Colloquia

Summary

Additive manufacturing using aluminium can be a valuable technology in industries that require lightweight parts, such as aerospace and automotive. However, state-of-art approaches that involve melting the feedstock material pose a challenge for many aluminium alloys due to metallurgical problems that occur during the melting and solidification phases. A solid-state approach that avoids the liquid phase and related problems is a potential solution.

Within the Production Technology chair, a new solid-state process has been developed, called Friction Screw Extrusion Additive Manufacturing (FSEAM). It employs a rotating screw in a stationary housing to deposit aluminium feedstock at much lower temperatures providing a fine-grained microstructure without porosity for a broad range of alloys. However, the influence of various parameters on microstructure and mechanical properties is still unknown. This work focuses on the effect of printing speed during manufacturing.

Four builds were successfully produced of AA6060T6 with printing speeds of 100, 150, 200, and 250 mm/min. The average temperature at the print head increased with printing speed. No macro-scale defects were observed, but SEM microscopy showed the presence of micro-scale defects. Furthermore, EBSD revealed substantial grain refinement for all samples.

The builds' hardness decreased by about 50% compared to the feedstock material. Tensile tests showed a decrease in yield and tensile strength but an increase in elongation at break after the additive manufacturing process. Tensile strength tended to increase with print speed. The mechanical properties differed significantly between samples extracted in the build and deposition directions. The former often showed premature failure related to unfavorable micro-scale defects, while the latter displayed consistent and relatively large ductility values, hardly or not affected by defects. Further process improvement is required to prevent interfacial defects and improve interlayer bonding.

A closer look at the stress-strain curves from builds with an average build temperature above 400°C (150-250 mm/min) revealed serrated stress-strain behavior in the plastic region. This behavior was ascribed to the dissolution of strengthening precipitates at elevated temperatures within the print head during deposition, with only partial recovery occurring in the subsequent deposition process.

Lastly, a 2D thermal model of the build was developed to gain a better understanding of the builds' thermal history. The model calculations confirmed the impact of deposition speed on heat generation and temperature development.