Spray and Spray Fusing Coating
Thermal spraying is a kind of metal surface treatment method which is used to spray the material in the molten condition and spray it on the surface of the parts by high-speed airflow. We refer to the special work surface as "coating", and the method of manufacturing coating is called "thermal spraying", which is the general term for spraying and fusing of various heat sources. There are two kinds of coatings for thermal spraying, spraying layer and spray fusing layer respectively. Following are the differences：
1. Workpiece heat conditions are different
Spray has no re-melting process, the workpiece surface temperature can always be controlled below 250 ℃, normally do not increase deformation and change the original organization of the workpiece. It has advantage to spray workpiece with complicated shape, thin wall, long axis and some important parts. Spray fusing need to make the powder layer melting, re-melting sintering workpiece and temperature can reach to more than 900 ℃, easily cause stress and deformation, many workpieces will occur annealing and unsafe annealing.
2. Bonding condition is different
The combination of spraying with substrate surface is mainly by mechanical lock, it also has micro-welding, bonding strength is not high, generally 20-65MPa. Spray fusing is through the powder layer melting with substrate surface to forming metallurgical bonding, the bonding strength can reach up to 343-441MPa.
3. Powder is different
The powder for the spray fusing must be self-fluxing powder, and the powder used for spraying is not restricted.
4. Spray layer structure is different
Spray fusing layer is uniform and dense, generally considered no pores and the spray coating has pore.
5. The bearing load performance is different
Spraying layer can’t bear impulse load and high contact stress, which can be applied to various surfac. The spray fusing coating has high bonding strength, it can bear impulse load and can be used in line contact application, and also can bear high contact stress.
When workpiece bearing high load, especially under the impulse load, coating should has high bonding strength or it can be used in corrosion medium, or high coating density requirement, spray fusing is advisable.
When the workpiece has high requirement of dimension accuracy and do not allow the deformation or not allowed to change its original quenching organization, and the workpiece is not affected by impulse load or only bear slightly impulse load, the bonding strength requirement is not high, spray method is advisable.
