Classification and characteristics of metal laser cladding powder

Laser cladding technology refers to the process of placing selected coating alloy powder on the surface of the substrate using different filling methods, using high-energy laser beam irradiation to act on the surface of the substrate, rapidly melting, expanding, and solidifying on the surface of the substrate, thereby forming a coating layer that is bonded with the substrate material. This newly generated covering layer can significantly improve or even reconstruct the substrate 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 significant impact 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 material composition. Among them, self fluxing alloy powder has the most research and application in reality.

1、 Self fluxing alloy powder

Self fluxing alloy powders can be divided into iron based (Fe), nickel based (Ni), and cobalt based (Co) alloy powders, which mainly contain boron (B) and silicon (Si), thus possessing self deoxidation and slagging properties; It also contains high chromium, which preferentially melts with oxygen in the alloy powder and surface oxides of the workpiece to generate low melting point borosilicate and other coatings on the surface of the molten pool, preventing excessive oxidation of the liquid metal, thereby improving the wetting ability of the melt to the base metal, reducing inclusions and oxygen content in the cladding layer, and improving the process formability of the cladding layer. Therefore, it has excellent corrosion resistance and oxidation resistance. It has good adaptability to various 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 can easily form at the interface, making the coating easy to peel off. Therefore, careful selection should be made.

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 prone to deformation. The matrix is mostly cast iron and low-carbon steel, and its biggest advantage is that it has a wide range of material sources, low cost, and good wear resistance. The disadvantages are high melting point, poor oxidation resistance, easy cracking of the cladding layer, and the production of gas pores. In the composition of iron based alloy powder, the hardness of the coating is adjusted by adjusting the alloy element content, 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. The iron based self fluxing alloy powder used for laser cladding can be divided into two types: austenitic stainless steel type and high chromium cast iron type

In recent years, many people have conducted experiments on laser cladding by adding other components to iron based powders. The results showed that the addition of rare earth improved the anti stripping ability of the passivation film on the surface of the cladding layer, reduced the corrosion weight loss of the material to varying degrees, and improved the corrosion resistance of the cladding layer.

02. Nickel based (Ni) self fluxing alloy powder

Ni based self fluxing alloy powder has been extensively studied and applied in laser cladding materials due to its excellent wettability, corrosion resistance, high-temperature self-lubrication, and moderate price.

Nickel based (Ni) self fluxing alloy powder is no longer suitable for use under severe sliding, impact wear, and abrasive wear conditions. 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.

03. Cobalt based (Co) self fluxing alloy powder

Cobalt based (Co) self fluxing alloy powder has excellent heat resistance, corrosion resistance, wear resistance, impact resistance, and high-temperature oxidation resistance, and is often used in industrial fields such as petrochemical, power, metallurgy, etc. for wear, corrosion, and high-temperature resistance. Co based self fluxing alloys have good wettability and lower melting points compared to carbides. After heating, Co element is first in the melting state, and when the alloy solidifies, it forms new phases with other elements, which is extremely beneficial for strengthening the cladding layer. At present, the main alloying elements used in cobalt based alloys are nickel, carbon, chromium, and iron. Among them, nickel element can reduce the thermal expansion coefficient of 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.

Comprehensive analysis shows that Ni or Co based self fluxing alloy powder systems have good self fluxing properties, corrosion resistance, wear resistance, and oxidation resistance, but their prices are relatively high; Although Fe based self fluxing alloy powder is cheap, it has poor self fluxing properties and is prone to cracking and oxidation. Therefore, in practical applications, the self fluxing alloy powder system should be reasonably selected according to the usage requirements.

2、 Composite powder

Composite powder mainly refers to the 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 matrix composite coatings using laser cladding technology, which organically combines the strength and toughness of metals, good processability, and excellent wear resistance, corrosion resistance, high temperature resistance, and oxidation resistance of ceramic materials. It can to some extent prevent carbonization from oxidation and decomposition, thus obtaining coatings with high wear resistance and hardness. This is currently a hot research and development topic in the field of laser cladding technology. Among them, carbide alloy powder and oxide alloy powder have the most research and application, mainly used in the preparation of wear-resistant coatings. The carbide particles in the composite powder can be directly added to the laser melt pool or mixed with metal powder to form a mixed powder, but more effectively added in the form of coated powder (such as nickel coated carbide or cobalt coated carbide).

In the process of laser cladding, the coated metal of the coated powder can effectively protect the core carbides, weaken the direct effect of high-energy laser and carbides, and effectively reduce or avoid phenomena such as burning loss, carbon loss, and volatilization of carbides.

3、 Ceramic powder

Ceramic powders mainly include silicide ceramic powders and oxide ceramic powders, with oxide ceramic powders (alumina and zirconia) being the main ones. Zirconia has lower thermal conductivity and better thermal shock resistance than alumina ceramic powder, so it is also commonly used in the preparation of thermal barrier coatings. Due to its excellent wear resistance, corrosion resistance, high temperature resistance, and oxidation resistance, ceramic powder is often used to prepare high-temperature wear and corrosion resistant coatings. At present, bioceramic materials are a hot research topic.

Disadvantages of ceramic powder: There are significant differences in thermal expansion coefficient, elastic modulus, and thermal conductivity between the ceramic powder and the base metal. The cladding layer is prone to defects such as cracks and holes, and is prone to deformation, cracking, peeling, and damage during use.

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

At present, research on laser cladding bioceramic materials mainly focuses on hydroxyapatite (HAP), fluoroapatite, and bioceramic materials containing Ca and Pr that are laser cladding on metal surfaces such as Ti based alloys and stainless steel. Hydroxyapatite bioceramics have good biocompatibility and have long been widely valued by scholars both domestically and internationally as human teeth. Overall, although the research on laser cladding bioceramic materials started relatively late, it has developed rapidly and is a promising research direction.

4、 Other metal powders

In addition to the above types of laser cladding powder material systems, the currently developed cladding material systems 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 alloy systems 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, Cu CuO, etc. By utilizing metallurgical properties such as liquid phase separation in the copper alloy system, a copper based composite powder material with laser cladding of copper based in-situ composite materials can be designed. Research has shown that there are a large number of self generated hard particle reinforcements in the laser cladding layer, which has good wear resistance. Shan Jiguo and others utilized the liquid phase separation of Cu and Fe and the metallurgical reaction characteristics between the base material and the surfacing material to prepare a copper based alloy composite cladding layer with Fe3Si dispersion distribution using laser cladding. Research has shown that during the laser cladding process, the Fe element that melts from the base metal and enters the molten pool is in a liquid phase separation state from the Cu alloy in the molten pool; The Fe entering the melt pool floats due to its low density. During the floatation process, it reacts with Si in the melt pool to generate Fe3Si, which is distributed in a dispersed gradient in the laser cladding layer α- In the Cu matrix.

2. Titanium based

Titanium based cladding materials are mainly used to improve the biocompatibility, wear resistance, or corrosion resistance of the substrate metal material surface. The titanium based laser cladding powder materials studied mainly include pure Ti powder, Ti6Al4V alloy powder, as well as titanium based composite powders such as Ti TiO2, Ti TiC, Ti WC, and Ti Si. Zhang Song et al. deposited a Ti TiC composite coating on the surface of Ti6Al4V alloy by laser cladding in an argon atmosphere. The study showed that small TiC particles were formed in situ in the composite coating, and the composite coating exhibited excellent friction and wear properties.

3. Magnesium based

Magnesium based cladding materials are mainly used for laser cladding of magnesium alloy surfaces to improve their wear resistance and corrosion resistance. J. Dutta Majumdar et al. fused magnesium based MEZ powder (composition: Zn: 0.5%, Mn: 0.1%, Zr: 0.1%, RE: 2%, Mg: Bal) onto ordinary commercial magnesium alloys. Research has shown that the microhardness of the cladding layer has increased from HV35 to HV 85-100, and due to grain refinement and redistribution of intermetallic compounds, the corrosion resistance of the cladding layer in 3.56wt% NaCl solution is significantly improved compared to that of the base magnesium alloy.

4. Aluminum based

SorinIgnat et al. used a 3kW Nd: YAG laser to laterally feed aluminum powder on two types of magnesium alloy substrates, WE43 and ZE41, and obtained a well bonded cladding layer. Research has found that the hardness value of the coating reaches HV0.05120-200, and the main reason for the increase in hardness is the presence of Al3Mg2 and Al12Mg17 metal compounds. ZMei et al. laser cladding aluminum based Al Zn powder on a magnesium based ZK60/SiC matrix obtained a metallurgical good cladding layer. Research has shown that the corrosion potential of the cladding layer is 300mV higher than that of the standard sample, while the corrosion current is at least three orders of magnitude lower.

5. Zirconium base

Laser cladding of zirconium based ZrAlNiCu alloy powder on a pure titanium substrate was carried out, 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, which has good mechanical properties; Adding 2wt% B and 2.75wt% Si to ZrAlNiCu alloy powder, it was found that the amorphous content in the coating increased, resulting in an increase in hardness. The highest hardness of the two coatings reached HV909.6 and HV1444.8, respectively.

5、 Summary:

The characteristics, prices, and performance of different cladding materials vary greatly. In practical use, alloy powders with different properties can be selected according to different processing needs. By laser cladding alloy powder on the surface of the workpiece (laser cladding), high-performance alloy surfaces can be prepared on inexpensive metal substrates without affecting the properties of the substrate, effectively reducing production costs and saving precious and rare metal materials. Compared with traditional surface treatment technologies such as surfacing, thermal spraying, and electroplating, laser cladding has advantages such as low dilution, dense structure, good adhesion between coating and substrate, multiple suitable materials for cladding, large changes in particle size and content, high processing quality, and good controllability (which can achieve three-dimensional automatic processing).

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

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

Overall, laser cladding technology is a high-tech surface modification technology and equipment maintenance technology, and its research and development have 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 from quantitatively designing alloy components according to the designed performance and application requirements of the cladding parts. Laser cladding materials have not yet formed serialization and standardization, and further research is needed.