| The traditional single-electrode gas tungsten arc additive manufacturing process is stable and flexible.It is a widely used additive manufacturing technology.However,the arc pressure is too high at a high deposition current,resulting in forming defects such as undercutting and humping.The deposition current is usually less than 200 A,leading to a low deposition rate,which is no more than 1 kg/h.Thus,a hybrid heat source with low arc pressure,twin-electrode gas tungsten arc is introduced to the additive manufacturing field.A double-wire twin-electrode gas tungsten arc additive manufacturing method is proposed.Forming characteristics of the depositied bead,flow field and temperature field of the molten pool,and composition regulating of the deposited metal are researched at the deposition current range of 200～650 A.This research provides theoretical support for expanding the process window of gas tungsten arc additive manufacturing and popularizing the double-wire twin-electrode gas tungsten arc additive manufacturing method.Firstly,the effects of wire feed angle on arc state,wire melting,and droplet transfer were explored.The optimal wire feed angle was obtained to make sure that the arc is symmetrical,and two droplets mixed before transferring into the pool.On this basis,the single-bead depositing experiments were conducted at different deposition currents,wire feed speeds,and travel speeds.The forming characteristics of beads were evaluated.The process window for well-formed beads was explored.It was found that the depositied bead is well-formed at 650 A deposition current when travel speed is between 5.0～7.0mm/s.The maximum deposition rate is up to 5.36 kg/h.On this basis,to realize the parameter planning of double-wire twin-electrode gas tungsten arc additive manufacturing,the regression models of bead width,bead height,penetration depth,and dilution rate on deposition current and travel speed are established by quadratic regression general rotation combination design.For this method,it is found that the bead has large width,shallow penetration and low dilution rate.To further reveal the mechanism of large width and shallow penetration in doublewire twin-electrode gas tungsten arc additive manufacturing,the three-dimensional transient flow and heat transfer characteristics of single-electrode and twin-electrode gas tungsten arc additive manufacturing molten pools were studied by numerical simulation.Based on the computational fluid dynamics software FLUENT,the phase interface grid layer between the metal and gas phases was marked in real-time,which provided an accurate loading position for arc heat and arc pressure.The dynamic evolution process of the flow field and temperature field in double-wire single-electrode and twin-electrode gas tungsten arc additive manufacturing molten pools were calculated.For the singleelectrode method,the main driving force of the molten pool flow is arc pressure.The molten metal on the pool surface flows from the edge to the center of the crater,leading to narrow width and deep penetration.For the twin-electrode method,the main driving force of the molten pool flow is surface tension.The molten metal on the pool surface flows from the center to the edge of the pool.This flow pattern increases the bead width and decreases the penetration.For multi-layer multi-bead deposition,the overlapping behavious of beads deposited by double-wire twin-electrode gas tungsten arc additive manufacturing were studied to improve the surface flatness of the deposited layer.Compared with the conventional additive manufacturing method,the profiles of beads deposited by double-wire twinelectrode gas tungsten arc additive manufacturing are various.The widely used curve models cannot fit them accurately.To quantitatively describe this geometric characteristic,the plumpness of the bead was defined,which is the ratio of the bead cross-section area to its circumscribed rectangle.The plumpness varies with deposition parameters,ranging from 0.576～0.718.Therefore,a universal power function curve fitting model was established.The profiles of bead cross-sections were accurately fitted.The overlapping distance of the beads is optimized on this basis.The surface flatness of the deposited layer is optimized to 0.40 mm,and the stability of multi-layer and multi-bead depositing process is improved.Based on the above researches on forming process of double-wire twin-electrode gas tungsten arc additive manufacturing,the dissimilar double-wire filling method was adopted aiming at the composition regulating of deposited metal.The mild steel wire H08Mn2 Si and high strength low alloy(HSLA)steel wire H06 Mn Ni3Cr Mo A were filled into the pool with different wire feed speed at the same time.The relationship between microstructure,properties,and composition of deposited metal was studied.The results show that with the increase of the HSLA steel wire proportion in the deposited metal,the microstructure is transformed from ferrite to granular bainite and acicular ferrite.The comprehensive mechanical properties are improved.The ranges of tensile strength,yield strength,and microhardness of deposited metal are 565～914 MPa,441～803 MPa,and HV205.7～324.5 respectively.To verify the theory and model proposed in this paper,mild-steel and HSLA-steel bimetal simulated composition-gradient crusher hammer and sliding bearing were designed and fabricated by double-wire twin-electrode gas tungsten arc additive manufacturing.The deposition rate reaches 2.49 kg/h.The gradient of composition,microstructure,and properties of the deposited parts was realized.The deviation between the measured microhardness distribution and the target is less than 5%.The standard error of the fabricated part geometries is 1.00 mm. |
PDF Full Text Request