Metal Additive Manufacturing Conference 2018

Aluminum alloys play an important role for lightweight components for automotive and aerospace industries. Al-Sc alloys exhibit excellent mechanical properties due to the presence of coherent intermetallic Al3Sc precipitates. Zr alloy additions lead to a Zr-rich shell around the small coherent Al3Sc precipitates that slows down precipitate growth and renders the alloy resistant to precipitate coarsening. In this study, two alloys with varied Sc and Zr contents are considered, namely the commercial Al4.5Mg0.66Sc0.4Zr Scalmalloy® and an Al1.0Sc0.6Zr alloy. Laser Metal Deposition (LMD) is a Laser Additive Manufacturing (LAM) process in which metallic powder is injected through a nozzle into the melt pool, created by a laser beam. Deposition of neighboring tracks and subsequent layers during the LMD process, leads to a cyclic reheating of already consolidated material, the so-called intrinsic heat treatment (IHT). The idea behind the present study is to exploit two key features of LAM processes, namely a high cooling rate to quench in a supersaturated solid solution and the IHT to trigger the precipitation reaction in this supersaturated solid solution. This way we were able to produce parts that are already precipitation hardened during manufacturing, eliminating the need for a post heat treatment. We achieved high number densities of intermetallic precipitates in the range of 10^21 3x10^23m 3 with sizes ranging from 5-40nm by exploiting the IHT. We analyzed different IHT conditions by lifting out Atom Probe Tomography (APT) tips from different layers of a multi-layer sample: the bottom layers received maximum IHT as a high number of subsequent layers were deposited while the top layers received a minimum IHT only from neighboring tracks. In the case of the commercial Al4.5Mg0.66Sc04Zr Scalmalloy®, the Al3Sc precipitates coarsened with increasing strength of the IHT. We found the reason for the coarsening in the absence of the expected Zr-rich shells around the precipitates. Instead, Zr was bound in micron-sized, primary precipitates and was therefore not available to form the Zr shell. In order to avoid the undesired coarsening, we optimized both, alloy composition and process. We used an alloy with increased Zr content and optimized the LMD process for a high cooling rate in order to keep enough Zr in solid solution to allow for the Zr shell formation. In the optimized alloy (Al1.0Sc0.6Zr), the precipitates that formed were of Al3(Sc,Zr) type and did not coarsen at all. Instead, with increasing strength of the IHT, a Zr-rich shell formed around the Sc-rich precipitate core that prevented coarsening. We evaluated mechanical properties of both alloys and related them to the precipitate number density, spatial distribution, and chemistry as found from APT and Transmission Electron Microscopy (TEM) experiments.