Fraunhofer ILT and the Max-Planck-Institute for Iron Research (MPIE) have developed a 3D printing method that can be used to produce a ferrous composite material of balanced hardness and toughness – similar to Damascus steel.
The material is produced using a precipitation hardening iron-nickel-titanium model alloy, which reacts to relatively short temperature spikes by forming precipitates that increase strength and hardness.
This means the microstructure of each layer can be modified by controlling the process temperature in a targeted manner – a technique known as intrinsic heat treatment.
By using the alloy as the build material in laser metal deposition (LMD), the researchers are able to adjust the temperature of each layer with great precision. Heat treatment can therefore be performed during 3D printing.
In conventional manufacturing, layered materials of this kind – such as Damascus steel – end up in a furnace for the final heat treatment process (in order to adjust the final materials mechanical properties). The material normally remains in the furnace for several hours at a high temperature to allow it to gradually harden.
‘With our method, this hardening process actually occurs during the 3D printing step. That eliminates the need for most, or even all, of subsequent heat treatment processes,’ said Markus Benjamin Wilms, a research fellow at Fraunhofer ILT.
The team does this by harnessing cyclic heating, where deposited layers are heated by the deposition of subsequent layers. Heat treatment in a furnace is still required in cases where the formation of precipitates in alloys proceeds too slowly.
The researchers also briefly pause the process after the deposition of each new layer to allow the metal to cool to below 195 degrees Celsius. ‘We need the austenite-to-martensite transformation,’ explained Weisheit. ‘And we can only get the precipitates to form by applying small, carefully dosed temperature peaks.’
The technique is not just limited to iron-based composites, according to the researchers. Experiments have shown that it also works with aluminium alloys, meaning the principle of intrinsic heat treatment using LMD can also be applied to other alloy systems.
Future plans
The researchers confirm that the method already works very well for LMD, but so far the process has mostly been used to produce simple geometric structures such as cubes. ‘By continuing to improve the method, we’ll be able to build more complex structures,’ said Wilms. ‘We’ve already incorporated complex forms into the material during printing, such as triangular and pyramid-shaped hardness profiles.’
The scientists also highlight the potential of using laser powder bed fusion (LPBF), sometimes referred to as SLM, to print the composite material. ‘If my goal is to build very complex parts with very high-resolution details, then I would obviously always go for LPBF,’ said Weisheit. ‘I would also lean toward LPBF if I’m looking for a very precise hardness profile in my 3D printing. But otherwise I would opt for laser material deposition.’ Another benefit of LMD is its suitability for hybrid processes, he added. For example, it offers complete freedom for building composite structures on free-form surfaces.
In addition, the scientists have already come up with new ideas to take the new method to the next level. According to Weisheit, it might be possible to control the process so skillfully that a third state could be achieved – for example a partially hardened area in addition to the fully hardened and unhardened layers. ‘So far we’ve stuck to working with the pause times. But the temperature profile could also be controlled using other process parameters such as laser output power,’ he said.
Weisheit looks forward to working with industry partners to bring new practical applications forward for the new 3D printing process.