| Aluminum alloy is a promising engineering material for lightweight manufacturing,and various pulsed gas metal arc welding(GMAW)are widely used in the field of aluminum alloy sheet welding because of its high efficiency and low cost.However,the influence mechanism of droplet impingement and wall constraint on the weld formation,microstructure and composition distribution during welding is not yet clear.To systematically study effect of the droplet impingement and the wall constraint on the weld profile,microstructure and properties,in the welding of 6061 aluminum alloy,different welding process parameters and conditions are used such as modulation of various current waveforms,application of different bottom wall supports,change of welding gun inclination angle,replacement of wire diameter.The main contributions and prime novelties of the dissertation are as follows:(1)The effect of fluid flow driven by droplet impingement in the cross section on the weld profile,grain morphology and element distribution of 6061(Al-Mg-Si)aluminum alloy was studied by adjusting the inclination of the welding torch and analyzing the distribution characteristics of elements in different regions.Through the welding experiment by doublepulsed current waveform with different low frequency,the influence mechanism of fluid flow driven by droplet impingement on the grain morphology and composition distribution in the longitudinal section was further explored.In order to more accurately characterize the change of weld composition and in view of the inconsistency between the existing calculation methods,a new method which can be used to calculate the dilution rate for the bead with multi-alloy composition is proposed.The results show that the grain morphology varies with the fluid stirring intensity in the molten pool,and the content of the main alloying composition in the corresponding region is also significantly different.Taking the finger-like penetration as an example,the grains inside the fusion line before and after sharp inflection point(SIP)are columnar and fine-grained respectively due to the different stirring strength;meanwhile,the content of Mg and Si before the SIP are about 229.17% and 75.83% of that after SIP.The grains also show a difference between the grain morphologies in the "downhill" and "uphill" areas at the bottom of the weld corresponding to the strong and weak pulse phase in double-pulsed GMAW(DP-GMAW),and the content of Mg and Si in the area is about 203.85% and 163.57% of the uphill area,respectively.The results of microstructure and composition analysis also show that as the heat input increases,the number of pores and crack defects in microstructure gradually increase,resulting in a gradual decline in mechanical properties,such as tensile properties from 276.13 MPa to 252.92 MPa,and the impact energy from 2.02 J to 1.57 J.(2)By modulating the pulse base current in a large interval range and analyzing the parameters of the upper/lower surface reinforcement,penetration depth and size of different base currents,the effect of wall constraint on the bead formation is researched,and the reinforcement forming coefficient Rc is proposed to evaluate the constraint ability of the bottom wall.The effect of wall constraint on the bead formation is further verified by imposing different support plate at different positions on the back of the weld pool.The results show that with the increase of the base current,the weld height and penetration depth are subjected to various forms.The wall surface of the molten pool has a "dividing" effect in the flow of molten metal.the dividing effect of the wall constraint can be reflected by the change of Rc value,which decreases from 0.30 to-0.09 as the bottom wall constrains weaken.After re-establishing the wall constraint on the back of the weld by adding the support plates at each position,taking the heat input 177 J/mm sample as an example,the upper reinforcement height increases by 3.82 times,and the Rc value rises from 0.05 to 0.24.The width of the underside pool is reduced by 40.66%.The weld bead formation is significantly improved due to the wall constraint,and the tensile strength is increased by 39.85% when the collapsed reinforcement is eliminate.(3)The profile characteristics under the influence of droplet impingement controlled by the peak current are studied by modulating the current waveform.The influence mechanism of droplet impingement on weld formation controlled by peak current was studied by further comparing and analyzing the variation of the weld profile shape and size between the variable peak current mode and the base current mode under the same heat input.By performing DPGMAW experiments using different low frequencies waveform,and by analyzing the weld profile features including surface reinforcement peak/valley difference,penetration peak/valley difference,and angle of penetration depth/valley,the influence mechanism of the combination of droplet impingement and wall constraint on the weld profile formation was studied.To further verify the mechanism,a fully penetrated welding experiment was carried out using double-pulsed waveform of 2.78 Hz parameters which has the maximum difference in penetration peak/valley.The results show that with the increase of peak current,the shape and size of the bead profile between variable peak current mode and the base mode are significantly different.In DP-GMAW,only when the low frequency is lower than 5.95 Hz,the penetration at the bottom of the molten pool exhibits periodic fluctuations corresponding to the low frequency.The penetration peak/valley difference and the reinforcement peak/valley difference increase with the decrease of low frequency;and the ratio of the differences is as high as 532.20% when the low frequency is 3.79 Hz.Corresponding to the strong and weak pulse phase transition points,the penetration peak angle is significantly different from the penetration valley angle.The maximum penetration peak angle is about 173.01% of the valley angle,and in the fully penetrated beads welded by the same frequency DP-GMAW,both the penetration peak/valley difference and the angle peak/valley difference disappear.(4)Bead-on-plate experiment was carried out using single factor experiment to investigate the influence of process parameters such as wire diameter,welding speed and shielding gas flow rate on weld profile formation characteristics,and its influence mechanism and mechanical properties were analyzed.On this basis,the welding process was optimized by the response surface method combined with the mechanical properties of the weld,and using the optimized process the DP-GMAW experiment was carried out.The results show that the welding speed has the greatest influence on the weld profile during the single factor experiment,followed by the wire diameter,and the gas flow has little effect on the weld profile.The regression equation between the penetration and the process factors is obtained by the response surface method,and the optimization process parameters were obtained by combining the mechanical properties.Compared with the single pulsed GMAW,the tensile and impact properties of the DP-GMAW under the same optimized parameters were increased by about 14.12% and 27.83%,respectively. |
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