e894a5i - ESNC 460/894 Assignment 5 Mar 31 2006 due Apr 7...

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ESNC 460/894 Assignment 5: Mar. 31, 2006 due Apr. 7, 2006 1. A light pulse of 7x10 7 W/m 2 is incident ont a semi-infintie slab of nickel. The energy is uniformly distributed over a circle of radius 1 mm. The nickel is coated to allow perfect absorption. Plot a graph of the temperature variation directly below the center of the spot at the surface and at 1 mm below the surface for times up to 0.1 sec. 2(a) Using the energy balance approach, estimate the power required to produce a spot weld 3 mm in diameter and 1 mm deep in low-carbon steel with a 1 sec pulse from a CO 2 laser. Use ρ = 7.86 g/cm 3 , α = 0.07 cm 2 /s, C = 0.7 J/gK, k = 0.4 W/cmK, T m = 1809 o K, L f = 13.8 kJ/mol, R=60% (b) Compare the diffusion depth given in part (a) with what is sometimes called the thermal penetration depth predicted on the basis of the uniform irradiance model. The thermal penetration depth is where the temperature changed is reduced by 1/e or about 37% of the surface temperature. (c) According to the uniform irradiance model, how long does it take to reach T m at the surface for the power in (a)? 3 Low carbon steel of 5 mm thickness is to be cut with a laser at a speed of 1.25 m/min. with a 0.5 mm
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