Figure 1. Shrink-fitting trial of CFC plats into a titanium vessel. The team carefully manoeuvred the heated titanium tube from the oven to the CFC plates, and lowered it precisely over the plates, keeping a careful eye on the time elapsed. Florence Thompson / CERN
By Nicola Solieri (CERN)
The successful shrink-fitting trial of the carbon-fibre-reinforced carbon (CFC) plates into the titanium vessel marked the final crucial milestone in the validation of the assembly process for the High-Luminosity (HiLumi) LHC beam dumps. This shrink fitting process will be a crucial step in the assembly of the beam dumps, and involves many challenges, presented by the high temperatures needed to expand the titanium tube to allow it to fit over the stack of CFC plates, the small time window to manoeuvre the hot tube over the plates, and the precision required during the lowering.
An interference fit is used to fit the parts together: at room temperature the CFC plates are larger than their housing in the vessel, so the titanium tube is heated to 450 °C and fitted over the plates. As it returns to room temperature, the two parts press tightly together, creating a strong mechanical hold and good thermal contact.
At 450 °C, a clearance of only 1 mm is created between components measuring 700 mm in diameter, and the fitting must be completed before the titanium cools and is unable to fit over the plates completely and without damage.
Achieving this requires several critical elements to be carefully controlled.
First, the manufacturing tolerances must be studied in detail. If the interference (strength of the mechanical hold between the plates and titanium) is too low, the contact between the plates and the vessel could be lost during the first beam-dump event in the HiLumi LHC machine, leading to unacceptable sliding of the plates. If the interference is too high, assembly would no longer be possible. Tolerance windows of only 0.1 mm therefore had to be specified, achieved, and thoroughly controlled for each of the assembled components. This was especially challenging – and was previously deemed unachievable – for the 12-mm-thick, 700-mm-diameter, vessel, given the relatively high flexibility of titanium.
Second, a robust assembly process had to be developed and validated through a series of tests performed in collaboration with the MME group. The maximum temperature was carefully assessed to demonstrate that the mechanical properties of the titanium vessel would be preserved, whilst at the same time minimising oxidation of the alloy. Dedicated tooling was also developed to facilitate and control the stacking of the plates within the tight tolerances required for assembly. Multiple tests were carried out to optimise the timing and keep the total assembly time below five minutes – a threshold beyond which the progressive cooling and contraction of the tube would make the operation too risky.
Despite the technical complexity, the trial was successfully completed. This provides a strong basis for the first series-production assemblies, scheduled to start in September, and represents an important step towards the delivery of the HiLumi LHC beam dumps.
Figure 2. The team after the successful trial. Florence Thompson / CERN
Watch the video of the HiRadMat experiment of beam dump materials