Machine Design


Tin is characterized by a low-melting point (450°F), fluidity when molten, readiness to form alloys with other metals, relative softness, and good formability. The metal is nontoxic, solderable, and has a high boiling point. The temperature range between melting and boiling points exceeds that for nearly all other metals (which facilitates casting). Upon severe deformation, tin and tin-rich alloys work soften.

Principal uses for tin are as a constituent of solder and as a coating for steel (tinplate, or terneplate). Tin is also used in bronze, pewter, and bearing alloys. Tin and tin alloys can be cast, rolled, extruded, or atomized. Because pure tin is too weak to be used alone for most mechanical applications, it is usually alloyed with elements such as copper, antimony, lead, bismuth, and zinc.

Tin and its alloys are cast using most conventional techniques, including gravity die casting, pressure die casting, and centrifugal casting. Tin-alloy castings are sound and dimensionally accurate because little shrinkage occurs on solidification. Essential to the casting of tin-rich alloys is the need to reduce microstructure segregation, which occurs during solidification. Part and mold designs should be such that ample metal is fed to remote regions of the mold cavity. Because of their low-melting points, tin-rich alloys can be cast in carbon-steel or rubber molds.

Pewter: Alloys contain from 1 to 8% antimony and 0.5 to 3% copper, and have excellent castability and workability. For spun pewter products, antimony content is usually below 7%, and pewter casting alloys contain 7.5% antimony and 0.5% copper. Because of the excellent drawing and spinning properties of tin, wrought parts are usually made from pewter that is first cast into slabs, then rolled into sheet.

Bearing alloys: These low-friction materials may contain high percentages of antimony (to 10%), copper (to 10%), or aluminum (to 80%). They include babbitt metal, bronze, and aluminum-tin. Although soft, conformable, and corrosion resistant, their low mechanical strength must be boosted by bonding to steel, cast iron, or bronze backing materials.

Die-casting tin-based alloys: Historically, these were the first materials to be die cast. Low melting point and extreme fluidity of these alloys produce sound, intricate castings inexpensively and with little wear on molds. Antimony, copper, and lead are the principal additions to tin in die-casting alloys. These alloys are mainly gravity or centrifugally die cast. Cast tin-alloy parts can be held to tolerances of 0.0005 in./in., with wall thicknesses down to 1/32 in. Shrinkage is negligible.

Fusible tin alloys: Melting temperatures for these alloys are usually below the solidus of eutectic-base tin-lead solders (361°F). Primary alloying elements include bismuth, lead, cadmium, and indium. Most of these alloys provide electrical or mechanical links in safety devices. Other applications include low-temperature solders, seals for glass and other heat-sensitive materials, foundry patterns, molds for low-volume production of plastic parts, internal support for tube bending, and localized thermal treatment of parts.

Tin and tin-alloy powders: Produced by atomization techniques, these powders are available in a number of mesh sizes. They are used in the manufacture of powder-metallurgy parts, in tinning and solder pastes, and in the spray metallization of surfaces.

In small amounts, tin is also combined with titanium, zirconium, and other metals to provide special properties. It is used as an alloy in nodular and gray irons to provide greater strength, increased and uniform hardness, and improved machinability. Tin-nickel and tin-zinc coatings are used in the braking systems of automobiles. The tin-nickel alloy is coated on disc-brake pistons because of its good resistance to wear and corrosion. Tin-zinc is used to plate master cylinders in automotive braking systems.

Noncritical parts such as costume jewelry and small decorative items such as figurines can be made by casting pewter and other low-melting tin-based alloys in rubber molds. Tin die-casting alloys are suitable for low-strength precision parts and bearings for household appliances, engines, motors and generators, and gas turbines. These bearings perform well even at start-up and run-down periods of operation, at which times they carry a heavy, unidirectional load without the benefit of a fully formed hydrodynamic film. Other applications for tin-based die castings include parts for food-handling equipment, instruments, gas meters, and speedometers.

TAGS: Materials
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