SPRAY DEPOSITION OF A SN-40 WT PCT PB ALLOY WITH UNIFORM DROPLETS

An apparatus capable of producing and depositing uniform droplets (100 to 200 mum in diameter) was developed and used to study the relations hip between spray deposition parameters and the microstructures of Sn- 40 wt pct Pb alloy spray deposits. The sprays used in the study consis ted of uniform droplets, either 103 or 178 mum in diameter, that were in identical thermal and solidification states as they impacted the su bstrate. The thermal and solidification states of the uniform droplets were determined as a function of the flight distance (the distance fr om the metal pouring orifice) by model calculations and calorimetric m easurements assuming equilibrium solidification. Although a fair agree ment was noted between the model and the calorimetric measurements at small flight distances, corresponding to large liquid fractions, the c alorimetric measurements indicated 10 to 20 pct higher liquid fraction s at larger flight distances. The resultant microstructures comprised either a mixture of the Pb-rich and Sn-rich phases, both in an equiaxe d morphology, or a lamellar eutectic structure with a small amount of the Pb-rich primary phase in a coarse spherical morphology. The mostly lamellar eutectic structure resulted from an excessive enthalpy flux and/or slow heat extraction from the deposit. Fine, equiaxed, two-phas e microstructures and high deposit density resulted from optimal combi nations of droplet enthalpy, deposition rate, droplet size, and deposi t cooling rate which gave short local solidification times.