Practical case of high speed machining of aluminum alloy die forgings
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发布日期:2017-06-16
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With the rapid development of the processing industry, high-speed aluminum alloy cutting has become the first choice for enterprises. High-speed cutting has always been limited to aluminum s..........
With the rapid development of the processing industry, high-speed aluminum alloy cutting has become the first choice for enterprises. High-speed cutting has always been limited to aluminum sheet-like wool. Due to the unevenness of aluminum alloy die forgings, high-speed machining and cutting are often discontinuous, and then intermittent cutting is formed. The tool wear is very serious, and the tool is broken and injured. The phenomenon of parts. Since the die forgings in the production of aerospace aluminum alloy parts are still the preferred wool material for design, in order to ensure the goal of doubling the numerical control machining capacity, it is imperative to use high-speed milling for such parts. This paper will analyze the feasibility of high-speed milling of die forgings by taking a machining plan of a machine as an example.

 

Parts introduction

For example, the part is an important part of the skeleton in a wing, and acts as a keel in the wing, which is an important force member. The material grade of the part is 7B04 T74, the outer dimension is 2430mm×236mm×42mm; there are 2 φ 12H9 process holes on the part for positioning during processing and assembly; the thickness of the web is 3~5mm, and there are 10 on the web. Φ76~116 through holes of different sizes; the thickness of the edge strip is 4.5mm; 13 pieces (thickness is 3mm) in the middle of the whole part.

 

Processability analysis of parts

The part material is a high-strength super-hard deformation aluminum alloy, which is an Al-Zn-Mg-Cu alloy, and is a heat-treppable reinforced aluminum alloy. It has the characteristics of high specific strength, good fracture toughness and excellent process performance. Its corresponding chemical composition (% by weight) is: 5.7% Zn, 2.3% Mg, 1.43% Cu, 0.2% Cr. The process of heating die forging is characterized by poor plasticity, poor fluidity, large adhesion, and easy forming of die forging; and it is very sensitive to deformation speed and deformation degree, and drops sharply with the increase of deformation speed; the forging temperature range is narrow. The initial and final forging temperatures are strictly required.

 

Forgings after quenching and artificial aging treatment σ b = 450~540MPa; due to the anisotropy between the metal crystals, due to the uneven heat and shrinkage of the various parts during the forging process and the volume change of the metallurgical structure transformation, the inside of the blank is produced quite Large internal stress. The internal stress of the blank is temporarily in a relatively balanced state. After cutting off some surface materials, the balance is broken and the internal stress is redistributed. At this time, the processed parts are obviously deformed.

 

Internal stress refers to the stress remaining inside the workpiece after the external load is removed. The internal stress is caused by uneven volume changes in the macroscopic or microscopic structure of the metal, and the external factors come from hot working and cold working. A part with internal stress is in an unstable state, and its internal organization has a strong tendency to return to a stable state without internal stress, even if the part is constantly changing at normal temperature until the internal stress disappears.

 

Residual stress refers to the influence of the process. In the absence of external force, the residual stresses in the parts are balanced with each other. The residual stress distribution of each part after the part is machined is uneven, which causes the parts to deform and affect the shape of the parts. And dimensional accuracy [2]. The reasons for this can be divided into three cases:

 

This residual stress is the most common occurrence of parts during processing. When an external load is applied, if a part of the part is unevenly plastically deformed, the part will generate residual stress after unloading; at the same time, since the residual stress must reach self-phase balance throughout the part, the plastic deformation does not occur in the part. Part of the area also produces residual stress.

 

This residual stress often occurs during hot working of parts due to uneven plastic deformation of the part during thermal processing and uneven volume changes. This residual stress is caused by chemical or physical changes from the surface of the part to the inside. The chemical heat treatment, electroplating, spraying, etc. of the metal material are examples.

 

This part belongs to a typical single-sided processing rib type. The part is machined with two holes on one side. The shape of the part is the outer edge of the wing theory. The shape of the part head has a closed angle of 33°. The inner shape of the part is relatively open, and it can be roughed by a tool with a diameter of φ 30 or more. The radius R of the bottom corner of the part is 4 mm, the radius R of the corner between the edge strip and the rib is 8 mm, and the distance between the local position boss and the rim is 8.5 mm.

 

Process design

In ordinary CNC milling, due to the low speed, low feed, large depth of cut, low speed machining, the part deformation is relatively large, the maximum side bending deformation after the rough machining of the parts is 6mm, the maximum warping deformation is 20mm, the maximum bending deformation It is 5mm.

 

Processing design

In order to meet the dimensional accuracy and processing quality of the product, a cumbersome process is designed to offset the effects of part deformation:

(1) rough milling the bottom plane, leaving a process allowance of 5mm;

(2) Make 2 φ12 process holes to φ 10H 9;

(3) Position the inner shape of the rough milling part with one hole and two holes (the inner shape has a process margin of 5mm, and the process margin of the web and the edge strip is 5mm);

(4) The heating of the parts corrects the flatness of the bottom surface of the web to ensure that it is within 2mm;

(5) Machining the bottom surface of the part, removing the process margin by 3mm (still retaining 2mm process margin);

(6) Once again, the inner shape of the milled part is positioned by two holes (the inner shape has a process allowance of 2 mm, and the process margin of the web and the edge strip is 1 mm);

(7) The heating of the parts corrects the flatness of the bottom surface of the web to ensure that it is within 1mm;

(8) Machining the bottom surface of the part to remove the process margin and the bottom surface of the web is in place;

(9) Expand the 2 process holes of the part to φ 12H 9;

(10) Position the inner part of the finished part with the bottom surface and the two process holes.

 

In order to meet the processing requirements, 3 sets of process equipment are used. One set is a drill mold for the two-process part process hole; one set is a vacuum milling clamp for numerical control, used for rough machining of parts; one set is vacuum milling clamp for numerical control, used for part finishing.

 

Processing quality analysis

(1) The bending deformation of the web is relatively large, and the flatness reaches 2 mm, which requires multiple heating corrections.

(2) The dimensional accuracy is within ±0.3mm, which basically meets the dimensional tolerance of the part.

(3) The surface roughness R a is mostly within 3.2 μm, and locally is 6.3 μm, or even worse, requiring a general polishing process by the fitter.

(4) Due to the influence of cutting force and cutting heat, the internal stress is relatively large, and the flatness of the parts after surface treatment is deteriorated, accompanied by side bending.

 

High-speed cutting process features

Cutting, which is usually 5 to 10 times higher than the conventional cutting speed, is called high-speed cutting. At present, the high speed milling machine tool processing aluminum alloy cutting line speed is 1000~7000m/min. In the high-speed cutting state, as the cutting speed increases, the cutting force decreases, and the quality of the machined surface increases; the cutting heat is mostly carried away by the chips, and the workpiece remains substantially cold. The tool life decreases as the cutting speed increases.

 

High-speed cutting has the following advantages over conventional cutting:

(1) High processing efficiency. As the cutting speed is greatly increased, the feed rate is also increased by 5 to 10 times. The metal removal rate is 3 to 10 times that of conventional cutting. At the same time, the rapid increase of the fast idle speed of the machine tool also reduces the non-cutting idle travel time, which greatly improves the productivity of the machine tool.

(2) The cutting force is reduced. After the cutting speed reaches a certain value, the cutting force can be reduced by more than 30%, especially the radial cutting force is greatly reduced, which is particularly advantageous for the precision machining of poorly-spaced parts such as thin-walled ribs.

(3) The thermal deformation of the workpiece is reduced. The high-speed cutting tool has good heat hardness. The cutting heat of 95%~98% is taken away by the chips quickly. The workpiece can be kept in a cold state. It can be used for high-speed dry cutting without coolant, reducing environmental pollution and enabling green processing. .

(4) The surface quality of the machined surface is high. At high-speed cutting, the machine's excitation frequency is particularly high, and it is far from the natural frequency range of the "machine tool - tool - workpiece" process system, and the operation is stable and vibration is small.

(5) It is beneficial to ensure the size and shape accuracy of the parts.

(6) It can keep the tool and workpiece at a low temperature and prolong the life of the tool. In high-speed cutting, the cutting amount is shallow, the cutting edge takes a short time, and the feed is faster than the heat propagation time. The radial force on the tool and spindle is low. It can reduce the wear of the spindle bearings, guide rails and ball screws, and has less impact on the spindle bearings. It is possible to use a tool with a long overhang and the risk of vibration is small.

(7) Processing costs are greatly reduced. High-speed machining improves machining efficiency and machining quality, and reduces grinding and finishing processes.

 

Process design

In high-speed machining, due to the high speed, small feed, small depth of cut, high-speed machining, the part deformation is small, the maximum side bending deformation after roughing is within 1mm, the maximum warping deformation is 3mm, the maximum bending deformation It is 1mm [3]. 6 The design of the machining process is to locate the inner shape of the finished part with the bottom surface and the two process holes. Here, a relatively simple process is designed:

(1) rough milling the bottom plane, leaving a process allowance of 2mm;

(2) Two φ12H 9 process holes for the parts;

(3) Position the inner shape of the rough milling part with one hole and two holes (the inner shape has a process allowance of 2mm);

(4) Correct the bottom surface to ensure a flatness of 1 mm;

(5) Machining the bottom surface of the part, removing the process margin by 2mm, and the bottom surface of the part is in place;

(6) Finishing the inner shape of the part.

 

Precautions in programming

(1) Set the maximum depth of cut for each layer, layered processing. These aluminum alloy parts have a depth of 3 to 5 mm per layer.

(2) Set the corner to force the arc transition. There is no right angle or acute angle in the path of the knife. Ensure the continuity and smoothness of the cutting process. 3 The difference between the “High Speed Milling” switch on/off (filler radius 1mm).

(3) Set the advance and retract tool macro command.

 

The high-speed milling ring often uses a spiral feed method. The spiral feed is the way in which the center of the tool moves along a spiral to the web surface of the part. This method reduces the resistance of the part to the tool during the machining process, and at the same time ensures that the bottom edge of the tool can cut off the part material on the moving track during processing.

Retracting can be performed by axially lifting the knife to the safety plane or tangential arc retracting.

 

Processing quality analysis

(1) The bending deformation of the web is relatively small, and the flatness is controlled within 0.3mm to meet the design requirements.

(2) The dimensional accuracy is within ±0.2mm, which meets the design requirements.

(3) The surface roughness R a is within 3.2 μm, which satisfies the design size requirements.

(4) The internal stress is relatively small, and the flatness remains stable after the heat meter process.

 

Comparison of 2 process plans

According to the above two process schemes, the analysis and comparison are made from two aspects of production efficiency and processing quality.

1 Comparison of production efficiency

According to the general CNC machining plan, 10 processes are required to complete, and high-speed milling only uses 6 processes. From the perspective of tooling, ordinary CNC machining requires 3 sets of tooling, while high-speed milling only uses 1 set of tooling; In terms of efficiency, the cutting time of high-speed milling is much lower than that of ordinary CNC milling.

2 Product quality comparison

The quality of the products processed by the two schemes is also very different. From the perspective of dimensional accuracy, the dimensional accuracy of parts after ordinary CNC cutting is within ±0.3mm, and the dimensional accuracy after high-speed cutting is within ±0.2mm. From the perspective of surface roughness, R a is 3.2~6.3μm after ordinary CNC cutting. The surface roughness R a of the parts after high-speed cutting is between 1.6 and 3.2 μm. The important reason for this difference is the influence of built-up edge and scales during the cutting process.

 

The effect of built-up edge on dimensional accuracy

In the case of cutting plastic metal such as steel or aluminum alloy, in the case where the cutting speed is not high and the band cutting is formed, there are often some metal cold welding (bonding) lamination from the chip and the workpiece. On the rake face, a wedge of high hardness is formed, which can replace the blade face and the cutting edge for cutting. This small piece is called built-up edge [2]. The built-up edge reduces dimensional accuracy and increases the roughness of the machined surface. The cutting speed is different, and the maximum size that the built-up edge can reach is also different.

 

The effect of scales on surface roughness

Scales are scaly burrs on a machined surface. Cutting high-alloy plastics with high-speed steel, carbide or ceramic tools at low and medium cutting speeds produces scales that follow the cutting direction and perpendicular to the machined surface. Scales are a major obstacle to obtaining a less rough surface. The cutting speed is high, and the height of the scale is greatly reduced in the case where the built-up edge is no longer generated. It is possible to obtain a processed surface roughness as small as R z 0.1 to 0.05 μm.

 

Future work direction

The high-speed milling of aluminum alloy die forgings will become an important means for the cutting of such parts in the future. The necessary conditions for realizing the work of the parts are good process structure, stable material blank reference, easy positioning and clamping, and high-efficiency CNC machining without manual intervention. program. The high-efficiency NC program without manual intervention can not be easily affected by individual differences of technicians. The cutting parameter database should be gradually established and improved according to the machine tool and its used tools to ensure that all technicians' procedures can adapt to the power and torque requirements of the machine tool. At the same time, it is necessary to maximize the cutting efficiency of the machine tool.


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