Análise comparativa do efeito do Al e Bi na liga Sn-9%Zn: parâmetros térmicos, microestrutura e propriedades mecânicas

Abstract

The search for viable solutions for replacing Sn-Pb alloys in soldered joints of electronic microcomponents has received special attention in recent years. In this sense, the Sn9wt.%Zn eutectic alloy is a promising alternative, as it has a eutectic temperature (198°C) similar to the Sn-38wt.%Pb alloy, low cost and excellent mechanical properties. However, these alloys exhibit disadvantages such as low corrosion resistance, which limits their practical use in soldering operations. One of the improvement alternatives is modification with aluminum (Al) and bismuth (Bi). These elements promote changes in microstructure and mechanical properties. Therefore, the present work aims to study the effect of adding Al (0.5% and 2.0% by mass) and Bi (3.0% and 6.0% by mass) on the thermal parameters of solidification (rate cooling rate-ṪL and growth rate-VL), as-cast microstructure, macrosegregation, mechanical properties and fracture modes of the directionally solidified (DS) Sn-9wt.%Zn-X alloys. The samples were characterized by Optical Microscopy (OM), Scanning Electron Microscopy (SEM), X-ray Fluorescence (XRF), Xray Diffraction (XRD), in addition to analysis of tensile mechanical properties. Thermodynamic calculations were performed by the CALPHAD method via Thermo-calc software, in order to evaluate the solidification sequence, phase compositions and phase transformation temperatures. The main results are highlighted: The additions of Al and Bi to the Sn-9wt.%Zn binary alloy increased the TL values and reduced the TE values, increasing the solidification range for the alloys examined. The Sn-9wt.%Zn-3.0wt.%Bi alloy was the only one to exhibit columnar growth in the macrostructure, with a ColumnarEquiaxial Transition (TCE) being observed in the rest of the alloys. Eutectic cell growth has been observed for the Sn-Zn-Al and Sn-Zn-Bi alloys, composed of the Sn+Zn+Al and Sn+Zn+Bi phases, respectively. Only the 2wt.%Al content was able to change the microstructural scale, subtly refining the eutectic cellular arrangement. On the other hand, all added Bi contents resulted in microstructural refinement. The morphologies of the Bi particles and the α-Zn phase (of the eutectic mixture) were shown to be dependent on the solute content and cooling rate. Al additions promoted a reduction in both the ultimate tensile strength (σu) and elongation-to-fracture (δ). On the other hand, Bi additions promoted an increase in σu and a reduction in δ. In comparison to the Sn9wt.%Zn alloy, it is noted that Al additions did not change the fracture mode, remaining ductile, while Bi additions induced a brittle fracture mechanism through cleavage.