Study on the Keyhole Behavior,Microstructure and Properties of the Welded Joints in K-TIG Welding

Author:Cui Shu Wan

Supervisor:shi yong hua


Degree Year:2019





Keyhole tungsten inert gas(K-TIG)welding is based on the traditional TIG welding,and the arc can penetrate the entire workpiece and achieve keyhole effect welding by using large welding current.For medium-thick metal plates,it can single-sided weld and double-sided form in a single pass,without opening grooves.The dynamic behavior of keyhole in the K-TIG welding process has a great effect on the quality of K-TIG welded joints.At present,the study on the dynamic behavior of keyhole during the K-TIG welding is rare.Duplex stainless steel and titanium alloy have great application prospects in the field of marine engineering.For example,the hull construction of polar condensate tanker also uses duplex stainless steel and titanium alloy materials.In this study,a passive vision sensing system was used to obtain the image of the keyhole exit on the back side of the workpiece,and the evolution law of the characteristic parameters of the keyhole with the welding process parameters was analyzed.On the basis of understanding the welding mechanism of K-TIG,the K-TIG welding experiments were carried out by using S32101 duplex stainless steel and TC4 titanium alloy respectively.The relationship between the properties of welded joints and microstructure evolution was established.In this paper,the welding parameters such as the gas flow rate and the distance between the tip of the tungsten needle and the surface of the workpiece to be welded(CTWD)are constant.The welding speed and the welding current are respectively changed to obtain the characteristic parameters of keyhole exit(length perpendicular to the welding direction YA,length along the welding direction XA and the area of the keyhole exit).The results show that under certain welding parameters,the mean value of the YA,XA and area of the keyhole exit gradually increase with the increase of welding current or the decrease of welding speed.The area of the keyhole exit is sensitive to the change of the weld profile.When the area of the keyhole exit is zero(when there is no keyhole),the weld is not penetrated and the pores are not formed.When the area of the keyhole is too large,the volume of the molten pool is excessively grown,and the weld seam collapses or even burns through.In the S32101 duplex stainless steel K-TIG plate welded joint,the microhardness of the heat-affected zone(HAZ)and the weld metal area(WM)are greater than that of the base metal zone(BM).The tensile strength of the WM is also greater than that of the BM,while the elongation and impact absorbed energy of the WM are less than that of the BM.According to the analysis data of microstructure in the welded joint,it shows that the content of austenite,the proportion of Σ3 coincidence site lattice(CSL)grain boundary and the random phase boundary proportion of ferrite and austenite have an effect on the impact toughness of WM.When the proportion of Σ3 CSL grain boundary increases,the random phase boundary ratio of ferrite and austenite increases,and the austenite content increases,the corresponding impact toughness also increases.The change in austenitic texture of the welded joint affects the plasticity of the WM.The study found that the morphology of the duplex stainless steel butt welds,the microstructure,mechanical properties and corrosion resistance of the welded joints have changed significantly under four different heat input conditions of 1.99 kJ/mm,2.14 kJ/mm,2.30 kJ/mm and 2.46 kJ/mm.With the increase of heat input,the microhardness and tensile strength of the WM are gradually decreased,while the elongation and impact absorbed energy of the WM are gradually increased.The intergranular corrosion resistance of the HAZ and WM are gradually increased with the increase of heat input.Under the same heat input condition,the intergranular corrosion resistance of the WM is better than that of the HAZ.The intergranular corrosion resistance of the welded joint is affected by both the proportion of the Σ3 CSL grain boundary and the content of chromium nitride.When the welding current is 510~550 A,as the welding current increases,the elongation and the impact absorbed energy of TC4 titanium alloy K-TIG WM increase gradually,and the tensile strength of the WM gradually decreases.In TC4 titanium alloy K-TIG welded joint,the microstructure of the HAZ and WM hardly changed,but the content of alloy elements such as Al,V and Ti in the BM and WM were less changed.When the proportion of the High angle grain boundary(HAGB)in α phase of the HAZ and WM in TC4 titanium alloy K-TIG butt welded joint increases,the proportion of the Σ9 CSL grain boundary also increases.The proportion of HAGB has a certain influence on the impact toughness of TC4 titanium alloy KTIG WM.The higher the proportion of HAGB,the better the impact toughness.