War machining

Repair, not replacement is the watchword as the US Army turns to rapid manufacturing technology to fix damaged helicopters. Jon Excell reports.

When the US began bombing Afghanistan in October 2001 its military planners expected stiff resistance from tough Taliban fighters equipped with shoulder-launched missiles. In the event the resistance quickly crumpled, but US forces still encountered a formidable adversary in the form of Afghanistan’s harsh climate. And it was sand rather than stingers that presented one of the biggest problems.

A major headache for the US was erosion on the massive blisks (bladed disks) within General Electrics’ T700 helicopter engines – the most widely used helicopter engine systems in the US Air Force. Sand is estimated to have reduced the operating life of the BlackHawk helicopters flying over Afghanistan from 2,500 hours to just 80.

Although not publicly admitted the problem is likely to be the same in Iraq.

Blisks, used widely throughout aircraft jet engines and industrial turbines, are bladed discs that have been machined from one part. In the past these disks consisted of individually attached aerofoils, but manufacturers began to replace these assemblies with a one-piece bladed disc. The advantage is that around 120 different parts are reduced to just one, and by eliminating the need for fixings, fairly substantial weight savings can be achieved.

The big problem with blisks is that damage to just one aerofoil, and this could be a chip on just one of the blades, typically means that the whole component has to be replaced, an exceptionally expensive repair. And the costs don’t stop here – a recent report carried out by the US Department of Defence illustrates that erosion on the blisks leads to increased fuel consumption, horsepower loss, higher operating temperatures and damage to the compressor and turbine hardware.

In response to this problem US forces are therefore trialling the Laser-Engineered Net Shaping process.

LENS uses a high-power computer-controlled laser to rapidly weld air-blown streams of metallic powders into custom parts and manufacturing moulds. Traditional machining processes (for example, milling) are generally subtractive techniques, meaning that they remove material to create components. LENS, however, is ‘additive’, with components built from scratch, layer by layer.

Dr. Martin Hedges is the European representative of Neotech, which is working with the US and is developing European markets for the LENS technique. He explained that because LENS is an additive process not only can it be used to manufacture entire components, but it can also rapidly repair or add to existing parts. ‘You can either add detail to forged parts, repair worn components, or if too much material has been machined away you can apply a new layer of metal and reclaim the part,’ he said.

Hedges explained that in Afghanistan the blades of helicopter blisks suffered from tip wear, wear on the leading edge and damage from foreign objects. He claimed that the ability of the technique to repair all three types of damage is currently being demonstrated to the DOD.

Repairing a blisk is an exceptionally challenging task, and although blade tipping techniques have been around for a while, Hedges explained that it is the precision of the LENS process that makes it so well suited to the job. ‘If you’re repairing on 0.2mm leading edges you’ve got to be careful you don’t put too much heat in and distort the part or damage the microstructure underneath. You’ve also got to be careful you don’t get any sink or undercut which you can do with some welding techniques,’ he said.

Hedges added that the technique is not simply suitable for adding 0.25mm of metal to build up slightly worn areas but that it can also be used to replace fairly substantial sections of damaged materials.

‘We’re talking about taking a whole section of damaged material out and putting a new section in,’ he claimed.

He was, however, unable to comment on the precise nature of the additive material used, revealing only that it is ‘a tough, wear-resistant alloy’. Hedges said that currently the average repair cost for an engine struck down by blisk trouble is around $110,000 (£65,000) per engine. This includes the cost of a new blisk at $8-$9,000 (£4,750-£5,000). He claimed that the cost of repairing a blisk using LENS is under $500 (£300).

He added that the repair material is more wear resistant than the original material, giving the repaired blisk a longer life and reducing the frequency of engine overhaul. Of course blisk repair is not just of interest to the army and the aerospace industry.

Gas turbine manufacturers also depend on them, and Hedges believes there are some big opportunities for the LENS process. ‘Everybody in the gas turbine market is interested in blisk repair and LENS is a potential way of doing that.’

Sidebar: Manufacture on the move

The US Army’s much-vaunted Mobile Parts Hospital (MPH) was recently deployed for the first time during the invasion of Iraq.

The hospital is essentially a trailer equipped with state-of-the-art rapid manufacturing equipment and communications technology that enables army engineers to manufacture functional components on the spot.

The mobile module is capable of making more than 150 different parts. Specifications for them are maintained in a database, and laser scanners can quickly generate specifications for additional pieces in the field.

The MPH is currently operating and producing vehicle replacement parts at Camp Arifjan, Kuwait, in support of American forces in Iraq. The announcement was made at the Association of the US Army (AUSA) Annual Meeting and Conference in Washington, DC.

‘Currently repairing vehicles in theatre is both time consuming and costly. When a vehicle breaks down it usually has to be transported back to the base and it can remain idle for weeks waiting for replacement parts,’ said Dennis Wend, executive director of the US Army National Automotive Centre.