FOLLOW US Twitter CONTACT US FTJ Email address Phone number
 
SIX ISSUE ANNUAL SUBSCRIPTION FROM JUST £195

Flying High – The Sky’s the Limit for Single Crystal Technology

Using single crystal technology, a world-leading turbine blade producer will continue to ensure its components are favoured by global aerospace manufacturers thanks to many years of investment and R&D.

The precision casting facility at Rolls-Royce plc in Derby (UK) has applied single crystal technology to the production of its nickel-based super alloy turbine blades since the 1980s and the technology is still helping the company to achieve maximum strength and resistance to hot corrosion for each of the 300,000 turbine blades it produces a year at the Derby facility. The company has five other foundries in various parts of the world, some of which adopt the single crystal approach.

Hollow components produced using single crystal technology ensures high strength materials with low thermal creep. The absence of grain boundaries provides an increase in yield strength and decreases the amount of creep which is critical for high temperature, close tolerance part applications. This makes it an ideal solution for the production of turbine blades.

“Air travel is set to double in the next twenty years,” the company’s Prof Paul Withey told Foundry Trade Journal. “Globally, I am not sure if the capacity is there whichever company wins those orders but I know that we are continuing to develop our facilities at Rolls-Royce to meet this growing demand and the technology we have put in at the Derby site is incredible.” This technology reduces the cost of the cast components whilst improving their performance in service.

Rolls-Royce is certainly setting very high (excuse the pun) standards for the industry as Withey explains: “At any given time there are 400,000 people being held up in the air by Rolls-Royce blades and engines and we are always striving towards helping the industry achieve fewer emissions in terms of aviation fuel usage.” This also drives the reduction in costs to the customer through reduced fuel usage.

A team of experienced personnel strive to ensure the highest standards are met, as demanded by this health and safety and performance critical industry sector. “The stress on the root of one of our blades is comparable to having a bus hanging on it,” Withey said. “Turbine blades have to last five million flying miles which is about ten years in ‘aviation speak’ and they are spinning at 10,000rpm.”

High temperatures for high demands
Blades near the combustor may be operating in gas path temperatures far exceeding their melting point and must be cooled to acceptable service temperatures to maintain integrity. Thus, the aim is to ensure the cast metal turbine remains below 1050°C in a gas stream of around 1600°C. As casting temperatures are around 1500°C, ceramic cores are required which won’t melt at such temperatures. They are one of the few things not made in-house at the foundry in Derby and are instead outsourced to local companies such as Ross Ceramics. 

The cores are placed inside a die and clamped ready for the wax to be injected and solidified. Colour coded chaplets are used to ensure wall thickness is maintained, and the pattern is formed through wax injection moulding. Platinum wires are also inserted to hold items in place; the platinum dissolves into the super alloy. The wax assembly room at the company’s precision casting facility is a hive of activity where mostly female operatives are employed to assemble and inspect the wax patterns. 

Once the patterns reach the shell room automation takes over and robots are used to load a multi tank system where several coats of a silica slurry are applied using a dip, rotate, drain system. The primary coat is the thinnest with between four and seven further coats applied in what equates to a two-hour cycle.
The top and bottom sections of the dipped assembly are then removed manually to allow access for the wax to be removed and a boilerclave is used so heat can be put in quickly to ensure the wax doesn’t expand too much. 

All in the details
The external surfaces of the wax melt off first before exposing the next layers. The remnant wax and other polymeric materials are burned off using a tunnel kiln furnace to form the mould which is then inspected to guarantee it has the correct integrity. The melting facility houses an impressive 36 small bore furnaces, three large chill furnaces and two other furnaces which amount to one of the largest collections of single crystal furnaces in the world. 

A vertical vacuum chamber furnace is then used to cast the mould which is placed on a copper chill and the metal charge is placed in the crucible at the top of the furnace. The mould is ramped up in vacuum to a central chamber where it is heated to around 1500°C. At this point the core and shell are at maximum strength, they reach thermal equilibrium and the metal is melted in the crucible before being poured into the mould.

The mould is then slowly withdrawn into the cold lower chamber to allow the controlled grain growth to occur. As the metal solidifies, and contracts, the shell mould cracks and in some cases falls from the component. This metal arrangement is allowed to cool to room temperature before the blades are cut off from the runner system and further processed.

The core is removed and the blade is heat treated at a local Bodycote facility before it returns to Rolls-Royce for inspection prior to shipping to the machining facilities. 

What you see is what you get
Inspection is an important aspect of production. Earlier on in the process, the wax pattern is only 100 per cent inspected if it is a new part; once it is in full production SPC batch inspection is applied. However, once the part becomes metal 100 per cent inspection to all parts is undertaken, including penetrant process, non-destructive testing, and flat plate x-ray. Although production is a complex process in itself, record keeping is particularly rigorous as Withey explains: “Legally we have to keep records for the life of the component which can be ten years. Although we know that the computer technology we use now will be around for the foreseeable future, we cannot predict it will still be used in ten years’ time. But we can be pretty sure paper will still be around so we have to keep paper records.”

The shape of things to come
So what will the company be like in ten years’ time? Many of the company’s annual apprentices - who hail from the Rolls-Royce Apprentice Academy - and its annual intake of 200+ graduates will most likely be engaged in shaping the future of Rolls Royce. The chances are that single crystal technology will still be offering the most appropriate results for the production of civil and military turbine blades because of the specific rigours they need to withstand. We will all probably be spending more time mid-air as our businesses and social lives become even more globalised.

With the investment already undertaken in existing employees and future generations of professional engineers it is clear to see that when the company first entered the aviation industry in 1915, the marque of quality was already emerging and is continuing to do so some one hundred years later so watch this space…

Contact: Prof Paul Withey, Rolls-Royce plc, P O Box 31, Derby DE24 8BJ UK. Tel: +44 (0) 1332 240718, web: www.rolls-royce.com