As far as spray forming is concerned, a large number of
particles with a high temperature and high velocity impact on
spray mould or solidified flattening particles, splashing, flattening,
solidifying and piling up to form a coating with a certain
porosity and layer interlaping structure. It can be divided
into three layers: initial layer, inter layer and back layer. The
rough back layer is nature-forming layer, which is available to
the post and reinforcing process. The inter layer which determines
the forming quality directly is regarded as a tie between
the initial layer and the back layer. The initial layer which contacts
spray mould and can duplicate its surface pattern exactly
is the working surface of formed-part, and its precision and
surface roughness are determined by pretreatment quality of
the spray mould. Interface bonding strength between coating
and spray mould should be properly controlled, and surface
quality of spray mould should be improved with the greatest
extent by assuring a certain deposit rate and demoulding
perfectly.
The microstructure of the coating cross-section is shown
in Fig. 7. The results show that due to the high temperature
and high velocity, atomizing particles are fully flattened
when impacted on spray mould, the coating is in shape of the
evident sandwich, and the texture is very compact. Because
the 3Cr13 belongs to martensite stainless steel, the M-pint
is about 300 ◦C, and spraying is a high-speed solidification
process, and then the degree of supercooling is very high, so
a great deal of martensite is produced instantaneously during
spraying.
Dimension together with velocity and temperature of
atomizing particles determine density and bonding strength
of the coating. Experimental results show that atomizing
particle size decreases and velocity increases with the
increasing of atomizing pressure, resulting in improving
coating quality. It can be seen in Fig. 8 that the layer structure
changes from wavilness to lamellae and its thickness
decreases with the increasing of atomizing pressure because