Study of heat and chemical treatments effects on the surface of ultra-precision machined discs for CLIC X-band Accelerating Structure Review (24 Nov. 2014) A. PÉREZ, S. ATIEH, A. CHERIF, D. GLAUDE and N. MOURIZ

Cleaning Introduction CLIC accelerate structures (AS) are composed by discs made out of oxygen free electronic copper (Cu-OFE). The geometry of CLIC discs: One side of the disc is flat and the other side features the cell geometry In the standard production of the CLIC AS the fabrication process can include a stress relief after the pre-machining.  Shape accuracy ±2.5 μm  Roughness Ra 25 nm 2 Pre-machining  Degreasing  Etching Shape accuracy of ± 2.5 µm Ra ≤ 25 nm Accomplish this is possible if the discs are machined by a combination of Ultra-Precise (UP) turning and milling; Raw material UP machining Stress relief annealing

To better characterize the microscopic changes on the surface of discs related with thermal and chemical treatments during the CLIC production process. Motivation and Objectives: 3 To achieve that: On five discs representatives of different fabrication routes some key features are compared before and after the etching process: A.The design of the edges and the presence of micro-burrs; B.The transitions between turned and milled regions; C.Precision losses due to the etching process. Characterization techniques/ equipements: Metallurgy  Micro-hardness and Scanning Electron Microscopy (SEM) Metrology  Coordinate Measuring Machine (CMM) and Interferometry Characterization techniques/ equipements: Metallurgy  Micro-hardness and Scanning Electron Microscopy (SEM) Metrology  Coordinate Measuring Machine (CMM) and Interferometry  Manufactured by the same company  From the same material batch (CERN)  ≠ stress-relief thermal treatments:  Manufactured by the same company  From the same material batch (CERN)  ≠ stress-relief thermal treatments:

4 A. The design of the edges and the presence of micro-burrs was studied qualitatively by SEM. Sector – I  0.02 mm radius Sector – II  chamfer of 0.02 mm x 0.02 mm Sector – III  sharp edge (90°) NOTE: The sectors are separated by three grooves of few millimeters width and depth. B. The transitions between turned and milled regions were studied qualitatively by SEM and numerical values of the step size were obtained by interferometry; Samples and experimental procedure: Sector –I Radius 0,02 Sector –II Chamfer 0,02x0,02 Sector –III Sharp edge Turned area Milled area C. Precision losses due to the etching process were observed by SEM and quantified by comparing CMM metrology measurements and interferometry results before and after the process. 5 discs with ≠ annealing cycle

Results: 1.Annealing effect on the microstructure Micro-hardness (HV 0,01): Annealing at T ≤ 250 °C  recovery At this state the physical and mechanical properties that suffered changes as a result of cold working tend to recover their original values, but the hardness remains constant or varies slightly Annealing at T = 500 °C  recrystallization The drastic decrease in hardness is typically observed when the recovery was finished and the recrystallization process is, at least, ongoing.

2.Presence of micro-burrs: SEM observation pointed the presence of burrs in all the edges after machining (independently of the annealing) Burrs were partly reduced by the etching. Smaller presence of micro-burrs in chamfer design compared to the other two designs (radius and sharp edge). Results: Radius Chamfer Sharp edge Before etchingExactly same sites after etching

3.Transition between different machining Results: After etching, the step size was reduced on discs nr 4 & 5. In the other cases the step becomed bigger. Before etching After etching The presence of steps (µm) was confirmed in all discs, before and after etching; The discontinuity before etching is smaller on discs annealed for a longer time or higher T (nr 4 & 5);

4.Precision losses due to the etching process Flatness is the parameter less affected staying even within the required tolerances for the CLIC discs after machining (≤ 5 μm); Shape variations (Δ µm) of the external diameter and the iris diameter of the disc are plotted as a function of the annealing temperature. Results:Flatness Disc nr T Cycle Before etching After etching (°C)(mm) In all the cases there is a certain loss of material but it is more critical on the lower temperatures. External Ø Iris Ø

Average roughness (Ra) as a function of the machining processes in extreme cases is plotted: disc nr 1  not subjected to thermal cycle before machining disc nr 5  annealed at 500 °C during 2 hours  Rise in Ra after the etching process in both discs independently of the machining process.  The increase is more acute in the case of disc nr 1 not subjected to thermal treatment.  The equality of Ra values after the etching even if the initial values were very different and with increments of even hundreds of nanometers. Results:  For higher annealing temperatures changes on Ra are less drastic.  In discs annealed ≤ 185 °C, the dislocations density is higher resulting in a more intense attack of the surface during the etching process. x 3 different machining sites

Micro-hardness test revealed that for annealing cycle with temperatures ≤ 250 °C, the recrystallization stage was not reached yet. Independently of the used thermal cycle, chamfer design is the one with less burrs presence. In all cases, the edges were smoothed and the presence of micro-burrs reduced considerably thanks to the etching process. Discs subjected to higher temperature annealing or longer holding time present a reduction of the transition step between UP turning and milling areas. Flatness is the parameter less affected by the etching process staying even within the required tolerances for the CLIC discs after machining. Conclusions:

The etching process degrades the shape accuracy of the discs in a heterogeneous way and up to various µm in the iris section. The removal is more intense in milled regions and at lower annealing temperatures. The annealing at low temperatures seems to be not enough for the rearrangement of defects in the microstructure and that results in a more severe attack of the surface during the etching like direct effect. This trend was confirmed with a bigger increase of Ra values after etching in discs nr 1 and nr 2. It is also remarkable the equality of Ra values after the etching even if the initial values were quite different and with increments of even hundreds of nanometers. Etching is a critical part of CLIC production process but it seems that other treatments like annealing can affect a lot to the final surface quality. Conclusions:

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Extra

The annealing, like a thermal mechanism to reduce damage caused by mechanical deformation makes dislocations mobile helping to their annihilation and rearrangement on the structure. The atoms in the core of dislocations possess a high free energy and like consequence the etch pits may be nucleated preferentially at the points of dislocation’s emergence. This preferential etching has been exploited, for example, in techniques for the investigation of dislocations density.