Nondestructive Evaluation of Programmed Defects in Ti-6Al-4V L-PBF ASTM E8-Compliant Dog-Bone Samples

Abstract

Nondestructive evaluation (NDE) was performed on laser powder bed fusion printed Ti-6Al-4V ASTM E8, Standard Test Methods for Tension Testing of Metallic Materials–compliant round dog-bone samples with and without intentional internal defects along the gauge section using X-ray computed tomography (CT) and microscopic-resolution immersion ultrasonic imaging methods. A total of six programmed defects with different dimensions in cubical and spherical shapes were equally spaced in the gauge section of the defect sample as opposed to no intentional defects in the nominal test sample. A total of 3,008 CT images were generated for each test sample with a resolution of 15 μm per slice over the entire length of the gauge section. For the ultrasonic testing (UT), a custom-made 20-MHz focused ultrasonic probe generating a 400-μm beam diameter at a focal distance of 25 mm was used along with a three-axis Cartesian coordinate scanning system capable of taking scan steps as small as 1 μm. According to the CT images, five out of six defects were detected, whereas all six defects were detected by the ultrasonic imaging method. One of the cubical defects missed by CT was found to be a 400-μm pancake-like defect with a less than 20 μm of volumetric separation between the two faces according to the ultrasonic images and the time of flight data. Detailed information on the location, size, and shape of each defect are discussed in this work along with a comprehensive comparison between the CT and UT images.