Process Development November 6 th and 7 th, 2013 Introduction - - PowerPoint PPT Presentation

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 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

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 Travel Speed

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

Pulse Ar + CO2 Shade Total # of Pores % Length Scattered Porosity # of Porosity Groupings Single Pores Pore Size Acceptability 0.7 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

Pulse Ar + CO2 Shade Total # of Pores % Length Scattered Porosity # of Porosity Groupings Single Pores Pore Size Acceptability 3.5 31 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

0.00 Pass W2 1.125 3/16 Argon+CO2 Pulse 1/16

• 20

10.94 Pass W3 0.72 5/16 Argon+CO2 Pulse 1/16

• 20

65.63 Fail (number) W4 0.72 3/16 Argon+CO2 Pulse 1/16

• 20

1.56 Pass W5 1.125 5/16 Argon Pulse 1/16

• 20

15.63 Fail (size) W6 1.125 5/16 Argon+CO2 CV 1/16

• 20

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

• f 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

Thank You

Marc Alan Purslow Arc Welding 614.688.5150 mpurslow.@ewi.org

References

1.

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