Fatigue properties of friction stir welded joint of ultrafine-grained 2024 aluminium alloy

Ultrafine-grained (UFG) 2024 aluminium alloy prepared by the equal channel angular pressing was friction stir welded (FSW). The high cycle fatigue and crack growth behaviour of the FSW joint were investigated in air and NaCl solution, respectively. This study demonstrated that FSW was a viable technique for joining UFG materials. The UFG microstructure was retained in the nugget zone (NZ). Compared with the UFG base metal (BM), FSW joint exhibited lower ultimate tensile strength and hardness, and the minimum hardness value was located in the heat affected zone (HAZ). NaCl solution significantly reduced the fatigue strength of FSW joint. Fatigue crack propagation rates in the NZ and HAZ were slower than that in the BM in the whole fatigue life.     

Critical review on friction stir welding of aluminium to copper

The status quo of aluminium-to-copper joining by friction stir welding (FSW) drastically changed in recent years, as a result of the increased interest of the scientific community on this subject. Actually, since 2006 a large increase in the number of research groups addressing Al–Cu FSW has been witnessed all over the world, together with a significant increase in the amount of published studies. A chronological perspective on the evolution in Al–Cu FSW research is provided in this work, highlighting the pioneer and original contribution of several researchers to the current knowledge on the subject. Detailed and comprehensive investigations on the material flow mechanisms, the phenomena governing the formation and distribution of intermetallic phases during Al–Cu FSW, their relations with the welding parameters and their impact on the morphological, structural and mechanical properties of the welds are thoroughly discussed. The main findings reported in the literature are summarised in thematic tables.     

Mechanical properties of 2024-T4 aluminium alloy joints in ultrasonic vibration enhanced friction stir welding

Ultrasonic vibration enhanced friction stir welding （UVeFSW） is a recent modification of conventional friction stir welding （FSW）, which transmits ultrasonic vibration directly into the localized area of the workpiece near and ahead of the rotating tool. In this study, a high strength aluminium alloy （2024-T4） was welded by this process and conventional FSW, respectively. Then tensile tests, microhardness tests and fracture surface analysis were performed successively on the welding samples. The tests results reveal that ultrasonic vibration can improve the tensile strength and the elongation of welded joints. The microhardness of the stir zone also increases.     

Numerical and experimental investigation on friction stir lap welding of aluminium to steel

Use of multimaterial fabrication such as aluminium to steel to reduce overall vehicular body weight has gained significant attention in the automotive industries. Since fusion welding of aluminium to steel is difficult, friction stir welding of the same is considered as an effective recourse. Quantitative studies on friction stir welding of aluminium to steel are thus important but scarce in the literature. We present here a numerical and experimental study on friction stir lap welding of AA6061 to high strength interstitial free coated steel sheets under different combinations of tool rotational speed and welding speed. The computed values of thermal cycle, torque and traverse force are found to be in good agreement with the corresponding experimentally measured values. The computed thermal cycles along the AA6061 to steel interfaces are related qualitatively with the experimentally measured trend and distribution in Fe–Al intermetallics along the weld joint interface.     

Effect of welding parameters on mechanical and microstructural properties of AA6082 joints produced by friction stir welding

The effect of processing parameters on mechanical and microstructural properties of AA6082 joints produced by friction stir welding was analysed in the present study. Different welded specimens were produced by employing a fixed rotating speeds of 1600 rpm and by varying welding speeds from 40 to 460 mm/min. The joints mechanical properties were evaluated by means of tensile tests at room temperature. In addition, fatigue tests were performed by using a resonant electro-mechanical testing machine under constant amplitude control up to 250 Hz sinusoidal loading. The fatigue tests were conducted in axial control mode with R = σ_min/σ_max = 0.1, for all the welding and rotating speeds used in the present study. The microstructural evolution of the material was analysed according to the welding parameters by optical observations of the jointed cross-sections and SEM observations of the fractured surfaces were done to characterize the weld performances.     

Microstructure,mechanical properties and failure behaviour of protrusion friction stir spot welded 2024 aluminium alloy sheets

The current study investigates the mechanical and microstructure properties of 2024 aluminium alloy welded by protrusion friction stir spot welding as a novel method to produce keyhole-free welds. Tool rotation speed and anvil protrusion height are used as effective variables of the process to obtain optimum conditions. Results illustrate that the keyhole-free welds with the joint show superior mechanical properties in protrusion friction stir spot welding compared to conventional friction stir spot welding. Failure mode changes from interfacial mode to circumferential mode by increasing the nugget zone depth and joint length, while the effect of nugget zone is considerable. Finally, welding at a rotation speed of 1600?rev?min^?1 and a protrusion height of 0.4?mm presents significant mechanical properties with more joint length.     

Analysis of process parameters effects on dissimilar friction stir welding of AA1100 and A441 AISI steel

Abstract

A prominent benefit of friction stir welding process is to join plates with dissimilar material. In this study, an attempt is made to find effects of tool offset, plunge depth, welding traverse speed and tool rotational speed on tensile strength, microhardness and material flow in dissimilar friction stir welding of AA1100 aluminium alloy and A441 AISI steel plates. Here, one factor at a time experimental design was utilised for conducting the experiments. Results indicated the strongest joint obtained at 1·3?mm tool offset and 0·2?mm plunge depth when the tool rotational speed and linear speed were 800?rev min^??1 and 63?mm min^??1 respectively. The maximum tensile strength of welded joints with mentioned optimal parameters was 90% aluminium base metal. Fracture locations in tensile test at all samples were in aluminium sides. Owing to the formation of intermetallic compounds at high tool rotational speed, the microhardness of joint interface goes beyond that of A441 AISI steel.     

Mechanical properties and microstructure analysis of refilling friction stir welding on 2219 aluminum alloy

The 2219 aluminum alloy under refilling friction stir welding (RF-FSW) was investigated. The micrographs showed that the bead could be divided into six zones, and the grain size and shape were greatly different in these zones. According to the microstructure analysis, the weld nugget zone and the shoulder stirring zone consisted of equiaxed grains, while the grains in the heat affected zone were seriously coarsened. It was obvious that bending deformation occurred in the thermo-mechanically affected zone. According to the microhardness analysis, the lowest hardness of the weld was at the thermo-mechanically affected zone, and the microhardness increased with the retraction of the stir-pin. The tensile strength and elongation of the bead were 70% and 80% of the base metal, respectively. The tensile strength was slightly different for the stable stage and the retraction stage, while the elongation decreased in the retraction stage. The mechanical properties and microstructure responded to different retraction speed were analyzed, and it showed that the elongation decreased with increasing retraction speed.     

Influence of aluminium alloy type on dissimilar friction stir lap welding of aluminium to copper

A heat-treatable (AA 6082) and a non-heat treatable (AA 5083) aluminium alloys were friction stir lap welded to copper using the same welding parameters. Macro and microscopic analysis of the welds enabled to detect important differences in welding results, according to the aluminium alloy type. Whereas important internal defects, resulting from ineffective materials mixing, were detected for the AA 5083/copper welds, a relatively uniform material mixing was detected in the AA 6082/copper welds. Micro-hardness testing and XRD analysis also showed important differences in microstructural evolution for both types of welds. TEM and EBSD-based study of the AA 5083/copper welds revealed the formation of submicron-sized microstructures in the stirred aluminium region, for which untypically high hardness values were registered.     

Macrostructures and mechanical properties of ultrasonic-assisted friction stir welding joint of 2024-T3 aluminium alloy

In friction stir welding of aluminum alloys, tunnel defect may occur due to insufficient plastic material flow caused by lower heat input in the weld region. The inadequacy in heat input is due to improper selection of friction stir welding tool and process parameters. Ultimately, such defects degrade the properties of weld and may pose serious concerns towards the integrity and safety of the weld component. In order to improve the properties of weld joints, an ultrasonic-assisted friction stir welding device has been configured where ultrasonic energy is transferred from an ultrasonic unit directly into one of the workpieces near the tool. Using this configuration, ultrasonic-assisted friction stir welding was conducted on 6 mm thick 2024-T3 aluminum alloy sheets. The macrostructure and mechanical properties of these welds were compared with the welds of this alloy prepared by conventional friction stir welding using identical process parameters. The results show that the welding speed can be increased while satisfactory weld quality is still ensured. The ultrasonic energy transferred in this configuration could enlarge the volume of weld nugget zone. Also, the influence of ultrasonic energy on the suppression or elimination of the tunnel defect is quite apparent. However, any beneficial effects of ultrasonic vibration on the tensile strength and the elongation of the joint were less obvious in this configuration.     

High strength lap joint of aluminium and stainless steels fabricated by friction stir welding with cutting pin

Abstract

Dissimilar lap joints of aluminium and stainless steel were first friction stir welded by the tool with a cutting pin. The results showed that sound joints could be obtained by this method. When the pin was inserted into the lower steel sheet, macrointerlocks were formed by the steel flashes plugging into the upper aluminium at both sides of the nugget bottom. At the aluminium/steel interface, a thin intermetallic compound (IMC) layer and the mechanical bonding of microinterlocks were formed. In addition, the aluminium near the interface was also strengthened by grain refinement and IMC particles. Therefore, the beneficial effect of the macrointerlocks provided by the steel flashes was removed, the shear strength of the joint reached 89·7 MPa, which was even higher than that of the base metal of aluminium.     

Comparison of 2A12 aluminium alloy joint by ultrasonic assisted friction stir welding and friction stir welding

Ultrasonic assisted friction stir welding （UAFSW） is a recent modification of conventional friction stir welding, which adds ultrasonic energy directly into the friction stir welding area by the pin. In this study, 2A12 aluminum alloy was welded by this process and conventional, respectively. The tensile tests, microstructure and fracture surface of FSW joint and UAFSW joint were analyzed. The research results show that the surface forming texture of ultrasonic assisted friction stir welding joint, compared with conventional, is finer and more uniform, showing metallic matte color. The grains are much finer in weld nugget zone, thermo-mechanically affected zone and heat-affected zone; S-phase particles size is much smaller and distribution is more homogeneous in the matrix. The tensile strength of UAFSW joint is 94. 13% of base metal, and the elongation is 11.77%. The tensile strength of FSW joint is 83.15% of base metal, and the elongation is 8.81%. The tests results reveal that ultrasonic vibration can improve the tensile strength and the elongation of welded joints.     

Achieving high property friction stir welded aluminium/copper lap joint at low heat input

Abstract

Aluminium and copper plates with 3 mm thickness were successfully friction stir lap welded at a lower rotation rate of 600 rev min^?1 using a larger pin 8 mm in diameter. Good metallurgical bonding on the Al/Cuinterface was achieved due to the formation of a thin, continuous and uniform Al–Cu intermetallic compound layer. Furthermore, many Cu particles consisting of pure Cu and intermetallic compound layers were generated at the lower part of the nugget zone, forming a composite structure with increased hardness. A lower rotation rate resulted in a decrease in annealing softening in the heat affected zone (HAZ), and a larger diameter pin increased the Al–Cu bonding area. These factors resulted in that the friction stir welded lap joint exhibited a high failure load of 2680 N with failure in the HAZ on the aluminium side.     

The effect of welding parameters on the corrosion behaviour of friction stir welded AA2024-T351

The effect of welding parameters (rotation speed and travel speed) on the corrosion behaviour of friction stir welds in the high strength aluminium alloy AA2024-T351 was investigated. It was found that rotation speed plays a major role in controlling the location of corrosion attack. Localised intergranular attack was observed in the nugget region for low rotation speed welds, whereas for higher rotation speed welds, attack occurred predominantly in the heat-affected zone. The increase in anodic reactivity in the weld zone was due to the sensitisation of the grain boundaries leading to intergranular attack. Enhancement of cathodic reactivity was also found in the nugget as a result of the precipitation of S-phase. The results were compared with samples of AA2024-T351 that had been heat treated to simulate the thermal cycle associated with welding, and with samples that had been exposed to high temperatures for extended periods to cause significant over-ageing.     

Comparative study of fatigue behaviour in dissimilar Al alloy/steel and Mg alloy/steel friction stir spot welds fabricated by scroll grooved tool without probe

Abstract

Aluminium alloy A6061-T6 or magnesium alloy AZ31 sheet was welded to steel sheet by a friction stir spot welding technique using a scroll grooved tool without a probe. The material flow in the nugget of the Mg/steel weld was less than that in the Al/steel one. The Al/steel weld exhibited higher static tensile–shear strength than the Al/Al weld, while the strengths of Mg/steel and Mg/Mg welds were comparable. Tensile–shear fatigue tests were performed using lap shear specimens of both dissimilar and similar welds. The dissimilar welds exhibited nearly the same fatigue strengths as the similar ones. The effective nugget size in the dissimilar welds was defined as the area where Al or Mg alloy remained on the steel side after static fracture. When the fatigue strengths of dissimilar welds were evaluated based on the effective nugget size, the normalised fatigue strengths of Al/steel and Mg/steel welds were comparable.     

CDRX modelling in friction stir welding of aluminium alloys

In the paper a numerical model aimed to the determination of the average grain size due to continuous dynamic recrystallization phenomena (CDRX) in friction stir welding processes of AA6082 T6 aluminum alloys is presented. In particular, the utilized model takes into account the local effects of strain, strain rate and temperature; an inverse identification approach, based on a linear regression procedure, is utilized in order to develop the proper material characterization.     

Investigation of weld defects in dissimilar friction stir welding of aluminium to brass by radiography

Abstract

The present study was focused on detecting weld defects in dissimilar joints made by friction stir welding. Radiography test and optical microscopy were used to evaluate the main defects. Coarse and continuous fragments of brass in aluminium matrix, tunnelling and void defects in vicinity of fragments were the main observed defects.     

Formation of interfacial microstructure in a friction stir welded lap joint between aluminium alloy and stainless steel

The interfacial microstructure produced through tool transit of a friction stir welded lap joint between an aluminium alloy and stainless steel was studied by transmission electron microscopy in order to clarify its early stages of formation. Transmission electron microscopy studies of the bottom surface of the exit hole revealed the presence of several mixed layers of an ultrafine intermetallic compound (IMC) and stainless steel. The joining between dissimilar materials was achieved through a continuous flow of the stirred aluminium alloy into the mixed layers and the resultant growth of the ultrafine IMCs due to the heat induced by the friction between the tool and the specimen. The continuous thin reaction layer finally produced at the interface was found to be stronger than the base aluminium alloy.     

铝合金搅拌摩擦焊的研究现状与展望

在介绍搅拌摩擦焊技术的基本原理及影响因素的基础上,综述了铝合金搅拌摩擦焊在接头金属塑性流动、显微组织以及性能等方面的国内外研究现状.研究现状表明,搅拌头形状和工艺参数对接头组织和性能具有重要影响,在合适的工艺参数下可获得综合性能良好的铝合金搅拌摩擦焊接头,并取得了一定规模的应用.此外,指出了搅拌摩擦焊技术的不足和接头性...