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Laser solid forming additive manufacturing TiB2 reinforced 2024Al composite: Microstructure and mechanical properties
Author’s Accepted Manuscript Laser solid forming additive manufacturing TiB2 reinforced 2024Al composite: microstructure and mechanical properties Xiaoli Wen, Qingzheng Wang, Qiang Mu, Nan Kang, Shang Sui, Haiou Yang, Xin Lin, Weidong Huang www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(18)31753-2 https://doi.org/10.1016/j.msea.2018.12.072 MSA37344
To appear in: Materials Science & Engineering A Received date: 16 November 2018 Revised date: 15 December 2018 Accepted date: 15 December 2018 Cite this article as: Xiaoli Wen, Qingzheng Wang, Qiang Mu, Nan Kang, Shang Sui, Haiou Yang, Xin Lin and Weidong Huang, Laser solid forming additive manufacturing TiB2 reinforced 2024Al composite: microstructure and mechanical properties, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2018.12.072 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.
Laser solid forming additive manufacturing TiB2 reinforced 2024Al composite: microstructure and mechanical properties
Xiaoli Wena,b, Qingzheng Wanga,b, Qiang Mua,b, Nan Kanga,b, Shang Suia,b, Haiou Yanga,b, Xin Lina,b*, Weidong Huanga,b
a
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University,
127 Youyixilu, Xi’an, Shaanxi 710072, PR China b
Key Laboratory of Metal High Performance Additive Manufacturing and Innovative Design,
MIIT China, Northwestern Polytechnical University, 127 Youyixilu, Xi’an,Shaanxi 710072, PR China
*
Corresponding author at: State Key Laboratory of Solidification Processing, Northwestern
Polytechnical University, 127 Youyixilu, Xi’an, Shaanxi 710072, PR China. [email protected] (X. Lin).
Abstract
The 2024Al alloy and 3%TiB2 reinforced 2024Al composite were fabricated using laser solid forming (LSF). Different from the LSF processed 2024Al with a large columnar grain and apparent preferential growth orientation, TiB2 reinforced 2024Al composite presents texture-less structure, consisting of the dendrite and fine equiaxed structures. Some TiB2 particles distribute within the Al matrix, others distribute along the grain boundaries and intertwine with the Al2Cu
phase. The incoporation of TiB2 gives rise to significant grain refinement, which is one of the important reasons for the improvement of mechanical properties. Moreover, the TiB2 reinforced 2024Al sample exhibits 284MPa high tensile strength, 163MPa yield strength, 108.5HV microhardness, and 18.7% excellent elongation.
Keywords: additive manufacturing; laser solid forming; TiB2 reinforced; 2024Al composite; microstructures; mechanical properties
1. Introduction
Metal-based additive manufacturing (three-dimensional (3D) printing) is a potentially disruptive technology, which can build up metallic components layer by layer. It provides a free design and a flexible manufacture[1]. Laser solid forming (LSF) is a typical laser direct energy deposition additive manufacturing technologies based on the powder feeding method with high deposition rate[2], which is a near-net-shape method and can fabricate highly complex geometric components with high performance in free-form forming. Due to its significant advantages in solidification process, LSF has become the frontier manufacturing and repairing techniques across multiple industries, including aerospace, automotive, biomedical, energy and power fields[3-5]. Aluminum matrix composites (AMCs) have attracted numerous attentions and been widely used in many fields of electronics, aerospace, military, automotive and transportation industries because of their excellent properties, such as improved specific strength, high specific stiffness, enhanced wear resistance and creep resistance, low density and good thermal stability[6-8], In recent years, the reports on laser additive manufactured aluminum alloy mainly focuse on AlSi10Mg and Al-12Si alloys, which are fabricated by selective laser melting(SLM) and reveals a
higher mechanical properties than those of fabricated by conventional methods[9, 10]. Whilst, other aluminum alloys are difficult to be fabricated due to the poor flowability and high laser reflectivity of their powders. They are also prone to producing large columnar grain during solidification process, which degenerate the alloy properties due to the produced periodic cracks. Nano or micro-particles decorated aluminum matrix composite is an effective way to suppress crack and improve properties by transforming columnar grain into equiaxed grain. A few reinforcement particles such as SiC, Al3Ti, Al2O3, TiB2 and ZrB2 have been incorporated in AMCs[11, 12]. P. Wang et al [13] fabricated TiB2 particle reinforced Al-3.5Cu-1.5Mg-1Si (wt.%) composite by SLM, and investigated the effects of the TiB2 particle on the microstructures and mechanical properties. Their result reveals that the uniform distribution of TiB2 particle represents a suitable reinforcement phase, which results in a grain refinement. The compressive yield strength of the TiB2/Al-Cu-Mg-Si composite was significantly improved by the grain refinement. J. H. Martin et al[1] assembled some particles onto 7075 and 6061 series aluminum alloy powder. These high-strength aluminum alloys were fabricated successfully by SLM. Crack-free and equiaxed structures were achieved. Among these samples, stock Al7075 almost retains no strength owing to the large hot tearing cracks. As a comparison, the Al7075+Zr sample acquired fine equiaxed and crack-free microstructure, and exhibited higher tensile strength (383-417MPa), yield strength (325-373MPa) and elongation (3.8-5.4%) because Al3Zr particles acted as grain refiner. X. P. Li et al[14] fabricated the nano-TiB2 decorated AlSi10Mg alloy by SLM. The material appears textureless microstructure, consisting of fine grains and cells, and showed a very high tensile strength ~530 MPa, excellent ductility ~15.5%. The nano-TiB2 particles were found at the grain and cell boundaries and inhibited the growth of grain. Therefore, these investigations indicate that
particle reinforced aluminum matrix composite synthesized by SLM can be an effective way to improve the mechanical properties of aluminum alloys. Among these particles, it has been demonstrated that TiB2 has excellent compatibility with aluminum alloys due to its good wettability with Al alloys and its excellent properties such as high hardness, high stiffness and melting point, superior wear resistance and considerable chemical stability [15, 16]. 2024Al alloy is a type of high strength, low density and heat treatable aluminum alloy, which is one of the most-used alloys in aerospace field. In recent years, it is difficult for the conventional manufacturing methods to meet the requirements of special structures in many practical applications. So far, a few works have been focused on laser additive manufacturing of 2024Al alloy [13, 17, 18]. The majority of them concentrate on the SLM technology. However, SLM has an obvious limitation in producing large-sized components. In contrast, LSF shows significant advantages in the free-design and flexible manufacture of metallic component in large scale with high production efficiency. In this work, the 2024Al alloy and TiB2 nano-particles reinforced 2024Al matrix composites are fabricated by LSF separately. The morphologies and distributions of the TiB2 particles are analyzed by XRD, SEM and EPMA. The influence of the TiB2 particles on the microstructures and mechanical properties of the 2024Al matrix composites are also investigated systematically.
2. Experimental Procedures
2024 aluminum alloy powder was prepared by gas atomization process. The scanning electron microscope (SEM) image of the powder is shown in Fig. 1(a). The powder has a spherical shape and a size range of 40–80 μm. The chemical composition of the alloy powder was measured by ICP-AES (inductively coupled plasma atomic emission spectrometry), and listed in Table 1.
The TiB2 powders were synthesized by high temperature solid state reaction method and the particle size is about 500nm (HAOXI Research Nanomaterials, Inc.). The 3%TiB2 reinforced 2024Al composite powders were mechanically milled in a ball mill at a 380 rpm rotation speed for 3h. The aim of the ball milling process is to disperse the TiB2 particles that can be adhered to the surface of the 2024Al particles as shown in Fig. 1(b and c), and keep a high flowability. Then these powders were dried in the vacuum oven at 120 °C for 1.5 hours.
Fig. 1 (a) SEM morphological image of 2024Al powder. (b) The morphology of nano-TiB2 decorated 2024Al powder. (c) Magnified image of nano-TiB2 decorated 2024Al powder.
Table 1 Chemical composition of 2024Al alloy (mass fraction, %)
Al
Cu
Mg
Mn
Si
Balance
4.52
1.56
0.6
0.15
Fe
0.21
Cr
Zn
Ti
0.02
0.05
0.02
Zr
<0.01
The experiment was then carried out in an LSF-VII laser solid forming system, consisting of a 6 kW semiconductor laser, a three-dimensional numerical controlled working table with a glove box, and a powder feeding system. The staggered scanning strategy was used and the schematic are shown in Fig. 2. A casted 2024Al was used as the substrate, which had been grounded with sandpaper and cleaned with acetone. In order to prevent cracks, the substrate was heated by laser
before shaping. 2024Al and 3%TiB2-2024Al block with dimensions of 70 mm×15 mm×15mm were built by LSF, The fabricated block is shown in Fig. 3(a), the tensile sample is shown in Fig. 3(b), with the dimension and sampling schematic diagrams are shown in Fig. 3(c) and (d). The longitudinal section (X-Z) plane is selected to observe the microstructure of as-deposited alloy, as shown in Fig. 3(d). The LSF processing parameters are shown as follows: 1200 W laser power, 2 mm spot diameter, 5 mm/s scanning speed, 5 g/min feeding rate, 45~50% overlap, and 0.1 ~ 0.3mm height of a single deposited layer.
Fig. 2 Schematic and scanning strategy of LSF
Fig. 3 (a) the block of LSFed alloy and (b) tensile sample and (c) geometrical properties (mm) and (d) sampling schematic
The microstructures of the powders and laser solid formed (LSFed) samples were investigated by SEM（Tescan VEGA//LMH). The phase analysis and element distribution were characterized by using X- ray diffraction (XRD, X’Pert PRO with Cu K target, operated at 40 kV 。
and 40 mA, 0.017 /min step size) and election probe microanalysis (Shimadzu, EPMA 1720). Microhardness was tested with hardness tester (Struers, DuraminA300 ) under a 0.2 kg load and a 15s dwell time. Room temperature tensile properties of LSFed samples were measured by using the floor model universal testing system (Instron 3382). The loading direction of the in situ tensile tests was perpendicular to the building direction. The tensile fracture surfaces were also characterized by using SEM.
3. Result and discussion
3.1 Microstructure and phase analysis
The microstructures of the 2024Al and 3%TiB2-2024Al sample were investigated by SEM. Fig. 4 shows the microstructures of 2024Al sample fabricated by LSF. Fig. 4(a), (b) and (c) show the detailed microstrutures of bottom, middle and top zone of the sample, respectively. Fig. 4(d) shows the magnified image of the columnar grains in the Fig. (b). As presented in Fig. 4(a), the bottom microstucture includes obviously three parts, the equiaxed grains in the substrate, the dendrite structure with the degradation of second-dendrite arm in the laser deposited zone and the columnar grains in the top. Moreover, the dendrite grain grows epitaxially along the deposition direction. The wide is about 85~90 m. Then the columnar grain begins to grow along the deposition direction. In the middle region, as shown in Fig. 4(b), it can be seen that there is a homogeneous layerwise structure in the longitudinal section, and these layers reveal melten pool
patterns and laser scan strategy of the whole deposited process. In addition, the large columnar grains with the wide about 160 m ~1.6 mm are also displayed with a growth direction along the deposition direction. And the microstructure indicates a fine granular feature in the columnar grains, as shown in Fig. 4(d). Although the structures become larger in the remelt layers, the columnar grain growth characteristics are not affected. On the top of the sample, Fig. 4(c) shows that the morphologies and growth directions of these grains are different. The area marked by the blue arrow is the interface of the two adjacent molten pool peripheries, and is also the laser re-melted area[17]. It is consisted of relative large dendrite grains with the growth direction toward to the core of the molten pool. The microstructure in other zones, by constrast, are mainly fine granular microstructures, and they grew randomly. The reason of these differences are mainly related to the direction of temperature gradient in solidification process. In the bottom and middle of the sample, the direction of the temperature gradient is perpendicular to the laser scanning direction and parallel to the building direction. Therefore, It induces the grains to grow along the building direction. But, the direction of the temperature gradient in the molten pool transforms into the laser scanning direction at the top region of the sample without any re-molten process. This is why the grain grows toward to the core of molten pool in the molten pool boundary.
Fig. 4 SEM images of 2024Al sample fabricated by LSF: (a) microstructure of bottom region and (b) microstructure of middle region and (c) microstructure of top and molten pool boundary and (d) magnified image of columnar grains of (b).
Fig. 5 shows the microstructures of the 3%TiB2-2024Al sample fabricated by LSF. Different from the microstructure of the 2024Al sample showed in Fig. 4(b), it can be seen from Fig. 5(b) that the large columnar grains disappear. Meanwhile, the microstructure in the middle region are different morphologies in different areas, and the grains become small, as shown in Fig. 5(a and c). The dendrite structure distributes in the re-melted layer zone in Fig. 5(a), which epitaxially grows along the deposition direction from the top grains of the previously deposited layers. And the fine equiaxed grains with the size ranging 20 ~ 35m distribute uniformly in the molten pool in Fig. 5(c). Fig. 5(d) shows a higher magnified feature, showing that the TiB2 particles embed into the Al matrix and grain boundaries. Their shapes are not uniform and their sizes range from 2 μm to 4 μm. Fig. 5(e) shows the microstructure of the bottom region of the sample. The dendrite grain epitaxially grows from the substrate along the deposition direction. And on the top region of the sample (see Fig. 5(f)), the structure is similar with that of the 2024Al sample, the morphologies and the growth directions of grains are different. Especially, the 2024Al sample exhibits a series of large columnar grains oriented parallel to the building direction and extending through multiple layers. But the 3%TiB2-2024Al sample mainly consists of the dendrite structures in the re-melted layers and fine equiaxed structures in molten pool. This result indicates that the columnar to equiaxed transition occurs in the TiB2
reinforced 2024Al sample, and all the grains become smaller. It is therfore deduced that the TiB2 particles should be effective for the grain refinement. The reason is that the TiB2 particle is an excellent grain growth inhibitor in Al matrix[14, 19, 20], which can provide a high density of low-energy-barrier heterogeneous nucleation sites ahead of the solidification front and induce a fine equiaxed structure[1].
Fig. 5 SEM images of TiB2 reinforced 2024Al sample fabricated by LSF: (a) microstructure of re-melted zone and layer-band; (b) microstructure of middle region; (c) microstructure in molten pool; (d) magnified images of (c); (e) microstructure of bottom region; (f) microstructure of top and molten pool boundary.
In order to investigate the distribution of the TiB2 particles, the corresponding EPMA mapping as shown in Fig. 6(a)-(f) was performed. The gray area is Al matrix, and the white basket net structures are Al2Cu reinforcement phase. Because the average Mg content in the mapping analysis is only 1.15wt. %. And the EPMA point analysis of the white basket area was made. The
Al-wt% , Cu-wt% and Mg-wt% are 51.3, 50.225 and 0.586, respectively. It is reasonable to consider that the white basket net structures are mainly Al2Cu phase due to the low Mg content in the area. In addition, it can be seen from the distribution of Ti and B elments in Fig. 6(e and f) that some TiB2 particles distribute in the Al matrix and act as the heterogeneous nucleation sites for Al to induce the fine equiaxed grains growth[1]. The others are pushed to the boundaries of the Al grains during the solidification process and intertwine with the eutectic Al2Cu. The magnified images of TiB2 particles distribution in different areas are shown in Fig. 5(d). In order to confirm that these are TiB2 particles, the corresponding EPMA point analysis was characterized. The Ti-At% and B-At% are 36.086 and 62.351 respectively, which is close to 1:2. Therefore, this result proves that they are TiB2 particles.
Fig. 6 EPMA map of TiB2 reinforced 2024Al sample fabricated by LSF
Fig. 7 shows the XRD spectra of the 2024Al powder and LSFed samples. The spectrum of the powder only exhibit the α-Al phase. As for the 2024Al and 3%TiB2-2024Al LSFed samples, weak peaks of the Al2Cu phase can be observed beside the main phase of α-Al. The diffraction peak of the TiB2 phase was not observed in the XRD spectrum of 3%TiB2-2024Al LSFed sample
since the TiB2 content in the powders is only 3wt. % and beyond to the XRD detecting limitation (typically 5wt. %). No other phases such as Al2CuMg and Al3Ti phases were tested. In the 2024Al sample, it is worth noting that Al (200) peak is much stronger than others, and even higher than the peak (111). It is implied that the grains prefer to grow along the orientation of the crystal plane (200). The result was also observed in Al-Cu-Mg alloy[17], AlSi10Mg[9] and Al-12Si parts[10] fabricated by SLM. Such texture is often attributed to the preferential solidification in the <100> crystal direction of fcc structure [9, 17].
Fig. 7 XRD patterns of 2024Al powder and the 2024Al and 3%TiB2-2024Al fabricated by LSF
3.2 Microhardness
Microhardness measurement of the 2024Al and the 3%TiB2-2024Al samples are shown in Fig. 8. It can be seen that the TiB2 reinforced 2024Al has the average microhardness value of 108.5HV, which is larger than the average microhardness 75HV of the 2024Al. And they are all higher than the average microhardness of the cast 2024Al (62HV). This is attributed to the rapid
solidification and the cooling rate of LSF processing, which result in the reduction of dendrite spacing and the soluted atoms enrichment in the Al matrix. Furthermore, the hardness of the TiB2 particle is about 1200 HV, which is much larger than that of the 2024Al mathix. As a result, these factors promote the improvement of the hardness.
Fig. 8 Microhardness of 2024Al and TiB2 reinforced 2024Al sample fabricated by LSF
3.3 Tensile behavior
The stress-strain curves of the LSFed 2024Al and the 3%TiB2-2024Al samples are shown in Fig. 9, which were tested at room temperature. It is clear that the TiB2 particle presents a significant influence on the tensile properties. Compared with the 2024Al (the tensile strength, yield stress and elongation are about 202MPa, 90MPa and 7% respectively), the 3%TiB2-2024Al specimen shows a higher tensile strength ~ 284MPa, yield strength ~ 163MPa and elongation
~18.7%. The improved tensile strength and yield strength would probably arise from the dislocation and refined crystalline strengthening mechanism. The fine equiaxed grains can limit the motion of dislocation because of a large amount of grain boundaries[21, 22], and provide additional strengthening[1]. The particles disperse inside the Al matrix and the grain boundary, which can provide the resistance to grain growth and additional strengthening for the pinning effects[1]. These reasons are believed to intrinsically improve the mechanical properties of the TiB2 particle reinforced 2024Al fabricated by LSF.
Fig. 9 Stress-strain curves of 2024Al and TiB2 reinforced 2024Al sample fabricated by LSF
Fig. 10 SEM images of the fracture surface of 2024Al and TiB2 reinforced 2024Al samples fabricated by LSF:(a) and (b) 2024Al alloy,(c) and (d) 3%TiB2-2024Al
Fig. 10 shows the fracture surfaces of the tensile tested specimens of the 2024Al and 3%TiB2-2024Al. Columnar grain features appear in the fracture surface, and the fracture zones locate at the grain boundaries as shown in Fig. 10 (a). Fig. 10 (b) shows that the dimples (as indicated by red coils) can be observed, indicating a ductile rupture. And some lack of fusion zones and porosities, which are indicated by the yellow coils and the blue arrows, are also observed. These lack of fusion zones and porosities could cause the appearance and propagation of the crack in the fracture face. Therefore, it is the important reason for the low tensile strength and ductility of 2024Al. Fig. 10 (c and d) show the micrographs of the TiB2 particles reinforced 2024Al, and columnar grain features are not observed. This is consistent with above
microstructural result. Moreover, dimples and some precipitated phases in dimples are also observed, which indicate the ductile rupture.
Conclusions
In summary, we have demonstrated a fabrication of 2024Al alloy and 3%TiB2 reinforced 2024Al composite by laser solid forming (LSF) technique. The 2024Al alloy has formed large columnar grains parallel to the building direction and appears apparent preferential growth orientation. Whilst, TiB2 reinforced 2024Al composite has a texture-less structure with dendrite and fine equiaxed structures. The added TiB2 particles provides the effect of grain refinement and causes the enhancement of mechanical properties. The mechanical parameters of the TiB2 reinforced 2024Al composite, including tensile strength, yield strength, elongation and microhardness, are measured to be 284 MPa, 163 MPa, 18.7% and 108.5 HV, respectively.
Acknowledgements
We thank for F. G. Liu, Y. L. Li, J. J. Xu Y. F. Zhang for the useful discussion. The work was supported by the National Key Research and Development Programme of China (Grant No. 2016YFB1100104).
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PII: DOI: Reference:
S0921-5093(18)31753-2 https://doi.org/10.1016/j.msea.2018.12.072 MSA37344
To appear in: Materials Science & Engineering A Received date: 16 November 2018 Revised date: 15 December 2018 Accepted date: 15 December 2018 Cite this article as: Xiaoli Wen, Qingzheng Wang, Qiang Mu, Nan Kang, Shang Sui, Haiou Yang, Xin Lin and Weidong Huang, Laser solid forming additive manufacturing TiB2 reinforced 2024Al composite: microstructure and mechanical properties, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2018.12.072 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.
Laser solid forming additive manufacturing TiB2 reinforced 2024Al composite: microstructure and mechanical properties
Xiaoli Wena,b, Qingzheng Wanga,b, Qiang Mua,b, Nan Kanga,b, Shang Suia,b, Haiou Yanga,b, Xin Lina,b*, Weidong Huanga,b
a
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University,
127 Youyixilu, Xi’an, Shaanxi 710072, PR China b
Key Laboratory of Metal High Performance Additive Manufacturing and Innovative Design,
MIIT China, Northwestern Polytechnical University, 127 Youyixilu, Xi’an,Shaanxi 710072, PR China
*
Corresponding author at: State Key Laboratory of Solidification Processing, Northwestern
Polytechnical University, 127 Youyixilu, Xi’an, Shaanxi 710072, PR China. [email protected] (X. Lin).
Abstract
The 2024Al alloy and 3%TiB2 reinforced 2024Al composite were fabricated using laser solid forming (LSF). Different from the LSF processed 2024Al with a large columnar grain and apparent preferential growth orientation, TiB2 reinforced 2024Al composite presents texture-less structure, consisting of the dendrite and fine equiaxed structures. Some TiB2 particles distribute within the Al matrix, others distribute along the grain boundaries and intertwine with the Al2Cu
phase. The incoporation of TiB2 gives rise to significant grain refinement, which is one of the important reasons for the improvement of mechanical properties. Moreover, the TiB2 reinforced 2024Al sample exhibits 284MPa high tensile strength, 163MPa yield strength, 108.5HV microhardness, and 18.7% excellent elongation.
Keywords: additive manufacturing; laser solid forming; TiB2 reinforced; 2024Al composite; microstructures; mechanical properties
1. Introduction
Metal-based additive manufacturing (three-dimensional (3D) printing) is a potentially disruptive technology, which can build up metallic components layer by layer. It provides a free design and a flexible manufacture[1]. Laser solid forming (LSF) is a typical laser direct energy deposition additive manufacturing technologies based on the powder feeding method with high deposition rate[2], which is a near-net-shape method and can fabricate highly complex geometric components with high performance in free-form forming. Due to its significant advantages in solidification process, LSF has become the frontier manufacturing and repairing techniques across multiple industries, including aerospace, automotive, biomedical, energy and power fields[3-5]. Aluminum matrix composites (AMCs) have attracted numerous attentions and been widely used in many fields of electronics, aerospace, military, automotive and transportation industries because of their excellent properties, such as improved specific strength, high specific stiffness, enhanced wear resistance and creep resistance, low density and good thermal stability[6-8], In recent years, the reports on laser additive manufactured aluminum alloy mainly focuse on AlSi10Mg and Al-12Si alloys, which are fabricated by selective laser melting(SLM) and reveals a
higher mechanical properties than those of fabricated by conventional methods[9, 10]. Whilst, other aluminum alloys are difficult to be fabricated due to the poor flowability and high laser reflectivity of their powders. They are also prone to producing large columnar grain during solidification process, which degenerate the alloy properties due to the produced periodic cracks. Nano or micro-particles decorated aluminum matrix composite is an effective way to suppress crack and improve properties by transforming columnar grain into equiaxed grain. A few reinforcement particles such as SiC, Al3Ti, Al2O3, TiB2 and ZrB2 have been incorporated in AMCs[11, 12]. P. Wang et al [13] fabricated TiB2 particle reinforced Al-3.5Cu-1.5Mg-1Si (wt.%) composite by SLM, and investigated the effects of the TiB2 particle on the microstructures and mechanical properties. Their result reveals that the uniform distribution of TiB2 particle represents a suitable reinforcement phase, which results in a grain refinement. The compressive yield strength of the TiB2/Al-Cu-Mg-Si composite was significantly improved by the grain refinement. J. H. Martin et al[1] assembled some particles onto 7075 and 6061 series aluminum alloy powder. These high-strength aluminum alloys were fabricated successfully by SLM. Crack-free and equiaxed structures were achieved. Among these samples, stock Al7075 almost retains no strength owing to the large hot tearing cracks. As a comparison, the Al7075+Zr sample acquired fine equiaxed and crack-free microstructure, and exhibited higher tensile strength (383-417MPa), yield strength (325-373MPa) and elongation (3.8-5.4%) because Al3Zr particles acted as grain refiner. X. P. Li et al[14] fabricated the nano-TiB2 decorated AlSi10Mg alloy by SLM. The material appears textureless microstructure, consisting of fine grains and cells, and showed a very high tensile strength ~530 MPa, excellent ductility ~15.5%. The nano-TiB2 particles were found at the grain and cell boundaries and inhibited the growth of grain. Therefore, these investigations indicate that
particle reinforced aluminum matrix composite synthesized by SLM can be an effective way to improve the mechanical properties of aluminum alloys. Among these particles, it has been demonstrated that TiB2 has excellent compatibility with aluminum alloys due to its good wettability with Al alloys and its excellent properties such as high hardness, high stiffness and melting point, superior wear resistance and considerable chemical stability [15, 16]. 2024Al alloy is a type of high strength, low density and heat treatable aluminum alloy, which is one of the most-used alloys in aerospace field. In recent years, it is difficult for the conventional manufacturing methods to meet the requirements of special structures in many practical applications. So far, a few works have been focused on laser additive manufacturing of 2024Al alloy [13, 17, 18]. The majority of them concentrate on the SLM technology. However, SLM has an obvious limitation in producing large-sized components. In contrast, LSF shows significant advantages in the free-design and flexible manufacture of metallic component in large scale with high production efficiency. In this work, the 2024Al alloy and TiB2 nano-particles reinforced 2024Al matrix composites are fabricated by LSF separately. The morphologies and distributions of the TiB2 particles are analyzed by XRD, SEM and EPMA. The influence of the TiB2 particles on the microstructures and mechanical properties of the 2024Al matrix composites are also investigated systematically.
2. Experimental Procedures
2024 aluminum alloy powder was prepared by gas atomization process. The scanning electron microscope (SEM) image of the powder is shown in Fig. 1(a). The powder has a spherical shape and a size range of 40–80 μm. The chemical composition of the alloy powder was measured by ICP-AES (inductively coupled plasma atomic emission spectrometry), and listed in Table 1.
The TiB2 powders were synthesized by high temperature solid state reaction method and the particle size is about 500nm (HAOXI Research Nanomaterials, Inc.). The 3%TiB2 reinforced 2024Al composite powders were mechanically milled in a ball mill at a 380 rpm rotation speed for 3h. The aim of the ball milling process is to disperse the TiB2 particles that can be adhered to the surface of the 2024Al particles as shown in Fig. 1(b and c), and keep a high flowability. Then these powders were dried in the vacuum oven at 120 °C for 1.5 hours.
Fig. 1 (a) SEM morphological image of 2024Al powder. (b) The morphology of nano-TiB2 decorated 2024Al powder. (c) Magnified image of nano-TiB2 decorated 2024Al powder.
Table 1 Chemical composition of 2024Al alloy (mass fraction, %)
Al
Cu
Mg
Mn
Si
Balance
4.52
1.56
0.6
0.15
Fe
0.21
Cr
Zn
Ti
0.02
0.05
0.02
Zr
<0.01
The experiment was then carried out in an LSF-VII laser solid forming system, consisting of a 6 kW semiconductor laser, a three-dimensional numerical controlled working table with a glove box, and a powder feeding system. The staggered scanning strategy was used and the schematic are shown in Fig. 2. A casted 2024Al was used as the substrate, which had been grounded with sandpaper and cleaned with acetone. In order to prevent cracks, the substrate was heated by laser
before shaping. 2024Al and 3%TiB2-2024Al block with dimensions of 70 mm×15 mm×15mm were built by LSF, The fabricated block is shown in Fig. 3(a), the tensile sample is shown in Fig. 3(b), with the dimension and sampling schematic diagrams are shown in Fig. 3(c) and (d). The longitudinal section (X-Z) plane is selected to observe the microstructure of as-deposited alloy, as shown in Fig. 3(d). The LSF processing parameters are shown as follows: 1200 W laser power, 2 mm spot diameter, 5 mm/s scanning speed, 5 g/min feeding rate, 45~50% overlap, and 0.1 ~ 0.3mm height of a single deposited layer.
Fig. 2 Schematic and scanning strategy of LSF
Fig. 3 (a) the block of LSFed alloy and (b) tensile sample and (c) geometrical properties (mm) and (d) sampling schematic
The microstructures of the powders and laser solid formed (LSFed) samples were investigated by SEM（Tescan VEGA//LMH). The phase analysis and element distribution were characterized by using X- ray diffraction (XRD, X’Pert PRO with Cu K target, operated at 40 kV 。
and 40 mA, 0.017 /min step size) and election probe microanalysis (Shimadzu, EPMA 1720). Microhardness was tested with hardness tester (Struers, DuraminA300 ) under a 0.2 kg load and a 15s dwell time. Room temperature tensile properties of LSFed samples were measured by using the floor model universal testing system (Instron 3382). The loading direction of the in situ tensile tests was perpendicular to the building direction. The tensile fracture surfaces were also characterized by using SEM.
3. Result and discussion
3.1 Microstructure and phase analysis
The microstructures of the 2024Al and 3%TiB2-2024Al sample were investigated by SEM. Fig. 4 shows the microstructures of 2024Al sample fabricated by LSF. Fig. 4(a), (b) and (c) show the detailed microstrutures of bottom, middle and top zone of the sample, respectively. Fig. 4(d) shows the magnified image of the columnar grains in the Fig. (b). As presented in Fig. 4(a), the bottom microstucture includes obviously three parts, the equiaxed grains in the substrate, the dendrite structure with the degradation of second-dendrite arm in the laser deposited zone and the columnar grains in the top. Moreover, the dendrite grain grows epitaxially along the deposition direction. The wide is about 85~90 m. Then the columnar grain begins to grow along the deposition direction. In the middle region, as shown in Fig. 4(b), it can be seen that there is a homogeneous layerwise structure in the longitudinal section, and these layers reveal melten pool
patterns and laser scan strategy of the whole deposited process. In addition, the large columnar grains with the wide about 160 m ~1.6 mm are also displayed with a growth direction along the deposition direction. And the microstructure indicates a fine granular feature in the columnar grains, as shown in Fig. 4(d). Although the structures become larger in the remelt layers, the columnar grain growth characteristics are not affected. On the top of the sample, Fig. 4(c) shows that the morphologies and growth directions of these grains are different. The area marked by the blue arrow is the interface of the two adjacent molten pool peripheries, and is also the laser re-melted area[17]. It is consisted of relative large dendrite grains with the growth direction toward to the core of the molten pool. The microstructure in other zones, by constrast, are mainly fine granular microstructures, and they grew randomly. The reason of these differences are mainly related to the direction of temperature gradient in solidification process. In the bottom and middle of the sample, the direction of the temperature gradient is perpendicular to the laser scanning direction and parallel to the building direction. Therefore, It induces the grains to grow along the building direction. But, the direction of the temperature gradient in the molten pool transforms into the laser scanning direction at the top region of the sample without any re-molten process. This is why the grain grows toward to the core of molten pool in the molten pool boundary.
Fig. 4 SEM images of 2024Al sample fabricated by LSF: (a) microstructure of bottom region and (b) microstructure of middle region and (c) microstructure of top and molten pool boundary and (d) magnified image of columnar grains of (b).
Fig. 5 shows the microstructures of the 3%TiB2-2024Al sample fabricated by LSF. Different from the microstructure of the 2024Al sample showed in Fig. 4(b), it can be seen from Fig. 5(b) that the large columnar grains disappear. Meanwhile, the microstructure in the middle region are different morphologies in different areas, and the grains become small, as shown in Fig. 5(a and c). The dendrite structure distributes in the re-melted layer zone in Fig. 5(a), which epitaxially grows along the deposition direction from the top grains of the previously deposited layers. And the fine equiaxed grains with the size ranging 20 ~ 35m distribute uniformly in the molten pool in Fig. 5(c). Fig. 5(d) shows a higher magnified feature, showing that the TiB2 particles embed into the Al matrix and grain boundaries. Their shapes are not uniform and their sizes range from 2 μm to 4 μm. Fig. 5(e) shows the microstructure of the bottom region of the sample. The dendrite grain epitaxially grows from the substrate along the deposition direction. And on the top region of the sample (see Fig. 5(f)), the structure is similar with that of the 2024Al sample, the morphologies and the growth directions of grains are different. Especially, the 2024Al sample exhibits a series of large columnar grains oriented parallel to the building direction and extending through multiple layers. But the 3%TiB2-2024Al sample mainly consists of the dendrite structures in the re-melted layers and fine equiaxed structures in molten pool. This result indicates that the columnar to equiaxed transition occurs in the TiB2
reinforced 2024Al sample, and all the grains become smaller. It is therfore deduced that the TiB2 particles should be effective for the grain refinement. The reason is that the TiB2 particle is an excellent grain growth inhibitor in Al matrix[14, 19, 20], which can provide a high density of low-energy-barrier heterogeneous nucleation sites ahead of the solidification front and induce a fine equiaxed structure[1].
Fig. 5 SEM images of TiB2 reinforced 2024Al sample fabricated by LSF: (a) microstructure of re-melted zone and layer-band; (b) microstructure of middle region; (c) microstructure in molten pool; (d) magnified images of (c); (e) microstructure of bottom region; (f) microstructure of top and molten pool boundary.
In order to investigate the distribution of the TiB2 particles, the corresponding EPMA mapping as shown in Fig. 6(a)-(f) was performed. The gray area is Al matrix, and the white basket net structures are Al2Cu reinforcement phase. Because the average Mg content in the mapping analysis is only 1.15wt. %. And the EPMA point analysis of the white basket area was made. The
Al-wt% , Cu-wt% and Mg-wt% are 51.3, 50.225 and 0.586, respectively. It is reasonable to consider that the white basket net structures are mainly Al2Cu phase due to the low Mg content in the area. In addition, it can be seen from the distribution of Ti and B elments in Fig. 6(e and f) that some TiB2 particles distribute in the Al matrix and act as the heterogeneous nucleation sites for Al to induce the fine equiaxed grains growth[1]. The others are pushed to the boundaries of the Al grains during the solidification process and intertwine with the eutectic Al2Cu. The magnified images of TiB2 particles distribution in different areas are shown in Fig. 5(d). In order to confirm that these are TiB2 particles, the corresponding EPMA point analysis was characterized. The Ti-At% and B-At% are 36.086 and 62.351 respectively, which is close to 1:2. Therefore, this result proves that they are TiB2 particles.
Fig. 6 EPMA map of TiB2 reinforced 2024Al sample fabricated by LSF
Fig. 7 shows the XRD spectra of the 2024Al powder and LSFed samples. The spectrum of the powder only exhibit the α-Al phase. As for the 2024Al and 3%TiB2-2024Al LSFed samples, weak peaks of the Al2Cu phase can be observed beside the main phase of α-Al. The diffraction peak of the TiB2 phase was not observed in the XRD spectrum of 3%TiB2-2024Al LSFed sample
since the TiB2 content in the powders is only 3wt. % and beyond to the XRD detecting limitation (typically 5wt. %). No other phases such as Al2CuMg and Al3Ti phases were tested. In the 2024Al sample, it is worth noting that Al (200) peak is much stronger than others, and even higher than the peak (111). It is implied that the grains prefer to grow along the orientation of the crystal plane (200). The result was also observed in Al-Cu-Mg alloy[17], AlSi10Mg[9] and Al-12Si parts[10] fabricated by SLM. Such texture is often attributed to the preferential solidification in the <100> crystal direction of fcc structure [9, 17].
Fig. 7 XRD patterns of 2024Al powder and the 2024Al and 3%TiB2-2024Al fabricated by LSF
3.2 Microhardness
Microhardness measurement of the 2024Al and the 3%TiB2-2024Al samples are shown in Fig. 8. It can be seen that the TiB2 reinforced 2024Al has the average microhardness value of 108.5HV, which is larger than the average microhardness 75HV of the 2024Al. And they are all higher than the average microhardness of the cast 2024Al (62HV). This is attributed to the rapid
solidification and the cooling rate of LSF processing, which result in the reduction of dendrite spacing and the soluted atoms enrichment in the Al matrix. Furthermore, the hardness of the TiB2 particle is about 1200 HV, which is much larger than that of the 2024Al mathix. As a result, these factors promote the improvement of the hardness.
Fig. 8 Microhardness of 2024Al and TiB2 reinforced 2024Al sample fabricated by LSF
3.3 Tensile behavior
The stress-strain curves of the LSFed 2024Al and the 3%TiB2-2024Al samples are shown in Fig. 9, which were tested at room temperature. It is clear that the TiB2 particle presents a significant influence on the tensile properties. Compared with the 2024Al (the tensile strength, yield stress and elongation are about 202MPa, 90MPa and 7% respectively), the 3%TiB2-2024Al specimen shows a higher tensile strength ~ 284MPa, yield strength ~ 163MPa and elongation
~18.7%. The improved tensile strength and yield strength would probably arise from the dislocation and refined crystalline strengthening mechanism. The fine equiaxed grains can limit the motion of dislocation because of a large amount of grain boundaries[21, 22], and provide additional strengthening[1]. The particles disperse inside the Al matrix and the grain boundary, which can provide the resistance to grain growth and additional strengthening for the pinning effects[1]. These reasons are believed to intrinsically improve the mechanical properties of the TiB2 particle reinforced 2024Al fabricated by LSF.
Fig. 9 Stress-strain curves of 2024Al and TiB2 reinforced 2024Al sample fabricated by LSF
Fig. 10 SEM images of the fracture surface of 2024Al and TiB2 reinforced 2024Al samples fabricated by LSF:(a) and (b) 2024Al alloy,(c) and (d) 3%TiB2-2024Al
Fig. 10 shows the fracture surfaces of the tensile tested specimens of the 2024Al and 3%TiB2-2024Al. Columnar grain features appear in the fracture surface, and the fracture zones locate at the grain boundaries as shown in Fig. 10 (a). Fig. 10 (b) shows that the dimples (as indicated by red coils) can be observed, indicating a ductile rupture. And some lack of fusion zones and porosities, which are indicated by the yellow coils and the blue arrows, are also observed. These lack of fusion zones and porosities could cause the appearance and propagation of the crack in the fracture face. Therefore, it is the important reason for the low tensile strength and ductility of 2024Al. Fig. 10 (c and d) show the micrographs of the TiB2 particles reinforced 2024Al, and columnar grain features are not observed. This is consistent with above
microstructural result. Moreover, dimples and some precipitated phases in dimples are also observed, which indicate the ductile rupture.
Conclusions
In summary, we have demonstrated a fabrication of 2024Al alloy and 3%TiB2 reinforced 2024Al composite by laser solid forming (LSF) technique. The 2024Al alloy has formed large columnar grains parallel to the building direction and appears apparent preferential growth orientation. Whilst, TiB2 reinforced 2024Al composite has a texture-less structure with dendrite and fine equiaxed structures. The added TiB2 particles provides the effect of grain refinement and causes the enhancement of mechanical properties. The mechanical parameters of the TiB2 reinforced 2024Al composite, including tensile strength, yield strength, elongation and microhardness, are measured to be 284 MPa, 163 MPa, 18.7% and 108.5 HV, respectively.
Acknowledgements
We thank for F. G. Liu, Y. L. Li, J. J. Xu Y. F. Zhang for the useful discussion. The work was supported by the National Key Research and Development Programme of China (Grant No. 2016YFB1100104).
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