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Description of a Technique to Attach Thermocouples to Aluminum Alloy Tensile Samples

Mary-Anne Kulas
03/12/01

Introduction

This technique presents another way to use a thermocouple to measure temperatures during our experiments. Previously, the usual way to measure temperatures was to attach a thermocouple of type K sheathed with Inconel to the sample with a Ni wire. There was only one point of contact of a thermocouple with the sample, and this contact was not firm. With this new method, we will assure a good, secure contact between our samples and the thermocouple.

Principle

The new technique involves the welding of a cylindrical piece of pure aluminum, 2.3 mm length and 1.3 mm diameter, on a sample. A thermocouple is placed inside a hole located in the center of the Al piece. Finally, soft Al is squeezed closed to maintain the thermocouple inside the Al piece.

Equipment

  1. Thermocouple: It consists of a junction, obtained by welding, of one wire of Alumel and one of Chromel, each one of 0.254 mm diameter. It is also a thermocouple of type K, as those used before.
  2. Piece of Al: This piece was obtained by cutting an extruded rod of pure Al with a diamond saw. The initial rod of pure Al had the following characteristics: 5 mm diameter and 99.999% purity. It is important to notice that the extruded Al piece is not perfectly circular.
  3. Tools used:
    1. A twist drill to maintain the Al piece during grinding
    2. A steel plate of around 2.5 mm thickness providing with a hole of 1.78 mm diameter
    3. A pointed tool and a hammer to make a hole in the center of the Al piece
    4. 1200 grit carbide paper and ethanol to clean the sample and Al piece
    5. Tweezers to hold the Al piece when we make contact between sample and Al piece for welding

Procedure

  1. With a diamond saw, cut a cylindrical Al piece of about 2.3 mm length.
  2. With the help of a twist drill, grind the 2 faces of the Al piece in order to have flat surfaces.
  3. Make a thermocouple.
  4. Put the Al piece inside the hole of the steel plate and, with the pointed tool and the hammer, punch! Try to make the hole as close to the center as possible.
  5. Maintain strongly the position of the pointed tool while punching and check with a thermocouple to assure that the hole is deep enough.
  6. Grind again the 2 faces of the Al piece and clean them with ethanol.
  7. Do step 6 with the sample on the surface which will be welded
  8. Put the sample in contact with the Al piece. Hold the Al piece with tweezers for sample placement in the welder, but take the tweezers out during welding. Welding is done at 100 Ws-1 and the indicative of load is set at 140.
  9. Put the thermocouple inside the hole of Al piece and squeeze in order to fix the thermocouple in the Al piece.

Control of the Accuracy of this New Method

The control was made by comparing different temperatures measured by the new thermocouple and the one used before. The thermocouples are attached on the sample next to each other and the sample is put in an alumina boat. Then, the assembly is inserted into the furnace and different temperatures are set. Below is the table which summarizes the temperatures obtained.

Tfurnace(°C)

28

100

300

500

Tin(°C)

28.6

95.9

289.9

500.1

Tn(°C)

28.3

95.2

287.8

505.8

Tin: temperature of the thermocouple sheathed with Inconel

Tn: temperature of the new thermocouple

The difference between the 2 thermocouples for each temperature is always less than 10%.

Conclusion

According to the comparison between the 2 thermocouples, we found that the new technique to measure the temperatures during our experiments is accurate and presents 2 more advantages compared to the old way. The first one is that aluminum is a better conductor than Inconel, and the second one is that we assure good contact between our sample and the thermocouple. Therefore, this new method to measure temperatures will be used for our next experiments.