Overcoming helium vessel weld re-design challenges for a critical MRI scanner application

Having spent many years successfully manufacturing the cryogenic pressure vessels crucial to encapsulating the liquid helium used in its long-term client Siemens’ MRI scanners, high-end precision fabricator LTi Metaltech knows all about the criticality of weld quality and compliance.

So, when LTi was investigating new innovative solutions that could further reduce process times in the manufacture of these vessel structures, while further improving consistency in quality and greater repeatability, their skills and expertise in this area were invaluable.

In meeting these objectives, this piece explores how LTi developed a new joint design that also needed to be compliant to the code requirements of AD2000 – Merkblatt. It also looks at the innovative Design for Manufacture (DFM) project that LTi delivered, which enabled them to automate the production of a critical sealing welded joint.

LTi’s work formed part of a much larger project aim for Siemens in cutting the overall manufacturing lead-times and costs on its MRI scanners, in order to help maintain its number one position in this marketplace.

A Re-Design That Needed to Meet Extreme Requirements

LTi needed to identify alternative methods of manufacturing the weld joint so it could be quicker, repeatable and more cost effective to produce, yet still capable of withstanding extremely low temperatures of -269.1C (4 degrees Kelvin) the essential condition where gaseous helium becomes liquid. It would also need to conform to their customer’s geometric tolerances and the code requirements of AD2000 – Merkblatt, which governs and sets the standard in the design and manufacture of pressure vessels.

The issue of repeatability in welding is critical, as consistency and quality must not be compromised for the sake of speed. But to achieve this continuously when fabricating identical structures, a good joint relies on being able to consistently maintain the same angle, speed, distance and accuracy of welding over and over again. Traditionally this has required highly skilled welders with many years of experience with regular ‘assessments’ to ensure these stringent requirements are always met. This would have been even more challenging to accomplish given the original joint design for this application. However, when repeatability is achieved it can be extremely valuable to cost saving, by significantly reducing the likelihood of any re-work.

In terms of the actual application, to take images of the body MRI scanners need to generate a very large magnetic field, using a super conducting magnet and many coils of wire through which an electric current is passed. This is why the liquid helium contained within the cryogenic pressure vessels is so crucial. Essentially it bathes and reduces the magnet wire coils to the super low temperatures needed to reduce their resistance to almost zero, allowing electricity to flow freely through them and start the superconductivity process. This is critical, as in order to produce a large enough magnetic field and work effectively, MRI scanners rely on superconductivity to produce the vast amounts of energy they need.

Fabricating a faster, more repeatable, higher quality joint design

The original joint design was based on a multi-pass double sided weld, which requires the welder to do a minimum of three weld passes; root, cap and second side cap. To weld on the second side of the joint, the welder would also need to dress the back of the original route run, perform a dye penetrant test to ensure there weren’t any surface flaws and then weld the second side cap.

There were two elements that LTi wanted to achieve:

1. Develop a single sided joint that eliminated the need for dressing and a third pass.
2. Automate the weld to produce a consistent robot weld.

Drawing on its considerable breadth of technical expertise, LTi was able to redesign from a double-sided weld into a single-sided weld that was more cost effective, yet still capable of withstanding the extreme conditions that it needed to operate within. This was achieved through the following steps:

• A detailed ‘design to cost’ exercise, which enabled LTi to identify an alternative manufacturing method that increased the material deposition rate by transferring from a manual TIG welding to an automated MAG welding process.
• The use of robotics and automated welding techniques: Having moved from a manual to an automated welding process, LTi was able to bring improved repeatability into their manufacturing and the benefits of increased productivity and cost saving efficiencies to their customer.
• LTi was also able to pass on part of its expertise directly to Siemens MRI, to help them generate similar performance efficiencies within their own production lines.

LTi successfully developed and delivered an automated single sided AD Mertblatt compliant welded joint that has significantly reduced the process times, improved consistency in quality and achieved greater repeatability.