| Aluminum alloy/stainless steel(referred to as "Al/steel")dissimilar metals composite structures on the basis of maintaining the high strength of steel and low density of Al alloy and other inherent excellent performance,give full play to the advantages of weight reduction,energy saving,emission reduction,cost reduction,etc.,have been widely used in the aerospace,automotive industry and other fields.The propellant delivery pipeline of the high thrust liquid launch vehicle is the typical Al/steel composite structure,however there are significant differences in the chemical and physical properties between the two materials.Traditional fusion welding processes are prone to forming a large number of brittle and hard intermetallic compounds(IMCs)at the weld interface,which seriously affect the joint performance.In recent years,as a typical solid-state welding process,inertial friction welding has attracted extensive attention from scholars at home and abroad because of its low welding temperature,concentrated energy input and accurate control,which effectively limits the formation of IMCs.2A14-T6 Al alloy rod and 304 stainless steel rods were used as the experimental materials,and inertial friction welding experiments were carried out.By changing the heat treatment state of Al alloy and the surface roughness state of stainless steel,the formation mechanism of IMCs in friction interface of Al/steel joints was comprehensively analyzed and evaluated.At the same time,alloying element were added into the friction interface by electroplating,the influence of alloying elements on the metallurgical reaction at the Al/steel interface were systematically summarized and evaluated and the feasibility of controlling the interfacial IMCs and joint properties by electroplating a layer of alloying elements was explored.In addition,the metallurgical control of the joint was carried out by adding an intermediate layer by welding,and the effect of the change of metallurgical environment on the formation process of interfacial IMCs and joint properties was analyze.The results of inertial friction welding show that the precipitate formed in the precipitation process of Al alloy during cooling strongly affected the formation of interfacial intermetallic compounds(IMCs),rather than its original pre-existing precipitates.At low heat input,the incomplete extrusion of oxide layer leads to the existence of nanoscale Fe-Al-O amorphous layer together with the nanoscale Al2CU phase precipitated at the interface in a semi-coherent form with Al.At high heat input,the enhanced coupling effect of heat and plastic flow of Al alloy,resulting in the possible reaction between Fe,Si and Al2Cu to form a Fe4Al13 type IMC,Al67.5Fe20.7Cu6.2Si5.6,with a lower formation enthalpy,but the non-uniform reaction caused the remnant Al2Cu in the submicron-scale IMCs layer,being non-uniformly distributed.Local tensile tests indicated that joint strength was negatively correlated with the thickness of the IMCs layer in which the joint with nanoscale Fe-Al-O amorphous layer showed the highest bonding strength.The effect of typical alloying elements on the microstructure and properties of Al/steel inertial friction welding interface was studied by adding alloying element layer by electroplating.The results showed that the addition of Cu element promoted the formation of Al-Cu and Al-Fe-Cu microstructure phases and inhibited the growth of IMCs between Fe-Al to a certain extent.The addition of Zn element inhibits the diffusion and migration of Al atom,and promotes the formation of Cr atom diffusion clusters,which form catalytic active structures.In addition,the smaller specific heat capacity of Zn element is conducive to the increase of interface temperature and promote the metallurgical reaction at the welding interface.The presence of Ni element changes the phase composition of IMCs,forming Ni-Al IMCs,inhibiting the formation of Fe-Al IMCs,but also acting as nucleation particle to promote the dissolution of Cu-rich phase in Al alloy base metal.There is no obvious difference in tensile strength between pure Al/steel and pure Al/coated steel,and the samples fracture on the side of pure Al base material.However,the tensile strength of the Al alloy/steel joint obtained by electroplating with alloyed element layer is relatively low.The tensile strength of the joint obtained by adding Zn and Ni elements is 204.9 MPa and 8.3 MPa under low heat input,and 195.9 MPa and 109.9 MPa under high heat input,respectively.However,adding alloy layer in electroplating is not conducive to improve joint strength as a whole,and the feasibility is poor.The co-deformability of Al alloy and Ni in 2A14 Al alloy/pure Ni/steel joint can effectively balance the stress distribution at the interface and reduce the interface temperature.which results in a significant change in the distribution of interfacial IMCs compared to the 2A14 Al alloy/steel joint,with the thickest IMCs layer in the edge region.Under the selected process window,adding Ni intermediate layer by welding method to adjust the strength of 2 A14 Al alloy/steel joint is applicable in most process,and the tensile strength of 2A14 Al alloy/steel joint is increased by about 15%to 355.6 MPa.Under the same welding parameters,adding Ni interlayer can increase the strength of 2A14 Al alloy/steel joint by up to 1.1 times,and all the joints fail at the 2A14 Al alloy/pure Ni interface,the stability of control is strong,and it is easy to obtain high-quality welded joints after regulation.The weld interface between different materials of 2A14 Al alloy/6061 Al alloy/polished steel joint is clear and straight,achieving a good bonding,where the 6061 Al alloy/polished steel interface is similar to the 6061Al alloy/polished steel joint interface in terms of the distribution of IMCs in different characteristic areas.The tensile strength of 2A14 Al alloy/6061 Al alloy/polished steel joint is significantly higher than that of 2A14 Al alloy/steel,and increases by about 60%up to 331.0 MPa under the same welding parameters.It is beneficial to obtain better welded joint by adding 6061 Al alloy intermediate layer to control 2A14 Al alloy/steel. |
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