Article
Requirements for Processing High-Strength AlZnMgCu Alloys
with PBF-LB/M to Achieve Crack-Free and Dense Parts
Steffen Heiland
1,2,
*, Benjamin Milkereit
3,4
, Kay-Peter Hoyer
1,2
, Evgeny Zhuravlev
3,4
, Olaf Kessler
3
and Mirko Schaper
1,2
Citation: Heiland, S.; Milkereit, B.;
Hoyer, K.-P.; Zhuravlev, E.; Kessler,
O.; Schaper, M. Requirements for
Processing High-Strength AlZnMgCu
Alloys with PBF-LB/M to Achieve
Crack-Free and Dense Parts. Materials
2021, 14, 7190. https://doi.org/
10.3390/ma14237190
Academic Editor: Amir Mostafaei
Received: 20 September 2021
Accepted: 12 November 2021
Published: 25 November 2021
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1
Chair of Materials Science, Paderborn University, 33098 Paderborn, Germany; hoyer@lwk.upb.de (K.-P.H.);
schaper@lwk.uni-paderborn.de (M.S.)
2
Paderborn Institute for Additive Fabrication (PIAF), 33100 Paderborn, Germany
3
Chair of Materials Science, University of Rostock, 18055 Rostock, Germany;
benjamin.milkereit@uni-rostock.de (B.M.); evgeny.zhuravlev@uni-rostock.de (E.Z.);
olaf.kessler@uni-rostock.de (O.K.)
4
Competence Centre
◦
CALOR, Department Life, Light & Matter, University of Rostock,
18055 Rostock, Germany
* Correspondence: heiland@lwk.upb.de; Tel.: +49-52-5160-5325
Abstract:
Processing aluminum alloys employing powder bed fusion of metals (PBF-LB/M) is becom-
ing more attractive for the industry, especially if lightweight applications are needed. Unfortunately,
high-strength aluminum alloys such as AA7075 are prone to hot cracking during PBF-LB/M, as well
as welding. Both a large solidification range promoted by the alloying elements zinc and copper
and a high thermal gradient accompanied with the manufacturing process conditions lead to or
favor hot cracking. In the present study, a simple method for modifying the powder surface with
titanium carbide nanoparticles (NPs) as a nucleating agent is aimed. The effect on the microstruc-
ture with different amounts of the nucleating agent is shown. For the aluminum alloy 7075 with
2.5 ma% titanium carbide nanoparticles, manufactured via PBF-LB/M, crack-free samples with a
refined microstructure having no discernible melt pool boundaries and columnar grains are observed.
After using a two-step ageing heat treatment, ultimate tensile strengths up to 465 MPa and an 8.9%
elongation at break are achieved. Furthermore, it is demonstrated that not all nanoparticles used
remain in the melt pool during PBF-LB/M.
Keywords:
grain refinement; crack reduction; laser beam melting; aluminum alloy; titanium carbide;
nanoparticle; PBF-LB/M
1. Introduction
High-strength aluminum alloys such as alloys of the 7xxx series are, due to their
low density and good mechanical properties (excellent strength-to-weight ratio), one of
the most important construction materials, in particular in the aerospace and aeronautic
industry [
1
,
2
]. However, the hot crack susceptibility of these alloys is one major challenge
that experts have been facing for decades, and they have been looking for the mechanism to
solve this challenge [
3
–
5
]. Moreover, poor flowability [
6
], high reflectivity, as well as high
thermal conductivity [
7
] are further challenges limiting the deployment of these materials
in laser-based powder bed fusion of metals (PBF-LB/M) [
8
,
9
], also known as laser beam
melting (LBM).
The phenomenon of occurring hot cracks is well-known. Regarding this, numerous
studies have been conducted for aluminum alloys with the propensity to hot cracking,
mainly for alloys of the 2xxx, 5xxx, 6xxx, and 7xxx series. In particular, aluminum al-
loys with the alloying elements copper (Cu) and magnesium (Mg) tend to have a higher
crack sensitivity [
10
–
13
]. From the material aspect, the alloy composition itself can also be
changed by evaporation of volatile alloying elements (e.g., zinc (Zn)) during PBF-LB/M [
4
].
Materials 2021, 14, 7190. https://doi.org/10.3390/ma14237190 https://www.mdpi.com/journal/materials
