key: cord-0288243-u2o3fzik
authors: Shukla, Pranjal; Dash, Balaram; Kiran, Degala Venkata; Bukkapatnam, Satish
title: Arc Behavior in Wire Arc Additive Manufacturing Process
date: 2020-12-31
journal: Procedia Manufacturing
DOI: 10.1016/j.promfg.2020.05.105
sha: 349be4da65bc2386f83d45e5ac9b8a8a945f46ed
doc_id: 288243
cord_uid: u2o3fzik

Abstract The welding arc behavior in a wire arc additive manufacturing process was studied while building a ten-layer wall over a base plate using a cold metal transfer (CMT) power source. The real-time recorded welding current, voltage, thermal cycles at the base plate, and the synchronized high-speed arc images with each deposited layer were used to understand the arc behavior. A gradual increase in the peak temperature was noticed from the first layer to the fifth layer, and the same decreased with the subsequent deposition of the layers. The intensity of the welding arc was high during the boost phase of the CMT cycle, where most of the base plate and the electrode melting happens. For a given process parameter, the welding arc intensity gradually increased from the first layer to the fifth layer while remained approximately similar from the fifth layer to the tenth layer. The effect of this arc intensity variation was reflected in the consistency of the deposited bead profile. It was observed that the bead width gradually increased from the first layer to the fourth layer and remained approximately the same from fifth to the tenth layer.

Wire arc additive manufacturing (WAAM) is the high productive variant of the additive manufacturing process, which uses the welding arc as a source to deposit the molten metal layer upon layer to build a three-dimensional part. Cold metal transfer (CMT) process is widely used with WAAM to control the heat input during the process. CMT is one of the advanced variants of the conventional gas metal arc welding (GMAW) process. The innovative droplet detachment by the wire feeding system during the short-circuiting mode reduces the overall heat input for a given deposition [1] . The flow of the deposited molten metal poses the difficulty in controlling the bead profile in the WAAM process. The resulting dimensional and geometrical inaccuracies lead to timeconsuming post-processing operations [1] . Understanding the arc behavior during the WAAM process helps in controlling the process more efficiently and improving product quality.

Xiong et al. [2] examined the influence of welding current, deposition speed, and heat input on the forming characteristics of multi-layer single-pass parts using a passive vision sensor in a gas metal arc (GMA) based WAAM process. It was reported that the arc current has the most significant effect on the forming appearance. The optimum current for a smooth bead appearance was reported to be between 100 and 180 A. Panda et al. [3] observed that the peak current significantly affects the deposited layer dimensions when compared to the wire feed and travel speeds in gas tungsten arc (GTA) assisted WAAM process.

Ghosh et al. [4] studied the variation in arc characteristics, with variation in pulse parameters during GMA weld deposition using Al-Mg filler wire. The increase of arc voltage up to about 25 V predominantly enhanced the arc length almost linearly. The velocity of plasma had significant influence on enhancement of the metal drop velocity within the arc environment. Kiran et al [5] discussed the behavior of the welding arc in the pulsed DC and AC GMA welding processes using real time recorded current, voltage waveforms, and synchronized high-speed video at different electrode negative (EN) ratios for a constant wire feed rate. For an approximately equal peak positive current, the increase in the pulse time enhanced the arc plasma distribution, and the arc root

Wire arc additive manufacturing (WAAM) is the high productive variant of the additive manufacturing process, which uses the welding arc as a source to deposit the molten metal layer upon layer to build a three-dimensional part. Cold metal transfer (CMT) process is widely used with WAAM to control the heat input during the process. CMT is one of the advanced variants of the conventional gas metal arc welding (GMAW) process. The innovative droplet detachment by the wire feeding system during the short-circuiting mode reduces the overall heat input for a given deposition [1] . The flow of the deposited molten metal poses the difficulty in controlling the bead profile in the WAAM process. The resulting dimensional and geometrical inaccuracies lead to timeconsuming post-processing operations [1] . Understanding the arc behavior during the WAAM process helps in controlling the process more efficiently and improving product quality.

Xiong et al. [2] examined the influence of welding current, deposition speed, and heat input on the forming characteristics of multi-layer single-pass parts using a passive vision sensor in a gas metal arc (GMA) based WAAM process. It was reported that the arc current has the most significant effect on the forming appearance. The optimum current for a smooth bead appearance was reported to be between 100 and 180 A. Panda et al. [3] observed that the peak current significantly affects the deposited layer dimensions when compared to the wire feed and travel speeds in gas tungsten arc (GTA) assisted WAAM process.

Ghosh et al. [4] studied the variation in arc characteristics, with variation in pulse parameters during GMA weld deposition using Al-Mg filler wire. The increase of arc voltage up to about 25 V predominantly enhanced the arc length almost linearly. The velocity of plasma had significant influence on enhancement of the metal drop velocity within the arc environment. Kiran et al [5] discussed the behavior of the welding arc in the pulsed DC and AC GMA welding processes using real time recorded current, voltage waveforms, and synchronized high-speed video at different electrode negative (EN) ratios for a constant wire feed rate. For an approximately equal peak positive current, the increase in the pulse time enhanced the arc plasma distribution, and the arc root

Wire arc additive manufacturing (WAAM) is the high productive variant of the additive manufacturing process, which uses the welding arc as a source to deposit the molten metal layer upon layer to build a three-dimensional part. Cold metal transfer (CMT) process is widely used with WAAM to control the heat input during the process. CMT is one of the advanced variants of the conventional gas metal arc welding (GMAW) process. The innovative droplet detachment by the wire feeding system during the short-circuiting mode reduces the overall heat input for a given deposition [1] . The flow of the deposited molten metal poses the difficulty in controlling the bead profile in the WAAM process. The resulting dimensional and geometrical inaccuracies lead to timeconsuming post-processing operations [1] . Understanding the arc behavior during the WAAM process helps in controlling the process more efficiently and improving product quality.

Xiong et al. [2] examined the influence of welding current, deposition speed, and heat input on the forming characteristics of multi-layer single-pass parts using a passive vision sensor in a gas metal arc (GMA) based WAAM process. It was reported that the arc current has the most significant effect on the forming appearance. The optimum current for a smooth bead appearance was reported to be between 100 and 180 A. Panda et al. [3] observed that the peak current significantly affects the deposited layer dimensions when compared to the wire feed and travel speeds in gas tungsten arc (GTA) assisted WAAM process.

Ghosh et al. [4] studied the variation in arc characteristics, with variation in pulse parameters during GMA weld deposition using Al-Mg filler wire. The increase of arc voltage up to about 25 V predominantly enhanced the arc length almost linearly. The velocity of plasma had significant influence on enhancement of the metal drop velocity within the arc environment. Kiran et al [5] discussed the behavior of the welding arc in the pulsed DC and AC GMA welding processes using real time recorded current, voltage waveforms, and synchronized high-speed video at different electrode negative (EN) ratios for a constant wire feed rate. For an approximately equal peak positive current, the increase in the pulse time enhanced the arc plasma distribution, and the arc root 48th SME North American Manufacturing Research Conference, NAMRC 48 (Cancelled due to dimensions. The fraction of the available arc energy rate supplied to the base plate was higher in positive pulse when compared to the negative pulse.

Wu et al. [6] developed an experimental system to implement vision-based observation of the weld pool behavior during high-speed GMAW process to control the weld pool humping defect. Kiran et al. [7] studied the arc behavior in two wire tandem submerged arc welding process. The arc characteristics were measured from the recorded arc images using Abel Inversion and Fowler Milner technique. Subsequently the physical model to estimate the interaction between the two arcs was developed. Wang et al. [8] reported that controlling the CMT process parameters like shortcircuiting duration and the wire feed motion can help in controlling the energy input of the process coupled with metal transfer behavior to design and optimize the weld properties for a given application.

WAAM technology uses arc welding process to supply the heat to the base plate and the solid electrode/filler wire. The arc welding processes normally applicable for WAAM include GTAW, Plasma Arc Welding (PAW), and GMAW processes. Although WAAM technology is able to achieve the mechanical requirements, it is necessary to choose the correct welding process depending on the material and application required. In this sense, it could be concluded that CMT is suitable for large size Stainless steel parts with low/medium mechanical requirements. GTAW and PAW processes could be used for small/medium size Titanium and Stainless-steel parts which have medium/high mechanical requirements as mentioned by Tabernero et al. [9] . The effect of electrode positive time cycle (% EP) of the alternating current GTAW process for the WAAM of linear walls made of Al was investigated by Ayarkwa et al. [10] . It was reported that the effective wall width was minimum at 20%EP with a corresponding maximum in layer height. It was also observed that increasing the% EP increased the electrode wear rate, which in turn affected the arc stability. Wu et al. [11] investigated the influence of heat accumulation on bead formation, arc stability, and metal transfer behaviour during the manufacture of Ti6Al4V with the GTA assisted WAAM process using localized gas shielding. It was reported that due to the various thermal dissipation paths along the building height, there exists a significant difference in temperature variation between substrate and in-situ layer. The formation and improvement of surface waviness for additive manufacturing of 5A06 aluminum alloy component with GTA assisted WAAM process was reported by Geng et al. [12] .

Xiong et al. [13] proposed a methodology based on a laser vision system to view the surface appearance on the side face of multi-layer single-pass low-carbon steel parts deposited using GMA based WAAM process. Yang et al. [14] compared the thermal cycles recorded while manufacturing a component using double electrode GMAW (DE-GMAW) and conventional GMAW based WAAM processes. It was reported that the peak temperature of the substrate during the deposition was lower with DE-GMAW when compared to the GMAW based WAAM process. Xiong et al. [15] reported that the increase in preheating of the base plate lowered the temperature gradient in the molten pool, and also reduced the temperature gradient difference in the molten pool of different layers.

In summary, detailed studies of the arc behavior during the CMT based WAAM process are not yet readily available in the open literature. The authors present here the comprehensive study to understand the variation in the layer dimensions, temperature distribution, and the arc behavior in a CMT assisted WAAM of a ten-layer wall over a base plate.

Experiments are performed in the WAAM platform developed at IIT Tirupati. Figure 1 shows the schematic representation of the WAAM platform. It includes a CMT advanced welding power source, Kuka KR16 robot with six axis, high speed camera (Nova S6 from Photron), and the data acquisition system (DAQ) to record the welding current and voltage waveforms as well as thermal cycles. Table 1 outlines the chemical composition of the base plate and the electrode wire. The process parameters used to deposit the wall over the base plate are given in table 2. Figure 2 depicts the schematic representation of the wall to be deposited over the base plate, and the position of high-speed camera and thermocouples. Ten layers are deposited one over the other to build a vertical wall. The arc images are recorded at a 5kHz sampling rate from the side orientation, as shown in Fig. 2(b) . A bandpass filter (690±10 nm) is used to eliminate the arc light interference. For few experiments ND filter is used to decrease the arc image saturation and to observe the variation in the intensity with in the arc column. The instantaneous welding current and voltage waveforms are recorded in synchronization with the arc images. The thermal cycles are measured using the K-type thermocouples fixed at a distance of 3 mm and 25 mm from the bottom and side face of the base plate, respectively [ Fig .2(b) ], and at a distance of 30 mm, 60 mm and 90 mm along the welding direction from the starting of the weld bead. The deposited wall dimensions are measured from the transverse section polished and etched with a 2% Nital solution and observing using a stereo-microscope. Figure 3 shows the thermal cycles measured using the thermocouples fixed at the locations (30, 60, and 90 mm) along the travel direction from the layer starting point, however, during the building of the first to tenth layers. The location of the thermocouples is also shown in Fig. 2(b) . It is clear from the recorded thermal cycles that the thermal history is measured in the region where the welding arc reached a quasi-steady state. The base plate experiences the repeated thermal cycles due to the heat input from the welding arc and the molten metal deposited from the first layer to the tenth layer. For instance, the temperature of the base plate at location A increases from room temperature to 266 0 C as the welding arc approaches the location, while the arc moves away from it the base plate starts cooling from 266 0 C to 159 0 C. Further, the temperature of the base plate at location A increases from 159 0 C to 355 0 C during the second layer deposition as the welding arc approaches near it and cools down to 248 0 C as the arc moves away from it. The heat accumulation by conduction from the first layer to fifth layer deposition increases the peak temperature from 266 0 C to 451 0 C. However, beyond the fifth layer, peak temperature decreases from 451 0 C to 413 0 C. As the wall height increases, the distance between point A and the welding arc, and the surface area of the wall enhances. This subsequently surges the heat loss by conduction and convection which result in a reduction in peak temperature. Fig. 4(b) -(e). It is evident that the already existing layer is remelted by the depositing layer, which results in forming a bow-shaped profile. As the welding arc energy and the pressure vary in a Gaussian distributed manner, their corresponding magnitudes are more near the arc center region. As a result, more amount of metal is remelted at the center portion of the already deposited bead.

The heat input and the molten metal deposited in the WAAM process are relatively more than that of the laserassisted AM processes. Along with the heat input, various driving forces in the weld pool like buoyancy force, Lorentz force, Marangoni force, shear stress from plasma jet, and arc pressure affects the fluid flow in the weld pool and the resulting bead profile. It is imperative to precisely control the heat input and the associated driving forces to maintain a consist layer dimensions in the WAAM process. Figure 5 depicts the terminology of the deposited layer dimensions measured in the present work. W1 and H1 represent the width and height of the first layer. From the second layer onwards, the width and height are calculated as

where i, h, and H refer to the layer number, the height of the i th layer, and overall height of the wall after depositing i th layer, respectively. It is observed that the width of the layer or bead gradually increases from the first layer (5.36±0.25mm) till the fourth (6.05±0.21mm) layer, while the height decreases simultaneously from 2.37±0.14 mm to 0.96±0.06 mm [also refer to Figs. 4(a), (d) and (e)]. The layer dimensions remain approximately consistent from the fifth layer onwards. The base plate acts as a heat sink to cool the deposited layers. The initial temperature of the base plate is at room temperature and it cools the first layer significantly. With each layer deposited the base plate temperature increases and its cooling efficiency on the newly depositing layers reduce. It is also evident in the thermal cycles presented in Fig. 3 . The magnitude of the variation in the peak temperatures experienced by the base plate from the fifth layer onwards reduces. This results in the diminished variation in the thermal gradient between the new layers and the base plate. Furthermore, the thermal balance between the heat loss by conduction and convection heat transfer could also be reached from the fifth layer onwards. This results in maintaining the consistent layer dimensions form the fifth layer to the tenth layer. The complete waveform is characterized by three phases. Phases I, II, and III refer to the boost, waiting, and short-circuiting aspects, respectively. The region between points 1 to 6 represents the boost phase, 6 to 8 corresponds to waiting phase, and the region beyond 8 depicts the short-circuiting phase. The arc behavior in Phases I-III is explained using the arc images chosen at different instances of the current waveform. Please note that ND filter is not used while recording the arc images presented in Fig. 7 .

The arc images corresponding to points 1, 2, and 3 explain the arc behavior during the current rising at the beginning of the pulse. As the welding current increases, the area enveloped by the anode spot enhanced at the tip of the electrode, and the plasma distribution between the electrode, and the base plate widened. Further, the increase in the heat input with the enhanced current also ionizes the shielding gas surrounding the existing plasma region and widens the arc.

The significant melting of the electrode wire and the workpiece happens during the boost phase, the droplet pendant generated at the tip of the electrode reaches the weld pool during the waiting phase. Here the droplet size doesn't grow due to the lower heat input associated with the waiting phase. Finally, in the short-circuiting region, the droplet pendant short-circuits with the molten pool surface at a relatively smaller magnitude of the welding current. The electrode wire moves opposite to the feeding motion at the instant droplet short-circuits with the molten pool allowing the droplet to detach without any spatter. Arc-light interference is decreased using ND filters along with the bandpass filter to capture the apparent variation in the intensity with the layer. It is observed that the arc intensity (in boost phase) increases gradually from the first layer to the fourth layer and remained consistent from fifth to tenth layers. The well-defined correlation between the arc intensity and the layer dimension is depicted in Fig. 8 . The layer width increases gradually from the first layer to the fourth layer and remains approximately the same from the fifth to the tenth layer. This gives us a clue that maintaining the consistent arc intensity by varying process parameters for each layer can result in consistent dimensions.

This paper presented the detailed experimental study on the CMT based WAAM of ten-layer wall built over a base plate. The results of this work can be summarized as follows:

• The base plate experiences repeated thermal cycles due to the heat input from the welding arc and the molten metal deposited from the first layer to the tenth layer. The heat accumulation by conduction from the first layer to fifth layer deposition increases the peak temperature of the base plate. Fifth layer onwards the peak temperature decreases due to dominant heat losses from conduction and convection. • The arc intensity increases from the first layer to the fourth layer and remains approximately similar in the subsequent layers. The possible reason can be the cooling effect of the base plate for the initial few layers. This arc intensity variation has a direct relation to the layer dimensions. • The width of the layer increased continuously with a simultaneous reduction in the layer height from the first layer to the fourth layer. The layer dimensions remained approximately consistent from the fifth layer onwards. The combined influence of the reduction in the variation of thermal gradient between the new layers and the base plate, and the thermal balance between the supplied heat input and heat losses by the conduction and convection from the fifth layer onwards results in stabilizing the fluid flow and maintaining the consistent layer dimensions from the fifth layer to the tenth layer.

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Forming appearance analysis in multilayer single-pass GMAW-based additive manufacturing

Evaluation of genetic programming-based models for simulating bead dimensions in wire and arc additive manufacturing

Arc characteristics and behavior of metal transfer in pulsed current GMA welding of aluminum alloy

Three dimensional finite element modeling of pulsed AC gas metal arc welding process

Visualization of hump formation in high-speed gas metal arc welding

Arc behavior in two wire tandem submerged arc welding

Characterization the contribution and limitation of the characteristic processing parameters in cold metal transfer deposition of an Al alloy

Study on arc welding processes for high deposition rate additive manufacturing

Assessing the effect of TIG alternating current time cycle on aluminum wire+arc additive manufacture

Effects of heat accumulation on the arc characteristics and metal transfer behavior in wire arc additive manufacturing of Ti6Al4V

Formation and improvement of surface waviness for additive manufacturing 5A06 aluminum alloy component with GTAW process

Influences of process parameters on surface roughness of multi-layer single-pass thin -walled parts in GMAW-based additive manfacturing

A comparative study of GMAW and DE-GMAW based additive manufacturing techniques: thermal behaviour of the deposition process for thin-walled parts

Finite element analysis and experimental validation of thermal beviour for thin-walled parts in GMAW based additive manfacturing with various subsrate preheating temperatures

The authors gratefully acknowledge the support of the DST/SERB Ramanujan research grant (Grant No. SB/S2/RJN-093/2015), Naval Research Board grant (NRB-436/MAT/18-19) and New Faculty Seed Grant from IIT Tirupati (ME/18-19/008/NFSG/DEGA)