Three-dimensional Unsteady-state Numerical Simulation And Process Investigation Of Double Pulsed Gas Metal Arc Welding

Lightweight is recognized as the development trend of advanced manufacturing technology in the future,aluminum alloy is an effective engineering material to achieve lightweight manufacturing.Double pulsed gas metal arc welding(DP-GMAW)is suggested as an advanced and efficient welding process for aluminum alloys,but the heat transfer and fluid flow behavior in the weld pool of DP-GMAW are not well understood.This dissertation releases a perspective on the whole physical process of DP-GMAW by experiments and numerical simulations,and then the welding process parameters are optimized.A three-dimensional unsteady-state heat transfer and fluid flow model of DP-GMAW is developed,then solidification structure and quality characteristics of DP-GMAW are comprehensively introduced,the mechanism of grain refinement by DP-GMAW is revealed.The mechanical properties of weld joint produced by DP-GMAW are superior to that produced by pulsed gas metal arc welding(P-GMAW),which is mainly owing to refined solidification structure.Effect of current amplitude,current frequency,and welding speed on DP-GMAW is systematically investigated,respectively.The mechanism of grain refinement during double pulsed arc additive manufacturing is revealed;modified DP-GMAW process,such as hybrid laser + double pulsed gas metal arc welding and hybrid cold metal transfer + pulse(CMT+Pulse)welding process are investigated.The main contributions and prime novelties of the dissertation are as follows:1.A three-dimensional unsteady-state heat transfer and fluid flow model of double pulsed gas metal arc welding is proposed.The physical process of heat transfer and fluid flow in the weld pool is discussed,the driving forces of fluid flow and their relative importance are studied,the governing equations,boundary conditions,and calculation methods for the numerical model are introduced.The model employs experimentally-collected welding current waveform as input data,so the heat input during the thermal pulse phase and thermal base phase is different.The model comprehensively considers the effect of momentum and heat of droplet on weld pool as well as the effect of convective heat transfer on weld pool.The model is validated by both P-GMAW and DP-GMAW experiments,so the solidification parameters calculated from the model are reliable.Results indicate that fluid flow in the weld pool directly affects the shape and size of the weld.The driving forces of fluid flow contain Marangoni force,electromagnetic force,and buoyancy,and the Marangoni force is dominant when welding current is not high.Heat transfer in the weld pool is mainly achieved by convective heat transfer through evaluating the dimensionless Peclet number.2.The mechanism of grain refinement by double pulsed gas metal arc welding is revealed,grain refinement by DP-GMAW is achieved by higher solidification growth rate and cooling rate.A perspective on characteristics and superiority of DP-GMAW is released.The weld pool behavior and solidification characteristics are investigated and the solidification parameters are calculated.Then the mechanism of grain refinement by DP-GMAW is revealed by the calculated solidification parameters.The microstructure and mechanical properties of the welds are studied.Results indicate that weld pool expands during the thermal pulse phase and shrinks during the thermal base phase.The expansion of the weld pool during the thermal pulse phase results in an unusual remelting and resolidification the previous solidified weld metal.DP-GMAW features higher solidification growth rate and cooling rate than P-GMAW on condition that the heat input is same.Thus,DP-GMAW features refined microstructure and improved mechanical properties than P-GMAW.DP-GMAW features diverse grain growth direction due to shrinkage of the weld pool during the thermal base phase.3.Effect of current amplitude,current frequency,and welding speed on DP-GMAW is systematically investigated,and the welding process parameters are optimized.The weld pool behavior and solidification characteristics of DP-GMAW with different current parameters are investigated numerically and experimentally.Then the grain dimension produced by DP-GMAW with different current parameters but same heat input are predicted by the computed solidification parameters,and the predictions are verified by the experiments.Results indicate that mean solidification growth rate and cooling rate can be increased by increasing current amplitude or current frequency within frequently-used parameter range on condition that the heat input is the same.With the increase of current amplitude or current frequency,both the grain size and dendrite size decrease,and the mechanical properties of the welds are improved.Effect of welding speed on DP-GMAW and sinusoid modulated pulse gas metal arc welding(SP-GMAW)are investigated.Results indicate that DP-GMAW can achieve stable welding at the welding speed of 16.6 mm/s without short circuit or arc extinguish,which confirmed that DP-GMAW is an efficient and stable welding process for aluminum alloys.As for SP-GMAW,the welding process is stable and the welding quality is favorable.Almost defect-free weld joints are produced,the fracture mode of the joint is ductile fracture.Weld produced by SP-GMAW features reduced porosity rate and refined grains in the fusion zone.Hardness of the weld is increased,so the mechanical property of the weld is improved.4.Modified DP-GMAW process,such as hybrid laser + double pulsed gas metal arc welding,double pulsed arc additive manufacturing,and hybrid cold metal transfer + pulse(CMT+Pulse)welding process are investigated,and the characteristics and advantages are indicated.The feasibility and high efficiency of double pulsed arc additive manufacturing are proved.Compared with single pulsed arc additive manufacturing,double pulsed arc additive manufacturing features higher solidification growth rate and cooling rate.When the heat input is same,double pulsed arc additive manufacturing produces refined grains due to higher cooling rate.A numerical model for CMT+Pulse welding is developed.Results indicate that heat input of CMT+Pulse welding can be adjusted by changing the number of pulses.As for CMT+Pulse welding,the weld pool size,maximum velocity in the weld pool,and peak temperature during the pulse phase are greater than that during the CMT phase.With more number of pulses,the weld pool size,maximum velocity in the weld pool,and peak temperature increase during both pulse phase and CMT phase.