1. Superalloy materials and their machinability
With the needs of science and technology and the advancement of human civilization, mechanical products are highly demanding in terms of high performance, versatility, and high quality. Product structure requirements are also more compact, and part sizes are being refined. To meet the above requirements, difficult-to-machine materials with high hardness, high toughness, and high wear resistance are used more and more in products.
Taking power generation equipment as an example, high-power and high-parameter equipment such as turbines ranging from 300,000 ordinary units to ultra-supercritical one million and gas turbines, and nickel-base superalloys or other hard-to-process materials are used for high-temperature, wear-resistant, and acid-resistant parts and materials. The proportion is increasing rapidly. According to incomplete statistics, when companies used conventional units as their leading products more than a decade ago, the high-temperature alloys and other difficult-to-machine materials involved were only a few of the GH132 parts. At present, due to the special requirements of new products such as thermal power, gas power, nuclear power, and wind power, more than ten types of materials such as high-strength stainless steels, low-temperature brittle metals, and high-temperature heat-resistant alloys have caused great difficulties in cutting processing. Among them, high-temperature alloys There are nearly ten brands and there are more than ten kinds of parts involved. Here, we will only discuss the processing of nickel-based superalloy holes.
Compared with common steels, the difficulties in cutting high-temperature alloys are mainly manifested in the following aspects:
(1) Work hardening tends to be large. For example, the matrix hardness of GH4169 un-strengthened is about HRC37, the hardened layer of about 0.03 mm is produced on the surface after cutting, the hardness increases to about HRC47, and the degree of hardening is as high as 27%. The work hardening phenomenon has a great influence on the tool life and usually produces severe boundary wear.
(2) Large cutting force. The strength of superalloys is more than 30% higher than that of alloy steels commonly used in steam turbines. At temperatures above 600°C, the strength of nickel-based superalloys is still higher than that of ordinary alloy steels. The non-reinforced superalloy has a unit cutting force of 4000N/mm2 or more, while the ordinary alloy steel has only 2500N/mm2.
(3) The material has poor thermal conductivity. The large amount of cutting heat generated when cutting high-temperature alloys is borne by the tool, and the cutting edge is subjected to cutting temperatures of up to 800-1000°C. Under high temperature and large cutting forces, the cutting edge will cause plastic deformation, adhesion and diffusion wear.
(4) Nickel-based alloys The main components of nickel-based alloys are nickel and chromium, in addition to a small amount of other elements: molybdenum, tantalum, niobium, tungsten, etc. It is worth noting that niobium, tantalum, tungsten, etc. are also used to make cemented carbide (or High-speed steel) The main component of the tool, the use of these tools for machining high-temperature alloys will produce diffuse wear and abrasive wear.
2. General features of hole processing
Hole machining is a difficult machining method in cutting processing, and it is semi-closed machining. Especially in the case of solid drilling, the cutting heat can easily stay near the cutting edge, and the cutting heat and cuttings can be discharged in a timely manner. This is the key to the tool life and must be given sufficient attention.
Boring and broaching machining is performed under pre-drilled conditions and is similar to that of turning. However, the rigidity of the shank system decreases with the increase of the depth-to-diameter ratio of the hole, and the arbor and broaching arbor system is usually reduced. Rigidity is much lower than turning and milling conditions. In order to ensure the hole processing accuracy and improve the chip removal conditions, the blade structure, the shank diameter and the tool bar strength must be reasonably matched.
3 high-temperature alloy hole processing technology difficulties
Compared to general steel cutting, the tool life of cutting high-temperature alloys is lower than 50%, the processing efficiency is very low, and the processing cost is much higher. The main difficulties of high temperature alloy hole processing are:
(1) The cutting force is large and the power consumption of the machine tool is large. For example, using an imported brand of φ68 machine clamp flat drill on the GH901 for solid drilling, power consumption is greater than 30KW, and with a considerable diameter of the composite drill in the 15CrMo cylinder drilled on the sub-surface power consumption is less than 10KW.
(2) Hole machining is semi-closed cutting. The chips with high cutting heat and chip breaking difficulty are difficult to discharge away from the tool tip in time, and the tool wear is more severe. For example, when a general twist drill is used to drill the GH901, it is processed at the normal material cutting amount. In just a few minutes, the tip of the tool shows a blue surface burn.
(3) It is difficult to ensure the precision of high-temperature alloy holes by ordinary drilling methods. The reason for this is that the axial force of drilling is large, and machining is performed using a device with poor rigidity such as a radial drilling machine. A tool such as a drill bit tends to generate a large bend, which results in deflection of the drill hole and influences the drilling accuracy. For example, when a GH901 stem φ14.5 radial hole is machined with a normal twist drill, the drill bit is bent under a large drilling force, and the blade wears extremely quickly and cannot be cut normally; instead, it is drilled in a boring machine and cut in other ways. Under the same conditions, the drilling task was successfully completed.
(4) In high-temperature alloy hole machining, the tool wear is much faster than that of ordinary steel, and it requires a cutting tool with better cutting performance. According to statistics of research data, the cost of cutting high-temperature alloy tools is 5 to 10 times that of ordinary steel.
4. Superalloy hole machining tool material
The tool materials that can be used for high-temperature alloy machining include CBN, ceramics, hard alloys, and high-speed steels.
The tool material for small and medium hole drilling is recommended to use a new type of coating (such as TiAlN, TiZrN) cemented carbide with ultrafine grained alloy as the matrix. This material has excellent wear resistance and low coefficient of friction. The most commonly used cutting tool material is currently difficult to machine. For example, a large number of solid carbide drills and indexable boring tools manufactured by some professional tool factories at home and abroad have used this technology in large quantities.
In high-temperature alloy drilling, expansion, and reaming, high-speed steel materials are still widely used. High-speed steel tool material has much higher toughness than cemented carbide. It does not require high carbide machine tools for machine tool accuracy and system rigidity. Therefore, it is advantageous in drilling large and medium-sized holes.
5. High-temperature alloy solid drilling
(1) The basic structure and requirements of the drill: When the nickel-based superalloy is used for solid drilling, the common cutter has an integral drill and a machine tool drill. Machine-tool drills include machine-tool deep-hole drills (such as machine clamp flat drills, indexable deep-hole drills) and indexable shallow-hole drills (for machining solid bores with an aspect ratio of less than 5). The overall drills include carbide drills and high-speed steel drills. Solid carbide drills are recommended for drill holes up to φ20. The use of solid carbide drills requires the machine to have sufficient feed accuracy, swivel precision, and rigidity. Ordinary boring and milling machines and drilling machine spindles generally exceed 0.05 mm, which can not meet the requirements of carbide drills. The drill bit should be preceded by a H6 cylindrical shank of tolerance class and be clamped with a precision force chuck or hydraulic chuck to increase the accuracy of the tool system.
As for the sharpening of solid carbide drills (and welded carbide drills), there is a need for CNC tool grinders and related grinding software. If the company does not have grinding conditions, high-speed steel drills can be recommended for high-temperature alloys.
Drilling high-temperature alloys requires better accuracy of drills. According to relevant data, some aerospace processing plants require that their radial runout should not exceed 0.02 mm when drilling such materials. The key to the drill bit is the structure of the drill tip, including the front angle, the width of the chisel edge, the rake angle, and the symmetry of the drill tip.
Increasing the front angle can reduce the chip contact length, reduce the heat of cutting, and improve the cutting conditions of the drill. The front angle of the high-temperature alloy drill is recommended to be 135° to 140°.
The cutting edge of the drill bit has a great influence on the drilling performance. Under normal circumstances, the cutting force at the cutting edge can account for about 60% of the cutting force of the drill bit. If the cutting edge is not properly treated, the drilling temperature is high. In alloy materials, the drill tip wears quickly. S-shaped (domestic called cross) grinding, is to increase the tangent angle at the edge of the plane, reducing the length of the chisel edge. For untrimmed drills, the rake angle at the flank edge is approximately -30°, and after the "cross-grinding", the rake angle at the traverse edge is greater than -15°. This is an effective way to reduce the bit torque and cutting heat, which is very beneficial to extend tool life and improve cutting conditions. Many new types of drill bits developed at home and abroad have this feature in the horizontal blades.
The drill tip for drilling high-temperature alloy holes must have sufficient symmetry, which is a necessary condition for controlling hole size and position accuracy. Machine grinding is the best way to control the sharpness of the bit and the symmetry of the cutting edge.
(2) Drilling cooling method: In order to facilitate chip evacuation, an internal cold hole drill bit is introduced on the market, which can supply sufficient water-soluble coolant or mist coolant to make the chip removal more smooth. The cooling effect of the cutting edge is also ideal. However, the drill requires the machine to have an internal cooling system or an external cold to internal cooling mechanism. At the same time, the cost of cutting tools for such drills is high, and the grinding of the tools generally requires re-grinding at the tool grinding center.
In general, external cooling can be used for drilling. The effective cooling of the machining superalloy is essential, and the water-based cutting fluid is the best cutting fluid for cooling effect. In order to improve the cooling conditions of the solid borehole, the nozzle should be as close as possible to the entrance of the drill bit, and the angle with the axis of the drill bit should be less than 30°. The direction of the spray should be consistent with the direction of the spiral groove of the drill bit.
(3) High-Speed ​​Steel Twist Drill: High-speed steel twist drill is a traditional tool for low-speed machining of high-temperature alloys. Its advantages are easy manual grinding, good toughness of tool materials, and relatively low requirements for machine tools. The following is an example of processing high-temperature alloy holes with high-speed steel twist drills:
Workpiece material: GH901; Machine tool: CNC boring boring machine TK6513, machine power 30KW; Drill: ordinary HSS twist drill, front angle 140°, horizontal cutting grinding to be narrow 0.2mm; drill size: φ12.3×80; cutting amount: Vc =3.864m/min, fn = 0.06mm/r; Cooling conditions: external cooling, Master water-based cutting fluid, 3mm per hole, immediate retraction, prevent the drill edge idling and the bottom of the hole repeated extrusion friction and increase the high temperature alloy The thickness of the work hardening layer of the material and the drill tip are all withdrawn, the purpose of which is to cool the drill bit and the hole to improve the cooling effect of the cutting fluid and improve the chip removal conditions. One hole processing time: 17 minutes; Tool wear: The edge has a slight wear; Processing effect: The straightness and surface roughness of the hole all meet the pre-drilling requirements (4) The indexable shallow hole drill can be indexed shallowly Hole drilling is an efficient drilling tool with indexable cemented carbide inserts. Its drilling efficiency is more than 4 times that of ordinary drill bits. Dongqi first used Bellardi CNC boring machines to process ordinary steel flanged bolt holes. It has been widely used in steel processing. Through testing the drill can also be used for drilling of high temperature alloy stem, but the knife has the following structural characteristics: 1 blade should have sharp cutting edge to reduce the cutting force; 2 blade chip breaking groove should be reasonable, reduce A large amount of cutting heat is caused by excessive curling deformation of chips; 3 Select the appropriate tool diameter according to the power of the machine; 4 Select the appropriate aspect ratio of the tool to increase the rigidity of the tool system as much as possible.
The analysis of the φ71×240 radial bore machining of high-temperature alloy shaft parts is now taken as an example.
1 Selection of Shallow Hole Drills: Compared with integral drills and cemented carbide weld drills, shallow hole drills have greater cutting forces. Basic problems such as system rigidity and machine power must be considered when selecting. In order to meet the requirements of high-temperature alloys on the drill bit, first select the three-dimensional shallow hole drill with good rigidity. In addition, when the diameter of the drill is selected to be φ68, the cutting power consumption is close to the rated power of the TK6513 CNC floor boring machine by 30 kW. To this end, in combination with existing resources, shallow hole drills of φ58 were selected. Specific specifications are as follows: Bit Specifications: φ58×174; Blade Model: P28479-7 WAP35
2 Selection of cutting amount: As mentioned above, the high-temperature alloy material has poor machinability, and the cutting amount in machining is much lower than that of ordinary materials. For example, the cutting amount is only Vc=25m when turning the GH901 stem. /min, fn = 0.15mm/r, ap = 2mm. The difficulty of drilling the solid alloy hole is even higher than that of normal turning. In addition, the space for chip evacuation in solid drilling tools is very limited, and the state of chip removal needs to be considered. Tests have shown that the "C" chips are conducive to chip removal. A reasonable amount of cutting is as follows (bit diameter is 29mm): Vc ≤ 20m/min, fn = 0.1mm/r.
3Drilling method: Because the machine has no internal cooling system, the chips cannot be discharged in time. In order to overcome this problem and reduce the effect of cuttings on the tool life, the same as the drilling of the twist drill, the gap feed method is adopted, ie, the depth is retracted at a depth of 5 mm per drill to prevent the drill idling and the bottom of the hole from repeated extrusion friction. While increasing the thickness of the work hardening layer of the high-temperature alloy material, the drill tip should be completely withdrawn, the purpose of which is to cool the drill bit and the hole, improve the cooling effect of the cutting liquid, and improve the chip removal conditions.
Due to the insufficient drilling depth of the drill, a separate drilling method is used in the machining, that is, drilling from one end to the working depth of the drill bit, and then turning the head to drill from the other end.
4 Requirements before cylinder drilling: Radial holes are drilled directly on the cylindrical surface, and drilling holes on the inclined surface will make the drill bit skew, affecting the position and dimensional accuracy of the hole. Therefore, before drilling, you must first mill a plane with a cutter larger than 80% of the diameter of the drill and smaller than the final diameter of the hole to ensure that the axis of the drill is perpendicular to the surface to be drilled. Shallow hole drilling will not skew the center to ensure that the drilled bottom hole is in the correct position.
6. Boring process
The finishing of the hole is generally done by two methods: reaming and boring. The reaming is a more efficient process, but the reamer is usually a sizing tool (even adjustable reamers can only fine-tune the hole size), only the hole The shape and size are accurately controlled and the hole position error cannot be corrected. A guillotine blade can meet the control of the accurate size in the larger size range. It can not only control the shape and dimensional accuracy of the hole, but also can not substitute the effect of reaming processing on the hole position accuracy.
(1) Blade selection The URMA boring system was selected for machining. The boring tool adopts the international standard series blade, and its advantage is that the indexable boring blade can be more widely selected according to the characteristics of the processing material.é•— The blade model is as follows: The rough boring blade is the same as the workshop's existing fine turning blade: CNMG1204; fine boring blade: CCMT09T3, CCMT06.
When selecting boring blades, the tool nose radius is a geometric parameter that must be taken into account because the tool nose radius influences the size of the cutting force. The turning radius of the tip of a general turning machine is 0.8 mm. For the boring processing, the rigidity of the tool system is limited. Usually, the tip radius is about 0.4 mm, and even when the boring is finished, a tip radius of 0.2 mm is recommended.
The above model blades can be found in many domestic and foreign professional factory samples, which gives us the choice to be more suitable for processing high-temperature alloy boring blades. The earliest use of the boring system for coarse-grained high-temperature alloy GH901 materials was the use of a trowel system with self-aligning inserts. These inserts are suitable for machining ordinary steel and do not meet the boring requirements at all. The tool life is very short. To this end, according to the existing conditions of the workshop, combined with the experience of turning and milling of high-temperature alloys, a number of standard fine boring blades manufactured by professional plants were selected for the URMA boring system. Representative materials include coated cemented carbides such as TT5030 and S05F. The selected inserts are more suitable for boring machining of high-temperature alloys, taking CNMG120408-MP and CCMT09T302-FA as an example. (2) Selection of rough boring 1 Cutting amount selection: Rough boring is commonly used for indexing double-edged boring tools The blade CNMG120408-MP was used. In the boring process, chip evacuation is an important step. Continuous swarf will wrap around the tool tip, affect the quality of surface machining, and have a poor chip removal effect, which will have a greater impact on tool life and machining surface quality. Short chips indicate that the chips are curled severely, resulting in increased cutting heat and poorer cutting process stability. After the selected blade, the cutting amount is related to the spare part of the workpiece and the chip breaker groove type. The rough groove shape of the blade is NP. The ideal condition for chip breaking is: ap=1～4mm, f=0.10～0.4mm/ r. For high-temperature alloy boring, there must be enough water-based cutting fluid for cooling, as far as possible to make cutting fluid near the cutting edge of the blade. If there is no internal cooling system, the cutting amount of boring high-temperature alloy hole should take the lower limit: Vc = 16m/min, ap = 2mm, f = 0.1mm/r.
From the drill size φ58 to the final hole size φ71, the final allowance of 0.05mm is left, and the unilateral allowance is 6.45mm. Therefore, it is necessary to go through four rough boring operations and the remaining amount is allocated as follows: φ62 → φ66 → φ70 → Φ70.9.
The aspect ratio of this hole is 3.5, but the rod length of the boring rod system used is φ50, the length of the shank is 250 mm, and the length-to-diameter ratio is 5. In order to increase the size and shape accuracy of the former hole, the last half is half. Fine boring, its purpose is to calibrate the processing error of the previous process. 2 Boundary wear and countermeasures: When cutting high-temperature alloys, the cutting temperature in the cutting zone is very high. Especially at the transitional cutting edge that is in contact with air, the tool material reacts with active elements such as oxygen, nitrogen, and hydrogen in the air. The edge of the blade is cracked or chipped. This type of wear is called boundary wear. In the machining of superalloys, boundary wear is the most common form of wear.
There are two basic methods for reducing the boundary wear. One is to use a small tool declination angle, and the other is to use a variable deep-cutting method. However, a small primary angle will increase the bending deformation of the boring bar, so the deep cutting is the most effective means to improve the effect of the boring hole work hardening on the tool. Variable deep processing refers to that the selected depth of cut is not the same as the previous depth of cut, so that the contact area between the cutting edge and the cutting surface of the workpiece is changed to improve the cutting boundary contact state. In numerical control machining, by continuously changing the cutting depth of the cutting tool, a variable cutting deep processing method can be realized in one pass, and the boundary wear of the tool can be more effectively relieved. This method requires that the boring bed has a function of radially controlling the movement of the cutting edge. In contrast, the first method is easy to operate.
(3) Fine boring hole The most critical part of the fine boring hole is the safety of the boring process. It needs to be cut across the hole. The parameters of the hole meet the quality requirements. The main concerns are two aspects, one is to maintain a stable cutting process and the other is to ensure that the tool life satisfies the cutting length of the hole.
Be careful to keep enough cutting fluid to assist chip removal and increase tool life. Generally, a smaller depth of cut is required and a stable and continuous fine spiral swarf is formed. This can improve the reliability of the fine boring process and maintain a stable cutting process. For high-temperature alloy fine boring, the cutting amount needs to take a low value. The experience of high-temperature alloy cars and milling machining tells us that the life of carbide cutting tools is usually within 50% of the cutting of alloy steels, and the tool life is generally around 30 minutes. For this reason, after selecting the cutting amount, it should be checked whether the tool can finish the machining of at least one hole by one knife.
According to the structural features of the selected carbide fine-grained boring blade CCMT09T302-FA, the reasonable cutting range of the blade can be found out. The final cutting amount was determined as follows: Vc=22 m/min, ap=0.05 mm, f=0.1 mm/r. According to the above cutting amount, the cutting time of one hole is within 25 minutes. Moreover, the machining quality of the hole fully meets the technical requirements of the drawings: the hole accuracy is within 0.02, and the surface roughness is within Ra1.6.
(4) Tool presetting In NC machining, the tool system usually requires external adjustment tool - tool presetting. Its purpose is to reduce the downtime of the CNC machine tool and increase the machine operation efficiency. Rotary tool presetting includes two parameters of the working length and tool diameter of the tool system. For rough boring tools, the tool parameters can be adjusted directly to the desired size. Under the action of cutting force, the file system will produce elastic bending deformation, the so-called knife. Roughness has little influence on the processing size of the knife, and the presetting can ignore the impact. However, when presetting the boring knife, it is necessary to fully consider the influence of the knife on the tool presetting dimensions. When boring the hole, the size of the knife is related to the material properties, cutting amount, and system rigidity. It is difficult to quantitatively determine the amount of the knife when the hole is boring.
The effective way to solve the effect of the knife is to pre-adjust the test and cut the knife. First, the boring tool is pre-adjusted to be 0.05mm smaller than the final dimension of the hole. For example, the final size of the machined hole is φ71±0.02, which can be pre-adjusted to φ70.95. The first test is performed to check the actual size of the test hole and the middle value of the hole tolerance. The difference δ, under the premise that the tool and the workpiece clamping system are not changed, adjust the precision trimming boring tool to a δ, and then perform the test cut until the hole final size requirement is satisfied. For a more intuitive view of the dimensions, a dial indicator (or dial indicator) can be attached to the tool tip in the direction of the tool diameter as a secondary test.
7. Conclusion
The machinability of high-temperature alloys is much lower than that of ordinary alloy steels, and their tooling costs and other processing costs are greatly increased. However, the use of high-temperature alloy materials has greatly improved the product's operating efficiency and has also greatly increased the added value of the products. This is what we should pay more attention to. In the processing of high-temperature alloys and other difficult-to-machine materials, as long as the correct understanding of the cutting process, a reasonable choice of tool structure and cutting parameters, you can minimize tool costs and improve processing efficiency. High-temperature alloy hole processing should pay attention to the following points: 1 whether it is drilling or boring, machine tool movement accuracy and rigidity of the process system should be as high as possible. 2 The tool clamping should use high-precision chucks, such as precision power chucks and hydraulic chucks. 3 The amount of cutting should be selected reasonably. One is to control the shape of the chip through the cutting amount to facilitate chip removal, and the other is to pay attention to the tool life that satisfies the hole processing length requirement. 4 When solid drilling uses external cooling method, in order to improve the cooling and chip removal effect, gap feed method is adopted, that is, the depth is retracted every few millimeters of drilling depth to prevent the drill edge from idling and the bottom of the hole is repeatedly squeezed and friction increases the high-temperature alloy. The thickness of the work hardening layer of the material, and the drill tip should all be withdrawn, the purpose of which is to cool the drill bit and the hole to improve the cooling effect of the cutting fluid and improve the chip removal conditions. In addition to chip evacuation, the key issue of 5-hole machining also needs to solve the high cutting temperature well, especially for machining high-temperature alloys. Full and effective cooling is an indispensable means, and preferred is the cooling of water-based cutting fluids. 6 When the boring blade is used, it is required that the cutting edge has a large rake angle, and the cutting edge should be as sharp as possible while maintaining the strength of the cutting edge. 7 The shape of the chip flute and the sharpening angle of the tool should be reasonable. The rough machining of the hole requires the chip to be a small “C†chip, improving the chip removal effect. The fine chip requires a short spiral swarf to keep the boring process stable. 8 The rough cut can be changed by deep cutting, which can reduce the tool boundary wear and extend the tool life.
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