INTRODUCTION OFSPOT WELDING

NA

WO

OTEC

.CO

.,LT

D

3-23, Jooan 5-Dong, Nam-Ku, Inchon, 402-205 KoreaTEL : 82-32-860-2242~4 / FAX : 82-32-866-9988Web Site : www.nawootec.com

Table of Contents

• Principle of resistance spot welding• 4 factors of spot welding• Factors that affect spot welding• Rated capacity and Duty of spot welder• Spare parts management of spot welder• Type of spot weld defects• Introduction to control equipment of spot welding

1. The principle of resistance spot welding

• Definition : The welding method which uses fusion heat generated at contact part due to metal’s peculiar resistance, and at the same time applies pressure.

• Principle : Using the Joule’s law

• Heat at the resistance, R :Q = 0.24 × I2 × R× t [cal]

I : welding current [A]R : resistance of work-piece [μΩ]t : welding time [sec]

<Secondary circuit of spot welder> <Electrical equivalent circuit>

• The resistance of metal and heat at unit volume

• Resistance : R = ρℓ/s

• Current densityδ = I/s

• Heat Q = 0.24 I2 R t = 0.24 δ 2 ρsℓt

• Heat per unit volumeq = Q/v = 0.24 δ 2 ρt

• Temperature per unit volumeΘ = q/(cρ) = 0.24 δ 2

ρt/(cρ)

ρ : resistance [μΩ-cm]

ℓ : length [cm]

s : unit area [cm2]

δ: current density [A/cm2]

q : heat per at unit volume[cal/cm2]

v : volume [cm3]

c : specific heat of metal[cal/g ]

σ : density of metal [g/cm2]

• Property of metals

Metal Resistance

[μ Ω -cm]

Temp. Coefficient of

resistance

[ ×10-3/]

(0 ~ 100)

Thermal conductivity

[cal/cm·

sec ]

Density [g/cm2]

Specific heat

[cal/g ]

Melting point

[]

Ag 1.63 4.1 1.0 10.49 0.056 960

Cu 1.67 3.93 0.94 8.96 0.092 1083

Al 2.81 4.03 0.53 2.7 0.215 660

Mg 4.6 4.20 0.38 1.74 0.25 650

Mo 5.14 4.20 0.35 10.2 0.061 2625

Zn 5.75 4.23 0.27 7.13 0.092 419

Ni 7.24 6.81 0.22 8.90 0.105 1455

Fe 9.71 6.51 0.18 7.87 0.11 1539

Pb 22 3.36 0.083 11.34 0.031 327

• Resistance vs temperature

• Relation between resistance and current according to the area of welding part

ρ= ρ0(1+αθ)

ρ : Resistance, when temperature is θ

ρ0 : Resistance, when temperature is θ0

α : Temperature coefficient of resistance

(Fe : 6.51×10-3/ )

Squeeze

state

Area(S) : small

Length(l) : long ⇒ Resistance(R) : high

Melting

state

Area(S’ ) : large

Length(l’ ) : short ⇒ Resistance(R) : low

time

resistance

2. 4 factors of spot welding

Resistance(R), Welding time(t), Welding current(A), Welding force(P)

• What is resistance of the welded part? This is the metal resistance. In spot welding, using this resistance, we weld metals without filler metal

<The curve of dynamic resistance during spot welding>

• Weldability according to Resistance(W)

• The material which has high melting point needs high heat• To release heat, the material which has low resistance needs high current• The material which has high thermal conductivity needs high current

< Weldability of metals>

Metal Resistance

[μ Ω -cm]

Thermal conductivity

[cal/cm· sec ]

Melting point

[] Weldability

Pure iron 9.71 0.18 1539 3.5

Stainless steel 70 0.038 1415 130

Al 2.81 0.53 660 0.8

Cu 1.67 0.94 1083 0.2

W =

ρ

F·K

F : Melting point[ ]K : Thermal conductivity[cal/cm/cm2/ ]ρ : Resistance[μΩ-cm]

• What is the welding time?

This is including charge to melt work-piece, squeeze time to solidify the work-piece, up-slope time, welding time and hold time.

The heat at welded part is proportional to

the welding time, but the heat wasted at

the near of welded part is proportional to

the welding time.

Mild steel plate : 1 mm

Welding force : 250 kg

Welding current : 8,800 A

<The effect of welding time that affects to welded strength>

• What is welding current?

Welding current is the energy source to melt work piece. Welding current control methods are constant current control, voltage compensation control, constant power control, etc.

Number of weld

Limit of separationInitial value

Limit of tip stuckChange of welding current

ConstantCurrent density

Current[KA]

Number of weld

• What is the welding force?

The welding force between electrode

needs to obtain specific current density,

when we weld, and affects directly the

formation of nugget.

This is the important factor to reduce weld

defects inside of melted part by cooled

with the state of pressing, after

transparent current.

Proper welding force in the spot welding

of mild steel plate

P = 2.75·10-6·I2 --- (An empirical formula) <Welding force vs welding current>

•What is heat ratio?•Heat ratio is the ratio of welding current to the maximum output current from

transformer, when maximum current output, this is the 100% of heat ratio.

•Heat ratio is changed, when affected by the change of input voltage

Input voltage down → Heat ratio up → Welding current reduce •Example, when used heat ratio is 80%, if input voltage is reduced by 30%,

current is reduced by the degree between extra ratio (20%) and voltage reduction

(30%).•Factor causing heat ratio to increase

- When welding current is higher than specification.

- When extra ratio is not enough in designing.

- When voltage drop is high at power cable. (from distribution panel to

transformer)

- When line voltage is reduced because of using with many welders.

- In case of using kickless cable, when the kickless cable wear happens highly.

Heat ratio = [%] Normal Heat ratio : 40 80 %∼Current

Max. current

• Relation between input voltage and heat ratio

Input voltage drop

Current supply ratio change

Welding current change

3. Factor that affects spot welding• From the equation, Q(heat) = 0.24 I2 R t between electrodes

Work-piece(resistance)

Welding

current

Welding force(distance between

electrodes)

Welding

time

End diameter

of electrodes

Q = 0.96·ρ·I2·ℓ·t/(π·d2)

When increased more than normal state When decreased less than normal state

Weldingcurrent

Expulsion occurs or tip stuck occurs Separation of the welded parts

Welding force Separation of the welded part Expulsion occurs or tip stuck occurs

Weldingtime

Expulsion occurs or tip stuck occurs Separation of the welded part

End diameterof electrode

Separation of the welded part Separation of the welded part

4. Rated capacity and Duty of spot welder

• Rated capacity : Rated capacity of welding transformer is said that it is not continuous usage but duty usage and duty cycle is 50 %.• Duty(α) : Duty is the ratio of total working per unit time to welding current

• Permitted duty : Permitted duty is the maximum duty that can be used with the regulated current. We should not weld above permitted duty

• If duty is over than permitted duty - Transformer can be damaged or the safety of welding current can not be obtained. - SCR(thyristor) in T/C can be damaged - Damage of input part and wear in advance at the secondary circuit(such as kickless cable, gun) can be occurred

α = · 100 %(Melting time per weld)·(Number of welds)

Total time

Permitted duty (α A) ≥ Duty (α)

• Relation between Permitted Duty and Current

Shape of electrode Dressing period Dressing tool Dressing method

120 hit / 1 time·Tip dressor·Auto tip dressor

·Manual·Auto

240 hit / 1 time·Tip dressor·Auto tip dressor

·Manual·Auto

1200 hit / 1 time·Tip dressor·Auto tip dressor

·Manual·Auto

180 hit / 1 time ·Auto tip dressor ·Auto

240 hit / 1 time · f ile ·Manual

5. Spare parts management of spot welder

• Management of electrode

• Management of Kickless cable in work field

Check point Check period Check method

1. Pressing state of terminal connector

Over 1time/month Tightly fasten bolt and nut Check the state with certain tool

2. Resistance of cable Over 1time/week Measurement by low resistance meter - Change cable when resistance is changed suddenly

3. Isolation resistance of cable Over 1time/week Measurement by mega tester

- Over 5MΩ

4. Flowing quantity of cooling water

Over 1time/month

Using the flow meter and thermometer - Cooling water flow : over 5 l/min

- Input temp. : less 25

- Output temp. : less 45

5. Wear state of cooling hose Over 1time/week Eye inspection

Name Remark

Over 100KVA 5ℓ /minTransformer

Less 100KVA 6ℓ /min

SCR of timer contactor (thyristor) 6ℓ /min

Kickless cable 5ℓ /min

Tip base 1.8ℓ /min(1500A)

Cap tip &Tip base

1.5ℓ /min(2000A)

Cap tip(Φ 13)

1.8ℓ /min(2900A)

Electrode

cap tip(Φ 16)

3ℓ /min(4100A)

Current in parenthesis iscontinuous equivalent

current

• Management of cooling water

6. Type of spot weld defects

• Type of defect

Condition Shape of nugget Cause

Expulsion

·Shortage of added pressure·Over supply of current and time· Impurity at work piece· Insufficient contacting area at the end of electrode

Surface flash· Insufficient contacting area at the end of electrode·Over current flowing to applied pressure

Indentation· Insufficient contacting area at the end of electrode·High welding force·High current

Blow hole

·Shortage of added pressure·Large area of end electrode·Shortage of hold time

Small nugget size·Shortage of current or time·High welding force

• Weld defects

Condition Shape of nugget Cause

Crack·Shortage of welding force·Shortage of hold time·Too enough at contacting area of electrode

Exess meltinginside

·High current, high cycle time·Using improper material of electrode

Unbalance ofmelting inside

·Unbalance of end shape of electrodes·Different material between electrodes

Nuggetdeformation

·Mismatch of electrodes·Mismatch of upper and lower tip

Shortage ofmelting inside

·Shortage of current or time· Insufficient contacting area at the end of electrode

• Weld defects

• The standard shape of nugget

INDENTATION

: G·M·T × 1/10

SHEET SEPARATION(max.)

: G·M·T × 1/10

G·M·T : Governing

Metal

Thickness

7. Introduction to control equipment of spot welding

Type and control functions of DECOMSTAR-70 series

RWC -

Type

70 72 73

76 78 79

Program entry

P : Panel entry

R : Remote entry C : Compact

Current sensor

C : Primary

T : Secondary

Network

N : Network type

S : Standard type

Specialfeatures

Type

Standardfunction

Separationof fault

messagesoutput

Detectionof tipstuck

Control ofelectrode

force

Detection ofwear of K-

cable

Detection ofhort of K-

cable

Autostepper

No. ofsequences

DECOMSTAR-70 4/15

DECOMSTAR-72 4/15

DECOMSTAR-73 4/15

DECOMSTAR-76 4/15

DECOMSTAR-78 4/15

DECOMSTAR-79 5/31