Study On Microstructure And Properties Of Sn - 58Bi Alloy Wire

With the increase of environmental awareness, the formulation of laws and the driven of electronic market demand, more and more people advocate to use the pollution-free and lead-free products. Nowadays, the most typical lead-free solder alloys are made based on Sn and add other alloy elements. Sn-Bi alloy is the four kinds of lead-free solders which are used to instead of the Tin-lead, has the advantages of low melting point, good weldability, high joint strength. But the ductility of Sn-Bi lead-free solder is poor, which greatly limits its application in the electronic packaging industry, especially in the field of low temperature lead-free solder. Based on the continuous extrusion technology, this paper produce a φ1mm～ φ3mm fine eutectic tin-bismuth alloy wire using the casted test rod of Sn-58Bi alloy after diffusion annealing. Respectively, the microstructure, hardness, mechanical properties, wetting properties and tensile fracture mode of each deformation zone and final products were observed. The test results show that:(1) The micro hardness in different deformation zone is on the rise in the process of continuous extrusion of Sn-58Bi. The grain become more and more refined and rounded.In the initial adhesion area, the grain are elongated in the deformation direction; The grain structure in upsetting region changed from strip into a round shape. The dynamic recrystallization phenomenon is started appear in extrusion nip; Expansion deformation occurs in the mold expansion zone, the original deformation grain was replaced by a small fine recrystallized grain. The grain of eutectic tin bismuth alloy wire are refined.(2) The hardness of Sn-58Bi alloy improved with the increase of extrusion ratio, while the brittleness of Bi phase decrease in size. In addition, with the increase of extrusion ratio, the tensile strength and elongation of eutectic tin-bismuth alloy increased. Compared with the cast Sn-58Bi alloy’s elongation of 9.1%, the elongation is 212% obviously improved, is nearly 20 times as the cast Sn-58Bi alloy.(3) Sn-58Bi alloy appeared different super plastic fracture mechanism in different strain rates. Compare the cast Sn-58Bi alloy with the continuous extruded Sn-58Bi alloy analysis the fracture mode, the results showed that:there are different directions of the "tear ridge" in the fracture morphology of cast eutectic Sn-Bi alloy, obviously, present the way of brittle cleavage fracture. The fracture model of continuous extrusion of Sn-58Bi alloy wire is transgranular fracture, the main reason is the phase separation between Sn and Bi and the brittle fracture of Bi phase. At low strain rate, the fracture mode of Sn-58Bi alloy is brittle cleavage fracture, while at high strain rate, the fracture mode of Sn-58Bi is transgranular fracture.(4) In conditions of different strain rate, the eutectic Sn-Bi alloy tensile strength increase with the increase of strain rate, while the elongation decrease with the increase of strain rate. In tensile test of Sn-58Bi alloy wire will appear a instable point. The instability point which bearing capacity reduced in tensile process of Sn-58Bi alloy wire appear in the 70% of uniform transition curve, from this point the stripping rate of material increased between phase and other phase.