A story about 3D printing with steel

In the INSIDE Metal AM project Sirris, CRM and BIL investigated the technological and economical feasibility of 3D printing with steel. Here is a short story about the demonstrator parts that were realised.

3D printing doesn’t always need to involve exotic materials or applications. In the INSIDE Metal AM project, we had a look at the number-one engineering material: steel. True, there are many types of steel. In this article, we don’t want to talk about the choice of material, which has already been covered in an earlier post (in Dutch). This article is about what is possible once you have decided on the steel type.

Example of a general rotor

The first example is a demonstrator part that we designed ourselves as part of the project. For illustrational purposes, we wanted to have a shape that is complex enough to make it interesting for 3D printing, but that was also printable with two different techniques: laser powder bed fusion (L-PBF) and laser metal deposition (LMD). From a number of candidate parts, we decided to go with what we have come to call a 'generalised rotor', printed with 17-4PH steel. The rotor has a total height of 255 mm,  a diagonal of 100 mm at the bottom and 80 mm at the top. It could be a pump screw, a type of gear, etc. A major advantage is a significant gain in weight, which can be important for fast acceleration/deceleration, applications in transportation or devices that need to be carried by personnel. We have also decided to integrate a graded lattice structure, which can account for a load gradient on the rotor, in this case with a higher load at the bottom as compared to the top.

We learned that printing with LMD puts a bit more constraint on the geometry in terms of the maximum inclination angle of walls, the use of support structures, etc. In addition, a careful choice of printing strategy is required in order to avoid hotspots. On the other hand, printing with L-PBF showed that we underestimated the residual stresses and that upfront process simulation would have been a smart choice. In the topology optimisation step, it was also clearly demonstrated, as expected, that the choice of boundary conditions is of utmost importance.

Finally, the printed demonstrators make for a good story and many lessons learned. Lessons which we are happy to share with you for your applications.

L-PBF manufactured rotor with graded internal lattice

LMD manufactured rotor (left) during deposition at CRM and (right) after tribo-finishing

Aluminium extrusion mould

The industrial advisory board of the INSIDE Metal AM was highly interested in our project and presented a few interesting cases. One was a case from aluminium extrusion company E-Max in Dilsen-Stokkem. In a four-cavity die (which can extrude 4 profiles simultaneously), one of the cavities typically fails. When this happens, the entire die needs to be replaced. If however 3D printed, replaceable inserts would be used, which could potentially reduce the down-time and save the die. It was decided to first test the performance of the 3D printed material, in this case H11 steel, with a simple test sample. This H11 steel requires specific equipment that can preheat the powder bed to 500 °C and thanks to VAC Machines in Bruges, Trumpf was willing to print a ring that could be built into the E-max extrusion die used for the extrusion of aluminium tubes. The ring has an outer diameter of 78 mm and a height of 10 mm. The 3D printed part turned out to have a good hardness, fine cellular microstructure and minimal porosity. It performed exceptionally well and is currently still being used in production. Unfortunately the tight tolerances on the extrusion channel turned out to be an insurmountable obstacle and further study of the cavity was abandoned. However, the excellent performance of the test ring opened up a new opportunity: printing of conformal cooling channels around the die could help improve the process stability for extrusion with recycled aluminium, something which has not yet been considered in this industry.

So you see, innovation does not always need to follow a predefined trajectory and creativity is key. This specific demonstration example, although not leading up to what we had initially expected, led to three important outcomes: (1) know-how on printing of H11, (2) the idea for cost reduction by using interchangeable inserts was discovered and remains valid, albeit with conventionally manufactured inserts and (3) use of conformal cooling as a new innovation idea for aluminium extrusion (with a sustainable touch).

Left: 4-cavity aluminium extrusion die, right: sketch of how inserts could be used

Replacement for an obsolete cast part

Together with our research partners and Engie/Laborelec we also discussed some potentially interesting cases for a wire-arc additively manufactured part (WAAM). Because part or tooling obsolescence can be important drivers for AM implementation, this issue was taken into account when selecting a large pump component as case study. A somewhat smaller version of the originally 1.6 x 0.8 m cast duplex steel component will be manufactured, using the LMD/WAAM robotic installation at CRM. The selection of the wire composition, relevant WAAM deposition strategies etc. have all been aspects that have been validated on smaller samples. Wall overhang, 'hot spots' during deposition and some path generation issues have proven to be potential risks for this approach. The developments and manufacturing related to this demonstrator are currently ongoing and hopefully by January this part will be ready for final analysis (deformation study, metallurgical analysis, …).

At Sirris, we are looking forward to also inspire you to take a small leap into the future. For more information on the project, or to discuss ideas/questions related to AM, contact us.

This project has been realised with the support of Vlaio and the Strategic Initiative Materials (SIM).