Additive manufacturing with the lightweight material aluminium alloy EN AW-7075

As a widely used additive manufacturing technique, the laser metal deposition process (LMD) also known as direct energy deposition (DED) is often used to manufacture large-scale parts. Advantages of the LMD process are the high build-up rate as well as its nearly limitless build-up volume. To manufacture large-scale parts in lightweight design with high strength aluminium alloy EN AW-7075, the LMD process has a disadvantage that must be considered. During the process, the aluminium alloy is melted and has therefore a high solubility for hydrogen. As soon as the melt pool solidifies again, the hydrogen cannot escape the melt and hydrogen pores are formed which weakens the mechanical properties of the manufactured part. To counter this disadvantage, the hydrogen must be successfully kept away from the process zone. Therefore, the covering of the process zone with shielding gas can be improved by an additional shielding gas shroud. Furthermore, the process parameters energy input per unit length as well as the horizontal overlapping between two single tracks can be varied to minimize the pore volume. Best results can be achieved in single tracks with an elevated energy input per unit length from 3000 to 6000 J/cm. To manufacture layers, a minimal horizontal overlapping will lead to lowest pore volume, although this results in a very wavy surface, as a compromise of low pore volume and a nearly even surface a horizontal overlapping of 30 to 37% leads to a pore volume of 0.95% ± 0.50%.


Introduction
Additive manufacturing technologies have become more and more relevant for industrial applications during the last years [1]. For metal additive manufacturing, various energy sources like laser beam, electron beam or arc are used with particular advantages and disadvantages [2]. Using a laser beam as energy source is beneficial due to the locally low heat input. For diverse applications, different laser beam systems can be used. Small-scale parts with complex geometries and high resolution can be additively manufactured via the powder-bed-based process laser beam melting (LBM). To reach a high resolution, a high beam quality with a small focus diameter is mandatory. For this fibre lasers with a comparatively low laser power up to 1 kW are used. If large-scale parts are desired, an openspace process like laser metal deposition (LMD) also known as direct energy deposition (DED) is advantageous. The process provides a much larger build-up volume only limited by the travels of the axis system. By using a defocused highpower laser with a larger spot diameter with laser power up to 10 kW high build-up rates up to 18 kg/h depending on the used material are possible [3]. In contrast to LBM, the LMD process enables not only the use of powder, but the usage of wire material as well. With wire, it is possible to reach nearly 100% of the material. The advantage of using powder instead of wire for additive processes is the possibility to mix several powders in situ for example for a graded material composition in the component [4]. As deposition, material metal powder or wire is often used. The powder-based techniques can be divided into powder-bed-based processes like laser beam melting (LBM) and open-space processes like laser metal deposition (LMD) also known as direct energy deposition (DED) [2]. Whereas LBM is commonly used to manufacture small parts with complex geometries in high resolution, LMD is more suitable to produce large-scaled parts. This is due to the high build-up rate during the LMD process and the nearly limitless build-up volume. This enables to manufacture large parts in comparatively short time and nearly without any restrictions by no need of use of a process chamber, as with LBM. A large variety of metal powders is suitable for additive manufacturing such as tool steel [5], aluminium alloys [6] and copper alloys [7].
Due to the development towards a more energy-efficient design through mass reduction, aluminium alloys are of importance for the additive production of innovative lightweight components. However, aluminium alloys show the disadvantage of pore formation during additive processing which must be considered. There are different approaches to overcome this disadvantage. On the one hand, postprocessing can be carried out, such as hot isostatic pressing or friction stir processing to reach fully dense parts [8], though these approaches are limited to simple geometries. On the other hand, there are several approaches to limit the formation of porosity already during the additive manufacturing process. It is known that pore formation in aluminium during laser welding is mainly affected by the surface conditions, the shielding gas and the used process parameters [9]. During the LMD process, the substrate material and the powder are melted and then undergo a rapid solidification [10]. While the aluminium alloy is in the molten state, it has a high solubility for hydrogen. Due to the rapid solidification and the concomitant large decrease in solubility for hydrogen, the gas cannot completely escape the melt pool; hydrogen pores remain in the weld bead [11]. The most important sources of hydrogen for porosity in aluminium alloy weld beads are the filler material, the shielding gas and the substrate material [11]. As even under unfavourable storage conditions, the hydrogen content in aluminium alloys increases only marginally [12]; the main source of hydrogen for porosity are the substrate material with its contaminants as well as oxide layer and the ambient gas. Cleaning the substrate's surface before starting the process can reduce hydrogen-caused porosity during the process [13]. Additionally, a reliable covering of the process zone with shielding gas can contribute to the effective avoidance of hydrogen pores. This is supported by aluminium alloy specimens manufactured by LBM. The use of a process chamber for LBM enables a defined and constant shielding gas coverage. With a sufficient energy density > 99.5%, dense aluminium parts can be manufactured [14]. Process parameters like laser power and welding speed during additive manufacturing are also known to be important influencing factors to minimise porosity [15]. Whereas the influence of the horizontal overlapping between single tracks during manufacturing layers out of aluminium alloy powder by LMD on porosity is not studied yet.
In this study, the influence of improved shielding gas coverage as well as the influence of the process parameters horizontal overlapping and energy input per unit length, as a quotient of laser power and welding speed, will be analysed. Therefore, an improved shielding gas coverage was developed. The analysis was carried out on single tracks and layers consisting of several single tracks for aluminium alloy EN AW-7075.

Equipment
For the LMD process, a lamp pumped Nd: YAG rod laser in cw mode has been used. The Trumpf HL 4006 D has a wavelength of 1064 nm and a maximum laser power of 4 kW. The beam was guided via an optical fibre with core diameter of 600 μm to the optical unit Trumpf BEO D 70 with a collimation lens and a focus lens with a focal length of 200 mm respectively. The powder was guided from the powder feeder Sulzer Metco Twin 10C to the coaxial three-jet powder nozzle made by Ixun Lasertechnik GmbH. Argon was used as carrier gas with a flow rate between 4.1 and 5.5 l/min. The distance from the nozzle to the substrate was 12 mm. The powder focus was located on the substrate's surface and was aligned to the laser beam. As traversing unit, a 3-axis CNC made by Foehrenbach Positioniersysteme GmbH driven by the control unit PA8000 made by Power Automation GmbH has been used.

Material
As powder material gas-atomised aluminium alloy, EN AW-7075 with a particle size D10 37 μm and D90 122 μm supplied by NMD New Material Development GmbH was used. The powder was sieved with 50-μm and 125-μm mesh size. Table 1 shows the results of a chemical analysis in accordance with EN 10204 3.1. As a substrate milled aluminium alloy, EN AW-5083 with 50 mm length and width and 10 mm height was used (see Table 1 for chemical composition according the supplier's (Amco GmbH) information). The substrate's surface was cleaned with ethanol before starting the process.

Influence of shielding gas concept
The influence of two shielding gas concepts on the pore volume was studied based on 35-mm-long single tracks. For shielding gas concept 1, the centric guidance in the powder nozzle for the laser beam was used to cover the process zone with shielding gas. For shielding gas concept 2, an additional shroud was used, which also supplies shielding gas (see Fig. 1). See Table 2, column "Influence of shielding gas concept" for specific process parameters.
The pore volume was measured via X-ray computer tomography with v|tome|x m 240 "research edition" pxSD09 made by GE Sensing & Inspection Technologies GmbH with a resolution of 40 μm.

Influence of energy input per unit length and horizontal overlapping
The influence of the quotient of laser power and welding speed, the energy input per unit length, was studied based on 35-mm-long single tracks. The laser energy was varied between 2 and 4 kW in 0.5 kW steps. The welding speed was not varied to have a constant powder mass flow per unit length (see Table 2 for specific process parameters).
The influence of horizontal overlapping on pore volume was studied based on layers out of three 35-mm single tracks. A unidirectional build strategy was chosen. The horizontal overlapping 16%, 23%, 30%, 37%, 44% and 51% was chosen (see Table 2 for specific process parameters). Each parameter combination was examined in randomized triple determination.
Additional experiments were done to determine whether the porosity is mainly formed due to the condition of the substrate material or feeding powder into the process zone. For this, tests were carried with feeding powder on the one hand and without feeding powder on the other hand. The process parameters can be found in Fig. 4.
The pore volume was determined in cross sections, which were wet sanded for 5 min with SiC abrasive paper P1200 grain and polished for 5 min and 10 min with 3-μm diamond suspension and 0.04 μm SiO x suspension, respectively. The software used to determine the pore volume was Olympus Stream Enterprise Desktop.

Influence of shielding gas concept
For the conventional shielding gas concept 1 with shielding gas through the centric beam guidance, a pore volume in a single track of 2.25% was measured by computer tomography. Increasing the shielding gas flow via the additional shroud (shielding gas concept 2) led to a decreased porosity of 1.78% in a single track. Figure 2 shows the pore distribution within the single tracks as a false-colour image.

Influence of energy input per unit length and horizontal overlapping
The influence of the energy input per unit length on the pore volume is shown in Fig. 3.
The pore volume decreased with increasing energy input per unit length. The lowest pore volume of 0.09% ± 0.07% was measured for highest energy input per unit length of 6000 J/cm with a laser energy of 4 kW and a welding speed of 400 mm/min. The powder usage efficiency of 82.0% ± 3.4% was not affected by varying the energy input per unit length between and 6000 J/cm.
During the additional experiments with and without feeding powder into the process zone, similar low porosity of 0.15% ± 0.05% was measured for the specimens without feeding powder. For the additive process with feeding powder, higher porosity of 0.52% ± 0.15% was measured (see Fig. 4).
The influence of the horizontal overlapping on the pore volume is shown in Fig. 5. The pore volume increased with increasing horizontal overlapping. The lowest pore volume measured in triple determination of 0.41% ± 0.18% was measured for horizontal overlapping of 23%. However, the low horizontal overlapping of 23% led to a wavy surface (see Fig. 6), as a compromise of low pore volume and a more even surface led a horizontal overlapping of 30 to 37% with a pore volume of 0.95% ± 0.50%. The powder usage efficiency of 82.8% ± 1.0% was not affected by varying the horizontal overlapping between 16 and 51%.
In the shown cross section of the layer with a horizontal overlapping of 30% in Fig. 6 pore clusters along the weld bead interfaces between two single tracks are visible. These clusters are also more or less present in the other layers with varying horizontal overlapping. Figure 7 shows these clusters very Fig. 2 False-colour image of pore distribution within single tracks. Concept 1: centric shielding gas supply. Concept 2: centric shielding gas supply and additional shielding gas shroud clearly between the first and the second weld bead in a cross section of a layer with 23% horizontal overlapping.

Discussion
An improved shielding gas coverage of the process zone with an additional shroud led to a decrease in porosity. This is due to an improved displacement of the ambient air from the process zone through the additional shielding gas flow. Since hydrogen is known to be a major source of pores in aluminium alloy weld beads, a more effective displacement of humid, hydrogen-containing ambient air will lower the pore volume [16]. Though, it is unclear whether an increased shielding gas flow only through the centric guidance would lead to the same decrease in pore volume or whether the large-area distribution of the additional shielding gas through the shroud is necessary. Despite the fact, that the values for the comparison of these two variations were done as single experiments and not triple determination, the results indicate that additional shielding gas supplied via the shroud will lead to lower porosity. A higher energy input per unit length leads to a larger and more overheated melt pool with a smaller spatial temperature gradient. As a result, the cooling rate and therefore the solidification rate is lower and gaseous elements such as hydrogen have more time for degassing, pore volume is decreasing. Additionally, the higher thermocapillary convection in the melt pool due to the higher energy input per unit length contributes the decrease of the cooling rate [17].
The lowest pore volume in specimens manufactured via LMD is reached in single tracks. Manufacturing layers out of several single tracks with a horizontal overlapping led to an increase in porosity. This is due to the oxide layer, which formed on the rough surface of the preceding weld bead with adhering powder particles. Besides others, the oxide layer on aluminium alloys in its hydrate form is considered an important source for pores in weld beads Fig. 3 Influence of energy input per unit length on pore volume (black circles) as well as on powder usage efficiency (black crosses) Fig. 4 Pore volume in single tracks with and without feeding powder into the process zone. Please note that a different laser source with adapted process parameters was used [18]. Since higher temperatures of aluminium alloy result in a thicker oxide layer [19], the surface of the preceding weld bead is a source for high porosity in the subsequent weld bead, which is welded with a high horizontal overlapping. This assumption is supported by clusters of pores along the weld bead interfaces between two single tracks (see Fig. 7).
The formation of porosity is affected by feeding the powder into the process zone. This is shown by the results of the additional studies in which the porosity of remelted specimens (without feeding powder) was determined. The significant lower porosity can possibly be explained by two different considerations. On the one hand, the shielding gas coverage could be disturbed by the powder flow. This enables humid, hydrogen-containing ambient air to affect the process zone and results in higher porosity. On the other hand, the results for the layers out of several single tracks showed that the interfaces of the preceding weld beads with their oxide layers significantly influenced the porosity. The powder particles are also surrounded by an oxide layer [20], which is why additional hydrogen is probably already introduced into the process zone with the powder.

Conclusions
The results can be concluded as follows: & The pore volume in additively manufactured EN AW-7075 specimens can be successfully reduced by adapting the shielding gas coverage of the process zone. Therefore, an additional shroud was developed. & Adapting the process parameters during LMD of EN AW-7075 towards higher energy input per unit length up to 6000 J/min, a significant lower porosity can be measured.  & It is preferable to set a low horizontal overlapping when manufacturing layers to reduce the impact of the preceding weld bead on porosity.