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Effects of the addition of silicon to 7075 aluminum alloy on microstructure, mechanical properties, and selective laser melting processability
Journal Pre-proof Effects of the addition of silicon to 7075 aluminum alloy on microstructure, mechanical properties, and selective laser melting processability Yuki Otani, Shinya Sasaki PII:
S0921-5093(20)30167-2
DOI:
https://doi.org/10.1016/j.msea.2020.139079
Reference:
MSA 139079
To appear in:
Materials Science & Engineering A
Received Date: 5 November 2019 Revised Date:
29 January 2020
Accepted Date: 6 February 2020
Please cite this article as: Y. Otani, S. Sasaki, Effects of the addition of silicon to 7075 aluminum alloy on microstructure, mechanical properties, and selective laser melting processability, Materials Science & Engineering A (2020), doi: https://doi.org/10.1016/j.msea.2020.139079. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Title Effects of the addition of silicon to 7075 aluminum alloy on microstructure, mechanical properties, and Selective Laser Melting processability
Author names and affiliations Yuki Otani1, (Given name) (Family name) 1. Department of Mechanical Engineering, Graduate school of Tokyo University of Science, 6-3-1 Niijuku Katsushika-ku Tokyo, Japan, [email protected] Shinya Sasaki2, (Given name) (Family name) 2. Department of Mechanical Engineering, Faculty of Engineering, Tokyo University of Science, 6-3-1 Niijuku Katsushika-ku Tokyo, Japan, [email protected]
Corresponding Author Yuki Otani, [email protected]
Abstract Aluminum alloys that can be processed using the selective laser melting (SLM) technique have been restricted to cast alloys based on the Al-Si binary system. To increase the selection of materials available for SLM, researches on SLM processability using wrought Al alloys (e.g. 2xxx, 7xxx series) have
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been conducted. The 7075 Al alloy has excellent mechanical properties among aluminum alloys; however, 7075 alloy has the problem of severe cracking occurring in parts fabricated through the SLM process. Our study demonstrated that additional silicon enables the fabrication of 7075 SLM parts without major defects and that the amount of silicon content changes the favorable processing conditions, morphology of microstructure, and mechanical properties. As the silicon content increased, voids and cracking in SLM samples were suppressed, and volumetric energy density for sufficient densification was reduced. Vickers microhardness and 0.2% proof strength were enhanced with increments of additional silicon content. In contrast, excessive silicon content resulted in brittleness, which appeared as slight ductility measured on tensile testing and breakage of samples during SLM process. These conflicting effects indicated that silicon content must be adjusted depending on the underlying alloy to yield better processability and a balance between strength and ductility. For the 7075 alloy, the optimal silicon content could be concluded to be 5%, being the lowest amount of silicon content needed for the elimination of cracking with enhanced tensile strength and acceptable ductility.
Keywords Additive manufacturing Selective laser melting Aluminum Processability
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Microstructure Mechanical properties
1. Introduction Selective Laser Melting (SLM) is a beam-based additive manufacturing technique that enables the fabrication of complex shaped metallic parts with higher dimensional accuracy than other additive manufacturing techniques. This advantage of SLM could realize the production of improved lightweight components by combining topology optimization with aluminum alloys. In general, the aluminum alloys that can be soundly processed using SLM has been restricted to the alloys based on Al-Si binary system [1] (e.g., AlSi7Mg0.6, AlSi9Mg0.3, AlSi10Mg, and AlSi12). These alloys are classified for cast materials which are designed for manufacturing through a melt and solidification process; thus, the formation of structural defects via their solidification hardly occurs. By contrast, their inferior specific strength results in moderate mechanical properties, which cannot satisfy the required performance for lightweight parts. Wrought aluminum alloys (e.g., 2xxx or 7xxx series) in particular are expected to be processed using SLM because their enhanced specific strength can provide exceptional lightweight components for automotive and aerospace industries. However, because wrought aluminum alloys are designed for cold-forming processes such as rolling and extrusion, these alloys form structural defects easily. Previous studies reported the occurrence of severe cracking in various wrought aluminum alloys and proposed special treatments to suppress the cracking. Karg et al., demonstrated the fabrication of 2219 alloy samples without cracks by
3
preheating to 200 ˚C and adding support structures between the samples and baseplate [2]. In their study, cracking was prevented by ensuring slower cooling rate during the SLM process due by reducing the heat transfer from the parts to the baseplate. However, they also reported that the porosity ratio and tensile property of the parts were highly sensitivity to the fabrication geometry. The samples having larger cross-sectional areas of the baseplate surface exhibited 5% porosity ratio, resulting in lower tensile properties. Koutny et al., explored the influence of the SLM process parameters (e.g. laser output, scanning velocity, scan strategy, and platform heating) on the relative density and mechanical properties of 2618 alloy [3]. They found extensive cracking, which was considered to be solidification cracks created by the stress due to high temperature differences between the solid and liquid phases during solidification. Reducing the thermal gradient by using a support structure showed the favorable effect of suppressing the cracks. In contrast, platform heating up to 400 ˚C and lower laser speed could not improve the quality of the samples, and inducing gas porosity. Reschetnik et al., reported the low mechanical properties of 7075 alloy parts fabricated by the SLM technique [4]. The reason of reduced mechanical properties is the occurrence of cracking, which appeared to be solidification cracks. The authors proposed changing the process parameters (i.e., laser output, scan distance, and scanning velocity) and the post-heat treatment for improving the mechanical properties, but this has not been realized in practice. Although these approaches have the definite effect of suppressing the cracks, it is necessary to study a method that can stably eliminate cracks without restricting the fabrication geometry. The 7075 alloy is one of the 7xxx series wrought aluminum alloys and has excellent static strength
4
among commercial aluminum alloys. Several researchers have investigated the influence of changes in processing conditions (e.g., laser parameters, scanning strategy, baseplate preheating) on the formation of defects in SLM parts using 7075 or analogous alloy. Qi et al. [5] have studied the relationships between melting mode transition and crack density using samples of 7050 alloy. According to their study, melt pool shapes could be altered using the scanning velocity and defocusing distance, and the shapes could be divided into three types: goblet, semicircle, and a combination of the two shapes. These are similar to keyhole, conduction, and transition modes in laser welding. The authors found that the keyhole melting mode results in the lowest crack density for a fabricated sample, but could not achieve the complete elimination of microcracks. Kaufmann et al. [6] have conducted the SLM processing of 7075 alloy under baseplate preheating at 200 ˚C, but significant improvement has not been achieved. Besides, previous studies have revealed that large inputs of heat owing to changing laser parameters and thermal conditions cause several drawbacks. The most harmful of these problems is the evaporation of alloying constituents caused by the high temperatures of molten metals. Mauduit et al. [7] observed the difference in chemical compositions of 7075 alloy before and after SLM processing, and reported a loss of 30% zinc and 19% magnesium content. Loss of Zn and Mg elements could decrease the mechanical properties of the 7075 alloy because these alloying constituents provide the effects of solution hardening and precipitation hardening by forming MgZn2 phase. Another approach that has been proposed is the addition of zirconium or scandium, which act as dispersoid forming elements for aluminum alloys. Martin et al. [8] have applied hydrogen stabilized
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zirconium nanoparticles for 7075 alloy powder. Zirconium nanoparticles form dispersed Al3Zr phases which provide nucleation sites for 7075 alloy during solidification, resulting in the formation of finely equiaxed grains. This is believed to suppress the microcracks by reducing the entrapped liquid phases between the grains and alleviating thermal stress due to thermal shrinkage. Scalmalloy®, which is the Al-Mg alloy of approximately 0.7% scandium and 0.4% zirconium, has been already used for aircraft components as the only high strength aluminum alloy processable by SLM. This material realized the processing of SLM parts made from high strength aluminum alloys without microcracks [9]. The peculiar precipitation hardening of the Al3Sc particles yields higher mechanical properties without the addition of conventional precipitation hardening elements that lead cause crack sensitivity. However, there are concerns for productivity of these material powders and scanty of metal resource because the scandium is one of the rare-earth elements. Silicon is a prevalent and inexpensive alloying constituent for aluminum alloy, and it prevents microcracks in 7075 alloy parts fabricated through SLM. Sistiaga et al. [10] have revealed that the use of a powder created by mixing 7075 powders and 4% silicon particles eliminates microcracks in samples. Aversa et al. [11] have succeeded in processing crack-free SLM samples from powder which was prepared by mixing 50% AlSi10Mg and 50% 7075 powder. However, the use of powder mixtures could cause inhomogeneous distributions of the alloying element, leading the inhomogeneous mechanical properties within fabricated parts. In our previous study, we investigated the microstructures and mechanical properties of SLM samples using pre-alloyed powders of 7075 alloy and 7075 with 5% of additional silicon alloy [12]. These microstructures were shown to be finely uniformed, and the tensile properties using 7075 with 5%
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additional silicon alloy were higher than those of the conventional AlSi10Mg alloy. This alloy system is expected to be applied to the fabrication of lightweight components via SLM; hence, more detailed measurements about the effect of additional silicon content are required. For this paper, we studied the effect of silicon content for SLM processabilities and the properties of fabricated samples. Pre-alloyed powders of 7075 with different amounts of additional silicon were used for SLM process. The effects of silicon content on the appropriate processing conditions were investigated, and the microstructures and mechanical properties of fabricated samples were compared.
2. Materials and Methods The ProX DMP 200 (3DSystems, USA) machine was employed to fabricate test samples. The material powders were pre-alloyed gas-atomized powders supplied by TOYO Aluminium KK. These powders are single alloy powders, not the mixture of 7075 alloy powder and Si particles. Tables 1 and 2 show the chemical compositions of material powders and their particle size distributions. The 7075 powder contains 0.05% silicon due to contamination during the powder manufacturing processes. Each of the modified 7075 powders contain different amounts of silicon ranging from 0.5 to 16 percent. To clarify the effect from the additional silicon, the concentrations of the other alloying constituents, Cu, Zn, and Mg, were kept constant. The powders were sieved to maintain identical particle size distributions.
Table 1 Chemical composition of material powders characterized by the powder supplier.
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Material
Element [mass%] Si
Cu
Zn
Mg
Fe
Cr
Mn
Ni
Ti
Al
7075
0.05
1.61
6.03
2.58
0.10
0.22
<0.01
<0.01
<0.01
Bal.
7075+0.5%Si
0.54
1.58
5.34
2.52
0.12
0.24
<0.01
0.01
0.01
Bal.
7075+1%Si
0.93
1.58
5.44
2.57
0.12
0.24
<0.01
0.01
0.01
Bal.
7075+2%Si
2.05
1.57
5.39
2.48
0.13
0.24
<0.01
0.01
0.01
Bal.
7075+3%Si
3.27
1.56
5.30
2.49
0.13
0.24
<0.01
0.01
0.01
Bal.
7075+4%Si
4.05
1.52
4.99
2.44
0.15
0.24
<0.01
0.01
0.01
Bal.
7075+5%Si
5.04
1.55
5.02
2.44
0.11
0.21
<0.01
0.01
<0.01
Bal.
7075+6%Si
6.07
1.49
4.96
2.35
0.16
0.23
<0.01
0.01
0.01
Bal.
7075+7%Si
7.05
1.73
4.90
2.40
0.16
0.22
<0.01
0.01
0.01
Bal.
7075+9%Si
9.20
1.42
5.07
2.35
0.15
0.20
<0.01
0.01
0.01
Bal.
7075+13%Si
12.98
1.36
4.84
2.31
0.16
0.18
<0.01
0.01
0.01
Bal.
7075+16%Si
16.39
1.30
4.69
2.15
0.18
0.17
<0.01
0.01
0.01
Bal.
Table 2 Particle size distribution of material powders characterized by the powder supplier. D10, D50, and D90 show the diameter for which 10%, 50%, and 90% of the particles are smaller than that when the particles are arranged from smallest to largest. D10 [µm]
D50 [µm]
D90 [µm]
7075
6.6
20.8
48.1
7075+0.5%Si
6.8
21.2
45.6
7075+1%Si
7.3
21.8
46.3
7075+2%Si
7.0
21.0
44.1
7075+3%Si
7.1
20.5
42.2
7075+4%Si
6.7
21.0
48.5
7075+5%Si
6.3
18.5
41.6
7075+6%Si
6.8
21.3
48.4
7075+7%Si
6.8
21.4
49.0
7075+9%Si
6.8
19.9
42.5
7075+13%Si
7.1
20.8
43.9
7075+16%Si
7.1
20.2
41.4
Material
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Cuboid SLM samples (15 mm × 15 mm × 10 mm) were fabricated using the processing parameters shown in Table 3. To assess the effects of laser conditions on the densification of the samples, thirty samples were fabricated from each material powder using different laser parameters, pairing six laser outputs with five scanning velocities. The optimal laser condition for providing the highest relative density to the fabricated samples was determined by density measurements based on the Archimedes principle. The laser output and scanning velocity were varied in the ranges of 169–234 W and 800–1600 mm/s, respectively. The layer thickness and scan distance were held at 30 µm and 100 µm. The build chamber was continuously purged with argon gas flow until the oxygen concentration was maintained at 0.1 % or less. An island scan strategy was employed for sample fabrication. This strategy divides the laser radiation region in each layer into several hexagonal areas with 10 mm of circumscribed diameter; thereafter, these regions were irradiated individually by back-and-forth laser scanning. SLM processing was conducted at room temperature; thermal condition controls including pre-heating the baseplate were not done.
Table 3 Ranges of studied processing parameters. Laser output [W]
169, 182, 195, 208, 221, 234
Scanning velocity [mm/s]
800, 1000, 1200, 1400, 1600
The structural defects and microstructures of fabricated samples were evaluated by microscopic observations using a Confocal Laser Microscope VK-X150 (KEYENCE, Japan) and a Scanning Electron Microscope TM3030Plus (Hitachi High-Technologies, Japan). Observations were carried out with the plane 9
being perpendicular to the baseplate surface. Cross-sections of the cuboid samples were finished to a mirror-surface by chemical mechanical polishing. These microstructures were revealed by etching for 2 min durations with an etching agent made by dissolving 2 g NaOH and 5 g NaF in 93 ml H2O. The differences in mechanical properties between materials were evaluated by Vickers microhardness tests. A Micro Vickers Hardness Tester HMV-G-FA-D (Shimadzu, Japan) was employed for hardness measurements. The indenter was forced to the mirror-polished sample surface at a load of 0.98 N and was held for 10 s. To minimize disturbance from defects, the measurements were repeated 25 times per sample and the top 10 values were compared. Tensile properties were evaluated by tensile testing with a Universal Tensile Testing Machine AG-Xplus (Shimadzu, Japan). Bar-shaped SLM samples were machined to tensile specimens with dimensions conforming to the ASTM E8 standard. Experiments were performed at room temperature and were repeated three times for each material.
3. Results 3.1 Sample fabrication and density measurements Figure 1 shows images of the powder particles. Significant differences in powder particle morphology between the material powders were not observed. Figure 2 shows the relationship between additional silicon content and the range between the maximum and minimum relative densities of the samples created using thirty different laser conditions. These relative densities were calculated by dividing the measured density by the theoretical density, which was obtained by multiplying the density and
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proportion of each element in the chemical composition of the material powders. The unmodified 7075 sample exhibited approximately 96.8% of maximum relative density, and its value was scattered due to the penetration of water into cracks during density measurements. Maximum relative density decreased once when the 0.5% silicon was added, but thereafter it started gradually increasing with the further addition of silicon. From 2% to 5% additional silicon content, maximum relative density increased abruptly until reaching 100.2%. It did not reduce below 99.9% until the amount of additional silicon increased to 9%. However, at 13% and 16%, the samples collapsed during the SLM process as shown in Figure 3; thus, the density measurements could not be performed for these cases.
Figure 1 Morphology of powder particles: (a) unmodified 7075 (b) 7075+5%Si (c) 7075+16%Si observed using SEM.
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Figure 2 Range between maximum and minimum relative densities of samples fabricated from 7075 with different amounts of additional silicon powder.
Figure 3 Appearance of samples fabricated from (a) 7075+13%Si and (b) 7075+16%Si powders.
Figures 4 and 5 show the effects of scanning velocity and laser output on density for 7075+5%Si samples. In these figures, the values when the laser output was set to 195 W or the scanning velocity was set to 1000 mm/s are plotted as square symbols, and other values are plotted as circles. The relative density was varied in the range of 94.4% to 100.2%, by changing the laser conditions. Regardless of laser output, a scanning velocity of 1000 mm/s provides enough density for the samples. The highest relative density, 100.2%, was obtained by using a combination of 195 W laser output and 1000 mm/s scanning velocity. 12
Figure 2 shows the relative densities of the samples processed under a laser output of 195 W and a scanning velocity of 1000 mm/s as a function of silicon content. All the compositions exhibited favorable relative densities under this laser condition; therefore, the combination of 195 W laser output and 1000 mm/s scanning velocity was defined as the optimal laser condition.
Relative density ％
100
98
96 Laser output 195W Other laser output
94
800 1000 1200 1400 1600 Scanning velocity [mm/s]
Figure 4 Correlations between relative density of the samples and laser conditions: as a function of scanning velocity.
13
Relative density ％
100
98
96 Scanning velocity 1000mm/s Other scanning velocity
94
180 200 220 Laser output [W]
240
Figure 5 Correlations between relative density of the samples and laser conditions: as a function of laser output.
3.2 Microscopic observations Figure 6 shows microscopic images of the samples at vertical cross-sections. Observations were carried out on the samples processed by the optimal laser condition in order to clarify the effect of additional silicon content. The figure illustrates the presence of defects including voids, macrocracks, and microcracks. No obvious precipitates or inhomogeneous structures were observed, but periodic traces of laser scanning were observed which were created by the difference of microstructure between the center and boundary regions of the trace. The voids and cracks were progressively suppressed as the amount of additional silicon increased. When the silicon content was lower than 2%, there were severe microcracks and macrocracks propagating through coarse defects. However, in the case of 4% silicon content, the sample contained minor microcracks and pores with a diameter of several tens of micrometers distributed on the boundary of trace. 14
When the silicon content reached 5%, microcracks were completely suppressed. The sample with 16% additional silicon also did not show major defects despite self-breakage during the SLM process.
Figure 6 Laser microscopic images for vertical cross-section of the samples processed under the optimal laser condition (195 W laser output and 1000 mm/s scanning velocity). (a) unmodified 7075 (b) 7075+2%Si (c) 7075+4%Si (d) 7075+5%Si (e) 7075+6%Si, and (f) 7075+16%Si.
Figure 7 shows SEM images on vertical cross-sections of the samples processed under the optimal laser condition. The microstructures consisted of bright and dark regions, and these morphologies were altered with additional silicon content. In the case of unmodified 7075, the morphology of microstructure was granular. As the silicon content increased, the microstructure was changed to a net-like shape; thus, the samples with 2 to 6% of additional silicon exhibited cellular-dendritic microstructure that elongated along a
15
building direction and connected at corresponding intersection areas. Regarding the sample with 16% of additional silicon, the microstructure was changed to a columnar-dendritic shape containing crystallized silicon particles. The significant alternations of the microstructure are attributed to the silicon content exceeding the solid solution limit and eutectic composition. The maximum solid solution limit of silicon in aluminum is approximately 1.6% at 577 ˚C; thus, the change in morphology between 0% and 2% of silicon content is due to increase in the volume of silicon that cannot be embedded in the aluminum matrix (the dark regions of the image). Similarly, silicon content in excess over the eutectic composition of the Al-Si binary system (Al-12%Si) cause a difference in the microstructure between the samples having 6% and 16% of silicon content. The 7075+16%Si alloy contains a sufficient amount of silicon to form a hypereutectic alloy for the Al-Si binary system. During the solidification process, the primary silicon phase appears first, and then solidification progresses subsequently.
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Figure 7 SEM images for vertical cross-section of the samples processed under the optimal laser condition (195 W laser output and 1000 mm/s scanning velocity). (a) unmodified 7075 (b) 7075+2%Si (c) 7075+4%Si (d) 7075+5%Si (e) 7075+6%Si, and (f) 7075+16%Si.
3.3 Measurements of Vickers microhardness and tensile properties Figure 8 shows the result of Vickers microhardness tests for the samples fabricated under the optimal laser condition. The Vickers microhardness grew proportionally as the amount of additional silicon increased. The unmodified 7075 samples exhibited 125HV0.1 of Vickers microhardness, and it was enhanced to 157HV0.1 in the case of 5% additional silicon. The maximum value, 218HV0.1, was measured when the additional silicon content was 16%. This is approximately double the hardness of the unmodified
Vickers microhardness HV0.1
7075.
220 200 180 160 140 120 100
0 2 4 6 8 10 12 14 16 Silicon content mass％
Figure 8 Vickers microhardness of the samples fabricated under the optimal laser condition (195 W laser
17
output and 1000 mm/s scanning velocity) as a function of silicon content.
Figure 9 depicts the tensile testing result for 0.2% proof strength, tensile strength, and elongation at break. The samples were fabricated using 3%, 5%, and 9% of additional silicon alloys, and the load was applied to be parallel to the baseplate surface. The result of using samples with 5% additional silicon and the laser condition for the fabrication of these samples are quoted from our previous paper [12]. The samples with 3% of additional silicon exhibited approximately 147 MPa of tensile strength and 0.4% of elongation. In this case, a 0.2% proof strength could not be obtained since the tensile specimens were broken before starting plastic deformation; furthermore, values of the elongation at break were obtained from one experiment because the range where it could be measured normally was not attained in two tests out of three. The samples with 5% of additional silicon showed 360 MPa of 0.2% proof strength, 537 MPa of tensile strength, and 9.7% of elongation, and for the samples with 9% of additional silicon, these were 402 MPa, 478 MPa, and 1.0%, respectively.
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Figure 9 Tensile property of the samples fabricated from 7075+3%Si, 5%Si, and 9%Si powder.
4. Discussion 4.1 Effect of additional silicon to densification of SLM samples The relationship between laser conditions and the densification of samples is usually interpreted using volumetric energy density (VED). As formulated below, VED expresses amounts of input heat against the per unit volume of material powder by laser irradiation. In this formula, P, v, d, and s represent laser output, scanning velocity, layer thickness, and scan distance, respectively. = Figure 10 shows the relative density of the samples with 5% of additional silicon as a function of VED. The typical convex upward curve was obtained. The samples with insufficient VED exhibit lower density due to a lack of fusion. The samples with excessive VED suffer from gas pores, which were formed
19
by trapping atmosphere as a result of the aggressive convection of molten metal, and keyhole porosity occurred from the evaporation of alloying elements having high vapor pressure. It was confirmed that the VED of the optimal laser condition (195 W laser output and 1000 mm/s scanning velocity), 65 J/mm3, corresponded to the global maximum of the curve. This VED is lower than that of the previous studies using 7075 with additional silicon. In the previous research using 7075 powder with silicon particles, the authors have reported that a laser condition containing 83.3 J/mm3 of VED (300 W of laser output and 1200 mm/s of scanning velocity) provides the highest density regardless of amount of additional silicon [10]. In a study using powder mixtures of 50% 7075 and 50% AlSi10Mg, the laser condition having 72.2 J/mm3 of VED (P: 195 W, v: 600 mm/s, d: 0.03 mm, s: 0.15 mm), was determined as the optimized condition [11]. One possible cause of this difference is the difference between powder mixtures and pre-alloyed powders. Powder mixtures must be completely melted and mixed together during laser irradiations and the subsequent liquid state. This process needs a long duration of melt; therefore, surplus VED may have been required for fabrication using powder mixture. The reduction of VED could prevent loss of the alloying element and reduce undesirable precipitations resulting from cyclic thermal input.
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Relative density ％
100 99 98 97
Optimal laser condition Other laser conditions
96
40 60 80 100 3 Volumetric energy density J/mm
Figure 10 Relative density of the 7075+5%Si samples as a function of VED.
The VED for sufficient densification of samples was affected by additional silicon content. Figure 11 shows the VED of the laser condition that gives the highest relative density for each material. A larger silicon content reduced the necessary VED for sufficient densification. Moreover, the minimum relative density was improved by including additional silicon as shown in Fig. 2. This indicates that the effects of additional silicon prevented the lack of fusion. This change could be explained in terms of the solidification speed. Additional silicon leads to a decrease of solidification speed because silicon releases the four times the latent heat of solidification relative to aluminum. Sales et al. [13] have demonstrated that increasing silicon content from 3% to 5% decreases the solidification speed and cooling rate of Al-Si alloys using directional solidification experiments. This behavior could offer additional time for entrapped bubbles to escape from molten metal before solidification. The changing of microstructure shown in Fig. 7 corresponded to this behavior. The morphology of the microstructure is determined by two factors: additional silicon content and ratio of thermal gradient (G) and solidification speed (R). When the silicon
21
content is between 2% to 6% (i.e. hypo-eutectic), the changes in the thermal kinetics would affect the microstructure. Granular, non-directional orientation is associated with a low G/R ratio, but columnar, directional orientation is correlated with a high G/R. Here, the changes of microstructure from granular to columnar may reflect the slower solidification speed, and could be one mechanism of porosity prevention by
Volumetric energy density J/mm3
the addition of silicon.
80 75 70 65 60 55 50
0 1 2 3 4 5 6 7 8 9 Silicon content mass％
Figure 11 Effects of silicon content on the VED of the laser condition that provides the highest relative density.
4.2 Effect of additional silicon to cracking and mechanical properties As shown in Fig. 6, the samples with less than 2% of additional silicon contained macrocracks which propagated through several layers. This arrangement means that the macrocracks happened during the progress of SLM fabrication, rather than immediately after laser irradiation. Therefore, the occurrence of
22
macrocracks depends on the intensity of metal expansion and contraction. One conceivable effect of additional silicon for inhibiting macrocracks is the reduction of thermal expansion. Nonmetallic elements including silicon have much lower coefficients of thermal expansion than metals; thus, adding silicon could reduce the changes of volume due to temperature change. This reduction leads to a reduction of the driving forces which promote cracking. Microcracks appeared in the samples with additional silicon of 4% or less. Microcracks that occur on wrought aluminum alloys are commonly attributed to solidification cracking and liquation cracking. The cause of solidification cracking is residuary thin-filmed liquid phase between primary crystallized grains. This isolated liquid hardly resists the thermal stress generated from thermal expansion during solidification, and eventually opens and forms cracks. Liquation cracking is caused by cyclic heat inputs as seen in multi-layer welding and layered fabrication. The heat inputs cause a reduction of strength mainly through the partial re-melting of eutectic phase or of compositions with lower melting points, and these weakened points work as starting points of cracking. To distinguish the microcracks occurred in the samples, microscopic observations at the top part of vertical sections were carried out. The surface layer is not affected by conduction heat from subsequent layers of fabrication; therefore, the solidification structure that is created by single laser irradiation has been preserved. The result of microscopic observations is shown in Fig. 12. Few microcracks appeared within the uppermost traces, but these subjacent layer traces contained microcracks. This positional relation indicates that the occurrences of severe cracking were promoted by cyclic heat input. Since precipitates or segregations, which might cause re-melting were not observed, the
23
severe microcracks seems to be motivated by the loss of strength due to high temperature. Kou [14] has reviewed the influence factors and remedies on the solidification cracking and liquation cracking appearing in welding. The author mentioned that the susceptibility of liquation cracks was affected by amount of heat input, tendency of weld-metal contraction, and degrees of restraint. As noted in section 4.1, additional silicon could reduce the VED for sufficient densification of samples, and suppress the thermal expansion of alloys; hence, it seems to contribute to the elimination of microcracks.
Figure 12 Laser microscopic images for the surface layer of fabricated samples. (a) unmodified 7075 (b) 7075+2%Si (c) 7075+4%Si (d) 7075+5%Si (e) 7075+6%Si, and (f) 7075+16%Si.
The Vickers microhardness and tensile properties were improved with the increase of silicon content. This tendency agrees with the case of Al-Si binary alloy reported in the previous research. Kimura
24
et al. [15] have investigated the relationships between silicon content and mechanical properties of Al-Si binary alloy samples fabricated by SLM method. They noted that the additions of silicon enhance the proof stress and ultimate tensile strength of SLM samples because the silicon elements enrich the proportion of the secondary phases, providing compositional reinforcement for the aluminum matrix. It is likely that a similar function improved the strength of the 7075 with additional silicon samples. However, this effect also emerged as brittleness of the alloy. As shown in Fig. 3, additional silicon of 13% or above resulted in failure of SLM processing due to self-breakage of the samples. The result of tensile testing for 7075+9%Si samples also showed especially lower ductility. These results imply that secondary phases hindered the movement of dislocations necessary for plastic deformation. Although the 7075+3%Si samples exhibited low elongation, the tensile test results did not indicate the intrinsic static strength of this material because microcracks were present before the measurement. If the samples having 3% of silicon content can be processed without microcracks, the ductility will be higher than that of the 7075+5%Si samples, based on the trend of Vickers microhardness. Besides, rupture of the wrought aluminum alloys is considered to progress through the formation of voids within the eutectic phases; thus, a large amount of silicon content appears to provide initiation sites for voids during the fracture process. Hence, it is preferable to reduce the amount of additional silicon within a range such that the SLM processability is maintained. The strength can also be increased by the other alloying constituents (i.e. Cu, Zn, and Mg). Actually, the 7075+9%Si samples exhibited higher 0.2% proof strength and tensile strength than the Al+10%Si alloy containing almost the same amount of silicon [15].
25
This fact indicates that the amounts of additional silicon must be adjusted to achieve both better processability and mechanical properties. As for 7075 alloy, it can be concluded that the 5% of additional silicon is optimal because the 7075+5%Si alloy showed the highest relative density at 100.2%, favorable hardness and strength, and acceptable ductility. Further silicon content induces lower ductility and the risk of sample breakage, but less silicon content cannot inhibit microcracks.
5. Conclusions This research aimed to investigate the effect of additional silicon for 7075 alloy on SLM processabilities, microstructure, and the mechanical properties of fabricated samples. The effects on processability were assessed using the density measurements and microscopic observations on samples fabricated from 7075 with additional silicon powders. Defect behavior clarified by microscopy was discussed with the changes of microstructure. The effects on mechanical properties were evaluated by Vickers microhardness tests and tensile tests. The main conclusions of this paper are as follows; ·
Densification of the SLM samples was promoted by additional silicon. As the amount of additional silicon increased, the maximum relative density of the samples obtained from each material was improved. The highest relative density of 100.2% was achieved by processing the 7075+5%Si alloy under the optimal laser condition (laser output of 195 W and scanning velocity of 1000 mm/s).
·
Additional silicon was effective for inhibiting defects forming during the SLM process. Microscopic observations proved the reduction of voids and cracks with increments of silicon content. This was
26
demonstrated through the elimination of microcracks in the 7075+5%Si sample fabricated under the optimal laser condition. ·
Additional silicon hardened the SLM samples. Values of Vickers microhardness were increased as the silicon content increased. The 7075+16%Si samples exhibited approximately two times higher hardness than the unmodified 7075. Tensile tests revealed that the 0.2% proof strengths of samples were enhanced by additional silicon. However, excessive silicon resulted in brittleness of the alloys, as slight ductility was observed by tensile tests and self-breakage of the 7075+13%Si and 7075+16%Si samples during the SLM process.
·
Interaction of additional silicon with other alloying constituents, including Cu, Zn, and Mg, could not be observed. This indicated that a similar approach might accommodate the other wrought aluminum alloys for the SLM technique. This approach could lead to the production of improved lightweight components by replacing assemblies by integrally manufactured parts with appropriate selection of aluminum alloys. Further research about the appropriate silicon content and laser conditions for other alloys will be expected.
Acknowledgements The authors would like to acknowledge financial supports from Public interest Satomi Scholarship Foundation.
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Author contributions Shinya Sasaki designed the study. The additive manufacturing processes and experiments were performed by Yuki Otani. The manuscript was prepared by Yuki Otani, and critically reviewed by Shinya Sasaki. All authors participated in the discussion and interpretation of the results. All authors have approved the final article.
Data availability statement The raw data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
References [1] Zhang, J.L., Song, B., Wei, Q.S., Bourell, D., Shi, Y.S., A review of selective laser melting of aluminum alloys: Processing, microstructure, property and developing trends, J. Mater. Sci. Technol. 35 (2) (2019) 270-284. https://doi.org/10.1016/j.jmst.2018.09.004 [2] Karg, M.C.H., Ahuja, B., Wiesenmayer, S., Kuryntsev, S.V., Schmidt, M., Effects of Process Conditions on the Mechanical Behavior of Aluminum Wrought Alloy EN AW-2219 (AlCu6Mn) Additively Manufactured by Laser Beam Melting in Powder Bed. Micromachines 8 (1) (2017) 11. https://doi.org/10.3390/mi8010023 [3] Koutny, D., Palousek, D., Pantelejev, L., Hoeller, C., Pichler, R., Tesicky, L., Kaiser, J., Influence of
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Scanning Strategies on Processing of Aluminum Alloy EN AW 2618 Using Selective Laser Melting. Materials 11 (2) (2018) 298. https://doi.org/10.3390/ma11020298 [4] Reschetnik, W., Brüggemann, J.P., Aydinöz, M.E., Grydin, O., Hoyer, K.P., Kullmer, G., Richard, H.A., Fatigue crack growth behavior and mechanical properties of additively processed EN AW-7075 aluminum alloy. Procedia Struct. Integrity 2 (2016) 3040-3048. https://doi.org/10.1016/j.prostr.2016.06.380 [5] Qi, T., Zhu, H.H., Zhang, H., Yin, J., Ke, L.D., Zeng, X.Y., Selective laser melting of Al7050 powder: Melting mode transition and comparison of the characteristics between the keyhole and conduction mode. Mater. Des. 135 (2017) 257-266. https://doi.org/10.1016/j.matdes.2017.09.014 [6] Kaufmann, N., Imran, M., Wischeropp, T.M., Emmelmann, C., Siddique, S., Walther, F., Influence of Process Parameters on the Quality of Aluminum Alloy EN AW 7075 Using Selective Laser Melting (SLM). Physics Procedia 83 (2016) 918-926. https://doi.org/10.1016/j.phpro.2016.08.096 [7] Mauduit, A., Pillot, S., Gransac, H., Study of the suitability of aluminum alloys for additive manufacturing by laser powder bed fusion. U.P.B. Sci. Bull., Series B 79 (4) (2017) 219-238. [8] Martin, J. H., Yahata, B. D., Hundley, J. M., Mayer, J. A., Schaedler, T., Pollock, T. M., 3D printing of high-strength aluminum alloys. Nature 549 (2017) 365-369. https://doi.org/10.1038/nature23894 [9] Schmidtke, K., Palm, F., Hawkins, A., Emmelmann, C., Process and Mechanical Properties: Applicability of a Scandium modified Al-alloy for Laser Additive Manufacturing. Physics Procedia 12 (2011) 369-374. https://doi.org/10.1016/j.phpro.2011.03.047 [10] Sistiaga, M.L.M., Mertens, R., Vrancken, B., Wang, X.B., Hooreweder, B. Van, Kruth, J.P., Humbeeck,
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J. Van, Changing the alloy composition of Al7075 for better processability by selective laser melting. J. Mater. Process. Technol. 238 (2016) 437-445. https://doi.org/10.1016/j.jmatprotec.2016.08.003 [11] Aversa, A., Marchese, G., Manfredi, D., Lorusso, M., Calignano, F., Biamino, S., Lombardi, M., Fino, P., Pavese, M., Laser Powder Bed Fusion of a High Strength Al-Si-Zn-Mg-Cu Alloy. Metals 8 (5) (2018) 300. https://doi.org/10.3390/met8050300 [12] Otani, Y., Kusaki, Y., Itagaki, K., Sasaki, S., Microstructure and Mechanical Properties of 7075 Alloy with Additional Si Fabricated by Selective Laser Melting. Mater. Trans. 60 (10) (2019) 2143-2150. https://doi.org/10.2320/matertrans.Y-M2019837 [13] Sales, R.C., Felipe, P., Paradela, K.G., Garcao, W.J.L., Ferreira, A.F., Effect of Solidification Processing Parameters and Silicon Content on the Dendritic Spacing and Hardness in Hypoeutectic Al-Si Alloys. Mat. Res. 21 (6) (2018) 8. https://doi.org/10.1590/1980-5373-mr-2018-0333 [14] Kou, S., Solidification and liquation cracking issues in welding. JOM. 55 (6) (2003) 37-42. https://doi.org/10.1007/s11837-003-0137-4 [15] Kimura, T., Nakamoto, T., Mizuno, M., Araki, H., Effect of silicon content on densification, mechanical and thermal properties of Al-xSi binary alloys fabricated using selective laser melting. Mater. Sci. Eng., A. 682 (2017) 593-602. https://doi.org/10.1016/j.msea.2016.11.059
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Credit Author Statement Yuki Otani: Methodology, Validation, Investigation, Writing - Original Draft Preparation, Visualization, Shinya Sasaki: Conceptualization, Resources, Writing - Review & Editing, Supervision, Funding acquisition,
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0921-5093(20)30167-2
DOI:
https://doi.org/10.1016/j.msea.2020.139079
Reference:
MSA 139079
To appear in:
Materials Science & Engineering A
Received Date: 5 November 2019 Revised Date:
29 January 2020
Accepted Date: 6 February 2020
Please cite this article as: Y. Otani, S. Sasaki, Effects of the addition of silicon to 7075 aluminum alloy on microstructure, mechanical properties, and selective laser melting processability, Materials Science & Engineering A (2020), doi: https://doi.org/10.1016/j.msea.2020.139079. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Title Effects of the addition of silicon to 7075 aluminum alloy on microstructure, mechanical properties, and Selective Laser Melting processability
Author names and affiliations Yuki Otani1, (Given name) (Family name) 1. Department of Mechanical Engineering, Graduate school of Tokyo University of Science, 6-3-1 Niijuku Katsushika-ku Tokyo, Japan, [email protected] Shinya Sasaki2, (Given name) (Family name) 2. Department of Mechanical Engineering, Faculty of Engineering, Tokyo University of Science, 6-3-1 Niijuku Katsushika-ku Tokyo, Japan, [email protected]
Corresponding Author Yuki Otani, [email protected]
Abstract Aluminum alloys that can be processed using the selective laser melting (SLM) technique have been restricted to cast alloys based on the Al-Si binary system. To increase the selection of materials available for SLM, researches on SLM processability using wrought Al alloys (e.g. 2xxx, 7xxx series) have
1
been conducted. The 7075 Al alloy has excellent mechanical properties among aluminum alloys; however, 7075 alloy has the problem of severe cracking occurring in parts fabricated through the SLM process. Our study demonstrated that additional silicon enables the fabrication of 7075 SLM parts without major defects and that the amount of silicon content changes the favorable processing conditions, morphology of microstructure, and mechanical properties. As the silicon content increased, voids and cracking in SLM samples were suppressed, and volumetric energy density for sufficient densification was reduced. Vickers microhardness and 0.2% proof strength were enhanced with increments of additional silicon content. In contrast, excessive silicon content resulted in brittleness, which appeared as slight ductility measured on tensile testing and breakage of samples during SLM process. These conflicting effects indicated that silicon content must be adjusted depending on the underlying alloy to yield better processability and a balance between strength and ductility. For the 7075 alloy, the optimal silicon content could be concluded to be 5%, being the lowest amount of silicon content needed for the elimination of cracking with enhanced tensile strength and acceptable ductility.
Keywords Additive manufacturing Selective laser melting Aluminum Processability
2
Microstructure Mechanical properties
1. Introduction Selective Laser Melting (SLM) is a beam-based additive manufacturing technique that enables the fabrication of complex shaped metallic parts with higher dimensional accuracy than other additive manufacturing techniques. This advantage of SLM could realize the production of improved lightweight components by combining topology optimization with aluminum alloys. In general, the aluminum alloys that can be soundly processed using SLM has been restricted to the alloys based on Al-Si binary system [1] (e.g., AlSi7Mg0.6, AlSi9Mg0.3, AlSi10Mg, and AlSi12). These alloys are classified for cast materials which are designed for manufacturing through a melt and solidification process; thus, the formation of structural defects via their solidification hardly occurs. By contrast, their inferior specific strength results in moderate mechanical properties, which cannot satisfy the required performance for lightweight parts. Wrought aluminum alloys (e.g., 2xxx or 7xxx series) in particular are expected to be processed using SLM because their enhanced specific strength can provide exceptional lightweight components for automotive and aerospace industries. However, because wrought aluminum alloys are designed for cold-forming processes such as rolling and extrusion, these alloys form structural defects easily. Previous studies reported the occurrence of severe cracking in various wrought aluminum alloys and proposed special treatments to suppress the cracking. Karg et al., demonstrated the fabrication of 2219 alloy samples without cracks by
3
preheating to 200 ˚C and adding support structures between the samples and baseplate [2]. In their study, cracking was prevented by ensuring slower cooling rate during the SLM process due by reducing the heat transfer from the parts to the baseplate. However, they also reported that the porosity ratio and tensile property of the parts were highly sensitivity to the fabrication geometry. The samples having larger cross-sectional areas of the baseplate surface exhibited 5% porosity ratio, resulting in lower tensile properties. Koutny et al., explored the influence of the SLM process parameters (e.g. laser output, scanning velocity, scan strategy, and platform heating) on the relative density and mechanical properties of 2618 alloy [3]. They found extensive cracking, which was considered to be solidification cracks created by the stress due to high temperature differences between the solid and liquid phases during solidification. Reducing the thermal gradient by using a support structure showed the favorable effect of suppressing the cracks. In contrast, platform heating up to 400 ˚C and lower laser speed could not improve the quality of the samples, and inducing gas porosity. Reschetnik et al., reported the low mechanical properties of 7075 alloy parts fabricated by the SLM technique [4]. The reason of reduced mechanical properties is the occurrence of cracking, which appeared to be solidification cracks. The authors proposed changing the process parameters (i.e., laser output, scan distance, and scanning velocity) and the post-heat treatment for improving the mechanical properties, but this has not been realized in practice. Although these approaches have the definite effect of suppressing the cracks, it is necessary to study a method that can stably eliminate cracks without restricting the fabrication geometry. The 7075 alloy is one of the 7xxx series wrought aluminum alloys and has excellent static strength
4
among commercial aluminum alloys. Several researchers have investigated the influence of changes in processing conditions (e.g., laser parameters, scanning strategy, baseplate preheating) on the formation of defects in SLM parts using 7075 or analogous alloy. Qi et al. [5] have studied the relationships between melting mode transition and crack density using samples of 7050 alloy. According to their study, melt pool shapes could be altered using the scanning velocity and defocusing distance, and the shapes could be divided into three types: goblet, semicircle, and a combination of the two shapes. These are similar to keyhole, conduction, and transition modes in laser welding. The authors found that the keyhole melting mode results in the lowest crack density for a fabricated sample, but could not achieve the complete elimination of microcracks. Kaufmann et al. [6] have conducted the SLM processing of 7075 alloy under baseplate preheating at 200 ˚C, but significant improvement has not been achieved. Besides, previous studies have revealed that large inputs of heat owing to changing laser parameters and thermal conditions cause several drawbacks. The most harmful of these problems is the evaporation of alloying constituents caused by the high temperatures of molten metals. Mauduit et al. [7] observed the difference in chemical compositions of 7075 alloy before and after SLM processing, and reported a loss of 30% zinc and 19% magnesium content. Loss of Zn and Mg elements could decrease the mechanical properties of the 7075 alloy because these alloying constituents provide the effects of solution hardening and precipitation hardening by forming MgZn2 phase. Another approach that has been proposed is the addition of zirconium or scandium, which act as dispersoid forming elements for aluminum alloys. Martin et al. [8] have applied hydrogen stabilized
5
zirconium nanoparticles for 7075 alloy powder. Zirconium nanoparticles form dispersed Al3Zr phases which provide nucleation sites for 7075 alloy during solidification, resulting in the formation of finely equiaxed grains. This is believed to suppress the microcracks by reducing the entrapped liquid phases between the grains and alleviating thermal stress due to thermal shrinkage. Scalmalloy®, which is the Al-Mg alloy of approximately 0.7% scandium and 0.4% zirconium, has been already used for aircraft components as the only high strength aluminum alloy processable by SLM. This material realized the processing of SLM parts made from high strength aluminum alloys without microcracks [9]. The peculiar precipitation hardening of the Al3Sc particles yields higher mechanical properties without the addition of conventional precipitation hardening elements that lead cause crack sensitivity. However, there are concerns for productivity of these material powders and scanty of metal resource because the scandium is one of the rare-earth elements. Silicon is a prevalent and inexpensive alloying constituent for aluminum alloy, and it prevents microcracks in 7075 alloy parts fabricated through SLM. Sistiaga et al. [10] have revealed that the use of a powder created by mixing 7075 powders and 4% silicon particles eliminates microcracks in samples. Aversa et al. [11] have succeeded in processing crack-free SLM samples from powder which was prepared by mixing 50% AlSi10Mg and 50% 7075 powder. However, the use of powder mixtures could cause inhomogeneous distributions of the alloying element, leading the inhomogeneous mechanical properties within fabricated parts. In our previous study, we investigated the microstructures and mechanical properties of SLM samples using pre-alloyed powders of 7075 alloy and 7075 with 5% of additional silicon alloy [12]. These microstructures were shown to be finely uniformed, and the tensile properties using 7075 with 5%
6
additional silicon alloy were higher than those of the conventional AlSi10Mg alloy. This alloy system is expected to be applied to the fabrication of lightweight components via SLM; hence, more detailed measurements about the effect of additional silicon content are required. For this paper, we studied the effect of silicon content for SLM processabilities and the properties of fabricated samples. Pre-alloyed powders of 7075 with different amounts of additional silicon were used for SLM process. The effects of silicon content on the appropriate processing conditions were investigated, and the microstructures and mechanical properties of fabricated samples were compared.
2. Materials and Methods The ProX DMP 200 (3DSystems, USA) machine was employed to fabricate test samples. The material powders were pre-alloyed gas-atomized powders supplied by TOYO Aluminium KK. These powders are single alloy powders, not the mixture of 7075 alloy powder and Si particles. Tables 1 and 2 show the chemical compositions of material powders and their particle size distributions. The 7075 powder contains 0.05% silicon due to contamination during the powder manufacturing processes. Each of the modified 7075 powders contain different amounts of silicon ranging from 0.5 to 16 percent. To clarify the effect from the additional silicon, the concentrations of the other alloying constituents, Cu, Zn, and Mg, were kept constant. The powders were sieved to maintain identical particle size distributions.
Table 1 Chemical composition of material powders characterized by the powder supplier.
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Material
Element [mass%] Si
Cu
Zn
Mg
Fe
Cr
Mn
Ni
Ti
Al
7075
0.05
1.61
6.03
2.58
0.10
0.22
<0.01
<0.01
<0.01
Bal.
7075+0.5%Si
0.54
1.58
5.34
2.52
0.12
0.24
<0.01
0.01
0.01
Bal.
7075+1%Si
0.93
1.58
5.44
2.57
0.12
0.24
<0.01
0.01
0.01
Bal.
7075+2%Si
2.05
1.57
5.39
2.48
0.13
0.24
<0.01
0.01
0.01
Bal.
7075+3%Si
3.27
1.56
5.30
2.49
0.13
0.24
<0.01
0.01
0.01
Bal.
7075+4%Si
4.05
1.52
4.99
2.44
0.15
0.24
<0.01
0.01
0.01
Bal.
7075+5%Si
5.04
1.55
5.02
2.44
0.11
0.21
<0.01
0.01
<0.01
Bal.
7075+6%Si
6.07
1.49
4.96
2.35
0.16
0.23
<0.01
0.01
0.01
Bal.
7075+7%Si
7.05
1.73
4.90
2.40
0.16
0.22
<0.01
0.01
0.01
Bal.
7075+9%Si
9.20
1.42
5.07
2.35
0.15
0.20
<0.01
0.01
0.01
Bal.
7075+13%Si
12.98
1.36
4.84
2.31
0.16
0.18
<0.01
0.01
0.01
Bal.
7075+16%Si
16.39
1.30
4.69
2.15
0.18
0.17
<0.01
0.01
0.01
Bal.
Table 2 Particle size distribution of material powders characterized by the powder supplier. D10, D50, and D90 show the diameter for which 10%, 50%, and 90% of the particles are smaller than that when the particles are arranged from smallest to largest. D10 [µm]
D50 [µm]
D90 [µm]
7075
6.6
20.8
48.1
7075+0.5%Si
6.8
21.2
45.6
7075+1%Si
7.3
21.8
46.3
7075+2%Si
7.0
21.0
44.1
7075+3%Si
7.1
20.5
42.2
7075+4%Si
6.7
21.0
48.5
7075+5%Si
6.3
18.5
41.6
7075+6%Si
6.8
21.3
48.4
7075+7%Si
6.8
21.4
49.0
7075+9%Si
6.8
19.9
42.5
7075+13%Si
7.1
20.8
43.9
7075+16%Si
7.1
20.2
41.4
Material
8
Cuboid SLM samples (15 mm × 15 mm × 10 mm) were fabricated using the processing parameters shown in Table 3. To assess the effects of laser conditions on the densification of the samples, thirty samples were fabricated from each material powder using different laser parameters, pairing six laser outputs with five scanning velocities. The optimal laser condition for providing the highest relative density to the fabricated samples was determined by density measurements based on the Archimedes principle. The laser output and scanning velocity were varied in the ranges of 169–234 W and 800–1600 mm/s, respectively. The layer thickness and scan distance were held at 30 µm and 100 µm. The build chamber was continuously purged with argon gas flow until the oxygen concentration was maintained at 0.1 % or less. An island scan strategy was employed for sample fabrication. This strategy divides the laser radiation region in each layer into several hexagonal areas with 10 mm of circumscribed diameter; thereafter, these regions were irradiated individually by back-and-forth laser scanning. SLM processing was conducted at room temperature; thermal condition controls including pre-heating the baseplate were not done.
Table 3 Ranges of studied processing parameters. Laser output [W]
169, 182, 195, 208, 221, 234
Scanning velocity [mm/s]
800, 1000, 1200, 1400, 1600
The structural defects and microstructures of fabricated samples were evaluated by microscopic observations using a Confocal Laser Microscope VK-X150 (KEYENCE, Japan) and a Scanning Electron Microscope TM3030Plus (Hitachi High-Technologies, Japan). Observations were carried out with the plane 9
being perpendicular to the baseplate surface. Cross-sections of the cuboid samples were finished to a mirror-surface by chemical mechanical polishing. These microstructures were revealed by etching for 2 min durations with an etching agent made by dissolving 2 g NaOH and 5 g NaF in 93 ml H2O. The differences in mechanical properties between materials were evaluated by Vickers microhardness tests. A Micro Vickers Hardness Tester HMV-G-FA-D (Shimadzu, Japan) was employed for hardness measurements. The indenter was forced to the mirror-polished sample surface at a load of 0.98 N and was held for 10 s. To minimize disturbance from defects, the measurements were repeated 25 times per sample and the top 10 values were compared. Tensile properties were evaluated by tensile testing with a Universal Tensile Testing Machine AG-Xplus (Shimadzu, Japan). Bar-shaped SLM samples were machined to tensile specimens with dimensions conforming to the ASTM E8 standard. Experiments were performed at room temperature and were repeated three times for each material.
3. Results 3.1 Sample fabrication and density measurements Figure 1 shows images of the powder particles. Significant differences in powder particle morphology between the material powders were not observed. Figure 2 shows the relationship between additional silicon content and the range between the maximum and minimum relative densities of the samples created using thirty different laser conditions. These relative densities were calculated by dividing the measured density by the theoretical density, which was obtained by multiplying the density and
10
proportion of each element in the chemical composition of the material powders. The unmodified 7075 sample exhibited approximately 96.8% of maximum relative density, and its value was scattered due to the penetration of water into cracks during density measurements. Maximum relative density decreased once when the 0.5% silicon was added, but thereafter it started gradually increasing with the further addition of silicon. From 2% to 5% additional silicon content, maximum relative density increased abruptly until reaching 100.2%. It did not reduce below 99.9% until the amount of additional silicon increased to 9%. However, at 13% and 16%, the samples collapsed during the SLM process as shown in Figure 3; thus, the density measurements could not be performed for these cases.
Figure 1 Morphology of powder particles: (a) unmodified 7075 (b) 7075+5%Si (c) 7075+16%Si observed using SEM.
11
Figure 2 Range between maximum and minimum relative densities of samples fabricated from 7075 with different amounts of additional silicon powder.
Figure 3 Appearance of samples fabricated from (a) 7075+13%Si and (b) 7075+16%Si powders.
Figures 4 and 5 show the effects of scanning velocity and laser output on density for 7075+5%Si samples. In these figures, the values when the laser output was set to 195 W or the scanning velocity was set to 1000 mm/s are plotted as square symbols, and other values are plotted as circles. The relative density was varied in the range of 94.4% to 100.2%, by changing the laser conditions. Regardless of laser output, a scanning velocity of 1000 mm/s provides enough density for the samples. The highest relative density, 100.2%, was obtained by using a combination of 195 W laser output and 1000 mm/s scanning velocity. 12
Figure 2 shows the relative densities of the samples processed under a laser output of 195 W and a scanning velocity of 1000 mm/s as a function of silicon content. All the compositions exhibited favorable relative densities under this laser condition; therefore, the combination of 195 W laser output and 1000 mm/s scanning velocity was defined as the optimal laser condition.
Relative density ％
100
98
96 Laser output 195W Other laser output
94
800 1000 1200 1400 1600 Scanning velocity [mm/s]
Figure 4 Correlations between relative density of the samples and laser conditions: as a function of scanning velocity.
13
Relative density ％
100
98
96 Scanning velocity 1000mm/s Other scanning velocity
94
180 200 220 Laser output [W]
240
Figure 5 Correlations between relative density of the samples and laser conditions: as a function of laser output.
3.2 Microscopic observations Figure 6 shows microscopic images of the samples at vertical cross-sections. Observations were carried out on the samples processed by the optimal laser condition in order to clarify the effect of additional silicon content. The figure illustrates the presence of defects including voids, macrocracks, and microcracks. No obvious precipitates or inhomogeneous structures were observed, but periodic traces of laser scanning were observed which were created by the difference of microstructure between the center and boundary regions of the trace. The voids and cracks were progressively suppressed as the amount of additional silicon increased. When the silicon content was lower than 2%, there were severe microcracks and macrocracks propagating through coarse defects. However, in the case of 4% silicon content, the sample contained minor microcracks and pores with a diameter of several tens of micrometers distributed on the boundary of trace. 14
When the silicon content reached 5%, microcracks were completely suppressed. The sample with 16% additional silicon also did not show major defects despite self-breakage during the SLM process.
Figure 6 Laser microscopic images for vertical cross-section of the samples processed under the optimal laser condition (195 W laser output and 1000 mm/s scanning velocity). (a) unmodified 7075 (b) 7075+2%Si (c) 7075+4%Si (d) 7075+5%Si (e) 7075+6%Si, and (f) 7075+16%Si.
Figure 7 shows SEM images on vertical cross-sections of the samples processed under the optimal laser condition. The microstructures consisted of bright and dark regions, and these morphologies were altered with additional silicon content. In the case of unmodified 7075, the morphology of microstructure was granular. As the silicon content increased, the microstructure was changed to a net-like shape; thus, the samples with 2 to 6% of additional silicon exhibited cellular-dendritic microstructure that elongated along a
15
building direction and connected at corresponding intersection areas. Regarding the sample with 16% of additional silicon, the microstructure was changed to a columnar-dendritic shape containing crystallized silicon particles. The significant alternations of the microstructure are attributed to the silicon content exceeding the solid solution limit and eutectic composition. The maximum solid solution limit of silicon in aluminum is approximately 1.6% at 577 ˚C; thus, the change in morphology between 0% and 2% of silicon content is due to increase in the volume of silicon that cannot be embedded in the aluminum matrix (the dark regions of the image). Similarly, silicon content in excess over the eutectic composition of the Al-Si binary system (Al-12%Si) cause a difference in the microstructure between the samples having 6% and 16% of silicon content. The 7075+16%Si alloy contains a sufficient amount of silicon to form a hypereutectic alloy for the Al-Si binary system. During the solidification process, the primary silicon phase appears first, and then solidification progresses subsequently.
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Figure 7 SEM images for vertical cross-section of the samples processed under the optimal laser condition (195 W laser output and 1000 mm/s scanning velocity). (a) unmodified 7075 (b) 7075+2%Si (c) 7075+4%Si (d) 7075+5%Si (e) 7075+6%Si, and (f) 7075+16%Si.
3.3 Measurements of Vickers microhardness and tensile properties Figure 8 shows the result of Vickers microhardness tests for the samples fabricated under the optimal laser condition. The Vickers microhardness grew proportionally as the amount of additional silicon increased. The unmodified 7075 samples exhibited 125HV0.1 of Vickers microhardness, and it was enhanced to 157HV0.1 in the case of 5% additional silicon. The maximum value, 218HV0.1, was measured when the additional silicon content was 16%. This is approximately double the hardness of the unmodified
Vickers microhardness HV0.1
7075.
220 200 180 160 140 120 100
0 2 4 6 8 10 12 14 16 Silicon content mass％
Figure 8 Vickers microhardness of the samples fabricated under the optimal laser condition (195 W laser
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output and 1000 mm/s scanning velocity) as a function of silicon content.
Figure 9 depicts the tensile testing result for 0.2% proof strength, tensile strength, and elongation at break. The samples were fabricated using 3%, 5%, and 9% of additional silicon alloys, and the load was applied to be parallel to the baseplate surface. The result of using samples with 5% additional silicon and the laser condition for the fabrication of these samples are quoted from our previous paper [12]. The samples with 3% of additional silicon exhibited approximately 147 MPa of tensile strength and 0.4% of elongation. In this case, a 0.2% proof strength could not be obtained since the tensile specimens were broken before starting plastic deformation; furthermore, values of the elongation at break were obtained from one experiment because the range where it could be measured normally was not attained in two tests out of three. The samples with 5% of additional silicon showed 360 MPa of 0.2% proof strength, 537 MPa of tensile strength, and 9.7% of elongation, and for the samples with 9% of additional silicon, these were 402 MPa, 478 MPa, and 1.0%, respectively.
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Figure 9 Tensile property of the samples fabricated from 7075+3%Si, 5%Si, and 9%Si powder.
4. Discussion 4.1 Effect of additional silicon to densification of SLM samples The relationship between laser conditions and the densification of samples is usually interpreted using volumetric energy density (VED). As formulated below, VED expresses amounts of input heat against the per unit volume of material powder by laser irradiation. In this formula, P, v, d, and s represent laser output, scanning velocity, layer thickness, and scan distance, respectively. = Figure 10 shows the relative density of the samples with 5% of additional silicon as a function of VED. The typical convex upward curve was obtained. The samples with insufficient VED exhibit lower density due to a lack of fusion. The samples with excessive VED suffer from gas pores, which were formed
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by trapping atmosphere as a result of the aggressive convection of molten metal, and keyhole porosity occurred from the evaporation of alloying elements having high vapor pressure. It was confirmed that the VED of the optimal laser condition (195 W laser output and 1000 mm/s scanning velocity), 65 J/mm3, corresponded to the global maximum of the curve. This VED is lower than that of the previous studies using 7075 with additional silicon. In the previous research using 7075 powder with silicon particles, the authors have reported that a laser condition containing 83.3 J/mm3 of VED (300 W of laser output and 1200 mm/s of scanning velocity) provides the highest density regardless of amount of additional silicon [10]. In a study using powder mixtures of 50% 7075 and 50% AlSi10Mg, the laser condition having 72.2 J/mm3 of VED (P: 195 W, v: 600 mm/s, d: 0.03 mm, s: 0.15 mm), was determined as the optimized condition [11]. One possible cause of this difference is the difference between powder mixtures and pre-alloyed powders. Powder mixtures must be completely melted and mixed together during laser irradiations and the subsequent liquid state. This process needs a long duration of melt; therefore, surplus VED may have been required for fabrication using powder mixture. The reduction of VED could prevent loss of the alloying element and reduce undesirable precipitations resulting from cyclic thermal input.
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Relative density ％
100 99 98 97
Optimal laser condition Other laser conditions
96
40 60 80 100 3 Volumetric energy density J/mm
Figure 10 Relative density of the 7075+5%Si samples as a function of VED.
The VED for sufficient densification of samples was affected by additional silicon content. Figure 11 shows the VED of the laser condition that gives the highest relative density for each material. A larger silicon content reduced the necessary VED for sufficient densification. Moreover, the minimum relative density was improved by including additional silicon as shown in Fig. 2. This indicates that the effects of additional silicon prevented the lack of fusion. This change could be explained in terms of the solidification speed. Additional silicon leads to a decrease of solidification speed because silicon releases the four times the latent heat of solidification relative to aluminum. Sales et al. [13] have demonstrated that increasing silicon content from 3% to 5% decreases the solidification speed and cooling rate of Al-Si alloys using directional solidification experiments. This behavior could offer additional time for entrapped bubbles to escape from molten metal before solidification. The changing of microstructure shown in Fig. 7 corresponded to this behavior. The morphology of the microstructure is determined by two factors: additional silicon content and ratio of thermal gradient (G) and solidification speed (R). When the silicon
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content is between 2% to 6% (i.e. hypo-eutectic), the changes in the thermal kinetics would affect the microstructure. Granular, non-directional orientation is associated with a low G/R ratio, but columnar, directional orientation is correlated with a high G/R. Here, the changes of microstructure from granular to columnar may reflect the slower solidification speed, and could be one mechanism of porosity prevention by
Volumetric energy density J/mm3
the addition of silicon.
80 75 70 65 60 55 50
0 1 2 3 4 5 6 7 8 9 Silicon content mass％
Figure 11 Effects of silicon content on the VED of the laser condition that provides the highest relative density.
4.2 Effect of additional silicon to cracking and mechanical properties As shown in Fig. 6, the samples with less than 2% of additional silicon contained macrocracks which propagated through several layers. This arrangement means that the macrocracks happened during the progress of SLM fabrication, rather than immediately after laser irradiation. Therefore, the occurrence of
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macrocracks depends on the intensity of metal expansion and contraction. One conceivable effect of additional silicon for inhibiting macrocracks is the reduction of thermal expansion. Nonmetallic elements including silicon have much lower coefficients of thermal expansion than metals; thus, adding silicon could reduce the changes of volume due to temperature change. This reduction leads to a reduction of the driving forces which promote cracking. Microcracks appeared in the samples with additional silicon of 4% or less. Microcracks that occur on wrought aluminum alloys are commonly attributed to solidification cracking and liquation cracking. The cause of solidification cracking is residuary thin-filmed liquid phase between primary crystallized grains. This isolated liquid hardly resists the thermal stress generated from thermal expansion during solidification, and eventually opens and forms cracks. Liquation cracking is caused by cyclic heat inputs as seen in multi-layer welding and layered fabrication. The heat inputs cause a reduction of strength mainly through the partial re-melting of eutectic phase or of compositions with lower melting points, and these weakened points work as starting points of cracking. To distinguish the microcracks occurred in the samples, microscopic observations at the top part of vertical sections were carried out. The surface layer is not affected by conduction heat from subsequent layers of fabrication; therefore, the solidification structure that is created by single laser irradiation has been preserved. The result of microscopic observations is shown in Fig. 12. Few microcracks appeared within the uppermost traces, but these subjacent layer traces contained microcracks. This positional relation indicates that the occurrences of severe cracking were promoted by cyclic heat input. Since precipitates or segregations, which might cause re-melting were not observed, the
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severe microcracks seems to be motivated by the loss of strength due to high temperature. Kou [14] has reviewed the influence factors and remedies on the solidification cracking and liquation cracking appearing in welding. The author mentioned that the susceptibility of liquation cracks was affected by amount of heat input, tendency of weld-metal contraction, and degrees of restraint. As noted in section 4.1, additional silicon could reduce the VED for sufficient densification of samples, and suppress the thermal expansion of alloys; hence, it seems to contribute to the elimination of microcracks.
Figure 12 Laser microscopic images for the surface layer of fabricated samples. (a) unmodified 7075 (b) 7075+2%Si (c) 7075+4%Si (d) 7075+5%Si (e) 7075+6%Si, and (f) 7075+16%Si.
The Vickers microhardness and tensile properties were improved with the increase of silicon content. This tendency agrees with the case of Al-Si binary alloy reported in the previous research. Kimura
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et al. [15] have investigated the relationships between silicon content and mechanical properties of Al-Si binary alloy samples fabricated by SLM method. They noted that the additions of silicon enhance the proof stress and ultimate tensile strength of SLM samples because the silicon elements enrich the proportion of the secondary phases, providing compositional reinforcement for the aluminum matrix. It is likely that a similar function improved the strength of the 7075 with additional silicon samples. However, this effect also emerged as brittleness of the alloy. As shown in Fig. 3, additional silicon of 13% or above resulted in failure of SLM processing due to self-breakage of the samples. The result of tensile testing for 7075+9%Si samples also showed especially lower ductility. These results imply that secondary phases hindered the movement of dislocations necessary for plastic deformation. Although the 7075+3%Si samples exhibited low elongation, the tensile test results did not indicate the intrinsic static strength of this material because microcracks were present before the measurement. If the samples having 3% of silicon content can be processed without microcracks, the ductility will be higher than that of the 7075+5%Si samples, based on the trend of Vickers microhardness. Besides, rupture of the wrought aluminum alloys is considered to progress through the formation of voids within the eutectic phases; thus, a large amount of silicon content appears to provide initiation sites for voids during the fracture process. Hence, it is preferable to reduce the amount of additional silicon within a range such that the SLM processability is maintained. The strength can also be increased by the other alloying constituents (i.e. Cu, Zn, and Mg). Actually, the 7075+9%Si samples exhibited higher 0.2% proof strength and tensile strength than the Al+10%Si alloy containing almost the same amount of silicon [15].
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This fact indicates that the amounts of additional silicon must be adjusted to achieve both better processability and mechanical properties. As for 7075 alloy, it can be concluded that the 5% of additional silicon is optimal because the 7075+5%Si alloy showed the highest relative density at 100.2%, favorable hardness and strength, and acceptable ductility. Further silicon content induces lower ductility and the risk of sample breakage, but less silicon content cannot inhibit microcracks.
5. Conclusions This research aimed to investigate the effect of additional silicon for 7075 alloy on SLM processabilities, microstructure, and the mechanical properties of fabricated samples. The effects on processability were assessed using the density measurements and microscopic observations on samples fabricated from 7075 with additional silicon powders. Defect behavior clarified by microscopy was discussed with the changes of microstructure. The effects on mechanical properties were evaluated by Vickers microhardness tests and tensile tests. The main conclusions of this paper are as follows; ·
Densification of the SLM samples was promoted by additional silicon. As the amount of additional silicon increased, the maximum relative density of the samples obtained from each material was improved. The highest relative density of 100.2% was achieved by processing the 7075+5%Si alloy under the optimal laser condition (laser output of 195 W and scanning velocity of 1000 mm/s).
·
Additional silicon was effective for inhibiting defects forming during the SLM process. Microscopic observations proved the reduction of voids and cracks with increments of silicon content. This was
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demonstrated through the elimination of microcracks in the 7075+5%Si sample fabricated under the optimal laser condition. ·
Additional silicon hardened the SLM samples. Values of Vickers microhardness were increased as the silicon content increased. The 7075+16%Si samples exhibited approximately two times higher hardness than the unmodified 7075. Tensile tests revealed that the 0.2% proof strengths of samples were enhanced by additional silicon. However, excessive silicon resulted in brittleness of the alloys, as slight ductility was observed by tensile tests and self-breakage of the 7075+13%Si and 7075+16%Si samples during the SLM process.
·
Interaction of additional silicon with other alloying constituents, including Cu, Zn, and Mg, could not be observed. This indicated that a similar approach might accommodate the other wrought aluminum alloys for the SLM technique. This approach could lead to the production of improved lightweight components by replacing assemblies by integrally manufactured parts with appropriate selection of aluminum alloys. Further research about the appropriate silicon content and laser conditions for other alloys will be expected.
Acknowledgements The authors would like to acknowledge financial supports from Public interest Satomi Scholarship Foundation.
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Author contributions Shinya Sasaki designed the study. The additive manufacturing processes and experiments were performed by Yuki Otani. The manuscript was prepared by Yuki Otani, and critically reviewed by Shinya Sasaki. All authors participated in the discussion and interpretation of the results. All authors have approved the final article.
Data availability statement The raw data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
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Credit Author Statement Yuki Otani: Methodology, Validation, Investigation, Writing - Original Draft Preparation, Visualization, Shinya Sasaki: Conceptualization, Resources, Writing - Review & Editing, Supervision, Funding acquisition,
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: