by clicking the arrows at the side of the page, or by using the toolbar.
by clicking anywhere on the page.
by dragging the page around when zoomed in.
by clicking anywhere on the page when zoomed in.
web sites or send emails by clicking on hyperlinks.
Email this page to a friend
Search this issue
Index - jump to page or section
Archive - view past issues
button in toolbar for more information.
Metal Casting Technologies : March 2009
TECHNICAL FEATURE Design of hot tearing test apparatus for cast alloys M. Nasresfahani, B. Niroumand Department of Materials Engineering, Isfahan University of Technology. email@example.com Abstract I n this research a new apparatus for quantitative assessment of hot tearing tendency in casting alloys and influence of casing conditions on it is introduced. In this method molten metal is cast in a T shaped mold cavity. The ends of the arms of the casting are hooked to the flask during the solidification in order to restrain its free contraction. One of the hooks is connected to a load cell which enables the real-time measurement of the contraction load during solidification and provides a load-time graph for each experiment. Temperature-time graphs are also plotted using a K-type thermocouple placed in the casting and connected to a data-logger. Using such a set up, the solid fraction at which the hot tearing occurs can be estimated. In order to investigate the accuracy of the method some experiments were performed on Al-9%Si alloy and the influence of mold thermal gradient on load-time graph and the occurrence of hot tearing were investigated. Microscopic study of the observed cracks indicated their hot tearing characteristics. Introduction The formation of microscopic cracks within semisolid casting has been the subject of metallurgical research since the 1940s. Hot tearing is a major defect in alloys with a large mushy zone. Hot tearing refers to cracks that can occur within the mushy zone at high fraction solid during cooling from the liquid to 42 www.metals.rala.com.au solid state in near-net-shape and ingot castings. This defect occurs in both ferrous and non ferrous alloys . This defect is inter dendritic and has a major band with many branches. The paths of cracks are through rough where dendritic morphology in surface of cracks can be seen . Hot tearing occurs in mushy zone near the solidus point where solidification is almost complete and only a thin film of liquid surrounds the grains . Due to the nature of this defect, the economic impact is often significant and can result in an immediate productivity loss. It is important for industry to be able to better predict the susceptibility of various alloys, casting geometry and / or process conditions to hot tearing. The majority of the experimental methods for determining hot tearing susceptibility aim to include the defect by means of constraining the solidifying casting and then quantifying the severity of tearing. Experimental designs have generally been of the ring type casting, the restrained bar type casting or variations thereof. The principle of the ring casting method is a central core that acts to resist thermal contraction during solidification and cooling, thereby causing tensile stresses to develop in the semisolid. Consequently, cumulative crack length or number of cracks has, in the past been, linked to alloy susceptibility [1, 4]. The restraint bar having other names - doge bone or black bone, have been based on preventing contraction of solidification. In this method both ends of the casting bar restraining and controlling cooling resulting in the end point of solidification being at the center of bar . Other approaches involve hot tearing test temperature of a casting sample arising to semisolid zone and using a tensile test to determine hot tearing susceptibility . This method provides quantitative data but it is difficult. Other methods have been explained in other references of these methods. A new T-shape method by Beckermann that simultaneous measurement contraction load, temperature and displacement of hot spot with a lot of sensors has recently been explained. This method produces complicated and possibly innacurate data where finding the relationships between the data is difficult because of the use of complicated instruments, Preparation for this method for each test is tedious and needs special experience . In this work a new apparatus is described for a quantitative determination of the hot tearing tendency of casting alloys that is based on measurement contraction load during solidification. This method involves a T-shape that is modified to simplify the explanation of problems. Early results of the apparatus work for Al-9%Si alloy are provided.