Welding in the World (2020) 0871-wRESEARCH PAPERDevelopment of a novel TIG hot-wire process for wire and arcadditive manufacturingE. Spaniol 1 & T. Ungethüm 1 & M. Trautmann 1 & K. Andrusch 1 & M. Hertel 1 & U. Füssel 1Received: 15 May 2019 / Accepted: 17 February 2020 / Published online: 10 March 2020# The Author(s) 2020AbstractThe presented work deals with the development of a novel TIG hot-wire process for the additive manufacturing of metalliccomponents, which, in contrast to previous arc processes, enables a significant increase in melting performance with simultaneously reduced heat input. This is achieved by means of an upstream resistance heating of the wire between two contact pointswithin the hot wire feeding system. The torch, hot wire feeder and gas nozzle are designed in such a way that a constant beadgeometry can be guaranteed regardless of the machining direction. On the one hand, this can improve the dimensional accuracy;on the other hand, an increase in productivity is achieved through a significant reduction of process times. Essential parts of thework include the simulation-supported development of the processing system, the design and implementation of an innovativeprocess control system and the testing of the new technology.Keywords WAAM . GTAW . TIG . Hotwire . SPH1 IntroductionThe increasing digitalization of industrial value chains requiresthe development of new, intelligent manufacturing technologies that are not only characterised by a high degree of automation but are also capable of producing a wide range ofcomplex component geometries in small quantities at low production costs. The demands for highly flexible, competitivesmall-series production can not be met sufficiently to a limitedextent by conventional primary shaping, forming and cuttingproduction processes, so that the development of additive production technologies is focused. They represent a complementary technology to existing, conventional processes and offerthe user completely new possibilities with regard to geometricdesign. Additive manufacturing technologies are based on theuse of a CAD/CAM environment that enables computer-aidedcomponent modelling and design to be increasingly networkedThis article is part of the collection Additive Manufacturing – Processes,Simulation and Inspection Recommended for publication byCommission XII - Arc Welding Processes and Production Systems* E. [email protected] Universität Dresden, 01069 Dresden, Germanywith component production. The layered production of partsand components, using additive manufacturing technologies,allows the realisation of resource-efficient, sustainable production through the consistent utilisation of the function-integratedlightweight construction potential (e.g. internal cooling channels, multi-material design) [1]. At the present, the majority ofadditive manufacturing technologies are based on the use of awire or powdered filler material that is melted by laser, electronbeam or arc processes [2–4], whereby the energy source ismoved over the substrate by a robot or a CNC system according to the specified component geometry [5]. While beambased additive manufacturing processes are particularly suitable for the production of highly complex components withsmall sizes [3, 4], arc-based processes are primarily suitable forthe production of large-volume components with moderatecomplexity [2, 6]. The melting rates of arc processes exceedthose of beam-based processes by many times, so that a veryhigh productivity can be achieved. At the same time, the investment, operating and maintenance costs of arc processes areonly a fraction of those of beam-based processes, being thereason why arc-based processes are particularly suitable forapplications in small and medium-sized companies [2].Despite the numerous advantages, additive arc processes arenot widespread in mass production so far, since the requirements for geometric dimensional accuracy and material properties (prevention of Widmannstätten structure) cannot be

1330sufficiently met [7]. The reason for this is the high local heatinput that is typical for welding processes. On one hand, thisleads to a change in the microstructure. On the other hand,there is a locally uneven expansion and contraction of themetallic material. As a result thermally induced stresses areintroduced into the component during the production process[8, 9]. These internal stresses cause distortion, so that the desired dimensional tolerances cannot be achieved. Martina et al.[10, 11] show the integration of mechanical rollers as a possible way for compensating residual stresses and distortion.The limitation of the introduced heat and thus the influencing of the component properties using arc processes for additive manufacturing can be realised in different ways.Depending on the arc type, GMAW processes introduce relatively much heat into the workpiece due to the coupling ofenergy and filler material supply. At the same time, the fillermaterial is fed coaxially and is therefore evenly moltenthrough the arc. To limit heat input and thus the penetration,GMAW processes with modified short arcs (e.g. ColdArc orCMT) are mainly used in additive production [12–17].However, the resulting process-related melting rates are limited. In contrast, GMAW welding processes with pulsed, sprayor rotating arc have high melting rates [18], but the higherlocal heating of the components has a pronounced influenceon the microstructure as well as the formation of residualstresses and distortion. This often means that the mechanicand geometric requirements cannot be met [19]. The resultinglong cooling times between the individual layers, which mustbe observed in order to prevent the components from collapsing due to excessive heating, also lead to a significant reduction of economic efficiency caused by very long process times[20].Compared to GMAW processes, GTAW hot wire processesare characterised by a separate supply of energy and fillermaterial. Therefore, it is possible to control the heat inputbetter [21]. This makes it possible to ignite the arc independently of the filler material supply by means of highfrequency or drawn arc ignition, which facilitates a stable,spatter-free starting process. TIG hot wire processes are mainly used in the additive production of highly reactive materialssuch as high-alloyed steels as well as titanium and nickelbased alloys [22–30]. However, a striking disadvantage ofprocesses with a non-melting electrode is that the torchis set vertically with a lateral hot wire feeding system.Consequently, there is no rotationally symmetrical supply ofthe wire material. Therefore, a constant adjustment of the hotwire feed to the torch travel direction is necessary.The varying geometry of the welding bead depends primarily on the feeding angle of the hot wire and the entryposition of the wire into arc and weld pool (leading, trailing)[31]. Up to now, it is possible to realise hot wire entry anglesof approximately 50 without inclination of the welding torch[32]. If more than one hot wire is used, it must be assumedWeld World (2020) 64:1329–1340that the melt pool is asymmetrical [33–36]. Variation of theweld pool geometry also influences the seam scales and thusthe surface roughness [37]. In the production of rotationallysymmetric components, the problem of missing direction independence can be solved by using turn-tilt tables [26, 27,29]. TIG hot wire processes have not yet reached the productivity of GMAW high-performance processes of more than8 kg/h [18, 38, 39]. The reduction of the mean melt pooltemperature cannot be compensated by the joule heatingdue to the short preheating distance between the hot wirenozzle and the melt pool, resulting in binding defects. As aresult, only small wire diameters and feeding rates can beachieved [40], resulting in low melting rates compared toother arc processes [18]. An increase of the melt pool temperature can be achieved by plasma processes, which arecharacterised by a significantly more concentrated heat input[41, 42]. Based on these findings, high-performance TIG hotwire processes have already been developed with the aim ofincreasing melting performance and welding speed.Shinozaki et al. [43–46] and Hori et al. [47] have shown that,depending on the wire material, wire feeding rates up toapprox. 9 m/min are possible by pulsing the hot wire current.Santangelo’s [48] and Henon’s [38] investigations also showthat drop separation and melting performance can be improved by additional mechanical oscillation of the hot wire.A further increase in the melting rate can be achieved byusing two hot wire sources. Depending on the current intensity, the melting rate can almost be doubled [49]. Recentdevelopments also show that it is possible to significantlyincrease the melting rate and thus also the welding speed byusing two-cathode torches in combination with one or twohot wire feeders [50]. Chen’s publication [51] shows that anincrease in hot wire feed and thus in melting rate can beachieved by connecting the hot wire source directly to theTIG torch so that the arc attaches the hot wire. Therefore,decoupling of heat input and filler material supply isachieved. For materials with low electrical resistance, a wirepreheating by an additional arc was also developed [52].2 Development goalsWithin the scope of the presented work, a novel GTAW hotwire system for wire-based additive manufacturing was developed, tested and evaluated, which is characterised by a highproductivity as well as a direction independence at automatedtraversing devices to guarantee a constant bead geometry. Thesystem allows the decoupling of melting rate and heat inputthrough the application of a preceding preheating unit.Therefore, no contact between the wire and the melt pool isrequired so that the process window can be enlarged significantly. Furthermore, the process stability is increased since thearc is not deflected by the hot wire feeding. Consequently, the

Weld World (2020) 64:1329–13401331Fig. 1 (a) cathode focussedGTAW hot wire welding (b) TIGhot wire welding with indirectohmic preheatingpenetration of the base material is lower compared to conventional hot wire process. Additionally, small contact angles canbe realised to avoid faulty connections between the individualbeads.2.1 Realisation of torch and controlling systemThe presented torch system includes a TIG hot wire supplyand a new type of process control. It consists of a TIGtorch and a preheating unit, which are built into a commongas nozzle, whereby very large hot wire feeding angles ofup to 70 , without an inclination of the TIG torch, can beachieved (see Fig. 1b). Therefore, a rotationally symmetrical heat input is guaranteed. TIG torch and hot wire feeding system have been designed in such a way that they arecharacterised by a high cooling performance and a veryslim shaft, whereby the wire feeding angle can be increased significantly. The TIG torch has a shaft diameterof 15 mm, whereas the hot wire system has a diameter of20 mm. The demonstrator has been designed in such a waythat it is also possible to vary the entry position of the hotwire into the melting pool. Furthermore, the shielding gasflow can be influenced by installing different gas nozzles.The process is characterised by indirect resistive heating ofFig. 2 Electrical conductivity as function of the temperature [55]the hot wire (see Fig. 1b). For this purpose, two consecutive contact points have been provided within the hot wirefeeding system in order to realise an adaptable preheatingcurrent. The process is controlled by a software, whichallows the adjustment of the TIG current and the hot wirecurrent depending on the position of the welding torch onthe workpiece. In addition, a further circuit for a joulepreheating can be integrated between the workpiece-sidecontact nozzle and the substrate. The heat input into thecomponent can be controlled independently of the selectedmelting rate due to the individually adjustable circuits ofthe TIG torch and hot wire preheating. It is also possible tovary the preheating length inside the hot wire system. Thecomplete automation of the presented TIG hot wire processenables the realisation of a spatter-free ignition process.2.2 Methodical approachIn order to prove the suitability and the potential of the developed TIG hot wire system, comparative investigations werecarried out with a cathode-focused GTAW hot wire highperformance process. The test arrangement implemented forthis purpose can be seen in Fig. 1a.Fig. 3 Applied model geometry

1332Weld World (2020) 64:1329–1340v W 5 m / m in&&&&&Base material: mild steel (S235JR)Filler wire material: mild steel (G3Si1)Filler wire diameter: 1.2 mmTravel speed: 0.3 m/ minArc length: 6 mmWithin the scope of the study, the influence of the following process parameters was examined to analyse and comparethe process behaviour of the developed hot wire technologywith conventional hot wire processes:&&&TIG currentWire feeding rateHot wire currentv W 7 m / m in1 Preliminary studies of cathode-focussed GTAW hot wirewelding using SPH methodFig. 4 Temperature distribution during cathode focussed GTAW hot wireweldingThe following evaluation criteria were considered for thevalidation of the novel TIG hot wire process:&&&Determination and comparison of the productivity of bothtechnologies by determining the maximum wire feedingratesChecking the direction independence by measuring thebead geometry using 3D digital microscopy when movingthe torch system in different directions, with constant position of the hot wire feedingDetermination of the penetration and the contact angle bymetallographic examinationsThe following materials and geometrical relations were applied for the investigations:For the further enlargement of the process boundaries ofhot wire processes, the modes of failure