Laser welding of Cu-Nb microcomposite wires for pulsed power applications

Abstract

The magnetic systems are quite simple in their construction and use low electrical power, therefore are popular in various fields of science and industry [1]. The key element of such magnetic systems is the solenoid. The most popular are multi-layer winded solenoids. The materials of conductors of such solenoids must be very strong and have good electrical conductivity. Magnetic fields over 45 T can be generated only in the form of short impulses, therefore the electric cables shall stand extreme impact and cyclic heating. For those purposes presently composite wires of new types are used: Cu–Nb, Cu–Ag, GlidCop, GlidCuSS and another [2, 3, 4]. Cu–Nb microcomposite is presently considered as the best-fit of all above-mentioned types because of the whole set of its unique properties. The strength of such wire is about 1,5 GPa, when electric conductivity is 67–70 % IACS [1]. This material is useful in different magnetic installations, but also in levitation transport, high-voltage power lines, induction welding, industrial equipment of thermal treatment. Presently the construction of solenoids of the most magnetic equipment is multisectional, therefore it needs many electric contact connections [5]. In electrical engineering, wire and cable connections may be destructive (screw) and nondestructive (welded, soldered and pressed). Nondestructive joints are preferable in such case, since microcomposite wires are characterised as of limited capability for deformation. The complexity of selection of welding technologies for those purposes occurs due to the Cu–Nb microcomposites structure and production specifics, as well as conditions of exploitation. The structure of microcomposites Cu–Nb consists of copper matrix where very thin Nb treads are integrated [6]. Traditional arc welding methods for connection of microcomposite wires may not be used because of inevitable microcomposite structure melting, overheat of the joined wires and the loss of unique properties. Therefore, one of the most important unsettled problems in the techniques of strong magnetic fields is creation of a reliable welded connection of microcomposite conductors. This problem theoretically may be solved using special methods of welding. The laser beam is one the highest power density sources suitable for industry. But unlike the electron beam, the vacuum space normally not used for laser welding. Laser welding offers many advantages: autogenous welding, welding of high thickness materials, welding of dissimilar materials, hybrid laser welding, remote laser welding, eco-friendly technology, variety of sources with wavelength from 10.6 μm till 0.532 μm, pulsed and continuous modes [7]. Of the numerous types of laser available today, two still dominate the materials area: CO2 and Nd: YAG lasers. The laser welding of Cu–Nb microcomposite wires was investigated. It was determined, that the joints structure does not have welding defects, while microscopic examination of joint cross-section showed, that microstructure of the Cu-rich weld consists mainly of a Cu-based solid solution strengthened by Nb-rich precipitations. This structure of joint provides insignificant increase in electrical resistance and sufficient ultimate strength and plasticity of the joint. The tensile strength of the laser welding joint is similar to the tensile strength of Cu‒18Nb (wt %) binary metal system, which is obtained after casting of Cu‒18Nb ingots. The laser welding technology in principle is applicable for electric contact connections of the Cu–Nb microcomposite wire.