Aluminum is a non-ferrous metal in the largest amount of use, the most widely used metal materials, and its scope of application is still expanding. The use of aluminum production of aluminum products is a wide variety, countless, according to statistics has more than 700,000 kinds, from the building decoration industry to the transportation industry and aerospace and other industries have different needs. Introduce the processing of aluminum products and how to avoid processing deformation.
The superiority and characteristics of aluminum are as follows:
1.Low density. The density of aluminum is about 2.7g/cm3. it is only 1/3 of the density of iron or copper.
2. High plasticity. Aluminum has good ductility and can be made into various supplies by extrusion, stretching and other pressure processing means.
3. Corrosion resistance. Aluminum is a highly negatively charged metal, in natural conditions or anodic oxidation of the surface will generate a protective oxide film, with much better corrosion resistance than steel.
4. Easy to strengthen. The strength of pure aluminum is not high, but its strength can be improved by anodic oxidation.
5. Easy surface treatment. Surface treatment can further improve or change the surface properties of aluminum. Aluminum anodic oxidation process is quite mature, stable operation, has been widely used in the processing of aluminum products.
6. Good conductivity, easy to recycle.
Stamping of aluminum products
1.Cold Punch
Material used: Aluminum pellets. Using an extruder table and die for one-time molding, it is suitable for column-shaped products or product shapes that are difficult to achieve with the stretching process, such as oval, square, and rectangular products.
The tonnage of the machine used is related to the cross-sectional area of the product, the gap between the upper die punch and the lower die tungsten steel is the wall thickness of the product, and the vertical gap between the upper die punch and the lower die tungsten steel to the lower dead center at the completion of the press fit is the top thickness of the product.
Advantages: shorter molding cycle, lower development cost compared to stretching molds.
Disadvantages: Longer production process, large fluctuation of product size during the process, high labor cost.
2.Stretching
Use material aluminum skin. Using continuous mold machine table and molds for multiple deformation to make it reach the demand of shape, suitable for non-cylindrical body (products with curved aluminum).
Advantages: more complex and multiple deformation products in the production process of dimensional control is stable, the product surface is more polished.
Disadvantages: high cost of molds, relatively long development cycle, high requirements for the selection and accuracy of the machine.
Surface treatment of aluminum products
1. Sandblasting (shot blasting)
The process of cleaning and roughening metal surfaces using the impact of a high-speed sand stream.
This method of surface treatment of aluminum parts enables the surface of the workpiece to obtain a certain degree of cleanliness and different roughness, so that the mechanical properties of the surface of the workpiece can be improved, therefore improving the fatigue resistance of the workpiece, increasing the adhesion between it and the coating, prolonging the durability of the coating film, and also contributing to the leveling and decoration of the coating. This process we often see in Apple’s various products.
2.Polishing
The use of mechanical, chemical or electrochemical effects, so that the surface roughness of the workpiece to reduce, in order to obtain a bright, flat surface processing methods. Polishing process is mainly divided into: mechanical polishing, chemical polishing, electrolytic polishing. Aluminum parts using mechanical polishing + electrolytic polishing can be close to the stainless steel mirror effect, the process, giving people a high-grade simple, fashionable future feeling.
3.Drawing
Metal drawing is the manufacturing process of repeatedly scraping lines out of aluminum sheets with sandpaper. Wire drawing can be divided into straight wire drawing, messy wire drawing, spinning wire drawing, thread drawing. The brushed metal process can clearly show each and every fine silk trace, thus making the metal matte with a fine hairline luster, and the product is both fashionable and technological sense.
4.High light cutting
The use of engraving machine will be diamond knife reinforced in the high-speed rotation (general speed of 20,000 rpm) of the engraving machine spindle to cut parts, in the product surface to produce localized highlights area. The brightness of the cutting highlights is affected by the speed of the milling drill, the faster the drill speed, the brighter the cutting highlights, and vice versa, the darker and prone to knife lines. Highlight highlight cutting in the use of cell phones in particular, such as iphone5, in recent years part of the high-end TV metal bezel using a high-gloss milling process, coupled with anodized and brushed process makes the TV set overall full of fashion sense and sharp sense of science and technology.
5.Anodic oxidation
Anodic oxidation refers to the electrochemical oxidation of metals or alloys. Aluminum and its alloys form a layer of oxidized film on aluminum products (anode) due to the action of applied current under the corresponding electrolyte and specific process conditions. Anodic oxidation can not only solve the defects of aluminum surface hardness and abrasion resistance, but also prolong the service life of aluminum and enhance the aesthetics, which has become an indispensable part of aluminum surface treatment, and it is the most widely used and very successful process at present.
6.Double color anode
Two-color anodizing refers to anodizing a product and giving different colors to specific areas. The two-color anodizing process is less used in the TV industry because of the complexity and high cost of the process; however, the contrast between the two colors better reflects the high-end and unique appearance of the product.
Process measures and operating techniques to reduce aluminum machining distortion
Aluminum parts processing deformation of many reasons, and materials, part shape, production conditions and so on have a relationship. There are mainly the following aspects: deformation caused by internal stress in the blank, cutting force, cutting heat caused by deformation, deformation caused by clamping force.
1.Reduce the internal stress of Mao Pei
Internal stresses in the blank can be partially eliminated by natural or artificial aging and vibration treatment. Pre-machining is also an effective process. For fat head and ears of the blank, due to the large margin, so the deformation after processing is also large. If pre-processing off the excess part of the blank, reduce the allowance of each part, not only can reduce the processing deformation of the later process, and pre-processing after placing for a period of time, but also can release part of the internal stress.
2.Improve the cutting ability of the tool
The material and geometric parameters of the tool have an important influence on the cutting force and cutting heat, and the correct selection of the tool is essential to reduce the deformation of the part machining.
(1) Reasonable selection of tool geometry parameters.
①Front angle: under the condition of maintaining the strength of the cutting edge, the front angle is appropriately chosen to be larger, on the one hand, it can sharpen the edge, and on the other hand, it can reduce the cutting deformation, make the chip removal smooth, and then reduce the cutting force and cutting temperature. Do not use negative front angle tool.
② rear angle: the rear angle size of the rear face wear and processing surface quality has a direct impact. Cutting thickness is an important condition for selecting the rear angle. Rough milling, due to large feed, heavy cutting load, heat generation, requiring good tool cooling conditions, therefore, the rear angle should be selected smaller. Precision milling, the requirements of sharp edges, reduce the friction between the rear face and the machined surface, reduce elastic deformation, therefore, the rear angle should be selected larger.
③ Helix angle: In order to make milling smooth and reduce milling force, the helix angle should be chosen as large as possible.
④ Main deflection angle: Appropriate reduction of the main deflection angle can improve the heat dissipation conditions, so that the average temperature of the machining area decreases.
(2) Improvement of tool structure.
① Reduce the number of teeth of milling cutter and increase the chip space. Due to the greater plasticity of aluminum materials, cutting deformation in the processing of large, need a larger chip space, so the bottom radius of the chip slot should be larger, fewer milling cutter teeth is good.
②Precision grinding cutter teeth. Roughness value of the cutting edge of the teeth to be less than Ra = 0.4 um. In the use of new knives before, should use a fine oil stone in front of the teeth, behind the knife gently ground a few times to eliminate the edge of the teeth of the knife when the residual burrs and slight serration pattern. In this way, not only can reduce the cutting heat and cutting deformation is relatively small.
③ Strictly control the standard of tool wear. After the tool wear, the surface roughness of the workpiece increases, the cutting temperature rises, and the deformation of the workpiece increases. Therefore, in addition to the selection of wear-resistant tool materials, tool wear standard should not be greater than 0.2mm, otherwise it is easy to produce chip tumors. When cutting, the temperature of the workpiece should generally not exceed 100 ℃ to prevent deformation.
3、Improve workpiece clamping method
For thin-walled aluminum workpieces with poor rigidity, the following clamping methods can be used to reduce deformation:
① For thin-walled bushing parts, if the three-jaw self-centering chuck or collet chuck is used to clamp from the radial direction, the workpiece is bound to be deformed once it is loosened after machining. At this point, you should use the rigidity of the better axial end face compression method. Positioned in the hole of the part, a threaded self-contained through the center of the shaft, set into the parts of the hole, with a cover plate on the end of the compression face and then backed with a nut tight. Processing of the outer circle can be avoided when the clamping deformation, thus obtaining satisfactory machining accuracy.
② thin-walled sheet workpiece machining, it is best to use vacuum suction cups, in order to obtain a uniform distribution of clamping force, and then a smaller amount of cutting to process, can be very good to prevent deformation of the workpiece.
In addition, the filling method can also be used. In order to increase the rigidity of the process of thin-walled workpieces, the workpiece can be filled with medium inside the workpiece to reduce the clamping and cutting process of the workpiece up to deformation. For example, to the workpiece filled with 3% to 6% potassium nitrate urea melt, after processing, the workpiece will be immersed in water or alcohol, the filler can be dissolved and poured out.
4、Reasonable arrangement of work processes
High-speed cutting, due to the large machining allowance and intermittent cutting, so the milling process often produces vibration, affecting the machining accuracy and surface roughness. Therefore, the CNC high-speed cutting machining process can generally be divided into: roughing – semi-finishing – clear angle machining – finishing and other processes. For parts with high accuracy requirements, it is sometimes necessary to carry out a second semi-finishing process, followed by finishing. After rough machining, the parts can be naturally cooled to eliminate the internal stresses generated by rough machining and reduce deformation. The margin left after rough machining should be larger than the amount of deformation, generally 1 ~ 2 mm. finishing, parts finishing surface to maintain a uniform machining margin, generally 0.2 ~ 0.5 mm is appropriate, so that the tool is in a smooth state in the machining process, you can greatly reduce the cutting deformation, to obtain a good surface finish quality, to ensure the accuracy of the product.
Operational techniques to reduce machining distortion
Aluminum parts material parts deformed during processing, in addition to the above reasons, in practice, the operation method is also very important.
1.For the machining of large parts, in order to make it in the process of machining have better heat dissipation conditions, to avoid heat concentration, processing, it is appropriate to use symmetric processing. If there is a 90mm thick plate needs to be processed to 60mm, if milling a good side immediately after milling the other side, a machining to the final size, then the flatness of 5mm; if the repeated feed symmetric machining, each side of the two times to the final size of the machining, can ensure that the flatness of 0.3mm.
2. If there are more than one cavity on the plate parts, processing, should not be used a cavity a cavity sequence of processing methods, which is likely to cause uneven force and deformation of the parts. The use of multi-layer processing, each layer as far as possible at the same time processing to all the cavities, and then process the next level, so that the parts are uniformly stressed, reducing deformation.
3.By changing the cutting amount to reduce the cutting force, cutting heat. In the three elements of the cutting amount, the back draft has a great impact on the cutting force. If the machining allowance is too large, the cutting force is too large, not only will make the parts deformed, but also affect the rigidity of the machine tool spindle, reduce the durability of the tool. If you reduce the amount of back draft, and will make a big difference in productivity. However, in CNC machining are high-speed milling, can overcome this challenge. In reducing the amount of back draft at the same time, as long as the corresponding increase in feed, increase the speed of the machine tool, you can reduce the cutting force, while ensuring the machining efficiency.
4.The order of tool walking should also be careful. Roughing emphasizes the improvement of processing efficiency, the pursuit of the removal rate per unit of time, generally can be used inverse milling. That is, the fastest speed, the shortest time to excise the excess material on the surface of the blank, the basic formation of the geometric contour required for finishing. And finishing emphasizes the high precision and quality, it is appropriate to use downstream milling. Because the cutting thickness of the cutter teeth gradually decreases from the maximum to zero, the degree of work hardening is greatly reduced, while reducing the degree of deformation of the parts.
5.Thin-walled workpieces in the processing of deformation due to clamping, even if the finishing process is difficult to avoid. In order to minimize the deformation of the workpiece, you can reach the final size in the finishing process before the pressurized pieces of loose, so that the workpiece is free to return to its original state, and then slightly pressed to just be able to clamp the workpiece shall prevail (entirely by feel), so that you can get the desired processing results. In short, the action point of the clamping force is best in the bearing surface, the clamping force should act in the direction of good rigidity of the workpiece, under the premise of ensuring that the workpiece does not loosen, the smaller the clamping force the better.
6. In the processing of parts with cavity, processing cavity try not to let the milling cutter like a drill directly down into the parts, resulting in the milling cutter chip space is not enough, chip removal is not smooth, resulting in parts overheating, expansion, as well as chipping, tool breakage and other unfavorable phenomena. To use the same size as the milling cutter or a larger drill bit to drill down the hole, and then use the milling cutter milling. Alternatively, CAM software can be used to produce a spiral undercut program.
