The Optimization Of Resistance Butt Welding Process Parameters And The Study On Joint Structure And Properties Of Closed Stirrup

Closed stirrup welding forming system is an important part of the manufacturing process of concrete prefabricated components.It conforms to the trend of "building industrialization and sustainable development of green building".And it is the key research direction of modern construction industry projects.Resistance butt welding technology has the advantages of high welding efficiency,high joint strength,low cost,simple and is easy to control.Resistance butt welding has become the mainstream method of closed stirrup welding.However,in the process of closed stirrup welding,the defects such as incomplete penetration,crack,brittle fracture and so on are often caused by the unreasonable setting of welding process parameters.In order to improve the application and strengthen development of resistance butt welding technology in the field of closed stirrup automatic welding,the closed stirrup automatic welding production line was developed by Institute of Building Mechanization/China Academy of Building Research and CABR Construction Machinery Technology Co.Ltd.In this paper,the principle,process and process parameters of resistance butt welding of closed stirrup were analyzed firstly.Secondly,the range of resistance butt welding process parameters of closed stirrup was obtained through tensile test of welded joints under different welding process parameters.Finally,on the basis of the range,the metallographic structure and mechanical properties of each welding process parameter on welded joints were studied.According to the actual production process,the welding parameters with better mechanical properties were found.When the steel bar with diameter of 6mm was automatically welded with closed stirrup,the range of the welding process parameters were as follows: the welding relative current was 76%～80%,the upsetting pressure was 0.2～0.3 MPa,and the welding time was 0.3～0.4 s.When the steel bar with diameter of 8mm was automatically welded with closed stirrup,the range of the welding process parameters were as follows: the welding relative current was 65%～68%,the upsetting pressure was 0.2～0.3 MPa,and the welding time was 0.5～0.6 s.When the steel bar with diameter of 10 mm was automatically welded with closed stirrup,the range of the welding process parameters were as follows: the welding relative current was 65%～68%,the upsetting pressure was 0.2～0.3 MPa,and the welding time was 0.8～0.9 s.The qualified welding joint of tensile test can be obtained by welding the closed stirrup with any value matching within the range of corresponding welding process parameters.In the range of welding parameters,with the increase of welding relative current,the grain size of each region increased gradually,and the content of widmanstatten structure increased gradually,which leaded to the decrease of toughness of the joint and prone to crack or brittle fracture.With the increase of upset pressure,the content of Widmanstatten structure and ferrite decreased,the grain was refined gradually,and the plastic toughness of the joint was enhanced.With the increase of welding time,the grain size of each region of the joint became larger and more uneven,which made the toughness of the joint decrease.Therefore,combined with the actual production process,on the premise of meeting the mechanical properties of the welded joint,the recommended welding relative current for the reinforcement with a diameter of 10 mm was 65% and 66%,the upset pressure was 0.3 MPa,and the welding time was 0.8 s.It laied a foundation for the digital control of closed stirrup automatic welding production line and the wide application of reinforcement welding equipment.