Elsevier

Ultrasonics

Volume 96, July 2019, Pages 75-82
Ultrasonics

Inspection of butt welds for complex surface parts using ultrasonic phased array

https://doi.org/10.1016/j.ultras.2019.02.011Get rights and content

Highlights

  • An ultrasonic multi-array method for variable curvature structure is proposed.

  • An iterative design and optimization method of wedge parameters is established.

  • The cracks depth of 0.2 mm in the linear friction weld can be detected.

Abstract

Detection of weld defects for complex surface parts has always been a difficult point in ultrasonic testing because the geometry complexity makes it difficult to arrange transducers and determine the propagation paths of acoustic beams. In this paper, the linear friction weld of the engine blade is taken as an example of the butt weld in complex surface parts, and the application of the ultrasonic array testing method is carried out. Firstly, the propagation properties of acoustic waves in the inspection area are analysed based on both the Snell's law and the acoustic pressure reciprocating transmittance (APRT). According to the inspection requirements, this study establishes a full-coverage inspection solution using multi-array transducers. Secondly, the whole inspection area is divided and the wedge parameters in each subarea are iteratively designed. Thirdly, based on the finite element method (FEM), a response simulation model of the ultrasonic array is established to testify the feasibility and validity of the inspection scheme. Lastly, experiments are conducted on the blade specimen welded by linear friction welding (LFW). The inspection results of different weld positions clearly identify the prefabricated crack defects, showing that the proposed method can fulfill the rapid and accurate inspection for the butt weld of complex surface parts.

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:sinαvα=sinβvβ=sinθvθHere, α, β are the wave angle of incidention and reflection, respectively; θ is refraction angle of longitudinal wave or shear wave; vα, vβ are the wave velocity of incidention and reflection, respectively; vθ 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.

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