This article covers in-line process monitoring of the metal additive manufacturing (AM) methods of laser and electron beam (e-beam) powder-bed fusion (PBF) and directed-energy deposition (DED). It focuses on methods that monitor the component directly throughout the build process. This article is organized by the type of AM process and by the physics of the monitoring method. The discussion covers two types of monitoring possible with the PBF process: monitoring the area of the powder bed and component and monitoring the melt pool created by the laser or e-beam. Methods for layer monitoring include optical and thermal methods that monitor light reflected or emitted in the visible and infrared wavelengths, respectively. Monitoring methods for laser directed-energy deposition (DED) discussed are those that measure the size and shape of the melt pool, the temperature of the melt pool, and the plasma generated by the laser as it interacts with the molten metal.

Acoustical holography is the extension of holography into the ultrasonic domain. The basic systems for acoustical holography are the liquid-surface type and the scanning type. This article discusses the applications for acoustical holography, including inspection of large composite parts, through-transmission breast imaging system, inspection of welds in thick materials, and inspection of sleeve-bearing stock. It describes the basic system for liquid-surface acoustical holography and scanning acoustical holography. A comparison between these techniques is also provided.

This article introduces the principal methodologies and some technologies that are being applied for nondestructive evaluation of composite materials. These include ultrasonic testing (UT), air-coupled UT, laser UT, ultrasonic spectroscopy, leaky lamb wave method, acousto-ultrasonics, radiography, X-ray computed tomography, thermography, low-frequency vibration methods, acoustic emission, eddy current testing, optical holography, and shearography. The article presents some examples are for fiber-reinforced polymer-matrix composites. Many of the techniques have general applicability to other types of composites such as metal-matrix composites and ceramic-matrix composites.

Holography is basically a two-step process for creating a whole three dimensional image of a diffusely reflecting object having some arbitrary shape. This article discusses the advantages, disadvantages and applications of using the optical holography method in nondestructive evaluation. It also discusses the types of acoustical holography, including liquid-surface acoustical holography and scanning acoustical holography. The article concludes by comparing liquid-surface and scanning systems.