The former of special form current pulses for micro resistance welding

The Former of Special Form Current Pulses for Micro Resistance Welding

Yuriy E. Paerand, Oleksandr F. Bondarenko, Yuliya V. Bondarenko

Donbas State Technical University (Alchevsk, Ukraine) [email protected], [email protected], [email protected]

Abstract—Conditions of obtaining high quality welded joints by

micro resistance welding are analyzed. Construction principle of the former of special form current pulses for micro resistance welding is offered. The former provides forming of weld current power pulse in accordance with electrophysic processes in welding contact. Offered pulse former included in experimental welding machine is used for welding details of important application. The results of experiments are given.

Index Terms—Micro resistance welding, current pulse former, special pulse schedule, reference pulse former, transistor linear mode, power feedback loop.

I. INTRODUCTION

Resistance welding is widely used to join details in electronics and instrument-making.

Resistance welding is realized by electrical current passing through the parts to be joined squeezed between electrodes. Current passing through the parts generates the heat in the resistance of weld area. Thus the temperature at the parts’ interface rises to melting point and the joint is formed as a result.

The heat generated during formation of a joint can be calculated by the following expression [1]:

∫ ⋅=Wt

0W

2WW dt)t(r)t(iQ , (1)

where iW(t) is weld current through the parts (in A), rW(t) is

an instant value of weld area common resistance (in Ω), tW is the weld time (in s).

The fig. 1a shows two surfaces of parts to be welded squeezed between electrodes. An instant value of weld area common resistance (fig. 1b) can be calculated by the following equation:

)t(r)t(r)t(r)t(r)t(r)t(r 3C2B2C1B1CW ++++= , (2)

where rC1(t) and rC3(t) are instant values of the contact

resistances between electrodes and details (in Ω), rC2(t) is an instant value of the contact resistance between details (in Ω), rB1(t) and rB2(t) are instant values of the details’ bulk resistances (in Ω) [2].

Each of the specified components of resistance rW(t) changes

during the weld process under its own complex law [1], [2]. The charts of the components’ changes are shown on the fig. 2, where RWo is the pre-weld (cold) common resistance, RC1o and RC3o are the pre-weld contact resistances between electrodes and details, RC2o is the pre-weld contact resistance between details, RB1o and RB2o are the pre-weld details’ bulk resistances, tC is the time in which RC2o becomes equal to zero (the joint is formed).

Fig. 2. Weld area common resistance and its components’ changes

during welding process

Fig. 1. Welding contact and its resistance components

396 2009 COMPATIBILITY AND POWER ELECTRONICS CPE2009 6TH INTERNATIONAL CONFERENCE-WORKSHOP

978-1-4244-2856-4/09/$25.00 ©2009 IEEE

We can see on the fig. 2 that the weld area common resistance changes most dynamically at the first moments of the weld current passing through the parts.

In the initial phase of welding the welded parts contact at some local points only, because of their surfaces’ microroughnesses (fig. 1c). Therefore, the weld current density at the contact points is very high. Later, under an effect of heating and electrode pressure the parts’ surfaces microroughnesses become destroyed, the contact areas increase and then become stabilized. But during the first moments of welding fast weld current rise may cause the excessive heating at the place of contact. It leads to the metal boiling and splash. In addition, the complex law of weld resistance change and extremely small duration of weld process may cause other defects in weld joint, such as: faulty fusion, weld nugget unwholeness, undesirable changes in metal structure [2].

It is necessary to note, that demands, made to the welded joints, are often limited to their strength only. But if details of important application are welded, in addition to strength it is required to provide some other parameters, such as: an absence of splashes, constancy of springiness, minimal metal structure changes etc.

The quality improvement of welded joints can be achieved by developing of special power supplies (current pulse formers), that realize the required welding pulse schedule [2] – [5]. Wrong schedule of weld current pulse may cause substantial worsening of welded joints quality. It is known that the best results in resistance welding may be obtained if the heat energy is input in the weld area gradually [2].

It is especially important to realize the right current pulse schedule in welding of small-scale parts, so called microwelding. Microwelding is joining of parts which have thickness or diameter in the range from several microns to 0.5 millimeter. Because of very small size of parts and very small welding duration (tenths of millisecond and less) great demands are made to pulse formers for micro resistance welding in providing high dynamic characteristics and possibility of pulse parameters regulation during the welding.

So, important role of pulse formers in providing the high quality welding joints is quite obvious. Because of that, it is highly topical to develop pulse former construction principle by maximal taking into account electrophysical processes in welding contact.

II. WELDING PULSE FORM

Welding joint quality improving is achieved by forming special form (schedule) weld current pulses, voltage pulses, power pulses or, sometimes, their combinations.

They use the following welding pulse schedules to obtain high quality joints [1]:

– pulse with smoothed leading edge; – dual pulse with preheating one; – complex form pulse derived from those described before. In works [4] – [6] it is shown that the best results may be

obtained with power pulse forming. Let’s consider basic electrophysical processes in welded contact to specify the power pulse schedule.

At work [6] it is explained that absence of splashes on the first stage of welding may be achieved by keeping constancy of heating intensity for true contact area unit:

constSQ

C

H = , (3)

where QH – common heating intensity (in J/s), SC – true

contact area (in m2).

Microroughnesses of welded parts’ surfaces may be modeled as pyramids with square bases. On the first stage of welding their sizes change in accordance with following expression [7]:

m)t/t()ya(yy Δ⋅−+= 00 , (4)

where t – a time moment (in s), m – degree index determined by

schedule of electrode pressure, tΔ – duration of current passing (in s), y – instant value of pyramid side width (in m), у0 – initial value of pyramid side width (in m), а – pyramid base width (in m).

Fig. 3 demonstrates how pyramid side width y and its area SC change on time interval tΔ. Those diagrams are built in accordance with expression (4).

Nonlinear functional dependence y(t) may be presented by degree function. The contact area SC increases proportionally to square side width of pyramid during microroughnesses crumpling. The common heating intensity QH is proportional to weld current power PW. So, the heating constancy (3) is kept if

n

W tkP ⋅= , (5)

where n = 2⋅ m, k – proportionality coefficient.

Therefore, on the welding initial stage the weld current power must be increased proportionally to time in n degree while heat energy amount necessary for welded parts contact area stabilization is allocated. Later, contact area SC does not almost change and it is required to keep weld current power PW on stable level for some time.

Fig. 3. Geometric parameters’ changes

of welding contact microroughnesses for tΔ.

SPECIAL POWER ELECTRONICS SYSTEMS AND APPLICATIONS 397

The weld current power pulse schedule on intervals of time described above may be presented by the following expression:

( )⎩⎨⎧

≤≤≤≤⋅

=;ttt,P

;tt0,tktP

WLmaxW

Ln

W (6)

where tL – weld pulse leading edge time (in s), tW – weld

pulse common time (in s), t – a time moment (in s).

Some forms of power pulse obtained by (6) with different degree indexes n are shown on the fig. 4.

It is quite difficult to define the most suitable index of degree n theoretically, so in practice the optimal schedule of weld current power pulse may be selected experimentally depending on concrete welding conditions (materials and configurations of parts and electrodes, electrodes pressure schedule etc.).

To obtain the best welded joints it is important to know the time moment when to finish the pulse leading edge forming and to begin the pulse flat top forming.

Due to the work of R. Holm [8] a fundamental principle of contact theory is known – the principle of voltage between weld electrodes correspondence to the welding contact temperature. Dependence between those variables is following:

2

0 2U)T2(L ⎟⎠⎞

⎜⎝⎛=⋅+⋅ θθ , (7)

where θ – contact overheating temperature in comparison to

environment temperature (in K), Т0 – environment temperature (in K), L – Lorentz function that for most metals at the temperature more than 200 К approximately equals to 2.4·10-8 (in (V/K)2), U – voltage between weld electrodes (V).

R. Holm introduced the following concepts: fritting voltage (destroy of oxide layer on metal surface), softening voltage, adherence voltage, melting voltage, boiling voltage. If the voltage between electrodes is lower than melting voltage the metal parts can not be welded. If the voltage is higher than boiling voltage splashes of molten metal are possible. Some threshold voltage between weld electrodes corresponds to the melting of parts’ surfaces microroughnesses. Consequently, the voltage between electrodes may be controlled during pulse leading edge forming, and when that voltage reaches threshold value from the interval "melting voltage – boiling voltage" it is time to begin forming the weld pulse flat top. At [8] R. Holm gave calculation formulas and data tables, which may be used in selecting necessary voltage interval and threshold voltage between electrodes for concrete materials.

So, it is necessary to develop special pulse former to realize power pulse schedule (6) with required degree index n and with control of voltage between electrodes.

III. PULSE FORMER TYPE CHOICE

For today there are a variety of current pulse former types for contact welding. Nippon Avionics (Japan), Seiwa Manufacturing (Japan), MacGregor Welding Systems (Great Britain), Miyachi (Japan), Unitek (USA), Peco (Germany) are leading world manufacturers of pulse formers for contact welding.

Most authors and manufacturers [9] – [12] select following basic types of pulse formers: AC Type, Capacitive Discharge Type, Linear DC Type or in some articles Transistor Type, Inverter Type.

Comparing pulse former types given above, it is necessary to note some their features in pulse forming.

Capacitive Discharge Type formers do not allow changing pulse form (schedule) during welding process. Required pulse form in that type of formers may be set only before beginning of welding and may be changed only by changing of weld circuit elements’ parameters. It is not convenient because there is no possibility to take into account contact resistance changing during welding process.

AC Type, Inverter Type and Linear DC Type formers, constructed as closed loop systems, keep the determined pulse form when contact resistance is changing. But AC Type and Inverter Type formers can not provide high regulation speed because of pulse mode of their work, which requires a presence of filters in feedback loop. It is a problem of AC Type former especially.

Linear DC Type formers are free of disadvantages discussed above. Linear work mode of transistors, that regulate weld current, provides the best regulating speed and accuracy. In addition to that Linear DC Type former may be constructed without inertial weld transformer. Due to that high pulse schedule accuracy may be achieved [13].

Fig. 4. Power pulse forms obtained by (6)


IV. WORK PRINCIPLES OF OFFERED PULSE FORMER

The block diagram of offered pulse former for micro resistance welding is presented on the fig. 5.

The former includes: weld current source, weld current regulator, current feedback signal sensor, voltage feedback signal sensor, multiplication block, comparing block, buffer amplifier, reference power pulse former, energy defining block, comparator 1, comparator 2, integrator, control block, threshold electrode voltage defining block.

Let’s consider former blocks purposes and its work principle.

When welded parts are placed at welding device and pressed by electrodes the reference power pulse former generates reference pulse, which defines weld current power pulse schedule (6). Reference power pulse enters one of the comparing block inputs. Then it enters the buffer amplifier and after amplifying it goes to the weld current regulator. It causes the weld current passing in weld circuit.

Weld current and electrodes voltage values are measured by current and voltage feedback signal sensors. Sensor signals are multiplied by the multiplication block. The multiplication block forms a signal that is proportional to an instant power value. The power signal is compared with reference pulse in the comparing block and the difference signal appears on its output. The difference signal amplified by buffer amplifier enters the weld current regulator, which increases or decreases weld current to keep pulse power correspondence to reference power pulse.

To define necessary pulse leading edge duration the check of voltage between electrodes is performed. Voltage sensor signal enters the comparator 2 for comparing with voltage value of threshold electrode voltage defining block. When the signals are equal the comparator 2 sends the signal to control block, which makes reference power pulse former to stop weld pulse leading edge forming and to begin pulse flat top forming (fig. 4).

From the multiplication block output power value enters the integrator input. The integrator treats the power signal and forms the signal that is proportional to energy located in weld contact. The energy value is compared by the comparator 1 with reference energy value necessary for welding. The reference energy value is set in the energy defining block. When the energy signals are equal the comparator 1 sends the signal to the control block, which makes the reference power pulse former to interrupt reference power pulse forming. Thus the weld current falls down to zero. The device returns to initial state and is ready to next weld pulse forming.

V. PULSE FORMER POWER CIRCUIT CONSTRUCTION FEATURES

The diagram of the offered pulse former power circuit is shown on the fig. 6. Power circuit is constructed in accordance with block diagram on the fig. 5.

The weld current source may be constructed as combined energy storage including battery GB1 and electrolytic capacitor C1 of great capacity. Due to that construction principle weld current source has great capacity and low own resistance, high speed of energy accumulating and delivering, constancy of parameters at frequent recharging [14]. Rechargeable lead-acid battery with capacity 50 Ah and nominal voltage 12 V and electrolytic capacitor with capacity 0.1 F were used in practical experiments. Ultracapacitors may be used instead, but they are more expensive yet.

AC line energy transforming for the weld current source charging is realized by the charge device. The weld current source charging is carried out in pauses between welding pulses, so the charge device influence on pulse forming process is excepted.

The weld current regulator may be constructed on n powerful transistors MOSFET VT1…VTn in linear mode connected in parallel and protected by shunting diodes VD1…VDn. MOSFET are preferable because their parallel connection as a rule does not require additional circuits to eliminate current and temperature unbalances between devices. The amount of transistors in weld current regulator is determined by necessary weld current value.

VI. REFERENCE POWER PULSE FORMER The reference power pulse former is an important part of the

pulse former for contact microwelding. The reference power pulse former constructing on the base

of microcontroller gives a possibility to set the necessary weld pulse schedule and to change the schedule for concrete welding conditions.

The block diagram of the reference power pulse former is shown on the fig. 7. It consists of microcontroller, digital-to-analog converter (DAC) and signal amplifier.

Let’s consider the purposes of the reference power pulse former blocks and the work principles of the former.

Fig. 5. Block diagram of the pulse former for micro resistance welding


Because of complex function computation difficulty by

microcontroller, a table of calculated values of reference pulse is created and located in microcontroller memory cells. To calculate reference pulse values the methods of digital integrating are used: continuous function is replaced by its discrete average at each time interval between samplings. Discrete binary values enter DAC by microcontroller memory cells scanning. DAC transforms them into analog values, which are amplified and enter the comparing block to be compared with feedback power pulse values (fig. 5).

Reference power pulse former scheme is shown on the fig. 8. Experimental researches with that scheme were carried out. In the researches microcontroller AT89C51 (clock rate 7.3728 MHz) and 8-bit DAC AD7801 were used. The table of calculated values of reference pulse was created by MathCAD application.

The diagrams of reference pulse formed in accordance with (6) are shown on the fig. 9. Degree indexes n were set as equal to 0.25, 0.5, 2 and 4.

The described approach to the reference pulse former constructing provides possibility of experimental weld pulse schedule selection depending on welding conditions.

VII. EXPERIMENTAL WELDING EQUIPMENT

Efficiency of offered technical decisions was tested by use of experimental weld machine including designed weld current pulse former, which provides regulated power schedule, for micro resistance welding. At the table 1 the former important technical characteristics are given.

The photo of experimental welding equipment is given on the fig. 10.

The welding machine was used for producing electromechanical filters’ oscillating systems (fig. 11). It joined cylindrical elinvar resonators (diameter 3 mm) with elinvar wires (diameter 0.22 mm). The following values of basic parameters were set: weld pulse energy – 0.5 J, threshold voltage between electrodes – 1.5 V (found experimentally), weld pulse leading edge time – 0.75...1.5 ms, common weld pulse time – 1...2 ms, weld current power amplitude – 300...550 W.

Quality of welded joints and springiness constancy of oscillating system stipulate the most important electromechanical filter characteristics, such as unevenness of frequency response, attenuation and selective property.

TABLE 1

WELD PULSE FORMER TECHNICAL CHARACTERISTICS

AC line input voltage, V 220 AC line input frequency, Hz 50 Max pulse energy, J 10 Max pulse power, W 1000 Max pulse duration, ms 10 Max voltage between electrodes, V 3 Max weld current, А 1000 Pulse form (schedule) set by operator Reference pulse sample time, μs 10

n = 0.25 n = 0.5

n = 2 n = 4

Fig. 9. Reference pulse diagrams

0.2 ms

1 V

0.2 ms

1 V

0.2 ms

1 V

0.2 ms

1 V

Fig. 6. The pulse former power circuit diagram

Fig. 7. Block diagram of the reference pulse former

Fig. 8. The reference pulse former scheme


While comparing parameters of electromechanical filters

produced by welding equipment, which included the offered pulse former, and parameters of filters produced by prototype equipment [6], 20 percent decrease of frequency response unevenness was noted [15] – [17].

VIII. CONCLUSION Analysis of electrophysic processes in welding contact has

shown that the best quality of welded joints may be obtained if the weld pulse schedule is following: on the welding initial stage the weld current power must be increased proportionally to time in degree n till the moment of equality of electrode voltage and threshold voltage from interval "melting voltage – boiling voltage", then the pulse power must be kept on the constant level while heat energy amount necessary for welding is allocated.

Pulse former, which is able to realize necessary schedule of weld current power pulse and to change it for concrete welding conditions, is offered.

The use of experimental welding equipment including the offered pulse former has allowed improving the welded joints quality.

Consequently, the offered pulse former may be recommended for precise resistance micro welding of important application devices’ details.

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[14] Ю.Э. Паэранд, Ю.В. Бондаренко и А.Ф. Бондаренко "Комбинированный накопитель энергии «аккумуляторная батарея – электролитический конденсатор» ", Технічна електродинаміка. Тем. вип. Проблеми сучасної електротехніки, vol. 6. pp. 76-79, 2008.

[15] Yu.E. Paerand and O.F. Bondarenko "Method for Control of Process of Contact Spot Welding", UA20927, B23K11/24, Feb. 15, 2007. [Online]. Available: http://v3.espacenet.com/textdoc?DB=EPODOC&IDX=UA20927U&F=0

[16] Yu.E. Paerand and O.F. Bondarenko "Device for Controlling the Process of Contact Spot Welding", UA21356, B23K11/24, Mar. 15, 2007. [Online]. Available: http://v3.espacenet.com/textdoc?DB=EPODOC&IDX=UA21356U&F=0

[17] Yu.E. Paerand and O.F. Bondarenko "Device for Control of Process of Resistance Spot Welding", UA79189, B23K11/24, May 25, 2007. [Online]. Available: http://v3.espacenet.com/textdoc?DB=EPODOC&IDX=UA79189&F=0

Fig.10. Experimental welding equipment

welded joints

Fig.11. Electromechanical filter oscillating system


The former of special form current pulses for micro resistance welding

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