China Powder Network News Gradient functional material is a new type of composite material which is composed of two or more materials and its composition is continuously gradient. It has received extensive attention in today's rapidly developing engineering field. However, the traditional gradient functional material preparation technology cannot meet the needs of aviation, medical, military and other industrial fields. As an emerging technology, additive manufacturing provides a new idea to solve the preparation problem of gradient functional materials.
1 Functionally graded materials
Functionally graded material (FGM) is an advanced engineering material with spatially graded composition, porosity or microstructure that maintains material properties and avoids part failure in harsh operating environments.
The concept of functionally graded materials was first proposed in 1984 by Japanese scholars Masako Niano and Toshio Hirai. The design idea is to use ceramic/metal functionally graded materials with continuously changing components along the thickness direction to solve the problem of thermal stress on the interface inside the material. Compared with conventional composite materials, functionally graded materials realize the continuous change of components in the gradient layer, reduce the difference in material properties such as thermal expansion coefficient and elastic modulus between layers, and reduce the interface stress, which not only improves the material performance and reliability, but also improves the reliability of the material. The composite properties of the material are guaranteed.
Today, functionally graded materials have been extended from the original high-temperature thermal insulation materials to biomedical engineering, nuclear industry, aerospace, semiconductor optoelectronics, defense and military industries, etc., and are increasingly valued by experts and scholars from all over the world.
With the wide application of functionally graded materials, traditional manufacturing techniques with long preparation cycles and complex procedures are increasingly difficult to meet the rapid preparation of customizable and complex-shaped functionally graded materials. A more flexible and efficient manufacturing technology is needed to promote functionally graded materials. development of. Laser additive manufacturing (LAM) technology is a new advanced manufacturing technology developed in the 1990s that integrates computers, numerical control, materials, and lasers.
2 The principle of laser additive manufacturing
Laser additive manufacturing technology is based on the idea of calculus, using laser as an energy source to perform layer-by-layer laser cladding on pre-set or synchronously fed metal powder, and prepare a solid layer by layer by adding materials. Components.
Laser Additive Manufacturing Schematic
The above picture shows the schematic diagram of laser additive manufacturing. First, carry out three-dimensional CAD modeling of the parts to be prepared by computer, and cut the CAD model into thin slices according to a certain thickness, so that the parts are discretized into a series of two-dimensional plane structures from complex three-dimensional three-dimensional structures, and then scan the trajectory of each two-dimensional contour. Design and transmit the processed data to the numerical control system to form the numerical control code. Finally, under the control of the computer program, the powder material is piled line by line and layer by layer according to the set route by the method of laser cladding, and finally a three-dimensional entity is formed. Parts or blanks that require only a small amount of machining.
3 Classification of Laser Additive Manufacturing
Because it is developed relatively independently in different research institutions around the world, laser additive manufacturing technology has many names, but the principles are basically similar. The most representative laser additive manufacturing technologies are Laser Melting Deposition (LMD) featuring synchronous powder feeding and Selective Laser Melting (SLM) featuring powder bedding.
The laser melting deposition technology uses a high-energy laser beam to irradiate the surface of the substrate to form a molten pool, and the powder feeder sends the metal powder into the molten pool to rapidly melt, thereby forming a metallurgical bonding layer with the metal substrate and covering the surface of the substrate to form a new metal powder. metal layer.
The difference between the laser selective melting technology and the laser melting deposition technology is the difference in the form of powder addition. Before the laser beam scans, the laser selective melting technology first uses a powder spreading roller to pre-spread a layer of metal powder on the substrate, and then uses the laser beam to pre-pow The set scanning trajectory is used to selectively melt the powder. After each layer of melting is completed, the forming cylinder descends to a preset height, while the powder spreading cylinder rises to a preset height, and the powder spreading roller evenly spreads a layer of powder. Final required parts.
4 Research progress in laser additive manufacturing of gradient functional materials
An important development direction of laser additive manufacturing technology is the preparation of high-performance gradient functional materials, which has attracted extensive attention from scholars at home and abroad. Research on the combination of metal materials/ceramic materials.
4.1 Laser additive manufacturing of metal/metal gradient functional materials
Metal materials have excellent comprehensive physical, chemical and mechanical properties unmatched by other engineering materials. To further improve the properties of metal materials by adjusting the microstructure has become the main direction of materials research in recent years. The introduction of gradient structures in metal materials breaks the originally coupled material properties, allowing one or more properties to be improved individually. The overall performance and service performance of the material can be greatly optimized and improved. Researchers at home and abroad have used advanced laser additive manufacturing technology to prepare different kinds of metal/metal gradient functional materials, and have made in-depth studies on their microstructure and properties.
4.2 Laser additive manufacturing of metal/ceramic gradient functional materials
Since the 1970s, people have been engaged in the preparation of ceramic coatings on metals. Until the end of the 1980s, the use of laser additive manufacturing technology to achieve metal surface modification gradually developed. At present, this method has become one of the most valuable and promising technologies in metal surface modification, and has been widely used in many fields.
Due to the different melting points of metal materials and ceramic materials, strong convection will be generated in the metal molten pool, which will easily destroy the bond between the metal and the ceramic. The reinforcement phase generated by the in-situ reaction has good wettability with the matrix, and the heat released by the reaction helps to increase the wettability of the ceramic and metal, so the in-situ self-generated ceramic reinforcement phase is more firmly combined with the matrix, which is a solution to the problem. An efficient approach to interface binding problems. Therefore, in-situ self-generated metal matrix ceramic composites have received widespread attention from researchers at home and abroad, and initial results have been achieved.
Reference source:
[1] Cui Xue, et al. Research status and prospect of laser additive manufacturing of high-performance gradient functional materials. Materials Engineering. 2020.
[2] Xia Xiaoguang, et al. Research progress and prospect of additive manufacturing technology for functionally graded materials. Materials Review. 2021.
[3] Li Qiqi, et al. Research progress on additive manufacturing technology of gradient functional alloys. Chinese Journal of Mechanical Engineering. 2021.
(Edited by China Powder Network/Xingyao)
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