A front-tracking measurement technique for in-situ columnar and equiaxed structure growth with controlled solidification

Growth rate is an important consideration in solidification as it influences features of the micro and macrostructure such as morphology and the columnar to equiaxed transition. Thus, accurately quantifying the growth rate is key to understanding the solidification process as well as verifying numerical simulations. In this work, an experimental apparatus has been developed to investigate in-situ directional solidification using a transparent analogue material, neopentyl glycol-35 wt%(D)camphor. The facility enables the control of the mushy zone growth rate as well as the imposed temperature gradient during solidification, similar to Bridgman type furnaces. Image processing techniques were developed for automated tracking of position and growth rate of the solidification front (SF) using background subtraction on photographic image sequences captured in experiments. Growth rate measurements using this technique and the traditional method (liquidus isotherm speed) have been benchmarked against manual measurements of position and growth rate. Results show that automatically tracked growth rate compares well throughout experiments with errors less than 2.7%. Contrastingly, isotherm speed was in error up to 19%. Three columnar and two equiaxed morphologies were investigated and SFs were characterised for their growth rates and thermal conditions.