Definition Introduction

Laser cladding technology refers to the process of placing selected coating materials on the surface of a coated substrate using different filler methods. After laser irradiation, a thin layer of coating material is melted simultaneously with the substrate surface and rapidly solidified to form a surface coating with extremely low dilution and metallurgical bonding with the substrate material. This significantly improves the wear resistance, corrosion resistance, heat resistance, oxidation resistance, and electrical properties of the substrate material surface.

Research Progress

Laser cladding technology is a new surface modification technology that emerged in the 1970s with the development of high-power lasers. It refers to the rapid heating and melting of alloy powder or ceramic powder with the substrate surface under the action of a laser beam. After the beam is moved away, self-excited cooling forms a surface coating with extremely low dilution rate, which is metallurgically bonded to the substrate material. This is a surface strengthening method that significantly improves the wear resistance, corrosion resistance, heat resistance, oxidation resistance, and electrical characteristics of the substrate surface. For example, after carbon tungsten laser cladding on 60 # steel, the hardness can reach more than 2200HV, and the wear resistance is about 20 times that of the substrate 60 # steel. After laser cladding CoCrSiB alloy on the surface of Q235 steel, its wear resistance was compared with the corrosion resistance of flame spraying, and it was found that the former had significantly higher corrosion resistance than the latter

Laser cladding technology is a new technology with high economic benefits. It can prepare high-performance alloy surfaces on inexpensive metal substrates without affecting the properties of the substrate, reducing costs, and saving precious and rare metal materials. Therefore, advanced industrial countries around the world attach great importance to the research and application of laser cladding technology

Equipment and process characteristics

The lasers used in laser cladding mainly include CO2 lasers and solid-state lasers (mainly including disc lasers, fiber lasers, and diode lasers. Old style lamp pumped lasers have gradually faded out of the market due to low photoelectric conversion efficiency and cumbersome maintenance). Scholars at home and abroad have conducted extensive research on continuous CO2 laser cladding The development of high-power solid-state lasers is rapid, mainly used for surface modification of non-ferrous alloys. According to literature reports, when CO2 laser is used for aluminum alloy laser cladding, the aluminum alloy substrate is prone to deformation and even collapse under CO2 laser irradiation conditions. Solid state lasers, especially disc lasers, have an output wavelength of 1.06 μ m, which is one order of magnitude smaller than CO2 lasers, making them more suitable for laser cladding of such metals.

Laser cladding can be divided into two categories based on the different powder feeding processes: powder pre setting method and synchronous powder feeding method. The two methods have similar effects. The synchronous powder feeding method has the advantages of easy automation control, high laser energy absorption rate, no internal pores, especially in the fusion of metal ceramics, which can significantly improve the anti cracking performance of the fusion layer and enable the uniform distribution of hard ceramic phases in the fusion layer.

1. Laser cladding has the following characteristics:

(1) The cooling rate is fast (up to 106K/s), belonging to a rapid solidification process, which can easily obtain fine crystal structures or generate new phases that cannot be obtained in equilibrium states, such as unstable phases, amorphous phases, etc.

(2) The dilution rate of the coating is low (generally less than 5%), and it forms a strong metallurgical or interfacial diffusion bond with the substrate. By adjusting the laser process parameters, a good coating with low dilution rate can be obtained, and the coating composition and dilution degree are controllable;

(3) The heat input and distortion are relatively small, especially when using high power density rapid melting, the deformation can be reduced to within the assembly tolerance of the parts.

(4) There are almost no restrictions on powder selection, especially when depositing high melting point alloys on low melting point metal surfaces;

(5) The thickness range of the cladding layer is large, with a single powder feeding coating thickness of 0.2~2.0mm,

(6) Capable of selective fusion deposition, with low material consumption and excellent cost performance ratio;

(7) Beam aiming can fuse hard to reach areas;

(8) The process is easy to automate.

Very suitable for repairing the wear and tear of common vulnerable parts in oil fields.

2. Differences and Similarities between Laser Cladding and Laser Alloying

Laser cladding and laser alloying are both fast reading fusion processes generated by high-energy density laser beams, forming alloy coatings with completely different compositions and properties on the surface of the substrate that fuse with the substrate. The two processes are similar, but there are essential differences, mainly as follows:

(1) During the laser cladding process, the cladding material is completely melted, while the melted layer of the substrate is extremely thin, thus having minimal impact on the composition of the cladding layer. Laser alloying, on the other hand, involves adding alloy elements to the melted cladding layer on the surface of the substrate, with the aim of forming a new alloy layer based on the substrate.

(2) Laser cladding essentially does not use the molten metal on the surface layer of the substrate as a solvent, but rather melts separately prepared alloy powder to become the main alloy of the cladding layer. At the same time, a thin layer of the substrate alloy also melts, forming a metallurgical bond with it. The preparation of new materials using laser cladding technology is an important foundation for the repair and remanufacturing of failed components under extreme conditions, as well as the direct manufacturing of metal components. It has received high attention from the scientific community and enterprises around the world.

Craft field

Laser cladding technology is an interdisciplinary high-tech that involves multiple disciplines such as optics, mechanics, electronics, computer science, materials science, physics, and chemistry. It was proposed in the 1960s and the first patent discussing high-energy laser cladding was born in 1976. In the 1980s, laser cladding technology developed rapidly, and the rapid prototyping technology combined with CAD technology added new vitality to laser cladding technology.

We have successfully carried out laser cladding of cobalt based, nickel based, iron-based and other self melting alloy powders and ceramic phases on the surfaces of stainless steel, mold steel, malleable cast iron, gray cast iron, copper alloy, titanium alloy, aluminum alloy and special alloys. Laser cladding of iron-based alloy powder is suitable for parts that require local wear resistance and are prone to deformation. Nickel based alloy powder is suitable for components that require local wear resistance, heat corrosion resistance, and heat fatigue resistance. Cobalt based alloy powder is suitable for parts that require wear resistance, corrosion resistance, and thermal fatigue resistance. Ceramic coatings have high strength, good thermal stability, and high chemical stability at high temperatures, making them suitable for parts that require wear resistance, corrosion resistance, high temperature resistance, and oxidation resistance. Under severe conditions of sliding wear, impact wear, and abrasive wear, pure nickel based, cobalt based, and iron-based alloy powders can no longer meet the requirements of operating conditions. Therefore, laser cladding of metal ceramic composite coatings on alloy surfaces has become a hot research topic for scholars at home and abroad. Various ceramic or metal ceramic coatings have been studied for laser cladding on steel, titanium alloys, and aluminum alloy surfaces.

The application of laser cladding is mainly in two aspects, namely corrosion resistance (including high temperature corrosion resistance) and wear resistance. It has a wide range of applications, such as the sealing surfaces of valves and valve seats in internal combustion engines, laser cladding of water, gas or steam separators, etc.

Simultaneously improving the wear resistance and corrosion resistance of the material, Co based alloys (such as Co Cr Mo Si system) can be used for laser cladding. The presence of Co3Mo2SI hard intermetallic phases within the composition range of the matrix ensures wear resistance, while Cr provides corrosion resistance.

Existing problems

The evaluation of the quality of laser cladding layers mainly considers two aspects. On a macro level, examine the shape of the overlay, surface roughness, cracks, pores, and dilution rate; Secondly, at the micro level, examine whether a good organization has been formed and whether it can provide the required performance. In addition, the types and distribution of chemical elements in the surface cladding layer should be determined, and attention should be paid to analyzing whether the transition layer is metallurgical bonding. If necessary, quality life testing should be carried out.

The focus of the research work is on the research and development of cladding equipment, melt pool dynamics, design of alloy composition, formation, propagation, and control methods of cracks, as well as the bonding force between the cladding layer and the substrate.

The main problems faced by the further application of laser cladding technology are:

① The main reason why laser cladding technology has not been fully industrialized in China is the instability of the quality of the cladding layer. During the laser cladding process, the heating and cooling rates are extremely fast, with a maximum speed of 1012 ℃/s. Due to the temperature gradient and thermal expansion coefficient difference between the cladding layer and the substrate material, various defects may occur in the cladding layer, mainly including pores, cracks, deformation, and surface roughness

② Detection and implementation of automated control in the process of photo deposition.

③ The cracking sensitivity of laser cladding layers remains a challenge for researchers both domestically and internationally, as well as an obstacle to engineering applications and industrialization. Although research has been conducted on the formation and propagation of cracks, the control methods are not yet mature.

Application and Development Prospects

Since the 1980s, laser cladding technology has developed rapidly and has become a hot topic in laser surface modification research both domestically and internationally. Laser cladding technology has great technical and economic benefits and is widely used in fields such as mechanical manufacturing and maintenance, automotive manufacturing, textile machinery, navigation and aerospace, and petrochemicals.

Laser cladding technology has achieved certain results and is in the initial stage of gradually moving towards industrial application. The future development prospects mainly include the following aspects:

(1) Basic theoretical research on laser cladding.

(2) Design and development of cladding materials.

(3) Improvement and development of laser cladding equipment.

(4) Establishment of theoretical models.

(5) Rapid prototyping technology of laser cladding.

(6) Automation of cladding process control.