1. Overview
Materials for laser cladding on the surface of titanium alloys mainly include: fusible alloy materials, composite materials and ceramic materials.
Among them, fusible alloy materials mainly consist of iron-based alloys, nickel-based alloys and cobalt-based alloys.
The main characteristic of these alloys is the inclusion of boron and silicon elements, which have strong deoxidizing and self-fusing actions.
During laser cladding, boron and silicon are oxidized to form oxides, creating a thin film on the surface of the coating layer.
This film not only prevents excessive oxidation of the alloy elements, but also forms borosilicate slag with the oxides of these elements, thus reducing the inclusion content and oxygen levels in the coating layer.
This process results in a laser coating layer with low oxide content and few pores. Boron and silicon can also lower the melting point of the alloy, improving the wettability of the melt towards the base metal, positively affecting the fluidity and surface tension of the alloy.
The hardness of self-fusing alloy increases with increasing boron and silicon content in the alloy. This is due to the increase in the amount of extremely hard borides and carbides formed by elements of boron and silicon with nickel, chromium and other elements in the alloy.
1. Nickel-based alloy powder
Nickel-based alloy powder has excellent wetting properties, corrosion resistance and self-lubrication at high temperatures.
It is mainly used in components that require wear resistance, thermal corrosion resistance and thermal fatigue resistance. The required laser power density is slightly higher than that of iron-based alloy cladding.
The alloying principle of nickel-based alloys involves austenitic solid solution strengthening with elements such as Fe, Cr, Co, Mo, W, precipitation strengthening of intermetallic compounds with Al, Ti, and grain boundary strengthening with B, Zr, Co.
The selection of nickel-based self-fusing alloy powder elements is based on these principles, while the amount of alloying elements added depends on the formability of the alloy and the laser cladding process.
At present, nickel-based self-fusing alloys mainly include Ni-B-Si and Ni-Cr-B-Si. The former is less hard but more ductile and easier to process, while the latter is formed by adding appropriate Cr to the Ni-B-Si alloy. Cr, soluble in Ni, forms a solid nickel-chromium solution increasing the strength of the coating layer, improving its resistance to oxidation and corrosion.
Cr can also form borides and carbides with B and C, increasing the hardness and wear resistance of the coating layer.
Increasing the content of C, B and Si in the Ni-Cr-B-Si alloy can increase the hardness of the coating layer from 25 HRC to about 60 HRC, but at the cost of reducing ductility.
Ni60 and Ni45 are the most used in this type of alloy. Furthermore, by increasing the Ni content, the cracking rate can be significantly reduced.
This occurs because Ni is a powerful expanding element of the austenite phase (γ). Increasing the Ni content in the alloy increases the toughness, thereby increasing the plastic toughness of the coating layer.
The increase in Ni content also reduces the coefficient of thermal expansion of the coating layer, thereby reducing the residual tensile stress of the coating layer and significantly reducing the generation of cracks and defects.
However, more Ni is not necessarily better, as an excessively high Ni content can damage the hardness of the coating layer, preventing it from achieving the required properties.
2. Cobalt-based alloy powder
Cobalt-based alloy powder offers excellent high-temperature performance and wear and corrosion resistance when laser applied to the surface of titanium alloys.
Currently, the cobalt-based self-fusing alloy powder used for laser cladding is developed based on Stellite alloys, with primary alloying elements of chromium (Cr), tungsten (W), iron (Fe), nickel (Ni) and carbon (C).
Furthermore, boron (B) and silicon (Si) are added to increase the wettability of the alloy powder to form a self-fusing alloy.
However, excessive boron content can increase the alloy's tendency to crack. Cobalt-based alloys exhibit superior thermal stability, with minimal evaporation and sublimation or noticeable degradation during plating.
Furthermore, the cobalt-based alloy powder has excellent wettability after melting, spreading evenly over the surface of the titanium alloy.
This leads to a dense, smooth and flat coating layer, increasing the bond strength between the coating layer and the base material.
The main constituents of cobalt-based alloy powder are cobalt (Co), chromium (Cr) and tungsten (W), endowing it with excellent high temperature performance and comprehensive mechanical properties.
Cobalt and chromium form stable solid solutions, and due to the low carbon content, various carbides such as metastable CrC, MC and WC, as well as borides such as CrB, are dispersed throughout the base, leading to an alloy with higher red hardness, high temperature wear resistance, corrosion resistance and oxidation resistance.
3. Iron-based alloy powder
Laser coating of iron-based alloy powder on the surface of titanium alloys is suitable for parts that are prone to deformation and require localized wear resistance. Its biggest advantage is low cost and good wear resistance.
However, it has a high melting point, low self-fusibility, low oxidation resistance, low fluidity and a coating layer that often contains a significant amount of porosity and slag inclusions, which limits its applications.
At present, the alloy design of the Fe-based alloy cladding structure mainly consists of Fe-CX (where X represents Cr, W, Mo, B, etc.), and the cladding layer structure is mainly composed of phases metastable, with the strengthening mechanisms being martensite strengthening and carbide strengthening.
Characteristics of self-melting alloy powder systems
| Self-Fusing Alloy Powder | Self-fuse | Benefits | Disadvantages |
| Iron-based | Poor | Cost benefit | Poor resistance to oxidation. |
| Cobalt-based | Pretty good | It presents superior resistance to high temperatures, excellent resistance to thermal shock and excellent resistance to wear and corrosion. | Relatively high cost. |
| Nickel-based | Good | It has good toughness, impact resistance, heat resistance, oxidation resistance and high corrosion resistance. | Below average performance at high temperatures. |
4. Composite Powders
Under severe conditions of sliding, impact wear and abrasive wear on titanium alloy surfaces, simple self-fusing alloys based on Ni, Co and Fe can no longer meet the usage requirements.
At this point, various carbides, nitrides, borides and high-melting point ceramic oxide particles can be added to the above-mentioned self-fusing alloy powders to create metal-ceramic composite coatings.
Among them, carbides (such as WC, TiC, SiC, etc.) and oxides (such as ZrO, AlO, etc.) are the most studied and used. The behavior of ceramic materials in cast titanium alloy includes: complete dissolution, partial dissolution and minor dissolution.
The degree of dissolution is controlled primarily by the type of ceramic and substrate and secondarily by the conditions of the laser cladding process.
During the laser cladding process, the molten pool remains at high temperatures for a very short period of time, leaving the ceramic particles insufficient time to melt completely. The coating layer consists of face-centered cubic γ phase (Fe, Ni, Co), unmolten ceramic phase particles and precipitated phases (such as MC, MC, etc.).
The laser cladding layer includes strengthening mechanisms such as fine grain strengthening, hard particle dispersion strengthening, solid solution strengthening, and dislocation accumulation strengthening.
Examples:
1. Through laser cladding of in-situ TiC or (TiB+TiC) reinforced titanium composite material coatings on the surface of titanium alloys, we can improve the surface hardness and wear resistance of titanium alloy, ensuring the at the same time a good adaptation of the coating material to the substrate.
2. The surface of titanium alloy is laser melted and different proportions of Ti-Cr binary alloy are deposited, preparing modified coatings on the surface that have high hardness and good compatibility with the substrate.
