Defects In Friction Stir Welding Research Articles

Phased Array Ultrasonic Testing for Post-Weld and OnLine Detection of Friction Stir Welding Defects

ABSTRACTNondestructive evaluation (NDE) techniques of phased array ultrasonic testing (PAUT) and digital X-ray radiography were employed on friction stir (FS)-welded Aluminum Alloy (AA)-2219-T87 specimens. PAUT intricacies required for scanning of FS-welded specimens with a 10-MHz 32-element transducer are discussed. The time corrected gain (TCG) calibration is required for scanning with an increase in index offset to compensate for decrease in A-Scan signal peak amplitude. Calibration techniques to find small defects with appropriate size tolerances are also established. The NDE technique of digital X-ray radiography is compared to PAUT, where it was found that a calibrated PAUT system is able to discover defects less than 0.2 mm where X-ray radiography could not. Incomplete penetration (IP), wormhole (WH), surface cavity (SC), and internal void (IV) defects are analyzed. Furthermore, an online PAUT system for FSW has been developed and successfully tested. The work provided herein will provide a gateway for an ultimate goal of an automated PAUT online sensing system.

Classification and identification of surface defects in friction stir welding: An image processing approach

Friction stir welding (FSW) is a new entrant in welding technology and getting a defect-free weld is the final objective. But different defects are generated due to various reasons and needs to be analyzed to eliminate them. The aim of the research work is to identify and classify different kinds of surface defects generally encountered during the FSW process using digital image processing techniques. The defects on the surface of the weld are identified using image pyramid and image reconstruction algorithms. Further, using these algorithms the defects can be classified into voids, grooves, cracks, key-hole and flash with the help of unique features of each kind of defect. Vertical intensity plot and the area plot of the defect blobs are represented for the proper localization and analysis of severity of defects.

Applying a nonlinear, pitch-catch, ultrasonic technique for the detection of kissing bonds in friction stir welds

Friction stir welding (FSW) is a promising technology for the joining of aluminum alloys and other metallic admixtures that are hard to weld by conventional fusion welding. Although FSW generally provides better fatigue properties than traditional fusion welding methods, fatigue properties are still significantly lower than for the base material. Apart from voids, kissing bonds for instance, in the form of closed cracks propagating along the interface of the stirred and heat affected zone, are inherent features of the weld and can be considered as one of the main causes of a reduced fatigue life of FSW in comparison to the base material. The main problem with kissing bond defects in FSW, is that they currently are very difficult to detect using existing NDT methods. Besides, in most cases, the defects are not directly accessible from the exposed surface. Therefore, new techniques capable of detecting small kissing bond flaws need to be introduced. In the present paper, a novel and practical approach is introduced based on a nonlinear, single-sided, ultrasonic technique. The proposed inspection technique uses two single element transducers, with the first transducer transmitting an ultrasonic signal that focuses the ultrasonic waves at the bottom side of the sample where cracks are most likely to occur. The large amount of energy at the focus activates the kissing bond, resulting in the generation of nonlinear features in the wave propagation. These nonlinear features are then captured by the second transducer operating in pitch-catch mode, and are analyzed, using pulse inversion, to reveal the presence of a defect. The performance of the proposed nonlinear, pitch-catch technique, is first illustrated using a numerical study of an aluminum sample containing simple, vertically oriented, incipient cracks. Later, the proposed technique is also applied experimentally on a real-life friction stir welded butt joint containing a kissing bond flaw.

Impact of tool profile on mechanical properties of AZ31B mg alloy during FSW using optimized parameters

In this experimental investigation, an attempt was made to understand the influence of the tool profile on the mechanical properties of the joints fabricated during friction stir welding (FSW) of AZ31B Mg alloy flat plates. When the specimens prepared from the successfully fabricated joints were subjected to tensile tests as per prescribed International Standards it was observed that employing a taper cylindrical pin profiled tool resulted in sound quality lap joints with improved mechanical properties. Moreover, the use of other tool profiles namely straight cylindricall&straight square could not produce defect free joints. Further, this investigation reveals us that the common FSW defects including low distortion, lack of cavity, tunnel defects etc are found to be completely eliminated while using taper cylindrical pin profiled tool.

Investigation of weld defects in friction-stir welding and fusion welding of aluminium alloys

Transportation industries are obliged to address concerns arising from greater emphasis on energy saving and ecologically sustainable products. Engineers, therefore, have a responsibility to deliver innovative solutions that will support environmental preservation and yet meet industries’ requirements for greater productivity and minimised operational costs. Aluminium alloys have successfully contributed to meeting the rising demand for lightweight structures. Notable developments in aluminium welding techniques have resolved many welding related problems, although some issues remain to be addressed. The present study attempts to give an overview of the key factors related to the formation of defects in welding methods commonly used with aluminium alloys. First, a concise overview of defects found in friction-stir welding, laser beam welding and arc welding of aluminium alloys is presented. The review is used as a basis for analysis of the relationship between friction-stir welding process parameters and weld defects. Next, the formation and prevention of the main weld defects in laser beam welding, such as porosity and hot cracking, are discussed. Finally, metallurgical aspects influencing weld metal microstructure and contributing to defects are tabulated, as are defect prevention methods, for the most common flaws in arc welding of aluminium alloys.

Ultrasonic Method for Rapid Detection of the Aluminum Friction Stir Welding Defects

It has been a difficult problem to detect the defects in aluminum alloy friction stir welding(FSW) joints, especially those in thin plates. Recently most of the methods adopt the transverse wave or the ultrasonic phased array S-scan. However, these means cannot present all of the defects in a single picture effectively and efficiently. Ultrasonic phased array technology is used to detect the inner defects just above the upper surface of the weld by C-Scan on different sorts of defects in plates with various thickness. A mechanical device is developed to accelerate the procedure of detection. What's more, the influence of the roughness of the FSW surface on its detectability is analyzed. The results of the experiments show that 0.3 mm-diameter side hole and 0.5 mm-diameter flat bottom holes in a 5 mm-thick flat plate can be detected effectively with the help of ultrasonic phased array.

Application of Taguchi Method in the Optimization of Friction Stir Welding Parameters of an Aeronautic Aluminium Alloy

The Friction Stir Welding (FSW) process is still an innovative solid state mechanical processing technology enabling high quality joints in materials previously considered with low weldability such as most of the aeronautic aluminium alloys. The Taguchi method was used to find the optimal FSW parameters for improvement mechanical behaviour of AA2024-T351. The Taguchi design is an efficient and effective experimental method in which a response variable can be optimized. The parameters considered were vertical downward forging force, travel speed and pin length. An orthogonal array of L9 (34) was used; ANOVA analyses were carried out to identify the significant factors affecting tensile strength (GETS), bending toughness (GEB) and hardness field. An algebraic model for predicting the best mechanical performance was developed and the optimal FSW combination was determined using this model. The results obtained were validated by conducting confirmation experiments. Significant interest has been shown in the use of advanced welding techniques for aircraft structures, particularly given the design and manufacturing benefits they afford over established mechanical joining methods. Whilst a variety of welding methods have been identified for airframe structures, friction stir welding is an important candidate technique that is distinctive in being a low energy, solid-state process [1]. Although the friction stir welding joints have a better quality compared to the fusion techniques, there are still some defects that may arise and which are very sensitive to small variations in process parameters. Typical defects that may arise in FSW joints result from: imperfect stir of the materials during the processing, inadequate surface preparation, lack of penetration of the pin and non-uniform vertical forging forces along the material thickness. Some characteristic FSW defects are lack of penetration (typically addressed as kissing-bond), root flaw (concerning weak or intermittent linking), voids on the advancing side and second phased particles and oxides alignment under the shoulder [2]. Advanced aerospace aluminium alloys have been required to allow high fracture toughness, higher fatigue performance, high formability, and superplasticity to meet the needs for lower structural weight, higher damage tolerance and durability [3].

Experimental Investigation of Material Flow and Welding Defects in Friction Stir Welding of Aluminum to Brass

In this research, the formation of a nugget zone in dissimilar friction stir welding (FSW) of brass to aluminum has been investigated. Also, the main parts of a stir zone (SZ) besides their dependency to the welding parameters are probed. The base materials play the role of marker to reveal the material flow pattern. Investigations carried out by optical microscopy and scanning electron microscopy verifies the formation of three different areas including pin affected zone, shoulder affected zone, and swirling zone within the SZ. Moreover, the various types of defects such as tunneling defect, coarse fragment defect created during dissimilar FSW are analyzed. Also, an attempt has been made to remove the defects by adjusting the welding parameters especially the offset value which features in defects formation.

The use of neural network and discrete Fourier transform for real-time evaluation of friction stir welding

This paper introduces a novel real-time approach to detecting wormhole defects in friction stir welding in a nondestructive manner. The approach is to evaluate feedback forces provided by the welding process using the discrete Fourier transform and a multilayer neural network. It is asserted here that the oscillations of the feedback forces are related to the dynamics of the plasticized material flow, so that the frequency spectra of the feedback forces can be used for detecting wormhole defects. A one-hidden-layer neural network trained with the backpropagation algorithm is used for classifying the frequency patterns of the feedback forces. The neural network is trained and optimized with a data set of forge–load control welds, and the generality is tested with novel data set of position control welds. Overall, about 95% classification accuracy is achieved with no bad welds classified as good. Accordingly, the present paper demonstrates an approach for providing important feedback information about weld quality in real-time to a control system for friction stir welding.

Lamb Waves Technique Applied to the Characterization of Defects in Friction Stir Welding of Aluminum Plates: Comparison with X-Ray and Ultrasonic C-Scan

Abstract Friction stir welding presents several advantages when compared with conventional arc welding processes, mainly in the welding of aluminum alloys. However, this welding technology leads to some degradation in the mechanical properties of welds, namely defect formation, which demands suitable nondestructive testing methods. The most common defects are mainly cold laps and voids, as a result of the large plastic deformation and hardening of the material as well as its complex flow behavior. In particular, the void appearance frequency may be correlated with the weld travel speed, though other welding parameters may contribute to the phenomenon. The purpose of this paper is to characterize such defects using conventional x-rays and ultrasonic C-scan, and a new method based on ultrasonic guided waves. The proposed new method presents as an attractive solution when large structures need to be inspected since propagation over long distances from a single probe position is possible with low attenuation. Additional characteristics such as straightforward inspection and testing fastness make the technique very cost effective. Test samples were fabricated using aluminum alloys of 3 mm in thickness, with different travel speeds and overlapping welds. This welding procedure gave rise to different defect sizes. Experimental results using both conventional and ultrasonic guided waves methods have confirmed the presence of the defects.

The investigation of typical welding defects for 5456 aluminum alloy friction stir welds

The external factors on the friction stir welding defects are so abundant that the experiments of friction stir welding were conducted for 5456 aluminum alloy. With the changes of the tool tilt angle and material condition, defects can be generated. These defects can be conventional ones (lack of penetration or voids), or lazy S, which are unique to friction stir welding. However, the origin of the defects remains an area of uncertainty. In this study, an attempt has been made to investigate the formation of these defects. The typical welding defects of friction stir welding joint for 5456 aluminum alloy were analyzed and discussed, respectively, by using optical microscopy (OM), energy-dispersive X-ray spectroscopy (EDS) and scanning electron microscope (SEM). The microscopic examination of the nugget zone and fracture location of the weld confirms that the tilt angle can change the plastic material flow patterns in the stir zone and accordingly control the weld properties. In addition, the oxide layer from the initial butt surface during FSW is dispersed at the grain boundary. These A1 2O 3 particles are actually the major cause of failure of the joint.