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Metal Casting Technologies : Whos who September 2011
METAL Casting Technologies September 2011 49 penetrant are typically red in color, and represent the lowest sensitivity. Fluorescent penetrants contain two or more dyes that fluoresce when excited by ultraviolet (UV-A) radiation. It is also known as black light. Since Fluorescent penetrant inspection is performed in a darkened environment, and the excited dyes emit brilliant yellow-green light that contrasts strongly against the dark background, this material is more sensitive to small defects. When selecting a sensitivity level, one must consider many factors; including the environment under which the test will be performed, the surface finish of the specimen, and the size of defects sought. The other inspection technique used in this research was the ultrasonic inspection. The principles of the ultrasonic inspection method are based on high frequency sound waves which are introduced into a material to be inspected and they are reflected back from surfaces or flaws. The reflected sound energy is displayed versus time, and the inspector can visualize a cross section of the specimen showing the depth of features that reflect sound. High resolution images can be produced by plotting signal strength or time-of-flight using a computer-controlled scanning system. As mentioned above, the speed of propagation of stress waves depends mainly on the density and the elastic constants of the solid. In Aluminum (Al) casting, variations in density can arise from non-uniform consolidation by the existence of defects. By determining the wave speed at different locations in the inspected structure; it is possible to make inferences about the uniformity of the Al sample. The compression wave speed is determined by measuring the travel time of the stress pulse over a known distance. Literal review The solidification rate in sand casting is of great importance in building up the microstructure of castings and so getting the material characteristics. Furthermore, the time the casting is left to solidify has great importance on the surface and subsurface defects which greatly affect the fatigue life of the castings and the surface finish requirements. Asta et. al.  reviewed the most important findings in the solidification science and technologically important area. Their review showed a great progress in this science and its great importance in the building material microstructure. The influence of casting defects on static and fatigue strength is investigated for a high pressure die cast aluminum alloy by Avalle et. al. in 2001 . They showed that the tensile and the fatigue strength decreased with the porosity defects range. Gunasegaram et. Al.  studied the basic process parameters affecting the size and location of a shrinkage pore defect in an aluminum alloy permanent mold casting. Using numerical simulations and design of experiments they found that mold coat thickness and mold temperature as the two most vital parameters. Of course these two factors relate directly to the solidification rate. The influence of porosity on the fatigue life of aluminum alloy was also studied by Mayer et. al. in 2003 . They concluded that porosity affects both surface and subsurface crack initiation. Vijayaram et. al.  reviewed and discussed the simulation process of casting solidification to analyze the casting defects which arose during solidification and heat transfer. They used casting solidification simulation technology to determine the solidification time and behavior of different materials accurately. Based on many assumptions, Vijayaram et. al. could generate time-temperature plot to explain the effect of under cooling of solidifying castings which reflects more on the inside microstructures responsible for material properties. Reis et. al.  presented a model that captures the difference in solidification behavior of long and short freezing materials. In their work, Reis et. al. found that the shrinkage defects in short freezing materials tends to be internal, as porosity, while in long freezing materials these defects tend to be external in the form of surface depressions. Cleary et. al.  simulated the casting process of aluminum to describe the sort shot formation. They supported their simulation results with some experimental results and produced some suggestions to reduce the shots in casting. In 2006, a numerical simulation of 3D temperature field of investment castings was developed by Zhang et. al.  to study the heat dissipation during casting. Their results agreed with experimental measurement ones. To avoid solidification defects in casting, Kang et. al.  redesigned the casting die, as they think it is a main factor in heating dissipation and so reducing casting defects. The heat dissipation during casting solidification was also studied by O'Mahoney and Browne in 2000 . They observed that there is significant variation of the alloy/mold heat transfer coefficient during solidification. O'Mahoney and Browne found it is highly dependent on the alloy type and on the vertical position below the initial free surface of the liquid metal. Upadhya et. al  modeled the casting process to calculate and detect the casting defects. Their approximate model could estimate the solidification time and predict the casting defects. They verified their model results by experimental results. Dahle and StJohn  developed a conceptual framework to explain the formation of defects during solidification process. They related the defects formation to the shear stresses during the microstructure growth and to the subsequent feeding THE COMPRESSION WAVE SPEED IS DETERMINED BY MEASURING THE TRAVEL TIME OF THE STRESS PULSE OVER A KNOWN DISTANCE.
Whos who September 2012