Study On Ultrasonic-assisted Soldering And Mechanism Of Mg,Cu And Mg/Cu Dissimilar Metals

Copper(Cu)is widely used in electronic packaging,construction industry,machinery manufacturing,defense industry and other fields due to its multiple slip systems,good ductility,good thermal conductivity and electrical conductivity.Magnesium(Mg)is widely used in light industry,heavy industry,and nuclear industry due to its multiple slip systems,soft and easy to deform and process.Bonding a magnesium alloy with Cu to form a part can reduce the weight of the part while retaining the good electrical and thermal conductivity of Cu.In order to meet usage needs and reduce costs,in this paper,according to the role of ultrasound in solid-liquid media,an external sound field was introduced,and the acoustic-thermal coupled ultrasonic-induced transient liquid phase bonding(U-TLP)was used to bond pure Cu matrix with tin(Sn)and low melting point solders of Sn-58Bi and Sn-52In.Using Ag-28Cu with a high-strength as the solder to bond MB8 magnesium alloy.Utilizing the characterization of zinc(Zn)and Mg can produce a low melting point eutectic phase,and Zn has a high solid solubility in Cu,AZ31B magnesium alloy/Cu dissimilar metals were bonded with Zn as a solder.In the Cu/Sn/Cu system,the effect of ultrasound on the microstructure of solder joints was analyzed.The growth of tin whiskers under ultrasonic and non-ultrasonic condition and different cooling methods by aging for 30 days at room temperature were also analyzed.When the ultrasound of 5 s,the solder joint is filled with Cu-Sn compounds.The sample treated with ultrasound must grow faster in speed and longer in length for the growth of tin whiskers.Ultrasonic energy qualitatively reflects the amount of stress acting on the solder joint.Under different cooling methods,the amount of tin whiskers grown under water cooling must be largest,followed by air cooling,and finally with furnace cooling.Chemical stress caused by chemical reactions,compressive stress caused by volume expansion of compounds,thermal stress caused by temperature rise,and thermal and compressive stress caused by the collapse of cavitation bubbles provide the driving force for the growth of tin whiskers.Due to the volume expansion of the compound,the oxide film partially ruptures to form a pressure gradient,which become the growth source of tin whiskers.Elemental diffusion of Sn provides the Sn atoms for continued growth of tin whiskers.In the Cu/Sn-58Bi/Cu system,the effect of ultrasonic vibration time on microstructure morphology,composition change and mechanic properties was analyzed.the presence of(Cu)solid solution,(Sn)solid solution,(Bi)solid solution,and Cu-Sn compounds is detected in the solder joint.At the ultrasound of 5 s,at Cu/Sn-Bi the interface,due to the presence of more brittle?(Cu₆Sn₅)compounds,the fracture occurs in the?(Cu₆Sn₅)region,the joint strength reaches a maximum of18.91 MPa.When the ultrasonic vibration time is extended to 20 s,fracture occurred between?(Cu₃Sn)and?(Cu₆Sn₅),and the joint strength decreases.In the Cu/Sn-52In/Cu system,the presence of(Sn)solid solution,?(Cu₃Sn),?(Cu₆Sn₅),Cu₁₀Sn₃,Cu₁₁In₉,Cu₉In₄,Sn-rich?,and In-rich?phases is detected in the solder joint.With the diffusion of Cu atoms,the?phase and the?phase are consumed,and Cu-Sn compounds and Cu-In compounds are formed.The coarse-grained bulk Cu₁₁In₉phase grow,partially of which detaches from the Cu/Cu-Sn-In interface,migrates to the center of the solder joint,and mixes with?phase and?phase.Under the action of ultrasound,the metastable?phase tends to transform into the steady-state?(Cu₃Sn)phase.The shear strength reaches a maximum of 22.76 MPa under ultrasonic vibration time of 30 s.Fracture occurs on the surface of the fine-grained?(Cu₆Sn₅)phase,and the fracture manner is a brittle fracture.In the MB8 Mg/Ag-28Cu/MB8 Mg system,the effects of ultrasonic vibration time and bonding temperature on the solder joint structure were studied,and the mechanical properties of the solder joint at room temperature and higher temperature(below 300?)were measured.With the extension of the ultrasonic time,a ternary eutectic reaction between the matrix metal and the interlayer occurs,and to form a solder joint structure composed of?-Mg solid solution,Ag Mg₃,Cu Mg₂intermetallic compound.With the further extension of the ultrasonic vibration time,the solder joint gradually closes,and the strength of the closed solder joint is the highest,which can reach 101 MPa.The fracture occurs in the base metal and belongs to a cleavage fracture.From 300? to room temperature,the shear strength increases with decreasing temperature.In the AZ31B/Zn/Cu system,the effects of pressure,temperature and ultrasonic vibration time on interface formation,microstructure and mechanical properties of solder joints were studied.Mg-Zn solid-liquid interface formation,?-Mg solid solution shedding and fusion growth behavior were analyzed.The evolution model of solder joint structure was established.With the increase of pressure,the point contact between the base metal and the interlayer increased,the interface metallurgical reaction was intensified.It is detected that solid solution,Mg-Zn,Mg-Cu,Cu-Zn,Cu Mg Zn intermetallic compounds exist in the solder joint.The micro-jet is generated due to cavitation bubble collapse and the gathering of Cu Mg Zn compound formed by metallurgical reacts at Mg side are reasons why the formation of a mountain-like interface and the necking of the?-Mg root until it leaves the base metal.In the solder joint,the morphology of the eutectic structure is changed from the mixed state of the granular and layered eutectic structure to a layered eutectic structure,and the joint strength is increased.There is a tendency for the adjacent?-Mg solid solution with different radii of curvature to merge and grow.The thickness of the compound increases as the temperature increases.The fracture occurs between the Cu Mg Zn and Cu₅Zn₈compounds formed on the Cu side,which is a brittle fracture.When the bonding temperature is 430? and the ultrasonic vibration time of 15 s,the joint strength is the highest,reaching to 80.75 MPa.