| Laser welding with high energy density laser beam as heating source can greatly improve the accuracy and efficiency of welding.With the development and application of new high-power laser,laser welding has been widely applied in aerospace,nuclear power,automobile and other industrial fields.The importance of laser welding quality optimization control has been becoming more and more prominent.However,in the actual application processes,it was found that conventional continuous laser welding method has many problems.On the one hand,it was difficult to control the weld forming and easy to produce porosity defects.On the other hand,due to smaller spot size,the melting zone heated by laser heat source was small and not suitable for joint form with larger fusion width.Laser welding quality optimization could be achieved by selecting the appropriate laser spot size and mode,shape shaping of spot contour,double beam welding and other static control methods,as well as dynamic control methods such as pulse laser and oscillating laser.The current research on pulse/oscillating laser regulation mode was not systematic,and the corresponding welding quality optimization mechanism was still unclear.In this paper,the laser energy distribution,weld formation and porosity defects,and molten-pool melting characteristics during pulse/oscillating laser welding process were systematically studied,and the quality optimization of narrow gap laser welding was further studied on the basis of the regulation of laser energy distribution.Firstly,the influence of the main pulse parameters on the laser energy distribution in the pulse laser regulation mode was studied,and the influence of different pulse welding conditions on the melting characteristics and welding quality were compared and analyzed.The pulse regulation mode could change the laser energy distribution form from one dimension,and produce many different laser energy distribution forms by changing the pulse frequency and duty cycle.With the increase of pulse frequency,the laser energy distribution could be divided into three distribution forms: complete separation of pulse energy,non-uniform superposition and uniform superposition.There was a critical pulse frequency Cf which can ensure the uniform and continuous laser energy distribution in the welding path.The critical pulse frequency was proportional to the welding speed,inversely proportional to the spot radius,and independent of the pulse duty cycle.Compared with continuous laser welding,pulse laser welding method could significantly improve the weld forming and internal porosity defects,and enhance the mechanical properties of the weld.According to the high-speed camera observation and analysis,the pulsed laser impact effect could cause the longitudinal periodic flow characteristics of metal molten liquid in the molten-pool,which is conducive to the metal molten liquid reflowing and filling the keyhole,and significantly reduces the occurrence of stomatal defects.When the pulse frequency was greater than 30 Hz,the weld internal porosity rate could be maintained below 0.24%.Secondly,the influence of the main oscillation parameters on the laser energy distribution in the oscillating laser regulation mode was studied,and the influence of different oscillating welding conditions on the melting characteristics and welding quality were compared and analyzed.The oscillation regulation mode could change the laser energy distribution form from two dimensions,and the actual motion speed of laser spot could be changed by adopting different oscillation trajectory,oscillation frequency and amplitude,thus affecting the laser energy distribution form during welding process.Similarly,there was a critical oscillation frequency CF which can ensure that laser energy distribution in the welding path was uniform and continuous.The critical oscillation frequency was proportional to the welding speed,inversely proportional to the spot radius,and independent of the oscillation amplitude.When the frequency was less than the critical oscillation frequency,the maximum laser energy flux density of energy distribution would decrease with the increase of the oscillation frequency,and the energy flux density would remain unchanged after reaching the critical oscillation frequency.At this point,different oscillation amplitude would correspond to a fixed energy distribution form,and with the increase of oscillation amplitude,the maximum laser energy flux density in the energy distribution would continuously decrease.With the increase of oscillation amplitude,the number and size of Invar alloy weld internal porosity defects decreased significantly.When the oscillation amplitude was greater than 4.0 mm,it was close to the absence of porosity defects,which was beneficial to improve the mechanical properties of oscillation weld.The tensile strength of Invar alloy weld increased from 361 MPa of continuous weld to 398 MPa of oscillation weld,while the maximum strain rate increased from 25% to 48%.With the increase of oscillation amplitude,the microstructure of the welds gradually changed from directional dendrites to equiaxed dendrites morphology.When the oscillation amplitude was 2.0mm,4.0mm and 6.0mm,the grain structure size was refined by 38.0%,50.1% and 54.6%,respectively.Furthermore,the temperature field distribution and flow characteristics of the molten-pool during oscillating laser welding process were studied in detail.The equivalent heat source models suitable for different oscillation amplitudes were respectively established based on the uniform and continuous laser energy distribution in the oscillating laser control mode.It was found that the lateral temperature gradient decreased with the increase of oscillation amplitude,and the molten metal in the local area of the molten-pool had the same temperature and even presented a negative temperature gradient.Combined with the observation and analysis of high-speed camera,the local rapid movement of laser spot could make the surface morphology of keyhole more stable without obvious fluctuation and spatter.With the increase of oscillation frequency and amplitude,the forced convection effect of laser beam on the molten metal in the molten-pool was gradually strengthened,which made the transverse periodic flow characteristics in the molten-pool more and more obvious,which was conducive to restrain the internal porosity defects in the weld and improve the uniformity of element distribution.Finally,the quality optimization of narrow gap laser welding based on energy distribution control was studied.According to the narrow gap groove form and weld quality requirements,on the basis of the study of oscillating laser energy distribution,the local interpolation method was used to accurately control the output power of laser so as to optimize and design the laser energy distribution,and the uniform laser energy distribution was obtained which is most suitable for narrow gap welding,improving the uniformity of welding heat input and weld formation in narrow gap groove.Combined with the observation and analysis of high-speed camera,it was found that the dynamic heating process of the filled wire by oscillating laser changed the melting transition behavior of the wire,which was conducive to the generation of “small drops”,“large drops”,“liquid bridge” melting transition forms,so as to ensure the stability of the welding process of oscillating laser welding with filler wire.The optimized oscillating laser welding with filler wire was used to realize the narrow gap multi-layer filling welding of 19.5mm thick Invar alloy,and the weld with uniform formation,fine microstructure and no obvious internal defects was obtained.The quality and mechanical properties of the weld meet the application requirements. |
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