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Safe and consistent method of spot-welding platinum thermocouple wires and foils for high temperature measurements

AIP Publishing
Review of Scientific Instruments
Authors:
Gilad Orr at Ariel University
Gilad Orr
Michael Roth at Hebrew University of Jerusalem
Michael Roth
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Abstract and Figures

A low-voltage (mV) electronically triggered spot welding system for fabricating fine thermocouples and thin sheets used in high-temperature characterization of materials' properties is suggested. The system is based on the capacitance discharge method with a timed trigger for obtaining reliable and consistent welds. In contrast to existing techniques based on employing high voltage DC supplies for charging the capacitor or supplies with positive and negative rails, this method uses a simple, standard dual power supply available at most of the physical laboratories or can be acquired at a low cost. In addition, an efficient and simple method of fabricating non-sticking electrodes that do not contaminate the weld area is suggested and implemented.
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All content in this area was uploaded by Gilad Orr on Sep 01, 2018
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160 µF
12 µm
9mΩ
GND
1
TR
2
Q3
R
4
CV
5THR 6
DIS 7
V+ 8
U1
LM555
..
SW1
Q1
IRF1104
+12
R7
1M
R8
100
C6
10nF
C4
0.1uF
C3
10nF
D2
1N914
C5
17.4mF
0 ÷ 40V
R9
10mΩ
DSO
21
U2A
40106
3 4
U2B
D1
1N914
C1
0.1uF
R2
100k
R1
100k
R6
20K MultiturnR5
1K
R4
10K
R3
10K
C2
3nF
1
2
3
4
5
6
4N25
OPTO SW3
R12
4.7K
R10
390 R11
10K
+12
Q2
2N3904
Logic In
Logic common
Analog Timer
Computer/Microcontroller/Lo gic Interface
Discharge Circuit
R12
18
12
Z2
6.2 V
1 2
Z1
6.2 V
dV/dt
Z1Z2
10 mΩ
100µs 2ms
C4
10µs
CuSO4·5H2O
20 mA/cm2
10 mΩ
Graphite anode without deposition
Monitored Voltage [V]
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Time [ms]
-2 0 2 4 6 8 10
-2 0 2 4 6 8 10
Measurement
Fit
τ = 3.72 ms
V0 = 0.69 V
I0 = 69 A
RT = 214 mΩ
Graphite anode with thin copper deposition
Monitored Voltage [V]
-0.5
0
0.5
1
1.5
2
-0.5
0
0.5
1
1.5
2
Time [ms]
-2 0 2 4 6 8 10
-2 0 2 4 6 8 10
Measurement
Fit
τ = 1.914 ms
V0 = 1.65 V
I0 = 165 A
RT = 110 mΩ
0.949J, 1.9ms
0.831J, 1.44ms
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
-1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0
Time [msec]
Voltage [V]
E=E0(1 −exp(−t
/τ)) τ
E0
Welding energy (J)
0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
0.8
1
Time (ms)
0 0.5 1 1.5 2
30µm 100µm
70µm 30µm
... A voltage difference occurs due to the temperature difference, and this correlation allows temperature measurement. A thermocouple is a simple and robust device suitable for wide temperature range measurements [6]. They have been used mainly for their accuracy and relatively easy setup [7]. ...
... As most situations in welding analysis involve a metallic component, the literature recommends thermocouple welding to the sample. The main reason for that is related to the fact that it does not introduce any third substance into the junction that could influence accuracy [6]. Despite being mechanically satisfactory, clips and bolts contribute to mass and volume increase and modify the heat diffusion distribution influencing the measured results. ...
... Even if the simulation is not validated, thermocouples would be required to evaluate material properties, such as thermal conductivity and thermal diffusivity, which are input parameters for the models [6]. An accurate program would consider the fact that thermal properties are a function of temperature and not constants. ...
Thermocouple positioning through capacitive discharge for temperature monitoring in laser welding
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Temperature measurement in welding constitutes relevant information for process understanding and simulation, as it can be used to validate and predict microstructure. Monitoring can be performed at specific points through transducers. The typical one is the thermocouple. In laser welding processes, due to the high energy concentration of the beam, the acquisition with thermocouples should be performed closer to the bead when compared to electric arc welding processes. This measurement is critical because of overheating by spatters, which can result in damage. The present work covers developing a Capacitive Discharge Welding device for thermocouple fixing and assessing possible temperature acquisition positions in keyhole laser welding. The study performed all tests with a fiber laser of 9 kW power and 3 m/min scanning speed. When placed at 2 mm from the weld centerline, the results showed a high probability of spattering due to the 71° range of incidence angle. Therefore, the thermocouple should be placed far from the weld centerline to minimize the spattering. However, it must be close enough to acquire temperatures above 727 °C. This temperature corresponds to the envisaged austenitization microstructure correlation. The study found the maximum distance at 3.5 mm based on an interpolation map, where the incidence spatters angle reduced to 59°. At a 2 mm distance from the weld centerline, the weld cross section temperatures reached more than 727 °C. However, the microstructural changes did not occur due to insufficient austenitization time. The Heat Affected Zone boundary was at 1.64 mm from the weld centerline. Therefore, to correlate microstructure with thermal cycles, it is necessary to place thermocouples at a distance below this value. However, spatter can be a problem since the range of incidence angle is about 75°. Therefore, this work recommends using more than one thermocouple for each position (redundancy) to increase valuable data.Graphic abstract
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  • Jan 2019
  • SOLDAGEM INSP
Resumo Os termopares são dispositivos elétricos de simples concepção utilizados na medição de temperatura, e possuem um grande uso na indústria. O termopar necessita de uma junção eficiente para os dois fios metálicos que o compõem, que comumente é feita pela soldagem. O processo de soldagem utilizado para construir um termopar pode ser feito de várias maneiras, e a mais comum e eficiente é a soldagem por descarga capacitiva, que irá fundir a liga e criar uma união, mantendo as propriedades dos metais envolvidos e resultar e uma união homogênea dos dois metais. Este trabalho tem como objetivo desenvolver um equipamento capaz de realizar automaticamente esta solda em ambiente inerte, por descarga capacitiva, cujo valor da energia a ser descarregada para a união do termopar é determinada utilizando o princípio da conservação da massa e energia em ligas metálicas e bitolas preestabelecidas de alguns tipos de termopares. Os resultados obtidos com a soldagem de diferentes tipos e bitolas de termopar foram analisados com relação ao aspecto visual e de softwares de análise de imagens, juntamente com curvas de calibração. Os termopares foram considerados de boa qualidade de acordo com os parâmetros de análise estabelecidos, e comparações com termopares comerciais.
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... The mass conservation of the alloys involved is important to assure the quality of the welding in a thermocouple, because there is no addition of a third alloy or element in the system. The inclusion of a third element between the pair of metallic in the welding process must be avoided what attribute a high efficiency and precision the measure equipment [3]. Similar equipment was developed by Zanstra [1] and G. Orr and M. Roth [3]. ...
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... The inclusion of a third element between the pair of metallic in the welding process must be avoided what attribute a high efficiency and precision the measure equipment [3]. Similar equipment was developed by Zanstra [2] and G. Orr and M. Roth [4]. In a market research, we found these welding equipment [5][6][7], in this equipment there is just a voltage regulator and there is no way of automation of the amount of energy to be used, depending of the thermocouple or alloy gauge, the welding process is almost completely done by humans, what makes the results susceptible to fails. ...
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... Connection consisted of thin platinum wires (φ70µm) which were carefully welded to the electrodes. The micro welding setup which was built for the task is discussed in in a previous article [15]. Special attention was given to electronic isolation from the surroundings in order to prevent leakage current. ...
High temperature, time domain measurement system for solid and molten materials
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A low cost time domain system for measuring dielectric relaxation in the frequency range of 1 Hz - 1 MHz of solid and molten material at high temperatures is described in detail. A parallel plate electrode sample cell is described that can hold molten material by capillary action. In order to avoid electrical breakdown or a change in electrode distance due to opposite charge buildup, the step voltage can be set to the mV range. At this level interference noise effects the measurements and active filtering is required. The resulting signal is captured using a digital storage oscilloscope. An algorithm for reducing the measured data prior to fitting is discussed. The system is calibration free and its upper temperature is limited only by the construction materials of the furnace and electrodes. Molten NaCl is presented as a test sample.
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Electrodeposition of platinum on metallic and nonmetallic substrates - Selection of experimental conditions
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The possibility for platinum electrodeposition on metallic and nonmetallic electrodes has been examined. Several materials were used to prepare the working electrodes for platinum electrodeposition: tungsten, titanium, rhenium, zirconium, stainless steel, glassy carbon (GC), polyaniline, and poly(2-hydroxy-3-aminophenazine). Efforts were made to establish experimental conditions under which the very initial, nucleation stage of the platinum electrodeposition could be studied. It was found that tungsten, titanium, and GC were suitable substrates for nucleation and growth studies in aqueous 0.1 M HClO4 solution containing K2PtCl6.
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The methods suitable for constructing thermocouple junctions are discussed, and descriptions are given of apparatus for resistance butt welding and arc welding wires.
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Spot welder for making small electrical contacts
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A capacitive-discharge spot welder is described which is primarily intended for making electrical connections between small (<25–150 μm diameter) wires and samples of metallic materials, although it can be used in many applications where miniature welds must be made. It has been particularly useful in allowing low resistance (several mΩ) contacts to be created on samples of rare earth and actinide intermetallic compounds which are highly resistant to soldering. The device has a minimum time constant of about 25 μs and can deliver a maximum of between 700 μJ and 400 mJ of energy to the weld region, depending on the resistance of the latter. A suitable electrode holder is also described, and considerations for choosing the electrode materials, selecting the contact wires, preparing the sample, and creating a weld are discussed. © 1998 American Institute of Physics.
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