**1.2 State of art**

The formation of coatings by thermal spraying in electric arc is carried out according to a well-precisely mechanism, presented in **Figure 3**, composed of the following stages, [18]:


Each stage is accompanied by a series of physical phenomena and interactions, such as: sequential melting of the coating material by Joule - Lentz effect; interaction between gas and molten metal, having as result the droplet dispersion ("atomization"), in micron size particles; propelling the particles formed onto the surface of the substrate; particle-substrate interaction (impact); solidification of the sprayed particles, [22].

**141**

spraying processes.

*Hard Alloys with High Content of WC and TiC—Deposited by Arc Spraying Process*

velocity of the particles towards the substrate surface [25, 26].

continuous decrease of speed and temperature.

and control system, which are expensive, [31].

The objectives of our study are the following:

• establishing the optimal process parameters.

(WC and TiC), as additional material.

The interaction between the molten metal and the uninsulated jet of gaseous fluid, under pressure, at ambient temperature, has as effect the heat transfer by forced convection between droplets (particles) and jet, fact which can determine the apparition of rapid solidification phenomenon in the droplet (particle).

It is known that the properties of the coatings produced by electric arc thermal spraying are closely related to the velocity and the temperature of the particles before the impact [23, 24]. The two parameters that characterize the particles from a thermodynamic point of view are mainly influenced by the intensity of the electric arc and the pressure of the carrier gas flow. The intensity of the electric current determines the melting/overheating temperature of the filler material. The carrier gas pressure, in close connection with its velocity, influences the displacement

The studies carried out by Zhao et al. [27], on the flow phenomenon of an uninsulated gas jet which pass through a nozzle, demonstrates the fact that the velocity and the temperature of the gaseous fluid decrease continuously along the jet. Due to the interaction between the fluid and the sprayed particles, it can be said that the molten particles inside the spray jet have a transient behavior, characterized by the

A large category of ductile metal materials, such as: aluminum, zinc, copper, bronze, steels as well as numerous wire drawing alloys can be sprayed by this technique, [28]. The exploitation of high temperature of the electric arc directed the scientific research towards the extension of the input materials range of use, by removing the technological barrier imposed by electrical ductility and conductivity. Thus, appeared the tubular wires, which have the exterior formed by a metal mantle characterized by high conductivity, and the interior is filled with powders of fragile materials, [29]. Recently, have been developed wires manufacturing technologies by crimping, in which the ductile material is folded and inside the folds are injected powders of hard materials. [30]. The widening of the spectrum of use of the input materials determined that the arc spraying process to be competing with the other spraying techniques (plasma, flame) in the technology of restoring of large surfaces. The advantages of the arc spraying process are [2, 18]: efficiency - from an energy point of view (power used between 5 ÷ 10 kW), high productivity (15 ÷ 45 kg/h), no need to preheat the substrate, uses cheap equipment compared to other

The main disadvantages of this spraying process are related to the high porosity of the deposits (over 18%) and to carrying out deposits resistant to abrasive wear (WC, TiC) only by using high-performance equipment, equipped with command

The purpose of our research is to present the technology of obtaining coatings of alloys resistant to abrasive wear - containing ultra-hard chemical compounds (WC and TiC), arc thermal the process using a classic spray device provided with a system of conical nozzles and tubular wire with a containing ultra-hard compound

• the realization of a system of conical nozzles able to melt the additional

• the characterization of the ultra-hard alloys coatings obtained with the aforementioned spraying device equipped with the conical nozzle system,

material (which contains ultra-hard compounds: WC and TiC) and to transfer, to the formed particles, the high speed necessary to obtain dense coatings,

*DOI: http://dx.doi.org/10.5772/intechopen.94605*

#### *Hard Alloys with High Content of WC and TiC—Deposited by Arc Spraying Process DOI: http://dx.doi.org/10.5772/intechopen.94605*

The interaction between the molten metal and the uninsulated jet of gaseous fluid, under pressure, at ambient temperature, has as effect the heat transfer by forced convection between droplets (particles) and jet, fact which can determine the apparition of rapid solidification phenomenon in the droplet (particle).

It is known that the properties of the coatings produced by electric arc thermal spraying are closely related to the velocity and the temperature of the particles before the impact [23, 24]. The two parameters that characterize the particles from a thermodynamic point of view are mainly influenced by the intensity of the electric arc and the pressure of the carrier gas flow. The intensity of the electric current determines the melting/overheating temperature of the filler material. The carrier gas pressure, in close connection with its velocity, influences the displacement velocity of the particles towards the substrate surface [25, 26].

The studies carried out by Zhao et al. [27], on the flow phenomenon of an uninsulated gas jet which pass through a nozzle, demonstrates the fact that the velocity and the temperature of the gaseous fluid decrease continuously along the jet. Due to the interaction between the fluid and the sprayed particles, it can be said that the molten particles inside the spray jet have a transient behavior, characterized by the continuous decrease of speed and temperature.

A large category of ductile metal materials, such as: aluminum, zinc, copper, bronze, steels as well as numerous wire drawing alloys can be sprayed by this technique, [28]. The exploitation of high temperature of the electric arc directed the scientific research towards the extension of the input materials range of use, by removing the technological barrier imposed by electrical ductility and conductivity. Thus, appeared the tubular wires, which have the exterior formed by a metal mantle characterized by high conductivity, and the interior is filled with powders of fragile materials, [29]. Recently, have been developed wires manufacturing technologies by crimping, in which the ductile material is folded and inside the folds are injected powders of hard materials. [30]. The widening of the spectrum of use of the input materials determined that the arc spraying process to be competing with the other spraying techniques (plasma, flame) in the technology of restoring of large surfaces.

The advantages of the arc spraying process are [2, 18]: efficiency - from an energy point of view (power used between 5 ÷ 10 kW), high productivity (15 ÷ 45 kg/h), no need to preheat the substrate, uses cheap equipment compared to other spraying processes.

The main disadvantages of this spraying process are related to the high porosity of the deposits (over 18%) and to carrying out deposits resistant to abrasive wear (WC, TiC) only by using high-performance equipment, equipped with command and control system, which are expensive, [31].

The purpose of our research is to present the technology of obtaining coatings of alloys resistant to abrasive wear - containing ultra-hard chemical compounds (WC and TiC), arc thermal the process using a classic spray device provided with a system of conical nozzles and tubular wire with a containing ultra-hard compound (WC and TiC), as additional material.

The objectives of our study are the following:


*Welding - Modern Topics*

ruption of a circuit, [20, 21].

**1.2 State of art**

following stages, [18]:

• heating the coating material, used as wire;

*Coatings formation mechanism by electric arc thermal spraying.*

• forming a drop of molten material;

formation.

The wire feed mechanism is positioned directly on the gun or outside of it and has the role of directing the wire from the coils towards the area of electric arc

The dispersion of the molten droplets determines the interrupting of the circuit respectively of the electric arc. The electric arc priming is carrying out by advancing the wires in the melting area, where the medium is strongly ionized. The phenomenon has a periodic character, being composed of melting sequences of the input material followed by interruptions. In this case, the electric arc is a short-circuit arc, intermediate between the usual arc and the "breaking" arc produced at the inter-

The formation of coatings by thermal spraying in electric arc is carried out according to a well-precisely mechanism, presented in **Figure 3**, composed of the

• transformation of the coating material into fine particles - atomization;

• displacement of the melted particles towards the surface of the substrate;

Each stage is accompanied by a series of physical phenomena and interactions, such as: sequential melting of the coating material by Joule - Lentz effect; interaction between gas and molten metal, having as result the droplet dispersion ("atomization"), in micron size particles; propelling the particles formed onto the surface of the substrate; particle-substrate interaction (impact); solidification of the sprayed

• the impact of the particles with the surface of the substrate;

• coating formation by successive particles depositions.

The spray head includes the wire guides and the nozzle through which the compressed air passes - symbolically called carrier gas, [2]. During the device functioning, the nozzle is placed in the lower part of the melting zone, before the contact point of the wire electrodes, usually called "arc point". The role of the spray head is to direct the entrainment gas in the area where the electric arc is formed, in order to produce the division (atomization) of the droplet of molten filler material

into particles, which it propels on the surface of the substrate, [19].

**140**

**Figure 3.**

particles, [22].
