cold-welding

name	cold-welding
description	Cold welding (pressure welding) — solid-state bonding without heat; vacuum cold welding mechanism (native oxide removal, asperity contact, adhesion); pressure welding of Al/Cu wire splices; explosive welding (flyer plate, collision angle, jetting condition, bond wave); roll bonding (reduction ratio, bond strength); ultrasonic metal welding; diffusion bonding; applications in space, electronics, and bimetallic joints; AWS and ASTM standards.
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Cold Welding (Pressure Welding) — Complete Skill

Cold Welding Fundamentals

Mechanism

Cold welding: solid-state bonding achieved by plastic deformation under high pressure at room temperature — no external heat applied Bonding mechanism: oxide layer on metal surfaces must be broken and displaced → clean metal-to-metal contact → metallic adhesion via electron sharing across interface

Requirements for cold welding:

1. Oxide film disruption: minimum deformation needed to break and extrude native oxide (Al₂O₃, Cu₂O) at asperities
2. Intimate contact: real contact area must approach apparent area (high pressure to flatten asperities)
3. No recontamination: joints must be made quickly or in inert/vacuum atmosphere
4. Ductile material: ductility enables plastic flow without fracture; cold weldability increases with ductility

Weldable metals (high cold weldability): Al, Cu, Au, Ag, Pb, In, Sn — high ductility, thin native oxide, FCC lattice Difficult: steel, Ti (tenacious TiO₂), Ni — require much higher deformation or vacuum

Pressure Welding (Wire Splicing)

Process: butt-joint wires pressed end-to-end; axial compression breaks oxides → bond forms Deformation required:

• Aluminum wire: ≥ 60% area reduction
• Copper wire: ≥ 70% area reduction Upset ratio (length reduction / original length): ≥ 0.5 for Al

Bond strength: σ_bond ≈ 0.6–0.9 × σ_UTS_parent (well-made joint) Shear strength: τ_bond ≈ 0.5 × σ_bond

Applications: telephone cable splicing (Kearney tool); ACSR aluminum conductor joints; bimetallic Al-Cu transitions (avoid galvanic corrosion in terminals)

Vacuum Cold Welding (Space Applications)

Spontaneous Bonding in Vacuum

In space (vacuum < 10⁻¹⁰ Pa): native oxide does not reform after disruption Metals with disrupted oxide films in contact will cold weld spontaneously Critical in: spacecraft deployment mechanisms, antenna hinges, docking mechanisms, bearing races after long storage

Prevention in space:

• Dissimilar material pairs (Al/Ti or Al/steel bearings)
• Hard coatings: TiN, DLC, Au flash plating
• PTFE/MoS₂ solid lubricant films
• Galling-resistant design: low contact stress

Fretting damage: micromotion causes cold weld/shear cycles → fretting wear → seizure risk in spacecraft mechanisms

Explosive Welding (Cladding)

Process Description

Explosive welding (EXW): controlled explosive detonation accelerates flyer plate onto base plate at oblique angle Jetting occurs at collision interface → cleans surfaces → metallic bond forms as wave front passes

Geometry: Base plate (static); flyer plate at standoff distance d_s ≈ 0.5–2 × flyer thickness t_f Explosive type: ANFO, Detasheet, PETN; detonation velocity V_D = 1,500–7,500 m/s Collision angle β: typically 2–15°; controlled by initial inclination angle α and dynamics

Jetting Condition (Birkhoff Criterion)

Jetting required for bonding: V_p × cos(β) < V_S (sound speed) / √3 [approximately; ensures supersonic collision avoidance; jetting requires specific velocity-angle combination]

Weldability window (Cowan-Holtzman): Plot V_flyer vs. β → weldable region bounded by: Lower limit: minimum deformation to achieve jet (rebound without bonding below) Upper limit: excessive melting / turbulence (above this → no solid bond)

Bond wave wavelength: λ ≈ k × d_s [k = empirical constant ≈ 2–4; wavy interface characteristic of explosive welds] Wavy interface increases real bond area → higher strength than flat interface

Explosive Weld Properties

Bond strength: Tensile strength: σ_bond ≥ σ_weaker_parent (meets or exceeds weaker component) Shear strength: τ ≈ 0.6 × σ_UTS_weaker

Microstructure: thin (<5 μm) melted interface pockets between wavy bond; parent metals unaffected beyond 1 mm from interface

Common combinations: Ti/SS 304 (aircraft heat exchangers; no other reliable joining method for Ti-SS) Al/SS (transition joints; marine applications) Cu/SS (electrical bus bars) Zirconium/SS (nuclear; corrosion-resistant lining) Tantalum/SS (chemical processing)

Standards: AWS C2.21 (Explosive welding); ASTM A263/A264/A265 (clad plate) Testing: ASTM A578 (ultrasonic bond integrity), shear test per AWS B4.0

Roll Bonding (Cold Roll Welding)

Process: sandwich of cleaned metal sheets passed through rolls; high reduction → oxide disruption → bonding Required reduction: ≥ 60% for Al/Al; ≥ 50% for Cu/Cu; ≥ 75% for dissimilar pairs Bond strength increases with reduction ratio and number of passes

Applications: Al-clad steel (vehicle trim, cookware) Bimetallic strips (thermostats — see bimetallic-strip skill) Al-Cu transition panels (battery terminals, bus bars) Ni-Al brazing clad sheets

Thickness ratio: typically 1:5 to 1:20 (clad:base); design to maintain during rolling Surface preparation: degreasing + wire brush to remove oxides just before roll pass

Diffusion Bonding

Diffusion bonding: applied pressure + elevated T (0.5–0.9 T_m) + time → atoms diffuse across interface No macroscopic plastic deformation; minimal distortion Applied pressure: 3–50 MPa; Temperature: 0.5–0.8 T_m; Time: 0.5–4 hours; vacuum or inert atmosphere required

Bond quality: voids at interface (unbonded) detected by ultrasonic C-scan or X-ray CT Ti-6Al-4V diffusion bonds: σ_bond/σ_parent = 0.85–1.0 (well-optimized; used in aerospace spars and fan blades) Defect tolerance: voids < 1 mm² acceptable for non-structural; < 0.5 mm for primary structure

Combined DB/SPF: see superplastic-forming skill for Ti aerospace applications

Ultrasonic Metal Welding

Principle: high-frequency (15–40 kHz) shear vibrations + moderate pressure → scrubbing → oxide removal → bonding No bulk melting; very localized interface heating (< T_melt) Particularly suited for: thin foil bonds, dissimilar metals (Cu-Al battery tabs), fine wire bonding in semiconductors

Wire bonding: gold wire (25–75 μm) on semiconductor pads; thermosonic (ultrasonic + heat + force) Power wire bonding: Al wedge bond on IGBT; bond force 1–10 N; frequency 60–120 kHz

Ultrasonic weld quality: Weld factor = shear strength / shear strength of weaker material → target > 0.7 for production

Standards and References

Standard	Scope
AWS C2.21	Specification for explosive welding
ASTM A263	Carbon/low alloy steel clad with stainless (EXW)
AWS C2.22	Ultrasonic welding of thermoplastics and metals
MIL-W-6858	Resistance welding for aerospace (related solid-state)
ECSS-E-ST-32-01	Space mechanisms — cold welding prevention

Output

Provide: base and clad material (alloys; composition; T_m [°C]; oxide type; cold weldability rating), process selection (cold pressure/explosive/roll bond/diffusion/ultrasonic — basis: geometry, scale, dissimilarity, temperature sensitivity), explosive welding (V_D [m/s]; flyer velocity V_p; standoff d_s [mm]; initial angle α [°]; jetting condition check; weldability window plot description), interface characterization (wavy bond; wavelength λ [μm]; melt pockets? TEM/SEM result), bond strength (σ_bond [MPa]; τ_shear [MPa]; ratio to parent; ASTM tensile test; ultrasonic C-scan defect map), surface preparation (oxide removal: wire brush/degreasing; time between prep and bonding < [min]; vacuum level if applicable [Pa]), post-weld treatment (if diffusion bond: HIP to close voids; T [°C]/P [MPa]/t [h]; if explosive: stress relieve annealing), applications (space cold weld prevention: coating + material pair selection; wire splice Kearney tool parameters; battery terminal Cu-Al), and applicable standard (AWS C2.21 for EXW; ASTM A265 for Ti clad; ECSS-E-ST-32-01 for space mechanisms).