Superplastic behavior and microstructural stability of friction stir processed AZ91C alloy

Superplastic behavior of a solution treated and friction stir processed (FSP) AZ91C alloy is studied. These studies are conducted in the temperature range of 300–375 °C and strain rates (SRs) in the range of 1 × 10^?4–3 × 10^?3 s^?1. Microstructural stability of the FSP alloy is also studied in comparison to the AZ31, AZ61, and AZ91 alloys processed by various routes. High SR sensitivity in the range of 0.33–0.39 and grain size stability till 350 °C is observed for the FSP alloy. The FSP AZ91C alloy showed better thermal stability in comparison to AZ31 and AZ61 alloys. Kinetics of superplastic deformation of the FSP alloy is found to be slower as compared to AZ31 and AZ61 alloys processed by various routes, which is due to the presence of significant amount of second phase precipitates, such as, β-Mg₁₇(Al,Zn)₁₂, Mg₂Si, and Al₈Mn₅ in the FSP alloy. However, these precipitates contributed for better thermal stability of the microstructure of FSP AZ91C alloy.     

Fatigue performance of friction stir welded Al2024 alloy in a different corrosive environment

The study examines the corrosion fatigue behavior of friction stir welded Al2024 alloy in the corrosive medium. The fatigue tests are conducted at a stress ratio of -1 in the different corrosive medium. The decrease in the fatigue life of welded joints in the corrosive environment is attributed to an increase in the crack initiation susceptibility in the presence of corrosive media. The fractured surfaces are investigated using the scanning electron microscopy (SEM). The formation of the corrosive compounds was studied using x-ray diffractometry.     

Material flow during friction stir spot welding of dissimilar Al2024/Al materials

In the present paper, the material flow and intermixing during friction stir spot welding of dissimilar Al2024/Al materials were investigated. The dissimilar materials had quite different strength. The microstructural evolutions taking place during a series of lap and butt welds were observed. The effect of penetration depths, dwell time, rotational speed and tool geometry were systematically investigated. The material flow and formation of the intermixed region were explained by a modified model.     

Process optimisation and microstructural evolution of friction stir spot-welded Al6061 joints

The joint strength and subsequent microstructural evolution of a friction stir spot-welded AA6061-T6 alloy was investigated according to the process parameters: tool rotation speed, dwell time and pin angle of the tool. A maximum tensile shear load of 2.78?kN was obtained from the joints generated under a combination of process variables like 1000 and 1500?rev?min^?1, 5?s, 5°. Under a fixed pin angle of 5° and a rotational speed of 1000?rev?min^?1, an increase in dwell time from 1 to 5?s resulted in a considerable increase in tensile shear load. An empirical process map under a fixed tool design is proposed to determine a feasible range of process conditions.     

Fatigue behaviour of Al2024-T3 friction stir welded lap joints

The aim of this work is the characterization of the fatigue behaviour of AA2024-T3 friction stir welded overlap joints produced at German Aerospace Center (DLR) of Cologne (D). In these joints, two crack-like unwelded zones are present at overlap ends. The stress intensity factor at the crack tip and the fatigue crack path have been studied using the FE code Franc2d and the lifetime has been estimated by integrating the material propagation law with the software AFgrow, in which the stress intensity factor calculated with Franc2d was introduced. The numerical results predict lifetimes shorter than the experiments. This fact is attributed to an initial propagation in shear mode for a short distance, which was indeed observed with an optical microscope and it is predicted by mixed mode “failure mechanism map”.     

Low temperature enhanced ductility of friction stir processed 5083 aluminum alloy

Commercial 5083 Al rolled plates were subjected to friction stir processing (FSP) with two different processing parameters, having 430 and 850 rpm tool rotational speed with a single traverse feed rate of 90 mm/min. These FSP conditions resulted in two fine grained microstructures of 0·95 μm (430 rpm) and 2·6 μm (850 rpm). Tensile elongations were measured at a relatively low temperature of 250°C at three strain rates, and demonstrated that a decrease in grain size resulted in significantly enhanced ductility and lower forming loads. The occurrence of a relatively high value of strain rate sensitivity, m of 0·45 for a grain size of 0·95 μm, suggests the operation of superplastic deformation under these present experimental conditions.     

Numerical simulation of fatigue crack propagation in friction stir welded joint made of Al 2024-T351 alloy

In this work, fatigue crack propagation in thin-walled aluminium alloy structure with two friction stir welded T joints has been simulated numerically. Crack propagation in stiffened part of the structure between two friction stir welded T joints is analysed by using the eXtended Finite Element Method (XFEM), including software ABAQUS, as well as MORFEO, for modelling and results display. Tensile fatigue loading is applied, with stress ratio R = 0, and maximum stress σ_max = 10 MPa. Material properties (Al 2024-T351, as used in aeronautical industry) in different welded joints zones are adopted from available literature data. Following results are obtained by numerical analysis: stress–strain and displacement state in the structure, position of the crack tip and value of stress intensity factor for every crack propagation step, as well as the structural life estimation, i.e. number of load cycles, N, also for each crack propagation step. Using these results the number of cycles at which the crack starts to propagate in an unstable manner is predicted.     

In vitro degradation behaviour of a friction stir processed magnesium alloy

In this study, the in vitro degradation behaviour of a friction stir processed AZ31 magnesium alloy was investigated. Electrochemical experiments in simulated body fluid suggest that friction stir processing marginally enhances the degradation resistance of the alloy, which could be attributed to the dissolution of secondary phase particles. Homogenisation of the microstructure reduces galvanic corrosion. It is envisaged that the beneficial effect would be more pronounced for magnesium alloys which contain high volume fraction of galvanic corrosion inducing secondary phase particles.     

Microstructure and mechanical behavior of friction stir processed ultrafine grained Al-Mg-Sc alloy

Twin-roll cast (TRC) Al-Mg-Sc alloy was friction stir processed (FSP) to obtain ultrafine grained (UFG) microstructure. Average grain size of TRC alloy in as-received (AR) condition was 19.0 ± 27.2 μm. The grain size reduced to 0.73 ± 0.44 μm after FSP. About 80% of the grains were smaller than 1 μm in FSP condition. FSP resulted into 80% of the grain boundaries to have high angle grain boundary (HAGBs) character. Uniaxial tensile testing of UFG alloy showed an increase in yield strength (YS) and ultimate tensile strength (UTS) (by ∼100 MPa each) of the alloy with a very marginal decrease in total and uniform elongation (total - 27% in AR and 24% in UFG and uniform - 19% in AR and 14% in UFG). A theoretical model predicted that the grain refinement cannot take place via discontinuous dynamic recrystallization. Zener pinning model correctly predicted the grain size distribution for UFG alloy. From work hardening behaviors in both the conditions, it was concluded that grain boundary spacing is more important than the character of grain boundaries for influencing extent of uniform deformation of an alloy.     

2024铝合金搅拌摩擦焊接头塑性变形行为研究

为了研究搅拌摩擦焊卷焊管坯的力学性能及接头塑性变形行为,以5 mm厚的2024退火态铝合金搅拌摩擦焊板坯为对象,采用网格法测试接头塑性变形分布,用EBSD测量接头各区域晶粒尺寸及位相,并结合SEM观察接头第二相的分布,研究了接头力学性能、应变分布与微观组织之间的关系.研究表明:搅拌摩擦接头强度与母材等强,延伸率下降44%;接头前进侧距离焊缝中心8～17 mm的母材较早出现了应变的集中,局部应变可达23%,而焊核区和接头返回侧母材发生的变形较小,平均应变分别为3%和11%,各区域应变的不均匀主要是由于接头各区晶粒尺寸及位相的差异造成的,导致接头整体延伸率的下降.     

An investigation of the microstructural effects on the mechanical and electrochemical properties of a friction stir processed equiatomic CrMnFeCoNi high entropy alloy

The electrochemical properties of a friction stir processed (FSPed) equiatomic CrMnFeCoNi high-entropy alloy (HEA) was investigated in an aerated 0.5 M Na2SO4 electrolyte solution at room temperature.The microstructural analysis reveals a highly refined stir zone (SZ) with an average grain size that decreases from the top region of the SZ to the bottom region of the SZ (also known as shear-processed zone;SPZ).However,the region below the SPZ,(i.e.below the plunge depth) experienced an increase in average grain size and dislocation densities compared to the other regions.There is no secondary phase observed in the FSPed region,however,the microstructural evolution in the FSPed region affects the electrochemical behavior of the HEA.Cr2O3 passive layer was observed to form on the FSPed HEA,leading to excellent corrosion properties from the polarization corrosion tests.Grain refinement in the SZ enhances the rapid formation of the passive layer,thus,leading to better corrosion properties in the front surface of the FSPed HEA.The localized corrosion behavior of the FSPed HEA was predicted to be caused by the micro-galvanic nature of the HEA,which leads to an increase in polarization at the anodic sites (pits).A numerical model was established using the corrosion parameters from the experiment to simulate the localized corrosion behavior on the surface of the FSPed HEA in a neutral environment.The predicted initial pitting potential and corresponding current density agree well with the experimental results.The model is also capable of tracking the dissolution of the pits over longer periods.     

Immersed friction stir welding of ultrafine grained accumulative roll-bonded Al alloy

In this research, ultrafine grained strips of commercial pure strain hardenable aluminum (AA1050) were produced by accumulative roll-bonding (ARB) technique. These strips were joined by friction stir welding (FSW) in immersed (underwater) and conventional (in-air) conditions to investigate the effect of the immersion method on the microstructure and mechanical properties of the joint, aiming to reduce the deterioration of the mechanical properties of the joint. Transmission electron microscopy and X-ray diffraction analyses were used to evaluate the microstructure, showing smaller grains and subgrains in the stir zone of the immersed FSW condition with respect to the conventional FSW method. The hardness and tensile properties of the immersed friction stir welded sample and ARBed base metal show more similarity compared to the conventional friction stir welded sample. Moreover, the aforementioned method can result in the enhancement of the superplasticity tendency of the material.     

Surface analysis of stationary shoulder friction stir processed AZ31B magnesium alloy

We propose a stationary shoulder friction stir process (SSFSP) to produce a smooth surface finish. The use of a stationary shoulder tool contributes to reducing the heat input during friction stir processing (FSP). Hence, a stationary shoulder tool is advantageous for FSP in heat sensitive alloys like magnesium. The present short communication investigates the surface finish of AZ31B magnesium alloy processed by SSFSP without using additional cooling. Surface analysis of the processed region was carried out by 2D and 3D surface mapping using digital microscopy. The surface mapping indicated that there was very little flash generation on the processed zone, while 3D mapping quantified the surface roughness in the longitudinal as well as transverse directions of the processing zone.     

Microstructural characterisation of friction stir processed aluminium

Abstract

Friction stir processing was carried out on commercially pure aluminium, and a detailed microstructural characterisation was performed by electron backscattered diffraction and transmission electron microscopy. Friction stir processing resulted in significant grain refinement with narrow grain size distribution. The microstructure showed fine and equiaxed grains, with some ultrafine grains being also observed. Electron backscattered diffraction studies showed majority of the boundaries to be high angle, confirming the occurrence of dynamic recrystallisation (DRX). Transmission electron microscopy observations revealed dislocation arrangement into subgrain boundaries, grains having different dislocation densities and in different stages/degrees of recovery. Electron backscattered diffraction analysis also revealed a progressive transformation of sub-grain boundaries into high angle grain boundaries. A multimechanism of dynamic recovery, continuous DRX and discontinuous DRX seems to be operating during the process. The microstructure is not affected by changing the rotation speed from 640 to 800 rev min^?1, except that the grain size was marginally larger for higher rotational speed.     

Microstructural aspects and mechanical properties of friction stir welded AA2024-T3 aluminium alloy sheet

In the present work, aluminium alloy AA2024-T3 thin sheets were joined by the Friction Stir Welding – FSW – process. Butt joints were obtained in 1.6 mm sheets, using an advancing speed of 700 mm/min. These joints were characterised by optical, scanning electron microscopy, tensile and fatigue mechanical tests. The results showed that the resulting microstructure is free of defects and the tensile strength of the welded joints is up to 98% of the base-metal strength. Fatigue tests result indicates an equivalent stress intensity factor (kt) of approximately 2.0 for the welded samples. Consequently, the FSW process can be advantageous compared to conventional riveting for airframe applications.     

Comparison of corrosion behaviour of friction stir processed and laser melted AA 2219 aluminium alloy

Dissolution of second phase particles (CuAl₂) present in AA 2219 aluminium improves the corrosion resistance of the alloy. Two surface treatment techniques, viz., solid state friction stir processing and fusion based laser melting lead to the reduction in CuAl₂ content and the effect of these processes on the corrosion behaviour of the alloy is compared in this study. Potentiodynamic polarization and electrochemical impedance spectroscopy tests were carried out to compare corrosion behaviour. The corrosion resistance achieved by friction stir processing is comparable to that obtained by the laser melting technique.     

Effect of friction stir processing on mechanical and microstructural properties of AM60B Magnesium alloy

Some AM60B magnesium alloys sheets produced by High Pressure die Casting were Friction Stir Processed and the mechanical and microstructural features are presented in the present study. The mechanical properties of the FSP material were analyzed in longitudinal direction respect to the processing one and compared with those of the base material. Tensile tests were performed at room and high temperature and different strain rates in the nugget zone, in order to analyse the superplastic properties of the recrystallized material and to observe the differences respect to the base material after the strong grain refinement effect due to the Friction Stir Process. The high temperature behaviour of the material was studied, in longitudinal direction, by means of tensile tests in the temperature and strain rate ranges of 175–250°C and 10⁻²–10⁻⁴ s⁻¹ respectively.     

Microstructure evolution of SIMA processed Al2024

In this paper, the strain induced melt activated (SIMA) process of A2024 was investigated systematically in order to provide a basis for semi-solid forming of the alloy. To delete the hydrostatic component of stress, the uniform uniaxial compression process is performed. The relation between the grain size and strain before heating is presented and a microstructure evolution model for SIMA is proposed on the basis of experiment. There include two steps in SIMA process. In the first step of plastic deformation, there are two mechanisms to control the deformation, slip or cross-slip or twinning in grain and grain rotation. The deviatory stress will benefit to slip or cross-slip or twinning in grains, and the hydrostatic stress will benefit to rotation of gain. In the second step of heating, the grain boundary will melt first and the shape of the grains will be globe.