Wire-based Directed Energy Deposition of AA7075: Using solid wire of AA7075 functionalized with TiC nanoparticles
Meneses Fuentes, William (2023)
2023
Materiaalitekniikan DI-ohjelma - Master's Programme in Materials Engineering
Tekniikan ja luonnontieteiden tiedekunta - Faculty of Engineering and Natural Sciences
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Hyväksymispäivämäärä
2023-09-21
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-202308187646
https://urn.fi/URN:NBN:fi:tuni-202308187646
Tiivistelmä
AA7075 is a heat treatable aluminum alloy widely used in aerospace and automotive applications due to its outstanding high strength-to-weight ratio. However, the implementation of this alloy in Additive Manufacturing (AM) processes has been limited due to its susceptibility to hot cracking. Moreover, selective evaporation of low boiling point elements Zn and Mg can cause gas porosity and diminish the mechanical properties of AM parts. Recent research revealed the effectiveness of nanoparticles additives to change the solidification behavior of high-strength aluminum alloys and improve their weldability/printability. In this Master’s thesis, the feasibility of Wire-DED process to fabricate samples of AA7075 enhanced with TiC nanoparticles was evaluated using two different power sources: CMT and coaxial laser.
Cross section analysis of single-bead samples revealed that CMT process is more prone to formation of gas porosity than coaxial laser process. However, the deposition rate of CMT is more than twice that of coaxial laser, as they were estimated to be 0.95 kg/h and 0.42 kg/h respectively. Due to persistence porosity above 2% in all CMT samples, characterization was limited to a macroscopic level. The effect of energy input and CMT variations on bead geometry and porosity was assessed. The presence of contamination and the influence of arc forces on the molten pool are possible reasons for the high porosity.
Regarding coaxial laser process, single and multi-layer samples were successfully created. The response of the samples to precipitation hardening was studied, evaluating the microstructure and the microhardness before and after T6 heat treatment. Specimens were characterized using optical and electron microscopy, electron backscatter diffraction (EBSD) and X-ray diffraction. Crack-free and virtually porosity-free samples were fabricated, which exhibit a refined equiaxed grain structure with grain size lower than 10µm. This confirms the ability of TiC nanoparticles to prevent columnar dendritic growth and promote heterogeneous nucleation. Energy Dispersive Spectroscopy (EDS) analysis showed that there are evaporation losses of Zn and Mg. When compared to wire composition, Zn and Mg contents show reductions of up to 20% and 14%, respectively. Considering the boiling temperatures of these elements, it is inferred that the molten pool reaches temperatures above 1090°C, and the partially melted zone temperature is between 907°C and 1090°C. Despite the losses of Zn and Mg, samples show a positive response to T6 heat treatment as microhardness values increased from 121 HV to 172 HV and were uniform across the sample.
Cross section analysis of single-bead samples revealed that CMT process is more prone to formation of gas porosity than coaxial laser process. However, the deposition rate of CMT is more than twice that of coaxial laser, as they were estimated to be 0.95 kg/h and 0.42 kg/h respectively. Due to persistence porosity above 2% in all CMT samples, characterization was limited to a macroscopic level. The effect of energy input and CMT variations on bead geometry and porosity was assessed. The presence of contamination and the influence of arc forces on the molten pool are possible reasons for the high porosity.
Regarding coaxial laser process, single and multi-layer samples were successfully created. The response of the samples to precipitation hardening was studied, evaluating the microstructure and the microhardness before and after T6 heat treatment. Specimens were characterized using optical and electron microscopy, electron backscatter diffraction (EBSD) and X-ray diffraction. Crack-free and virtually porosity-free samples were fabricated, which exhibit a refined equiaxed grain structure with grain size lower than 10µm. This confirms the ability of TiC nanoparticles to prevent columnar dendritic growth and promote heterogeneous nucleation. Energy Dispersive Spectroscopy (EDS) analysis showed that there are evaporation losses of Zn and Mg. When compared to wire composition, Zn and Mg contents show reductions of up to 20% and 14%, respectively. Considering the boiling temperatures of these elements, it is inferred that the molten pool reaches temperatures above 1090°C, and the partially melted zone temperature is between 907°C and 1090°C. Despite the losses of Zn and Mg, samples show a positive response to T6 heat treatment as microhardness values increased from 121 HV to 172 HV and were uniform across the sample.