Material extrusion additive manufacturing of steel 316L: Mechanical properties’ optimisation and the effect of load rate on in-plane compressive performance of honeycomb structures

This thesis presents a novel experimental investigation focused on the compressive behaviour of additively manufactured (AM) Steel 316L honeycomb structures subjected to varying quasi-static in-plane compression. Particular attention is devoted to material extrusion (ME) based Ultrafuse Steel 316L honeycomb structures. The ME metal fabrication process offers a low-cost alternative to the laser powder bed fusion (LPBF) AM process owing to metal-polymer filaments, such as the BASF Ultrafuse Steel 316L. However, metal parts production through the ME process is still in the infancy stage compared to other AM metal fabrication processes.

In order to determine the suitability of the Ultrafuse Steel 316L for the production of honeycomb structures and to investigate the effect of primary manufacturing parameters on subsequent mechanical performance and shrinkage behaviour, an experimental study was performed. The effect of primary manufacturing parameters, namely layer height, print speed, and raster angle, on the mechanical performance of the material, has been examined. Analysis of variance statistical tool has been used to investigate the relationship between these manufacturing parameters and the resulting mechanical and shrinkage properties. The failure mechanisms of the specimens were examined, with digital image correlation (DIC) employed to map local strain fields during plastic deformation. Experimental results and analysis confirm a dependence of shrinkage and mechanical properties, including plastic anisotropy, on ME manufacturing parameters. Visual representations of the strain evolution in the specimens during deformation confirmed the existence of a non-uniform deformation even at low tensile strain. 

Building upon initial findings, to gain further insight into the macroscopic characteristics of the material in focus—Ultrafuse Steel 316L, the potential application of the immersion ultrasonic testing (IUT) method for quality assurance was explored. An Ultrafuse Steel 316L specimen was manufactured with artificial defects using the optimum manufacturing parameters from the initial experimental study. The obtained inspection results are promising in terms of the capability of the IUT method to detect and measure the defects. It was found that the quality of obtained IUT images is not only probe frequency dependent but also sensitive to the part characteristics. 

Finally, building upon the overall understanding of the capabilities of the Ultrafuse Steel 316L material, ME Steel 316L honeycombs with varying cell sizes were then produced with optimised manufacturing parameters. For a general comparison of compressive performance, LPBF honeycombs were also produced. The AM honeycombs were subjected to varying quasi-static loading rates in the in-plane direction. The honeycomb structures exhibited sensitivity to varying loading rates in terms of deformation mechanisms and compressive performance. Furthermore, it was found that even within the quasi-static loading regime, adiabatic conditions can be induced at higher loading rates. Empirical models were used to predict the in-plane compressive performance of the honeycomb structures. Particularly for the plastic collapse stress, an empirical formula incorporating viscoplastic dependency on the cell wall material within the quasi-static loading regime has been derived, producing accurate predictions.