November 6th and 7th, 2013
Advanced Robotic GMAW Cladding
Process Development
Introduction
Stainless steel cladding is common for carbon steel components used in commercial and military ships
Porosity defects have been reported as a significant issue in automated gas metal arc welding (GMAW)
Commercially available electrodes are preferred over custom-made products to reduce cost
Productivity requirements demand long arc-on times making extended contact-tip-life an important consideration
Objectives
Develop stainless-steel GMAW cladding procedures to ─ minimize porosity using commercially available ER308L and
ER309L stainless steel electrodes
─ maximize arc-on time by increasing contact tip life.
Approach
Majority of development work conducted using ER308L stainless steel electrodes
─ Assumed that porosity mitigation techniques would apply to ER309L stainless steel electrodes
Laser-diode illuminated high-speed video
─ GMAW-P
─ Commercially available waveforms
─ EWI-developed waveforms
─ CV GMAW
DOE approach to identify critical variables
Porosity prediction model
DOE validation trials
Electrode chemistry analysis
Effect of travel angle and electrode diameter on dilution
Contact-tip-life trials
Pulse Waveform
Evaluation and Selection
Four Commercially available GMAW-P stainless steel waveforms ─ Three 0.063-in. waveforms
─ One 0.045-in. waveforms
─ 100% Argon shielding gas
─ Necking with poor droplet transfer
─ Forceful, columnar arc
─ Significant puddle depression
─ 0.35-in. arc length required
─ Shorter arc lengths resulted in excessive shorting and spatter
─ Poor wetting and inconsistent bead width on a carbon steel
─ Improved wetting on subsequent layers
─ 99.75% argon/0.25% CO2 shielding gasses
─ Necking with marginally improved droplet transfer
─ Arc length could be reduced slightly
─ Significantly improved wetting on carbon-steel substrates
Pulse Waveform Evaluation
and Selection (cont.)
One waveform of each diameter selected
0.045-in. stainless steel waveform
Wire feed speed: 360 ipm
Average current: 202 amps
Pulse frequency: 203 Hz
0.063-in. stainless steel waveform
Wire feed speed: 200 ipm
Average current: 221 amps
Pulse frequency: 153 Hz
0.045-in. Waveform
0.063-in. Waveform
EWI Pulse Waveform
Development
Higher pulse frequencies to improve droplet transfer
0.045-in. stainless steel waveform
Wire feed speed: 360 ipm
Average current: 194 amps
Pulse frequency: 312 Hz (+54%)
0.063-in. stainless steel waveform
Wire feed speed: 200 ipm
Average current: 246 amps
Pulse frequency: 322 Hz (+110%)
0.045-in. Waveform
0.063-in. Waveform
12-layer Build-ups
All four GMAW-P waveforms were used to create 12-layer build-ups ─ Shielding gas: 100% argon ─ CTWD: 3/4-in. ─ Travel speed: 6 ipm ─ Weave width: 0.75-in. ─ Weave frequency: 1.3 oscillations per minute─ Bead overlap: 3/8-in.
Evaluated with radiography (RT)─ Both 0.045-in. waveforms resulted in significant levels of porosity
and poor droplet transfer─ The commercially available 0.063-in. waveform had the fewest
number of pores ─ The EWI-developed 0.063-in. pulse waveform had the largest
number of pores─ The commercially available pulse waveforms were selected for use
in all subsequent trials.
Diode-laser-illuminated
high-speed video
Used to observe the effect of welding mode, CTWD, and arc length on puddle depression
CTWD significantly affects the depth of the puddle depression─ Increasing the CTWD increases the
resistive heating of the electrode
─ The required current is reduced
─ The required pulse frequency is reduced
─ Results in a less-focused arc with a larger footprint
─ Current density is reduced
─ Puddle depression is more shallow
GMAW-P, 0.75” CTWD, 294 Amps
GMAW-P, 1.125” CTWD, 230 Amps
Diode-laser-illuminated
high-speed video
GMAW CV arc is more conical
Results in a larger-diameter puddle depression
Decreases the current density “seen” by the molten puddle when operating at the same current level
CV GMAW, 1.125” CTWD, 300 Amps
Weld Mode CTWD Pulse Frequency Average Current
GMAW-P 1.25 175 230
GMAW-P 0.72 294 294
CV GMAW 1.25 N/A 300
DOE
In preliminary trials, stringer beads contained more porosity than welds made with a weave
When a weave was used, the majority of porosity was found at the penetration spike located at the dwells
Assumptions─ Stringer beads represent a “worst-case-scenario” regarding porosity
─ Methods of reducing porosity in stringer beads will be effective in weave welds
Fractional factorial DOE design based on a HadamardMatrix
A resolution V design, allowing the estimation of the main effects of each variable, as well as the interactions between variables (1)
48 weld beads
DOE Levels
Two levels required for each of the eight variables selected for investigation
Based on end-user requirements and/or EWI experience: ─ Electrode diameter: 0.045-in., 0.063-in.
─ Shielding gas: 100% Argon, 99.75% Argon + 0.25% CO2
─ Weld mode: GMAW-P, CV GMAW
Scaling trials were used to select the following levels:─ Travel speed: 8 ipm, 12 ipm
─ Part inclination: -10° (downhill), 0°
─ Travel Angle: -20° (drag), 0°
─ CTWD: 3/4-in., 1 1/2-in.
─ Arc length: 3/16-in., 5/16-in.
DOE Level Selection Criteria
Setting must produce a visually acceptable bead for the majority of variable combinations
─ Example:
─ Travel speeds up to 16 ipm were acceptable with a 3/16-in arc length
─ The maximum travel speed with a 5/16-in. arc length was 12 ipm
─ The upper travel speed level was 12 ipm
Less penetration is preferred
Parameters selected to test the widest range possible
36
8 ipm
-10° 0° +10°
Part Inclination
Tra
vel S
peed
36
Example
X
DOE Level Selection
Weld beads evaluated with radiography
Porosity evaluation criteria─ Size
─ Shade of indications
─ Acceptability per end-user supplied criteria
─ Total number of pores
─ Number of groups of pores
─ Percent of weld length containing scattered porosity
─ Number of isolated pores
Numerical model created to predict porosity level─ “Acceptability scale” from 0 to 4
─ 0: no pores
─ 4: porosity far exceeding the acceptable level
Numerical Prediction Model
Model Inputs
Wire Diameter
(in.)
Arc
Length CTWD (in.)
Travel Speed
(ipm)
Travel Angle
(deg.)
Part Inclination
(deg.)
Weld
Mode
Shielding
Gas
0.0625 Long 1.125 12 -20 0 Pulse Ar + CO2
Shade Total # of Pores % Length Scattered Porosity # of Porosity Groupings Single Pores Pore Size Acceptability
0.7 0 0 0.0 1 0.6 0.0
(0-5) (count) (% Length) (count) (count) (0-4) (0-4)
Summary - Porosity Measurements
Model Inputs
Wire Diameter
(in.)
Arc
Length CTWD (in.)
Travel Speed
(ipm)
Travel Angle
(deg.)
Part Inclination
(deg.)
Weld
Mode
Shielding
Gas
0.0625 Long 0.072 12 -20 0 Pulse Ar + CO2
Shade Total # of Pores % Length Scattered Porosity # of Porosity Groupings Single Pores Pore Size Acceptability
3.5 31 0 1.2 2 2.9 3.7
(0-5) (count) (% Length) (count) (count) (0-4) (0-4)
Summary - Porosity Measurements
0.72
Model predicts CTWD as the most significant variable─ Verified in validation trials
Also predicted that short arc lengths and 100% Argon shielding gas would increase porosity─ Disproved in validation trials
X
Validation Trials
X
Weaving Validation Trials
Six additional weld build-ups made using a weave
DOE model predictions ─ W1, W2, and W6 would have minimal to no porosity
─ W4 and W5 would have an acceptable amount of porosity
─ W3 would have porosity far exceeding the acceptance criteria
5 results were consistent with the model predictions
W5 failed due to pores exceeding the size limit
Weave
Set CTWD
Arc
Length Gas
Weld
Mode
Wire
Diameter
Travel
Angle
Part
Inclination
# of Pores
per 100
Inches Pass/Fail?
W1 1.125 5/16 Argon+CO2 Pulse 1/16 -20 0 0.00 Pass
W2 1.125 3/16 Argon+CO2 Pulse 1/16 -20 0 10.94 Pass
W3 0.72 5/16 Argon+CO2 Pulse 1/16 -20 0 65.63 Fail (number)
W4 0.72 3/16 Argon+CO2 Pulse 1/16 -20 0 1.56 Pass
W5 1.125 5/16 Argon Pulse 1/16 -20 0 15.63 Fail (size)W6 1.125 5/16 Argon+CO2 CV 1/16 -20 0 3.13 Pass
Effect of Current Density
At 300 amps, the build-up made using CV GMAW had less than 5% of the number of pores contained in the GMAW-P build-up made at an equal average current
Indicates that porosity is not only related to current level, but also to current density
CV GMAW Build-ups
Additional build-ups made to evaluate whether CV GMAW would consistently reduce porosity
Twelve-layer build-up created using CV GMAW─ Over 550 inches of linear inches of weld
─ 0.0625-in. electrode
─ 10-degree push angle
─ 1.125-in. CTWD
─ Only two pores were found, both within the size limit
─ 0.36 pores per 100 linear inches of weld
Effect of Electrode Chemistry
Five heats of 308L were used in welding trials
Material certifications were studied to identify whether chemical elements could be correlated to porosity formation─ Data presented is of welds made with GMAW-P, since a larger number of
samples were created with GMAW-P than with CV GMAW
Effect of Chromium
Strong correlation between chromium level and porosity level Chromium affects the solid solubility of nitrogen Nitrogen that cannot be absorbed by the weld pool must escape
before solidification occurs, or porosity will result Increased levels of chromium correlate to decreased porosity
Effect of Chromium
308L: 19.5% to 22% chromium
309L: 23% to 25% chromium
309L build-ups had fewer pores than 308L build-ups
Effect of Sulfur
Strong correlation between sulfur level and porosity level
Surface-active element that creates a layer on the surface of the weld pool
Acts as a barrier to degassing, increasing porosity levels.
Effect of Electrode Diameter
and Travel Angle on Dilution
Lowest dilution with a 0.045-in. electrode at a -20° travel angle─ More porosity was observed than with a 0.063-in. electrode
Decreased dilution with the welding arc located on the weld pool
Contact-tip-life Trials
Compared GMAW-P to CV GMAW
Improvement in arc stability and a significant decrease in contact tip wear with CV GMAW
GMAW-P
CV GMAW
Conclusions
Porosity can be reduced in 308L and 309L clad layers by ─ manipulating key process parameters
─ selecting electrode heats with ideal levels of chromium and sulfur
These findings suggest that porosity occurs via two distinct mechanisms
Mechanism 1 - Current Density─ The forceful, columnar arc common to GMAW-P produces a deep
puddle depression, driving pores to the bottom of the penetration spike
─ Current density at the surface of the molten weld pool has a significant effect on porosity level
─ Welding in CV mode results in a more conical arc shape that reduces the current density and the severity of the depression in the weld puddle
─ Welding with an extended CTWD further reduces the current density as the increased resistive heating experienced by the electrode decreases the current required to melt the electrode.
Conclusions (cont.)
Mechanism 2 - Electrode Chemistry
─ Porosity level is a function of electrode chemistry
─ Increased levels of chromium correlate to decreased porosity because chromium increases the solubility of nitrogen in the weld puddle
─ Electrodes with higher levels of chromium allow absorption of higher levels of nitrogen, minimizing the level of degasification required to allow pores to escape the weld pool before solidification
─ Decreased levels of sulfur correlate to decreased levels of porosity because sulfur is a surface-active element which creates a layer on the weld pool surface that acts as a barrier to degassing
In addition to reduction in porosity, contact-tip-life and arc stability were both significantly improved when using CV GMAW over GMAW-P
References
1. Diamond, William J., Practical Experiment Designs for Engineers and Scientists, 1981.
