Taking the volute of the aluminum alloy special-shaped structural part in the power unit as an example, a process analysis was carried out on the basis of the thin-walled hollow structural characteristics and the main technical difficulties and a plan Complete process has been formulated including roughing, finishing and finishing. aging treatment, during the product treatment process. The internal stress is released several times, which effectively controls product deformation while ensuring product accuracy.
1 Preface
Special-shaped aluminum alloy structural parts are often used in fields such as military industry, aerospace and high-precision molds. Their most notable features are high precision and complex shapes. Most of the materials chosen are 2A12-T4 aluminum alloy. 2A12-T4 aluminum alloy has good machining performance because its main characteristic is that it is easy to deform.[1]therefore, processing special-shaped structural parts is difficult. The following takes volute, a special-shaped structural part made of aluminum alloy in a power plant, as an example to discuss its processing method.
2 Structural characteristics and main technical difficulties
2.1 Structural characteristics
The volute shown in Figure 1 is a complex structural part of special shape and is a thin-walled hollow part. The structure of the volute is shown in Figure 2. From the perspective of product precision, the volute is a key structural component of precision output; From the perspective of the assembly structure, the volute is the assembly support of the entire structure.
a) Face it squarely
b) Side view
Figure 1 Case of the volute
a) obverse b) reverse
c) Looking sideways d) Looking up
e) Looking down
Figure 2 Volute structure
2.2 Main technical difficulties
(1) Based on the analysis of the design drawing, the key dimensional accuracy of the volute is shown in Figure 3. There are many requirements for geometric tolerance.
a) Main view
b) Top view
c) Sectional view
Figure 3 Critical dimensional accuracy of volute
(2) From the point of view of material, 2A12-T4 aluminum alloy has good machining performance, and its biggest feature is that it is easy to deform.
(3) From the perspective of product structure, the wall thickness of the local physical connection part is only 1-1.5mm, which conforms to the structural characteristics of thin-walled parts.
(4) From the perspective of processing technology, it is particularly important to control the deformation of the product while ensuring its accuracy.
3 Blank manufacturing and tightening plan
3.1 Manufacturing of prosthesis blanks of special shapes
The model of the special-shaped denture blank scheme is shown in Figure 4, in which the green color represents the workpiece and the yellow circle is the denture blank. The design idea of the prosthesis blank is as follows.
a) Front view b) Side view c) Top view
Figure 4 Blank plan model of a specially shaped prosthesis
(1) When analyzing and designing prosthetic blanks from the shape of the part, square blanks and round blanks are generally given priority. Since the square blank is convenient for vice clamping, the round blank is convenient for self-centering chuck clamping, and the shape of the volute is closer to a circle, it is initially determined that it This is a round prosthesis blank.
(2) According to the analysis of the processing method, according to the structural composition of the workpiece, a five-axis machining center is required to complete the processing. During the five-axis multi-station rotation process, the circular prosthesis blank is more suitable. the angle of oscillation of the axis of rotation.
(3) From the analysis of the length of the processing tool overhang, starting from the center of the workpiece, relative to the uneven overhang of the square blank, the Round blank is processed with equal radius and the tool overhang length will not be longer or shorter.
In summary, based on the principle of maximizing the part of the workpiece, focusing on the rigidity when processing the workpiece, the plan of the prosthetic blank was finally determined. The circular blank was used to increase the bottom positioning reference plane, and the height was the same as the upper surface of the small boss at the bottom.
3.2 Formulation of the tightening plan
Design 2 red φ10mm pin holes (see Figure 5), 4 purple M5 threaded holes and 5 blue φ4.2mm circular through holes at the appropriate positions of the denture blank. Ideas for formulating the tightening plan are as follows.
Figure 5 Reference hole and locking hole
1) The purpose of adding two red φ10mm pin holes is to form the classic clamping and positioning mode of “two pins on one side” with the large yellow denture plane to prepare the design at the same time subsequent tooling. of each process must follow the processing requirements of the reference unification principle.[2]。
2) The purpose of adding 4 purple M5 threaded holes is to fix the part body by tightening the screws backwards to prepare for later tooling design.
3) The purpose of adding 5φ4.2mm blue circular through holes is to fix the workpiece body by positive screw locking to prepare for subsequent tooling design.
In summary, the tightening plan has been determined and the process plan will be developed accordingly.
4 rough machining plans
4.1 First rough machining of the reverse side
The first rough machining of the back face is shown in Figure 6. Processed using a three-axis machining center and clamped with a vice.
a) Back roughing area b) Simulation effect after back roughing
Figure 6 First rough machining of the reverse side
1) The vise clamps the bottom of the blank in a parallel clamping position and makes the original round blank rough with a large margin.
2) Leave a margin of 0.5mm on the large flat surface at the top, and process pin holes, threaded holes and circular through holes everywhere to meet the size requirements of the parts.
3) Leave a margin of 0.3 mm on one side of the green area of the part.
Process analysis: ① Remove the large margin on the back of the part to release the initial stress on the back of the product. ② Since the green area in Figure 6a has a certain strength through the assistance of the prosthesis blank, a small margin can be left on one side, with a margin of 0.3mm on one side .
4.2 First rough machining of the front face
The first rough machining of the facade is carried out using a three-axis machining center. The front special tooling design is shown in Figure 7. The first rough machining of the front face is shown in Figure 8. All green areas are rough machined, leaving a margin of 0.5mm on one side.
a) Internal sectional view of the workpiece b) Three-dimensional view of the front tooling c) Actual front tooling
Figure 7 Front Special Tooling Design
a) Front blank processing area b) Simulation effect after front blank processing
Figure 8 The first rough machining of the front face
Process analysis: ① Remove the large margin on the front side to release the initial stress on the front side of the product. ②Use three-axis machining center to remove blank margin to reduce quality and save five-axis machining time for subsequent five-axis machining.
The design ideas for special front tooling are as follows.
1) Positioning surface design. Follow the principle of clamping two pins on one side, as shown in Figure 7b. The green side is the positioning surface and the two red pins are used for limiting.
2) Tooling chip removal design. As shown in Figure 7a, due to the special structure of the internal cavity of the workpiece, a large amount of chips will inevitably be generated when processing the internal cavity using a T-shaped milling cutter. With the increase in chips, it does. It is very likely that if the chips are not discharged smoothly, to cut the T-shaped knife, two purple through holes are designed on the tooling (see Figure 7b) to facilitate chip evacuation and sinking .
3) Design of tightening method. According to the positions of the 4 M5 threaded holes on the prosthesis of the workpiece, design the locking position of the tooling accordingly, and use 4 M5 hex socket head screws to tighten the workpiece upside down to fix the workpiece. The four blue parts in Figure 8a are M5 hex socket head screws.
4.3 Second draft of the front panel
The second rough machining of the front face is shown in Figure 9. It is processed using a five-axis machining center and special tooling and clamping. As shown in Figures 9a and 9b, perform secondary processing on all red areas of the part, leaving a margin of 0.3 mm on one side; treat all green areas of the part in place (i.e. the area here in the final product; is treated in place, leaving no margin)[3]. The treatment simulation effect is shown in Figures 9c and 9d.
a) Front treatment area b) Treatment area rotated by a viewing angle of 180°
c) Processing simulation effect d) Simulation effect of 180° rotating viewing angle
Figure 9 Second draft of the front panel
Process analysis: Based on the product structure and dimensional accuracy analysis, it is concluded that the green area in Figure 9a and 9b is the weight reduction area, so the processing is in place and the internal stress on the front of the product is also released twice.
5 anti-aging treatments
In order to eliminate internal stresses during part processing and to stabilize the matrix structure of the part, the part is aged. The oven inlet temperature is room temperature, the heating speed is 127℃/h, the holding temperature is (185±10)℃, the holding time is 4-5 hours, the speed of cooling is 43℃/h, the cooling method is cooling with oven, and the outlet temperature is room temperature.
6 Finishing Solutions for Key Parts
6.1 Correction of the reference plane
(1) Correct the inverted reference plane (see Figure 10) and design the special tooling for the soft jaw, as shown in Figure 10b. A three-axis machining center is used for processing. The vise is used with special tooling to clamp the workpiece. The maximum outer circle of the part is slightly tight, and the large red surface can only be processed when exposed to light. In the free state, the reference plane is exposed to light.
a) Machining area b) Soft jaw tool c) Clamping diagram
Figure 10 Back data correction
Process analysis: This process is in fact the correction of the reference plane after heat treatment. The free state of the part is a state supported by rigid conditions. Generally speaking, this means that the part has good body resistance in its current state. For high precision structural parts, the workpiece must be rigid before finishing. It is necessary to create rigid conditions for the room before finishing. Under the support of rigid conditions, the reference plane is corrected. The correction level should be low and the reference plane can be exposed to light. This is the meaning of the correction of the reference plane in the free state. Since the workpiece has a certain degree of rigidity before removing the circular prosthesis blank, the reference plane must be corrected based on this so that the plane is flat, so this process is very critical.
The design idea of special tooling for soft jaws (see Figure 10b) is as follows.
1) The starting point of the design. Since the reference plane is corrected in a free state, the part can only withstand force in the radial direction and cannot withstand force in the vertical direction, because the force in the vertical direction will cause elastic deformation of the room.
2) Design of the positioning surface. As shown in Figure 10b, the three small red surfaces are the positioning surfaces of the part, simulating point contact to support the part, thus avoiding uncertain factors of surface-to-surface contact. Since it is the reference surface correction, the lower surface of the part focuses on the support rather than the adjustment, especially the positioning surface of the part after heat treatment. Therefore, the design method of point contact positioning surface is here. is another key point in repairing the reference surface.
3) Design of tightening method. As shown in Figure 10c, soft claw tooling is designed for the outer circumference of the workpiece dummy, imitating the lathe clamping method, and integrated to clamp the outer circumference of the workpiece.
(2) Correction of the front reference plane and finishing of each hole (see Figure 11). Use a five-axis machining center and special tooling to process the large red surface and the two purple surfaces marked on the front of the part. it lights up to guarantee the surface. The surface-to-surface distance tolerance is ±0.02mm. Finish each round hole, threaded hole, and side through hole marked in red on the front of the part to the finished size.
Figure 11 Correction of the front reference plane and finishing of each hole
The analysis of the process is as follows.
1) Correction of the front reference plane. ① Since the elastic deformation of the rear surface of the workpiece is eliminated after correction in the natural state, when correcting the front reference surface, the force is allowed in the vertical direction. As shown in Figure 11, the back reference surface is close to the. workpiece and the rear reference surface is close to the workpiece. Simply tighten the piece against the screw. ②The function of correcting the front reference plane is to prepare the design of subsequent tooling and ensure the consistency of the positioning surface. As shown in Figure 11, be sure to select the large red surface and the upper purple surface as the tooling positioning surface. ③Why do we need to correct the purple surface in the middle. Suppose that after the circular prosthesis blank is removed from the workpiece during post-processing, secondary deformation of the back reference surface occurs and post-processing cannot be connected. In this case, two purple surfaces are needed as positioning surfaces, and. the tooling is used to perform secondary deformation on the rear reference surface. The importance of this correction is to prepare a plan for the process plan itself and to manage the uncertainty of product deformation. This is also important.
2) After the reverse datum plane is corrected and the main body of the workpiece is in good condition after heat treatment, the round holes and threaded holes can be processed throughout the workpiece to the finished size, and the hole processing will not affect the repair of the front reference plane.
6.2 Reverse finishing
The design of special tooling for reverse finishing is shown in Figure 12. Three-axis machining center processing, special tooling and clamping. The back of the part is finished as shown in Figure 13. First, use 6 M5 hex socket head screws to tighten the part. Complete the red area of the room. Use the cavity method to process the plane, the surface of the step. concave cavity and each round hole in the red area to the size of the finished product, then the program is paused, as shown in Figure 13b, the inner cavity. Add a circular pressure plate and a clamping hexagon socket head screw to the red area, and process the green area to the finished size, use trimming (cutting and milling) to process the larger side surface of the workpiece (see Figure 13c). and use φ2mm end mill for processing. From appearance to finished size, remove the denture blank.
a) Tooling locating surface and limit pin b) Tooling entity
c) Tightening instructions
Figure 12 Special tooling design for reverse finishing
a) Finishing the red plane and the cavity b) Finishing the green groove
c) Finishing the appearance of the part
Figure 13 Reverse finishing
The analysis of the process is as follows.
1) Design special tooling ideas for reverse finishing. ① Positioning surface design ideas. The tightening principle of unified process standards is always followed, as shown in Figure 12a. The two green surfaces are positioning surfaces and the two red pins are used for position limits. ②Design idea of tightening method. According to the position of the round hole in the circular prosthesis blank of the part, 6 hexagon socket head screws (see the 6 blue components in Figure 13a) are designed to lock the exterior of the part. In order to prevent the shape of the workpiece from moving during cutting and sawing, find the appropriate hole position according to the structure of the product itself, and add 4 hexagon socket head screws and 1 circular pressure plate (see the 5 blue pieces inside the workpiece in Figure 13c). Lock the inside of the workpiece.
2) Judging from the distribution of the finishing allowance of the product, as shown in Figure 13b, the red area of the part only has a one-sided allowance of 0.3mm, and the green area is only partially cleaned. no major damage occurred. Excess cutting will not produce excessive internal stress and affect the overall accuracy of the part, so this process can be fully handled.
3) Focus on analyzing suppression treatment and treatment effects. ①Use a method similar to slot milling to drop the workpiece from the blank, usually leaving a margin of 0.1-0.15mm on the bottom surface. The importance of cutting processing is to avoid the final deformation of the workpiece caused by large margin local milling due to internal stress concentration. Blanking processing is often used when there is significant material removal in the exterior shape, interior cavity and interior hole. ② As shown in Figure 13c, the workpiece blank is removed by cutting. The treatment content is less and no excessive thermal stress will be generated. ③After removing the prosthetic blank from the workpiece, the measured deformation of the reference plane on the reverse side is 0.016mm, and the flatness required by the drawing is 0.03mm, which meets the requirements of the drawing and can be processed by post-processing.
6.3 Facade finishing
The front finishing of the product is shown in Figure 14. Five-axis machining center, designed with special tooling and clamping for front finishing. As shown in Figure 14c, the planes, outer circles, walking surfaces, and the 4 small round holes marked in red on the front side of the part are finished to the finished size.
a) Tooling locating surface and limit pin b) Tightening instructions
c) Front finishing area
Figure 14 Facade finish
The analysis of the process is as follows.
1) Design ideas for special tools for finishing facades. ① Positioning surface design ideas. Still following the principle of two-pin clamping on one side, as shown in Figure 14a, the green side is the positioning surface and the two red pins are used for limiting. ②Design idea of tightening method. According to the structural characteristics of multiple tabs at the bottom of the part, as shown in Figure 14b, a multi-pressure plate is used to clamp the tabs at the bottom of the part.
2) As shown in Figure 14c, the one-sided margin of the red areas on the workpiece is only 0.3 mm. The processing content is small, it will not generate too much internal stress and affect the overall accuracy of the part. the process can be handled entirely on site.
7Conclusion
After the special-shaped precision volute is processed according to the above process plan, it is tested strictly according to the requirements of the drawing, and the test results meet the requirements of the drawing.
In the whole development process of aluminum alloy special-shaped structural parts volute, the technical key points are mainly reflected in four aspects: ① From the structural characteristics of aluminum alloy special-shaped structural parts aluminum, establish a reasonable and regular prosthetic blank, and use the prosthetic blank to transform into The positioning and clamping reference of the workpiece facilitates subsequent tightening. ② Focus on the force direction of the product during the tightening process, and design the tooling in accordance with the principle of unified references. ③ Let the workpiece be clamped in a free state to eliminate the deformation of the workpiece reference plane. ④During product processing, internal stresses are released several times. The four aspects complement each other to complete high-precision special-shaped part products.
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