I am trying to weld a k-thermocouple on a Ti-6Al-4V sample but the thermocouple comes off every time at 950 °C (after four attempts). Is k-thermocouple suitable for titanium alloys at high temperatures? How can I obtain a good thermocouple's weld?
That is because Ti react with iron, if you check Ti-Fe phase diagram, you will find that. I tried to weld k thermocouple on ti64, it fall off at 950C. Can't use k at that high temperature.
It is difficult to attach chromel and alumel wires to Ti alloys for high temp measurement. The use of smaller diameter wires (0.003-0.005") does allow for a stronger weld (as opposed to larger diameter 0.010 or 0.032"), but when you are getting up to 900C and hotter you willl always encounter detachment. When spotwelding fine wires the key is to determine the proper combination of heat input, spring force, and wire diameter.
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He is an expert in disk laser welding of such titanium alloy
Absorption of oxygen by Ti-Al-V alloys leads to the formation of an extremely hard and brittle Alpha oxygen stable phase as a layer upon heating in air ,which can probably act against k-thermocouple welding, so I suggest the use of friction stir welding or arc melting under vacuum.
Welding of a thermocouple on to the TI6Al4-alloy specimen might affect the thermal-electrical properties of the couple, thereby influencing its signal that depends on the compositional differences between the two couple threads. With reference to earlier answers I would suggest temperature measurements based on non-material contacts, e.g. radiation.
Yes, Ti and Fe will react at that temperature. You have to look for other type of thermocouple.
Some additional details are required: what is the diameter of thermocouple wires, the welding technique? and if the failure occurs in the weld junktion of theh thermocouple to the specimen or in the thermocouple wire side? What is the max. measutred temperature you expect?
I think the better way to insert thermocouple is to drill a small blind hole in your work plate equal to that of thermocouple wire diameter (after joining two of its junctions i.e. Ni and Chroml wire) and then try to insert thermocouple wire into this small hole and pucnh it from top so that it make proper contact with the surface.
the thermocouple wire diameter was 0.08 mm, it was welded using the capacitive discharge welding, the failure occurs in the weld junction between the thermocouple and the sample. I expect to reach 1100 °C.
Use different type of TC for temperature you want to reach. I would suggest S type. Also if sample is not very large, you can tie wires around it and tighten them down to ensure good contact. Sometime we also drilled small hole to insert wires in and then hammer it down for contact. It all depends on the sample size and later usage.
we used also the method you suggest Murshid Imam, but it even doesn't work. In fact, I check on the Ti-Ni phase diagram (as a professor I asked suggested) and I found that there is an eutectic transformation at about 940 °C, this transformation may cause embrittelement and fall off the weld junction.
what Murshid Imam suggested is in the right approach. In addition, it may be a good idea to drill a bigger hole and embed the thermo-couple wire inside the hole and fill the hole with some metal (alloy) which has the similar or better thermo conductivity than the Ti alloy but do not reaction with K thermo couple (chromel and alumel), What metal or alloy will be (for example copper / copper alloy)?
I recommend the method suggested by (Murshid Imam) you may insert it mechanically and use some high temperature contact grease to improve the accuracy of measurements
I don,t think that you required any metallurgical aspects ( for example seeing phase diagram) for inserting thermocouple. Its a simple mechanical concept. since your wire diameter is very small, it may likely happen that at higher temperature it is getting melted because its a very small diameter. If possible use some larger diameter wire, automatically thermal resistance will be high and use mechanical concept to get it to attached.
Mr Murshid Imam :
Whether the wire been melt or not should not depend on its diameter if not consider the diffusion of the other element and the reaction on the surface
Yes Zhang you right just i want to say that use some larger diameter wire so that you can fix it with the workpiece with the process what i have suggested earlier.
I think spot welding it would help. Do not try very thin diameter thermocouple as spot welding such thin ones would not help. try thick thermocouples by spot welding them.
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