Multi-dimensional Additive Manufacturing

Lasers are employed for manufacturing (laser manufacturing) in a wide variety of applications, including shape control of metal materials by laser processing and welding, as well as modification of microstructure and functionality through laser quenching. Fundamental research is currently being conducted on laser manufacturing and its industrial applications. In this context, selective laser melting (SLM) can be considered as a culmination of laser manufacturing technology, which is a method for manufacturing three-dimensional objects with complex shapes using the advantages provided by lasers. This chapter introduces the fundamental information on additive manufacturing technology, which has garnered considerable attention in recent years. In particular, the fundamental and potential applications of SLM, which uses lasers as a heat source are showcased.

During the additive manufacturing (AM) process by laser melting, laser irradiation solidifies the surface layer of metal powder by the generated heat. Subsequently, another powder layer is added thinly on the irradiated surface. The sequence is repeated to obtain the designed three-dimensional structure. This represents the basic routine of laser processing such as selective laser melting (SLM) and selective laser sintering (SLS). The heated part can be solidified by melting or sintering. Both heating processes connect individual metal particles and add them to the processing structure, ultimately fabricating the designed object. The products made by this laser process have a similar performance to the conventionally processed metal parts. This is the most important characteristic of laser processing of metal materials compared to any other AM processes. These laser processes are introduced in this section, and their advantages and disadvantages are described.

Selective electron beam melting (SEBM) is a type of additive manufacturing (AM), which is categorized into powder bed fusion (PBF) in ISO52900 as well as selective laser melting (SLM). SEBM is denoted by PBF-EB, meaning electron beam-based PBF. Formerly, SEBM was denoted by the term “E-PBF,” but currently “PBF-EB” is used. To emphasize that the process is for metal, the term “PBF-EB/M” is used to indicate SEBM. Overviews of SEBM and the characteristics of the metal microstructures of EBM-built parts have been already published in books [1, 2] and commentary articles [3‐8]. The term “electron beam melting (EBM)” is used for meaning SEBM by the company Arcam, which developed this technology. The type of manufacturing method employing an electric beam, where the material is added through a process similar to build-up welding, and a metal wire is used instead of metal powder (i.e., Sciaky Co. Electron Beam Additive Manufacturing: EBAM [9]), is not dealt with here.

Electron beam melting (EBM) is one of the powder bed fusion (PBF) processes, and it is characterized by the usage of an electron beam as a heat source. In this process, an electron beam is precisely controlled with high-speed scanning. This scanning is also utilized to preheat the powder bed for the regression of thermal stress during building. The building is performed in a vacuum, so that oxidation can be minimized. Thus, the PBF-EB process is preferable for building reactive metals as a matrix or second phase. In contrast, the relatively rough surface due to the use of large powder and the raveling of sintered powder remain issues with this process. Herein, the trend of research and development of PBF-EB is reviewed from the scientific papers published from 2007 to 2016.

Three-dimensional additive manufacturing using stereolithography and nanoparticle sintering techniques was investigated to create micro-components with geometrically designed patterns composed of functional ceramic materials. Nanometer-sized ceramic particles with biological, electronic, or dielectric properties were dispersed in photosensitive liquid resins. The mixed slurries were solidified using laser scanning or micro-patterning. These composite precursors were dewaxed and carefully sintered, and micro-lattice structures were successfully obtained. Subsequently, the irradiation power of the scanning laser was increased systematically to process the ceramic components directly through dewaxing by ultraviolet decompositions, while sintering by electromagnetic absorption fevers. These components showed dendrite structures with periodic arrangements of micro-lattices to effectively control modulate liquid and gaseous material fluid flows and electromagnetic wave propagations. The technological details of the ceramic-free forming and applications of the functional dendrite structures will be reviewed.

The coating of various types of materials, i.e., metals, ceramics, polymers, and composites, with ceramic layers and films is one of the key technologies employed for prolonging the lifetime of infrastructural materials by protecting them from harsh environments. This process also enhances properties and adds multi-functionalities to materials. Thermal barrier coatings (TBCs) on turbine blades, hard coatings on cutting tools, and corrosion-protective coatings on steel pipes and containers are applied on an industrial scale, while biocompatible coatings on artificial bones, growth of semiconductor films, and depositions of nano-layers and particles for catalysts have been developed to produce high-performance functional materials. Vapor-phase deposition techniques, such as physical vapor deposition (PVD) and chemical vapor deposition (CVD), are advantageous, as they create finely controlled microstructure in films and deposits, compared to liquid- and solid-state processes. In TBCs fabricated by the electron beam evaporation method, the growth of columnar crystals with nanostructures, such as nano-pores and gaps, contributes to the relaxation of thermal stress at the interface between coatings and basal materials, owing to the difference in thermal expansion coefficients. CVD, an industrially important vapor-phase deposition technique, has been employed for the growth of silicon semiconductors, thin film processing of integrated circuits, insulating coatings, and hard coatings on cutting tools. In CVD, regulation of the process parameters, such as deposition temperatures, pressures, and instrumental systems of heating and supplying precursor vapors, has enabled us to produce various types of deposits with well-controlled microstructures and unique textures. Single crystal-like highly pure films can be grown, while coatings with thick layers can be deposited. CVD is also employed for the synthesis of nanomaterials, such as graphene and carbon nanotubes (CNTs) and for the infiltration in manufacturing of fiber-reinforced ceramic composites (ceramic matrix composite: CMC). CVD techniques combined with high-power laser irradiation have enabled the fabrication of thick layers with discriminating microstructure at significantly high deposition rates. Although the typical applications of CVD techniques are for the deposition of films on flatter surfaces of plate-shaped substrates like wafers, CVD can also deposit coating layers and nanoparticles on particulate and porous substrates with complex surface configurations because of its excellent step coverage. These CVD techniques utilizing laser processing and powder technologies are expected to contribute to the further development of additive manufacturing. In this chapter, the basis of conventional CVD processing is briefly outlined; then high-speed deposition and refinements of microstructure by CVD using the auxiliary energy of high-powered lasers and the deposition of coatings and nanoparticles on particulate substrates by a rotary CVD technique are introduced.

Research and development in the field of material science is aiming for further advancements, and expectations in the joining of materials and composite technology are progressively increasing. In particular, a new concept proposed for the control of interface phenomena with the aim to combine materials that are difficult to bond and composite has been attracting increasing interest. This concept has been accelerating progress in the interdisciplinary research area of material development. In this study, joining and compositing of various materials are carried out by precisely controlling the crystal growth and the interface phenomenon in an aqueous solution. Although joining of certain materials has been difficult in the past, these materials are now held together in nano- or micro-sized regions in order to control the microstructure (Masuda et al, Bull Ceram Soc Jpn 49(5):356–360, 2014; Masuda, Bull Ceram Soc Jpn 47(12):929–934, 2012). In particular, a composite structure of a tin oxide (SnO₂) nanosheet assembly and a polymer material is prepared. Oxide materials, such as tin oxide, have been synthesized by high-temperature firing on the order of several hundred degrees and faced many difficulties in bonding and compositing the tin oxide structure to the surface of the polymer material with low heat resistance. Using the proposed method, synthesis of tin oxide crystals can be realized at ordinary temperatures by determining a suitable condition for tin oxide crystallization in an aqueous solution, thereby facilitating bonding with the polymer. In the aqueous solution, nucleation of tin oxide and crystal growth is controlled on the substrate surface to form a nanostructure. Furthermore, crystal growth in SnO₂ is precisely controlled to form a tin oxide nanosheet, and an integrated structure composed of tin oxide nanosheets is synthesized. Moreover, the principle of this method can be applied to control the crystal growth and nanostructure in other materials. The wide applications of this method contribute to the development of various composite materials, and further progress in research on the interface phenomena of　different materials is expected. We aim to develop a method to join different materials, control their nano/microstructure, and achieve patterning (Masuda et al, Bull Ceram Soc Jpn 49(5):356–360, 2014; Masuda, Bull Ceram Soc Jpn 47(12):929–934, 2012). Particularly, a composite material consisting a SnO₂ nanosheet assembly and a polymer film is developed via crystal growth in the aqueous solution.

Recently, the aerosol deposition (AD) has attracted attention in the field of coating technologies. In this process, dry solid particles of ceramics or metals are conveyed by gas and sprayed from nozzles toward substrates, forming thick coatings at low temperature at high deposition rates. The novel features of these processes are regarding the deposition of fine or ultrafine particles on a solid state, instead of molten or semi-molten states as in a conventional thermal spray method. They have significant impact on the coating technology from both principle and application perspectives, and they exhibit a large potential for use in additive manufacturing (AM) technology. Therefore, it is a natural consequence that these solid particle impact processes lead a breakthrough in coating technologies, and are expected to bring new insight to coating technologies such as the AM technology. In this context, we would like to introduce the AD method capable of depositing ceramic coatings at room temperature and present the method’s applicability to the AM technology.

Among various types of surface treatments available, thermal spraying techniques have been used for the past century in many industrial fields owing to their ability to produce coatings of several tens of micrometers to several millimeters in thickness. Such techniques use flames, arcs, plasma, and other methods to heat and accelerate particles. In contrast, cold spray (CS) techniques [1‐7], which have been attracting attention during the past 20 years, apply a deposition technology based on impacting particles at high velocities. That is, accelerating the particles using a relatively low-temperature and high-velocity working gas results in a significant difference from other thermal spraying methods. Because the gas temperature is lower than the melting point of the material particles, this technique is called a “cold” or low-temperature spray. It has also been called a kinetic spray, because of the coating formation through the kinetic energy of the particles. In this chapter, as an outline of the CS method, the basic principle, equipment, features, materials, application examples, and issues in the use of a cold spray are described. In addition, CS technology has been systematized to a certain extent, and readers are encouraged to refer to other specialized materials [2‐7] focusing on this area. In addition, as an additive manufacturing technique, a nearly net-shaped formation is also being studied using a thick coating as a possible feature of CS technology, which is briefly explained in the following.

The cold spray (CS) is a solid-state metal particle deposition technique developed by Russian researchers, Alkhimov and Papyrin, et al. (Ide in Light Mater Weld 40:27–34, 2002) during the 1980s. In this technique, metal microparticles are accelerated from subsonic to supersonic speeds using compressed gases, e.g., air, nitrogen, or helium). The particles collide with a substrate in their solid phase without melting, thus forming a film. The current CS technique is classified as the thermal spray (TS) technique, wherein metal or ceramic particles are melted using plasma, laser etc., and collide with a substrate to create a film. However, the CS technique is a completely different process from traditional thermal spraying, as the former does not involve melting the particles, but instead deposits them in a solid state. There are many advantages of the CS technique, some of which are listed below (Japan Welding Society in Welding and joining handbook, Japan Welding Society, 2003; Alkhimov et al in U.S. Patent No. 5, 302, 414; April 12, 1994; The Mechanical Social Systems Foundation, Research report on innovative parts manufacturing using high-speed particle collision 2005; Karthikeyan in International status and USA efforts, ASB Industries Inc., 2004): (1) Ability to apply fine airborne coatings, (2) Ability to inhibit oxidization, heat effect, and thermal stress, (3) Ability to apply thick films (on the order of tens of cm) (Sasaki in Therm Spray Technol 20:32–41, 2000), (4) Ability to apply a compressive residual stress coating (Sasaki in Therm Spray Technol 21:29–38, 2002), (5) Relatively compact nature of the equipment required. Because of these advantages, the CS technique is expected to replace traditional TS technique as a surface reforming technology. As the name implies, CS technique is not a cold processing technique. The term “cold” refers to the processing temperature, from 500 to 1000 ℃ for the working gas, which is lower than that used in traditional thermal spraying (performed at 5000 ℃ (Pattison in Int J Mach Tools Manuf 47:627–634, 2007) in case of plasma spraying). However, temperature of the gas is maintained at a high level to expand the gas and disperse particles at a high speed. During this process, the temperature of the metal particles still remains significantly lower than their melting point. It is possible to deposit the particles without melting or half-melting, and without taking into consideration the effect of significant oxidation on the particles in a solid state. In the TS technique, because the particles are melted and dispersed, residual stress is generated during the cooling process, making it difficult to form thick films. In contrast, it is possible to form thick films with the CS technique, as shown in Fig. 10.1, considering the materials and deposition conditions are conducive. This particular ability is expected to have potential applications not only in producing two-dimensional coatings, but also in three-dimensional molding technology. Figure 10.1 shows a fine copper powder deposited on an aluminum pipe at a depth of 50–60 mm. Later, the flange is processed, the holes are opened, and threads are cut. Though the particles are not melted at the time of deposition, there is a great cohesive strength between them, and machine operations can be performed in a similar way as with standard bulk materials. Figure 10.2 shows a schematic diagram of the CS equipment. The equipment consists of a nozzle to accelerate the particles to collide with the substrate at a high speed, a gas heater for heating the working gas and accelerate the particle by expanding the volume of a gas, and a powder hopper for supplying particles to the nozzle. CS techniques are generally categorized as low-pressure and high-pressure processes based on the pressure and temperature of the working gas. In the low-pressure process, the working gas pressure is maintained at 1 MPa or lower, and the maximum working gas temperature is maintained at a lower temperature of 500 °C (for a specified temperature range of 500–1000 °C). Therefore, in the low-pressure technique, the particle velocity is slower than the high-pressure process, making it difficult for metal materials (with a high melting point) to deposit on a surface. However, the low-pressure process can be applied to deposit relatively soft materials such as aluminum and copper, and the deposit of certain ceramic and polymer materials are also possible, as discussed below. However, as the low-pressure CS equipment produces a relatively slower particle velocity than the high-pressure equipment, it has significantly less efficiency. However, the equipment is very compact and can use the compressed air as a working gas—its two foremost advantages. Hence, the low-pressure process holds great potential to perform on-site repairs. The “high-pressure” process uses a working gas pressure in the range of 1–5 MPa and a maximum working gas temperature of 1000 °C. Using the high temperature and pressure to increase the particle temperature and velocity, it is possible to deposit metal materials with high melting points, such as stainless steel and nickel-based superalloys, with high efficiency. For soft materials such as pure copper, a deposition efficiency above 95% is reported. However, the equipment required to achieve such a specified high pressure is relatively large, and is not suitable to perform on-site repairs, which is a significant disadvantage for the process. High-pressure and low-pressure CS processes have their pros and cons, and it is advisable to select the process for a given task that is based on the purpose and materials. Here, we present an example of a metal particle deposition mechanism featuring aluminum particles deposited by applying low-pressure CS, as well as examples from analyses conducted by ceramic deposition and ultra-high-molecular-weight polyethylene deposition by applying low-pressure CS.

The thermal spray technology has been employed in practice for around 100 years, and the use of various feedstock as the utility of the thermal sprayed film has been extended. Metallic wires, and metallic and ceramic powders have been used since this process was developed. However, recently, liquid feedstocks such as suspension and solution precursors, etc., have been employed. The former process is referred to as the suspension plasma spray (SPS), whereas the latter is named precursor spray (PS) in above thermal spray methods using liquid precursors. The PS, which is introduced in this chapter, is a process employing the solution precursor as feedstock. In this process, high rates deposition of metal films and ceramics films are conducted by utilizing chemical reactions of feedstock materials in thermal plasma environment. Although this process was mainly known as plasma jet CVD or thermal plasma CVD upon its development, recently this process is referred to as precursor spray or solution precursor plasma spray (SPPS), because the SPPS equipment is the same as that of the conventional plasma spray except for the feedstock feeding system. As the conventional CVD and plasma CVD, PS can deposit films with various components and various microstructures such as lamellar, columnar, porous, dense, etc., by controlling deposition conditions. Moreover, the film deposition rate of PS is much higher than those of CVD and plasma CVD. Therefore, PS is expected to function as a deposition process for functional film. In this chapter, combustion flame CVD, known as the high rate diamond synthesis method, is introduced as a PS.