Lätt UHS

Repair spot welding

Anna de Try och Arne Melander

Swerea KIMAB

Aim:

• To determine how different conditions typical to a repair work shop influence spot welding of UHSS.

• To produce corresponding welding diagrams

Five different conditions were investigated in this project:

• The effect of shunt spots • The effect of grinding of the surface of electro coated

steel sheets • The effect of epoxy and rubber-based adhesives• The effect of different misalignments between two

sheet material combinations.• The effect of different electrode forces

Equipment used

MFDCCurrent 0-13kAForce 0-5.5kN

Shunt spot effecs

Study with one, two and three spots

with different distance between spots:

(10*t= 15, 20*t=30, 30*t=45, t=1,5mm).

DP1000 Z100 1,5mm

DX54 Z100 1,5mm

Influence of distance between the three spots

DP 1000 Z100 1,5mm

5

5.2

5.4

5.6

5.8

6

6.2

6.4

6.6

6.8

5.8 6 6.2 6.4 6.6 6.8 7

Weld current [kA]

Plu

gg

dia

met

er [

mm

]

0*t one spot

10*t three spots

20*t three spots

30*t three spots

Approved plugg

Linjär (0*t one spot )

Linjär (10*t three spots)

Linjär (20*t three spots )

Linjär (30*t three spots)

DX54 Z100 1,5mm (Three spots)

3

3.2

3.4

3.6

3.8

4

4.2

4.4

4.6

4.8

5

5.2

5.4

5.6

5.8

6

6.2

6.4

6.6

6.8

7

7.2

7.4

7.6

7.4 7.6 7.8 8 8.2 8.4 8.6 8.8

Weld current [kA]

Plu

gg

dia

met

er [

mm

]

0*t one spot

10*t three spots

20*t Three spots

30*t Three spots

Approved plugg

Linjär (0*t one spot)

Linjär (10*t three spots)

Linjär (20*t Three spots)

Linjär (30*t Three spots)

-0,30

-0,20

-0,10

0,00

0,10

0,20

0,30

0,40

0,50

0,60

0,70

0,80

0,90

1,00

1,10

1,20

str

pun

Avs

*pun

Avs

pun*

str

Avs

*str

Eff

ects

Investigation: svets (MLR)Effects for pluggdiameter

N=52 R2=0,910 R2 Adj.=0,898DF=45 Q2=0,880 RSD=0,1233 Conf. lev.=0,95

Weld

cu

rren

t

Nu

mb

er

of

weld

sp

ots

Effect plot of weld current: Dogal 1000

Nu

mb

er

of

weld

sp

ots

Weld

cu

rren

t

Dis

tan

ce

betw

een

w

eld

sp

ots

cu

rren

t*D

ista

nce

Effect plot of weld current: DX54

Compensation for shunts

• UHSS less sensitive to shunt effects than mild steel• Compensation must however be made when going

from data without shunt to data with shunt• Compensation by increasing current

Effect of ED coat on outside surfaces on spot welds

Two sheets of Dogal 1000DPX Z100 fully ED coated

The contacting surfaces between sheets contain ED coat

The outside surfaces towards electrodes are in three conditions:

Moderately groundWell ground

MG NG NG MG

- NG NG -

- NG NG MG

•1

ED coat condition

Effect of different levels of grinding DP1000 Z100 1,5mm

5

5,2

5,4

5,6

5,8

6

6,2

6,4

6,6

6,8

7

6,2 6,4 6,6 6,8 7 7,2

Weld current [kA]

Plu

gg

dia

me

ter

[mm

]

One spot parent material

Good grinded

Bad grinded

Combination good and bad grinded

Approved plugg

• It is possible to weld with ground outer surface and ED beween sheets but

• Spatter observed originating between sheets in all cases. This is main cause of scatter and difficulty to reach large spot size.

Two cases • Case I

– TDPX1000Z100(1,5mm)-DPX1000Z100(1,5mm)

– Welding through cured and non cured adhesive (one shunt)

• Case II– DPX1000Z100(1,5mm)-

DPX1000Z100(1,5mm)-DX54(0,8mm)

– Welding through cured adhesive ( two shunts)

Effects of epoxy adhesives for DP1000

1,2,3 = weld sequence

Coupon without adhesive Dogal 1000 DPX Z100 1,5mm

Hardening of adhesive

1 2 3

Shunt spot for spot no 2

• Elektrode force = 2,4kN • Elektrode force = 5,0kN

6,8kA/200ms/450ms

D=3mm, Interfacial, ss

6,8kA/200ms/450ms

D=6,0mm, Interfacial, s

8kA/200ms/450ms+6,8kA/200ms/450ms

D=6,9mm, Interfacial, s

8kA/200ms/450ms+6,8kA/200ms/450ms

D=7,8mm, Interfacial, ss

Adhesive – DP1000 Epoxy

• It is possible to weld through adhesive ( cured and non cured) if shunt exist

• Non cured adhesive usually no problem• Problems with spatter for cured adhesive• Larger electrode force -> less risk of spatter• Interfacial failure frequent.• Pre pulse with large current to ”evaporate” cured

adhesive or is often required

1,2,3 = weld sequence

Coupon without adhesive Dogal 1000 DPX Z100 1,5mm

Coupon without adhesive DX54 Z100 0,8mm

Hardening of adhesive

1 2

Shunt spot for spot no 1

Shunt spot for spot no 2

3

Epoxy adhesive – 2*DP1000(1,5mm) + DX54(0,8mm)

Weld data • Electrode force: 2,4kN• Upslope: 200ms• Weld pulse: 700ms• Hold: 400ms • Current: 6,8 – 8,2kA

DX54 0,8mm

DP 1000 Z100 med härdat epoxy lim + två

shuntpunkter

Svetsat utan lim Svetsat i ohärdat lim

Svetsat i härdat lim, treplåts komb.

Three sheet material combination with harden epoxy

4

4,2

4,4

4,6

4,8

5

5,2

5,4

5,6

5,8

6

6,2

6,4

6,6

6,8

7

7,2

7,4

7,6

6,8 7 7,2 7,4 7,6 7,8 8

Weld current [kA]

Plu

gg

dia

met

er [

mm

]

Epoxy

Approved plugg

Spatter avoided at weld time 700ms

Tripple sheet with epoxy between UHSS

• Possible to weld through cured epoxy with long weld time

Misalignment

DPX 1000 Z100 1,5mm

Effect of dissplacement on plugg diameter

4

4,2

4,4

4,6

4,8

5

5,2

5,4

5,6

5,8

6

6,2

6,4

6,6

6,8

5,8 6 6,2 6,4 6,6 6,8 7

weld current [kA]

Plu

gg

dia

met

er [

mm

]

a=1mm

a=1,5mm

a=2mm

a=2,5mm

a=0

Approved plugg

Spatter interval a=1,5mm

Spatter interval a=1mm

Spatter interval a=2,0mm

Spatter interval a=2,5mm

Misalignment gives spatter which gives scatter in weld size

Misalignment

• Misalignment gives problems with spatter which can produce scatter in weld size due to loss of weld size

• Increase electrode force might be solution

Magnitude of electrode force

DPX 1000 Z100 1,5mm

High electrode force gives small weld size

Effect of different levels of elektorde force

4

4,2

4,4

4,6

4,8

5

5,2

5,4

5,6

5,8

6

6,2

6,4

6,6

6,8

5,8 6 6,2 6,4 6,6 6,8 7

Weld current [kA]

Plu

gg

dia

met

er [

mm

]

190daN

240daN

350daN

550daN

Approved nugget

Electrode force

• Higher electrode force is not always solution to good results since contact resistance is reduced with smaller weld size as result

• Higher electrode force can however be solution to reduce misalignment or reduce spatter from obstacles between sheets such as ED or cured adhesive

Conclusions•Dogal 1000DPX not sensitive to number (1 or more) or distance between shunt spots

•DX54 more sentitive to shunt effect than Dogal 1000DPX

•It is possible to weld through ED coat on inside. No large effect of quality of grinding on outside for Dogal 1000DPX. Large risk of spatter with loss of weld size.

•It is possible to weld through cured and non cured epoxy adhesive if shunt spot is present. Pre pulse or long weld pulse can be needed for cured adhesive. Large risk of spatter

•Misalignment produces problems with spatter

•Difficult to obtain large spot sizes with high electrode force only