Milling and cutting of titanium alloy materials and its advantages

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In many aerospace applications, titanium and its alloys are replacing traditional aluminum alloys. Today, the aerospace industry consumes about 42 per cent of global production of titanium, and demand for Titanium is expected to continue to grow at double-digit rates between now and 2010. The new generation of aircraft needs to make full use of the performance provided by titanium alloys. Both commercial and military aircraft markets are promoting the demand for titanium alloys. New models such as the Boeing 787, Airbus A380, F-22 Raptor Fighter, and F-35 Joint Attack Fighter(also known as Lightning II) all use a large amount of titanium alloy material.

Advantages of Titanium Alloy Materials
Titanium alloy has high strength, high fracture toughness and good corrosion resistance and weldability. With the increasing use of composite structures in aircraft fuselage, the proportion of titanium-based materials used in fuselage will also increase, because titanium and composite materials are far better than aluminum alloys. For example: Compared with aluminum alloy, titanium alloy can increase the life of the fuselage structure by 60 %.
The extremely high intensite/density ratio of titanium alloys(up to 20:1, that is, weight can be reduced by 20 %) provides a solution for reducing the weight of large components(this is the main challenge for aircraft designers). In addition, the inherent high corrosion resistance of titanium alloys(compared with steel) can save the cost of daily operation and maintenance of aircraft.
Need for greater processing capacity
Because it is more difficult to process than ordinary alloy steel, titanium alloys are generally considered to be difficult to process materials. The metal removal rate of a typical titanium alloy is only about 25 % of that of most ordinary steel or stainless steel, so it takes about four times as long to process a titanium alloy workpiece.
In order to meet the increasing demand for titanium alloy processing in the aviation manufacturing industry, manufacturers need to increase their production capacity and therefore need to better understand the effectiveness of titanium alloy processing strategies. The processing of typical titanium alloy workpieces starts from forging until 80 % of the material is removed to obtain the final workpiece shape.
With the rapid growth of the aviation parts market, manufacturers have felt overwhelmed. In addition, the increased processing demand due to the low processing efficiency of titanium alloy workpieces has led to a significant tension in the processing capacity of titanium alloy. Some leading companies in the aviation manufacturing industry even openly question whether the existing machining capabilities can complete the processing tasks of all new titanium alloy workpieces. Since these workpieces are usually made of new alloys, it is necessary to change the processing methods and tool materials.
Titanium alloy Ti-6Al-4V
Titanium alloys have three different structural forms: α-titanium alloys, α-βtitanium alloys, and β-titanium alloys. Commercial pure titanium and α-titanium alloys can not be heat-treated, but usually have good weldability; Alpha-β-titanium alloys can be heat-treated, and most of them are also weldable; Beta and quasi-β-titanium alloys can be completely heat-treated and are generally weldable.
Most common α-β-titanium alloys used in turbine engines and fuselage components are Ti-6Al-4V(Allvac Ti-6 -4, Ti-6 -4). Ti-6 is used in this article to represent ATI Allvac. Titanium alloys, The company is a major supplier of titanium alloys(recently signed a $2.5 billion long-term supply contract with Boeing for titanium alloys). In addition, ATI Stellram, which cooperates with ATI Allvac to develop processing solutions, also uses these titanium alloy codes to describe processing requirements.
Ti-6 has excellent strength, fracture toughness and fatigue resistance, and can be made into various product forms. Ti-6 in the deactivated state can be widely used in structural parts. Through minor changes in chemical composition and different thermal mechanical treatment processes, Ti-6-4 can be used to produce components for various uses.
Titanium alloy Ti-5Al-5V-5Mo-3Cr
The Ti-5Al-5V-5Mo-3Cr(Ti-5-5-5-3) is a new titanium alloy with market influence. Compared with β-titanium alloys and α-β-titanium alloys, this quasi-β-titanium alloy can provide the fatigue fracture toughness required in applications that require higher tensile strength.
Compared with traditional titanium alloys(such as Ti-6 and Ti-10 -2 -3), The Ti-5-5-5-3's malleable shape, heat treatment and final tensile strength of up to 180 ksi(thousands of pounds per square inch) make it the most promising material for the manufacture of aircraft advanced components and landing devices.
Ti-5 -5 -3 can obtain excellent mechanical properties by dissolving heat treatment below the β-transition temperature or annealing above the β-transition temperature while appropriately controlling the grain size and precipitation in the microstructure. The β-transition temperature is the specific temperature of the compound, at which the alloy changes from an α-β microstructure to a full β-microstructure.
The change of chemical properties and microstructure makes it possible to obtain a wide range of performance combinations of titanium alloys, and thus it is widely used in aerospace components. The processing difficulty of Ti-5-5-5-3 has increased by about 30 % compared to Ti-6-4. Therefore, parts manufacturers applying this new alloy are committed to developing corresponding processing processes that do not shorten the tool life and do not extend the production cycle.
When processing titanium alloy, material hardness is a key factor. If the hardness value is too low(<UNK> 38HRC, titanium alloy will become sticky, cutting edge is easy to produce accumulation tumor. The titanium alloy with higher hardness value(> 38HRC) will wear off the tool material and wear the cutting edge. Therefore, the correct selection of machining speed, feed and cutting tool is crucial.


Requirements for cutting tools
In order to meet the requirements of production costs, processing quality and scheduled delivery, new workpiece materials and spare parts designs have increased the pressure on aviation parts manufacturers. The processing of these new materials has changed the requirements for cutting tools. Improving the metal removal rate, tool life, product quality, and the unbreakable life of the tool can be crucial for efficient and safe processing. "Hard to process" is a relative concept. Through the correct combination of cutting tools and processing parameters, efficient productivity can also be achieved.

In processing aero-grade titanium alloy workpieces, cutting tool manufacturers use methods such as increasing matrix density, designing special tool geometry, adopting accurate cutting edge grinding techniques, and developing new coating technologies to control cutting heat generated by the knife-work interface., The performance of the tool is greatly improved.
In milling process, one of the important characteristics of titanium alloy is very poor thermal conductivity. Due to the high strength and low thermal conductivity of titanium alloy materials, extremely high cutting heat can be generated during processing(up to 1200 °C if not controlled). Heat is not discharged with the cuttings or absorbed by the workpiece, but is concentrated on the cutting edge. Such high heat will greatly shorten the tool life.
With special processing technology, it is possible to improve the tool performance and life(using the correct processing technology to control the temperature, the temperature can be reduced to 250-300 °C).
Reduce heat generation
Cutting heat can be controlled by reducing the radial and axial engagement of the cutter and workpiece. For titanium alloys, the adjustment period for velocity, feed, radial, and axial joints is very short before the accumulation tumor is generated due to overheating. In order to achieve an appropriate tool life, only a maximum of 15 % of the "joint arc length" is required for the processing of titanium alloys. In contrast, the joint arc length is 50 % to 100 % when processing ordinary steel. Reducing the contact arc length can increase the cutting speed and improve the metal removal rate without losing the tool life.
The use of a cutting tool with a cutting angle of 45 ° or a thinning chip can increase the contact length of the cutting edge and the cutting chip, thereby reducing the local high temperature and extending the cutting edge life, while also allowing higher cutting speed.
Geometrical Design of Blade
When cutting titanium alloy, it is very important to use the circumferential grinding blade to minimize the cutting pressure and friction with the finished surface. The blade geometry must have a positive angle, but this is not enough to ensure the best performance. If a small initial angle of higher strength is used to enhance the first part of the cutting edge, then the use of a larger secondary angle(to obtain a larger positive and inverted edge) is the best geometric design for enhancing the pressure resistance of the blade and extending the tool life. In addition, minor passivation also helps to protect the cutting edge, but passivation dimensions must be coordinated with the cutting process and maintain strict tolerances. When processing titanium alloy, it is necessary to use sharp cutting edge to cut materials, but cutting edge too sharp can easily lead to collapse edge and shorten tool life. Proper passivation can protect the cutting edge from premature collapse. The correct geometrical parameters of the blade can reduce the stress and pressure on the tool material, so that the tool can obtain a longer life and improve the processing efficiency.
The cutting angle of the cutter body and blade must be positive to achieve progressive cutting effect and to avoid cutting the entire cutting edge impact and can not obtain the desired shear effect. If this is not done, the workpiece structure may be deformed, making processing impossible.
Concave milling and screw interpolation milling
In concave milling and spiral interpolation milling, internal cooling tools must be used, and if possible, a constant pressure coolant should be used, which is particularly important for deep concave cavity or deep hole processing.
When deep concave cavities are processed, the high density cemented carbide lengthening tool with a modular cutting head can improve rigidity and reduce deflection deformation, and obtain the best machining effect.
The function of the coolant is to remove the chip from the cutting area to avoid secondary cutting that may cause early failure of the tool. At the same time, the coolant also helps to reduce the temperature of the cutting edge, reduce the geometric deformation of the workpiece, and extend the tool life.
Spiral interpolation of milling holes with milling cutter can reduce the use of other tools(such as drills, etc.) in the cutter library. A diameter milling cutter can be used to process different sizes of aperture.
With the increasing application of titanium alloy in aerospace industry, the cutting technology supporting the high efficiency processing of titanium alloy is also developing continuously. The workshop or manufacturer that uses the most efficient processing technology will benefit first due to the large demand for the processing capacity of titanium alloy parts.
Internal integration produces new solutions
The combination of Allegheny Technologies, a multi-domain manufacturer whose business includes both metal smelting and metal cutting, gives the company an advantage in developing new methods of processing advanced materials such as titanium alloys.
ATI Stellram is a business unit of ATI Metalworking Products, a subsidiary of Allegheny Technologies. It is responsible for testing the processing performance of all new materials developed by ATI Allvac to determine the best blade design, tool geometry, Matrix and coating structure, and cutting parameters. This enables these new materials to be processed economically and efficiently before they are publicly available for sale. In addition, as a representative of Allvac, Stellram is a major aviation manufacturer and a leading supplier of aerospace machinery parts that can meet the common needs of both workpiece materials and cutting tools.
The comprehensive understanding of the inherent structure of materials gives ATI Stellram an advantage in the design of unique formulas for tool bases. One of its achievements is X-Graade technology, which ATI Stellram stated has proved to be a reliable solution for processing difficult materials. Through the research and development of X-Graade technology, a new hard alloy brand has been created that can effectively cut difficult materials at extremely high metal removal rates under unstable processing conditions.
X-Graade Blade Technology(Matrix and Coating)
The X-Graade blade uses a ruthenium / cobalt alloy matrix that can resist the generation and expansion of thermal cracks and achieve high metal removal rates. The matrix has a strong crystal binding matrix structure, which improves the toughness of the cutting edge. According to ATI Stellram, this matrix material combines with new tool geometry and coating to provide excellent tool combinations for processing aerospace alloys. The use of X-Graade blades can be achieved: 1. The metal removal rate is doubled; 2 The tool life has increased to three times; 3 The finish of the processing surface increased by 30 %.
The available X-Graade blades include three brands(X400, X500, and X700), each of which is designed for specific difficult cutting processes. They can use a standard blade type, and most of them can be installed in the blade groove of the standard blade body. However, ATI Stellram said that the best solution is to use specially designed tools to optimize the performance of the X-Graade blade. The tool slot design of these tools can achieve maximum sawdust, reinforced groove and optimal cooling. The two types of knives in this series include: 1 7710VR anti-turning button milling cutter: a patented locking system with round blades and preventing blade displacement during large feed cutting; 2 7792VX high feed milling cutter: Compared with traditional tools, the metal removal rate can be increased by 1 times. In addition to the surface milling by Gaojin, the 7792VX series of tools can also be used for milling cavities, milling slots, and milling. Since the cutting force is directly transmitted to the spindle along the shaft, the friction of the spindle can be reduced and the cutting stability can be improved.
Case study of aerospace titanium alloy parts processing
The following are two examples of the processing of aerospace titanium alloy parts using ATI Stellram tools and X-Graade blades.
(1) Examples of processing 1
Processed Parts: Titanium Alloy Overlay(Military)
Workpiece material: Ti-6Al-4V(Allvac Ti-6 -4 alloy)
Workpiece size: 110 "x 18"
Processing description: The ATI Stellram 7792VX high advance milling cutter with XDLT-D41 convertible blade was used to process the milling cutter, and the tool life of rough milling processing reached 156 minutes.
Milling cutter: C7792VXD12-A3.00Z5R; Number of slots: 5
Blade: XDLT120508ER-D41; Brand: X500(using X-Graade technology