| GH909 low expansion superalloy is widely used in aerospace,electric power industry,military weapons and other fields due to its advantages of high strength,low expansion coefficient and good cold/thermal fatigue properties.With the increasingly complex component structure and the gradual improvement of service performance requirements,the quality of GH909 medium-thick plate welded joints is posed a serious challenge.Laser welding has many advantages,but there are problems such as poor joint formation,obvious pores,and many hot cracks in GH909 medium-thick plate laser welding.In recent years,the heat source oscillating strategy has shown significant advantages in stabilizing the welding pool,reducing welding defects,and improving the microstructure.In this paper,the effect of beam oscillating on stable molten pool flow and dynamic behavior of keyhole was studied.The causes of formation of hump,pores and hot cracks in laser welding of GH909medium-thick plate,as well as the suppression effect and mechanism of beam oscillation on defects were analyzed.The regulation law of beam oscillation on microstructure evolution and the mechanism of joint strengthening and toughening were summarized.Finally,a penetration welded joint with excellent mechanical properties was obtained.The main conclusions are as follows:(1)Through the study of single laser welding of GH909 medium-thick plates,it was found that the molten pool fluctuated violently,and defects such as spatter and undercut were easy to occur.The melt that continued to flow to the bottom of keyhole has a strong impact force,and it was easy to form a root hump.The violently evaporated metal vapor could easily cause the instability of the keyhole wall,and the continuous impact of the melt flow on the rear wall of the keyhole could easily cause it to collapse and form pores.The large temperature gradient of the thermal cycle,severe element segregation in the fusion zone and excessive growth of the γ/Laves eutectic phase together lead to the formation of crystalline cracks and liquefaction cracks in the fusion zone and heat-affected zone.(2)Through the laser oscillating welding process research and numerical simulation,it was found that the beam oscillation could effectively promote the heat flow diffusion and melt flow in the molten pool,reduce the temperature gradient and liquid flow fluctuation in the molten pool,and improve the stability of the molten pool and keyhole.The laser oscillation eliminated the continuous impact of the melt in the bottom area of the keyhole and lowered the melt temperature there to increasing the surface tension,which ultimately eliminated joint forming defects and slumping.(3)Through high-speed camera synchronous observation combined with numerical simulation,it was found that beam oscillaing enlarged the opening of keyhole,suppressed the continuous impact of the melt flow on the rear wall of the keyhole,and reduced the tendency of the keyhole to collapse to form pores.The keyhole oscillating with the laser beam could effectively absorb the bubbles in the surrounding molten pool,and induce multiple dynamic swirling liquid flows to promote the escape of bubbles in the molten pool.Under the optimized parameters,a weld with a porosity of less than 1% could be obtained.(4)By analyzing the microstructure of the welded joint and combining the characteristics of welding thermal cycle,it was found that the beam oscillating promote the liquid flow exchange in the molten pool,effectively suppresse the segregation of components in the fusion zone,reduce the temperature gradient of the molten pool,weaken the driving force of the γ/Laves eutectic reaction,and promote the diversification of the growth orientation of γ dendrites which avoided the excessive growth of the eutectic phase at the end of the dendrites.As a result,the crystallization cracks in the fusion zone and the liquefaction cracks in the heat-affected zone were effectively eliminated.(5)On the basis of eliminating welding defects by beam oscillation,the formation of equiaxed grains in the joint was promoted.The joint strengthening and toughening mechanism of Nb,Ti atomic solid solution strengthening,fusion zone grain refinement strengthening,second phase strengthening of coherent interface eutectic phase and in-situ precipitation(Nb,Ti)C particles and joint slip tendency reduction were generated.The maximum chamber/high temperature tensile strength,average microhardness and maximum notched bending force of the joint reach 95.2%,97.9%,90.3% and 94.6% of the base material,respectively. |
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