3D Printing Technology with Liquid Metal
Liquid metal can be formed by a variety of printing methods for three-dimensional structure formation, and can be combined with non-metal and bio-ink materials to achieve composite printing. It has important application prospects in the fields of biomedical equipment, three-dimensional electronics, and functional device manufacturing.
3D printing, also known as additive manufacturing. As an advanced manufacturing technology, it is considered as an important engine of the "third industrial revolution" to meet people's individualized and customized needs for physical objects through the processing method of "layered manufacturing and incremental molding". Metal 3D printing technology, as a cutting-edge technology with the most engineering application potential in the 3D printing manufacturing system, is one of the key development directions to accelerate the development of new technologies and new equipment for intelligent manufacturing. Among them, the innovative R&D and achievement transformation of special metal printing materials, process technology level, manufacturing equipment and core devices are the key technology nodes for the development of 3D printing. In recent years, liquid metal has emerged in the field of advanced manufacturing, showing unique technical advantages.
The unique advantages of liquid metal 3D printing technology
Originating from the exploration and practice in the field of liquid metal for more than ten years, the author's laboratory was the first in the world to realize the great value of liquid metal in 3D printing and advanced manufacturing at a wider level, and took the lead in proposing relevant academic concepts and technologies. Thought, and developed a series of manufacturing equipment and even launched market-oriented products [1]. So far, domestic and foreign research and development activities around liquid metal 3D printing and even functional device manufacturing have entered a stage in full swing. It can be said that an advanced manufacturing field with great vitality is taking shape. The advantage of combining liquid metal with 3D printing technology is that due to the use of low-melting liquid metal ink, the entire manufacturing process can be alternately printed with non-metallic materials, thereby realizing the rapid development of various electronic circuit functional devices such as wearable devices. Forming makes it possible to automatically manufacture and assemble the target terminal equipment. The corresponding research results are expected to change the manufacturing rules of traditional electronics and integrated circuits, and provide a transformative way for the development of the modern electronics industry, and the whole process is fast, green, and low-cost; at the same time, such technologies will also significantly improve flexible intelligent machines and bionics. The development process of robots is also extremely valuable for the development of flexible power sources and mechanical power systems that go beyond the traditional ones. It is not difficult to see that, based on the technical challenges in the fields of materials, equipment and applications faced by the existing 3D printing industry, efforts to promote the research and development of beyond the traditional liquid metal 3D printing technology, raw material ink and popular desktop printing equipment will meet the needs of aerospace industry. , biomedical, cultural and creative fields and other major needs, promote 3D metal printing from the niche to the public, and then realize the explosion of the industry to provide sufficient power and material basis.
Liquid metal laser 3D printing technology
For low melting point liquid metal, the target object can be formed by laser additive manufacturing. The specific manufacturing process includes 5 steps: preparation of low melting point metal powder, surface modification of powder particles, printing model design, laser additive manufacturing, Forming and demolding. Among them, methods such as ball milling, airflow atomization, ultrasonic atomization, microfluidics and jet fracture based on continuous electrowetting have been developed for the preparation of low melting point metal powders [2]. Micron homogeneous metal powder. Particle surface modification is mainly to modify the electrical and magnetic properties of particles by coating, doping and other methods. The process of model design and additive manufacturing is consistent with the traditional high-melting-point metal manufacturing method: ① Use slicing software to slice the model of the printed object layer by layer and convert it into a computer control program for the laser motion path; ② Place the substrate on a liftable print On the platform, the printing cavity is filled with inert gas to discharge the oxygen in the cavity; ③ Use a roller to spread a layer of metal powder with a thickness of less than 100 microns on the substrate; ④ The laser beam moves and scans the powder on the substrate under the control of the computer layer, the powder in the place where the light spot passes is sintered and formed; ⑤ The printing substrate is lowered by one layer, and the roller is re-applied with a layer of powder for the next printing. After the structure is formed and demolded, the printed structure needs to be surface treated, mainly including chemical treatment (such as cleaning with hydrochloric acid or sodium hydroxide solution), electrochemical polishing, physical sandblasting, coating deposition technology, etc.
Liquid metal fused deposition printing technology
Among many 3D printing technologies, fused deposition printing is a basic and widely used printing method, which is to deposit and solidify molten ink on a substrate layer by layer, and print objects from bottom to top. Fused deposition printing methods can be divided into extrusion and inkjet printing. The ink used in extrusion printing is filament, which is fed into the nozzle by the feeding roller, while the inkjet printing uses the ink directly in the nozzle. Heat and deposit drop by drop. For inkjet printing, the printing speed is related to the moving speed of the nozzle and the time interval of ink droplet ejection. When the interval is short, the ink is continuously ejected and no individual ink droplet appears. The inks commonly used in fused deposition printing technology include plastics, metals and metal matrix composites. Among different types of metal inks, low melting point liquid metal has unique advantages, which is due to its low melting point, easy to achieve the melting and solidification of the ink, and the research on this printing process also provides a variety of materials for composite printing and function The direct fabrication and assembly of sexual terminal devices opens up new directions.
Liquid metal liquid phase 3D printing technology
The cooling environment of traditional metal 3D printing is often air or vacuum, which can be called "dry printing", and the cooling speed is slower. In order to speed up the cooling of metal parts, the author's laboratory has established a liquid phase cooling 3D printing method [5]. The principle is that the deposition and molding process of metal is carried out in a liquid phase fluid. The electrolyte of water ethanol, acid or alkali, and the metal used for printing is liquid metal. Using this printing mechanism, the 3D metal structure can be rapidly formed.
Compared with traditional gas phase fluid cooling, the liquid metal liquid phase 3D printing method has the following advantages: ① It can be printed quickly and flexibly. Due to the fluid control mechanism used in the printing process, various three-dimensional structures can be printed, and the temperature and flow rate of the cooling fluid can also be flexibly controlled. For example, by adjusting the flow rate and direction of the cooling fluid, some unique three-dimensional structures (such as rotating body). ②A higher cooling rate can be achieved, and the oxidation of the metal ink can be effectively avoided or reduced, which is a technical problem that is difficult to overcome in traditional printing. ③ The printing energy consumption of metal parts will be greatly reduced. Compared with traditional high-melting metal printing, the difficulty of manufacturing will be greatly reduced. In the future, the combination of driving pump array and printing nozzle array can be used to realize the target object from zero dimension to zero dimension. 3D Rapid Manufacturing. Liquid-phase 3D printing is a conceptual innovation to traditional printing technology, which will raise many fundamental technical problems.
Liquid metal suspension 3D printing technology
In the process of exploring liquid metal printing technology, the author's laboratory established a suspension 3D printing method [6]. In the absence of external field effects, higher surface tension and lower fluid viscosity are the main factors that determine the extrusion process and shape of the liquid metal through the nozzle. These characteristics make the liquid metal often hang on the tip of the nozzle in the form of droplets after extrusion, and the fusion phenomenon between the contact droplets is easy to occur, which restricts the liquid metal to form a macroscopic three-dimensional structure. In order to overcome the influence of surface tension and viscosity on the formation of liquid metal, self-recovery hydrogel is used as the supporting material, and the material properties of the gel can be freely converted between the fluid and solid state, so that the printing nozzle can be in the gel supporting environment according to the material properties. The pre-set path is free to reciprocate, and the liquid metal is continuously extruded. The gel material is used to support and fix the shape of the extruded liquid metal, and a macroscopic three-dimensional structure with a complex shape is formed by layer-by-layer accumulation. During the movement of the nozzle, the gel material is partially liquefied by the extrusion of the nozzle, which allows the nozzle to be easily inserted into the gel and move freely. When the nozzle passes through, the liquefied gel will quickly solidify and return to a stable form .
During the printing process, the liquid metal is continuously extruded through the printing nozzle, and the high surface tension makes the extruded liquid metal hang on the tip of the nozzle in the form of spherical droplets. The metal droplets are necked and finally disconnected from the nozzle, wrapped and fixed by the support gel, leaving a series of independent liquid metal microspheres on the path passed by the printing nozzle, so that the liquid metal can pass through the liquid metal in the gel support environment. The layer-by-layer accumulation of microspheres finally forms a three-dimensional structure. We use this method to print liquid metal three-dimensional circuits, and rely on extruded liquid metal microspheres to realize the circuit connection of electronic devices, thereby forming three-dimensional three-dimensional circuits. The experimental results show that the liquid metal microspheres have good electrical conductivity and can still achieve effective connection of electronic devices in the support gel.
Metal-non-metal composite printing technology
Metals and non-metals such as plastics and polymers are often formed by different printing methods due to the large differences in material properties. So, can different printing methods or materials be combined for composite printing? The answer is yes. The author's laboratory first proposed and established a composite 3D printing technology of liquid metal and non-metal [7]. The "composite" here can be a mixture or combination of different structures, different inks, and different printing methods. The metal and non-metal materials here are selected as bismuth indium tin alloy and 705 silicone rubber respectively: the melting point of bismuth indium tin alloy is about 60 ℃, and it is solid at room temperature; 705 silicone rubber is a neutral and transparent one-component room temperature vulcanization Silicone rubber, which can be cured by absorbing moisture in the air at room temperature, has non-toxic, anti-corrosion, insulation, anti-arc, excellent bonding properties, etc. The role of stabilizing device performance; good compatibility between bismuth indium tin alloy and 705 silicone rubber. The composite printing process of the two materials is as follows: first, a layer of silicone rubber with a thickness of several millimeters is printed on the base of a circular petri dish. A layer of metal structure was printed with bismuth indium tin alloy ink, and then a layer of 705 silicone rubber was printed again. After curing for 5 hours, the printed object was removed from the petri dish. This method can be used to print plane and three-dimensional structures, such as printing a three-layer LED three-dimensional circuit, which consists of 6 LED lights and 6 current-limiting resistors. The circuit is divided into 3 layers, and the LED light-emitting colors are red, yellow, green. The entire printed object includes a three-layer metal structure and a four-layer non-metal structure, as well as connecting posts between adjacent metal layers. When connected to the DC power supply, the LED lights in the circuit emit bright colors. The performance of the three-dimensional circuit depends on the properties of the two printing materials. The mechanical properties of the circuit such as hardness and shear strength mainly depend on the 705 silicone rubber, and the electrical properties depend on the bismuth indium tin alloy ink.
Composite printing can be the interactive printing of multiple inks, or the combination of multiple printing methods. The printing temperature of low melting point metals, especially liquid metals at room temperature (such as gallium-based and bismuth-based alloys) is usually lower than 150 °C, close to Printing inks such as plastics and biological materials can be easily combined with these materials for low-cost one-stop manufacturing of terminal objects [8]. It is foreseeable that composite printing will be an important direction for the development of 3D printing technology in the future. Combining 3D printing technology with liquid metal materials can explore more newer printing technologies and processes and expand a wider range of applications. It will bring more innovations to the development of industrial manufacturing, biomedicine and many other fields.
