Visual Detection Of Arc Plasma,droplet Transfer Process And Parameter Optimization In Compulsively Constricted Wire Arc Additive Manufacturing

Wire arc additive manufacturing(WAAM),which uses welding arc as heat source and metal wire as feed material,is a directly forming technology,based on computer 3D modeling technology,taking molten droplets as the basic unit.WAAM has the advantages of high deposition rate,high material utilization,low equipment cost and flexible design,and is suitable for the manufacture of large and simple structures,which has been used in aerospace and automobile fields.However,there are still two unresolved problems:one is the large residual stress and distortion due to the excessive heat input caused by high arc temperature;the other is the low forming accuracy caused by "step effect" and surface roughness.For GMAW,adopting the way of igniting an indirect arc can effectively reduce heat input in the welding process.However,it is easier to produce the situation of weld incomplete fusion and unstable droplet transfer process.Therefore,this paper used a ceramic nozzle to constrict the indirect arc in order to reduce excessive heat input and obtain stable droplet transfer in the WAAM process,which was proved to be feasible in subsequent experiments.In view of the problems of poor process repeatability and too much weld spatter existed in that technology,the reason lies in the lack of understanding of work principle of welding gun,arc behaviors and droplet transfer laws under different parameters.Therefore,this paper mainly investigated the physical process of the technology and then obtained improved weld forming.The main research contents and conclusions are as follows:(1)A water-cooled copper nozzle with a high-temperature resistant glass tube was designed and produced.The high-speed video technology was used to explore generation process of arc plasma and molten droplets without nozzle constriction.The arc shapes and droplet transfer laws under different current,voltage and shielding gas flow parameters were studied.The arc and droplet transfer process under high electrical parameters was more stable and suitable for CC-WAAM,and 300 A/18.3 V,gas flow 5 L/min was one set of suitable parameters.(2)Through intuitive comparison of droplet transfer images,two different characteristics of droplet transfer were summarized,one was directly single droplet transfer,corresponding to 80 A/14.9 V--240 A/18.3 V,the other firstly formed a liquid column then split into several single droplets,corresponding to 250 A/18.3 V--300 A/18.3 V.At 80 A/14.9 V--240 A/18.3 V.And these phenomena were qualitatively explained by analyzing the forces applied on the droplet that was about to leave the wire(3)Three sets of experiments were designed to explore the arc behaviors and droplet transfer with constriction of nozzle by using high speed filming technology.It was found that using ceramic nozzle to constrict indirect arc could generate plasma flow,if distinguishing the ejecting flow by brightness,the brighter was called main arc and the darker was plasma flow.And the plasma flow could change the shape of main arc and promote droplet transfer.(4)The improved process parameters were used to obtain free-spatter single layer weld with good process repeatability;attempts were made to form single-pass multi-layer weld,due to the imperfection of welding torch structure,the forming was not good;finally,several shortcomings of welding torch and suggestions for improvement were proposed.