Improved Detection of Near-back Side Defects Using High-density Alumina Slurries

In recent years, technologies such as metal additive manufacturing have advanced, leading to an increase in metal components with complex geometries. Conventional ultrasonic non-destructive testing often suffers from strong surface reflections, making it difficult to detect internal defects close to the rear surface. This study proposes an ultrasonic inspection technique using a high-density alumina slurry, whose acoustic impedance is close to that of metal. Experiments on test specimens containing artificial defects near the back side were conducted by placing their back surfaces in contact with a 50 vol% alumina slurry. As a result, the energy reflection coefficient at the back surface was reduced to approximately 0.2. This reduction enhanced the signal contrast from defects, leading to improved detection performance.