Interface Control And Performance Analysis Of Aluminum/Steel TIG Brazing Joints

Due to the difference in the properties of aluminum and steel,a large amount of brittle Al-Fecompounds will be produced in the joint,which will lead to the deterioration of performance.Therefore,the regulation of the microstructure and properties of the welding interface between aluminum and steel is a key problem that needs to be solved in lightweight engineering.In the existing research,most of the single component or binary alloy powders or foils were used to improve the interface microstructure,but the effect was not ideal.So it was proposes to use multi-component alloy powder and foil as the interlayer to regulate the interface of aluminum and steel fusion brazing joint in this study.AlSi5 welding wire was used as the solder in the experiment.Firstly,the influences of groove size,welding current and welding speed on joint morphology and interface behavior were studied.Next,the regulation of the interface by adding CoCrNi mixed powder and CoCrNi medium entropy powder through coating and prefabricated was further studied.At the same time,the effect of powder as an intermediate layer on the interface of the joint under the lap structure was compared and discussed.Finally,the influences of CoCrNi alloy foil on the macrocosmic and microcosmic morphology and interface behavior of TIG welding-brazing joint were studied.The experimental results of aluminum and steel TIG welding-brazing process show that the variations of process parameters have limited regulation and control on the types and properties of interface products,and various Al-Feintermetallic compounds are formed at the interface.The groove angle shows a greatest influence on the weld formation that the equal grooves on both sides facilitate the flow of the molten pool metal,and can also increase the contact area of the brazing interface,contribute to the improvement of the tensile strength of the joint.But too large groove size will affect the spread of molten pool metal on the surface of steel.Welding current and welding speed have a great influence on the thickness and morphology of IMC.With the increase of heat input,the thickness of IMC increases,and the shape of IMC develops from fine needles or flocs to thick rods,zigzags,and large scallops.The current variation has the greatest effect on the IMC thickness.Considering the joint forming and tensile strength comprehensively,the groove angle of 45°,the current of70A and the welding speed of 2.5mm/s are the optimal process parameters.The results of adding a powder intermediate layer show that the types of intermetallic compounds generated at the interface are reduced.And the strength of the joint with powder is higher than that without powder,as well as the strength of the joint by prefabricated is higher than that of the coating method.The addition of CoCrNi medium entropy alloy powder can significantly regulate the interface performance and improve the joint strength than the mixed powder.When CoCrNi powder coated on the surface of the steel,the joint formability is poor.Because the mixed powder aggregates in the middle and lower part of the brazing interface,the uniformity of the interface structure is poor.The IMCs formed at the interface is not densely connected to the steel side interface,which mainly composed of FeAl,FeAl₃,Al-Ni,FeCo and Al-Si-Fe.The shape of IMC changes from neat small blocks near the steel side to short rods or slender tentacles growing toward the weld.The joint has good formability without obviously layered of IMCs when the mixed powder interlayer is preformed.And the thickness of IMCs is thinner than mixed powder direct coating.Compared with non-powder,the types of Fe-Alare reduced,the gray IMC layer near the weld seam side composed of FeAl₃ and Al₈Fe₂Si is formed,and the black IMC layer near the steel side composed of Fe₃Aland Al-Ni has no special morphology.Under the condition of coating MEA powder,the weld reinforcement is high and accompanied by incomplete penetration in the lower part.The brazing interface without obvious IMC layer is mainly composed of Fe-Al,Al-Ni,Al-Cr and Al-Co,and unmelted ellipsoid CoCrNi particle layer on the interface acts as an enhances phase to a certain extent.The welded seam of the prefabricated MEA powder is well formed with the gray IMC layer of FeAl₃ and Al₈Fe₂Si near the weld side,and the black IMC layer of Fe₂Al₅,FeAl,Fe(Cr,Ni),Al-Ni near the brazing interface,which the tensile strength is up to 83.6 MPa.The bonding interface of the lap welding joint has poor continuity and does not form a homogenized structure.The experimental results of arc prefabricated CoCrNi medium entropy alloy foil intermediate layer show that the joint is bonded well.The maximum hardness of the joint is lower than that of coated,prefabricated powder and unpowdered,and the maximum tensile strength is up to 97.6MPa when the foil thickness is 1.5mm.Compared with that with and without powder,the joint strength is greatly improved.Because of the best control effect on the interface that makes the formation of brittle phase such as Al-Feobviously inhibited,prefabricated CoCrNi medium entropy alloy foil is considered as the best metal sandwich based on improved mechanical properties of joints,although the complexity of the generated phase increases.As the thickness of foil increases,the thickness of prefabricated layer and IMC layer increases.The greater the thickness of prefabricated layer,the stronger the diffusion hindrance of Feelement.