CURRENT LEADS FOR RESISTIVE CCCM HEATERS

Abstract

The article justifies the necessity of taking into account the transient electrical resistance of the welding contact between the titanium current lead and the resistive element of the heater made of carbon-carbon composite material (CCCM) during its manufacturing.
 Carbon-carbon composite material possesses a unique set of technical characteristics that make it suitable as a structural material for manufacturing resistive heaters operating in the temperature range from 400°C to 2500°C in a vacuum or protective atmosphere. The use of CCCM heaters in the thermal chambers of electrothermal engines to convert electrical energy into thermal energy allows them to be employed for temperature measurement inside the engine's thermal chamber. The utilization of an integrated temperature assessment of the CCCM heater, which relies on the resistance change of the heater with increasing temperature and essentially involves measuring the electrical current passing through the heater, requires ensuring accurate values of both the electrical resistance of the heater itself and the electrical resistance at the point of contact with the current lead.
 The design of the heater has a particular feature, which is the need to supply electrical current to two lamellas of the resistive element of the CCCM heater, located on one side of the heater. For welding the joints between the titanium wire and the resistive element of the heater, TIG welding with a surface arc in an argon environment was applied, with argon being supplied between the welding point and the non-consumable tungsten electrode.
 Laboratory tests revealed that detachable connections of the titanium wire to the resistive element of the CCCM heater and separable connections (such as bolts, screws, and wedges) cannot meet the requirements for contact resistance stability for various reasons. Among non-detachable connections, only welded joints demonstrated stable electrical contact resistance characteristics.
 The electrical resistance of the heater with current leads should be 3.8±0.2 Ohms. The tolerance value for resistance is determined by several factors, including the physical properties of CCCM, the technology of mechanical processing, the method of applying protective coatings, the resistance of the welding joint, and others.
 During mechanical tests of the welding joint between the titanium wire and the heater, it was observed that the wire detachment from the heater occurs with residues of CCCM and a transitional layer of titanium carbide, which forms during welding and can contribute to additional electrical resistance of the heater. Thermal deformation of the CCCM heater in the welding zone does not occur.
 The results of the conducted research demonstrate that the installation of titanium current leads by welding increases the resistance value of the resistive element of the CCCM heater due to the chemical reaction between the titanium alloy and the resistive element, leading to the formation of an additional layer of titanium carbide. Therefore, the electrical resistance of the resistive element of the CCCM heater during its manufacturing, before welding two titanium wires for current leads, should be reduced, taking into account the transitional electrical resistance of the two welding contacts.