0 Preface
With the continuous increase of the total number of automobiles in my country, especially the rapid growth of personal vehicles in cities, the pollution caused by automobile exhaust emissions to the city has become increasingly serious, so clean fuel vehicles have developed rapidly. Vehicles fueled by hydrogen have zero emission pollution and fully meet the current requirements for the development of non-polluting and lightweight vehicles. Due to the environmentally friendly hydrogen preparation and supply method for high-pressure vehicles: the technology to produce hydrogen with a purity of more than 99.9% and P=30MPa from pure water has broken through the bottleneck, and the hydrogen engine manufacturing technology used has matured. This method provides hydrogen energy for vehicles as power. , which is environmentally friendly and has a large amount of available resources, so hydrogen-fueled vehicles will develop rapidly. The pressure vessel containing hydrogen has developed from a steel gas cylinder to a gas cylinder with a fully wound aluminum liner fiber. This type of high-pressure hydrogen cylinder has excellent performances such as high pressure bearing capacity, light weight and strong corrosion resistance, and has become the current hydrogen storage for vehicles. Bottle of choice.
The vehicle-mounted aluminum hydrogen gas storage cylinder is made of 6061 aluminum tubing billet by strong spinning to reduce the wall, close the mouth, wind the carbon fiber, heat treatment, chemical polishing, passivation and coating. It is still in its infancy in my country. Aluminum bottles in developed countries have been put into use. In the early stage, my country has manufactured some aluminum alloy materials instead of steel materials to manufacture natural gas storage cylinders. After more than ten years of use, it is safe and light. Since the internal pressure of the aluminum hydrogen gas storage cylinder is about 1.5 to 2 times that of the natural gas cylinder, the technical requirements of the tube blank are higher.
1 Introduction to high-pressure hydrogen storage cylinders for automobiles
Ordinary cars can become hydrogen fuel vehicles by adding a set of modification systems. It is the development trend of modern automobiles and belongs to a kind of non-polluting vehicles. Compared with natural gas and gasoline, hydrogen fuel has the following advantages: low calorific value per unit mass, high It is about 2.7 times the low calorific value of gasoline; the concentration range of the combustible mixture is wide, and hydrogen is easy to achieve lean combustion; the auto-ignition temperature is high, which is conducive to improving the compression ratio; the ignition energy is low, and the minimum can be as low as 0.02m J; The combustion reaction proceeds according to the chain reaction mechanism; the flame propagation speed is 7.72 times that of gasoline; the anti-knock performance is better than that of gasoline during engine combustion, and a higher compression ratio can be used; the diffusion coefficient in the air is very large, which is the highest in gasoline. 12 times as much, so hydrogen is easier to mix with air than gasoline to form a homogeneous mixture, and the combustion efficiency is high. The characteristics of hydrogen fuel vehicles compared with gasoline vehicles are shown in Table 1. Vehicle-mounted hydrogen fuel is generally stored in high-pressure gas cylinders. At present, high-pressure hydrogen gas storage cylinders are mostly made of steel composite materials. Due to their large proportion, it is difficult to popularize and use high-pressure aluminum gas storage cylinders. Select objects.
1.1 Aluminum gas cylinder
The high-pressure aluminum gas storage cylinder is a closed container formed by a large-sized aluminum alloy tube blank through a spinning head. It has the characteristics of economy, safety, lightness, large storage capacity, and fast filling. The pressure in the cylinder is generally 35MPa. The pressure of the bottle is not less than 70MPa.
The material of the tube blank is 6061-T1, and various technical indicators are required to be high: seamless, thin-walled, smooth surface, high performance after heat treatment, small eccentricity, small curvature, large outer diameter, and fixed length.
1.2 Market Demand Status of Aluminum Tube Billets
Statistics show that the demand for aluminum auto parts in the new energy vehicle industry is growing rapidly. According to data, in 2016, my country produced 517,000 new energy vehicles, a year-on-year increase of 52%, and car sales accounted for 1.84%, which is still a relatively small proportion of the market. With the continuous introduction of subsidy policies by the government, it is expected that the production of new energy vehicles will maintain a compound annual growth rate of about 35% by 2021, and will reach 2.32 million in 2021. Within ten years, the proportion of hydrogen-fueled vehicles in new energy vehicles will rise from less than 1% now to 30%, and the required aluminum billet will reach more than 100kt (the market demand in 2017 is 5000t).
2 Development difficulties and analysis
2.1 Features of the tube blank
(1) The tube blank is seamless. It is required to use hollow ingot extrusion or solid ingot perforation extrusion on a large extruder, and the theoretical calculation of the extrusion force is more than 8000t.
(2) The outer diameter of the tube blank is large, reaching ϕ260~ϕ400mm, and the wall is thin, 8~20mm.
(3) The technical indicators of the tube blank have high requirements, all exceeding the national standard GB 44371-2000 (high precision), see Table 2.
2.2 Difficulties in the development of tube blanks
(1) The wall of the tube blank is thin and the inner and outer diameter tolerances are small. It is required that the center of extrusion cylinder, extrusion shaft, hollow gasket and die hole should not be offset by 0.5mm, and the deformation caused by stretching straightening and sawing should not be larger than 0.5mm.
The large-diameter gas cylinder tube blank can only be produced by a horizontal extruder with a pressure of more than 80MN. The factors that determine the outer diameter, tolerance and surface quality of the tube blank are the mold, and the factors that determine the inner diameter of the tube blank are the needle support, the extrusion needle, and the needle tip. The needle support, the extruding needle and the needle tip are connected by a thread, and the needle support is connected with the body of the extruder to form a cantilever beam by the thread. The beam length of a large extruder is generally about 5m and the weight is about 4~5t. Under its own weight, due to the existence of thread fit clearance and elastic deformation, it is impossible for the needle tip to coincide with the mold center. 5~10mm down.
It can be seen from this that it is impossible to use a horizontal extruder to produce gas cylinders with a wall thickness of 8~20mm and a deviation of less than 1.5mm, and the eccentricity is unavoidable, so a vertical extruder must be used. However, at present, the extrusion force of the vertical extruders for extruding aluminum alloys in China and even in the world is not large, no more than 5000t, and the vertical stroke is not enough.
The tolerance zone of the inner diameter of the tube blank is within 3mm, the size of the die tip is difficult to determine, the shrinkage of the thin-walled pipeline is different from that of the bar and thick-walled tube, and there is no thin-walled tube data before. , without reference data.
(2) The curvature and ellipse are difficult to control. In the national standard GB 4437.1-2000, only pipes with an outer diameter of less than 250mm are checked for curvature, so pipes with a diameter greater than 250mm are not subjected to the stretching and straightening process, but the curvature of the gas cylinder tube blank is less than 1mm/m, and must be stretched Straighten. Stretching mandrel, single weight 120~200kg, manual operation, easy to produce ellipse and uneven wall thickness deformation during stretching.
Because the working surface of the jaws is flat, the tube blank is mainly stressed by only the upper and lower lines during stretching, and the tube blank appears elliptical and has uneven wall thickness deformation; the tube blank is easily deformed during sawing and sampling, and an ellipse phenomenon occurs.
(3) The quality of the inner wall of the surface is high. Scratches on the inner wall ≤ 0.05mm, which are equivalent to extrusion lines or scratches, and feel no touch. The pipe material is soft, and the σb of the 6061 alloy in the hot state (about 400 °C) is between 17 and 25 MPa, so it is difficult to control the scratches on the inner wall surface ≤ 0.05 mm in production, and the ingot requires good metallurgical quality.
(4) The structure and properties of the tube blank are high, and the composition of the ingot must be strictly controlled, and the best extrusion process must be required.
The chemical composition of the hydrogen gas storage cylinder tube blank material is strictly required. Within the scope of the national standard, the ingredients are prepared according to the special composition control requirements of a company's 6061 alloy, and the content of individual harmful impurity elements Pb and Bi is strictly controlled.
(5) Extruded products all have the problem of uneven metal deformation due to the influence of tools and processes. The gas cylinder tube blank has high requirements in terms of surface, size, structure, performance, etc., so the existing process must be improved and optimized. .
3 Preparation process
Due to the high requirements of various indicators of this product, the difficulty of production, and the need to overcome a variety of technical problems, a series of measures must be taken in production, such as: adjusting the center of the extrusion needle through the center of the extrusion cylinder, reducing the size of the extrusion gasket and the The matching tolerance of the inner sleeve of the cylinder can shorten the length of the ingot; when extruding, the ingot and gasket are installed first, and after the extrusion shaft pushes the ingot and gasket in place, the perforation needle is advanced, the special guide is manufactured, and each The process is strictly soft contact management.
After the process was determined, combined with the implementation of the above series of measures, the strong-spin 6061 aluminum tubing for hydrogen gas storage cylinders was finally successfully developed and mass-produced. The following is a brief introduction to some of the typical specifications of ϕ375.5mm×20mm×4000mm produced by a company. The product is shown in Figure 1.
(1) Tool preparation: ϕ650mm extrusion cylinder, ϕ360mm extrusion needle, ϕ335mm needle tip, ϕ375.5mm tube die; prepare soft contact pads and spreaders.
(2) Ingot preparation: ingredients are prepared as required, smelted and cast into ϕ630mm×1250mm rods, uniformly fired, machined with holes ϕ630mm×ϕ370mm×1250mm, and the inner and outer surfaces are free of oil and dust.
(3) The extrusion process parameters are shown in Table 3.
(4) Extrusion requirements: Extrusion is upsetting at a slow speed, and the surface of the inner cavity is checked for each material, and any problems are dealt with in time.
(5) Stretching: according to the relevant regulations, the head and tail are stretched after cutting, and the stretching rate is 1.8%~2.0%.
(6) Cut the head and tail, sample and cut to length.
(7) Inspection and packaging.
4 Development results
According to the national standard GB 4437.1-2000 standard (high precision) and the special technical agreement, the developed 6061 aluminum tubing hydrogen cylinder tube blank was tested, and the results are as follows:
(1) Performance index: σb≥310MPa, σ0.2≥270MPa, δ=14%.
(2) The low magnification structure has no defects that are not allowed in the standard and the protocol, and the grain size is grade 1, see Figure 2.
(3) The outer diameter of the tube blank is ϕ374.5mm~ϕ376.2mm, the inner diameter is ϕ336.2mm~ϕ337.6mm, the wall thickness difference is 0~1.5mm, and the inner diameter tolerance is 0.7~2.1mm.
(4) The inner surface is smooth and clean, without scratches with a depth greater than 0.05mm.
(5) The curvature is within 1mm/m.
The above results all meet the index requirements of the technical agreement between the two parties. A few months after the first batch of products was released, follow-up orders increased one after another, so the development of 6061 aluminum tubin blanks for hydrogen cylinders has begun to be marketed.
5 Conclusion
Strong spinning 6061 aluminum tubing blanks for high-pressure hydrogen cylinders have been successfully developed in China, and mass production has been achieved. It is the process developed in this paper that is used, which proves that the production process is correct, reasonable and feasible.
It is feasible to properly adjust the tools and molds of large horizontal presses above 80MN and take certain corresponding measures to produce hydrogen gas cylinder tube blanks with a nominal diameter of ϕ260mm or more, wall thickness of 10~20mm, high precision and high quality.
