Design And Properties Of New Fusible Alloys

Fusible alloys are the alloys with the melting point below 232°C (meltingpoint of Tin), and normally composed by bismuth, lead, tin, and indium elementsin different proportions depending on the desired properties. These alloys arewidely used as solders, electric fuses, safety plugs in building sprinkler systemsand other special applications. Up to now, there is not so much information aboutthe basic aspects of fusible alloys, so it is quite necessary to investigate the design,phase diagram, properties of these alloys.According to the phase diagram found and orthogonal design methods,optimal composition of Bi-Pb-Sn-Cd alloys and Bi-Sn-In alloys are designed byconsidering the characteristics of heat-response applications and the requirement forthe environmental protection. The microstructure, melting behaviors, hardness,shear strength and tensile strength of Bi-Pb-Sn-Cd alloys and Bi-Sn-In alloys havebeen determined by optical microscopy, DSC, microhardness tester, XRD, andSEM techniques. The results are shown that Bi-Pb-Sn-Cd alloys are composed ofBi, Pb7Bi3, Sn and Cd phases;whereas Bi-Sn-In alloys are composed of Bi, Sn,BiIn,γ, β,Bi3In5, BiIn2 and InBi phases. The liquidus and melting range ofBi-Sn-Pb-Cd alloys decrease with the increase of Cd content. The effect of alloycompositions on the melting range, shear strength and tensile strength ofBi-Sn-Pb-Cd soldering joints are reduced by the order of Cd, Sn and Pb element.Bi-Sn-In alloys designed have a short melting range. The tensile strength ofsoldering joints of Bi-Sn-In alloys rises linearly with the decrease of In contentand the increasing of Sn content.By optimizing of alloy compositions, 50Bi-24Sn-20Pb-6Cd alloys with low Pband Cd content and Pb(Cd)-free 42.6Bi-40.4Sn-17.0In alloys are developed. Themelting behaviors, hardness and tensile strength of these two alloys are superior tothe available Bi-Pb-Sn-Cd eutectic alloys, and these two alloys can fulfil totallythe requirement of 74℃ automatic sprinkler.