Spot WeldabilityQuality and Performance

Spot Weldability

Lesson ObjectivesWhen you finish this lesson you will understand:• visual identification of discontinuities• how to develop & use lobe curves• electrode life• peel test & failure modes

Learning Activities1. View Slides; 2. Read Notes, 3. Listen to lecture4. Do on-line

workbook5. Do Homework

KeywordsCurrent level Current Range Lobe Curve Electrode lifeNugget Dia Button Dia. Peel Test Tensile ShearCross tension Shunt current

Process Requirements• Electrode Materials

• Electrode Geometry

• Welding Force

• Welding Cycle

– Squeeze time

– Welding time

– Hold time

• Welding Current

W. Stanley, Resistance WeldingMcGraw-Hill, 1950

Visual Inspection of Quality

ab

cd

W. Stanley, Resistance WeldingMcGraw-Hill, 1950

Visual Inspection of Quality

a

b

c

d

e

Process Characteristics as a Measure of Weldability

• Current Level

• Current Range & Weldability Lobe

• Process Deterioration

– Characterization of the loss in weld size at constant welding conditions

– Characterization of the variation in process requirements for maintaining welds of an adequate quality

Current Level

Material Welding Current (kA)

Uncoated Steel 9

Hot-Dipped Galvanized 13

Electro Galvanized 12

Galvannealed 10

Fe-Zn Electro Coated 10

Simple Current Levels for a Range of 0.8 mm Sheet Steels

(6.1 mm Electrodes & 12-14 Cycles of Welding Time)

Current Range & Weldability Lobe

ExpulsionAcceptableNuggets

Small Nuggets

Minimum Nugget Diameter

Weld Current

Weld Current

ExpulsionLevel

TimeA Smaller

“Brittle” Nuggets

AcceptableNuggets

LobeCurve

Nu

gge

t Dia

met

er

We

ld T

ime

Time A

Weld Current

Wel

d Ti

me

Effect of Weld Time on Current Range (Weld Lobe)

Weld Current

Wel

d Ti

me

5 CycleHold Time

60 CycleHold Time

BrittleButtons

Effect of Hold Time on Lobe Curve

Low Force

Higher Force

Weld Current

Wel

d Ti

me

Indentation, UnstableVery High Force

Lobe moves toHigher CurrentLonger Times

Effect of Electrode Force on Lobe Curve

Effect of Electrode Misalignment on Lobe Curve

Karagoulis, “Process Control in Mfg”,AWS Sheet Metal Conf V, 1992

Turn to the person sitting next to you and discuss (1 min.):•

Pareto Charts Relate Importance of Process Parameters

• Current Density

• Weld Force

• Weld Time

• Current Density

• Weld Force

• Weld Time

• Weld Spacing

• Surface Conditions

Standard Approach Graeco-Latin Approach

What is a Pareto Chart ?

• In our case, A Pareto Chart is a graphical comparison of process variables vs. weld quality. In other words it ranks the process variables in the order of their potency (amount they change weld quality).

A B C D E F G

Process Parameter

Pot

ency

of

a pr

oces

s pa

ram

eter

in

infl

uenc

ing

wel

d qu

alit

y

Scharfy & Kuhnash “Pareto Chart Development for Resistance Welding”Senior Capstone, OSU, 2000

Defining Window Size• Develop weld windows for each process

parameter• Determine the affect of changing the parameter

has on the weld window

Weld

Nugget

Process parameter

Expulsion

Average

Initiation

Steep slope

Stable slope

Window Size = expulsion - initiation

Scharfy & Kuhnash “Pareto Chart Development for Resistance Welding”Senior Capstone, OSU, 2000

Weld Force WindowsCYCLE TIME 11, FORCE 500 LBS

0

1

2

3

4

5

6

7

8

6.5 7 7.5 8 8.5 9 9.5 10

WELD CURRENT (kA)

BUTT

ON S

IZE (m

m)

CYCLE TIME 11, FORCE 800 LBS

0

1

2

3

4

5

6

7

8

6.5 7 7.5 8 8.5 9 9.5 10

WELD CURRENT (kA)

BUTT

ON S

IZE (m

m)

Scharfy & Kuhnash “Pareto Chart Development for Resistance Welding”Senior Capstone, OSU, 2000

Weld Time Windows CYCLE TIME 8, FORCE 650 LBS

0

1

2

3

4

5

6

7

8

6.5 7 7.5 8 8.5 9 9.5 10

WELD CURRENT (kA)

BUTT

ON S

IZE

(mm

)

Scharfy & Kuhnash “Pareto Chart Development for Resistance Welding”Senior Capstone, OSU, 2000

Window Size Pareto Chart

0

0.2

0.4

0.6

0.8

1

1.2

Increasingforce

Decreasingforce

Increasingcycle time

Loweringcycle time

% change window / % change parameter

Scharfy & Kuhnash “Pareto Chart Development for Resistance Welding”Senior Capstone, OSU, 2000

Ne g

a tiv

e %

Ch

ang e

Current Median Pareto Chart

0

0.1

0.2

0.3

0.4

0.5

0.6

Decreasingforce

Increasingcycle time

Loweringcycle time

Increasingforce

% change of midpoint /% change in parameter

Scharfy & Kuhnash “Pareto Chart Development for Resistance Welding”Senior Capstone, OSU, 2000

Turn to the person sitting next to you and discuss (1 min.):•

Process Characteristics as a Measure of Weldability

• Current Level

• Current Range & Weldability Lobe

• Process Deterioration

– Characterization of the loss in weld size at constant welding conditions

– Characterization of the variation in process requirements for maintaining welds of an adequate quality

Process Deterioration

MinimumAcceptableDiameter

Number of Welds

Wel

d D

iam

eter

SurfaceExpulsion

Low Electrode Force

Hirsch, R & Leibovitz, R, “Improved Weld Quality and Electrode Life in Resistance Welding” Practical Welding Today, Nov-Dec, 1997

Low Electrode Force

Hirsch, R & Leibovitz, R, “Improved Weld Quality and Electrode Life in Resistance Welding” Practical Welding Today, Nov-Dec, 1997

Mechanical Properties as a Measure of Weldability

• Button Size as a Measure of Weld Quality

• Simulative Mechanical Tests– Tensile shear loads

– Peel loads

– Cross tension

• Modes of Failure in Spot Welds– Full button

– Irregular button

– Interfacial failure

Button Size as a Measure of Weld Quality

ButtonDiameter

Simulative Mechanical Tests

Tensile Shear Peel Cross Tension

Introduction to Peel Testing Technique

Geometric Effects on The Qualified Peel Test

W = 1”

W = 1.5”W = 2”

W

x

Load

450

350

250

1500 1/4 1/2 3/4 1 1-1/4

Distance x, in

Max

. Lo

ad,

lbs

Modes of Failure in Spot Welds

Full Button Irregular Button

Interfacial Failure

Microstructural Measures of Weldability

• Weld Geometry– Degree of weld penetration– Actual diameter of weld

• Weld Structural Integrity

• Weld Hardness

Weld Geometry

Weld Diameter

Weld Diameter

Weld Penetration

Weld Penetration

Full Size Weld

Sub-Size Weld

Weld Structural Integrity

Fine Weld Porosity

Residual DendriticStructure

Weld Nugget

Workpieces

Weld Hardness

Ha

rdne

ss

Distance (mm)

(a) Weld Morphology (25X)

(B) Variation in Weld Hardness

Turn to the person sitting next to you and discuss (1 min.):•

Other Factors Effecting Weld Geometry

• Shunt Current• Electrode Radius

Effect of Shunt Current on Spot Weld Quality

International Recommendations• Spot Spacing = 16 (sheet thickness)• Spot Spacing = 3 (recommended electrode face dia.)

Shunting Currents

Howe, Spot Spacing Effect on Buton Size”AWS Sheet Metal Conf. VI, 1994

International 16 t Formula

Conclusion: Above the internationally recommended spot spacing, there is little effect on button size

Howe, Spot Spacing Effect on Buton Size”AWS Sheet Metal Conf. VI, 1994

Effect of Electrode Radius

RSW Certification Training Class, Boeing

RSW Certification Training Class, Boeing

RSW Certification Training Class, Boeing

Spot Weldability

Assignment 2