Process Development and Metallurgical Characterization of Wire-based Direct Energy Deposition for Aluminium-Magnesium Alloys
von Martin FröndAdditive manufacturing is a production technology based on the layered addition of
material to produce a component, typically using powder or wire raw material. Wirebased
additive manufacturing stands out for its potential to realize high material deposition
rates of several kilograms, rather than a few hundred grams per hour, compared
to powder-based processes. The melt to solidification behaviour is important, since it
determines the resulting solidification microstructure and chemical composition. Thus, it
affects the mechanical properties of the processed component. Besides the physical properties
of the material, the melt to solidification behaviour is dependent on the process
temperature and cooling rates, which can be controlled by targeted adjustment of the
process energy input.
Due to their excellent physical and mechanical properties, Al-Mg alloys are among the
most used light-weight construction materials in transportation industry. They are used
for large-scale components and could be processed economically with wire-based highthroughput
laser metal deposition. However, wire-based laser metal deposition of Al
alloys is poorly understood so far. Complex challenges such as the high reflectivity of Al,
pore and crack development, or loss of volatile elements impede its industrial implementation.
The aim of this work is to methodically address these challenges in order to enable the processability
of Al-Mg alloys by wire-based laser metal deposition. Special attention is paid
to the process energy to solidification microstructure relationship, since the microstructure
of non-heat-treatable Al-Mg alloys strongly determines the mechanical properties of
the generated structure.
For this purpose, theoretical considerations on important thermophysical properties are made and relevant process energies in wire-based laser metal deposition are identified and discussed with regard to their influence on the process temperature. Moreover, frequently observed defects during the processing of Al-Mg alloys are identified and discussed.
Knowledge gained is used to develop two approaches enabling the achievement of defect-free Al-Mg wall-like structures in wire-based laser metal deposition.
The first approach is based on an externally generated affection of the process temperature by pre-heating the substrate during deposition. This allowed for high deposition velocities and showed positive effects on the fusion characteristics. However, metallographic characterization and microhardness testing revealed the development of grain coarsening along the height of the deposited wall-like structures, which was also accompanied with a decrease of tensile strength.
The second approach investigated in the thesis consists of the selection of low deposition velocity to achieve stable melting properties combined with a systematic adjustment of the laser beam irradiance, which was identified to an important process energy that strongly affects the process temperature and melt-to-solidification behaviour during deposition.
To investigate the process-to-part-property relationships, the deposition of wall-like structures using different laser beam irradiances was analysed by a thermal monitoring system.
The wall-like structures were characterized in order to link the results of thermal analysis to distinct microstructural features and mechanical properties. Homogeneous microstructure evolutions along the height of the deposited structures were achieved. Moreover, it was found that a targeted control of the laser beam irradiance can significantly affect the process temperature and enable the achievement of distinctly different solidification microstructures and mechanical properties.
material to produce a component, typically using powder or wire raw material. Wirebased
additive manufacturing stands out for its potential to realize high material deposition
rates of several kilograms, rather than a few hundred grams per hour, compared
to powder-based processes. The melt to solidification behaviour is important, since it
determines the resulting solidification microstructure and chemical composition. Thus, it
affects the mechanical properties of the processed component. Besides the physical properties
of the material, the melt to solidification behaviour is dependent on the process
temperature and cooling rates, which can be controlled by targeted adjustment of the
process energy input.
Due to their excellent physical and mechanical properties, Al-Mg alloys are among the
most used light-weight construction materials in transportation industry. They are used
for large-scale components and could be processed economically with wire-based highthroughput
laser metal deposition. However, wire-based laser metal deposition of Al
alloys is poorly understood so far. Complex challenges such as the high reflectivity of Al,
pore and crack development, or loss of volatile elements impede its industrial implementation.
The aim of this work is to methodically address these challenges in order to enable the processability
of Al-Mg alloys by wire-based laser metal deposition. Special attention is paid
to the process energy to solidification microstructure relationship, since the microstructure
of non-heat-treatable Al-Mg alloys strongly determines the mechanical properties of
the generated structure.
For this purpose, theoretical considerations on important thermophysical properties are made and relevant process energies in wire-based laser metal deposition are identified and discussed with regard to their influence on the process temperature. Moreover, frequently observed defects during the processing of Al-Mg alloys are identified and discussed.
Knowledge gained is used to develop two approaches enabling the achievement of defect-free Al-Mg wall-like structures in wire-based laser metal deposition.
The first approach is based on an externally generated affection of the process temperature by pre-heating the substrate during deposition. This allowed for high deposition velocities and showed positive effects on the fusion characteristics. However, metallographic characterization and microhardness testing revealed the development of grain coarsening along the height of the deposited wall-like structures, which was also accompanied with a decrease of tensile strength.
The second approach investigated in the thesis consists of the selection of low deposition velocity to achieve stable melting properties combined with a systematic adjustment of the laser beam irradiance, which was identified to an important process energy that strongly affects the process temperature and melt-to-solidification behaviour during deposition.
To investigate the process-to-part-property relationships, the deposition of wall-like structures using different laser beam irradiances was analysed by a thermal monitoring system.
The wall-like structures were characterized in order to link the results of thermal analysis to distinct microstructural features and mechanical properties. Homogeneous microstructure evolutions along the height of the deposited structures were achieved. Moreover, it was found that a targeted control of the laser beam irradiance can significantly affect the process temperature and enable the achievement of distinctly different solidification microstructures and mechanical properties.