Machining Methods and Surface Integrity Control Technology for Aerospace Titanium Alloys

Analysis of Titanium Alloy Machining Processes Based on Machining Characteristics, Tools, Fixturing, and Cutting Parameters, with an Introduction to Surface Integrity Control Techniques

Senior Engineer Huang Qiang

1. Introduction

In recent years, the demand for titanium alloys in the aviation manufacturing industry has increased significantly. Titanium alloys are widely used in large aircraft. As an excellent manufacturing material for aircraft and engines, titanium alloys feature high structural strength, light weight, and good corrosion resistance. The machinability of titanium alloy materials often results in poor surface integrity of the workpiece after machining. Below, the machining methods and surface integrity control technologies for aerospace titanium alloys are introduced from the aspects of machining characteristics, cutting tools, fixturing selection, and cutting parameters.

2. Characteristics and Applications of Titanium Alloys

In the aviation industry, titanium alloys are mainly used to manufacture components such as engine compressor discs, hollow fan blades, turbine discs, and casing shells, as well as structural parts like large aircraft landing gear, outer wing sections, fuselage skins, doors, hydraulic systems, and rear fuselage sections. Currently, the usage proportion of titanium alloys in the aviation industry has increased from 6% to over 15%. The Boeing 777 uses 7%–9% titanium alloy parts; to achieve a 20% reduction in fuel consumption, approximately 2 billion RMB was invested in the development of the Boeing 787 specifically for researching the replacement of aluminum alloys with titanium alloys in certain parts of the aircraft, resulting in a titanium alloy content of 15% in the Boeing 787 airframe. In domestic large aircraft projects, the usage of titanium alloys has gradually increased from 4.8% in the regional jet ARJ21 to over 9% in the trunk liner C919.

The demands for structural lightweighting and high strength in the aviation field make it increasingly reliant on titanium alloys. Based on strength and high-temperature performance, titanium alloys can be classified into α titanium alloys, β titanium alloys, α+β titanium alloys, and titanium-aluminum intermetallic compounds, among which α+β titanium alloys (like Ti6Al4V) are the most widely used. α titanium alloys have good thermal weldability and strong oxidation resistance, but average toughness; β titanium alloys have better forgeability, cold formability, and heat treatment strengthening capability; α+β titanium alloys possess good toughness, are weldable and can be strengthened by heat treatment, and have good fatigue resistance.

The material composition of Ti6Al4V mainly includes Ti, Al, V, Fe, O, C, Si, Cu, and small amounts of N, H, B, and Y. Titanium alloys have excellent comprehensive mechanical properties, low density, and good corrosion resistance. As a high-strength alloy material, they have been continuously promoted for use in aeroengines and the aviation industry. However, the high temperatures and high cutting forces during the machining of titanium alloys lead to severe work hardening on the machined surface, exacerbating tool wear and resulting in poor machinability. These factors are detrimental to achieving good surface quality and affect the service life of titanium alloy components and engine performance. Below, using Ti6Al4V as the research subject and combining experience accumulated in production practice, the cutting performance, machining methods, and surface inspection techniques for titanium alloy parts are introduced.

3. Titanium Alloy Machining Methods

3.1 Tool Selection

Tool materials for machining titanium alloys should have characteristics such as good toughness, hot hardness, heat dissipation, and wear resistance. Additionally, tools should meet requirements like sharp cutting edges and a smooth surface. When machining titanium alloy materials, carbide tools with good thermal conductivity and high strength are preferred, featuring a small rake angle and a large relief angle. To prevent chipping and breakage of the tool tip, the cutting edge at the tip should have a rounded transition. The cutting edge should be kept sharp during machining to facilitate timely chip removal and avoid chip adhesion.

When machining titanium alloys, to prevent affinity reactions between the tool substrate/coating and the titanium alloy, which would accelerate tool wear, titanium-containing carbides and titanium-based coating tools are generally avoided. Years of production practice have found that although titanium-containing carbide tools are prone to adhesion and wear, they possess excellent anti-diffusion wear capability, especially during high-speed cutting, where their performance is significantly better than YG-type carbide tools.

Major global tool manufacturers have introduced cutting inserts specifically for machining titanium alloy parts. Continuous improvements in tool materials and coating materials have enhanced the cutting efficiency of titanium alloy materials and promoted the development of the titanium alloy industry. For example, ISCAR's IC20 inserts, with sharp cutting edges, are suitable for finishing titanium alloy workpieces. Its IC907 inserts effectively improve wear resistance, suitable for roughing and semi-finishing. SECO's CP200 and CP500 for machining titanium alloys are high-hardness, ultra-fine grain insert materials using Physical Vapor Deposition (PVD) technology. Walter's WSM30, WSM20, and WAM20, using TiCN, TiAlN, TiN, and Al₂O₃ coatings, offer strong resistance to deformation and wear. Commonly used tools and coatings for titanium alloy machining are shown in Table 1.

According to statistics, the aviation manufacturing sector largely relies on imported tools, and the dependence is even higher for difficult-to-machine materials like titanium alloys. Therefore, promoting the development and application of domestic tools and coating materials is an effective way to fundamentally solve the problem of titanium alloy machining in China.

3.2 Tool Wear and Solutions

When machining titanium alloys at high cutting speeds and large depths of cut, a crater wear (flank wear) forms on the rake face at the point of highest cutting temperature, with a distinct land between the crater and the cutting edge. The width and depth of the crater gradually expand as wear progresses, reducing the rigidity of the cutting edge, potentially leading to chipping if the tool continues to be used. Electron micrographs of insert wear are shown in Figure 1.

a) Crater wear with chipping phenomenon.    b) Flank wear

c) Built-up edge

During titanium alloy machining, severe friction between the insert and workpiece causes wear on the relief face near the cutting edge, forming a small wear land with zero relief angle, known as flank wear. Additionally, due to work hardening of titanium alloys, the cutting thickness at the tool nose on the minor cutting edge gradually decreases, causing the cutting edge to skid, which also leads to significant wear on the relief face.

After tool wear occurs, cutting parameters like cutting speed and feed rate can be adjusted by observing chip morphology and color, as well as machine tool force, sound, and vibration, to control abnormal rake face wear. Using positive rake angle insert geometries, selecting wear-resistant insert materials or coatings, can improve tool life.

Built-up edge (BUE) is prone to form during titanium alloy machining. When the BUE is stable, it can protect the tool by acting as the cutting edge. However, when the BUE grows to a certain extent, its top extends beyond the cutting edge, increasing the actual working rake angle. The accumulation and detachment of the BUE directly affect machining accuracy. BUE fragments adhering to the machined surface of the titanium alloy form hard spots and burrs, affecting surface quality. The irregular shedding and regeneration of BUE cause fluctuations in cutting force, leading to chatter and affecting tool life. Common methods in production practice to reduce or avoid BUE formation in titanium alloy cutting include: increasing cutting speed, gradually increasing depth of cut to the optimum; using PVD-coated insert materials; employing high-pressure cooling systems, etc.

In cutting operations, due to the low plasticity of titanium alloys, the contact area between the chip and the rake face is small, and tool wear mainly occurs on the rake face of the turning tool. Therefore, cutting inserts should be selected with a small rake angle, typically 0° to 5°. A small rake angle effectively increases the contact area between the chip and the rake face, helping to dissipate heat concentrated near the cutting edge. Selecting a relief angle of 5° to 10° can reduce friction between the tool and the part. Choosing a V-shaped contact surface combination between the insert base and the tool holder, a robust clamping structure design, can effectively improve the clamping rigidity of the tool holder, eliminate tool vibration, and improve the surface quality of the machined titanium alloy workpiece.

3.3 Fixture Selection

When positioning and clamping titanium alloy workpieces, the interaction between the clamping force of the fixture and the supporting force on the workpiece can cause stress deformation in the free state. The cutting force resistance during titanium alloy machining is significant, so the process system must have sufficient rigidity. The positioning structure and dimensions of the workpiece need to be analyzed, selecting stable and reliable datum points, and adding auxiliary supports or using over-constraint if necessary to increase part rigidity. Since titanium alloys are prone to deformation, the clamping force should not be excessive; a torque wrench can be used if necessary to ensure stable clamping force. Furthermore, when using fixtures to position and clamp titanium alloy parts, ensure good fit between the fixture's locating surface and the workpiece's locating surface, and balance the fixture's clamping force with the workpiece's supporting force. For relatively large clamping surfaces, a distributed clamping method should be used as much as possible to avoid deformation caused by concentrated pressure. The clamping points of the fixture clamps should be as close as possible to the machined surface of the workpiece to reduce vibration generated during titanium alloy cutting.

The use of fixtures, measuring tools, or various temporary tooling containing lead, zinc, copper, tin, cadmium, or low-melting-point metals is strictly prohibited for titanium alloy machining. Equipment, fixtures, and tooling used for titanium alloy should be kept clean and uncontaminated. Titanium alloy workpieces should be cleaned promptly after machining, and residues of lead, zinc, copper, tin, cadmium, low-melting-point metals, etc., are not allowed on titanium alloy surfaces. Special transfer containers should be used when moving and handling titanium alloy workpieces to avoid mixing and storing them with workpieces of other materials. When inspecting and cleaning finely machined titanium alloy surfaces, wear clean gloves to prevent oil contamination and fingerprints, which could cause stress corrosion cracking and affect the service performance of the titanium alloy workpiece.

3.4 Cutting Parameters

The main cutting parameters for titanium alloys are cutting speed, feed rate, and depth of cut, with cutting speed being the primary factor affecting its machinability. Comparative tests between constant rotational speed cutting and constant surface speed cutting of titanium alloy workpieces indicate that constant rotational speed cutting performs worse than constant surface speed cutting. When the cutting speed vc = 60 m/min, feed rate f = 0.127 mm/rev, and depth of cut ap = 0.05–0.1 mm for titanium alloys, a hardened layer is rarely found on the titanium alloy surface.

Since the hardened layer mainly appears on the workpiece surface after finishing, the depth of cut during finishing should not be too large, otherwise it will generate significant cutting heat. Accumulation of cutting heat can cause changes in the metallographic structure of the titanium alloy surface, easily generating a hardened layer on the part surface. An excessively small depth of cut can cause friction and extrusion on the workpiece surface, leading to work hardening. Therefore, during the machining of titanium alloy workpieces, the depth of cut for finishing must be greater than the size of the tool's hone (edge preparation).

The selection of feed rate for titanium alloys should be moderate. If the feed rate is too small, the tool cuts within the hardened layer during machining, leading to faster wear. The feed rate can be selected based on different tool nose radii. Finishing generally selects a smaller feed rate because a large feed rate increases cutting forces, causing the tool to heat up and bend or chip. Table 2 shows common parameters for cutting titanium alloys with different types and materials of tools.

3.5 Cooling System

The requirement for cutting fluid in titanium alloy cutting is low misting. High-pressure cooling tools should be selected for titanium alloy machining,配合机床高压泵,冷却压力可达(60–150) × 10⁵ Pa (approximately 60–150 bar). Using high-pressure cooling tools to machine titanium alloys can increase cutting speed by 2–3 times, extend tool life, and improve titanium alloy chip morphology. When applying cutting fluid during titanium alloy machining, the cutting force is reduced by 5%–15% compared to dry cutting of titanium alloy, radial force is reduced by 10%–15%, cutting temperature is reduced by 5%–10%, and the surface morphology of the machined titanium alloy is better with less massive adhesion, which is conducive to obtaining higher surface quality.

The currently used Trim E206 chemical emulsion, mixed from 8% concentrate and 92% pure water, with a concentration of 7%–9%, achieves good machining results in titanium alloy material processing and can be used in turning, milling, and grinding operations. Trim E206 contains special additives that effectively control the formation of built-up edge. The cutting fluid contains tiny emulsified molecules, improving the stability of the cutting fluid and reducing carry-off during machining, making it easier for the cutting fluid to enter the cutting zone. Additionally, Trim E206 has strong resistance to oil contamination, and residues from the cutting fluid are easily soluble in water and the working fluid, helping to maintain the cleanliness of equipment and machined part surfaces.

4. Titanium Alloy Surface Integrity

4.1 Microstructure Inspection of Titanium Alloy Forgings

Titanium alloy microstructure inspection involves examining the surface of an etched titanium alloy part under an electron microscope to observe the morphological characteristics, distribution, etc., of the material's microstructure, used to check whether the metallographic structure of the titanium alloy complies with relevant standards and drawing specifications. The steps for microstructure inspection of titanium alloy forgings are: rough machining of the forging → surface polishing → surface etching → cleaning → drying → microscopic inspection. The microscopic inspection of Ti6Al4V titanium alloy is shown in Figure 2.

a) Surface polishing     b) Surface etching

c) Rinsing with water   d) Microscopic examination

The purpose of rough machining the forging is to completely remove the α case. The titanium alloy surface is polished using alumina sandpaper with grit sizes 400#–800#, and the surface roughness must reach Ra = 0.025 μm or higher grade requirements. Etching uses Kroll's reagent, prepared as a 2% HF, 4% HNO₃ aqueous solution. An appropriate amount of Kroll's reagent is applied to the polished titanium alloy surface until the desired clear structure is obtained, then rinsed in water and dried. A handheld electron microscope is used to inspect the titanium alloy surface. The structure should contain 10%–50% primary α. The microstructural morphology of Ti6Al4V titanium alloy shown in Figure 3 represents a qualified metallographic structure.

a) Primary α in β-transformed matrix      b) Discontinuous α at β grain boundaries

c) Lamellar α in β grains

4.2 Blue Anodizing Corrosion Inspection for Titanium Alloys

During titanium alloy machining, when the tool's flank wear occurs, the tool's impact resistance gradually decreases, leading to work hardening on the machined surface of the titanium alloy due to extrusion and overheating. The blue anodizing corrosion method is commonly used to detect hardening and other defects. The surface of a titanium alloy workpiece after blue anodizing corrosion is shown in Figure 4. After post-treatment dissolution of the anodized titanium alloy workpiece, the color of a qualified oxide film should be a uniform light blue (see Figure 4a). Work-hardened titanium alloy workpieces, after corrosion inspection, show a dark blue surface (see Figure 4b) or localized darker areas (see Figure 4c), with uneven color distribution across different areas.

a) Uniform light blue     b) Dark blue     c) Localized dark blue

After blue anodizing corrosion, for parts exhibiting work hardening, methods such as adjusting the cutting tool material, coating, and cutting angles for machining titanium alloy, optimizing tool paths and cutting parameters, can be used to control and eliminate work hardening.

4.3 Surface Finishing of Titanium Alloys

To remove surface defects from titanium alloy compressor discs, hubs, impellers, shafts, and rotor spacers, and improve part service life, after completing all mechanical machining operations on the titanium alloy workpiece, manual flap disc finishing can be used for surface finishing. Flap disc finishing requires the use of finishing tools shown in Figure 5: a rotary air tool (speed 18,000 rpm), a polishing mandrel, and alumina or silicon carbide abrasive cloth (specification 10mm × 20mm, grit 120#).

a) Rotary air tool      b) Polishing mandrel       c) Abrasive cloth

The internal groove finishing of a titanium alloy workpiece is shown in Figure 6. To achieve good finishing results, the following methods can be used:

1. Fold the alumina abrasive cloth lengthwise and firmly insert it into the clamping slot at the front end of the polishing mandrel. Tighten it in the direction opposite to the rotation direction of the mandrel. Change to a new abrasive cloth after finishing each workpiece surface area (see Figure 6a).

2. The rotating abrasive cloth should reciprocate over the titanium alloy surface for one or two cycles, each cycle lasting 10–30 seconds, with a reciprocating speed of about 1.57 mm/s (see Figure 6b).

3. When finishing different surfaces of the titanium alloy workpiece, change the abrasive cloth between cycles. During manual finishing, use an appropriate stop wrench or mechanical depth stop device to control the passage of the rotating abrasive cloth.a) Abrasive cloth installation      b) Rotary polishing

5. Conclusion

Titanium alloy is a typical difficult-to-machine material. Due to high cutting forces, high cutting temperatures, and severe tool wear during machining, selecting reasonable tool materials and insert geometries is the primary challenge in titanium alloy machining. Ti-containing carbide tools have good anti-diffusion wear performance. During cutting, a stable titanium alloy adhesion layer forms on the tool surface, which can inhibit wear. With the development of domestic tools, the machining efficiency of titanium alloys has gradually improved, saving machining costs and playing a positive role in realizing the overall localization of engines. In production practice, titanium alloy machining should be based on existing enterprise conditions regarding technology, equipment, management, and cost. Reasonable positioning fixtures should be selected, and cutting parameters should be optimized using the enterprise's information data platform, gradually moving away from the extensive machining concept of selecting parameters based solely on experience and analogy.

By conducting microstructure inspections on titanium alloy forgings, the metallographic structure of roughly machined titanium alloy can be compared and evaluated. Finishing machining can effectively remove machining and material defects on the titanium alloy surface, improving workpiece service life. Blue anodizing corrosion inspection can effectively identify defects like work hardening that occur during titanium alloy machining. Effectively controlling the surface integrity of machined titanium alloy is of great significance for stabilizing titanium alloy machining quality and improving the service life of titanium alloy workpieces.

This article was published in Metal Working (Cold Working), Issue 7, 2021, pages 1–5, authored by Huang Qiang from AECC Xi'an Aero-Engine Ltd., originally titled "Machining Methods and Surface Integrity Control Technology for Aerospace Titanium Alloys".