Tunnel Defects Research Articles

A combined wavelet packet and Hilbert-Huang transform for defect detection and modelling of weld strength in friction stir welding process

The objectives of the current research work is two folded. First, to develop effective methodology for identification of internal defects in friction stir welded samples. Identification of tunnel type of defect is attempted through the analyses of real-time force signals and analyzing the signals with a proposed combination of wavelet packet transform with Hilbert-Huang transform. The analyses results in two features namely instantaneous phase and instantaneous frequency that efficiently describe the presence of defects in the welded samples. The study presented for tunnel defect can as well be implemented in detection of other types of defects in friction stir welded samples. The second objective of this work is to develop a data driven model for accurate estimation of ultimate tensile strength of the welded samples. For this purpose support vector machine learning based support vector regression model is developed. The model parameters are optimized using the grid search method to control the overfitting during training of the model. The accuracy of the developed model leads to the impression that it can be further modified for real-time welding operations for accurate modelling of ultimate tensile strength of the joints.

Decoherence spectroscopy with individual two-level tunneling defects.

Recent progress with microfabricated quantum devices has revealed that an ubiquitous source of noise originates in tunneling material defects that give rise to a sparse bath of parasitic two-level systems (TLSs). For superconducting qubits, TLSs residing on electrode surfaces and in tunnel junctions account for a major part of decoherence and thus pose a serious roadblock to the realization of solid-state quantum processors. Here, we utilize a superconducting qubit to explore the quantum state evolution of coherently operated TLSs in order to shed new light on their individual properties and environmental interactions. We identify a frequency-dependence of TLS energy relaxation rates that can be explained by a coupling to phononic modes rather than by anticipated mutual TLS interactions. Most investigated TLSs are found to be free of pure dephasing at their energy degeneracy points, around which their Ramsey and spin-echo dephasing rates scale linearly and quadratically with asymmetry energy, respectively. We provide an explanation based on the standard tunneling model, and identify interaction with incoherent low-frequency (thermal) TLSs as the major mechanism of the pure dephasing in coherent high-frequency TLS.

The weld formation of self-support friction stir welds for aluminum hollow extrusion

Friction stir welding has made significant strides to weld aluminum hollow extrusion thanks to the advancement of tool design. Research has concentrated on weld defect in aluminum hollow extrusion self-support friction stir welds and the mechanism of weld formation. Three distinct material flow zones are involved in weld formation. The formation of inner material-loss defects depends on migration ability of plastic material which corresponds to the effectiveness of lower shoulder and welding parameters. The size of defect decreases and formation position shifts downward with increasing welding speed and lower shoulder diameter. Materials flow characterization left around the tunnel and pore defect is used to study plastic material flow patterns. The formation quality can be improved by increasing welding speed and lower shoulder diameter, which affects deformation ability and flow of plastic material around the tool.

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.

Effect of geometrical parameters on Friction Stir Welding of AA 5083-H111 T-joints

The aim of this research is to study the effect of three different tool geometries and two joint geometries on quality of AA 5083-H111 T-welds done using the friction stir welding process (FSW). All the tools have concave shoulder with different pin geometries: tapered and threaded, quadrangular pyramidal and progressive pin, part threaded cylindrical and part pyramidal. T-lap and T-butt joints configurations have been studied. Tunnel and kissing-bond type defects have been found in joints produced with a pyramidal pin tool, while welds produced with the tapered pin tool only show presence of oxide lines. Sound welds were produced with the progressive tool. No significant change in hardness has been observed in all combinations of tool and joint geometry. The tensile strength efficiency in joints welded with the progressive pin tool has been found to be 100%. The fatigue results(R=0) show a higher fatigue strength of FSW T-joints than T-joints welded by conventional process as Metal Inert Gas (MIG).

Effects of Tool Pin Design on Formation of Defects in Dissimilar Friction Stir Welding

Friction stir welding is an automatic process, falls under the category of solid state welding processes. In this study, friction stir welding was carried out on dissimilar copper to aluminum materials by nine different tool designs. Present investigation provides an insight on formation of defects under the effect of different tool pin designs. The tool pin profiles such as cylindrical, triangular, square and hexagonal were used in the present experimental investigation by keeping other parameters constant. These tool pin profiles were designed based on its static and dynamic constant areas from best suitable cylindrical profile. Besides, the defects were analyzed through visual observation, macrostructure, microstructural investigation and scanning electron microscope. The results revealed that, the copper particles detached from base material were large and irregular in case of polygonal pin designs. Maximum irregular and large copper particles were reported in welds made by triangular pin profiles. Additionally, polygonal pin profiles were generating defects such as voids, tunnel, cracks and fragmental defects irrespective of its static and dynamic constant areas. Furthermore, defects were decreased as the polygonal edges increases. Defect free macro joint was reported for cylindrical tool pin profile.

Investigation on the effect of tool pin profile on the joint quality of Friction Stir Welded aerospace grade Aluminium Alloy

Abstract Present paper explores the effect of the various tool pin profiles viz. cylindrical, threaded cylindrical, threaded cam and threaded triflute on the joint quality of Friction stir welded aerospace grade aluminium alloy (AA7475). 2.5 mm thick plates of AA7475-T761 are welded using Friction stir welding (FSW) and subsequently, the welds are characterized using optical microscopy (OM) and mechanical testing. Microstructural examinations reveal that the tools with threaded cylindrical, threaded cam, and threaded triflute pin profile lead to the formation of tunneling defect and joint line remnant (JLR) in the welds probably due to imbalance in material movement caused by insufficient plunge and sticking of base material around and on the bottom of the pin. Further, it is also found that cylindrical pin profile results in the maximum tensile strength (291.8 MPa) and threaded cam pin profile gives minimum tensile strength (247.5 MPa). Low tensile strength is observed due to sticking of 0.1 mm thick layer of aluminium on the bottom of the pin leading to reduction in the active plunge causing defects formation and deteriorating the joint quality.

Defect Analysis in Friction Stir Welding Process using Wavelet Transformation Technique

Vibration sources are found in machining operations, which can predict the machining characteristics and operating conditions. These sources were found in mixed form with machine spindle excitation and external disturbances. This contaminates the monitoring predictability. This study deals with the independent component analysis to separate the source signals for identifying the pre-induced defects in the friction stir welding process. Experiments were carried out for defect induced and defect free work piece of Al6082 aluminum alloy with the acquisition of vibration signals during the friction stir welding process. These signals were analyzed using the proposed method with the decomposition of the acquired mixed component signals using continuous wavelet transform called pseudo channel signals. These signals were used to acquire the independent component signals by ICA analysis. This signal is denoised using discrete wavelet transform to achieve the redundant signal with the removal of white Gaussian noise. Modal analysis is carried using finite element method (FEM) to extract the natural frequency of the source components, which impacts the welding process. The projected technique is used for examining and diagnosing the tunnel defects in the FSW process. It is established that the separated source components successfully expose the defects induced in the aluminum alloy work-piece. This proves the separation process to be more robust and accurate for Friction stir welding based process monitoring.

Microstructural and Mechanical Properties of Friction Stir Welded Nickel-Aluminum Bronze (NAB) Alloy

In this study, the applicability of friction stir welding to cast NAB alloy (i.e., C95800) with a thickness of 9 mm has been investigated. The joint performance was determined by conducting optical microscopy, microhardness measurements, and mechanical testing (e.g., tensile and Charpy impact tests). The effect of stir intensity on joint performance was also determined. A grain refinement (equiaxed fine grain structure) as well as evolution of a Widmanstatten structure was achieved within the stir zone of all the joints produced. Thus, all of the joints produced exhibited higher proof stress (i.e., between 512 and 616 MPa) than that of the base material, i.e., 397 MPa. On the other hand, only half of the specimens exhibited higher tensile strength values than that of the base plate (i.e., 794 MPa), whereas the other specimens displayed lower tensile strength than the base plate due to the existence of weld defects, namely cold bonding and/or tunnel defect.

Elimination of tunnel defect in ultrasonic vibration enhanced friction stir welding

Through observing the marker material (MM) distribution and the exit holes left by the “tool sudden stop action” technique, the formation mechanism of tunnel defect in friction stir welding (FSW) is elucidated. Based on the MM distribution on the transverse cross-sections, the weld nugget zone is divided into three subzones, i.e., shoulder affected zone (SAZ), pin affected zone (PAZ), and weld bottom zone (WBZ). There is a time-delay feature of material flow which is considered as the inducement of tunnel defect formation. When a tunnel defect occurs in FSW joints, three subzones connect with each other and there exists a converging point. The insufficient material flow from retreating side to advancing side in the PAZ and the shortage of downward material transfer in the SAZ are the primary and secondary reasons for tunnel defect occurrence, respectively. Under the same welding conditions, ultrasonic vibration enhanced friction stir welding (UVeFSW) can eliminate the tunnel defect in the weld nugget zone where the SAZ and WBZ are separated by the PAZ, because the exerted ultrasonic vibration improves the material flow in the PAZ and reduces or even suppresses the time-delay of material flow in the PAZ.

Effect of Double Shoulder Tool Rotational Speed on Thermo-Physical Characteristics of Friction Stir Welded 16mm Thick Nylon6

Friction stir welding (FSW) of polymers is relatively a new concept among modern polymer joining techniques. This study demonstrates the applicability of FSW on 16mm thick nylon-6 plates at constant welding rate of 25mm/min and varying rotational speed between 300 to 1000RPM. A special designed tool was fabricated which has double shoulder and right-hand threaded pin profile. It has shown excellent results at relatively lower rotation speeds. Visual inspection and microstructural examination of cross sections showed that the cavities and tunnel defects appeared only at higher rotational speeds. A linear relationship was observed between temperature and rotation speed.

Mechanism of soft ground tunnel defect generation and functional degradation

This paper deals with the mechanism of soft ground tunnel defect generation and functional degradation. The subway mileage has increased dramatically worldwide, especially in China, in the past decades and will be continuously increasing in the next decades driven by the demands on underground space usage and the advancement of tunneling technique. Subway tunnels are vulnerable to a variety of defects which, individually or interactively, deteriorate the tunnel function for providing passengers with a safe and comfortable transportation means. Understanding the mechanism of tunnel defect generation and functional degradation and providing effective maintenance measures can slowdown the tunnel defect generation and prevent the tunnel defects from developing into catastrophic structural failure. This paper summarizes typical tunnel defects and major contributing factors to the defect generation based on the findings from an inspection program of 130km of soft ground tunnels in east China. A detailed example of the tunnel inspection and rehabilitation is presented. A framework is developed for analyzing the tunnel defects. The mechanism of tunnel functional degradation is explored associated with five critical links: environment, structure, components, joints, and materials, within a tunnel operation system. The five links individually deteriorate with time and interactively degrade the tunnel function. The research findings from this paper lay a foundation for developing practice guidance for tunnel maintenance.

Study on the effects of friction stir welding process parameters on the microstructure and mechanical properties of 5086-H34 aluminum welded joints

In this study, the effects of tool rotational and traverse speeds on the mechanical properties and the microstructure of the friction stir welded joints from Al 5086-H34 were studied. Insufficient heat generation and inadequate metal transportation at very low rotational speed and high turbulence in the plasticized metal at very high rotational speed emerged tunnel and worm hole defects into the weldment. It was discovered that as rotational and traverse speed increased, the average grain size of the weldment decreased due to more dynamic recrystallization and amplified stirring effect. An increase in the rotational speed increased the ultimate tensile strength and microhardness of the specimens. At the best welding condition, by employing the rotational and traverse speeds of 1250 rpm and 80 mm/min, respectively, 51 % enhancement in the elongation and 8 % increase in the microhardness of the welded samples were obtained. Moreover, the ultimate tensile strength of this welded joint reached to 85 % of the base metal.

Investigations on tunneling and kissing bond defects in FSW joints for dissimilar aluminum alloys

In this paper an attempt has been made to investigate the effect of two Friction Stir Welding (FSW) parameters i.e. tool pin offset and tool plunge depth on the formation of defects such as tunnel (tunneling defect) and kissing bond (KB) during welding of dissimilar aluminum alloys. 4.75 mm thick plates of AA5083-H116 and AA6063-T6 were welded using a novel work-fixture developed in-house which, apart from clamping the plated also imparted continuous variation of offset on both side of the faying line. The tunneling defect was modeled as a function of offset and plunge depth. The welds were characterised using optical microscopy (OM), scanning electron microscopy (SEM) and mechanical testing. The causes of such defects have been analyzed and discussed and recommendations have been made to prevent their occurrence. The findings of the study have revealed that the tunneling defects are formed at all offset (including zero offset) values towards stronger material (advancing side). And the cross-section of the tunnel varied with the amount of offset. Further, KBs are formed at the interface for all pin offset values except 0.5 mm towards weaker material and high plunge depth resulting in the poor mechanical properties.

MECHANICAL STRENGTH OF DISSIMILAR AA7075 AND AA6061 ALUMINUM ALLOYS USING FRICTION STIR WELDING

The present study focuses on the effects of material position and tool rotational speed on the tensile strength of dissimilar AA7071 and AA6061 welded aluminum alloys with a thickness of 2.0 mm in using a conventional milling machine. Ten joints were produced by varying tool rotational speeds and by changing the fixed position of the material on the advancing and retreating sides. The results show that the maximum tensile strength of 207 MPa was achieved for Sample E when AA6061 aluminum alloys were placed on the advancing side at a rotational speed of 1000 rpm with seamless surface appearance and no inner defect across the weld area, while the lowest tensile strength of 160 MPa was obtained for Sample F when AA6061 was placed on the retreating side with severe tunnel defects across the weld area contributing to crack propagation. Thus, in dissimilar welding, weaker materials should be placed on the advancing side to trigger heat from the tool rotation and smooth the material flow formation in the stirred zone.

English

Welding is a process of joining similar metals by the application of heat and pressure. Welding process is used commonly to get the advanced properties of the metals like Mild steel, Aluminum, Copper, Magnesium etc. Among these metals Magnesium has the lowest density (1.78 g/cm3) and it has good mechanical properties like high strength to weight ratio, good damping capacity, and good corrosion resistant compared to Mild steel. But Magnesium is highly flammable which requires gas shielding to prevent the oxidation, which forms Magnesium oxide, an undesirable compound. Friction Stir Welding, a Solid State Welding process uses the heat developed by the rotating tool, due to the friction the heat developed is used to join the metals, where the weld zone temperature zone is comparatively lower than conventional welding .The retention of the mechanical characteristics is achieved in FSW process. But problems in FSW like Pin, Tunnel and Flash defects are produced in weld zone. These problems and Weld quality, microstructure, mechanical characteristics of the weld zone can be controlled by varying the FSW parameters such as Tool rotational speed, Tool traverse speed, Tool Axial force, Tool shoulder to tool pin diameter ratio, tool profile etc. In this project we are going to study the effects of these parameters in Friction Stir Welding of Magnesium alloys.

Friction Stir Welding of AA2024-T3 plate – the influence of different pin types

Abstract. Some aluminium alloys are difficult to join using traditional fusion (melting and solidification) welding techniques. Friction Stir Welding (FSW) is a solid-state welding technique that can join two plates of material without melting the workpiece material. This proecess uses a rotating tool to create the joint and it can be applied to alumium alloys in particular. Macrostructure, microstructure and micro hardness of friction stir welded AA2024-T3 joints were studied. The influence of tool pin profile on the microstructure and hardness of these joints was examined. Square, triflute and tapered cylinder pins were used and results from each weldment are reported. Vickers micro hardness tests and grain size measurements were taken from the transverse plane of welded samples. Distinct zones in the macrostructure were evident. The zones were identified by transitions in the microstructure and hardness of weld samples. The zones identified across the sample were the the unaffected parent metal, the Heat Affected Zone (HAZ), the Thermo-Mechanicaly Affected Zone (TMAZ), and the Nugget Zone (NZ). Measured hardness values varied through each FSW zone. The hardness in each zone was below that of the parent material. The HAZ had the lowest hardness across the weld profile for each pin type tested. The cylindrical pin consistently produced tunnel and joint-line defects. Pin profiles with flat surface features and/or flutes produced consolidated joints with no defects.

Influence of rotational speed on the formation of friction stir processed zone in pure copper at low-heat input conditions

The aim of this work is to study the influence of rotational speed on the formation of friction stir processing (FSP) zone in commercial pure copper at low-heat input conditions. The experiments were conducted using K-type thermocouples to record the peak temperature history at different locations on the workpiece. The results suggest that the temperature achieved during processing plays an important role in determining the microstructure and properties of the processed metal. FSP produced very fine and homogenous grain structure and it is observed that smaller grain size structure is obtained at lower rotational speed whereas a tunnel defect was formed at lower speed of 250rpm. It is also observed that the hardness of the processed copper depends strongly on the heat input during FSP. Tensile tests were carried out and the tensile strength of the FSPed samples was compared to that of the base metal. For a successful FSP at low-heat input condition, the minimum rotational speed was found to be 350rpm.

Effect of Tool rotation speed and feed rate on the formation of tunnel defect in Friction Stir Processing of AZ31 Magnesium alloy

Friction Stir Processing (FSP) is a relatively new material processing technique, which is used to refine the grain structure of metals and to impart improved material properties. In the current research, FSP of magnesium alloy AZ31 was conducted at different parameters. Two type of tool profiles, namely cylindrical and truncated conical probe tool, were used to analyze the defects produced at two different feed rate viz. 40mm/ min and 50mm/min. Studies indicate that thedefects produced with truncated conical tool were lesser than the cylindrical tool. Also the tunnel defect increases with the weld length. Negligible tunnel defect was found during FSP with the truncated conical tool at the rotational speed of 2000rpm and welding speed of 40mm

Effect of Pin Profile on Defects of Friction Stir Welded 7075 Aluminum Alloy

In this work, effect of the pin profile on defects of FSWed 7075 alloy was investigated. Three pins with cylindrical, square and triangle geometry were used for welding. Microstructure of the welding zone showed that the tunnel hole produced by triangle pin has smaller dimensions compared to cylindrical pin. On the other hand, results of optical microscope indicated that the size of grains resulted from square pin is smaller than the other kinds of tools. Also, the results showed that when the pin is cylindrical, tunnel, kissing bond, and zigzag line defects are formed. On the other hand, when the pin is triangle, original joint line with severe plastic deformation and crack defects are created.