1992 Volume 33 Issue 6 Pages 611-617
In Japan, Sn–Pb alloys have been used as materials for utensils and vessels. However, Pb is undesirable in the utensils and vessels because of its toxicity. For developing Pb-free Sn alloys having excellent machinability, high strength, and pear-like surface morphology, Sn–Bi alloys were studied from the viewpoints of hardness, microstructure, surface morphology, and machinability.
The hardness equivalent to that of Sn–Pb alloys is obtained by the addition of 1 mass% Bi. Hardness of Sn–Bi alloys is attributable to solid-solution hardening up to 2 mass% Bi, but thereafter to dispersion hardening. Microstructure of Sn–Bi alloys depends upon solidification rate. In rapid cooling fine Bi particles crystallize out uniformly, while in slow cooling Bi particles crystallize out along grain boundaries. When the Sn–Bi alloys solidified under the latter condition are etched with HNO3, the etched surface results in pear-like morphology. Sn–Bi alloys exhibited an excellent chip-disposability because Bi particles promote the crack propagation in chip. Surface roughness decreases as Bi content increases up to 2 mass%, but thereafter yields to a saturation behavior. The decrease in surface roughness is caused by matrix-embrittlement due to the addition of Bi.
