In-situ analysis of process characteristics in Directed Energy Deposition

• In recent years, the interest in Additive Manufacturing on an industrial scale has risen due to the various new processes and the increase in potential use cases. In this thesis, the two Additive Manufacturing processes, Wire Arc Additive Manufacturing and Laser Metal Deposition were investigated. Both processes belong to the Directed Energy Deposition processes in Additive Manufacturing and are already used in different industrial areas such as automotive, aviation, railway, or medical engineering. A major challenge for the industrialization of Additive Manufacturing is the insufficient quality and repeatability of the manufactured parts due to the complexity of the processes and the lack of process knowledge. Therefore, this work focused on a deeper understanding of the process characteristics and their correlations with different sensors used for in-situ analysis.  One of the sensors used for in-situ analysis was high-speed imaging. High-speed imaging during Wire Arc Additive Manufacturing revealed that different lead angles of the welding torch have an influence on fluctuation effects in the manufactured structures. To avoid such fluctuation effects, which mainly origin from too low or too high interlayer temperatures, it was found that a pushing Wire Arc Additive Manufacturing process with a slightly tilted lead angle is working best. Apart from fluctuation effects, oxidation of aluminium can be also critical for the process stability of Wire Arc Additive Manufacturing. It was found that the surface oxidation on aluminium parts changed from an amorphous oxide layer into a white duplex oxide layer during the process. It was also found that oxidation anomalies, which can occur during processing due to process instabilities or lack of shielding gas, can be detected by light emission spectroscopy during manufacturing as peaks in the light spectrum arise when they occur. Another challenge in Wire Arc Additive Manufacturing of aluminium is the porosity in parts as it can have a significant impact on the resulting mechanical properties. It has been observed that as the shielding gas flow rate increases, the porosity in aluminium parts also increases due to the rapid solidification of the melt pool by the forced convection of gas flow. In addition, it has been shown that gas inclusions escaping from the melt pool leave cavities on the surface that can be observed by process imaging, which reveals information about the porosity of the part. Another promising sensor for in-situ analysis of the process characteristics are microphones that capture acoustic emissions. For Wire Arc Additive Manufacturing, the investigations showed that the main acoustic emissions origin from the plasma expansion of the arc. The acoustic emissions and the process anomalies that occur correlate mainly with the size of the arc because that is essentially the ionized volume that leads to the air pressure and causes the acoustic emissions. For Laser Metal Deposition, it was found that the main acoustic emissions are created by the interaction between the powder particles and the laser beam, because they create an air pressure when the particles expand from the solid state to the liquid state while melting. Another major area investigated in this thesis was multi-material Additive Manufacturing, as process and material characteristics can change significantly. For processing of multi-material parts in Wire Arc Additive Manufacturing, it was found that the strength was limited by the properties of the individual aluminium alloys and not by those of the material transition zones. Process monitoring algorithms have been investigated to determine the chemical composition of the processed material. It was shown that the voltage, current, acoustic, and spectral emission data can be used for in-situ analysis of the chemical differences between two aluminium alloys. For Laser Metal Deposition, the design freedom of Additive Manufacturing with multiple materials was also demonstrated. It was shown that material transitions can be implemented discrete and graded, but the gradual material transition showed advantages in avoiding cracks in the material transition zones. In summary, all scientific papers contributed to a deeper process understanding of the process, the processed materials, and the resulting mechanical properties. In addition, the contributions provided crucial insights into the interrelationships of the process characteristics and the physical principles of various sensors used for in-situ analysis of the processes. 

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Materialteknik -- Bearbetnings-, yt- och fogningsteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Materials Engineering -- Manufacturing, Surface and Joining Technology (hsv//eng)

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Produktionsutveckling

Manufacturing Systems Engineering

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