Res Welding Section 1
Res Welding Section 1
Res Welding Section 1
Section 1 - Introduction
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also to miniature welding applications, although equipment and settings are more specific. The guide
comprises three sections:
production engineers
equipment operators
To achieve the required quality, it is necessary to have an understanding of the fundamentals of the
process and of the various factors which control the quality of spot welds. The guide is planned to
provide this information so that errors can be avoided and the best process tolerance may be achieved.
Competitiveness in manufacturing demands improvements and consistency in product quality, with
simultaneous increases in productivity. Both these requirements emphasise the importance of good
operating practice and the application of relevant quality standards.
The process has changed little over the years but developments in welding equipment, controllers and
automation have resulted in a reliable technique, ideal for modern production methods (Fig.1.1).
However, demands for cost reduction and improved productivity inevitably lead to short cycle times and
non-ideal settings. This can cause a reduction in process tolerance.
Resistance spot welding is the most widely used joining process for fabricating components from formed
sheet. Joining is achieved by electrical resistance heating under forging pressure. The cross section of a
typical spot weld is shown in Fig.1.2.
The key features of spot welding are:
single point joints
Main uses
The primary application of resistance spot welding is high speed welding of products formed from sheet
material. The process can accommodate a wide range of materials and thicknesses. The sheet thicknesses
most commonly welded fall in the range 0.5-5mm. Materials that can be spot welded include:
Coated steels
Zinc, tin, terne (Pb/Sn), aluminium and alloy coatings can all be satisfactorily welded,
although electrode life is shorter than for uncoated steel due to pick-up of the coating on the
electrode.
Stainless steels
Most grades are weldable but embrittlement can occur in some materials as a result of grain
growth (high Cr ferritic grades) or hardening (martensitic grades). High electrode forces are
needed.
Nickel alloys
The high hot strength materials need higher electrode forces. Care must be taken to control
cracking.
Aluminium alloys
High welding currents are required and rapid electrode wear occurs. The surface oxide
condition has a critical effect on weld formation.
Other materials include titanium, copper and its alloys, magnesium and some refractory metals.
Applications cover a range of industry sectors including white goods, automotive, heating and
ventilation, and construction.
Further information on material type and thickness is given in Section 3 of the guide.
Resistance spot welding joins sheet metal parts in a localised area by application of heat and pressure
(see Fig.1.3). Heat is generated by passing a short pulse of high electric current through the metal parts
using shaped, copper alloy electrodes. The electrodes also clamp the parts together at a pre-set force.
The amount of heat generated depends on the level of current, the time of current flow and the electrical
resistance in the circuit, i.e. the resistivity of the sheet materials and the resistance of the contact area
between the electrodes and the workpiece.
The process is described in more detail in Section 2 of the guide.
As the welding current is passed through the material via the electrodes, heat is generated, mainly in the
material at the interface between the sheets. As time passes, the heating effect creates a molten pool at
the joint interface which is contained by the pressure at the electrode tips. Once the welding current is
switched off, the molten pool cools under the continued pressure of the electrodes to produce a weld
nugget. The sequence is shown in Fig.1.3.
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Process variants
Projection welding
Hybrid processes
Stitch welding: spot welding to produce a row of overlapping spots to give a continuous, leak-tight joint.
Resistance seam welding: a continuous process which essentially produces over-lapping spot welds to
give a leak-tight seam. The seam is formed by using a rotating wheel or roller mechanism in place of
single point electrodes on the welding machine (Fig.1.4). This process is used for welding containers
such as domestic radiators, drums and cans. A consumable copper wire may also be used as the
contacting material. This technique is used particularly for welding coated steels such as in the
manufacture of cans and vehicle fuel tanks.
Projection welding: embossed 'dimples', machined projections or natural component features such as
edges are used to produce localised current concentrations and localised heating when two pieces to be
joined are placed in contact. Thus, the shape of the component determines the weld position, as opposed
to the shape of the electrode in spot welding (as illustrated in Fig.1.5).
Fig.1.5 Examples
of projection
welded
components:
a) Embossed
projection for
joints involving
sheet material
b) Machined or
formed stud to
plate joint
c) Annular ring
projection for
sealing a
threaded boss to
sheet
Hybrid processes: Weld-bonding is an example in which resistance spot welding is carried out through
a layer of adhesive at the joint interface. During the process, the electrode force locally squeezes out the
adhesive to make electrical contact, resulting in the formation of a spot weld (Fig.1.6). The adhesive used
is normally of the single part, heat curing type (so that its viscosity does not change during assembly) and
is subsequently cured in a separate operation using an oven or induction heater. Weld-bonding is
typically used when the following are required:
sealing
- in cases where a low strength inter-weld sealant is sufficient and an elastomeric or polyurethane
type adhesive can be used
stiffening
- for better structural performance which can be achieved using a structural adhesive such as an
epoxy
Further information on Weld-bonding (Knowledge summary).
Equipment
The main features of spot welding equipment are:
a frame structure, which provides the machine's mechanical rigidity
a force application system, usually comprising a controlled air supply to a cylinder, but can be
electrical, hydraulic or spring operation
a timer/controller to control the welding sequence, switch the welding current on and off, and also
provide fine current control
a transformer to reduce the mains supply voltage to the low voltage (2-20 volts) and provide the
high current (several thousand amps) used for welding
The main equipment variants for resistance spot welding include:
pedestal machine
robot system
Costs
The capital cost of resistance spot welding equipment depends on the type and the specific application
requirements. The following gives a guide to relative costs:
Cost range
Type of equipment
(k)
manual pedestal machine 5 - 40+
portable welding gun 5 - 20+
multi-welder 10 - 100+
robot system 60 - 80+
Operating costs for resistance spot welding depend on the application but are generally around 0.01 per
spot for thin sheet joining. Factors which determine operating costs include:
power supply
water supply
welding electrodes
compressed air
Operators should visually inspect all equipment (e.g. electrical cables, pneumatic hoses) for
obvious defects prior to use and report defects to the appropriate supervisor.
Mechanical hazards involve the risk of trapping fingers or other parts of the body between
electrodes or other moving parts. Safety devices include various types of guard, interlocked
two-hand button operation and low force electrode approach. Where practicable, spot welding
electrodes should have a working gap of no more than 6mm.
Safety hardware/software interlocks and trip devices must be fully functional at all times and must
never be overridden.
Long hair, items of loose clothing and jewellery present the risk of entanglement from inward
running nips, particularly with seam welding equipment. Neckties, rings and bracelets, etc should
not be worn; long hair must be tied back and covered.
Splash metal may be expelled under pressure from the weld. Burns or lacerations may result from
careless handling of hot assemblies or materials with burrs or sharp edges. Operators must observe
requirements for wearing personal protective equipment (PPE), e.g. eye protection and gloves.
The equipment must have a clearly identifiable and readily accessible means of isolation from its
Electrical hazards result from inadvertent contact with live terminals. Exposed conductors do not
normally exceed 20V, but mains voltage is connected to the control cabinet and to the transformer
taps and primary windings. The machine should be installed and enclosed to the appropriate
standards, using the correctly rated cables and protection devices.
Equipment should be isolated and locked off at the mains before removing covers or opening
panels, such as for the purpose of changing internal transformer taps. Such panels should be
provided with safety interlocks and only trained and approved personnel should be permitted to
undertake such tasks.
Fume is created by the vaporisation or burning of metal or organic coatings on materials being
welded, or from interweld adhesives, sealants, etc. This is not normally a major problem and
adequate ventilation is usually sufficient. Local extraction may be required in some cases,
depending on the type and concentration of the fume.
A magnetic field is produced around the welding machine, created by the flow of high current
from the transformer through the machine arms. This magnetic field may interfere with medical
electronic devices, such as heart pacemakers. Advice should be obtained on safe exposure levels
from the manufacturers and those responsible for implanting such devices.
Exposure to magnetic fields, such as those produced by resistance welding equipment, does not
have a proven link to long-term biological hazards. However, guidelines limiting exposure to
electric and magnetic fields have been established by the UK National Radiological Protection
Board (NRPB) and the International Commission on Non-ionising Radiation Protection (ICNIRP).
Research continues on the biological effects and the measurement of magnetic fields.
Periodic inspection, examination and maintenance of the equipment's safety, mechanical, electrical
and pneumatic/hydraulic systems and ancillary equipment (fixtures, etc) must be carried out by a
competent person. The results of any inspection must be recorded, and where there is a
maintenance log for the equipment, that log must be kept up to date.
Electromagnetic compatibility
Resistance welding equipment can cause disturbances to the mains supply and emit electromagnetic
radiation which may interfere with other equipment. The limits of such emissions, representing
electromagnetic compatibility (EMC) requirements, are defined in the Draft Standard pr EN 50240.
Further information
JoinIT offers numerous FAQs on resistance spot welding.
Also available: Resistance spot welding - technology file (provides a brief summary of TWI's expertise
BS 1140 Specification for resistance spot welding of uncoated and coated low
carbon steel
EN ISO 14554 Quality requirements for welding - Resistance welding of metallic
materials
Part 1 - Comprehensive quality requirements
Part 2 - Elementary quality requirements
EN 1418 Welding Personnel - Approval testing of welding operators for fusion
welding and resistance weld setters for fully mechanised and automatic
welding of metallic materials
EN ISO 10447 Welding: Peel and chisel testing of resistance spot, projection and seam
welds
EN ISO 14270 Specimen dimensions and procedure for mechanised peel testing
resistance spot, seam and embossed projection welds
EN ISO 14271 Vickers hardness testing of resistance spot, projection and seam welds
(low load and microhardness)
EN ISO 14272 Specimen dimensions and procedure for cross tension testing resistance
spot, seam and embossed projection welds
EN ISO 14273 Specimen dimensions and procedure for shear testing resistance spot, seam
and embossed projection welds
EN ISO 14324 Resistance spot welding - Destructive tests of welds - method for the
fatigue testing of spot welded joints
EN ISO 14329 Destructive testing of welds - Failure types and geometric measurements
for resistance spot seam and projection welding
EN ISO 14454 Quality requirements for welding - Resistance welding of metallic
materials
Part 1 - Comprehensive quality requirements
Part 2 - Elementary quality requirments
EN ISO 17653 Destructive tests on welds in metallic materials - torsion of resistance spot
welds
EN ISO 17654 Destructive tests on welds in metallic materials - internal pressure test on
continuous seam welds
BS 2630 Specification for resistance projection welding of uncoated low carbon
steel sheet and strip using embossed projections
EN 28167 Projections for resistance welding (ISO 8167:1989)
BS 7670 Steel nuts and bolts for resistance projection welding
Part 1 - Dimensions and properties
Part 2 - Specification for welding of weld nuts and bolts
BS 6265 Specification for resistance seam welding of uncoated and coated low
carbon steel
BS 4129 Specification for welding primers and weld-through sealants, adhesives
and waxes for resistance welding of steel sheet
ANSI/AWS/SAE/D8.9-97 Recommended practices for test methods for evaluating spot welding
behaviour of automotive sheet steel materials
BS 499 Welding terms and symbols
Part 1 - Glossary for welding brazing and thermal cutting
EN 22553 Welded brazed and soldered joints - Symbolic representation on drawings
(ISO 2553)
BS 4204 Specification for flash welding of steel tubes for pressure applications
BS 6944 Specification for flash welding of butt joints in ferrous metals (excluding
pressure piping applications)
Electrode related
EN 25827 Specification for spot welding - electrode backups and clamps (ISO 5827)
EN 27286 Graphical symbols for resistance welding equipment (ISO 7286)
EN 28430 Specification for resistance spot welding - electrode holders (Parts 1, 2, 3)
EN 29313 Specification for resistance spot welding equipment - cooling tubes
Equipment - general
pr EN ISO 8166 Resistance welding - Procedure for the evaluation of the life of spot welding
electrodes using constant machine settings
pr EN ISO 14327 Resistance welding - Procedure for determining the weldability lobe for
resistance spot, projection and seam welding
BS ISO 14373 (draft) Welding - Resistance spot welds - Procedure for spot welding uncoated and
coated low carbon and high strength steels
pr EN ISO 15607 Specification and approval of welding procedures for materials - General rules
pr EN ISO 15609 Specification and approval of welding procedures for materials - Part 5:
Resistance welding
pr EN ISO 15611 Specification and approval of welding procedures for materials - Approval
related to previous welding experience
pr EN ISO 15613 Specification and approval of welding procedures for materials - Approval by a
pre-production welding test
pr EN ISO 15614 Specification and approval of welding procedures for materials - Welding
procedure test - Part 12: Spot, seam and projection welding
pr EN ISO 18278-1 Resistance welding - Weldability - Part 1: Assessment of weldability for spot,
seam and projection welding of metallic materials
pr EN 50240 Electromagnetic compatibility (EMC) - Product Standard for resistance
welding equipment
Definitions
Information presented in the table below is not intended to be comprehensive but includes the most
frequently encountered terms used in resistance welding, grouped by subject area.
Term Definition
Resistance welding Welding in which, at some stage in the process, force is applied to
surfaces in contact and in which the heat for welding is produced by
the passage of electric current through the electrical resistance at, and
adjacent to, these surfaces.
Spot welding Resistance welding in which a weld is produced at a spot in the
workpiece between electrodes, the weld being of approximately the
same area as the electrode tips, or as the smaller of tips of differing
sizes. Force is applied to the spot, usually through the electrodes,
continuously throughout the process.
Squeeze time The period of time between the normal application of force by the
electrodes to the work and the first passage of current. Note: In timers
complying with National Electrical Manufacturers Association
specifications, this is the period between the initiating of the welding
operation and the first passage of current.
Weld time The total time between the start and finish of welding current during
the making of one weld. Note: A seam weld is considered to be one
weld.
Heat time (On-time) In pulsation and seam welding, the duration of each successive
welding current impulse.
Cool-time (Off-time) In pulsation and seam welding, the period of time between two
successive heat times in the same welding cycle.
Hold time (Dwell time) The period of time between the cessation of current in a welding cycle
and the cessation of electrode force.
Welding force The force, at the abutting surface of the workpiece, used to make a
weld.
Electrode force The force applied to the electrode when making a spot or projection
weld.
Welding pressure The pressure (force per unit area) resulting from the welding force.
Electrode tip pressure The pressure (force per unit area) resulting from the electrode force.
Welding current The current (excluding preheating current) used to bring the workpiece
to, and maintain it at, welding temperature.
Synchronous control An electronic control system which ensures that the instant of closing
the circuit to a resistance welding transformer is always at a pre-set
electrical angle to the instant of zero voltage of the ac supply.
Non-synchronous control A control system that does not ensure that the instant of closing the
circuit to a resistance welding transformer is repetitively at the same
instantaneous voltage of the ac supply.
Phase shift The alteration of the phase relationship between two ac voltages.
Generally this is used to control the period of conduction of the
thyristor contactors in each half cycle.
Phase angle (Firing Angle) The electrical angle between the instantaneous zero voltage applied to
the thyristor and the point at which it conducts in each half cycle.
Weld nugget A zone in a resistance weld where the metal has been melted.
Weld plug/slug A piece of metal pulled from one sheet when a spot or projection weld
is prised apart. Note: The diameter of the plug may be used for weld
quality assessment.
Kickless cable A cable in which the feed and return conductors are so arranged that
the current pulse does not cause the cable to jump or kick.
Contents
The Best Practice Guide has two more sections:
TWI information:
Information and advice from TWI are provided in good faith and based, where appropriate, on the best engineering knowledge available at the time and
incorporated into TWI's website in accordance with TWI's ISO 9001:2000 accredited quality system. No warranty expressed or implied is given regarding the
results or effects of applying information or advice obtained from the website, nor is any responsibility accepted for any consequential loss or damage.
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