# Delamination Detection in Bimetallic Composite Using Laser Ultrasonic Bulk Waves
## 1 Introduction  
Bimetal products have become an important structural material in modern industrial production, due to their good comprehensive properties and low cost [1, 2, 3]. Copper-aluminum bimetallic products are compounded by a copper layer and an aluminum layer, which are widely used in electricity and communication [4, 5, 6]. The main production process of copper-aluminum bimetallic composites is the sandwich rolling method [7]. In the process of sandwich rolling, a variety of uncertain factors will cause interfacial delamination defects, which will significantly reduce the structural strength and fatigue life of the bimetallic products. Although the interface delamination defects seriously affect the quality of bimetal products, on-line delamination detection is still quite difficult. The on-line detection of defects can not only record the compound quality of all products and optimize the sandwich rolling process, but also provide real-time feedback control signals for the rolling mill. Therefore, on-line nondestructive detection of bimetallic products is highly needed.  
Ultrasound is very sensitive to interface bonding characteristics, so it is widely used in the detection of interface delamination in multilayer composite materials [8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18]. Conventional piezoelectric ultrasonic transducers need couplant [19], which cannot realizenon-contact detection in the sandwich rolling process. Although air-coupled ultrasonic transducer and electromagnetic ultrasonic transducer can realize non-contact detection, the sensitivity and resolution of air-coupled ultrasonic transducer are low [20], and electromagnetic ultrasonic transducers cannot adapt to high-temperature environments. As a new detection technology, laser ultrasound has the characteristics of non-contact and high spatial-temporal resolution, which can excite and detect ultrasound a long distance in a high-temperature environment. Laser ultrasonic technology has been successfully used in the on-line inspection of the strip rolling process and the defects detection of multilayer composite. [PERSON] et al. measured the thickness of the seamless tube using the time-of-flight of laser ultrasonic echo at about 1000 \({}^{\circ}\)C, which was used for feedback control of rolling mill and reduced the thickness error of the seamless tube, and proposed an on-line measurement method of austenite grain based on laser ultrasonic [21]. In the hot rolling workshop, [PERSON] et al. used lasers to excite and detect ultrasonic waves on high-speed, high-temperature hot-rolled strips to characterize the transformation state from austenite to ferrite [22]. [PERSON] et al. measured the grain size of high-strength steel using laser ultrasound, which was in good agreement with the scanning electron microscope-electron backscatter diffraction (SEM-EBSD) results [23]. [PERSON] et al. characterized the disbonds in a steel-lead structure using laser ultrasound and proposed a quantitative method to assess the sizes of disbands [24]. [PERSON] et al. proposed an ultrasonic spectroscopy method to quantitatively analyze the influence of internal delamination in laminates on ultrasonic attenuation coefficient [8]. Consequently, laser ultrasonic testing technology can be used for on-line delamination detection of bimetallic composite in the sandwich rolling process.  
In this paper, the interaction between laser ultrasonic bulk waves and interface separation defects in Cu/Al bimetallic composite was systematically studied, and a new method to detect delamination defects is proposed. The effect of the position and size of the delamination defects on the peak value of different bulk waves was explored experimentally and numerically. The reflected shear wave from the composite interface is a sensitive feature to evaluate the delamination defects at the interface. Finally, the position and size of the delamination defects are given intuitively by the C-scan imaging results.