Study On Automatic TIG Welding Of Invar Alloy At Vertical Position

In recent years,the air pollution problems such as fog haze are increasingly serious in our country.As a kind of clean energy,the exploitation and utilization of Liquefied Natural Gas(LNG)have a significant place in energy saving and environmental protection.The LNG ships play an important role in the marine transportation of liquefied natural gas,thus the manufacturing technology of LNG ships has gradually been paid much more attention in recent years.As a typical alloy with low expansion coefficient,Invar alloy is a main material in manufacturing the inwall of thermal insulation layer of LNG ships.But invar alloy welding process window is narrow,the microstructure of HAZ is coarse etc.Therefore,this paper carried out the invar alloy automatic gas tungsten arc welding research technology at vertical.It both has certain theoretical significance and important practical value.The invar alloys used in this study are developed by Shanghai Baosteel Special Steel Co,LTD.Invar alloy plates in different thickness was assembled and welded by DC GTAW and Pulsed GTAW respectively.The influence of welding parameters on the microstructure,the appearance and mechanical properties of the lap joints are studied.The results show that: during the process of DC GTAW welding of invar alloys at vertical position with a thickness combination of 0.7 mm/1.5 mm,the weld formation shows a great sensitivity to the transverse offset;the weld forming feature size La and P decreases with the increasement of the welding speed while increases as the welding current increasing.When the transverse offset,welding speed and welding current are set as-0.4 mm,220-260 mm/min and 68-74 A,respectively,well-formed lap joint could be obtained.The fusion zone,as a weak zone when the lap joint,has large difference of the microstructure.The weld zone of the lap joint is composed of coarse columnar grain,which contains cellular crystal substructure that is beneficial for the enhancement of the weld hardness.The HAZ shows a lowest mechanics properties which is caused by the coarse equiaxed grain.All the specimens fractured near the fusion zone,Throughout the heat affected zone and presented a ductile fracture mode.In the process of Plused GTAW welding at vertical position with a thickness combination of 0.7 mm/1.0 mm,the characteristic values of weld formation,La and P,are in direct proportion to the peak current,background current and duty ratio.Among them,the peak current and duty cycle play a more important role.When the pulse frequency are 3 Hz-10 Hz,the value of La and P will increase.However,if the pulse frequency exceed 10 Hz,it will be difficult to play the advantage of Plused GTAW.When the peak current,background current,duty ratio and the pulse frequency are set as 100 A,5A-10 A,30%-50% and 5 Hz-10 Hz,respectively,a well-formed lap joint could be obtained.The Plused GTAW technology can not only make the grain size and width of HAZ and fusion zone obviously decreased,but also improve the mechanical properties of lap joint.Experimental results show that,the fracture failure position is in the base metal with 0.7mm thickness during tensile test,if the characteristic values of the weld seam are in line with the standard requirements for the plused GTAW lap joint.