Since the powder feeding system of traditional L–PBF equipment is evenly applied in the same layer through scraper/roller type, it is difficult to accurately deposit multiple materials in the same layer or different layers point-to-point, which has become a major problem limiting the formation of multiple materials by L–PBF. Therefore, how to design the powder feeding system of the traditional powder bed has become the key to the preparation of multi-materials, and the existing multi-material powder feeding system is divided into a scraper powder feeding system, ultrasonic auxiliary powder feeding system, electronic photographic powder feeding system, and scraper + ultrasonic auxiliary powder feeding system. This paper introduces the main factors affecting the interfacial bonding strength of multi-material additive manufacturing technology and the method of eliminating defects; introduces the current different powder feeding methods and the advantages and disadvantages of each powder feeding method; discusses the influence of process parameters on multi-material additive manufacturing; and finally summarizes the current bottleneck problems of the technology and looks forward to the main research directions in the future.
1. Scraper feeding system
The scraper powder feeding system was the first device to realize multi-material printing and two or more powder containers can store and convey two or more powders separately. However, this powder feeding system can only manufacture multi-material components along the deposition direction, which makes it difficult to achieve multi-material deposition with the same sedimentary layer, and the applicable material system is limited, so it does not have development potential. Researchers in Singapore upgraded the powder feeding system of standard L–PBF and successfully formed copper-stainless steel bimetals using dual powder silos, which have a sharp material interface where material mutations can easily produce defects such as porosity, cracks, and delamination at the material interface, making it difficult to achieve commercial applications.
2. Ultrasonic auxiliary powder feeding system
The ultrasonic-assisted powder feeding system is an advanced technology for precise control of powder delivery, which has a wide range of application prospects in the field of material forming. Through ultrasonic vibration, dry powder particles can be fed to the powder bed with micron-level precision, realizing high-precision control of formed parts, and providing an effective means for manufacturing high-quality parts. Researchers at the University of Manchester have made a breakthrough innovation in an ultrasound-assisted powder feeding system that successfully delivers dry powder particles from a variety of materials precisely onto the powder bed, culminating in laser-forming two-dimensional multi-material components. This innovation not only expands the range of applications of ultrasonic-assisted powder feeding systems but also opens up new ideas and paths for manufacturing higher precision and high-quality parts.
3. Electronic photographic powder feeding system
The application of electrophotography technology in the industrial printing industry is very successful, it is the use of electrostatic force to selectively transfer the material to the substrate, therefore, some scholars propose to apply electrophotography technology in 3D printing to achieve multi-material selective additive manufacturing. While the system can convey multi-material powders quickly and accurately, stacking them into three-dimensional parts is challenging because the electric field decreases as the thickness of the deposits increases. Stichel et al. developed an electrophotographic powder feeding system based on a powder bed, which proved that the powder feeding system is feasible to manufacture multiple materials, and achieved a constant potential between the surface of the printed powder layer and the photoconductor, reducing the thickness correlation of the deposited layer. At present, electrophotography technology still has shortcomings, such as improper process parameters causing accidental drops of powder, which is easy to cause powder contamination, in addition to the need to classify the deposited powder in terms of material and particle size.
4. Scraper + ultrasonic auxiliary powder feeding system
The scraper + ultrasonic assisted powder feeding system is currently the ideal solution, proposed by researchers at the University of Manchester, which integrates a blade and ultrasonic powder dispenser and uses a vacuum powder suction device to accurately remove the powder. The start and stop of the powder flow are precisely controlled by the ultrasonic vibration excitation signal, which can improve the dry powder flow, destroy the powder agglomeration, and ensure a stable and continuous powder flow. The device was used to successfully prepare a series of 316L stainless steel and Cu10Sn copper alloy composed of a variety of metal materials, which proved that the system not only has feasibility, but also improves the powder deposition efficiency, but the powder feeding efficiency still needs to be further improved.
Different from the traditional single-material design, multi-material forming software should have the function of designing the mechanical properties, spatial distribution, and geometry of the material, and the 3D CAD software based on voxel modeling can achieve these functions, but it needs to establish a complete material science database to match, and it still does not have the professionalism of multi-material additive manufacturing forming. The only software on the market, such as ParaMatters and Monolith and other multi-material additive manufacturing tool software, is mainly used to form polymers, and there are fewer applications for metal materials, so multi-material additive manufacturing software needs to be explored and designed in the future.
