Introduction: Robotic arc directed energy deposition (DED) additive manufacturing (AM) provides a wide range of potential benefits for building structures, adding features to structures, and repairing structures. Prior investigation developed pulse gas metal arc (GMA-P) DED procedures for building single pass wall and multi-bead multi-pass (MBMP) block sections. These DED parameters were used to fabricate standard qualification builds (SQBs) and evaluate DED procedure qualification scheme requirements for new technical publication . Prior GMA-P DED investigation evaluated SQBs with two bead sizes and two inter-pass temperatures using ER308L electrode where tensile properties met expectations in the x-, y- and z-directions. The deposition rates of the ER308L GMA-P DED procedures were 7.1 lbs/hr for large bead and 8.7 lbs/hr for small bead parameters.
To improve DED AM economics and deposition rate for making larger shapes, multi-wire variants of GMA-P were reviewed in this investigation. Based on this review, the twin wire - gas metal arc - pulse (TW-GMA-P) process was selected for procedure development. The twin wire torch is more omni-directional compared to tandem torches making it more attractive for DED. In addition, the twin wire process only requires a special torch and wire feeder. Twin wire GMA-P shares a common power supply making it more affordable than tandem wire GMA-P systems that require two power supply setups.
Experimental Approach: The TW-GMA-P DED AM process was setup to use two 0.040-inch diameter ER308L electrodes and argon-CO2 shielding gas. 304L stainless steel was used for the build platforms. Maximum inter-pass temperature was 350°F for each bead. The contact tip to work distance was set at 0.625-inch and all tests were performed in the flat position with no torch or work angle. No angles were used to simplify parameter modeling in robotic computer aided manufacturing (CAM) software packages that solve build plans for making DED structures. Mini-wall and mini-block parameter build tests were used to develop the TW-GMA-P DED procedures. A systematic parameter development method was used to develop parameters at a constant bead size by varying total wire feed speed (WFS) to travel speed (TS) ratios from 30 to 45. A final set of WFS and TS combinations were tested at the WFS/TS ratios of 35 and 42 to determine fusion quality from insufficient fusion to excessive dilution/penetration and loss of fairness.
Standard qualification builds (SQB) must pass both ultrasonic and radiographic inspections and have acceptable surface and edge fairness. Therefore, the main acceptance criteria used to guide parameter development was the ability to make straight and fair walls and blocks that are completely fused and defect-free. Tests that met fairness requirements were sectioned, etched, and examined for fusion quality and bead structure. Preferred parameters that met the above criteria and maximized deposition rate were selected for making MBMP block builds and testing properties.
Preferred parameters were used to build the SQB is shown in Figure 1. The SQB is 28-in. long. The build contains both single pass wall and MBML block features to permit tensile specimens in x- and z-, and x-, y- and z-directions, respectively. All passes were made in the x-direction. The first pass of each layer is produced down the center of the build width. Additional passes in each layer in multi-pass layers were alternated about the center pass of the layer (bi-symmetric build). All passes in each layer are produced in the same direction. The direction of passes alternated direction in every new layer to level the start and stop areas.
Figure 1. ER308L TW-GMA-P DED Wall and Block Property Build on Non-integrated 304L Build Platform
Results and Discussion: The objective of this investigation was to develop high deposition TW-GMA-P DED parameters and benchmark the preferred parameters against single-wire GMA-P DED procedures. As noted, the deposition rates were 7.1 lbs/hr for large bead and 8.7 lbs/hr for small bead GMA-P procedures. The preferred parameters were a result of the pulse waveform melting rate, pool stability while depositing DED walls and blocks, surface fairness, and soundness. Pool stability at higher travel and wire feed speeds was also limited by arc plasma-jet phenomena. High travel speed tests that had deep finger penetration were more susceptible to plasma-jet induced porosity.
Higher melting rate (wire volume melting from anode and resistive heating) processes produce lower heat inputs and require higher travel speeds to achieve the same level of fusion (base metal dilution) for a given bead deposit size. The TW-GMA-P process uses smaller electrodes that provide higher resistive heating and melting rate. From a plasma jet perspective, tandem and twin wire arcs that are the same polarity are attracted to each other and couple. A coupled arc results in lower arc pressure compared to single wire arcs at the same current and are very resistant to pool keyhole formation. In addition, a coupled arc improves melting rate since each arc provides additional electrode extension heating to the opposing electrode. The net affect was the TW-GMA-P arc and puddle were very stable at significantly higher travel speeds and wire feed speeds compared to the prior investigation. The deposition rate evaluated for making block builds varied from 12 lbs/hr to 30 lbs/hr. At the latter deposition rate, the deposit was stable on the first layer, but as build height increased the stability of the build deposits was lost. The preferred parameters for WFS/TS ratios of 35 and 42 were:
• 700-ipm WFS combined at 20-ipm TS; 15 lbs/hr
• 1050-ipm WFS combined at 25-ipm TS; 23 lbs/hr
The same parameters were used for all beads at the 15 lbs/hr deposition rate. When the deposition rate was increased to 23 lbs/hr, edge bead and single bead procedures needed modified to lower rate settings to ensure pool stability and surface fairness.
Conclusion: Preferred TW-GMA-P DED parameters provided two to three times the deposition rate of single wire GMA-P DED procedures for making MPML block builds. Stable builds were made at deposition rates up to 23 lbs/hr. The TW-GMA-P DED process offers significant benefits and should be considered for large-scale DED AM of structures.