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\title{High Strength Aluminium Alloys in Laser-Based Powder Bed Fusion - a Review }


\author{Julie Langedahl Leirmo, ${ }^{\mathrm{a}, *}$\\
${ }^{a}$ Department of Manufacturing and Civil Engineering, NTNU - Norwegian University of Science and Technology, Teknologiveien 22, 2815 Gjøvik, Norway\\
*Corresponding author. Tel.: +47 48202281.E-mail address: julie.1.leirmo@ntnu.no}
\date{}


\begin{document}
\maketitle


\begin{abstract}
Despite the difficulties of processing high strength aluminium alloys in laser-powder bed fusion additive manufacturing, there is a growing interest in these types of alloys. In this paper, a brief literature review is presented, aiming to give an overview of different approaches to enable laser-powder bed fusion additive manufacturing of high strength aluminium alloys in the 2xxx and 7xxx series. Relevant literature was collected and analysed. The analysis found that adjusting the scan speed is the most investigated approach for aluminium alloys in both the $2 \mathrm{xxx}$ and $7 \mathrm{xxx}$ series. Layer thickness is the least investigated approach, and never investigated for alloys in the 7xxx series.
\end{abstract}

(C) 2021 The Authors. Published by Elsevier B.V.

This is an open access article under the CC BY-NC-ND license (\href{https://creativecommons.org/licenses/by-nc-nd/4.0}{https://creativecommons.org/licenses/by-nc-nd/4.0})

Peer-review under responsibility of the scientific committee of the 54th CIRP Conference on Manufacturing System

Keywords: Additive Manufacturing; Powder Bed Fusion; Aluminium

\section*{1. Introduction}
There has been a growing interest in additive manufacturing $(\mathrm{AM})$ of aluminium (Al) in recent years in various industries such as automotive and aerospace. Low weight combined with relatively high strength makes $\mathrm{Al}$ popular material. In combination with AM technology, parts can become even lighter while still obtaining, or even enhance the required strength and mechanical properties compared with the cast counterpart [1]. However, $\mathrm{Al}$ in $\mathrm{AM}$ and especially laserpowder bed fusion (LPBF) faces challenges due to the nature of $\mathrm{Al}$ and the LPBF technology. Al powder has high reflectivity and low laser absorption and is therefore difficult to melt with a laser, which makes it difficult to produce parts with satisfying quality [1-4].

Several investigations have been performed regarding $\mathrm{Al}$ in LPBF, and in recent years there has been a particular interest for high strength alloys Al alloys in LPBF. In the current study, literature on the processability of high strength $\mathrm{Al}$ alloys in the $2 \mathrm{xxx}$ and $7 \mathrm{xxx}$ series has been collected and analysed. The aim is to give a brief overview of the current trends in research for the successful production of high strength Al parts with LPBF.

A brief theoretical background is given in the next section before the methodology is described in section 3. The results are presented in section 4 and discussed thereafter. Finally, conclusions and proposed future work is presented in section 6.

\section*{2. Theoretical background}
\subsection*{2.1. Additive manufacturing}
ISO/ASTM 52900:2015(E) [5] defines AM as a "process of joining materials to make parts from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing and formative manufacturing methodologies." The interest in AM is predominantly due to the possibility to produce lightweight parts with complex geometries that cannot be obtained with traditional technologies $[1,6]$. The possibility to produce complex parts also enables the fabrication of parts in one piece whereas conventional technologies would yield several components to be assembled [7].

\subsection*{2.2. Powder bed fusion}
According to ISO/ASTM 52900:2015(E) [5], PBF is an "additive manufacturing process in which thermal energy selectively fuses regions of a powder bed". PBF is separated into subcategories that use different types of energy and is suitable for different types of materials $[8,9]$. In the remainder of this paper, the abbreviation LPBF is used to refer to laser powder bed fusion of metals only. Common for all of the PBF technologies is that a thin layer of powder is deposited onto the build surface whereas the desired areas are fused before another thin powder layer is deposited on top of the previous [10].

\subsection*{2.3. Laser-powder bed fusion for metals}
The build process starts with the deposition of a thin layer of the metal powder which then is scanned with a laser beam to fuse the powder particles. The process is repeated in a layerwise manner until the desired part is formed [11]. The powder in the successive layer must not only be melted but also fused into the former layer to ensure a solid part $[9,12]$. According to Ahuja, et al. [12], a melting depth of three-layer thicknesses is the most suitable.

In LPBF, the process is conducted in a controlled atmosphere where the build chamber is filled with an inert gas, usually argon $(A r)$, to prevent oxidation [1, 9]. The metal powder particles are fused with a high-power laser beam to form near-net-shape parts that require minimal post-processing $[9,13,14]$. In addition to the possibility of complex parts with little post-processing, LPBF can produce parts with high accuracy compared to other AM techniques [15].

\subsection*{2.4. Aluminium in additive manufacturing}
High strength combined with low weight is one of the properties that makes $\mathrm{Al}$ favourable in industries such as automotive and aerospace $[1,16]$. Combined with the possibility of complex geometries and topology optimization, even lighter parts can be produced that still have the required strength, making the technology even more attractive [4, 7, 17]. However, at this point, only a few $\mathrm{Al}$ alloys can reliably be processed by AM [18]. Mostly Al-Si-Mg alloys and other alloys with Silicon $(\mathrm{Si})$ as the main alloying element is used in AM because they are relatively easy to melt with a laser beam due to their near-eutectic composition [11, 12]. This composition gives a short solidification range, compared with some high strength $\mathrm{Al}$ alloys $[12,19,20]$. Because $\mathrm{Si}$ is a nonmetallic element, the thermal expansion coefficient is much lower than for metals, and the addition of $\mathrm{Si}$ can reduce this effect and consequently prevent cracking [15].

One of the factors that make it hard to additively manufacture $\mathrm{Al}$ alloys is the high reflectivity of $\mathrm{Al}$ and its low laser absorption $[1,21,22]$. The poor flowability of the $\mathrm{Al}$ alloy powder also makes it hard to process with LPBF technology [22]. Another challenge with $\mathrm{Al}$ in $\mathrm{AM}$ is that these alloys are prone to hot cracking. The main factor for hot cracking is the chemical composition of the $\mathrm{Al}$ alloy [23].

In general, $\mathrm{Al}$ alloy parts produced by LPBF show a higher hardness than their cast counterparts. It is believed that this is due to the rapid cooling rate found in the LPBF process, which results in a microstructure refinement [24]. The ductility of the $\mathrm{Al}$ alloys produced by LPBF however, seems to decrease [25].

\subsection*{2.5. High strength aluminium alloys}
High strength Al alloys in the $2 \mathrm{xxx}$ and $7 \mathrm{xxx}$ series are wrought alloys intended for cold-forming processes. Consequently, they easily form defects when exposed to heat from e.g. a laser beam in the LPBF process [15].

Some of the main issues are that these alloys are prone to solidification cracking, liquid cracking, and hot cracking. Moreover, some of the alloying elements in these alloys, such as $\mathrm{Zn}, \mathrm{Mg}$, and $\mathrm{Li}$, easily evaporate in the process and are therefore not very processable by LPBF $[22-24,26]$. Evaporation of these alloying elements can reduce metallurgical integrity [23].

\subsection*{2.5.1. $2 x x x$ series alloys}
The $\mathrm{Al}$ alloys in the $2 \mathrm{xxx}$ series have copper $(\mathrm{Cu})$ as the main alloying element besides Al. Al-Cu alloys in this series are not suitable for welding, hence challenging to produce by LPBF [17]. They are prone to hot cracking, which is connected to the solidification interval [17]. However, these Al-Cu alloys are ductile and therefore reduces the stress peaks which results in predicted plastic deformation rather than failure [27].

\subsection*{2.5.2. $7 x x x$ series alloys}
$\mathrm{Al}$ alloys in the $7 \mathrm{xxx}$ series have Zink $(\mathrm{Zn})$ as their major alloying element [28]. While all 7xxx alloys are prone to hot cracking, $\mathrm{Al} 7075$ is found to be especially susceptible when used in LPBF $[15,29]$. The 7xxx alloys are also not weldable because they are prone to liquidation cracking. This occurs due to a thin liquid film at the boundaries of the grains, which cannot follow the solidification shrinkage [26].

\section*{3. Methodology}
The literature was collected through searches in online search engines and databases such as Google Scholar and Web of Science. A total of 27 papers were collected in this study, 16 regarding $2 \mathrm{xxx}$ series alloys and 11 regarding $7 \mathrm{xxx}$ series alloys. The investigated approaches can be divided into three different categories as shown in Figure 1, namely additives, process parameters, and heat treatment. The category additives contain approaches where new alloying elements are added, the amount of an alloying element is increased, or where an alloy has been mixed with another alloy. The category process parameters contain approaches where process parameters have

\begin{center}
\includegraphics[max width=\textwidth]{2024_04_13_392391e00aace0077bd8g-2}
\end{center}

Fig. 1. Approaches investigated in this study\\
been adjusted. The category heat treatment contains different heat treatments on LPBF produced parts. Tx includes T1, T2, ..., T10 treatment [30]. Only those who explicitly said they did a Tx treatment are placed in the Tx category, while the rest is placed in the category others. Al alloys specially developed for LPBF are not included in this study, but the interested reader is referred to Aversa, et al. [22] which provides an extensive review on this topic.

It is known that there is a significant number of papers on the topic that is published in other languages, especially in German. Due to the author's insufficient skillset in this language, they are not included in this study.

\section*{4. Results}
The publishing year for all the collected literature in this study is presented in Figure 2. The first year of publication in this topic was in 2014, and except for 2015, there have been publications every subsequent year. Please note that the number of publications in 2021 only includes publications in January, due to the work being finalised in January of 2021.

Table 1 gives an overview of the number of studies that investigated the different approaches. In general, adjusting process parameters are the most investigated approach. However, these are often adjusted in combination with other approaches. For instance, the effect of adding additives to the $\mathrm{Al}$ alloy in combination with changing the scan speed has been investigated by many. Of all the process parameters, layer thickness was the least investigated parameter and was investigated in only two studies.

Seven different additives were investigated in the collected literature, which can be seen in Table 2 and Table 3 for $2 \mathrm{xxx}$ series alloys and 7xxx series alloys respectively. Additionally, Aversa, et al. [26] made a new alloy mix by mixing $50 \%$ $\mathrm{Al} 7075$ with $50 \% \mathrm{AlSi10Mg}$.

While scan speed is a common parameter, an alternative measure was used by Ahuja, et al. [12] and Stopyra, et al. [31] who looked at the point to point distance and exposure time. The scan speed, however, is by far the most investigated parameter, which was investigated in 20 different studies, but it was usually investigated together with other parameters such as laser power or hatch spacing.

One article mentioned that they did annealing [32], eight performed Tx treatment, while six performed other types of heat treatment that do not fulfil the requirements of any of the Tx treatments.

\begin{center}
\includegraphics[max width=\textwidth]{2024_04_13_392391e00aace0077bd8g-3}
\end{center}

Fig. 2 Year of publications regarding high strength $\mathrm{Al}$ alloys

\section*{4.1. $2 x x x$ series}
Table 2 shows the different approaches investigated regarding $2 \mathrm{xxx}$ series alloys in the collected literature. Among these alloys, the scan speed is the most investigated approach, followed by laser power and hatch spacing. As can be seen in Table 2, these parameters are often investigated together, where more than one of the parameters are adjusted in the same experiment. It is observed that investigations regarding additives generally adjust fewer process parameters.

Hatch spacing was investigated in six studies, but always in combination with other parameters. However, they were never investigated in combination with additives. Post heat treatment was performed in eight investigations, whereas five of them explicitly said they did a T6 or T4 treatment.

The least investigated approach is layer thickness, which was investigated in two studies. In the category Other we find the study [12], that investigated exposure time and point to point distance instead of scan speed.

\section*{4.2. $7 x x x$ series}
Different approaches investigated for $\mathrm{Al}$ alloys in the $7 \mathrm{xxx}$ series is presented in Table 3. The most investigated parameter was scan speed with a total of nine investigations, while the second most investigated approach was additives with seven investigations. The most investigated additive for the $7 \mathrm{xxx}$ series alloys was Si. Instead of adding one or two alloying elements to the alloy powder, Zhou, et al. [21] created a mix of $50 \% 7075$ and $50 \% \mathrm{AlSi} 10 \mathrm{Mg}$, which increased the total $\mathrm{Si}$ $\mathrm{wt} \%$ of the alloy composition in the final part.

In all but one of the studies that investigated scan speed, the laser power was also investigated. The laser power on the other hand was never adjusted without the scan speed also being adjusted. In the study by Qi, et al. [45] however, different defocusing distances was investigated. Post heat treatment was performed in seven studies, where three said explicitly that they performed a T6 treatment. For alloys in the 7xxx series, none investigated the layer thickness.

It can also be noted that in the studies on alloys in the $7 \mathrm{xxx}$ series, all but one of the investigated alloys was Al7075 with slightly different notations depending on the standard that was

Table 1. Number of investigations of the respective approaches

\begin{center}
\begin{tabular}{llll}
\hline
Approaches &  & References & Total \\
\hline
Additives &  & $[2,15,18,21,33-38]$ & 10 \\
\hline
\begin{tabular}{l}
Process \\
parameters \\
\end{tabular} & \begin{tabular}{l}
Layer- \\
thickness \\
\end{tabular} & $[39,40]$ & 2 \\
 & \begin{tabular}{l}
Hatch \\
spacing \\
\end{tabular} & $[12,17,26,31,35,39,41-44]$ & 10 \\
 & Scan speed & $[2,15,17,26,29,31,33-37,39-$ & 20 \\
 &  & $47]$ & 15 \\
\cline { 2 - 4 }
 & Laser power & $[2,12,15,17,26,29,31,35-37$, & 15 \\
\hline
\multirow{2}{*}{}\begin{tabular}{ll}
Heat \\
treatment \\
\end{tabular} & Annealing & $[32]$ & 1 \\
\cline { 2 - 4 }
 & Tx treatment & $[18,21,27,38-40,43,46]$ & 6 \\
\cline { 2 - 4 }
 & Others & $[2,26,31,35,36,48]$ & 1 \\
\hline
Others &  & $[12]$ & 6 \\
\hline
\end{tabular}
\end{center}

Table 2. Approaches 2xxx series alloys

\begin{center}
\begin{tabular}{|c|c|c|c|c|c|c|c|c|}
\hline
 & Alloy & Additives & \begin{tabular}{l}
Laser \\
power \\
\end{tabular} & \begin{tabular}{c}
Scan \\
speed \\
\end{tabular} & \begin{tabular}{l}
Hatch \\
spacing \\
\end{tabular} & \begin{tabular}{c}
Layer \\
thickness \\
\end{tabular} & \begin{tabular}{c}
Heat \\
treatment \\
\end{tabular} & Other \\
\hline
Nie, et al. [33] & $\mathrm{Al}-4.24 \mathrm{Cu}-1.97 \mathrm{Mg}-0.56 \mathrm{Mn}$ & $\mathrm{x}(\mathrm{Zr})$ &  & $\mathrm{x}$ &  &  &  &  \\
\hline
Ahuja, et al. [12] & AW-2219 and AW-2618 &  & $\mathrm{x}$ &  & $\mathrm{x}$ &  &  & $x^{*}$ \\
\hline
Wang, et al. [46] & $\mathrm{Al}-3.5 \mathrm{Cu}-1.5 \mathrm{Mg}-1 \mathrm{Si}$ &  & $\mathrm{x}$ & $\mathrm{x}$ &  &  & $\mathrm{x}(\mathrm{T} 6)$ &  \\
\hline
Casati, et al. [32] & 2618 &  &  &  &  &  & $\mathrm{x}$ (annealing) &  \\
\hline
Zhang, et al. [41] & $\mathrm{Al}-\mathrm{Cu}-\mathrm{Mg}$ (close to AA2024) &  &  & $\mathrm{x}$ & $\mathrm{x}$ &  &  &  \\
\hline
Zhang, et al. [34] & \begin{tabular}{l}
$\mathrm{Al}-\mathrm{Cu}-\mathrm{Mg}(4.24 \mathrm{Cu}, 1.97 \mathrm{Mg}$ \\
$0.56 \mathrm{Mn})$ \\
\end{tabular} & $\mathrm{x}(\mathrm{Zr})$ &  & $\mathrm{x}$ &  &  &  &  \\
\hline
Qi, et al. [42] & 2195 &  &  & $\mathrm{x}$ & $\mathrm{x}$ &  &  &  \\
\hline
Rasch, et al. [39] & AW-2024 &  & $\mathrm{x}$ & $\mathrm{x}$ & $\mathrm{x}$ & $\mathrm{x}$ & $\mathrm{x}(\mathrm{T} 4)$ &  \\
\hline
Karg, et al. [17] & AW-2022 and 2024 &  & $\mathrm{x}$ & $\mathrm{x}$ & $\mathrm{x}$ &  &  &  \\
\hline
Zhang, et al. [48] & $\mathrm{Al}-\mathrm{Cu}-\mathrm{Mg}$ &  &  &  &  &  & $\mathrm{x}$ &  \\
\hline
Karg, et al. [27] & EN AW-2219 (AlCu6Mn) &  &  &  &  &  & $\mathrm{x}(\mathrm{T} 6)$ &  \\
\hline
\begin{tabular}{l}
Raffeis, et al. \\
$[36]$ \\
\end{tabular} & AA2099 & \begin{tabular}{l}
x (Ti-alumide and \\
Al) \\
\end{tabular} & $\mathrm{x}$ & $\mathrm{x}$ &  &  & $\mathrm{x}$ &  \\
\hline
Qi, et al. [40] & 2195 &  &  & $\mathrm{x}$ &  & $\mathrm{x}$ & $\mathrm{x}(\mathrm{T} 6)$ &  \\
\hline
Tan, et al. [38] & 2024 & $\mathrm{x}(\mathrm{Ti})$ &  &  &  &  & $\mathrm{x}(\mathrm{T} 6)$ &  \\
\hline
Tan, et al. [47] & 2024 &  & $\mathrm{x}$ & $\mathrm{x}$ &  &  &  &  \\
\hline
Pekok, et al. [44] & AA2024 &  & $\mathrm{x}$ & $\mathrm{x}$ & $\mathrm{x}$ &  &  &  \\
\hline
\end{tabular}
\end{center}

*Ahuja, et al. [12] looked at time exposure and point to point distance instead of scan speed

followed. The only alloy different from A17075 was an AlZnMgScZr alloy, investigated by Zhou, et al. [21].

\subsection*{5.1. Additives}
\section*{5. Discussion}
The collected literature indicates that there has been less research on 7xxx series alloys than 2xxx series alloys. For both the series, the scan speed is the most investigated process parameter. This may be because the scan speed is relatively easy to adjust, and a change in scan speed can greatly influence whether the alloy powder is fully melted or not. A high laser power is required to enable full melting of the Al powder, and with limitations in the machine parameters, there is a limit to how high the laser power can be adjusted. By reducing the scan speed, the volumetric energy density will increase which in turn can ensure full melting of the powder. However, by increasing the volumetric energy density, some elements with a low melting point can evaporate.

Of the collected literature, among the 7xxx series alloys investigated, all but one was Al7075. This indicates that this alloy is of great interest within AM. This can be due to the highly regarded properties of this specific alloy.

The amount of an alloying element added into an alloy can change the composition of the alloy to the extent where it can no longer be categorised as the original alloy. When additional elements are added, the new composition does not necessarily meet the specific alloy requirements of the initial composition. Therefore, the new alloy blend may not qualify as an existing alloy but is rather considered a new alloy. This is closely related to the issue of alloying elements evaporating during the LPBF process. If a significant amount of an alloying element evaporates, the alloy composition in the final part might not qualify as the initial alloy. In the study by Kaufmann, et al. [29] the chemical composition of the LPBF produced parts differed from the composition of the $\mathrm{Al}$ alloy powder. This results in the amount of both $\mathrm{Zn}$ and $\mathrm{Si}$ no longer be inside the $\mathrm{min} / \mathrm{max}$ requirement for AW-7075, according to NS-EN 573-3:2019. Kaufmann, et al. [29] concludes that it must be either an evaluation of trade-off or a different alloy composition is needed to compensate for the loss of alloying elements.

Instead of adding one or two alloying elements to the alloy powder, Zhou, et al. [21] created a mix of 50\% Al7075 and

Table 3. Approaches 7xxx series alloys

\begin{center}
\begin{tabular}{|c|c|c|c|c|c|c|c|c|}
\hline
 & Alloy & Additives & \begin{tabular}{c}
Laser \\
power \\
\end{tabular} & \begin{tabular}{c}
Scan \\
speed \\
\end{tabular} & \begin{tabular}{l}
Hatch \\
spacing \\
\end{tabular} & \begin{tabular}{c}
Layer \\
thickness \\
\end{tabular} & \begin{tabular}{c}
Heat \\
treatment \\
\end{tabular} & Other \\
\hline
Montero-Sistiaga, et al. [2] & $\mathrm{A} 17075$ & $\mathrm{x}(\mathrm{Si})$ & $\mathrm{x}$ & $\mathrm{x}$ &  &  & $\mathrm{x}$ &  \\
\hline
Martin, et al. [18] & $\mathrm{A} 17075$ & $\mathrm{x}(\mathrm{Zr})$ &  &  &  &  & $\mathrm{x}(\mathrm{T} 6)$ &  \\
\hline
Qi, et al. [45] & $\mathrm{A} 17075$ &  &  & $\mathrm{x}$ &  &  &  & $x^{*}$ \\
\hline
Aversa, et al. [26] & 7075 & $\mathrm{x}(\mathrm{AlSi} 10 \mathrm{Mg}) * *$ & $\mathrm{x}$ & $\mathrm{x}$ & $\mathrm{x}$ &  & $\mathrm{x}$ &  \\
\hline
Kaufmann, et al. [29] & AW 7075 &  & $\mathrm{x}$ & $\mathrm{x}$ &  &  &  &  \\
\hline
Zhou, et al. [21] & $\mathrm{AlZnMgScZr}$ & $\mathrm{x}(\mathrm{Sc}+\mathrm{Zr})$ &  &  &  &  & $\mathrm{x}(\mathrm{T} 6)$ &  \\
\hline
Otani and Sasaki [15] & 7075 & $x(\mathrm{Si})$ & $\mathrm{x}$ & $\mathrm{x}$ &  &  &  &  \\
\hline
Wu, et al. [37] & $\mathrm{A} 17075$ & $\mathrm{x}$ (TiN nanoparticles) & $\mathrm{x}$ & $\mathrm{x}$ &  &  &  &  \\
\hline
Stopyra, et al. [31] & AA 7075 &  & $\mathrm{x}$ & $\mathrm{x}^{* * *}$ & $\mathrm{x}$ &  & $\mathrm{x}$ &  \\
\hline
Li, et al. [35] & AL7075 & $\mathrm{x}(\mathrm{Si})$ & $\mathrm{x}$ & $\mathrm{x}$ & $\mathrm{x}$ &  & $\mathrm{x}$ &  \\
\hline
O E, et al. [43] & $\mathrm{Al7075}$ &  & $\mathrm{x}$ & $\mathrm{x}$ & $\mathrm{x}$ &  & $\mathrm{X}(\mathrm{T} 6)$ &  \\
\hline
\end{tabular}
\end{center}

time exposure and point to point distance\\
$50 \% \mathrm{AlSi} 10 \mathrm{Mg}$, which will drastically change the alloy composition in the final part. It can, however, be debated whether this should be regarded as an additive or rather a new alloy or alloy blend. Only half of the alloy is now A17075 which belongs to the wrought alloy $7 \mathrm{xxx}$ series, and the other half is $\mathrm{AlSi10Mg}$ which is a cast alloy. Therefore, it can also be questioned if the new alloy blend qualifies as a high strength alloy and if so, if it would belong to the $7 \mathrm{xxx}$ series.

\subsection*{5.2. Layer thickness}
It seems from the collected literature that layer thickness is of least interest, only investigated in two studies, both investigating alloys in the $2 \mathrm{xxx}$ series. However, the layer thickness can influence the laser power and scan speed required to fully melt the powder. As stated by Ahuja, et al. [12] melting depth of three times the layer thickness, would be the most suitable, and therefore it can be argued that a thinner layer thickness can decrease the required laser power and/or increase the scan speed. However, how thin the powder layer can be, is limited by the particle size of the powder.

An issue with thinner layers is that the production time might increase. However, higher scan speed can decrease the build time, and it can be assumed that with thinner powder layers, the scan speed can be increased, and still result in fully melting the powder. Also, insufficient melting of powder is an issue that can lead to defects such as pores [36], and with thinner powder layers, it may be easier to fully melt the powder.

\subsection*{5.3. General notes}
Figure 2 shows that there is a growing interest in LPBF of high strength alloys, despite the difficulties associated with these alloys. However, there might be more research published that has not been included in this study. Therefore, this overview can be used as an indicator of the current trend, but the numbers cannot be used as an absolute. Also, this study was finalized in January 2021, so the numbers for 2021 are only up to this point and more literature on this topic can be published within this year. However, it is interesting to note that the number of publications in January 2021 is already equal to the number published in 2018. This indicates that there may be an exponential growth in interest in this topic.

All the reviewed studies report promising results to different extents. Some were able to produce fully dense parts with close to no defects, while others were able to increase the density and reduce the number of defects. However, different approaches can lead to different challenges.

It is known that more literature on this topic exists in other languages, predominantly German, in which the author lacks a sufficient skillset. Therefore, these are not included in the study, which means that this study does not give a complete picture of what approaches are the most investigated. However, it is intended to give an indicator of the current trends in research of high strength $\mathrm{Al}$ alloys in the 2xxx and 7xxx series.

This study is also limited to a subset of approaches selected for this study. This means that there can be other promising approaches that enable successful processing of high strength $\mathrm{Al}$ alloys that are not considered in this study.

\section*{6. Conclusion}
Relevant literature on high strength $\mathrm{Al}$ alloys in the $2 \mathrm{xxx}$ and 7xxx series in LPBF has been collected to give an overview of the current state of research on this topic. The different approaches considered in this paper was adding additives to the alloy powder, adjusting the process parameters, (i) laser power, (ii) scan speed, (iii) hatch distance and (iv) layer thickness, as well as heat treatment. Some main conclusions can be drawn from this:

\begin{enumerate}
  \item For Al alloys in the $2 \mathrm{xxx}$ series, scan speed was the most investigated approach. However, it was always investigated in combination with other approaches.

  \item For Al alloys in the 7xxx series, scan speed was the most investigated approach, and laser power being the second most investigated approach.

  \item Layer thickness was investigated in only two studies and was the least investigated approach in the collected literature.

\end{enumerate}

There is a significantly larger amount of research efforts towards $\mathrm{Al}$ alloys in the $2 \mathrm{xxx}$ series than in the $7 \mathrm{xxx}$ series. Also, Al7075 was the only alloy investigated in the $7 \mathrm{xxx}$ series, except for one study. Therefore, more research efforts on $7 \mathrm{xxx}$ series alloys, especially other alloys than A17575 could be valuable for this field of study.

Insufficient melting of the $\mathrm{Al}$ alloy powder is an issue in LPBF, and layer thickness may influence this. As this was the least investigated approach, this parameter should be considered in future research efforts.

\section*{Acknowledgements}
The KPN VALUE project at NTNU, co-funded by industry and the Norwegian Research Council.

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