Correlation between precipitate microstructure and mechanical properties in AA7075‐T6 aluminum alloy friction stir welded joints

Microstructural evolution and mechanical properties of friction stir welded AA7075‐T6 aluminum alloy were examined. Grain structure and precipitate evolution in the stir zone and heat‐affected zone were evaluated using optical microscope and differential scanning calorimetry. A significant grain refinement and dissolution of η′ precipitates in the stir zone were found, but chromium‐bearing dispersoids remained nearly unchanged. The main particles in the stir zone and heat‐affected zone were η precipitates as well as Guinier‐Preston zones formed during post‐weld natural aging. The small recrystallized grains were observed in the thermo‐mechanically affected zone next to the stir zone. A W‐shaped hardness distribution where soft region was produced in the heat‐affected zone at a short distance from the stir zone were obtained. Hardness profiles of the welds were explained by precipitate distributions. Friction stir welding resulted in the reversion and coarsening of η′ precipitates. The formation of Guinier‐Preston zones in the stir zone and some parts of the heat‐affected zone during post‐weld natural aging increased the hardness. In transverse tensile specimens, fracture occurred in a location with the minimum hardness at either advancing or retreating side randomly. Further, influences of welding parameters on mechanical properties were investigated.     

Identification of the pin offset effect on the friction stir welding (FSW) via Taguchi – Grey relational analysis: A Case study for AA 7075 – AA 6013 alloys

The aim of this work is to present a case study relating to the dissimilar friction stir welding (FSW) ability of AA 7075‐T651 and AA 6013‐T6 by applying pin offset technique. An orthogonal array L18 was conducted to perform the overlapped weld seams using three different values of pin offset, welding speed and tool rotational speed along with two different pin profiles determine the impact of welding parameters on the tensile properties of friction stir welded joints. The nugget zone for each of overlapped weld seams exhibited a complex structure and also, the pin offset and profile also were found to have a great impact on the microstructural evolution of the nugget zone. The ultimate tensile strength, elongation at the rapture and bending strength of welded joints were measured in the ranges of 194–215 MPa, 1.79–3.34 % and 203–352 MPa. From the Taguchi based Grey relational analysis, the optimum welding condition was determined for the welded joint performed using a single fluted pin profile with the zero pin offset, tool rotational speed of 630 min^?1 and welding speed of 63 mm/min. Microstructural and macro‐structural observations revealed that welded joints exhibiting lower tensile strength are consistent of various types of defects (e. g. cracks, tunnels and cavities). The fracture location of welded joints was found to be on the heat affected zone and between the heat affected zone and AA 6013‐base metal. The tool and pin wear was not observed during the welding applications     

Thermal cycles and their effects during friction stir welding of AA7075 thicker plates with and without in-process cooling

Friction stir welding of AA 7075 plates in three different thicknesses such as 10, 16 and 25 mm at natural convection condition was carried out successfully without defects. Water cooled friction stir welds were also produced on 16 mm thick plates. The thermal cycles at different locations of the plate, during the friction stir welding process, were predicted using a three-dimensional thermal model. Mechanical properties of the welds were evaluated using tensile and hardness tests. Weld microstructures were also examined with optical and transmission electron microscopes. The weld hardness values and tensile properties were found to decrease with increase in plate thickness. The use of water cooling was found to improve the weld properties to some extent, although not to the level of base metal. The reasons for this behavior are discussed, correlating thermal cycles, mechanical properties, fracture locations and precipitate morphology.     

Corrosion study and quantitative measurement of crystallite size of high strength aluminum hybrid composite developed via friction stir processing

This present study is centered on the corrosion behaviors and structural integrity of high strength aluminum-alloy 7075-T651 hybridized with carbonaceous coconut shell ash (CSA) and α+β titanium alloy powder in ratio of 50 : 50, fabricated during friction stir processing (FSP). The study examined the base metal – AA7075-T651 as well as the friction stir processed AA7075-T651 as control experiments. The Processing parameters used were plunge depth of 0.3 mm, travel speed of 20 mm/min, the tilt angle of 3° and rotational speed of 1500 min⁻¹. The corrosion characteristics were examined with the Potentiodynamic Polarization method in 3.5 % NaCl medium while crystalline phases were studied with x-ray diffraction (XRD). Results show that fabricated aluminum hybrid composite (AHC) AA7075-T651/coconut shell ash (CSA)/titanium-alloy has the highest percentage of inhibition performance efficiency (IPE) of 84.81 % resulting to 0.76938 mm/year corrosion rate and 356.51 Ω polarization resistance, whereas inhibition performance efficiency (IPE) for friction stir processed (FSPed) AA7075-T651 was 63.99 % while the base metal – AA7075-T651 was taken as the reference point. It was also revealed that the hybridized coconut shell ash (CSA)/titanium-alloy powder on AA7075-T651 enhanced the structural integrity, producing 38.5 nm crystallite size over friction stir processing (FSP) AA7075-T651 with 41.2 nm and the base metal with 48.7 nm crystallite size.     

Friction stir welding of thick plates of aluminum alloy matrix composite with a high volume fraction of ceramic reinforcement

The microstructure and mechanical properties of joints conducted by friction stir welding, FSW, at different rotational speeds in thick plates of a composite material with a high volume fraction of reinforcement, namely 2124Al/25vol%SiC_p, are studied. Original particle-free regions vanish during the stirring process, leading to a homogeneous particle distribution. Occasional breakage of some large particles occurs. Tunnel defects appear at low rpm, and disappear at high rotational speeds. The size of the thermo mechanically affected zone, TMAZ, increases with increasing rpm. Ductility of the welds in the range of 10–15% is achieved in compression tests whereas a rather brittle behavior is obtained in tension. A strength difference, SD, effect between compression and tensile test is obtained. This accounts for the little detrimental effect of the FSW process on the matrix–reinforcement interface. The SD effect is attributed to the presence of a microscopic residual stress.     

Fracture toughness and fatigue crack growth in Ti‐6Al‐4V friction stir welds

Friction stir welding of titanium holds the promise for producing joints with microstructures and mechanical properties that are more comparable to wrought material than traditional fusion welding processes. Extensive data exist on the microstructure and static mechanical properties of titanium friction stir welds, but very little are available on the durability (fatigue) and even less on the damage tolerance (fracture toughness and fatigue crack growth). This paper presents the results of an investigation into the damage tolerance of friction stir welds made in 6 mm thick Ti‐6Al‐4V after a post‐weld heat treatment. It was found that the apparent fracture toughness was lower than the wrought base material, 7–25% depending on the crack orientation relative to the weld, but the crack growth performance (ΔK vs. da/dN) of the weld in the absence of weld‐induced residual stresses was identical to the base material.     

Comparative study of fatigue and fracture in friction stir and electron beam welds of 24 mm thick titanium alloy Ti‐6Al‐4 V

In this study, friction stir welding of Ti‐6Al‐4 V was demonstrated in 24 mm thickness material. The microstructure and mechanical properties, fatigue, fracture toughness and crack growth of these thick section friction stir welds were evaluated and compared with electron beam welds produced in the same thickness material. It was found that the friction stir welds possessed a relatively coarse lamellar alpha transformed beta microstructure because of slow cooling from above the transus temperature of the material. The electron beam welds had a fine acicular alpha structure as a result of rapid solidification. The friction stir welds possessed better ductility, fatigue life, fracture toughness and crack growth resistance than the base meal or electron beam welds. Thus, even though friction stir welding is a relatively new process, the performance benefits it offers for the fabrication of heavy gage primary structure make it a more attractive option than the more well‐established electron beam welding method.     

Microstructural Study and Mechanical Properties of Dissimilar Friction Stir Welded AA5083-H111 and AA6082-T6 Reinforced with SiC Nanoparticles

Dissimilar friction stir welds were produced in 3 mm thick plates of AA6082-T6 and AA5083-H111 aluminum alloys using SiC as reinforcing material. The optimum weld presents a good distribution of nanoparticles in the weld nugget and mechanical mixing of the two alloys as well as further grain refinement compared to the one without nanoparticles. Higher hardness in the weld nugget is also evidenced, followed by enhanced ultimate tensile strength and elongation values. All specimens, after the tensile test, were lead to fracture at the heat affected zone of AA6082-T6 and specifically at the region of the lowest hardness.     

转速对 7075-T651 铝合金搅拌摩擦焊接件内部残余应力分布的影响

目的掌握铝合金搅拌摩擦焊接板内部残余应力的分布,为控制焊接残余应力、改进焊接工艺和提高焊接件质量。方法以13.6 mm厚的7075-T651铝合金为研究对象,用短波长X射线衍射技术,对在不同搅拌头转速下搅拌摩擦焊接板内部的残余应力进行了无损测试,并对焊接接头在板厚中心层上的微观组织和显微硬度进行了研究。结果在垂直于焊缝截面上的显微硬度均呈"W"型分布,焊核区的显微硬度高于其两侧的热机械影响区和热影响区,但低于母材区的硬度;随着转速的增大,接头硬度的最小值减小,低硬度区的范围越大。横向残余应力绝对值整体小于焊接方向;焊核区为正应力,热机械影响区残余应力减小且变化梯度最大;残余应力的极大值位于热影响区和热机械影响区的交界处;残余应力极大值与硬度最小值的位置重合。结论通过残余应力的无损检测分析,不仅可以直接获得加工件内部应力分布,还可以间接获得加工件内部的加工缺陷情况,为改进加工工艺、提高成形精度提供依据。     

The influence of low‐plasticity burnishing process on the fatigue life of friction‐stir‐processed Al 7075‐T6 samples

The effects of low‐plasticity burnishing (LPB) on the fatigue life of friction‐stir‐processed (FSP) Al 7075‐T6 plates were examined experimentally and numerically. Aluminum samples were taken from plates to test fatigue response in the presence of heat‐affected zone (HAZ) at different loading magnitudes. Finite element method was employed to numerically evaluate fatigue life of FSPed samples by means of the Smith–Watson–Topper (SWT) model. Through numerical analysis, the FSP and its cooling procedure were modelled on the basis of the arbitrary Lagrangian–Eulerian technique, and then, the effect of the LPB to assess fatigue response of samples was examined. Aluminum samples undergoing friction‐stir process presented lower‐fatigue life as stresses were highly concentrated within FSP regions. Involvement of LPB regained fatigue durability through compressive residual stress induced on the aluminum samples. The higher applied force over the LPB promoted compressive residual stress on the sample surface and improved fatigue life of samples. The predicted life results were found twice more in magnitude than those of experimentally obtained.     

Microstructure and residual stress distributions in friction stir welding of dissimilar aluminium alloys

The aim of this investigation was to study the effect of welding heat input and postweld natural aging on residual stress, microstructure, and precipitation distribution in different zones of dissimilar friction stir welding of 8 mm thick plates of AA6082-T6 and AA7075-T6. It was found that atomic diffusion occurs at the interface of the materials in the stir zone of the joints. Transmission electron microscopic investigations showed that reprecipitation of fine Guinier–Preston zone, β′, and η′ precipitates resulted in increased micro-hardness in the SZ after natural aging. An increase in welding heat input resulted in decreased maximum tensile residual stress and increased size of the tensile residual stress region. Natural aging within the SZ and thermo-mechanical affected zone resulted in 15–20 MPa reduction of the residual stress in these zones.     

Analysis of the effect of structural defects on the fatigue strength of RFSSW joints using C‐scan scanning acoustic microscopy and SEM

Solid‐state refill friction stir spot welding (RFSSW) technology offers significant benefits in the fabrication of aluminium structures in the transport and aerospace industries. In this paper, the joining of 1.6‐mm‐thick Alclad 7075‐T6 aluminium alloy sheets is investigated. High‐cycle fatigue strength tests of single‐lap welded joints were carried out on an Instron E10000 testing machine with a limited number of cycles equal to 2 × 10⁶. The welding of overlap fatigue specimens was conducted using an RPS100 spot welder by Harms & Wende GmbH & Co KG. C‐mode scanning acoustic microscopy (C‐SAM) and scanning electron microscopy (SEM) were utilised to evaluate the joint quality and characterise the microstructure. The paper discusses the effect of the maximum load force and defects (voids, hook, kissing bond, bonding ligament, etc) associated with the material flow in the weld on the failure mechanism. Insufficient plasticisation of sheet material and mixing of the material in the weld area are crucial defects that influence the number of destructive cycles. The weld defects in the joint structure are a source of a decrease in the fatigue life compared with the fatigue life of defect‐free welds. It was also found that RFSSW joint defects can be effectively detected by the nondestructive C‐SAM method.     

AA7075 bit for repairing AA2219 keyhole by filling friction stir welding

In the present study, 7.8 mm thick AA2219 rolled plates were successfully filling friction stir welded (FFSW) without keyhole using a semi-consumable tool. The influences of the bit’s geometric parameters and the plunge speed on the joint’s mechanical properties were investigated. Microstructure of the joint, especially at the interface, was observed. The results revealed that the AA7075 bit’s employment was able to decrease the shedding bit material effectively. During tensile tests, the maximum ultimate tensile strength (UTS) and elongation of the joint were 179.6 MPa and 13.7%, equivalent to 96.6% and 99% of the original defect-free friction stir welding (FSW) joint, respectively. The defect-free FFSW joints were produced at lower plunge speeds, and the fracture locations were at the softened region within the heat affected zone (HAZ) adjacent to the thermo-mechanically affected zone (TMAZ) on the retreating side. With increasing the plunge speed, the fracture location was more mainly dependent on the interface strength instead of the hardness distribution.     

Fatigue properties of AA6060‐T6 butt welds made by hybrid metal extrusion & bonding

The present investigation is concerned with high‐cycle axial fatigue testing of a 2‐mm AA6060‐T6 hybrid metal extrusion & bonding (HYB) butt weld produced in the solid state using AA6082 filler metal addition. The results complement the three‐point bend testing and the tensile testing done in two previous studies. In this study, optical microscope and scanning electron microscope examinations have been carried out to reveal the joint macro/microstructure and document possible surface and root defects deemed to affect fatigue life. In the as‐welded condition, the HYB weld suffers from surface irregularities at the weld face and ‘kissing’ bond formation in the root region. Despite of this, the subsequent testing shows that the fatigue properties exceed those reported for comparable AA6082‐T6 gas metal arc butt welds and matching those reported for corresponding high‐strength laser beam and friction stir weldments.     

Fatigue of friction stir welds in aluminium alloys that contain root flaws

Although the vast majority of friction stir welds will be free of flaws, it is not always possible to assume that they are. The properties of welds with flaws are needed to enhance confidence in the design and application of friction stir welded joints. The monotonic strength and fatigue behaviour of single-sided butt welds in 6–7 mm thick AA5083-O, AA5083-H321 and AA6082-T6, both without and with root flaws, was investigated.

Examination of the root flaw faces showed that there was bonding between the flanks of the flaws but the bonding was of poor quality and incomplete. This meant that the strength and ductility of the flaws were lower than the surrounding material. However, the comparison of the mechanical test results suggests that root flaws up to a certain size are tolerable without a significant loss in performance when compared to nominally flaw-free welds. These data also suggest that even friction stir welds with root flaws exceed the design life for equivalent fusion welds set out in the draft Eurocode 9 and that a higher rating may be warranted. Limited test results produced for this work need to be supplemented with a wider range of tests.     

Dissimilar friction stir lap welding of AA 5754-H22/AA 6082-T6 aluminium alloys: Influence of material properties and tool geometry on weld strength

Dissimilar friction stir welding (FSW) of heat (AA 6082-T6) and non-heat (AA 5754-H22) treatable aluminium alloys, in lap joint configuration, was performed in this work. The base material plates were 1 mm thick. Welds were performed combining different plates positioning, relative to the tool shoulder, in order to assess the influence of base materials properties on welds strength. Three different tools were tested, one cylindrical and two conical, with different taper angles. Welds strength was characterized by performing transverse and tensile–shear tests. Strain data acquisition by Digital Image Correlation (DIC) was used to determine local weld properties. The results obtained enabled to conclude that the dissimilar welds strength is strongly dependent on the presence of the well-known hooking defect and that the hooking characteristics are strongly conditioned by base materials properties/positioning. By placing the AA 6082-T6 alloy, as top plate, in contact with the tool shoulder, superior weld properties are achieved independently of the tool geometry. It is also concluded that the use of unthreaded conical pin tools, with a low shoulder/pin diameter relation, is the most suitable solution for the production of welds with similar strengths for advancing and retreating sides.     

Microstructure and mechanical properties of friction stir welded 18Cr–2Mo ferritic stainless steel thick plate

In this study, microstructure and mechanical properties of a friction stir welded 18Cr–2Mo ferritic stainless steel thick plate were investigated. The 5.4 mm thick plates with excellent properties were welded at a constant rotational speed and a changeable welding speed using a composite tool featuring a chosen volume fraction of cubic boron nitride (cBN) in a W–Re matrix. The high-quality welds were successfully produced with optimised welding parameters, and studied by means of optical microscopy (OM), scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD) and standard hardness and impact toughness testing. The results show that microstructure and mechanical properties of the joints are affected greatly, which is mainly related to the remarkably fine-grained microstructure of equiaxed ferrite that is observed in the friction stir welded joint. Meanwhile, the ratios of low-angle grain boundary in the stir zone regions significantly increase, and the texture turns strong. Compared with the base material, mechanical properties of the joint are maintained in a comparatively high level.     

Einfluss der Mikrostruktur auf das Verschleiß‐ verhalten der hartanodisierten Aluminium‐legierungen EN AW‐6082 und EN AW‐7075

Microstructural effect on the wear behaviour of the hard‐anodised aluminium alloys EN AW‐6082 and EN AW‐7075 The suitability of hard‐anodising of high‐strength Al alloys (EN AW‐7075‐T651) for the fabrication of protective coatings which are also applicable on screws was investigated. A medium‐strength AlSi1MgMn alloy (AA60682‐T6), generally rated as applicable for anodising, was used as reference material. After possible setting phenomena of a screw joint, the load‐bearing surface of the screw can be subjected to an oscillating relative movement. The damaging tribological load was simulated in an oscillation wear test. The resulting wear appearances have revealed that the untreated oxide coatings on the EN AW‐6082 substrate are not capable of providing protection against tribological load. Since hot‐water sealing increases the hardness of the coating but also contains the technology‐induced risk of softening the substrate material, other tribological protection methods have been looked for. The analysis of the tribological tests (characterisation of the structure and the resulting properties of the material, measurement of the wear amount and analysis of the wear appearance) have shown that the films sealed with wax emulsion on both substrate materials are the most promising candidates for the application of devices under oscillation wear. The obtained roughness, friction coefficients and hardness values confirm the positive behaviour of the anodically oxidised EN AW‐7075‐T651 alloy under the chosen tribological load.     

工具形状及工艺过程对搅拌摩擦增材成形及缺陷的影响

采用2mm厚的2195-T8铝锂合金作为增材板条,利用5种不同形状的搅拌工具进行搅拌摩擦增材工艺实验。利用金相观察和硬度测试的分析方法,重点探讨搅拌工具形状与工艺过程对增材成形、界面缺陷及硬度分布的影响。结果表明:圆柱状和三角平面圆台状搅拌针下增材界面上下材料无明显混合,偏心圆柱状和三凹圆弧槽状搅拌针有利于增材界面上下材料混合及减小界面钩状缺陷;增材前进侧界面形成致密无缺陷冶金连接,而后退侧界面材料混合不充分,钩状缺陷易伸入焊核区,且弱连接缺陷起源于此。四层增材中,相邻两层焊接方向相反的增材工艺使除顶层增材外其他增材两侧钩状缺陷向焊核区外侧弯曲,弱连接缺陷得到改善;顶层增材后退侧钩状缺陷伸入焊核区。增材焊核区有明显软化现象,但不同增材工艺下焊核区硬度分布均匀,表明搅拌摩擦增材制造可获得性能均匀的增材;相比于单道焊接工艺,来回双道焊接工艺使单层增材焊核区进一步软化;四层增材中,越靠近顶部的增材,其焊核区平均硬度越大。     

The study on defects in aluminum 2219-T6 thick butt friction stir welds with the application of multiple non-destructive testing methods

The present study focused on the relationship between primary friction stir welding process parameters and varied types of weld-defect discovered in aluminum 2219-T6 friction stir butt-welds of thick plates, meanwhile, the weld-defect forming mechanisms were investigated. Besides a series of optical metallographic examinations for friction stir butt welds, multiple non-destructive testing methods including X-ray detection, ultrasonic C-scan testing, ultrasonic phased array inspection and fluorescent penetrating fluid inspection were successfully used aiming to examine the shapes and existence locations of different weld-defects. In addition, precipitated Al₂Cu phase coarsening particles were found around a ‘kissing-bond’ defect within the weld stirred nugget zone by means of scanning electron microscope and energy dispersive X-ray analysis. On the basis of volume conservation law in material plastic deformation, a simple empirical criterion for estimating the existence of inner material-loss defects was proposed. Defect-free butt joints were obtained after process optimization of friction stir welding for aluminum 2219-T6 plates in 17–20 mm thickness. Process experiments proved that besides of tool rotation speed and travel speed, more other appropriate process parameter variables played important roles at the formation of high-quality friction stir welds, such as tool-shoulder target depth, spindle tilt angle, and fixture clamping conditions on the work-pieces. Furthermore, the nonlinear correlation between weld tensile strengths and weld crack-like root-flaws of different lengths was briefly investigated.