23-Ind-B2 Manufacturing Processes · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2015 — 98-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. — material selection, casting, metal-cutting theory, welding processes, polymer processing, statistical process control; Montgomery, Introduction to Statistical Quality Control, 8th ed. — acceptance sampling, control charts, the Deming/Taguchi quality philosophies.
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 choice of welding process for a given job is driven by: base-material type and thickness (weldability, melting point, thermal conductivity — thin sheet favours a low-heat-input process, thick sections favour a high-deposition process); joint design and access (position, joint geometry, and whether the joint is accessible from one or both sides); required joint quality/service duty (structural, pressure-vessel or fatigue-critical joints demand processes with well-controlled, code-qualified procedures — in Canada, welding procedures and welder qualification for most structural and pressure work fall under CSA W47.1/W59 and the ASME code where applicable); production volume and rate (a manual process suits low-volume/field work, an automated resistance or arc process suits high-volume production); equipment and operating cost, including consumables, shielding gas and skilled-labour availability; and working environment (indoor/shop versus outdoor/field, since some processes are impractical in wind or require ventilation for fumes).
(1) Oxyacetylene welding burns a mixture of acetylene and oxygen to produce an intense flame (up to about 3480°C) that melts the base metal and filler rod directly, with no electrical energy involved: $2\text{C}_2\text{H}_2+5\text{O}_2\rightarrow4\text{CO}_2+2\text{H}_2\text{O}+\text{heat}$ for the complete (neutral-flame) reaction. It is portable, inexpensive and gives the operator good control of heat input, but heats a relatively large zone slowly, producing more distortion and a larger heat-affected zone than arc processes, and is now used mainly for repair, brazing and cutting rather than production welding.
(2) Arc welding uses an electric arc struck between an electrode and the workpiece to melt the base metal and (in consumable-electrode processes) the electrode itself, which becomes filler metal; shielded metal arc, gas metal arc (GMAW/MIG) and gas tungsten arc (GTAW/TIG) are the common variants, differing mainly in how the arc and molten pool are shielded from atmospheric oxygen and nitrogen (flux coating, inert/active shielding gas, or flux core). Arc processes concentrate heat into a small zone, giving faster travel speed, deeper penetration and a smaller heat-affected zone than oxyfuel welding, and dominate production and structural welding.
(3) Resistance welding (e.g. resistance spot welding) forces electrical current through the two overlapping workpieces held together under electrode pressure; resistive (Joule) heating is generated at the faying-surface interface, $H=I^2Rt$ (heat generated proportional to current squared, contact resistance, and current duration), melting a small nugget that fuses on cooling under continued electrode force. No filler metal or shielding gas is required, cycle times are very short, and the process is highly automatable — it is the dominant joining method for automotive sheet-metal body assembly.
The basic difference is the mechanism of metal removal: oxyfuel gas cutting first preheats the steel with a fuel-gas/oxygen flame to its ignition (kindling) temperature, then directs a jet of pure oxygen at the hot spot, which chemically oxidizes (burns) the iron and blows the resulting molten oxide/slag out of the kerf — it is fundamentally a chemical oxidation process and works well only on materials that oxidize readily and whose oxide has a lower melting point than the base metal (mainly carbon and low-alloy steels). Arc cutting, by contrast, uses an electric arc to melt (and, in plasma cutting, also mechanically blow away with a high-velocity ionized gas jet) the base metal directly — it is a thermal-melting process, not a chemical-oxidation one, so it can cut materials that oxyfuel cannot (stainless steel, aluminum, and other non-ferrous/non-oxidizing metals).
The main types of arc cutting are: plasma arc cutting (PAC), which constricts the arc through a small nozzle to produce a high-velocity, high-temperature ionized-gas jet capable of cutting any electrically conductive material at high speed; air carbon arc cutting/gouging (CAC-A), which melts the metal with a carbon-electrode arc and blows the molten metal away with a jet of compressed air, commonly used for gouging out welds or defects; and shielded metal arc cutting, a slower, less precise method using a covered electrode much as in manual arc welding, generally reserved for field repair where plasma or oxyfuel equipment is unavailable.
