Speed metal

The designer’s dream of producing functional alumunium components direct from CAD models could soon become a reality, given recent advances in rapid prototyping technology. Jon Excell reports.

Because of its light weight, aluminium is an increasingly popular metal for designers, particularly in the aerospace and automotive industries. Just imagine then, that if instead of relying on tooling, dies and traditional metalworking methods, functional aluminium parts could be created directly from CAD models.

That prospect that is becoming increasingly likely thanks to a number of recent breakthroughs in rapid prototyping technologies. In the most recent development, researchers from the University of Queensland in Australia have created a process that could pave the way for the fast, inexpensive production of aluminum parts with the same properties as ordinary cast aluminum.

The system is based on laser sintering technology which uses a laser to fuse powder and ‘grow’ 3D objects layer-by-layer. The chief advantage of laser sintering is that it is pretty much unconstrained by geometric complexity. However, while engineers have successfully produced functional components from steel and titanium using the technology, aluminium is a notoriously tricky material to sinter. This is because it conducts heat quickly and is highly reflective, making it difficult to turn from powder to liquid with lasers.

Professor Graham Schaffer, who heads the Queensland project, explained how the new process works. ‘We make an aluminium powder/nylon powder preform using selective laser sintering (SLS). Then we burn out the resin, react the aluminium powder with nitrogen to form a rigid aluminium nitride skeleton and infiltrate this with a second, different aluminium alloy,’ he said. The last three steps, he added, are done in a single furnace treatment.

Schaffer is convinced that this approach represents a breakthrough. ‘This is the first method I know of for making aluminium parts using a direct rapid prototyping system,’ he said, ‘and because we infiltrate one aluminium alloy with a second, we end up with properties that approach those of conventional aluminium castings. It can therefore be used to fabricate real components.’

The system could be ready for commercial use in one or two years, according to Schaffer, who added that he is talking to RP giant 3D Systems about commercialisation.

But while Schaffer’s system waits nervously in the wings an alternative direct-metal process is striding purposefully onto the commercial stage.

Originally developed at North America’s Sandia National Laboratories for the low-volume production of nuclear weapons components, LENS (Laser-Engineered Net Shaping) is on the verge of becoming a fully-fledged commercial production technique. The process uses a high power computer-controlled Nd:YAG laser to weld air-blown streams of metallic powder into parts and manufacturing moulds.

Nozzles direct a stream of metal powder at a central point which is simultaneously heated by a laser beam. The laser and powder jets are manipulated relative to the part in up to seven axes, continually adding material to recreate a CAD model.

Dr. Martin Hedges, whose German company Neotech Services MTP is developing European markets for the technique, said that although LENS can be used for prototyping he prefers to see it as a rapid manufacturing process.

One of the chief differences between LENS and sintering is that LENS is a freeform building process; there’s no bed of powder. ‘For rapid manufacture, you don’t want to be building a bed of powder, lasering, putting down another layer and then lasering that – you want to be building as fast as possible and controlling the process economics,’ said Hedges.

LENS is now establishing itself as a trusted rapid manufacturing process, at least when it comes to steel, superalloys and titanium. The parts are essentially fully functional and made from real engineering materials, not using plastics or coated materials. For example, in the case of titanium, superalloys and steels, properties are equivalent to, and in some cases exceed, those of forged material.

Aluminium however, presents now-familiar problems. ‘It reflects the laser light, so you have to put a lot of power into the part,’ said Hedges, ‘and when you do this you’re faced with the high thermal conductivity of aluminium – it’s a hard process to control.’

An additional problem with aluminium, according to Hedges, is that when you work from a powdered material you must contend with the fact that powder manufacturers typically passify the materials (to stop them exploding) by putting a thick oxide layer on the surface. This can end up in the component and result in mediocre mechanical properties.

Thus, so far, the LENS process has been focussed – with considerable success – on producing functional components from titanium, superalloys and steel. Unable to name names, Hedges said components made with LENS are now being investigated as a production solution by a number of manufacturers in the aerospace, gas turbine, motorsport and medical implant markets.

Hedges said that in some motorsport applications, LENS-manufactured components are already just one-third the cost of the traditionally manufactured alternatives. And he added that within a year one aerospace manufacturer might be able to make a certain LENS-produced component for less than the cost of its raw materials today. But what about aluminium? LENS has been used to produce aluminium components but, stressed Hedges, it depends what you want to use the part for.

‘We get good tensile strengths but low ductilities. If you’re concerned about thermal properties and light weight it’s pretty good, but it’s perhaps not the best way to go for complex aluminium load-bearing components in a high-stress environment.’

This is not to say that LENS will never get round the problems associated with aluminium. Indeed, Hedges is confident that if the commercial demand continues, the work can be done to improve aluminium parts.

While LENS is very different from laser sintering, the two techniques do have lasers in common and it appears that the use of lasers is one of the biggest stumbling blocks to producing high-quality aluminium components.

US company Solidica has therefore approached the problem from a different angle and developed a process that makes fully dense aluminium parts from layers of ultrasonically fused sheets of aluminium.

Solidica’s system, dubbed Form-ation, combines this bonding process with a simultaneous milling and finishing operation. Thus, while successive layers of aluminium tape (0.15mm thick x 23mm wide) are ultrasonically bound, milling tools driven by 3D CAD data produce tools, cores and cavities.

Solidica chief executive Dr. Dawn White said that ultrasonic consolidation (UC) generates a friction that produces true metallurgical bonds between layers of material without the formation of molten metal. As the parts are produced from standard alloys without binders, burn-out or infiltrates, the properties of Form-ation components are pretty impressive.

Scheduled for a 2004 European launch, the system is said to be ideal for producing aluminum tooling for plastic injection moulding, investment casting and sand casting. For example, while traditional manufacture of a 10in x 12in x 3in prototype injection moulding tool typically requires multiple processes over a two to four-week period at a cost of up to $20,000 (£11,000), Solidica said it could build the same tool in seven days for just $5,000 (£3,000).

Form-ation is now in the final stages of its Beta testing, with machines operating at a number of companies, including Raytheon and 3D Systems, in the US.

So whether you want to build functional aluminium components with lasers or ultrasound, the technology is there. Perhaps in the not-too distant future RP could not only stand for Rapid Prototyping but also Real Parts.