TY - BOOK

T1 - Link between Microstructure and Fracture Mechanical Properties of Scandium-modified Aluminium Alloys

AU - Schimbäck, David

N1 - embargoed until 03-04-2028

PY - 2023

Y1 - 2023

N2 - For several years now, Additive Manufacturing (AM) of metallic materials has been a rapidly growing area of research in materials science. Among the many different processes included in the term ¿additive manufacturing¿, Laser Powder Bed Fusion (LPBF) has attracted the interest of many industries, especially aerospace companies who are pushing the limits of this manufacturing process with ambitious projects to make the advantages of LPBF accessible on an industrial level. For this reason, tailor-made alloy concepts are being brought to market to accommodate the special processing needs of LPBF. One of the first alloys to be tailored to LPBF¿s processing environment is an AlMgMnScZr alloy developed by Airbus based on Al5083 (commercially known as Scalmalloy®). However, to qualify the material for fatigue-loaded applications in aircraft structures, extensive knowledge of the alloy system in terms of processability, microstructure, and behavior under mechanical loading is necessary. The microstructure of additively manufactured materials is highly dependent on the chemical composition of the alloy. For this reason, an initial study was conducted with the different alloy compositions of Scalmalloy® (Mg as the main alloying element), Scancromal® (Cr as the main alloying element) and Scantital® (Ti as the main alloying element) to investigate the influence of the main alloying element on the microstructure. Despite the high Sc and Zr content of all alloys and thus the strong grain refining effect of the primary precipitates, epitaxial grain growth was observed in the Cr alloy system. Thus, by varying the main alloying element, three different microstructures were adjusted and investigated: ultrafine grain (Scantital®), bimodal (Scalmalloy®) and epitaxial grain growth (Scancromal®). In addition to the chemical composition of the material, the adaptation of process parameters is a critical point for the production of high-performance components. In particular, the high Mg content of Scalmalloy® - an element that has a low vapour pressure and thus evaporates significantly due to the high processing temperatures - makes a specific scan strategy necessary to reduce the interaction between the metal vapour plume and the laser as much as possible. The combination of this scan strategy and an optimized welding profile produced extraordinary material quality, whose fatigue properties exceed those of conventional aerospace Al alloys. Furthermore, by combining this scan strategy with a heated build platform, it was possible to demonstrate ¿ first through simulations and then through experiments ¿ that strength-increasing secondary precipitates form during the manufacturing process. As a result, the as-built condition already exhibits strengths in the range of the peak-aged condition. Thus, subsequent heat treatment could be fully avoided. Since the process studied here builds up material layer by layer using a welding-like process, an increased probability of defects has to be expected. In order to increase the understanding of the failure mechanisms and to be able to estimate the effects of defects, a fracture mechanics analysis of the material is essential. It is shown that the bimodal microstructure has a strong influence on the quasi-static behaviour as well as on the fatigue properties and fatigue crack growth. Likewise, the consequences of process defects are described and predicted with the aid of fracture mechanic tools.

AB - For several years now, Additive Manufacturing (AM) of metallic materials has been a rapidly growing area of research in materials science. Among the many different processes included in the term ¿additive manufacturing¿, Laser Powder Bed Fusion (LPBF) has attracted the interest of many industries, especially aerospace companies who are pushing the limits of this manufacturing process with ambitious projects to make the advantages of LPBF accessible on an industrial level. For this reason, tailor-made alloy concepts are being brought to market to accommodate the special processing needs of LPBF. One of the first alloys to be tailored to LPBF¿s processing environment is an AlMgMnScZr alloy developed by Airbus based on Al5083 (commercially known as Scalmalloy®). However, to qualify the material for fatigue-loaded applications in aircraft structures, extensive knowledge of the alloy system in terms of processability, microstructure, and behavior under mechanical loading is necessary. The microstructure of additively manufactured materials is highly dependent on the chemical composition of the alloy. For this reason, an initial study was conducted with the different alloy compositions of Scalmalloy® (Mg as the main alloying element), Scancromal® (Cr as the main alloying element) and Scantital® (Ti as the main alloying element) to investigate the influence of the main alloying element on the microstructure. Despite the high Sc and Zr content of all alloys and thus the strong grain refining effect of the primary precipitates, epitaxial grain growth was observed in the Cr alloy system. Thus, by varying the main alloying element, three different microstructures were adjusted and investigated: ultrafine grain (Scantital®), bimodal (Scalmalloy®) and epitaxial grain growth (Scancromal®). In addition to the chemical composition of the material, the adaptation of process parameters is a critical point for the production of high-performance components. In particular, the high Mg content of Scalmalloy® - an element that has a low vapour pressure and thus evaporates significantly due to the high processing temperatures - makes a specific scan strategy necessary to reduce the interaction between the metal vapour plume and the laser as much as possible. The combination of this scan strategy and an optimized welding profile produced extraordinary material quality, whose fatigue properties exceed those of conventional aerospace Al alloys. Furthermore, by combining this scan strategy with a heated build platform, it was possible to demonstrate ¿ first through simulations and then through experiments ¿ that strength-increasing secondary precipitates form during the manufacturing process. As a result, the as-built condition already exhibits strengths in the range of the peak-aged condition. Thus, subsequent heat treatment could be fully avoided. Since the process studied here builds up material layer by layer using a welding-like process, an increased probability of defects has to be expected. In order to increase the understanding of the failure mechanisms and to be able to estimate the effects of defects, a fracture mechanics analysis of the material is essential. It is shown that the bimodal microstructure has a strong influence on the quasi-static behaviour as well as on the fatigue properties and fatigue crack growth. Likewise, the consequences of process defects are described and predicted with the aid of fracture mechanic tools.

KW - Additive Manufacturing

KW - Laser Powder Bed Fusion

KW - Aluminium Alloy

KW - Scalmalloy

KW - Metallurgy

KW - Microstructure

KW - Fracture Mechanics

KW - Fatigue

KW - Additive Fertigung

KW - Laser Pulverbett Schmelzen

KW - Aluminium Legierung

KW - Scalmalloy

KW - Metallurgie

KW - Mikrostruktur

KW - Bruchmechanik

KW - Ermüdung

M3 - Doctoral Thesis

ER -