Study of Various Process Parameters on Bead Penetration and Porosity in Wire Arc Additive Manufacturing (WAAM) of Copper Alloy Cu1897

Abstract

Copper-based alloys are widely known for their high thermal and electrical conductivity. Although the use of these alloys in powder-based additive manufacturing (AM) shows significant promise, applying this method in wire arc additive manufacturing (WAAM) processes poses various considerable challenges, including porosity, delamination, surface oxidation, etc. The limited research on WAAM of copper alloys, especially Cu1897, highlights the need for a more in-depth investigation. This study addresses the effects of process parameters in pulse cold metal transfer (CMT)-based WAAM of Cu1897, i.e., pulse correction (PC) and arc length correction (ALC), on bead penetration and porosity. The results showed that as PC was increased from −5 to +5, weld bead penetration increased from 2.38 mm to 3.87 mm. To further enhance penetration and reduce the porosity, the ALC was varied from +30% to −30% with a step size of 15%. The results showed that weld bead penetration increased to 4.47 mm by altering the ALC from +30% to −30%. Additionally, as the ALC varied within this range, porosity decreased significantly from 3.98% to 0.28%. Overall, it is concluded that a lower value of ALC is recommended to improve bead penetration and reduce porosity in WAAM of Cu1897.

1. Introduction

Additive manufacturing (AM) is defined as a process of manufacturing three-dimensional components from a computer-aided design (CAD) file by adding materials layer-by-layer. As per International Organization for Standardization (ISO) [1], which provides standard terminology for AM, it can be broadly categorized into seven processes: one of them is directed energy deposition (DED). DED is one class of AM in which a focused thermal energy source such as a laser, arc, or electron beam is used to melt the material, and they are selectively deposited. The DED process utilizes metallic material in the form of powder or wire as feedstock. Wire arc additive manufacturing (WAAM) is one type of DED process that uses an electric arc as thermal energy to melt the feedstock material in wire form. WAAM is renowned for its high deposition rate, cost-effectiveness, and ease of material handling [2]. It is possible to achieve a near-net shape with WAAM, and with the addition of the machining process, a net shape can be manufactured, and the process becomes hybrid [3]. A major advantage of using WAAM is the broad scope of material processing. However, there exist various challenges with WAAM in terms of residual stress due to high heat input [4,5], porosities [6,7], humping [8], corrosion [9], and other mechanical properties. The WAAM process can be categorized into three prominent categories based on the type of arc used to melt the wire electrode, i.e., gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), and plasma arc welding (PAW).

Cold metal transfer (CMT) welding is an advanced version of the GMAW process based on the short-circuit metal transfer mode. This process was developed by Fronius in 2004. The CMT process is renowned for producing lower heat input and higher arc stability through an innovative wire feed setup and a digital control system. In this process, droplet detachment occurs mechanically. Furthermore, when the electrode wire tip comes into contact with the metal’s molten pool, “robacter drive”, the servomotor reverses the filler wire by digital process control. Thus, it causes the wire to retract and generate droplet cutting; meanwhile, the current value reduces swiftly to nearly zero, and in this way, CMT causes reduced heat input and minimized spattering during material deposition [10]. The CMT process can be further categorized into five types: CMT-conventional, CMT-advance, CMT-pulse (CMT+P), CMT-pulse advance, and CMT-dynamic.

The CMT+P process merges a pulse cycle with a CMT cycle, producing more heat. However, various parameters in the CMT+P influence the properties of the molten metal pool in different ways, affecting the micro/macrostructure of the final weld bead. These parameters of CMT+P include pulse correction (PC) and arc length correction (ALC). PC affects the droplet’s detachment forces in the process, and in Fronius—TPS 5000 machine (Fronius, Wels, Austria-based company), it ranges from −5 to +5. Specifically, few pulses per second (low frequency), high current, and high droplet detachment forces describe +5 PC. On the other hand, more pulses per second (high frequency), low current, and low droplet detachment forces describe −5 PC.

ALC can be defined as the percentage of electrode wire oscillation that alters the arc length and, hence, the arc formation. Arc length can be defined as the distance between the tip of the wire electrode and the substrate, as described in Figure 1. Arc length affects the heat transfer to the substrate [11], the bead’s penetration behavior [12], and spatters [13] during the material deposition process. In the case of the short arc length, the droplets may come into contact with the molten pool while still on the electrode wire and, hence, may lead to a short circuit [14]. Furthermore, if the arc length is too long, the arc’s stability may be affected, and oxidation of the molten metal may occur [15]. This may lead to different defects in the weld bead, such as porosity, penetration, etc. These parameter settings must be adjusted in accordance with the material properties. The WAAM of copper-based alloys presents various challenges, such as porosity, delamination, oxidation, humping, etc.

Copper (Cu) has medium mechanical strength, exceptional ductility, outstanding electrical and thermal conductivity, and robust corrosion resistance, rendering it an indispensable material for numerous industrial applications [6,10]. Processing pure copper and its alloy with WAAM is challenging due to its high thermal conductivity, reflectivity, and oxide formation on the surface [7]. High thermal conductivity leads to faster heat dissipation during the deposition, causing undesirable residual stress and eventually distorting or delaminating the deposited layer [7]. The formation and accumulation of oxides on pure copper during the WAAM process leads to reduced wettability and diminished flowability of the molten metal [7]. The reflectivity of copper material is another challenge, and it requires high heat input, causing spatter, porosity, and microcracks [7,16].

Porosity is a common issue in WAAM for some materials such as aluminum and copper. Specifically, achieving adequate penetration with the substrate is another challenge for copper and its alloys due to its high thermal conductivity [8]. Tomar et al. [6] investigated the mechanical strength of copper and discovered that WAAM produced superior results compared to other AM techniques. Furthermore, the thermal conductivity of copper processed with CMT-based WAAM was comparable to that obtained through the TIG-based WAAM process, with enhanced grain structure observed [6,17]. In another study by Deshmukh et al. [7], copper deposition using WAAM resulted in hardness comparable to other studies and improved tensile strength. There is limited research on Cu1897 deposition using WAAM, but porosities, penetration depth, and its area have not been explored, highlighting a significant research gap. The current research study addresses these issues by varying the PC and ALC in the CMT+P mode.

2. Materials and Methods

2.1. Materials

The wire material used in the current study is Cu1897, also known as ERCu. It has a 1 mm diameter, and its composition is given in

Table 1. Chemical composition of Cu1897.

Chemical Composition of Cu1897 (%)
Cu	Ag	Mn	Sn	Ni	Zn	Fe	P	Pb	Al
Rem.	0.86	0.065	0.003	0.001	0.006	0.002	0.014	0.001	0.001

. The substrate was mild steel (S235JR), with dimensions of 250 × 200 × 10 mm³, and its composition is presented in

Table 2. Chemical composition of S235JR (%).

Chemical Composition of S235JR (%)
Fe	C	Mn	P	S	N	Cu
Rem.	0.190	1.5	0.045	0.045	0.014	0.55

. Mild steel was chosen as the substrate for Cu1897 deposition to create a bi-material structure that harnesses the mechanical strength of mild steel alongside the superior thermal conductivity of Cu1897. A Fronius—TPS 5000 welding machine with CMT+P welding technology was used for material deposition. Pure Argon was used as a shielding gas to protect the molten pool from the surrounding environment.

2.2. Specimen Preparation and Analysis

An initial investigation was conducted to compare CMT+P with CMT-conventional, which confirmed the suitability of CMT+P for the deposition of Cu1897. After successful trials, wire feed speed (WFS) and torch speed (TS) of 9000 mm/min and 300 mm/min were selected for material deposition, respectively. Furthermore, a gas flow rate (GFR) of 14 L/min was utilized, which was identified as optimal through the initial study, as a high gas flow rate can reduce the bead’s penetration [18]. An interlayer temperature of 100 °C was kept for depositing the second and third layers of Cu1897. The reason for depositing the second and third layers was to study the effects of PC and ALC on porosity in WAAM of Cu1897, as in the first layer, very minute porosity was observed due to the strong intermixing of copper and steel.

Three distinct values were initially used to investigate the effects of PC on bead penetration, which are −5, 0 and +5. Three layers were deposited on top of each other for each value of PC, and the resulting samples are shown in Figure 2.

The samples were cut through a wire—electric discharge machine at three sections, as shown in Figure 2, and mounted using PolyFast powder as mounting material. For the specimens to achieve a mirror-like surface finish, their surfaces were ground with an automatic grinding and polishing machine using a series of abrasive papers of SiC (silicon carbide) from 400 to 2000 grit and eventually with diamond suspensions. The specimen at PC = −5 after grinding and polishing is shown in Figure 3.

These specimens were then observed under an optical microscope, ZEISS Axio Imager.M2m (from Carl Zeiss AG, a company based in Jena, Germany), to study the effects of PC on bead penetration. The optimal PC was then selected and kept constant while the ALC was varied to investigate its effects on bead penetration and porosity. Arc length is influenced by ALC, and variations in ALC can lead to various changes in bead characteristics. In the Fronius CMT welding machine (model—TPS 5000), the ALC ranges from −30% to +30%. Five different values of ALC were used for investigation in this study, which are −30, −15, 0, +15, and +30 and the specimens deposited using these parameters are presented in Figure 4.

To study the effects of ALC on bead penetration and porosity, samples were cut at three sections as shown in Figure 4 and prepared in a similar way to that in the PC study. The study of bead penetration and porosity was performed in ImageJ, version 1.54j (an open-source software for image processing and analysis).

3. Results and Discussion

3.1. Effects of Pulse Correction on Bead Penetration

Bead penetration is crucial in WAAM as it directly influences the mechanical and structural integrity of the deposited material. Adequate penetration ensures strong interlayer bonding and proper fusion between the deposited material and the substrate. Lesser penetration of the bead may reduce the final part’s intended performance; hence, it needs to be investigated in WAAM of Cu1897.

In CMT+P, PC impacts the droplet’s detachment force. Generally, when the PC is 0 for a specific synergic line, the droplet detachment force is equal to an initially programmed value, and in such cases, the weld behavior is governed by WFS and TS in the absence of any other correction factors.

Changing PC from −5 to +5 allows diversion from the pre-set detachment force. A lower PC value, i.e., −5, results in a higher pulse frequency and small size drops [19] and, hence, lower penetration. For the samples presented in Figure 2, grinding and polishing were performed, and it was observed under an optical microscope for penetration study. The results showed that by changing PC from −5 to +5 at intervals of 5, penetration depth increased, and the average of three measured values for bead penetration are plotted in Figure 5.

This increase in penetration depth with an increase in PC value is attributed to the fact that higher PC results in large droplets and, hence, deep penetration. A similar trend was observed in WAAM of magnesium alloy [19], where an increase in penetration was noted at the higher values of PC.

The porosity study is focused on the dilution and reinforcement area of the weld bead. The actual images obtained from the optical microscope are presented in Figure 6a, and porosity numbering and its distribution using ImageJ are depicted in Figure 6b (the large-size pores are marked with a red color, and small pores are overshadowed by numbers), and the corresponding porosity area (in %) is presented in

Table 3. Variation of porosity with pulse correction.

PC (Unitless)	−5	0	+5
Penetration (mm)	2.38	2.89	3.87
Porosity area (%)	3.35	6.69	3.39

. In the dilution zone, the porosity is lower, and as the second and third layers of Cu1897 are added, the porosity increases. This behavior is due to the chemical composition of the base material, as there is less chance of porosity in WAAM of mild steel compared to copper-based materials, as it is less prone to oxidation. It can be seen in Figure 6b that with the addition of second and third layers, the Cu1897 amount increases as more layers are added, and hence, porosity increases.

The problem of porosity persists, and further investigations were made to reduce the porosity by varying the ALC. At +5 PC, penetration was significantly improved, as tabulated in Table 3. Therefore, +5 PC was selected to carry out further investigation by varying the ALC to mitigate the porosity.

3.2. Effects of Arc Length Correction on Bead Penetration and Porosity

3.2.1. Effects of Arc Length Correction on Bead Penetration

ALC is used to adjust the arc length. The change in ALC affects the heat transfer, arc shape, and droplet properties [11]. Additionally, changes in ALC also affect the mechanical properties [20]. For this investigation, ALC was varied at intervals of 15% from +30% to −30% for specimen preparation using CMT+P. The resulting five deposited samples after grinding and polishing were observed under the microscope, and the results are presented in Figure 7.

Weld bead penetration was measured using ImageJ; the results are plotted in Figure 8. As the ALC was altered from +30% to −30%, the bead penetration increased. A similar trend was observed in CMT-based welding of AISI 316L by Kannan et al. [21], where the penetration depth increased as the ALC was changed from −5% to −20%. Moreover, Zhai et al. [12] also observed in mild-steel welding that as the arc length increases, the penetration depth reduces. This is because the arc length reduces with a reduction in ALC, and the arc cone is concentrated on a smaller area [22]; hence, the bead’s penetration increases. The highest penetration of weld bead was observed at ALC = −30%, which is 4.47 mm.

Furthermore, the penetration area of the weld bead, as shown in Figure 9, was examined by altering the ALC. These measurements are presented in

Table 4. Variation of weld bead penetration area with arc length correction.

ALC (%)	+30	+15	0	−15	−30
Penetration Area (mm2)	7.29	11.34	16.55	18	20.63

, and it can be observed that as ALC reduces, the penetration area increases. The results indicate that at ALC +30%, the lowest penetration area of 7.29 mm² was observed, compared to the maximum penetration area of 20.63 mm² at ALC −30%.

3.2.2. Effects of Arc Length Correction on Porosity

Since an increase in ALC will result in higher arc length, the arc stability is reduced, and the chances of molten material being oxidized increases. Copper-based alloys with higher copper content have higher chances of oxidation during WAAM process [7]. The oxide layer formation contributes to porosity in Cu1897 WAAM. Thus, porosity analysis was conducted for different values of the ALC using ImageJ software, and the resulting images after analysis are depicted in Figure 10. The resulting total area, consisting of pores in terms of percentage, is tabulated in

Table 5. Variation of porosity area (in %) with arc length correction.

ALC (%)	+30	+15	0	−15	−30
Porosity Area (%)	3.98	3.81	3.39	0.62	0.28

.

It was observed that the porosity concentration is higher at +30% ALC and reduces significantly at ALC −30%. This is due to the fact that the arc is concentrated at a smaller surface area, and the molten pool can be better shielded from surrounding gases and other contamination in the air [22].

Furthermore, it was observed that as ALC varied from +30% to −30%, spattering also diminished, as illustrated in Figure 11. The gradual decrease in spattering can also be observed in Figure 4 with variation in ALC from +30% to −30%.

4. Conclusions

This study shows that WAAM of Cu1897 (ERCu) is possible by using CMT+P synergic lines. However, in CMT+P lines, there are higher chances of porosity due to higher heat input. In these synergic lines, PC and ALC play a vital role in bead penetration and porosity. Hence, PC and ALC were varied, and the valuable findings can be summarized as follows:

• Variations in the PC from −5 to +5 can be used to vary the penetration of the bead, and higher penetration was observed when the PC value is +5. This is attributed to the fact that in the case of a higher PC value, the droplet size increases.
• The ALC study on bead penetration demonstrated that reducing the ALC to −30% significantly enhances penetration. This is because with a higher ALC, more heat is lost to the surroundings compared to the short arc length, and hence, there is less penetration of the bead. Whereas in a shorter ALC, the arc is concentrated at a small area, and less heat is lost to the surroundings, so the bead penetration is higher. Thus, in the case of the highest ALC value, i.e., +30%, the lowest penetration of 2.56 mm was achieved. On the other hand, in the case of the lowest ALC value, i.e., −30%, the maximum penetration of 4.47 mm was achieved.
• Additionally, the area of penetration also increases with a reduction in ALC. This can be explained as that in the case of the lower value of ALC, the arc cone is concentrated at a small area, and the heat loss to the surroundings is reduced; hence, concentrated heat results in a larger penetration area.
• Interestingly, as the ALC was reduced from +30% to −30%, porosity decreased significantly, from 3.98% to 0.28%. This is due to the fact that the distance from the tip of electrode to substrate reduces with the reduction in ALC, and hence, the molten metal is more protected from the surrounding environment.

This study briefly discusses the effects of PC and ALC on bead penetration and porosity. This study can be used to investigate the mechanical, thermal, and metallurgical properties of the deposited material by varying PC, ALC, and other related parameters along with their interaction effects.