Laser Surface Modification Technology of Titanium Alloy

Laser surface quenching mainly uses a high-energy laser beam to irradiate the surface of a metal or alloy, and the thermal effect generated makes the surface of the substrate form a solid heating process that does not exceed the melting point. At the same time, the phase transformation is strengthened on the metal surface by using the presence of isomeric transformation in the matrix material combined with laser heating and material self-chilling effect. For titanium alloys, there have been studies in this area at home and abroad since the beginning of the 20th century. Dai Zhendong et al. significantly improved the hardness of TC11 titanium alloy by means of surface scanning laser quenching, and its friction coefficient could be reduced to the original 0.2 ~ 0.3, and the fretting wear resistance was increased by 123 times, which greatly improved the alloy surface properties. The surface structure and properties of TC11 titanium alloy were optimized by Zhang Hong et al. The results show that laser quenching can refine the surface structure obviously, and improve the hardness and wear resistance. Zhang Qi et al., through the study of various titanium alloys treated by laser self-quenching and rapid solidification, confirmed that the self-quenching treatment not only refines the α and β grain structure of the alloy, but also makes the surface chemical composition more homogeneous, and the segregation ratio after quenching can be reduced from the original state of 1.28 to 1.04, and no holes, cracks and other defects are found in the quenching layer. A smooth and uniform alloy surface can be obtained.

Laser surface remelting is a method to melt and solidify the surface of the substrate rapidly by radiating the surface of the material under the protection of argon atmosphere, so as to refine the structure and improve the performance of the material. Guo Chun et al. carried out laser remelting treatment on the surface of TC4 alloy by laser beam. After microscopic observation, the surface structure of the matrix was refined, and the surface properties such as hardness and wear resistance were also significantly improved. In addition, some researchers used Nd∶YAG laser to remelt the surface of TiNi alloy, and the cladding layer and the matrix metallurgical combination is good, can form a continuous and dense passivation film, and the corrosion resistance is significantly enhanced. By laser surface remelting treatment of TA2 industrial pure titanium, Dai Jingjie believes that the improvement of surface wear resistance is due to the lattice distortion, fine crystal strengthening and dislocation strengthening caused by the melting process. However, surface remelting does not improve the performance of all titanium alloy materials, and its performance is also possible to deteriorate. The results show that the β grains formed by laser surface melting of TA15 titanium alloy are abnormally coarsened.

Laser surface repair can be classified as a branch of laser forming repair technology, and it is also a synthesis of laser forming technology and laser cladding technology, which is a further application and development in the field of metal parts repair. The surface defects of titanium and titanium alloys can be eliminated by using laser surface repair technology. The cracks on the surface of titanium alloy can be healed by laser repair. After laser repair, the hardness value of the matrix around the modified zone increases, and the hardness curve between the modified zone and the heat-affected zone is relatively flat.

Laser surface alloying is a method that uses high-energy laser beam to rapidly heat and melt the surface of the material to promote the surface alloying reaction, so as to improve the surface properties of the alloy, which can be divided into surface gas alloying and surface powder alloying.

The gas introduced by gas alloying is mainly N2 or its mixture, also known as laser gas nitriding. It is in a nitrogen atmosphere, the use of high energy laser beam to activate nitrogen atoms, high temperature action to melt the surface of the material, the active N atom and the liquid phase of the metal molten pool Ti alloying reaction, forming a hard phase TiN.

Laser surface cladding can also be classified as a surface modification technology, is the basis of laser surface repair. It is the use of high energy density laser beam to add the cladding material and the substrate surface together melting technology, on the base surface to form the cladding material and the substrate good metallurgical combination of cladding layer.

The process of laser cladding is accompanied by laser alloying, but compared with the simple laser alloying, the cladding layer material is not fully mixed with the matrix to alloying reaction, which can better reflect the special properties of the cladding material. At present, there are many material systems used for laser cladding of titanium and titanium alloys, including C, B, N, Si and Ni. According to the composition and properties of the cladding layer, the prepared coatings can be divided into wear-resistant coatings, high-temperature oxidation resistant coatings, biological coatings and thermal barrier coatings.

5.1 Wear-resistant coating

The wear resistance of titanium alloy is poor compared with other properties, so the laser surface modification focuses more on improving the wear resistance of the matrix. In general, the higher the hard phase content in the wear-resistant coating, the higher the hardness and the better the wear resistance. There are many cladding materials that can improve the wear resistance of titanium alloys, including B, C, Ni, Si, B4C, Cr2C3, TiC, BN, SiC, TiB, TiB2 and Al2O3.

5.2 Antioxidant coating

Structural parts for engineering applications are often in long-term service under high-temperature conditions. In order to reduce or avoid the chemical or electrochemical reaction between O, S, N and other elements in the high-temperature working atmosphere and the matrix, a dense high-temperature protective layer is generally constructed on the surface to protect the matrix from being destroyed.

5.3 Thermal barrier coating

The operating temperature in aerospace, gas turbine engines and other environments has reached the limit temperature of superalloy materials. The thermal barrier coating of alloy materials combines the performance of metal materials with the advantages of high-temperature resistance of ceramic materials to play the role of thermal insulation of ceramic materials so that parts can work normally under high-temperature conditions.

5.4 Biological Coatings

A bioactive coating is deposited on the surface of titanium alloy by laser cladding technology, which makes the titanium alloy implantation show better biocompatibility.

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