Study Of Microstructure And Properties Of 45 Steel Welding Joint With Different Brazing Joint Clearance

In actual production,there is a welding problem of low joint strength when the low-strength brazing material encountered is brazed in the large-area brazing high-strength base metal in the butt joint form,and the related research,especially the gap of the brazing seam,is insufficient.In this paper,low-strength pure copper brazing filler metal is used to braze high-strength 45 steel.The joints are in the form of large-area butt joints.Under the same conditions of other welding processes,the brazed joints are obtained by designing 6 sets of gap values including 0.02mm,0.06mm,0.20mm,0.40mm,0.50mm and 0.70mm,and the mechanical properties and microstructure analysis of the welded joints were studied.The influence of the gap between the joints on the microstructure and properties of the joints was studied in detail.The following conclusions can be drawn:(l)The test piece was heated to a temperature of 20?/min to 1150?,and held for 15 min,and then under the welding process conditions of the furnace cooling,a better quality welded joint was obtained.The joint between the brazing seam and the base metal in the joint is mismatched,the combination is tight,the interface is well organized,and there are no defects such as porosity,cracks,and no brazing,and the joint forming effect is good;(2)The interdiffusion of the base metal and the brazing filler metal is sufficient.The Cu in the solder penetrates into the intergranular crystal and forms a uniform iron-based solid solution strip parallel to the brazing seam on the base material near the interface,the thickness of which is about 8 ?m,and the interdiffusion between the base metal and the brazing filler metal Under the combined action of dissolution,some of the narrow strips are separated in a small block form and migrate toward the center of the brazed joint to form an "island"structure.The Fe in the base material is sufficiently dissolved to the brazing filler metal and forms a uniformly distributed free iron element and a copper-solute solute iron-copper solid solution in the brazing joint after the weldment is cooled;(3)The tensile test at room temperature shows that the tensile strength of the joint increases first and then decreases with the increase of the gap between the joints.The tensile strength of the joint with the gap of 0.40mm is the highest,reaching 629.62 MPa,which is about the resistance of the base metal.The tensile strength is 99.56%,the brazing tensile strength is 3.61 times,and in the gap range of 0.06mm to 0.50mm,the tensile strength of the joint is not less than 600MPa.The strength of the joint in this range is very close to the strength of the base metal.The bearing capacity of the joint is basically the same as that of the base metal,which is in full compliance with the strength requirements of the welded joint.The tensile fracture sites of the specimens are at the center of the brazing joint,not at the interface.The fracture type is brittle and ductile hybrid fracture,and a large number of iron-copper solid solution with copper as solute is found in the fracture,which is beneficial to the joint strength;(4)The hardness test results show that the hardness of the center of the brazing joint is basically about 80HV regardless of the width of the brazing joint gap,which is basically the same as the hardness of the brazing material itself.The hardness of the base metal is basically in the range of 150 to 230HV,and the hardness of the joint interface.The value is between the base metal and the brazing seam,which is the result of the mutual diffusion of the parent metal and the brazing filler metal at the interface;(5)The fatigue test results show that the fatigue strength of the joint increases first and then decreases with the increase of the gap between the joints,and the fatigue strength of the joint with the gap of 0.06 mm is the highest.Under the condition of 2×106 cycles,the fatigue strength of joints with a gap of 0.40 mm under conditions of 50%and 95%respectively was 277.61 MPa and 258.94 MPa,respectively,and the fatigue performance of the joint was good.The fatigue specimens are all broken in the brazed joint,and the crack originates from the processing defects on the surface of the specimen,and then expands from the joint interface to the center of the brazed joint;(6)Based on the study of the microstructure and mechanical properties of joints with different brazing gaps,the optimum brazing gap value is 0.06mm when using pure copper to braze 45 steel.Under this gap value,the joint strength,plasticity and toughness are good.