Study On Brazing Filler Metals And Brazing Methods For Magnesium,aluminum And Mg To Al Dissimilar Metals

Radar system is widely used in aerospace,weapon systems and civil airport facilities,which is an important part of air-search systems.As a main functional component in the radar system,a seeker made of aluminum/magnesium alloy is a multi-cavity complex integral component,which is bonded as a whole by means of brazing.However,a defect of cold joint is existed in the brazing seam constituted by aluminum/magnesium alloy.Brazed joint rate is so difficult to improve that the brazing yield is generally too low.Signal transmission quality will be lowered due to defects in brazing.System normal operation will be also affected in severe cases.Research concerning too-low continuity of brazing seam is conducted in this paper on the background of integral bond of millimeter wave(MMW)radar seeker.Started from the reaction between filler metal and base metal,active element will be introduced to the filler metal for design and preparation of new filler metal;Defects in the brazing seam can be eliminated and brazing rate of the brazing seam can be improved with the effects of filler metal and base metal,ensuring reliable conduction of electrical signal.Vacuum brazing and Contact-reaction brazing are adopted to discuss formation mechanisms and organization characteristics of brazing seam as well as to study the generation,distribution and evolvement rules of compounds in the interfacial region.Mg2Si composite was added to Al-Si filler metal for aluminium alloy vacuum brazing via in situ synthesis.Spreading area method was used to measure the wettability of the brazing alloy on the parent metal in vacuum.The filler metal wetting area increased with increasing Mg2Si content.The compound within the interface was examined with an energy spectrum and the following results were obtained:Mg2Si content of the brazing interface was lower than that of filler metal,Mg2Si was distributed into both sides of the interface and arranged in block,grain and short rod-like shapes.Si crystal had a primarily needle-like structural arrangement.The elemental distribution on both sides of the interface was analysed using thread scanning method.A solid solution of Mg was formed in the interior parts of the grain in the parent metal.The solid solution content decreased as the distance from the interface increased.A AlSi-Mg2Si self-fluxing brazing filler metal was obtained by activating AlSi brazing filler metal with the Mg2Si phase and applying a metamorphism treatment of Mg2Si and Si crystals with Sr and SmO.A good wetting performance between the modified AlSi-Mg2Si and 3003 Al alloys was achieved in the vacuum brazing without Mg vapour as the activator.As measured by STA409Pc differential scanning calorimetry.A scanning electron microscope was used to analyse the microstructure and the component distribution of the brazing filler.The results indicate that the change in morphology of the Mg2Si phase is remarkable after metamorphism.The analyses of the microstructure indicated that the Mg2Si and Si phases presented small needle-type and granular morphologies.The following were found to occur:intergranular penetration of Mg and Si on the base metal,a large amount of granular eutectic structure,and a significant grain boundary effect.The shear fracture of the lap joint presents cleavage fracture;in addition,with the decrease of the Mg2Si phases,the fracture morphology transformed from a rock candy shape to a steam shape,and cleavage steps appeared.The rock candy-shaped fracture areas enlarged after metamorphism treatment.Compared to 3003 aluminum alloy,the surface of AZ31 magnesium alloy is more difficult to be wetted by liquid filler metal based on analysis of the nature of the alloy surface oxidation film.AgCuZn alloy and AlCuSi alloy are respectively adapted to be the interlayer with zinc as an element of lowering the melting point,so as to discuss bonded problems of magnesium/aluminum composite waveguide and to improve structural state of a joint made of magnesium/aluminum dissimilar materials.Contact-reaction brazing method is employed to bond with AZ31 with an investigation of surface bonding situation.On this basis,a bond is conducted on AZ31 magnesium alloy and 3003 aluminum alloy.The results indicate that a part of the interlayer is dissolved,And a composite joint is formed.SEM metallographic analysis shows that the reaction layer is formed by the magnesium side.Contact-reaction brazing is used for welding the side of the magnesium alloy,while diffusion welding for the side of the aluminum alloy.Energy spectrum analysis reveals that alloy phase of the reaction layer is an intermetallic compound(IMC)formed by magnesium-based solid solution as well as Mg,Ag,Cu and Zn.The shear strength of brazed joints in shear is 89.3 MPa.Mixed fracture characteristics including intergranular fracture and trans-granular fracture are showed on the tensile shear fracture.When holding time of brazing is 10min,the interlayer is dissolved completely to form a joint of contact-reaction brazing.A large amount of A13Mg2 and Ali2Mg17intermetallic compounds are appeared on the reaction layer.The shear strength of brazed joints in shear is 30 MPa.Fracture is occurred in the aluminum side.The join of interlayer has changed organizational state of Mg/Al joint.Holding time of brazing exerts a tremendous effect on joint mechanical properties.