Evolution of carbon nanotubes and their metallurgical reactions in Al-based composites in response to laser irradiation during selective laser melting

Aluminium-based composites reinforced with carbon nanotubes are widely sought for their outstanding metallurgical and structural properties that largely depend on the manufacturing route. In this work, the process-structure-property relationship for a composite made from high-energy-ball-milled pure Al and multi-walled carbon nanotubes (MWCNT) processed by laser powder-bed-fusion additive manufacturing was investigated. The strong good bond between MWCNT (high laser absorptivity) and Al (low laser absorptivity) allowed the energy transfer from the former to the latter and maintained the heat for longer allowing for better melt efficiency hence improved processability. The response of MWCNT to laser irradiation and their interfacial reactions with Al were probed in a holistic investigation. X-ray diffraction confirmed the partial transformation of C into Al-carbides in addition to the presence of some nano-crystalline graphitic materials. Microscopy revealed evidence of carbides segregation at the melt pool boundaries as well as migration along the build direction. Raman spectroscopy showed that laser irradiation promoted re-graphitisation in MWCNT, reducing the amount of defects introduced by milling. Two types of Al4C3 formed as a result of the metallurgical reaction between Al and MWCNT. These were needle-like and hexagonal Al4C3 and their mechanisms of formation, direct precipitation and dissolution-precipitation, respectively, were explained in light of the thermal profile experienced by the material during melting and solidification. Large scale electron backscatter diffraction showed that there is no distinctive texture developing during melting and solidification. Micro- and nano-indentation testing showed uniform mechanical properties.