2. Profile Grinding and Polishing of Superalloys
2.1 Super alloy Components and Manual Blending
Superalloys are widely used, in the aerospace industry for example, to meet the following demanding engineering requirements:
• High strength-to-weight ratio
• High fatigue resistance
• High corrosion resistance
• Superior high-temperature strength
Jet engine turbine vanes and blades are often made of Inconel materials.
However, these materials have poor machinability, long recognised by manufacturers. Figure 1 shows the schematics of a high-pressure turbine (HPT) vane.
The vane consists of an airfoil having concave and convex surfaces, an inner buttress and an outer buttress. After operating in a high-temperature and high-pressure environment, vanes incur severe distortions as large as 2 mm in reference to the buttress. On the airfoil surface there are hundreds of cooling holes. After a number of operational cycles, defects such as fully or partially blocked cooling holes, micro cracks and corrosions begin to occur.
Because of the high cost of the components, it is common practice to repair
these parts instead of scrapping them. The repairing process starts with cleaning and covering the defective areas with the braze material. The purpose of brazing is to fill up the defects, but unavoidably, the brazed areas will be higher than the original surface. Figure 2 shows a cross section of the airfoil brazed with a repair material.
inner buttress outer buttress
baffle
outer platform concave side of mateface
featherseal slots
airfoils (concave side) inner platform
mounting lugs
Figure 1 Schematics of a HPT Vane.
Leading edge
Braze material
Cavities
Parent material
Cooling holes
^ \ railing edge |
Figure 2 Turbine airfoils repaired with braze material.
Table 1 summarises the overhaul conditions of typical jet engine high- pressure turbine vanes. The wall thickness of the airfoil ranges from 0.8 mm in the trailing edge to 2 mm in the leading edge. The braze material, similar to the parent material in composition, is laid down on the airfoils manually, and its thickness is about 1 mm. The compositions of the braze
material are shown in Table 2. The three major chemical elements of both parent and braze materials are cobalt (Co), chromium (Cr) and nickel (Ni).
Table 1 Overhaul conditions of jet engine turbine vanes.
Items
Airfoil material Airfoil curvature Braze material Hardness Machinability Part dimension Part weight Wall thickness Part distortion Braze thickness Braze pattern Braze coverage Brazing operation
Conditions Inconel
3D, concave and convex Similar to parent material 20 to 30 HRC
Poor
150 x 140 x 80 mm (max.) 0.68 kg (max)
0.8 (trailing edge) to 2 mm (leading edge) Up to 2.0 mm
0.5-1.5 mm Defined sets
About 80% of airfoil surface Manually done with dispenser
Table 2 Compositions and properties of braze material.
Chemical elements Cobalt Chromium Nickel Tungsten Tantalum Titanium Carbon Baron
Atomic Number 27 24 28 74 73 22 6 5
Atomic Weight 58.93 51.996 58.693 183.84 180.95 47.90
12.01 10.811
Density (g/cm3) 8.90 7.19 8.902 19.30 16.654 4.54 3.513 2.34
Weight Percentage 45.6 23.75 25 3.5
1.75 0.1 0.3 1.48
Blending can be defined as the material removal process to achieve the desired fmish profile and surface finish roughness. The process is often
employed to remove excessive material on surfaces of new jet engine parts or overhaul turbine airfoils. Within the blending process, we further define rough grinding as the process step to remove the excessive material with the profile generation as the primary aim. The aim of fine polishing is to achieve the desired surface roughness. In this sense, the term blending is often interchangeable with grinding and polishing.
In the aircraft overhaul industry, the blending process is intended to remove excessive braze material for the brazed area to be flush with the original surface within a tight tolerance. Current manual blending (also called belt polishing) uses belt machine to remove the braze, within tolerated undercuts and overcuts, as illustrated in Figure 3 (a). After belt polishing, the part is polished with a flap wheel, as shown in Figure 3 (b), to achieve the final surface finish. Table 3 lists quality requirements of the blending operation. They are achieved with the operator's skills and knowledge:
• Manipulate the part correctly in relation to the tool head.
• Exert correct force and compliance between the part and the tool through wrists, and control the force interaction based on process knowledge.
• Adapt to part-to-part variations through visual observation and force feedback.
Figure 3 Manual polishing of brazed airfoils with (a) belt polishing tool, (b) flap wheel.
One can imagine that a possible automation solution is to develop a machine which can mimic the operator's capabilities. In an abrasive machining process such as belt polishing, the amount of material removed
not only depends on tool position, but also the contact force between the tool and workpiece. Such an automation system requires position control as well as force control so that the desirable amount of material can be removed to avoid excessive overcut or undercut. The complexity of such automation further escalates in consideration of process dynamics and part- to-part variations typified by near-net-shape new parts and overhaul parts.
The first hard choice is what material removal process is most suitable for the intended automation system. Hence, the machining processes for superalloy materials must be carefully evaluated.
Table 3 Quality requirements of airfoil blending.
Items Overcutting Undercutting Trailing edge Leading edge Wall thickness Surface roughness Transition from brazed to non-brazed area Blending path Part integrity
Specifications
< 100 microns
< 100 microns
Absolutely no overcutting
0 to 200 microns gap from the template.
Smooth curvature.
Greater than minimum wall thickness at specified check points
< 1.6 microns Ra
No visible transition lines
No visible path overlapping marks No burning marks