ABSTRACT

Droplet-Based Manufacturing (DBM) process can produce a functional part directly from raw material. The DBM process consists of generating and depositing molten metal micro-droplets. Fundamental to the deposition process is droplet spreading on the substrate surface, for it determines geometric definition and properties of the product. 

The Uniform Droplet Spray (UDS) process is a novel method of DBM which generates droplets of a uniform size. The UDS process can accurately control the dynamic and thermal states of droplets, thereby creating an excellent opportunity to study the droplet impact behavior. 

The measurement techniques available today, however, are inadequate to monitor the high-speed impact behavior of micro-droplets due to limits on the temporal and spatial resolutions. A novel technique to capture the transient spreading of molten metal droplets has been developed using the Very Large Scale Integration (VLSI) technology. The microsensor consists of thin conducting lines (Au) finely spaced and mounted on a nonconducting base (Si). 

In order to verify the UDS process capability to control the impact conditions of droplets, molten tin droplets were sprayed with varied conditions such as orifice diameter, ejection pressure, and charging voltage. Velocity of the droplets was measured using a high speed camera at 6,000 frames per second. The simulation results were compared with the experimental results and the comparison showed fairly good agreement between them. 

Spreading behavior of a molten tin droplet was measured by depositing a 356 5m diameter droplet at the velocity of 4.84 m/s onto the microsensor. The initial speed of contact area expansion was found to be about 2.4 times the impact velocity. The spreading speed decreased dramatically afterwards and the spreading process lasted over 450 5s. The experimental results verified that the microsensor had satisfactorily monitored the transient spreading behavior of molten metal droplets.

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