Conference Paper

Processing of High Strength Al-Cu alloy Using 400W Selective Laser Melting – Initial Study

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Abstract

The proposed paper deals with development of production parameters of high strength Aluminium (Al–Cu–Mg–Fe–Ni) alloy using 400W selective laser melting system. The AW2618 high-strength aluminium alloy is typically used in aerospace and military components, engine pistons, parts of turbochargers due to its ability to work in higher temperature applications. The advantage is the stability of mechanical properties after heating even over 100 °C due to the Ni and Fe content. Due to high energy input of SLM, high heating and cooling rates are induced during the melting/solidification process which gives the ability to process this typically difficult to weld material. First stage of experiments with different values of laser power (LP) and laser scanning speed (LS) were conducted to describe the processing window. Single track scans (STS) with LP 100-400W and LS 200-1400mm/s were processed to find the optimal energy density for Al-Cu alloy. Layer thickness and other parameters stayed unchanged. Continuity and quality of STS were analyzed with non-contact 3D optical profilometer. Second stage of experiments was aimed on processing of multiple layers and homogeneity of the material. The μCT scanning of the samples was used in order to obtain qualitative and quantitative information about the sample porosity. Results show, that with the higher laser power (400W) a relative density higher than 99% can be reached, however with large amount of cracks. To reach the fully dense material free of cracks further experiments are necessary.

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... Recently, a considerable focus has been made on SLM of aluminium alloys, building single tracks, multitracks, and blocks. Crack-free samples of Al-Cu-Mg [51][52][53], Al-Cu [39], Al-Cu-Mg-Si [62], Al-Cu-Mg-Ni [66], and Al-Cu-Mg-Fe-Ni [65] with high relative densities have been built by SLM process. However, they are still susceptible to cracking, and porosities are likely to be formed at different sets of processing parameters. ...
... The results suggest that input energy density cannot be considered as the only affecting parameter in processing materials, and relative density varies greatly at different combinations of speeds and powers. This is also confirmed by Koutny et al. [65]. In Figure 2(c), wider ranges of laser powers and speeds were used for processing Al2618 alloy. ...
... • Preferentially precipitation of second phases at grain boundaries was observed, which caused local softening [59] EBM • Processability of Al2024 alloy by applying different beam currents, scan speeds, line offsets, and baseplate preheating temperatures • The highest relative density achieved was about 90.6%, and samples had lots of porosities and cracks [60] • Fabricated defect-free Al2024 cubes with high relative densities • Optimising the processing parameters and preheating the spread fresh powder before scanning each layer with a defocused beam Al-Cu-Mg-Mn SLM • Improving the processability and performance of fabricated parts by adding Zr and modifying chemical composition • Improved flowability and molten pool stability [63] Al-Cu-DMD • No indication of solidification cracking was found when an optimised set of parameters was used • Excellent response to precipitation strengthening for an alloy having 6%-8% Cu and 1%-4% Ag, and containing 0.3% Mg, Ti, and Zr [64] Al-Cu-Mg-Fe-Ni (Al2618) SLM • Using lasers with high powers (400 W) can increase both heating and cooling rates, and therefore, widening the processing window [65] • No direct correlation between volumetric energy density and relative density of samples was observed • Melting depth equals about three layer thicknesses was the most suitable, concerning densification [66] • Developing a process window; the best result was achieved when samples fabricated on support structures using meander scanning strategy In a similar study, Nie et al. [53] studied the formation of single tracks in a selective laser melted Al-Cu-Mg alloy using 180 and 200 W of laser power and scanning speeds in the range of 5-60 m min −1 . They reported the significant effect of laser power and scan speed on the process, resulting in different morphologies such as unstable tracks and tracks with or without cracks, as shown in Figure 3. ...
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Aluminium alloys are commonly used in different areas and considerable interest has been given to additive manufacturing (AM) aluminium alloys recently. However, a few research studies succeeded in fabricating high-strength 2xxx alloys owing to porosity formation and their high cracking susceptibility. AM of these alloys is still suffering from different challenges. This review aimed at giving an insight into the current state-of-the-art of Al–Cu alloys AM and discusses the processes, challenges, and approaches in improving the integrity and performance of the parts manufactured in recent years. First, an introduction about the AM processes is given. Then, the main techniques used to fabricate parts are presented, and major defects found in additively manufactured parts are discussed afterward.
... Popovich et al. [23] showed that the different process parameters together with the scanning strategy strongly affects grain orientation and the resulting mechanical properties of Inconel 718, thus functionally graded materials can be produced with this approach. This paper builds mainly on the findings of the initial study of high power processing of EN AW 2618 proposed in two articles, Koutny et al. [24] and Koukal et al. [25]. In these studies, a SLM ...
... The aim of this study is a detailed examination of the process parameter window found in previous studies [24,25]. Larger cube samples which have been built and evaluated to explore the influence of different scanning strategies and other SLM process parameters on relative density and mechanical properties, have not yet been investigated. ...
... Volumetric cube samples The aim of single-track welds is to find the areas with a suitable combination of main process parameters that would be prospective for the building of low porosity volumetric samples. Because of non-consistent results in the initial study [24], single-track experiments were performed again for better accuracy. To ensure uniformity of the coated powder layer, a manual recoating device was used. ...
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This paper deals with various selective laser melting (SLM) processing strategies for aluminum 2618 powder in order to get material densities and properties close to conventionally-produced, high-strength 2618 alloy. To evaluate the influence of laser scanning strategies on the resulting porosity and mechanical properties a row of experiments was done. Three types of samples were used: single-track welds, bulk samples and samples for tensile testing. Single-track welds were used to find the appropriate processing parameters for achieving continuous and well-shaped welds. The bulk samples were built with different scanning strategies with the aim of reaching a low relative porosity of the material. The combination of the chessboard strategy with a 2 × 2 mm field size fabricated with an out-in spiral order was found to eliminate a major lack of fusion defects. However, small cracks in the material structure were found over the complete range of tested parameters. The decisive criteria was the elimination of small cracks that drastically reduced mechanical properties. Reduction of the thermal gradient using support structures or fabrication under elevated temperatures shows a promising approach to eliminating the cracks. Mechanical properties of samples produced by SLM were compared with the properties of extruded material. The results showed that the SLM-processed 2618 alloy could only reach one half of the yield strength and tensile strength of extruded material. This is mainly due to the occurrence of small cracks in the structure of the built material.
... LBM of EN AW-2022, EN AW-2024, EN AW-2618A and EN AW-2219 with ρrel > 99.9% [29,30] and results from tensile testing of EN AW-2618A [31] were published by the authors. Others followed to publish on LBM of 2xxx series Al-Cu wrought alloys [32,33]. ...
... [29,30] and results from tensile testing of EN AW-2618A [31] were published by the authors. Others followed to publish on LBM of 2xxx series Al-Cu wrought alloys [32,33]. ...
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Additive manufacturing is especially suitable for complex-shaped 3D parts with integrated and optimized functionality realized by filigree geometries. Such designs benefit from low safety factors in mechanical layout. This demands ductile materials that reduce stress peaks by predictable plastic deformation instead of failure. Al-Cu wrought alloys are established materials meeting this requirement. Additionally, they provide high specific strengths. As the designation "Wrought Alloys" implies, they are intended for manufacturing by hot or cold working. When cast or welded, they are prone to solidification cracks. Al-Si fillers can alleviate this, but impair ductility. Being closely related to welding, Laser Beam Melting in Powder Bed (LBM) of Al-Cu wrought alloys like EN AW-2219 can be considered challenging. In LBM of aluminium alloys, only easily-weldable Al-Si casting alloys have succeeded commercially today. This article discusses the influences of boundary conditions during LBM of EN AW-2219 on sample porosity and tensile test results, supported by metallographic microsections and fractography. Load direction was varied relative to LBM build-up direction. T6 heat treatment was applied to half of the samples. Pronounced anisotropy was observed. Remarkably, elongation at break of T6 specimens loaded along the build-up direction exceeded the values from literature for conventionally manufactured EN AW-2219 by a factor of two.
... However, these studies in general only changed the processing parameters used in L-PBF, e.g. laser power, scanning speed, hatch spacing, layer thickness and scanning strategy (Kaufmann et al. 2016;Koutny et al. 2015Koutny et al. , 2018Nie et al. 2018;Hu et al. 2019Zhang et al. 2016). The cracks become a bottleneck that restricts the development of L-PBF high strength aluminium. ...
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The influence of the defocusing distance on melt pool morphology, defect, microstructure and mechanical properties of the laser powder bed fusion (L-PBF) of high strength Al–Cu–Mg–Mn alloy was studied. It is found that because of the different laser energy distribution, the defocusing distance has significant effects on melt pool morphology, defects, microstructure and mechanical properties of the L-PBF parts using same process parameters and laser spot size. When defocusing distance in the range of −3.0 to 0 mm, specimens with cracks, fine grains and keyhole or transition melt mode are obtained. When defocusing distance is positive or −3.0 mm, specimens with no cracks, columnar grains and conduction mode are obtained. The best mechanical properties are obtained when defocusing distance is 1.0 mm in this experiment. This paper provides a novel method for tailoring the microstructure and mechanical properties of L-PBF parts.
... The hypothesis put to experimental falsification in this work is: size variation of free-flowing metal microparticles does not allow stabilizing mixtures against segregation. The system of Al-Cu is chosen because of its challenging mass density ratio of 3.3, and because it attracts increasing scientific interest with regards to LBM [25,27,[48][49][50][51][52][53][54][55][56][57][58][59][60], since processability has been shown [61,62]. The range of free-flowing particle sizes is extended to particles < 20 µm by dry coating with SiOx nanoparticles [46]. ...
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Up to now, minimizing segregation of free-flowing, microscale metal powder mixtures driven by different mass density is an open challenge. In this work, effects of particle size variation on homogeneity of Al-Cu mixtures, with a density ratio of 3.3, are examined. Dry coating Al particles with 0.3 wt% fumed silica SiOx nanoparticles significantly decreases interparticle attraction. This enlarges the range of free-flowing Al particle sizes to < 20 µm. Powder mixture homogeneity is examined optically in vibrated bulk powder and thinly spread layers. From various powder mixtures, solid samples are built layer by layer with the Additive Manufacturing (3D printing) technology Laser Beam Melting in metal powder bed (LBM). Chemical homogeneity of solids is evaluated via energy-dispersive X-ray spectroscopy, backscattered electron microscopy, metallographic analysis and tensile tests. Persistent homogeneity of Al-Cu powder mixtures and LBM solids is found only with particles < 20 µm dry coated with SiOx nanoparticles. Observed segregation phenomena are explained with a decrease in particle mobility at increasing local concentration and the decreasing effectiveness of mass in smaller particles. The main effects are based on geometry, so they are expected to be transferrable to other nanoparticles, alloying components and powder bed technologies, e.g., binder jetting.
... Clearly, Zr addition is one approach to tailoring the chemical composition of an Al alloy to make it suitable for AM processes. A preliminary study on the effect of using higher laser power of the order 400 W for SLM of AA 2618 showed that while a relatively high relative density around 99% could be achieved, the parts showed microcracking, indicating the need for further research [116]. Another study reported that solution treatment (530°C, 1 h) and ageing (200°C, 3 h) significantly enhanced the mechanical properties of SLM 2618 with the YS increasing from 211 as built to 316 MPa [110]. ...
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Previous work has shown that the processing of aluminium alloys by selective laser melting (SLM) is difficult, with reasonable components only being produced with high laser powers (minimum 150 W) and slow laser scanning speeds. The high laser power is a significant problem as it is higher than that used in many SLM machines. Also, the combination of high power and low speed creates a large melt pool that is difficult to control, leading to balling of the melt and possible damage to the powder distribution system. Even when processing is carried out successfully, the high power and slow scan speed significantly increase build time and the manufacturing costs.
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Purpose The purpose of this paper is to study the effects of particle size distribution, component ratio, particle packing arrangement, and chemical constitution on the laser sintering behaviour of blended hypoeutectic Al‐Si powders. Design/methodology/approach A range of bimodal and trimodal powder blends were created through mixing Al‐12Si and pure aluminium powder. The powder blends were then processed using selective laser sintering to investigate the effect of alloy composition, powder particle size and bed density on densification and microstructural evolution. Findings For all of the powder blends the sintered density increases with the specific laser energy input until a saturation level is reached. Beyond this saturation level no further increase in sintered density is obtained for an increase in specific laser energy input. However, the peak density achieved for a given blend varied significantly with the chemical constitution of the alloy, peaking at approximately 9 wt% Si. The tap density of the raw powder mixture (assumed to be representative of bed density) was also a significant factor. Originality/value This is the first study to consider the usefulness of silicon as an alloying element in aluminium alloys to be processed by selective laser sintering. In addition the paper outlines the key factors in optimising processing parameters and powder properties in order to attain sound sinterability for direct laser sintered parts.
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Many studies have shown that certain biomaterials with specific porous structures can induce bone formation in non-osseous sites without the need for osteoinductive biomolecules, however, the mechanisms responsible for this phenomenon (intrinsic osteoinduction of biomaterials) remain unclear. In particular, to our knowledge the type of pore structure suitable for osteoinduction has not been reported in detail. In the present study we investigated the effects of interconnective pore size on osteoinductivity and the bone formation processes during osteoinduction. Selective laser melting was employed to fabricate porous Ti implants (diameter 3.3mm, length 15 mm) with a channel structure comprising four longitudinal square channels, representing pores, of different diagonal widths, 500, 600, 900, and 1200 μm (termed p500, p600, p900, and p1200, respectively). These were then subjected to chemical and heat treatments to induce bioactivity. Significant osteoinduction was observed in p500 and p600, with the highest observed osteoinduction occurring at 5mm from the end of the implants. A distance of 5mm probably provides a favorable balance between blood circulation and fluid movement. Thus, the simple architecture of the implants allowed effective investigation of the influence of the interconnective pore size on osteoinduction, as well as the relationship between bone quantity and its location for different pore sizes.
Wohlers Report 2014: 3D Printing and Additive Manufacturing State of the Industry Annual Worldwide Progress Report. Wohlers Associates
  • T T Wohlers
Wohlers, T T. 2014. Wohlers Report 2014: 3D Printing and Additive Manufacturing State of the Industry Annual Worldwide Progress Report. Wohlers Associates. https://books.google.cz/books?id=iCamoAEACAAJ.