Prompt: We are currently focused on metals, their classification, and fabricatio
ID: 1766998 • Letter: P
Question
Prompt: We are currently focused on metals, their classification, and fabrication methods. While additive manufacturing methods have gained popularity in the past two decades, casting remains the most predominant method of producing structural metals for forming, joining, etc. In sand casting, however, there are some rules that must be followed to ensure that the casting is free of defects, the number one of which is shrinkage-driven and micro-porosity. In this term paper, please do the following: 1. 2. 3. Explain what these two forms of porosity are and why they are so deleterious to castings. What are risers (aka feeders)? Why is its solidification prior to the casting bad? What are geometrical reasoning methods in casting design, most notably Chvorinov's and Pellini's rules? What are some of the methods used to keep the riser warm relative to the casting to ensure that it doesn't solidify before the casting? 4.Explanation / Answer
1. Shrinkage related porosity is caused when section of casting solidifies later than than the surrounding due to insufficient molten metal. Microporosity is caused due to presence to small air bubbles and gas entrapment in the moulds.Air is present in the tool cavity and is sasily trapped as metal fills the cavity.
These are harmful as it reduces the stree bearing capacity and lessen the strength of the cast object. It hampers the fatigue and other mechanical properties of the cast.
2. A riser is a reservoir that is placed in the mould to compensate the shrinkage of metal during solidification from freezing temperature to ambient room temperature. Its soldification before cast is bad as it will cause shrinkage cavity due to insufficient metal flow.
3. Chvorinov rule relates surface area and volume to solidification time of casting. Time of solidification , t = k*(V/A)^n , where k = constant
V= volume of the casting , A= Area of the casting
4. Different materials such as silica, rice hull , exothermic materials and insulating materials are used to cover the top of the riser that keeps it warm when casting solidifies. These materials are called hot topping. Now a days floatinv cover lids are also being used to reduce the heat loss from the riser.
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