3D Printing of Multi-materials metal

Multimaterial Electric Motor Stator and Rotor designed by Computational Engineering

What is multi-material additive manufacturing?

While all additive manufacturing technologies have their advantages, not all of them can be used for multi-material additive manufacturing (MMAM), nor do they offer the same capabilities. Firstly, the choice of technology depends on the material (metal, ceramic, polymer). Secondly, the complexity of the part and the distribution of materials also influence the choice of technology.

Multi-Metal piston head by AddUP

So far, we have identified five different types of FA technology that can use this procedure: Material Extrusion, Powder bed fusion (PBF), Material jetting, Binder jetting, Directed Energy Deposition (DED).

In the laser-sintering process, each layer can only be made of powder with a homogeneous composition and is only distributed in a single direction. With Fused Deposition Modelling (FDM), the composition can be varied in multiple directions, but this is more complex to achieve and will be discontinuous.

In principle, the possibilities with MMAM are virtually limitless, both in terms of geometric complexity and material distribution complexity.

Start your 3D print multi-material here

What are the challenges with multi-material additive manufacturing ?

The new opportunities come with challenges. With MMAM, data preparation is not as simple as with AM. In addition, process monitoring and post-processing are challenging.

1. Data preparation of MMAM?

Process data must be prepared during the pre-process. Typically, STL models are created and sliced into layers. This data is linked to material-specific process parameters such as laser power and scan speed. With MMAM, this is not as simple as with the STL model, it is not possible to associate two different sets of parameters with one model. The STL model can only represent surface information.

Engineers are therefore sometimes forced to store multiple meshes per material, which can make data preparation very complex and error-prone.

Current data preparation for MMAM parts

Data formats such as AMF (Additive Manufacturing File Format) can include information beyond the surface representation and allow different parameter sets to be assigned within a part. However, these formats are typically not compatible with LPBF machines.

2. Process monitoring

To ensure that the process runs correctly and to achieve the desired part quality, it is necessary to control the defined process parameters. It is also important to monitor the resulting dynamic process characteristics, such as melt pool or scan track. For mono-material production, more and more machines are equipped with quality control tools. As multi-material production can result in not only pores and cracks, but also defects such as cross-contamination of the different powders, this places new demands on quality control tools. This means that LPBF machines must be adapted or new methods incorporated to meet these requirements.

Thermography can be used. It exploits the fact that objects emit electromagnetic radiation, known as thermal radiation. The key to distinguishing between materials is that different materials have different emissivities. Therefore, materials at the same temperature emit thermal radiation at different intensities.

Powder bed monitoring using thermography

Detecting cross-contamination is the main challenge in powder bed monitoring. Cross contamination of powders can promote the formation of defects such as cracks. It can lead to changes in melt pool dynamics.

The challenges in melt pool monitoring lie mainly in the interpretation of the recorded process emissions. In particular, the morphology of the transition regions of multi-material components plays an important role in defining the mechanical and functional properties and requires accurate monitoring.

3. Post-processing of multi-material additive manufacturing

The post-processing of multi-material LPBF parts differs from that of mono-material parts. For example, the thermal treatment must be adapted to the material combinations. This means developing new process parameters for treatments that are compatible with both materials. The same applies to the surface treatment of multi-material parts.

The powders must also be separated and reconditioned in a recycling process. The principles of separation depend on differences in physical, chemical, thermal, electrical or other material properties. Therefore, for powder materials used in multi-material LPBF, there is a need for an overview of the potential differences in these properties.

Some companies are trying to separate the powders using a magnet, but this is still at the research stage.

Industrial application of MMAM

A large number of sectors are seeking to optimize the properties of materials in parts to enhance their properties. Just think of hardening the teeth of steel gears, chroming an object to make it resistant to rust or scratches, or overmolding TPU rubber on tool handles to make them more comfortable.

Multi-material additive manufacturing is particularly interesting for the aerospace, automotive and medical industries. Some aircraft engine parts are made in FA.

1. Copper and nickel-base for heat exchange

Fraunhofer did some research about fluid-driven heat exchange of multi-material components. They used with Powder bed fusion a nickel-base (Ni) and a copper-base (Cu) alloy in one process to create a burner tip component.

The multi-material component is compared to its mono-material counterpart made entirely from the Ni-base alloy. A computational fluid dynamics simulation is used to calculate the maximum component temperature differences between both components as-built and heat-treated. This led to a theoretical thermal improvement of 36% in the burner tip and 32% experimentally.

Fraunhofer also produced a liquid/liquid heat exchanger with Leap 71.

Heat exchanger (Leap 71)

The Fraunhofer system is based on a SLM Solutions powder bed metal printer with a custom designed recoater system. Each layer is recoated twice (once for each material). A magnetic system sorts the metal powders for repeated use. The result is an object that combines both materials in a complex geometry.

The inner tubes are copper to maximise thermal conductivity and the outer shell is stainless steel to ensure structural integrity and non-reactivity. Steel is also used for the spiral and pipework where no heat exchange is desired.

2. Multi-material telescopic crown containing gold

Researchers were able to successfully process the material combination of 18kt gold and CoCrMo on an AMCM machine.

Precise and selective material deposition using a robotic arm enabled us to process precious metals in multi-material applications. A defined amount of a second metal is applied using a nozzle-based powder applicator moved by a robotic arm. Thanks to a fully digitised process chain, the gold mating surfaces were automatically machined and immediately met the requirements.

Conclusion

Multi-material additive manufacturing enables the production of functionally graded parts with complex geometries, while reducing cycle times and costs. It’s an extremely fast and accurate technology that is constantly evolving.

Multi Material additive manufacturing can be found in all sectors, but is of particular interest to the aerospace, medical, tooling and prototyping industries.

However, a lack of data and suitable software still makes it difficult to use. However, multi-material additive manufacturing is still in its infancy and has a promising future.

For simple applications, multi-material deposition can be used to deposit virgin powder alongside recycled powder. This can allow only the virgin powder to be used to build the parts, with the rest of the bed and the support being filled with recycled used material. This angle is perhaps more ready for industrial adoption.

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