Metal technology powers onward for efficiency

Tom Shelley reports on metallurgy at the leading edge, especially where it delivers energy efficiency and power

Advances in metals technology keeps British made aircraft engines ahead of the competition and it is helping meet some of the immense challenges posed by global climate change. Yet ­­other British advances have been embraced enthusiastically by overseas competitors and are now being sold back to us. For example, friction stir welding of very long aluminium seams, a technology invented by TWI, allows Hitachi to make its 140mph Class 395 trains conform to UK crash regulations while being lighter and able to carry more passengers than its competitors. The bottom line is that the trains cost less to run and cause less rail wear, and are extraordinarily quiet.

So the choice of who got the contract ended up being a 'no brainer' when South Eastern looked for replacement commuter trains to run on the Channel Tunnel high speed rail link. A recent conference – Cambridge Materials Science around the world in the 21st Century –hosted by the University of Cambridge Department of Materials Science and Metallurgy showed that some UK companies are taking advantage of the metal and alloy technology advances being made in the UK.

Foremost amongst the department's industrial partners is Rolls Royce. Dr Neil Glover, who is the company's head of materials capability acquisition department, described its aero engine capabilities as now being, 'limited by metallurgy'. "Historically," he says, "fuel consumption has been reduced by 70%, but to go further will be much more difficult. A lot of the easy challenges have already been met." Present state of the art alloy manufacture allows aircraft engines to operate with peak combustion temperatures of 2,300°C, turbine blade tip speeds of 1,000mph and tip loads of some 100tonnes force. Compressor and fan blades are fabricated from titanium alloys, while those at the rear of the engine are single crystal nickel alloys with cooling passages and thermal barrier coatings. To go further, Dr Glover mentioned the possibility of controlling the turbine blade crystal orientation by seeding and coating blades with alternating layers of vapour deposited nitrides and softer cushioning materials as a way of reducing dust erosion.

Hardide Coatings based in Oxfordshire has recently developed a chemical vapour deposition (CVD) process for depositing tungsten carbide nano particles in a tungsten matrix. This is thought to be suitable for both steel and titanium blades for ground based gas turbines. Hardnesses can be varied from 400 to 3000HV. The company describes test results as 'very promising' and adds that, while the liquid droplet erosion resistance is estimated to approach that of titanium nitride, Hardide can be deposited in layers up to 25 times as thick. Dr Glover and Rolls Royce are, however, looking even further forward to new materials that have been developed and demonstrated, but which have yet to have their long term reliability totally assured. One idea is to make turbine blades and disks as a single unit – called a blisk – which saves about 30% weight.

Rolls Royce has made one out of a titanium aluminium vanadium alloy for the Joint Strike Fighter project by friction welding the blades to the disk. The stage after that would be to make a bladed ring – or bling – which would potentially save up to 70% weight. As a possible blade material, Dr Glover mentions titanium aluminide, which has a specific gravity of only 4g/cm3, is 30% stiffer than conventional titanium alloys and has good erosion resistance. However, it is described as 'very brittle'. The barriers to its implementation are the uncertainties, says Dr Glover. "We need to know there are no other failure mechanisms." And, haunted by the RB211 composite fan failure fiasco that almost destroyed the company, he added: "We can't just replace it. If it turns out to be no good, we would have to redesign the entire engine."

Iron and steel alloys also have their places in jet engines and this aspect is being pursued by the Phase Transformations and Complex Properties Research Group. It has developed an iron alloy for gas turbines which has a creep strength of 600MPa at 600°C and a tensile strength of 1600MPa. It is said to be packed with intermetallics and PhD student Yan Pei says the alloy was lighter than a nickel alloy and much cheaper. Arijit Sahah Podder, another research student says: "The group is still discovering things about welding. For example, if there are groups of fine crystals in the weld or heat affected zones adjacent to the weld that are of similar orientation they behave like large crystals.

This is not good." These kinds of developments and discoveries are of particular importance to the power generating industry, where the need to generate more electricity is becoming ever more urgent. Professor John Knott from the University of Birmingham, who spent some years at the Central Electricity Research Laboratories, noted the UK is more dependent on electricity than ever and pointed out that options are becoming somewhat limited.

He said that using advanced nickel alloys in conventional steam raising power generation at steam pressures of 300bar and temperatures of 700 to 720°C would only increase efficiencies to about 47% from the 44% achieved today by running at 600°C. Alternatives are gas turbines, which are about 32% efficient, and combined cycles, where the exhaust gas is used to generate steam, which raises efficiency to 55%.

Author
Tom Shelley

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