molten metal flame spraying, a thermal spraying process variation
in which the metallic material to be sprayed is in the molten
condition. (1)
Photo courtesy of Westaim Ambeon
It has long been recognized that fluids may
be broken up into very fine particles by a stream of high velocity
gas emanating from a nozzle. Early experiments using this atomizing
approach appear to have been directed at producing metallic powders
rather than coatings. It was left to Schoop to appreciate the
possibility that a stream of metallic particles, formed from a
molten source, could produce a coating. Myth has it that Schoop
developed the concept when playing "soldiers" with his
son and observing the deformation of lead pellets being fired
from a toy cannon against a brick wall. Whatever the rationale,
it can be stated that the pioneer work of Schoop resulted in the
discovery and development of metal spraying and subsequently the
"Thermal Spray Process".
The first spray technique developed by Schoop
was the outcome of experiments in which molten metal was poured
into a stream of high velocity gases. Schoop's apparatus consisted
of a compressor supplying air to a heated helical tube. The heated
air was used to pressurize a crucible filled with molten metal
and eject it out as a fine spray that would adhere to a suitable
surface. This system was bulky, primitive and inefficient; however,
the concept did lead to the development of portable and user friendly
equipment.
There are no further accounts of molten metal
spraying by Schoop, it appears that his efforts were directed
at developing and improving powder and wire flame spraying. However,
work by others continued as a 1924 Dutch patent, describing equipment
for spraying low melting point metals, was granted to Jung and
Versteeg (2). Mellowes Ltd commercialized the process
in the UK. Their system consisted of a gun, a furnace, an air
compressor and a fuel supply. The gun had many air and gas valves,
a heating chamber (burner), nozzle, handle and a melting pot.
The pot was bulky having the ability to store 1.8 kg (4 lb) of
molten lead. The pot sat atop the heating chamber, which was similar
in construction to a Bunsen burner. Compressed air, fed to the
burner, intensified the flame. The handle jutted out and downward
from the pot; it was insulated using wood and asbestos. Metal
exited the pot through a front orifice where it was directed into
a nozzle. Compressed air surrounded the nozzle, atomizing the
molten metal and propelling it to the surface to be coated.
The molten metal process has advantages and
disadvantages. Advantages include: cheap raw materials; use of
inexpensive gases; and, gun design is very basic. Noteworthy disadvantages
are: gun is cumbersome to use in the manual mode, can only be
held in a horizontal plane; high maintenance due to high temperature
oxidation and molten metal corrosion; and, useful only with low
melting temperature metals.
Uses for the molten metal thermal spray process
include the fabrication of molds, masks and forms for the plastics
industry, using low melting point bismuth based alloys (the Cerro
family of alloys); the deposition of solder alloys to joints that
would be coalesced using torches or ovens; and, the production
of metal powders.
