Application of additive manufacturing processes and more precisely of Laser Powder Bed Fusion (LPBF) is growing since a few decades. This growth is due to the many benefits offered by this process compared to the conventional manufacturing methods. However, LPBF, by its nature, causes the appearance of specific processing-induced flaws that affect the mechanical properties of parts produced, thus limiting their industrial use Among these defects, porosity is particular since it cannot be completely eliminated and its detection is difficult and costly. Knowing that a residual level of porosity is unavoidable and that it affects the mechanical properties of the parts, it becomes crucial to be able to detect and characterize pores in order to determine if their presence is critical in a particular application.
In order to give a full picture of this problematic and provide an answer useful for the scientific and engineering community, this master thesis will be divided as such:
The first chapter presents a literature review of the additive manufacturing market, LPBF process, characteristics of Ti-6Al-4V (Ti64) parts produced by LPBF, damage tolerant design and fatigue and crack propagation mechanisms in parts produced by LPBF.
The second chapter takes the shape of a scientific article and describes the methodology that was used to manufacture, using LPBF, three groups of Ti64 specimens with purposely- induced porosity. The specimens were then subjected to analyses of their chemical composition, microstructure and porosity and also to tensile, fatigue and crack propagation testing. The study showed that the three groups of specimens manifested similar characteristics with the exception of porosity. Chemical composition analysis reveals that elements distribution are within acceptable ranges for an LPBF-formed Ti64 alloy. The microstructure of all groups is composed of prior β-columnar grains 120 μm wide and about 500 μm high. After heat treatments, the columnar grains are composed mostly of 2 μm thick α lamellae, 10% globular α, 10 μm wide, and 4% inclusions of β phase. The OED group has the lowest porosity level (0,003 %) composed mostly of small to medium seized pores (√□ 0,075 mm) relatively spherical (aspect ratio ≈ 0.65). The level of porosity in the HED and LED groups is ten times higher (0.033% and 0.044%, respectively), but while pores in the HED group are relatively similar to those in the OED group (√□ 0,075 mm and aspect ratio ≈ 0,75), pores in the LED group are larger and more elongated (√□ 0,085 mm and aspect ratio ≈ 0,55). With respect to mechanical properties, tensile strength (Sy = 890 MPa ; Su = 990 MPa), long crack propagation threshold (ΔKth = 3,9 MPa m½) and long crack propagation behavior are minimally influenced by variations in the porosity characteristics. However, elongation at break (εOED = 14,93 % ; εHED = 13,64 % ; εLED = 11,30 %) and fatigue strength at N = 107 cycles (Δσf, OED = 495 MPa ; Δσf, HED = 450 MPa ; Δσf, LED = 360 MPa) are highly influenced by the presence of defects. More specifically, the criticality of the pores was attributed to their size and proximity to the surface. The Kitagawa-Takahashi diagram, constructed from the results of this study, appeared to be an adequate, but conservative, tool for damage-tolerant design of LPBF-formed Ti64 samples.
The third chapter presents a study of the fatigue resistance of complex Ti64 parts produced by LPBF. In this chapter, the part manufacturing process, the test bench, the finite elements model used to predict stresses and strains in the parts, the testing protocols and the results obtained are described and discussed. Fatigue testing reveals that the fatigue strength of the complex parts is one-tenth of that of the standard specimens for the same stress level. The lower fatigue strength was attributed to the effect of the as-fabricated surface finish and secondary damage mechanisms.
Finally, in the conclusion, the main elements of the master thesis are summarized and the recommendations for further study are made.
| Date | 27 Apr 2023 |
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| Original language | French |
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| Awarding Institution | - École de technologie supérieure
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| Supervisor | Vladimir Brailovski (Supervisor) |
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