Laser welding looks simple: point a beam at a joint and move along the seam. But inside the metal, a lot happens in a very short time. This guide explains the process in plain language. You will learn what the laser does, how the melt pool forms, and why small changes in settings can change the weld a lot.

Quick answer (the short version)

Laser welding works by focusing a strong laser beam onto the joint between two metal parts. The beam heats the metal in a very small area. The metal melts and forms a small liquid “pool.” As the beam moves, that pool moves with it. When the pool cools, it turns back into solid metal. That solid metal becomes the weld.

The most important idea is simple: laser welding is controlled heat in a tiny spot. When the spot is small, the heat is strong. That can give fast welding, deep penetration, and low distortion. But it also means your settings and your joint fit-up must be stable.

Plain-language terms used in this article

Melt pool: the small area of liquid metal created by the laser.
Penetration: how deep the weld melts into the joint.
Heat-affected zone (HAZ): metal next to the weld that got hot but did not melt.
Shielding gas: gas that protects the hot metal from oxygen in the air.

The big idea: energy in a tiny spot

If you want to understand laser welding, you only need one core concept: energy density. This means “how much energy is pushed into a small area.” A laser can put a lot of energy into a very small spot. That spot can be smaller than a grain of rice.

Think of it like this: A candle has heat, but it spreads out. A magnifying glass can focus sunlight into a tiny point. When the point is tiny, paper can burn. Laser welding is similar. The beam is focused. The heat is local. So the metal can melt quickly.

Why “tiny spot” matters

• More heat goes into the joint, not the surrounding part.
• The weld can form fast, so the part may warp less.
• The heat-affected zone can be smaller than with some older methods.

What can go wrong

• If the spot is not aimed right, the weld can miss the joint.
• If the joint gap is large, the melt pool can fall through or pull apart.
• If gas coverage is weak, the hot metal can oxidize fast.

Step-by-step: what happens during laser welding

Step 1: The beam is created and guided to the weld

A laser welding machine creates a laser beam. The machine then guides the beam to the welding head. In many systems, the beam travels through an optical path. The head helps aim and focus the beam onto the joint.

For the operator, this part is “invisible.” But it matters because stable beam delivery helps stable weld quality. If the beam is stable, your results are more repeatable.

Step 2: The beam is focused to a small spot

Inside the head, optics focus the beam. Focusing turns a wider beam into a small spot. This step changes the energy density. A tighter focus usually means higher energy density.

This is why focus position matters so much. If the focus is too high or too low, the spot becomes bigger. A bigger spot spreads the energy. That can reduce penetration. It can also change how the melt pool behaves.

Step 3: The surface heats up and starts to melt

When the beam hits the metal, the surface absorbs some energy. That energy turns into heat. The surface temperature rises quickly. The metal starts to melt. Now you have a small melt pool at the joint.

At this moment, the melt pool is the main actor. Almost every weld “problem” is really a melt pool problem. If the pool is stable, the weld is usually good. If the pool is unstable, defects appear.

Step 4: The melt pool flows, mixes, and connects both parts

Liquid metal moves. It flows due to heat, surface tension, and gravity. It also reacts to any gap between the parts. If the joint is tight, the pool bridges it easily. If the joint has a gap, the pool has to “stretch.” That makes it more likely to form holes or undercut.

If you feed filler wire, the wire melts into the pool. That adds metal. Added metal can help fill gaps. It can also change the final shape of the bead.

Step 5: The beam moves, and the weld becomes a line

Welding is not one spot. Welding is a moving process. As you move, the beam keeps making a melt pool. The pool travels along the seam. Behind the pool, the metal cools and solidifies. This creates a continuous weld bead.

Step 6: Cooling and solidification form the final weld

When the heat source moves away, the pool cools. The liquid turns into solid metal. The final shape depends on how stable the pool was. It also depends on how fast heat leaves the area. Thick parts pull heat away faster. Thin parts heat up faster. This is why thin-to-thick welding needs extra care.

Want the practical setup guide (parameters, power choice, real use cases)? See: Laser Welding Machine Guide 2025.

Two modes: conduction vs keyhole

Laser welding has two main “ways” it can run. These are often called conduction mode and keyhole mode. You do not need advanced physics to understand them. Just remember this: conduction is gentler and shallow. keyhole is stronger and deeper.

Conduction mode (shallow melt)

In conduction mode, the laser heats the surface and melts it. The melt pool stays mostly on the top. The weld is usually wider and not very deep.

• Often smoother bead on the surface
• Lower risk of deep “burn-through” on thin sheet
• Good for thin parts and appearance work

Think: “heat spreads through the metal, like a hot pan.”

Keyhole mode (deep penetration)

In keyhole mode, the beam is intense enough to create a deep cavity in the melt pool. This cavity is called a “keyhole.” The keyhole lets the laser energy go deeper into the joint. That can make a narrow, deep weld.

• Deeper penetration
• Can weld thicker joints faster
• More sensitive to fit-up and stability

Think: “a tiny tunnel in the liquid metal that lets energy go down.”

Why this matters in real work
If you expect deep penetration but your setup runs in conduction mode, you may see lack of fusion. If you push too hard into keyhole mode on thin sheet, you may see burn-through, spatter, or an unstable bead. The best results come from choosing the right mode for the joint, then keeping it stable.

The 5 knobs that control weld results

Many people think laser welding is “magic.” It is not. It is controlled energy. In daily shop work, most weld results are controlled by five main knobs: power, speed, spot size, focus position, and shielding gas.

If you learn how each knob changes the melt pool, you can fix most problems faster. You do not need to guess. You can test one change at a time and see what the pool does.

One simple rule that helps beginners
If the weld is not fusing, you need more energy in the joint. You can get that by increasing power, reducing speed, or improving focus/aim. If the weld is burning through or warping the part, you need less heat per length. You can get that by reducing power, increasing speed, widening heat with wobble, or improving fit-up.

Why shielding gas matters

Most metals react with oxygen when they are hot. During welding, the metal is extremely hot. If oxygen reaches the melt pool, the metal can oxidize. Oxidation can change color, weaken the surface, and make the bead rough.

Shielding gas creates a protective “bubble” around the weld. It helps keep air away. It can also help push out unwanted fumes near the pool. This is why stable gas coverage is a big part of repeatable results.

Common signs of poor gas coverage

• Strong rainbow color on stainless steel
• Black soot-like surface near the weld
• Small pores or pinholes after welding
• Weld bead that looks dry, sandy, or crusty

Simple gas tips that help in real shops

• Keep distance steady. If your nozzle is too far, coverage gets weak.
• Keep angle steady. If you tilt too much, the gas can miss the pool.
• Shield the start and the end. Many pores form when coverage is not stable at the start/stop.
• Clean the surface. Oil and paint can make fumes and pores even with good gas.

If you want a practical guide that includes real-world notes on discoloration, gas, and parameter windows, see Laser Welding Machine Guide 2025.

Wobble and filler wire (simple explanation)

Two features often confuse new users: wobble welding and filler wire. You do not need complex terms. Here is the simple idea.

What is wobble welding?

Wobble means the beam does not move in a straight line only. It also moves side to side in a small pattern. So the energy is spread across a wider area. This can make the bead wider and smoother. It can also help with small gaps because the heat covers more of the joint.

When wobble helps most

• When your fit-up is not perfect and you have small gaps
• When you want a nicer surface bead on visible seams
• When thin sheet is sensitive and you want a gentler heat spread

What filler wire does

Filler wire adds metal into the melt pool. This can help in three common cases: you have a gap, you want a stronger bead shape, or you want to change the final chemistry of the weld (for example, to reduce cracking risk in some jobs). For many simple seams on clean, tight joints, you may not need wire. But for real shop work, wire can make results more forgiving.

Thin-to-thick welding is where wobble and fit-up matter most. If you often weld thin sheet to a thicker bracket, read: Mixed-Thickness Laser Welding Guide.

Joint fit-up: the hidden reason welds fail

Many people blame settings first. But in laser welding, joint fit-up is often the real root cause. A laser weld pool is small. It cannot “jump” big gaps. If your parts do not touch well, the pool has to stretch to bridge the gap. That is when you see holes, undercut, and weak fusion.

What “good fit-up” looks like

• Parts sit flat and do not rock
• Edges meet with a small, consistent gap (or no gap)
• Clamps hold the joint steady during welding
• Surface is clean (no oil, paint, heavy rust, thick oxide)

Why clamps matter more than people expect

When metal heats up, it expands. As it cools, it shrinks. This movement can open a gap while you weld. A good clamp keeps the joint stable. It reduces stress and distortion. It also helps the melt pool stay centered on the seam.

Common beginner trap
If you keep increasing power to “solve” lack of fusion, but the real issue is a gap, you may end up with burn-through and spatter. First check fit-up. Then tune settings.

How different metals behave

Laser welding does not feel the same on every metal. Metals differ in how fast they move heat away and how much light they reflect. That changes how easy it is to start a stable melt pool. Below are simple, practical notes.

Mild steel (carbon steel)

Mild steel is often one of the easier metals to weld. It usually absorbs heat well and does not reflect as much as some shiny metals. Rust, mill scale, and oil can still cause problems. Clean the joint and keep your travel steady.

Stainless steel

Stainless often welds cleanly with laser. Many shops like it because the weld can look smooth and need less grinding. But stainless also shows oxidation color fast when gas coverage is weak. If you care about appearance, gas control and cleaning matter a lot.

Aluminum

Aluminum is common in modern fabrication, but it is less forgiving. It moves heat away fast. It also has an oxide layer on the surface. These factors can make it harder to keep the pool stable, especially for beginners. The keys are clean surface, stable aim, and enough process margin.

Galvanized steel (zinc-coated)

Zinc coating can vaporize during welding. This can cause pores and spatter. It can also produce fumes that are not safe to breathe. If you must weld galvanized parts, prepare the joint properly and use strong fume control. Always follow safety rules and local regulations.

Copper and brass

Copper and brass can reflect a lot of laser light. They also move heat away fast. This can make starting a weld harder. These jobs often need careful tuning, good surface prep, and stable shielding. For thick copper, many shops use special process setups.

If you want a more practical buying-and-use view (what power level fits which common jobs), you can also check: Laser Welding Machine Guide 2025.

Common defects and what causes them

Most laser welding defects come from one of three root causes: (1) unstable melt pool, (2) poor joint fit-up, or (3) poor protection and cleanliness. Below are the most common issues and simple ways to think about them.

1) Lack of fusion (weld sits on top)

This happens when the joint does not melt enough to join both parts. The bead may look “glued” on top. Often the root cause is low energy in the joint or the beam not centered.

• Check aim: is the beam centered on the seam?
• Check fit-up: is there a gap pushing the pool away?
• Tune heat: add energy per length (more power or slower speed) in small steps.

2) Burn-through (hole in thin metal)

This happens when heat is too high for a thin edge. The melt pool grows too large and drops through. It often appears on corners, edges, or start/stop points.

• Reduce heat per length: lower power or increase speed.
• Improve support: clamp and back up thin sheet.
• Use wobble wisely: spreading heat can reduce sharp overheating in some seams.

3) Undercut (groove along the weld edge)

Undercut often means the melt pool is pulling away from the edges while it cools. Too much heat, poor travel control, or a large gap can make it worse.

• Check travel angle and steady movement.
• Reduce heat slightly or use a wider heat pattern.
• If you have a gap, add filler wire or improve fit-up.

4) Porosity (small holes inside the weld)

Pores often come from trapped gas. That gas can come from surface oil, paint, rust, or coating. It can also come from unstable shielding gas coverage.

• Clean first: remove oil, paint, heavy oxide.
• Improve shielding: stable distance, stable angle, stable coverage.
• Reduce stop/start chaos: pores often form at the start and end of a bead.

5) Heavy discoloration (especially on stainless)

Color is usually oxidation. Oxidation means oxygen reached hot metal. Sometimes this is only a surface appearance issue. Sometimes it also signals that protection is weak and quality may suffer.

• Improve shielding gas coverage and nozzle control.
• Reduce overheating (less heat per length).
• Clean the joint area well before welding.

A simple “first weld” checklist

If you are new to laser welding, do not try to learn everything at once. Use a simple checklist. This makes your learning faster and safer.

Before welding

• Clean the joint. Wipe off oil. Remove paint. Brush off rust or heavy oxide.
• Fit the parts well. Reduce gaps. Use clamps. Keep edges aligned.
• Plan the path. Know where you will start and stop. Avoid shaky stops.
• Set shielding gas. Confirm flow, nozzle position, and coverage area.
• Wear proper PPE. Eye protection and skin protection matter.

During welding

• Keep a steady speed. Do not speed up and slow down in the same bead.
• Keep a steady distance. Distance changes beam focus and gas coverage.
• Watch the melt pool. A stable pool is your best sign of a stable weld.
• Stay centered. Aim at the seam, not just at the top surface.

After welding

• Inspect the bead. Look for uniform width, smooth edges, and good tie-in.
• Look at start/stop. Many defects show up there first.
• Log what you did. Write down settings and observations. Repeat the test.

Simple learning method
Change only one thing at a time. If you change power, speed, focus, and gas all at once, you will not know what fixed the issue. One change, one test bead, one note. This is how shops build stable presets.

Safety basics

Laser welding is powerful. It can be fast and clean, but it is still welding. Safety is not optional. You must follow the safety rules for your machine, your shop, and your local laws.

Key safety points

• Eye protection: Use proper laser safety eyewear for your system.
• Skin protection: Hot metal and bright light can burn skin.
• Fume control: Many metals and coatings create harmful fumes.
• Fire risk: Clear flammable items from the work area.
• Training: Do not rely on “guessing.” Learn from the machine manual and trained staff.

This article is for general education. It is not a replacement for professional training, machine manuals, or workplace safety rules. Always follow the safety guidance from your equipment supplier and your local regulations.

What to read next (parameters, mixed thickness, machines)

This page focused on how laser welding works. If you want to go deeper into real shop problems and practical setup, these pages will help:

• Laser Welding Machine Guide 2025 — a practical guide with real-world tips, selection logic, and common use cases.
• Mixed-Thickness Laser Welding Guide — how to weld thin sheet to thick brackets with fewer defects.
• GWEIKE M-Series (6-in-1 metal laser workstation) — cut + weld + clean + mark in one workflow (useful for small shops that want one system for many steps).
• Portable Laser Welding Machine for Metal — overview of handheld laser welding options.

FAQ

Does laser welding melt the metal or vaporize it?

In most cases, laser welding melts the metal to form a melt pool. In keyhole mode, some metal can vaporize inside the pool and create a small cavity. That cavity helps energy go deeper. But the final weld is still formed by melted metal that cools and becomes solid again.

Why can laser welding cause less distortion?

Distortion comes from heat spreading through the part. Laser welding can concentrate heat in a small spot and move fast. That can reduce total heat put into the part. Less total heat often means less warping, especially on thin sheet.

Why is fit-up so important for laser welding?

The melt pool is small. If there is a large gap, the pool cannot bridge it well. That can lead to holes, undercut, or lack of fusion. Tight fit-up and good clamping make the process much more stable.

What does shielding gas actually do?

It protects the hot metal from oxygen in the air. Without protection, the weld can oxidize and change color. Poor protection can also increase pores and surface roughness. Stable gas coverage is a big part of consistent weld quality.

What is wobble welding in simple terms?

Wobble means the beam moves side to side while traveling forward. This spreads heat wider. It can make the bead wider and smoother. It can also help when fit-up is not perfect and you have small gaps.

Is laser welding always better than TIG or MIG?

Not always. Laser welding can be faster and cleaner on many thin-to-medium metal jobs. But TIG and MIG can be better for some thick joints, outdoor work, or heavy filler needs. The “best” method depends on your parts, your quality needs, and your workflow.

Why is aluminum harder for beginners?

Aluminum moves heat away fast. It also has an oxide layer on the surface. These factors can make the melt pool less stable if the joint is not clean or the settings are not in a stable window. Good cleaning and stable technique matter a lot.

What should I read if I want real parameter guidance?

Start here: Laser Welding Machine Guide 2025. If you often weld thin-to-thick joints, also read: Mixed-Thickness Laser Welding Guide.