Micros

Converting thermal analysis information into microstructure information
D.A. Sparkman MeltLab Systems, Winchester, Virginia Copyright 2010 David Allen Sparkman ABSTRACT Thermal Analysis has been used in the past to determine phase diagrams, which describe temperatures and energies produced from crystallization or the reversed melting of various crystal substances. Physics describes howexothermic and endothermic reactions relate to microstructure. As the hardware and software for thermal analysis improve, it can be expected that the refinements will allow better understanding of casting solidification and microstructure. Currently the standard consists of extracting a microstructure sample from the casting or creating a test coupon in the mold or an external mold, and examining it underthe microscope. If thermal analysis could be improved and made more sensitive, then the speed of analysis and the manpower associated with microstructure analysis could be greatly reduced. This paper examines ways of improving that sensitivity and the results from those improvements in finding carbides, shrinkage, and oxides in iron, beta crystals, gas, copper, and magnesium silicide in aluminum,and phosphorus in copper. INTRODUCTION Thermal analysis has been used for some time to measure iron flowability (carbon equilivant) and more recently to measure carbon, silicon and solidus. Work has been done to document calculating chill/inoculation, ferrite/pearlite, magnesium, and shrinkage potential. In aluminum, thermal analysis is used to measure the degree of inoculation and the degree ofmodification. In addition, grain size, percent silicon, freezing range, and the coherency point have been measured. Dendritic arm spacing has been studied1, as well as inoculation of hyper-eutectic aluminums by phosphorus, using thermal analysis.2 These have been documented in many other papers and are common knowledge in the foundry industry. But there are many other features on a thermal analysiscurve that can also benefit the production foundry. The past problem with thermal analysis practiced as a manufacturing tool in the foundry was that it was not sensitive enough to pick up things like pearlite, carbides, nodularity, nodule count, and vermicular in iron, or beta crystals, gas and grain boundary precipitates in aluminum. So a time consuming process of creating a “micro” became astandard operation in the foundries. With Iron, vermicular or nodularity checks are performed in short times, and recently, ultrasonic nodularity checks on iron samples have been introduced3. This is an improvement over the older ultrasonic testing on castings after the molding, cleaning and fettling processes because it can lead to faster corrective actions. Efforts have also been made to remove thehuman element and standardize these results by using a computer to make multiple readings under a microscope and average the results. Carbides in iron are usually detected by more careful polishing of a micro in the lab. Shrinkage is usually found after the fact, leading to costly scraping of entire lots of castings or ultrasonic or x-ray tests to find the shrink Still, these tests are very timeconsuming and labor intensive and there can be problems in isolating bad castings because of the normal delays involved in testing. This paper looks at the problems involved in improving thermal analysis and how far it might be able to go in providing a quicker feedback on casting quality. This paper presents a new way of looking at microstructure through thermal analysis. The validity of thismethod starts with the development of a smoothing algorithm to see beyond the noise that has stymied previous researchers. Once the validity of the smoothing method is accepted, then one can move beyond the data preparation method, and begin interpreting the meaning of the small arrests. In addition, the importance of the 4th derivative as a means of detecting and qualifying the small arrests will...