ADVANCED MATERIALS & PROCESSES | MAY/JUNE 2024 31 porosity (gas or shrinkage)[11] is also a challenge when casting high-tin bronze for cymbal production, both at the surface and throughout the internal dendritic microstructure. Figures 8 and 9 show typical surface defects (i.e., slag impurities, uneven cooling, underweight pours, and unbalanced bowls) Fig. 5 — As-cast bun, top surface. Fig. 6 — As-cast bun, bottom surface. Fig. 7 — Hot-worked blank exhibiting edge cracks; as-rolled, first pass. Fig. 8 — Casting defects observed on buns. Fig. 9 — Bronze buns exhibiting surface conditions relating to pour temperature and cooling rate. Top row from left: Bun 1 temperature too low at pour; bun 2 and bun 4 exhibit tin-rich phase at surface (inverse segregation); bun 3 color suggests low Sn content. Bottom row: Various smaller cast buns for finger cymbals or medallions. Fig. 10 — As-cast, bun cross-section, unetched, 50×. Heavy porosity is evident. Courtesy of Fredrick A. Skinner, metallurgical consultant. Fig. 11 — As-cast, etched, 50×. Dendritic microstructure, porosity at surface and subsurface. Microhardness, ~ 95-98 HRB. Courtesy of Fredrick A. Skinner, metallurgical consultant. and varying surface colors relating to variations in temperature and pouring conditions. Figures 10 and 11 show porosity in an as-cast (8.0 lb) bun, cross-sectioned approximately 13.0 mm from center of bun. Excessive porosity trapped within the cast bun, or located at surface (surface voids), may manifest during downstream processing and cause surface defects (i.e., pits), which hammering or lathing cannot remove. HAND HAMMERING AND ROLLING INGOT INTO A BLANK Starting with a cast bun of proper weight and surface condition, the arduous task of working the thick metal bun into a thinner shape, and eventual flat blank, begins. In ancient times, a group of metalworkers armed with hammers hot forged the castings into the shape of a blank, at which point the metal met cymbal criteria (thickness, diameter, and weight). Ancient metalworkers learned high-tin bronze must be in a red-hot state (~800°C) when forging. The practice involved heating the bun, hot working (hammering) within a narrow temperature range, reheating, and repeating. If heated properly, the thick shape of the cast buns (a result of the bowl molds) helps retain heat
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