• 01/10/2022
  • Report
  • Technology & Processes

Hot at Last!

Standard tool steel H11/H13 (1.2343/ 1344) can now be produced in a process-reliable manner using metal 3D printing. This opens up completely new possibilities for the tool- and die making industry.

500 degree preheating

For more than five years, I have been helping companies to establish conformal cooling using metal 3D printing. But even though it was known that actually only tool inserts made of 1.2709 can be produced by metal 3D printing as standard, at almost all trade shows, lectures or tool making events, the first question was always: "Can you also produce the steel H11/H13 by metal 3D printing?" Today, I can answer "Yes".

Until now, the higher carbon content had prevented the process-reliable production of components of these materials in metal 3D printing. But by heating the substrate plate to 500 degrees Celsius, these very materials can be printed with process reliability.

Preheating of 200 degrees Celsius was the industrial standard in metal 3D printing. This was a compromise: Preheating induces less residual stress. On the other hand, it has the disadvantage that it makes powder recycling more difficult and needs longer cooling time at the end of the build-job.

 

No More Compromise Necessary

Metal 3D printing in powder bed has become the most successful, industrial additive process for metals over the past 15 years. However, with the gradual establishment in series production, the demands of industrial companies on the process are also increasing. In addition to the general desire for shorter production times, the main demands are for higher component quality and reliable initial production - even complex parts should therefore succeed right from the start without approximation tests.

With the new TruPrint 5000 from TRUMPF in Ditzingen, the compromise between less residual stress and recyclability is no longer necessary. This machine was designed and developed for 500 °C applications from the very beginning, so it can maintain the necessary process stability even with steels containing carbon.

Trials at TRUMPF were able to confirm that, thanks to 500-degree preheating, high-carbon alloys such as H11 (1.2343) and H13 (1.2344) can really be processed reliably with metal 3D printing. This is particularly interesting for the tool and die industry, as this sector prefers to use these steels and can derive a particularly large benefit by integrating conformal cooling into printed inserts.

In the following, I would like to show how 500-degree preheating with 3D printing in the powder bed can reliably process high-carbon alloys for the first time.

Further, I would like to describe how preheating technology can be integrated into the manufacturing process in such a way that maximum utilization of the metal 3D printer is achieved and - despite the high temperatures - the residual powder can be recycledwithout any problems.

Table showing the relation between preheating and deflection
500-degree Celsius preheating reduces deflection by about 95% (measured at the cantilever) compared to the industry standard of 200 degrees Celsius.

This is How Preheating Works

Residual stresses and distortion in metal 3D printing with carbon-containing steels are always an issue, especially when manufacturing large, high-volume components. In particular, when there are large jumps in the cross-section of the geometry (volume jumps), there are large temperature differences and thus non-uniform heat dissipation. This leads to thermally induced residual stresses in the component: there is a risk that the component will warp. During the metal 3D printing process or afterwards it delaminates (i.e., detaches from the carrier plate by bulging or lifting off) and sometimes even cracks.

An effective countermeasure is to keep the top of the substrate plate at a temperature of 500 degrees Celsius throughout the build process. On the one hand, the increased preheating temperature reduces the thermal gradients, i.e. the temperature drops and increases, for example at the edges. On the other hand, the yield strength is lowered. The combination of these two factors means that residual stresses are already reduced during additive manufacturing.

Image showing how preheating reduces deflection
Preheating the substrate plate to 500 degrees Celsius reduces deflection of the Ti64 cantilever by 95 % (top) compared to 200 degrees Celsius (bottom).

Studies have shown that 500-degree Celsius preheating in metal 3D printing reduces deflection by 95 percent compared to the current industry standard of 200-degree Celsius preheating. The lower thermal stress thus increases geometry accuracy, and this has positive effects both before and after the printing process: in the design phase, many support structures and simulation steps that were previously necessary to prevent deformation, delamination and cracking are eliminated. This increases the design freedom of the parts and subsequently also reduces the post-processing effort, as fewer supports have to be removed.

The benefits of 500-degree Celsius preheating apply to all part geometries: Although the heat distribution in the part varies depending on the part geometry, TRUMPF was able to show in tests that high-temperature processing has the same effects in all cases.

Residual stresses reduction
If a component is 3D printed with a preheating of 500 degrees Celsius, the residual stresses in the Ti6AI4V component are significantly reduced (top). This opens up completely new possibilities in design, especially in the production of more massive components with volume jumps. At 200 degrees Celsius, the residual stresses are significantly higher (bottom). This leads to a higher preparation and post-processing effort, as more simulations and support designs are required.

Application in Tool and Mold Making

The many advantages of manufacturing tools or mold inserts using metal 3D printing are widely known: It is often the only possible process for such tasks, especially to incorporate complex cooling channels that improve the cooling properties of tools and dies. But until now, there has been one problem: The industry prefers carbon steels because they are wear-resistant and polishable. If H11/H13 is printed with a preheating of 200 degrees Celsius, as has been common practice up to now, hard and brittle martensite forms during the short cooling phase. As a result, cracks often form in the component. Many components made of H11/H13 could therefore only be printed at great expense and thus unprofitably.

The 500 degree Celsius preheating removes this limitation. The higher base temperature slows down the cooling process, making it smoother and thus preventing the formation of undesirable martensite. Microscopic examinations showed that 3D-printed H11/H13 components have a density of up to 99.99 percent. They also come close to conventionally produced H11/H13 components in terms of strength and hardness. There is also no difference in polishability.

Preheating at 500 degrees Celsius now makes it possible to print H11/H13 in a process-safe manner and to further.

Cracking and crack-free material
Left: Cracking in 1.2343 and 200-degree Celsius preheating. Right: Crack-free at 500 degrees Celsius. In a downstream step, remaining small defects can be corrected by annealing.

High Machine Availability and Powder Recycling

Higher preheating also means a longer cooling phase – at the end of the build-job - up to 20 hours, depending on the volume of the printed component. However, a suitable overall concept can prevent these long machine downtimes.

With the TruPrint 5000, TRUMPF relies on the proven exchange cylinder principle, which is capable of temperatures up to 500 degrees Celsius. The additive process takes place in an exchangeable build cylinder. Once the 3D printing process is complete, the build cylinder is moved to a separate cooling station. The machine can be immediately loaded with a new build cylinder (and a full powder supply cylinder, if needed) and run the next build job without interruption, while the previous build-job cools down externally.

 
Highly polished core of H11 printed at 500 degrees Celsius
Highly polished core of H11 printed at 500 degrees Celsius by the plastic injection molding company Reinhard Bretthauer GmbH: The component is crack-free and has a density of more than 99.9 %. The polishability is correspondingly high - there is no discernible difference to conventional production. Thanks to the integrated cooling channels, stable production of plastic parts in injection molding was possible and the cycle time was significantly reduced.

Another possible disadvantage of the 500-degree Celsius preheating could be the poorer recyclability of the powder: As a higher temperature leads to more oxidation, this could reduce the recyclability of H11 powders. TRUMPF was also able to develop a counterstrategy for this and prove its effectiveness.

Both the process chamber and the exchangeable containers are flooded with argon before production begins. This creates a system atmosphere with low residual moisture and a residual oxygen content at a very low level of a few ppm. In compression tests with H11, chemical tests showed that the powder had the same oxygen content as new powder, even after several build cycles: due to the low oxidation, the powder remained very free-flowing, and the particles did not adhere to each other. The powder could therefore be easily removed from cooling ducts, for example, without leaving any residue.

 

Conclusion

The 500-degree Celsius preheating for the metal 3D printing process increases part quality but also design freedom. It also reduces post-processing – true to the motto “first time right”. With preheating, tool steels containing carbon can be reliably printed for the first time. Replaceable construction cylinders ensure high machine availability despite longer cooling times at the end of the build-job, and recycling and powder flowability are not significantly altered by high-temperature processing.

These results for tool steels and titanium alloys are just the beginning. TRUMPF and its customers are already working on the further materials and components that will benefit from preheating to 500 degrees Celsius or can be processed in this way for the first time.

 
Message icons on a smartphone.

The EUROGUSS 365 newsletter

Don't miss our best content and news around the die casting industry. Sign up for the newsletter now for free.

Subscribe now