Technological Method To Prevent Deformation Of Aluminum Alloy Workpiece

At present, aluminum alloy materials are widely used in aviation, aerospace, automobile, machinery manufacturing, shipbuilding and chemical industry.

For the research on the processing technology of aluminum alloy workpiece, most of it is how to control the deformation of aluminum alloy, so as to further improve the accuracy of the product.

Conventional processing methods of aluminum alloy workpieces

(1) Blanking processing

Also known as sleeve processing, that is, similar to the method of milling slots to let the workpiece fall off from the blank, generally leaving a small amount of 0.10~0.15mm on the bottom surface.

This machining method is often used when there is a large amount of material removal in the shape, inner cavity and inner hole, which can avoid the final deformation of the workpiece caused by heat and internal stress concentration caused by large margin roughing.

Blanking processing plays an important role in preventing the deformation of aluminum parts, especially with the sucker, it can solve the problems of deformation and clamping of a large class of thin-walled parts, which is very suitable for the processing of plate parts.

The blanking process is shown in figure 1.

blanking processing

Fig. 1 blanking processing

(2) Vacuum sucker method

The positioning surface of the workpiece is adsorbed on the sucker, and the workpiece is processed with the pressing plate.

Sucker clamping, combined with blanking processing method, as long as there is a surface in the product that can be adsorbed, even the bevel and curved surface are also applicable.

The use of sucker and blanking processing, can complete a variety of plate thin-walled parts processing, even if the plate thickness is only 0.5mm can also be processed.

The general-purpose sucker is shown in figure 2, and the special suction cup tooling designed according to the product is shown in figure 3.

universal sucker

Fig. 2 universal sucker

Special sucker tooling designed according to the product

Fig. 3 special sucker tooling designed according to the product

(3) Design stiffener and process head

If the structural strength of the product is insufficient to support the subsequent finishing, the stiffener shall be left during roughing, so as to ensure the clamping strength before finishing, and no excess heat and stress will be generated when removing the stiffener.

The overall dimension of the workpiece using this method should not be too large.

For example:

① The stiffener is designed to facilitate vise clamping and machining the back of the workpiece (see Fig. 4).

② The process head is designed to facilitate subsequent positioning (see Fig. 5).

③ Design process head + stiffener to facilitate subsequent positioning and processing (see Fig. 6).

design stiffener to facilitate vise clamping and machining the back of workpiece

Fig. 4 design stiffener to facilitate vise clamping and machining the back of workpiece

design process head to facilitate subsequent positioning

Fig. 5 design process head to facilitate subsequent positioning

design process head + stiffener to facilitate subsequent positioning and processing

Figure 6 design process head + stiffener to facilitate subsequent positioning and processing

An example of comprehensive application of conventional machining methods is shown in Figure 7.

The process implementation method is as follows:

① According to the analysis of the structure of the front and back of the product, the processing content of the back of the workpiece is less, which is suitable for the positioning surface (see Fig. 7a and Fig. 7b).

② The positioning surface is patched to facilitate the subsequent suction cup clamping (see Fig. 7C).

③ The product is thin and easy to deform. Stiffeners are designed on the front to avoid rough machining deformation of the product and finishing deformation of the positioning surface at the same time (see Fig. 7D).

④ After finishing the positioning surface, finish the front of the product through the special suction cup tooling (see Fig. 7e and Fig. 7F).

comprehensive application example of conventional processing method

Fig. 7 comprehensive application example of conventional processing method

Aging treatment

(1) indirect aging treatment

That is to say, rough machining is used as a separate process to replace aging treatment.

The workpiece is processed on the machine tool, and the workpiece is removed after the rough machining is completed, and then the finishing process can be carried out without interval time (if it is a mass-produced product, the finishing can be carried out after unified roughing, and there is no interval aging time in between).

Note: the workpiece needs to be completely opened, and the overall opening leaves a margin of 0.5mm on one side before finishing.

The roughing process of the blank is an indirect aging treatment, and the secondary loosening and clamping process of the workpiece is the indirect stress release process.

This method is suitable for the machining of most precision structures and aeronautical structures.

(2) Aging treatment

① Artificial aging:

For aluminum alloy workpiece, most methods are to place it for 24 ~ 48h after roughening.

Mainly for large parts and thin-walled parts, the process is slow.

Heat treatment aging:

Compared with the artificial aging process, it is faster and more used in closed parts with complex structure and large allowance.

Both methods are used in the processing of aluminum alloy workpieces, and are mostly used to remove products with large margin and high precision requirements. The purpose is to remove internal stress and obtain stable structure.

If the product belongs to power parts in structure (see Figure 8), aging treatment is required for most products:

Firstly, aging treatment can effectively eliminate internal stress and ensure the dimensional stability of subsequent processing;

Secondly, while removing the internal stress, stable tissue can be obtained, so as to avoid the deformation of the originally qualified product again due to the continuous heat generated during the movement.

products belonging to power parts in structure

Figure 8 products belonging to power parts in structure

Method for preventing datum plane deformation of aluminum alloy products

In the process of machining, it is often encountered that the datum plane has been deformed before finishing and cannot be used as the finishing datum.

Moreover, because the deformation of aluminum alloy workpiece is elastic deformation, there is springback after loosening whether pressing or clamping, resulting in dimensional deformation and out of tolerance after finishing.

The methods to prevent deformation of aluminum alloy product datum are summarized as follows.

1) If the workpiece can only be milled, the datum plane must be machined on the premise that the main strength of the workpiece is high enough, so that the datum plane of finish milling can be flat.

2) If the workpiece can only be milled and the workpiece is thin, it is also necessary to repeat several times, that is, turn the surface several times, before effectively eliminating the deformation.

However, the final surface obtained by this method will still have some deformation.

3) When the structure of the workpiece allows, turning is also a good method, and the surface of the turning is smoother than that of milling.

Because 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 processed the datum plane of a square product, which always deforms during milling. Even if the feed speed of the tool is very slow, because the tool is in surface contact with the workpiece, it will inevitably produce heat and stress.

Finally, a single acting chuck was used to clamp the square workpiece, and the qualified datum plane was turned out.

After all, turning is the point contact of the tool tip, and there will be no large-area heat and stress concentration. Examples are as follows.

The products to be processed are shown in Figure 9.

The workpiece material is 2A12-T4, and the blank size is 120mm×400mm×12mm, product size 370mm×110mm×7mm, the parallelism of the two major surfaces is required to be 0.03mm and the flatness is required 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 Clamp with lathe and single acting chuck, and see light on the turning side.

② On the machining center, the bright surface is used as the positioning surface. On the premise of ensuring that the workpiece is not damaged, threaded holes are made around the blank.

③ Make special tooling for lathe, tighten the workpiece with screws in reverse direction, and turn the thickness of the product to meet the drawing requirements.

blank and structure of products to be processed

Fig. 9 blank and structure of products to be processed

Clamping ideas for preventing deformation of aluminum alloy products

In addition to the suction cup clamping to prevent deformation, how to clamp products that cannot be clamped with suction cups?

It can be said for sure that except that the vise can be used for clamping during roughing, the vise is not allowed to directly contact the workpiece in most subsequent processes.

Because the force of the vise is horizontal, and the workpiece material itself is a deformable material, the clamping force of the vise will cause elastic deformation of the workpiece, so the workpiece needs a reasonable downward clamping force, that is, similar to the clamping method of pressing plate or screw.

In fact, the design idea of aluminum alloy workpiece processing tooling is to consider the direction of clamping force. The reference methods are as follows:

① Design special tooling + special pressing plate and thread reverse drawing tooling (see Figure 10).

② Inner hole fit + small boss limit (see Figure 11).

③ Design soft jaws with pins (see Figure 12).

④ Design V-shaped positioning tooling with limit (see Figure 13).

⑤ Design single piece tooling block and multi module tooling matched with shape (see Figure 14).

⑥ Design tooling for special pressing plate (see Figure 15).

design special tooling + special pressing plate and thread reverse drawing tooling

Figure 10 design special tooling + special pressing plate and thread reverse drawing tooling

inner hole fit + small boss limit

Fig. 11 inner hole fit + small boss limit

design of soft jaw with pin

Figure 12 design of soft jaw with pin

design of V-shaped positioning tooling with limit

Fig. 13 design of V-shaped positioning tooling with limit

single piece tooling block and multi module tooling matched with shape

Fig. 14 single piece tooling block and multi module tooling matched with shape

tooling for designing special pressing plate

Figure 15 tooling for designing special pressing plate

Process improvement measures for plastic deformation of cast aluminum parts

Cast aluminum products have certain particularity.

Because it is a casting, the product accuracy requirements are usually very high.

Some unimportant parts of its shape have been cast, but there are some disadvantages while it is convenient:

① The structure of the casting itself has been lack of rigidity, especially the clamping rigidity during processing.

② The casting is a stress concentration body in the processing process. With a little processing, the internal stress will exist.

③ Plastic springback is very easy to occur in the datum plane and datum hole of most cast aluminum parts.

Figure 16 shows the processing process of a cast aluminum part. There is no rigid clamping that causes hard deformation, but the dimensional accuracy and geometric tolerance of the processed product can not meet the requirements.

processing process of a cast aluminum part

Figure 16 processing process of a cast aluminum part

The main accuracy requirements of the product: the limit deviation of the two vertical holes is imagemm, the flatness of the large plane is 0.02mm, and the perpendicularity and parallelism of the large plane with the two vertical holes and horizontal holes are 0.03mm.

The processing technology is improved as follows.

1) First improvement:

During the processing, due to the size retraction after the hole processing, the retraction is 0.005mm on one side, so the fine boring size is increased by 0.003mm on one side.

In the boring method of holes, two empty knives are added. The tolerance of the tested hole is qualified, but the geometric tolerance of the product is out of tolerance.

2) Second improvement:

In another way, a light knife is added to the large plane and the opposite plane respectively, and the light knife allowance is 0.08mm, which successfully ensures the product size and geometric accuracy. It can be seen that the plastic deformation of cast aluminum parts should be paid attention to.

Easy deformation of aluminum alloy material

The machinability of two commonly used aluminum alloy materials is as follows.

(1) 2A12 aluminum alloy

It can be strengthened by heat treatment. It is often used in high-end products such as military, aerospace and other high-load parts and high-precision molds.

2A12-T4 is the most widely used duralumin alloy.

2A12 aluminum alloy has good machinability, but its biggest feature is easy deformation. It is a representative material to test the level of technicians.

(2) 6061 aluminum alloy

Although the strength can not be compared with 2-Series or 7-series aluminum alloys, it has excellent welding characteristics, electroplating and good corrosion resistance, and is widely used in industry.

The output has exceeded 1 / 3 of the total amount of non-ferrous metal materials. It is commonly used in various process structural parts with high strength and corrosion resistance requirements, such as ships, trams and mobile phone cases.

6061 aluminum alloy has excellent machining performance and is not easy to deform. However, once deformed, it is almost scrapped, and it is very difficult to calibrate the shape, except for products with special structure.

Open hard filling and chemical filling

(1) Open hard fill (see Figure 17)

Figure 17a shows the state after roughening, with workpiece inside and blank outside, and open blank.

The reason why closed blanks are not made is that cutting will cause a large amount of stress concentration in the product.

The rigidity is improved through the pin hole connecting block. After rough machining, the connecting block can be removed. After releasing the stress, it can be connected again for subsequent finishing machining, so as to reduce the deformation of the workpiece.

It is worth mentioning that one of the most important functions of filling is to enhance the rigidity, and the other is to increase the timeliness of the product.

schematic diagram of open hard filling

Figure 17 schematic diagram of open hard filling

After the inner arc surface of the workpiece is processed to meet the dimensional requirements, the supporting tooling is filled with the inner arc surface to process the external allowance to enhance the rigidity.

(2) Chemical filling

Mainly used for finishing.

I have only used this method once at present. It is to chemically fill a thin-walled arc aluminum alloy blade, and it also needs the assistance of tooling.

Chemical substances are filled into the processed arc surface. With the solidification of chemical substances, it is a seamless support for the clamping surface, which can not only prevent deformation, but also avoid knife vibration due to too thin products in processing.

This method is not a popular general method, but a special process scheme for special products.

Fitter calibration

Fitter calibration is mostly aimed at plate parts, which is a great test of fitter’s skills and experience.

And fitter calibration is mechanical correction after all, after all, products that can be calibrated need at least a good body as a prerequisite.

In addition, I don’t think most of the calibrated products are of high value.

Fitter calibration is generally divided into rough correction and finishing correction.

The skilled fitter calibrates the plate products, and the leveling accuracy is about 0.1mm.

Final words

There are many process methods to prevent the deformation of aluminum alloy workpiece, and it is especially important to change the way of thinking.

In the actual processing, we should grasp the structure and material characteristics of the parts to find the breakthrough point, adopt reasonable clamping methods and cutting routes, eliminate stress deformation through aging treatment, and use simple methods to solve complex problems.

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