Resistance Welding

Introduction

Resistance Welding = Group of welding processes where heat is generated by resistance to electric current flow through the workpieces

Principle: Joule's Law

• Heat = I²Rt
• Where: I = current, R = resistance, t = time

Basic Principle

1. Pressure applied between electrodes holding workpieces
2. High current (thousands of amperes) passed through
3. Resistance heating at interface melts metal
4. Pressure maintained during cooling
5. Solid-state weld formed (no filler metal)

Heat Generation

Total Heat Generated: Q = I²Rt

Where:

• Q = Heat (Joules)
• I = Current (Amperes)
• R = Resistance (Ohms)
• t = Time (seconds)

Resistance Components:

• Contact resistance (highest at interface)
• Bulk resistance of workpieces
• Electrode resistance

Heat Distribution:

• Maximum heat at workpiece interface (highest resistance)
• Less heat in bulk material
• Minimal heat at electrodes (copper, low resistance)

Types of Resistance Welding

1. Resistance Spot Welding (RSW)

Process:

• Two overlapping sheets
• Electrodes apply pressure from both sides
• Current passed through
• Localized weld nugget formed

Applications:

• Automotive body panels
• Sheet metal fabrication
• Appliances
• Electronics

Advantages:

• Fast (0.1-1 second per spot)
• No filler metal
• Minimal distortion
• Automated easily
• High production rate

Limitations:

• Lap joints only
• Thickness limited (typically <3 mm each sheet)
• Electrode maintenance
• Not suitable for all materials

Weld Quality Factors:

• Nugget size: Diameter of fused zone
• Penetration: Depth into each sheet
• Indentation: Electrode marks on surface

Process Parameters:

• Current: 5,000-20,000 A
• Time: 0.1-1 second (squeeze, weld, hold)
• Pressure: 200-500 kg
• Electrode force: Critical for quality

2. Resistance Seam Welding (RSEW)

Process:

• Continuous or intermittent weld along seam
• Rotating wheel electrodes
• Overlapping spot welds create seam
• Can be leak-tight

Types:

• Continuous seam: Overlapping spots, leak-tight
• Intermittent seam: Spaced spots
• Mash seam: Thin material, flat surface

Applications:

• Fuel tanks
• Cans and containers
• Pipes and tubes
• Automotive mufflers

Advantages:

• Leak-tight joints possible
• Continuous operation
• Good for cylindrical parts

Limitations:

• Lap joints only
• Electrode wear
• Power consumption high

3. Resistance Projection Welding (RPW)

Process:

• Embossed projections on one part
• Current concentrated at projections
• Multiple welds simultaneously
• Projections collapse during welding

Projection Types:

• Button projections: Circular raised areas
• Elongated projections: For seam-like welds
• Annular projections: For studs, nuts

Applications:

• Welding nuts to sheet metal
• Attaching studs
• Cross-wire welding
• Fastener attachment

Advantages:

• Multiple welds at once
• Less electrode wear (flat electrodes)
• Better heat concentration
• Consistent weld quality

Limitations:

• Requires projection forming
• Part design critical
• Projection height accuracy needed

4. Flash Butt Welding

Process:

• Parts held in clamps (electrodes)
• Light contact, current flows
• Arcing (flashing) heats interface
• Rapid upset (forging) pressure applied
• Flash expelled

Stages:

1. Flashing: Arcing heats surfaces
2. Upsetting: Rapid pressure, forge weld
3. Flash removal: Trim excess material

Applications:

• Welding rails
• Pipe manufacturing
• Chain links
• Tool joints

Advantages:

• Welds dissimilar metals
• Large cross-sections
• High quality welds
• Self-cleaning (flash removes oxides)

Limitations:

• Flash must be removed
• Alignment critical
• Power consumption very high

5. Upset Butt Welding

Process:

• Parts held in clamps
• Pressure applied first
• Current passed through
• Interface heats and forges together
• No arcing (unlike flash welding)

Applications:

• Wire and rod welding
• Small diameter parts
• Chain manufacturing

Advantages:

• No flash (or minimal)
• Suitable for small parts
• Lower power than flash welding

Limitations:

• Clean surfaces required
• Limited to small cross-sections
• Precise alignment needed

Electrodes

Electrode Materials

Copper Alloys (most common):

• Class 1 (Pure copper): High conductivity, low strength
• Class 2 (Chromium copper): Good balance
• Class 3 (Refractory metals): High temperature applications

Properties Required:

• High electrical conductivity
• High thermal conductivity
• Adequate strength at high temperature
• Wear resistance

Electrode Shapes

Spot Welding:

• Pointed: Thin materials
• Domed: General purpose
• Flat: Thick materials, projection welding

Seam Welding:

• Wheel electrodes: Various profiles

Electrode Maintenance

• Dressing: Reshape tip regularly
• Cleaning: Remove contamination
• Replacement: When worn beyond limits
• Cooling: Water cooling for high production

Process Parameters

Current

• Range: 1,000-100,000 A (depending on process)
• AC or DC: AC most common (transformer-based)
• Waveform: Single phase AC, three phase DC

Time

Spot Welding Cycle:

1. Squeeze time: Electrodes close, pressure applied
2. Weld time: Current flows (cycles or milliseconds)
3. Hold time: Pressure maintained, no current
4. Off time: Electrodes open

Typical Times:

• Squeeze: 0.1-0.5 s
• Weld: 0.1-1 s (or 1-30 cycles at 50/60 Hz)
• Hold: 0.1-0.5 s

Pressure (Electrode Force)

• Range: 100-1000 kg (depending on material and thickness)
• Too low: Expulsion, poor weld
• Too high: Excessive indentation, reduced nugget size

Heat Balance

For dissimilar materials or thicknesses:

• Adjust electrode size
• Use different electrode materials
• Modify surface preparation
• Adjust current path

Advantages of Resistance Welding

1. High speed: 0.1-1 second per weld
2. No filler metal: Cost savings
3. Minimal distortion: Localized heating
4. Automation: Easily automated
5. Consistent quality: Repeatable process
6. No flux or gas: Cleaner process
7. Energy efficient: Heat only where needed
8. Operator skill: Less critical than arc welding

Limitations of Resistance Welding

1. High equipment cost: Expensive machines
2. Lap joints primarily: Limited joint types
3. Thickness limitations: Typically thin materials
4. Electrode maintenance: Regular dressing needed
5. Surface preparation: Clean surfaces required
6. Power requirements: High current needed
7. Material limitations: Not all metals suitable
8. Accessibility: Both sides must be accessible

Materials Welded

Easily Welded:

• Low carbon steel
• Stainless steel
• Aluminum alloys
• Nickel alloys

Difficult:

• Copper (high conductivity)
• Aluminum (oxide layer, high conductivity)
• Coated steels (galvanized)

Not Suitable:

• Cast iron (brittle)
• Very thick sections

Weld Quality and Testing

Visual Inspection

• Surface appearance
• Indentation depth
• Expulsion (metal splash)

Destructive Testing

• Peel test: Separate sheets, check nugget
• Chisel test: Break weld, inspect nugget
• Tensile-shear test: Measure strength

Non-Destructive Testing

• Ultrasonic: Nugget size detection
• Radiography: Limited use

Quality Criteria

• Nugget diameter: Typically 4-6√t (t = thickness)
• Penetration: 20-80% into each sheet
• No expulsion: Metal splash indicates poor quality

Defects in Resistance Welding

Expulsion:

• Metal expelled from joint
• Causes: Excessive current, low pressure, contamination

Insufficient Penetration:

• Small nugget
• Causes: Low current, short time, high pressure

Cracking:

• Cracks in nugget or HAZ
• Causes: Rapid cooling, high carbon content, contamination

Surface Damage:

• Excessive indentation
• Causes: High pressure, worn electrodes

Sticking:

• Workpiece sticks to electrode
• Causes: Contamination, excessive current

Applications by Industry

Automotive: Body panels, brackets, exhaust systems Aerospace: Fuel tanks, structural components Appliances: Washing machines, refrigerators Electronics: Battery tabs, contacts Construction: Wire mesh, reinforcement Rail: Rail welding (flash butt)

Comparison: Resistance Welding Types