Application And Development Of Metal Additive Manufacturing Technology in Weapons And Equipment

Abstract Metal additive manufacturing technology can achieve rapid overall manufacturing of complex-shaped metal components, and has become a new method for the design and manufacturing of complex components for high-performance weaponry. This paper reviews the metal additive manufacturing methods and features used in the manufacture of weapons and equipment, and lists the typical applications of metal additive manufacturing technology in the field of weapons and equipment manufacturing according to the classification of materials. Based on the analysis of the current situation, the future development trend of metal additive manufacturing technology in the field of weapons and equipment is prospected.

Keywords metal additive manufacturing; weapon and equipment; application; challenges; development trend

With the development of light weight, strong protection, high damage, information technology and intelligence of weapons and equipment, the structure and function of their components are gradually becoming more complex and diversified, and the traditional machining processes such as casting, forging and welding are difficult to meet the manufacturing and repair needs. The rapid development of metal additive manufacturing technology in recent years has provided a new method for manufacturing and repairing complex components of weapons and equipment. Compared with traditional manufacturing processes, metal additive manufacturing technology does not require tooling, can reduce manufacturing processes, shorten manufacturing cycles, and can achieve lightweight and structurally integrated design and manufacturing of complex components.

In this paper, the authors mainly introduce the metal additive manufacturing technology and features used in the manufacture and repair of weapons and equipment, outline the current situation of the application of metal additive manufacturing technology in the field of weapons and equipment manufacturing, analyze the current problems in the application of metal additive manufacturing technology, and outlook the development trend of metal additive manufacturing technology in the application of weapons and equipment.

1 Metal additive manufacturing technology

Metal additive manufacturing technology is an advanced manufacturing technology that uses metal wire, bar or powder as raw materials, and achieves the overall forming of components through sintering, melting, spraying, etc. by stacking layer by layer according to the predetermined route after model dispersion. At present, the metal additive manufacturing technologies mainly used in the development of domestic and foreign weapons and equipment include laser, electric arc, electron beam, cold spray, stirring friction additive manufacturing technology, etc., classification and working principle as shown in Figure 1.

1.1 Laser additive manufacturing technology 

Laser additive manufacturing technology uses a high-energy laser as the heat source to melt powder or wire under the protection of inert gas and accumulate layer by layer to realize direct forming of parts. Laser additive manufacturing technology includes laser-powder additive manufacturing and laser-fused wire additive manufacturing, of which laser-powder additive manufacturing technology is divided into laser-selective melting additive and laser-coaxial powder feeding additive manufacturing. Compared with other additive manufacturing technologies, laser additive manufacturing technology, especially laser-selective melting additive manufacturing technology has high forming accuracy and is suitable for the overall manufacturing of complex fine structure parts of weapons and equipment. However, laser-selective melting additive manufacturing technology is limited by the size of the inert gas chamber and the cost of equipment and powder, which is not suitable for the rapid and economic manufacturing of large and complex components. In addition, due to the high thermal conductivity of materials such as aluminum alloys and their high reflectivity to laser light, defects such as cracks and porosity are prone to occur during laser-selective melting additive manufacturing . Compared with laser-powder additive manufacturing technology, laser filament additive manufacturing technology has fast deposition rate, high material utilization, low cost, high density of additive components, and easy storage of filaments, but it is not suitable for fine structured parts and metal materials where filaments are difficult to prepare.

1.2 Electron beam additive manufacturing technology

E-beam additive manufacturing technology uses a high-energy density electron beam as the heat source to melt metal filler materials such as wire or powder in a vacuum environment and deposit them according to a pre-planned path to produce metal parts or blanks. Compared with laser additive manufacturing technology, electron beam additive manufacturing technology has a faster deposition rate and can produce refractory metals. Because it is performed in a vacuum environment, it not only avoids material contamination by oxygen, hydrogen and nitrogen, but also has a vacuum melting effect on the metal, so e-beam additive manufacturing technology can meet the demand for additive manufacturing of metals that are very active at high temperatures, such as titanium alloys. In addition, the electron beam can quickly scan over the surface of the stacked metal before subsequent metal stacking to preheat it and reduce residual stress and deformation during the additive manufacturing process. Compared with electron beam selective melting powder additive manufacturing technology, electron beam fused wire additive manufacturing technology has fast deposition efficiency, high component density, low material cost, high utilization rate, and is suitable for rapid manufacturing of large components. However, due to the small electron beam spot and concentrated energy, it is easy to interrupt the additive manufacturing process when the wire deviates from the electron beam spot area due to thermal deformation or poor diameter uniformity during the electron beam filament additive manufacturing process.

1.3 Arc filament additive manufacturing technology 

Arc filament additive manufacturing technology (hereinafter referred to as "arc additive manufacturing technology") uses metal filaments as fillers, melts the filaments through the electric arc, and builds up layer by layer according to the set route to achieve the overall formation of metal components. Similar to arc welding, arc additive manufacturing technology can be divided into melting electrode and non-melting electrode arc additive manufacturing technology according to the electrode type. Among them, non-melting arc additive manufacturing technology includes tungsten argon arc and plasma arc . Compared with laser and electron beam powder-based additive manufacturing technology, arc additive manufacturing technology is less likely to produce defects such as unfused, high manufacturing efficiency, high material utilization, low wire and equipment costs, and is suitable for the overall rapid manufacturing of large, more complex weapon and equipment components. However, compared with laser or electron beam powder-based additive manufacturing, arc additive manufacturing technology has lower manufacturing accuracy and requires subsequent mechanical processing, which makes it difficult to realize the manufacturing of complex and fine structure parts. In addition, arc additive manufacturing technology is not suitable for poor plastic deformation ability, difficult to prepare into wire metal materials.

1.4 Cold spray additive manufacturing technology 

Cold spray additive manufacturing technology will be metal powder or metal/non-metal mixed powder at supersonic speed sprayed onto the surface of the substrate, the powder and the substrate collision plastic deformation and adhesion to the surface of the substrate, layer by layer stacking to form a solid component . In the cold spray additive manufacturing process, the powder is not melted, and only relies on kinetic energy to adhere and accumulate into a block. Cold spray additive manufacturing technology has the advantages of high deposition rate, low residual thermal stress, and less oxidation of the material, which is suitable for metal materials that are prone to oxidation and poor thermal stability, such as magnesium alloy. However, the cold spray additive manufacturing process of metal materials is prone to holes, mechanical bonding between metal particles and low bonding force, resulting in mechanical properties, corrosion resistance is difficult to meet the target demand, the need for post-treatment such as hot isostatic pressing of the additive components . At present, cold spray additive manufacturing technology is mainly applied to the surface modification and repair of components of weapons and equipment.

1.5 Stirring friction additive manufacturing technology 

Stir friction additive manufacturing technology heats up metal powder or bar material to a thermoplastic state through the heat generated by mechanical friction, and metallurgical bonding is produced by layer-by-layer accumulation to realize the overall rapid forming of components. The stir friction additive manufacturing process does not involve the melting of metallic materials and is a solid-state additive manufacturing forming process. Compared with other melting additive manufacturing technologies, stir friction additive manufacturing technology has lower residual stress, higher density, lower susceptibility to defects such as porosity, and finer organization, and better mechanical properties and corrosion resistance. In addition, stir friction additive manufacturing technology works in an open atmosphere and is not limited by the size and working conditions of the powder bed or vacuum system, so it can be used for the overall manufacturing of large weapon and equipment components, especially for melting additive manufacturing of aluminum alloys, magnesium alloys and other lightweight alloys with high defect sensitivity. However, stir friction additive manufacturing technology is difficult to achieve the manufacturing of complex, fine structure or high melting point metal parts due to the low manufacturing accuracy and limited heat source heat.

2 Domestic and international application status

In recent years, domestic and foreign attach great importance to the application and promotion of additive manufacturing technology in weapons and equipment. In 2016, the U.S. Department of Defense released the "Additive Manufacturing Technology Roadmap", which analyzes the needs of national defense for additive manufacturing technology and elaborates the development goals of design, materials, processes and value chain and other technology areas. 2017, the U.S. Navy released the "Navy Additive Manufacturing Implementation Plan", which determines Long-term development goals for additive manufacturing technologies. In order to promote the application of additive manufacturing on ships, the Navy Additive Manufacturing Plan was released in 2018, which added additive manufacturing for ships to increase maritime security capabilities. The U.S. Air Force has proposed an Additive Manufacturing Printing Strategic Plan to introduce key technologies, development strategies, and goals for Air Force additive manufacturing, with the hope of establishing a global manufacturing network in the future to enable on-demand printing processes, reduce costs, and effectively improve military flexibility. The U.S. Army refined the domain requirements based on the DoD Additive Manufacturing Technology Roadmap, developed the Army Additive Manufacturing Technology Roadmap, and introduced the application needs and goals of additive manufacturing technology in the areas of Army maintenance and support, acquisition and deployment of new components/systems, and expeditions. 2021 the U.S. Department of Defense released the Additive Manufacturing Strategy, which introduces the meaning of additive manufacturing technology and its impact on national defense strategy In 2021, the U.S. Department of Defense released the Additive Manufacturing Strategy, which introduced the meaning of additive manufacturing technology and its impact on national defense strategy, described in detail the key development areas and path planning of additive manufacturing, and proposed future development directions. In 2015, China released the strategic plan of "Made in China 2025", which takes additive manufacturing and other advanced manufacturing technologies as the key development direction. 2017, the Ministry of Industry and Information Technology and other twelve departments issued the "Action Plan for the Development of Additive Manufacturing Industry (2017-2020)", which clarifies the development goals of additive manufacturing industry, introduces the key tasks and safeguard measures for the development of additive manufacturing. In 2020, China launched the "Additive Manufacturing Standard Pilot Action Plan (2020 -2022)", proposing to basically establish an additive manufacturing standard system based on national conditions and dovetailing with international ones by 2022. After decades of rapid development, metal additive manufacturing technology has been applied to the development, production and repair of weapons and equipment at home and abroad, greatly shortening the development and repair cycle of complex parts, reducing manufacturing and maintenance costs, increasing the freedom of structural design and manufacturing, and improving the comprehensive technical and combat capabilities of weapons and equipment. At present, metal additive manufacturing technology for weapons and equipment parts manufacturing and repair of the material types involved include special steel, titanium alloys, aluminum alloys, high-temperature alloys, magnesium alloys and refractory alloys.

 2.1 Special Steel 

In February 2019, the U.S. Army Laboratory successfully printed and delivered a high-strength steel impeller fan for the turbine engine of the Abrams M1 main battle tank using laser-selective melting additive manufacturing technology. The UK's BAE used arc additive manufacturing to achieve the overall manufacturing and application of high-strength steel artillery shell casings.

Shenyang Aircraft Design Institute and Beijing University of Aeronautics and Astronautics realized laser additive manufacturing of ultra-high-strength steel A100 aircraft landing gear and passed the trial ahead of others .In 2018, Ningbo Branch of China Academy of Weapons Science used electric arc additive manufacturing technology to realize the overall manufacturing of heat-resistant steel head cones for weapons and equipment, and successfully passed the installation assessment. Nanjing University of Science and Technology used arc additive manufacturing technology to realize the overall manufacturing of ultra-high strength steel artillery shell body.

2.2 Titanium alloy 

In 2019, General Dynamics Land Systems, Inc. and General Additive Manufacturing, Inc. cooperated to realize the monolithic manufacturing of titanium alloy cable shroud, which successfully replaced the original 18 pieces of steel welded parts of the U.S. Army ground combat vehicles with 85% mass reduction. AeroMet used laser additive manufacturing technology to manufacture F-22 fighter joints, F-18 fighter wing root reinforcements and landing gear connecting rods, which have been installed in aircraft. The F-22 joint has achieved more than twice the required fatigue life, the wing root stiffener has achieved more than four times the required fatigue life, and the landing gear linkage has exceeded 30% of the fatigue life of the original part. In addition, AeroMet has used laser additive manufacturing technology to repair broken titanium alloy components in military helicopters. BAE and Cranfield University collaborated to manufacture the titanium wing beam of Typhoon GR4 fighter jet using arc additive technology.

Wang Huaming broke through the key technologies of large titanium alloy laser additive manufacturing process, engineering equipment, internal quality and mechanical property control of components, and produced a large titanium alloy main bearing frame, which successfully optimized the original multiple mechanical connection structure into a single component and has been applied in a certain type of fighter jet. The titanium alloy parts made by Gong Shuihui and others using electron beam fusion wire additive manufacturing have been applied to a certain type of fighter jet. The high-temperature titanium alloy products made by Xinjinghe team through laser deposition additive manufacturing technology have been installed in an aircraft model.

 2.3 Aluminum alloy 

In 2016, the U.S. Navy used additive manufacturing technology to produce a new aluminum alloy connector chassis, which solved the fracture of missile connectors while significantly shortening the design and manufacturing cycle. 2020, the U.S. Meldmanufacturing Company used stir friction additive manufacturing to achieve the overall manufacturing of aluminum alloy parts with a diameter of 3.05 m, and is currently working on the 5th generation and future warplane aluminum alloy components. The research of stirred friction additive manufacturing repair technology for 5th generation and future warplane aluminum alloy components is underway.

In 2017, the Ningbo Branch of the Chinese Academy of Weapons Science has used arc additive manufacturing technology to print complex aluminum alloy components such as artillery gun frames, missile hulls and brackets, etc., which have greatly shortened the development cycle while achieving light weight through installation assessment and application. launch vehicle joint ring prototype.

2.4 High-temperature alloys 

In 2017, SAFRAN Group of France used laser-selective melting additive manufacturing technology to manufacture nickel-based alloy turbine nozzles to replace the original chromium-nickel-iron alloy castings, successfully reducing the original eight components to four, reducing the mass by 35%. 2017, Rockstar of the United States and the U.S. Air Force collaborated to print AR1 rocket engine high-strength ablation-resistant nickel-based high-temperature alloy components using additive manufacturing technology, which is similar to the RD-180 high-temperature alloy parts prepared by Russia. This printed part does not require metal coating compared to the previously prepared Russian RD-180 engine part. GE has successfully produced TiAl alloy blades using electron beam additive manufacturing technology, which have been applied to the GE9X engine, with a 50% mass reduction compared to traditional nickel-based high-temperature alloys. 2019 Orbex used nickel alloy as a raw material to print small rocket engines using laser-selective melting additive manufacturing technology, with a structure that reduces mass by 30% and can withstand extreme temperature and pressure fluctuations. Manufacturing time was reduced by 90% and cost savings were over 50% compared to traditional machining. The U.S. Air Force and GE used cobalt-chromium alloy as a raw material for laser additive manufacturing of the F-15 and F-16 combat aircraft F110 jet engine oil sump.

2.5 Magnesium Alloys 

In 2012, the U.S. Army repaired a magnesium alloy gearbox housing for a helicopter gunship through cold spray additive manufacturing. 2021, the U.S. Army Laboratory achieved the fabrication of a rare-earth magnesium alloy micro lattice structure through optimized laser-selective melting additive manufacturing to investigate compression behavior and fracture patterns, and is currently demonstrating the use of rare-earth magnesium alloy micro lattices in ultra-lightweight UAS and robotic vehicle components. Validation.

2.6 Refractory alloys such as tungsten and molybdenum 

In 2019, the Max Planck Institute in Germany broke through the laser-selective melting additive manufacturing process for pure tungsten and successfully printed a pure tungsten honeycomb structure for a magnetic confinement fusion device. 2020, Oak Ridge National Laboratory in the United States used electron beam additive manufacturing to prepare TiC-reinforced molybdenum-based composites that can withstand extreme temperatures and are suitable for use in high-temperature aerospace environments.

3 Problems and development trend

The development and application of metal additive manufacturing technology provides a new process for the manufacture and repair of complex metal components, which effectively complements the traditional manufacturing process and provides technical support and guarantee for the design, manufacture and service of complex parts of advanced weapons and equipment. However, metal additive manufacturing technology is still facing many problems and challenges in materials, processes and equipment, organization and performance, quality inspection and evaluation when applied to the manufacture and repair of weapon and equipment components. Figure 2 shows the development trend of metal additive manufacturing technology for weapons and equipment. Further corresponding basic and application research is needed in the future, as follows.

3.1 Design and preparation of high-performance metal materials for additive manufacturing of weapons and equipment

Melting-type additive manufacturing such as laser, electron beam and electric arc is a rapid heating and cooling process that forms microstructures and defect characteristics different from those of traditional manufacturing processes under the action of complex heat-force cycles.

First, under the action of high-temperature heat source, the burnout of alloying elements causes the overall alloying elements of the additive manufacturing stack to deviate from the target composition; during the solidification process of metal additive manufacturing, the redistribution of solute elements causes segregation, which leads to the deviation of local alloying elements of the additive manufacturing stack from the target composition, and the deviation of overall and local alloying elements content affects the performance of the component. Second, melt fluidity and surface oxidation directly affect the formability and defect sensitivity during the metal additive manufacturing process. In addition, under the thermal cycle of "warming-cooling-warming" and the stress cycle of "compressive stress-tensional stress-compressive stress" caused by the multi-layer stacking of additive manufacturing, the additive manufacturing components are prone to solidification cracks, reheat cracks, liquefaction cracks and other defects. Therefore, the design of metallic material composition for additive manufacturing needs to take into account the effects of metallurgical processes such as alloy element burnout, segregation, melt flow and oxidation, and heat-force cycling. However, many metal powders and filaments used in current additive manufacturing technology are often made of the same composition as castings and forgings, without considering the requirements of the above metallurgical processes and process characteristics of additive manufacturing on the type and content of alloying elements, resulting in poor formability, high defect sensitivity, and low performance of many metal materials for additive manufacturing (even after heat treatment), making it difficult to meet the manufacturing and service requirements of high-performance weapon and equipment components. The demand of high performance weapon and equipment components manufacturing and service. The current problems faced by the additive manufacturing of various metallic materials.

(1) Special steels: Special steels, especially ultra-high-strength steels, are susceptible to surface oxidation during the arc additive manufacturing process, resulting in inclusions in the components. Compared with the variable form or deformation heat treatment state, the ultra-high-strength steel made by additive manufacturing has coarse organization, serious segregation and low toughness, which requires heat treatment to improve toughness. However, in order to obtain a good match of strength and toughness, the heat treatment process of ultra-high strength steel is complicated, and the segregation of alloying elements in additive manufacturing increases the difficulty of heat treatment. Such as laser additive manufacturing secondary hardening ultra-high tensile steel stacked state is low, the need for heat treatment, the stacked body at 1150 ℃ insulation 0.5 h, furnace cooling to 900 ℃ insulation 1 h, air-cooled to room temperature; then at 680 ℃ for 16 h, air-cooled to room temperature; followed by austenitizing at 885 ℃ for 1 h, oil cooling to room temperature immediately after fast cooling to -73 ℃, placed 1 h; finally at 482 ℃ tempering 5 h, the heat treatment process is very complex, increasing energy and time costs at the same time, increasing the difficulty of process control . In addition, ultra-high strength steel has high hydrogen embrittlement sensitivity, poor plastic deformation ability, high difficulty in wire preparation, low yield, and high tendency of cold cracking in the process of additive manufacturing.

(2) Titanium alloy: Because titanium alloy has high activity at high temperature, it can react with oxygen, hydrogen, nitrogen and other elements, so it needs inert gas or vacuum protection in the process of titanium alloy additive manufacturing. In addition, the current titanium alloy system has a high tendency to form columnar crystals in the additive manufacturing process, resulting in anisotropic mechanical properties.

(3) Aluminum alloy: The solubility of hydrogen decreases in a stepwise manner when the aluminum alloy goes from the liquid phase to the solid phase. Therefore, a large amount of hydrogen precipitates during the solidification process of aluminum alloy in additive manufacturing, and the fast solidification speed of aluminum alloy causes the hydrogen to escape in time, which makes it easy to produce porosity defects. Due to the large thermal conductivity of aluminum alloy, the heat dissipation during the additive manufacturing process is fast, which easily leads to defects such as unfused holes. On the other hand, the coefficient of thermal expansion of aluminum alloys is large, and the stress and deformation are large during the additive manufacturing process, which is easy to produce thermal cracking defects, especially for high-strength aluminum alloys. In addition, because of the high oxidation tendency of aluminum alloy, the oxide film is formed on the surface during the arc additive manufacturing process, and if the oxide film does not float in time during the subsequent multi-layer and multi-lane stacking process, it will easily remain between the layers and channels of the deposited metal, increasing the impurity content in the stacked body. For aluminum alloy with high alloy content, on the one hand, due to the redistribution of solute elements in the solidification process of additive manufacturing, serious segregation of alloy elements and coarse incipient phases appear, which are easy to cause liquefaction cracks under the action of subsequent additive manufacturing thermal cycles; on the other hand, the addition of high content alloy elements expands the solid-liquid two-phase area of aluminum alloy, and the tendency of solidification cracks increases.

 (4) High-temperature alloys: High-temperature alloys are prone to the formation of defects such as unfused and metal vapor holes during the additive manufacturing process, which are closely related to the size of heat input . On the other hand, high-temperature alloys contain a large number of alloying elements, and the solid-liquid two-phase solidification process has a wide temperature range, which is prone to solidification cracking under the action of thermal stress. During the solidification of high-temperature alloy additive manufacturing, the impurity elements and alloying elements are segregated between dendrites and grain boundaries, and the segregation of alloying elements causes coarse primary phases between dendrites or grain boundaries. In the subsequent additive manufacturing thermal cycle fast hot and cold, impurity element segregation and incipient phase caused by the dendrites or grain boundaries to produce liquefaction cracks .

(5) magnesium alloy: magnesium alloy evaporation temperature is low, high vapor pressure, oxidation tendency, in the laser, arc and other heat sources easy to evaporate, the formation of unfused holes and pores and other defects, low density, and easy to be oxidized to produce inclusions . For magnesium alloys with high alloy content, the process of additive manufacturing is also prone to the formation of segregation and coarse primary phases. Defects, segregation and incipient phase formation reduce the mechanical properties of the additive manufactured magnesium alloy. In addition, due to the easy oxidation of magnesium alloy, powder is easy to burn and explode, plastic deformation capacity is poor, resulting in magnesium alloy powder and wire preparation is difficult and costly, oxygen, hydrogen and other impurities content is too high, the yield is low.

 (6) tungsten and molybdenum and other refractory alloys: Due to the high melting point of tungsten and molybdenum and other metals, it is difficult to melt in additive manufacturing, easy to form defects such as unfused and cracked . In order to ensure the formability of additive materials and reduce the number of defects, refractory metals such as tungsten and molybdenum need to be preheated at high temperatures on the substrate before additive manufacturing. The results of a study conducted by the Max Planck Institute in Germany showed that the preheating temperature of the substrate for laser additive manufacturing of tungsten alloys needs to reach 1000 °C in order to obtain high density. The high preheating temperature and process increase the difficulty of forming and quality control of the additive material, and also place higher demands on the additive equipment.

From the above analysis, it can be seen that the research on the design and preparation of special metal materials for additive manufacturing, the establishment of metal material systems and preparation process specifications with good formability and serviceability for additive manufacturing is of great significance for the application and promotion of metal additive manufacturing technology in weapons and equipment, in view of the manufacturing and application requirements of metal parts for weapons and equipment. In recent years, a number of studies have been conducted at home and abroad on metal materials dedicated to additive manufacturing, and breakthroughs have been achieved. The Royal Melbourne Institute of Technology (RMIT) of Australia has designed a new Ti-Cu alloy, which can form equiaxed, ultrafine crystal organization during the rapid solidification process of laser additive manufacturing, overcoming the problem of forming coarse columnar crystals during the laser additive manufacturing of Ti-6Al- 4V and other titanium alloys, with better strength and elongation. Ltd. developed a new aluminum alloy powder material suitable for laser additive manufacturing process by adding silicon and optimizing other alloying elements, which effectively solved the problem of thermal cracking in laser additive manufacturing of aluminum alloy. The University of Oxford, UK, designed and prepared a high-temperature alloy powder with low crack sensitivity for laser additive manufacturing for high-temperature alloys by calculations and experimental research for crack defects. Based on the metallurgical characteristics of arc additive manufacturing, Ningbo Branch of China Academy of Armament Science has designed and developed high-performance aluminum alloys, heat-resistant steels and (ultra) high-strength steel wires, which have solved the key problems of defect sensitivity of aluminum alloy additive manufacturing, tissue stability of heat-resistant steels and toughness control of (ultra) high-strength steels, and the related products have been successfully applied to the overall manufacturing of large complex metal components in the fields of armament and aerospace. Although the development of special metal materials for additive manufacturing has received attention, there is a single type of material, and most of the work is still in the laboratory research, and individual types of materials are at the stage of small batch trial production, which cannot meet the application and promotion needs of additive manufacturing technology in the field of weapons and equipment manufacturing. There is no clear criteria and basis for determining whether metallic materials are suitable for additive manufacturing.

Factors to be considered in the design of metal materials for additive manufacturing

(1) Additive manufacturing formability: The metal material system used for additive manufacturing must first have good additive manufacturing formability, including defect sensitivity, melt flow formability, dimensional accuracy and surface quality. The selection and design of wire alloys for arc additive manufacturing should also take into account surface oxidation and slag removal during the additive manufacturing process to avoid residual oxidation on the surface of the molten metal in the multi-layer continuous stacking process, which may cause arc drift and slag and inclusion problems.

 (2) Powder or filament preparation formability: metal materials for additive manufacturing should have good powder and filament formability, and the indicators include powder and filament size and shape, uniformity, impurity content, internal defects and material utilization.

(3) Additive manufacturing stack mechanical processing performance: At present, metal components often require mechanical processing to ensure the size and shape accuracy after additive manufacturing, so the selection and design of metal material systems for additive manufacturing should take into account the mechanical processing formability of the material.

 (4) Service performance: In order to meet the actual service environment requirements, additive manufacturing metal materials need to have good service performance, such as static/dynamic mechanical properties, corrosion resistance and oxidation resistance, etc., while requiring the performance control process to be as simple and easy to operate as possible.

 (5) Cost-effectiveness: As the basis of metal additive manufacturing technology, the cost-effectiveness of metal materials largely determines whether the technology can ultimately be applied in the field of weapons and equipment parts manufacturing. Therefore, the design and preparation of metal materials for additive manufacturing should take into account the cost performance of the raw alloy, powder or wire preparation, additive manufacturing process (such as whether special protection or auxiliary temperature control devices are required), machining, heat treatment process, and other aspects of the whole process.

3.2 Design and preparation of high-performance metallic materials based on additive manufacturing process

The harsh service environment of weaponry requires metal materials with excellent specific strength and properties such as impact resistance, corrosion resistance, high temperature resistance or wear resistance. Therefore, the design and preparation of high-performance metallic materials is a long-term concern and research hotspot in the field of weaponry. The development and application of high-performance metallic materials such as gradient materials, metal matrix composites, and high-entropy alloys have significantly increased the service performance and service life of weapons and equipment in terms of light weight, protection against strikes and efficient destruction. For example, the application of aluminum matrix composites on pistons has significantly improved the power and service life of engines while meeting the lightweight design of military vehicles; the high-temperature alloy/copper alloy gradient materials (outer high-temperature alloy and copper alloy lining) for rocket engine combustion chambers can simultaneously ensure the high-temperature strength of the outer layer and the thermal conductivity and heat dissipation needs of the lining. At present, gradient materials, metal matrix composites, high entropy alloys and other high-performance metal materials are mainly prepared by traditional smelting and casting or powder metallurgy methods. In the casting process, it is difficult to control the organization, structure and performance of gradient materials and composites, and the reinforcing phase is easily biased, while high-entropy alloys are prone to alloy element bias, which seriously affects the mechanical properties and corrosion resistance of materials and components. In addition, smelting and casting and powder metallurgy methods are difficult to manufacture complex structure of gradient materials, composite materials and high-entropy alloy materials. Therefore, it is important to explore new methods for the preparation of gradient materials, metal matrix composites, high entropy alloys and other high performance metal materials for their application and promotion in weaponry, and to improve the comprehensive performance of weaponry.

 The results of a large number of studies in recent years have shown that metal additive manufacturing technology can be used to prepare high-performance metal materials such as high-entropy alloys, gradient materials and composite materials, in addition to manufacturing or repairing complex structures and homogeneous material parts . Wenzhou University has achieved a high entropy alloy with a compressive strength of 2.8 GPa and a compressive plastic strain of 41.8% by using arc additive manufacturing technology and stranded welding wire as raw material. By adjusting the dwell time between layers of laser-selective melting additive manufacturing to control the number of nanoscale precipitated phases, the German Max Planck Institute successfully prepared high-strength martensitic aging steel with Damascus knife microstructure characteristics, with tensile strength above 1300 MPa and elongation at break greater than 10%. Nanyang Technological University, Singapore, used laser additive manufacturing technology to prepare TiC particle-reinforced 316L austenitic stainless steel with tensile and yield strengths of 1.03 GPa and 832 MPa, respectively, while maintaining an elongation at break of 29%. Compared with traditional manufacturing methods such as powder metallurgy and smelting and casting, additive manufacturing technology can realize the controlled design and preparation of complex organization and structure of metal gradient materials and composites, and can effectively reduce the tendency of agglomeration of gradient or composite particles or fiber reinforcement and elemental segregation of high-entropy alloys during the preparation process of smelting and casting, showing better performance.

The gradient materials that can be prepared by additive manufacturing technology can be divided into homogeneous and heterogeneous gradient materials. Homogeneous gradient materials are single-phase materials with a gradient distribution of microstructure characteristics or macrostructure size by adjusting the process and structural design parameters using the same raw materials for additive manufacturing, including solid solution elements, grain size, crystal orientation, dotted structure gradients and their hybrid gradient materials, as shown in Figure 5a~d . Solid solution element gradient materials are formed by adjusting the mixing ratio of protective gas in additive manufacturing to form materials with the same matrix phase type and gradient distribution of solid solution gas element content (e.g., adjusting the ratio of nitrogen in argon gas in the process of high nitrogen steel additive manufacturing) (Figure 5a). Grain size and crystal orientation gradient materials are prepared by adjusting the additive manufacturing process parameters (heat input, scan path, scan rate, etc.) to produce materials with a gradient distribution of grain size and crystal orientation (Fig. 5b, c). The key to additive manufacturing of materials with gradients in grain size and crystal orientation is to clarify the correspondence between grain size and crystal orientation characteristics and the additive manufacturing process, and to realize the control of crystal growth and recrystallization behavior under the action of thermal cycling during the solidification process of additive manufacturing. Dot structure gradient materials are materials with a gradient distribution of dot structure size prepared by additive manufacturing technology through structural design (Figure 5d). The change of the structure of the gradient material in the additive manufacturing process is likely to cause the change of stress distribution, resulting in local stress concentration, leading to structural deformation and even cracking, and reducing the load-bearing capacity of the material. Therefore, the structural design optimization and additive manufacturing stress and deformation control of the gradient material of the dot matrix structure are very critical.

Heterogeneous gradient material is a phase material with gradient distribution of alloying elements or second phase by adjusting the additive manufacturing process, powder feeding rate or wire feeding rate of two or more powders or wires, including continuous composition, gradient composition, refractory particles and fusible particles gradient, etc., as shown in Figure 5e~h . The gradient material is a multiphase material with a gradient distribution of alloying elements by adjusting the powder feeding rate or wire feeding speed of two or more powders or wires in the additive manufacturing process, as shown in Fig. 5e and f. The difference between the continuous type and the stepped type is that the former is to gradually adjust the feeding rate or wire speed of two or more powders or wires during the additive manufacturing process to form a multi-phase alloy with a gentle gradient in composition, while the stepped type is to ensure that the metallurgical situation is to adjust the feeding rate or wire speed of two or more powders or wires abruptly during the additive manufacturing process to form a multi-phase alloy with a steep gradient in composition. The difference between refractory and fusible particle gradient materials is that the particles used in refractory particle gradient materials do not dissolve and react with the substrate under heat source, while the particles in fusible particle gradient materials do dissolve and react with the substrate by adjusting the ratio between the reinforcing particles and the substrate powder during the additive manufacturing process.

The metal matrix composites that can be prepared by additive manufacturing technologies include laminates, particle reinforced, short fiber reinforced, bionic composites and combinations of these composites. Figure 6 shows the schematic diagram of additive manufacturing metal matrix composites. The laminate composite is a composite material that combines the mechanical properties and functions of multiple materials through the alternate deposition of multiple materials with different mechanical properties or functions during the additive manufacturing process (Figure 6a). Particle- or short-fiber-reinforced composites are composites with diffuse distribution of particle- or short-fiber reinforcements by directly mixing particle- or short-fiber-reinforced powders into metal powders or by in situ generation of particle- or short-fiber reinforcements using metallurgical reactions during the additive manufacturing process (Fig. 6b, c). Biomimetic composites are composites with graded, layered, and porous structures prepared by additive manufacturing techniques that mimic the structure of natural biological materials (Fig. 6d). For the above metal matrix composites prepared by additive manufacturing technology, the matrix can also be designed and prepared as a graded material.

In addition to the preparation of high-performance metallic materials, additive manufacturing technology can also be used as a tool for high-throughput design and optimization of metallic materials. The traditional weapon and equipment metal material development is based on the trial-and-error method of "experience-guided testing", which involves designing and preparing a large number of samples with different compositions, involving smelting, forging, rolling, heat treatment and other processes, and optimizing the alloy composition through structural characterization and performance testing, which consumes a lot of time, manpower and materials and is difficult to meet the needs of advanced weapons and equipment. It is difficult to meet the urgent demand for high-performance metallic materials for advanced weapons and equipment. Therefore, the efficient, reliable and economic design and optimization of the components in the development of high-performance metallic materials is crucial to the rapid development of weapons and equipment. The emergence of metal additive manufacturing technology provides new ideas and methods for the design, rapid preparation and optimization of new materials for weapons and equipment. Take laser melting deposition additive manufacturing as an example, by adjusting the mixing ratio of multiple raw materials (metal powder or wire), the metallurgical reaction occurs by melting under the heat source of additive manufacturing to achieve high throughput design and preparation of different alloy components, effectively reducing the investment of labor and materials in the process of new material development and shortening the development cycle. Figure 7 shows the principle of high-throughput preparation of metallic materials based on laser melting deposition additive manufacturing. The Raabe team of Max Planck Institute, Germany, used laser coaxial powder feeding additive manufacturing technology to rapidly prepare Fe-19Ni-xAl ultra-high strength steel materials with Al mass fraction of 0-25% by continuously increasing the Al powder feeding rate and adjusting the ratio with Fe-19Ni alloy powder, and determined the critical value of Al content through microstructure observation and hardness testing to realize the laser additive manufacturing of Fe-19Ni-xAl. The Al content threshold was determined by microstructure observation and hardness testing, and the rapid design and optimization of Fe-19Ni-xAl ultra-high strength steel powder for laser additive manufacturing was achieved. The additive manufacturing and application of high performance materials such as high entropy alloys, gradient materials and composites are not only expected to improve the comprehensive performance of weapons and equipment such as impact resistance, wear resistance and light weight, but also this concept of material design and preparation based on additive manufacturing technology has important theoretical guidance significance for the material design and performance control of future weapons and equipment. However, at present, due to the lack of special metal materials for additive manufacturing and the limitation of equipment forming size and precision, metal additive manufacturing technology can only realize the preparation of some small and medium-sized gradient materials and composite materials, and the high throughput design and optimization for metal materials are still in the laboratory research stage. In addition, for heterogeneous gradient materials and composites, the differences in thermal properties such as melting point, thermal conductivity and thermal expansion coefficient between different materials and the formation of brittle intermetallic compounds increase the difficulty of additive manufacturing . Therefore, in the future research for the demand of high-performance materials in the field of weapons and equipment, to carry out research on the design, preparation and tissue property regulation and evaluation of high-performance metals and their composites based on additive manufacturing technology can help expand the application scope of additive manufacturing technology and enhance the R&D capability of metal materials for weapons and equipment.

3.3 Perception, prediction and control of additive manufacturing process of metal components

In order to ensure the size and performance requirements in use, the "control of shape and sex" of metal additive manufacturing components is very critical. The "shape control" mainly includes component deformation, cracking, structural size and shape, etc. The "property control" mainly ensures the service performance of the final component through microstructure and structural control, which involves grain size, weave, degree of segregation, second phase size, porosity or density, defect characteristics and phase transition pattern, etc. These include grain size, weave, segregation, second phase size, porosity or density, defect characteristics, and phase transition patterns. The current mode of quality evaluation of "shape and property control" of metal additive manufacturing components is mainly to test and evaluate the dimensional specifications and performance through nondestructive or destructive testing after the additive manufacturing. Once a component has serious dimensional deviations, deformations and internal defects, if it cannot be repaired, the entire component will be scrapped, resulting in wasted material, labor and time, especially for large and complex metal components. Therefore, to realize the sensing, prediction and control of macrostructure and microstructure characteristics during the additive manufacturing process of large and complex metal components can help to control the shape and sex of additive manufactured parts of weapons and equipment and reduce material and labor costs.

The application and development of simulation technology in additive manufacturing research in recent years has provided new methods and approaches for predicting and controlling the macrostructure and tissue evolution characteristics of metal additive manufacturing processes. Taking laser melting deposition additive manufacturing technology as an example, numerical simulation is used to study the characteristics of gas-powder flow field, heat- and mass-transfer processes in the melt pool, non-equilibrium solidification and solid-state phase change organization characteristics, and stress and deformation of the components in the additive manufacturing process, to establish the correspondence between alloying elements of metal materials, additive manufacturing process and microstructure characteristics, stress and deformation, and to clarify the control of microstructure characteristics, stress and deformation of additive manufacturing components. The method of controlling the microstructure characteristics, stress and deformation of the additive manufacturing components will be developed from the aspects of material, additive process and structure design. However, due to the complexity of the thermal-force process of additive manufacturing and the macro-micro coupling of simulation, the simulation of additive manufacturing is still at the stage of basic laboratory research, and most of the work is still a qualitative or semi-quantitative description of the additive manufacturing process, and the simulation results still differ greatly from the actual macrostructure and microstructure characteristics of additive manufacturing. The expected role in the prediction and control of the structure and structure of the actual additive manufacturing has not been fully exploited, and further basic theoretical and applied research is needed in the future for the actual metal additive manufacturing characteristics. Figure 8 shows the simulation of metal laser melting deposition additive manufacturing.

3.4 Quality inspection and evaluation of metal additive manufacturing components 

The quality of metal additive manufacturing components directly determines the performance and lifetime of weapons and equipment, and the quality inspection and evaluation of additive manufacturing metal components are very critical. As mentioned in Section 3.1, the forming pattern, microstructure, defect characteristics and corresponding mechanical properties of metal additive manufacturing are different from those of traditional manufacturing processes, which increases the difficulty of quality inspection and evaluation such as determination of tissue characteristics, defect identification and rating, performance testing and failure analysis of components. For example, laser additive manufacturing of metal dot matrix structure has excellent buffering and energy absorption, which has potential application prospects in tank protective armor and naval structure underwater blast resistance. The internal defects of metal dot matrix structures are easily missed by conventional detection methods, and the stacked interface of additive manufacturing has a serious scattering effect on signals such as ultrasound, which affects the identification of defects . At present, the inspection of metal dot matrix structures for additive manufacturing mainly relies on CT, which leads to high inspection and application costs. Therefore, achieving rapid and reliable inspection and evaluation of the quality of additively manufactured complex metal parts and forming a standard system for inspection and evaluation of the quality of additively manufactured parts for weapons and equipment applications are essential for the reliability and safety of future weapons and equipment additively manufactured parts in service.

3.5 Large-size high-efficiency and high-precision metal additive manufacturing technology 

With the development of large and complex weapons and equipment parts, the development of additive manufacturing technology has put forward higher requirements for forming size and efficiency, manufacturing accuracy and cost performance. On the one hand, the development and production of large and complex metal components require high forming efficiency while ensuring accuracy and quality, but the current metal additive manufacturing technologies used for weapon and equipment components are difficult to take into account both forming efficiency and accuracy. For example, laser-selective melting additive manufacturing technology has high forming accuracy, but the forming efficiency is low and the size of manufactured components is limited by the inert atmosphere chamber, which is suitable for the manufacture of small-sized complex components. Arc additive manufacturing technology has high forming efficiency and can manufacture large-size components, but the forming accuracy is low, and mechanical processing is required to ensure the size and shape accuracy, which is mainly used for the overall manufacturing of large and complex components. On the other hand, explosion, impact, heat and humidity and other harsh service environment requires weapons and equipment parts with excellent static and dynamic mechanics and corrosion resistance and other properties. Although the performance of a small number of additive manufactured metal parts is close to or even better than forgings, most of them are still below the level of forgings, which is difficult to meet the needs of weapons and equipment in extreme environmental service. In addition, in order to meet the application and installation needs of weapons and equipment, parts manufacturing technology needs to have a good cost performance, and the current metal additive manufacturing technology using wire and powder materials and equipment costs are high, which to a certain extent limits its application and promotion in the field of manufacturing complex parts of weapons and equipment. Therefore, the development of high-efficiency, high-precision, high-performance and cost-effective additive manufacturing technology for large complex metal components is of great military significance for the development of lightweight, strong protection and high destruction of weapon and equipment structures.

In recent years, the development of metal additive and subtractive manufacturing technology has made it possible to manufacture complex metal components of weapons and equipment with high efficiency and high precision, which has received wide attention and research at home and abroad. Metal additive and subtractive manufacturing technology is the introduction of traditional machining processes such as milling, grinding and polishing in the additive manufacturing process to remove the formed components, which can ensure the quality and accuracy of the surface of the component bore, closed space walls and so on. Metal additive and subtractive manufacturing technology not only has the advantages of fast forming speed, high material utilization and easy forming of complex structures, but also has the advantages of high quality and high precision of machining. The representative one is the arc additive and subtractive manufacturing technology, which has the characteristics of high deposition efficiency, high material utilization, low cost and small size limitation of parts. In addition, the introduction of auxiliary processes such as forging or rolling and high-energy ultrasound in the additive manufacturing process has significantly improved the performance of metal parts. Therefore, deformation or ultrasound-assisted arc additive and subtractive manufacturing technology is one of the important development directions for high-efficiency, high-precision and cost-effective additive manufacturing of large weapons and equipment parts.

 3.6 4D printing technology and application 

At present, the metal additive manufacturing technology used in the manufacture of weapons and equipment parts at home and abroad is mainly 3D printing, that is, through layer-by-layer stacking to create the required shape of the structural parts, focusing on the shape and mechanical properties of the structural parts, their shape, performance and functional requirements for stability. In recent years, 4D printing technology has been developing rapidly and has become an important development direction for additive manufacturing technology. Compared with 3D printing, 4D printing introduces a time dimension in three-dimensional space and enables controlled changes in the structure, performance and function of components in space and time dimensions through active design and manufacturing of materials and structures. 4D printed components with controlled changes in structure, performance and function provide new ideas for the development of advanced weaponry. For example, NASA has proposed the concept of intelligent variant aircraft design, where the shape of the aircraft can change adaptively with the external environment, deforming to different shapes during cruise, takeoff, landing and hovering, thus changing the maneuverability, speed and range of the aircraft. This concept is also applicable to the design and manufacturing of weaponry such as UAVs and amphibious unmanned platforms. The structural and functional design and manufacturing of metal components of weapons and equipment can be realized through 4D printing, and the structure and function can be changed accordingly to enhance the long-range combat, environmental adaptation, and high survivability of weapons and equipment under the rapidly changing conditions of the battlefield, such as adjusting the mobility and range of amphibious unmanned platforms through shape changes, creating "mimicry" camouflage, and adapting to the surface of water and land. "and adapting to amphibious environments. However, at present, the types of metal materials that can be used for 4D printing technology are relatively few, limited to a few metal materials such as Ni-Ti-based shape memory alloys and Cu-based shape memory alloys, and the functions realized are mainly shape changes of components under the action of temperature, which are still in the demonstration stage of the phenomenon . In order to realize the application of metal 4D printing technology in advanced weapons and equipment, we need to carry out in-depth research on 4D printing material development, process and equipment development, as well as structural-functional design and evaluation of metal components for weapons and equipment in the future.

4 Conclusion

Metal additive manufacturing technology provides new ideas and opportunities for the development of lightweight, strong protection, high damage, information and intelligence of weapons and equipment, and has been applied to the overall rapid manufacturing and repair of complex parts of special steel, aluminum alloy, titanium alloy, high-temperature alloy, magnesium alloy and refractory metals, which has greatly improved the comprehensive performance of weapons and equipment and shortened the development, production and maintenance cycle. In addition, metal additive manufacturing technology has successfully realized the design and preparation of high-performance metal materials such as high-entropy alloys, gradient materials and composite materials, which have broad application prospects in the fields of high temperature resistance, impact resistance and structural lightweighting of weapons and equipment. However, the current metal additive manufacturing technology faces bottlenecks such as limited variety of special high-performance metal materials, difficulty in both manufacturing size and precision and efficiency, lack of perception prediction and control of additive manufacturing process, and imperfect quality inspection and evaluation standards of additive manufacturing components, which seriously restrict its application and promotion process in advanced weapons and equipment. Therefore, in the future, further basic and applied research work is needed to address the bottlenecks of metal additive manufacturing technology, break through the key scientific or technical problems in metal additive manufacturing materials, processes, testing and performance control, actively develop new theories and methods of advanced metal additive manufacturing technology such as 4D printing, improve the whole process industry chain and standard specification system of metal additive manufacturing technology for weapons and equipment parts, and provide a better solution for the development and production of advanced weapons and equipment. To provide strong technical support and guarantee for the development, production, operation and maintenance of advanced weapons and equipment.