Weld of difference

A low temperature friction technique – used to weld aluminium alloys – heralds a revolution for designers and the machine tool industry. Sue Stuckey reports

Aluminium’s potential as an engineering material is not disputed. Light and inherently strong, it is an obvious rival to steel in cars, lorries, planes and trains where weight, fuel costs and payload can have real commercial impact. But a big disadvantage is the difficulty of welding the metal.

Little surprise then that research organisation TWI claims to have invented a world-beater with a pioneering technique which can produce welds in aluminium that are strong and almost free of distortion.

Or, for that matter, that the world’s biggest welding equipment producer – Esab of Sweden – has had plenty of enquiries for its new SuperStir welding machine from aluminium extruders, aerospace and automotive companies, as well as shipyards and the makers of telecommunications pylons.

Esab is a partner in a £750,000 low-budget project to develop and commercialise a technique called friction stir welding. TWI holds the patents and Esab, along with 14 international partners, mainly aerospace, are funding the work.

US automotive parts maker AO Smith Corporation of Milwaukee is one of them. Last year the $850m supplier to Ford, Chrysler and GM achieved what many in the industry consider to be a breakthrough, using aluminium to replace steel in a structural component. The aluminium engine cradle it developed jointly with Kaiser Aluminium of California is fitted to a Chrysler demonstration vehicle. It went on to win the International Aluminium Extrusion Design Competition. But without the friction stir welding technology, it is doubtful the development could have succeeded.

Smith builds around 500,000 steel cradles a year. These support the engine and the front suspension and also damp road noise. Smith’s brief was to reduce the weight while keeping the performance, design dimensions and fit of the existing steel cradle.

The aluminium cradle is made up of 12 extrusions. Some of these need to be large – up to 600mm across – which caused a major problem since the maximum extrusion size acceptable to the car industry is about 250mm. A handful of extruders produce larger parts, but to get rigidity the component wall thickness and hence the relative weight increases, while bigger presses increase costs. Using its expertise in stir welding, Smith fabricated a cradle weighing 18kg, some 9kg lighter than the steel version.

The cradles were tested using the industry-standard Chrysler tests including fatigue and engine inertia. The aluminium version passed all five tests using one prototype cradle for all of them. When testing a steel prototype, each test is done on a different cradle because of the stresses the tests impose.

Road performance tests were even more encouraging. `The way we designed and welded the aluminium extrusions had a tremendous impact on lateral stiffness,’ says Dr Peter Fritz, head of AO Smith’s corporate research centre. Stiffness is double that of the steel cradle, improving handling and providing better response in emergencies.

Stir welding is a low-temperature process used to create butt or lap joints, carried out below 500oC when the aluminium alloy is still in its solid phase. Other more usual welding methods, such as MIG and TIG welding, are higher temperature 700-800oC liquid phase processes involving electric arc (fusion welding) techniques.

Chris Dawes, TWI project leader for friction stir welding, with colleague Wayne Thomas, has perfected a variation of forge welding, a process he says blacksmiths have always understood for welding steel. Similarly, early Eygptian goldsmiths knew how to cold forge joints in the metal by beating it with a hammer. What happens is the extension and rupture of the prepared surfaces expose virgin metal and cause molecular bonding between the parts.

Key to all this is a metal surface free from impurities which prevent the molecules knitting. It is a bit like having dust on an adhesive surface which reduces the bonding area. Metal oxides are the equivalent of dust and they form most readily at high temperatures. Forging actually disperses those oxides.

Another undesirable side effect of high temperature welding is the effect on alloying elements. These, notably copper, manganese, silicon and zinc, improve strength or hardness. But they tend to be burned off, leaving a material that is up to 80% weaker than the virgin metal, according to Peter Fritz.

TWI and Esab have each built special purpose stir welding machines and Esab has sold its first one to contract shipyard and offshore extrusions company Marine Aluminium of Norway. The modular design means that the 19m long by 6m wide bed, capable of welding extrusions up to 16m long, can be varied to suit the size of the fabrication.

The Edison Welding Institute in Columbus, Ohio, is using a converted Giddings & Lewis milling machine for its research work. TWI has appointed EWI, its partner organisation in the States, to serve the US-based industrial partners.

Karl-Erik Knipstrom, Esab’s welding machine project manager, says it is essential to have a machine that is exceptionally rigid, with special clamping arrangements for the workpiece that can only be designed into a machine from scratch.

Like Esab’s, the TWI machine is modular but is set up to take parts 3.5m long by 2m high and almost any width. TWI’s friction stir welding cell can be hired by companies wanting to evaluate the potential without committing themselves to buying a machine.

Welding speeds of the TWI machine are said to match those of the MIG process.

Knipstrom says that welding speeds of 750mm/min are possible on a 6mm thick 6082-T6 silicon alloy. On a much harder magnesium alloy of the same thickness, the welding speed is more like 130mm/min.

At TWI, sections up to 25mm thick have been welded without significant distortion. The thinner the section the more likely distortion is, but down to 1.5mm it is said to be negligible compared with other processes. Systematic studies of welds up to 13mm thick have been done using ultrasonic and X-ray tests plus standard mechanical tests to measure bend, tensile and fatigue capabilities.

What nobody will talk about is the all-important design of the stir welding tool.

A friction stir weld is formed by plunging a rotating pin tool into the joint to almost the full weld depth. The pin has a larger diameter shoulder that wipes along the surface of the weld giving a neat appearance. Friction causes the material to plasticise and then consolidate behind the rotating pin.

If the size, quality and performance of aluminium fabrications can be improved using stir welding, then other factors such as cost, design freedom and the environment are equally important.

Aerospace companies, for example, mill aluminium parts from solid because of welding difficulties and up to 80% of the material can be wasted. Stir welding could mean that parts such as fuselage rings are fabricated, opening up new design possibilities.

The technology potential should not be lost on machine builders. One drawback of the process is that it produces a straight line weld. Curved and especially complexly curved surfaces are, as yet, out of the question.

Knipstrom says Esab is looking at the problem while TWI’s Dawes says he could envisage a freeform hexapod-type machine tool doing the job. However, both agree that the present generation of robotic machines are nowhere near rigid enough.

So a new generation of combined cutting and welding machines could emerge.

`I would have thought the machine tool industry would be delighted with the prospect,’ says Dawes.

BENEFITS OF STIR WELDING:

1. Cost

Unlike gas welding, there are no consumables such as filler wire and shielding gas. There is no need for commercial cleaning of parts or for special finishing of the edge to be joined. The process is 50% more energy-efficient than laser welding, the latter tending to introduce porosity and stress cracking which does not happen with stir welding.

2. Design

The very low distortion of the fabrication method makes it economically possible to produce larger parts such as the engine cradle. By the same token it is possible to produce small batches and compound shapes.

3. Environment and safety

The process is clean and free from chemical emissions. There are no fumes and no forms of radiation such as arc or laser light. The only safety requirement is for general machine tool guarding.