Effect of Bi on the Properties of Tin Alloys for Utensils and Vessels

In Japan, Sn-Pb alloys have been used as materials for utensiles and vessels. However, Pb is undesirable in the utensiles 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 view-point of hardness, microstructure, surface morphology, and machinability.Hardness equivalent to that of Sn-Pb alloys is obtained by the addition of 1%Bi. Hardness of Sn-Bi alloys is attributable to solid-solution hardening up to 2%Bi, but thereafter to dispersion hardening. Microstructure of Sn-Bi alloys depends upon solidification rate. In rapid cooling fine Bi particles crystallize uniformly in Sn matrix, while in slow cooling Bi particles crystallize along grain boundaries. When the latter is 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%, but thereafter yields to a saturation behavior. The decrease of surface roughness is caused by matrix-embrittlement due to the addition of Bi.