Experimental development of microstructure, microsegregation and tensile mechanical properties of non-ferrous alloys during upward and downward unsteady-state directional solidification

Abstract

The solidification processes of metals are pre-programmed in order to produce components according to the project demands. These requirements are regularly associated with as-cast microstructure features. It is well known that melt flow affects the final as-cast structure. The flow continues even after pouring procedures, essentially moving in the interdendritic/intercellular channels or in the open liquid where transformation temperature is not reached. Interdendritic melt convection may be originated due to the thermal and solutal gradients immediately ahead the solidification front. This melt flow may interfere in the local solute distribution not only close to the liquid/solid interface but also between the dendritic arms. The melt interacts with the mush and its solutal and thermal diffusion layers, resulting in a situation where both temperature and solute concentration gradients play significant roles. Another key parameter to be considered for solidified components is the wetting behavior of a melted alloy against a solid surface or mold. During soldering, for instance, alloy melts and wets over a required substrate area. The wettability defines the alloy performance and wetting conditions during soldering. Good wettability means uniform, smooth, unbroken and adherent coating of base metal. The final as-cast structures of lead-free solder alloys like Sn-Cu, Sn-Zn and Sn-Bi are strongly dependent on metal/mold physicochemical affinity. The wettability can be connected with the molten alloy fluidity. What is meant by fluidity in the present work (and in the foundry sense) is the ability to fill a refractory or metallic mold, and not the reciprocal of viscosity. The present purpose aims to provide experimental facilities and methods which can lead to a better comprehension of: i. microsegregation evolution in Al-Cu and Sn-Pb alloys castings directionally solidified with and without induced convection under unsteady-state heat flow conditions; ii. the thermal and microstructure effects of nickel additions in alternative solder Sn-Cu and Sn-Ag-Cu alloys with the use of several mold materials like copper, stainless steel, low carbon steel and tin plate. (AU)