Evaluation of polychromatic X-ray radiography defect detection limits in a sample fabricated from Hastelloy X by selective laser melting

摘要

Selective laser melting is a rapidly maturing additive manufacturing technology ideally suited to the net-shape fabrication of high value metallic components with complex shapes. However, if the processing conditions are poorly controlled, internal defects such as cracks or pores filled with metal powder may be present and impair the properties. As a result, a non-destructive defect detection method needs to be found that is suited to this application. In this work, a staircase sample was designed and fabricated from Hastelloy X by selective laser melting with step thicknesses ranging from 0.8 mm to 10 mm and with each step containing the same series of custom-made spherical, rod-shaped and coin-shaped defects arranged in different orientations and ranging from 0.2 mm up to 2 mm in size. The sample was exposed to various X-ray radiography testing and analysis methods. In particular, a theoretical and experimental evaluation of defect detection limits by polychromatic X-ray absorption radiography was performed based on the measurable contrast, which depends on both defect size and shape and slab thickness. The experimental data suggest that the minimum detectable contrast is about 1-2% when using X-rays with a very broad spectrum. This equates to a minimum detectable defect size of about 0.2 mm for a Hastelloy X slab thickness of < 2 mm. The experimental findings are in good agreement with theoretical expectations. The theoretical framework provides a criterion for estimating contrast, which is useful for optimising the experimental conditions. Polychromatic X-ray absorption radiography represents a simple and effective non-destructive investigation technique. Methods for further improving the defect detection limits are also discussed and examples relative to computed tomography are reported.