The Role of Tool Design in Influencing the Mechanism for the Formation of Friction Stir Welds in Aluminum Alloy 7020

Design of the required tool is a key and important parameter in the technique of friction stir welding (FSW). This is so because tool design does exert a close control over the quality of the weld. In an attempt to optimize tool design and its selection, it is essential and desirable to understand the mechanisms governing the formation of the weld. In this research study, few experiments were conducted to systematically analyze the intrinsic mechanisms governing the formation of the weld and to effectively utilize the analysis to establish a logical basis for design of the tool. For this purpose, the experiments were conducted using different geometries of the shoulder and pin of the rotating tool in such a way that only tool geometry had an intrinsic influence on formation of the weld. The results revealed that for a particular diameter of the pin there is an optimum diameter of the shoulder. Below this optimum shoulder diameter, the weld does not form while above the optimum diameter the overall symmetry of the weld is lost. Based on experimental results, a mechanism for the formation of friction stir weld is proposed. A synergism of the experimental results with the proposed mechanism is helpful in establishing the set of welding parameters for a given material.

Structural characterisation of reaction zone for friction stir welded aluminium-stainless steel joint

In the present investigation, commercially pure Al has been joined with 304 stainless steel (SS) by friction stir welding. The assembly finds widespread application in the field of cryogenics, nuclear, structural industries and domestic appliances. Microstructural characterisation was carried out using scanning and transmission electron microscopes. It has been found that diffusion of Fe, Cr and Ni is substantial within Al; however, diffusion of Al within 304SS is limited. Owing to interdiffusion of chemical species across the bondline, discrete islands of Fe3Al intermetallic form within the reaction zone. The rubbing action of tool over the butting edge of 304SS removed fine particles from 304SS, which were embedded in the stirring zone of Al matrix. Subsequently, austenite underwent phase transformation to ferrite due to large strain within this grain. Fracture path mainly moves through stirring zone of Al alloy under tensile loading; however, in some places, presence of Fe3Al compound has been also found.

Forming limit investigation of friction stir welded sheets: influence of shoulder diameter and plunge depth

The main aim of the present work is to analyze the influence of shoulder diameter and plunge depth on the formability of friction stir welded sheets. The base material used for welding and forming was AA6061-T6. Formability evaluation was performed through limiting dome height tests. The forming limit curve, FLC (only in the stretching region), thickness distribution, and strain hardening exponent of the weld region were monitored during formability studies. It is found from the work that the forming limit of friction stir welded sheets is better than unwelded sheets. In general, with an increase in shoulder diameter and plunge depth, the forming limit is found to improve considerably. With a decrease in thickness gradient severity and an increase in strain hardening exponent (n) of the weld region, the forming limit is found to increase. The increase in n value of the weld region is believed to occur because of the reduction in dislocation density. The maximum thickness difference is higher in the retreating side, rather than in the advancing side, of the weld. This is due to the differential straining and hardness levels attained by both sides during friction stir welding.

Friction Stir-Welded Dissimilar Aluminum Alloys: Microstructure, Mechanical Properties, and Physical State

A356 and 6061 aluminum alloys were joined by friction stir welding at constant tool rotational rate with different tool-traversing speeds. Thermomechanical data of welding showed that increment in tool speed reduced the pseudo heat index and temperature at weld nugget (WN). On the other hand, volume of material within extrusion zone, strain rate, and Zenner Hollomon parameter were reduced with decrease in tool speed. Optical microstructure of WN exhibited nearly uniform dispersion of Si-rich particles, fine grain size of 6061 Al alloy, and disappearance of second phase within 6061 Al alloy. With enhancement in welding speed, matrix grain size became finer, yet size of Si-rich particles did not reduce incessantly. Size of Si-rich particles was governed by interaction time between tool and substrate. Mechanical property of WN was evaluated. It has been found that the maximum joint efficiency of 116% with respect to that of 6061 alloy was obtained at an intermediate tool-traversing speed, where matrix grain size was significantly fine and those of Si-rich particles were substantially small.

Influence of external weld flash on the in-plane plane-strain formability of friction stir welded sheets

The main aim of the present work is to analyze the influence of external weld flash on the formability of friction stir welding sheets through in-plane plane-strain formability tests. The load-extension behavior and forming limit strains are measured to quantify the formability. The influence of friction stir welding parameters on the height of weld flash was also studied. The base materials used for welding trials are AA6061T6 and AA5052H32 alloy sheets of 2.1-mm thickness. It is observed that the influence of external weld flash on the maximum load and total extension for all the friction stir welding conditions is negligible. The effect of weld flash on the limiting major strain is also insignificant. But the presence of weld flash has changed the limiting minor strain, more toward plane-strain condition, indicating the change in strain-path toward plane-strain. This is due to the strain taken by weld flash, along with the major strain, minor strain, and thickness strain in the friction stir welding sheet plane because of constancy of volume. The formation of weld flash and its height are affected synergistically by the axial force and temperature development during friction stir welding. The higher the axial force and temperature, the higher the flash height.

Influence of tool rotation speed and feed rate on the forming limit of friction stir welded AA6061-T6 sheets

In this study, the influence of tool rotation speed and feed rate on the forming limit of friction stir welded Al 6061-T651 sheets has been investigated. The forming limit curve was evaluated by limit dome height test performed on all the friction stir welded sheets. The welding trials were conducted at a tool rotation speed of 1300 and 1400 r/min and feed rate of 90 and 100 mm/min. A third trial of welding was performed at a rotational speed of 1500 r/min and feed rate 120 mm/min. It is found that with increase in the tool rotation speed, from 1300 to 1400 r/min, for a constant feed rate, the forming limit of friction stir welded blank has improved and with increase in feed rate, from 90 to 100 mm/min, for a constant tool rotation speed, it has decreased. The forming limit of friction stir welded sheets is better than unwelded sheets. The thickness gradient after forming is severe in the cases of friction stir welded blanks made at higher feed rate and lower rotation speed. The strain hardening exponent of weld (n) increases with increase in tool rotation speed and it decreases with increase in feed rate. It has been demonstrated that the change in the forming limit of friction stir welded sheets with respect to welding parameters is due to the thickness distribution severity and strain hardening exponent of the weld region during forming. There is not much variation in the dome height among the friction stir welded sheets tested. When compared with unwelded sheets, dome height of friction stir welded sheets is higher in near-plane-strain condition, but it is lesser in stretching strain paths.

Failure modes and fatigue behavior of conventional and refilled friction stir spot welds in AA 6061-T6 sheets

The fatigue behavior of conventional friction stir spot welding (FSSW) and friction stir spot welding refilled by the friction forming process (FSSW-FFP) in aluminum 6061-T6 lap shear specimens, are investigated based on the experimental observations. Optical micrographs of the welds after fatigue failure in both the cases are examined to study the fatigue crack propagation and failure modes. Experimental results indicate that the fatigue strength of the FSSW-FFP weld samples is higher than that of the conventional FSSW samples at all loads. Fracture surfaces are analyzed in detail using the scanning electron microscope. (C) 2013 Elsevier Ltd. All rights reserved.

Static shear strength and fatigue life of refill friction stir spot welded AA 6061-T6 sheets

This study was aimed at evaluating the static shear strength and fatigue properties of the newly developed refilled friction stir spot welded AA 6061-T6 joints. The keyhole, the process disadvantage of conventional friction stir spot welding, was refilled successfully, using an additional filler plate, with specially designed tools. Two different tool profiles, namely, convex and concave, were used for the refilling process. Sound and defect free joints were obtained by the refilling process. Joints refilled with convex tools showed better static shear strength than those with the concave ones. The variation of microhardness in different regions of the weld was analysed. Fatigue tests were conducted on the lap shear specimens at a stress ratio of R=0.1. The optical micrographs of the welds after fatigue failure in both the conventional and refilled processes were examined to study the fatigue crack propagation and failure modes.

Evolution of microstructure and texture in friction stir processed Al-Mg-Mn alloy

Friction stir processing was carried out on the Al-Mg-Mn alloy to achieve ultrafine grained microstructure. The evolution of microstructure and micro-texture was studied in different regions of the deformed sample, namely nugget zone, thermo-mechanically affected zone (TMAZ) and base metal. The average grain sizes of the nugget zone, TMAZ and base metal are 1.5 mu m +/- 0.5 mu m, 15 mu m +/- 8 mu m, and 80 mu m +/- 10 mu m, respectively. The TMAZ exhibits excessive deformation banding structure and sub-grain formation. The orientation gradient within the sub-grain is dependent on grain size, orientation, and distance from nugget zone. The microstructure was partitioned based on the grain orientation spread and grain size values to separate the recrystallized fraction from the deformed region in order to understand the micromechanism of grain refinement. The texture of both deformed and recrystallized regions are similar in nature. Microstructure and texture analysis suggest that the restoration processes are different in different regions of the processed sample. The transition region between nugget zone and TMAZ exhibits large elongated grains surrounded by fine equiaxed grains of different orientation which indicate the process of discontinuous dynamic recrystallization. Within the nugget zone, similar texture between deformed and recrystallized grain fraction suggests that the restoration mechanism is a continuous process.

A bottom-up approach for optimization of friction stir processing parameters; a study on aluminium 2024-T3 alloy

Friction stir processing (FSP) is emerging as one of the most competent severe plastic deformation (SPD) method for producing bulk ultra-fine grained materials with improved properties. Optimizing the process parameters for a defect free process is one of the challenging aspects of FSP to mark its commercial use. For the commercial aluminium alloy 2024-T3 plate of 6 mm thickness, a bottom-up approach has been attempted to optimize major independent parameters of the process such as plunge depth, tool rotation speed and traverse speed. Tensile properties of the optimum friction stir processed sample were correlated with the microstructural characterization done using Scanning Electron Microscope (SEM) and Electron Back-Scattered Diffraction (EBSD). Optimum parameters from the bottom-up approach have led to a defect free FSP having a maximum strength of 93% the base material strength. Micro tensile testing of the samples taken from the center of processed zone has shown an increased strength of 1.3 times the base material. Measured maximum longitudinal residual stress on the processed surface was only 30 MPa which was attributed to the solid state nature of FSP. Microstructural observation reveals significant grain refinement with less variation in the grain size across the thickness and a large amount of grain boundary precipitation compared to the base metal. The proposed experimental bottom-up approach can be applied as an effective method for optimizing parameters during FSP of aluminium alloys, which is otherwise difficult through analytical methods due to the complex interactions between work-piece, tool and process parameters. Precipitation mechanisms during FSP were responsible for the fine grained microstructure in the nugget zone that provided better mechanical properties than the base metal. (C) 2014 Elsevier Ltd. All rights reserved.