Laser additive manufacturing is a kind of additive manufacturing technology with laser as energy source, which can completely change the processing mode of traditional metal parts. LAM is mainly divided into laser selective melting (SLM) characterized by powder bed and laser direct deposition (LDMD) characterized by synchronous powder feeding. For example, General Electric Company (GE)' s SLM aero-engine fuel nozzle and Beihang's LDMD aircraft titanium alloy frame are typical application cases.
Judging from the development of metal LAM technology at home and abroad, the technical direction of industrialization is still a minority, because the accumulation of basic theory, the breakthrough of key technologies, the maturity of engineering application technology, and the commercialization and popularization of technology research and development have restricted the industrial application of LAM technology to varying degrees. At present, the research at home and abroad mainly focuses on control research, focusing on basic research such as porosity, cracks, microstructure characteristics and anisotropy [5~9]. There are few research reports on shape control, testing and product standards, which also shows that metal LAM is in the development stage of transition from technical research to industrial application.
Through literature review, on-the-spot investigation and questionnaire survey, this paper systematically combs the development status and trend of research and application in the field of metal LAM, analyzes the gap at home and abroad, theoretical research and application requirements, and puts forward the core key technologies and bottleneck processes involved in industrial application, with a view to promoting the development of industrial application of metal LAM technology in China.
Second, the metal laser additive manufacturing demand analysis
LAM, based on digital-analog slicing, realizes near-net-shape manufacturing of metal parts by stacking layer by layer, which is especially suitable for manufacturing complex parts, gradient materials and performance components, composite parts and difficult-to-machine materials, and is favored in advanced manufacturing directions such as aerospace. On the one hand, the shapes of related parts are complex and changeable, with high material performance requirements, great processing difficulty and high cost; On the other hand, new aircraft are developing in the direction of high performance, long life, high reliability and low cost, so it is urgent to adopt complex and large overall structure.
The precision of parts formed by SLM is high, but the size of parts is limited by the machining room. Therefore, SLM is mainly used for the precision forming of small and medium-sized complex precision structures, and the functional attributes of the corresponding product structures are generally greater than the bearing attributes. In order to meet the overall performance requirements, the fuel nozzle (with complex internal oil circuit, gas circuit and cavity), bearing seat, control housing, blades, aircraft door bracket, hinge, grille structure intake valve, exhaust valve and satellite bracket of aero-engine need structural innovation design, which becomes the applicable application object of SLM technology.
The mechanical properties of parts formed by LDMD are good, but the dimensional accuracy is relatively low. LDMD is mainly used to manufacture medium-sized or large-scale complex load-bearing structures, and the load-bearing performance of the corresponding product structure is generally greater than the functional performance. Various types of aero-engine casings, compressor/turbine blisks and other structures have complex shapes, and even require heterogeneous or functionally graded materials to improve efficiency. In order to reduce the weight and improve the bearing efficiency, it is necessary to optimize the structural topology of the bearing components such as aircraft joint, landing gear, bearing frame, pulley frame and grid structure bearing skeleton of high-speed aircraft wing/pneumatic rudder. The outstanding complexity and manufacturing difficulty of this structure put forward a clear demand for LDMD technology.
In addition, it is difficult to ensure the local configuration and performance of some load-bearing components of aircraft and engines with special structures such as local bosses and lugs by forging process. The oversize titanium alloy bearing frame of large aircraft has exceeded the upper limit of processing capacity of existing forging equipment. This puts forward a clear demand for the composite manufacturing technology of forging+additive manufacturing/additive connection.
Third, the development status of foreign metal laser additive manufacturing
Present situation of technical research
1. Laser selective melting technology
Related enterprises have prepared SLM powder by vacuum-induced gas atomization (VIGA), crucible-free electrode-induced molten gas atomization (EIGA), plasma rotary atomization (PREP), plasma torch (PA) and other methods, which have the capacity of mass supply and occupy the main market in the world [10].
The focus of LAM process research is mainly on the control of microstructure and properties. People have done a lot of research on the microstructure, defects and properties of SLM and their relationship with process parameters. For example, for the stainless steel part SLM, increasing the laser power and reducing the scanning speed is beneficial to improve the density [11]; High surface roughness and porosity will reduce the corrosion resistance of AlSi 10Mg aluminum alloy SLM, and the formed oxide film can improve the corrosion resistance; Cracks perpendicular to the direction of adding materials occurred in SLM samples of AW7075 aluminum alloy, but preheating aluminum powder had no effect on crack control, and the fatigue life caused by internal cracks was much lower than that caused by traditional technology [7].
Energy density has obvious influence on SLM structure and defects of Ti-6Al-4V titanium alloy [5, 12, 13]: low energy density leads to flaky α+β phase structure, which is easy to cause porosity and poor fusion; High energy density leads to the α′ structure of acicular martensite, which promotes the segregation of aluminum and the formation of α2 -Ti3Al phase. The fatigue strength of as-deposited Ti-6Al-4V alloy is about 80% lower than that of forgings [6]. Hot isostatic pressing can reduce porosity and improve properties. For CMSX486 single crystal alloy SLM, low energy density can reduce cracks and high energy density can reduce porosity [8]. The longitudinal section of CM247LC alloy SLM is mainly composed of columnar γ grains. The segregation of Hf, Ta, W and Ti increases precipitates and residual stress, which leads to internal cracking of the parts [14]. The microcracks in SLM of IN738LC superalloy are related to the enrichment and segregation of Zr at grain boundaries [15]. The dendrite of IN7 18 alloy can be refined by adding proper amount of Re, but excessive Re is unfavorable to fatigue strength [14]. After heat treatment, Hastelloy-X alloy of SLM forms equiaxed crystals, and its yield strength decreases. After hot isostatic pressing, the tensile strength returns to the as-deposited level, and the elongation can be increased by15% [16].
For metal LAM process, many detailed studies have been carried out abroad. It is understood that it takes 6~8 months for German equipment manufacturers to develop a new material SLM technology and adjust more than 70 parameters. Realizing structural lightweight design through topology optimization is also the focus of SLM application research, and foreign countries have put forward new concepts such as design guiding manufacturing and function priority. A special bracket design technology is also developed, which makes it unnecessary to cut parts from the substrate, thus effectively shortening the picking cycle.
In addition, the research and formulation of metal forest standards have been developing simultaneously with the application of technology. In 2002, the United States issued the "Laser Deposition Products of Annealed Ti-6Al-4V Titanium Alloy", and then issued the relevant standard 19, covering the annealing and hot isostatic pressing system of products, aging system, stress relief annealing system in the manufacturing process and many other aspects. The timely formation of standards has played a fundamental supporting role in the industrial application of LAM technology.
2. Laser direct deposition technology
1995, Johns Hopkins University, Pennsylvania State University and MTS System Co., Ltd. jointly developed the LDMD technology of large-size titanium alloy parts based on high-power CO2 laser, and the deposition rate was 1~2 kg/h, which promoted the application of LDMD parts in aircraft [12].
The research of LDMD technology mainly includes forming process and microstructure and properties. The mechanical properties of LDMD formed parts prepared by Sandia National Laboratory and Los Alamos National Laboratory are close to or even better than traditional forged parts. The relationship between the stability, part precision, microstructure, mechanical properties and process parameters during LDMD repair of single crystal blades was studied by the Federal Institute of Technology in Lausanne, Switzerland. The repair technology formed has been applied to engineering.
The LDMD technology of Ti-6Al-4V alloy has been deeply studied by foreign scholars, revealing the relationship between process parameters and the microstructure and mechanical properties of additive manufacturing, and clarifying the adjustment effects of process adjustment and hot isostatic pressing on the microstructure and properties [13, 17~ 19]. LDMD technology provides greater freedom for controlling the microstructure of materials: by adjusting the nucleation and growth conditions of LDMD, the expected single crystal and polycrystalline structures can be obtained [9]; The LDMD technology developed by the National Aeronautics and Space Administration (NASA) can make the performance of parts vary with different parts. By combining LAM technology with traditional cutting methods, German enterprises can process complex parts that are difficult to manufacture by traditional technology, and the product precision and surface roughness are improved [1 1].
Second, the current situation of equipment development
Economical and efficient LAM equipment is the basis for the popularization and application of LAM technology. SLM equipment development is concentrated in Germany, France, Britain, Japan, Belgium and other countries, while LDMD equipment development countries mainly include the United States and Germany.
1. Laser selective melting equipment
Germany is the first country to study SLM technology and equipment. SLM equipment imported from EOS company has certain technical advantages. The related equipment is applied to the manufacture of fuel nozzle of LEAP aero-engine of GE Company, and the quality of manufactured products is further improved by monitoring the additive manufacturing process. Realizer GmbH's all-round design and parts stacking technical solutions are unique; Concept Laser's equipment is famous for its huge building size; SLM Solutions is in the leading position in laser technology and airflow management technology. American 3D Systems Company relies on the technical advantages of its special powder deposition system to form accurate detailed features. British Lei Nishao Company has technical characteristics in the flexibility of material use and convenience of replacement.
2. Laser direct deposition equipment
American EFESTO Company has technical advantages in large-size metal LAM, and the size of the LDMD equipment studio developed can reach1500mm1500mm2100mm. The studio space of LDMD equipment introduced by Optomec Company of the United States is 900 mm 1500 mm 900 mm, equipped with a 5-axis movable table, and the maximum molding speed is 1.5 kg/h ... The laser integrated processing system provided by German enterprises is also the mainstream LDMD equipment.
In recent years, additive and additive compound processing equipment has become a new hot spot in the market. Japan's DMG company introduced LDMD equipment with 2 kW laser and 5-axis CNC milling machine. The forming speed is 20 times higher than that of ordinary powder bed, and the inaccessible part of the final part can be milled in the manufacturing process. The related equipment imported from Japan Mazak Company can carry out 5-axis turn-milling compound machining, and its application objects include polygonal forgings or castings, rotary parts and complex special-shaped parts.
(iii) Application status
Titanium alloy LAM has achieved important applications in the aviation field. The United States took the lead in applying LDMD titanium alloy load-bearing parts to carrier-based fighters; Carpenter technology company uses high-strength customized stainless steel made of additive to produce advanced aviation gears; The maintenance of F-22 fighter adopts SLM corrosion-resistant bracket, which significantly shortens the maintenance time. Britain has successfully applied LDMD technology to the whole frame manufacturing of UAV.
SLM technology has been widely used in manufacturing complex parts of aero-engines. GE company in the United States took the lead in applying SLM technology to the production of high-pressure compressor temperature sensor shell. This product has been approved by the Federal Aviation Administration (FAA) and equipped with more than 400 GE90-40B aero engines. The fuel nozzles of GE LEAP series aero-engines are also produced by SLM technology (the production capacity in 2020 is 44,000 nozzles per year). Pratt & Whitney Company of the United States uses SLM technology to produce borescope sleeves, which are equipped with PW 1 100G-JM aero engines. The titanium alloy front bearing assembly (including 48 airfoil guide vanes) of Trent ·XWB-97 aero-engine is manufactured by rolls royce Company of England using SLM.
LAM technology has been applied to the manufacture of spacecraft since 20 12. NASA uses LAM technology to manufacture the bending joint of RS-25 rocket engine. Compared with the traditional method, the number of parts, welds and processing procedures are reduced by about 60%. If the hydrogen-oxygen rocket engine adopts the integrated design and manufacturing method, the total number of parts will be reduced by 80%. Terez Group of France manufactured the supporting parts (aluminum alloy) of TT&C antenna for Koreasat5A and Koreasat7 communication satellites through SLM technology, which reduced the mass by about 22% and saved about 30% of the funds.
The popularization and application of LAM technology accelerates the structural topology optimization and lattice structure design of aerospace vehicles. The aluminum alloy mounting bracket of Eurostar E3000 satellite platform telemetry/telecontrol antenna of Astrium Company in Europe is made by LAM Company, which reduces the weight by about 35% and improves the structural stiffness by about 40%. Cobra Aero Company of the United States cooperated with Renishaw PLC Company of the United Kingdom to manufacture the integral engine component LAM with complex lattice structure. In addition, the compound processing technology of adding and subtracting materials began to be applied. Virgin Orbit Company of the United States used the addition/subtraction hybrid machine tool to manufacture and finish the components of rocket engine combustion chamber, and completed 24 engine test runs in 20 19.
(D) Development experience and enlightenment
It is an important experience to review the development process of metal forest technology in the world, promote technical research and equipment development with industrial development, and improve market competitiveness through industry integration. Application enterprises pay attention to the manufacturing quality and production cost of their own products. As the main body and the biggest beneficiary of technological development, they can integrate materials, processes, equipment, verification, standard research and personnel training, thus promoting the development of LAM industry more efficiently. For example, the industrial application of LAM by GE Company in the United States is in the leading position in the world, mainly due to the establishment of manufacturing quality control company and additive manufacturing equipment company in strategic acquisitions of industry consolidation to strengthen the integrity of LAM industrial chain; Product manufacturing uses more than 300 kinds of industrial-grade manufacturing equipment around the world. Foreign companies attach importance to the personnel training of LAM product manufacturing. For example, GE has an additive manufacturing training center equipped with special equipment, which can train hundreds of engineers every year.
Fourth, the development status and gap analysis of domestic metal laser additive manufacturing
development status
1. Metal forest technology
There are a lot of researches on the structure, defects, stress and deformation control of LDMD in China [1 1, 13, 14]. Beihang University has developed key technologies such as LDMD internal defects and quality control of titanium alloy large structural parts [20]. Northwestern Polytechnical University has completed the LDMD manufacturing of aircraft super-large titanium alloy flange, and the forming accuracy and deformation control have reached a high level. Shenyang University of Aeronautics and Astronautics put forward the method of sectional scanning forming, which effectively controlled the deformation and cracking of parts in LDMD process. Youyan Engineering Technology Research Institute Co., Ltd. has broken through the problems of interface quality control and complex shape integration control of different materials such as TC 1 1, TA 15/Ti2AlNb in the impeller and inlet, and the products have passed the test.
In China, the research on precise control of shape and size and surface roughness focuses on the direction of SLM technology. Xi' an platinum laser forming technology co., ltd. uses SLM method to process the runner parts with the minimum aperture of about 0.3 mm, and the minimum wall thickness of thin-walled parts is about 0.2 mm; ; The overall dimensional accuracy of the parts reaches 0.2 mm, and the roughness Ra is not more than 3.2 micron meters ... Nanjing University of Aeronautics and Astronautics takes SLM precision manufacturing as the main line and improves the comprehensive performance of the parts through the whole process control. Xi Jiaotong University applies LAM to the manufacture of hollow turbine blades, aviation propellers and auto parts. [ 1 1].
China Hangfa Beijing Institute of Aeronautical Materials has completed the comprehensive study of LAM technology: the integral bladed disk of nickel-based double-alloy turbine manufactured by LDMD has passed the over-rotation test, and the landing gear of Il -76 aircraft has been repaired by adding materials in batch; The LAM ultrasonic scanning evaluation system was developed, and the detection standard and reference block were established. The technical achievements of evaluation and nondestructive testing are applied to batch inspection of aircraft pulley frames and frames.
In terms of SLM powder, domestic products basically meet the requirements of molding process. The Institute of Metals of Chinese Academy of Sciences has broken through the clean preparation technology of ultrafine titanium alloy and superalloy powder for SLM, and its properties have reached the level of imported products. Titanium alloy and superalloy powder products developed by Xi 'an Ouzhong Materials Technology Co., Ltd. have been applied in engineering.
2. Metal forest equipment
Domestic LDMD and SLM devices have strong R&D capabilities and gained a certain market share. Xi Anbo Park Jung Su Laser Forming Technology Co., Ltd. independently developed SLM series equipment and laser high-performance repair series equipment. Nanjing Zhongke Chen Yu Laser Technology Co., Ltd. has developed core equipment such as automatic zoom coaxial powder feeder, remote powder feeder and high-efficiency inert gas circulation purification box. , and formed the metal LDMD serialization equipment. In addition, Beijing Yijia 3D Technology Co., Ltd. and Beijing Xinghang Electromechanical Equipment Co., Ltd. have made good progress in the small batch production of industrial and small metal SLM equipment, and Shanghai Aerospace Equipment Manufacturing General Factory Co., Ltd. has made good progress in the development of standard and large format SLM equipment and robot LDMD equipment.
3. Metal forest application
LDMD is mainly used to manufacture load-bearing structures. The main bearing frame, main landing gear and other components manufactured by Beihang University have been applied to aerospace vehicles, gas turbine engines and other equipment. Shenyang Aircraft Design and Research Institute of aviation industry has promoted the maturity of LDMD technology through engineering application verification, and realized the aircraft application of 8 kinds of metal materials and 10 kinds of structural parts. The First Aircraft Design and Research Institute of Aviation Industry has realized the installation and application of LDMD components of the outer main flap pulley frame and tail rudder arm of large aircraft. The LDMD manufacturing and application of large-scale thin-walled skeleton cockpit structure have been realized by Beijing Institute of Electromechanical Technology.
SLM is mainly used for manufacturing complex parts. In the aviation field, China Aviation Manufacturing Technology Research Institute has realized the installed application of SLM products; Chengdu Aircraft Design and Research Institute of Aviation Industry uses SLM to assist the grid structure of the intake/exhaust valve of the power cabin on the aircraft. The design and research institute of aviation industrial helicopter has realized the installation and application of SLM components in the aspects of inlet ventilation grille structure, rain-proof sealing structure and multi-cavity structure. In the aerospace field, SLM products such as tank intermittent bracket, space radiator and guide device of Shanghai Aerospace Equipment Manufacturing General Factory Co., Ltd. have been installed and applied; SLM products such as cockpit structural parts and control surfaces of Beijing Xinghang Electromechanical Equipment Co., Ltd. passed the ground test and flight test. Beijing Institute of Electromechanical Technology has realized SLM manufacturing of small and complex parts, and the technical maturity of control surfaces, supports and other products has reached level 5; New Crystal Laser Technology Development (Beijing) Co., Ltd. used SLM to manufacture large-size thin-walled titanium alloy lattice sandwich structure (heat collection window frame), which met the strict technical requirements of deep space exploration aircraft.
In addition, Xi 'an Platinum Laser Forming Technology Co., Ltd. can provide more than 8,000 parts for the aerospace field every year by using SLM technology; Huazhong University of Science and Technology has made gradient material dies with conformal cooling channels by adding and subtracting materials, which have been widely used in industry.
The gap faced
1. There is a gap between the design and preparation technology of metal forest materials.
The design theory and method system of LAM special materials in China is still weak, and the design work of special materials is few and scattered. Material genomics technology shortens the research and development cycle and reduces the research and development cost, and has been successfully applied to related material design abroad. The research on material genome technology and its application in improving the properties of LAM special materials are relatively weak in China.
In terms of powder preparation, the domestic vacuum argon atomization technology is relatively mature, and the properties of the prepared stainless steel and nickel-based alloy powders basically meet the requirements of the molding process. However, there is a big gap in the preparation of ultrafine powder of titanium alloy and aluminum alloy. The main problem is that the powder sphericity is poor and the yield of fine powder is low, which can not meet the requirements of SLM molding, so the practical application still depends on imports.
2. There is a gap between the design and manufacturing technology of metal forest equipment.
The gap between China and LAM technical powers such as the United States and Germany mainly lies in technology and equipment. Most SLM equipment used in China is imported from Germany, while SLM equipment used in large-scale projects mainly depends on imports. Domestic enterprises lack self-research ability in core components such as lasers and vibrating mirrors, and the processing size, stability and accuracy of domestic equipment need to be improved urgently. The control software used for monitoring and forming processes such as powder flow state and molten pool state in China is not perfect.
3. Insufficient research on metal bonding technology
With the continuous improvement of material properties of important equipment such as turbine engines and aircraft, the manufacturability of materials has declined. The research on LAM technology of aviation backbone materials in China is insufficient, and effective methods such as stress deformation and crack control have not yet been formed. The internal tissue defects of parts have not been cured, the mechanical properties of parts are uniform and consistent, and the batch stability is not good. However, the research on LAM process of ultra-high temperature structural materials required by advanced aero-engines and high-speed aircraft is even more lacking.
4. The dimensional accuracy and surface roughness of the product do not meet the technical requirements.
Generally speaking, there is machining allowance for LDMD aircraft structural parts, and dimensional accuracy and surface roughness are not necessarily the key limiting factors. However, turbine engine parts are mostly complex structural parts with internal flow channels and cavities, and the corresponding SLM forming dimensional accuracy is about 0. 1 mm, and the surface roughness Ra is about 6.3, which is still far from the precision castings. Related products also face the problem of insufficient research on molding and internal surface processing.
5. Metal LAM lacks guiding standards.
At present, the common problem faced by LAM industry in China is the lack of quality control standards, which leads to the lack of acceptance basis for metal LAM products in design, materials, technology, testing, organizational performance and dimensional accuracy. As the application basis of parts, nondestructive testing, mechanical properties, metallographic map and other basic data. Due to the lack of sorting, it is difficult to formulate product standards and the guarantee of industrial application and promotion is insufficient.
5. Analysis of key technologies of metal laser additive manufacturing in China.
1. Design and manufacture of core devices such as laser processing head.
Develop core devices with independent intellectual property rights, focus on improving the quality and performance of basic devices such as processors, memories, industrial controllers, high-precision sensors, digital-to-analog converters, and design and manufacture core devices and key components of process equipment; Research and development of high-beam quality laser and beam shaping system, high-power laser scanning galvanometer, dynamic focusing mirror and other precision optical devices, high-precision nozzle processing and other core components.
2. Scanning strategy, parameter programming and online monitoring.
Breakthrough the software technology of data design, data processing, process library, process analysis and process intelligent planning, online detection and monitoring system, adaptive intelligent control of forming process, etc. And build a LAM core supporting software system with independent intellectual property rights.
3. LAM material design optimization based on material genome
A technical model of Qualcomm quantity for special materials far from equilibrium conditions is developed, and a multi-scale simulation algorithm suitable for Qualcomm quantity calculation is developed. The preparation technology of powder materials with controllable composition and microstructure was studied, and the material gene bank was established by Qualcomm experiment. Through the cooperation of Qualcomm calculation, experiment and database, the LAM special material with excellent performance was developed rapidly.
4. LAM controllability and shape control of typical structures of main materials.
Aiming at some key materials and typical parts, the key technologies and engineering applications of LAM controllability and shape control of parts are studied. Master the factors that affect the final quality in the manufacturing process of parts and the solutions, and form a LAM technical system that can be used in engineering, involving raw material control, process equipment, forming process, heat treatment, machining, surface treatment, nondestructive testing and verification test. Pay attention to the uniformity and batch stability of LAM parts, which meets the requirements of practical engineering application.
Conclusion of intransitive verbs
In order to catch up with metal LAM technology and its engineering application, the development of LAM in China should follow the objective law of "technology-product-industry", consolidate the technical foundation of organizational performance control, fill the shortcomings of core equipment in hardware/software R&D and integration, strengthen product quality control, standards and verification, and steadily promote industrial application.
(1) Consolidate the research foundation of laser additive manufacturing, and give play to the role of technological exploration and tackling key problems in universities and research institutes. Industrial departments or application units will take the lead in developing LAM process and verifying the performance of products, and gradually expand from conventional metals to advanced materials such as intermetallic compounds and niobium-silicon ultra-high temperature alloys based on the principle of easy first and difficult later.
(2) Research on application of orderly propulsion engineering. Select representative products in aerospace field in advance to carry out LAM quality control, standards and verification, and realize product mass production and engineering application as soon as possible; Then it gradually expanded to high-value products with complex structure, harsh working conditions and poor processability, and was popularized and applied in advanced manufacturing fields such as nuclear industry, weapons, automobiles and electrical equipment.
(3) Carry out the research and formulation of the quality control standard of Lim products. Accumulate basic data such as nondestructive testing, mechanical properties, metallographic map and fatigue life of LAM defects. , determine the acceptance basis of materials, technology, nondestructive testing, microstructure and mechanical properties, dimensional accuracy, surface roughness, etc. And formulate technical standards for LAM products in China.
(4) According to the actual needs of the industry, LAM-related majors are added in colleges and universities and vocational and technical colleges to train professional and technical personnel for enterprises. LAM training centers should be set up in enterprises with advanced technology to provide specialized training for designers, technicians and equipment operators in many industries in China, thus providing intellectual support for the development of LAM industry.