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Great Designs in Steel is Sponsored by:

ArcelorMittal Dofasco, ArcelorMittal USA, Nucor Corporation,

Severstal North America, Inc. and United States Steel Corporation

Effects of Material Properties and Weld

Geometry on Fatigue Performance of DP780

and Mild Steel GMAW Lap Joints

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and Mild Steel GMAW Lap Joints

Justin Hunt, Jack Sang

AET Integration Inc.

Dave Anderson

American Iron & Steel Institute

Presentation Content

• Project Background

• Welding Process Development

• Fatigue Test Results & Analysis

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• Fatigue Life Modeling

• Concluding Remarks

Project Objective

• Conduct a focused study on 2.0mm mild steel

and 2.0mm DP780 steel to evaluate the

effects of the following factors on weld

fatigue life of GMAW lap joints

– Weld Geometry

– Steel strength

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– Steel strength

– Gaps

• Investigate the fundamental mechanism that

influences GMAW fatigue performance

(collaboration with Oak Ridge National Lab)

Project Sponsors

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AET Integration Inc.

AET Integration, Inc. is a technology firm dedicated to

R&D and Engineering in materials welding and joining,

as well as related material engineering services.

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2007 Henry Ford

Technology Award

Related AET Capability

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AET Fatigue Testing Capability

22Kips 110 Kips

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Approach

• To compare weld geometries, four different geometries were used for each material

• To compare steel strengths, two matching geometries were used that had very similar penetration, leg size, toe angle, and toe radius for both materials

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radius for both materials

• Very similar weld parameters were used for all matching geometries

Welding Process Development

• It was found in previous AISI studies that low toe angle and high toe radius increases fatigue performance

• In general, the mild steel tested had a tendency to produce higher toe angle and lower toe radius welds than DP780 at production travel speeds (45-70 IPM)

• The mild steel tested shows high susceptibility to undercutting. Travel speed and voltage needed to be unusually low to eliminate undercut and produce the theoretical “best” geometry.

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theoretical “best” geometry.

Toe Radius

Toe Angle

DP780 & Mild Steel Joints

with Same Weld Parameters

Mild DP780

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High travel speed and voltage causes very different

weld geometries between mild steel and DP780

Improved Matching Geometry

Mild DP780

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Low travel speed and voltage allows almost identical

weld geometries to be obtained for both materials.

Improved Matching with Gap

Geometry

Mild DP780

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Similar weld parameters as the Improved

Matching Geometry, with a 0.5 mm gap.

Acceptable Production Geometry

Mild DP780

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High travel speed welds could not be matched

perfectly. Toe radius and angle are similar.

Improved Production Geometry

Mild DP780

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Mild steel geometry was improved slightly over the Acceptable

Production weld. DP780 showed a much greater improvement. No

attempt was made to match geometry. These were the best

achievable geometries at higher travel speeds

Weld Geometry Variations

IP-MildIP-DP780IM-MildIM-G_MildIM-G_DP780IM-DP780AP-MildAP-DP7802.01.51.00.50.0 GGGGeeeeoooommmmeeeetttt rrrr yyyy //// SSSStttt eeeeeeeellll GGGGrrrr aaaaddddeeee

TTTTooooeeee RRRRaaaaddddiiiiiiii ((((mmmmmmmm))))VVVVaaaarrrr iiiiaaaatttt iiiioooonnnn ooooffff TTTTooooeeee RRRRaaaaddddiiii iiii

IP-MildIP-DP780IM-MildIM-G_MildIM-G_DP780IM-DP780AP-MildAP-DP78070656055504540 GGGGeeeeoooommmmeeeetttt rrrr yyyy //// SSSStttt eeeeeeeellll GGGGrrrr aaaaddddeeeeTTTTooooeeee AAAAnnnngggglllleeee ((((DDDDeeeeggggrrrreeeeeeee

)))) VVVVaaaarrrr iiiiaaaatttt iiiioooonnnn ooooffff TTTTooooeeee AAAAnnnnggggllll eeee

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IP-MildIP-DP780IM-MildIM-G_MildIM-G_DP780IM-DP780AP-MildAP-DP780

6.05.55.04.54.0 GGGGeeeeoooommmmeeeetttt rrrryyyy //// SSSStttt eeeeeeeellll GGGGrrrraaaaddddeeeeHHHHoooorrrriiiizzzzoooonnnnttttaaaallll LLLLeeeegggg LLLLeeeennnnggggtttthhhh ((((mmmmmmmm))))

VVVVaaaarrrr iiii aaaatttt iiii oooonnnn ooooffff HHHHoooorrrr iiii zzzzoooonnnnttttaaaallll LLLLeeeegggg LLLLeeeennnnggggtttthhhh

Weld Parameters

Geometry Material Current (A) Voltage (V)Travel Speed

(ipm)

Torch

Angle (°)*

Wire Displacement

(mm)

Improved Matching DP780 118 20.9 20 40 1.5

Improved Matching Mild 120 20.0 20 35 1.0

Improved Matching - Gap DP780 118 18.7 20 40 0.0

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Improved Matching - Gap DP780 118 18.7 20 40 0.0

Improved Matching - Gap Mild 120 20.0 20 35 0.0

Acceptable Production DP780 280 19.7 65 45 0.0

Acceptable Production Mild 225 17.0 45 30 0.0

Improved Production DP780 190 23.0 45 35 2.0

Improved Production Mild 170 17.5 35 30 1.0

Schematic of Weld Fatigue

Test Specimen

152.4 mm Spacer

25.4 mm

152.4 mm Spacer

25.4 mm

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25.4 mm

152.4 mm

Spacer

Spacer

25.4 mm

152.4 mm

Spacer

Spacer

Fatigue Life, DP780 / Mild

Improved Matching Geometry

ad (lb) R=0.1

2500

2200

1900

1500

1200

Mild

DP780

Improved Matching Geometry

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Cycles

Peak Loa

10000000100000010000010000

1200

1000

900

800

700

DP780 shows up to 3 times higher fatigue life and

25% higher run-out load than mild steel

Fatigue Life, DP780 / Mild

Improved Matching with Gap Geometry

ad (lb) R=0.1

2500

1900

1500

1200

Mild

DP780

Improved Matching with Gap Geometry

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Cycles

Peak Loa

10000000100000010000010000

900

700

600

Both steels show similar high cycle fatigue life

and run-out loads

Fatigue Life, DP780 / Mild

Acceptable Production Geometry

ad (lb) R=0.1

2500

1900

1500

1200

Mild

DP780

Acceptable Production Geometry

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Cycles

Peak Loa

10000000100000010000010000

1000

800

600

Both steels show similar fatigue life and equal

run-out loads

Fatigue Life, DP780 / Mild

Improved Production Geometry

d (lb) R=0.1

2500

1900

1500

Mild

DP780

Improved Production Geometry

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Cycles

Peak Load

10000000100000010000010000

1200

1100

1000

800

DP780 shows up to 4 times higher fatigue life and

38% higher run-out load than mild steel

Run-out Loads

Geometry Mild Run-out (lb) DP780 Run-out (lb)

Improved Matching 800 1000

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Improved Matching 800 1000

Improved Matching with Gap 600 700

Acceptable Production 800 800

Improved Production 800 1100

Mild Steel S/N Curves

ak Load (lb) R=0.1

2 5 0 0

1 9 0 0

1 5 0 0

1 2 0 0

1 0 0 0

9 0 0

Im p r o v ed M atc h in g

Im p r o v ed M atc h in g w ith G ap

A c c ep tab le P r o d u c tio n

Im p r o v ed P r o d u c tio n

M i ld

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Cy c le s

Pea

10 00000 01 0000 001000 00100 00

9 0 0

8 0 0

7 0 0

6 0 0

Fatigue life was increased by up to 4 times and run-out

load was increased 33% by eliminating gaps and

improving weld geometry

DP780 S/N Curves

ak Load (lb) R=0.1

2500

2200

1900

1500

1200

1100

1000

Im p ro v ed M atc h in g

Im p ro v ed M atc h in g w ith G ap

A c c ep tab le P r o d u c tio n

Im p ro v ed P r o d u c tio n

DP7 8 0

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Cy c le s

Pea

10000000100000010000010000

1000

900

800

700

Fatigue life was increased by up to 10 times and run-out

load was increased 57% by eliminating gaps and

improving weld geometry

Fatigue Specimen Fracture

Locations

• Most mild steel samples (with the exception if the acceptable production geometry) failed at the weld root, indicating that fatigue life could not be further improved by weld toe improvements.

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• Most DP780 specimens (with the exception of the improved production geometry) failed at the weld toe, indicating further weld toe improvements may increase fatigue life.

Fatigue Specimen Fracture

Locations

DP780 Examples Mild Steel Examples

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Tensile Specimen Fracture

Locations

• All mild steel samples failed in the base metal far from

the weld.

• DP780 samples failed in the HAZ or weld metal near

the fusion line.

DP780 Examples Mild Steel Examples

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DP780 Improved Weld Toe SCF: 6.5

DP780 Baseline Weld Toe SCF: 8.55

Crack Initiation Life Modeling

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Weld Toe SCF: 6.5 Weld Toe SCF: 8.55

Oak Ridge-AET Model

)()2()2()(

2

Ei

c

if

b

i

mfNNNN

E≤+

−=

∆ε

σσε

Crack Propagation Life

Modeling

Calculate Stress

Intensity Factor at

various crack length

),()( KKKKda

<<∆<<∆∆= βα

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Oak Ridge-AET Model

),()( ICth KKKKdN

<<∆<<∆∆= α

Experiment Data vs.

Predicted Data

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1000000010000001000001000020001000

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1000000010000001000001000020001000

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ExperimentPrediction2222....0000mmmmmmmm MMMMiiii lllldddd SSSStttt eeeeeeeellll WWWWeeeellllddddssss,,,, IIII mmmmpppprrrr oooovvvveeeedddd PPPPrrrr oooodddduuuucccctttt iiii oooonnnn GGGGeeeeoooommmmeeeetttt rrrr yyyy10000000100000010000010000

20001000CCCCyyyycccclllleeeessss tttt oooo FFFFaaaaiiiilllluuuurrrreeee

MMMMaaaaxxxx.... LLLLooooaaaaddddExperimentPrediction2222....0000mmmmmmmm MMMMiiii lllldddd SSSStttt eeeeeeeellll WWWWeeeellllddddssss,,,, IIII mmmmpppprrrr oooovvvveeeedddd PPPPrrrr oooodddduuuucccctttt iiii oooonnnn GGGGeeeeoooommmmeeeetttt rrrr yyyy

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1000000010000001000001000020001000

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1000000010000001000001000020001000

CCCCyyyycccclllleeeessss tttt oooo FFFFaaaaiiiilllluuuurrrreeeeMMMMaaaaxxxx.... LLLLooooaaaadddd ((((llllbbbbssss))))ExperimentPrediction2222....0000mmmmmmmm DDDDPPPP777788880000 WWWWeeeellllddddssss,,,, AAAAcccccccceeeepppptttt aaaabbbblllleeee PPPPrrrr oooodddduuuucccctttt iiiioooonnnn GGGGeeeeoooommmmeeeetttt rrrr yyyy

1000000010000001000001000020001000

CCCCyyyycccclllleeeessss tttt oooo FFFFaaaaiiiilllluuuurrrreeeeMMMMaaaaxxxx.... LLLLooooaaaadddd ((((llllbbbbssss))))ExperimentPrediction2222....0000mmmmmmmm DDDDPPPP777788880000 WWWWeeeellll ddddssss,,,, IIII mmmmpppprrrroooovvvveeeedddd PPPPrrrr oooodddduuuucccctttt iiii oooonnnn GGGGeeeeoooommmmeeeetttt rrrr yyyy

1000000010000001000001000020001000

CCCCyyyycccclllleeeessss tttt oooo FFFFaaaaiiiilllluuuurrrreeeeMMMMaaaaxxxx.... LLLLooooaaaadddd ((((llllbbbbssss))))ExperimentPrediction2222....0000mmmmmmmm DDDDPPPP777788880000 WWWWeeeellll ddddssss,,,, IIII mmmmpppprrrroooovvvveeeedddd PPPPrrrr oooodddduuuucccctttt iiii oooonnnn GGGGeeeeoooommmmeeeetttt rrrr yyyy

Regression Analysis

Regression with Life Data

Response Variable: Cycles

Censoring Information Count

Uncensored value 118

Right censored value 14

Censoring value Censor = C

Estimation Method Maximum Likelihood

Distribution: Weibull

Regression Table

95.00%

Standard Normal CI

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Standard Normal CI

Predictor Coef Error Z P Lower Upper

Intercept 17.6836 0.229438 77.07 0.000 17.2339 18.1332

Steel Grade

Mild -0.427812 0.13237 -3.23 0.001 -0.687253 -0.168371

Geometry

IM 0.340398 0.177553 1.92 0.055 -0.007599 0.688395

IP 0.614269 0.185467 3.31 0.001 0.25076 0.977777

Gap

With Gap -0.979513 0.172222 -5.69 0.000 -1.31706 -0.641964

Peak Load -0.003127 9.83E-05 -31.8 0.000 -0.00332 -0.002934

Shape 1.5141 0.106781 1.31863 1.73854

Log-Likelihood = -1590.796

Anderson-Darling (adjusted) Goodness-of-Fit

Standardized Residuals = 4.146

Concluding Remarks

• Weld geometry can be improved by welding parameter

adjustments. The DP780 steel selected for this project

shows significant advantages over the mild steel in terms of

achieving improved geometries using production welding

parameters.

• In general, DP780 welds had fatigue lives that were similar

to or higher than mild steel for a given geometry.

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• With the exception of the Acceptable Production geometry,

all DP780 geometries had higher run-out loads than mild

steel.

• Weld geometry needs to be taken into consideration when

comparing weld fatigue life of various steel grades.

• Base metal strength can affect weld fatigue life

• Weld geometry improvement has a more significant

impact on DP780 weld fatigue life than on mild steel.

• Additional improvement in weld toe geometry may not

improve mild steel weld fatigue life because most mild

steel specimens fractured at the weld root.

Concluding Remarks

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• Gaps appear to have a greater effect in reducing

fatigue life of DP780 joints, compared to mild steel.

• Improvement in weld geometry is important to take full

advantage of DP780.

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Great Designs in Steel is Sponsored by:

ArcelorMittal Dofasco, ArcelorMittal USA, Nucor Corporation,

Severstal North America, Inc. and United States Steel Corporation