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CHSC LAB 8 - Mike Borton CHSC 2205 SET 2A Jan 22/07...

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Mike Borton CHSC 2205 SET 2A Jan 22/07 ENGINEERING MATERIALS LAB NO.8 ALUMINUM PRECIPITATION HARDENING; CAST IRONS
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OBJECTIVES: Harden and strengthen AA 2024 aluminum alloy rods by precipitation hardening treatment. Also test the specimens and study the changes in mechanical properties that take place at various stages during the heat treatment process. Observe and sketch the microstructures of cast iron microstructures, paying particular attention to the various forms of carbon present in different types of cast irons. THEORY: The purpose of precipitation hardening treatment is to produce a lot of fine, but strong particles of a second phase to act as obstacles to make it more difficult to move dislocations and cause slip. Slip results when plastic deformation occurs as a metal is stressed beyond its yield strength. Dislocations are usually present in virtually all metals and alloys which makes slip occur very easily. The AA 2024 aluminum alloy contains aluminum and approximately 4.5% copper. In its soft annealed condition, there are a few fairly large particles of a copper-rich second phase, CuAl 2 . These two phases should be present at room temperature. The precipitation hardening process takes place in two steps: 1. Solution treatment involves heating the 2024 alloy to approximately 490°C to dissolve the copper and form the solid solution, and then quenching in water to keep the copper trapped in the solid solution.
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2. Aging involves re-heating the alloy to 190°C for a specified length of time. The increased molecular vibration from the heat accelerates the diffusion rate in the alloy. This allows the copper atoms to move closer to each other and eventually form a multitude of tiny precipitate particles. These can effectively block dislocations, preventing slip and resulting in high yield strength, high hardness and low ductility. If the alloy is overheated or aged for to long, a much smaller number of widely spaced large particles will form making it easy for slip to occur.
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