Metal laser cladding powder classification and characteristics

    Laser cladding technology refers to the process of placing the selected coating alloy powder on the surface of the substrate in different filling methods, irradiating it with a high-energy laser beam, causing it to act on the surface of the substrate, rapidly melting, expanding and solidifying on the surface of the substrate, and then forming a covering layer combined with the base material. This newly generated covering layer can improve or even regenerate the base material, enabling it to achieve wear resistance, heat resistance, corrosion resistance, oxidation resistance and other target properties.

    Laser cladding technology is a complex physical and chemical metallurgical process, and the setting of laser parameters has a great influence on the quality of the cladding layer. In addition, the selection of alloy powder is also an important factor. Laser cladding alloy powder can be divided into self-fluxing alloy powder, composite powder and ceramic powder according to the material composition. Among them, self-fluxing alloy powder is more studied and applied in reality.

    1. Self-fluxing alloy powder

    Self-fluxing alloy powder can be divided into iron-based (Fe), nickel-based (Ni), and cobalt-based (Co) alloy powders. Its main characteristics are that it contains boron (B) and silicon (Si), so it has self-deoxidation and slag-making properties; it also contains a high level of chromium, which preferentially melts with the oxygen in the alloy powder and the oxide on the surface of the workpiece to form low-melting-point borosilicates, etc., which cover the surface of the molten pool to prevent excessive oxidation of the liquid metal, thereby improving the wetting ability of the melt to the base metal, reducing the inclusions and oxygen content in the cladding layer, and improving the process forming performance of the cladding layer, so it has excellent corrosion resistance and oxidation resistance. It has good adaptability to a variety of substrates such as carbon steel, stainless steel, alloy steel, and cast steel, and can obtain a cladding layer with low oxide content and low porosity. However, for sulfur-containing steel, due to the presence of sulfur, a low-melting-point brittle phase is easily formed at the interface, making the coating easy to peel off, so it should be selected with caution.

    01 Iron-based (Fe) self-fluxing alloy powder

    Fe-based self-fluxing alloy powder is suitable for parts that require local wear resistance and are easy to deform. The matrix is ​​mostly cast iron and low-carbon steel. Its advantages are wide material sources, low cost and good wear resistance. The disadvantages are high melting point, poor oxidation resistance, easy cracking and pores in the cladding layer. In the composition of iron-based alloy powder, the hardness of the coating is adjusted by adjusting the content of alloy elements, and the hardness, cracking sensitivity and residual austenite content of the cladding layer are improved by adding other elements, thereby improving the wear resistance and toughness of the cladding layer. Iron-based self-fluxing alloy powder for laser cladding is divided into two types: austenitic stainless steel type and high chromium cast iron type

   
    iron-based alloy powder

    . In recent years, many people have conducted experiments on adding other components to iron-based powder in the research on laser cladding. The results show that the addition of rare earth improves the anti-stripping ability of the passivation film on the surface of the cladding layer, reduces the corrosion weight loss of the material to varying degrees, and improves the corrosion resistance of the cladding layer.

    02Nickel-based (Ni) self-fluxing alloy powderNi

    -based self-fluxing alloy powder has been widely studied and applied in laser cladding materials due to its good wettability, corrosion resistance, high-temperature self-lubricating effect and moderate price.Nickel

    -based alloy powderNickel

    -based (Ni) self-fluxing alloy powderUnder the conditions of severe sliding, impact wear and abrasive wear, the simple self-fluxing alloy powder is no longer competent for the use requirements. At this time, various high-melting-point carbides, nitrides, borides and oxide ceramic particles can be added to the self-fluxing alloy powder to make a metal composite coating.03Cobalt

    -based (Co) self-fluxing alloy powderCobalt

    -based (Co) self-fluxing alloy powder has excellent heat resistance, corrosion resistance, wear resistance, impact resistance and high-temperature oxidation resistance. It is often used in wear-resistant, corrosion-resistant and high-temperature resistant occasions in petrochemical, electric power, metallurgy and other industrial fields.Co-based self-fluxing alloy has good wettability and its melting point is lower than that of carbide. After heating, the Co element is first in a molten state, and when the alloy solidifies, it first forms a new phase with other elements, which is more beneficial to the strengthening of the cladding layer. At present, the alloying elements used in cobalt-based alloys are mainly nickel, carbon, chromium and iron. Among them, nickel can reduce the thermal expansion coefficient of the cobalt-based alloy cladding layer, reduce the melting temperature range of the alloy, effectively prevent cracks in the cladding layer, and improve the wettability of the cladding alloy to the substrate. A comprehensive analysis

    of cobalt-based alloy powders

    shows that Ni-based or Co-based self-fluxing alloy powder systems have good self-fluxing and corrosion resistance, wear resistance, and oxidation resistance, but the price is relatively high; Fe-based self-fluxing alloy powders are cheap, but have poor self-fluxing properties and are prone to cracking and oxidation. Therefore, in practical applications, the self-fluxing alloy powder system should be reasonably selected according to the requirements of use.

    2. Composite powder

    Composite powder mainly refers to a powder system formed by mixing or compounding various high-melting point hard ceramic materials such as carbides, nitrides, borides, oxides and silicides with metals. Composite powder can be used to prepare ceramic particle reinforced metal-based composite coatings with the help of laser cladding technology, which organically combines the toughness, good processability of metals with the excellent wear resistance, corrosion resistance, high temperature resistance and oxidation resistance of ceramic materials, and can protect carbides from oxidation and decomposition to a certain extent, thereby obtaining coatings with high wear resistance and hardness. This is the current research and development hotspot in the field of laser cladding technology. Among them, carbide alloy powders and oxide alloy powders are widely studied and applied, and are mainly used to prepare wear-resistant coatings. The carbide particles in the composite powder can be directly added to the laser molten pool or directly mixed with metal powder to form a mixed powder, but it is more effective to add them in the form of coated powders (such as nickel-coated carbides and cobalt-coated carbides). In the process of laser cladding

    of nickel-based tungsten carbide powder

    , the coated metal of the coated powder can effectively protect the core carbides and reduce the direct effect of high-energy lasers on carbides, which can effectively reduce or avoid the occurrence of carbide burning, carbon loss, volatilization and other phenomena.

    3. Ceramic powder

    Ceramic powder mainly includes silicide ceramic powder and oxide ceramic powder, among which oxide ceramic powder (alumina and zirconia) is the main one. Zirconia has lower thermal conductivity and better thermal shock resistance than alumina ceramic powder, so it is also often used to prepare thermal barrier coatings. Because ceramic powder has excellent wear resistance, corrosion resistance, high temperature resistance and oxidation resistance, it is often used to prepare high-temperature wear-resistant and corrosion-resistant coatings. At present, bioceramic materials are a hot topic of research.

    Zirconia ceramic powder

    Ceramic powder disadvantages: The thermal expansion coefficient, elastic modulus and thermal conductivity of the base metal are quite different, and the cladding layer is prone to defects such as cracks and holes. It is easy to deform, crack, peel off and damage during use.

    In order to solve the cracks in the pure ceramic coating and the high-strength bonding with the metal substrate, some scholars have tried to use an intermediate transition layer and add CaO, SiO2, TiO2 and other materials with low melting point and high expansion coefficient to the ceramic layer to reduce internal stress and alleviate the crack tendency. However, existing studies have shown that the crack and peeling problems of pure ceramic coatings have not been well solved, so further in-depth research is needed.

    At present, the research on laser cladding bioceramic materials is mainly focused on hydroxyapatite (HAP), fluoroapatite and bioceramic materials containing Ca and Pr that are laser clad on the surfaces of metals such as Ti-based alloys and stainless steel. Hydroxyapatite bioceramics have good biocompatibility and have long been widely recognized by relevant scholars at home and abroad as human teeth. In general, although the research on laser cladding bioceramic materials started late, it has developed very rapidly and is a research direction with broad prospects.

    4. Other metal powders

    In addition to the above types of laser cladding powder material systems, the cladding material systems that have been developed and researched also include: copper-based, titanium-based, aluminum-based, magnesium-based, zirconium-based, chromium-based and intermetallic compound-based materials. Most of these materials utilize certain special properties of the alloy system to achieve one or more functions such as wear resistance, friction reduction, corrosion resistance, conductivity, high temperature resistance, and thermal oxidation resistance.

    1. Copper-based

    Copper-based laser cladding materials mainly include copper-based alloy powders and composite powder materials such as Cu-Ni-B-Si, Cu-Ni-Fe-Co-Cr-Si-B, Cu-Al2O3, and Cu-CuO. Using the metallurgical properties of the copper alloy system such as the liquid phase separation phenomenon, copper-based composite powder materials for laser cladding copper-based self-generated composite materials can be designed. Studies have shown that there are a large number of self-generated hard particle reinforcements in the laser cladding layer, which has good wear resistance. Dan Jiguo et al. used the liquid phase separation of Cu and Fe and the metallurgical reaction characteristics of the parent material and the surfacing material to prepare a copper-based alloy composite cladding layer with Fe3Si dispersed distribution by laser cladding. Studies have shown that during the laser cladding process, the Fe element that melts into the molten pool from the parent material is in a liquid phase separation state with the Cu alloy in the molten pool; the Fe that enters the molten pool floats due to its low density, and reacts with Si in the molten pool to form Fe3Si during the floating process. Fe3Si is dispersed in the α-Cu matrix in the laser cladding layer.

    2. Titanium-based

    Titanium-based cladding materials are mainly used to improve the biocompatibility, wear resistance or corrosion resistance of the surface of the base metal material. The titanium-based laser cladding powder materials studied are mainly pure Ti powder, Ti6Al4V alloy powder and titanium-based composite powders such as Ti-TiO2, Ti-TiC, Ti-WC, and Ti-Si. In an argon atmosphere, a Ti-TiC composite coating was laser clad on the surface of Ti6Al4V alloy. The study showed that tiny TiC particles were formed in situ in the composite coating, and the composite coating had excellent friction and wear properties.

    Titanium-based alloy powder (TC)

    3. Magnesium-based

    Magnesium-based cladding materials are mainly used for laser cladding on the surface of magnesium alloys to improve the wear resistance and corrosion resistance of the magnesium alloy surface. Magnesium-based MEZ powder (composition: Zn: 0.5%, Mn: 0.1%, Zr: 0.1%, RE: 2%, Mg: Bal) was clad on ordinary commercial magnesium alloys. Studies have shown that the microhardness of the cladding layer is increased from HV35 to HV85-100, and because of grain refinement and redistribution of intermetallic compounds, the corrosion resistance of the cladding layer in 3.56wt% NaCl solution is greatly improved compared with the base magnesium alloy.

    Magnesium-based alloy powder

    4, aluminum-based cladding

    aluminum powder was clad on two magnesium alloy substrates WE43 and ZE41 using a 3kW Nd:YAG laser with lateral powder feeding to obtain a cladding layer with good bonding performance. The study found that the hardness value of the coating reached HV0.05120-200, and the main reason for the increase in hardness was the presence of Al3Mg2 and Al12Mg17 metal compounds. ZMei et al. laser clad aluminum-based Al-Zn powder on a magnesium-based ZK60/SiC substrate and obtained a metallurgically good cladding layer. Studies have shown that the corrosion potential of the cladding layer is 300mV higher than the standard sample potential, while the corrosion current is at least 3 orders of magnitude lower.

    Aluminum-based alloy powder

    5. Zirconium-based

    laser cladding of zirconium-based ZrAlNiCu alloy powder on pure titanium substrate, and the coating was studied and analyzed. It was found that the coating is composed of intermetallic compounds with high specific strength and high hardness and a small amount of amorphous phase, and has good mechanical properties; adding 2wt% B and 2.75wt% Si to the ZrAlNiCu alloy powder, it was found that the amorphous content in the coating increased and the hardness increased. The high hardness of the two coatings reached HV909.6 and HV1444.8 respectively.

    Zirconium-based alloy powder

    5. Summary:

    The characteristics, prices and performance after cladding of different cladding materials vary greatly. In actual use, alloy powders with different performances can be selected according to different processing requirements. By laser cladding the alloy powder on the surface of the workpiece (laser cladding), a high-performance alloy surface can be prepared on a cheap metal substrate without affecting the properties of the substrate, effectively reducing production costs and saving precious rare metal materials. Compared with traditional surface treatment technologies such as surfacing, thermal spraying, and electroplating, laser cladding has the advantages of low dilution, dense organization, good combination of coating and substrate, suitable for a variety of cladding materials, large changes in particle size and content, high processing quality, and good controllability (three-dimensional automatic processing can be achieved).

    At present, it is mainly used for material surface modification (such as hydraulic columns, rollers, gears, gas turbine blades, etc.), product surface repair (such as rotors, molds, bearing inner holes that fail due to wear, etc.), the strength of the repaired parts can reach more than 90% of the original strength, and the repair cost is less than 1/5 of the cost of replacing the product. More importantly, it shortens the maintenance time and effectively solves the problem of rapid repair of rotating parts of major complete sets of equipment in large enterprises.

    In addition, by laser cladding the surface of key components with wear-resistant and corrosion-resistant alloys, the service life of the components can be greatly improved without deformation of the surface of the components. Laser cladding treatment of the mold surface not only improves the mold strength, but also reduces the manufacturing cost by 2/3 and shortens the manufacturing cycle by 4/5.

    In general, laser cladding technology is a high-tech surface modification technology and equipment maintenance technology, and its research and development has important theoretical significance and economic value.

    Laser cladding materials are the main factor restricting the development and application of laser cladding technology. Although some progress has been made in the development of laser cladding materials, there is still a long way to go to quantitatively design the alloy composition according to the performance and application requirements of the designed cladding parts. Laser cladding materials are far from being serialized and standardized, and more efforts need to be made for in-depth research.