In high speed rail transport, reducing weight whilst maintaining performance is all important. Railcar structures are often fabricated from extruded aluminium alloy sections, which offer lightweight performance from a combination of high specific strength and stiffness.
In fabrication, metal inert gas (MIG) welding or its variants are often used. However, high heat inputs can lead to strength loss. Cumulative seam weld related distortion, over railcar body lengths, typically of 25m, can also be a problem. Strength loss is compensated for by local thickening of material, but this adds weight to the vehicle. The industry is also using lower heat input friction stir welding (FSW), but local thickening of material in the joint region can still be required, to resist the tooling forces encountered in FSW.
TWI supported one of its Member companies, Nippon Sharyo, in developing hybrid laser-MIG welding as a high speed, low heat input, non-contact welding process, with improved fit-up gap tolerance when compared with conventional laser welding. TWI has used a 7kW Yb fibre laser in this work. Yb fibre lasers are commercially available with high powers, as well as being more efficient, having a smaller footprint, and being cost competitive when compared with other lasers, and having the processing flexibility of optical fibre beam delivery.
Using this laser with standard MIG welding equipment, welding trials have been carried out on 6000 series aluminium alloy extruded plates. These trials have demonstrated that the process is capable of producing low porosity content,high quality butt welds, at speeds of up to 5m/min in 3mm thickness material. In the same work, fit-up gaps of up to 1mm width (if constant width gaps) or up to 1.5mm width (if tapering gaps) have been bridged successfully.
As a follow on, welding trials have shown that high quality welds can also be made between extruded sections, at least in close-fitting butt joints, using a full penetration self-supporting joint configuration.
In ongoing work, joint configurations and welding procedures are now being developed for producing similar high quality welds between extruded sections with fit-up gaps.
The hybrid process is capable of producing high quality welds in a variety of materials and thicknesses, including steels, stainless steels, titanium alloys and nickel alloys, typically up to 10mm in thickness (depending on the laser source), as well as aluminium alloys. For further discussion with an expert, please contact email@example.com.