Additive manufacturing of fatigue resistant materials: Challenges and opportunities

Abstract

This overview article focuses on the current state of knowledge pertaining to the mechanical characteristics of metallic parts fabricated via additive manufacturing (AM), as well as the ongoing challenges and imminent opportunities in fabricating more fatigue resistant materials.Current experimental evidence suggests that the mechanical properties of laboratory AM specimens may not be representative ofthose associated with parts, due primarily to differences in geometry/size which influence the thermal histories experienced during fabrication, and consequently, microstructural features, surface roughness and more. In addition, standards for mechanical testing methods, specimen design procedures, post-manufacturing treatments, etc., may need to be revisedfor AM parts. Standardizing the AM processmay only be accomplished by strengthening the current understanding of the interrelationships among process parameters, thermal history, solidification, resultant microstructure, and mechanicalbehavior ofthe part.Having the ability to predict variation in mechanical behavior based on resultant microstructure, or matching the best conceivable properties of a part in accordance with the loading critical plane,are some possible solutionsfor making AM a more reliable means for producing functional parts. Developingmicrostructure-property modelsis arguably the first, necessary step towarddesign optimization and the more efficient, accurate estimation of the structural integrity ofAM parts.

A Review of the As-Built SLM Ti-6Al-4V Mechanical Properties towards Achieving Fatigue Resistant Designs

“…Additionally, obtaining more details on the plane orientations that are more susceptible to damage and developing and testing methods of predicting these orientations can be very assistive for design purposes. As proposed by Yadollahi and Shamsaei [54], weaker planes can be aligned with areas expecting lower loads, which will improve the multiaxial fatigue resistance of the material.…”

“…Microstructure features, such as the grain size and morphology, can influence the crack growth of the material [54]. Increased grain boundaries due to larger grain size can create resistance to crack propagation, therefore, varying the size of the grains is likely to influence crack growth.…”

“…The orientation of the grains will also affect crack initiation, especially if crack initiation occurs in slip bands within the grains [68]. This is due to the fact that different build directions have the maximum resolved shear direction at different orientations to planes which contain the easiest and most common slip systems [54]. In a comprehensive analysis of the HCF results of wrought Ti-6Al-4V, obtained from 21 different published research results, it was shown by Wu et al [69] that a bimodal microstructure will have superior HCF resistance over a lamellar microstructure, followed by a equiaxed microstructure.…”

Laser powder-bed fusion of Inconel 718 to manufacture turbine blades

“…Namely, laser powder-bed fusion (LPBF) is a solid freeform fabrication technique based on laser irradiation of metal powder [2]; in this frame, selective laser sintering (SLS) and melting (SLM) are even referred to [3,4]. Assemblies are turned into single parts; hence, streamlining of manufacturing and potential elimination of tooling are benefited [5]; moreover, customization is allowed and the mechanical strength of the bulk is achieved upon proper setup. Nevertheless, the resulting surface quality may limit the application if compared with conventional metal manufacturing.…”

VHCF response of AM materials: A literature review

“…In the last few years, the rapid and continuous improvements of the additive manufacturing (AM) processes and technologies have permitted to significantly enhance the quality of AM parts, which are currently being employed in an increasing number of applications (eg, turbine palette, heat exchanger, and biomedical applications). However, there are still some issues regarding the structural integrity of components produced with different AM processes . In particular, there are many concerns on the fatigue response of AM parts, because of the formation of large defects during the AM process, which represent a critical site for crack initiation.…”