NAWOOTEC.CO.,LTD
INTRODUCTION OF SPOT WELDING
3-23, Jooan 5-Dong, Nam-Ku, Inchon, 402-205 Korea TEL : 82-32-860-2242~4 / FAX : 82-32-866-9988 Web 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]
•
The resistance of metal and heat at unit volume •
Resistance : R = ρℓ/s
•
Current density δ = I/s Heat Q = 0.24 I2 R t = 0.24 δ 2 ρsℓt Heat per unit volume q = Q/v = 0.24 δ 2 ρt Temperature per unit volume Θ = 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 -3 [× 10 /℃] (0 ~ 100℃)
Thermal conductivity [cal/cm·
Density 2 [g/cm ]
Specific heat [cal/g ℃]
Melting point [℃]
sec ℃]
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 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 /℃)
• Relation between resistance and current according to the area of welding part Squeeze Area(S) : small state
Length(l) : long
Melting
Area(S’ ) : large
state
Length(l’ ) : short
⇒ Resistance(R) : high
⇒ Resistance(R) : low
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
resistance
time
•
Weldability according to Resistance(W)
W=
• • •
F : Melting point[℃] K : Thermal conductivity[cal/cm/cm2/℃] ρ : Resistance[μΩ-cm]
ρ F·K
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 ℃]
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
Melting point [℃]
Weldability
• 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
• 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.
Current[K Change of welding A] current Initial value Current density
Limit of tip stuck
Limit of separation Number of Constant weld
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)
•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 =
Current Max. current
[%]
Normal Heat ratio : 40∼80 %
•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.
• 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)
Weldin g current
Welding force (distance between electrodes)
Weldin g time
Q = 0.96·ρ·I2·ℓ·t/(π·d2)
End diameter of electrodes
When increased more than normal state
When decreased less than normal state
Welding current
Expulsion occurs or tip stuck occurs
Separation of the welded parts
Welding force
Separation of the welded part
Expulsion occurs or tip stuck occurs
Welding time
Expulsion occurs or tip stuck occurs
Separation of the welded part
End diameter of 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
α=
(Melting time per weld)·(Number of welds) · 100 % Total time
• Permitted duty : Permitted duty is the maximum duty that can be used with the regulated current. We should not weld above permitted duty
Permitted duty (α A) ≥ Duty (α) • 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
• Relation between Permitted Duty and Current
5. Spare parts management of spot welder • Management of electrode Shape of electrode
Dressing period
Dressing tool
Dressing method
120 hit / 1 time
· Tip dressor · A uto tip dressor
· M anual · A uto
240 hit / 1 time
· Tip dressor · A uto tip dressor
· M anual · A uto
1200 hit / 1 time
· Tip dressor · A uto tip dressor
· M anual · A uto
180 hit / 1 time
· A uto tip dressor
· A uto
240 hit / 1 time
· f ile
· M anual
• Management of Kickless cable in work field C h eck p o in t
C h eck p erio d
C h eck m eth o d
1 . P ressin g state o f term in al T ig h tly fasten b o lt an d n u t O v er 1 tim e/m o n th co n n e cto r C h eck th e state w ith certain to o l 2 . R esistan c e o f cab le
M easu rem en t b y lo w resistan ce m eter O v er 1 tim e/w eek - C h an g e c ab le w hesistan en r ce is ch an g ed su d d en ly
M easu rem en t b y m eg a tester 3 . Iso latio n resista n c e o f cab leO v er 1 tim e/w eek - Ov er 5 MΩ U sin g th e flo w m eter a n d th erm o m eter - C o o lin g w ater flo w : o v er 5 l/m in 4 . F lo w in g q u an tity o f co o lin g O v er 1 tim e/m o n th- In p u t tem p . : less ℃2 5 w a ter ℃45 - O u tp u t tem p . : less 5 . W ear state o f co o lin g h o se O v er 1 tim e/w eekE y e in sp ectio n
• Management of cooling water Name Transformer
Over 100KVA Less 100KVA
SCR of timer contactor (thyristor) Kickless cable
Remark 5ℓ /min 6ℓ /min 6ℓ /min 5ℓ /min
Tip base
1.8ℓ /min (1500A)
Cap tip & Tip base
1.5ℓ /min (2000A)
Cap tip (Φ 13)
1.8ℓ /min (2900A)
cap tip (Φ 16)
3ℓ /min (4100A)
Electrode
Current in parenthesis is continuous equivalent current
6. Type of spot weld defects • Type of defect
• Weld defects Condition
Shape of nugget
Cause
Expulsion
· S hortage of added pressure · O v er supply of current and time · I m purity at work piece · I nsufficient contacting area at the end of electrode
Surface flash
· I nsufficient contacting area at the end of electrode · O v er current flowing to applied pressure
Indentation
· I nsufficient contacting area at the end of electrode · H igh welding force · H igh current
Blow hole
· S hortage of added pressure · L arge area of end electrode · S hortage of hold time
Small nugget size
· S hortage of current or time · H igh welding force
• Weld defects Condition
Crack
Shape of nugget
Cause
· S hortage of welding force · S hortage of hold time · Too enough at contacting area of electrode
Exess melting inside
· H igh current, high cycle time · U sing improper material of electrode
Unbalance of melting inside
· U n balance of end shape of electrodes · D ifferent material between electrodes
Nugget deformation Shortage of melting inside
· M ismatch of electrodes · M ismatch of upper and lower tip · S hortage of current or time · I nsufficient contacting area at the end of electrode
• 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
Program entry
Current sensor
Network
70 72 73
P : Panel entry
C : Primary
76 78 79
R : Remote entry C : Compact
T : Secondary
N : Network type
Special features Standard function Type
Separation of fault messages output
Detection of tip stuck
Control of electrode force
Detection of wear of Kcable
DECOMSTAR-70
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DECOMSTAR-72
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DECOMSTAR-73
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DECOMSTAR-76
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DECOMSTAR-78
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DECOMSTAR-79
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Detection of hort of Kcable
S : Standard Auto typeNo. of stepper
sequences 4/15
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4/15 4/15 4/15 4/15 ○
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