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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.31 No.5 pp.245-254
DOI : https://doi.org/10.3740/MRSK.2021.31.5.245

Improving Mechanical Properties of Wire Arc Additively Manufactured Ti-6Al-4V Alloy by Ultrasonic Needle Peening Treatment

Hui-Jun Yi1, Jin-Woo Kim1, Young-Lak Kim2, Sangyong Shin3
1Defense Manufacturing Engineering Team, Hyundai-Rotem Company, Changwon, 51413, Republic of Korea
2Strategy and Planning Division, KISWEL LTD., Seoul, 04627, Republic of Korea
3School of Materials Science and Engineering, University of Ulsan, Ulsan 44610, Republic of Korea
Corresponding author E-Mail : yi.h.jun@gmail.com (H.-J. Yi, Hyundai-Rotem Co.) sshin@ulsan.ac.kr (S. Shin, Univ. of Ulsan)
March 17, 2021 April 20, 2021 April 21, 2021

Abstract


Wire arc additive manufacturing (WAAM) is being considered as a technology to replace the conventional manufacturing process of titanium alloys. However, coarse β grains, which can extend through several deposited materials, result in strong textures and anisotropy. As a solution, we study the plastic deformation effects of ultrasonic needle peening (UNP) on the microstructure. UNP treated materials deform plastically and the dislocation density increases. Fine α+α' grains with low aspect ratio are observed in the UNP treated specimens. UNP treated WAAM Ti-6Al-4V alloys have higher strength and lower elongation than those characteristics of WAAM Ti-6Al-4V alloys. Due to UNP treatment, the z-axis directional specimens exhibit a greater effect of reducing elongation than do the x-axis directional specimens. The UNP treatment produces fine grains in proportion to the number of times UNP is performed, thereby increasing strength. UNP processes produce a large number of dislocations in the WAAM Ti-6Al-4V alloys, with the most dislocations being formed at the surface.



초록


    © Materials Research Society of Korea. All rights reserved.

    This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

    1. Introduction

    Titanium alloys are widely used in aerospace applications due to titanium's high strength/weight ratio, exceptional corrosion resistance, and good fatigue resistance. Titanium possesses a variety of highly desirable properties due to its high ductility, strength, and thermal resistance; however, titanium is also rendered difficult to machine.1,2) Subtractive manufacturing is inefficient with respect to time and material waste. Near net shape fabrication of metallic components via additive manufacturing (AM) is an important new technological area with many potential applications in the aerospace industry. Wire arc additive manufacturing (WAAM) is one such AM process that uses a consumable wire as the feedstock and employs an electric arc or plasma welding torch as the heat source. The WAAM process yields a much higher deposition rate than most other metal additive manufacturing techniques. This low cost process is thus suited toward producing large scale parts with less complex geometries.3,4)

    The current concerns and issues associated with additively manufactured aerospace components with alloys such as Ti-6Al-4V alloy are that coarse-columnar prior β grains are nearly always observed in AM processes. With wirebased AM, β grains are often as tall as the build height and with larger components can be tens of centimeters long.5-8) The strong tendency to form coarse-columnar β grain AM structures with Ti-6Al-4V alloys is difficult to avoid because the structures result from a combination of solidification conditions in a small heated moving melt pool, where there is a steep positive thermal gradient at the solidification front and the metallurgical characteristics of the alloy itself. When combined with the high partition coefficients of Al and V in Ti, this limits the degree of possible constitutional supercoiling and nucleation ahead of the solidification front, which also becomes very difficult.9-11) As a result, homo-epitaxial re-growth takes place within each melted layer, allowing coarse directional grain structures to develop and grow through many deposited layers. Additionally, due to the preferred <001> growth direction in cubic metals, the large columnar grains tend to have a strong β <001> fiber texture parallel to the average solidification direction within a particular AM process.9,10)

    Several methods have been proposed to solve the problems caused by epitaxial grain growth in the build direction. The first method involved changing the texture by applying plastic deformation to the deposited metal through plastic deformation processing applied in this study.11-15) The second method involved controlling the chemical composition of materials. There was a method for reinforcing solids and improving mechanical properties by injecting an element such as boron.16) Finally, the proposed method involved controlling the cooling rate. To control the cooling rate, research has been performed that involves quenching via liquid CO2 after welding.17-18)

    In this study, an alternative approach has been investigated to improve the large columnar β grain structures and strong textures typically seen in WAAM processes. This involved the introduction of a small sequential deformation step with the deposition of each layer. The deformation step was applied using an ultrasonic needle peening device such that each deposited layer could be lightly deformed prior to the addition of a new layer of material. The aim of this approach was to determine if it was possible to introduce sufficient plastic deformation into each layer such that β grain refinement could occur during re-heating. Moreover, the plastic deformation behavior of deposited metals by ultrasonic needle peening (UNP) treatment and the effect of grain refinement on the mechanical properties was investigated. It can be applied in the field of surface coating to further improve the hardness or strength on the metal surface.

    2. Experimental Procedure

    2.1 WAAM Ti-6Al-4V alloys

    The material used in this study was Ti-6Al-4V alloy with a chemical composition of Ti-5.61Al-3.89V-0.13Fe- 0.11O-0.2C-0.03N-0.004H (wt%). To control UNP treatment during the WAAM process, a fixture was used, as shown in Fig. 1. To verify the effects of plastic deformation caused by UNP treatment, an origin specimen, a 1L specimen with UNP treatment applied at every layer, and a 3L specimen with UNP treatment applied every 3 layers was fabricated. The welding conditions for the WAAM process to fabricate deposited metal can be found in Table 1, the interpass temperature was 100 °C and UNP treatment was performed at 100 °C. The 1L specimen was subjected to the UNP treatment for each layer subjected to the WAAM, and the greatest amount of heat and deformation was applied, and the change in microstructure was greatest compared to the O specimen subjected to the WAAM only. The 1L specimen requires a lot of cost and time compared to the O specimen, but the microstructure and mechanical properties will change the most. The 3 L specimen was subjected to the UNP process for every three layers subjected to the WAAM, so that less heat and deformation than the 1L specimen were applied, so that the change in the microstructure was less than that of the 1L specimen. The 3L specimen will cost less and less time than the 1L specimen, but will have less variation in microstructure and mechanical properties.

    2.2 Microstructural analysis and mechanical testing

    The macro and microstructure of WAAM Ti-6Al-4V alloys were characterized via optical microscopy, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). The side planes of the WAAM Ti- 6Al-4V alloys were polished and etched with 5 % HF, 10 % HNO3, and 85 % distilled water for 10 s. EBSD analysis was conducted with a field emission scanning electron microscope. EBSD measurements were performed after micro polishing using a 1 μm diamond suspension under an applied potential of 15 kV with a 0.25 μm step size. The data were then interpreted using orientation imaging microscopy analysis software provided by TexSEM Laboratories, Inc.

    Tensile specimens were obtained in accordance with ASTM E8M-05 for two directions: the build direction (zaxis) and the travel direction (x-axis). Tensile tests were performed only once at room temperature at a strain rate of 10−3 s−1 using a universal testing machine with a 100 kN capacity (Instron 8501). Vickers hardness tests were performed with a micro-Vickers hardness tester according to ASTM E92-82(2003), under a 0.5 kg load with a dwell time of 10 s.

    3. Results

    3.1 Microstructure

    Fig. 2 shows the microstructure of the top, middle, and bottom positions of all the specimens via optical microscopy. It was possible to readily distinguish coarse columnar prior β grains. Epitaxial grain growth of the coarse columnar prior β grains up through many deposited layers was clearly observed in Fig. 2. Columnar prior β grains formed via epitaxial grain growth were observed in the 3L and 1L specimens; however, the length was 10 ~ 20 mm, shorter than that of the O specimen and prior β grains formed in a polygonal shape close to the sphere in some areas. In the O specimen, the grain boundary α was thick. On the other hand, in the 3L and 1L specimens, the grain boundary α was narrower than that for the O specimen. Fig. 3 shows the microstructure of the top position of all the specimens via optical microscopy. The microstructure of the bottom position of the O and 1L specimens can be seen in Fig. 4 via SEM. The O specimen possessed a larger grain boundary α and a larger size of α grains formed inside the Widmanstätten structure compared to the 1L specimen. It could be seen that the grain boundary α grew into the Widmanstätten structure.

    The additively manufactured metals formed via welding processes were subsequently heat affected by subsequent welding processes. Regions that did not receive subsequent welding heat input were classified as deposited weld metal (ADWM) regions and regions that received subsequent welding heat input were classified as reheated weld metal (RWM) regions. The microstructures of the ADWM and RWM regions that formed near the fusion line were observed via SEM and can be seen in Fig. 5. The grains were larger in the RWM region than in the ADWM region in all the specimens because the grains in the heated RWM region grew due to subsequent welding. Comparing the RWM region, the α grains were larger in the O specimen than in the 1L specimen. In all the specimens, α grains grew in the same direction at the grain boundary α, resulting in a thicker and sharper grain boundary α.

    3.2 Tensile properties and Vickers hardness results

    Fig. 7 shows the tensile properties of all the specimens. The z-axis directional specimens possessed a lower yield strength and tensile strength and higher elongation compared to the x-axis directional specimens. In the same direction, the yield and tensile strength values were high in the order of the 1L, 3L, and O specimens, with a low elongation. In other words, specimens to which UNP treatment was applied to a greater extent possessed higher strength and lower elongation. With regard to the x-axis directional specimens, the yield strength increased by 1 ~ 5 % with UNP treatment; with regard to the z-axis directional specimens, the yield strength increased by 1 ~ 2 % with UNP treatment. In other words, the effect of increasing yield strength was greater in the x-axis directional specimens compared to the z-axis directional specimens with UNP treatment. In the elongation results, elongation was reduced by 8 ~ 14 % with UNP treatment in the z-axis directional specimens and elongation was reduced by 8 ~ 12 % with UNP treatment in the x-axis directional specimens. In other words, the z-axis directional specimens possessed a greater effect of reducing elongation than the x-axis directional spe7cimens as a result of UNP treatment. Fig. 8 shows the Vickers hardness test results for all the specimens. Vickers hardness increased in the order of the 1L, 3L, and O specimens as a result of UNP treatment.

    4. Discussion

    Fig. 9 shows the diameter, area, and aspect ratio for the α + α’ grain distribution at the top region of the O, 1L, and 3L specimens. The diameter and area of the α + α’ grains for the 3L and 1L specimens were smaller than that for the O specimen. The aspect ratio of α + α’ grains was similar for all the specimens. In the UNP treated specimens, when the ultrasonic needle hit the surface of the deposited metal, mechanical energy from the ultrasonic needle was transferred to the surface of the specimen, which rendered plastic deformation and the generation of dislocations on the surface. At this time, the plastic deformation layer or the plastic strain area that formed depended on the quantity of mechanical energy. To observe the plastic deformation layer or the plastic strain area that formed on the deposited metal surface by UNP treatment, EBSD analysis was performed on the surface of the deposited metal with applied UNP treatment. Figs. 10(b1) and (c1) show the inverse pole figure (IPF) map and Kernel average misorientation (KAM) map of the inner region of the deposited metal without UNP treatment. Figs. 10(b2) and (c2) show the IPF map and KAM mapof the surface of the deposited metal with applied UNP treatment. As seen in the red region of Fig. 10(c2), heavily strained areas appeared on the surface of the deposited metal with applied UNP treatment. The depth of the surface was approximately 5 μm.

    The α-β titanium alloy Ti–6Al–4V is particularly sensitive to the thermal history as it can develop a range of microstructures depending on temperature and deformation. 19,20) Large quantities of dislocations existed in the strained area where plastic deformation occurred in the surface region of the deposited metal with applied UNP treatment; these dislocations could serve as heterogeneous nucleation sites for β and α grains in subsequent WAAM processes. In other words, the high dislocation density region formed by UNP treatment was inhibited by the growth of columnar prior β grains for epitaxial grain growth. Therefore, in the specimens with applied UNP treatment, smaller prior β grains and fine α grains were formed compared to the O specimen, resulting in high strength. However, the elongation of the deposited metal was lower than that of the O specimens in the specimens with applied UNP treatment.

    From the results, UNP treatment yielded fine grains in proportion to the number of times that UNP treatment was performed, thereby increasing the strength. It is welldocumented that the presence of grain boundary α phase tends to reduce elongation in conventionally processed Ti–6Al–4V by furnishing a preferential path for damage accumulation along the prior β grain boundaries.2,21) Fig. 11 shows the effect of UNP treatment and WAAM processing on recrystallization and grain growth. The coarse grain boundary α and Widmanstätten structures (α or α' grains) formed through the 2nd pass welding process. Subsequent 2nd pass UNP processes produced a large number of dislocations with the most dislocations being formed in the surface region. In subsequent 3rd pass welding processes, the microstructure of the 2nd layer was refined because recrystallization was caused by dislocations created during the 2nd pass UNP process. Finally, grains in the 2nd layer grew through the 4th pass welding process. In this way, the microstructure of all the layers of the 1L specimen formed and the microstructure of the 3L specimen exhibited less of a peening effect than the 1L specimen. Coarse grains formed in the O specimen because no recrystallization occurred due to dislocations or a strained area.

    5. Conclusions

    WAAM Ti-6Al-4V alloy was fabricated with UNP treatment and their microstructure was analyzed to investigate the effects of UNP treatment on the microstructure and tensile properties of WAAM Ti-6Al-4V alloys.

    • (1) The grains of specimens treated with the UNP process were smaller than those of the origin specimen. When the ultrasonic needle hit the surface of the deposited metal, the mechanical energy of the ultrasonic needle was transferred to the surface of the specimen, rendering plastic deformation and generating dislocations on the surface. Dislocations could serve as heterogeneous nucleation sites for β and α grains in subsequent WAAM processes.

    • (2) Specimens to which UNP treatment was applied were higher in strength and lower in elongation. The zaxis directional specimens exhibited a greater effect of reducing elongation than the x-axis directional specimens due to UNP treatment.

    • (3) UNP treatment produced fine grains in proportion to the number of times UNP was performed, thereby increasing strength. UNP processes produced a large amount of dislocations, with the most dislocations being formed at the surface. In subsequent welding processes, the microstructure of the layers was refined because recrystallization was caused by dislocations created by the UNP process.

    Author Information

    Hui-Jun Yi

    Deputy General Manager, Defense Manufacturing Engineering Team, Hyundai-Rotem Company

    Jin-Woo Kim

    Assistant Manager, Defense Manufacturing Engineering Team, Hyundai-Rotem Company

    Young-Lak Kim

    Sales Director, Strategy and Planning Division, KISWEL

    Sangyong Shin

    Associate Professor, School of Materials Science and Engineering, University of Ulsan

    Figure

    MRSK-31-5-245_F1.gif

    Schematic drawings of ultrasonic needle peening (UNP) treatment and the wire arc additive manufacturing (WAAM) process.

    MRSK-31-5-245_F2.gif

    Optical microstructure of the origin WAAM specimen (O), the specimen with UNP treatment applied at every layer (1L), and the specimen with UNP treatment applied every 3 layers (3L).

    MRSK-31-5-245_F3.gif

    Optical microstructure of the top region of the O, 1L, and 3L specimens.

    MRSK-31-5-245_F4.gif

    SEM microstructure of the bottom region of the O and 1L specimens.

    MRSK-31-5-245_F5.gif

    SEM microstructure of (a) and (b) as deposited weld metal (ADWM) and (c) and (d) reheated weld metal (RWM) regions of the O and 1L specimens.

    MRSK-31-5-245_F6.gif

    EBSD results of the inverse pole figure maps for the O, 1L, and 3L specimens.

    MRSK-31-5-245_F7.gif

    Tensile properties of the O, 1L, and 3L specimens.

    MRSK-31-5-245_F8.gif

    Vickers hardness distribution of the O, 1L, and 3L specimens.

    MRSK-31-5-245_F9.gif

    Distributions of (a) diameter, (b) area, and (c) aspect ratio for α+α’ grains at the top region of the O, 1L, and 3L specimens.

    MRSK-31-5-245_F10.gif

    EBSD results of the (a) image quality map, (b) inverse pole figure map, and (c) kernel average misorientation map of the 1L specimen.

    MRSK-31-5-245_F11.gif

    Illustration of the effect of ultrasonic needle peening (UNP) treatment and the wire arc additive manufacturing (WAAM) process on recrystallization and grain growth.

    Table

    Welding parameters of the WAAM process.

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