By Khalid Nor
CSR Zhuzhou Electric Locomotive Co. Ltd (CSRZ) in China contacted TWI to commission a friction stir welding (FSW) procedure development study for rail carriage panels. When using the process in the past, the company had encountered the ‘lazy S’ issue in the weld zone and ‘hook’ defects in the interface of some of the joints. CSRZ approached TWI in the hope of improving its processes and eliminating these problems.
The project began with a procedure development study at TWI’s UK headquarters using aluminium alloy AA6005-T6 for four different panel designs.
The work included the design and manufacture of FSW tools, the development of optimum welding parameters, mechanical assessment of weld quality through metallography, tensile, bend and fatigue tests, and production of evaluation panels as project deliverables. The targeted welding speed was 500–600mm/min.
This initial phase of the project successfully optimised weld parameters for all four panel designs, eliminated hooking, and, in welds in which a residual ‘lazy S’ was visible, showed that the residual ‘lazy S’ had no influence on weld failure locations
Two TWI FSW engineers then spent five days at a CRSZ site in China, deploying the developed FSW procedures and training CRSZ staff. After assessing the onsite FSW equipment, TWI supervised CSRZ engineers implementing the new processes, providing direction where necessary.
During this stage of the project, CSRZ requested a higher welding traverse speed to be tested, of up to 1400mm/min. In production, 500mm/min is considered to be low – especially for high-volume manufacturing. TWI accordingly raised welding speeds to 1400mm/min, but at this speed the FSW machine started to vibrate while traversing. The speed was reduced to 1200mm/min, which was found to be the ‘sweet spot’ for the machine.
The project concluded with CSRZ satisfied with both the welding speed and the mechanical properties of the resulting welds.
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