Electrical discharge machining (EDM) and electrochemical machining (ECM) can only process conductive metallic materials and cannot work with non-conductive non-metallic materials.
In contrast, ultrasonic machining (USM) not only allows the processing of brittle and hard metallic materials such as hard alloys and hardened steel, but is also more suitable for working with non-conductive non-metallic materials such as glass, ceramics, semiconductors. , germanium and silicon wafers.
Additionally, USM can be used for applications such as cleaning, welding and non-destructive testing.
Ultrasonic welding uses high-frequency vibration waves transmitted to the surfaces of two objects that need to be welded. Under pressure, the two surfaces rub against each other, resulting in fusion between the molecular layers.
The components required for ultrasonic welding include an ultrasonic generator, a converter, a booster, and welding tools.
I. Principles of Ultrasonic Machining
The main components of an ultrasonic welding system include a trio set of ultrasonic generator, transducer, boosters, welding head, mold and frame.
Ultrasonic welding involves converting a current of 50/60 Hz into electrical energy of 15, 20, 30 or 40 KHz using an ultrasonic generator. The converted high-frequency electrical energy is transformed back into mechanical movement of the same frequency via a transducer.
Subsequently, the mechanical movement is transferred to the welding head through a set of reinforcement devices that can change the amplitude. The welding head transmits the received vibrational energy to the joint of the part to be welded. In this region, vibrational energy is converted into thermal energy through friction, fusing the areas that need to be welded.
Ultrasound can be used not only to weld thermosetting metals and plastics, but also to process fabrics and films.
Specifically, ultrasonic machining (USM) is a method of processing hard and brittle materials through the use of ultrasonic vibration on the end face of the tool, combined with the action of abrasive suspension.
USM is the result of the combined effects of mechanical impact and abrasive grinding caused by the vibration of abrasives under ultrasonic waves, with the continuous impact of abrasives being the main factor.
During the ultrasonic machining process, a suspension of liquid and abrasive mixture is introduced between the tool head and the workpiece. Light pressure is applied in the direction of vibration of the tool head.
The ultrasonic frequency generated by the ultrasonic generator is transformed into mechanical vibrations by the transducer. The amplitude is amplified to 0.01-0.15 mm by the amplitude rod and then transmitted to the tool.
The end face of the tool is driven to vibrate ultrasonically, causing the abrasive particles in the suspension to continuously impact and grind the surface of the workpiece at high speed. This results in the material being crushed in the machining area into fine particles, which are then removed from the material.
Although each impact removes a small amount of material, the high frequency of over 16,000 impacts per second allows for some processing speed.
At the same time, the hydraulic impact and cavitation phenomenon caused by ultrasonic vibration at the end of the tool result in the liquid penetrating into the cracks in the workpiece material, accelerating the destruction process.
The hydraulic impact also forces the suspension working fluid to circulate in the machining gap, ensuring timely renewal of worn abrasive particles.
1) Principle of ultrasonic metal welding
The principle of ultrasonic metal welding involves utilizing mechanical vibrational energy at ultrasonic frequencies (exceeding 16KHz) to connect identical or dissimilar metals in a unique way.
During the ultrasonic welding process, no current is transmitted to the workpiece, nor is a high-temperature heat source applied. Vibrational energy is simply converted into frictional work and strain energy between parts, along with a limited increase in temperature, under static pressure.
Thermochemical bonding between joints is a solid-state welding process that occurs without melting the original material. As such, it effectively overcomes the spatter and oxidation problems that occur during resistance welding.
Ultrasonic metal welding machines can perform single-point welding, multi-point welding and short strip welding on thin wires or thin sheets of non-ferrous metals such as copper, silver, aluminum and nickel. They are widely used for welding thyristor cables, fuse strips, electrical cables, lithium battery pole pieces and polar ears.
2) Principle of ultrasonic plastic welding
When ultrasonic waves act on the contact surface of thermoplastic materials, they generate high-frequency vibrations tens of thousands of times per second. This high-frequency vibration, upon reaching a certain amplitude, is transmitted to the welding area through welding, converting ultrasonic energy into heat.
The sound resistance at the junction of the two welds in the welding area is high, generating high local temperatures. Due to the poor heat conduction of plastics, heat cannot be dissipated immediately and accumulates in the welding area, causing the contact surfaces of the two plastics to melt quickly.
By applying a certain amount of pressure, the plastics are fused into one. When the ultrasonic waves cease, pressure is maintained for a few seconds to allow solidification, forming a robust molecular chain to achieve the purpose of welding. The welding strength can approach the strength of the original material.
The quality of ultrasonic plastic welding depends on three factors: the amplitude of the transducer welding head, the applied pressure and the welding time. Both welding time and welding head pressure can be adjusted, while amplitude is determined by the transducer and amplitude rod.
These three factors interact and have an optimal value. When the energy exceeds this ideal value, the amount of melted plastic is high and the welded material is subject to deformation.
If the energy is too low, the resulting weld will not be firm and the pressure applied should not be too high. The ideal pressure is the product of the edge length of the welded part and the ideal pressure per millimeter of the edge.
II. Features of ultrasonic machining
1. Wide range of applications:
The. It can process traditionally difficult-to-machine metals and non-metallic materials such as hardened steel, stainless steel, titanium, alloys, and especially non-conductive non-metallic materials such as glass, ceramics, quartz, silicon, agate, precious stones and diamonds. It can also process hard conductive metallic materials such as hardened steel and hard alloys, although with lower productivity.
B. Suitable for processing deep holes, thin-walled parts, slender rods, low-rigidity components and complex-shaped parts with high requirements.
w. Ideal for precision machining of high precision components and low surface roughness.
2. Low cutting force and energy consumption:
Due to the instantaneous localized impact, ultrasonic machining imposes a minimal macroscopic cutting force, resulting in reduced cutting stress and heat.
3. High machining precision and low surface roughness:
Ultrasonic machining can achieve high machining precision (dimensional accuracy of up to 0.005-0.02 mm) and low surface roughness (Ra value of 0.05-0.2). The process leaves no residual stresses or burn marks on the machined surface, making it suitable for thin walls, narrow gaps and low stiffness components.
4. Suitable for processing complex shaped cavities and molded surfaces.
5. Tools can be made of relatively soft materials with complex shapes.
6. Ultrasonic machining equipment generally has a simple structure, making it easy to operate and maintain.
III. Ultrasonic Machining Process Laws
1. Machining speed and influencing factors:
Machining speed refers to the amount of material removed per unit of time and is expressed in mm 3 /min or g/min.
Factors that influence machining speed include tool amplitude and frequency, feed pressure, abrasive particle type and size, workpiece material, and abrasive suspension concentration.
The. Influence of tool amplitude and frequency:
Excessive amplitude and high frequency can subject the tool and amplitude rod to high internal stresses. The amplitude is generally between 0.01-0.1 mm and the frequency is between 16,000-25,000 Hz.
In actual machining, it is necessary to adjust the resonance frequency according to different tools to obtain maximum amplitude and achieve higher machining speed.
B. Influence of supply pressure:
The tool must have adequate supply pressure during machining. Very low pressure increases the gap between the end face of the tool and the surface of the part, reducing the impact force of the abrasive on the part.
Increasing the pressure reduces the clearance, but when the clearance decreases to a certain extent, it will reduce the circulation and renewal speed of the abrasive and working fluid, thus decreasing productivity.
w. Influence of abrasive type and particle size:
Different abrasives can be selected for materials with varying resistance during machining. Higher abrasive resistance results in higher machining speed, but cost must also be considered. To process materials such as gemstones or diamonds, diamond abrasives must be used.
Boron carbide is suitable for machining hardened steel and hard alloys, while aluminum oxide abrasives are used to process materials such as glass, quartz, silicon and germanium.
d. Influence of the part material:
Hard and brittle materials are easier to remove during machining, while materials with good toughness are more difficult to process.
It is. Influence of abrasive suspension concentration:
Lower abrasive suspension concentration means fewer abrasive particles in the machining gap, which can lead to a significant decrease in machining speed, especially for large surface areas and deep depths.
Increasing the concentration of abrasives improves the machining speed, but an excessively high concentration may affect the circulation and impact of abrasive particles in the machining area, leading to a decrease in the machining speed.
2. Machining accuracy and influencing factors:
The accuracy of ultrasonic machining is influenced by machine tool and fixture accuracy, as well as abrasive particle size, tool accuracy and wear, magnitude of lateral vibration, machining depth, and workpiece material properties.
3. Surface Quality:
Ultrasonic machining provides excellent surface quality, without generating surface layers or burn marks. The surface roughness mainly depends on the size of the abrasive particles, the ultrasonic amplitude and the hardness of the workpiece material.
Smaller abrasive particle size, lower ultrasonic amplitude and harder workpiece material lead to improved surface roughness, as the roughness value is mainly determined by the size and depth of the grooves left by the impact of each abrasive particle on the workpiece material.
Although the productivity of ultrasonic machining is lower compared with electrical discharge machining and electrochemical machining, its machining accuracy and surface quality are superior.
Importantly, it can process hard and brittle semiconductor and non-metallic materials such as glass, ceramics, quartz, silicon, agate, gemstones and diamonds, which are difficult to machine using other methods.
Furthermore, it is often employed in the final finishing stages of hardened steel, hard alloy molds, drawing dies, and plastic molds processed by electrical discharge machining, to further reduce surface roughness.
4. Ultrasonic Machining Applications
1. Cavity and mold machining:
Ultrasonic machining is mainly used to process circular holes, shaped holes, cavities, sockets and micro holes in brittle and hard materials.
2. Cutting Processing:
Ultrasonic machining is suitable for cutting brittle and hard materials such as ceramics, quartz, silicon and gemstones, which are difficult to cut using conventional methods. It offers advantages such as thin slices, narrow cuts, high precision, high productivity and cost-effectiveness.
3. Ultrasonic cleaning:
This method is based on the cavitation effect produced by the cleaning solution under the action of ultrasonic waves. The strong impact generated by cavitation acts directly on the surface to be cleaned, causing the debris to disintegrate and detach from the surface.
This method is mainly used for precision cleaning of small and medium-sized precision parts with complex geometries, where other cleaning methods are less effective, especially for deep holes, micro holes, curved holes, blind holes, grooves, narrow slots, etc. . Provides high productivity and purification rates.
Currently, it is applied to cleaning semiconductor and integrated circuit components, instrument parts, electronic vacuum devices, optical components and medical instruments.
4. Ultrasonic Welding:
Ultrasonic welding uses ultrasonic vibration to remove the oxide film from the surface of parts, exposing the surface of the base material. The high-speed vibratory impact between the two surfaces of the welded part causes heating and frictional bonding.
It can be used to weld nylon, plastic and aluminum products prone to oxide film formation. It can also be used to apply tin or silver to the surfaces of ceramics and other non-metallic materials, improving their solderability.
5. Composite machining:
To increase machining speed and reduce tool wear when machining hard metallic materials such as hard alloys and heat-resistant alloys, combined ultrasonic-assisted machining with electrochemical or electrical discharge machining is employed.
This is often used to machine narrow holes or slots in components such as fuel injectors and wire drawing plates, resulting in significantly improved productivity and quality.
Ultrasonic vibration cutting (e.g. turning, drilling, thread cutting) has also been developed over several decades as a new technology for precision machining and difficult-to-cut materials, reducing cutting forces, surface roughness, tool wear and increasing productivity.
Some commonly used applications include ultrasonic vibration turning, ultrasonic vibration grinding, ultrasonic machining of deep holes, small holes, and threading, among others.
