Additive manufacturing - From 3D-printing to 4D-printing

Additive manufacturing (AM), or 3D-printing, allows for direct production of components of unprecedented geometrical complexity. Structures are build layer-wise, wires or powders can be used as base materials. In principal, polymers, ceramics and metals can be processed, however, recently metals came into focus of academia and industry.

For metals, powder bed techniques such as selective laser melting (SLM) and selective electron beam melting (EBM) are most frequently used. Most studies available so far focus on processability of materials for structural applications, e.g. Ti-6Al-4V, stainless steel 316L or Ni-base superalloys. From these studies process-microstructure-property relationships can be deduced, however, damage evolution only has been addressed scarcely. Most importantly, pathways for direct microstructure design elaborated for these materials open up new perspectives for AM processing of functional alloys.

Direct manufacturing of complex shaped components with tailored microstructure is a promising way for processing of shape memory alloys (SMAs). Thus, recently numerous activities in this field are seen. As the shape of SMAs upon AM can be changed due to the inherent shape memory effects, a further degree of freedom is opened up, thus, 3D-printing evolves to 4D-printing.

Besides Ni-Ti, Cu-based and Fe-based SMAs have been studied so far. In the case of SLM processing of Ni-Ti, process induced cracking is seen as the major issue. Thus, data mostly report on compression testing. State-of-the-art and future perspectives for AM of SMAs are presented and discussed. Current limitations and roadblocks to application as well as potential pathways for future developments will be highlighted.