Non-destructive evaluation of additively manufactured metal (AlSi10Mg) brackets using laser Doppler vibrometry and acoustical resonance spectroscopy

Abstract

The use of additively manufactured metal parts has increased dramatically in the past few years. This has drawn considerable attention to the as-built mechanical properties of these parts and their ultimate durability. Additive manufactured parts have a wider variation in properties than parts made with classical techniques. These variations are dependent on several parameters including the specific additive manufacturing technique used, the material, build variables, part orientation during the build, and secondary operations required to remove support structures required for the additive build. Non-destructively verifying the quality of these parts is especially important to aerospace, automotive and defense applications where failure can be catastrophic. This paper describes an ongoing research project that utilizes nondestructive techniques to detect defects, damage, and other variations of mechanical properties in additively manufactured metal parts that could reduce the quality of the part. The dynamic properties (frequencies and modes of vibration) provide a characteristic “signature” for all parts. If a part has any significant variations in elastic modulus, density, dimensions, microstructure, internal flaws or defects, the vibrational “signature” will change, and this variation can be detected. The monitoring process used combines Laser Doppler Vibrometry with acoustical resonance spectroscopy. Multiple spectra measured for different excitation and testing conditions are combined into a single spectrum, which is then compared with finite element analysis predictions. Any variation in the spectrum pattern is an indicator of damage. This non-destructive technique was used to successfully detect damage in a series of metal parts manufactured with predefined defects using a Selective Laser Melting (SLM) technique with two 400-watt lasers to microweld the metal powders at 30-micron layers.