Study On The Equipment And Process Of Heat Assisted Friction Stir Welding Of Steel

For the problem of friction stir welding is difficult to weld high strength and high melting point material like steel,and the pin wears easily,special pin for thin steel plate and heat assisted device was designed in this paper.The effects of process parameters on the forming,microstructure,hardness and mechanical properties of Q235 and 304 stainless steel heat source assisted friction stir welding were studied.The fracture morphology and fracture mechanism of the joint were analyzed.The formation and strengthen mechanism of the strength organization were revealed.By rating Q235 and 304 stainless steel weld surface appearance,the results prove that on under normal circumstances,with increasing the rotational speed and the welding current or decreasing welding speed,the amount of melting metal near surface flash and ahead of key hole increased,otherwise it decreased.When the rotational speed is too high or too low,the change of the amount of melting metal ahead of key hole was not obvious.Because when the rotational speed is too low,FSW heat production is not enough to increase amount of the TIG arc affected melting material.When the rotational speed is too high,the friction coefficient reduced,pin heat production verge to stable value,the total heat input run to stable value.When the welding heat input is too large,the back side of the weld near the advanced side appears groove.The results of the research on the macroscopic morphology of the TIG affected area show that the effect of FSW severe plastic deformation can increase the penetration of the TIG arc affected zone.FSW heat and resistance heat and arc thermal superposition make arc melting depth increase;FSW severe plastic deformation make soften the material filled to the gap,forming plastic extrusion welding area,the higher the speed,the greater plastic extrusion welding area thickness.By research on microstructure and microhardness of Q235 steel and 304 stainless steel weld,the results of the show that the weld microstructure of cross section is basically divided into 5 zones,include the shoulder stirring zone,thermo-mechanically-affected-zone,heat affected zone,resistance heating zone and base metal.Surface fine crystal area is ferrite or austenite recrystallization layer,the "rhizome" organization is recrystallization of ferrite or austenite grain.The "root hair" organization in Q235 weld is martensite.The overall hardness of the weld is higher than that of the base metal,the hardness of Q235 steel near surface area is up to 252.9HV,and the maximum hardness of the center line of the 304 stainless steel thickness is 360HV.With higher the rotational speed and welding current or lower welding speed,the hardness of the joint will be higher.The results of research on the mechanical properties show that when the rotational speed is 800r/min,the welding speed is 40mm/min,the current is 60A,,the Q235 weld tensile strength is up to 514MPa,which is 20%higher than that of the base metal,because the fine grain zone and the martensite transition layer provide fine grain strengthening and phase transformation strengthening.The Influence level of orthogonal test parameters on tensile strength of 304 stainless steel joints is welding current>welding speed>rotational speed;When the rotational speed is 800r/min,the welding speed is 50mm/min,the welding current is 60A,the maximum tensile strength of 304 stainless steel joints is up to 650MPa,close to base matal strength.