Aluminum is the most widely used non-ferrous metal, and its range of applications continues to expand. There are over 700,000 types of aluminum products, which cater to various industries, including construction, decoration, transportation, and aerospace. In this discussion, we will explore the processing technology of aluminum products and how to avoid deformation during processing.
The advantages and characteristics of aluminum include:
- Low Density: Aluminum has a density of about 2.7 g/cm³, which is roughly one-third that of iron or copper.
- High Plasticity: Aluminum has excellent ductility, allowing it to be formed into various products through pressure processing methods, such as extrusion and stretching.
- Corrosion Resistance: Aluminum naturally develops a protective oxide film on its surface, either under natural conditions or through anodization, offering superior corrosion resistance compared to steel.
- Easy to Strengthen: Although pure aluminum has a low strength level, its strength can be significantly increased through anodizing.
- Facilitates Surface Treatment: Surface treatments can enhance or modify the properties of aluminum. The anodizing process is well-established and widely used in aluminum product processing.
- Good Conductivity and Recyclability: Aluminum is an excellent conductor of electricity and is easy to recycle.
Aluminum product processing technology
Aluminum product stamping
1. Cold stamping
The material used is aluminum pellets. These pellets are shaped in a single step using an extrusion machine and a mold. This process is ideal for creating columnar products or shapes that are challenging to achieve through stretching, such as elliptical, square, and rectangular forms. (As shown in Figure 1, the machine; Figure 2, the aluminum pellets; and Figure 3, the product)
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 made of tungsten steel determines the wall thickness of the product. Once the pressing is complete, the vertical gap from the upper die punch to the lower die indicates the top thickness of the product.(As shown in Figure 4)
Advantages: Short mold opening cycle, lower development cost than stretching mold. Disadvantages: Long production process, large fluctuation of product size during the process, high labor cost.
2. Stretching
Material used: aluminum sheet. Use continuous mold machine and mold to perform multiple deformations to meet the shape requirements, suitable for non-columnar bodies (products with curved aluminum). (As shown in Figure 5, machine, Figure 6, mold, and Figure 7, product)
Advantages: The dimensions of complex and multi-deformed products are controlled stably during the production process, and the product surface is smoother.
Disadvantages: High mold cost, relatively long development cycle, and high requirements for machine selection and precision.
Surface treatment of aluminum products
1. Sandblasting (shot peening)
The process of cleaning and roughening the metal surface by the impact of high-speed sand flow.
This method of aluminum surface treatment enhances the cleanliness and roughness of the workpiece surface. As a result, the mechanical properties of the surface are improved, leading to better fatigue resistance. This improvement increases the adhesion between the surface and any coatings applied, extending the durability of the coating. Additionally, it facilitates the leveling and aesthetic appearance of the coating. This process is commonly seen in various Apple products.
2. Polishing
The processing method employs mechanical, chemical, or electrochemical techniques to reduce the surface roughness of a workpiece, resulting in a smooth and shiny surface. The polishing process can be categorized into three main types: mechanical polishing, chemical polishing, and electrolytic polishing. By combining mechanical polishing with electrolytic polishing, aluminum parts can achieve a mirror-like finish similar to that of stainless steel. This process imparts a sense of high-end simplicity, fashion, and a futuristic appeal.
3. Wire drawing
Metal wire drawing is a manufacturing process in which lines are repeatedly scraped out of aluminum plates with sandpaper. Wire drawing can be divided into straight wire drawing, random wire drawing, spiral wire drawing, and thread wire drawing. The metal wire drawing process can clearly show every fine silk mark so that the matte metal has a fine hair luster, and the product has both fashion and technology.
4. High light cutting
Highlight cutting uses a precision engraving machine to reinforce the diamond knife on the high-speed rotating (generally 20,000 rpm) precision engraving machine spindle to cut parts and produce local highlight areas on the product surface. The brightness of the cutting highlights is affected by the milling drill speed. The faster the drill speed, the brighter the cutting highlights. Conversely, the darker the cutting highlights are, the more likely they are to produce knife marks. High-gloss cutting is particularly common in mobile phones, such as the iPhone 5. In recent years, some high-end TV metal frames have adopted high-gloss CNC milling technology, and the anodizing and brushing processes make the TV full of fashion and technological sharpness.
5. Anodizing
Anodizing is an electrochemical process that oxidizes metals or alloys. During this process, aluminum and its alloys develop an oxide film when an electric current is applied in a specific electrolyte under certain conditions. Anodizing enhances the surface hardness and wear resistance of aluminum, extends its service life, and improves its aesthetic appeal. This process has become a vital component of aluminum surface treatment and is currently one of the most widely used and successful methods available.
6. Two-color anode
A two-color anode refers to the process of anodizing a product to apply different colors to specific areas. Although this two-color anodizing technique is rarely employed in the television industry due to its complexity and high cost, the contrast between the two colors enhances the product’s high-end and unique appearance.
There are several factors that contribute to the processing deformation of aluminum parts, including material properties, part shape, and production conditions. The main causes of deformation include: internal stress present in the blank, cutting forces and heat generated during machining, and forces exerted during clamping. To minimize these deformations, specific process measures and operating skills can be implemented.
Process measures to reduce processing deformation
1. Reduce the internal stress of the blank
Natural or artificial aging, along with vibration treatment, can help reduce the internal stress of a blank. Pre-processing is also an effective method for this purpose. For a blank with a fat head and large ears, significant deformation can occur during processing due to the substantial margin. By pre-processing the excess parts of the blank and reducing the margin in each area, we can not only minimize the deformation that occurs during subsequent processing but also alleviate some of the internal stress present after pre-processing.
2. Improve the cutting ability of the tool
The tool’s material and geometric parameters significantly affect cutting force and heat. Proper tool selection is essential to minimize parts’ processing deformation.
1) Reasonable selection of tool geometric parameters.
① Rake angle: Under the condition of maintaining the strength of the blade, the rake angle is appropriately selected to be larger. On the one hand, it can grind a sharp edge, and on the other hand, it can reduce cutting deformation, make chip removal smooth, and thus reduce cutting force and cutting temperature. Avoid using negative rake angle tools.
② Back angle: The size of the back angle has a direct impact on the wear of the back tool face and the quality of the machined surface. Cutting thickness is an important condition for selecting the back angle. During rough milling, due to the large feed rate, heavy cutting load, and high heat generation, the tool heat dissipation conditions are required to be good. Therefore, the back angle should be selected to be smaller. During fine milling, the edge is required to be sharp, the friction between the back tool face and the machined surface must be reduced, and elastic deformation must be reduced. Therefore, the back angle should be selected to be larger.
③ Helix angle: In order to make milling smooth and reduce milling force, the helix angle should be selected as large as possible.
④ Main deflection angle: Appropriately reducing the main deflection angle can improve the heat dissipation conditions and reduce the average temperature of the processing area.
2) Improve tool structure.
Reduce the Number of Milling Cutter Teeth and Increase Chip Space:
Since aluminum materials exhibit high plasticity and significant cutting deformation during processing, it is essential to create a larger chip space. This means that the radius of the chip groove bottom should be greater, and the number of teeth on the milling cutter should be reduced.
Fine Grinding of Cutter Teeth:
The roughness value of the cutting edges of the cutter teeth should be less than Ra = 0.4 µm. Before using a new cutter, it is advisable to gently grind the front and back of the cutter teeth with a fine oil stone several times to eliminate any burrs or slight sawtooth patterns left from the sharpening process. This not only helps in reducing cutting heat but also minimizes cutting deformation.
Strictly Control Tool Wear Standards:
As tools wear down, the surface roughness of the workpiece increases, the cutting temperature rises, and the workpiece can suffer from increased deformation. Therefore, it is crucial to choose tool materials with excellent wear resistance, and ensure that tool wear does not exceed 0.2 mm. If wear exceeds this limit, it can lead to chip formation. During cutting, the temperature of the workpiece should generally be kept below 100°C to prevent deformation.
3. Improve the clamping method of the workpiece. For thin-walled aluminum workpieces with poor rigidity, the following clamping methods can be used to reduce deformation:
① For thin-walled bushing parts, using a three-jaw self-centering chuck or a spring collet for radial clamping can lead to the deformation of the workpiece once it is loosened after processing. To avoid this issue, it is better to use an axial end face clamping method that offers greater rigidity. Position the inner hole of the part, create a threaded through-mandrel, and insert it into the inner hole. Then, use a cover plate to clamp the end face and secure it tightly with a nut. This method helps prevent clamping deformation when processing the outer circle, ensuring satisfactory processing accuracy.
② When processing thin-walled sheet metal workpieces, it is advisable to use a vacuum suction cup to achieve a uniformly distributed clamping force. Additionally, using a smaller cutting amount can help prevent the deformation of the workpiece.
Another effective method is to fill the interior of the workpiece with a medium to enhance its processing rigidity. For example, a urea melt containing 3% to 6% potassium nitrate can be poured into the workpiece. After processing, the workpiece can be immersed in water or alcohol to dissolve the filler and then pour it out.
4. Reasonable arrangement of processes
During high-speed cutting, the milling process often generates vibration due to large machining allowances and intermittent cutting. This vibration can negatively impact machining accuracy and surface roughness. As a result, the CNC high-speed cutting process is typically divided into several stages: roughing, semi-finishing, angle cleaning, and finishing. For parts that require high precision, a secondary semi-finishing may be necessary before finishing.
After the roughing stage, it is advisable to allow the parts to cool naturally. This helps to eliminate the internal stress generated during roughing and reduces deformation. The machining allowance left after roughing should be greater than the expected deformation, generally between 1 to 2 mm. During the finishing stage, it is important to maintain a uniform machining allowance on the finished surface, typically between 0.2 to 0.5 mm. This uniformity ensures that the cutting tool remains in a stable state during processing, which significantly reduces cutting deformation, enhances surface quality, and ensures product accuracy.
Operational skills to reduce processing deformation
Aluminum parts deform during processing. In addition to the above reasons, the operation method is also very important in actual operation.
1. For parts that have large processing allowances, symmetrical processing is recommended to improve heat dissipation during machining and to prevent heat concentration. For example, when processing a 90mm thick sheet down to 60mm, if one side is milled immediately after the other side, the final dimensions may result in a flatness tolerance of 5mm. However, if a repeated feed symmetrical processing approach is used, where each side is machined to its final size twice, the flatness can be improved to 0.3mm.
2. When there are multiple cavities on sheet parts, it is not advisable to use the sequential processing method of addressing one cavity at a time. This approach can lead to uneven forces on the parts, resulting in deformation. Instead, use a layered processing method where all cavities in a layer are processed simultaneously before moving on to the next layer. This ensures even stress distribution on the parts and minimizes the risk of deformation.
3. To reduce cutting force and heat, it’s important to adjust the cutting amount. Among the three components of cutting amount, the back-cutting amount significantly impacts the cutting force. If the machining allowance is excessive and the cutting force during a single pass is too high, it can lead to deformation of the parts, negatively affect the rigidity of the machine tool spindle, and reduce tool durability.
While decreasing the back-cutting amount can enhance tool longevity, it can also lower production efficiency. However, high-speed milling in CNC machining can effectively address this issue. By reducing the back-cutting amount and correspondingly increasing the feed rate and machine tool speed, cutting force can be lowered without compromising machining efficiency.
4. The sequence of cutting operations is important. Rough machining focuses on maximizing machining efficiency and increasing the material removal rate per unit of time. Typically, reverse milling is utilized for this phase. In reverse milling, excess material from the surface of the blank is removed at the highest speed and in the shortest time possible, effectively forming a basic geometric profile for the finishing stage.
On the other hand, finishing prioritizes high precision and quality, making down milling the preferred technique. In down milling, the thickness of the cut gradually decreases from the maximum to zero. This approach significantly reduces work hardening and minimizes deformation of the parts being machined.
5. Thin-walled workpieces often experience deformation due to clamping during processing, a challenge that persists even during the finishing stage. To minimize this deformation, it is advisable to loosen the clamping device before the final size is achieved during finishing. This allows the workpiece to return to its original shape, after which it can be gently reclamped—sufficient only to hold the workpiece in place—based on the operator’s feel. This method helps achieve the ideal processing results.
In summary, the clamping force should be applied as close as possible to the supporting surface and directed along the workpiece’s strongest rigid axis. While it is crucial to prevent the workpiece from coming loose, the clamping force should be kept to a minimum to ensure optimal results.
6. When processing parts with cavities, avoid allowing the milling cutter to penetrate directly into the material as a drill bit would. This approach can lead to insufficient chip space for the milling cutter, causing problems like unsmooth chip removal, overheating, expansion, and potential chip collapse or breakage of the components.
Instead, first, use a drill bit that is the same size or larger than the milling cutter to create the initial cutter hole. After that, the milling cutter is used for milling operations. Alternatively, you can utilize CAM software to generate a spiral-cutting program for the task.
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Post time: Nov-27-2024