Comparison of GMAW and GMAW–C Arc Welding

Processes for Welding on Galvanized Steels

Vaidyanath B. Rajan

Yen-Chih Liao

The Lincoln Electric Company, Cleveland, Ohio

Special Acknowledgement to Daniel Galiher and Allan Gwinn at

Tower International for all the tensile testing, microscopy and

analysis presented in this study

Outline

• Introduction

• Experimental Procedure

• Visual Examples of Welds (X Ray and Surface)

• Examination of Metrics

− % Porosity

− % Porosity vs. Wire Type at Different Travel Speeds

− Joint Strength

− Methodology and Description of Fracture Modes

− Fatigue Life

− Fatigue Life as a function of Porosity

− Weld Cross Sections and Microhardness Traces

• Conclusions

Introduction

• Auto industry is focused on light weighting to meet fuel

efficiency standards

• One light weighting strategy in chassis apps is thickness reduction

with AHSS

− Problem: Mass reduction has led to corrosion concerns

− Solution: Zinc coating in concert with e-coating for chassis applications

• Corrosion solution via Zn for chassis applications presents a

joining concern

− Problem: GMAW of HDG steel causes increased spatter, internal porosity,

arc instability, & throughput degradation

− Solution: FCAW has been proven to be successful to reduce porosity and

maintain throughput

Introduction

• Weld manufacturers continue to investigate alternative welding

solutions for Zn coated materials

− Why?

− To reduce cost and/or eliminate slag on weld while still avoiding

porosity

− How?

− Via delivery methods (e.g. Pulsed GMAW, Waveform customization,

Wire manipulation methods)

− Via Metal Cored wire solutions (GMAW-C)

Experimental Procedure

• Lap welds on HDG HSLA steels

− Base Material: 80 ksi HSLA Steels 60G/60G 3 mm thick

− 657 MPa (95 ksi) Yield; 722 MPa (105 ksi) Tensile; Total Elongation 19.7%;

Uniform Elongation 8.8%, ~10 µm of HDG coating; N-value 0.106

− Filler Metal & Process

• Metrics − % Porosity Content of welded joint (via X-ray analysis) − Tensile Strength of welded joint (max load) − Fatigue lives of samples with varying levels of porosity − Ancillary data: Micro-hardness and Metallography

Goal: Understand weld quality & weld process robustness of HDG 80 ksi HSLA

material using different weld wires & travel speeds

AWS Spec Welding Filler & Process Process Description Gas

ER70S-6 (S) 0.045 SuperArc L56 + Precision Pulse GMAW 85%Ar/15%CO2

ER80S-D2 (D) 0.045 SuperArc L-90 + Precision Pulse GMAW 85%Ar/15%CO2

E70C-GS (M) 0.040 Metalshield Z with Rapid Z AC GMAW-C 85%Ar/15%CO2

• 2 Factor Multi-Level DOE with 8 replicates

− Factors: Weld Wire, Travel Speed

− Main Responses: % Porosity, Tensile Force

• 3 Weld Wire Types and Processes

• 3 Weld Travel Speeds at 2F orientation

− 25 ipm

− 35 ipm

− 45 ipm

• 8 samples at 35 ipm at 45°downhill relative to 2F orientation

Experiment Description: Factors and Responses

8 samples ea = 72 samples

24 Samples

Experiment Description: Fixture & Orientation

45 deg downhill config

8” and

Rolling

Direction

Direction

of Gravity

in Side

View

12”

2F Orientation

1”

ER70S-6 GMAW Weld (S) @ 35 IPM Travel Speed

Porosity Area 8.26 %

Porosity Area 0.35 %

The same procedure can produce different amounts of internal

porosity. High porosity inside weld is not reflected in surface quality.

ER80S-D2 GMAW Weld (D) @ 35 IPM Travel Speed

Porosity Area 8.26 %

Porosity Area 0.41 %

The same procedure can produce different amounts of internal

porosity. High porosity inside weld is not reflected in surface quality.

Metal-Core AC GMAW-C Weld (M) @ 35 IPM Travel Speed

Porosity Area 0.84 %

Porosity Area 0.27 %

Metal cored AC GMAW-C weld shows excellent surface and interior quality with very little

porosity. Internal porosity is very low and predictable.

% Porosity vs. Wire Type at Different Travel Speeds

Metal Core AC GMAW-C provides < 1.6 % porosity across all speeds & orientations.

Solid Wire GMAW welds provide wide range of porosity up to 12 %.

Wire Wire

D

M

S

D

M

S

% Porosity

0 1.6 3.2 4.8 6.4 8.0 9.6 11.2

Wire 25

35

45

45 deg/35

25

35

45

45 deg/35

25

35

45

45 deg/35

Speed (ipm) Wire

% Porosity vs. Wire Type at Different Travel Speeds

D M S Weld Wire/Process

D

M

S

12

10

8

6

4

2

0

D M S Weld Wire/Process

Interquartile Range for Weld Porosity - Metal Cored AC GMAW-C < Solid wire GMAW

Porosity Summary

• Metal Cored AC GMAW-C welds shows lower medians & less variation

in % porosity than solid GMAW welds in every scenario.

• Metal Cored AC GMAW-C welds exhibit a normal distribution of %

porosity.

• Solid wire GMAW welds exhibit a wide distribution of % porosity. Also,

the % porosity range obtained is unpredictable.

• 2F orientations typically have lower medians than downhill

orientation; however, variability is not particularly distinguishable by

orientation.

• 45 downhill orientation shows higher minimum porosity levels relative

to 2F for all wires.

− Metal cored AC GMAW-C welds downhill minimum is still small (0.49%) and is ¼

the value of the solid wire GMAW weld minimums.

• Tensile Test done in longitudinal direction of

material (rolling direction).

• Picked a Best & Worst dogbone tensile sample

based on porosity X-rays from each welded

parent sample. Two from each welded condition

totaled 192 tensile tests.

Tensile Test General Information

Example of Worst/Best

Tensile Fracture Mode – Pure HAZ (H) Sample: Metal-Core Weld (M)

Top View of the Weld Section.

6.7X magnification.

Polished and Etched Side of the Weld.

12X magnification

Tensile Fracture Modes – Partial Weld (W) Sample Solid Wire (D)

Top View of the Weld

6.7X magnification.

Polished and Etched Side of

the Weld. 12X magnification

View of the Fracture Surface.

6.7X magnification.

Fracture occurs at pore near the side.

Tensile Fracture Modes – Partial Weld (W) Sample Solid Wire (D)

Top View of the Weld

6.7X magnification.

The white arrows indicate the

direction of viewing the section.

Polished Section of the Weld taken at

the blue line.

Away from the pore, fracture tunnels into the HAZ.

Fracture s were either 100% HAZ fractures , 100% Weld fractures or mixtures of the two.

Fracture Mode Categorization

29%

27%

44%

Relative Number of HAZ Fracture

Samples

S

D

M

41%

45%

14%

Relative Number of Weld Fracture

Samples

S

D

M

0

10

20

30

40

50

60

S D M

Nu

mb

er

of

HA

Z

Fa

ilu

res

Number of HAZ Failures vs. Wire

0

10

20

30

40

S D MNu

mb

er

of

Fa

ilu

res

in W

eld

Number of Failures in Weld vs. Wire

Metal Core (AC GMAW-C) has the most HAZ fractures and the least Weld Fractures.

This is consistent with AC GMAW-C process showing the least amount of porosity.

Recall tensile dogbones were pre-chosen as a “best” & “worst” for each sample

Peak Load vs. Wire Type and Fracture Mode

For HAZ or Full/Partial Weld fractures, peak load median is independent of wire/process.

For Full/Partial Weld fractures, the scatter in Peak loads is significantly greater than in pure

HAZ failures.

5500500045004000350030002500

Fracture Type Wire Type

H

W

D

M

S

D

M

S

Peak Load (lbf)

H

W

Type

Fracture

H = Pure HAZ Failure; W = Partial or Full Weld Failure

Parent Metal = 6189 lbs

Dotplot of Peak Load (lbf)

D

M

S

D

M

S

H

W

Fracture Type

Wire Type

1. Full or partial weld fractures showed more variability & lower

median peak loads than pure HAZ failures.

2. Full or partial weld fractures all exhibited porosity on the

fracture surface.

3. Because of (1) & (2), and since porosity is minimized (in

median and in variation) with AC GMAW-C, we believe AC

GMAW-C provides the best joints.

4. 80 ksi GMAW wire does not provide stronger joints for this weld

joint compared to Metal Core wire (rated at 70 ksi min).

Tensile Testing Summary

• Fatigue Testing of the lap joints conducted in tension-tension mode

• Tests conducted at

− 1000 lbs max load (16% of Ultimate Tensile load)

− 1500 lbs max load (25% of Ultimate Tensile load)

− R=0.1

− Frequency of 10 Hz

• Fatigue “Dog Bone” Samples were chosen from the ER70S-6 GMAW

welds with varying levels of porosity from 0 to 25%

• Fatigue samples were identical in size to the Tensile Samples

Fatigue Testing General Information

Fatigue Life as a Function of Porosity

1,000

10,000

100,000

1,000,000

0 5 10 15 20 25 30

Fa

tigu

e L

ife

(cyc

les)

% Porosity

Fatigue Life (@10 HZ, R =0 .1) vs % Porosity

1550 lbs Peak Load 1000 lbs Peak Load

Lower Bound

(LB) 0%P

50% drop in fatigue life vs. LB

45% drop in fatigue life vs. LB

25% drop in fatigue life vs. LB Lower Bound

(LB) 0%P

Porosity has a dramatic effect on Fatigue life.

For <7% porosity, up to 25% drop in fatigue life vs lower bound (LB) of life at 0% porosity.

For >7% porosity, up to 50% drop in fatigue life vs lower bound (LB) of life at 0% porosity.

Fatigue Fracture Surface Fatigue Fracture Surface @1550 lbs,

16,345 cycles 10.2% porosity

Fatigue Fracture Surface @1000 lbs,

73,894 cycles 10.3% porosity

Crack at toe

Pore

Weld Root

Side View

1 mm

Pore

Weld Root

Side View

1 mm

Pore Pore Weld Root

1 mm

Fracture Surface

Pore Pore Weld Root

1 mm

Fracture Surface

Fatigue failure in the presence of porosity occurs preferably in the root.

10% porosity in the weld reduces fatigue life by up to 50%.

• Porosity has dramatic effect on Fatigue life.

− Fatigue Life decreases drastically with % porosity.

• At 1000 lbs max load (16% of UTS load)

− 1 -7 % porosity can decrease the fatigue life by about 25%.

− >7% porosity can decrease the fatigue life by about 50%.

• At 1550 lbs max load (25% of UTS load)

− <5% porosity didn’t have significant effect on fatigue life.

− >10% porosity can decrease fatigue life by about 45%.

• Fatigue failure in samples with no porosity could occur either at the

weld root or the toe.

• Fatigue failure in samples with porosity preferably occurs at the weld

root.

Fatigue Testing Summary

• General Info

− Reported hardness values are Vickers Micro-hardness.

− Sample X-Section pictures for each wire at 35 ipm are in subsequent

slides.

− Micro-hardness measurements were taken at each speed/orientation for

welds with each wire (12 total micro-harness sweeps).

• Summary

− HAZ hardness degradation relative to parent metal is approx 15%*.

− All weld metals except ER70S-6 are harder than the HAZ.

* Mean degradation for each wire ( S = 14%, D=14%, M=13%)

MicroHardness

MicroHardness Summary

S-Weld:

HAZ & Weld hardnesses are indistinguishable

Parent Metal is harder than both Weld and

HAZ

D-Weld & M-Weld:

Parent Metal & Weld Hardness levels are

indistinguishable

HAZ is significantly softer then either Parent or

Weld areas.

H – HAZ

P – Parent Metal

W - Weld

H P W H P W H P W

D M S

Location

Weld Wire/Process

MicroHardness Traverse S Weld

175

200

225

250

275

300

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Har

dnes

s (H

V)

Point #

HAZ HAZ

Weld hardness and HAZ hardness < base metal hardness

MicroHardness Traverse D Weld

175

200

225

250

275

300

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Har

dn

ess

(H

V)

Point #

HAZ HAZ

HAZ hardness < weld and base metal hardness

MicroHardness Traverse M Weld

175

200

225

250

275

300

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Har

dnes

s (H

V)

Point #

HAZ HAZ

HAZ hardness < base metal hardness < weld hardness

• GMAW-C metal-core wire weld with AC waveform exhibits dramatically

less porosity and porosity variation than GMAW solid wire welds in all

tested scenarios.

− GMAW-C % porosity exhibits a normal distribution while the GMAW solid wires do not.

• Surface quality is not necessarily indicative of internal porosity.

• Porosity adversely affects joint strength.

− Weld fractures had lower median strengths than pure HAZ failures.

− All full or partial weld fractures had visible porosity present in the joint.

− Weld fractures had significantly greater variability in peak load than pure HAZ failures.

• Porosity has a dramatic effect on fatigue life. Porosity higher than 7%

can decrease fatigue life by up to 50%. Below 7% porosity, the effect

depends on the load levels, but can result in up to a 25% reduction in

fatigue life.

Conclusions

1. Clasius, R., 2014. 80 Day Cyclic Corrosion Test Results

ArcelorMittal Report #: RAC2014-006

2. Dragolich, F. and Geisler, R,. 2010. Lincoln Electric Technical

Report 1007010005G

3. Galiher D. et al., 2014. Solutions for Coated Steels. FabTech

2014.

4. Marttila, W., 2007. Arc Welding of Coated Steels. Internal Chrysler

Welding Engineering Doc.

5. Narayanan, B. et al. 2014. Solutions for Welding Zinc Coated

Steels. AWS SMWC.

6. Narayanan, B., 2014. Lincoln Electric Technical Report TH-216B

Bibliography

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