This article mainly introduces the development, research status and future development trend of ultrasonic machining technology from the perspective of industrial application needs.
Ultrasonic machining technology is a special machining technology for difficult-to-process materials (hard brittle materials, composite materials, difficult-to-process metal materials, etc.), which has broad application prospects in aerospace, automotive, semiconductor, 3C, medical and other fields.
Ultrasonic machining technology can realize the precise removal of difficult-to-machine materials through ultrasonic vibration energy.
In recent years, with the joint efforts of many universities, research institutes and enterprises at home and abroad, ultrasonic machining technology has developed rapidly, and a large number of typical applications have been realized in the precision machining scene of many kinds of difficult machining materials.
As the representative of special machining technology in advanced manufacturing technology, ultrasonic machining technology will provide important support for the improvement of the manufacturing level.
In recent years, advanced engineering materials, such as titanium alloys, superalloys, engineering ceramics, ceramic matrix composites and honeycomb composites, have been emerging in aerospace, automotive, semiconductor, 3C, medical and other manufacturing fields.
These materials have excellent performance, but their machinability is poor.
They are typical hard-to-machine materials.
There are some bottlenecks in precision machining these materials with traditional mechanical manufacturing technology.
A new manufacturing technology, ultrasonic machining (UM), has received more and more attention and been widely used.
Ultrasonic machining technology is a process technology that realizes the precise removal of difficult machining materials through ultrasonic vibration energy.
This technology focuses the ultrasonic vibration energy on the working area of the tool through a series of structural transmission and transformation, thus forming the impact removal effect of the material to be cut, and thus improving the machinability of many difficult machining materials.
This technology has many advantages in the machining process, such as: reducing cutting force and cutting heat, reducing tool wear and edge collapse burr, optimizing chip morphology, improving surface quality, reducing sub-surface damage, and improving machining efficiency (the specific improvement effects of each machining process are different due to different ultrasonic tools, materials, processes, etc.).
Ultrasonic machining technology is a special machining technology developed based on power ultrasonic technology. It is essentially a physical removal process, and does not involve the change of material properties.
With the increasingly strong market demand, commercial standardized systems in ultrasonic machining technology have also become the focus of current market demand.
Relevant ultrasonic machining technologies have begun to go out of the laboratory and have been applied in the precision machining of many typical difficult machining materials, such as optical glass, sapphire, ceramics, alumina ceramics, titanium alloys, superalloys, carbon fiber composites and aluminum based silicon carbide composites, Its application fields and typical cases are shown in Fig. 1.
Many scientific research institutions and manufacturing enterprises have begun to apply ultrasonic machining technology in the industry.
Fig. 1 application fields and typical cases of ultrasonic machining
2. Development status of ultrasonic machining technology
“If you want to do a good job, you must first sharpen your tools”.
Ultrasonic machining technology is a sharp tool for the precision machining of difficult materials.
In most cutting fields, the more precise name of ultrasonic machining should be “ultrasonic assisted precision machining”, that is, to assist ultrasonic vibration in traditional cutting technology, so as to achieve special material removal effect.
However, in some special cases, ultrasonic vibration will also become the main or even the only cutting power.
This kind of ultrasonic machining technology can be directly called ultrasonic machining.
For example, the ultrasonic scalpel often used in the medical field is used for bone cutting;
The ultrasonic Dagger Knife, which is widely used in the processing of aviation honeycomb parts, can achieve efficient and green processing.
As early as the 1920s, scientists from the United States, Japan, Germany and the Soviet Union began the basic research of vibration machining, and the early research mainly focused on the realization of material chip breaking by improving cutting conditions.
The main application is also in the field of ultrasonic turning.
The main feature of this stage is low-frequency vibration machining, whose frequency is quite different from the current ultrasonic frequency (above 15KHz).
After entering the 21st century, the machine tool manufacturer demarchesen precision machine launched a commercial ultrasonic machining machine, becoming the first enterprise to commercialize ultrasonic machining technology.
Around 2000, due to the endless emergence of all kinds of difficult-to-process materials, domestic universities and research institutes set off an upsurge in the research of ultrasonic machining technology.
It involves equipment design, control technology, ultrasonic machining system and ultrasonic technology in ultrasonic machining technology.
Ultrasonic machining technology has experienced a budding stage from its inception to about 2000, and then a rapid development stage of more than 20 years.
In the past five years, the rapid and large-scale application of difficult-to-process materials has accelerated the commercial development of ultrasonic machining technology.
At present, the technologies of ultrasonic machining equipment, ultrasonic vibration control and technology at home and abroad are gradually mature, which also promotes people’s in-depth understanding of the basic requirements, working mechanism, process characteristics and application fields of ultrasonic machining technology.
At the same time, more and more universities and research institutes are carrying out research on ultrasonic machining technology, and the application fields of ultrasonic machining are more and more extensive.
3. Basic principle of ultrasonic machining technology
At present, the understanding of ultrasonic machining technology at home and abroad is still in the process of continuous development, and there is no unified standard and specification.
Conventional ultrasonic machining system mainly includes ultrasonic drive controller, ultrasonic transmitter, transducer, horn, clamping structure and cutter, as shown in Fig. 2.
This article will introduce the characteristics of ultrasonic machining technology from the classification of ultrasonic machining system, typical structure of ultrasonic machining system, ultrasonic drive control technology and ultrasonic process technology.
Fig. 2 main components of ultrasonic machining system
3.1 Classification of ultrasonic machining system
According to the form of ultrasonic vibration, it can be divided into one-dimensional ultrasonic machining, two-dimensional ultrasonic machining and three-dimensional ultrasonic machining.
The vibration forms include longitudinal (axial) ultrasonic vibration machining, torsional ultrasonic vibration machining, elliptical ultrasonic vibration machining and composite ultrasonic vibration machining, among which composite ultrasonic vibration involves the combination of different ultrasonic vibration forms.
According to the ultrasonic vibration starting materials, it can be divided into electrostrictive ultrasonic machining and magnetostrictive ultrasonic machining.
At present, electrostrictive ultrasonic machining is mainly based on the vibration starting principle of piezoelectric ceramics, which is driven by applying voltage to achieve ultrasonic vibration.
Its structure is simple and its technology is mature, but its vibration power is limited, which is more suitable for light load cutting conditions;
Magnetostrictive type is based on the magnetic field driving principle of magnetostrictive or giant magnetostrictive materials.
Its vibration power capacity is large, but its structure is complex, which is more suitable for heavy-duty cutting conditions.
According to the energy transmission mode, it can be divided into wired energy transmission ultrasonic machining and wireless energy transmission ultrasonic machining.
Wire energy transmission ultrasonic machining is often used in low speed machining without rotating motion or using brushes;
Wireless energy transmission ultrasonic machining is generally used in rotary ultrasonic machining (RUM).
The realization of wireless energy transmission is based on the loose coupling non-contact method, which can be divided into fully coupled ultrasonic machining and partially coupled ultrasonic machining.
In terms of the current technological development, considering the problem of automatic tool change of machine tools, the partially coupled wireless energy transmission mode is the most adaptable one at present.
According to the process types, it can be divided into ultrasonic milling, ultrasonic turning, ultrasonic drilling, ultrasonic grinding, ultrasonic polishing, ultrasonic cutting and ultrasonic hardening (strengthening).
Each process has special requirements for the size of ultrasonic energy and vibration form.
According to the machining accuracy of parts, it can be divided into precision ultrasonic machining and ultra precision ultrasonic machining.
The main difference between the two lies in the amount of material removal per unit cutting.
Generally, the removal with accuracy requirements less than 1μm is considered as ultra precision ultrasonic machining.
The more typical is elliptical ultrasonic machining, which is more suitable for micro nano cutting.
According to the cutting speed, it can be divided into traditional ultrasonic machining and high-speed ultrasonic machining.
High speed ultrasonic machining generally refers to ultrasonic machining with linear speed up to 400m / min.
3.2 Typical ultrasonic machining machine structure
The structure of a typical ultrasonic machining machine tool is shown in Fig. 3.
It mainly includes an ultrasonic drive controller, a radio energy transmission module, an ultrasonic tool holder, a tool, a numerical control system and a machine tool body.
The ultrasonic drive controller includes an ultrasonic generator, an ultrasonic power amplifier, an ultrasonic feedback detection and an ultrasonic controller.
The ultrasonic transmitter and the ultrasonic receiver constitute a radio energy transmission module.
Ultrasonic tool holder is the main functional part of ultrasonic machining system, which is generally composed of ultrasonic receiver, transducer and horn.
The ultrasonic machining system and the CNC system of the machine tool should maintain a certain communication control capability to ensure the smooth progress of the whole machining process.
Fig. 3 structure of typical ultrasonic machining machine
3.3 Driving control technology of ultrasonic machining system
The driving control technology of the ultrasonic machining system is the soul of realizing the advantages of ultrasonic machining technology, and the performance of the controller is the key to reflecting the performance of ultrasonic machining.
In the actual machining process, the ultrasonic drive controller needs to control a lot of parameters.
In addition to the most basic and important ultrasonic frequency and power, the frequency resolution, response speed, amplitude fluctuation, etc. of the ultrasonic machining system also play a key role in the machining process.
Many control parameters jointly determine the ultimate cutting capacity of the ultrasonic machining system.
These parameters are the key to determining whether the material processing is effective.
It is also the technical condition for studying ultrasonic machining technology.
In the actual machining process, the process of cutting tools into and out of materials is a typical strong time-varying load process, that is, the cutting force changes greatly in a very short time.
In this process, the impedance characteristics of the ultrasonic machining system and the electrical characteristics of the control system will all undergo great changes.
Therefore, it is very important to ensure the stability of the amplitude in this process.
The resolution and accuracy of the frequency of the ultrasonic controller and the response speed of the system are the key parameters in this process.
Fig. 4 shows the change of state characteristics of the system during a typical ultrasonic cutting process.
It can be seen from Fig. 4 that when the tool starts to enter the machining state, the cutting force increases instantaneously during the machining process.
At this time, in order to ensure the stability of the ultrasonic amplitude during the cutting process, the system will adjust the internal control parameters (such as frequency, power, etc.), so that the vibration amplitude is stable during the machining process.
A similar process also occurs when the tool cuts out the material to be processed.
Fig. 4 change of state characteristics of ultrasonic machining system
In the process of ultrasonic machining, the change of load and temperature will cause the system characteristics to change greatly in the process of machining.
Fast frequency tracking is the key to achieve effective ultrasonic machining.
Common methods include maximum current method, phase locked loop method, maximum power method, etc.
In addition, ultrasonic power adaptive control technology is also an important part of ultrasonic machining.
Its goal is to solve the problem that the tool can not complete effective cutting due to the suppression of the amplitude after being loaded.
These tests all require the ultrasonic machining system to realize the feedback detection of the real-time state of the ultrasonic actuator.
Therefore, the feedback detection of ultrasonic vibration is also an important part of achieving stable ultrasonic machining.
Now many advanced algorithms have been applied in this process.
Common algorithms include PID, fuzzy algorithm, artificial neural network, etc.
The core of the algorithm is to identify the state in the machining process, so as to ensure the robustness of the machining process.
4. Process characteristics and application of ultrasonic machining technology
For the materials with different characteristics, the material removal mechanism and effect of ultrasonic machining are quite different.
Typical hard working materials can be divided into hard brittle materials, composite materials and hard working metal materials.
Hard and brittle materials mainly include glass, ceramics, tungsten steel and ceramic-based materials.
These materials are often processed with diamond tools.
In the process of processing, we need to consider the hardness of the material to be processed and how to reduce the cutting force and improve the processing quality during ultrasonic processing.
Composite materials mainly include carbon fiber reinforced composite materials, aramid fiber composite materials and honeycomb weak stiffness composite materials.
The processing of such materials mainly needs to select appropriate ultrasonic vibration forms and ultrasonic amplitudes according to material characteristics, so as to reduce cutting force and slow down tool grinding.
Difficult-to-machine metal materials mainly include titanium alloy, high-temperature alloy, high-strength steel and other metal materials with a certain toughness.
The processing of such materials mainly needs to solve the problem of sticking to the tool, reduce the cutting temperature and slow down the wear of the tool.
Its ultrasonic vibration requirements are quite different from those of hard and brittle materials.
Generally, torsional vibration or longitudinal torsion are used and the amplitude requirements are relatively large.
It is the most ideal machining situation for metal materials to realize complete intermittent cutting of materials and efficient cooling and lubrication in the cutting area.
The main process characteristics of different materials during ultrasonic machining are as follows.
(1) Hard brittle material
Glass, ceramics (alumina, zirconia, silicon carbide and silicon nitride), ceramic based materials, glass ceramics and other materials with high hardness and brittleness.
The main processing difficulties are large surface damage, serious tool wear and low processing efficiency.
Ultrasonic machining is beneficial to improve the cutting state, so as to improve the tool life, improve the surface quality and improve the machining efficiency.
The average cutting force comparison between ultrasonic machining and ordinary machining of semiconductor silicon carbide (SIC) is shown in Fig. 5.
Fig. 5 Comparison of average cutting force between ultrasonic machining and ordinary machining of silicon carbide
(2) Composite material
The composites reinforced with carbon fiber and aramid fiber are prone to surface damage, tearing and delamination, low processing efficiency and fast tool wear during processing.
After ultrasonic machining, the cutting ability of the tool will be greatly enhanced, the burr will be reduced, and the service life of the tool will be prolonged.
The ultrasonic-processed aviation honeycomb material is shown in Fig. 6.
a) Disc cutter machining
b) straight edge cutter machining
Fig. 6 ultrasonic machining of aviation honeycomb materials
(3) Difficult to process metal materials
Metal materials with certain toughness, such as titanium alloy, high-temperature alloy and high-strength steel, are prone to problems such as tool sticking and serious tool wear due to high processing temperature.
Ultrasonic machining can reduce the cutting force, reduce cutting temperature, improve chip shape, reduce sticking and prolong tool life.
The comparison of tool wear between ultrasonic machining and ordinary machining of titanium alloy is shown in Fig. 7.
a) Comparison of cutting tools under different removal amounts
b) Comparison and improvement ratio of tool wear under different removal amounts
Fig. 7 Comparison of tool wear between ultrasonic machining and ordinary machining of titanium alloy
5. Future development trend of ultrasonic machining technology
At present, ultrasonic machining technology is in the stage of rapid development.
The use of various new materials provides a strong application foundation for ultrasonic machining.
Ultrasonic machining technology has also become a powerful weapon for precision machining of such materials.
In the future, ultrasonic machining technology will be rapidly developed with the promotion of the whole industry.
At the same time, the following research directions may become the hotspots of future scientific research and engineering applications.
5.1 Advanced ultrasonic composite energy field technology
Multi dimensional ultrasonic machining technology will be applied more and more, and the matching control technology and process technology will be the focus of future research.
In addition, the composite technology of ultrasonic, laser, plasma and other processes is an important direction for the development of ultrasonic machining in the future.
Different energy fields will play their unique roles in the processing of difficult to process materials, making processing easier.
5.2 Fast, accurate and stable control algorithm and control system
The actual cutting process is quite different from the static or quasi-static process.
Different materials, tools and cutting parameters will have a great impact on the cutting force, cutting temperature and system impedance characteristics during the machining process. Especially under the strong time-varying load, it is easy to cause the instability of the ultrasonic machining system.
Ensuring the stability of the ultrasonic machining system through fast, accurate and stable control algorithms is the key to achieve efficient and precise machining.
In the future, more control algorithms will be introduced into the ultrasonic machining process.
5.3 High speed and efficiency of ultrasonic machining
In traditional precision machining, it is difficult to have both machining quality and machining efficiency, especially for complex structural parts in the aerospace field.
In order to ensure their machining quality, the machining efficiency is usually low.
When ultrasonic machining is used to process difficult-to-machine materials in this field, such as titanium alloy, superalloy and carbon fiber composite materials, its unique intermittent separation mode can realize the periodic opening of the cutting area during the cutting process, thereby improving the cooling and lubrication effect during the machining process, and improving the cutting speed through the force reduction and temperature reduction of the cutting area, so as to achieve high-quality and efficient precision machining.
5.4 Formulation of ultrasonic machining standardization system
Process capability is the intuitive embodiment of the advantages of ultrasonic machining technology.
A full and profound understanding of process capability is helpful to give full play to the role of ultrasound.
With the deepening of technical research, more special tools, fixtures and matching process parameters for ultrasonic machining will gradually emerge, and the ability of ultrasonic machining will be gradually brought into full play.
However, up to now, ultrasonic machining technology has not formed a unified standard.
“If you want to know straightness, you must follow the standard; If you want to know the surrounding area, you must follow the rules”.
Speeding up the formulation of industrial standards and national standards will help eliminate technical barriers and promote the promotion and application of ultrasonic machining technology and the development of the industry.
It is worth mentioning that ultrasonic machining technology is not a universal machining technology for difficult-to-process materials.
Different types of ultrasonic machining also have their own adaptability and limitations.
Only by being familiar with the characteristics of ultrasonic machining technology and understanding the process requirements can we apply this technology well.
Ultrasonic machining technology is a new type of advanced manufacturing technology with great development potential, which is an important technical support for manufacturing power.
With the large use of difficult-to-machine materials, ultrasonic machining technology will be rapidly applied and developed.
In the future, ultrasonic machining technology will solve the precision machining problems of more difficult-to-machine materials.