Titanium alloy high efficiency milling technology

Milling titanium alloy parts with other difficult materials in common is due to a small increase cutting speed results in faster tool wear and the cutting edge.
The difference is that due to the high strength and high viscosity of the titanium alloy, it is easier to generate and accumulate heat in the cutting zone during cutting, and the thermal conductivity is poor, which may cause burning when milling with a large amount of cutting. This is why milling of titanium alloy parts must not be able to choose high cutting speeds.
However, the speed of processing titanium alloy parts can be improved. That is, when the cutting speed is kept constant, the processing speed of the parts is improved by increasing the metal removal rate. Achieving this goal does not include the use of more powerful or high-end machines, but rather tools that take full advantage of the cutting capabilities of existing machines. It also compensates for certain deficiencies in the machine, such as poor rigidity.
Kennametal is a leading tool manufacturer specializing in the experimental research of titanium milling processes. In the company, Mr. Brian Hoefler, a technical consultant and milling product manager who has received many consulting users of titanium milling technology. This article highlights his extensive experience in titanium milling.
Why is the milling of titanium alloys attracting special attention? There are at least two reasons. First, titanium alloys are mainly used for high-end parts, not only for the manufacture of aircraft fuselage and engine parts, but also for the manufacture of many parts in medical devices. Especially for some of the growing American manufacturing companies, they must shift to high-end products, and often encounter technical problems in the milling of titanium alloy parts.
Another reason is that not every workshop can achieve high feed rate machining. Therefore, when the material is difficult to process in titanium milling or the cutting speed is not high during the machining process, it is urgent to achieve high efficiency through machining. The problem solved has caused the manufacturer to attach great importance to it.
Use high toughness tools
The correct choice of cutting tool material will be the first important issue in the efficient milling of titanium alloys, Mr. Hoefler said. Carbide tools can be the right choice, and machine shops are often accustomed to using cemented carbide as the best cutting tool material, especially in almost all difficult machining applications. For titanium alloy processing, a new generation of high speed steel will be a good alternative to cemented carbide.
It is reasonable to say that cemented carbide tools with good wear resistance can achieve high cutting speeds at reasonable processing costs. However, this reasonable processing cost is based on the "very high toughness" that the tool must have or the ability to withstand impact and resist fracture. Unfortunately, the hard alloys that are commonly used are much more brittle than high-speed steel.
This is of great importance in milling titanium alloys. In general, the main cause of failure of cemented carbide tools is not the wear of the cutting edge, but the fracture of the blade. Secondly, the increase in cutting heat during milling of titanium alloys also makes it impossible for cemented carbide tools to perform at high cutting speeds. Because it is processed at high cutting speed, a large amount of coolant needs to be added. Under this alternating action of heat and cold, a strong thermal shock is generated between the tool and the workpiece, which will quickly cause the cutting edge of the carbide tool with high brittleness. broken. The above two technical problems need to be solved by the inherent high toughness of the tool itself. Or ordinary carbide tools are far from competent. The cutting test proves that using a high-toughness tool, such as milling a titanium alloy workpiece with a high-speed steel tool, there is no need to worry about causing impact during cutting and cracking of the cutting edge. Especially for machining on smaller rigid machines, high toughness high speed steel tools can achieve high metal cutting rates by increasing the depth of cut rather than increasing the cutting speed.
Not only that, but also a wide range of high toughness high speed steel tool materials are available for users to choose from. Most workshops don't all know this. They also don't know that high-speed steel knives sold on the market can be subjected to special treatment procedures, such as high-speed steel smelting (such as increasing cobalt content) to increase the composition of certain elements for heat treatment (multiple stage quenching and tempering), or The high-speed steel material is strictly controlled by its manufacturing process to form a powder metallurgy high-speed steel with uniform metallographic structure. Therefore, expensive high-cobalt high-speed steel and powder metallurgy high-speed steel are ideal tool materials for efficient milling of titanium alloys. [next]
High cutting temperature control
Carbide tools can sometimes be used, and titanium alloy parts can be cut with a small radial incision method to achieve amazingly high speeds (see section "10% and 100%"). In these cuttings, the tool not only solves the problem of wear resistance under normal conditions, but also solves the problem of the wear resistance of the tool at high cutting temperatures. This is very important, and it is necessary to use a coated carbide tool for processing.
According to Mr. Hoefler introduced, titanium aluminum nitride (of TiAlN) coated cemented carbide tools, for machining titanium is usually the best choice. Among many basic tool coating types, TiAlN has a good effect on maintaining the overall mechanical properties of the tool and maintaining the high temperature cutting performance of the tool as the temperature increases. In fact, high cutting temperatures also provide some protection for the coating. The aluminum molecules are released from the coating by the processing energy in the cutting process, forming an aluminum oxide protective layer on the surface of the tool. This layer of alumina protective layer reduces heat transfer and chemical element diffusion between the tool and the workpiece. At the same time, in addition to the formation of this protective coating, more aluminum molecules are continuously added to keep the chemical reaction forming the protective layer of alumina continued (see section "New Aluminum-rich Coatings").
However, TiAlN coatings are not suitable for applications where vibration is strong. At this time, titanium carbonitride (TiCN) is used, which prevents the coating from peeling off due to vibration. “When you use a replaceable blade and a powerful cutting machine on a less rigid machine, trying TiCN is probably the best option,” Mr. Hoefler said.
More cutting edges participate in cutting
Even if the cutting speed during cutting, the feed per tooth and the depth of cutting of the milling cutter remain unchanged, production efficiency can sometimes be improved. The solution here is to have more cutting edges participate in the cutting.
For example, for a helical milling cutter, choose a small pitch tool (such as a spiral corn end mill) as much as possible. The use of this tool enables high speed steel knives to have more cutting edges. The former is more widely used because it can provide more cutting edges than carbide tools.
Another way to get more cutting edges into the cut is to mill in different directions. Through the "Plunge Milling Roughing" (sometimes referred to as Drilling Rough Cut) method, a set of milling cutters is used, as if drilling along the Z-axis, and the end and side teeth of the tool are in accordance with the assembled machining program. Perform lap processing. Therefore, the production efficiency is high and the chip removal is convenient.
This method can only be used for roughing because there are still some scalloped raw metal left between every two lap joints. However, since the cutting edge roughing has many cutting edges to participate in the cutting, the feed rate per minute can be greatly improved when the feed amount per tooth of the tool is kept constant. Furthermore, the Z-axis feed for the roughing of the plunge milling has the advantage of being able to take advantage of the high rigidity of the machine, since the various connection mechanisms along the main shaft (such as the toolholder interface) are bound to produce along the X or Y axis. Flexing produces compression in the Z-axis direction, which gives the machine a high stiffness along the Z-axis. This means that the feed per tooth of the tool can be increased.
Mr. Hoefler said, “Plunge milling is the best solution for efficient machining of high-strength metals. It is recommended to use this machining solution in titanium milling.”
Vibration elimination measures
It is also important to study the cause of the deflection of the tool during cutting and to eliminate the problem, because it will lead to a very important technical problem - vibration. Vibration in titanium alloy milling, there are two disadvantages: First, the generation and increase of cutting force, will trigger and increase vibration; on the other hand, the spindle speed of the machine tool seems to be independent of vibration, so can not find An "ideal" speed that tuned the vibration.
In fact, vibration determines the productivity of most titanium milling operations. A large number of cutting tests have proved that in the titanium alloy milling process, the maximum metal cutting rate is obtained not at the maximum power output of the machine tool, but at the beginning of the great vibration. This is why it is necessary to establish and also establish a time to control the vibration program. Mr. Hoefler suggested that to improve the productivity of titanium milling, you must also pay attention to the following technical problems: [next]
The connection between the stiffness tool and the toolholder, the coupling between the toolholder and the spindle must be such that it is as rigid as possible. For the tool holder, the thermal expansion and contraction type provides the best solution. For the spindle, the HSK quick change tool holder provides the best stiffness compared to the conventional taper interface.
Damping design the tool with an eccentric relief angle or a “edge” tip structure that provides good damping to suppress vibrations generated during cutting. When the tool is flexed, the knives with the eccentric relief angle will contact and rub against the workpiece. Not all materials are better able to rub against the workpiece, and the aluminum alloy has a tendency to adhere. For titanium milling, the “edge” sharpened on the cutting edge of the tool also acts as a good shock absorber. Changing the flute space between the cutting edges For many types of tool design and anti-vibration measures, many workshops may not be familiar. When the tool rotates at a high speed, the cutting edge regularly strikes the workpiece, thereby generating vibration. If the chip flute space of the milling cutter is designed to be irregularly arranged, the cutting test proves that it will have a good damping effect. For example, when the first and second cutting edges of the milling cutter are separated by 72°, the second and third cutting edges should be separated by 68°, and the third and fourth cutting edges should be separated by 75°, which is unevenly distributed. . A patented anti-vibration measure designed by Kennametal is designed to achieve a good damping effect by designing the cutting edges of the milling cutters into unequal axial rake angles.
New aluminum-rich coating
The "Al" molecule is the most active in the TiAlN coating and has a large effect on the cutting performance of the coated tool. It forms an aluminum oxide protective film on the surface of the tool. In the coating, the content of the "Al" molecule is increased to make this effect more effective.
Of course, thanks to the continuously improved vapor deposition process for the production of coatings, the content of "Al" molecules in TiAlN continues to increase, resulting in a newly formed TiAlN coating without sacrificing toughness. , which greatly improves the red hardness of the coating (tool). Kennametal has developed this new aluminum-rich TiAlN coated tool in the first half of this year.
10% and 100%
At present, some of the more advanced workshops have been able to use carbide coated tools to cut titanium alloy parts by a small radial cutting method. The main purpose is to solve the technical problem of high cutting temperature generated in titanium alloy processing. The cutting principle is to use a small radial incision cutting process to select a radial cutting depth that is much smaller than the radius of the tool for radial cutting. Since the cutting depth is chosen to be small, the cutting speed can be greatly increased. As a result, the cutting time of each cutting edge is greatly reduced, that is, the machining time of the cutting edge is reduced, and the non-cutting time is extended, that is, the cutting edge is increased. The cooling time is excellent for controlling the cutting temperature.
According to Mr. Brian Hoefler of Kennametal, the use of small radial cutting methods for cutting titanium alloy parts provides excellent control of cutting temperatures and high speed machining. Small radial depth of cut does not result in high metal removal rates, but this method can be used in the factory to improve machining accuracy.
The cutting test conducted by Mr. Hoefler proved that in the milling of titanium alloy parts, the machining with small radial cutting method will follow the following rules:
When the radial depth of cut is less than 25% of the diameter, the cutting speed (sfm) of 50% can be increased, generally exceeding the rated speed for heavy cutting.
When the radial depth of cut is less than 10% of the diameter, the cutting speed (sfm) can be increased by 100%.

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