At present, aluminum alloys are widely utilized in industries such as aviation, aerospace, automobiles, machinery manufacturing, shipbuilding, and the chemical sector.
The research on the processing technology of aluminum alloy workpieces primarily focuses on controlling their deformation to enhance product accuracy.
Conventional processing methods of aluminum alloy workpieces
Also known as sleeve processing, this method involves milling slots to release the workpiece from the blank. A small amount of material, usually 0.10-0.15mm, is typically left on the bottom surface.
This machining method is frequently used when there is a significant amount of material removal required in the shape, inner cavity, or inner hole. This helps to avoid the final deformation of the workpiece caused by heat and internal stress concentration resulting from roughing with large margins.
Blanking processing plays a crucial role in preventing deformation of aluminum parts, especially when a sucker is used. It effectively solves the problems of deformation and clamping in a large class of thin-walled parts, making it an ideal process for plate part machining.
A representation of the blanking process is shown in Figure 1.
Fig. 1 blanking processing
(2) Vacuum sucker method
The workpiece is positioned on the sucker surface through adsorption and processed with the pressing plate.
The sucker clamping method, in combination with blanking processing, is applicable to any surface of the product that can be adsorbed, including beveled and curved surfaces.
The use of sucker and blanking processing techniques allows for the processing of a variety of thin-walled parts, even if the plate thickness is as low as 0.5mm.
Figure 2 depicts a general-purpose sucker, while figure 3 displays a special suction cup tooling that is designed specifically for the product.
Fig. 2 universal sucker
Fig. 3 special sucker tooling designed according to the product
(3) Design stiffener and process head
If the structural strength of the product is inadequate to support the finishing process, the stiffener must be left in place during roughing to ensure sufficient clamping strength before finishing. This will prevent excessive heat and stress from being generated during the removal of the stiffener.
The workpiece size should not be overly large when using this method.
① The stiffener is designed to aid in vise clamping and machining the back of the workpiece (refer to Figure 4).
② The process head is designed to aid in subsequent positioning (refer to Figure 5).
③ The design of the process head and stiffener combined facilitates subsequent positioning and processing (refer to Figure 6).
Fig. 4 design stiffener to facilitate vise clamping and machining the back of workpiece
Fig. 5 design process head to facilitate subsequent positioning
Figure 6 design process head + stiffener to facilitate subsequent positioning and processing
An example of the comprehensive application of conventional machining methods is shown in Figure 7.
The process implementation method is as follows:
① The analysis of the product’s front and back structure reveals that the processing content on the back is minimal, making it suitable for the positioning surface (refer to Fig. 7a and Fig. 7b).
② The positioning surface is patched to ease the subsequent suction cup clamping (refer to Fig. 7C).
③ The product is thin and prone to deformation. To prevent rough machining deformation and finish deformation of the positioning surface, stiffeners are designed on the front (refer to Fig. 7D).
④ After completing the positioning surface, the front of the product is finished using the special suction cup tooling (refer to Fig. 7e and Fig. 7F).
Fig. 7 comprehensive application example of conventional processing method
(1) indirect aging treatment
Rough machining is used as a separate process to replace aging treatment. The workpiece is processed on the machine tool and, once rough machining is completed, it is removed. The finishing process can then be carried out without any interval time (if the workpiece is part of a mass-produced product, the finishing can be performed after unified roughing, with no interval aging time in between).
It’s important to note that the workpiece must be completely opened, leaving a 0.5mm margin on one side before finishing.
The roughing process of the blank serves as an indirect aging treatment, while the secondary loosening and clamping process of the workpiece is an indirect stress release process.
This method is suitable for machining most precision structures and aeronautical structures.
(2) Aging treatment
① Artificial aging:
For aluminum alloy workpieces, most methods involve placing the workpiece for 24 to 48 hours after roughening. This process is slower, especially for large and thin-walled parts.
② Heat Treatment Aging:
This method is faster compared to artificial aging and is commonly used for closed parts with complex structures and large allowances.
Both methods are used in the processing of aluminum alloy workpieces and are mainly applied to remove internal stress and achieve stable structure in products with large margins and high precision requirements.
If the product belongs to a power structure (as shown in Figure 8), aging treatment is required for most products. This is because:
Aging treatment effectively eliminates internal stress and ensures dimensional stability during subsequent processing.
By removing internal stress, stable tissue is obtained, avoiding deformation of the originally qualified product due to the continuous heat generated during movement.
Figure 8 products belonging to power parts in structure
Method for preventing datum plane deformation of aluminum alloy products
In the machining process, it is common to encounter situations where the datum plane has become deformed prior to finishing and cannot be used as the finishing datum. Additionally, when working with aluminum alloy workpieces, the elastic deformation of the material results in springback after being pressed or clamped, leading to dimensional deformation and going out of tolerance after finishing.
To prevent deformation of the aluminum alloy product datum, the following methods can be implemented:
1）When the workpiece can only be milled, the datum plane must be machined under the premise that the main strength of the workpiece is high enough, so that the finish milling can achieve a flat datum plane.
2）When working with thin workpieces that can only be milled, it is necessary to repeat the milling process several times, turning the surface several times, before effectively eliminating deformation. However, some deformation may still be present on the final surface obtained by this method.
3）When the structure of the workpiece allows, turning is a good method to prevent deformation. The surface of the turning is smoother than that of milling and since almost no heat is generated in the turning process, the datum plane can be turned in place at one time without semi-finishing after rough machining.
I once worked on processing the datum plane of a square product, which always deformed during milling, even if the tool’s feed speed was very slow. Due to the tool’s surface contact with the workpiece, heat and stress inevitably formed.
To overcome this issue, we used a single acting chuck to clamp the square workpiece, and successfully turned out the qualified datum plane. Turning involves point contact of the tool tip, which prevents large-area heat and stress concentration.
For reference, the products to be processed are shown in Figure 9. The workpiece material is 2A12-T4, and the blank size is 120mm × 400mm × 12mm, with a product size of 370mm × 110mm × 7mm. The parallelism of the two major surfaces needs to be 0.03mm, and the flatness needs to be 0.025mm.
Initial process method:
The machining center with good precision is used to repeatedly turn over milling to remove excess allowance, and the geometric tolerance of thickness can not meet the requirements.
Improved process method:
1）Mount the workpiece on the lathe using a single acting chuck and ensure that the turning side is properly illuminated.
2）When using a machining center, use the bright surface of the workpiece as the positioning surface. Before making threaded holes around the blank, make sure that the workpiece is not damaged.
3）Create specialized tooling for the lathe, fasten the workpiece with screws in the reverse direction, and turn the thickness of the product to meet the specifications outlined in the drawing.
Fig. 9 blank and structure of products to be processed
Clamping ideas for preventing deformation of aluminum alloy products
In addition to using suction cups to prevent deformation, how can products that cannot be clamped with suction cups be secured?
It is important to note that although a vise can be used for clamping during roughing, it should not come into direct contact with the workpiece during most subsequent processes.
This is because the force exerted by the vise is horizontal, whereas the workpiece material is typically deformable. As a result, the clamping force of the vise can cause elastic deformation of the workpiece. Therefore, the workpiece requires a reasonable downward clamping force, which can be achieved through methods such as using a pressing plate or screw.
In fact, the design of tooling for processing aluminum alloy workpieces takes into account the direction of the clamping force. The following methods are recommended for reference:
1）Design special tooling, including a special pressing plate and thread reverse drawing tooling (refer to Figure 10).
2）Ensure an inner hole fit and small boss limit (refer to Figure 11).
3）Create soft jaws with pins (refer to Figure 12).
4）Develop V-shaped positioning tooling with a limit (refer to Figure 13).
5）Construct a single piece tooling block and a multi-module tooling set to match the shape (refer to Figure 14).
6）Design tooling for a special pressing plate (refer to Figure 15).
Figure 10 design special tooling + special pressing plate and thread reverse drawing tooling
Fig. 11 inner hole fit + small boss limit
Figure 12 design of soft jaw with pin
Fig. 13 design of V-shaped positioning tooling with limit
Fig. 14 single piece tooling block and multi module tooling matched with shape
Figure 15 tooling for designing special pressing plate
Process improvement measures for plastic deformation of cast aluminum parts
Cast aluminum products have certain peculiarities. Due to the casting process, high precision is usually required for these products. Although some unimportant parts of the product’s shape may be cast, there are also some disadvantages that come with this convenience:
① The casting structure lacks rigidity, especially during processing where clamping rigidity may be an issue.
② The casting is a body that experiences stress concentration during processing, which can result in internal stress with even minimal processing.
③ Cast aluminum parts are prone to plastic springback in the datum plane and datum hole.
Figure 16 illustrates the processing process of a cast aluminum part. Without rigid clamping, hard deformation may occur, resulting in a processed product that does not meet the dimensional accuracy and geometric tolerance requirements.
Figure 16 processing process of a cast aluminum part
The main accuracy requirements for the product are as follows: the maximum deviation between the two vertical holes should be within imagemm, the flatness of the large plane should be 0.02mm, and the perpendicularity and parallelism of the large plane with the two vertical holes and horizontal holes should be 0.03mm.
The processing technology has been improved as follows:
1. First improvement:
During processing, due to size retraction after hole processing, there is a retraction of 0.005mm on one side. To address this, the fine boring size has been increased by 0.003mm on one side. Additionally, in the boring method of the holes, two empty knives have been added. Although the tolerance of the tested hole is qualified, the geometric tolerance of the product is still out of tolerance.
2. Second improvement:
Another method was implemented, where a light knife is added to the large plane and the opposite plane, respectively. The light knife allowance is 0.08mm, which has successfully ensured the product’s size and geometric accuracy. This also highlights the importance of paying attention to the plastic deformation of cast aluminum parts.
Easy deformation of aluminum alloy material
The machinability of two commonly used aluminum alloy materials is as follows.
(1) 2A12 aluminum alloy
The material can be strengthened through heat treatment and is commonly utilized in high-end products such as military, aerospace, and other high-load parts, as well as high-precision molds.
The 2A12-T4 is the most extensively used duralumin alloy.
The 2A12 aluminum alloy exhibits good machinability, but its most significant characteristic is its susceptibility to deformation. It serves as a representative material to evaluate the skill level of technicians.
While it may not possess the same strength as aluminum alloys like the 2-Series or 7-Series, this material does offer excellent welding characteristics, electroplating, and corrosion resistance, making it a popular choice within industry.
In fact, it accounts for over one-third of all non-ferrous metal materials used today and is commonly employed in a variety of structural components that require high strength and corrosion resistance, such as ships, trams, and mobile phone cases.
Additionally, the 6061 aluminum alloy displays exceptional machining capabilities and is resistant to deformation. However, once it does deform, it becomes almost unusable, making calibration incredibly difficult and often rendering the product unsuitable for anything other than specialized structures.
Open hard filling and chemical filling
(1) Open hard fill (see Figure 17)
Figure 17a depicts the state after roughening, with the workpiece inside and the blank outside, as well as an open blank.
Closed blanks are not produced due to the large amount of stress concentration cutting can cause in the product.
The rigidity is improved through the use of a pin hole connecting block. Once the rough machining is completed, the connecting block can be removed. After releasing stress, it can be reconnected for subsequent finishing machining to reduce workpiece deformation.
It is worth mentioning that filling serves two essential functions: enhancing rigidity and increasing the timeliness of the product.
Figure 17 schematic diagram of open hard filling
Once the inner arc surface of the workpiece has been processed to meet the required dimensions, the supporting tooling is used to fill the inner arc surface and process the external allowance. This process helps to enhance the rigidity of the workpiece.
(2) Chemical filling
This method is mainly used for finishing. I have used this method only once so far. It involves chemically filling a thin-walled arc aluminum alloy blade, and it requires the assistance of tooling.
The process involves filling chemical substances into the processed arc surface. Once the chemical substances solidify, they provide seamless support for the clamping surface. This prevents deformation and also avoids knife vibration that can result from processing products that are too thin.
It is important to note that this method is not a commonly used general method, but rather a special process scheme designed for specific products.
Fitter calibration is primarily focused on plate parts, which tests the fitter’s skills and experience. As fitter calibration involves mechanical correction, calibrated products require a good foundation, at minimum.
Furthermore, I believe that most calibrated products are not particularly valuable. Fitter calibration typically involves both rough and finishing corrections.
A skilled fitter can calibrate plate products with leveling accuracy up to 0.1mm.
There are several process methods available to prevent the deformation of aluminum alloy workpieces, and it is particularly crucial to alter the mindset.
During the actual processing, it is essential to understand the structure and material properties of the parts to identify the key factors. We should utilize appropriate clamping methods and cutting routes, remove stress deformation via aging treatment, and employ straightforward techniques to address complicated issues.