| FV520B steel is used as the manufacturing material of the impeller,which is the core component of centrifugal compressors,due to its high strength,good toughness and corrosion resistance.Laser welding has become one of the most promising technical means in the field of high-end equipment manufacturing due to its advantages of high efficiency,low heat input,and small joint deformation.The advantages of laser welding can just make up for the shortcomings of the traditional welding method of the impeller,thereby improving the quality of the weld and the accuracy of the parts.In this paper,FV520 B stainless steel was used as the base metal,and the laser welding process parameters were optimized under the larger process window by using orthogonal design and analysis methods,and the rule of the influence of each welding parameter on the responses such as penetration was obtained.The optimized parameters were used to weld the test plates,and the microstructure and performance of the welded joint were investigated.Aging treatment was carried out at different temperatures to explore the evolution of the microstructure of the weld after heat treatment,and the microhardness,impact toughness and corrosion resistance of the weld under different heat treatments were investigated.The CMT welding was used to fill the upper surface of the laser welding,and the microstructure and performance of the joint after filling were analyzed.Research indicates:(1)When performing welding on a 12 mm steel plate,with the increase of the laser line energy,the penetration shows a tendency of increase,and presents a power function relationship with the line energy.Within the selected parameter range,the degree of influence of each factor on penetration is: laser power> welding speed> spot diameter> defocus.After the initial selection and reselection of parameters,the optimized parameter combination is: laser power 4.2 kW,welding speed 35 mm/s,spot diameter 1.2 mm.(2)The microstructure of weld fusion zone(FZ)is mainly composed of low-carbon lathy martensite and ? ferrite.There are a small amount of carbides on the martensite,and the ? ferrite exhibits worm-like and lath-like morphology.As the line energy density increases,the microhardness value of the weld decreases slightly.Under the optimized parameters,the hardness of the fusion zone of the butt weld is slightly lower than that of the base metal,and the softening of the heat affected zone is obvious.After removing sagging and surface defects,the strength of FZ is higher than that of the base metal.Compared with the heat-treated FV520 B steel,the strength of the weld is higher,but the plasticity and toughness are lower.(3)Aging treatment was performed on the laser welded joint of FV520 B steel plate at a heating temperature of 300 ?~900 ? and a time range of 9 min~12 h.When aging at 550 ?,with the increase of holding time,spheroidization of ? ferrite in FZ occurs,and its amount gradually decreases,and it almost disappears at 12 hours of aging.When heated to a higher temperature,? ferrite spheroidizes and disappears faster.After aging at 550 ? for 12 h,the obtained microstructure contains a small amount of reversed austenite.With the increase of aging temperature and holding time,the volume fraction of reversed austenite increases.However,after aging at 650 ? for 12 h,all the reversed austenite in the weld disappeared.It can be seen that the amount of reversed austenite at room temperature is not only related to the volume fraction of reversed austenite obtained at the aging temperature,but also to the stability of the reversed austenite during the cooling process.The hardness and impact toughness of the weld mainly depend on the balance between the dispersion strengthening effect of the Cu-rich precipitates and the softening effect of the tempered microstructure and reversed austenite.The corrosion resistance of the weld is higher than that of the base metal.With the increase of aging temperature and aging time,the passivation behavior of the weld in NaCl solution weakens or even disappears.(4)CMT filler welding will generate secondary heating on the microstructure of the laser welding seam,so that the microstructure of the laser welding area near the fusion line changes from columnar crystals to equiaxed crystals.With the increase of heat input of CMT welding,the equiaxed crystal area expands and even the laser welded area is all transformed into equiaxed crystals.In addition,the crystal grains also grow.After filling,the hardness distribution of the weld cross section horizontally from the laser welding area to the base metal is more uniform.The long-term heat input of the welding leads to a decrease in the strength of the base metal.While the impact toughness of the laser weld fusion zone increases with the increase of the heat input of the filler welding and is higher than the base metal.The corrosion resistance of the laser welding area is higher than that of the base metal. |
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