Powder Spreadability and Characterization of Sc- and Zr-modified Aluminium Alloys processed by Selective Laser Melting: Quality Management System for additive manufacturing

Abstract

Additive manufacturing technologies such as Selective Laser Melting have reached a level of maturity, which allows the direct production of functional parts for many industrial applications. For certain industrial sectors such as for instance the Space-, the Aerospace- and the turbine industry, high quality requirements are put on structural parts. These requirements can address aspects of mechanical material properties, material microstructure, part- and surface quality and accuracy, respectively. However, the variety of influencing parameters in the SLM-processing chain makes it difficult to directly qualify parts for such applications, giving the need to qualify each AM-part separately after the additive build process. For this reason, a Quality Management System for Selective Laser Melting, and more generally for additive manufacturing processes, is essential. The implementation of a Statistical process control (SPC) methodology is therefore suggested, enabling the consideration of various different influencing parameters, and giving insights into the dependencies of input and output parameters, as-well as cross-correlations between different parameters. As a contribution to a Quality Management System, two influencing parameters in the SLM-process chain are investigated in more detail. Powder flowabilty is an essential parameter affecting the capability of a SLM-machine to create the thin powder layers required in a repeatable and high quality. As the traditional flowability measurement techniques, such as e.g. the Hall flow meter, are not capable of measuring flowabilty sufficiently close to the SLM-processing conditions, a new quantitative method is developed, which is based on the statistical analysis of avalanches taking place when a powder flows freely in a rotating drum. This measurement approach is closer to the conditions in a SLM-machine when powders are spread across the build platform. Therefore, the flowability measurement technique can also be considered as an assessment of the powder spreadability, and the quantitative results can be correlated with the quality of the generated powder layer. In addition, the alloy system and –composition also affects the quality of the final parts by the various metallurgical phenomena taking place when a small melt-pool cools down. A Sc- and Zr-modified 5xxx aluminium alloy powder is used to develop the basic understanding of the consolidation phenomena taking place during SLM. The evolving SLM-processed unique microstructure consists of a bi-modal grain size distribution, with grain sizes in the range of [200 nm, < 2 um] in the fine-grained area, and about [1 um, 15 um] in the coarser regions, respectively. Hence, such grain sizes are a factor of 5 to 10 smaller compared to traditional SLM-processed Al-alloys. Due to the complete absence of any preferential grain orientation in the fine-grained areas, alongside with the small grains, this alloy system shows almost no mechanical anisotropy. A further advantage of this fine-grained microstructure is a reduced tendency for hot-cracking. Even in the heat treated condition, the microstructure is almost not affected and grain growth only takes place in very limited regions of coarser grains when additional energy is available from a HIP processing. Consequently the mechanical material properties can be significantly improved to values > 500 MPa alongside with high ductility, detrimental without suffering from influences of the heat treatment. The metallurgical origins of this advantageous microstructural behaviour are related to the formation of a high density of intermetallic particles with a similar lattice structure as aluminium, and with a very small lattice misfit to the Al-matrix. The Al3Sc- and Al-Mg-oxide seed crystals are having a high thermal stability in the melt, and are leading to the fine-grained fraction of the microstructure. However, due to very high melt-pool temperatures reaching > 2’000 °C during processing, such seed crystals go into solid solution in the coarse-grained regions, giving rise to the formation of a coarser grained microstructure. Al3Sc particles dissolve at temperatures > 800 °C, whereas Al-Mg-oxides can sustain somewhat higher temperatures. The dissolution of such intermetallic phases also leads to a reduction of the density of grain boundary particles, thereby reducing the stabilizing effect of such particles in the coarser grained regions. Due to the very high cooling rates of up to 1.5 x 10^6 Ks-1, such particles cannot be precipitates during the melt-pool cool-down. Therefore, in melt-pool regions where the temperatures exceed about 800°C primary Al3Sc seed-particles are not precipitated. This results in the formation of a coarser grained material, whose local grain sizes depend on the local density of Al-Mg-oxides, and the competing grain growth. Due to the high amount of Sc remaining in solid solution during processing, a post-process heat treatment can be applied, enabling the controlled precipitation of fine dispersed, coherent < 5nm Al3Sc particles. These particles are responsible for the significantly improved mechanical properties compared to the as-built condition. Due to the high thermal stability of the Al3Sc- and Al-Mg-oxide particles being located at grain boundaries, the as-processed microstructure is stabilized, and not affected by the heat treatment, which makes the alloy a good candidate for high temperature applications. The microstructural principles discussed in this thesis show that by a tailored alloy-design the typical SLM-process-specific disadvantages, such as e.g. anisotropic mechanical properties, can be overcome. However, there remains a potential to further improve the alloy system as the microstructure of the investigated alloy system is not homogeneous after SLM-processing, aiming at a homogeneously fine-grained microstructure. In this context two options are discussed, which also can be combined: a) Alloy design Alloy design should enable the formation of a high density of seed-crystals in the metallic melt. These seeds should have an as high thermal stability as possible, which can be assessed by an appropriate Scheil-simulation. Furthermore, in order for these particles to show a high seed-crystal potential, they should have a similar lattice structure, and an as small lattice parameter misfit as possible. b) SLM-process optimization Given a suitable alloy composition, the reduction of the speed of the solidification front in the metallic melt would allow the formation of primary precipitates, which could serve as seed crystals during the solidification of the melt-pool. This approach longs for a suitable SLM-process optimization, for which a two-laser beam technology could be used. Thereby, the second laser beam would follow the main laser beam, and enable a more controlled, slower cool-down of the melt-pool. Therefore, this thesis shows that the alloy system used, and its specific composition, along with the flowability of the powder used significantly influence the SLM-process, and the resulting final part quality.