21-Mat-B7 Structure and Properties of Polymers · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, May 2016 — 10-Met-B7, Physical Metallurgy of Non-Ferrous Metals and Alloys. Three hours, closed book, approved Casio/Sharp calculator only. Eight questions of 20 marks each; the rubric states that any five questions constitute a complete paper (100 marks total) and that only the first five appearing in the answer book are marked. All eight are answered here, because this set is a study resource rather than an exam script. The rubric explicitly notes that most questions require an essay-format answer and that clarity and organization are marked, so the answers below are written as structured prose rather than as note form.
Reference texts. The answers below are keyed to the works normally recommended for this syllabus code:
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.
Wrought alloys use a four-digit system, $Axxx$, in which the first digit names the principal alloying addition and therefore the alloy family: 1xxx (99%+ pure aluminum), 2xxx (copper — age-hardenable, aerospace sheet/plate), 3xxx (manganese — non-heat-treatable, can body and general sheet), 4xxx (silicon — filler wire and forgings), 5xxx (magnesium — non-heat-treatable, marine and structural sheet), 6xxx (magnesium and silicon, forming Mg&sub2;Si — age-hardenable, extrusions), 7xxx (zinc, with magnesium and copper — the highest-strength age-hardenable aerospace alloys), and 8xxx (other elements, e.g. lithium or iron/nickel). The second digit indicates a modification of the original alloy (0 = original), and the last two digits either identify the specific alloy within the 1xxx series (purity) or are arbitrary identifying numbers in the other series.
Casting alloys use a related but distinct three-digit-plus-decimal system, $Axx.x$: the first digit again identifies the major alloying group (1xx.x pure Al, 2xx.x Cu, 3xx.x Si with Cu and/or Mg, 4xx.x Si, 5xx.x Mg, 7xx.x Zn, 8xx.x Sn), the next two digits identify the specific alloy, and the decimal digit distinguishes castings (.0) from ingot compositions intended to be remelted for casting (.1 or .2). Silicon-bearing casting alloys (3xx.x/4xx.x) dominate because silicon additions dramatically improve fluidity and feeding during solidification, which wrought compositions rarely need and casting compositions depend on.
F — as fabricated. No special control has been exercised over thermal or strain-hardening condition during shaping (e.g. rolling, extrusion or casting); mechanical properties are not guaranteed and vary with the specific fabrication history.
O — annealed. The wrought product has been given a full anneal to produce the lowest-strength, highest-ductility, most formable condition; it is the recrystallized, stress-relieved state used as the starting point for heavy forming operations or as the temper of choice where maximum ductility matters more than strength.
H — strain hardened. Applies only to non-heat-treatable alloys (1xxx, 3xxx, 5xxx); strength is developed by cold working, optionally followed by a partial (stabilizing or softening) anneal, and is specified by two or three digits — the first indicating the basic operation (H1 = strain hardened only, H2 = strain hardened and partially annealed, H3 = strain hardened and stabilized) and the second the degree of strain hardening (H12–H18 running from quarter-hard to full-hard, i.e. roughly 75 percent cold reduction equivalent).
T — thermally treated to a stable temper other than F, O or H. Applies to the heat-treatable alloy families (2xxx, 6xxx, 7xxx and some cast alloys); the digit following T specifies the exact sequence — T4 (solution heat treated, then naturally aged at room temperature), T5 (artificially aged only, from an as-fabricated condition close enough to solution temperature), T6 (solution heat treated and artificially aged to peak strength), and T7 (solution heat treated and overaged/stabilized to trade some strength for improved dimensional stability or corrosion resistance), with further suffixes (e.g. T651, T73) denoting an added stress-relief step or a specific overaging practice.
