Effect of Heat Input on Porosity Defects in a Fiber Laser Welded Socket-Joint Made of Powder Metallurgy Molybdenum Alloy.

Miao-Xia Xie,Yan-Xin Li,Xue-Wu Wang,Xiang-Tao Shang,Jun-Yu Pei

doi:10.3390/ma12091433

Abstract

Porosity defects are still a challenging issue in the fusion welding of molybdenum and its alloys due to the pre-existing interior defects associated with the powder metallurgy process. Fiber laser welding of end plug and cladding tube made of nanostructured high-strength molybdenum (NS-Mo) alloy was performed in this work with an emphasis on the role of welding heat input. The distribution and morphology of porosity defects in the welded joints were examined by computed tomography (CT) and scanning electron microscopy (SEM). Preliminary results showed that laser welding of NS-Mo under low heat input significantly reduced the porosity defects in the fusion zone. The results of computed tomography (CT) showed that when the welding heat input decreased from 3600 J/cm (i.e., 1200 W, 0.2 m/min) to 250 J/cm (i.e., 2500 W, 6 m/min), the porosity ratio of the NS-Mo joints declined from 10.7% to 2.1%. Notable porosity defects under high heat input were related to the instability of the keyhole, expansion and the merging of bubbles in the molten pool, among which the instability of the keyhole played the dominant role. The porous defects at low heat input were generated as bubbles released from the powder metallurgy base metal (BM) did not have enough time to overflow and escape.

Highlights

Due to its high fusion point, excellent mechanical properties and low neutron absorption cross-section [1,2,3], nanostructured high-strength molybdenum alloy (NS-Mo) has been selected as one of the candidate materials of the generation Accident Tolerant Fuel (ATF) to avoid accidents such as that occurring at the Fukushima Nuclear Power Station in 2011
Tomography (CT)indicated test indicated that welding input had impact on the number and the size of the porosity defects in fusion welding joints of Figure significant impact on the number and the size of the porosity defects in fusion welding joints of NSreveals that 7higher heat input results moreresults porosity defects and larger porosity size. porosity
Results of the scanning electron microscopy (SEM) test indicated that the welding heat input had significant impact on the shape of the porous defects in the joints of NS-Mo

Summary

Introduction

Due to its high fusion point, excellent mechanical properties and low neutron absorption cross-section [1,2,3], nanostructured high-strength molybdenum alloy (NS-Mo) has been selected as one of the candidate materials of the generation Accident Tolerant Fuel (ATF) to avoid accidents such as that occurring at the Fukushima Nuclear Power Station in 2011. High-quality and reliable welding technology of NS-Mo must be developed before it can be widely applied in various structures. The appearance of a new molybdenum alloy NS-Mo and its wide application prospects have aroused a research upsurge in welding technology of molybdenum alloys [5,6,7,8,9,10]. The welded seam produced using brazing shows poor high-temperature performance, which does not satisfy the service requirements of cladding tubes at high temperature. Farrell et al [13] found that electrode sticking and molten metal expulsion

Methods

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Discussion

Conclusion