Joining Methods

A modern BIW contains 3,000-5,000 spot welds, plus hundreds of other joints. As material variety increases, so does the complexity of joining them together.

Why Joining Matters

The joint is often the weakest link in a structure:

Resistance Spot Welding (RSW)

How Spot Welding Works

Spot Welding Process:
        Electrode
            │
            ▼
    ┌───────●───────┐  ← Force (clamp)
    │   ┌───────┐   │
    │   │ WELD  │   │  ← Current flows
    │   │ NUGGET│   │     through stack
    │   └───────┘   │
    └───────●───────┘  ← Force (clamp)
            ▲
        Electrode

Time: 0.1-0.5 seconds
Current: 8,000-15,000 A

Spot Weld Parameters

Spot Weld Quality

A good spot weld creates a nugget that's:

• Diameter: 4-8 mm (typically 5√t where t = thickness)
• Penetration: 20-80% of each sheet
• Mode of failure: Plug pullout (not interfacial fracture)

Spot Weld Challenges with UHSS

High-strength steels create spot welding challenges:

Resistance Seam Welding

Seam Welding:
    Rolling electrode
          ○─────→
    ┌─────────────────┐
    │=================│  ← Continuous weld
    └─────────────────┘
          ○─────→
    Rolling electrode

Seam Weld Applications

• Fuel tank seams
• Wheelhouse flanges
• Any location requiring leak-tight joints

MIG/MAG Welding (GMAW)

MIG Welding:
       Wire feed
          │
          ▼
    ┌─────●─────┐
    │   ╱ ╲     │ ← Shielding gas
    │  ╱   ╲    │
    │ ╱~~~~~╲   │ ← Arc + weld pool
    ├─────────────┤
    │   PARTS     │
    └─────────────┘

MIG Weld Types in BIW

MIG vs. Spot Welding

Laser Welding

Laser Welding:
        Laser beam
            │
            ▼
    ┌───────┴───────┐
    │    ┌───┐      │ ← Keyhole
    │    │   │      │    (vapor)
    │    └───┘      │
    └───────────────┘

Laser Weld Advantages

Laser Weld Applications

• Roof-to-body side (eliminates visible flange)
• Tailor-welded blanks
• Transmission components
• EV battery enclosures

Remote Laser Welding

• Scanner head moves beam without moving robot
• Cycle time reduction: 50-70% vs. conventional laser
• Flexibility: Multiple weld patterns without tool change

Laser Brazing

Laser Brazing (roof ditch example):
        ┌─────────────┐
        │   ROOF      │
        └──┬───────┬──┘
           │BRAZE  │
        ┌──┴───────┴──┐
        │  BODY SIDE  │
        └─────────────┘

Laser Braze Advantages

• Excellent appearance — smooth, paintable surface
• No finishing required — unlike MIG weld bead
• Lower heat input — less distortion

Used primarily for visible joints like roof-to-body side.

Structural Adhesives

Adhesive Types

Weld-Bonding

Weld-Bonded Joint:
    ┌─────────────────────┐
    │████●████●████●████│  ← Adhesive (█) + spot welds (●)
    └─────────────────────┘

Benefits:

• Increased stiffness — adhesive transfers load continuously
• Improved fatigue — less stress concentration at welds
• Better sealing — moisture protection
• Crash energy — adhesive absorbs energy before weld failure

Adhesive Area in Modern BIWs

Mechanical Fasteners

When welding isn't possible (especially with aluminum), mechanical fasteners provide alternatives:

Self-Piercing Rivets (SPR)

Self-Piercing Rivet:
    Before          After
    ┌─────┐        ┌──●──┐
    │     │        │ ╱ ╲ │
    ├─────┤   →    ├╱   ╲┤
    │     │        │╲   ╱│
    └─────┘        └──●──┘
                   (flared)

Flow Drill Screws (FDS)

Flow Drill Screw:
    ┌─────────┐
    │    ↓    │  ← Rotating screw
    │   ╲│╱   │     generates heat
    │    ●    │
    └────│────┘
         │
    (threads form)

Clinching

Clinch Joint:
    ┌─────────────┐
    │    ╱   ╲    │  ← Punch pushes
    │   │     │   │     material down
    │───┴─────┴───│
    │   ╲     ╱   │  ← Die shapes
    └─────────────┘     interlock

Multi-Material Joining Summary

Key Takeaways

• Spot welding dominates (3,000-5,000 per BIW) but faces UHSS challenges
• Laser welding/brazing enables design flexibility and clean surfaces
• Structural adhesives increase stiffness 25-35% in modern BIWs
• Weld-bonding combines benefits of welding and adhesives
• Mechanical fasteners (SPR, FDS, clinch) enable aluminum and mixed-material joints
• Multi-material BIWs require diverse joining strategies

---