Quantitative Thermography and Image Quality in Additive Manufacturing of Metal

This thesis presents work on quantitative thermography in the additive manufacturing of metals process. The work is motivated by a need for accurate, spatiotemporally resolved measurements of the thermal fields near the heat source, which is usually 50-500 µm in size. This level of detail requires a high spatial sampling rate, which can be provided by near infrared sensitive silicon-based instruments. The high spatial sampling rate means that the resolution of the instruments is limited by the imaging components. The imaging performance is characterised by the spatial transfer function. In this work three distinct silicon based thermographic instruments were designed and constructed. The three instruments were trialled in additive manufacturing of metals applications. The three trials were: a low-cost smart-phone-sensor system used on a commercial direct energy deposition machine; a high-performance sensor system with a telephoto lens used on a modified commercial machine; and a high performance, high magnification system used on a custom built process replicator. The performance of the three systems for their applications was assessed. The three instruments have provided valid research data which paves the way for future studies using these technologies. The instrument used for thermography on the process replicator could resolve previously unseen levels of thermal detail in the process, having an instantaneous field of view of 3 µm. The measurement field of view of this instrument was found to be a circle of 130 µm diameter. The cooling rates in the process replicator for the alloy (Ti-6-4). were measured to be 0.06- 0.14 °C µs -1 , which is consistent with literature for this material. The spatial transfer function of the instruments was calculated using methods developed for this thesis. Measurements of the spatial transfer function were used to reconstruct the thermal fields and a method for validating the reconstruction was devised. A reconstruction method devised for this work was found to outperform the standard reconstruction methods used in literature, for scenes similar to those found in the additive manufacture of metals.