New welding methods start to stick
PROCESSES which improve the productivity and quality of welds are continuing to emerge from research institutions across Australia and overseas. Although in their infancy, many of these techniques are starting to become commercially viable, and experts are predicting widespread adoption in years to come.
Dr Colin Chipperfield, CEO of the Cooperative Research Centre for Welded Structures (CRC-WS) told Manufacturers’ Monthly laser welding and laser hybrid welding are amongst the most scientifically challenging areas of current development.
“Laser welding provides a very intense source of light which has been used for many years for cutting materials…it is increasingly being used for welding mainly because it is an intense and localised source of heat and provides an opportunity to reduce distortion and the extent of metallurgical damage when you’re welding. This is particularly becoming evident in large scale construction.
“Lasers are being increasingly used for example in ship building and in Europe, and increasingly being looked at to combine with an arc welding process. You get a combination of intense laser light combined with arc welding process in the same weld. This is for higher productivity, so you are basically combining the benefits of those processes,” he said.
In hybrid laser and arc welding processes, the laser beam is used to provide an intense heat source which punches through the material being welded. The arc welding process is then used to add consumable to the weld combining the penetration capability of the laser with the high productivity deposition rates of arc welding.
“Obviously this complicates the whole process and one of the key impediments for laser welding is the cost of the capital equipment. Laser welding is used a fair bit in automotive manufacturing but the cost is perhaps an impediment to its general uptake in manufacturing,” Chipperfield said.
Despite continuing high costs, laser welding is becoming more viable as a result of developments in laser efficiency and the introduction of diode lasers.
“The problem historically with lasers has been they have quite poor electrical efficiency whereby you only get 3% of the actual power generated in terms of light to the workpiece, so you’ve got to get rid of a lot of heat elsewhere in the process. This has required fairly large coolers or refrigeration devices to cool the power supply systems, however with the introduction of diode lasers in recent times, we now have the possibility for portable devices. These have efficiencies of 30% to 40% rather than 3 or 4%,” Chipperfield said.
Currently there is only one such portable laser in Australia, owned jointly by University of Wollongong and Swinburne University of Technology, but Chipperfield predicts potential future use by service companies, welders and welding subcontractors.
Weld Monitoring
Chipperfield claims another area of research focus is real-time weld process monitoring.
“Essentially, in its simplest form, one is looking at monitoring amps, volts, travel speed and other parameters and trying to analyse those in a real time sense to identify defects or deviations from a control range,” he said.
Monitoring systems would alert the operator to potential defects that would be evident by visual inspection of the final weld, and would also identify potential subsurface flaws. This is useful for critical welds in the automotive industry for example, where quality must be assured.
Chipperfield said trials of weld monitoring systems are currently being conducted at the BlueScope Steel mill in Wollongong on a processing line in which steel coils are joined together to form a continuous process. “It is very important for them to have a very high quality weld because of the line speed and the fact that that weld subsequently goes through rolling operations at very high speed,” Chipperfield said.
Friction Stir Welding
Industry is now beginning to feel the impact of friction stir welding. Chipperfield explained this is a non-fusion joining process in which a rotating tool is plunged into the gap between two material sheets to be joined. The rotating action of the tool generates heat and joins the materials. The process is said to produce no fumes, noise or radiation.
The technique is particularly useful for joining metals with low melting points, such as aluminium alloys.
“Steel has been attempted and done successfully, however, of course one has to have very much higher temperatures and this places extra onerous applications on the actual tool material itself that is rotating and generating the heat and friction,” Chipperfield said.
According to Welding Technology Institute of Australia manager technical panels, Professor Ian Henderson, friction stir welding is particularly suitable for joining aerospace alloys such as 2000 and 7000 series high strength alloys which are difficult, if not impossible to fusion weld.
Henderson reported that ductile materials including magnesium alloys, copper, zinc and lead have also been successfully joined using friction stir welding. He added that the process has been commercially applied to railway rolling stock, automotive components, in ship building and for copper canisters used for storing nuclear waste.
High Current GTAW
Henderson said welding developments are also emerging is the area of gas-tungsten arc welding (GTAW), with two new variants known as high current buried-arc mode, and keyhole mode.
These techniques are said to match the quality and cleanliness of conventional GTAW with deeper penetration than is possible with conventional welding techniques.
Henderson said that buried-arc mode GTAW is suitable for light metals such as aluminium alloys, while the keyhole mode is better for lower conductivity materials such as stainless steel and titanium alloys.
The high current process is significantly faster than conventional GTAW. Henderson reported a West Australian company which uses the technology for circumferential butt welding is able to join its 250mm walled titanium pipes in seven minutes using a two-pass butt welding process, compared to eight hours which would be required using conventional multi-pass GTAW.
8-Oct-2004