Research On Interfacial Fractureing Behaviors And Adjusted Method Of IMC Layer For The Laser Welded–brazed Al/Steel

In this thesis,DP590 dual-phase steel and 6061-T6 aluminum alloy which are used in the automible are adopted as base metals while the AlSi12 filler is employed as filling metals.The 2-mm-thick Al/steel dissimilar butted joint is successfully joined by the laser welding-brazing technology.The processing features of the laser welding-brazing are studied and the influence of processing parameters on weld formation and tensile strength is clarified.The fracture behaviors of Al/Fe interface and the affected regulation of inhomogeneous distribution of interfacial IMC on the joint strength are revealed,the IMC morphology and thickness which is most beneficial for the bonding strength of the interface is determined.An adjusting method of dual-spot laser is proposed based on the analysis of growing regulation of different IMC and features of thermal fields.The interfacial IMC and thermal field are both adjusted and an Al/steel joint with high tensile strength is obtained.For the laser welding-brazing of Al/steel dissimimar metals,wettability and spreadability of molten fillers along steel side has a great relationship with weld formation and joint strength.The experimental results show that a smooth and continuous weld formation on the front and back surfaces can be obtained under these parameters:V-shaped grooves at steel side,1.0mm welding gap and 0.3mm bias to A1 side for the center of laser spot.The corresponding tensile strength of produced joint is as highest as 142MPa.In addition,tensile strength of joints obtained at different thermal conditions also show variations in spite of similar weld formations and this indicates that the different interfacial IMC structure due to various heat inputs will also have a great influence on the tensile strength of the joints.Inhomogeneous distribution of IMC thickness and components occurs according to the observed results of interfacial micro structure.The IMC in the top,middle and bottom regions along brazing interface are 0-Fe(Al,Si)3+?5-Fe1.sAl7.2Si,?-Fe(Al,Si)3+?5-Fe1.8Al7.2Si and solo ?5-Fe1.8Al7.2Si phase and their corresponding thickness are 8.6?m?4.9?m andl.?m.?-Fe2(Al,Si)5+?-Fe(Al,Si)3+?5-Fe1.8Al7.2Si phase with the thickness of 12?m will also be generated in the high laser power.Different IMC bring different fractured behaviors of interface.Interface bonded with?-Fe(Al,Si)3+?5-Fe1.8Al7.2Si phase will fracture in the interface between ?-Fe(Al,Si)3 and steel while interface bonded with solo ?5-Fe1.8Al7.2Si phase will fracture in the interface between ?5-Fe1.8Al7.2Si and steel.The in-situ tensile tests results show that interface with highest bonding strength is produced when the interface is joined with 2-3?m serration-shaped solo ?5-Fe1.8Al7.2Si phase.Lower interfacial residual stress,serration-shaped morphology are responsible for its highest bonding strength and this can provide an adjusted aim for the homogenization of interfacial IMC.The generated regulation of different interfacial IMC and diffused mechanism of element Si in interface are analyzed.The generation of interfacial IMC is both determined by Gibbs free energy and generated temperatures.The orders of Gibbs free energy values for three IMC is:?-Fe2(Al,Si)5>?5-Fe1.8Al7.2Si>?-Fe(Al,Si)3 while the order of generated peak temperature for three IMC is:?-Fe2(Al,Si)5>?-Fe(Al,Si)3>?5-Fe1.8Al7.2Si.A thermodynamic model is developed to calculate the chemical potential of element Si.The results show that a smaller chemical potential of Si element will be produced in interfacial IMC and this makes contribution to the preferential diffusion of element Si to IMC and finally leads to the aggregation of Si.This aggregation of element Si in interface will result in the generation of long and ordered Fe3(Al,Si)phase in the ?-Fe2(Al,Si)5/Fe interface.The corresponding thermal fields of solo ?5-Fe1.5Al7.2Si phase are calculated based on the developed FEM(Finite Element Method)model.The corresponding thermal fields for the generation 2-3?m solo ?5-Fe1.8Al7.2Si phase is:the interfacial peak temperature should keep about 1000? and duration at high temperature should keep about 1.64-2.63s.According to guidance of numerical results,the interfacial thermal fields and IMC homogeneous adjustments are both obtained by adopting dual-spot laser heating source:a homogeneous distribution of solo?5-Fe1.8Al7.2Si phase with the thickness of 1.8-2.7?m along brazing interface.The spots configuration is parallel and the energy ration between two spots is 5:5,the obtained peak temperatures along brazing interface is 990-1036? while the duration at high temperature is 1.9-2.2s.Tensile strength of the obtained joint is as much high as 196MPa.