Parâmetros térmicos de solidificação, microestrutura e resistência mecânica de ligas eutéticas Sn-0,7%Cu-(xNi)

Abstract

Sn-Pb traditional solder alloys were overly utilize in electronic devices industry, since their properties and features use were adequate and well known. In the last years many restrictions and directives were and have been designed with a view to minimize or eliminate the use of this metal in the electronic industry. Thereby, several lead-free solder alloys emerged. Among these alloys, eutectic Sn-0.7wt%Cu (TE=227°C) appears as potential candidate to replace eutectic Sn-37wt%Pb alloys, because present low cost when compared with the other lead-free solder alloys. Fatigue resistance and fluidity are quite similar compared with those obtained for the traditional Sn-Pb alloys. Small alloying additions of Ni in Sn-Cu eutectics between levels 20-1000ppm Ni can improve welding properties such as good fluidity, reliability, fine degree of wetting, ensuring suitable mechanical resistance of solder joint. Thus, the main goals of present study are investigate the influence of the Ni microadditions on solidification thermal parameters (transient interfacial heat transfer coefficient - hi), eutectic growth rate v and cooling rate - Ṫ), microstructure features (dendritic and cellular spacings, λ1 and λC) and tensile properties in directionally solidified Sn-0.7wt%Cu, Sn-0.7wt%Cu-0.05wt%Ni e Sn-0.7wt%Cu-0.1wt%Ni alloys under unsteady-state conditions against AISI steel carbon 1020 water-cooled bottom part. Measurements of hi coefficient showed that Ni microadditions (500 and 1000ppm of Ni) in the eutectic Sn-0.7wt%Cu affected significantly the fluidity levels. The microstructures obtained for the Sn-0,7wt%Cu-(xNi) alloys presented eutectic colonies/cells along the casting length, with dendritic regions only on first positions close to the bottom. Small Ni additions promoted increase on ultimate tensile strength (σu) and elongation (δ), with the Sn-0.7wt%Cu-0.05wt%Ni alloy corresponding to the better combination of σu and δ, 36.6MPa and 12.1%, respectively.