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TECHNICAL PAPERS

# Experiments on Remelting and Solidification of Molten Metal Droplets Deposited in Vertical Columns

[+] Author and Article Information
M. Fang, S. Chandra, C. B. Park

Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Ontario M5S 3GB, Canada

J. Manuf. Sci. Eng 129(2), 311-318 (Sep 20, 2006) (8 pages) doi:10.1115/1.2540630 History: Received January 01, 2006; Revised September 20, 2006

## Abstract

Experiments were done to determine conditions under which vertical columns could be built by metal droplets landing sequentially on top of each other. Molten tin droplets ($0.6mm$ diameter) were deposited using a pneumatic droplet generator on an aluminum substrate. The primary parameters varied in experiments were those found to most affect bonding between droplets: droplet temperature $(250–345°C)$, substrate temperature $(60–200°C)$, and deposition rate $(1–15Hz)$. At lower deposition rates the substrate cooled down too much to induce remelting whereas at higher rates the tip of the column remained liquid and surface tension forces pulled it into a spherical mass. Assuming one-dimensional conductive heat transfer in a column a simple analytical model was developed to calculate the temperature at the tips of the column. It predicts that deposition frequency should be decreased as column height increases to hold the tip temperature constant. Droplet coalescence was best achieved when the tip temperature of a column was maintained at the melting point of the metal. Columns fabricated following the deposition frequency predicted by the model show good bonding between droplets and uniform diameter.

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## Figures

Figure 1

Schematic diagram of the experimental system

Figure 2

Schematic diagram of pneumatic droplet generator

Figure 3

Schematic diagram of the structure used to supply nitrogen gas to prevent oxidation

Figure 4

Photograph of the experimental system

Figure 5

Schematic diagram and photograph of the thermocouple support

Figure 6

Columns built with 40 droplets at temperatures of (a) 250, (b) 300, and (c) 345°C. Substrate temperature and deposition frequency were 60°C and 1Hz, respectively. Scale divisions are 0.5mm apart.

Figure 7

Columns built with 40 droplets at substrate temperatures of (a) 60, (b) 100, (c) 125, (d) 150, and (e) 200°C. Droplet temperature and deposition frequency are 300°C and 1Hz respectively. Scale divisions are 0.5mm apart.

Figure 8

Columns built with 40 droplets at deposition frequencies of (a) 1, (b) 2, (c) 4, and (d) 8Hz. Droplet temperature and substrate temperature are 300 and 60°C, respectively. Scale divisions are 0.5mm apart.

Figure 9

Model for droplet deposition on the tip of a column

Figure 10

Temperature distributions of a column and its surrounding air

Figure 11

Variation of deposition frequency required to keep the column tip temperature at Tm with column height. Thermal contact resistance Rt,c=10−4m2K∕W, droplet temperature Td=250°C and substrate temperature Tb=100°C.

Figure 12

Columns built with 100 droplets deposited with frequency (a) varying from 15to2Hz, following the curve of Fig. 9, (b) 2 Hz, and (c) 15Hz. Droplet temperature Td=250°C and substrate temperature Tb=100°C in all cases. Scale divisions are 1mm apart.

Figure 13

Cross sections of columns with (a) poor bonding and (b) good bonding

Figure 14

Deposition frequency variation with column height for various values of droplet temperature (Td). Substrate temperature Tb=60°C.

Figure 15

Columns built with 100 droplets (0.6mm) with droplet temperatures (Td) of (a) 250, (b) 275, (c) 300, and (d) 325°C. The substrate temperature was 60°C in all cases. Scale divisions are 0.5mm apart.

Figure 16

Columns built with 200 droplets (0.35mm) at droplet temperatures of (a) 250, (b) 275, (c) 300, and (d) 325°C Substrate temperature was 65°C for all cases. Scale divisions are 0.5mm apart.

Figure 17

Diameter prediction of the columns built with 0.6mm droplets and 0.35mm droplets, respectively

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