Characterization of zinc based high temperature solder alloys with chromium and molybdenum additions

Abstract

Initiative of development and characterization of Pb free high temperature solders was taken. Two systems such as Zn-xMo (x= 0.4, 0.6 and 0.8 wt% Mo) and Zn-xCr (x= 0.2, 0.4 and 0.6 wt% Cr) were chosen for this study. Samples of the selected compositions were fabricated through casting in a permanent metal mold. Their microstructural, thermal, mechanical and electrical characteristics were determined using several analytical techniques such as Scanning Electron Microscope (SEM), Differential Thermal Analyzer (DTA), X-Ray Diffractometer (XRD), Thermomechanical Analyzer (TMA), Universal Testing Machine (UTM), Hardness tester, Conductivity Meter etc. Experimental findings revealed that with the addition of Mo and Cr particles grain refinement occurred in both Zn-xMo and Zn-xCr systems which was evident in the scanning electron micrographic images. Energy-dispersive X-ray spectroscopy (EDS) spots in several zones of the microstructure confirmed the presence of Mo and Cr particles throughout the respective structures. XRD analysis showed a very little or no shift in the peak pattern compared to standard Zn pattern indicating no new phase formation. However, crystallite size calculation using Scherrer equation showed that, average crystallite size decreases with increasing Mo content and it ranged from around 34.0-29.4 nm in case of Zn-xMo system, while the average crystallite size of Zn-xCr system ranged between 29.7 and 28.8 nm. Brinell hardness showed an incremental trend with increasing Mo content. Similar results were obtained in Zn-xCr system. Brinell hardness number of Zn-xCr increased from 33.98 in Zn-0.2 Cr to 42.74 in case of Zn-0.6 Cr alloy. Similarly, hardness increased from 29.71 to 41.28 in case of Zn-0.4 Mo to Zn-0.8 Mo respectively. Tensile strength of unalloyed zinc increased from 31.09 MPa to 33.04 MPa when alloyed by 0.8% Mo. In a similar manner, strength of Zn-Cr solders increased from 40.53 to 47.07 MPa due to an increase in 0.4% Cr than that of the former. Fractographic analysis of the tensile fracture surfaces with SEM indicated mixed (neither completely ductile nor completely brittle) fracture mode of both Zn-xMo and Zn-xCr systems. Coefficient of thermal expansion (CTE) of Zn-Cr system was found to vary from 4.5-5×10-5 (1/ºC) while in Zn-Mo system it was 4.14-2.64×10-5 (1/ºC). Electrical conductivity of both Zn-xMo and Zn-xCr decreased with increasing Mo and Cr contents respectively. The relative conductivity of Zn-xMo and Zn-xCr varied from 27.35 to 26.55 and 25.3 to 22.7 %IACS (International Annealed Copper Standard) respectively in the selected composition range. DTA results suggested that onset of melting started at temperature below the melting temperature of pure Zn. It was found that, the onset temperature of melting of all three compositions of the Zn-xMo system was around 416ºC whereas, the finishing temperature of melting was around 423ºC indicating a melting range of around 7ºC. On the other hand, DTA graphs exhibited that, the onset of melting of the Zn-xCr alloys started at around 412ºC and it finished near about at 418ºC showing a melting/solidification range of about 6ºC. From the experimental findings it can be said that Zn-xMo as well as Zn-xCr systems might be promising candidate for high temperature solders in replacement of Pb based solders.