Rebuilding track crossings using additive manufacturing

One of the key components of rail networks, whether it be high-speed or urban rails, is the railway crossing. Located in the centre of the switches and crossing is the frog, its purpose is to ensure the continuity of crossing routes. As a result of its function, this part of the track is subject to significant stresses such as high loads/pressure and impact, resulting in wear and deformation of the rail.

Once wear is evident on the rails, these components can be repaired on site, following a very precise welding procedure. The primary objective being to rebuild the desired geometry by arc welding, thanks to a deposition of material, and to then proceed with grinding the rail back to its original profile before returning to service.

In general, when the track crossing has undergone several repairs on site, additional arc welding repairs are no longer authorized by the network manager, as there is a risk that the initial mechanical characteristics of the crossing will have deteriorated too much (due to the different thermal cycles occurring during welding operations). The worn crossing is then discarded (to be recycled) and replaced by a new one.

With additive manufacturing, we can rethink the life cycle of these track crossings

Metal 3D printing has been successfully used in industry for several decades now, with different processes (arc, laser, etc.). These manufacturing technologies make it possible to produce parts with very often complex geometries, by adding material and overlapping the necessary amount of layers.

This new way of designing and manufacturing opens up the field of possibilities by bringing significant advantages in terms of production time, but also costs.

In this article, the process used is based on the use of laser beam welding with filler metal in the form of cored wire. Commonly referred to by the acronyms WLAM (Wire Laser Additive Manufacturing) or Ded-Clad-w (Wire Directed Energy Deposition), this process consists of melting using a beam laser focused on a metal wire, in order to form a deposit in which the dimensions are completely controlled.

This highly productive technique is used to manufacture large parts (several meters). The use of a cored wire makes it possible to increase productivity, thanks to an increased deposition rate of the filler metal (cored wire melts quicker than a full section of solid wire). It is also possible to adapt and adjust the weld metal chemistry by tweaking the chemical composition of the wire (while staying within reasonable production lot quantities).

1. Reconstruction – an environmentally friendly alternative

Reconstruction by WLAM additive manufacturing extends the life cycle of the crossing. Therefore, instead of being sent to a steel mill to be recycled, the crossing can be completely reconditioned in a specialised workshop.

The first step is to remove the part of the crossing that has been affected by wear, ensuring that only the original ‘healthy’ and unaffected material remains.

After checking the quality of the base metal of the remaining material, a 3D capture is performed using a portable metrology tool. This makes it possible to simulate the wire laser additive manufacturing approach in order to reconstruct the original shape of the crossing. It is then just a matter of applying the appropriate welding procedure (parameters, thermal cycles, etc.) in order to superimpose the necessary layers of optimal quality, according to the desired geometry.

2. The new design – technical-economic alternative

To date, track crossings are casted parts with a chemical composition that varies according to the networks and needs. By overlaying the necessary layers on a base support (e.g. group 1 C-Mn steel), additive manufacturing will make it possible to produce large crossings of variable geometry in the alloy chosen by the customer:

• low and medium alloy steel providing significant hardness (350-600HB);
• high-alloy steel to respond to significant stresses/wear;
• austenitic Cr-Mn or Cr-Ni-Mn steel with work hardening properties;
• stacking of layers with several alloys to respond to stresses as precisely as possible.

The use of the Ded-Clad laser-wire process in additive manufacturing offers several advantages:

• reduction of production costs;
• component customization;
• overall performance improvement.

Irepa Laser(1) and Welding Alloys France(2) have combined their knowledge and skills in laser welding and additive manufacturing to produce a demonstration.

Some key information:

The track crossing used was scrap and ready to be recycled

→ Base material = 400 HB

→ Part dimension: 1,870 x 800 x 370 mm (370 kg)

→ Welding Alloys cored wire = work hardening HARDFACE AP-G

→ Preparation = Arc-air gouging and grinding

→ Generation of a 3D model using a white light 3D scanner = 30 min

→ Offline programming (definition of approach, parameters and post-processing simulation) = 1 hour 30 minutes

→ Reconstruction time by Ded-Clad laser-wire process = 11 hours

→ Quantity of cored wire used = 13 kg

(1) Irepa Laser is a cooperative community-oriented enterprise (SCIC) specialized in laser processes and materials. It offers its customers tailor-made industrial solutions to support them in their product development and industrialization projects. It acts at the national level with a European focus. The nature of its activities and its footprint are at the heart of societal issues that relate to people, the environment or ethics. Faced with these challenges, Irepa Laser and its partners aim to become leaders in innovative and responsible manufacturing processes using lasers.

(2) Welding Alloys France SAS is a subsidiary of the Welding Alloys Group, a family group established in more than twenty countries on all continents. Its purpose is to improve the durability of its customers’ industrial equipment. Its competence: surfacing by welding of wear-resistant materials. To do this, it develops and manufactures its own welding wires and its own automatic welding machines, but it also offers, thanks to teams of specialized welders, the repair of its customers’ installations / parts in situ, or in its workshops.