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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, [email protected]
2 Politehnica University of Timioara, [email protected]
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)