What Is Laser Welding and How Does It Work? 

The term “laser welding” covers two physically distinct processes: conduction-mode welding, where the material surface is heated to melting without vaporization, and keyhole-mode welding, where the beam vaporizes a narrow channel through the thickness and the surrounding molten pool solidifies behind it. The direct answer to how laser welding works is that it concentrates electromagnetic radiation at a wavelength absorbed by the workpiece onto a spot 50–500 µm in diameter, raising the material locally to its melting point or beyond, and the relative motion between the beam and the workpiece produces a continuous fusion zone. Which mode you operate in, and whether the resulting weld meets structural requirements, is determined by four parameters: power density, welding speed, beam quality, and shield gas type and flow.

The Physics of Laser-Material Coupling

A laser beam is a coherent, monochromatic light source. When this light strikes a metal surface, a fraction of the photon energy is absorbed by free electrons in the conduction band. These electrons thermalize within femtoseconds, transferring energy to the lattice through electron-phonon coupling within picoseconds. The surface temperature rises at a rate proportional to the absorbed power density.

The absorbed power density is:

I_abs = (P × A) / (π × r²)

Where P is incident laser power (W), A is absorptivity (dimensionless, 0–1), and r is the focused spot radius (mm). For a 2 kW fiber laser at 1070 nm focused to a 50 µm spot (r = 0.025 mm), with an absorptivity of 0.35 for steel, the absorbed power density is:

I_abs = (2000 × 0.35) / (π × 0.025²) = 700 / 0.00196 ≈ 357,000 W/mm² ≈ 3.6 × 10⁷ W/cm²

This value exceeds the threshold for keyhole formation by an order of magnitude. The physics of the process changes qualitatively at the keyhole threshold.

Keyhole-Mode vs. Conduction-Mode Welding

The Yihai manual, while focused on laser cutting, describes laser-material interactions that are directly transferable to welding. The fundamental distinction is between conduction and keyhole modes.

Conduction-mode welding occurs when the power density at the surface is sufficient to melt the material but insufficient to cause vaporization. Heat flows from the surface into the bulk by conduction, producing a hemispherical weld pool. The weld is wide and shallow, with a width-to-depth ratio typically greater than 1.5:1. Conduction mode is used for thin sections, cosmetic welds, and applications where full penetration is not required. The surface remains intact; there is no vapor cavity.

Keyhole-mode welding occurs when the power density exceeds approximately 10⁶ W/cm². At this threshold, the surface temperature reaches the vaporization point. The recoil pressure of the escaping metal vapor depresses the molten surface, creating a narrow, deep cavity—the keyhole. The keyhole is filled with metal vapor and plasma, which absorbs and scatters the incident beam, but also transmits energy to the walls of the cavity through multiple reflections and Fresnel absorption. As the beam moves relative to the workpiece, the keyhole travels with it. Molten material flows from the front of the keyhole around the sides and solidifies at the rear, forming a deep, narrow fusion zone. Width-to-depth ratios in keyhole welding can be as low as 0.2:1.

The table below compares the two modes across key metrics:

Characteristic	Conduction-Mode	Keyhole-Mode
Power density (W/cm²)	<10⁶	>10⁶
Width-to-depth ratio	>1.5:1	0.2:1 to 1:1
Surface vaporization	None	Yes (keyhole cavity)
Penetration depth (typical)	<1 mm per pass	Up to 20 mm in a single pass
Weld pool shape	Hemispherical	Hourglass or nail-head
Shielding gas requirement	Moderate (prevents oxidation)	Critical (displaces plasma, protects keyhole)
Process stability	Stable, low spatter	Sensitive to parameter balance
Typical applications	Thin sheet, cosmetic, electronics	Structural, automotive, shipbuilding, pipelines

The transition from conduction to keyhole mode is not always desirable. An unstable keyhole—oscillating between open and collapsed states—produces porosity. The weld pool solidifies around a collapsed vapor cavity, trapping gas that cannot escape. Porosity is the most common defect in keyhole-mode welding and is controlled by maintaining a stable keyhole through consistent power delivery, travel speed, and shield gas coverage.

Wavelength and Material Absorption

The laser wavelength determines the fraction of incident energy absorbed by the workpiece. Absorption is a function of wavelength, material, temperature, and surface condition. Fiber lasers operate at approximately 1070 nm. CO₂ lasers, historically dominant but now largely displaced for metal welding, operate at 10,600 nm.

The table below lists room-temperature absorptivity values for common metals at the two wavelengths:

Material	Absorptivity at 1070 nm (Fiber)	Absorptivity at 10,600 nm (CO₂)
Steel (mild, polished)	0.35–0.40	0.12–0.15
Stainless steel (304)	0.30–0.35	0.10–0.12
Aluminum (1050)	0.05–0.10	0.02–0.04
Copper	0.03–0.05	0.01–0.02
Titanium	0.40–0.50	0.15–0.20

Three observations follow from this data:

1. Fiber lasers are absorbed 2–3 times more efficiently than CO₂ lasers across all metals. This is the primary reason fiber lasers have replaced CO₂ lasers in metal welding.
2. Aluminum and copper have low absorptivity at both wavelengths, explaining why these materials require higher laser power or pre-heating for equivalent penetration.
3. Absorptivity increases with temperature. Once a material melts, its absorptivity rises significantly. Once a keyhole forms, multiple internal reflections trap the beam, increasing effective absorption to near 100% regardless of the initial room-temperature value. This is why high-power fiber lasers can weld aluminum and copper despite low cold absorptivity.

Beam Quality and Its Effect on Penetration

Beam quality, quantified by the beam parameter product (BPP, in mm·mrad), describes how tightly a laser beam can be focused. A lower BPP indicates a beam that can be concentrated into a smaller spot at a given focal length.

Single-mode fiber lasers produce BPP values below 0.4 mm·mrad. Multi-mode fiber lasers produce BPP values from 1 to 10 mm·mrad. The focused spot diameter is:

d_spot = 4 × BPP × (f / D)

Where f is the focal length of the focusing optic, and D is the beam diameter at the optic. A single-mode laser with BPP = 0.35 mm·mrad, f = 200 mm, and D = 20 mm produces a spot of approximately 14 µm. A multi-mode laser with BPP = 3 mm·mrad under the same conditions produces a spot of 120 µm.

The effect on welding is direct: a smaller spot produces a higher power density for the same laser power, achieving the keyhole threshold at lower power or producing a narrower keyhole and a finer weld. Single-mode lasers are used for precision welding of thin sections, medical devices, and electronics. Multi-mode lasers, with their larger spots and higher total power (often 6–20 kW), are used for structural welding where penetration depth and productivity dominate.

Shielding Gas and Its Three Functions

The shielding gas in laser welding performs three functions that are distinct from its role in cutting:

1. Plasma suppression. The metal vapor and ionized gas in and above the keyhole form a plasma that can absorb and defocus the laser beam. A shield gas, typically helium or argon, displaces this plasma from the beam path. Helium is most effective due to its high ionization potential (24.6 eV vs. 15.8 eV for argon), but it is expensive. Argon is used for steel welding. Nitrogen is sometimes used for stainless steel but can cause nitride formation.
2. Weld pool protection. The shield gas prevents atmospheric oxygen and nitrogen from reacting with the molten weld pool. Oxidation causes discoloration, porosity, and reduced mechanical properties. Nitrogen can dissolve in the weld pool and form nitrides, which embrittle the joint.
3. Keyhole stabilization. The gas flow helps maintain a stable keyhole by applying pressure to the molten surface and assisting in the removal of vapor and spatter from the beam path. Gas flow that is too high can disrupt the keyhole; too low allows plasma buildup.

The Yihai manual’s guidance on gas selection for cutting—using nitrogen for oxide-free edges, and the purity requirements—does not transfer directly to welding. For laser welding, the gas selection is:

Material	Recommended Shield Gas	Reason
Carbon steel	Argon, or Ar/CO₂ mix (2–5% CO₂)	CO₂ stabilizes the arc-equivalent and improves wetting
Stainless steel	Argon, or Ar/He mix	Helium suppresses plasma and increases penetration
Aluminum	Argon, or Ar/He mix (50/50)	Helium increases penetration; argon provides coverage
Titanium	High-purity argon (99.999%)	Prevents any interstitial contamination
Copper	Helium	Highest plasma suppression; argon insufficient at high power

Gas flow rate for laser welding is typically 10–25 L/min, delivered through a coaxial nozzle or a side jet. The coaxial nozzle is integrated into the welding head and provides omnidirectional coverage. A side jet, positioned behind the weld pool, provides additional plasma suppression for deep-penetration welding above 4 kW.

Laser Welding Parameters and Weld Geometry

The Yihai manual and the HypCut manual are primarily cutting references, but the fundamental laser-material interaction principles apply to welding. A weld is characterized by three geometric parameters: penetration depth (D), bead width (W), and reinforcement height (R, the height of the weld above the parent material surface). These are determined by five adjustable parameters:

Parameter	Symbol	Effect of Increasing
Laser power	P	Increases penetration depth and width linearly above the keyhole threshold
Welding speed	v	Decreases penetration; above a critical speed, keyhole collapses
Focus position	z	Optimal when focal point is at or slightly below the surface (0 to –1 mm)
Spot diameter	d	Larger spot reduces power density, widens the weld, and reduces penetration
Shield gas flow	Q	Insufficient flow causes porosity and oxidation; excessive flow disrupts keyhole

The relationship between these parameters is captured in the specific energy input:

E_spec = P / v (J/mm)

Specific energy determines the total heat input per unit length of weld. For a given material and thickness, there is a minimum specific energy required to achieve full penetration. Above that threshold, increasing specific energy widens the weld but does not increase penetration beyond the through-thickness maximum.

Representative parameters for fiber laser keyhole welding of common materials at 2 kW are:

Material	Thickness (mm)	Speed (m/min)	Specific Energy (J/mm)	Shield Gas
304 SS	1.0	3.0–4.0	30–40	Argon
304 SS	2.0	1.5–2.5	48–80	Argon
304 SS	3.0	0.8–1.5	80–150	Argon
Carbon steel	1.5	2.0–3.0	40–60	Ar/CO₂
5052 Al	1.0	2.0–3.0	40–60	Ar/He
5052 Al	2.0	1.0–2.0	60–120	Ar/He
Copper	1.0	1.5–2.5	48–80	Helium

The penetration depth achievable with a given laser power depends on the thermal diffusivity of the material—the same property that governs laser cutting power requirements, as analyzed in the previous article on aluminum’s thermal conductivity. Materials with low diffusivity (stainless steel, titanium) achieve deeper penetration at a given power than materials with high diffusivity (aluminum, copper).

Common Laser Welding Defects and Their Parameter Corrections

The defects observed in laser welding map directly to parameter imbalances. The table below, adapted from the general principles of the Yihai manual’s cutting defect diagnostics, provides a systematic elimination method:

Defect	Appearance	Probable Cause	Parameter Correction
Porosity (spherical voids)	Rounded cavities on X-ray or cross-section	Unstable keyhole collapse; contaminated shield gas	Increase shield gas flow; check for leaks in gas line; reduce welding speed for stability
Lack of fusion	Weld bead sits on surface, no penetration	Insufficient power density; speed too high	Increase power or reduce speed; check focus position
Undercut	Groove at weld toe, reduced cross-section	Excessive speed; insufficient filler if used	Reduce speed; add filler wire if joint fit-up is poor
Spatter	Metal droplets on surface adjacent to weld	Excessive power density; unstable keyhole	Reduce power; increase welding speed; adjust focus position
Cracking (solidification)	Centerline crack in weld bead	High solidification rate; unfavorable chemistry	Reduce speed (slower cooling); use filler metal with crack-resistant composition
Excessive penetration (burn-through)	Weld metal drips from underside	Power too high; speed too low; keyhole too wide	Reduce power; increase speed; reduce spot size
Oxidation/discoloration	Blue, brown, or black surface on weld	Inadequate shield gas coverage	Increase flow rate; check gas nozzle alignment; verify gas purity

The diagnostics are sequential. Porosity, the most common defect, should be addressed first by verifying the shield gas system—leaks, purity, flow rate—before adjusting power or speed.

Continuous vs. Pulsed Laser Welding

Laser welding can be performed with continuous wave (CW) or pulsed output. CW welding delivers constant power and is used for most structural and production applications. Pulsed welding delivers energy in discrete pulses, with controllable pulse duration, peak power, and repetition rate.

Pulsed welding provides finer control of heat input. Each pulse melts a small volume that solidifies before the next pulse arrives. Heat-affected zone (HAZ) width can be reduced to 0.1 mm or less. Pulsed welding is specified for:

• Thin foils and wires where CW would cause burn-through
• Heat-sensitive components near electronics
• Medical devices where precise energy delivery is regulated
• Dissimilar metal joints where intermetallic formation must be controlled

The trade-off is speed. Pulsed welding is inherently slower than CW because the average power is lower. A laser capable of 2 kW CW may deliver only 200–400 W average power in pulsed mode, at typical duty cycles of 10–20%.

Laser welding vs Tig and Mig

Examples of Laser Welding Machines

Laser Welding Machine: Full Walkthrough of Components & Different Weaving Modes

Frequently Asked Questions

What is the difference between laser welding and TIG welding?
Laser welding uses a focused beam of light as the heat source; TIG welding uses an electric arc between a tungsten electrode and the workpiece. Laser welding produces a narrower heat-affected zone (typically 0.5–2 mm vs. 3–10 mm for TIG), higher depth-to-width ratios (up to 10:1 vs. 1:1), and higher welding speeds (1–10 m/min vs. 0.1–0.5 m/min). Laser welding requires a tighter joint fit-up (gap <0.1 mm vs. <0.5 mm) and is more sensitive to part positioning. TIG is more tolerant of imperfect fit-up and is the preferred process for small-batch, high-mix production where fixturing costs for laser would be prohibitive.

Can laser welding replace MIG welding in production?
In specific high-volume applications, yes. The automotive industry has replaced MIG welding with laser welding for roof seams, door assemblies, and tailor-welded blanks because laser welding is 5–10 times faster and produces a weld that requires no post-processing before painting. However, laser welding cannot replace MIG where gaps exceed 0.1 mm (MIG tolerates up to 1 mm with filler), where thick sections (>10 mm) require multiple passes with filler metal, or where capital cost is the dominant constraint. A 6 kW fiber laser welding system costs 3–5 times more than a MIG power source.

What is the maximum thickness that laser welding can join in a single pass?
With current high-power fiber lasers (20–30 kW), single-pass keyhole welding can join steel up to 20–25 mm and aluminum up to 12–15 mm. Beyond these thicknesses, the keyhole becomes unstable, and hybrid laser-arc welding (combining a laser with a MIG/MAG arc) is used. Hybrid welding can join steel up to 30 mm in a single pass, with the arc providing additional heat and filler metal to bridge the gap.

Why is aluminum more difficult to laser weld than steel?
Three reasons. First, aluminum’s low absorptivity at 1070 nm (5–10% at room temperature) means less energy is coupled into the workpiece until a keyhole forms. Second, aluminum’s high thermal diffusivity (4–7 times that of steel) conducts heat away from the weld zone, requiring higher power for equivalent penetration. Third, aluminum’s low viscosity when molten makes the keyhole more prone to collapse and porosity. The solution is higher power density (single-mode fiber laser, tight focus) and helium-based shield gas to suppress plasma.

References

1. Steen, W.M., Mazumder, J., Laser Material Processing, 4th ed., Springer, 2010, Chapters 6–7 (Laser Welding Physics and Process Parameters).
2. Ion, J.C., Laser Processing of Engineering Materials, Elsevier, 2005, pp. 260–310 (Keyhole Dynamics and Weld Formation).
3. Yihai Laser Cutting Machine Operation Manual, Rev. 2, Sections 5.1–5.5 (Laser-Material Interaction Principles applicable to welding).
4. AWS D1.1/D1.1M:2020, Structural Welding Code — Steel, American Welding Society, Sections 3–4 (Weld Design and Qualification).
5. ISO 13919-1:2019, *Electron and laser-beam welded joints — Guidance on quality levels for imperfections — Part 1: Steel.*
6. Dawes, C., Laser Welding: A Practical Guide, Abington Publishing, 1992, Chapters 2–3 (Laser Types, Beam Delivery, and Shielding).