Microstructure,Mechanical Properties Of Aluminum/Steel Laser Welded Joints And First-principles Calculation Of Intermetallic Compounds

The future manufacturing industries,which tend to be more and more integrated,efficient production and large scale,has urged to improve the current welding technology to fit with this trend.With the development of the science and technology,various new welding technologies have emerged;laser welding was born under this background.The laser welding,with high welding accuracy and speed,can meet the requirements of efficient production,and has been wildly used to connect the higher specific strength aluminum/steel dissimilar metal material composite structures in the industry.This can be of great significance for the energy conservation and emission reduction.However,the laser welding of aluminum/steel dissimilar metals is more complex,for the quite different physical properties of the two.When the molten pool is solidified,a large number of intermetallic components are formed and micro cracks are generated in the joints,leading to the seriously deteriorates of the joints mechanical properties.As a result,the observation of the joint microstructure,the control and properties study of intermetallic compounds have attract more attentions.In this paper,the three types of joints of aluminum/steel dissimilar metal laser welding,including edge welding,lap welding and T-welding were studied the microstructure and mechanical properties by using low power Nd:YAG pulsed laser welding machine and high power fiber laser welding machine.The first-principles calculations were performed for the mechanical properties and electronic structures of the base material and several intermetallic compounds.The first,this paper uses Nd:YAG pulsed laser welding machine to conduct the aluminum/steel dissimilar metal edge welding.This position focuses on the optimization of laser welding parameters,and then observes the weld seam morphology,microcracks,intermetallic compounds and joint tensile shear forces changes with the welding line energy,analyzes the importance of various parameters for the welding quality.The results show that the parameters with welding current 110A,pulse width 4 ms,frequency 10 Hz,welding speed 200 mm/min is the optimized,and the maximum tensile shear force per unit length is 19.79 N/mm.The importance of the parameters shows the order of pulse width>frequency>current>speed.The intermetallic compounds mainly distributed near the fusion line preforms the shape of bulk and acicular.In the tensile shear test,the joint fractured in the form of brittle fractures,and localized plastic fracture features.Further,it has been understood that the intermetallic compound is formed due to the large amount of aluminum element entering into the molten pool during the welding process;an intermediate layer method is proposed to serve as a physical barrier to reduce the diffusion of it,thereby avoiding the over growth of intermetallic compound.In that case,the Nd:YAG pulsed laser welding machine was selected to weld the copper-clad aluminum alloy and stainless steel.By the use of SEM,EDS and XRD,the evolution of interface morphology,element distribution pattern,intermetallic compound species and fracture position of the joints with or without copper coating were revealed.The addition of the Cu plating layer effectively decreased the diffusion of Al into the molten pool,and then resulted in the decrease of the cracks and brittleness zone.At the same time,the joint toughness and tensile shear strength increased for the effectively suppression of Fe₂Al₅.With the increase in the cooper coating thickness,the fracture locations of the joints shifted from inside of the molten pool to the Al substrate,and the fracture mode changed from cleavage fracture to the mixed fracture consisting of quasi-cleavage and ductile fracture modes.At the same time,by the use of fiber laser welding machine,the aluminum/steel dissimilar metal welding of T shape joints was conduct to reveal the mechanical properties of intermetallic compounds.The microhardness and elastic modulus of the intermetallic compound region were measured using a more sophisticated nanoindentation instrument.Finally,the spatial structure of each intermetallic compound was optimized by using the Castep software,and the first-principles calculations were performed on some mechanical and electronic properties,such as elastic modulus,hardness,and density of states.The results show that the hardness of intermetallic compounds zones tested by the nanoindentation is much higher than that of other zones.However,due to the loosely distribution and the relatively soft weld metal,the measured values of intermetallic compounds zones are lower than the calculated values.Compared with the base metal,the unit cell of FeAl₂,Fe₂Al₅ and FeAl₃ are larger,complex and no symmetry.Besides,their elastic modulus,bulk modulus,shear modulus and hardness are large,which shows the higher hardness and harder to be compression and occur plastic deformation.The electronic density of states shows that the Fe₂Al₅ and FeAl₂exhibit a strong covalent bond and the FeAl₃ has a relatively low stability.