Comparison of Digital Radiography and Computed Tomography as Nondestructive Testing Techniques for the Assessment of Lack of Fusion Defects in Additively Manufactured SS316L Coupon

Abstract

Additive manufacturing has been playing a significant role in the manufacturing of components with complex geometries for aerospace applications recently. Comprehensive nondestructive testing techniques (NDT) are vital for the successful quality evaluation of critical components in this domain. Appropriate selection of the NDT scheme is essential for the qualification of such components. Major NDT techniques are designed based on the interaction of electromagnetic radiation and the response of the sound or heat energy transmission or reflection from the test object. The common defects noticed in the components made through additive manufacturing (AM) routes are pores, clusters of porosities, micro-cracks, lack of fusion and layer delamination. Considering the morphology and the complications in the geometry of aerospace components, many conventional NDT techniques are unsuitable for the inspection of AM components. Detection of unfused powder in the AM components by conventional radiography is difficult due to the low radiation attenuation coefficient gradient between the unfused and fused metallic regions. Also, the detection of defects in the radiography technique depends entirely on the beam path. Multiple radiography images with different beam angles and film combinations are essential to get the maximum information on the defects by conventional radiography techniques. In this aspect, computed tomography, a noncontact NDT technique provides a better solution for determining embedded defects such as lack of fusion and layer separation due to presence of unfused powder in the AM components. The present study compares the capability of computed tomography and 2D digital radiography for the identification of lack of fusion defects in stainless steel SS316L specimens fabricated through the Laser powder bed fusion AM route.