Experimental study of disruption of columnar grain growth during rapid solidification

Abstract

Over the years, many studies have been conducted to study and analyze the grain structures of metal alloys in order for them to have superior structural and mechanical properties. In particular, columnar grains are observed predominantly during rapid solidification of molten metal. This leads to lower mechanical properties and requires expensive secondary heat-treatment processes. This study is aimed at disrupting the formation of columnar grain growth during rapid solidification using ultrasonic vibration and analyzes the effects on grain structure and mechanical properties. A MIG welder mounted on a low cost metal 3D printer was used to deposit ER70S-6 mild steel layers on a plate. A contact type ultrasonic transducer with control system to vary the frequency and power of the vibration was used. The effects of ultrasonic vibration were determined from the statistical analysis of microstructure using ImageJ and micro-indentation techniques on the deposited layer and heat affected zone. It was found that both frequency and interaction between frequency and power had significant impact on the refinement of average grain size up to 10.64% and increased the number of grains by approximately 41.78%. Analysis of micro-indentation tests showed that there was an increase of approximately 14.3% in micro-hardness and 35.77% in Young's modulus due to the applied frequency during rapid solidification. Along with the results from this study, further efforts in modeling and experimentation of multi directional vibrations would lead to a better understanding of disrupting columnar grains in applications that use mechanical vibrations, such as welding, metal additive manufacturing, brazing, and the likes.