Resistance Welding Resistance Welding By: Majid Pouranvari Materials Science and Engineering Department, Sharif University of Technology Fall, 2014 A little about me… Education Authorship Reviewership Faculty PhD, Materials Engineering, Major in Welding, Sharif University of Technology, 2013 Sharif University of Technology 55 ISI-WoS Papers 22 National & Inter. Papers in Scientific Journals 40 Papers in conferences Reviewer for MSEA, STWJ, JALC, JMAD, JMPT, JMEP, …
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Resistance WeldingResistance Welding
By:Majid Pouranvari
Materials Science and Engineering Department,
Sharif University of Technology
Fall, 2014
A little about me…Education
Authorship
Reviewership
Faculty
PhD, Materials Engineering, Major in Welding,Sharif University of Technology, 2013
Sharif University of Technology
55 ISI-WoS Papers
22 National & Inter. Papers in Scientific Journals
Resistance welding is a fusionwelding process in whichcoalescence of metals isproduced at the faying surfacesby the heat generated at thejoint by the resistance of thework to the flow of electricity.
Force is applied before, during,and after the application ofcurrent to prevent arcing at thework piece.
Melting occurs at the fayingsurfaces during welding.
Resistance WeldingHeat generation is expressed as
Q = I2R T,Q = Heat generated.
Resistance welding depends onthree factors:Time of current flow (T).Resistance of the conductor (R)Amperage (I).
Principal Types of Resistance WeldsElectrodesor WeldingTips
Electrodesor WeldingWheels
Electrodesor Dies
ProjectionWelds
Electrodes or DiesSpot Weld Seam Weld Projection Weld
Upset Weld Flash Weld
After Welding After Welding[Reference: Resistance Welding Manual, RWMA, p.1-3]
The dominant process for welding of automotivesheets
Resistance Spot Welding
Resistance Spot Welding
Professor T. W. Eagar, MIT“ The more one studies the resistance welding process, the more
one appreciates how complex the process is ”
RSWEquipments
Heat Generation in Resistance Spot Welding
Block Diagram of Single-PhaseSpot Welder
Spot Weld
Main Power Line
Contactor
N = n p /n s
V s= V p /N
I s = I p N
Typically, the turns ratio in resistance weldingtransformers is about 100 to 1. Therefore, if 480 V, 200A power is available at the primary, 4.8 V at 20kA willbe available at the secondary (neglecting losses).
Temperature Readings of A Spot Welding Process
Workpiece
This illustration was takenabout 4/60th of a secondafter the welding currentstarts.
(Note: Temp at Electrode Sheet Interface Higher than Bulk)
Adaptability for Automation in High-RateProduction of Sheet Metal Assemblies
No filler metal Economical Dimensional Accuracy
Process Disadvantages and Limitations
Higher equipment coststhan arc welding
Power line demands Nondestructive testing Low tensile and fatigue
strength Not portable Electrode wear Lap joint requires additional
metal Difficulty for repair
Macrostructural Features of a RSW
Weld Size Requirements
Weld Penetration Requirements
In general, a large penetrationis acceptable if it does notcreate a large indentation. Therequirements on penetrationare often applied together withthose on weld size.
Factors Affecting Heat Generation (Q):
Welding time
Welding Current
Welding current is most effective inheat generation compared to welding time
Q = I2Rt
Factors Affecting Heat Generation (Q):
Resistance
Bulk Resistance vs. Contact Resistance
R=f (Materials Properties and Pressure)
Factors Affecting Heat Generation (Q):
Resistance
Surface Condition
Steel
Steel
Steel
Steel
Oils/DirtOxide
OxideOils/Dirt
(a) Pickled Conditions
(b) Rusted Conditions Rusty
PickledPolished
Electrode Force
Res
istiv
ity
Resistance Varies withPressure
Low Pressure Medium Pressure High Pressure
(a) (b) (c)
Expulsion: A common Phenomena inResistance Welding
Expulsion: A common Phenomena inResistance Welding
Zhang et al, “Expulsion Modeling in RSW of Steel and Al Alloys”,AWS Sheet Metal Conf VIII, 1998
Effect of Welding Parameters on Expulsion
Welding Current Welding time Electrode force
Effect of Expulsion
Void Excessive electrode indentation
Operating Window - Lobe Curve
Current (1000’s of amperes)
Tim
e (c
ycle
s of
cur
rent
)
ExpulsionNuggettoo small
Acceptablenugget
size
Constant electrodeforce
Effect of Process variable onOperating Window - Lobe Curve
Materials properties
Some welding problems
Some welding problems
Some welding problems
Some welding problems
Some welding problems
Some welding problems
Some welding problems
Some welding problems
Heat Balance
In RSW, heat balance can be defined as a conditionin which the fusion zones in both pieces being joinedundergo approximately the same degree of heatingand applied pressure. It describes the ideal situationwhen a symmetric weld (with equal depth of nuggetpenetration) is made.
Heat balance is influenced by the relative thermaland electrical conductivities of the materials to bejoined, the geometry of the weldment, and thegeometry of the electrodes.
Dissimilar Thickness Welding
In the case of dissimilar thickness, bulk resistance ofthicker sheet is lager than that of the thinner sheet. Thisleads to an asymmetric weld nugget (i. e. penetration of theweld nugget into the thicker sheet is larger than that of thethinner sheet).
Dissimilar Thickness Welding
Solution for Heat Unbalance indissimilar Thickness Welding
This can be overcome by using electrodes of two differentdiameters or by inserting a high-resistivity tip in one electrode.The smaller electrode or the one with high-resistivity insertshould be placed against the thinner of the two sheets
Dissimilar Metal Welding
Shunting Effect
Previously made welds may affect the subsequent welding if the weldsare spaced close to each other due to electric current shunting.
The welding current may be diverted from the intended path by thepreviously made welds. As a result, the current or current densitymay not be sufficient to produce a quality weld.
Shunting Effect
Shunting Effect
Shunting Effect
Shunting Effect
The shunting effect is a strong function of the bulk resistivity of thesheet material. A high conductive metal, such as aluminum, requires alarge space between the welds.
This should be taken into account when welds are designed intostructures, as putting too many welds close to each other may notprovide the intended strength.
electrode
electrode
Electrodes
Electrode Functions
Conduct the welding current to the work
Transmit the proper electrode pressure orforce to the work in order to produce asatisfactory weld
Help dissipate heat from the weld zone
Material Requirements forElectrode in Resistance Welding High electrical conductivity High Thermal conductivity High temperature mechanical strenght
Effect of Strengthening Mechanism on theElectrical Resistivity?
RWMA Electrode Material Standards
Group A - CopperBase Alloys RWMA Class 1
Zirconium Copper Cadmium Copper Chromium Copper
RWMA Class 2 Chromium-Zirconium
Copper Chromium Copper
RWMA Class 3 Cobalt-Beryllium
Copper Nickel-Beryllium
Copper Beryllium-Free Copper
RWMA Class 4 Beryllium Copper
RWMA Class 5 Aluminum Copper
RWMA Electrode MaterialStandards (CONT.) Group B - Refractory
Metals and RefractoryMetal Composites RWMA Class 10
Melting Point (F)Copper 1980Brass Down to 1710Zinc 787
MeasuredElectrode Face Temp (F)
Bare 1000-1200Galvanized 1500-1700
Cu Zn
~45% ~60% ~85%
Uncoated
Hot Dipped Galvanized
Spot Weld Mechanical Properties
Interfacial Mode
Pullout Mode
Weld With Expulsion Failed in Pullout Mode
Dickinson, “Welding in Auto Industry,AISI, 1981
Workshop Tests
AWS Spec D8.9-97, 1997
Advantages of Peel Test• Ease of Performance• Low Cost• Ability to use on Shop Floor as quality control test
Disadvantages of Peel Test• A qualitative rather than a quantitative
Dickinson, “Welding in Auto Industry,AISI, 1981
Chisel Test
Turn to the person sitting next to you and discuss (1 min.):• Why do automotive manufactures prefer the chisel test onthe production line over the peel test?
Mechanical Properties of RSW
Mechanical Properties of RSWTensile-Shear Test
Mechanical Properties of RSWCoach-Peel Test
Mechanical Properties of RSWCross-Tension Test
Mechanical Properties of RSWTensile-Shear Test
Strength Requirements for Steels
Strength Requirement for Mg & Al
Heuschkel, “Expression of Spot Weld Properties”,Weldign Journal Oct 1952
FTS=Dt[-(C+0.05Mn)] BM
Failure Mode of Spot Welds
مود شکست جوشهاي مقاومتی نقطه اي
یک قطر دکمه بحرانی وجود دارد که در مقادیر پایین تر از آن، شکست فصل مشترکی
Stage 1: Resistance (Joule) Heating help ease of hotplastic deformation
Stage 2: Application of extra upsetting pressure causesmaterial extrudes out-wards forming an upset.
Upset Resistance Welding
Upsetting
The main mechanism to remove thecontaminants is the formation of the upset.
By changing the welding conditions to generatemore upset the amount of contaminants will bereduced, although more material will be lost andmore energy will be consumed.