Non-combustible Mg alloy components fabricated by laser powder bed fusion in as-built conditions have an average ultimate tensile strength (UTS) of 320 MPa, a significantly larger value than its casting counterparts, which present an average UTS of 200 MPa. In addition, it was determined that stable crack extension always starts at the outer surface due to the coarsened microstructure regions present in the area. Therefore, this paper will use fracture mechanics to predict the UTS value by determining the size of the coarsened microstructure region and considering it as a surface crack with the √area parameter. Then, by using a fixed fracture toughness value, the UTS will be predicted. Furthermore, a processing parameter known as contour, which is used for remelting the outer surface of the specimen, can also smoothen the microstructure and potentially increase the UTS value. Results showed that the √area of the surface crack responsible for fracture was 730 μm for the no-contour specimen and 630 μm for a contour specimen. Subsequently, using Murakami’s theory, the predicted UTS is 320 MPa and 345 MPa respectively. Finally, tensile testing was performed to confirm the prediction, showing similar results with an average deviation of 2.9%.
EXTENDED ABSTRACT
Themes: Fatigue and Fracture of Additively Manufactured Materials
FAILURE CHARACTERIZATION IN 17-4PH STAINLESS STEEL ACROSS MULTIPLE MANUFACTURING METHODS
Accurate models of additively manufactured (AM) materials require extensive mechanical testing for proper calibration and verification/validation. The process-structure-property relationships in 17-4PH stainless steel from multiple manufacturing modes were examined via mechanical testing across several strain rates and post-mortem characterizations of the fracture surfaces and microstructure. Under all manufacturing modes and testing conditions, optical and scanning electron microscopy showed ductile failure characteristics. Higher porosity concentration (determined by density measurement) resulted in lower ultimate strength in cast samples; the pores often acted as crack initiation points. Strain-rate dependence and failure modes were also affected by process-dependent anisotropy in the microstructure, which was quantified through electron backscatter diffraction (EBSD) imaging. This data will be used to inform models of failure in the 17-4PH for multiple manufacturing forms.
EXTENDED ABSTRACT
EVALUATION OF STRENGTH CHARACTERISTICS FOR NON-COMBUSTIBLE MAGNESIUM ALLOY PRODUCTS FABRICATED BY LASER POWDER BED FUSION UNDER AS-BUILT CONDITION
It is difficult to evaluate fracture toughness according to ASTM standards for non-combustible magnesium alloy fabricated by Laser Powder Bed Fusion (LPBF) in as-built conditions. The reason is its microstructure duality between inner and outer surfaces. The microstructure duality can be eliminated by heat treatment. However, heat treatment reduces the strength of the material by around 11%. Therefore, heat treatment was not performed. In addition, the greatest advantage of LPBF is maximized when it can be used immediately without post-processing. Therefore in this study, the as-built condition was targeted. In the case of non-combustible Mg products, the mechanical properties of the inner and outer microstructures have a non-negligible difference. The difference is expected to affect the fracture behavior, so it is important to consider the difference in microstructure in strength evaluation. Therefore, this paper explains why ASTM standards are difficult to apply to non-combustible magnesium products fabricated by LPBF in as-built conditions with their microstructure differences. Furthermore, the alternative methods for measuring the fracture toughness of metals fabricated by the LPBF in as-built conditions with these characteristics are introduced and discussed.
EXTENDED ABSTRACT
MODELING OF MIXED-MODE CRACK GROWTH BEHAVIOR IN LB-PBF TI-6AL-4V USING A CRITICAL PLANE FRAMEWORK
Many service loading conditions are multiaxial, and small cracks have been shown in many situations to grow in mode II or mixed-mode due to the orientation of defects and microstructural effects, particularly in additively manufactured metals. This paper uses fracture mechanics with a critical plane framework to predict crack growth rates using only mode I constants from the literature.
EXTENDED ABSTRACT
REDUCING LOW CYCLE FATIGUE LIFE SCATTER OF ADDITIVE MANUFACTURED ALSI10MG USING LASER SHOCK PEENING
Additive manufactured (AM) alloys are still prone to critical manufacturing flaws, such as gaseous bubble entrapment. These defects can lead to early crack initiation reducing fatigue life and increasing scatter, especially when near surface. This research investigated the effect of femtosecond laser shock peening (FLSP) on the fatigue life of AM AlSi10Mg. Due to the low penetration of the FLSP, fatigue life remained consistent between treated and untreated specimens. Of equal importance though, the scatter was found to be reduced in the FLSP treated samples. From the high resolution DIC results, the average strain per grain in the untreated specimens showed a higher increase of strain from initial loading to final fracture as compared to the FLSP samples. Implementing the use of FLSP onto AM materials could lead to more consistent fatigue life despite the presence of porosity, leading to a path of easier certification and improved confidence in their behavior.
EXTENDED ABSTRACT
SIGNIFICANCE OF INTRA-BUILD DESIGN VARIABLES ON THE FRACTURE TOUGHNESS PROPERTIES OF ELECTRON BEAM MELTED TI6AL4V
Structurally reliable materials are essential for adopting additive manufacturing (AM) metals in safety-critical applications. Limited data on the damage-tolerance properties of metal AM materials exists, hindering the acceptance of AM metals in fracture-critical applications. A design of experiments (DOE) is used in this study to investigate the role of build space and part design parameters on the fracture toughness properties of Electron Beam Melted (EBM) Ti6Al4V. ASTM E399 tests were performed on over 100 compact tension (CT) samples in the as-built and machined conditions to obtain fracture toughness properties and evaluate the influence of part size and location within 80% of the build space. Results were comparable to wrought annealed titanium, with less than 10% variation in overall fracture toughness. Specimen location within the build envelope contributed to the observed variation, with an increase in properties with build height and specimens located in the center of the build envelope. The location-dependent properties result from changes in microstructure and porosity throughout the build space. While the experimental EBM Ti6Al4V fracture toughness properties are promising for future applications, it is crucial to consider the variation in properties due to build space location and design parameters when designing for consistency.
EXTENDED ABSTRACT
COMPUTATIONAL MODELING FOR IDENTIFYING VOIDS IN ADDITIVELY MANUFACTURED AL-SI10-MG
Additive manufacturing (AM) is a quicker and more cost-effective technique to produce complex parts that can perform similar to or better than conventionally manufactured parts. However, due to the dissimilar microstructure compared to conventional parts, there is a lack of understanding in the physical and mechanical response of AM alloys under different loading conditions and strain rates, and thus the suitability of using AM parts is uncertain. Notably, the presence of voids in AM metal alloys is more prevalent. By developing a computational model that can represent plasticity and track fracture initiation at the void sites in AM alloys such as Al-Si10-Mg, the failure response can be predicted. Therefore, the objective of this research is to use in-situ micro-computed tensile testing to identify individual voids or networks of voids that are likely to cause fracture initiation in an AM Al-Si10-Mg alloy.
EXTENDED ABSTRACT
FACTORS GOVERNING THE FATIGUE PERFORMANCE OF AM TI-6AL-4V COMPONENTS
Before an additively manufactured component can be safely used in a load bearing application, its mechanical performance must be qualified. Traditional qualification approaches, involving the fabrication and testing of many identical components, negate one of the greatest benefits of additive manufacturing, i.e. the ability to quickly and cheaply fabricate one-off components. Thus, qualification methods that rely less on mechanical testing and more on predictive modeling are of value. This is most true for high cycle fatigue performance, where mechanical testing requires significant resources and produces stochastic results.
High cycle fatigue failure is difficult to predict because it can depend nonlinearly on many parameters, e.g. part geometry, residual stresses, surface characteristics, material defect characteristics, grain and dislocation structures, mechanical and environmental loading characteristics and their history. This has motivated a succession of fatigue models with ever increasing mechanistic fidelity, with some now diving down to the atomic scale. This raises the question of: what level of mechanistic detail is required to sufficiently predict the performance of AM Ti-6Al-4V components? In this talk, I will give my perspective on this question, building from a decade of AM Ti-6Al-4V fatigue modeling and experimentation across scales.
EXTENDED ABSTRACT
FINITE ELEMENT MODELLING IN PREDICTING THE EFFECT OF DEFECTS ON STRESS CONCENTRATION AND FATIGUE LIFE OF L-PBF ALSI10MG ALLOY
The elastic-plastic finite element analysis is performed to obtain the stress field around pores and evaluate their resultant effects on fatigue life for L-PBF (Laser Powder Bed Fusion) produced AlSi10Mg alloy. The stress field is calculated for both single and multiple pore models, where stress concentration is evaluated as a function of the pore location and its size. A multi-scale finite element (FE) model is proposed based on the inherent porosity data from Computed Tomography (CT) to predict the overall fatigue life with high (90%) accuracy. The predicted fatigue life (cycles) are calculated using the rainflow counting algorithm in fe-Safe software using the stress-strain data obtained from the proposed FE model developed using the Abaqus software. Using the proposed model, it is possible to generate S-N curves for any loading condition for a given porosity characteristic (porosity density and average pore size).
EXTENDED ABSTRACT
IMPACT OF MICRO AND MESOSTRUCTURE ON THE FAILURE RESISTANCE OF LASER POWDER BED FUSION-PROCESSED MATERIALS
Engineering materials processed using additive manufacturing (AM) techniques such as laser powder bed fusion (LPBF) often exhibit unique microstructures and defects that must be controlled to obtain peak performance in mechanical properties and as such a level of damage-tolerance that cannot be achieved in cast alloys. However, our understanding of how processing conditions control micro- and mesostructure and, in turn, mechanical performance, particularly regarding failure resistance, is weak. Furthermore, heat treatments that have been designed to achieve peak performance in cast alloys are often not optimized for alloys that have been processed using AM techniques. Here, we report our work on the effect of processing parameters such as layer thickness, hatch spacing, and scan strategy on crack resistance curve (R-curve) behavior in different orientations of LPBF-processed AlSi10Mg and correlate mechanical performance with meso- and microstructural features such as melt pool arrangement, cell morphology, grain size, grain orientation, and texture. Compared to that we show how heat-treatments impact fracture resistance as well as their anisotropy in two orthogonal orientations in an LPBF-processed 18Ni-300 maraging steel.
EXTENDED ABSTRACT