Titanium alloy has low thermal conductivity. Therefore, during Titanium alloy machining, the cutting temperature is high. Under the same processing conditions, the cutting temperature of TC4 titanium alloy is far more than twice that of 45 steel.
The heat generated during processing is difficult to dissipate through the workpiece. Titanium alloy also has a low specific heat, causing the local temperature to rise quickly during machining.
As a result, the cutting tool tip reaches very high temperatures. This leads to rapid wear and reduced tool life.
This alloy has a low modulus of elasticity, causing the machined surface to be prone to rebound. This rebound is especially serious when processing thin-walled parts.
The rebound can cause strong friction between the back face and the machined surface. As a result, the tool experiences wear and chipping.
Titanium’s chemical activity is very strong at high temperatures. It easily reacts with oxygen, hydrogen, and nitrogen.
This interaction increases its strength but causes a decline in plasticity during heating and forging. As a result, an oxygen-rich layer forms, which makes machining more difficult.
Titanium alloy material cutting principle
In the processing of the selected tool materials, cutting conditions and cutting time will affect the efficiency and economy of titanium alloy cutting and processing of its processing principles are as follows.
(1) Select reasonable tool materials
For titanium alloy material properties, processing methods, processing technology conditions reasonable selection of tool materials.
Cutting tool materials should be selected more commonly used, lower prices, good wear resistance, high thermal hardness and sufficient toughness of the material.
(2) improve cutting conditions
The rigidity of the system of machine tool – fixture – tool should be good. The gap of each part of the machine tool should be adjusted and the radial runout of the spindle should be small.
The fixture should be fastened to the workpiece and have sufficient rigidity.
The cutting part of the tool should be as short as possible, and the thickness of the cutting edge should be increased as much as possible to improve the strength and rigidity of the tool if the chip capacity is sufficient.
3) Reasonable tool geometry parameters
Ensure the durability of the tool so that the cutting is carried out smoothly to obtain reasonable processing quality and efficiency.
4） Select reasonable cutting dosage
Cutting speed should be low.
The cutting speed has a great impact on the temperature of the cutting edge. As the cutting speed increases, the cutting edge temperature rises dramatically.
The temperature of the cutting edge directly affects the tool’s life. Therefore, it is important to choose an appropriate cutting speed.
The depth of cut should be large.
Experiments have proved that the depth of cut has a small effect on the cutting edge temperature. Therefore, using lower cutting speeds and increasing the depth of cut when cutting titanium alloy is favorable.
This is especially true during rough turning. Increasing the depth of cut ensures that the blade fully engages the surface of the titanium alloy being cut. This helps prevent wear or chipping of the tool.
Do not stop the tool during cutting.
If the tool stops during cutting, the cutting edge and the titanium alloy being cut will rub together for a long time under high load.
This can easily cause hardening of the titanium alloy. It may also lead to sintering and extrusion cracking. As a result, the tool can be damaged.
5) Appropriate heat treatment of the machined material
Through heat treatment to change the properties and metallographic organization of titanium alloy materials to improve the machinability of materials.
6） Emphasis on chip control
Due to the titanium alloy material properties and cutting characteristics of the machining process of chip control is very important. To have reliable chip breaking measures in order to carry out cutting smoothly.
Machining technology
Titanium alloy machining includes turning, drilling, milling, tapping, grinding and EDM. Foreign countries continue to study titanium alloy processing technology and have achieved some results.
British Callender aviation parts company is a professional production of aviation, power generation and other major industries, engineering parts, assembly and supply solutions for engineering companies.
It finished machined the front turbofan blades for two variants of the JSF Joint Strike Fighter from monoblock titanium forgings in 1998.
It has also extensively developed “insert milling” techniques with excellent results.
Steven Waddington, the company’s senior blade machining engineer, says that “the development of insert milling has allowed Callender to speed up its roughing beats by a factor of three to four.”
The company’s most recent attempt at insert milling a 1.3m blade diameter, 43cm long part used 217/220.79 cutters with diameters ranging from 32mm to 100mm.
Integral carbide milling cutters from Jabro Cutting Tools were also used for this project.
The cutting fluids, tool materials, tool geometries and machining process parameters used in the machining of titanium alloys are described below.
Cutting fluid
The use of cutting fluid helps remove heat from the cutting edge and flush away chips, which reduces the cutting force.
Therefore, effective cooling and reasonable application of cutting fluid are key ways to improve productivity.
hey also play an important role in enhancing the surface quality of machined parts.
Generally used cutting fluid has three categories namely water or alkaline water solution water-based soluble oil solution and non-water-soluble oil solution.
To increase the productivity of machining titanium alloys and improve the stability of cutting tools, Russian technicians use halogenated lubricating coolants.
They also pour large quantities of coolant to cool the machined parts.
The application of halogen coolant will lead to the generation of a layer of salt crust on the surface of titanium alloy parts in the rising temperature and stress caused by the combined effect of salt corrosion.
Therefore, the use of the coolant cooling processed parts to be removed after processing the surface thickness of 0.005mm ~ 0.010mm of the tempering pickling.
However, the use of this coolant is not permitted for the machining of titanium alloy parts in the assembly process.
India’s N. Suresh KumarReddy and others have recently developed a solid lubricant for machining.
Whether this lubricant can be introduced into the machining of titanium alloys remains to be traced.
Rodrigo Daun Monici et al. from Brazil used 5% synthetic emulsion and pure oil as cutting fluids, respectively.
They conducted grinding tests using a CBN grinding wheel with these fluids. The results show that the combination of pure oil and the CBN grinding wheel can improve grinding efficiency.
Cutting tool materials
Titanium alloy has low thermal conductivity, low plasticity, and a tendency for work hardening. These characteristics result in high cutting forces and elevated cutting temperatures during machining.
The increased temperature leads to accelerated tool wear and reduced tool life. Therefore, it is important to select tool materials with high hardness and good wear resistance.
Cutting tool materials commonly used in the machining of titanium alloys are as follows.
(1) high-speed steel grade
– Aluminum-containing high-speed steel
W6Mo5Cr4V2Al; – high cobalt high-speed steel W2Mo9Cr4VCo8; – powder metallurgy high-speed steel HRC65-70.
(2) Carbide grade
– YG8, YG8W, YG10H for rough machining; – YG8W, YP15 (YGRM) for fine machining.
3) Coated tools
Coated tools have strong oxidation resistance and anti-bonding properties, thus have good wear resistance and resistance to crescent pit wear. Processing titanium alloy effect is better.
France’s M. Nouari and Indonesia’s A. Ginting using multi-layer CVD coated carbide tools end milling titanium alloy Ti-6242S and accordingly to study the wear characteristics and function of the tool.
4) Cubic Boron Nitride (CBN) Tools
Cubic Boron Nitride (CBN) cutters have very high hardness and thermal hardness.
CBN tools are ideal for high-speed finishing or semi-finishing of hardened steel, cold-hard cast iron, and high-temperature alloys.
They also perform well in the machining of titanium alloys.
Britain’s E. O. Ezugwu and others and Brazil’s Machado has used CBN tools on Ti-6Al-4V alloy for turning test cutting speed up to 250m/min.
Z.G. Wang and others in Singapore developed a new type of tool material called binder-free cubic boron nitride (BCBN).
They used BCBN tools for high-speed milling of the titanium alloy Ti-6Al-4V. These tools maintain a long tool life even at higher milling speeds.
(5) Polycrystalline diamond (PCD) tools
Polycrystalline diamond (PCD) material has high hardness, high wear resistance, high thermal conductivity and low friction number and other characteristics.
PCD tools can realize high-speed, high-precision, high-stability processing of titanium alloy materials. The machining effect of titanium alloy is good.
Geometric parameters of tools
1) Turning tool
Roughing requires good tool rigidity, so the front and rear angles of the tool should be smaller.
Such as rough machining of titanium alloy TC4 cutting force is larger cutting generally do not grind the front angle of the crumbs groove selected for 0 ° ~ 3 ° angle.
In the finishing process, good surface integrity and dimensional accuracy are essential. To achieve this, the tool must be sharp. Therefore, the front angle, rear angle, and helix angle should be larger.
Additionally, using tools with more densely spaced teeth is recommended. The cutting edge should have no chamfer or, at most, a minimal negative chamfer.
Such as finishing TC4 titanium alloy when the front angle should be 8 ° ~ 15 ° at this time can be processed out of the roll chip.
Other turning tools used for machining titanium alloy, such as cylindrical turning tools and boring tools, should follow specific geometry.
The front angle should be between 10° and 15°. The rear angle should range from 8° to 14°.
The tip radius should be between 0.2 mm and 0.6 mm, with the maximum value used for finish turning.
The front angle of the thread turning tool is 0° and the rear angle is 10°. The front angle of the forming cutter is 5° and the rear angle is 10°.
2) Drill
The design of the drill bit should ensure smooth chip discharge. This is important to achieve effective chip removal and cooling.
A larger helix angle of 25° to 30° should be used. The spiral groove of the drill bit requires polished processing. The core thickness should be about one-quarter of the drill bit’s diameter.
In order to provide the drill with sufficient strength and centering effect, the crosscut can be sharpened according to the actual situation.
In order to increase the cutting thickness and cutting width, the top angle of the drill is 135°～140° and the back angle of the drill is 12°～15°.
3) Milling cutter
The design of the milling cutter mainly considers the cutting characteristics of titanium alloy.
Take the end mill as an example, the front angle of 6 ° ~ 8 ° back angle of 6 ° ~ 12 ° helix angle of 35 ° ~ 40 ° helix angle of the front section of 3 ° (in the 1mm ~ 1.2mm on the) tip of the arc radius R for 0.5mm ~ 0.6mm.
4) Tap
Tapping is in a semi-closed state cutting cutting fluid is difficult to reach the cutting zone heat dissipation and lubrication effect is poor.
Due to the small modulus of elasticity of titanium alloy cutting rebound is easy to “hold cone” so that the friction torque increases and cause the tap chipped teeth or twisted.
To solve the above problems, it is necessary to choose the appropriate geometric parameters.
Front angle of 7 ° ~ 10 ° cutting cone part of the back angle of 6 ° ~ 12 ° correction of the tooth surface shovel back angle 1 ° cutting cone angle 5 ° ~ 7 ° 30 ′.
M6 below thread tapping should be used cutting cone 7 ° 30 ′ corrected tooth taps.
5) Reamer
Reamer design mainly consider the characteristics of titanium alloy modulus of elasticity is small reamer back angle of 10 ° front angle of 3 ° ~ 5 ° cutting edge width of 0.15 mm.
Cutting process parameters
1） Turning
Under the condition of continuous cutting, the recommended cutting speed for turning titanium alloy workpiece with hard skin with YG8 turning tool is as follows:
V=15 m/min～28 m/minf=0.25mm/r～0.35mm/rap=1mm～3mm.
Under the condition of continuous cutting, the recommended cutting dosage for precision turning of titanium alloy workpieces with YG3 tool is as follows:
V = 50m/min ~ 70m/min f = 0.1mm / r ~ 0.2mm / rap = 0.3mm ~ 1mm.
Turning of titanium alloy cutting dosage is shown in Table 1.
With YG6X turning tool turning TC4 (hardness of HB320 ~ 360) ap = 1mmf = 0.1mm / r when the best cutting speed of 60mm / min. On this basis, different amount of travel and depth of cut under the cutting speed is shown in Table 2.
(2) drilling
Titanium alloy drilling is more difficult often in the process of burning and broken drill phenomenon.
This is mainly due to poor drill sharpening, untimely chip removal, poor cooling and poor rigidity of the process system and other reasons.
The following should be done when drilling.
-Return the tool frequently and remove the chips in time. Pay attention to the shape and color of the chips.
If the chips appear feathery or the color changes during drilling, it indicates that the drill bit is blunt and should be sharpened in time.
-If necessary, add OLTIP special drilling and tapping oil.
-The drill template should be fixed on the table. The drill template should be guided as close as possible to the working surface.
-When manual feed is used, the drill must not be in the hole without moving in and out, otherwise the drill edge will rub against the machined surface and cause hardening and dulling of the drill.
Drilling example. With molybdenum high-speed steel drill in the TC4 titanium alloy workpiece drilling processing drill diameter D = 6.35mm hole depth H = 12.7mm.
Selected cutting parameters: V = 11.6 m / min f = 0.127 mm / r using emulsion cooling.
Tool durability T to wear width h = 0.38mm for the standard each drill can drill 260 holes with excellent results.
3) Milling
Because milling is an intermittent cutting process, the milling cutter is prone to chipping. As a result, its durability is low.
Additionally, chips can easily bond to the cutter teeth. This bonding can lead to severe chipping or even damage to the cutter teeth.
Shun milling provides a smoother cutting process. The cutting path of the cutter teeth is shorter than that of reverse milling.
Cutting from thick to thin helps reduce the chip bonding phenomenon. It also improves milling cutter wear and enhances tool durability. Additionally, it can reduce surface roughness.
Therefore, when the machine tool and cutter allow, shun milling should be used whenever possible.
Milling titanium alloy cutting dosage is shown in Table 3.
4) Tapping
Titanium alloy tapping priority selection of a jump in place of the number of teeth of the tap should be less than the standard tap is generally 2 to 3 teeth.
Cutting taper angle should be large taper part of the general 3 to 4 buckle thread length.
In order to facilitate chip removal can also be ground in the cutting cone part of the negative inclination.
Try to use a short tap to increase the rigidity of the tap.
The inverted cone part of the tap should be appropriately larger than the standard to reduce the friction between the tap and the workpiece.
When machining the bottom hole of the thread, first rough drill and then ream the hole with a reaming drill to reduce the work hardening of the bottom hole.
For the pitch of 0.7mm ~ 1.5mm threaded hole size can be processed to the national standard provisions of the standard threaded hole on the difference and allow to increase 0.1mm.
If not subject to the location of the screw hole and the shape of the workpiece as far as possible to use the machine tapping to avoid manual tapping feed uneven stops and processing hardening caused by.
5) Grinding
A common problem in titanium alloy grinding is the clogging of the grinding wheel and burns on the surface of the parts due to sticky chips.
The reason is that titanium alloy has poor thermal conductivity. This causes the grinding zone to generate high temperatures.
As a result, bonding, diffusion, and strong chemical reactions occur between the titanium alloy and the abrasive.
As a result, the surface of the workpiece is burned and the fatigue strength of the part is reduced. The following measures were taken to solve this problem:
Grinding wheel material: green silicon carbide TL wheel hardness: ZR1 wheel particle size: 60 wheel speed: 10m / s ~ 20m / s slightly smaller feed with emulsion sufficient cooling.
Pin parts to be in 600 ℃ ~ 650 ℃ vacuum furnace annealing stress relief.
Parts machined surface residual stress treatment
After machining the surface of titanium alloy parts generated stress the size of this stress is related to the cutting and processing conditions.
Stress relief annealing treatment is conducted at temperatures of 400 ℃, 450 ℃, 500 ℃, 550 ℃, 600 ℃, and 650 ℃.
The heating and holding times are 5 min, 15 min, 30 min, 45 min, 60 min, 120 min, and 240 min, respectively.
After heating, the parts are air-cooled to complete stress relief annealing. To avoid hydrogen absorption during annealing, titanium parts should be treated in a vacuum furnace.
This ensures both stress relief and hydrogen removal. The annealing of titanium alloy parts must also be performed in a vacuum furnace to prevent hydrogen absorption.
Conclusion
From the above discussion, we draw the following conclusions.
(1) cutting and machining of titanium alloy parts require good heat dissipation of tool materials and good resistance to high temperature performance;
(2) Cutting and machining titanium alloy parts require reasonable tool geometry parameters;
(3) cutting titanium alloy parts must strictly control the cutting amount;
(4) cutting titanium alloy parts must be sufficient cooling;
(5) machining of finished parts to be 600 ℃ ~ 650 ℃ in the vacuum furnace annealing to eliminate stress.