applied sciences Article
Microstructure and Mechanical Properties of Ti-6Al-4V Fabricated by Vertical Wire Feeding with Axisymmetric Multi-Laser Source Jie Fu 1 , Lin Gong 1, *, Yifei Zhang 2 , Qianru Wu 1 , Xuezhi Shi 1 , Junchao Chang 2 and Jiping Lu 1 1
2
*
School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China;
[email protected] (J.F);
[email protected] (Q.W.);
[email protected] (X.S.);
[email protected] (J.L.) Shenyang Siasun Robot Automation Co., Ltd., Shenyang 110168, China;
[email protected] (Y.Z.);
[email protected] (J.C.) Correspondence:
[email protected]; Tel.: +86-10-6891-5017
Academic Editor: Peter Van Puyvelde Received: 10 January 2017; Accepted: 23 February 2017; Published: 28 February 2017
Abstract: Vertical wire feeding with an axisymmetric multi-laser source (feeding the wire vertically into the molten pool) has exhibited great advantages over LAM (laser additive manufacturing) with paraxial wire feeding, which has an anisotropic forming problem in different scanning directions. This paper investigates the forming ability of vertical wire feeding with an axisymmetric multi-laser source, and the microstructure and mechanical properties of the fabricated components. It has been found that vertical wire feeding with an axisymmetric multi-laser source has a strong forming ability with no anisotropic forming problem when fabricating the complex parts in a three-axis machine tool. Most of the grains in the samples are equiaxed grains, and a small amount of short columnar grains exist which are parallel to each other. The microstructure of the fabricated samples exhibits a fine basket-weave structure and martensite due to the fast cooling rate which was caused by the small size of the molten pool and the additional heat dissipation from the feeding wire. The static tensile test shows that the average ultimate tensile strength is 1140 MPa in the scanning direction and 1115 MPa in the building direction, and the average elongation is about 6% in both directions. Keywords: vertical wire feeding; laser additive manufacturing; Ti-6Al-4V titanium alloy
1. Introduction Ti-6Al-4V titanium alloy was widely used in the aerospace and military industries due to its excellent specific strength, high shear modulus, low density and thermal expansion coefficient [1–3]. However, traditional manufacturing methods of Ti-6Al-4V titanium alloy, such as forging or cutting, which have the disadvantages of a high buy-to-fly ratio and low manufacturing flexibility, are not economical. The laser additive manufacturing (LAM) process which fabricates Ti-6Al-4V titanium alloy with a near net shape has no restriction in the component shapes. It has been popular in recent studies [4–7] and has been widely used to fabricate complex components in aerospace and other industries [1]. The typical grain structure in the fabricated Ti-6Al-4V titanium alloy samples is coarse columnar prior β grains, which arise due to high temperature gradients and distribute approximately parallel to the building direction [8]. Wire and powder are the two main material forms of the Ti-6Al-4V in LAM process [6,9]. SLM (Selective Laser Melting) is a typical LAM method which uses powder as the feedstock. It has the advantages of high manufacturing capacity and accuracy. Compared with powder, using wire as the feedstock form has the great advantages of high efficiency, low cost and simple handling and storage
Appl. Sci. 2017, 7, 227; doi:10.3390/app7030227
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storage storage operation operation during during the the manufacturing manufacturing process process [9,10]. [9,10]. Therefore, Therefore, it it has has received received more more attentions in recent research activities [11–16]. operation during the manufacturing process [9,10]. Therefore, it has received more attentions in recent attentions in recent research activities [11–16]. LAM with wire feeding can be divided into vertical wire research activities [11–16]. LAM with wire feeding can be divided into vertical wirefeeding and paraxial wire feeding [9,17]. feeding and paraxial wire feeding [9,17]. LAM with wire feeding evolved from laser wire feed welding, so paraxial wire feeding accounts for LAM with wire feeding can be divided into vertical wire feeding and paraxial wire feeding [9,17]. LAM with wire feeding evolved from laser wire feed welding, so paraxial wire feeding accounts for the majority of the wire feeding methods, and the schematic drawing of it is indicated in Figure 1a. LAM with wire feeding evolved from laser wire feed welding, so paraxial wire feeding accounts for the majority of the wire feeding methods, and the schematic drawing of it is indicated in Figure 1a. Although it is easy to implement, there is an anisotropic forming problem in the deposition process the majority of the wire feeding methods, and the schematic drawing of it is indicated in Figure 1a. Although it is easy to implement, there is an anisotropic forming problem in the deposition process due to different wire kinds of in Although it is easy to feed implement, there [18]. is[18]. an There anisotropic forming problem infeed the orientations deposition process due to different wire feed orientations orientations There are are three three kinds of wire wire feed orientations in the the deposition process, which are shown in Figure 2. The depositions from front feeding and side due to different wire feed orientations [18]. There are three kinds of wire feed orientations in the deposition process, which are shown in Figure 2. The depositions from front feeding and side feeding have a smoother surface than back feeding [19], and the smoothest surface can be obtained deposition process, which are shown in Figure 2. The depositions from front feeding and side feeding feeding have a smoother surface than back feeding [19], and the smoothest surface can be obtained when the wire is positioned at the leading edge of the molten pool in front feeding [20]. Due to the have a smoother surface than back feeding [19], and the smoothest surface can be obtained when the when the wire is positioned at the leading edge of the molten pool in front feeding [20]. Due to the different forming morphologies in different wire feed orientations, different layer thicknesses may wire is positioned at the leading edge of the molten pool in front feeding [20]. Due to the different different forming morphologies in different wire feed orientations, different layer thicknesses may appear in the same layer, especially when forming large complex components. forming morphologies in different wire feed orientations, different layer thicknesses may appear in appear in the same layer, especially when forming large complex components. Vertical wire feeding with an axisymmetric multi‐laser source can be realized by the same layer, especially when forming large complex components. Vertical wire feeding with an axisymmetric multi‐laser source can be realized by exchanging exchanging the position of laser head wire as paraxial wire Vertical feeding anand axisymmetric multi-laser source with can bethe realized by exchanging the the position wire of the the laser with head and wire feeder, feeder, as compared compared with the paraxial wire feeding. feeding. The schematic diagram is shown in Figure 1b. It is obvious that the wire feeding orientation will not position of the laser head and wire feeder, as compared with the paraxial wire feeding. The schematic The schematic diagram is shown in Figure 1b. It is obvious that the wire feeding orientation will not change no matter what the scanning direction is in the deposition process. Therefore, there will be no diagram is shown in Figure 1b. It is obvious that the wire feeding orientation will not change no change no matter what the scanning direction is in the deposition process. Therefore, there will be no anisotropic problem in process, but few done matter whatforming the scanning direction isthe indeposition the deposition process. Therefore, there have will bebeen no anisotropic anisotropic forming problem in the deposition process, but few studies studies have been done to to characterize the forming ability of vertical wire feeding with an axisymmetric multi‐laser source. forming problem in the deposition process, but few studies have been done to characterize the forming characterize the forming ability of vertical wire feeding with an axisymmetric multi‐laser source. Thus, aim this paper to the ability of wire feeding ability of vertical with an axisymmetric multi-laser source. Thus, the aim of this with paper is Thus, the the aim of of wire this feeding paper is is to investigate investigate the forming forming ability of vertical vertical wire feeding with an an axisymmetric multi‐laser source as well as the microstructure and mechanical properties of the to investigate the forming ability vertical wire with an axisymmetric multi-laser source as axisymmetric multi‐laser source ofas well as the feeding microstructure and mechanical properties of the fabricated components. well as the microstructure and mechanical properties of the fabricated components. fabricated components.
Figure 1. The schematic of laser additive additive manufacturing with: (a) paraxial wire feeding; (b) vertical Figure 1. The schematic of laser Figure 1. The schematic of laser additive manufacturing with: (a) paraxial wire feeding; (b) vertical manufacturing with: (a) paraxial wire feeding; (b) vertical wire feeding. wire feeding. wire feeding.
(a) (a)
(b) (b)
Wire feeding direction Wire feeding direction
Top view Top view Wire feeding direction Wire feeding direction
Wire feeding direction Wire feeding direction
(c) (c) Scanning direction Scanning direction
Figure 2. The schematic of wire feed orientations: (a) front feeding; (b) back feeding; (c) side feeding. Figure 2. The schematic of wire feed orientations: (a) front feeding; (b) back feeding; (c) side feeding. Figure 2. The schematic of wire feed orientations: (a) front feeding; (b) back feeding; (c) side feeding.
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2.2. Experiment Experiment 2.1. Deposition Process 2.1. Deposition Process The LAM equipment is shown in Figure 3. The additive manufacturing device is mounted on the The LAM equipment is shown in Figure 3. The additive manufacturing device is mounted on three-axis CNC machine (Shenyang Siasun Robot Automation Co., Ltd., Shenyang, China) which uses the three‐axis CNC machine (Shenyang Siasun Robot Automation Co., Ltd., Shenyang, China) which Simens 828D control control system system (SIEMENS, Berlin, Germany) as shown Figurein 3a. The stroke uses Simens numerical 828D numerical (SIEMENS, Berlin, Germany) as inshown Figure 3a. ofThe stroke of each axis is 1000 mm (X), 800 mm (Y) and 600 mm (Z), all of which are driven by servo each axis is 1000 mm (X), 800 mm (Y) and 600 mm (Z), all of which are driven by servo motor (Shenyang Siasun Robot Automation Co., Ltd., Shenyang, China) and ball screw (Shenyang Siasun motor (Shenyang Siasun Robot Automation Co., Ltd., Shenyang, China) and ball screw (Shenyang Robot Automation Co., Ltd., Shenyang, China). Siasun Robot Automation Co., Ltd., Shenyang, China). The additive manufacturing device consists of four functional modules: laser focusing module, The additive manufacturing device consists of four functional modules: laser focusing module, vertical wire feeding module, shielding gas module and water-cooling module as shown in Figure 3b. vertical wire feeding module, shielding gas module and water‐cooling module as shown in Figure 3b. InIn the laser focusing module, three laser beams are focused onto the wire at a certain angle (in this the laser focusing module, three laser beams are focused onto the wire at a certain angle (in this paper, angle between the laser 55°). In vertical wire feeding module, paper, thethe angle between thethe wirewire andand the laser beambeam is 55◦is ). In vertical wire feeding module, the wire isthe wire is fed vertically to the substrate. The additive manufacturing device driven by the actuator fed vertically to the substrate. The additive manufacturing device driven by the actuator moves moves along the arranged path, thus the components can be formed. Argon gas is injected around along the arranged path, thus the components can be formed. Argon gas is injected around the feeding the tofeeding to oxidation prevent metal oxidation at high temperatures. The water provides cooling cooling module to wire preventwire metal at high temperatures. The water cooling module provides cooling to the laser head to ensure long‐term stable operation. The operating temperature the laser head to ensure long-term stable operation. The operating temperature of the laser head is of the laser head is 10 to 25 °C. 10 to 25 ◦ C.
Figure 3. (a) The schematic of the laser additive manufacturing equipment; (b) The equipment of Figure 3. (a) The schematic of the laser additive manufacturing equipment; (b) The equipment of vertical wire feeding with an axisymmetric multi‐laser source; (c) the additive manufacturing device vertical wire feeding with an axisymmetric multi-laser source; (c) the additive manufacturing device of of vertical wire feeding. vertical wire feeding.
Two components were fabricated in this experiment: one for the forming ability test and the Two components were fabricated in this experiment: one for the forming ability test and the other other for the microstructural and mechanical properties tests. The shape of the component fabricated for the microstructural and mechanical properties tests. The shape of the component fabricated for for the forming ability test consists of circles and polyline in all directions to test the forming ability and anisotropic problem of vertical wire feeding method. The tool‐path of it ability is: first, the forming abilityforming test consists of circles and polyline in all directions to test the forming and scanning a circle with a diameter of 50 mm; twelve angle star inscribed anisotropic forming problem of vertical wiresecond, feedingscanning method.a The tool-path of it which is: first, scanning circle is 55 mm and circumcircle is 95 mm in diameter; third, scanning a circle with a diameter of a circle with a diameter of 50 mm; second, scanning a twelve angle star which inscribed circle is 55 mm 100 circumcircle mm; finally, additive manufacturing device is by 0.8 mm and ofrepeat the finally, first and is the 95 mm in diameter; third, scanning a raised circle with a diameter 100 mm; three steps. the additive manufacturing device is raised by 0.8 mm and repeat the first three steps.
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For convenience, a cuboid block with the size of 81 mm × 40 mm × 40 mm was fabricated for the 4 of 11 microstructure observation and mechanical properties tests. The tool‐path of the cuboid block is shown in Figure 4. For cuboid block with size of 81 mm × 40 alloy mm ×substrate 40 mm was fabricated The convenience, components awere fabricated on the a Ti‐6Al‐4V titanium with the size for of the microstructure observation and mechanical properties tests. The tool-path of the cuboid block is 150 mm × 150 mm × 5 mm. The process parameters used of the two components in the deposition Appl. Sci. 2017, 7, 227 4 of 10 shown in Figure 4. process are given in Table 1. For convenience, a cuboid block with the size of 81 mm × 40 mm × 40 mm was fabricated for the The components were fabricated on a Ti-6Al-4V titanium alloy substrate with the size of microstructure observation and mechanical properties tests. The tool‐path of the cuboid block is Table 1. Process used in parameters vertical wire feeding with axisymmetric multi‐laser 150 mm ×shown in Figure 4. 150 mm × 5parameters mm. The process used of the twoan components in the deposition source process. The in components process are given Table 1. were fabricated on a Ti‐6Al‐4V titanium alloy substrate with the size of Appl. Sci. 2017, 7, 227
150 mm × 150 mm × 5 mm. The process parameters used of the two components in the deposition Unit Parameter Set process are given in Table 1. Table 1. Process parameters used in vertical wire feeding with an axisymmetric multi-laser
Laser wavelength Laser power (total) source process. Diameter of laser beam Deposition speed Laser wavelength Laser wavelength Wire‐feed speed Laser power (total) Laser power (total) Diameter of laser beam Wire feed angle Diameter of laser beam Deposition speed Deposition speed Layer thickness Wire-feed speed Wire‐feed speed Diameter of wire Wire feed angle Wire feed angle Layer thickness Deposit spacing Layer thickness
μm 976 kW 1.65 mm 3 Set Unit Parameter Unit Parameter Set mm/min 500 µm μm 976 976 mm/min kW 1.65 kW 1.65 2200 mm 3 ° mm 3 55 mm/min 500 mm/min 500 0.8 mm mm/min mm/min 2200 2200 ◦ mm 1.2 ° 55 55 mm 0.8 mm mm 0.8 3
source process. Table 1. Process parameters used in vertical wire feeding with an axisymmetric multi‐laser
Diameter of wire Diameter of wire Deposit spacing Deposit spacing
mm mm mm
mm
1.2 3
1.2 3
layer thickness
3mm 3mm
0.8mm layer thickness 0.8mm
81mm 81mm
substrate substrate
Figure 4. The tool path of the cuboid sample block. Figure 4. The tool path of the cuboid sample block.
Figure 4. The tool path of the cuboid sample block.
2.2. Mechanical Characterization
2.2. Mechanical Characterization 2.2. Mechanical Characterization The scanning direction (T direction) and building direction (L direction) of the block sample are shown in Figure 5a. Three tensile specimens were obtained from the (L middle of the block in each The scanning direction (T direction) and building direction direction) of the block sample The scanning direction (T direction) and building direction (L direction) of the block sample are direction by wire‐cutting techniques (Figure 5b,c). Static tensile tests at room temperature were are shown in Figure 5a. Three tensile specimens obtained the middle ofblock the block in shown in carried out on an INSTRON 5966 material testing machine (INSTRON, Boston, MA, USA). The max Figure 5a. Three tensile specimens were were obtained from from the middle of the in each each direction by wire-cutting techniques (Figure Static tensile tests atacquired room temperature direction by wire‐cutting techniques (Figure 5b,c). Static tensile tests at room temperature were load capacity of it is 10 kN and the drive type is 5b,c). powerful motors. The strain is by video‐extensometer (INSTRON, 5966 Boston, MA, USA). The machine cross‐head (INSTRON, displacement Boston, speed was were carried out on an INSTRON material testing MA, USA). carried out on an INSTRON 5966 material testing machine (INSTRON, Boston, MA, USA). The max 0.01 mm/s. The load capacity of it is 10 kN and the drive type is powerful motors. strain is acquired load max capacity of it is 10 kN and the drive type is powerful motors. The The strain is acquired by by video-extensometer (INSTRON, Boston, MA,USA). USA).The Thecross‐head cross-head displacement displacement speed speed was video‐extensometer (INSTRON, Boston, MA, was 0.01 mm/s. 0.01 mm/s.
Figure 5. (a) The block sample fabricated by vertical wire feeding with an axisymmetric multi‐laser source; (b) tensile specimen obtained from the sample; (c) the dimensions of tensile specimens.
Figure 5. (a) The block sample fabricated by vertical wire feeding with an axisymmetric multi‐laser Figure 5. (a) The block sample fabricated by vertical wire feeding with an axisymmetric multi-laser source; (b) tensile specimen obtained from the sample; (c) the dimensions of tensile specimens. source; (b) tensile specimen obtained from the sample; (c) the dimensions of tensile specimens.
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2.3. Microstructure and Morphology 2.3. Microstructure and Morphology The samples samples for for microstructural microstructural and and macrostructural macrostructural characterization characterization can be obtained obtained from from The can be each direction. direction. The paper (180, (180, 360, 360, 600, 600, 1000, 1000, 2000), 2000), then then each The samples samples were were firstly firstly polished polished with with SiC SiC paper etching for 20 s using a solution consisting of 2% HF (hydrofluoric acid) and 6% HNO 3 (nitric acid). etching for 20 s using a solution consisting of 2% HF (hydrofluoric acid) and 6% HNO3 (nitric acid). Light microscopy pictures were taken with a Leica DM4000M optical microscope (Leica Microsystems, Light microscopy pictures were taken with a Leica DM4000M optical microscope (Leica Microsystems, Wechsler, Hesse, Germany). And macrostructural pictures were taken with a Canon DSLR Camera Wechsler, Hesse, Germany). And macrostructural pictures were taken with a Canon DSLR Camera (NIKON CORPORATION, Tokyo, Japan). All the grains sizes were determined by ImageJ software (NIKON CORPORATION, Tokyo, Japan). All the grains sizes were determined by ImageJ software (2.1.4.7, National Institutes of Health, Bethesda, MD, USA). (2.1.4.7, National Institutes of Health, Bethesda, MD, USA). 3. Results 3. Results 3.1. Macrostructure 3.1. Macrostructure The component for the forming ability test is shown in Figure 6. It is obvious that the surface The component for the forming ability test is shown in Figure 6. It is obvious that the surface morphology Moreover, morphology of of the the circles circles was was smooth smooth and and there there was was no no difference difference in in all all directions. directions. Moreover, the formation was steady in all the corners, where the surface morphology was smooth as well. well. the formation was steady in all the corners, where the surface morphology was smooth as The deposition process of the component was stable, as observed. Thereby, there was no anisotropic The deposition process of the component was stable, as observed. Thereby, there was no anisotropic forming problem in vertical wire feeding with an axisymmetric multi-laser source process and it had forming problem in vertical wire feeding with an axisymmetric multi‐laser source process and it had aa strong forming ability which can form the complex structure in a three‐axis machine tool. strong forming ability which can form the complex structure in a three-axis machine tool. The surface of the sample block shows shiny areas where the molten pool was arranged like fish The surface of the sample block shows shiny areas where the molten pool was arranged like fish scales, as shown in Figure 5a. The size of the single track was about 5 mm in width and 0.8 mm in scales, as shown in Figure 5a. The size of the single track was about 5 mm in width and 0.8 mm in height under the present process parameters. height under the present process parameters.
Figure 6. The forming part fabricated by laser additive manufacturing with vertical wire feeding. Figure 6. The forming part fabricated by laser additive manufacturing with vertical wire feeding.
3.2. Grains 3.2. Grains Figure 7 shows the macrostructures of the samples in the T (scanning direction) and L (building Figure 7 shows the macrostructures of the samples in the T (scanning direction) and L (building direction) directions. In the L direction, there were fine equiaxed grains evenly distributed with an direction) directions. In the L direction, there were fine equiaxed grains evenly distributed with average size of 81.5 μm, as shown in Figure 7a,c. In the T direction, most of the grains were equiaxed an average size of 81.5 µm, as shown in Figure 7a,c. In the T direction, most of the grains were equiaxed grains as well, but some columnar grains mixed with them, as shown in Figure 7b. The columnar grains as well, but some columnar grains mixed with them, as shown in Figure 7b. The columnar grains were all short columnar grains (the aspect ratio of was less than five), as observed in Figure 7d; grains were all short columnar grains (the aspect ratio of was less than five), as observed in Figure 7d; the average size of the short columnar grains was 338.6 μm in length and 122.5 μm in width. the average size of the short columnar grains was 338.6 µm in length and 122.5 µm in width. The average size of the equiaxed grains in the T direction was 101.9 μm (Figure 7e). A very The average size of the equiaxed grains in the T direction was 101.9 µm (Figure 7e). A very important important phenomenon could be observed as well: the position of the short columnar grains phenomenon could be observed as well: the position of the short columnar grains coincided with the coincided with the moving path of the molten pool edge from one side to the other in the T direction, moving path of the molten pool edge from one side to the other in the T direction, and the distance and the distance between the two parallel short columnar grains was 3 mm, which was the same as between the two parallel short columnar grains was 3 mm, which was the same as the deposit spacing. the deposit spacing.
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Figure 7. Macrostructure of the fabricated samples: (a) all fine equiaxed grains evenly distributed in the L direction (building direction); (b) most of the grains in the T direction (scanning direction) are equiaxed grains and a small amount of short columnar grains (aspect ratio less than five) which are parallel to each other. Optical micrographs showing the microstructure of the fabricated samples: (c) equiaxed grains in the L direction; (d) short columnar grains and (e) equiaxed grains in the T direction. Figure 7. Macrostructure of the fabricated samples: (a) all fine equiaxed grains evenly distributed in Figure 7. Macrostructure of the fabricated samples: (a) all fine equiaxed grains evenly distributed the L direction (building direction); (b) most of the grains in the T direction (scanning direction) are in the L direction (building direction); (b) most of the grains in the T direction (scanning direction) 3.3. Microstructure equiaxed grains and a small amount of short columnar grains (aspect ratio less than five) which are are equiaxed grains and a small amount of short columnar grains (aspect ratio less than five) which parallel to each other. Optical micrographs of microstructures of the sample in showing the two the directions are shown in Figure samples: 8 with a areThe parallel to each other. Optical micrographs showing themicrostructure microstructure ofthe the fabricated samples: fabricated (c) equiaxed grains in the L direction; (d) short columnar grains and (e) equiaxed grains in the magnification 500 times. microstructures of the two directions (α’) (c) equiaxed of grains in the LThe direction; (d) short columnar grains and (e)exhibited equiaxed martensite grains in the T direction. morphology T direction. (Figure 8a,b), which differs from the Ti‐6Al‐4V component fabricated by laser
deposition with powders which exhibits a coarse α phase [21,22]. The martensite morphology 3.3. Microstructure proves that the cooling rate is faster compared with other AM process. 3.3. Microstructure The diameters of the laser beams in References [9] and [21] were 4.7 mm and 5 mm, which The microstructures sample the two directions are Figure 8 with a The microstructures of of thethe sample in thein two directions are shown in shown Figure 8in with a magnification means the size of the molten pool will be at least about 7 mm [9,21]. However, the size of the molten magnification of 500 times. The microstructures of the two directions exhibited martensite (α’) of pool was smaller in this paper, about 5 mm; in this case, the heat can dissipate quickly. Moreover, 500 times. The microstructures of the two directions exhibited martensite (α’) morphology morphology (Figure 8a,b), which differs from the Ti‐6Al‐4V fabricated laser (Figure 8a,b), which differs from the Ti-6Al-4V component fabricated component by laser deposition with by powders the wire was inserted into the molten pool in the deposition process, which can take away part of the deposition with powders which exhibits a coarse α phase [21,22]. The martensite morphology which exhibits a coarse α phase [21,22]. The martensite morphology proves that the cooling rate is heat. Thereby, the cooling rate was faster in vertical wire feeding with an axisymmetric multi‐laser proves that the cooling rate is faster compared with other AM process. faster compared with other AM process. source using the current process parameters, resulting in a martensite morphology. The diameters of the laser beams in References [9] and [21] were 4.7 mm and 5 mm, which means the size of the molten pool will be at least about 7 mm [9,21]. However, the size of the molten pool was smaller in this paper, about 5 mm; in this case, the heat can dissipate quickly. Moreover, the wire was inserted into the molten pool in the deposition process, which can take away part of the heat. Thereby, the cooling rate was faster in vertical wire feeding with an axisymmetric multi‐laser source using the current process parameters, resulting in a martensite morphology.
Figure 8. Optical micrographs showing the microstructure of the fabricated samples in: (a) T direction; Figure 8. Optical micrographs showing the microstructure of the fabricated samples in: (a) T direction; (b) L direction. (b) L direction.
The diameters of the laser beams in References [9] and [21] were 4.7 mm and 5 mm, which means the size of the molten pool will be at least about 7 mm [9,21]. However, the size of the molten pool was smaller in this paper, about 5 mm; in this case, the heat can dissipate quickly. Moreover, the wire was Figure 8. Optical micrographs showing the microstructure of the fabricated samples in: (a) T direction; (b) L direction.
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inserted into the molten pool in the deposition process, which can take away part of the heat. Thereby, the cooling rate was faster in vertical wire feeding with an axisymmetric multi-laser source using the Appl. Sci. 2017, 7, 227 7 of 10 current process parameters, resulting in a martensite morphology.
3.4. Mechanical Properties 3.4. Mechanical Properties The results of the static tensile tests at room temperature are shown in Figure 9. The average The results of the static tensile tests at room temperature are shown in Figure 9. The average ultimate tensile strength was 1140.33 MPa with standard deviation of 11.59 in the T direction ultimate tensile strength was 1140.33 MPa with thethe standard deviation of 11.59 in the T direction and and 1114.67 MPa with the standard deviation of 20.81 in the L direction. However, the increase in 1114.67 MPa with the standard deviation of 20.81 in the L direction. However, the increase in the the ultimate tensile strength resulted in a low strain (about 6%). The high strength and low ductility ultimate tensile strength resulted in a low strain (about 6%). The high strength and low ductility of the of the samples were mainly caused by the martensite. samples were mainly caused by the martensite. The components components fabricated fabricated by by LAM LAM with with paraxial paraxial wire wire feeding feeding usually usually exhibit exhibit anisotropic anisotropic The mechanical properties properties different directions which is caused by the development of coarse mechanical in in different directions which is caused by the development of coarse columnar columnar grains [23,24]. grains [23,24]. Due to the large number of equiaxed grains, it is obvious that although the samples obtained Due to the large number of equiaxed grains, it is obvious that although the samples obtained from the T direction a better strength and ductility the obtained samples from obtained from the from the T direction hadhad a better strength and ductility than the than samples the L direction, L direction, the difference between the two directions became smaller when compared with the the difference between the two directions became smaller when compared with the components components fabricated by the paraxial internal wire feeding method [9]. Therefore, the anisotropy fabricated by the paraxial internal wire feeding method [9]. Therefore, the anisotropy problem of the problem of the mechanical properties has been greatly improved. mechanical properties has been greatly improved.
Figure 9. Ultimate tensile strength and strain of the block sample fabricated by vertical wire feeding Figure 9. Ultimate tensile strength and strain of the block sample fabricated by vertical wire feeding with an axisymmetric multi‐laser source. with an axisymmetric multi-laser source.
4.4. Discussion Discussion ItIt is observed that the sample block shows the novel grain morphology. All the columnar grains is observed that the sample block shows the novel grain morphology. All the columnar grains are short columnar grains, and the position of the short columnar grains coincides with the edge of are short columnar grains, and the position of the short columnar grains coincides with the edge of the the molten pool one from one to the other in the T The direction. The distance the regions molten pool from side to side the other in the T direction. distance between thebetween regions where short where short columnar grains occur is the same as the track spacing. According to our knowledge, columnar grains occur is the same as the track spacing. According to our knowledge, this phenomenon this phenomenon has never been found in LAM or other AM process. The explanation is as follows. has never been found in LAM or other AM process. The explanation is as follows. There are are two two dominant dominant solidification solidification mechanisms mechanisms in in the the local local molten molten pool pool during during the the LAM LAM There process, namely the heterogeneous nucleation on partially melted wire for equiaxed grains and the process, namely the heterogeneous nucleation on partially melted wire for equiaxed grains and the epitaxial growth growth from the pool bottom for columnar grains. The competition between them epitaxial from the pool bottom for columnar grains. The competition between them determines determines the component grain morphology [25]. The growth direction of the columnar grains is in the component grain morphology [25]. The growth direction of the columnar grains is in the same the same direction as the temperature gradient in the molten pool. The high temperature gradients direction as the temperature gradient in the molten pool. The high temperature gradients promote the promote the growth of coarse columnar grains and make them grow throughout the sample [26]. growth of coarse columnar grains and make them grow throughout the sample [26]. In the vertical wire feeding process, the width of the molten pool was about 5 mm, and the diameter of the wire was 1.2 mm, which is nearly a quarter of the width of the molten pool. Therefore the wire can take away part of the heat flow, and the temperature gradient of the molten pool was disturbed in this way. When the wire was inserted into the molten pool during the deposition process, the wire could conduct a portion of the heat flow, as shown in Figure 10. The temperature guidance was disturbed, thus preventing the epitaxial growth of the columnar grains at the bottom. Hence, the heterogeneous nucleation on partially melted wire for equiaxed
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In the vertical wire feeding process, the width of the molten pool was about 5 mm, and the diameter of the wire was 1.2 mm, which is nearly a quarter of the width of the molten pool. Therefore the wire can take away part of the heat flow, and the temperature gradient of the molten pool was disturbed in this way. When the wire was inserted into the molten pool during the deposition process, the wire could conduct a portion of the heat flow, as shown in Figure 10. The temperature guidance was disturbed, thus preventing the epitaxial growth of the columnar grains at the bottom. Hence, Appl. Sci. 2017, 7, 227 8 of 10 the heterogeneous nucleation on partially melted wire for equiaxed grains can be the main solidification mechanism inthe themain molten pool, and itmechanism grows at the between and the molten pool, grains can be solidification in interface the molten pool, the and wire it grows at the interface resulting in the formation of Figure 10a. between the wire and the molten pool, resulting in the formation of Figure 10a. However, for for the the edge edge of of the the molten molten pool pool where where the the impact impact of of the the wire wire was was small, small, a a high high However, temperature gradient still existed, leading to the formation of short columnar grains. Thereby, when the temperature gradient still existed, leading to the formation of short columnar grains. Thereby, when molten pool solidifies, short columnar grains will be formed on both sides of the molten pool and the molten pool solidifies, short columnar grains will be formed on both sides of the molten pool and equiaxed grains formed in the middle. When the molten pool moves the next track, remelting equiaxed grains will will bebe formed in the middle. When the molten pool to moves to the next track, will occur,will which makes the short columnar of thegrains original molten pool molten remelt, pool contributing remelting occur, which makes the short grains columnar of the original remelt, to the formation of equiaxed grains. New, short columnar grains will be formed at both sides of the contributing to the formation of equiaxed grains. New, short columnar grains will be formed at both molten pool, as shown in Figure 10b. sides of the molten pool, as shown in Figure 10b.
Figure 10. (a) The schematic of the grain solidification in molten pool; (b) the schematic of the Figure 10. (a) The schematic of the grain solidification in molten pool; (b) the schematic of the formation formation of grain morphology. of grain morphology.
When the Ti‐6Al‐4V block is deposited layer by layer, the morphology (see Figure 11) appears When the Ti-6Al-4V block is deposited layer by layer, the morphology (see Figure 11) appears at last. So the block sample shows the grain morphology (Figure 7), where short columnar grains are at last. So sample showsThat the grain morphology 7), where short the columnar grains parallel to the the block building direction. is the reason why (Figure the distance between two parallel are parallel to the building direction. That is the reason why the distance between the two parallel columnar grains is the same as the deposit spacing. columnar grains is the same as the deposit spacing.
formation of grain morphology.
When the Ti‐6Al‐4V block is deposited layer by layer, the morphology (see Figure 11) appears at last. So the block sample shows the grain morphology (Figure 7), where short columnar grains are parallel to the building direction. That is the reason why the distance between the two parallel Appl. Sci. 2017, 7, 227 9 of 11 columnar grains is the same as the deposit spacing.
Figure 11. The schematic of the grain morphology of the fabricated sample. Figure 11. The schematic of the grain morphology of the fabricated sample.
5. Conclusions In this paper, the forming ability of LAM with coaxial wire feeding and the microstructure and mechanical properties of the fabricated component were investigated. Vertical wire feeding with an axisymmetric multi-laser source can eliminate the anisotropic forming problem in the deposition process. No matter whether the tool path is round or polyline, the forming morphology of the sample deposited by the vertical wire feeding method is smooth and there is no difference in all directions. A conclusion can be drawn that vertical wire feeding with an axisymmetric multi-laser source has a strong forming ability. The samples fabricated by the vertical wire feeding method at the current process parameters exhibited fine equiaxed grains and some short columnar grains (parallel to each other). The average size of the equiaxed grains was about 91.7 µm. The average size of the short columnar grains was 339.6 µm in width and 122.5 µm in height. The microstructure of the samples exhibited fine basket-weave and martensite morphology. The static tensile tests showed that the average strain of the samples was about 6%, and the average ultimate tensile strength of the L direction and T direction was 1140.33 MPa and 1114.67 MPa, respectively. The ultimate tensile strength difference of the two directions (L and T) becomes smaller. Therefore, vertical wire feeding with an axisymmetric multi-laser source at the current process parameters can improve the ultimate tensile strength, and the anisotropy of the mechanical properties of the fabricated parts can be largely eliminated. Acknowledgments: This research is supported by the National Natural Science Foundation of China (No. 51405018), the Discipline Group Innovation Collaboration Project of China Association for Science and Technology (No. 2016XKYL05), Fundamental research of the State Ministry (No. JCKY2016602B007) and the Excellent Young Scholars Research Fund of the Beijing Institute of Technology (No. 2016CX04026). The authors would like to thank the anonymous reviewers for their helpful comments and suggestions to improve the quality of this paper. Author Contributions: Jie Fu, Lin Gong, Yifei Zhang conceived and designed the experiments; Junchao Chang and Yifei Zhang performed the experiments; Lin Gong and Xuezhi Shi analyzed the data; Jiping Lu contributed reagents/materials/analysis tools; Jie Fu, Lin Gong and Qianru Wu wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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