A study on the effect of print parameters on the internal structural quality of 316 L samples printed via laser powder bed fusion : experimental and algorithmic approach

Abstract

This work aims to establish a quality assurance methodology for additively manufactured (AM) samples, produced from laser powder bed fusion (LPBF) method. The method incorporates resonance frequency method (RFM), where reference samples from wrought 316 L will be used to establish a data-base with a set of reference frequencies. The data-base is enhanced further with simulated frequencies, via FEM method, which was carried out on samples with the same dimensions and material properties as those of the reference. The quality of LPBF samples were benchmarked against this database. Four sets of LPBF samples (termed as A, B, C, and D) were printed with different parameters, and their densities were measured to understand deviations from the reference database. It was observed that Set-C had the least drop in density of approx. 0.65% when compared to the wrought samples. Microscopic analysis revealed that the melt pools were clearly visible in all the samples, with no significant effect from different print parameters. Subsequently RFM was performed on all the sets and clear shifts in frequencies observed. Set-C had the least deviation when compared to the reference (averaged at 200 Hz), whereas it was 250, 300, and 400 Hz for Set-D, Set-A and Set-B respectively. There are several reasons for the frequency shift, the presence of porosity being one of them. Set-B had the highest concentration of porosity in the -YZ plane. An algorithm was developed to sort the samples based on the frequency shifts seen from those of the samples from wrought 316 L. The sorting methodology was based on the shift frequencies, and the farther the sift is from the wrought the worst it get in terms of quality. The algorithm, which is programmed based on this methodology, was tested on a new set of LPBF samples and its effectiveness validated.