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Nonconventional Technologies Review 2013 Romanian Association of Nonconventional Technologies Romania, April, 2013

RESEARCH ON WELDING WITH STORED ENERGY IN THE CAPACITOR OF

THE AMORPHOUS ALLOYS IN RIBBON FORM

Burc Mircea1, Codrean Cosmin2 1 Politehnica University of Timioara, mircea.burca@mecupt.ro

2 Politehnica University of Timioara, cosmin.codrean@mecupt.ro

ABSTRACT: Amorphous alloys often exceptional characteristics not associated in such way at other known class of materials.

Amorphous alloys are industrially produced as ribbons with thicknesses below 60 m, which makes them difficult to be welded. Resistance spot welding with stored energy in capacitors is applied to thin sheet or foil due to very precise energy input in the

components at the capacitors discharge This paper presents the results of experimental research on spot welding with stored energy

in the capacitor of metallic amorphous ribbons with a thickness of 50m, in order to establish of welding technologies. KEY WORDS: spot welding, stored energy in capacitors, amorphous alloys, ribbons

1. INTRODUCTION

The thin sheet welding of foil type, s

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The capacitors battery BC is charging by

thyristor T1 with a controlled voltage U by

potentiometer Pc. The resistance R limits the current

initial shock that occurs in the initial stage, at the

battery charging start. To order for welding, the

thyristor T1 is blocked and immediately start up the

thyristor T2, and the stored energy in capacitors BC

is transferred in TS welding transformer primary and

in the secondary of the transformer is obtained the

welding current.

Advantages:

- symmetric charge when using a three-phase bridge

rectifier PRT;

- constant welding parameters as the battery voltage

is stabilized at voltage fluctuations and the value of

the capacitors do not fluctuate.

- by adjusting the battery voltage U and its capacity

C can obtain a very precise dosage of energy E,

which is very important to micro welded joints:

E = CU2/2 (1)

- power consumption in charging phase is 50 ... 100

times smaller than a standard spot welding machine

which welds in the same conditions because the

welding time, ts, can be more than 50 ... 100 times

smaller than the battery charge time:

ts = tinitial + tforging + tbreak (2)

Disadvantages:

- more expensive machine because battery high

capacity capacitors BC: C = 5000 ... 100000F;

- lower efficiency than a standard spot welding

machines;

- welding time cannot be adjusted and depends on

the total welding circuit inductance and the battery

capacity.

There are two waveforms on the capacitors

discharge depending on the electrical characteristics

of for welding circuit, Figure 2:

- aperiodic discharge, if C

LR tt 2 , Figure 2a;

- periodically amortized discharge, if C

LR tt 2

Figure 2.b.

where Rt, Lt is the total resistance and inductance of

the electrical circuit.

At first variant is required the presence of an

inverter for reversing the direction of the electric

current passing through the transformer after each

weld point to avoid transient phenomena related to

transformer magnetic remanence.

Second variant requires the presence of the diode

D for negative alternation conduction.

Aperiodic

discharge Periodically amortized

discharge

a) b)

Figure 2. The waveforms on the capacitors discharge

The capacitors are special electrolytics with high

capacity 5000 ... 100,000 F and resists at sudden shock discharges.

3. WELDING EQUIPMENT

Figure 3 shows the spot welding machine

MSCIPT-2, modernized and made by self

endowment in the Welding Department of

Politehnica University of Timisoara.

Resistance spot welding machine with stored

energy in capacitors consists of the following parts:

frame, welding transformer, outside circuit,

actuation system of mobile arm, capacitors,

electrical installation of welding and control circuit.

The main technical characteristics of the

equipment are:

- supply voltage: 230V, 50Hz;

- maximum voltage for battery capacitors

charging: 500V;

- maximum energy accumulated in capacitors:

1.25 kJ (at total capacity of 10000F);

- the main battery capacity: 4000F, plus 6000F auxiliary battery capacity which is adjustable by technological needs;

- the maximum pressing force on electrode:

600N;

- displacement of mobile electrode: 100mm;

- operation mode of the mobile electrode:

pneumatic at pmax = 8bar;

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The battery voltage capacitor is adjusted in 6

steps by changing the number of the transformer

primary. Capacity of the capacitors can be adjusted

by putting in parallel capacitors of different

capacities. The clamping force of the electrodes is

controlled using the pressure regulator FRU group

by changing the compressed air pressure. The

transformation ratio of welding transformer is set by

connecting in series or parallel of spiral coils of the

primary winding in the following steps: 150-125-

100-75-50-25.

Figure 3. Spot welding machine MSCIPT-2

4. EXPERIMENTAL RESEARCHES

4.1 Basic material

Amorphous structure is characterized by

disordered arrangement of atoms in space. In

amorphous metal case, the distribution of atoms in

space is not entirely random, but grouping of

neighbouring atoms complies a certain arrangement

due to chemical or topological constraints.

Consequently, the amorphous structure is

characterized by the absence of long-range atomic

ordering and some ordered distribution on short

interatomic distances [4].

Amorphous metal alloys often exceptional

characteristics not associated in such way at other

known class of materials. The absence of

crystallinity leads to high values of tensile strength,

ease magnetization, extremely low attenuation of

acoustic waves and electrical resistivity appreciated.

Also, some structural and compositional

homogeneity lead to higher electrochemical

corrosion resistance. Unlimited solubility of

chemical compounds contained in metal glasses, in

comparison with limited solubility encountered in

most crystalline alloy systems, underlying the

electronic transport properties at low temperatures

not found at any other classes of materials [2, 3, 4].

In this paper as base material was used

amorphous alloy Ni68Cr7Fe3B14Si8 prepared by

planar flow casting method as a geometrically

uniform ribbon,with 50 m thickness and 100 mm width, Figure 4.

Figure 4. Amorphous alloy ribbon

To certify the structure of obtained amorphous

alloy, it was subjected to structural analysis by X-ray

diffraction (XRD), Figure 5.

Figure 5. The diffraction spectrum of

Ni68Cr7Fe3B14Si8 alloy

The diffraction spectrum is characterized by the

absence of net intensity peaks, which indicates an

amorphous structure.

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4.2 Establishing welding technology

Literature has relatively little information on the

resistance spot welding with energy stored in the

capacitor of thin foils, especially for amorphous

metals ribbons, which determined the extension of

the experiments to a large numbers of samples in

order to determine a feasible welding technology. In

micro welded jonts case it requires very precise

dosage of energy to charge capacitors E, which

means knowing how to set and control it.

The adjustment of welding technology enables

the following options, alone or combined, Figure 6.

1. change the battery charging voltage U, Figure 6.a;

2. changing battery capacity C, Figure 6.b;

3. change the transformer transformation ratio

k = W1i/W2 (where E '= E''), Figure 6.c

a) b)

c)

d)

Figure 6. Set of the welding technology

For a given energy on can change load voltage

U or battery capacity C or both. If the base material

is sensitive to welding it is recommended increase

discharge time, tdesc. For a constant energy, E, one

can act through transformation ratio, k or a

combination of, C and U, increasing capacity C, and

reducing voltage U. Welding machines are equipped

with three types of adjustment.

The machine programmer is relatively simple

because the welding time cannot be adjusted, and

therefore it must ensure only lowering and

maintaining time of mobile electrode, respectively

forging time and pause time between two points, see

Figure 6.d cyclorama.

Therefore specific technological parameters of

the welding process are:

- charging voltage of the capacitors U;

- capacity of the capacitors C;

- the pressing force of welding components F;

- stored energy in capacitors E.

Experimental investigations have focused

mostly on two aspects, on the one hand to obtain

acceptable weld points from qualitative point of

view, without defects such as material breakthrough

or material sticking on electrodes, with surface

damage (aesthetic) of welded point, and on the other

hand to provide the mechanical resistance of welded

point.

This was achieved by changing for welding

successive energy E, for different values of capacity

or charging voltage of capacitor and keeping

constant the pressure and transformation ratio. If

welding energy is too low there is only a surface

solder of the components, without mechanical

strength, and if energy is too high it produce

breakthrough of components and ribbons brazing to

the copper electrodes.

The welding technological parameters that gave

the best results are:

- charging voltage: U = 300V;

- capacity: C = 40F capacitors; - pressing force: F = 0.5 kN;

- compressed air pressure: p = 4 bar;

- welding energy: E = 1.8 J.

The aspect of weld points is shown in Figure 7.

Figure 7. Aspect of welded points

There is a good reproducibility of the welded

points, barely discernible marks without damaging

the ribbon surface, which shows a very good dose of

energy input in welding process.

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Mechanical resistance of welds was examined

by cross-tension test of welded points. Acceptance

welding by this method is if the plucking of welded

point takes place from a component without its

detachment.

Figure 8 shows the fracture appearance after

cross-tension test in welded joints.

It is found that in all cases the breakage of

material around the welded point which

demonstrates the very good quality of welds,

respectively the welding technology feasibility

established from experimental tests.

Figure 8. Breakage by cross-tension test

Microscopic examination of welded point after

cross-tension test shows a welded joint even

without evidence of heat affected zone, Figure 9.

There is a continuity of material in welded joints

without to notify a distinction between basic

materials.

It is noted that breaking after cross-tension test

occurred around welded point, therefore the chosen

welding technology is adequate.

Figure 9. Microscopic aspect of the weld point

5. CONCLUSIONS

Thin sheet as foil type welding requires the use

of processes which allow a very precise control of

energy introduced in the components to avoid the

material burning with its breakthrough.

Energy stored in the capacitor welding process

is ideal for spot welding ribbons because of very

precise dosage of energy and adjustment

possibilities in very large limits.

Welding of amorphous metal ribbons presents

a further difficulty mainly because the particular

properties and high electrical resistivity.

Experimental research, analyzes and tests

carried out, confirmed that the 50m thickness ribbons of amorphous alloy material

Ni68Cr7Fe3B14Si8 can successfully welding using

resistance spot welding with stored energy in

capacitor, which are prerequisites widening and

furthering of the research in the future.

6. REFERENCES

1. Fukushima, S., Spot welding of amorphous alloys, Welding International, Vol. 5, Iss.8, pp.

654-659, (1991).

2. Schuh, Christopher A., Hufnagel, Todd C., Ramamurty, Upadrasta, Mechanical behaviour

of amorphous alloys, Acta Materialia Vol.55,

pp.40674109, (2007). 3. Nowosielski, R., Babilas, R., Ochin, P.,

Stoklosa Z, c., Thermal and magnetic

properties of selected Fe-based metallic glasses,

Archives of Materials Science and Engineering

Vol.3, No.1, pp. 13 -16, (2008).

4. Ashby, M., Greer, A., Metallic glasses as structural materials, Scripta Materialia, Vol.

54, pp. 321-326, (2006).

5. Xiaowei, W., Haobin Z. and Xiangqian Xu,

Stored Energy Welding Technology of Ultra-

thin Sheet Stainless Steel, Transactions of

JWRI, Special Issue on WSE 2011, pp. 31-32,

(2011)

6. *** American Welding Society, 1991, Welding

Handbook, Vol. 2, pp. 541, 8th

edition, (1991)

7. Popovici, Vl., Negoitescu, St., Glita, Gh.,

Echipamente pentru sudare-Indrumator pentru

lucrari de laborator, Editura UPTVT Timioara, (1985)