Microstructure and Properties of Cobalt-Based Alloy Encapsulation Workpiece by Laser Arc Hybrid Welding AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY25010017

Chinese Library Classification Number:TG44

Citation Information:

The laser arc hybrid welding technology was used to weld the cladding structure of the radioisotope heat source，and the effects of different pulse frequencies on the microstructure and mechanical properties of the welded joint of L605 cobalt-based alloy cladding structure were studied in this paper. During the welding process，a high-speed camera was used to observe the changes in the arc morphology in laser arc hybrid welding when the pulse frequency was 250 Hz. Under the combined action of laser and arc，the laser heat source provides efficient local heating and had a significant attraction to the arc，causing the front end of the arc to tend to the laser irradiation area，thereby effectively suppressing the sputtering phenomenon and improving the stability of the molten pool. The edges of the macroscopic morphology of the weld of the cladding workpiece were more flush，and the surface was well-formed. On the cross-section of the cladding structure，the end plug cover and the outer cladding tube wall were well fused，without obvious welding defects such as pores or unfused areas，and the molten pool presented a "bell jar" shape. The upper part of the molten pool was greatly affected by the arc and had a wider area，while the lower part was deeply affected by the laser and had a deeper melting zone. The overall molten pool cross-section showed the typical characteristics of laser and arc hybrid heat sources. The pulse frequency had a significant effect on the microstructure of the welded joint. With the change of the pulse frequency，the microstructure morphology and grain size of the welded joint changed significantly. When no pulse current was applied，the upper part of the welded joint showed an equiaxed crystal structure，while the lower part of the welded joint was a long columnar crystal. This was because a temperature gradient was formed from top to bottom during the cooling of the molten pool：the cooling rate of the upper part was higher，and the grains grew in an equiaxed morphology；the lower part was close to the substrate，the cooling rate was lower，and the grains grew perpendicular to the heat flow direction to form columnar crystals. After the pulse frequency was introduced，the grains of the welded joint were further refined as the pulse frequency increases，and the fusion zone was almost completely composed of equiaxed crystals. At the same time，in order to further study the diffusion behavior of alloy elements at the interface of the weld zone and the fusion zone under different pulse frequencies，the diffusion of alloy elements at the interface of the fusion zone and the weld zone after welding was analyzed by scanning electron microscopy （SEM），energy spectrum analysis （EDS） and X-ray diffraction （XRD） and other test methods. The results showed that CrFeNi and FeNi phases existed in the weld zone at different frequencies，and W element was also found in the weld zone. With the change of pulse frequency，the physical phase in the weld zone did not change significantly，and W-rich particles were precipitated in the base metal zone. Compared with direct current，pulse current significantly increased the content of Cr element and reduced the content of Co element. The effect of pulse current on the hardness of the weld joint was small when no pulse current was applied，and when pulse current was applied，the hardness value in the depth direction of the weld showed a downward trend. The average hardness of the weld surface area was about HV 450，and the hardness of the heat-affected zone was HV 320，which was close to the hardness of the base metal （about HV 300）. Microstructure analysis showed that the increase in pulse frequency promoted grain refinement and the transformation of columnar crystals into equiaxed crystals，thereby improving the mechanical properties. After direct current welding，the fracture extended along the crystal cleavage plane，and there was no obvious dimple structure on the tensile fracture，showing cleavage fracture；after the introduction of pulse current，a large number of dimples gradually appeared on the tensile fracture，showing ductile fracture. With the increase of pulse frequency，the number of dimples on the fracture gradually increased，and was mainly concentrated at the fracture interface.