Niobium superconducting rf cavity fabrication by electrohydraulic forming

Superconducting rf (SRF) cavities are traditionally fabricated from superconducting material sheets or made of copper coated with superconducting material, followed by trim machining and electron-beam welding. An alternative technique to traditional shaping methods, such as deep-drawing and spinning, is electrohydraulic forming (EHF). In EHF, half-cells are obtained through ultrahigh-speed deformation of blank sheets, using shockwaves induced in water by a pulsed electrical discharge. With respect to traditional methods, such a highly dynamic process can yield interesting results in terms of effectiveness, repeatability, final shape precision, higher formability, and reduced springback. In this paper, the first results of EHF on high purity niobium are presented and discussed. The simulations performed in order to master the multiphysics phenomena of EHF and to adjust its process parameters are presented. The microstructures of niobium half-cells produced by EHF and by spinning have been compared in terms of damage created in the material during the forming operation. The damage was assessed through hardness measurements, residual resistivity ratio (RRR) measurements, and electron backscattered diffraction analyses. It was found that EHF does not worsen the damage of the material during forming and instead, some areas of the half-cell have shown lower damage compared to spinning. Moreover, EHF is particularly advantageous to reduce the forming time, preserve roughness, and to meet the final required shape accuracy.

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Figure 1

Equipment used for electrohydraulic forming [8, 13].

Figure 2

EHF tank.

Figure 3

Setup of expansion test for characterization of metals at high strain rates [20].

Figure 4

Experimental and simulated velocity versus time curves for tube expansion test.

Figure 5

Pressure evolution inside the water tank after the electrical discharge between the electrodes: after $50\mu \mathrm{s}$ from the discharge (a), after $100\mu \mathrm{s}$ (b), and after $150\mu \mathrm{s}$ (c).

Figure 6

Pressure evolution inside the water tank for different shots during forming of niobium.

Figure 7

Strain distribution through thickness after EHF of a niobium half-cell.

Figure 8

Microstructure of Niobium (Ningxia) used for EHF of half-cells: (a) surface of the metal sheet (RD indicates the rolling direction and TD the transversal direction), (b) through thickness microstructure (Ningxia), (c) through thickness microstructure (Plansee).

Figure 9

Images of EHF niobium: (a) example of specimens which were extracted along the profile of the half-cell; (b) section of a half-cell.

Figure 10

Vickers hardness (HV 10) measured on the external and internal surfaces for half-cells formed by spinning and EHF.

Figure 11

(a) Half-cell of niobium showing circumferential and radial zones of cutting of RRR specimens, (b) sample holder used for RRR specimens.

Figure 12

RRR values obtained for nonformed niobium (NF—Nb) and for half-cells formed by EHF or spinning. “Circ.” indicates specimens which were extracted circumferentially close to the iris.

Figure 13

RRR values after heat treatment at different temperatures for 5 h for specimens obtained in circumferential direction (circ.) for a half-cell formed by EHF.

Figure 14

Close neighbors defined for the calculation of KAM ($3\times 3$) in a pixel inside a grain (a) and close to a grain boundary (b) [31].

Figure 15

KAM ($3\times 3$) maps for nonformed specimen (a) and for formed specimens extracted from SRF half-cells formed by electrohydraulic forming and by spinning: (b) EHF equator; (c) spinning equator; (d) EHF iris; (e) spinning iris. The external surface of the half-cell is on the right of the image, while the internal surface of the half-cell is on the left of the image. White zones inside the KAM maps represent nonindexed points.

Figure 16

Local misorientation distribution for specimens extracted from half-cells formed by EHF and spinning: (a) specimens obtained from equator; (b) specimens obtained from iris.

Figure 17

Local misorientation map (KAM $3\times 3$) through thickness close to external and internal surfaces: (a) external surface equator of EHF half-cell; (b) external surface equator of half-cell formed by spinning; (c) external surface iris of EHF half-cell; (d) external surface iris of half-cell formed by spinning; (e) internal surface equator of EHF half-cell; (f) internal surface equator of half-cell formed by spinning; (g) internal surface iris of EHF half-cell; (h) internal surface iris of half-cell formed by spinning. White zones inside the KAM maps represent nonindexed points.

Figure 18

Local misorientation distribution at the external and internal surfaces for specimens extracted from half-cells formed by EHF and spinning: (a) specimens obtained from equator; (b) specimens obtained from iris.

Figure 19

Bright field TEM images of specimens extracted through thickness at the equator of the EHF niobium half-cell: (a) specimen obtained close to external surface; (b) specimen obtained close to internal surface.

Figure 20

Weighted average KAM ($⟨\mathrm{KAM}⟩$) and average hardness values measured through the whole thickness: (a) specimens extracted from equator; (b) specimens extracted from iris.

Figure 21

Weighted average KAM ($⟨\mathrm{KAM}⟩$) and average hardness values measured in the first $200\mu \mathrm{m}$ close to the external and internal surfaces: (a) specimens extracted from equator; (b) specimens extracted from iris.