Kinetics of precipitation hardening phase in aluminium alloy AA 6110

Abstract

The solid solution treated aluminium alloy AA 6110 was investigated using a differential scanning calorimetry (DSC) with different heating rates. Kinetic parameters, e.g. the activation energy and the Avrami exponent, were calculated. The peak temperatures of the hardening phase β″ from each heating rate were collected to calculate the activation energy of the aluminium alloy AA 6110 using various mathematical models: the Kissinger, Ozawa and Boswell models. It was found that the activation energies from each model were within the range of 106–114 kJ mol^?1. These activation energies were found to be lesser than those for the bulk diffusion of aluminium, magnesium and silicon atoms in aluminium matrix. The average Avrami exponent, n of 2·36 was calculated using the developed Matusita model and corresponded to a one-dimensional linear growth of fresh nuclei of needle-like β″ in this aluminium alloy AA 6110.     

A thermomechanical process for grain refinement in precipitation hardening AA6xxx aluminum alloys

A new thermomechanical processing method, consisting of conventional rolling and a continuous non-isothermal annealing process, has been designed to achieve substantial grain refinement through cost-efficient routes in heat treatable aluminum alloys. The method has been implemented on an alloy of interest for automotive applications and a highly-stable fine-grained microstructure with a very weak texture has been achieved.     

Influence of notch severity on the impact fracture behavior of aluminum alloy 7055

In this paper, the conjoint influence of notch severity and test temperature on the impact behavior of an Al-Zn-Mg-Cu alloy 7055 in the T7751 microstructural condition is presented and discussed. Notch angles of 45°, 75° and 90° were chosen for a standard charpy impact test specimen containing two notches. For a given angle of the notch the increase in dynamic fracture toughness, with test temperature, is most significant for the least severe of the notches, i.e. 45°. At a given test temperature, the impact toughness of the T7751 microstructure decreased with an increase in notch severity. An increase in notch severity resulted in essentially Mode I dominated fracture at all test temperatures. The influence of localized mixed-mode loading is minimal for the alloy has low dynamic toughness. The impact fracture behavior of the alloy is discussed in light of alloy microstructure, mechanisms governing fracture and the deformation field ahead of a propagating crack.     

Examination of precipitation in the aluminum alloy AA6111 using electrical resistivity measurements

The evolution of electrical resistivity during the aging of aluminum alloy AA6111 has been measured over a wide range of annealing temperatures. From these measurements, the solubility product for the Q phase in aluminum has been proposed and using a new approach, the contribution of precipitates to the overall resistivity is quantified.     

Strain hardening and orientation hardening/softening in cold rolled AA 5052 aluminum alloy

The tensile properties of cold rolled sheets were measured for the hot band and annealed hot band of AA 5052 aluminum alloy. The variation in yield strength with rolling true strain was used to represent the hardening rate of cold rolled sheets. The Taylor factor (M?) of cold rolled sheets in tension along the rolling direction was calculated based on the measured orientation distribution functions. The strain hardening and orientation hardening/softening produced by cold deformation were analyzed. The results show that the contribution to the hardening rate of cold rolled sheets comes largely from the deformed microstructure and partly from the texture change. For the annealed hot band the orientation softening occurs at strains below 0.5, while the orientation hardening occurs at strains over 0.5. For the hot band the dM?/dε value is always positive, indicating that orientation softening does not occur.     

Cyclic hardening behavior of roller hemming in the case of aluminum alloy sheets

A method to investigate the cyclic hardening behavior of roller hemming in the case of aluminum alloy sheets is described in this approach. Roller hemming hardening behaviors are studied with numerical simulation. The minimum sets of uniaxial experiments are established for the determination of accurate constitutive parameters. A special specimen holder, together with modified uniaxial specimens with two side fins is used in uniaxial tension and compression tests to determine combined hardening parameters. The material parameters are verified by pre-hemming roll-in/out data. It is also demonstrated that the combined hardening parameters derived from the minimum set of uniaxial tests with the aid of special specimen holder and modified specimens are accurate and believable.     

Effects of heat treatment on the microstructure and mechanical properties of AA2618 DC cast alloy

Direct chill (DC) cast ingot plates of AA2618 alloy have been increasingly used for large-mold applications in the plastics and automotive industries. The effects of different heat treatments on the microstructure and mechanical properties of AA2618 DC cast alloy were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and hardness and tensile testing. The as-cast microstructure contained a considerable amount of coarse intermetallic phases, including Al₂CuMg, Al₂Cu, Al₇Cu₄Ni, Al₇Cu₂(Fe,Ni) and Al₉FeNi, resulting in poor mechanical properties. Solution treatment at 530 °C for 5 h dissolved the first three phases into the solid solution and consequently improved the mechanical properties of the alloy. By utilizing the appropriate aging temperature and time, different combinations of strength and ductility could be obtained to fulfill the design requirements of large-mold applications. The strengthening of AA2618 DC cast alloy under the aging conditions studied was caused by GPB zones and S′ precipitates. The evolution of both precipitates in terms of their size and density was observed to have a significant effect on the mechanical properties of the alloy.     

Through-thickness texture gradient in AA 7055 aluminum alloy

Through-thickness texture gradient in AA 7055 aluminum alloy rolled plate has been investigated using the electron back-scattered diffraction (EBSD) technique. Quantitative analyses of texture in five layers from the surface to the center of the plate were performed. A pronounced texture variation through the plate thickness was found. In the center layer, a typical β fiber texture running from {112} <111> orientation through {123} <634> orientation to {011} <211> orientation was found. Near the surface, in contrast, shear type textures including {001} <110> orientation, {112} <110> orientation and {111} <110> orientation were dominating. In particularly, when the shear type textures reached the maximum in both intensity and content, the β fiber became minimums.     

Extrusion Processing of High-Strength Al Alloy 7055

The effects of a combination of extrusion processing parameters and aging schedules on the microstructure and mechanical properties of the 7055 Al alloy were investigated. A safe extrusion processing zone is determined through a limit diagram constructed over the experimental initial billet temperature ranging from 380° to 420°C, extrusion ratio from 10:1 to 40:1, and the ram speed ranging from 1 to 15 mm s^-1. Microstructural characterization of as-extruded, solution-treated, and artificially-aged materials was carried out using polarized light microscopy (for grain structure) and transmission electron microscopy (for precipitate morphology). A combination of hardness and tensile tests was used to evaluate mechanical properties. It is shown that in 7055 Al alloy, the optimization of alloy composition, extrusion processing parameters, and peak aging treatment results in reproducible tensile properties of 0.2% P.S. = 725 MPa, UTS = 750 MPa, and % elongation = 12.9. In order to improve the stress-corrosion resistance of peak aged material, retrogression and reaging (RRA) temper was established. A strength-electrical conductivity relationship has been established for the RRA temper between 36% and 37% International Annealed Copper Standard (IACS) electrical conductivity to enable selection of suitable combination of properties.     

A transmission electron microscopy study of microscopic causes for localized-corrosion morphology variations in the AA7055 Al alloy

By Using (scanning) transmission electron microscopy, localized-corrosion morphology variations of the AA7055 AlZn(Cu)Mg alloy with different thermal processes and their underlying microscopic causes were investigated systematically. Our study shows that the corrosion resistance of the nanoscale precipitates varies with their structure type and Cu-content. Just like the Al-matrix, the early-stage precipitates are corrosion resistant, as compared with the η_p/η-precipitates without high Cu-content. With a high Cu-content, however, the η-precipitates become most corrosion resistant among all phases involved. Hence, tailoring the precipitate microstructure and chemistry though thermal processes may change the overall corrosion morphology and improve corrosion resistance property of the alloy.     

少量Sc对7055铝合金组织与性能的影响

利用拉伸试验、光学金相、X射线物相分析、SEM及TEM等实验方法,研究了添加0.2%Sc(质量分数)对7055铝合金组织与性能的影响.实验结果表明,添加0.2%Sc可以显著细化7055铝合金铸态晶粒并减少晶界非平衡共晶相数量,促进非平衡共晶相在均匀化退火时的溶解,从而提高合金固溶度;由于Sc的添加可以提高7055Sc合金的溶质原子固溶度、形成更为均匀弥散分布的Al3ScZr粒子、以及抑制变形组织再结晶和有效细化固溶处理后的(亚)晶粒尺寸,因而显著提高7055Sc合金综合力学性能.     

Effects of scandium addition on electrical resistivity and formation of thermal hillocks in aluminum thin films

This study investigates the effects of doping aluminum (Al) films with minor amounts of scandium (Sc) on the electrical resistivity and the formation of thermal hillocks. The pure Al and Al-Sc films, prepared via sputtering deposition, before and after isochronal annealing are examined using a scanning electron microscope and a transmission electron microscope. In-situ thermal stress analyses of the films are also carried out. The grain size of the as-deposited films is reduced by addition of Sc. Moreover, the Sc can immobilize the grain boundaries, retarding grain growth and re-crystallization of the films during annealing. Although the as-deposited Al-Sc films show higher resistivity than that of a pure Al film, the former is significantly decreased after annealing at 300 °C. The hillock density dramatically reduces with increasing the Sc concentration in the films. Average size of the hillocks in Al-Sc films clearly increases when the temperature is elevated.     

Improvement in formability of aluminum alloy sheet by enhancing geometrical hardening

Individual grains in a polycrystal rotate during plastic deformation. This leads to a change in the crystallographic texture, and results in an increase or decrease of the macroscopic flow stress of the material. Such a change of strength as a result of grain rotations is called geometrical or texture hardening/softening. In the present study, for textured aluminum alloy sheets, the geometrical hardening/softening effect in the in-plane plane-strain stretching mode is numerically investigated using a generalized Taylor-type polycrystalline model. It is found that the cube texture () exhibits significant geometrical hardening when the major stretching direction is inclined at 45° relative to the orthotropic axes, and that a cube texture rotated about the normal direction (ND) shows a notable degree of geometrical hardening for any in-plane orientation of the sheet. Using the Marciniak-Kuczyński-type approach, forming limits for these textured sheets are analyzed. It is found that geometrical hardening definitely enhances the formability. It is, therefore, strongly suggested that texture control guided by the present results may be highly effective in producing aluminum alloy sheets with higher formability.     

Examination of an aluminum alloy behavior under different routes of twist extrusion processing

Experiments were conducted to evaluate the effects of loading path during twist extrusion processing. The samples of aluminum 8112 were processed by different routes of twist extrusion. Two consecutive clockwise dies (route I) and alternative clockwise-counterclockwise dies (route II) were used. The grain sizes created by route II were significantly finer than those created by route I. Furthermore, the mechanical properties, including the strength and hardness, not only enhanced but also distributed more homogeneously across the transverse cross-section of the samples.     

Texture evolution of AA3003 aluminum alloy sheet produced by accumulative roll bonding

The accumulative roll bonding process was carried out on an AA3003 aluminum alloy sheet up to eight cycles. The electron backscattering diffraction (EBSD) method was employed to investigate the microtextural development in the ARB processed sheets. The results indicate that with increasing the number of cycles, the overall texture intensity increases even up to the eighth rolling pass and a strong texture develops. The main textural components are the copper and Dillamore components of which the intensities increase with increasing number of cycles. Measurement of microhardness and lamellar spacing of grains in the processed sheets revealed that the presence of second phase particles in this aluminum alloy can promote the occurrence of dynamic recovery during the ARB process.     

Effect of heat treatment on the behavior of an AA7055 aluminum alloy during ballistic impact

Effect of heat treatment on mechanical properties and ballistic resistance of AA 7055 aluminum alloy plates has been studied. Large differences in the static mechanical properties are observed in the plates subjected to three different heat treatment schedules namely under-aging, peak aging and over-aging. The peak-aged plate shows the maximum strength and hardness followed by the under-aged and over-aged plates. The ballistic resistance of these heat treated plates of thicknesses 5 and 10 mm are evaluated by impacting deformable projectiles at a velocity of 820 ± 10 m/s and at a normal angle of attack, by using a thick backing technique. The under-aged and peak-aged materials exhibit similar ballistic penetration resistances which are superior to the over-aged material. Metallographic examination of the impacted plates shows formation of adiabatic shear band (ASB) induced cracks and few transformed ASBs. Peak-aged 7055 aluminum alloy plate shows much improved ballistic properties compared to commercially available 7017 aluminum alloy plate in terms of the depth of penetration.     

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.     

Microstructural evolution in friction self-piercing riveted aluminum alloy AA7075-T6 joints

Friction self-piercing riveting(F-SPR)is an emerging technique for low ductility materials joining,which creates a mechanical and solid-state hybrid joint with a semi-hollow rivet.The severe plastic deforma-tion of work materials and localized elevated temperatures during the F-SPR process yield complex and heterogeneous microstructures.The cut-off action of the work materials by the rivet further compli-cates the material flow during joint formation.This study employed the F-SPR process to join AA7075-T6 aluminum alloy sheets and systematically investigated the microstructural evolutions using electron backscatter diffraction(EBSD)techniques.The results suggested that as the base material approached the rivet,grains were deformed and recrystallized,forming two distinct fine grain zones(FGZs)surround-ing the rivet and in the rivet cavity,respectively.Solid-state bonding of aluminum sheets occurred in the FGZs.The formation of FGZ outside the rivet is due to dynamic recrystallization(DRX)triggered by the sliding-to-sticking transition at the rivet/sheet interface.The FGZ in the rivet cavity was caused by the rotation of the trapped aluminum,which created a sticking affected zone at the trapped aluminum/lower sheet interface and led to DRX.Strain rate gradient in the trapped aluminum drove the further expansion of the sticking affected zone and resulted in grain refinement in a larger span.     

Precipitation hardening and hydrogen embrittlement of aluminum alloy AA7020

AA7020 Al–Mg–Zn, a medium strength aluminium alloy, is used in welded structures in military and aerospace applications. As it may be subjected to extremes of environmental exposures, including high pressure liquid hydrogen, it could suffer hydrogen embrittlement. Hydrogen susceptibility of alloy AA7020 was evaluated by slow strain-rate tensile testing, and delayed failure testing of hydrogen-charged specimens of air-cooled, duplexaged, and water-quenched duplex agedmaterials. The resistance to hydrogen embrittlement of the alloy was found to be in the order of air-cooled duplex aged alloy > as-received (T6 condition) > water quenched duplex aged material.     

Effect of corrosion on the fatigue life and fracture mechanisms of 6101 aluminum alloy wires for car manufacturing applications

An innovative solution for the automotive industry is to replace the copper used for wiring harnesses with aluminum alloys, such as the aluminum–magnesium–silicon 6101 alloy. Wiring harnesses are composed of thin strand arms obtained by a wire drawing process. These strands are susceptible to exposure to a corrosive environment and fatigue solicitations simultaneously. The fatigue endurance of this alloy was studied using the stress-life approach for three metallurgical states representative of three cold-drawing steps. Fatigue tests performed in corrosive media tests highlighted a strong decrease of the 6101 alloy lifetime due to fatigue–corrosion interactions and a modification of failure modes.