| Aluminum alloys are important structural material for energy conservation and emission reduction,which are widely used in aircraft,automobile and high-speed train manufacturing.However,due to the special physical and chemical characteristics,the metallurgical defects of laser welding aluminum alloys,especially the porosity defects,are difficult to be eliminated and it is hard to obtain the ideal joint performance.How to achieve their high quality,high efficiency welding has been the research focus in the welding and joining field.By means of the interaction of laser and arc,high quality aluminum alloy weld can be achieved by Laserarc hybrid welding.However,high arc current must be used to avoid the pore formation within the weld,which will cause large heat input to reduce the weld strength and toughness.Thus,this paper puts forward to a new idea named as oscillating laser beam-arc hybrid welding based on high frequency laser beam oscillator to overcome this question.The effects of laser beam oscillation on weld morphology,porosity,microstructure,plasma interaction and metallurgical behavior of molten pool were studied.The results are follows:The effect of three beam oscillating patterns(transverse,longitude and circle)and oscillating amplitude and frequency on weld morphology and process stability were studied.The weld surface morphology defects of transverse oscillation are dominated by small particle spatter bands(diameter<0.3 mm)with a small amount of large spatters(diameter>0.3 mm).Large spatters and rough ripples are easily appear on the longitudinally oscillating weld surface.The surface quality of circularly oscillating welds are the best with the least surface defects.As the increase of the oscillating amplitude and frequency,the cross-section morphology of the transversely and circularly oscillating welds are greatly affected by the variation of the oscillating parameters,and transform from goblet shape to rectangle and semicircle shape,respectively.The circular oscillation has the best sound weld morphology with the fine ripples and without spatters at the optimized oscillating parameters,which are 0.4 mm?A?1.0 mm and 200<f?500 Hz.The effects of beam oscillation on plasma interaction and droplet transfer characteristics were studied by means of high-speed photography and spectral analysis.The interaction can be enhanced and the electrical resistivity of the Arc zone can be reduced by laser beam oscillation.The circular oscillation has the strongest interaction.Compared with nonoscillation hybrid welding,the spectrum strength of the Laser zone and the arc zone respectively increases by 30% and 33%,electrical temperature respectively increases by 9% and 8.5%.The increase of electrical density is mainly in Arc zone,which increases by 43%.The reason of interaction enhancing mainly depends on the increase of the area of plasma interaction and the exchange of charge particles,which promotes to melt the weld wire and refine the droplet diameter.It is beneficial to stabilize the welding process and improve weld morphology.The best laser-arc plasma interaction can be achieved by circular oscillation,which forms the minimum droplet diameter and fine diameter spray projected droplet transfer.The droplet transfer modes of transverse and longitudinal oscillation are “medium diameter projected or short circuit” transfer.The plasma interaction of circular oscillation increases at first then decreases with the increase of oscillating amplitude,accordingly the droplet transfer experiences “large-fine-large” diameter spray projected transfer.The plasma interaction increases with the increase of oscillating frequency and keeps stable until f?200 Hz and the droplet transfer changes to fine diameter spray projected transfer.The effect of laser beam oscillation on porosity was studied.It can be found that the circular oscillation can significantly reduce porosity.The process bottleneck of the conventional non-oscillating hybrid welding is broken through,which must use more than 250 A welding current to eliminate porosity and obtain the grade B weld,and the threshold current is reduced to 100 A.Thus,the welding process window is greatly expanded.There is a frequency threshold for eliminating the porosity under different amplitudes,and the relationship between them can be quantitatively expressed by the formula(f=391.53A2-1143.7A+886.64).The frequency threshold of eliminating porosity decreases with the increase of current.When the welding current is 100 A,150 A,200 A,250 A and 300 A,respectively,the corresponding porosity elimination threshold is 500 Hz,400 Hz,350 Hz,150 Hz,and 100 Hz.The observation of keyhole behavior and the characteristics of molten pool flow indicate that the high-frequency oscillation keyhole can “capture” the bubbles in the molten pool and the pores in the solidification front.It also enlarges the diameter of the keyhole and increases the keyhole stability,changing the melt flow characteristic,ultimately avoiding or reducing porosity.The influence of beam oscillation on the microstructure of the welds was studied.It is found that the increase of the oscillating amplitude and frequency can reduce the solute segregation of the weld.When A?0.8 mm and f?300 Hz,the weld macrosegregation is reduced to less than 0.28%,which is nearly 300% lower than that of the weld without oscillation.The main reason of solute segregation reduction is that the convection of the upper and lower layers of melt flow and solute diffusion can be improved by laser beam oscillation,which enlarges the transition angle of laser-arc zone and forms the effect of stirring melt and vortex pumping.Moreover,it also found that the melt flow stirring reduces the temperature gradient at the front edge of the solid-liquid interface of the molten pool,enhancing the forced convection and melt-flow permeation between the dendrites in the mushy zone,which remelts and breaks up the weld columnar dendrites.The columnar-equiaxed grain transformation can be promoted and the weld grain refinement is achieved.When A?0.6 mm and f?300 Hz,the equiaxed grain content of the oscillating weld is more than 80%,which is more than 53% higher than the weld without oscillation at the same current(200 A),and the columnar crystal length and the equiaxed grain diameter are reduced by 73% and 56%,respectively.For AA6082-T6 aluminum alloy welds,with the increase of oscillating amplitude and frequency,the equiaxed grain content dropped to 48% from 75% without oscillation,and equiaxed grain content increased from 25% to 52%;the length of the columnar grain decreases from 300 ?m to 90 ?m,equiaxed diameter decreases from 120 ?m to 55?m.The enhancement mechanism of the weld mechanical properties under the optimized process parameters was discussed.For AA6082-T6 aluminum alloy,under the condition of eliminating macro-porosity defects,the tensile strength of all standard weld samples are about 230 MPa,whose tensile fracture locations and low hardness areas both are located in the HAZ.For notched welds,the tensile strength of the fusion zone of oscillating weld is 260 MPa,which is 8% higher than weld without oscillation.For AA2219-T6 aluminum alloy,the tensile strength of the oscillating weld is nearly 20 MPa higher than that of the weld without oscillation,and the elongation is increased by nearly 40%,but the corresponding heat input is reduced by 10%.The beam oscillation refines the weld grain when eliminating the macroscopic porosity and reduces the micro-defects such as weld microcracks or shrinkage,which is the main reason for the improvement of the strength and toughness of the weld. |
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