Non-destructive testing of the parts manufactured by Direct Metal Laser Sintering

Abstract

Interest in Additive Manufacturing (AM) has grown considerably in the past decades and industry has gained great benefits from this type of technology. The main advantages are: geometrical freedom that allows the design of parts with complex shape, which are difficult or impossible to produce by conventional technology; shortened design-to-product time; customization and the possibility to use several materials in one process. Direct Metal Laser Sintering (DMLS) is one of the most promising AM technologies that utilizes metal powders. Due to the layer-by-layer nature of powder delivery used in DMLS, the drawbacks are: surface quality and accuracy, high residual stress in as-built parts and porosity – all of which depend on the powder material, process-parameters, scanning and building strategies. This can result in a substantial deterioration of the mechanical properties of the products and their performance characteristics. For this reason, it is very important to identify defective parts before enrolling into service. Non-destructive testing (NDT) is effective for detection of internal defects without causing damage. NDT also covers a wide group of methods of analysis used to evaluate the properties of a material. NDT techniques like visual, acoustic, ultrasonic, thermal, X-ray and 3Dcomputed tomography (CT) inspections are now widely used for various industrial applications. For the analysis of material properties and the detection of defects, each of these methods uses different physical principles that have their advantages and disadvantages. In this study, some of the NDT techniques are evaluated in terms of their applicability to the inspection of parts manufactured by DMLS technology: Visual, Ultrasonic, Computed Tomography and Acoustic Emission inspection. Artificial defects were used to determine the feasibility of each NDT method. DMLS samples were produced containing a range of artificial defects. These samples were than subjected to each method and the results compared. A comparison between the amount of defect information obtained is made. It was shown that the nature of the sample; shape, size, material and the type of defects present plays a vital role in the selection of testing methods. Ultrasonic-Total Focus Method indicated that some defects are present upon testing relatively big samples with simple geometry. X-ray Computed Tomography showed some limitations with regard to the possibilities and the amount of defect detail, the only drawback being the cost and time involved. Acoustic Emission showed to be a promising method for production parts although it requires an initial time investment; thereafter it is a simple and easy way of detecting defective samples.