Date of Award
2024-12-01
Degree Name
Master of Science
Department
Engineering
Advisor(s)
Francisco Medina
Abstract
Additive manufacturing (AM) processes include Laser-Based Powder Bed Fusion of Metals (PBF-LB/M), which enables the making of complex metal parts, making it an attractive prospect to industries such as aerospace and automotive. However, PBF-LB/M faces challenges regarding defect detection and part quality inconsistencies, often due to hidden parameters such as polygon delay that impact component quality beyond the standard quality control capabilities. Traditional PBF-LB/M qualification processes use machined specimens, including tensile, fatigue, and chemical coupons; these usually cannot pick up key features like scanning control errors, surface roughness, and geometric accuracy.
This thesis explores using as built tensile coupons from the GTADExP program’s quality test artifact (QTA) to address these limitations. As built coupons maintain inherent surface roughness and near-edge defects, better representing process-induced variations with lower cost input. This is supported by the specific tensile testing methodology used in this research on cylindrical-shaped sub-size samples by emphasizing 2.5 mm gauge diameter condition according to ASTM E8 Type 5 and 4 mm according to ASTM E8 Type 4. Examples of analyses included but were not limited to ANOVAs on YS, UTS, and strain at break to assess what process variables affect the mechanical properties of the tensile coupons.
Lack of fusion (LoF) defects present within UNS N07718 QTAs (UTEP 18.01 and UTEP 19.01) resulted in a strain at break reduction of 5% for UTEP 18.01, indicating ductility was reduced as a consequence of laser power ramping; however, YS and UTS values did not significantly vary between QTAs, suggesting that tensile testing can detect defects but lacks the specificity to identify them as LoF defects in PBF-LB/M coupons. Further experiments were conducted with Sc-modified AlSi alloy coupons in fractional factorial design experiments (DOE). For non-heat-treated samples, the most influencing factors in tensile properties were the hatch distance and laser power. A power setting of 370 W yielded an ultimate tensile strength (UTS) of 394 ± 11.7 MPa, in contrast to the UTS of 382 ± 15.2 MPa observed at a power setting of 300 W. The yield strength (YS) and elongation parameters were found to be influenced by the hatch distance. Specifically, a hatch distance of 0.06 mm resulted in a YS of 300.4 ± 11.9 MPa and an elongation of 25.7 ± 2.6%. Conversely, a hatch distance of 0.12 mm produced a YS of 287.8 ± 8.6 MPa and an elongation of 21.6 ± 3.3%.
In the published work presented in Chapter 4, the applicability of tensile testing towards machine qualification is evaluated via a full factorial experiment examining coupon size variation, surface roughness variation, and scanner control variation. Machined and as built conditions, standard-size coupons ASTM E8 Type 3, and sub-size Types 4 and 5 were tested. Amplified polygon delay 300 µs introduced defects that increased defect density an order of magnitude of 10 while maintaining porosity below the NASA-STD-6030 limit (0.25%). Standard coupon geometry (Type 3, 6 mm gauge diameter) in both as built and machined conditions presented a statistically significant difference in UTS under an idealized model (R^2>0.9) of less than 1%, which was too small to be considered relevant. Subsequent alternative tests showed that Type 5 as built samples were susceptible to keyhole defects with pairwise t-tests indicating a significantly lower strain at break 22 ± 4% compared to the nominal parameters of 28 ± 3%.
Despite these findings, sub-size coupons showed high variance in UTS due to surface roughness, introducing uncertainty in load-bearing area measurements. Consequently, tensile testing is limited as a qualification metric and is better suited for assessing material properties after qualification. Further research is recommended to evaluate rough sample geometry, explore alternative qualification methods based on geometrical accuracy, and improve subsystem testing and scanner control.
Language
en
Provenance
Recieved from ProQuest
Copyright Date
2024-12-01
File Size
141 p.
File Format
application/pdf
Rights Holder
Ernesto Gamboa Tiscareno
Recommended Citation
Gamboa Tiscareno, Ernesto, "Mechanical Testing Of Laser Beam Metal Powder Bed Fusion Tensile Specimens: Insights Of Factors Affecting Part Quality And Material Properties" (2024). Open Access Theses & Dissertations. 4241.
https://scholarworks.utep.edu/open_etd/4241