Research On Dissimilar Inertia Friction Welding Of High-strength Low-alloy Steel 30CrMnSiNi2A And 45 Steel

High-strength low-alloy(HSLA)steels 30 CrMnSiNi2 A have been widely applied in aircraft and military parts due to their high tensile strength,excellent fatigue performance,and good plasticity and toughness.Considering that medium carbon steel can be easily machined and is relatively inexpensive,it is broadly used to manufacture components such as axle and shaft parts and gears.Combining HSLA steel with carbon steel allows the benefits of both metals to be taken advantage of,which could have broad applications in the field of welding.It is necessary to obtain a sound joint between dissimilar steels because of their different thermal conductivity values and chemical compositions.HSLA steel 30 CrMnSiNi2 A often needs to be welded with the same material or with different steels(such as 45 steel)in practical engineering applications,it is imperative to suppress the formation of a decarburized layer and carbides at the carbon-steel side of the weld interface during long welding processes.Short welding times and relatively low heat inputs are preferable for decreasing the softened zone near the weld joint.Therefore,inertia(IFW)friction welding as a typical solid-state welding technique has been successfully used to join similar or dissimilar steels.30CrMnSiNi2 A and 45 steel are used as the base metals,Based on the study of30 CrMnSiNi2 A weldability,the inertia friction welding technology of 30 CrMnSiNi2A/45 steel was studied,and the influence of heat treatment process on the joint structure and performance was studied.The main research results are as follows:(1)30CrMnSiNi2 A has good adaptability of inertia friction welding.With the increase of initial welding kinetic energy and the increase of heat input,the metallurgical bonding of the joint becomes sufficient.The microstructure of welding rod from surface to inside showed the changes of fine-coarse-fine.Martensite appeared in the peripheral area,coarse martensite appeared in the mid-radius area owing to phase transition.As the distance move toward the central area,the martensite gradually disappears and the ferrite increases.Cementite and ultrafine ferrite particles occurred in the central area.Post-weld heat treatment greatly improves the mechanical performance of 30 CrMnSiNi2 A joint.With an initial kinetic energy of 15.30 KJ,a defect-free IFW joint can be obtained,the tensile strength of the joint is up to 1730 MPa,the elongation is 12%,joint efficiency reaches 95%,the fracture location is at the base metal,the fracture is mainly plastic fracture.Compared with the as-weld state,it not only improves the strength by nearly 2.5 times,but also maintains good toughness,and the joint has good mechanical properties.Therefore,30 CrMnSiNi2 A joint welding performance is excellent with energy of 15.30 KJ,it provides weldability reference for30 CrMnSiNi2A/45 dissimilar material IFW.(2)45 steel and 30 CrMnSiNi2 A were joined successfully using inertia friction welding.The effects of the rotational speed on the microstructure and mechanical properties of the joints were investigated.With increasing rotational speed,the tensile strength first increased and then decreased.The joint has the best mechanical properties at a rotational speed of 2200 rpm,with tensile strength of 713 MPa,elongation of 15.3%and impact toughness of 28.8J /cm2.The fracture of the tensile sample is in the base material,and the highest joint efficiency is 97%.The increase of rotational speed(2800rpm)increases the width of welding heat affected zone in 45 steel,making the fracture position appear in the heat affected zone.The hardness profiles also indicated that the weld joint had higher hardness than that of either of the two base metals,and the highest hardness of the joint was 581 HV at 30 CrMnSiNi2 A.The high strength and toughness of the weld joint were attributed to the phase transformation of lath martensite,bainite,pearlite,ultrafine ferrite,and cementite.Welding defects were observed in the 1000-rpm sample as the low heat input and inclusions at the weld interface caused incomplete metallurgical bonding,resulting in the fracture occurring at the weld seam.(3)The microstructure of the joint of 45 steel and 30 CrMnSiNi2 A becomes relatively homogeneous after heat treatment.The tempered martensite occurred in the side of 30 CrMnSiNi2 A,the pearlite and ferrite occurred in the side of 45 steel.After heat treatment,the joint still has good mechanical properties at 2200 rpm,and the tensile specimen breaks at 45 steel base metal.However,when the rotational speed is up to2800 rpm,coarse Widmannstatten structure appears in 45 steel,which reduces the mechanical properties of the joint,the fracture location is Thermo-mechanical affected zone in 45 steel.(4)Good inertia friction welding joint is mainly formed by thermo-mechanical coupling,element diffusion and phase transition of the microstructure.The structure in the plastic deformation zone is dynamically recrystallized and bonded together by mechanical interlocking under frictional pressure.The diffusion of elements at the joint and the phase transition of the structure make the joint bond more closely and achieve a chemical connection.