Inspection of butt welds for complex surface parts using ultrasonic phased array
Introduction
With the development of manufacturing technology, advanced welding technology has been increasingly used in the manufacture of complex surface components. The blisk’s blade, a lightweight structure with complex surface, has been widely applied to the new generation of aero engines [1]. Among the existing blisk manufacturing methods, linear friction welding (LFW) is the most important method that first used by Rolls-Royce in 2000 because of its excellent welding quality and high machining efficiency [2]. Meanwhile, due to the complicated welding procedure and the poor work condition, defects in the blade weld often inevitably occur during the processes of manufacturing and the fatigue damage in usage, causing severe security incidents and economic losses [3]. Compared with other kinds of defects, the crack in the weld has a greater impact on the engine blade’s performance, and it has a higher probability of occurrence. However, as the butt weld area is located on the root of the blade, it is difficult to achieve this structure’s in-service inspection by traditional ultrasonic inspection methods because of the complex surface, the narrow space and the high requirement of inspection precision [4], [5]. Accordingly, a new inspection method for the blade welded by LFW has become one of the urgent needs for non-destructive testing (NDT) area to ensure the aeroengine security.
The ultrasonic phased array technique has the characteristics of compact transducer and a high inspection precision because of properly time-delayed pulses of array elements [6]. Therefore, it has some advantages over other non-destructive testing methods for complex structures inspection [7], including turbine rotors [8], corner-shaped components [9] and nuclear power plant components [10]. The research team at the University of Bristol developed an ultrasonic phased array testing system to inspect fir tree roots of turbine blade, whose sensitivity was measured by using an analytical model to predict the propagation of acoustic waves in anisotropic metal alloy materials [11]. Charlesworth designed an integral wedge that could be fully coupled with the roots of the low-pressure turbine rotor blade to meet the in-situ crack inspection requirements in narrow space, and offer higher levels of coverage [12]. Different from the above blade of fir tree tenon/mortise structure, the weld area of the LFW blade to be detected varies greatly in curvature and thickness, which lead to complex ultrasonic propagation path. Therefore, it is of great theoretical and practical significance to develop the ultrasonic phased array inspection method for butt weld of complex surface parts, such as the linear friction weld on the areoengine blade.
The structural characteristic of the blade’s variable curvature and constraints on detection conditions causes the ultrasonic refraction path is very complex and difficult to move the transducer during the detection process. Therefore, it is difficult to obtain defects characteristic information with only one set of ultrasonic array transducer. In addition, it is unrealistic to use the mode of multiple reflection to achieve full coverage of the ultrasonic beam in the detection area because the torsion rate of the component surface is not equal to 0. Therefore, it will be a tedious and time-consuming task to design a set of transducers and wedges at different locations on the blade, at the same time, the accurate blade geometry information is required for design support. In order to design the full coverage inspection scheme of ultrasonic linear multi-array transducer based on the shape information of complex variable curvature structure and iteratively generate the wedge parameters, this paper is composed of the following sections. To begin with, the feasibility of using ultrasonic array method for the inspection of LFW blade is analyzed by combining the ultrasonic theory and the blade’s properties. Then, the full-coverage inspection scheme of ultrasonic multi-array transducers is presented, and the parameters of transducers and wedges for each acoustic cross section are designed and optimized using the iterative method. Subsequently, a FEM model is established to analyze the correlation between the inspection parameters and the crack depth. Finally, experiments for the blade specimen of LFW are conducted using 5 MHz, 32 elements linear transducers, and the artificial crack defects (5 mm × 0.2 mm × 0.2 mm) are preformed in a different weld area. The inspection results have proved the validity of the designed ultrasonic phased array method and have shown that this technique effectively improves the efficiency and accuracy of inspection for blades’ butt weld in practical applications.
Section snippets
Propagation of ultrasonic in detection area
In the case of a two-dimensional plane, the propagation of ultrasound between different media follows the Snell's law, as shown in expression:Here, , are the wave angle of incidention and reflection, respectively; is refraction angle of longitudinal wave or shear wave; , are the wave velocity of incidention and reflection, respectively; is the refraction velocity of longitudinal wave or shear wave.
The surface of the blade is a hyperboloid, and the basic theory
Area division and information extraction for blade
The weld joint runs through the entire blade, and has the length of about 160 mm. There are different curvature and thickness variations at different positions of weld, so parameter extraction is a necessary prerequisite for designing an ultrasonic array inspection scheme. Assuming N-group transducers are to be designed, it is necessary to divide N sub-regions accurately and extract N-group 2D cross-section NURBS (Non-uniform rational basis spline) curves on the CAD model. Although it is
Experimental validation
The specimen is an integral blade that has been welded with the root using LFW and contains a butt weld, as shown in Fig. 11. Six rectangular artificial defects (5.0 mm × 0.2 mm × 0.2 mm groove) were machined at different curvatures of the weld position on the upper and lower surfaces, respectively, which the length direction is the same as the extension direction of the weld. Each artificial defects are numbered in order to facilitate the expression of the inspection process and results. As
Summary and conclusions
The full-coverage inspection scheme is presented based on ultrasonic phased array technique to meet the fast and accurate inspection requirements of butt weld for the aeraengine blade. The ultrasonic theory in complex interface of wedge and blade is analyzed as well. The appropriate transducers and wedges are designed, which can be applied to inspection scheme design for complex surface parts.
The propagation properties of ultrasonic wave based on Snell's law in the complex curved surface for
Acknowledgements
This work is supported by National Natural Science Foundation of China (NSFC) No. 51775026.
References (16)
- et al.
Evaluation of corrosion fatigue crack propagation life at low-pressure steam turbine rotor groove
Eng. Fract. Mech.
(2006) - et al.
Phased array ultrasonic signal compressive detection in low-pressure turbine disc
NDT and E Int.
(2017) - et al.
Using phased array ultrasonic technique for the inspection of straddle mount-type low-pressure turbine disc
NDT and E Int.
(2009) - et al.
Ultrasonic arrays for nondestructive evaluation: a review
NDT and E Int.
(2006) - et al.
Material damage diagnosis and characterization for turbine rotors using three-dimensional adaptive ultrasonic NDE data reconstruction techniques
Ultrasonics
(2014) - et al.
Numerical simulation and experiment for inspection of corner-shaped components using ultrasonic phased array
NDT and E Int.
(2014) - et al.
Development of an ultrasonic phased array system for non-destructive tests of nuclear power plant components
Nucl. Eng. Des.
(2002) A review on BLISK technology
Int. J. Innovat. Res. Sci., Eng. Technol.
(2013)