Friction screw extrusion additive manufacturing of an Al-Mg-Si alloy

Abstract

The Friction Screw Extrusion Additive Manufacturing (FSEAM) process is a newly created process for additive manufacturing of low weight-high strength aluminum and magnesium alloys in the solid state which are unsuited for many fusion-based approaches. The process is based on a rotating threaded tool located within a stationary housing that is equipped with a feeding mechanism. The dimensions and shape of the deposited layers can be controlled through a dedicated printhead design. This work reports the results on the fabrication of rectangular structures composed of AA6060 T6 feedstock. The study mainly focused on the influence of the feed ratio on the quality, the microstructure and mechanical properties of the fabricated builds. The feed ratio, defined as the fraction of volume of material deposited per unit of time relative to the volume of material necessary per unit of time for a given cross sectional shape, was varied between 0.995 and 1.7. Solid builds free from macroscopic defects were fabricated at feed ratios of 1.3 and above. Tensile tests performed on samples from the interior of the structure in the build direction showed values for ultimate tensile strength and homogeneous elongation in excess of 100 MPa and 12.5 %, respectively. At feed ratios close to one, layers were formed with fabrication defects, such as macro voids and insufficiently bonded areas, that caused a significant reduction in the elongation values to typically below 5 %. The average grain size of the deposited layers was 3 – 4 micrometers for all builds. The hardness of the builds was reduced from 80 HV to about 40 HV which was ascribed to the thermo-mechanical processes taking place during transport of the feedstock material by the threaded tool and the subsequent deposition. The appearance of the builds and the occurrence of fabrication defects could be explained using a descriptive model by the way the feedstock material was distributed underneath the printhead during deposition. Lateral plastic deformation occurred both within the current layer being built and in the previously deposited layers. Further exploratory tests of the FSEAM process showed that the deposition speed can be increased to 490 mm/min at a favorable feed ratio of 1.3, corresponding to a build rate of about 400 cm3/hour, while maintaining good deposition without macroscopic defects demonstrating the future potential of the process.