23-Ind-B2 Manufacturing Processes · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2018 — 17-Ind-B2 Manufacturing Processes. Closed book; Casio or Sharp approved calculators only. Any five of the seven questions constitute a complete paper; all questions are of equal value (20 marks each). Answers are written in point form but fully, with all calculations shown, as instructed. Complete answers to all seven questions follow.
Reference texts: Groover, Fundamentals of Modern Manufacturing: Materials, Processes, and Systems, 6th ed. — engineering-material property overview, casting processes, polymer/composite processing, metal-forming theory, and metal-cutting theory.
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
The dominant reason is fatigue resistance. A forged (wrought) crankshaft has a continuous grain flow that follows the contour of the part, and the forging process closes up porosity and shrinkage voids inherited from solidification while also refining the coarse as-cast grain structure — so a forged crankshaft is both mechanically stronger and, critically, far less likely to contain internal defects (porosity, inclusions, shrinkage cavities) that act as fatigue-crack initiation sites. A crankshaft experiences many millions of cyclic bending and torsional load reversals over its service life, so fatigue strength — not just static strength — is the governing design requirement, and this is exactly the property in which wrought (forged/stamped) parts most decisively outperform castings of the same alloy. Casting's usual advantages (shape complexity, lower tooling cost per part at high volume) do not outweigh this fatigue-life difference for a safety-critical, cyclically loaded component like a crankshaft.
Work-hardening (strain hardening) is the increase in a metal's flow stress — its resistance to further plastic deformation — that occurs as the metal is plastically deformed at a temperature below its recrystallization temperature (cold working). As deformation proceeds, dislocations multiply and increasingly tangle and interact with one another and with other lattice obstacles, which progressively impedes further dislocation motion; consequently more and more stress is required to continue deforming the material. The practical result is that a cold-worked metal becomes stronger and harder, but correspondingly less ductile, as strain accumulates, until it eventually fractures if deformation continues too far without an intervening anneal.
Advantages. Flow stress is much lower at elevated temperature, so large shape changes can be achieved with lower forming forces and smaller-capacity equipment; ductility is substantially higher, permitting large deformations without cracking; continuous recrystallization during (and immediately after) deformation restores a fine, strain-free grain structure, so the part does not retain the strain hardening or the internal residual stress that a cold-worked part would; and hot working can break up and homogenize the coarse, non-uniform dendritic grain structure inherited from ingot casting, while also helping to close internal porosity.
Disadvantages. Dimensional accuracy and surface finish are poorer than cold working, owing to thermal expansion and contraction on cooling and, for many metals, oxide-scale formation on the hot surface; tooling wear and die cost are higher, since dies operate at elevated temperature under repeated thermal cycling; the need to heat (and often reheat) the workpiece adds energy cost, cycle time, and process complexity; and, because recrystallization removes strain hardening as it occurs, a hot-worked part does not gain the strength benefit that a cold-worked part of the same material would retain after forming.
