Research On Forming Technology And Thermal Process Of GMA Additive Manufacturing With GTA Bypass

Gas metal arc additive manufacturing(GMA-AM)has been brought into focus due to its advantages,such as low production cost and high deposition rate.At present,the common solution to heat accumulation problem in GMA-AM process is to reduce deposition current or delay inter-layer cooling time,which undermines its own advantages to a certain extent.Based on this,the paper presents the energy regulation in GMA-AM process.Through introducing the bypass arc,the heat input of the deposited part can reduce without change of the deposition rate.This paper adds the gas tungsten arc(GTA)as the bypass arc in the GAM-AM,and further investigates the forming characteristics,deposition dimension control,thermal process and microstructure and properties of the deposited parts.A GAM-AM system is first established with a GTA bypass to decompose the deposition current.The effect of GTA torch position on the forming process is studied.When the GTA torch is at the front and the GMA torch is at the rear,the forming appearance will be better.The process parameter ranges for excellent forming appearance are obtained through single-layer single-bead deposition experiments.Tests show that with the increase of deposition current,the bypass current range for well forming becomes larger with higher deposit velocity.With the increase of the bypass current,the deposition width decreases,the deposition height increases and the depth of fusion also decreases.In different deposition current,the introduction of bypass arc can reduce the melted base metal effectively and the area of the heat affected zone.As the bypass current increases,the reduction of the heat input becomes more obvious.According to the area of the melting metal,the largest reduction of the heat input to base metal can exceed 20%,with the same deposition current and deposit velocity.The forming characteristics of multi-layer single-bead deposition of GMA-AM with GTA bypass are analyzed.With the low deposition current(160A),the bypass current should be controlled within a reasonable range(40A-80A)for good surface quality.While introducing the bypass arc,the height difference between the arc striking points and arc extinguishing points reduces and the coefficient of utilization in materials increases.In the same inter-layer temperature,the inter-layer cooling time also decreases by the bypass arc and it can reduce the cooling time by 15% with bypass current of 80 A.With large deposition current(more than 200A),the forming appearance of the multi-layer single-bead GMA deposition is bad due to overflow of liquid metal caused by the larger arc pressure,droplet impact force and gravity of molten pool.Lowering the inter-layer temperature or increasing the deposit velocity has no effect on the deposition of large current.Adding the bypass arc can reduce the heat input of the deposited part and arc pressure.The forming appearance can be better with deposition current of 250 A during the multi-layer single-bead deposition with bypass arc.It expands the range of the deposition current with well forming appearance.A quadratic regression model between the process parameters(including bypass current,deposit velocity and arc voltage)and deposition dimensions(including deposition width,deposition height and depth of fusion)is established.It can be used to analyze the influence of the process parameters on the deposition dimensions and predict the deposition dimensions with different parameters.On this basis,the thin-wall parts with same width are deposited in two ways——adjusting the bypass current or deposit velocity.The result shows that deposition with increasing the bypass current can reduce the deposition layer by 35% and shorten the deposition time by 25%,compared with improving the velocity.The thin-wall parts with variable width can be fabricated by the adjusting of the bypass current with change range of 6mm-8.5mm.The dynamic thermal process of the multi-layer single-bead GMA deposition with GTA bypass is anlayzed by the actual measurement of thermal infrared camera and the finite element numerical simulation respectively.The results of the experiments and simulation show that,the bypass arc in GMA-AM is conducive to the reduction of the heat accumulation in the deposited thin-wall parts without a corresponding decrease in the deposition rate.With the same deposition current and inter-layer cooling time,the dimensions of the molten pool decrease with the bypass arc and the length of the molten pool can reduce 25%.With the increase of deposition height,the high temperature zone become larger and the heat accumulation is more serious.Introducing the bypass arc,in the same deposition layer,the metal of the high temperature zone decreases.In the higher deposition layer,the high temperature zone can reduce 15%.With the same inter-layer cooling time,the mean temperature of the deposited part is lower due to the bypass arc.Finally,the high-strength steel of H06 MnNi3CrMoA with yield strength of 800 MPa is deposited by GMA-AM with GTA bypaass.In this multi-layer single-bead deposition process,the cooling time of the deposited metal such as T8/5 and T8/3 decreases through introducing the bypass arc.It will lead to the change in the microstructure and properties of the deposited parts.In lower layer of the final thin-wall parts,the deposited metal consists of granular bainite and martensitic microstructure.With the increase of the deposition height,the martensitic decreases and acicular ferrite microstructure comes into being due to the poor heat dissipation and the post-heating.Therefore,the hardness of the deposited parts reduces fast with the increase of deposition height firstly,and then tends to be a stable value.Introducing the bypass arc can reduce the heat input of the deposited part.It leads to the increase of the martensitic microstrcture in the thin-wall part.The tensile strength and yield strength of the depostied part are both improved with bypass arc.The yield strength of the depostied part without the bypass arc only can be improved to 680 MPa through lowering the inter-layer temperature or increasing the deposit velocity,not achieving the actual requirement.It also prolongs the deposition time and reduces the deposition rate.By comparison,the yield strength of the depostied part with bypass current of 90 A can increase to 789 MPa,meeting the actual technical requirements.Meanwhile,in this condition,the deposition time can reduce 17% with no reduction of deposition rate.