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1.INTRODUCTIONThe 1950s saw the development of titanium and its alloys, which are now significant metal structural materials. The aerospace, chemical machinery, pharmaceutical engineering, and leisure industries have all made extensive use of TC4 titanium alloy because of its high specific strength, excellent corrosion resistance, and superior overall performance [1–3]. At present, the welding of titanium alloy is usually carried out by traditional tungsten argon arc welding, melting argon arc welding, or plasma arc welding. However, these welding methods need to be filled with welding materials. Due to the limitations of the protective atmosphere, purity, and protective effect, it is easy to cause defects such as embrittlement of the weld zone, welding porosity, and cracks. Compared with other fusion welding, electron beam welding technology has the advantages of high power density, large depth-width ratio, small deformation of the welding zone, low energy consumption, easy control, and automation, and has been highly valued and applied in various industries [3–6]. The use of an energy-concentratedted electron beam as the heat source for TC4 titanium alloy welding has undoubtedly become one of the preferred methods for TC4 titanium alloy welding. However, with this high-energy forming method, welding cracks are also an extremely common and serious defect, which has always been a research hotspot in the field of material processing [7-9 ]. A satellite gas cylinder is a high-pressure vessel. All parts are titanium alloy TC4, which is formed by electron beam welding. The thickness of the base metal on both sides of the weld between the gas cylinder nozzle and the gas cylinder shell is 5 mm, and the welding is not filled with solder. The weld quality requirements reach the ‘GJB1718A-2005’electron beam welding’I level. After welding, the pressure verification test, XYZ three-direction transportation test, and random vibration test were carried out on the gas cylinder, and no obvious abnormality was found. However, in the blasting test of the gas cylinder, the actual blasting pressure was only 24 MPa, which was much lower than the design blasting pressure of 40 ~ 50 MPa. The low-pressure cracking occurred, and the cracking position is shown in Figure. 1. In this paper, the cracking reason of the electron beam weld of the TC4 pressure vessel was determined by analyzing the metallographic structure, microfracture characteristics, microhardness, oxygen, and hydrogen content of the weld. 2.TEST PROCESS AND RESULTSThe gas cylinder’s failure due to weld cracking is the result of numerous circumstances. The typical macroscopic and microscopic analytical techniques, including energy spectrum analysis, metallographic analysis, microhardness, oxygen and hydrogen content, and macroscopic inspection, are employed in this work to identify the reasons for weld cracking [10]. 2.1Macroscopic examination of fractureThe crack position of the nozzle is located in the center of the weld. No obvious deformation or mechanical damage is observed on the inner and outer surfaces. The surface of the weld is a bright metal color, and no obvious signs of oxidation are observed. Multiple particles can be seen at the edge of the inner surface weld, and no obvious particles can be seen at the edge of the outer surface weld, as shown in Figure. 2. According to the fact that the crack appears in the weld and has intergranular characteristics, it cracks longitudinally along the weld, and it is judged that the crack is a crystalline hot crack [11]. The macroscopic morphology of the crack is shown in Figure. 3. The whole section has a metallic luster, and a bright band with a width of about 1.5 mm can be seen in the middle of the section. The crystalline structure of the bright band area is obviously different from that of the two sides of the section. There are many small reflective facets in the bright band area, and the sections on both sides show obvious coarse columnar crystals. 2.2Micro-analysisThe fracture analysis was carried out by scanning electron microscopy. The fracture in Figure. 4 was cleaned and observed under a scanning electron microscope. The bright band in the middle of the fracture section showed quasi-cleavage morphology with short-range river-like patterns. Cracks were formed in local areas, and short-range propagation was formed in this area, forming a large number of short and curved tearing edges. At the same time, there was a concentrated distribution of looseness at the edge of the bright band, as shown in Figure. 4. Under additional magnification, the shallow dimples on the surface of the columnar crystals, which comprise the two sides of the section, may be observed, as illustrated in Figure.5. The morphological features demonstrate that brittle cracking is the nozzle weld’s mode of breaking [12]. 2.3Metallographic analysisIn order to observe the microstructure, metallographic samples were prepared from the crack area, the weld position close to the crack, and the weld position far from the crack area. Figure.6 displays the sampling position and number. There was no obvious misalignment in the base metal on both sides of the four samples, and three obvious welding traces were observed in the 1 #, 2 #, and 3 # samples. An obvious welding trace can be seen on the inner and outer surfaces of the #4 specimen, as shown in Figure.7. The weld microstructure of the four samples is relatively coarse. As illustrated in Figure.8, among them, there are evident loose welding defects on the inner surface of the #1 sample’s first welding mark position, but no other samples have clear welding defects like porosity or porosity in the weld area. The structure of the nozzle and the cylinder body is α + β dual-phase structure, and the cylinder body shows obvious deformation streamline, as shown in Figure. 9. 2.4Hardness testingBoth the metallographic analysis and the microhardness test sampling were completed. The weld area and the base metal area on both sides of the sample were subjected to the microhardness test. Table 1 presents the results, which suggest that the excessive H and O concentration in the titanium alloy may be the reason why the weld area’s hardness is higher than the base metal’s on both sides. Table 1Microhardness test results
2.5Weld oxygen and hydrogen content measurementThe hardness and brittleness of titanium alloys are significantly influenced by the H and O contents of the substance. A hydrogen and oxygen gas analyzer located close to the weld was used to determine the H and O concentrations. Table 2 shows that the amount of hydrogen and oxygen in the weld did not surpass the acceptable level, indicating that the excessive hydrogen and oxygen content was not the reason for the brittle cracking in the weld. Table 2Test results of oxygen content and hydrogen content in welds (mass fraction/%)
2.6Energy spectrum analysisThe energy spectrum analysis of the base metal on each side of the weld was performed in order to have a better understanding of the reason behind weld cracking. As seen in Figure.10, the energy spectrum of the cross section was compared and examined. The findings demonstrate that the primary alloying components and contents of the materials used on both sides of the base metal fulfill the standards of the TC4 brand. In addition to the matrix elements, it was discovered that the cross section included elements of Fe, Cr, and Ni. The content of these elements was higher on the cross section close to the inner surface than it was on the cross section close to the outer surface. Figure.11 and 12 display the energy spectrum. The elements Fe, Cr, and Ni in the weld area are anomalous, according to the findings of the energy spectrum analysis of the matrix and the weld area. It can be the result of the stainless steel material melting into the weld area, according to the element content study. On the inner surface of Figure.2, the morphology of the particles close to the weld was noted. The findings demonstrated that the particles’ surfaces were molten and that some of them had visible grinding scratches, suggesting that the particles ought to spatter during the welding process. The energy spectrum analysis results indicate that the majority of the splashes are TC4 matrix splashes, with some of the splashes primarily including elements of Fe, Cr (18.3%), and Ni (8.1%). Based on a preliminary comparison with the energy spectrum of stainless steel, which is displayed in Figure. 13, it is determined that the splashes should be composed of 18-8 stainless steel. Special tooling is utilized for the electron beam welding of the tank’s bottom head and valve nozzle. Figure.14 shows the structure. The material is 18-8 stainless steel, and before welding, the tooling is qualified. After the welding was finished and the aberrant elements were identified by energy spectrum analysis, the tooling was examined. The inside surface of the grove of the stainless steel tooling, known as the “support block,” was discovered to have deposits of both blue and black color. As a result, it may be said that the particles at the inner surface’s weld border are splashes caused by the beam breaking through the weld during the welding process and colliding with the “support block.” 3.ANALYSIS AND DISCUSSION OF THE RESULTSPrior to the electron beam welding of the lower head and valve nozzle weld, the welding, X-ray inspection, and evaluation of the simulated specimen were finished. The welding records demonstrate that the valve nozzle and the lower head of the pressure tank’s electron beam welding process characteristics match those of the simulated specimens and satisfy the process specifications. The tank wreckage was examined following the appearance of the weld crack. The broken weld did not exhibit partial penetration, incomplete fusion, or undercut, and the degree of weld misalignment complied with the grade I standards of GJB1718A-2005. The results of the energy spectrum analysis show that the main alloy elements and contents of the base material used in the weld of the storage tank meet the chemical composition requirements of TC4 titanium alloy raw materials, which further proves that the chemical composition of the raw materials meets the standard requirements. The weld area’s energy spectrum analysis results, however, indicate that the Fe, Cr, and Ni elements are anomalous introduction elements. Additionally, there is a high concentration of anomalous elements in the vicinity of the inner surface weld area, along with a large amount of granular spatter. The composition of certain granular spatters is 18-8 stainless steel, according to the results of the energy spectrum study. Melting stainless steel results in a weld that is both extremely hard and brittle because Fe and Ti elements combine to generate FeTi brittle intermetallic complexes. 4.CONCLUSIONDuring the welding process, due to the abnormal discharge of the welding torch of the welding machine, the electron beam current penetrates the weld and hits the support block of the welding tooling. The raw material of the support block is stainless steel, and the resulting spatter adheres to the surface of the weld, resulting in the integration of stainless steel material into the weld. Therefore, during welding, for the tooling that is heterogeneous with the base metal, groves should be opened at the position where the beam passes to prevent splashing from polluting the weld. 5.ACKNOWLEDGMENTThank you to China Aerospace Science and Technology Corporation CAST Fund Project (2023Y11-012). 6.6.REFERENCELiu Hua,
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