October AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 9 2 4 detectability of inhomogeneities (such as cavities and cracks) below the spa- tial resolution of µ-CT over large assess- ment areas. Both techniques are used successfully to characterize defect pop- ulations in AM alloys [1, 2] . Additional nondestructive tech- niques used at BAM for quality assess- ment and microstructural analysis of AM components include high-resolu- tion ultrasonic testing using immersion technology, which is suitable to local- ize macroscopic inhomogeneities and to investigate mesoscale material dis- tributions. BAM immersion technology systems are available for three differ- ent component sizes. The immersion technique achieves a significantly high- er resolution compared with the tradi- tional contact technique. Through the complementary use of x-ray techniques and ultrasonic testing, different length scales of features (defects, grains, and pores) and sample sizes can be inves- tigated for additively manufactured components. To complement defect character- ization, BAM uses its extensive experi- ence of residual stress analysis using lab x-ray, synchrotron x-ray, and neu- tron diffraction applied to laser-based additively manufactured materials. The combined techniques enable non- destructive evaluation of the residual stress state of a component from the surface through to the bulk [2,3] . FATIGUE, DAMAGE TOLERANCE OF METALLIC AM COMPONENTS The effects of fatigue damage on the load-bearing capacity and integri- ty of additively manufactured metal- lic structural components limits their suitability for use in fatigue-related ap- plications. Despite intensive research, this problem cannot yet be regarded as solved. The most important factors that influence overall service life, fatigue strength, and fatigue crack growth (in- cluding the threshold value) are surface integrity including roughness, cavities such as pores, and, much less favor- able, unfused regions of material. Fur- thermore, material inhomogeneity and anisotropy, as well as the way in which material texture affects the crack prop- agation path, such as through build-up direction and hatching strategy, play an important role. Also, residual stress- es are expected to influence fatigue be- havior, whereby redistribution and, in the best case, relaxation of these stresses under cyclic loading must be considered [4] . In the BAM project AGIL (micro- structure development in additively manufactured metallic components), fatigue behavior is being investigated by means of fracture-mechanics meth- ods, whereby theoretical modeling and experiments are combined. In this approach, both long and short crack growth are considered, which together with classical crack-propagation and re- sidual service-life analysis, should also enable determining total service life and fatigue strength. In addition to de- termining fatigue strength, the fracture behavior of additively manufactured alloys is being studied using fractog- raphy to understand the interaction of fatigue cracks with the character- istic additively manufactured micro- structure. Electron backscatter diffrac- tion (EBSD) is widely used to char- acterize the grain structure of addi- tively manufactured alloys, reveal- ing both the phase and orientation of the material crystal structure (Fig. 3). EBSD maps are also used as input for microstructure-based simu- lation of the mechanical behavior of ad- ditively manufactured materials. Real grain structures as characterized by EBSD are transformed into virtual grain structures by 3D reconstruction, and deformations and stresses are calculat- ed using crystal plasticity modeling. INFLUENCE OF THE AM PROCESS ON THE ENVIRONMENT Currently, there is not enough knowledge to comprehensively and conclusively assess and balance the positive and harmful effects of AM processes on the environment. Envi- ronmental risks in production are as- sociated with unwanted ingestion, inhalation, and contamination due to improper handling of powder materials and with particles and harmful gases re- leased uncontrolled from the manufac- turing area into the direct vicinity of the manufacturing plant. The removal and mechanical-chemical post treatment of components can also be considered as air-hygienic environmentally relevant emission sources. Associated potential risks can be characterized and quanti- fied using appropriate measurement Fig. 3 — EBSD grain orientation map of laser powder bed fusion (LPBF) 316L austenitic stainless steel looking perpendicular to the building direction measured by K. Sommer.