HPT). The alloy was prepared in two different initial conditions: (i) in a T651 annealed condition and (ii) after a solution treatment followed by over-aging and subsequent processing by equal-channel angular pressing (ECAP). All samples were processed by HPT through 1/2, 1, 2, 5, and 10 turns and then the microstructures were examined using electron backscattered diffraction (EBSD). Significant grain refinement was achieved after processing by HPT through 5 turns with measured grain sizes of ~0.5 μm in both types of alloy. Microhardness measurements were conducted to evaluate the evolution of hardness after HPT for the two initial conditions. It is demonstrated that there is a difference in the hardness values between these two initial conditions, and this difference remains almost constant after processing by HPT.     

均匀化过程中铸态GH742合金的组织转变

铸态GH742合金中存在复杂的第二相组织和严重的枝晶偏析,Nb、Ti大量富集于枝晶间和MC碳化物、Laves相、δ相、(γ γ′)等析出相中.本文确定了γ基体的熔化温度为1160℃,并通过不同的均匀化处理确定了γ′相、(γ γ′)共晶、MC碳化物、Laves相、δ相、Ni_5Ce相及含氧硫稀土相等析出相的溶解温度.由于稀土相在1120℃发生熔化,提出一种GH742合金低温预处理加高温扩散的二次均匀化制度来扩大其均匀化温度区间,提高扩散退火温度,从而获得均一的组织.元素扩散计算结果表明,提高均匀化温度可以提高元素的扩散系数;Nb的偏析系数比Ti的小得多,达到完全均匀化需要很长的时间.     

The kinetics of transformation in amorphous germanium alloy films

The kinetics of the amorphous-to-crystalline transformation were studied in amorphous germanium (a-Ge) and its alloys with aluminium, silicon, indium, copper, iron, nickel, gallium and tin by using isochronal and isothermal annealing of the room temperature resistivity, differential thermal analysis and electron microscopy measurements. It was shown that the transformations in a-Ge and a-Ge alloy films are nucleation and growth controlled. The activation energy of 3.0 eV obtained for the amorphous-to-crystalline transition in a-Ge was found to decrease on alloying with metals, and the decrease depended on the nature of the alloying element.     

MB15在半固态等温处理中的组织和成分演变

为考查等温处理技术制备半固态成形用MB15镁合金非枝晶锭料的可能性,借助于液淬技术、光学和电子显微分析,研究了MB15镁合金的微观组织和成分演变.结果表明:MB15镁合金在半固态等温热处理过程中能形成晶粒细小、晶粒粗化速度缓慢的球状组织,但其圆整程度低于AZ91D合金.随着温度的升高和保温时间的延长,Zn向晶界扩散速度加快,晶界处的Zn含量逐渐增加,晶内Zr偏析逐渐扩散.冷却时,α-Mg与Zr在晶内富Zn处发生包晶反应,晶界处发生共晶反应和共析反应.     

Effect of self-support friction stir welding on microstructure and microhardness of 6082-T6 aluminum alloy joint

In this paper, 5-mm-thick 6082-T6 aluminum alloy was joined by means of self-support friction stir welding (SSFSW). Here we report the grain structure and second phase particles in various regions including the welding nugget zone (WNZ), thermo-mechanically affected zone (TMAZ), and heat affected zone (HAZ). In the upper part of the joint, microhardness in the TMAZ in proximity of the UWNZ was the highest (average 89.4 HV) due to the severe plastic deformation. The similar result was also found in the lower part of the SSFSW joint. The microstructural development in each region was a strong function of the local thermo-mechanical cycle experienced during welding. Some coarse equiaxed grains which were produced in incomplete dynamic recrystallization process and dissolution of some precipitates have been observed in TMAZ. The HAZ retained the same grain structure as the base material, however, the grain size decreased with increasing distance of the weld centerline.     

Investigation of bainitic isothermal transformation products in unalloyed ductile cast iron

The properties of the products of isothermal transformation of undercooled austenite into bainite in unalloyed ductile cast iron were investigated using X-ray diffraction. The following parameters were investigated: the fraction of austenite in the cast iron matrix, the crystal lattice parameter, and the width of the diffraction lines of the α and γ phases. The structures were studied using a TEM. It was observed that the temperature T_A and time τ_A of the isothermal transformation significantly influence the nature of the α and γ phases. The transformations are determined by the diffusion of carbon, and the maximum carbon content is approximately twice the equilibrium carbon content at the austenitising temperature. The lattice parameter of the α phase in the range of T_A studied decreases with increasing cooling time but increases in the upper bainite range. The increase in this lattice parameter results from the typical process of bainitic transformation during the retained austenite eutectoidal reaction (stage III). The crystal structure of the γ phase in the upper bainitic region is more perfect than in the lower range. Within the investigated temperature range of T_A, bainitic ferrite continually improves its crystal structure.

MST/3104     

Evolution of microstructure and texture in an ultrafine-grained Al6082 alloy during severe plastic deformation

The evolution of microstructure and texture in Al6082 precipitation-hardened alloy during equal-channel angular pressing (ECAP) was studied. It was found that although the dislocation density and the subgrain size saturated after 1 pass, the size of grains bounded by high angle boundaries reached its minimum value only after 4 passes. Furthermore, the grain orientation distribution changes between 4 and 8 passes, indicating the development of grain boundary structure even after the saturation of the parameters of the microstructure. As a result of this evolution, the initial texture of the commercial alloy was diminished after 8 passes and the grain orientation distribution became to be close to random case.     

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.     

Investigation of the trivalent-chrome coating on 6063 aluminum alloy

In this work, the green trivalent chromium coating on 6063 aluminum alloy surface was made and studied by means of trivalent chromate compound (KCr(SO₄)₂) as inhibitor. The coating was evaluated in 3.5 wt.% NaCl at room temperature using polarization curve, AC impedance spectroscopy (EIS). The results show that the trivalent chromium compounds treated surface presents better corrosion behavior in chloride media than the original material surface. The morphologies and composition and valence state of the coating were examined by scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) and X-ray photoelectron spectroscopy (XPS), respectively. The result indicates that the trivalent chromium chemical conversion coating was made on aluminum alloy surface.     

Electric resistance variation during isothermal deformation of a titanium-nickel alloy

Changes in the electric resistance and strain were simultaneously measured during isothermal torsion of a titanium-nickel alloy specimen. It was established that the inelastic deformation of the specimen in the austenite state is not accompanied by reconstruction of the alloy lattice.     

铝合金板材热塑性变形行为研究

为了研究铝合金板材的热塑性变形行为,进行了热态胀形试验,获取了不同温度及压力率下的胀形压力-高度曲线,分析了压力率对胀形高度的影响规律.基于同一压力率、不同温度下胀形压力与等效应变之间的关系,提出胀形压力关于等效应变及压力率的拟合方程,同时获得压力率和应变率之间的函数关系.试验结果表明,板材充液热成形工艺过程中,压力率对金属材料的变形行为影响显著,同时压力率能够表征材料成形过程中的变形快慢.     

Modeling of dynamic microstructure evolution of EN AW-6082 alloy during hot forward extrusion

The aim of this work is to present briefly a model for predicting and simulating the evolution of microstructure, in particular the evolution of grains, during hot forming processes of aluminum alloy EN AW-6082 and give a comparison with the experimental results. The model is a physically motivated phenomenological model based on internal state dependent variables. The microstructure evolution is a temperature dependent process and is simulated in a fully coupled thermo-mechanical process by help of Finite Element software Abaqus. The results are compared and verified with experimental results obtained by EBSD measurement of a small-scale extrusion process established for scientific purposes. The simulation results are in reasonable agreement with experiment.