EFFECT OF DEGASSING ON HOT TEARING TENDENCY OF A206 ALUMINUM CAST ALLOY Mohamad Reza Nasresfahani Department of Materials Science and Engineering, Meybod University, Yazd, Iran Behzad Niroumand Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran Copyright Ó 2019 American Foundry Society https://doi.org/10.1007/s40962-019-00378-1 Abstract Effect of melt degassing on the hot tearing susceptibility of A206 alloy was examined using the ICTC apparatus. The results of thermal and load analyses, as well as macro- scopic examinations, were then used to evaluate predic- tions of Davies’s criterion, Load Rate criterion and NNC. Load Rate criterion and NNC predicted that degassing increased the hot tearing susceptibility due to increasing the contraction load of the alloy which was consistent with the results of macroscopic investigation of the castings. Davies’s criterion predicted that degassing decreased the hot tearing susceptibility which was in contrast to the other two criteria and the actual observations. Keywords: hot tearing, degassing, criterion, thermal analyses, load analyses Introduction Formation of large fissures in the semisolid temperature range of cast alloys has been the subject of various researches since 1940. This defect, known as the hot tear, solidification crack, hot shortness and hot brittleness, is one of the major defects in the wide freezing range alloys. 1–3 The hot tearing occurs in the mushy zone near the end of solidification when a solid continuous network has already been formed. 3 There are several techniques for assessing hot tearing, which the most typical of which are visual observation and measuring the length of the crack. 4,5 These methods are not sufficiently precise. Newer techniques have been estab- lished which use load measurement equipment, ultrasonic waves or X-ray microtomographic observations to assess the hot tearing tendency. 6–8 The equipment used in these techniques are relatively expensive, and their results are complex. On the other hand, there are hot tearing criteria which evaluate the results of these techniques and quantify them for a better comparison. One of the most commonly employed criteria in this field is that of Clyne and Davies. In this criterion, the end stage of solidification is considered. Davies’s criterion is presented in the form of Eqn. 1, where t V is the vulnerability time at the end of solidification where the tearing possibility is very high and t R is the time available for the stress release process (repair of initial tears); during this time, feeding is carried out in the form of liquid or mass feeding. Fur- thermore, t 0.99 , t 0.9 and t 0.4 are solidification time at solid fraction (f s ) of 0.99, 0.9 and 0.4, respectively. 9–11 HCS ¼ t V t R ¼ t 0:99 À t 0:9 t 0:9 À t 0:4 Eqn: 1 NNC is a more recent hot tearing criterion. This criterion uses both mechanical and metallurgical terms and is believed to provide more accurate predictions. The required mechanical and metallurgical terms are obtained from an instrumented constrained T-shaped casting (ICTC) apparatus. This apparatus measures the contraction load generated during solidification. Also, two thermocouples record temperature changes in the T-junction. Then the thermal and load analyses are performed on the graphs. 12 NNC corresponding criterion is given in Eqn. 2. 13 NNC ¼ C 3 gL 2 f 2 L k 2 2 P S Â r min t min À t 0 Eqn: 2 International Journal of Metalcasting