This project aims to combine a custom Spatially Resolved Acoustic Spectroscopy (SRAS) imaging with Laser Powder Bed Fusion (LPBF) technique to integrate the SRAS detecting system into a LPBF equipment to realise a real-time closed loop feedback system with synchronous detecting and printing capacity, shown in Fig. 1. On one hand, by integrating microstructure inspection via SRAS system with LPBF in an on-line capacity, it would be possible to directly link the processing parameters with an immediate change to the material microstructure to deposit parts with desired material characteristics. On the other hand, the printed material performance can not only equal but surpass that builds in traditional manufacture even with complex processing procedures, and the period of materials development compresses years into days even hours of build time, and it may achieve the so-called integrated shaping with less and even no postprocessing.
Fig. 1 An integrated feedback loop system with online printing and detection. SRAS enables to characterise the microstructure and their crystallographic orientation. This information can be feedback to the LPBF system to adjust for defects or reduce the grain size.
Materials used in performance engineering (e.g., metals and alloys) are made up of crystals (or grains) and the size, orientation and distribution of these crystals (microstructure) plays a significant role in determining the material characteristics. To be more specific, the mechanical responses and physical properties of manufactured parts basically depend strongly on microstructures, then they will decide the service performance of these safety-critical system parts.
LPBF is one of the mature metal advanced manufacturing technologies with the introduction of extremely high cooling rate (~106 K/s), and new material microstructure would be formed due to the great thermal gradients from the bottom to the top of the melt pool. However, this complex interaction between the laser and material, the undesired metallurgy defects will occur and accumulate thermally accompanying with the laser working, and the column grains with uncontrollable sizes will be generated, they will introduce the obvious anisotropy in the parts, and finally impair their mechanical responses, including tensile properties, fatigue responses.
Controlling this microstructure is one of the pivotal technologies that underpins advanced manufacturing. Traditionally this control is achieved through complex forging, processing and heat treatments and so the development of new materials is a slow and highly expensive business.
SRAS, one of the Non-Destructive Testing (NDT) technologies, up to now can determine the elasticity, crystalline orientation and grain distribution of the single crystal elasticity matrix of polycrystalline materials in a fast and easy measurement  but with the capacity of high accuracy.
This project presents an amazing opportunity whereby we develop techniques to “print” the microstructure we require for optimal performance on demand. There are two major challenges to this: the first is to monitor the microstructure down to the orientation of a single grain as the material is built into polycrystals during LPBF, and the second is to be able to tweak the build to steer the microstructure in the right direction.
 Paul Dryburgh, Wenqi Li, Don Pieris, Rafael Fuentes-Domínguez, Rikesh Patel, Richard J. Smith, Matt Clark, Measurement of the single crystal elasticity matrix of polycrystalline materials,
Acta Materialia, Volume 225, 2022, 117551.