Comment machine à faire des alliages en titane à droite?

We know about titanium alloys’ high strength-to-weight ratio, excellent mechanical properties, et résistance à la corrosion, are ideal materials for aircraft and engine manufacturing. Cependant, their poor machinability has long limited their application. With the advancement of machining technology, titanium alloys have been widely used in the manufacturing of compressor sections of aircraft engines, engine nacelles, exhaust systems, and structural frame components such as the main beams and bulkheads of aircraft.

Machinability and General Principles of Titanium Alloys

Titanium alloys are classified based on their metal structure into α phase, β phase, and α+β phase, represented by TA, TB, and TC, respectively. In our company’s new engine, TA and TC materials are used. Generally, castings and forgings use the TA series, while bar materials use the TC series.

Characteristics and Machinability

• High Strength-to-Weight Ratio: With a density of only 4.5 g/cm³, titanium alloys are much lighter than iron, while their strength is comparable to that of ordinary carbon steel.
• Good Mechanical Properties: The melting point of titanium alloys is 1660°C, higher than that of iron, providing high thermal strength. They can work below 550°C and generally exhibit good toughness at low temperatures.
• Excellent Corrosion Resistance: Below 550°C, titanium alloys form a dense oxide film on the surface, preventing further oxidation. They have high corrosion resistance to the atmosphere, seawater, steam, and some acids, bases, and salt media.

D'autre part, titanium alloys have poor machinability due to:

• Poor Thermal Conductivity: This leads to high cutting temperatures, reducing tool durability.
• Oxidation at High Temperatures: Above 600°C, a hard oxide layer forms on the surface, causing severe tool wear.
• Low Plasticity and High Hardness: This increases the shear angle, resulting in a small contact length between the chip and the rake face, causing high stress on the rake face and making the cutting edge prone to damage.
• Low Elastic Modulus: This causes large elastic deformation and significant rebound of the workpiece surface near the flank face, leading to severe wear due to large contact area with the flank face.

These characteristics make machining titanium alloys very difficult, leading to low processing efficiency and high tool consumption.

General Principles of Machining

Considering the properties of titanium alloys and the characteristics during machining, the following aspects should be considered during processing:

1. Tool Material: Use carbide tools as much as possible. Tungsten-cobalt carbide has low chemical affinity with titanium alloys, good thermal conductivity, and high strength. For low-speed intermittent cutting, impact-resistant ultrafine grain carbide can be used. High-speed steel with good high-temperature performance can be used for forming and complex tools.
2. Cutting Angles: Use a smaller rake angle and larger relief angle to increase the contact length between the chip and the rake face, reduce friction between the workpiece and the flank face, and adopt a rounded edge to increase strength, avoiding burning and chipping.
3. Sharp Cutting Edges: Keep the cutting edge sharp to ensure smooth chip removal and avoid edge chipping due to chip sticking.
4. Cutting Speed: Use a low cutting speed to avoid excessive cutting temperatures. The feed rate should be moderate—too high will burn the tool, too low will cause rapid wear due to working in the hardened layer. The cutting depth can be relatively large to work below the hardened layer, improving tool durability.
5. Cooling: Use sufficient coolant during machining to keep the temperature down.
6. Rigidity of the Machining System: Ensure the machining system has enough rigidity due to the high cutting resistance of titanium alloys. Since titanium alloys are easily deformed, clamping forces should not be excessive, especially in some finishing operations where auxiliary supports may be necessary.

Better Ways to Machine Titanium Alloys

Milling

Milling titanium alloys is more challenging than turning because milling involves intermittent cutting, and the chips tend to adhere to the cutting edge. When the adhered chips re-enter the workpiece, they detach and take a small portion of the tool material with them, causing chipping and significantly reducing tool life. To address these issues, three measures are taken for milling titanium alloys:

1. Milling Method: Generally, down milling is used.
2. Tool Material: High-speed steel M42 is preferred.
3. Increasing System Rigidity: Improve the rigidity of the process system from workpiece clamping and equipment aspects.

It is important to note that down milling is not typically used for general alloy steel machining due to the backlash in the machine tool lead screw and nut. In down milling, the milling cutter acts on the workpiece in the same direction as the feed force, causing intermittent movement and potentially damaging the tool. En plus, down milling initially causes the cutting edge to hit the hard skin, leading to tool damage. Cependant, with titanium alloys, the issue of chip adherence and chipping is more pronounced in up milling, where chips are thin to thick, causing severe friction at the initial contact point. Donc, down milling is preferred despite its drawbacks.

To ensure smooth milling of titanium alloys, the following points should also be noted:

• Cutting Angles: Compared to standard milling cutters, the rake angle should be smaller, and the relief angle should be larger.
• Cutting Speed: Keep the milling speed low.
• Cutter Type: Use sharp-tooth milling cutters and avoid fluted cutters.
• Smooth Cutting Edge: Ensure a smooth transition at the cutting edge.
• Cutting Fluid: Use plenty of cutting fluid.
• Milling Depth and Width: To improve production efficiency, increase the milling depth and width appropriately. Rough milling depth is typically 1.5-3.0mm, and finishing milling depth is 0.2-0.5mm.

Threading

To facilitate threading in titanium alloys, follow these guidelines:

1. Negative Rake Angle: Grind a negative rake angle on the cutting cone portion to facilitate chip removal. Use short taps to increase tap rigidity.
2. Back Taper: The back taper portion of the tap should be appropriately enlarged beyond the standard to reduce friction between the tap and the workpiece.
3. Pre-Drilling: Pre-drill the bottom hole with rough drilling followed by reaming to reduce work hardening. For threads with a pitch of 0.7-1.5mm, the bottom hole size can be processed to the upper tolerance of the standard thread bottom hole size and allowed to increase by an additional 0.1mm.
4. Machine Tapping: If possible, use machine tapping to avoid work hardening caused by uneven feed and mid-operation pauses during manual tapping.

Titanium alloys, with their high strength-to-weight ratio, excellent mechanical properties, et résistance à la corrosion, are ideal for aircraft and engine manufacturing. Despite their poor machinability, advancements in machining technology have allowed titanium alloys to be widely used in the manufacture of components such as aircraft engine compressors, nacelles, exhaust systems, and structural frames like main beams and bulkheads.