Session: 02-01-02: 7th Annual Conference-Wide Symposium on Additive Manufacturing: Metals II

Paper Number: 100079

100079 - Characterization of Defects in Additively Manufactured Materials From Mechanical Properties 

Additively manufactured metallic parts of metal material using laser powder bed fusion (LPBF) frequently exhibit transversely isotropic characteristics. Due to the recurrent melting and solidification, resulting large thermal gradients lead to the existence of cracks and pores in additively manufactured metal. With the ability to accurately detect, characterize, and predict defects, it is possible to produce low defect metal additive manufacturing (AM) builds. In this work, an approach to characterize the solidification cracking and gas porosity of additively manufactured materials from their mechanical properties is developed. The approach mainly utilizes a micromechanics analytical model based on the Mori Tanaka technique. In micro-mechanics, the Mori-Tanaka homogenization scheme is one of the main techniques that is used for the prediction of the effective properties of a multi-phase material. Many developments have been made based on the original model. In this work, two such formulations are used, first, the model proposed by Qiu Et al. [1] is used to consider multiple types of defects in a medium, having each defect population defined by a specific aspect ratio. The obtained stiffness tensor represents a material with multiple types of aligned defects. However, defects are rarely perfectly aligned but are oriented according to a probability distribution function inside the medium. To account for distribution of orientations, the formulation developed by Schjodt Et al. [2] is utilized. Thus, the stiffness tensor of a medium with aligned defects is transformed and then followed by spatial averaging over a specific representative volume element. This creates a relationship between defect attributes and mechanical properties, which is then inverted so that inferences on the defect population can be made from commonly measured mechanical properties. Aluminum 6061 samples manufactured with LPBF containing distinct defect morphologies are then characterized using the mechanical properties from simple tensile testing done for different processing conditions with varying scan speeds and laser powers. 3D Digital Image Correlation (3D-DIC) is applied to get accurate strain data on the sub-scale specimens; the longitudinal and the transverse moduli are measured from samples manufactured in different orientations. Additionally, Poisson’s ratio is measured and also fed into the analytical model. By inputting the two moduli and Poisson’s ratio into the analytical model, the corresponding defect characteristics within the material can be obtained. The defect characteristics quantified are the total volume of defects, volume of cracks vs. pores, and the orientation distribution function of the cracks. The defect descriptors obtained through mechanical testing and the newly developed model are then compared to the same quantities as observed from Micro-CT. The model uncertainty is then quantified for each output and compared to that of Micro-CT. The mechanical property and analytical modeling prove to be a simple cost-effective method to get information about the defect population of a medium without the need for any additional imaging, giving the ability to make faster decisions about further additive manufacturing repetitions.

References

[1] Qui YP, Weng GJ. On the application of Mori–Tanaka’s theory involving transversely isotropic spheroidal inclusions. International Journal of Engineering Science 1990;28(11):1121–37

[2] Shjødt-Thomsen J, PyrzR. The Mori–Tanaka stiffness tensor: diagonal symmetry, complex fibre orientations and non-dilute volume fractions. Mech Mater 2001;33:531–44.

Presenting Author: Rimah Aridi University of South Carolina

Presenting Author Biography: Rimah Al Aridi is a PhD student in the department of Mechanical Engineering at the University of South Carolina. His primary interest is in the track of design, material mechanics, and manufacturing. This work interlocks two of his research interests, additive manufacturing with micromechanics.

Authors:

Rimah Aridi University of South Carolina
Sivaji Karna University of South Carolina
Zhang Tianyu University of South Carolina
Vincent Dinova Savannah River National Laboratory
Timothy Krentz Savannah River National Laboratory
Dale Hitchcock Savannah River National Laboratory
Lang Yuan University of South Carolina
Andrew Gross University of South Carolina