Research On Microstructure And Mechanical Properties Of Friction Stir Welded 1500 MPa Martensitic High-Strength Steel Plates

Friction stir welding(FSW)is a new solid-phase welding technology that has been widely used in the welding of light metals such as aluminum alloys and magnesium alloys.With the development of high wear resistance stirring tools,the rapid development of high-strength steel FSW has been promoted.In this paper,FSW experiments are carried out on 1500 MPa martensitic high-strength steel using traditional FSW,water-cooled FSW and traditional FSW + post-weld heat treatment.In the traditional FSW experiments,the effects of different welding speeds on the microstructure and mechanical properties of the joint and materials flow around the keyhole tissue were studied.Due to the serious softening of heat affected zone(HAZ)in traditional friction stir welding joints,it is hoped that water cooling and post-weld heat treatment can reduce the softening effect of HAZ and improve the mechanical properties of joints.The microstructure and mechanical properties of water-cooled joint and post-weld heat treatment joint were studied,and the strengthening mechanism of these two joints was explored.The main conclusions are as follows:In this study,martensitic steel plates were friction stir welded at various welding speeds of 40-120 mm/min and a constant rotation speed of 300 rpm.A kissing bond was found in the joint welded at 100 and 120 mm/min.It was revealed that the peak temperature exceeded Ac3 for all the welding conditions and martensitic structures were fnally formed in the stir zone of the joints.A signifcant decrease in hardness was located in the heat-afected zone,which had a transitional microstructure from tempered martensite near base metal to a mixed structure containing hard martensite,soft ferrite and bainite near stir zone.For the sound joints,the specimen was fractured in the heat-afected zone during tensile tests and the highest tensile strength could reach about 1058 MPa.In this study,martensitic steel plates were successfully friction stir welded at welding speed of 40 mm/min and rotation speed of 500 rpm both in traditional and water cooling.The microstructure and mechanical properties of the water-cooled and traditional joints were comparatively studied.The results indicated that the peak temperature surpassed Ac3 under all welding conditions and formed a martensitic structure in the stir zone of the joints.The microstructure of the HAZ consisted of tempered martensite,ferrite and martensite of water-cooled joints,compared with the tempered martensite,ferrite,bainite and martensite structure of the traditional joints.Softening of the heat-affected zone was substantially suppressed and the ultimate tensile strength of the joints was obvious enhanced via water cooling from 1002 MPa to 1235 MPa,which was attributed to the grain refinement,the narrow softened zone and fewer carbide precipitation of heat affected zone.In the traditional FSW+ post-weld heat treatment experiments,the rotation speed was 500 rpm and the welding speed was 40 mm/min.The joints were heated at three temperatures of 760 ℃,820 ℃ and 880 ℃ for 30 min,and then water-cooled for post-weld heat treatment.The microstructure and mechanical properties of the joints were studied.The results showed that the microstructure of the joint was martensite and ferrite after treatment at 760 ℃ and 820 ℃.And at 820 ℃,the ferrite content of the joint was less than that at 760 ℃.At 880 ℃,the microstructure of joint was fully martensitic.After post-weld heat treatment,the microstructure of joint tends to be uniform in all areas.Due to the effect of weld thinning,the joints fractured in the stirring zone after different temperature treatments.At a post-weld heat treatment temperature of 880 ℃,the tensile strength of the joint could reach up to 1472 MPa.