Cladding Crack in Laser Cladding: a Review

Method	Principle	Application	Advantage	Limitation
Visual testing	Using the light that reflected or transmitted from the measured object to image in the eye or photosensitive device	General cladding coating and macroscopic crack	Cheap, simple, and wide application	Only evaluate surface condition, need an effective light source and it must be accessible during detection
Penetrant testing	The liquid containing visible or fluorescent substances is applied to the surface of the workpiece, and the capillary phenomenon acts it into the crack	General cladding coating and macroscopic crack	Cheap, sensitive, general and easy to operate	Only evaluate surface condition. The surface condition must be relatively smooth and free of contaminants
Magnetic particle testing	After magnetizing the tested workpiece, fine magnetic powder is applied to the coating surface, and the magnetic powder is arranged at the crack	Magnetic material and medium crack	Fast,sensitive and general	Only evaluate magnetic material’s surface condition
Radiographic testing	The film is exposed when the X-ray passes through the inspected object, and the crack has an effect on the exposure	General cladding coating and medium crack	Permanent recording, high sensitivity, and wide application	Detection range is related to the material density. Radiation pollution
Ultrasonic testing	When ultrasonic propagates in the workpiece, it will reflect when crack occurs	Simple shape and good surface roughness, and medium or micro crack	Fast, accurate, sensitivity. Obtainable location and defect properties	Limiting to shape. Coupling agent required
Eddy current testing	Local current field is produced in the coating under the action of electromagnetic induction	Conductive materials, and medium or micro crack	Fast, general, sensitive and non-contact, suitable for automatic inspection	Limiting to material
Infrared thermal imaging testing	Measure the temperature change of coating with temperature sensor, detector or camera	General cladding coating and macroscopic crack	Sensitive to micro temperature changes, permanent records provided	It can’t effectively detect the cracks in thick parts, and the evaluation is difficult
Acoustic emission testing	When a crack occurs, the energy is released and propagates in the form of stress wave, which can be detected by sensors	General cladding coating and medium or micro crack	The ability to monitor the damage of large area and effectively predict the failure	The sensor must contact the surface to be detected. Defect location requires multiple sensors