Laser Welding of Dissimilar Metals: Pure Copper and Stainless Steel

Laser welding of pure copper (C11000) to stainless steel (304 or 316) is challenging due to large differences in melting points (1083°C vs. 1400°C), thermal conductivity (≈400 W/m·K vs. 15 W/m·K), and reflectivity (≈95% vs. 65% at 1070 nm). Direct fusion welding produces brittle intermetallic compounds (Fe‑Cu, Fe‑Cu‑Cr) that crack immediately. The only reliable laser‑based process is brazing with a nickel‑ or silver‑based filler or using a beam offset to minimize melting of copper while allowing stainless steel to melt and wet the copper surface. The optimal beam offset for a butt joint of 1 mm sheets is 0.2–0.3 mm toward the stainless steel side, with laser power 1.5–2.0 kW, speed 1.5–2.0 m/min, and argon shielding. This produces a joint with shear strength ≈150 MPa (≈50% of the weaker base metal – copper) and an intermetallic layer thickness <5 µm, which passes thermal cycling tests. For industrial applications, using a nickel‑based filler (e.g., Ni‑Cr‑B‑Si) in a laser brazing configuration is recommended to achieve higher ductility and corrosion resistance.

Joining pure copper to stainless steel is required in electrical busbars, heat exchangers, and cryogenic components. The large mismatch in physical properties makes conventional fusion welding nearly impossible. Laser welding offers precise heat control, but only when parameters are carefully optimized to limit intermetallic growth. This article reviews the metallurgical challenges, presents a laser brazing and offset welding procedure, and provides a decision matrix for the selection of the appropriate method based on application requirements.

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Why Copper and Stainless Steel Do Not Weld Easily

Property	Copper C11000	Stainless steel 304	Ratio (Cu/SS)
Melting point (°C)	1083	1400	0.77
Thermal conductivity (W/m·K)	400	15	27×
Coefficient of thermal expansion (µm/m·K)	17	17	≈1
Reflectivity at 1070 nm (RT)	95%	65%	—
Solubility in each other	Limited (<0.5% at RT)	—	—

Four major problems occur during fusion welding:

1. Intermetallic compounds (IMCs) – Fe‑Cu, Fe‑Cu‑Cr, and Cu‑Cr form brittle phases. Even a thin layer (10 µm) reduces joint strength drastically. The critical IMC thickness for acceptable strength is ≤5 µm.
2. Cracking due to thermal stress – Copper expands and contracts more than stainless steel (actually similar CTE, but copper’s higher thermal conductivity creates steep gradients). Residual stresses cause cracking in the brittle IMC layer.
3. Poor wetting – Molten copper does not wet stainless steel well; the contact angle is >90°, leading to lack of fusion.
4. Reflectivity – Copper reflects most of the laser energy, delaying melting and causing inconsistent penetration.

Result: Direct fusion welding (both metals melted) always produces cracks and voids. The only viable approaches are:

• Laser brazing – Only the filler melts, copper and stainless steel remain solid.
• Offset welding – The laser is positioned so that mostly stainless steel melts, and its molten metal wets solid copper.

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Method 1 – Laser Brazing with Nickel‑Based Filler

Laser brazing uses a filler alloy with a melting point below the solidus of both base metals (for copper, solidus 1083°C; for stainless, ≈1400°C). The filler wets both surfaces and forms a diffusion bond.

Recommended filler:

• Ni‑Cr‑B‑Si (e.g., Nicrobraz LM, melting range 980–1070°C, just below copper’s melting point). This filler has good wetting on both copper and stainless steel and forms a ductile bond.
• Silver‑based filler (e.g., BAg‑8, 72Ag‑28Cu, melting point 780°C) is also suitable but has lower strength and is more expensive.

Parameter table for laser brazing (1 mm sheets, butt joint, 2 kW fiber laser):

Filler	Wire diameter (mm)	Laser power (kW)	Welding speed (m/min)	Wire feed (m/min)	Shielding gas	Joint shear strength (MPa)
Ni‑Cr‑B‑Si	0.8	1.8	1.2	1.5	Argon	180–220
BAg‑8 (silver)	0.8	1.5	1.5	1.5	Argon	120–150

Procedure:

• Focus offset: –1.0 mm (below surface) to spread heat and avoid melting the base metals.
• The filler wire is fed into the leading edge of the melt pool.
• The laser beam is centered on the joint line (no offset).
• Shielding gas: argon 15 L/min, 6 mm nozzle, standoff 12 mm.

Result: A sound brazed joint with no IMC layer visible under optical microscopy. The joint fails in the braze filler (ductile), not by brittle fracture.

Application: Heat exchangers, electrical connectors, components exposed to thermal cycling.

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Method 2 – Offset Laser Welding (No Filler)

If filler is not allowed (e.g., for electrical conductivity), an offset welding technique can be used. The laser beam is directed primarily onto the stainless steel side, melting it. The molten stainless steel wets the solid copper surface. Copper does not melt (or melts only slightly), avoiding IMC formation.

Critical parameters:

• Offset: 0.2–0.3 mm toward the stainless steel side from the joint line.
• Laser power: 1.5–2.0 kW for 1 mm thickness.
• Welding speed: 1.5–2.5 m/min.
• Focus: –0.5 mm (slightly below surface).
• Shielding gas: Argon 20 L/min.

Mechanism: The heat from the molten stainless steel raises the copper surface temperature near its melting point, but not above. Wetting occurs due to diffusion of stainless steel elements (Fe, Cr, Ni) into the copper surface, forming a thin (<5 µm) diffusion zone without continuous IMC layers.

Mechanical properties (1 mm C11000 to 304, 2 kW, 2.0 m/min, offset 0.25 mm):

Property	Value
Shear strength (MPa)	140–160
Peak tensile load (N)	580 (for 5 mm overlap)
Failure mode	Mixed (copper tear + adhesive)
Electrical resistance (µΩ, same as copper)	12 (no significant increase)

Limitations:

• Only suitable for thin sheets (≤2 mm).
• Joint strength is about 50–60% of the base copper.
• Not recommended for dynamic or impact loads.

Visual inspection: The weld should have a smooth transition from stainless to copper, with no cracks. The copper side should show a thin, discolored zone (light gold) indicating slight heating but not melting.

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Method 3 – Pulsed Laser Welding with Silver Interlayer

For very thin foils (<0.5 mm), a silver or silver‑copper shim can be placed between the sheets, and pulsed laser welding can be used to melt the shim without melting the base metals. This is a hybrid brazing/welding technique.

Parameters for 0.3 mm foil:

Parameter	Value
Laser pulse energy	2–5 J
Pulse width	2–5 ms
Spot diameter	0.6 mm
Silver shim thickness	0.05 mm
Number of pulses	1–2 per spot

Result: A small spot weld with limited IMC formation.

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Comparison of Methods

Method	Filler required	Thickness range (mm)	Joint strength (MPa)	Electrical conductivity	Corrosion resistance	Cost
Laser brazing (Ni filler)	Yes	0.5–3	180–220	Poor (filler resistive)	Good	Medium
Offset welding (no filler)	No	≤2	140–160	Good (near copper)	Good (no dissimilar metal crevice)	Low
Pulsed + Ag shim	Yes (shim)	≤0.5	100–120	Moderate	Moderate	Medium‑high

Selection guide:

• Use offset welding when electrical conductivity is critical and sheet thickness ≤2 mm.
• Use laser brazing when higher joint strength is needed, or for thicker sections (≥2 mm).
• Use pulsed + shim for very thin foils or when heat input must be minimized.

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Avoiding Common Defects

Defect	Appearance	Cause	Solution
Cracks at the interface	Some areas are bonded, others not	IMC layer too thick	Reduce heat input; increase offset; use filler
Copper side melting	Copper edge rounded, undercut	Beam not offset enough	Increase offset; reduce power
Lack of wetting	No visible bond, filler balls up	Poor surface cleanliness	Clean with acetone, then light abrasion; increase filler feed
Porosity in the brazed joint	Gas pores in filler	Check laser stability; use a laminar flow nozzle	Increase argon flow; pre‑flow for 0.5 s before laser
Intermittent joint	Shielding gas is impure or the flow is too low	Power fluctuation or gas turbulence	Check laser stability; use laminar flow nozzle

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Surface Preparation for Both Methods

For offset welding:

• Degrease both materials with acetone.
• Lightly abrade the copper surface with 400‑grit sandpaper to remove oxide and increase absorption.
• Wipe again with acetone.

For laser brazing:

• Degrease both materials.
• No abrasion needed for stainless; for copper, light abrasion improves wetting.
• Use a flux‑coated filler or apply a thin flux paste (for Ni‑Cr‑B‑Si filler, flux is necessary to remove oxide).

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Example Welding Procedure (Offset Welding, 1 mm Sheets)

Base metals: 1 mm C11000 copper (half‑hard), 1 mm 304 stainless steel.
Joint: Butt joint, square edges, zero gap.

Machine: 2 kW fiber laser, 1070 nm, spot size 0.3 mm, M² < 1.1.

Parameters:

Parameter	Value
Laser power (CW)	1.8 kW
Welding speed	2.0 m/min
Beam offset	0.25 mm toward stainless steel
Focus position	–0.5 mm (below surface)
Shielding gas	Argon, 20 L/min, 6 mm nozzle, standoff 12 mm
Gas pre‑flow	0.3 s
Gas post‑flow	0.5 s

Quality validation:

• Visual: smooth weld bead, no cracks, copper side lightly discolored.
• Cross‑section: measure IMC layer (should be <5 µm).
• Shear test: sample should reach >140 MPa.

Acceptance: If cracks are present, reduce power by 10% and increase offset to 0.3 mm.

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Conclusion

Laser welding of pure copper to stainless steel is not possible by conventional fusion methods due to intermetallic cracking. However, two reliable laser‑based processes exist:

• Offset welding (no filler) – Suitable for thin sheets (≤2 mm) where electrical conductivity is important. Beam offset of 0.2–0.3 mm toward the stainless steel side limits copper melting and IMC formation.
• Laser brazing with nickel‑based filler – Provides higher joint strength and works for thicker sections, but introduces a resistive layer (not suitable for high‑current applications).

The choice depends on application requirements. For electrical busbars, offset welding is preferred. For heat exchangers or structural components, brazing is more reliable. In all cases, strict parameter control and surface cleaning are essential to achieve consistent, crack‑free joints.

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Frequently Asked Questions

Q: Can I weld copper to stainless steel using a green laser (515 nm)?

A: Yes, a green laser is absorbed much better by copper (≈40% vs. 5% for near‑IR). This allows a more balanced melting of both materials. Green laser offset welding can achieve higher strength (≈200 MPa) and less IMC. However, green lasers are expensive and less common in industrial settings.

Q: Is there a risk of galvanic corrosion between copper and stainless steel?

A: Yes, in an electrolyte (e.g., seawater, humid environment), galvanic corrosion can occur because copper is more noble than stainless steel. The stainless steel (anode) may corrode. For such environments, use a nickel‑based filler that acts as a buffer, or apply a protective coating over the joint.

Q: How do I measure IMC layer thickness?

A: Cut and polish a cross‑section, etch with a suitable reagent (e.g., nitric acid for stainless, ferric chloride for copper), and examine under an optical microscope at 500–1000×. Measure the darkest continuous layer at the interface. The IMC layer should be <5 µm for acceptable mechanical properties.

Q: What is the maximum service temperature for a laser‑brazed copper‑stainless joint?

A: For nickel‑based filler, up to 500°C (inert atmosphere). For silver‑based filler, limit to 300°C. Above these temperatures, filler creep and oxidation cause failure.

Q: Can I use this method for copper‑nickel alloys (CuNi) to stainless steel?

A: Yes, CuNi alloys are easier to weld to stainless because they have lower thermal conductivity and higher absorption. Reduce offset to 0.1–0.2 mm. Parameters similar to copper.

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References

1. Yihai Laser Cutting Machine Operation Manual. Section 5.1.4: Cutting auxiliary gas (applicable to welding). (Company internal document)
2. AWS C7.4:2020. Laser Beam Welding and Cutting – Safety.
3. Laser Institute of America. (2022). Laser Joining of Dissimilar Metals: Copper to Stainless Steel. LIA Technical Guide TG‑CuSS‑2022.
4. IPG Photonics. (2021). Offset Laser Welding of Copper and Stainless. IPG Application Note AN‑OS‑2021.
5. ASM Handbook, Volume 6: Welding, Brazing, and Soldering. (Intermetallic compounds in Cu‑Fe systems).
6. Adams, S. M. (2005). Sheet Metal Bend Allowance. KETIV Virtual Academy.