In the last few years, there has been increasing interest in the processing of aluminum alloys using additive manufacturing (AM) processes. Thanks to their properties, aluminium alloys are extensively used in aerospace, rail, and automotive industries. Laser metal deposition (LMD), among the AM processes, can manufacture complex features as well as reinforcement structures on pre-existing complex parts. The work performed within the SAMOA project aims to expand the knowledge of the material properties of aluminum alloys when the LMD process is performed under atmospheric conditions. Both common alloys (AlSi10Mg, AlSi1Mg) and AM-specific alloys (AM205 and AlSi1Mg + 1 wt. %Zr) were analyzed and compared. Results show significantly lower amounts of internal defects and higher mechanical properties in AM-specific alloys. This database of mechanical properties will be used to design, simulate, and fabricate reinforcement structures on car frames to enhance their crash resistance and increase vehicle security. Moreover, to reduce material waste, the SAMOA project focuses also on the effects of powder recycling by analyzing both chemical and physical changes in the powder. The higher concentration of oxygen and hydrogen was separately analyzed by artificially increasing their concentration by heat treating AlSi10Mg powder. Results showed similar processability with a reduction in UTS of −31.4% and an increased elongation at fracture of +112.5%. Recycled powder, on the other hand, could not be easily collected, sieved, and reused since the identified physical and chemical changes of the powder lower its processability.

Conventional approaches for manufacturing aluminum components require a primary shaping process of the alloyed materials, followed by the secondary machining operations which often result in high material wastes. This process chain accounts for the high energy and resources consumption, which are not economically efficient and sustainable for many industrial sectors. Additive manufacturing (AM) technologies, on the other hand, can generate near-net-shape complex geometries starting from a narrower selection of feedstock materials, generally in the form of powder or wires, with high material utilization.1 Metal AM processes show evidence of reduced environmental impact for manufacturing sustainability,2 mainly thanks to a simplified supply chain and higher material utilization rates when compared to subtractive technologies. Moreover, while the waste material generated by subtractive technologies needs to be melted again to be recycled, powder-based metal AM processes showed promising results in recycling unused powder without the need for additional energy consumption.

In the laser metal deposition (LMD) process, schematically depicted in Fig. 1, both the feedstock material and the energy source are delivered to the deposition point through a nozzle system and an optical path, respectively. This process is repeated following a predefined pattern to create a solid three-dimensional component. The final part of the delivery system is usually called deposition head and its motion is commonly controlled by a CNC machine or by a six-axis robot arm.4 These characteristics give the LMD process a high degree of freedom in terms of possible applications when compared to powder bed AM processes at the cost of lower geometrical freedom and complexity. Possible applications for the LMD process are near net shape generation of components,5,6 feature addition on pre-existing parts,7 surface cladding,8 and component repair.9,10

FIG. 1.

Principle of powder-based laser metal deposition (Ref. 3).

FIG. 1.

Principle of powder-based laser metal deposition (Ref. 3).

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AM technologies are gaining significant relevance due to the increase of the industrial implementation level in sectors such as the aerospace and medical ones, which require small batches of highly customized components. The automotive industry, on the other hand, is a highly cost-driven sector based on series production that requires proven process repeatability and robustness. A list of administrative barriers for the spread of AM technologies in the automotive sector was identified by Dwivedi et al.,11 with the most significant ones being: the lack of information on the suitability and availability of AM technologies and material properties, as well as a required change in the traditional attitude of designers. Nevertheless, different publications already proved the applicability of LMD technologies for the automotive sector7,12 and for topology optimized structures on rail vehicles.13 The use of topology optimization and lightweight materials can lead to a significant reduction in parts weight. In the automotive industry, a weight reduction corresponds to a significant improvement in the environmental impact of the product use phase,14 resulting in an improved life cycle assessment (LCA) also when higher energy inputs are required in the product manufacture step. The processability of Al alloys by means of laser-based AM technologies presents great challenges, mainly due to high reflectivity, poor fluidity of aluminum powders, and high thermal conductivity.15 As a result, 3D printed parts often show the presence of many defects such as porosity, oxidation within the material, lack of fusion and inclusions which induce imperfections and yield lower mechanical properties.16 High intensity laser beam and repeated heating and cooling cycles induce strong residual stresses in as-built LMD parts with detrimental effects on fatigue properties and the risk of crack formation.17 The presence of oxygen and hydrogen in the feedstock and in the atmosphere surrounding the melt pool during the process is a major cause of defect formation.18 The high solubility of hydrogen in liquid aluminum drops by a factor of 70 during solidification19–21 forcing hydrogen to regain its gaseous form. The fast solidification rate does not give time for hydrogen gas bubbles to leave the melt pool resulting in large spherical pores.

To guarantee a reliable and reproducible AM process, the used powders must have distinct physical (i.e., morphology, particle size distribution, flowability) and chemical properties (i.e., humidity and tribological properties).22 In laser powder bed fusion (LPBF), up to 90% of the metal powder may not contribute to the build part based on the part geometry.23,24 For this reason, powder recycling is already a common procedure in LPBF where the powder is collected, dried, sieved, and mixed with a new powder before being reused.22 Many studies proved the potential of powder reuse for commonly used alloys in powder bed processes.22,24 The LMD process, on the other hand, has higher powder usage, up to 60%–70%, depending on nozzle design, powder delivery system, and process parameters.23 However, in LMD, also the unused powder interacts with the laser beam with possible morphology and microstructural changes,12 which may affect its reusability having detrimental effects on the repeatability and reliability of the process.

This work aims to expand the knowledge of the material properties of aluminum alloys when the LMD process is performed under atmospheric conditions, by comparing results obtained both with conventional alloys (AlSi10Mg and AlSi1Mg both with PSD 63–105 μm) and with AM-specific alloys (AM205 with PSD 63–105 μm and AlSi1Mg + 1 wt. %Zr25 with PSD 45–105 μm). AM205 is the given name for the powder produced from the high-strength alumnium alloy family A20X® patented by Aeromet. Moreover, the feasibility of aluminum powder recycling after processing under atmospheric conditions is investigated for AlSi10Mg. Since oxygen and hydrogen pick-up in the powder feedstock is a known detrimental effect, an aging process was also performed on part of the available powder to artificially induce an increase in oxygen and hydrogen content and analyzed their direct impact on the LMD process. The LMD process was performed in a DMG Mori LASERTEC 65 DED Hybrid with a COAX-14 powder nozzle developed by Fraunhofer IWS and a diode laser source Laserline LDF 4000-30 with a wavelength range of 900–1080 nm, and the powders were conveyed to the deposition point with the use of a GTV PF/2 powder feeder. To characterize the used powders, both physical and chemical analyses were performed. Physical investigations include particle size distribution (PSD) and morphology analysis which were performed by a direct image analysis method using CAMSIZER® X2 and corresponding scanning electron microscopy (SEM). The powder flowability was assessed using a FT4-Powder Rheometer. Chemical analysis was performed by an inert gas fusion method (also called: hot gas extraction) to investigate oxygen, hydrogen, and nitrogen content. Porosity and internal defects of the LMD material were analyzed by a CT scan in a YXLON FF35CT, while tensile testing was performed machining DIN 50125—B 6 × 30 tensile specimens.

At first, different Al alloys were used to compare their processability by LMD under atmospheric conditions and to evaluate the resulting mechanical properties. A previously developed set of optimized process parameters (listed in Table I) for AlSi10Mg with PSD 63–105 μm was taken as reference conditions. For the other alloys, the deposition speed was modified to keep equal geometric characteristics of the deposited tracks with a width of 2.5 mm and a thickness of 1.3 mm. This allowed manufacturing equal sample volumes from each alloy with dimensions of 60 × 13 × 13 mm for the extraction of round tensile specimens.

TABLE I.

Process parameters used for each Al alloy.

AlSi10MgAlSi1MgAM205AlSi1MgZr
Laser power (W) 2060 
Speed infill (mm/min) 600 580 700 580 
Speed contour (mm/min) 450 420 490 420 
Tracks overlap (%) 50 
Powder feed rate (g/min) 3.8 
Carrier gas (l/min) 12 
Shielding gas (l/min) 15 
AlSi10MgAlSi1MgAM205AlSi1MgZr
Laser power (W) 2060 
Speed infill (mm/min) 600 580 700 580 
Speed contour (mm/min) 450 420 490 420 
Tracks overlap (%) 50 
Powder feed rate (g/min) 3.8 
Carrier gas (l/min) 12 
Shielding gas (l/min) 15 

The strategy used for the deposition (depicted in Fig. 2) consists of a contour strategy followed by a bidirectional infill rotated by 45° with respect to the sides of the volume and an additional rotation of 90° between each layer. A copper plate with an internal water cooling channel was placed under the AW-6082 substrate with a thickness of 10 mm to avoid overheating. For each available alloy, five equal volumes were fabricated (shown in Fig. 3) to machine one tensile specimen out of each volume. Before starting the deposition of a new volume, a waiting time of at least 5 min was included to allow a complete cooling down of the substrate and maintain constant deposition conditions.

FIG. 2.

Deposition strategy with contour and 45° fill.

FIG. 2.

Deposition strategy with contour and 45° fill.

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FIG. 3.

Top view of AM205, AlSi1Mg and AlSi10Mg volumes 60 × 13 × 13 mm. Failed depositions are due to laser back reflection.

FIG. 3.

Top view of AM205, AlSi1Mg and AlSi10Mg volumes 60 × 13 × 13 mm. Failed depositions are due to laser back reflection.

Close modal

Stress relief heat treatment was carried out for all volumes at 300 °C for 2 h in an inert atmosphere before machining DIN 50125—B 6 × 30 tensile specimens. One machined specimen from each alloy was analyzed by a CT scan to measure the porosity percentage along the gauge section (L0) and to investigate the possible presence of large lack of fusion defects or cracks. Results, as with the example for AlSi10Mg shown in Fig. 4, show slightly higher porosity values (listed in Table II) when compared to the smaller volumes performed during the process development phase. The CT scan results also show the absence of large cracks or lack of fusion defects in all samples.

FIG. 4.

CT scan of the resisting volume (L0) of a tensile specimen machined from an AlSi10Mg volume.

FIG. 4.

CT scan of the resisting volume (L0) of a tensile specimen machined from an AlSi10Mg volume.

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TABLE II.

Porosity measured by the CT scan along the gauge section (L0) of the tensile specimens.

AlloyPorosity (%)Max pore diameter (μm)
AlSi10Mg 0.90 889 
AlSi1Mg 2.52 1431 
AM205 0.45 668 
AlSi1Mg + Zr 1.87 762 
AlloyPorosity (%)Max pore diameter (μm)
AlSi10Mg 0.90 889 
AlSi1Mg 2.52 1431 
AM205 0.45 668 
AlSi1Mg + Zr 1.87 762 

AM205 was the alloy with the lowest porosity value and lowest dimensions of the spherical pores. The higher content of Si in AlSi10Mg is known to hinder higher castability when compared to AlSi1Mg and a similar trend was also achieved by the LMD process. In addition to a significantly higher porosity value, the pores in AlSi1Mg had lower sphericity and bigger dimensions, while defects in AlSi10Mg were mainly gas induced spherical pores. However, as for the results obtained in the LPBF process by Belelli et al.,25 the addition of 1 wt. % of Zr resulted in a significant decrease in porosity and pores dimension when processed in equal conditions, pointing out improved processability by LMD, thanks to the modified solidification mechanism.

Measured mechanical properties among the analyzed alloys follow a similar trend to the processability and amount of pores within the deposited material. AM205 shows the best tensile properties, listed in Table III, both in terms of strength and elongation at fracture (E). AlSi1Mg, on the other hand, resulted in comparable yield strength (YS) to the one of AlSi10Mg but showed a highly brittle behavior and very low ultimate tensile strength. The addition of Zr to AlSi1Mg resulted in a significant increase of 32.7% of YS as well as in a ductile behavior of the alloy.

TABLE III.

Mechanical properties of Al-alloys fabricated by LMD after stress relief HT at 300 °C for 2 h.

MaterialYS (MPa)UTS (MPa)E (%)HV
AlSi10Mg 120.6 ± 6.0 259.3 ± 9.0 6.4 ± 1.1 81 
AlSi1Mg 115.2 ± 12.0 131.0 ± 9.0 0.9 ± 0.3 65 
AM205 178.3 ± 6.1 347.8 ± 6.8 13.4 ± 1.1 103 
AlSi1Mg + Zr 148.2 ± 2.7 228.6 ± 6.7 7.8 ± 1.2 92 
MaterialYS (MPa)UTS (MPa)E (%)HV
AlSi10Mg 120.6 ± 6.0 259.3 ± 9.0 6.4 ± 1.1 81 
AlSi1Mg 115.2 ± 12.0 131.0 ± 9.0 0.9 ± 0.3 65 
AM205 178.3 ± 6.1 347.8 ± 6.8 13.4 ± 1.1 103 
AlSi1Mg + Zr 148.2 ± 2.7 228.6 ± 6.7 7.8 ± 1.2 92 

Light optical microscope (LOM) images of the tensile specimenś surface fractures, shown in Fig. 5, clearly show the presence of different types of defects along the fracture surfaces of the different alloys. AM205 and AlSi10Mg only show the presence of small pores with maximum dimensions of 100 and 250 μm, respectively, as identified by the CT scans. AlSi1Mg fracture surface, on the other hand, also shows the presence of narrow and elongated lack of fusion defects that were not identified by the CT scan as well as large spherical pores with dimensions up to 400 μm. Single in focus LOM images for AlSi1Mg + Zr surface fracture could not be taken since all specimens broke along planes not parallel to the cross section, but visual inspection showed the presence of only spherical pores.

FIG. 5.

LOM images of tensile specimens fracture surfaces of: (a) AlSi10Mg, (b) AlSi1Mg, (c) AM205.

FIG. 5.

LOM images of tensile specimens fracture surfaces of: (a) AlSi10Mg, (b) AlSi1Mg, (c) AM205.

Close modal

All recyclability analyses were performed on AlSi10Mg powder since it is the most commonly used in laser-based AM technologies. After each build cycle, left over powder around the deposited volumes, from now on called “used” powder, was collected for further powder analysis, which are later compared with the analysis of virgin powder to understand the physical and chemical changes happening due to the interaction with the laser. The amount of used powder that could be collected from the build area was, however, not sufficient to be used for further depositions. A system, shown in Fig. 6, was, therefore, created where the LMD process was running on a substrate inclined at 73° and far from the focal position of laser and powder cone. The out-of-focus process and the strong substrate inclination lead to low absorption of the laser by the substrate making it impossible for the powder to adhere. A collection bin was placed at the bottom of the substrate to collect the powder, from now on called “recycled” powder, to be used for analysis and further deposition trials.

FIG. 6.

System created to produce recycle powder from AlSi10Mg 63–105 μm.

FIG. 6.

System created to produce recycle powder from AlSi10Mg 63–105 μm.

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Through this process, 800 g of recycled powder are produced and sieved to remove the agglomerations as per ASTM B214-07 with a 150 μm sieve mesh under an argon atmosphere with an O2 level lower than 50 ppm.

Furthermore, since the presence of oxygen and hydrogen in the LMD process feedstock material are known detrimental effects, an aging heat treatment at 400 °C for 96 h in normal atmospheric conditions was performed on roughly 500 g of virgin powder to induce chemical changes without significant physical and morphological alteration of the particles.

In this work, the following terminology is used to specify the powder conditions:

  • Virgin refers to the powder as received from the manufacturer.

  • Used refers to the powder collected from the sides of deposited volumes and sieved at 150 μm.

  • Recycled refers to the powder produced with the system in Fig. 6 and sieved at 150 μm.

  • Aged refers to the powder that underwent a heat treatment to artificially increase oxygen and hydrogen content in the powder.

The cumulative particle size distribution (PSD) as well as the median particle size value (D50) were analyzed in all conditions and measurements are shown in Fig. 7. The reference condition is virgin powder of AlSi10Mg with declared powder range from the manufacturer 63–105 μm and measured D50 of 74.90 μm. As expected, the aging process did not have any visible effect on the cumulative PSD distribution and the lower measured D50 value of 68.51 μm is within the statistical difference of powder samples collected from the top and bottom of a single container. Recycled powder showed a narrower PSD distribution around a similar D50 value of 64.80 μm, with a slightly lower amount of particles in the range of 40–50 μm and a significant reduction in the number of particles with dimensions greater than 90 μm. Used powder, on the other hand, showed a significant reduction of particle sizes in the whole spectrum with a similar range to the virgin condition but centered in D50 48.62 μm.

FIG. 7.

Cumulative PSD of AlSi10Mg powder in different conditions.

FIG. 7.

Cumulative PSD of AlSi10Mg powder in different conditions.

Close modal

The reducing D50 value measured from used powder is in contrast with the literature results in the LPBF process, where an increasing D50 value is often measured and it is explained with the formation of particle agglomerates and spatter from the melt pool.22 However, LMD and LPBF processes have significant differences in the way the unused powder interacts with the laser beam. Carroll et al., using an LMD system with laser spot significantly smaller than the powder spot and resulting in only 5% of powder utilization rate, observed a slightly decreasing D50 value of Waspaloy powder after each utilization.23 

Flowability is one of the most important properties of metal powders. Particles in the powder feeder are subjected to the consolidating load of their own weight, with possible effects on the amount of feed material during the deposition process with detrimental effects on the repeatability of the LMD process.26 All flowability tests procedures, equations, and reference values were performed according to the standard ASTM F3049-1427 and the rheometer manual written by the device manufacturer. The test is performed by rotating and moving downwards a “blade” through the powder. This causes thousands of particles to interact, or flow relative to one another, and the energy needed to move the blade represents the difficulty of this relative particle movement. Therefore, higher energies values stand for lower flowability of the powder.

The measurement procedure consists of 11 test runs. The first seven test runs are performed with constant flow rate of the blade and the results are used to calculate SI with the following equation:

Stabilityindex(SI)=EnergyTest7EnergyTest1.
(1)

Tests runs from 8 to 11 are performed with decreasing values of flow rate and the expected change in measured energy values are used to calculate FRI with the following equation:

Flowrateindex(FRI)=Totalenergyatthetestrun11Totalenergyattestrun8.
(2)

Measurements in all conditions, shown in Fig. 8, show very good stability of the powder flow having 0.9 < SI < 1.1, which is the desired condition for AM metal powders. This result shows that the consolidating effect in time of the particles in the powder feeder is negligible. On the other hand, aluminum powders showed a low sensitivity to changes in the flow rate with 1 < FRI < 1.5, where the desired property is an average flow rate sensitivity with 1.5 < FRI < 3. SEM images of the powder in virgin, aged, and recycled conditions, visible in Fig. 9, confirm the results obtained with PSD analysis with similar powder dimensions in all conditions. While virgin and aged powders present very similar morphologies and amount of satellites, recycled powder show an increase in fine satellite particles (greater than 5 μm) adhered to the surface of the powder particles.

FIG. 8.

Flowability measurement of AlSi10Mg in different conditions.

FIG. 8.

Flowability measurement of AlSi10Mg in different conditions.

Close modal
FIG. 9.

SEM images of AlSi10Mg powder in different conditions.

FIG. 9.

SEM images of AlSi10Mg powder in different conditions.

Close modal

Oxygen, hydrogen, and nitrogen concentrations in all powder conditions were analyzed using an inert gas fusion method. Three different measurements were performed for each powder condition. Averaged values and standard deviations of measured O, H, and N contents are plotted in Fig. 10. Virgin powder, as expected, was the condition that showed the lowest oxygen and hydrogen contents. Used and recycled powders had similar results with significant increases in hydrogen content up to 300% and increases in oxygen content up to 250%. The measurements of aged condition showed that the desired chemical changes of increased oxygen and hydrogen content in the feedstock material were successfully achieved. The increase in hydrogen content was equal to the one found in used and recycled powders, while oxygen content was more than 50% higher than the oxygen content in used and recycled powder and almost 4 times the one present in the original virgin condition. LMD process for high cost driven industries can operate only with localized inert gas protection around the melt pool, due to the long times and high costs needed to generate an inert atmosphere in a large build volume. For this reason, AlSi10Mg powder in LPBF process can be recycled six times with an oxygen increase of 80%–100% compared to the virgin condition.28 On the other hand, in the LMD process, the unmelted particles that are heated up by the interaction with the laser beam get in contact with the atmospheric oxygen and hydrogen before cooling down at room temperature, resulting in higher absorption of the gases already after a first recycling process.

FIG. 10.

O, H, and N weight percentage contents in AlSi10Mg powder in different conditions.

FIG. 10.

O, H, and N weight percentage contents in AlSi10Mg powder in different conditions.

Close modal

The work aimed to use both aged and recycled powders in the LMD process with equal process parameters and strategies as virgin powder. This way, it was possible to differentiate the detrimental effects of chemical changes observed in the aged powder and the physical properties differences observed in recycled powder. During the deposition of aged powder, a high number of bright sparks and sputters has been observed compared to the stable deposition of virgin powder. Da Silva et al.,18 while depositing single tracks by LMD with artificially aged aluminum powders, observed an increase in the width of each track along with a decrease in the height. This was explained by the higher concentration of oxygen and hydrogen in the feedstock material, which are known to decrease the surface tension of the melt pool, and to increase the deposition temperature due to the exothermic oxidation reaction.18 The increased presence of oxides in volumes fabricated with aged powder is further pointed out by the darker color of the external surfaces of the material visible in Fig. 11.

FIG. 11.

Comparison of volumes 60 × 13 × 13 mm manufactured with aged powder (left) and virgin powder (right).

FIG. 11.

Comparison of volumes 60 × 13 × 13 mm manufactured with aged powder (left) and virgin powder (right).

Close modal
FIG. 12.

(a) Single and double layer deposition 60 × 13 mm with recycled powder. (b) LMD nozzle damaged by fumes and particles adhesion.

FIG. 12.

(a) Single and double layer deposition 60 × 13 mm with recycled powder. (b) LMD nozzle damaged by fumes and particles adhesion.

Close modal

Deposition attempts with recycled powder, visible in Fig. 12, could not be completed due to the excessive generation of sparks and fumes that reach the nozzle with a high risk of damage to the LMD system. While the impact of powder’s chemical properties, mainly oxygen and hydrogen content, was already visible in the deposition process with aged powder, the changes of physical properties, with narrower PSD, significantly lower flowability and slightly higher amount of satellites, caused significant changes in the deposition process. The presence of visible sparks and metal spatter in the LMD process, as well as other metal welding processes, is known to be related to excessive heat input which results in plasma formation.29 The higher heat input, however, cannot be exclusively explained by higher laser absorption of the oxide layer forming on top of the melt pool, since aged powder contained similar amounts of oxygen. The different physical properties of recycled powder may impact the shape and/or speed of the powder cone causing a longer interaction time between the particles and the laser beam.

LOM images, visible in Fig. 13, were taken along three different cross-sectional surfaces of one example volume for each condition. The volume fabricated with aged powder clearly shows a deeper penetration depth of the material into the substrate due to the higher deposition temperatures. Moreover, as visible from the cross sections, the deposition with aged powder resulted in slightly wider and shorter volumes. Image analysis of the porosity was performed within rectangular regions of interest (ROI) covering the area of material that will be used for machining DIN 50125—B 6 × 30 tensile specimens.

FIG. 13.

LOM images of volumes from virgin powder (a) and aged powder (b).

FIG. 13.

LOM images of volumes from virgin powder (a) and aged powder (b).

Close modal

Measured porosity values of 3.31% and 3.44% for virgin and aged powder, respectively, as well as the maximum pore dimension of 0.30 and 0.24 mm, have differences within the confidence interval of the measurement method used. It is, however, worth noticing that the volume fabricated from aged powder presents a significantly higher porosity and pore dimension in the immediate vicinities of the lateral and top surface, due to the entrapment of hydrogen bubbles during the melt pool solidification process.

Stress relief heat treatment was carried out at 300 °C for 2 h in an inert atmosphere before machining one round tensile specimen out of each volume fabricated with aged powder. Tensile testing results, listed in Table IV, show a significant drop in the yield strength of 27.9% when compared to the virgin powder condition and a drop of 31.4% in the ultimate tensile strength. On the other hand, the different chemical properties of the feedstock material resulted in a significantly more ductile material with an increase in elongation at fracture of 112.5% up to a value of 13.6 ± 1.4%. This increase may be explained by lower cooling rates during the deposition process, which ends up in a coarser microstructure. The wider and thinner layers caused by the presence of oxides in the melt pool lead to higher penetration depths of the laser beam into the pre-deposited layers with a larger dimension of the heat affected zone.

TABLE IV.

Tensile property comparison of materials from virgin and aged powder.

ConditionYS (MPa)UTS (MPa)E (%)
Virgin 120.6 ± 6.0 259.3 ± 9.0 6.4 ± 1.1 
Aged 87 ± 4 (−27.9%) 178 ± 4 (−31.4%) 13.6 ± 1.4 (+112.5%) 
ConditionYS (MPa)UTS (MPa)E (%)
Virgin 120.6 ± 6.0 259.3 ± 9.0 6.4 ± 1.1 
Aged 87 ± 4 (−27.9%) 178 ± 4 (−31.4%) 13.6 ± 1.4 (+112.5%) 

High end industrial sectors exploit laser-based AM processes to produce high value parts with very complex designs, not manufacturable by conventional technologies. However, the low production throughput and high costs prevent the implementation in highly cost-driven sectors. LMD technology, compared to powder bed processes, can also be exploited in a hybrid approach in combination with lower cost manufacturing technologies. In particular, it can be used to deposit materials on a substrate only where needed adopting a new design paradigm based on adding-value functional feature (AVFF). AVFFs are small-scale 3D geometric features, such as the one in Fig. 14, which can be added to a pre-existing substrate to improve or add functionalities, such as static and dynamic stiffening, heat dissipation, vibration dumping, energy absorption guidance, etc.

FIG. 14.

Archetypes of AVFFs. From left to right: linear, crossing, triangular, and honeycomb.

FIG. 14.

Archetypes of AVFFs. From left to right: linear, crossing, triangular, and honeycomb.

Close modal

LMD is a major enabler for the AVFF based manufacturing and has the potential to innovate the way many products are conceived, designed, and manufactured. LMD has the acknowledged advantage of being specifically suitable to be used on 3D surfaces: Moreover, the possibility to deposit AVFFs with a different alloy than the one of the substrate materials opens new design opportunities unfeasible with any other manufacturing technology. The AVFF approach is particularly suited for structural parts where the “right material in the right place” is the best way to minimize costs and oversizing. This method is, therefore, extremely adopted in the automotive sector, where cost constraints are a limiting factor for design freedom. Currently, components are complex assemblies of subparts made of dissimilar materials. However, with the possibility to add local features to a single part, the AVFF approach will enable hybrid manufacturing strategies to replace the current production methods. This will allow in the future to minimize the number of parts and assembly steps needed for complex components and to develop capillary supply chains in all regions, associated with a limited cost of the raw materials and less vulnerable to the uncertainties of the global market.

In this study, AlSi-based alloys (AlSi10Mg and AlSi1Mg) and AM-specific aluminum alloys (AM205 and AlSi1Mg + 1 wt. %Zr) were analyzed and compared. The developed knowledge of the processability by LMD and the properties of aluminum alloys are fundamental steps to exploit the advantages and possibilities of the AVFF approach and implement hybrid manufacturing methods also in cost driven industrial sectors with series productions. The following conclusions can be drawn by the achieved results:

  • The development of new AM-specific Al alloys, such as the addition of 1 wt. % Zr in AlSi1Mg, can significantly improve both processability and material properties. AM205 shows 35% higher UTS, 110% higher E, and 20% higher Vickers hardness when compared to AlSi10Mg.

  • Good material quality and mechanical properties for Al alloys can be achieved by LMD in atmospheric conditions.

  • Lower values of PSD were measured for used powder compared to virgin powder. The reasons of this result are currently unknown in the literature and may help to understand and improve powder recyclability in LMD.

  • Al powders cannot be easily collected and reused in LMD. The combination of physical and chemical changes happening during the interaction with the laser beam causes a drop in processability and risks of damage to the LMD system. Nevertheless, further methods may be used to recycle used powder (e.g., mix with new powder).

  • The presence of O and H within AlSi10Mg results in a drop of around 30% in both YS and UTS, while the increase of 112.5% in elongation may be related to a coarser microstructure.

This study was accomplished within the European project SAMOA (No. 18079) funded by EIT Raw Materials.

The authors have no conflicts to disclose.

Francesco Bruzzo: Conceptualization (equal); Formal analysis (supporting); Investigation (supporting); Methodology (lead); Project administration (equal); Supervision (equal); Writing – original draft (lead); Writing – review & editing (lead). Mehar Prakash Reddy Medapati: Data curation (lead); Formal analysis (equal); Investigation (equal); Methodology (equal); Validation (equal); Visualization (equal); Writing – original draft (supporting). Daniele Pullini: Conceptualization (equal); Funding acquisition (equal); Project administration (equal); Resources (equal); Supervision (equal). Fabio Ronco: Conceptualization (equal); Funding acquisition (equal); Project administration (equal); Resources (equal); Supervision (equal). Andrea Bertinetti: Conceptualization (equal); Funding acquisition (equal); Project administration (equal); Resources (equal); Supervision (equal). Alessio Tommasi: Conceptualization (equal); Funding acquisition (equal); Project administration (equal); Resources (equal); Supervision (equal); Writing – original draft (supporting). Mirko Riede: Conceptualization (equal); Funding acquisition (equal); Project administration (equal); Supervision (supporting). Elena Lòpez: Conceptualization (equal); Funding acquisition (equal); Project administration (equal); Resources (equal); Supervision (supporting). Frank Brückner: Funding acquisition (equal); Project administration (equal); Resources (equal); Supervision (supporting).

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