Research On Ultrasonic Vibration Or Mechanical Stir Of Molten Pool In The CMT+P Welding Process Of TC4 Alloy

TC4 alloy has the advantages of high specific strength and good comprehensive mechanical properties,so it is widely used in aerospace and petrochemical industries.However,due to its high melting point and low thermal conductivity,the weld metal stays at high temperature for a long time during welding,making the weld grain coarse,thereby the mechanical properties of the joint reduced.In the present work,the ultrasonic vibration and mechanical stir of molten pool were introduced to refine the weld grains in the CMT+P welding of TC4 alloy,thereby improving the mechanical properties of the welded joint,which could provide new methods and theoretical foundation for the research of grain refinement in the welding of TC4 alloy.Firstly,through offsetting the tungsten needle,the parameter range of the ultrasonic-assisted arc welding was determined to ensure the stability of the welding process.Secondly,under different offsets of the tungsten needle,grain size,microstructure and mechanical properties of weld metal were tested and analyzed,to explore the grain refinement effect by ultrasonic vibration,and the influences of ultrasonic vibration on the microstructure and mechanical properties of joints.Afterwards,the bead-on-plate experiment with mechanical stir of molten pool was conducted,in which weld penetration depth,weld width and grain size were surveyed,the characteristics for the mechanical stir of molten pool were analyzed,and the influences of rotation speed of tungsten needle on the weld penetration,weld width and grain size were investigated.Finally,the butt welding experiment with the mechanical stir of molten pool was conducted.The eccentric design was done to increase stir radius.The influences of different stirring parameters on the microstructure and mechanical properties of the weld zone were analyzed and investigated.The experiment results showed that when the tungsten needle was inserted into the molten pool from the front side of the weld,tungsten needle burning or metal adhesion would be induced with too small or too large offsets of tungsten needle.Under the experimental conditions,the suitable offset selection range of the tungsten needle was from 2.5mm to 3.5mm.The microstructure in weld zone obtained was columnar grains.The ultrasonic vibration of molten pool could inhibit the growth of the columnar grains.When the offset of tungsten needle was 3.5mm,the grains were finer.The microstructure of the weld metal was mainly the basket-weave structure,which consisted of?phase,?'phase and?phase.The ultrasonic vibration of molten pool increased the acicular martensite in the weld zone,forming some Widmanst?tten structure,and influenced the grain growth direction of the secondary phase.The ultrasonic vibration of molten pool increased the microhardness of the weld metal and improved the tensile properties of the joints.The fractures of the joints were located at the weld zone,which mode was a mixed fracture mode of brittle fracture and ductile fracture.The mechanical stir of molten pool increased the weld penetration depth and weld width in the bead-on-plate experiment.With the increase in rotation speed of tungsten needle,the weld width gradually increased,but the weld penetration depth had no apparent trend.The mechanical stir of molten pool could refine the grains.At the range of 750r·min^-1 to 2000r·min^-1,the grains in the weld metal gradually decreased with the increase in rotation speed.The mechanical stir of molten increased the amount of acicular martensitic,thereby increasing the microhardness of the weld metal.The mechanical stir of molten inhibited the growth of the columnar grains in the weld metal,thereby improving the tensile properties of the joints.When the eccentric mechanical stir fixture was utilized,the grains would be finer,and the tensile properties of the joints were better.