Effect of Reinforcements on the Structure and Properties of Lead Free Tin Zinc Alloys

Abstract

In the present study 3.0 weight percent Bi is added in the Sn-9Zn eutectic solder alloy to investigate the effect of Bi on the microstructure, mechanical and thermal properties of Sn-9Zn alloys. Effect of micron size Ni powder addition on the microstructure, mechanical and thermal properties of both the Sn-9Zn and Sn-8Zn- 3Bi alloy systems are investigated. Nickel is added in three different weight percentages of 0.25, 0.5 and 1. The melting temperature and solidification range of the solder alloys are determined by Differential thermal Analysis (DTA). Microstructure of all the alloys are investigated with an optical microscope and a scanning electron microscope. Energy dispersive X-ray (EDX) analysis is used to analyze the phases present in the structure. X-ray Diffraction (XRD) is performed to determine the compounds present in the solder alloys. To determine the mechanical properties at first Brinell Hardness test is carried out. Then tensile tests are carried out on an Instron testing machine (Instron 3369 Universal Testing Machine) at a rate of 3.00 mm.min-1 at 298K. Shear strength of soldered joint is measured using an Instron testing machine (Instron 3369 Universal Testing Machine) at a rate of 5.00 mm.min-1 at 298K. It is found that melting temperature of Sn-9Zn solder alloy decrease with the addition of 3% Bi. Melting temperature is increased with the addition of Ni in both the Sn-9Zn and Sn-8Zn-3Bi alloy. Solidification range of the alloys is increased with the addition of both the Bi and Ni. The structure of newly developed solder alloys are fine needle-like α-Zn phase with some Zn spheroids distributed in the β-Sn matrix. Bi rich particle are observed in Bi containing alloys. The size of the Zn needle are found to increase with the addition of Bi in Sn-9Zn alloy. It is also found that addition of small amount of Ni (0.25%) refines the Zn needles of both the Sn-9Zn and Sn-8Zn-3Bi alloys. However, coarsening of Zn needles occurs with increasing Ni content in both the alloys. From XRD analysis it is found that all the alloys consist of only two phases i.e. a body centered tetragonal β-Sn matrix phase and a secondary phase of hexagonal Zn. But both the Bi and Ni addition causes the Zn precipitation in the β-Sn ix matrix. Additon of Bi in Sn-9Zn alloy increases the hardness and ultimate tensile strength. Ni addition in both the Sn-9Zn and Sn-8Zn-3Bi alloys also increases the hardness and ultimate tensile strength. However, for higher than 0.5% Ni addition, hardness is slightly decreased for both the Sn-9Zn and Sn-8Zn-3Bi alloys. It is also found that the tensile strength decreases for higher Ni addition. The maximum ultimate tensile strength is observed for 0.25% Ni addition in Sn-9Zn alloy and 0.5% Ni addition in Sn-8Zn-3Bi alloy. The improvement in tensile strength is achieved with the sacrifice in elongation. Shear strength of soldered joint is found maximum for 0.5 wt.% Ni addition in both the Sn-9Zn and Sn-8Zn-3Bi alloys. The results indicate that the addition of Bi improves the mechanical properties and lowers the melting point of Sn-9Zn alloy. The addition of Ni in both the Sn-9Zn and Sn-8Zn-3Bi alloy systems initially refines the Zn needles and improves mechanical properties. From the present study it can be suggested that a better combination of thermal and mechanical properties may be achieved by the addition of 0.25% Ni in Sn-8Zn and 0.5% Ni in Sn-8Zn-3Bi alloy.