Paper presented at BINDT Annual Conference 2004, Torquay, 14-16 Sept.
Friction stir welding (FSW) is a solid-state bonding technique. Because it is an automated technique it is a controlled and reliable process. However, with changes in material condition, dimensions or welding parameters, flaws can be generated. These flaws can be conventional (lack of penetration or voids) or unique to this welding technique, such as 'hooking' flaws in a lap weld arrangement. The latter flaw, also referred to as 'thinning', can be difficult to detect with NDT methods due to the compactness of the two adjacent faces caused by the forces involved in the welding process. However, the shape, position and extent of the flaw can have a dramatic impact on the resulting mechanical properties of the joint, both because of the effective thinning of the joint cross-sectional area and as a fatigue-crack initiator.
This paper illustrates the results of a study carried out at TWI on ultrasonic phased array inspection of FSW lap welds. The technique successfully identified geometrical features of the weld and volumetric defects, showing very promising results for 'hooking' flaw identification and characterisation. Thorough destructive testing successfully validated the inspection results, indicating that the technique could be improved further through the application of signal processing and material grain noise analysis.
Following the successfully completed Qualistir TM European-Sponsored Project  , TWI moved onto assessing the capabilities of Ultrasonic Phased Array in inspecting lap Friction Stir Welds. The knowledge gained from Qualistir [3,4] , was channelled into the development of the inspection technique for the lap-joint configuration. The lap-joint configuration used in the project presented new challenges due to the different geometry and to the different types and geometry of possible flaws. Because in Qualistir the weld quality assessment was based on grain noise analysis, the weld material was also changed from Al7000 series to Al5000 series in order to assess adaptability and applicability of the Phased Array inspection technique to different grain structures.
The project involves two recently developed techniques both in the welding field and in the NDT field. The former is friction stir welding and the latter is ultrasonic phased array inspection.
2.1. Friction Stir Welding
Friction stir welding is a continuous process that involves plunging a portion of a specially shaped rotating tool between the abutting faces of the joint. The relative motion between the tool and the substrate generates frictional heat that creates a plasticised region around the immersed portion of the tool. The tool is moved along the joint line, forcing the plasticised material to coalesce behind the tool to form a solid-phase joint. As the weld is a solidphase weld, different types of weld flaws can be present and hence novel inspection techniques are required to find them.
Previous published work and discussions with the aircraft industry indicated that conventional flaws, e.g. voids and lack of penetration, could be detected by current ultrasonic methods. The flaws that were more characteristic of lap-joint configuration FSW were 'hooking' defects. The latter flaw, also referred to as 'thinning', can be difficult to detect with NDT methods due to the compactness of the two adjacent faces caused by the forces involved in the welding process (see Figures 1&2). However, the shape, position and extent of the flaw can have a dramatic impact on the resulting mechanical properties of the joint, both because of the effective thinning of the joint cross-sectional area and as a fatigue-crack initiator.
3.1. Phased Array Ultrasonic Inspection
Phased array equipment was recently developed for the nuclear industry. It involves the use of a specialised probe containing a large number of miniature crystals. The ultrasonic beam angle and focal distance are controlled by a sophisticated electronic system, which is computer programmable (see Figure 3). This enables a large volume of data to be stored and analysed.
The benefits of the system over normal A-scan techniques are:
- Improved sizing capability.
- Pictorial presentation of the data.
- Shorter inspection times through electronic scanning.
- Permanently recorded results.
- Post acquisition data manipulation (e.g. statistical analysis).
2. Inspection technique development
The phased array probe arrangement and scanning pattern are illustrated in Figure 3.
The inspection angles were chosen from geometrical considerations, and the focal laws for the PA probes were derived after modelling of the beam.
3. Work performed
Six lap-joint configuration FSW were manufactured for the project using 6mm thick Al5083 plates. The joints had various degrees of hooking flaws present and were manufactured using different FSW tools for lap welds, with the welding parameters outside the correct welding envelope. All the welds were scanned using two phased array probes: 15MHz L-wave at 0°, and 10MHz S-wave at 35° and 45°. After PA scanning, all the welds were destructively tested, obtaining: micrographs, macrographs, full tensile tests results and fatigue life results.
The main results from the PA scanning of the same weld shown in Figures 1 & 2 are illustrated below in Figures 5, 6 & 7.
Legend (also applicable to micro and macrograph shown in Figures 1 & 2):
- A and B indicate the corners of the lower plate.
- C and D indicate the corners of the upper plate.
- E indicates the interface between the plates at the edge of the weld nugget in the direction of the upper plate.
- F indicates the interface between the plates at the edge of the weld nugget in the direction of the lower plate.
- G indicates the 'hook' crack in the direction of the upper plate.
- H indicates the 'hook' crack in the direction of the lower plate.
- P indicates porosity or voids.
- X indicates an ultrasonic artefact (UT signal not corresponding to any physical feature created by mode conversion/refraction/reflection in the sample).
The first display from the left in each Figure is a D-Scan, or 'end view'. The centre display is a B-Scan or 'side view' and the last display is a C-Scan or 'top view'.
All the main features could be identified from the high gain PA scans for most of the weld samples, and most of the 'hooks' could also be seen. Low gain scans highlighted the main features in the welds (such as geometrical features, porosity or voids) but had difficulty in locating the 'hooks'.
From the PA scans it appeared that all the lap welds exhibited the same main features including the 'hook' cracks. It has to be understood that, due to the geometry of a lap weld, an interface between the two plates, getting tighter as the weld nugget is approached, will always be present. The question is whether this interface is 'stirred' into the weld nugget itself as a tight metallurgically unbonded region or whether it stops at the boundaries of thenugget.
All the manufactured welds contained some degree of hooking but the degree of hooking was controlled to create a varying set of welds.
Although there was a variation in the performance in terms of fatigue endurance across the specimens, destructive testing results show that the majority of fatigue results from the trial welds fell significantly below the design curve  . In addition to a region of fatigue cracking, most lap welds exhibited evidence of unbonded regions on the fracture face, with the fatigue crack initiating from the tip of the unbonded region. The unbonded region of the fracture faces is characteristic of the 'hooking effect'.
Ultrasonic phased array testing of lap FSW gave very promising results, identifying most of the relevant features of the welds with consistency. This indicates that direct ultrasonic detection of 'hook' cracking is possible, once the inspection procedure has been developed and optimised. UT results were confirmed by the destructive testing performed on the welds, and by micrographic evidence.
6. Future Development
From the promising initial results, ultrasonic phased array inspection procedure for lap FSW will be developed further. Optimisation of probe design and inspection parameters will increase sensitivity of the technique for detection of 'hook' cracking and other flaws in the joint. A possible route for further inspection development would be the application of signal processing to the ultrasonic data for evaluation of the weld nugget shape, extent and quality, as successfully employed in the Qualistir project. TWI is currently running a Joint Industry Project on phased array inspection of FSW joints of different alloys and geometry.
The authors would like to thank Mr David Staines (TWI) and Mr Kim Hayward (TWI) for their invaluable help in successfully running the project.
References and Footnotes
- Qualistir Project, 'Development of Novel Non Destructive Testing Techniques and Integrated In-line Process Monitoring for Robotic and Flexible Friction Stir Welding Systems. Work Programme Version 4.0', CRAFT-1999-70641, September 2002.
- C R Bird 'Ultrasonic phased array inspection technology for the evaluation of friction stir welds', January 2004, Insight - The journal of the British Institute of Non Destructive Testing.
- D Kleiner and C R Bird, 'Signal Processing for Quality Assurance in Friction Stir Welds', February 2004, Insight - The Journal of the British Institute of Non-Destructive Testing.
- C R Bird, 'The ultrasonic phased array technology for the evaluation of friction stir welds', Workshop on the Non-Destructive Inspection of Friction Stir Welds, DLR Cologne, December 2003.
- BS 8118: Part 1:1991: 'Structural Use of Aluminum. Part 1: Code of practice for design'. British Standards Institution 1991.