티타늄 합금 부품의 CNC 터닝

자동차용 티타늄 합금 부품의 CNC 터닝

하지만, CNC turning processing of titanium alloy parts is very difficult, making mechanical technologists daunting. They believe that the super performance of titanium alloy greatly weakens its machinable “ability”, making its CNC machining extremely challenging.

Although this view is reasonable, it is not comprehensive. This article discusses the turning strategy of titanium alloys, which will be helpful for mechanical technologists to process such a wide range of difficult-to-machine materials and to apply new cutting techniques.

Titanium alloy producer Bill Headland, senior project expert at RTI International Metals, pointed out that although many processing shops regard titanium alloy parts processing as a daunting way. But in fact, titanium alloys include a wide range of varieties, and you must know which grade you are processing. There are many grades of titanium alloy, some grades are extremely difficult to CNC machining, while others are relatively easy to CNC machining. Commercial pure titanium (CP grade) is a non-alloy material commonly used in the manufacture of medical parts, heat exchangers and spectacle frames. CP grades have excellent corrosion resistance and are relatively easy to process. But compared with other titanium alloys, its strength is very low, and it is sticky and soft.

After adding an alloy to pure titanium, its crystal phase (crystal structure) is changed, and the characteristics and machinability of the material are also changed. Alpha titanium alloys and quasi-alpha titanium alloys contain additives such as nickel, 알류미늄, and vanadium. The CNC machinability of these intermediate grade titanium alloys is quite good. α-β titanium alloy grades may contain more aluminum and vanadium. The mainstream industrial titanium alloy Ti6Al4V is an α-β titanium alloy grade, which contains about 6% aluminum and 4% vanadium. Ti6Al4V and its variants account for about 50%-70% of the titanium alloys currently used.

Beta-grade titanium alloys with iron and chromium added are one of the most difficult grades to process. Due to its high fracture toughness and excellent resistance to high cycle fatigue, the machinability of the β grade is similar to that of Hastelloy nickel-based alloys and similar materials. A typical application example is the manufacture of light springs, which are used to trigger the foldable tail of tactical missiles launched underwater.

Various titanium alloys show different CNC turning performance. Some people believe that the time required to process a Ti6Al4V workpiece is usually three times that of CNC machining a steel part;
Some people say that CNC machining a Ti5553 difficult-to-machine β brand workpiece takes twice as long as machining a Ti6Al4V workpiece.

In turning titanium processing, the most important characteristic of titanium alloys is poor thermal conductivity. Because the high temperature generated during turning is difficult to be absorbed by the workpiece and concentrated on the cutting edge of the tool, excessive heat promotes a chemical reaction between the cutting edge and the chips and produces crescent wear.

Titanium alloys also have a tendency to work hardening, so it is important to remove titanium by shearing rather than extrusion. 게다가, although titanium alloys have high strength, they also have a low modulus of elasticity. This means that compared with other materials, titanium alloys are relatively more elastic and easier to leave the cutting edge (especially when cutting with light loads). Considering these characteristics of titanium alloys comprehensively, in order to successfully realize the turning of titanium alloys, the key is to achieve the balance of cutting speed, feed rate and cutting depth.

의료용 티타늄 합금 부품의 CNC 터닝

CNC cutting speed is the primary factor affecting cutting heat generation. Stefan Gyllengahm, a turning expert at Sandvik Coromant, spent three and a half years developing tool grades for tool manufacturers. 이 기간 동안, he conducted Ti6Al4V and TC4 cutting tests in the laboratory. The results showed that the choice of cutting speed must be very careful:
In some cases, when the cutting speed is increased by 10%-15%, the tool life will be reduced from 40 pieces for CNC machining to 6 조각, which indicates that the adjustment range of the cutting speed is too large. He also found that when turning at a cutting speed that does not shorten the life of the tool, if the feed rate is increased, a critical temperature that will impair the life of the tool is reached. Because there is a limit range where there is too much heat.

Tool geometry plays a key role in controlling chip shape to dissipate heat. The wider and thinner chip enlarges the contact area between the forming chip and the cutting edge, thereby reducing the accumulation of heat on the cutting edge. If the chip is thinner and generates less heat, the cutting speed can be faster.

예를 들어, when using a C-type (80°) diamond insert with a standard lead angle of -5° for rough machining, the chip thickness and the feed rate are approximately equal; The use of a square blade with a 45° lead angle allows the cut metal (and cutting heat) to spread along the longer cutting edge. The round blade in theory takes this concept to its extreme (but only when the depth of cut is small). Gyllengahm said that usually when the depth of cut is small, a round blade can be used to obtain very thin chips.
하지만, considering that the effective lead angle of a round blade with an inscribed circle of 12.7mm will decrease when the cutting depth is greater than 2mm, it is better to use a square blade. Because at the same depth of cut, the square blade still has a 45° lead angle.

Bill Skoretz is the manager of the CNC machining division of Patriot Forge. The company provides various raw materials ranging from low-alloy steel and stainless steel to special grades of aluminum alloy and titanium alloy. Sometimes it also CNC machining titanium alloy parts for the company’s customers. When discussing the challenges of turning titanium alloys, he focused on reviewing the experience of coolant titanium alloy nozzles used in turning rolling mills. This is a corrosion-resistant titanium alloy grade. Due to the resilience of titanium alloy, a positive tool geometry angle must be used, and attention must be paid to the shape of the tool head. If the bottom or rake angle of the tool is too small, the tool will begin to generate tension, which will cause many problems. 그러므로, the best balance must be found between the positive geometric angle of the tool head and the support for the cutting edge.

Skoretz described the development status of CNC cutting tools from a historical perspective. Before the development of cemented carbide tools, high-speed steel tools were mainly used to cut steel. The emergence of cemented carbide tools makes it possible to adopt a positive geometric angle, but the machine tool must have sufficient power. The negative rake angle blade can only fold or squeeze the titanium alloy material, but it is difficult to remove it. But he also warned that if the front angle of the blade is too large, it will also cause tension on the titanium alloy material. 그러므로, a balance must be found between compressive stress and tensile stress. He mentioned that he had used inserts with 0.1mm or 0.13mm T-shaped land on the cutting edge in the past. “Mainly for the safety of the blade, a cutting edge that is too sharp cannot be used because it will not last long.” He also uses oil-based cutting fluids in processing, but mainly uses its lubricating properties rather than cooling capabilities.

Other processing workshops also have different titanium alloy turning methods, because there are various solutions for any material removal processing.

My company has 30% of the business is processing car parts, many of which are expensive titanium parts. Because the top racing teams are willing to pay extra for light-weight and high-strength parts to keep their cars at the minimum weight allowed by the rules. But at the same time, it can still maintain control over the distribution of its entire body weight. 예를 들어, reducing the mass of rotating parts and non-suspension parts (such as wheels and brake components) can effectively improve acceleration and handling performance. The titanium alloy parts processed by our company include parts that run between the brake caliper and the rotor.

Rayco president Greg Cox also stated that in order to successfully CNC turning titanium alloys, a balanced approach is needed. He believes that when selecting cutting parameters, it is important not to squeeze the titanium workpiece material during processing, otherwise it is easy to produce work hardening and cause great troubles in processing. The processing parameters usually used by Rayco are: cutting speed 120sfm, feed amount 0.13-0.20mm.

The depth of cut is also important. Similarly, a moderate depth of cut is the best choice. The maximum depth of cut used by Rayco is 0.8-1mm. Rayco’s titanium alloy parts can be processed in batches as high as 200 조각, but most of them are between 5-20 조각. Cox said that Rayco is also continuing to improve the process, but must be careful when it comes to cutting parameters, because titanium alloys are quite expensive, so as not to cause parts to be scrapped. The price of titanium alloys has risen rapidly, from US$47/lb to US$68/lb last year. The high price of titanium alloy has also tightened the inventory of workpiece materials.

Scott Holland, general manager of the R&D and manufacturing branch of diving equipment manufacturer Atomic Aquatics, said: When processing underwater breathing apparatus with titanium alloy, “we always try to continuously improve processing efficiency to shorten processing time, process more workpieces and extend tool life.” But when they tried to machine more parts, the tool suddenly broke. 그러므로, Holland hopes to reach an optimal balance point, but this balance is not just numbers and procedures. Holland has nearly ten years of experience in titanium alloy processing. He also relies on observing the shape of workpieces and tools and listening to cutting noise to master this balance. Holland said that processing titanium alloys can also be relatively simple. “If you use sharp tools and change the tools according to your estimated time, you can only process titanium alloys within a limited range. You can try it your own way, but it doesn’t necessarily work. Titanium alloy processing has certain rules, if you master these rules, you can be handy. ”

The increasing use of titanium alloys has promoted the development of cutting technology, which focuses on effectively turning titanium alloys. Sandvik processing management and technology emphasized that when turning titanium alloys, chemical wear is the main tool failure mechanism, and high temperature cutting accelerates chemical wear.

When the hot chips scratch the rake face of the tool, they actually “pulls” the cobalt from the blade. Mills introduced a two-step method to reduce the cutting temperature: The first step is to use tool geometry design (such as square or round blades with a 45° lead angle) to reduce the chip thickness to reduce the cutting temperature and reduce the crater wear caused by this. 마지막으로, a higher feed rate can be used and tool life can be extended; The second step is to use a high-pressure coolant with a special shape. This coolant not only has a high pressure, but also presents a very precise laminar jet pattern, which can form a “water wedge” between the chips and the rake face of the blade to hold up the chips. Minimize the contact with the rake face, so as to prevent the tool from crater wear.

Sandvik’s Jetbreak cooling system has a pressure of 1000-3000 psi, a nozzle with a diameter of 1.27 mm and a standard emulsion coolant. It not only has a cooling effect, but also can generate lift to hold the chips, which can reduce the friction and temperature between the chips and the rake face. Using this system can increase the cutting speed by 50%.

Mills described the effect of combining the above two cooling strategies: Using CNMG inserts with -5° lead angle to optimize cutting parameters (절단 속도: 40m/분, feed rate: 0.3mm/회전) when machining titanium alloys, the tool life is about 20 분; When machining with round inserts or square inserts with 45° lead angle, the cutting speed can be increased to 50-60m/min, the feed rate can be increased to 0.4mm/rev, and similar tool life can be maintained. Since more workpiece material can be removed in the same time, the productivity can be doubled by using only the square blade. If the high-pressure cooling system is used again, the cutting speed can be increased by another 50%.

The high-pressure coolant must be transported through the channel inside the machine spindle (not the external pipe). The coolant flows through a special connector on the Sandvik Capto quick-change tool chuck, and Capto controls its pressure. When installing the machine tool, you can easily install this high-pressure cooling system.

For workshops that often process titanium alloy parts (especially expensive large aerospace parts), 그만큼 50% increase in productivity is worth their investment in special tool chucks and machine tools equipped with high-pressure cooling systems. This high-pressure cooling system has a unique advantage when turning titanium alloys, because it will not produce crescent wear like CNC machining other workpiece materials.