Work Carried Out
The first task required the processes and requirements of the project to be defined, including a literature survey and a gap analysis for additive manufacturing applications for railway crossings. This survey identified a justification for continuing with the research project with initial deposition trials to take place on S460NL steel grade in 100mm thickness x 50mm width x 300 mm length dimensions.
A gas metal arc welding variant known as cold metal transfer, plasma arc welding, and laser powder deposition processes were all investigated for the initial deposition trials, and filler wire and metal powders were selected in accordance with AWS A5.21: Specification for bare electrodes and rods for surfacing. Once initial trials were completed, one of the three deposition processes would be selected for further trials.
Visual inspection, bend tests, charpy impact testing, Vickers hardness tests as well as macrostructure and microstructure tests were carried out in accordance with relevant standards. The initial trials also examined the suitability of feedstock.
The results found that the metal active gas (MAG) welding process was most able to deposit weld layers of acceptable quality on the carbon steel substrate using AMS flux cored filler wire as a feedstock. The plasma arc welding trials proved inconclusive as the filler wire was deemed unsuitable for this process, while the laser powder deposition trials were incomplete. As a result, MAG process was chosen as the best process to undergo further trials.
Process Optimisation and Production of Demonstrator Parts
Network Rail supplied TWI with a mock-up railway crossing to be used as a deposition trial test block. The weld deposit was built directly onto the crossing ‘nose’ and ‘wing’ profiles with a welding procedure developed using a minimum preheat temperature of 50°C applied using ceramic heating mats and a maximum interpass temperature of less than 300°C. Five 25mm weld layers of approximately 210mm and 125mm lengths were built using multi-layer passes with the robotic MAG system. Mechanical tests were then conducted in accordance with the requirements of EN ISO 15614-1:2017 and BS EN 15689:2009 and fatigue tests were also performed to assess the overall deposit weld quality.
The tests determined that the chosen process could successfully produce an additive manufactured weld deposit using an AMS metal-cored filler wire as the feedstock. The deposit hardness and the fatigue performance exceeded the minimum requirements and the non-destructive examination (NDE) and mechanical tests were deemed acceptable to BS EN 15689 and ISO 5817 standards. However, the impact bend tests did not meet the BS EN 15689 requirements, which indicated that the weld deposit impact toughness falls short of the specification required for the rail/wheel contact area.
With the process optimisation complete, Network Rail provided a crossing mock-up with a plane surface (without the ‘nose’ and ‘wing’ profiles machined on it). Meanwhile TWI continues work to produce a demonstrator part using the optimised welding procedure with a few modifications, including building the ‘wing’ and ‘nose’ profiles directly onto the plane crossing surface and a minimum preheat temperature of 100°C to reduce the peak hardness in the HAZ of the substrate. Offline programming (OLP) software was used to develop a robot program for controlling the weld deposition path.