Effect of preheating on the microstructure and properties of fiber laser welded girth joint of thin-walled nanostructured Mo alloy

et al. 2020

Metals

Owing to its potential application prospect in novel accident tolerant fuel, molybdenum alloys and their welding technologies have gained great importance in recent years. The challenges of welding molybdenum alloys come from two aspects: one is related to its powder metallurgy manufacturing process, and the other is its inherent characteristics of refractory metal. The welding of powder metallurgy materials has been associated with issues such as porosity, contamination, and inclusions, at levels which tend to degrade the service performances of a welded joint. Refractory metals usually present poor weldability due to embrittlement of the fusion zone as a result of impurities segregation and the grain coarsening in the heat-affected zone. A critical review of the current state of the art of welding Mo alloys components is presented. The advantages and disadvantages of the various methods, i.e., electron-beam welding (EBW), tungsten-arc inert gas (TIG) welding, laser welding (LW), electric resistance welding (ERW), and brazing and friction welding (FW) in joining Mo and Mo alloys, are discussed with a view to imagine future directions. This review suggests that more attention should be paid to high energy density laser welding and the mechanism and technology of welding Mo alloys under hyperbaric environment.

Section: Laser Weldingmentioning

confidence: 99%

Research Status and Progress of Welding Technologies for Molybdenum and Molybdenum Alloys

Zhu¹,

et al. 2020

Metals

“…In addition, it was confirmed that the oxygen contents at the GBs within the FZ of joint 2 was significantly higher than that of joint 1. Generally, MoO 2 will segregate on GBs after laser welding no matter which heat input is used [5,6,17,23]. Then, TEM was used to identify the phase in the FZ of each joint, and the TEM results are shown in Figure 14a,b, respectively.…”

Section: Composition Of Grain Boundary Segregationmentioning

confidence: 99%

“…The reliable welding technology of the NS Mo alloy is the premise of its wide application in various complex structures. Therefore, the weldability of this new type of molybdenum alloy has been widely studied in recent years [5][6][7][8][9][10].…”

Section: Introductionmentioning

confidence: 99%

“…Laser welding not only has the advantages of high-power density and low-heat input, but also can be carried out in an open environment without vacuum chamber [24][25][26][27][28][29][30][31]. Therefore, laser welding of molybdenum alloy has been studied by many researchers [5][6][7][8][9][10]32]. However, welding of molybdenum alloys still faces many unsolved issues, especially those related to the newly developed molybdenum alloys.…”

Section: Introductionmentioning

confidence: 99%

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Microstructure and Mechanical Properties of a Fiber Welded Socket-Joint Made of Powder Metallurgy Molybdenum Alloy

et al. 2019

Metals

Self Cite

Fiber welding of socket-joints made of nanostructured high-performance molybdenum alloy (NS Mo) was carried out to get a better understanding of the role of welding heat input. It was found that low heat input (i.e., high welding speed) resulted in significantly refined grains in the fusion zone (FZ) of fiber laser welded NS Mo joints. When welding heat input decreased from 3600 J/cm (i.e., 1.2 kW, 20 cm/min) to 250 J/cm (i.e., 2.5 kW, 600 cm/min), the tensile strength of welded joints increased from about 250 MPa to about 570 MPa. It was confirmed by energy spectrum analysis that the higher the welding heat input, the higher the oxygen contents at the grain boundary (GB) within the FZ. In addition, the most important reason for poor strength of welded joints of Mo alloys was reported as being that MoO2 was segregated on the grain boundary. Therefore, it was concluded that welding under low heat input (i.e., high welding speed) was able to reduce the segregation degree of MoO2 at the grain boundary by refining grains and increasing the total area of GBs, thus improving the strength of welded joints and reducing the proportion of the intergranular fracture zone in tensile fractures.

“…Zirconium alloy is widely used to produce nuclear fuel claddings and core structural parts [1][2][3]. However, when the temperature exceeds 1200 • C, zirconium can react with water vapor to produce large amounts of hydrogen, easily causing explosions, and release large amounts of heat, further accelerating the melting of a reactor core [4,5].…”

Section: Introductionmentioning

confidence: 99%

Rotary Friction Welding of Molybdenum without Upset Forging

et al. 2020

Materials

A large instantaneous axial forging load is required to be applied for the final stage of rotary friction welding (RFW), which is usually conducive to obtaining clean, compact, and high-quality joints. However, for slender fuel claddings made of molybdenum (Mo) with low stiffness, the instantaneous axial forging load cannot be applied at the final stage of welding. This study carried out RFW tests without upset forging on Mo in the atmospheric environment and investigated the effects of welding time on joint morphology, axial shortening, microstructures, microhardness, tensile strength, and tensile fracture morphology. It found that the excessive and abrupt burning and a lot of smoke were generated around the weld zone during welding and spiral flashes were observed after welding. Under welding pressure of 80 MPa and spindle speed of 2000 r/min, the minimum average grain size and maximum tensile strength can be obtained in 4 s when the welding time is between 2–5 s. Scanning electron microscope (SEM) results show that there were morphologies of a large number of intergranular fractures and a small number of transgranular fractures in the fracture. The above results demonstrated that it is feasible to use RFW without upset forging to seal the last weld spot on upper end plugs of fuel claddings made of Mo in high-pressure inert gas, which would not only obtain reliable welding quality but also seal high-pressure inert gas in cladding tubes. The research results have a practical guiding significance of manufacturing accident-tolerant Mo nuclear fuel cladding.