summary
Conventional laser beam welding (LBW) is difficult to achieve deep penetration of titanium alloy thick plates in atmospheric environments, because the laser beam energy is easily dissipated in the air. In this study, LBW was proposed in a low vacuum environment (100 Pa) to achieve a deep weld penetration of 16 mm of Tie6Al e4V alloy thick plate, which was more than 1 times deeper than that of atmospheric joints (7 mm). In addition, the content of impurity gas elements (N, H, O) almost does not change after vacuum LBW (VLBW), which ensures the welding quality. The fusion zone is mainly composed of needle-like A0 martensite, which enhances the strengthening of the A0/A0 interface, resulting in a higher microhardness than the base metal. The average tensile strength of the joint reaches 1010 MPa, and the strength efficiency is 100%. The mechanism of microstructure evolution during VLBW is discussed. This study provides a reference for the engineering application of titanium alloy thick plate VLBW.
introduction
Titanium alloy has the advantages of high strength, low density and excellent corrosion resistance, and is widely used in aviation, nuclear industry, aerospace, shipbuilding and other industrial fields. With the application of titanium alloys in the industrial field, welding has attracted more and more attention. To date, the main welding methods to achieve deep penetration of titanium alloys are arc welding and high-energy beam welding, such as electron beam welding (EBW) and laser beam welding (LBW). When arc welding of titanium alloy medium and heavy plates or plates (more than 10mm), porosity and cracks are easy to occur in the fusion zone. Moreover, multi-layer, multi-pass filler wire welding reduces welding efficiency. In addition, arc welding causes large residual stress and coarse microstructure, which reduces the mechanical properties of welded joints. For electron beam welding of titanium alloys, it has been reported that the titanium alloys have been successfully welded with a strength efficiency of more than 90%. However, due to the limitation of the vacuum chamber, it is generally difficult to realize the welding of large structural parts by electron beam welding. In addition, the electron beam welding process requires high vacuum and is susceptible to electromagnetic field interference, resulting in increased processing costs and welding difficulties. In addition, it is often difficult to integrate with other auxiliary equipment such as robotic arms in the electron beam welding process, resulting in poor flexibility and difficulty in successfully connecting some complex-shaped parts that require special welding angles.
Compared with arc welding, LBW, as a mature high-energy beam welding method, has a series of advantages such as fast welding speed, less residual stress and deformation, no need to groove before welding, and no need to fill the wire during welding. In addition, LBWs offer lower costs and greater flexibility than EBWs, as LBW devices are often easy to integrate with other ancillary equipment such as robotic arms, and LBWs enable excellent welding of complex, irregularly shaped large structures. Parts at any welding angle. Therefore, LBW has been widely used to join titanium alloys. However, in conventional titanium alloy LBW, the welding process is usually carried out in an atmospheric environment. When a laser beam irradiates a sheet, metal vapor and a plasma plume are typically generated. In this case, most of the energy is dissipated during the LBW process, which makes it difficult to achieve deep penetration welding of titanium alloy thick plates. Therefore, conventional LBW is mostly used for welding titanium alloy sheets and medium plates with a thickness of less than 10mm, which limits the engineering application of LBW on titanium alloy plates.
In recent years, Katayama et al. have proposed Vacuum Laser Beam Welding (VLBW), which provides a new direction for the application of LBW. The LBW process is carried out in a vacuum chamber and has been used to weld thick plates. In addition, unlike EBW, which requires a high vacuum, VLBW can be advanced under a low vacuum [19,20,22e26]. Therefore, VLBW technology greatly improves the welding efficiency, saves the processing cost, and has great application potential in the welding of titanium alloy thick plates in important industrial fields such as aerospace, automotive, and shipbuilding.
Compared with LBW in the atmospheric environment, VLBW will effectively inhibit the plasma plume and improve energy utilization, thereby increasing the weld penetration [. However, VLBW technology is still in its infancy in the welding of titanium alloy thick plates, and its application still lacks sufficient data, such as process optimization, joint structure and performance, etc., which limits its engineering application. Moreover, the liquid flow and solidification of titanium alloy thick plates in the weld pool is more complex than that of thin plates and medium plates. It should be noted that titanium alloys exhibit lower thermal conductivity compared to aluminum and copper alloys. Therefore, it is difficult to achieve deep penetration welding of titanium alloy thick plates.
At present, only a few studies have successfully achieved the welding of Tie6Ale4V alloy using VLBW technology, and good joints are often obtained under critical vacuum (<10 Pa), which paves the way for the application of this technology. VLBW titanium. However, only the microstructure of VLBW joints has been briefly characterized, and the detailed microstructure characterization and microstructure evolution mechanism of VLBW joints need to be further studied. Moreover, compared with aluminum, steel and copper alloy joints, titanium alloy joints are much more sensitive to atmospheric elements such as nitrogen, hydrogen and oxygen (N, H and O), but the content of N, H and O is not reported in the above literature, which may affect the performance of VLBW titanium alloy joints. In addition, it should be pointed out that the vacuum level of VLBW has a great influence on the penetration of the weld. Increasing the vacuum degree can increase the weld penetration to a certain extent [27,28], while decreasing the vacuum degree can improve the welding efficiency. Therefore, it is of great significance to further study the VLBW of titanium alloy thick plates under low vacuum.
Based on the unique characteristics of VLBW, in order to better understand the difference in weld penetration of titanium alloy LBW joints in atmospheric and vacuum environments, Tie6Ale4V alloy was used as a typical allotrope phase (a+b), and the weld was selected by LBW in atmospheric and low vacuum (100 Pa) environments.
In addition, the microstructure evolution mechanism of VLBW joints was analyzed in detail. The purpose of this study is to explore the feasibility of achieving deep penetration in a low vacuum environment, and to study the relationship between the microstructure and mechanical properties of VLBW Tie6Ale4V alloy joints, so as to provide a reference for future industrial applications.
Experimental setup
In this study, in order to study the influence of the welding process on the penetration of the joint weld, the material received was a 100 mm thick rolled annealed Tie 6 Al 4V alloy plate. Before welding, the base material (BM) is treated as follows. The BM was mechanically polished and chemically cleaned with acetone to remove the surface oxide film, and then LBW in an atmospheric and low-vacuum environment (100 Pa), a schematic of the VLBW process is shown in Figure 1a. The same LBW welding parameters were performed in an atmospheric and vacuum environment, with an output power of 6 kW, a welding speed of 1 m/min, and a defocusing distance of 0 mm.
Fig. 1 Schematic diagram of (a) VLBW process and (b) tensile specimen
Metallographic samples are cut through the center of the LBW joint, and each sample is mechanically ground and polished to characterize its cross-sectional structure. Samples for macro- or microstructure observation were etched using Kroll reagent (consisting of 5 ml HNO3, 2 ml HF, and 100 ml H2O), and then characterized by (om, Axio observer Z1), SEM (SEM, ZEM-15), electron backscatter diffraction (EBSD, Oxford symmetry) in 0.06 mm steps, and transmission electron microscopy (TEM, Talos F200). In this study, TD, ND, and WD represent lateral, normal, and welding directions, respectively. All optical and EBSD micrographs are taken in the end plane. EBSD data were analyzed using AZtec crystal software. Samples for EBSD and transmission electron microscopy were prepared by dual jet electropolishing using a solution of 6 vol % HClO4 or 34 vol % CH3OH or 60 vol % C4H9OH at a voltage of 20 V and a temperature of 25 to 31 In addition, oxygen, nitrogen, and hydrogen (N, H, and O) content in the substrate and VLBW joints were measured by an oxygen/nitrogen/hydrogen analyzer (TCH600). The elemental analyzer uses inert gas melting technology, and the sample can be melted in a graphite crucible at a temperature of more than 3000°C. The amounts of nitrogen, hydrogen and oxygen are then determined by infrared absorption. The microhardness of the joint was tested with a force of 500 g and a loading time of 15 s. Dog bone tensile specimens with a nominal length of 10 mm, a width of 4 mm, and a thickness of 2.5 mm were prepared and stretched at room temperature with an Instron 8801 universal testing machine.
Picture of the paper
Fig.2. Macroscopic view of a typical cross-sectional view of a titanium alloy LBW joint in (a) atmospheric and (b) vacuum environments
Fig. 3 (a) OM macroscopic image of VLBW, SEM magnified micrograph of VLBW joints in regions (b), (c)d, (d)E in Figure 3a, and regions (E) A and (f) B in Figure 2b
Fig. 4 EBSD analysis of the phase distribution of FZ in regions A and B in Figure 2b: (A) top and (c) bottom layers, (b) reverse pole diagram (IPF) pattern of top and (d) bottom layers
Fig.5 Misorientation angle fractions of the top layer and bottom layer of FZ(a) and (b).
Fig.6. TEM images of (A) top and (B) bottom layers of FZ in regions A and B in Figure 2b and diffraction patterns of selected regions
Fig.7 Fractions of the thickness of (a) the top and (c) bottom layers, and the fractions of the aspect ratios of (b) the top and (d) bottom layers in FZ
Fig.8. Microhardness of the welded layer of VLBW joints
Fig.9. Engineering stress-engineering strain curves of VLBW joints and BM and specific sampling locations
Fig.10. Fracture locations of the top layer (a) and (b) bottom layer of VLBW joints
Fig.11 Schematic diagram of the evolution of the microstructure of the VLBW joint: (a) the microstructure before welding, (b) and (c) the solid-state transition during welding, and (d) the final solidification after welding
conclusion
In this work, thick plates of titanium alloy were welded in an atmospheric and vacuum environment, and the following conclusions can be drawn.
(1) The penetration depth of the VLBW joint (16 mm) is greater than that of the atmospheric pressure joint (7 mm), and the VLBW deep penetration welding of titanium alloy thick plate is successfully realized.
(2) FZ is mainly composed of acicular martensite, which is unevenly distributed in the welded layer, but there is no obvious preference orientation.
(3) VLBW fittings have a higher hardness than BM fittings. The average tensile strength of the joint reaches 1010 MPa, the strength efficiency is 100%, and the fracture position is in BM. In addition, the content of the impurity gas elements (N, H and O) barely changed after VLBW.
Thesis information
Realizing deep penetration and superior mechanical properties in a titanium alloy thick plate joint via vacuum laser beam welding
https://doi.org/10.1016/j.jmrt.2023.08.059
The copyright of this article belongs to the original author, only for communication and learning, and the final interpretation right belongs to this official account (laser manufacturing research).
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