Non-Destructive Testing of Components Made by Selective Laser Melting

Selective laser melting (SLM) is increasingly recognised as an important industrial additive manufacturing technology for the net-shape fabrication of complex metallic components. It is of particular interest to the aerospace industry since it allows weight-saving components to be fabricated in a range of metals directly from CAD files. Also known as 3D printing, the SLM process offers significant design freedoms to enable more complex 3D geometries to be fabricated compared to what is possible by conventional manufacturing. However, if the powders and processing parameters are poorly controlled, then defects such as porosity or cracking may result. For aerospace applications, it vital that such defects are either avoided or carefully controlled within acceptable limits. It is therefore important to understand the defect detection capabilities of various non-destructive testing methods and to develop appropriate inspection techniques and strategies for components made by SLM.

This paper reports on the inspection of various Ni-based and Al-based samples that were fabricated by SLM to deliberately contain various types of defects such as internal porosity, internal cracks or holes of various sizes. In particular, a staircase sample was designed and fabricated from Hastelloy X with steps ranging from 0.8 mm to 10 mm in thickness and with each step containing the same series of custom-designed spherical, coin-shaped and rod-shaped defects in various orientations and sized from 0.2 to 2 mm. These samples were then exposed to a range of computed radiography and tomography techniques using both X-rays and neutrons. Various experimental and modelling techniques were employed and minimum detectable defect sizes were determined for conventional X-ray radiography. The results are discussed in the context of defect inspection strategies for aerospace components made by SLM.