1 Introduction Nickel-based superalloys (such as In718, Waspaloy, etc.) are characterized by good thermal stability, high temperature strength and hardness, corrosion resistance, and wear resistance. They are typical hard-to-machine materials and are often used to make key components such as turbine disks. Because the turbine disk is one of the key components of the aeroengine, it is prone to fatigue failure under stress, temperature, and harsh working environment conditions. Therefore, the turbine disk material and manufacturing technology are the key to the development of high-performance aeroengines. Because the shaped hole on the turbine disk is composed of several arcs and straight lines, the shape is complex, and the position of each component segment is required to be accurate during machining, and the transition is smooth without producing turning marks. The surface roughness meets the process requirements, so the high-temperature alloy shaped hole Machining is a difficult point in the processing of turbine disks. At present, aero-engine manufacturers all use electric discharge machining to machine nickel-chromium alloy special-shaped holes, but the heat-affected layers generated during EDM are difficult to remove by ordinary grinding and grinding methods, and often require special uses such as abrasive jets. The process removes the degraded layer, resulting in low processing efficiency and high production costs. Therefore, more and more attention has been paid to the research on the processing method of high-performance and low-cost nickel-based superalloy shaped holes. In this paper, through the different combinations of drilling, milling and grinding processes, the use of new coating tools and appropriate processing parameters processing of nickel-based superalloy shaped holes in the process test, discusses the use of milling and grinding machining method instead of electric spark machining nickel The feasibility of base superalloy shaped hole. 2 Process test and analysis Test conditions The cutting test was performed on the machining center. The shape and size of the shaped holes machined are shown in Fig. 1: The cross-section of the special-shaped hole consists of 6 arcs and 2 straight lines, and the hole depth is 10mm. The following techniques were used in the tests: 1 Drill Ø6mm round holes → Mill special holes; 2 Drill Ø6mm round holes → Grinding shaped holes; 3 Drill Ø6mm round holes → Milling shaped holes → Grinding shaped holes. The processing conditions and process parameters for the three different processes are shown in Table 1.
Table 1 Process Tool Cutting Parameters Cutting Speed
(m/min) Feed rate
(mm/min) Depth of cut
(mm) Drilling, Boring, Milling Drilling Ø6mm Carbide Coated Drills 22 58 - Milling Milling Hole 1 Ø4mm Multilayer PVD Coated Ball Milling Cutter, 2 Edges, Blade Length 25mm, Total Milling Cutter Length 100mm, Shank Ø6mm, Straight shank 52 333 0.1 Milling hole 2 104 666 Drilling boring grinding Drilling Ø6 mm carbide coated drill 22 58 - Grinding cylindrical Ø4 mm, 6 mm long alumina chrome corundum, grade RA120, shank Diameter Ø3mm 188 333 0.05 Drilling ↓ Milling ↓ Grinding Drilling Ø6mm Carbide Coated Drills 22 58 - Milling Milling Holes 1 Ø4mm Multilayer (TiAlN, TiCN, TiN) PVD Coated Ball Mills, 2 Edges, Edges Length 25mm, cutter length 100mm, shank diameter Ø6mm, straight shank 52 333 0.1 Milling hole 2 104 666 Grinding Ø4mm, 6mm long cylindrical alumina wheel (chrome corundum), grade RA120, shank diameter Ø3mm 188 333 0.05 Workpiece Material: In718 Nickel Base Superalloy Coolant: 9% emulsion, pressure 30 Bar
Figure 1 Cross-sectional shape and size of shaped hole
Fig. 2 Surface roughness of irregular holes obtained by different processes
The wear of the milling cutter and grinding wheel was measured using a tool microscope and an image acquisition system, respectively, and the wear profile was recorded. The surface roughness Ra of the hole was measured along the axis of the irregular hole with a Taylor-Hobson Surtronic 3p type surface roughness meter. Results and Analysis The surface roughness of the shaped holes obtained from the three machining processes were compared. The results are shown in Figure 2. In the three processes, drilling → milling → grinding (drilling Ø6mm round holes → low Milling Machining Shaped Holes → Grinding Shaped Holes The surface roughness of the shaped holes obtained by the process is the smallest, while the surface of the shaped holes obtained by drilling → grinding (Ø6 mm hole drilling → grinding irregular holes) is rough The greatest degree. Tests have proved that: Under this test condition, special holes satisfying the requirements of surface roughness can also be obtained by milling; after drilling, the precision of the surface roughness of the shaped holes obtained after the milling process is lower than that of the hole after milling; Grinding can improve the surface roughness accuracy of irregular hole machining to some extent, but it will increase the cost and reduce the efficiency. Milling cutter wear and breakage under different machining conditions: During the drilling-milling process, after milling one hole, severe groove wear and damage occurred at the corners of both milling cutters. When milling holes with low cutting rates (v=52m/min, f=333mm/min), the cutters produce more pronounced breakage (see Figure 3a); when milling with irregular holes at high cutting rates (v=104m/min) ,f=666mm/min), the groove wear of the cutter is more pronounced (see Figure 3b).
(a) Milling tool for milling hole 1
(b) Milling cutter for milling hole 2
Fig. 3 Wear and damage appearance of the milling cutter (after milling one hole) Since the In718 nickel-based superalloy is susceptible to work hardening in the cutting process, the g', g" strengthening phases in the alloy and the hard phases such as WC, WN, etc. The phase still retains high hardness at high temperatures and sculpts the tool face and edge at high speeds, causing the tool to wear grooves.In addition, nickel-based superalloys are prone to lateral plastic flow during cutting and are separated at the cutting edge of the tool. It produces jagged burrs and burrs on the workpiece that cause high-speed, high-frequency impact tooling to cause micro-cracks and spalling under periodic thermal stress, which results in cutting zones during high-volume milling. The high temperature causes severe wear and damage to the cutter, which increases the surface roughness of the machined hole.From the experiment, it can be seen that when using an alumina grinding wheel to grind In718 nickel-based superalloy, the grinding wheel wears rapidly, and after grinding one irregular hole, the grinding wheel Into a conical shape, the surface has severe adherence (see Fig. 4), which is due to the high grinding force, high grinding temperature, etc. at the higher grinding temperature and larger grinding of nickel-based superalloys. Under the action of normal force, the material being grinded in the grinding zone produces severe plastic deformation and adheres to the surface of the grits, and this deformation and adhesion causes the grind force to further increase, with the adhering substance in the shear force. Under the effect of shedding, the abrasive grains of the grinding wheel may be damaged or even fall off and prematurely lose the cutting ability, resulting in an increase of the surface roughness of the workpiece (even larger than the roughness of the surface of the workpiece processed by milling.) In summary, according to the above three kinds of processing Compared with the machining efficiency and processing effect of the inhomogeneous holes in the Ni-base superalloy workpiece of In718, it is feasible to use the drilling-milling process instead of the electric spark method to machine the nickel-based superalloy workpiece shaped hole.
Fig. 4 Wear appearance of grinding wheel after grinding a special hole
Figure 5 Shaped hole of the workpiece
3 Drilling→Milling Machining and Inspection of Nickel-based Superalloy Shaped Holes Workpieces and Materials: The workpiece material is Waspaloy nickel-chromium high-temperature alloy (hardness 38HRC). The main components are shown in Table 2.
Table 2 Chemical Composition of Waspaloy Ni-Cr Superalloy Ni Cr Al Ti Fe Zr Mo Co CB Content (wt.%) 57 19.5 1.4 3 1 0.7 4.3 13 0.05 0.01 The shaped hole in the turbine disk is a 15° depth with a depth of 19mm. Hole, the top of the hole is arced. In the milling test, in order to simulate the machining process of the irregular hole on the turbine disk, the test piece was machined into an arc surface with a 15 degree slope from the bottom surface, the hole depth was 19 mm (see Fig. 5), and five workpieces were used as a test group. Machining and inspection: The special hole machining process includes: 1 milling Ø6mm center hole plane; 2 drilling processing Ø6mm round hole; 3 milling processing irregular hole (processing conditions shown in Table 3). Measure the wear of the cutter and the surface roughness of the irregular hole; Measure the size and profile change of the shaped hole at the depths of 3mm, 6mm, 9mm, 12mm and 15mm using a three-coordinate measuring machine (with a probe diameter of Ø2mm); The microhardness of the surface of the first and last deformed holes was processed in order to study the degree of work hardening of the profiled holes.
Table 3 Drilling→Milling Processing Conditions Machining Process Tool Cutting Parameters Cutting Time
(min/hole) Cutting speed
(m/min) Feed rate
(mm/min) Depth of cut
(mm) Milling Ø6mm Center Hole Plane Ø6mm Carbide Milling Cutter 18 47 3 1.3 Boring Ø6mm Hole Ø6mm Carbide Coated Bit 18 47 - 0.63 Milling Hole Ø4mm Multilayer PVD Coating (TiAlN, TiCN, TiN) Milling cutters, 2 blades, blade length 19 mm, total cutter length 75 mm, shank Ø6 mm, straight shank 25 200 0.1 7.58 Coolant: emulsion with a concentration of 9%, pressure 30 Bar Test results and discussion Geometrical accuracy Measuring with a coordinate measuring machine As a result (as shown in Fig. 6), all the hole diameters of the milling process are reduced along the depth direction of the axial direction, the axial direction of the shaped hole is tapered, and the maximum taper is 0.19°, indicating that the size of the hole in the X and Y directions varies. Milling cutter wear is significantly reduced.
(a) X direction
(b) Y direction
Fig. 6 The actual size of the profiled hole in the milling process is compared with the dimensional tolerance of a company's D shaped hole (X direction: 7.65 to 8.25 mm; Y direction: 6.35 to 6.85 mm). The variation in the size of the five holes in the milling test group is within this tolerance. Within the range, meet the processing accuracy requirements. The surface roughness is shown in Fig. 7. The surface roughnesses Ra and Rz of the special-shaped hole machined by the 2-flute coated cutter were varied within the ranges of 0.30 to 0.40 μm and 2.3 to 3.64 μm, respectively; the wear of the milling cutter was increased as the milling time increased. With the increase, the workpiece surface roughness Rz tends to increase. It can be seen that if a four-edge coated carbide cutter is used and the milling parameters are further optimized to reduce the tool wear, it is expected to directly obtain a shaped hole that satisfies the requirements of the surface roughness, eliminating the need for subsequent finishing operations, reducing costs and improving Processing efficiency. Machining surface microhardness Figure 8 shows the variation of the microhardness of the machined surface of the first and last holes in the five shaped holes milled by the same cutter. As shown in Fig. 8, the softening phenomenon of the processed surface layer (thickness is about 60 μm) appears in both shaped holes, and its microhardness is even lower than that of the substrate. As the microhardness of the subsurface layer increases, the substrate hardness recovers when the depth reaches about 140μm to 180μm. The surface softening may be related to the large plastic deformation of the surface layer of the nickel-base superalloy with poor thermal conductivity and the high temperature. As the milling cutter wears, the temperature in the cutting zone increases, and the deformed hole is softened and hardened by the processed surface.
Fig. 7 Relationship between the surface roughness of profiled holes and milling time
Fig. 8 Change of microhardness of machined surface
Milling efficiency Under the conditions of this test (see Table 3), the machining time for machining the center plane of the shaped hole, the drilling center hole and the milling hole is 1.3 min, 0.63 min and 7.58 min, respectively, plus the milling hole The total machining time for a special hole is approximately 17 minutes for the arc chamfer at both ends. In the past, the same parts were processed using the two processes of EDM and abrasive jet. The process time was about 40 minutes and 6 minutes respectively, and the total work time was 46 minutes. After comparison, the milling process used in this experiment can reduce the processing time by 58%. Moreover, all the processes can be completed in one machining center, and the auxiliary time is small, which saves the total working hours. Because the thickness of the degenerated layer produced by milling is much smaller than the thickness of the burned layer produced by EDM, even if the abrasive water jet machining process is added after the milling process to improve the integrity of the machined surface, the abrasive water jet can be processed. Working hours are greatly shortened. 4 Conclusions Through the comparison of the processing technology of the turbine disk shaped hole, the surface roughness of the shaped hole obtained by drilling → low-volume milling → grinding process is the smallest, while the surface roughness of the shaped hole processed by the drilling → grinding process is the largest. The drilling-milling process of nickel-based superalloy turbine disk shaped holes can meet the workpiece geometric precision and surface roughness requirements; compared with the EDM → abrasive jet process, can significantly reduce the processing time. Under the premise of guaranteeing the processing precision, by optimizing the cutting parameters, the machining efficiency using the drilling-milling process can be further improved.
Air fryer 8.0 - 11.0 Liter
Large Air Fryer,Air Fryer Healthy Cooking
Ningbo GOLMA Electric Appliance Co., Ltd. , https://www.golmahome.com