Preventing formation of intermetallic compounds in ultrasonic-assisted Sn soldering of Mg/Al alloys through pre-plating a Ni coating layer on the Mg substrate

Magnesium and aluminum alloys are widely used in various industries because of their excellent properties,and their reliable connection may increase application of materials.Intermetallic compounds(IMCs)affect the joint performance of Mg/Al.In this study,AZ31 Mg alloy with/without a nickel(Ni)coating layer and 6061 Al alloy were joined by ultrasonic-assisted soldering with Sn-3.0Ag-0.5Cu(SAC)filler.The effects of the Ni coating layer on the microstructure and mechanical properties of Mg/Al joints were systematically investigated.The Ni coating layer had a significant effect on formation of the Mg_(2)Sn IMC and the mechanical properties of Mg/Al joints.The blocky Mg_(2)Sn IMC formed in the Mg/SAC/Al joints without a Ni coating layer.The content of the Mg_(2)Sn IMC increased with increasing soldering temperature,but the joint strength decreased.The joint without a Ni coating layer fractured at the blocky Mg_(2)Sn IMC in the solder,and the maximum shear strength was 32.2 MPa.By pre-plating Ni on the Mg substrate,formation of the blocky Mg_(2)Sn IMC was inhibited in the soldering temperature range 240–280℃and the joint strength increased.However,when the soldering temperature increased to 310℃,the blocky Mg_(2)Sn IMC precipitated again in the solder.Transmission electron microscopy showed that some nano-sized Mg_(2)Sn IMC and the(Cu,Ni)_(6)Sn_(5)phase formed in the Mg(Ni)/SAC/Al joint soldered at 280℃,indicating that the Ni coating layer could no longer prevent diffusion of Mg into the solder when the soldering temperature was higher than 280℃.The maximum shear strength of the Mg(Ni)/SAC/Al joint was 58.2 MPa for a soldering temperature of 280℃,which was 80.7%higher than that of the Mg/SAC/Al joint,and the joint was broken at the Mg(Ni)/SAC interface.Pre-plating Ni is a feasible way to inhibit formation of IMCs when joining dissimilar metals.

Discharge Characteristic of Micro-Arc Oxidation on Aluminum Alloy under the Changing Electrolyte Temperature

The electrolyte temperature has a great influence on the performance of the coating prepared by micro-arc oxidation (MAO). The behavior of MAO discharge in the changing electrolyte temperature has been investigated. Compared to constant electrolyte temperature in conventional MAO process, the process has different discharge characteristics under the changing electrolyte temperature. The amplitude of pulse voltage was detected to study the change of discharge characteristic under the constant-current control of MAO power supply. Three successive discharge stages were differentiated by the variable the pulse voltage versus process time. Since there were significant changes in the sound, the sound signals were acquired and the audio analysis was used to describe the changing of the MAO discharge at different stages. Optical emission spectroscopy (OES) was employed in situ to unveil how the micro-discharge changed with the temperature increasing. Scanning electron microscopy (SEM) was used to characterize the morphology of the coatings on 6N01 aluminum alloy prepared by normal process with the constant-temperature control of the MAO electrolyte and by the process under the changing electrolyte temperature. A mode of film growth and micro-discharge was given to describe the effects of the changing electrolyte temperature in the whole MAO process.