Microstructural features of Sc- and Zr-modified Al-Mg alloys processed by selective laser melting

Significance Statement

Additive manufacturing techniques, more specifically selective laser melting (SLM), are necessary techniques which can be adopted for the manufacture of metallic components with high complexity. Selective laser melting is defined by the metal powder layers’ creation, as well as selective laser scanning of successive cross-sections of the components under consideration. Laser radiation is responsible for full melting of these powders, and therefore, almost fully dense metal components can be built.

This presents new opportunities for creation of improved products for almost all industrial applications in automotive, tooling, and general mechanical engineering. Above all, these technologies can be applied to manufacture lightweight aerospace and space applications including brackets for satellites, and planetary rovers etc using aluminium alloys.

In a recent paper published in Materials and Design Adriaan Spierings and colleagues presented an explanation model for the microstructure evolution of a SLM-processed Al-Mg-Sc-Zr alloy, which combined the outcomes of the microstructural analyses with selective laser melting processing temperature simulations in the melt pool, and simulations of precipitation and solidification reactions from the melt.

The authors used Scalmalloy powder material having a hypereutectic composition of Sc and Zr, and produced additively 10x10x10mm3 cube specimens using two levels for the applied energy density based on layer thickness, hatch distance, and laser power. The scan strategy was implemented using bi-directional scanning with or without turning of the direction of scanning from layer to layer. The researchers used all the materials in the as-processed condition in a bid to get insights into microstructural formation during the process.

The selected laser melting conditions create a microstructure that is very distinct from the materials processed conventionally. The conditions entail rapid melting of a thin layer of powder that forms a melt-pool situated on top of consolidated cold materials. Once the laser beam is passed, the melt-pool solidifies immediately owing to the high rate of cooling. Microstructures formation is therefore initiated by the high cooling rate and direction, and reactions that take place in the pool before and after solidification.

Selective laser melting processed microstructure consists of two distinct zones. One of the zones had very fine grained microstructure with preferential grain orientation. The other zone had slightly larger columnar grains that grew along the temperature gradient. This appears unique when compared to other aluminum alloys, therefore, a profound analysis of the microstructure and related formation mechanisms during selective laser melting is necessary. The authors observed a number of microstructural effects in the selective laser melting processed Scalmalloy®. These reactions entail precipitation of various particles including Al3Sc and Al-Mg-oxides, being intra- and intergranular, thereby stabilizing the resulting microstructure.

The fine grained zones were caused by the high number density of seed crystals that were present in a thin region around the pool, therefore, favoring instantaneous growth of very fine grains. The coarser grained region was related to the steep temperature gradient associated with low number density of the aluminum-magnesium oxide seed crystal that favored a columnar and coarser grain structure.

Such an aluminum microstructure is advantageous due to the outstanding grain refinement and related recrystallization prevention up to high temperatures.

Microstructural features of Sc- and Zr-modified Al-Mg alloys processed by selective laser melting 1- advances in engineering

Figure 1: SLM scan strategy for additive manufacturing of a cube sample. Microstructure with last consolidated layer showing columnar grain growth, and EBSD scan from an inner region of a small SLM-cube

Microstructural features of Sc- and Zr-modified Al-Mg alloys processed by selective laser melting 2- advances in engineering

Figure 2: a) xy-, xz- and yz-EBSD-images of the cube b)EBSD of the xz-plane of the cube shown in a) and inverse pole figure (IPF) map. The [001] direction refers to the additive build direction. c) [001] Pole figure from single FG area. d) [001] Pole figure of the complete scan area with fine and coarse grained material. The points are weighted to the grain size, hence the large points relate to coarse grains and the fine “background” is created by the fine grained material.

About The Author

Adriaan B. Spierings finished his studies in mechanical engineering and business administration at ETH in Zurich in 1999. Since 2005 he is leading the research group at inspire AG – icams. He focus on additive manufacturing technologies, specifically on the development of quality management systems for the Selective Laser Melting. In his research he addresses the complete SLM-processing chain, from quantitative powder requirements, alloys for SLM and material characterization, to process simulation and monitoring solutions and new SLM machine concepts.

He is lecturer at the ETH and universities of applied sciences for additive manufacturing, and editor in the Springer Journal “Progress in additive manufacturing”. A. Spierings is board member of several platforms on additive manufacturing, and member of the Swiss Academy of Engineering Sciences. Next to being head of R&D in SLM, he finishes his PhD in 2017 in the above mentioned topics.

Reference

A.B. Spierings1, K. Dawson2, T. Heeling3, P.J. Uggowitzer4, R. Schäublin4, F. Palm5, K. Wegener3. Microstructural features of Sc- and Zr-modified Al-Mg alloys processed by selective laser melting. Materials and Design, volume 115 (2017), pages 52–63.

Show Affiliations
    1. Innovation Centre for Additive Manufacturing, INSPIRE-AG, Lerchenfeldstrasse 5, CH-9014 St. Gallen, Switzerland
    2. Centre for Materials and Structures, School of Engineering, University of Liverpool, Liverpool L69 3GH, UK
    3. Institute of Machine Tools and Manufacturing, Department of Mechanical and Process Engineering, ETH Zurich, CH-8092 Zurich, Switzerland
    4. Laboratory of Metal Physics and Technology, Department of Materials, ETH Zurich, CH-8093 Zurich, Switzerland
    5. Airbus Group, Innovations, TX2, D-81663 Munich, Germany

 

 

Go To Materials and Design