Thermal Analysis—Theory and Applications in Metalcasting

Abstract

Monitoring and adjusting the chemical composition and the nucleation potential of casting alloys melts is critical to the production of castings of consistent high quality. While other methods such as fractured test samples (e.g., the wedge test for gray iron) and spectrographic chemical analysis deliver useful information, thermal analysis (TA) (also called cooling curve analysis, CCA) can provide a more complete insight in the dynamic changes occurring upon melting and during melt treatment of casting alloys. Initially TA was used for the rapid evaluation of the carbon equivalent (CE) in cast iron, or the silicon content in Al-Si alloys. Extensive research then extended the capabilities of TA to the understanding of the solidification changes induced by compositional variations such as the Mn/S ratio, Ce and Mg additions. With the advent of ever faster computers, the first derivative of the cooling curve, which is the cooling rate, was added to the arsenal of data provided by TA, and a new technique, Computer Aided Cooling Curve Analysis (CA-CCA), was born. This technique evolved then into Differential Thermal Analysis (DTA) without a reference sample. With this tool in hand the metallurgist ventured to predict not only the chemistry of the melt but also the nucleation potential (degree of inoculation), the shrinkage propensity, the dendrite arm spacing and the grain size, the graphite shape, the solidification microstructure, and even the room temperature microstructure. Currently, different TA techniques are used worldwide in the daily production of all grades of cast iron, as well as in monitoring the melting of aluminum and steel alloys. From this short introduction it should be obvious that the story of DTA/CCA is a long and exciting one. This paper will try to summarize the fascinating development and extraordinary success of this technique in the casting industry.