Repair and refurbishment of parts
In sensitive and strategic industries where the replacement of damaged parts is not possible, the preference of industries is to refurbish and extend the lifespan of such parts rather than replacing them. Nowadays, various methods are used to repair or refurbish industrial parts, each with its own advantages and disadvantages. For example, welding methods, especially Tungsten Inert Gas (TIG) welding and the modern welding method known as Plasma Transferred Arc (PTA) welding, as well as thermal spray methods, especially the Atmospheric Plasma Spray (APS) and High-Velocity Oxy-Fuel (HVOF) flame spray methods can be used.
1.Tungsten welding method with TIG neutral gas
According to the American Welding Society (AWS) nomenclature, the GTAW welding method is also known as TIG welding. In this process, a non-consumable electrode made of tungsten or its alloys is used to transfer electricity between the workpiece and the electrode and create an electric arc. In TIG welding, the molten pool is fully visible, and there is a possibility of oxidation in the heated area and even the electrode. Therefore, to protect the molten pool from oxidizing and destructive atmospheric elements, inert gases are used. Argon is the most commonly used inert gas in this welding process. That’s why it is also called Argon welding. Argon TIG welding can be performed both with and without a filler metal (welding wire). This method is suitable for a wide range of thicknesses of both ferrous and non-ferrous metals. Due to its many advantages, such as high welding quality, clean and delicate welds and the ability to weld thin sheets, Argon welding is widely used in various industries.
2. APS plasma spraying method
Plasma spraying is one of the most widely used methods in the industry and is a suitable method for creating wear-resistant and corrosion-resistant coatings with appropriate quality and durability. For example, it is commonly used to coat various ceramics on roller surfaces. The operation of this method is as follows: a plasma gun is made up of a copper anode and a tungsten cathode, as well as a material nozzle. First, a potential difference is established between the anode and the cathode, and then the gas between them is ionized. The gas is heated and its volume is increased by passing a current through it. At this point, the gas is ejected from the spray gun, and if the coating material is in powder form, the particles are fed into the flame through the nozzle, while if it is in the form of wire, it is fed into the plasma section from the back of the gun by several rollers. As the particles enter the plasma flame, they melt or semi-melt depending on their size and strike the substrate surface at high speed to form the coating. The ideal conditions are such that the temperature of the particles on the surface is equal to their melting point.
HVOF coatings have been used for over forty years to create hard and dense coatings for industrial applications. In this process, a combination of oxygen and various fuels such as hydrogen, propane, and kerosene is used. This method is one of the most advanced spray methods in which the fuel and oxygen are mixed together at a specific ratio and, after ignition, are expelled from the gun’s nozzle under high pressure and velocity. By using the shockwave inside the internal combustion chamber, the carbide particles are applied at a speed of about 1200 to 1500 meters per second (almost five times the speed of sound) onto the surfaces of the desired parts. The energy released from this reaction is converted into heat and pressure, which melts the powder and accelerates the powder particles. After stabilizing the explosion conditions, the powder, together with a carrier gas such as nitrogen or argon, is injected into the gun by a powder injection device at a controlled rate and is accelerated by the propelling gases due to continuous explosions. These accelerated particles reach the opposite surface, which is at an optimal distance in a very short time and create a completely smooth and resistant surface.
Among these methods, the use of high-energy radiation, such as laser, has much greater capabilities than conventional methods. Laser is used in various surface modification techniques, and very desirable results have been obtained. However, numerous studies are still being conducted to make more extensive use of this relatively clean process. Some of the problems with conventional methods include high porosity, improper adhesion to the substrate, and excessive heating of the substrate, leading to the disruption of the microscopic structure, distortion, and, ultimately, in some cases, fracture of the part.
Laser surface processes offer higher controllability in creating metallurgical structures, considering the geometry and surface characteristics of the parts. The process speed is relatively high, which creates a smaller heat-affected zone on the parts. Therefore, it has become a suitable method for coating and repairing industrial parts. Of course, laser cladding is also considered a high-yield process in the field of industrial part repair. In this process, an alloy of the same substrate or an alloy with better properties than the substrate is used to modify and repair the surface or defective area.
