Research Of Microstructure And Properties Of Sn-58Bi Alloy Prepared By Continuous Method Of Upward-casting/drawing

Sn-58 Bi alloy is a promising low-temperature lead-free solder because of its low melting point,non-toxic,high tensile strength,shear strength,creep resistance and fatigue life,and the properties of Sn-58 Bi are equal to Sn-Pb eutectic solder in most conditions.It is difficult to explore the efficient method for the fabrication of Sn-Bi rod/ wire,due to the many brittle primary Bi phase existed in the Sn-58 Bi alloy prepared by the traditional casting method.It is important to develop new low cost and high efficient way to prepare the Sn-Bi alloy wire material.This paper used the upward continuous casting method for the preperation of Sn-58 Bi alloy rod with the diameter of 8mm.Compare with the microstructure and mechanical properties of the same composition alloy prepared by traditioal casting method,it shows that the upward continuous casting method efficiently refine the microstructure and ductility.The large deformation and muti passes drawing of Sn-58 Bi prepared by upward continuous casting was performed by the selfdesigned heating drawing device.The change of microstructure and properties of this material during the drawing process were investigated.And the effects of heat treatment parameters on the properties of the material were also studied.The main results are as follows:(1)The material of Sn-58 Bi alloy was prepared by the upward-casting method,and the effect of heattreatment on the microstructure and properties was investigated.Compared with the material prepared by the direct casting method,the microstructure of the alloy fabricated by upward casting was effectively refined,the Sn phases connected with each other forming the network microstructure,the primary round Sn phases disappeared,and the parallel primary dendritic Bi phases along the vertical direction formed.The markedly change of microstructure influenced the fracture mechanism,which caused the significantly increases of Sn-58 Bi alloy elongation to the value of 42.5%,meanwhile the elongation of the material prepared by the direct method is only 5%.The effect of heat treatment on the microstructure and properties of the upward casting alloy was greatly.The heat preservation promoted the Bi elements precipitated from the Sn-rich phase,and coarsen Sn phases started to appear.Heat preservation at high temperature or at low temperatue for long time both lead to the decreases of the elongation.Only the heat presesvation at low temperature for short time caused to the increase of elongation.(2)The multi-pass drawing of(37)8 mm→(37)7 mm→(37)6.5 mm→(37)6 mm→(37)5.5 mm under a certain temperature was performed by the optimum of the processing parameters.The influences of temperature and drawing speed on the drawing process of 8mm→7mm was studied.The optimum drawing temperature range was 45 ℃—55 ℃,while the optimum of drawing speed was 250 mm/min—500 mm/min.On this base,the effect of processing parameters on the multi-pass drawing of(37)7 mm→(37)6.5 mm→(37)6 mm→(37)5.5 mm was investigated as well.The multi-pass drawing process of(37)8 mm→(37)5.5 mm was steadily performed at last.(3)The annealing parameters on the Sn-58 Bi alloy after drawing was studied.Within the temperature range of 40—60 ℃,the strength of the drawing Sn-58 Bi alloy after aging increased with the time,while the elongation of this material increase at first then decreased.Through 50 ℃/0.5 h heat treatment on the alloy with the diameter of 5.5 mm,the single-pass drawing process of(37)5.5 mm→(37)5 mm was performed successfully.(4)By method of decreses of single-pass deformation and increases of the number of passes,the muti-pass of from(37)5.5 mm to(37)3.8 mm was performed steadily,and the effect of heat treatment on the drawing Sn-58 Bi alloy with the diameter of 5.5 mm on the following drawing process was investigated as well.