Abstract: This paper introduces the development of laser cladding technology, which began in the 1970s and is now widely used in material surface modification technology. According to the different ways of supplying cladding materials, the process methods are divided into laser cladding alloy pre-setting method and alloy synchronous powder feeding method. The cladding materials are usually added in three forms: powder, wire and plate, with powder being the most commonly used. Laser cladding powder is mainly divided into self-fluxing alloy powder, ceramic powder and composite powder according to its composition. Self-fluxing alloy powders are commonly used in Ni-based, Co-based and Fe-based; ceramic powders are mainly silicide ceramic powders and oxide ceramic powders, which have strong wear and corrosion resistance and are often used to prepare high-performance cladding layers.
1. Laser cladding technology
The research on laser cladding technology began in the 1970s, when AVCO of the United States conducted relevant research on many easily worn parts of automobile engines. In 1981, Rolls.Royce of the United Kingdom successfully coated the cobalt-based alloy surface on the jet engine blades and significantly improved their wear resistance. Since this new technology has great development potential and can generate great economic benefits, it has been widely promoted and applied in production.
Laser cladding is to add materials of different components to the substrate, use high-energy laser beam to irradiate the substrate, and form a thin layer of cladding powder and substrate. This thin layer melts quickly and solidifies, and the dilution of the substrate to the cladding layer is extremely low. Therefore, the cladding layer and the substrate have good metallurgical bonding, and can prepare surface protective coatings with heat resistance, corrosion resistance, wear resistance, oxidation resistance, fatigue resistance or optical, electrical and magnetic properties.
2. Laser cladding process method
According to the different supply methods of cladding materials, laser cladding process methods are divided into the following two types:
2.1 Alloy pre-setting method
The alloy pre-setting method is to place the pre-coated material on the surface of the substrate by some methods, and then use high-energy laser beam irradiation. The coating surface absorbs energy to rapidly heat up, vaporize and melt the cladding part. After the laser beam leaves, the cladding layer and the substrate show good metallurgical bonding.
The cladding materials are usually added in three forms: powder, wire, and plate, among which powder is the most commonly used. The presetting method generally includes bonding method and thermal spraying method. For powder materials, both presetting methods can be used. The main advantages of thermal spraying are high spraying efficiency, easy control of uniformity of deposition thickness, and firm bonding with the substrate, but the powder utilization rate is low, it is limited by the shape of the workpiece and the cost is relatively high. The bonding method is to use a binder to mix the powder into a paste on the surface of the base material. The disadvantage of this method is that it is inefficient and it is difficult to obtain a coating with uniform thickness. It may hinder melting or cause transitional dilution; at the same time, due to the poor thermal conductivity of the deposited layer, it will consume more energy; it is usually only applicable to workpieces with a small cladding area and is mostly used in laboratories.
For wire alloy materials, both the presetting bonding method and the thermal spraying method can be used for spraying, but the plate alloy materials mainly use the presetting bonding method.
2.2 Alloy synchronous powder feeding method
The alloy synchronous powder feeding method is to directly feed the material into the laser working area so that the feeding and cladding are completed at the same time. By using the laser, the cladding material and the substrate are melted together and then condensed into a cladding layer. This method can make full use of the laser energy, greatly reduce the unevenness of the cladding layer, and also reduce the thermal effect of the laser on the substrate.
The alloy synchronous powder feeding method is relatively simple, with less consumables and good controllability. It is a good method in practical applications. Compared with the preset method, the synchronous powder feeding method is the development trend of laser cladding technology.
3. Current status of laser cladding material system
Laser cladding powder is mainly divided into self-fluxing alloy powder, ceramic powder and composite powder according to the different material composition.
3.1 Self-fluxing alloy powder
Self-fluxing alloy powder refers to alloy powder with Si, B and other elements with strong deoxidation and self-fluxing effects. Currently, Ni-based, Co-based and Fe-based self-fluxing alloy powders are commonly used.
Ni-based alloy powder is widely used, with reasonable cost performance and good material properties, such as good toughness, oxidation resistance and other properties, so it is the most studied and widely used in laser cladding materials. Ni-based self-fluxing alloy powder can be divided into two alloy series: Ni-B-Si and Ni-Cr-B-Si. Ni-based self-fluxing alloy powder is mainly suitable for components that require local wear resistance and heat corrosion resistance. The power density of Ni-based cladding is higher than that of iron-based. The disadvantage of Ni-based alloy powder is that it has poor high temperature resistance. Ni60 and Ni45 are commonly used in Ni-based alloy powders.
Co-based alloy powder has good high temperature resistance, strong wear resistance and corrosion resistance, and is often used in petrochemical and metallurgical fields. In addition, cobalt-based powder alloy has good wettability when melted, and its melting point is lower than that of carbide. After heating, the Co element melts first and forms a new phase first when the alloy solidifies, obtaining a smooth and flat cladding coating, which improves the bonding strength between the cladding layer and the substrate. At present, the main elements of the commonly used Co-based alloys are Ni, C, Cr and Fe, among which Ni is used to reduce the thermal expansion coefficient of the Co-based alloy cladding layer, reduce the melting temperature range of the alloy, effectively inhibit the cracking of the cladding layer, and improve the wettability of the cladding layer to the substrate. The disadvantage of Co-based alloy powder is that it is relatively expensive.
Fe-based alloy powder is suitable for wear-resistant parts with low temperature requirements (temperature less than 400℃), and the substrate is mostly cast iron and low-carbon steel. Its biggest advantage is low cost and strong wear resistance. The main elements of Fe-based alloys are Ni, B, Si and Cr, among which B, Si and Cr are used to improve the hardness and wear resistance of the cladding layer, and Ni is used to improve the crack resistance of the cladding layer.
Since iron-based alloys are low in cost, they are often used instead of nickel-based alloys. Compared with Ni-based alloys, the disadvantage of iron-based alloys as laser cladding layers is that the cladding layer has slightly poor toughness.
In summary, Ni-based or Co-based alloys have good self-fluxing and oxidation resistance, and high corrosion resistance. Ni-based or Co-based alloy powders have better self-fluxing properties than Fe-based alloy powders, but their prices are also higher than Fe self-fluxing alloy powders. Although Fe-based alloy powders are cheaper than Ni-based or Co-based alloy powders, they have poor self-fluxing properties and poor oxidation resistance. When using them, self-fluxing alloy powders should be selected reasonably.
3.2 Ceramic powders
There are two main types of ceramic powders: silicide ceramic powders and oxide ceramic powders, of which oxide ceramic powders are the most commonly used. Ceramic powders have many advantages as cladding layers, such as strong wear resistance and corrosion resistance, so ceramic powders are often used to prepare high-performance cladding layers; currently, research on bioceramic materials is also a hot topic. Laser cladding metal ceramics can be clad on the metal surface with a layer of ceramics through the action of a high-energy laser beam. However, the thermal expansion coefficient, elastic modulus and thermal conductivity of this material are quite different from those of the substrate, and the mismatch of these properties will cause cracking and voids in the cladding layer.
