Research On Arc Stability And Welding Quality Of PP-GMA Welding-brazing Of Aluminum To Steel

Conventional fusion welding is undesirable due to the low solubility of iron in aluminum,large difference in physical and chemical properties,and excessive formation of brittle intermetallic compounds(IMCs).In order to achieve high quality welding of aluminum/steel,MIG welding-brazing method was used.However,current researches were mostly focused on pulsed MIG welding-brazing of aluminum to steel,the investigation of pulse on pulse MIG welding-brazing process was fairly rare.In this work,the welding system of pulse MIG arc welding-brazing of aluminum alloy to stainless steel was established.The effects of pulsing parameters on arc shape,metal transfer,weld formation and mechanical properties was discussed.Meanwhile,a set of evaluation methods of stability on the pulse MIG arc welding-brazing process were developed.The results show that the pulse on pulse was composed of high energy pulse and low energy pulse.For high-energy pulse,the effects of pulse on pulse frequency on arc characteristics and behavior of metal transfer are significant.However,for low-energy pulse,the effects are insignificant.The external oscillation frequency of the molten pool induced by variation of arc force and impingement of metal droplets is closer to its natural oscillation frequency when the pulse on pulse frequency is closer to 5 Hz;the weld pool is more intensively oscillated.This oscillation results in the formation of fine and uniform equiaxed grain structures,and thin brazed interface layers,which leads to excellent mechanical properties.The aluminum alloy/stainless steel dissimilar metal joints consist of three parts: welded seam formed by molten Al alloy and Al-Si filler wire,a brazed interface layer and solid-state stainless steel.The thickness of brazed interface layers varied from about 5 to 9 ?m,and the thinner the layer is,the better the mechanical property of Al alloy/SS joints will be.When pulse on pulse frequency is less than or equal to 5.3 Hz,the joints fractured at heat-affected zone of 6061 Al alloy with highest strength of 117.4 MPa up to 94.7 % of the tensile strength of Al alloy.However,the joints fractured at the brazed interface layer with much lower strength when the pulse on pulse frequency is larger than 5.3 Hz.While MIG arc brazing method provides significant potential for joining aluminum alloy to stainless steel,the welding process is still hard to examine and control quantitatively.Pulse on pulse can be applied to improve the stability of conventional pulse gas metal welding-brazing process.The calculation results of welding voltage and current based on time domain and frequency domain analysis demonstrate that the welding process stability of pulse on pulse gas metal welding-brazing(PP-GMWB)far outweighs that of pulse gas metal welding-brazing(P-GMWB)process.The validated experimental results show that the metal transfer mode of P-GMWB was the blend of one drop per pulse(ODPP)and multiple drop per pulse(MDPP).However,for high energy pulse of PP-GMWB,the droplet transfer mode is primarily ODPP with a few number of two drop per pulse(TDPP);for low energy pulse,the metal transfer maintains ODPP through the whole welding process.The weld formation and mechanical properties of PP-GMWB was better than that of P-GMWB.