China's ancient welding process facilitates the development of new high thrust-to-weight ratio engines
Release time: 2015-08-07 17:26:38 Views: 1530
"After the high-pressure turbine guide blades have been tested in a long-term high temperature environment, the double welds are intact and fully meet our design requirements!" After a new engine intermetallic compound guide vane failure test, the testers were very moved. This double weld is the result of the new technology of Transient Liquid Phase (TLP) diffusion welding adopted by AVIC Manufacturing. This technology is also the result of AVIC's adherence to the old process innovation. This old craft is renewing its vitality and becoming an important fulcrum of aviation manufacturing technology.
Brazing, diffusion welding technology is an ancient process. The dissertation on brazing in China was first found in Han Book written by Ban Gu in the Han Dynasty. AVIC has started research on brazing, diffusion welding technology and special welding equipment since the 1960s. The materials involved include aluminum alloys, copper alloys, carbon steel, stainless steel, high temperature alloys, titanium alloys, ceramics, hard alloys, Diamond, etc., is one of the earliest units in China to carry out research on brazing and diffusion welding technology. The researched brazing and diffusion welding technologies, centrifugal impellers, engine blades, heat exchangers, honeycomb structures, thrust chambers, jet disc components, steering gear skeleton components, plunger pump sliding shoe components and other products are widely used in aerospace and civil fields .
With the improvement of technical specifications of new aviation equipment, the demand for high temperature resistant, lightweight materials and high-performance new structures continues to increase. The honeycomb sealing structure is a typical structure that improves the performance index of aero engine. This structure has strict requirements on the small grid honeycomb core forming / welding technology and brazing technology, and has been subject to long-term embargoes in Europe and the United States. The technicians of the professional group of brazing and diffusion welding of the manufacturing plant bravely burdened themselves, analyzed the characteristics of the small grid honeycomb core, and successfully researched the technology and equipment of automatic forming and automatic tailor welding of small grid honeycomb cores, and won national awards; Welding process and its matching with heat treatment research, forming honeycomb seal structural component manufacturing technology, the honeycomb seal structure is widely used in all aero engines, aerospace engines and gas turbines, which significantly improves engine performance and breaks through foreign technical blockades. In order to meet the requirements of high thrust-to-weight ratio engines for air tightness and high operating temperature, new manufacturing technologies such as high temperature resistant honeycomb seals, brush seals, and fingertip seals were developed to form air seals. Strict manufacturing technology system.
TLP diffusion welding technology is a new technology developed for the structure and performance characteristics of new high-temperature materials such as single crystals, intermetallic compounds, and oxide dispersion-strengthening alloys, combined with the advantages of brazing and diffusion welding. Through the design and development of special solders that match the metallurgy of the base material and optimize the welding process to obtain welded joints with the same or similar microstructure and properties as the base, it can meet the requirements of high temperature resistance and bearing capacity of high temperature parts. In the research of guide vane welding, problems such as inaccurate gap clearance control and excessively large steps on both sides of the weld have also occurred, which affect weld strength and local dissolution. After a large number of process experimental researches, we independently designed and developed a variety of high-performance nickel-based and cobalt-based solders, and have obtained 2 invention patents. The Ni3Al directional solidification alloy TLP diffusion welding joint has a high-temperature endurance strength of more than 90% of the substrate. Using these new solders and high-strength welding technology, a high (low) pressure turbine guide vane double (triple) assembly with high thrust-to-weight ratio was developed for installation tests. The MGH956 alloy "super-cooled" porous laminate flame was developed. After the combustion chamber bench test, the cylinder has a cooling efficiency of more than 0.85 and a wall temperature that is lower than that of the film cooling structure by more than 70K. It shows excellent cooling effect and uniformity of wall temperature, laying a solid technical foundation for the development of new engines.
In order to solve the welding problem of the load-bearing parts of components such as powder alloys and ceramics with poor weldability, the research team introduced the phenomenon of rapid atom diffusion under field-promoting conditions into diffusion welding and powder coating sintering. The temperature control mode of the DC pulsed power source was used to directly heat the ceramic / Metal, powder alloy / single crystal and other materials with poor weldability can be researched by discharge plasma diffusion welding technology, which can achieve fast welding of these difficult-to-weld materials in 20 minutes. The high-temperature tensile strength of the welded joint is similar to that of the substrate. It can be used for ceramics and Welding and manufacturing of ceramic-based composite material components, integral leaf rings, and turbine integral disks. Direct current sintering of abrasion-resistant ceramic coatings using the current control mode of a DC pulsed power supply, and research on electrical contact sintering technology, can significantly increase the bonding strength and cohesive strength of ceramic coatings, and greatly increase the service life of abrasion-resistant layers. These exploratory studies provide technical support for future high-performance engine development.
The metal honeycomb wall structure has light weight, high specific strength, high specific rigidity, high temperature resistance, corrosion resistance, sound insulation, heat insulation and other excellent properties, and is widely used in the aerospace field, such as aircraft fuselage, wings, engine door, Engine nacelles, etc. In recent years, China Aviation Industry Research Institute has developed new technologies such as precision machining of flexible honeycomb cores such as titanium alloys, stainless steels, and high-temperature alloys. New technologies such as large-area variable cross-section and variable curvature honeycomb wall slab structures with high weld rate brazing and non-destructive testing make flexibility The machining accuracy of the honeycomb core is within 0.05mm, and the brazing welding rate of the honeycomb wall panel structure is more than 90%. The honeycomb wall panel structure such as the front fuselage cover, the rear fuselage side panel, and the starter air inlet are developed. The components have been successfully applied to different types of aircraft, and at the same time laid a technical foundation for the development of high-speed aircraft's anti-insulation light-weight and high-temperature structure.
Special equipment is the carrier of technology. AVIC Manufacturing has always focused on research and development of special technology equipment. Since the 1970s, vacuum brazing furnaces, vacuum diffusion welding furnaces, and vacuum arc brazing equipment have been developed, providing different types of vacuum furnace equipment and welding process technology for the aviation, aerospace, electronics, petrochemical and civilian fields. The engineering form contributes to the development of advanced technology in the military and civilian industries. At present, AVIC has all the capabilities to provide research and development of brazing and diffusion welding and welding of parts for all sectors of society.
Facing the tide of China's aerospace science and technology development in the new century, AVIC is committed to brazing and transition liquid phase diffusion welding of intermetallic compound materials, single crystal superalloys, ceramics and composite materials, single crystal / powder alloys, and ceramics. / Metal, metal-based composite materials in the field of discharge plasma diffusion welding to carry out research work on brazing / diffusion welding, breaking through honeycomb wall panels, turbine blades, porous laminates, integral leaf disks, gas seals, etc. It is a new structural engineering manufacturing problem for key manufacturing technologies, and achieves mass production of parts such as honeycomb seals, brush seal rings, honeycomb wall panels, turbine guide vanes, thrust chambers and jet disks for rocket engines, and repair of guides. Provide stronger support for aerospace technology research and development and model production.
Brazing / Diffusion Welding: Legacy and Innovative Solid Phase Welding Technology
Brazing refers to a method of joining non-melting base materials through molten solder or liquid phase under high temperature to form a metallurgical bonding connection. Diffusion welding refers to a connection method in which the surface of materials in contact with each other diffuses between atoms under the action of high temperature and pressure, thereby forming a reliable connection.
Although brazing / diffusion welding has been around for a long time, its development has been slow. Before the 20th century, it was just a craft in hand workshops. After the 1930s, with the rapid development of the aviation, aerospace, energy and nuclear industries, in order to meet the requirements of light weight, high strength, high rigidity, high electrical conductivity and thermal conductivity of components, and the use of certain harsh conditions, a large number of new materials The new structure was designed and put into production, and it also greatly promoted the development and wide application of modern brazing / diffusion welding technology.
The principle of the brazing process is shown in (Figure 1). The brazing material and the component to be brazed whose melting point is lower than the base material are uniformly heated to the brazing temperature at the same time. The brazing material is melted and the component to be brazed remains solid. The molten solder wets the base metal and fills the joint gap under the action of capillary. In the subsequent heat preservation process, the liquid solder composition diffuses into the base metal, causing a slight amount of dissolution in the near-seam area of the base material and mutual diffusion with the solder, and the joint forms a metallurgical bond during the cooling process.
Before brazing parts, we must first do joint design work, including the following aspects:
Joint form: The commonly used forms of brazed joints are lap joint, butt joint, miter joint and T-joint (Figure 2). Lap joints usually have the highest strength, followed by miter joints, and the worst is butt joints.
Overlap length: If the joint overlap length is too long, it will consume materials and increase the weight of the component; if the overlap length is too short, it cannot meet the strength requirements. In production practice, the overlap length is usually 3 to 4 times the thickness of the brazing metal, but rarely exceeds 15 mm.
Assembly gap: The assembly gap is one of the key factors affecting the tightness of the joint and the strength of the joint. If the gap is too small, the inflow of the solder will be hindered. If the gap is too large, the capillary action of the solder will be destroyed. The solder cannot fill the gap of the joint, resulting in a reduction in the strength of the joint. Generally speaking, the weaker the interaction between the solder and the base metal, the smaller the gap is required; the stronger the interaction between the solder and the base metal, the larger the gap is required.
Solder material selection: The melting point of the selected solder should generally be tens of degrees lower than the melting point of the base metal, which can well wet the base metal and fill the gap between the solder at the brazing temperature; To obtain a strong joint; the solder should not contain elements harmful to the base metal, which can meet the mechanical, physical and chemical performance requirements of the solder joint. In addition, the economics of the solder must also be considered, and rare or precious metals should be used as little as possible.
When brazing parts, surface treatment of the parts to be brazed is required, including degreasing, deoxidizing film, and plating on the surface of the parts. Then, tight fitting, spot welding, and fixture positioning are used to assemble and fix the brazed parts. Weld parts and solder to ensure the required brazing gap. After the solder is placed, a flux stopper should be applied around it to prevent the molten solder from spreading or adhering the weldment to the brazing fixture. The main process parameters to be controlled during the brazing process are brazing temperature and holding time to ensure that the brazing material can fully wet the base metal and fill the brazing gap to form a dense brazed joint.
After the part is brazed, the subsequent diffusion heat treatment can be performed on the brazed part according to the requirements of the part to improve the mechanical properties of the joint. For solder joints, the joints need to be cleaned after soldering.
The principle of the diffusion welding process is that in a vacuum or a protective atmosphere environment, the surfaces of the parts to be welded that are in contact with each other undergo microscopic plastic deformation under a certain temperature and pressure to form a tight bond, metal atoms at the interface diffuse each other, and finally a metallurgical bonding joint is formed. In order to meet the high-performance connection requirements of hot-end components such as turbine blades, based on the advantages of traditional brazing and diffusion welding technologies, a new type of transition liquid phase diffusion welding (TLP) technology has been developed. The basic principle is that an intermediate layer with a similar composition but a lower melting point than the base metal is placed in advance on the surface to be welded. During the insulation process, the intermediate layer melts and diffuses with the base material, causing the joint to isothermally solidify during the insulation process. In the subsequent diffusion process, the composition and the structure are homogenized, and finally a high-performance metallurgical combination is obtained. The basic operation process of transition liquid phase diffusion welding between brazing and diffusion welding is similar to brazing, but the joint performance is significantly improved, so it has been widely used in the aerospace field.
Due to the overall heating of the workpiece or the large area around the joint during the brazing / diffusion welding, the deformation of the workpiece after welding is small, and it is easy to ensure the size of the workpiece. As long as the process is selected properly, the joint performance is similar to that of the base material, and it is not necessary Processing is "seamless", so it is very suitable for highly reliable connection of precision parts. In addition, according to the melting point of the base metal, a brazing material having a lower melting temperature than the base metal can be selected for welding, thereby avoiding the change of the microstructure characteristics of the base metal by the brazing heating, so it is also very suitable for dissimilar metals, even between metals and non-metals, non-metals. The connection between metal and non-metal.
Parts of different structural materials require different brazing methods, and different brazing methods correspond to different brazing equipment. In the 1950s and 1960s, the materials used were mainly carbon steel, aluminum alloy, stainless steel, etc. The corresponding brazing methods were salt bath brazing, flame brazing, brazing in a protective atmosphere furnace, and so on. With the continuous advancement of science and technology, advanced titanium alloys and superalloy materials are gradually promoted and applied in the industrial field. In order to achieve the requirements of small deformation, non-oxidation and high-performance connection of parts, new methods such as vacuum brazing and transition liquid phase diffusion welding are mainly used. Brazing / diffusion welding method, the main equipment is vacuum brazing furnace. At the same time, in order to meet specific needs, corresponding new brazing processes and equipment have been developed, such as induction brazing under vacuum or gas protection conditions, vacuum arc brazing, electron beam brazing, and laser brazing.
Aerospace engines are one of the most widely used areas for brazing / diffusion welding. The major aero engine manufacturing companies in Europe and the United States attach great importance to the research of brazing / diffusion welding technology. Through the development of new brazing materials and intermediate layer materials, the development of new brazing / diffusion welding processes and equipment, the brazing / diffusion welding technology has gradually changed from engine cooling. The non-stressed part is enlarged to the key part which is stressed at the hot end. For example, Pratt & Whitney JT3D and JT8D engine compressor stator rings, JT9D engine turbine blades, fuel manifolds, and stainless steel heat exchangers, UK Rolls-Royce Spey engine high-pressure primary turbine blades, RB211 and V2500 engine titanium alloy fan blades are all Prepared by brazing / diffusion welding. Among them, there are more than 220 brazed components on JT3D engines alone.
With the development of advanced propulsion systems, the manufacture of heterogeneous metal materials, ceramics and metals, intermetallic compounds, directional solidification, single crystal blades, and film-cooled high-temperature components of complex structures increasingly depends on the continuous innovation of brazing / diffusion welding technology. Development is the general trend of technological progress since entering the 21st century, so it can be foreseen that the ancient technique of brazing / diffusion welding will surely usher in a long-lost spring in the tide of technological progress in the 21st century.