ABSTRACT

Ultrasonic metal welding is a solid state welding process, and here ultrasonic vibrating motion is used to join two materials. The two work pieces are held in between the anvil and sonotrode. The work piece on the anvil is hold stationary while the other part is moved to and fro due to the vibrating effect of the sonotrode. This movement the oxide disperses layer in between them and atoms of the work pieces are diffused from one part to another. When vibration is stopped a pure metallurgical bond is obtained.
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CONTENTS

1. INTRODUCTION
2. WORKING
3. MECHANISM
4. PARTS OF APPARATUS
5. WELDING PARAMETERS
6. COMPARISON
7. ADVANTAGES OF ULTRASONIC WELDING
8. LIMITATIONS
9. APPLICATIONS
10. CONCLUSION
11. REFERENCES
1. INTRODUCTION

In 1938, Lludwig Bergman and some colleagues were experimenting with ultrasonic Waves and their effects on metal. He found that many metals could be combined by using ultrasonic welding that could not be joined by any other method. It was also found that any metal could be strengthened by subjecting the metal in its molten state to ultrasonic vibrations. The ultrasonic effect upon the molten metal generates a smaller grain size, giving the metal more strength. Ultrasonic welding combines pressure and high frequency vibration motions to form a solid state bond. The range ultrasonic frequency used in this welding is from 20kHz to 60kHz. This cool, strong weld capable of joining such combination as aluminium to steel, aluminium to tungsten, aluminium to molybdenum and nickel to brass. Ultrasonic welding has also made it possible to join metals with vastly different melting temperatures, making strong rigid joints. Thus many applications previously considered unweldable can now revaluated.



2. WORKING

The circuit of UMW is as shown below.

It consists of an ultrasonic generator, which generates a frequency of 20 kHz to 40 kHz from a supply of 220v/50Hz. The converter transforms the high frequency electric energy produced by the generator into mechanical energy. The booster serves as an amplitude transformer for the required amplitude range as well as a general stabilizer for the oscillations of the transducer system. The sonotrode or horn is the working tool of the ultrasonic metal welding.

The pieces to be welded are clamped between the welding tip called sonotrode and anvil. Both tip and anvil are faced with high-speed steel, since considerable wear can occur at the contacting surface. The process of ultrasonic welding is fairly simple. It begins when the parts those are to be welded, such as two multi-strand copper wires for example, are placed together in the welding unit. The system then compresses the wires together with a force of between 50 and several 100 pounds per square inch to form a close connection between the two pieces. Next, the ultrasonic horn is used to vibrate the two pieces together at a rate of around 20000 or 40000 Hz, depending on the application.



The system that is used to scrub the pieces together consists of four major components. The first of these is the anvil. This is simply a piece of the machine, usually with a replaceable head, that holds one of the components still while the other is rubbed against it. The “business end” of the ultrasonic system consists of three major parts. The first of these is the ultrasonic transducer. This component takes an electric signal from a power supply that is providing a 20 kHz AC (by using an ultrasonic generator) signal and converts it to a mechanical motion at the same frequency. The vibration that results is at a frequency that is appreciably above the range of human hearing, hence the name ultrasonic.

Although this motion is very strong, it has a very low amplitude or stroke length. This is not suitable for welding. The next part of the system, appropriately called the booster, increases the amplitude of the motion, at the cost of some of its force.

This motion is then passed into the ultra sonic horn. This is the portion of the system that actually vibrates the work piece. In addition to providing the interface between the ultra sonic generator and the work piece, the horn also further amplifies the amplitude of the motion, again reducing its force. Like the anvil, the horn ends in a replaceable head.

Before the interaction of the pieces at the interface can be explained, some basic molecular physics must be reviewed. The first principal is that when two clean pieces of metal are placed in intimate contact, they will begin to share electrons, thus welding together, second, at atomic scale even surfaces those that look perfect and smooth are very rough and impure. The majority of these impurities are in the form of metal oxides that were produced when the bare metal was exposed to the atmosphere. The second part of contamination is in the form of ordinary dirt and oils. These impurities form a layer that prevents the electrons in the two parts from passing between them, thus preventing them from welding together. In addition, the rough surface prevents the metals from being in intimate contact, which also prevents the exchange of electrons.




3. MECHANISM

The two work pieces are held stationary on the anvil. The ultra sonic vibrating force acting on the upper piece and this gives to and fro movement to the upper part, while the other held stationary.



When the upper part is moved back and fro along the surface of the lower work piece, the vibration combined with the pressure, first rubs the impurities and oxides of the surfaces of the work piece when they are in contact. When the complete oxide layer is dispersed, then atoms start to diffuse from one part to another. When the vibration is stopped a pure metallurgical bond is formed in between them. The bond we get is very strong and may be stronger than the parent metal.



4. PARTS OF APPARATUS

The parts of the welding unit are as follows,
1) Ultra sonic generator
2) Ultra sonic transducer system
a) Converter
b) Booster
c) Sonotrode
3) Anvil and fixtures

4.1 ULTRA SONIC GENERATOR

The ultra sonic generator operates with ac line voltage (220/110 V). It generates a sinusoidal wave of 20 kHz from the supplied power. The ultra sonic generator has three components,

Automatic tuning adjustment for constant welding parameters

Advanced ultra sonic generators feature automatic tuning adjustments to optimize the generator to the transducer system, even if there is a change in the mechanical or electrical properties during the welding process. The automatic tuning adjustment compensates for changes and ensures optimal efficiency. Above all, it guarantees that welding parameters remain constant. This is extremely important for ultra sonic metal welding.

Keeping the amplitude constant

The welding phase when molecules inter mingle will require a much high mechanical load. The rise in mechanical load must not, under any circumstances, influence or reduce the amplitude. The electronic controls in the generator must detect this condition and maintain constant amplitude by providing appropriate energy. Constant amplitude during the welding process is decisive for a successful weld. The required welding amplitude is based on the application characteristics and / or tests of the weld strength. The physical amplitude generally varies between 0.015 mm and 0.045 mm at half of the travel distance.

Generators with amplitude adjustment

Different geometrical shapes of transducer will provide different values of amplitude. The amplitude for specific welds however can be varied by electric or electronic variation means. This is why ultra sonic generators are preferably equipped with electronic regulation control for amplitude adjustments. By using a selector switch up to 10 steps or a regulating potentiometer, the amplitude can be adjusted in fine increments.

4.2 ULTRASONIC TRANSDUCER SYSTEM

The ultrasonic transducer system consists of three parts; they are converter, booster and sonotrode

.



4.2.1 THE CONVERTER

The converter transforms the high frequency electric energy produced by the generator into mechanical energy. In the past, Ferro magnetic materials with magnetostrictive characteristics have been used for this purpose. Ultrasonic electrical frequency current induces a periodic alternative magnetic field inside the magnetostrictive material, which changes its dimensions as a function of the frequency of the excitation.

Highly efficient ring shaped piezoceramics are used to transform electric energy into mechanical energy with an efficiency of approximately 95%. The ceramics are sintered from crystalline powder similar to the one used for the manufacture of china. The reverse piezoelectric effect is almost exclusively used to produce oscillations. Piezoelectric materials periodically change under the effect of an electric alternating voltage.

4.2.2. THE BOOSTER

The booster serves as an amplitude transformer for the required amplitude range as well as a general stabilizer for the oscillations of the transducer system. Certain design featured or the geometrical shape of the booster achieves amplitude magnification or reduction.

The resonance frequency of the booster has to match the nominal frequency (working frequency) of the generator. The coupling surfaces to the converter and to the sonotrode have to be completely leveled and lapped so as to prevent a loss of energy during its transfer. If this is not achieved, an undesirable temperature rise, annoying noise and damage of the device may occur. The connection to the converter and the sonotrode is assured through the high tensile threaded pins and by tightening with a defined torque.

4.2.3 THE SONOTRODE

The sonotrode is the welding tool of the ultra sonic welding. Unlike sonotrodes for ultrasonic plastic welding, those used for ultra sonic metal welding devices are not made from aluminum or titanium, but are made exclusively from tool steel. The hardening process requires that special procedures be observed. The manufacture of reliable steel sonotrodes used to be a serious obstacle when ultrasonic metal welding was still in its in fancy. Steel has to expand and contract 20000 times per second without heating up too much, tearing or even breaking. One of the considerations, which date back to the early days, is the idea of designing the working area of the sonotrode with a replaceable tip as a separate part. The sonotrode did not consist of one single piece but of three pieces, including the fastening bolt, all with very different masses. Strongly tightened, this bracing constituted a feasible compromise when low amounts of energy and low amplitude were involved.

With high energies or high amplitudes, as often is the case with ultra sonic metal welding; the three different masses often produced a disharmony in the oscillation pattern of the sonotrode. The uncoordinated oscillations sooner or later led to destruction of the tip, the bolt or the sonotrode itself. The sonotrode must be designed to suite the requirements of a solid weld and the geometrical shapes of the parts to be welded. The work surfaces of the sonotrode are usually rough or structured to permit setting the work piece in high frequency acceleration without slip relative motion.

4.3 THE ANVIL AND FIXTURES

The parts to be welded are inserted between anvil and sonotrode and overlap each other. The anvil keeps the work piece in position and secures it against any incontrollable movement. The anvil absorbs the pressure and oscillations forces introduced by the sonotrode. Its sturdy designed must insure that a constant pressure is applied to the welding area during the whole welding process. The immediate welding area of the anvil, as well as the work surface to the sonotrode, is roughened to enable a secure gripping of the parts to be welded. This prevents slippage of the work pieces and thus an efficient oscillating energy is delivered. An accommodating fixture, such as pneumatically operated jaws or other machine parts, should enable easy insertion and removal of parts to be welded.

5. WELDING PARAMETERS

The manufacturer before a decision on the design of the tool completes a determination of optimum welding parameters and the fixture is made. The parameters can vary depending on variation in materials, dimensions, and surface contaminations.

The selected welding parameters for ultrasonic welding are:

• Clamping pressure

This is the pressure between anvil and the sonotrode applied on the parts to be welded. The contact pressure must remains constant during the whole welding process. It is usually produced pneumatically and totals up to approximately 400 – 1500 N.

• Welding time

The welding time is defined as the duration of the ultrasonic and can be both a, constant parameter and a variable parameter adjusted with the help of quality-controlled devices for an optimum weld. Depending on the application the welding time can be between 0.1 and approximately 1second.

• Trigger point

The starting point of ultrasonic welding is called trigger point. It is usually selected as a function of the clamping and compaction of parts, sometimes also as a function of stroke length.

• Amplitude

The amplitude constitutes the oscillation length of the welding tool. The amplitude should remain constant through out the welding process. The booster defines wit modern welding equipment a relatively large amplitude range mechanically. With in this range, the fine adjustment of the amplitude is made electrically with a selective switch.

• Welding tools

The design of the welding tools requires a considerable amount of process engineering know-how and is often the decisive factor for determining the quality of weld. If the surface structure on the work surface of the sonotrode begins to wear, it may be judicious to change one or several parameters until a reconditioning or exchange of the tool become necessary.

• Ultrasonic frequency

The frequency, measured in cycles per second, remains largely constant and depends on the generators working frequency. It is also decisive for the construction of the machine and the complete transducer system. Most equipment has a working frequency of 20 kHz, but for small-scale applications, equipment between 35 and 40 kHz is available.



6. COMPARISON

6.1 COMPARISON-1

Soldering Resistance Welding Crimping Crimping & Soldering Ultrasonic Welding
Investment 5 3 4 4 2
Lifetime Of Tools 3 2 3 3 5
Required Energy 3 1 5 2 4
Process Time 1 3 5 1 4
Environmental Factors In Production Area 1 2 4 1 4

5-very good/advantageous, 4- good, 3- satisfactory
2-suffiocient, 1- defective/ disadvantageous

Here three types of factors of different metal joining processes are compared. In first comparison ultra sonic welding has low investment, very long lifetime for tools, required energy for welding is less and pollution by welding is less


6.2 COMPARISON- 2

Soldering Resistance
Welding Crimping Crimping
And
Soldering Ultra sonic
Welding
Flexibility of production
(Time for tool change,
Required tools) 5 3 2 2 4
Complexity of welding
Parameters influencing
the weld 5 2 3 3 4
Consumables 1 5 1 1 5
Quality assurance of the
Weld 1 3 2 2 5
Consistency of measuring v
values 2 3 3 4 4

5-very good/advantageous, 4- good, 3- satisfactory
2-suffiocient, 1- defective/ disadvantageous

In this comparison table second type of factors are compared. Here ultrasonic welding has good flexibility of production, complexity of welding parameters influencing the weld are less, no consumable electrodes are used, quality assurance of the weld is very good, very high consistency of measuring values when compared to other joining processes.





6.3 COMPARISON- 3


Soldering Resistance
Welding Crimping Crimping
And
Soldering Ultrasonic
Welding
Long term durability
Of weld 3 3 2 4 5

Stability against
Vibration 4 3 2 4 4

Stability against Bending
(Brittleness after
Heat built up) 5 1 5 5 5
Compactness, density
Of the weld (corrosion) 2 4 1 2 4
Electric conductivity 3 4 3 3 5

5-very good/advantageous, 4- good, 3- satisfactory
2-suffiocient, 1- defective/ disadvantageous

In third comparison we can see that ultra sonic welding has a long term durability of weld, good stability against vibration, very good corrosion resistant to the welded part, very good electrical conductivity to the welded joint when compared to other joining processes.



7. ADVANTAGES OF ULTRASONICMETAL WELDING

1) Surface preparation is not critical.
2) No defects are produced from the arc, gases and filler metals.
3) Dissimilar metals having vastly different melting points can be joined.
4) Minimum surface deformation results.
5) Very thin materials can be welded.
6) Thin and thick sections can be joined.
7) No fumes or odours.
8) No filler metal is used.
9) Metals can be inserted into plastic using ultrasonic metal welding.




8. LIMITATIONS

1) Due to fatigue loading the life of equipment is short.
2) The thickness of the work pieces is restricted, that is the maximum is about 3mm aluminum and 1mm for harder metals.
3) Very ductile metals will yield under ultrasonic strain without sliding.
4) Materials being welded tend to weld to the tip and anvil.
5) Hard materials will fatigue under the stresses necessary for welding.




9. APPLICATIONS

1) Joining of electrical and electronic components.
2) Welding aluminum wire and sheets.
3) Fabricating nuclear fuel elements.
4) In aircraft structural applications.




10. CONCLUSION

In conclusion ultrasonic welding has many advantages over other welding techniques for many type e of connections. These include speed, cost, reliability and many others. However, it should also be made clear that it is only ideal for a relatively small proportion of all of the possible joints. Ultrasonic welding should not be seen as a replacement for other techniques, such as GTAW, resistance, laser etc; but rather as an option in situations for which it is well situated.






11. REFERENCES

1) O.P.Khanna “WELDING TECHNOLOGY”.
2) Staple ultrasonic corporation “Principles and applications of high grade bonding technology”.
3) Richard L Little “WELDING AND WELDING TECHNOLOGY”.