Compressive deformation and forging behavior of Ti14 alloy in semi-solid state

Deformation behavior and formability of Ti14, a new typical α + Ti₂Cu alloy in the semi-solid state, were investigated by compressive tests in temperature range between 1223 and 1473 K and by forging tests between 1223 and 1373 K. Tensile tests of the forged alloys were also performed to study the effect of forging on mechanical properties. The results show that the maximum compressive stress was greatly dependent on the fraction solid, and that a transition in stress occurred at a solid fraction between 0.94 and 0.98 (corresponding to the temperature between 1323 and 1373 K). This transition was related to the decrease in amount of solid bridges between grains. Excellent formability and grain refinement due to dynamic recrystallization were achieved during the semi-solid forging. Furthermore, tensile tests showed a high ultimate tensile strength and yield strength after semi-solid forging at more than 1273 K, which was attributed to the grain refinement caused by the semi-solid forging. Ductility was also improved by changing the forging ratio. These results indicate that both good formability and tensile properties can be simultaneously attained by the compressive processing in the semi-solid state.     

Evaluation of low strain rate constitutive equation of 7075 aluminium alloy at high temperature

Abstract

The 7075 aluminium alloy is one of the most important engineering alloys utilised extensively in aircraft and transportation industries due to its high specific strength. In the present research, the flow behaviour of this alloy has been investigated using hot compression test at strain rates of 0·001, 0·01, 0·1 and 1 s^?1 and temperatures of 350, 400 and 450°C. The results reveal that dynamic softening occurred in these temperatures and strain rates. The activation energy, strain rate sensitivity and two constitutive equations (hyperbolic sine law and the power law) are derived from the results. It is shown that the hyperbolic sine law has a better agreement with the experimental results.     

Grain structure of laser remelted 7075 aluminium alloy in presence of Al2O3 particles

Abstract

The effect of inert particles on grain structure development from alloy melt during laser rapid solidification has been investigated. It is found that the presence of Al₂O₃ particles may disrupt the usual epitaxial grain structure evolution of an aluminium 7075 alloy when processed by laser surface remelting. This result, in addition to observations of crystal termination at the particles and grain refinement in particle dense regions, indicates that a mechanism of particle restricted grain growth operates.     

Residual stress relief in the aluminium alloy 7075

The residual stresses in heat-treated 7075 aluminium alloy blocks have been characterised using two neutron diffraction strain scanning instruments. The influence of uniaxial cold compression on relieving the residual stress has been determined. Systematically increasing the magnitude of cold compression from 1 to 10% has been shown to have a small beneficial effect on the final residual stress distribution, by reducing the magnitude of the range in the triaxial residual stresses. The effect of an overaging precipitation treatment on the residual stress has also been characterised, and this was found to have a significant stress relieving influence (25–40%). A relationship between the width of the {311} diffraction peaks and the amount of cold compression was also observed.     

Composition segregation in semi-solid metal cast AA7075 aluminium alloy

The composition distribution and microstructural evolution associated with the characteristic globular grain structure in semi-solid metal cast AA7075 aluminium alloy has been investigated. The primary globular grains possess uniform composition over nearly 90% of their diameter with fluctuations only occurring in the near surface of the individual grains. The inter-globular regions are solute rich and contain non-equilibrium eutectic. The segregation pattern in semi-solid cast structures occurs over a much greater length-scale than the dendritic pattern prevalent in conventional castings and it is proposed that the distinction arises from predominant planar growth of the α-primary globular grains during the SSM conditioning stage. Examination of the in-grain structure using electron backscattered diffraction revealed a quasi-periodic fluctuation in misorientation that may have arisen from local perturbations on the planar solid interface as solidification progressed. Homogenisation prior to artificial ageing is severely compromised by the undesirable segregation pattern.     

Multistep induction heating regimes for thixoforming 7075 aluminium alloy

Abstract

Thixoforming involves processing alloys with a spheroidal microstructure in the semisolid state. Commercially it is applied to conventional casting alloys, and one of the scientific challenges is to extend its application to high performance aluminium alloys such as 7075. Aluminium alloy 7075 is readily available in extruded form, and one route to a spheroidal microstructure is to reheat extruded material into the semisolid state to obtain recrystallisation, with the liquid penetrating the recrystallised boundaries. Here this route has been followed, but it has been found that the presence of pinning particles in the microstructure inhibits recrystallisation. To overcome this, a multistep induction heating regime has been developed consisting of a 1 min hold at 475-500 °C, a 1 min hold at 575-600 °C and a shorter 20 s hold for the final step at 620 °C.     

Influence of heat treatment and cutting speed on chip segmentation of age hardenable aluminium alloy

Abstract

A change in chip shape has been observed as a function of age hardening and cutting speed during high speed milling of the aluminium alloy 7075. In order to study this effect systematically, the aluminium alloy was heat treated to produce different precipitation states and machined under carefully controlled conditions at cutting speeds between 1000 and 7000 m min^-1. The underaged state shows local shearing producing segmented chips. The degree of segmentation increases with cutting speed. In contrast, the overaged state shows continuous chips up to the highest cutting speeds. The chips obtained with the peak aged state show a fluctuation between segmented and continuous parts. These results can be understood in terms of the differing work softening/hardening behaviour of the under- and overaged states owing to the specific interactions between dislocations and precipitates during chip formation.     

Statistical characterization of the geometric properties of particles in 7075‐T6 aluminium alloy

Corrosion in aluminium alloys is initiated and sustained at constituent particles within the metal matrix via a localized galvanic process. These particles are also known to play a critical role in fatigue crack initiation and growth. Consequently, statistical characterization of particle geometrical features is critical when modelling corrosion and fatigue. A key statistic is particle size distribution, which was extensively modelled here via imaging of unstressed and fatigued 7075‐T6 aluminium alloys. Fatigued samples were obtained from the outer wing panels of teardown specimens from retired military aircraft. The purpose of this effort was therefore to analyse extensive sets of particle geometry data obtained via microscopy using advanced multimodal statistical modelling and to appropriately characterize the properties of constituent particles and fatigue cracks found in these specimens. The resulting distribution functions for the underlying modes are assumed to be three‐parameter Weibull distribution functions.     

Influence of low magnitude plastic strains on residual stress in 7075 aluminium alloy

The effect of post quench delay on residual stress has been determined by subjecting four rectilinear blocks of quenched 7075, containing large residual stress magnitudes, to small (<1.0%) plastic strains applied by cold compression. One experiment kept the plastic strain the same in all samples (～0.12%) by cold compressing simultaneously in one pressing. The second pressed the blocks separately to the same strain (～0.45%). The post quench delays were <10, 60, 120 and 240 min. The residual stresses were characterised using neutron and X-ray diffraction. It was possible to detect the impact of the small plastic strains on all samples, but the diffraction measurement techniques could not conclusively isolate the influence of the post quench delay.     

S-N-P curves in 7075 T7351 and 2024 T3 aluminium alloys subjected to surface treatments

The S-N-P (stress, number of cycles, failure probability) curves for 2024 T3 and 7075 T7351 aluminium alloys were obtained. Previously, surface treatments of degreasing and different types of anodizing were applied to samples, evaluating the influence of these treatments on the fatigue life of the alloys. The determination of the S-N-P curves was done using Maennig's method. Rotary fatigue was used because this technique produces greater stress on the sample surfaces, the zone in which it is important to evaluate fatigue resistance. Both the transition range and the finite life range were evaluated, calculating the 1, 50 and 99% fracture probability. SEM was performed in order to characterize the fracture micromechanism. The conclusions were that Maennig's method is useful to evaluate fatigue life of these materials in a fast and efficient manner. Moreover, surface treatments produce a decrease in the fatigue life of both alloys, associating this effect with the surface damage produced on each sample during the treatment.     

Microstructural evolution and mechanical properties of back-extruded Al 7075 alloy in the semi-solid state

The thixoforming process is a new method for manufacturing complicated and net shape components through which high strength materials can be formed more easily. In this study 7075 Al alloy which has low extrudability has been thixoformed by backward extrusion process. The recrystallisation and partial melting (RAP) route was used to obtain the semi-solid feedstocks for thixoforming. Microstructural evolution during partial remelting was studied at temperatures for times. Results showed that a fine and globular microstructure can be obtained by the RAP route. The results showed that high semisolid isothermal temperature would increase the liquid volume fraction and accelerate the spherical processing of the solid particles. Furthermore at long holding time, the globular grains coarsened slightly and the average grains size are increased. The experimental results showed that when the semisolid billet is hold at 580°C with the holding time, less than 30 min, the microstructure of the billet is composed of spherical grains and remnant liquids, the average grain size are smaller than 100 μm. So the remelted billet is suitable for thixoforming. In this paper, a back-extruding of 7075 Al alloy with a high solid fraction in the semi-solid state at 580°C for 10 min was performed. Mechanical properties of thixoformed components at room temperature were examined. Tempering treatment T6 has been applied after thixoforming to investigate the effects of heat treatment on mechanical properties of thixoformed parts. The tensile properties and low hardness values in the as-thixoformed 7075 Al alloy were improved by subsequent heat treatment. Post-forming heat treatment is one of the key parameters for improving the mechanical properties of thixoformed parts.     

Deformation behavior of semi-solid A356 Al–Si alloy at low shear rates: Effect of fraction solid

Conventional casting was employed to produce semi-solid metal, SSM, billets of A356 Al–Si alloy. The rheological behavior of the as-cast SSM billets was studied using the parallel plate compression viscometry. The compression tests were carried out at different applied pressures, billet temperatures and fractions solid. The rheological studies were conducted assuming the SSM slurry behaving as Newtonian or non-Newtonian fluid. The calculated viscosity numbers confirmed the pseudoplastic behavior of the SSM billets and were correlated to fraction solid and shear rate, through an empirical equation, to highlight the effect of metallurgical and process parameters on the viscosity of the SSM A356 alloy.

This equation is valid at very low shear rates, less than 0.01 s⁻¹.     

Fatigue crack initiation life prediction for aluminium alloy 7075 using crystal plasticity finite element simulations

An experimentally-validated approach for predicting fatigue crack initiation life of polycrystalline metals is developed based on crystal plasticity finite element (CPFE) simulations. In this approach, the microstructure used in the simulations possesses statistically the same grain size and crystallographic orientations as those obtained from electron back-scatter diffraction experiments. A backstress model is incorporated into the CP constitutive model to describe the mechanical behaviour of aluminium alloy (AA) 7075 under cyclic loading. The key variables of the prediction model, the energy efficiency factor and plastic strain energy density, are calibrated using a fatigue test on a round-notched AA7075 specimen at room temperature. The proposed approach is then validated by using another fatigue test to predict 69.1–87.3% of the experimentally measured fatigue crack initiation life. The effects of the microstructure and texture on the energy efficiency factor and fatigue life prediction are quantitatively determined. It is shown that for a given range of energy efficiency factors a similar range of life prediction is obtained. Since the proposed approach considers the heterogeneity of the microstructure, it can well capture the grain scale deformation localisation and therefore improve the precision of fatigue life prediction.     

Damage and fracture mechanism of aluminium alloy thick-walled cylinder under external explosive loading

The damage and failure mechanism of aluminium alloy were investigated by means of the radial collapse of a thick-walled cylinder under high-strain-rate deformation. Adiabatic shear bands (ASBs) initiated at the inner surface of the aluminium alloy cylinder, and most of them were in spiral form along the cross-section of the cylinder towards counterclockwise direction. Tip of a shear band propagated along the direction of the maximum shear stress. The propagating trajectory of small shear band rather than developed shear band could be affected by grain boundary. The morphologies of ASBs such as bifurcation, crossing and annihilation were observed, and the evolution of ASBs was affected to a great extent by stress state. The nucleation of microcrack was mostly observed at the weak microstructure in ASBs. The coalescence of microcracks formed the crack within ASBs, and when the critical crack length is reached catastrophic fracture occurs.     

Fracture mechanisms of aluminium alloy AA7075-T651 under various loading conditions

The fracture behaviour of the aluminium alloy AA7075-T651 is investigated for quasi-static and dynamic loading conditions and different stress states. The fracture surfaces obtained in tensile tests on smooth and notched axisymmetric specimens and compression tests on cylindrical specimens are compared to the fracture surfaces that occur when a projectile, having either a blunt or an ogival nose shape, strikes a 20 mm thick plate of the aluminium alloy. The stress state in the impact tests is much more complex and the strain rate significantly higher than in the tensile and compression tests. Optical and scanning electron microscopes are used in the investigation. The fracture surface obtained in tests with smooth axisymmetric specimens indicates that the crack growth is partly intergranular along the grain boundaries or precipitation free zones and partly transgranular by void formation around fine and coarse intermetallic particles. When the stress triaxiality is increased through the introduction of a notch in the tensile specimen, delamination along the grain boundaries in the rolling plane is observed perpendicular to the primary crack. In through-thickness compression tests, the crack propagates within an intense shear band that has orientation about 45° with respect to the load axis. The primary failure modes of the target plate during impact were adiabatic shear banding when struck by a blunt projectile and ductile hole-enlargement when struck by an ogival projectile. Delamination and fragmentation of the plates occurred for both loading cases, but was stronger for the ogival projectile. The delamination in the rolling plane was attributed to intergranular fracture caused by tensile stresses occurring during the penetration event.     

铝合金激光表面强化的研究进展

综述了铝合金表面激光强化的工艺方法和特点，阐述了铝合金激光表面强化层的组织特征、硬度变化及其耐磨、耐蚀性能，总结了国内外在此研究领域的最新进展．     

The influence of aluminium alloy anisotropic texture on crack-tip plasticity

Crack‐tip plasticity in textured aluminium alloys was numerically analysed using an anisotropic yield function and an isotropic hardening law as the material constitutive response. Four real textures obtained from extrusions of rectangular and square shapes as well as five ideal textures typical of rolled products were considered and predicted crack‐tip plastic zones were compared with those obtained using the isotropic von Mises yield criterion. The use of the anisotropic yield function revealed important differences in crack‐tip plasticity compared with the isotropic case. The plastic zone features obtained for different textures were compared to single crystal results published in the open literature and similar crack‐tip plastic strain localization was observed.     

The effect of retrogression and re‐aging treatment on mechanical and tribological features of AA7075 aluminium alloy

In this study, T651‐applied AA7075 alloy was subjected to retrogression and re‐aging (RRA) process. Various retrogression temperatures (180 °C, 280 °C, 370 °C) and times (15 min, 30 min, 90 min) were used to determine the effects of temperature and time on the mechanical and tribological properties of the AA7075 alloy. All re‐aging stages were performed at 120 °C for 24 hours. Retrogression and re‐aging‐applied specimens were characterized by scanning electron microscope, transmission electron microscope, x‐ray diffraction, Charpy V‐notch impact and tensile tests. Brinell hardness measurements and ball‐on‐disc type tribometer measurements by using AISI 316 ball as a counterpart have also been conducted. Grain boundary precipitates in the T651‐applied specimen was transformed from continuous to the discontinuous structure after retrogression and re‐aging process. Continuous MgZn₂ precipitates at grain boundaries were disintegrated and re‐precipitated along the grain boundaries. The sizes of intragranular precipitates have become coarsened by comparison with the T651 condition. Hardness, tensile strength and wear resistance were decreased whereas impact toughness values were increased with increasing retrogression temperature and time. The best wear resistance was obtained in the sample treated at 180 °C for 15 minutes.     

A study of fatigue crack growth of 7075-T651 aluminum alloy

Both standard and non-standard compact specimens were employed to experimentally study the crack growth behavior of 7075-T651 aluminum alloy in ambient air. The effects of the stress ratio (R), overloading, underloading, and high–low sequence loading on fatigue crack growth rate were investigated. Significant R-ratio effect was identified. At the same R-ratio, the influence of specimen geometry on the relationship between crack growth rate and stress intensity factor range was insignificant. A single overload retarded the crack growth rate significantly. A slight acceleration of crack growth rate was identified after a single underload. The crack growth rate resumed after the crack propagated out of the influencing plastic zone created by the overload or underload. A parameter combining the stress intensity factor range and the maximum stress intensity factor can correlate the crack growth at different stress ratios well when the R-ratio ranged from −2 to 0.5. The parameter multiplied by a correction factor can be used to predict the crack growth with the influence of the R-ratio, overloading, underloading, and high–low sequence loading. Wheeler’s model cannot describe the variation of fatigue crack growth with the crack length being in the overload influencing zone. A modified Wheeler’s model based on the evolution of the remaining affected plastic zone was found to predict well the influence of the overload and sequence loading on the crack growth.     

The stress–life fatigue behaviour of aluminium alloy foams

The tension–tension and compression–compression nominal stress versus fatigue life responses of Alulight closed cell aluminium alloy foams have been measured for the compositions Al–1Mg–0.6Si and Al–1Mg–10Si (wt %), and for relative densities in the range 0.1–0.4. The fatigue strength of each foam increases with the relative density and with the mean applied stress, and is greater for the transverse orientation than for the longitudinal orientation. Under both tension–tension and compression–compression loading the dominant cyclic deformation mode appears to be material ratchetting; consequently, the fatigue life is highly sensitive to the magnitude of the applied stress. A micromechanical model is given to predict the dependence of life upon stress level and relative density. Panels containing a central hole were found to be notch insensitive for both tension–tension and compression–compression fatigue loading: the net-section strength equals the unnotched strength.