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.