Advanced Robotic GMAW Cladding Process Development. November 6 th and 7 th, 2013

Transcription

1 Advanced Robotic GMAW Cladding Process Development November 6 th and 7 th, 2013

2 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

3 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.

4 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

5 Pulse Waveform Evaluation and Selection Four Commercially available GMAW-P stainless steel waveforms Three in. waveforms One 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

6 Pulse Waveform Evaluation and Selection (cont.) One waveform of each diameter selected in. stainless steel waveform Wire feed speed: 360 ipm Average current: 202 amps Pulse frequency: 203 Hz in. stainless steel waveform Wire feed speed: 200 ipm Average current: 221 amps Pulse frequency: 153 Hz in. Waveform in. Waveform

7 EWI Pulse Waveform Development Higher pulse frequencies to improve droplet transfer in. stainless steel waveform Wire feed speed: 360 ipm Average current: 194 amps Pulse frequency: 312 Hz (+54%) in. stainless steel waveform Wire feed speed: 200 ipm Average current: 246 amps Pulse frequency: 322 Hz (+110%) in. Waveform in. Waveform

8 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 in. waveforms resulted in significant levels of porosity and poor droplet transfer The commercially available in. waveform had the fewest number of pores The EWI-developed in. pulse waveform had the largest number of pores The commercially available pulse waveforms were selected for use in all subsequent trials.

9 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, CTWD, 230 Amps

10 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 Weld Mode CTWD Pulse Frequency Average Current GMAW-P GMAW-P CV GMAW 1.25 N/A 300 CV GMAW, CTWD, 300 Amps

11 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 Hadamard Matrix 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

12 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: in., in. Shielding gas: 100% Argon, 99.75% Argon % CO 2 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.

13 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 Travel Speed 8 ipm Part Inclination X Example

14 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

15 Numerical Prediction Model 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 Model Inputs Wire Diameter (in.) Arc Length CTWD (in.) Travel Speed (ipm) Travel Angle (deg.) Part Inclination (deg.) Weld Mode Shielding Gas Long Pulse Ar + CO2 Summary - Porosity Measurements Shade Total # of Pores % Length Scattered Porosity # of Porosity Groupings Single Pores Pore Size Acceptability (0-5) (count) (% Length) (count) (count) (0-4) (0-4) Model Inputs Wire Diameter (in.) Arc Length CTWD (in.) Travel Speed (ipm) Travel Angle (deg.) Part Inclination (deg.) Weld Mode Shielding Gas Long Pulse Ar + CO2 Summary - Porosity Measurements Shade Total # of Pores % Length Scattered Porosity # of Porosity Groupings Single Pores Pore Size Acceptability (0-5) (count) (% Length) (count) (count) (0-4) (0-4) X

16 Validation Trials X

17 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 # of Pores per 100 Inches CTWD Arc Length Gas Weld Mode Wire Diameter Travel Angle Part Inclination Pass/Fail? W /16 Argon+CO2 Pulse 1/ Pass W /16 Argon+CO2 Pulse 1/ Pass W /16 Argon+CO2 Pulse 1/ Fail (number) W /16 Argon+CO2 Pulse 1/ Pass W /16 Argon Pulse 1/ Fail (size) W /16 Argon+CO2 CV 1/ Pass

18 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

19 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 in. electrode 10-degree push angle in. CTWD Only two pores were found, both within the size limit 0.36 pores per 100 linear inches of weld

20 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

21 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

22 Effect of Chromium 308L: 19.5% to 22% chromium 309L: 23% to 25% chromium 309L build-ups had fewer pores than 308L build-ups

23 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.

24 Effect of Electrode Diameter and Travel Angle on Dilution Lowest dilution with a in. electrode at a -20 travel angle More porosity was observed than with a in. electrode Decreased dilution with the welding arc located on the weld pool

25 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

26 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.

27 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

28 Thank You Marc Alan Purslow Arc Welding

29 References 1. Diamond, William J., Practical Experiment Designs for Engineers and Scientists, 1981.

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