Implementing an on-line bond quality inspection system for cold roll bonded Al/Al-Sn/Al/steel strips using guided wave EMATs

A prototype on-line bond inspection system for Al/Al-Sn/Al/steel strips is introduced to detect bond defects that occur in the cold roll bonding (CRB) process of strips. The transmitting and receiving ElectroMagnetic Acoustic Transducer (EMAT) probes inspect 100% material volume whilst the strip is processed continuously. The inspection signals are processed in real-time and are stored for post-processing. Serial production strips with good bond as well as with defects along the bond line were inspected for rigorous testing of the prototype machine. The inspection results were statistically analysed to determine an alarm threshold for serial inspection. The most important results are that the guided waves are sensitive enough for practical non-destructive testing (NDT) to detect all bond defects with only 0.5% false alarm rate, which nowadays can only be detected with a destructive peel-off test. The detection of all kinds of bond defects would lead to unreasonable levels of scrap due to false alarms. The application of this prototype EMAT inspection system to an industrial strip processing line shows that on-line detection of significant defects in CRBed Al/AlSn/Al/steel strips in a harsh serial production environment is possible. Design improvements are proposed to address the issues that occurred during inspection trials to design a more robust mechanical machine for the industrialisation of an EMAT inspection system.