01
Dissertation Introduction
The concept of functionally graded materials (FEMs) was first proposed in Sendai, Japan in 1980. In gradient materials, when the chemical composition or microstructure changes gradually in one or more directions, the properties change smoothly and abruptly from one side to the other. In material selection, FGMs with gradient interfaces are used instead of dissimilar joints or common composites with sharp interfaces, as they offer several advantages, such as higher toughness and lower residual stress levels due to gradual changes in physical, metallurgical and mechanical properties between adjacent layers, resulting in improved structural performance and longer service life. As a result, FGMs are widely used in applications where different parts of a particular component require different and sometimes conflicting characteristics, such as industries such as energy, aerospace, automotive, and optoelectronics. Gradient materials can be classified according to two criteria: size and structure. Depending on the size, the gradient is either thin (similar to a surface coating) or large blocks, which are processed differently. According to the structure, they are divided into two categories: continuous and discontinuous. In discontinuous gradients of material (Figure 1a) where the chemical composition or microstructure changes step by step, the interface is usually detectable and visible. In contrast, the chemical composition or microstructure of a material with a continuous gradient (Figure 1b) is constantly changing with position, so it is almost impossible to see a clear boundary as an interface throughout the gradient structure.
Figure 1. Schematic diagram of (a) discontinuous and (b) continuous gradient materials.
Laser & Electron Beam Processing
02
Methods for manufacturing gradient metal materials
The manufacturing method plays an important role in meeting the design requirements in terms of the geometrical characteristics, chemical composition, and microstructure of the gradient structure, as well as the performance and performance of the FGM. In addition, it is important to choose a manufacturing method based on economic aspects (cost and time) and environmental aspects (consumption and pollution). Chemical/physical vapor deposition technology, thermal spraying, powder metallurgy, and discharge plasma sintering are among the common manufacturing methods for gradient materials. However, geometric dimensions, energy consumption, and environmental pollution have hindered the development of gradient materials produced by these traditional manufacturing methods.
In additive manufacturing technology, the final shape of a part is created by adding material, layer by layer. For this reason, it is also known as layered manufacturing technology. The principle of layered manufacturing is based on the fact that any object, regardless of its geometric complexity, can be cut into several layers and reconstructed by connecting layers. The unique nature of the additive manufacturing process offers a number of advantages over traditional processes. In the additive manufacturing process, complex parts can be produced in a single step, very close to the intended design, and without the limitations of traditional manufacturing methods. In addition, in this process, the number of component components can be significantly reduced by eliminating or reducing the need to assemble multi-component parts. In addition, with the help of additive manufacturing technology, parts can be produced on demand, reducing the need for parts storage and transportation.
Laser & Electron Beam Processing
03
Metal-to-metal gradient materials
3.1 Titanium-based gradient alloys
One of the most striking metals in FGM additive manufacturing is titanium and its alloys, because although they have some unique properties, their use in sensitive and highly advanced applications requires on the one hand to improve their service life and efficiency, which are the most critical design requirements; On the other hand, their compatibility with other engineering materials and alloys is also very challenging. Table 1 summarizes the research on additive manufacturing of titanium-based gradient alloys. One of the main concerns in the additive manufacturing of titanium alloys and other alloys is the formation of brittle intermetallic compounds, which means that these structures can easily fail prematurely during processing.
Table 1.Research on additive manufacturing of titanium-based gradient alloys.
3.2 Iron-based gradient alloys
Other notable metals in FGM additive manufacturing are iron-based alloys, especially stainless steels, because, in addition to being cost-effective, they combine various properties such as high strength, good thermal stability, and excellent resistance to oxidation, corrosion, and wear. For example, in some research work, the development of gradient structures of stainless steel-nickel-base superalloys has been studied, instead of the dissimilar joints they use in important power plants and in the oil refining industry. Table 2 summarizes some of the findings on additive manufacturing of iron-based gradient alloys.
Table 2.Summary of research on additive manufacturing of iron-based gradient alloys.
3.3 Other gradient alloys
Table 3.Summary of research on additive manufacturing of various other gradient alloys.
Laser & Electron Beam Processing
04
summary
With the advent of additive manufacturing technology, the trend towards functionally graded materials has accelerated. DED and LPBF technologies make up the majority of FEM additive manufacturing technologies. In the additive manufacturing of metal-to-metal gradient materials, in many cases, the tensile properties of the gradient structure are often comparable to those of the weakest metal, and fractures occur within the component, indicating a proper metallurgical bond at the interface of the base metal. Fragile intermetallic compounds often form inevitably in gradient structures and are highly sensitive to cracking.
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Reprinted by Chen Changjun of the Yangtze River Delta G60 Laser Alliance