Session: 16-01-01: Poster Session: NSF-Funded Research (Grad & Undergrad)

Paper Number: 100095

100095 - Fracture Mechanisms in Additive Manufactured 17-4 Steel 

Additive manufacturing provides exceptional geometrical freedom to the designers and enables the production of parts that cannot be made through subtractive processes. However, the AM parts suffer from microscale defects that cause considerable variability in their ductility. A good understanding of the fracture initiating mechanism is necessary to alter the AM processing and build parameters to control the defects responsible for both the fracture and its variability. Several studies in the past revealed the presence of microscopic cups and cones on the fracture surfaces of AM specimens which is indicative of a ductile fracture. Currently, there are limited studies that focused on the fracture initiating mechanisms in AM steels.  The following questions will be addressed in this study: 1) what is the fracture initiating mechanism in AM steel; 2) how do the AM defects grow after the deformation in the metal; 3) how to statistically simulate the defects in as-printed metallic specimens; 4) how do sudden geometrical features change the local porosity in AM samples, and 5) what is the characteristic length over which the damage should accumulate for the fracture to occur in AM steels.     

To address the above questions, we additively manufactured one unnotched and three notched specimens from 17-4 steel powder using direct metal laser sintering. A layer thickness of 40 microns  and a build angle of 90° has been used. The notches are designed to simulate the stress concentrations that are usually seen in geometrically complex components. In the first step, Micro-CT analysis is conducted on the untested specimens to characterize the defects in the as-printed specimens. Subsequently, the AM fracture specimens are loaded until fracture. A displacement rate of 0.02 mm/s is used in the mechanical tests. The Micro-CT analysis is once again conducted on the fractured specimens to quantify the growth in the initial AM defects. Statistical analysis is performed on the defect sizes, shapes, and void-to-void distances for both unfractured and fractured specimens. Based on the Micro-CT analysis, the initial defects in the unfractured specimens grew in volume when the fractured specimens are tested. Furthermore, higher defect volume was noticed in notched specimens especially in the notched regions indicating that the geometrical complexities could lead to more defect volume in AM specimens. We observed coalescence of the AM defects leading to the formation of void colonies after the specimens were deformed. The presence of microvoid coalescence leading to void colonies in the fractured specimens in the vicinity of the fracture surface is direct evidence of ductile fracture. The sizes of these void colonies range between 350-600 microns  suggesting that the voids coalesce over this characteristic length to initiate and sustain a fracture in AM specimens. The Micro-CT results including the AM defects before and after fracture along with their statistical features will be presented in this poster.   

Presenting Author: Anik Das Anto North Dakota State University

Presenting Author Biography: Anik Das Anto is a graduate student in Civil, Construction & Environmental Engineering at North Dakota State University, USA. His current research interest is to model fracture in additive manufactured steel.

Authors:

Anik Das Anto North Dakota State University
Ravi Kiran North Dakota State University