by
Christian Henry Passow
S.B. in Mechanical Engineering
Massachusetts Institute of Technology
1990
Submitted to the Department of Mechanical Engineering
in Partial Fulfillment of the
Requirements for the Degree of
MASTER OF SCIENCE IN MECHANICAL ENGINEERING
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 1992
ABSTRACT
An apparatus has been built that generates sprays
of uniformly sized, electrically charged, molten metal droplets.
All of the droplets have the same velocities and thermal histories.
For a given set of spraying conditions, the mass and enthalpy flux of the
spray can be controlled by varying the charge on the droplets and the distance
from the orifice. The droplets are created by the controlled break-up
of a laminar jet, a technique commonly used in ink-jet printers.
Models have been assembled that describe the different segments of the
process and characterization experiments have been performed to verify
the models.
In spray forming, the deposit porosity, deposit
microstructure, and process yield depend on four parameters: the
mass flux of the spray, the enthalpy flux of the spray, the substrate conditions,
and the impact states of individual droplets. It is easier
to control these parameters in uniform droplet sprays than it is in the
gas atomized sprays that are typically used in spray forming. A series
of uniform droplet deposition experiments was performed to determine the
effect of droplet impact state and spray mass flux on deposit characteristics.
It was found that fully dense deposits with fine grained microstructures
(5-10 u m) can be formed using 103 and 178 u m diameter Sn40wt40%Pb droplets
with enthalpies corresponding to solid fractions around 29% and 17 %, respectively,
and at depostion rates of 0.17 and 0.25 mm/sec, respectively.