Typical Applications of Laser Welding Explained

Generally, any material that can be welded by traditional welding methods can also be welded by laser welding, which offers better quality and higher efficiency. Laser welding can also be used to weld dissimilar materials of many ferrous and non-ferrous metals.

Laser Welding of Steel

Welding of Non-alloy Steel and Low-alloy Steel

Low carbon steel and low-alloy steels have good weldability, but the rapid heating and cooling rates during laser welding can increase the sensitivity to welding cracks and notches.

1) Weldability of Low Carbon Equivalent Steel with Laser Welding

Low carbon equivalent steel has good laser weldability. As long as the selected welding parameters are appropriate, a joint with mechanical properties equivalent to the parent material can be obtained.

2) Weldability of Steel with a Carbon Equivalent Above 0.3% by Laser Welding

When the carbon equivalent exceeds 0.3%, welding becomes more challenging, cold cracking sensitivity increases, and the tendency for brittle fracture under fatigue and low-temperature conditions also increases. The joint design should consider that the weld should have a certain amount of shrinkage, which helps reduce residual stress and cracking tendency in the weld and heat-affected zone.

Using pulse laser welding can reduce heat input, lowering the tendency for crack formation and reducing welding deformation. At the same time, measures such as reducing the quenching rate can be taken to decrease the tendency for cracking.

3) Steel with a Sulfur and Phosphorus Mass Fraction Above 0.04% Tends to Produce Hot Cracks During Laser Welding

Steel with a surface treated by carburizing tends to crack in the carburized layer due to its high carbon content. Nitrided steel is also prone to produce pores and cracks during laser welding, so such steel is generally not welded by lasers. For undeoxidized steel such as boiling steel, laser welding is not suitable unless the oxygen content in the steel is very low; otherwise, the bubbles formed during gas escape can easily lead to porosity.

For lap structures of galvanized steel, laser welding is generally challenging. This is because zinc’s vaporization temperature (903℃) is much lower than the melting point of steel (1535℃). During welding, the vapor pressure produced by the evaporation of zinc causes a large amount of zinc vapor to be expelled from the molten pool, carrying out some molten metal, which can result in severe porosity and undercut in the weld seam.

Welding of Stainless Steel

Stainless steel exhibits good laser weldability. The thermal conductivity of austenitic stainless steel is only one-third of carbon steel, while its absorption rate is slightly higher than carbon steel. Therefore, the melt depth of austenitic stainless steel is 5% to 10% deeper than ordinary carbon steel.

For instance, when welding austenitic stainless steel with a CO2 laser, under conditions of 5kW power, welding speed of 1m/min, and a spot diameter of Φ0.6mm, the light absorption rate is 85%, and the melting efficiency is 71%. Due to the fast welding speed, the overheating phenomenon and the adverse effects of large linear expansion coefficient during stainless steel welding are mitigated.

Thermal deformation and residual stress are relatively small, the weld has no pores or inclusions, and the joint strength is comparable to the parent material. Practice shows that when the Cr/Ni equivalent in steel is greater than 1.6, austenitic stainless steel is more suitable for laser welding; when the Cr/Ni equivalent is less than 1.6, the tendency to produce hot cracks in the weld will significantly increase.

When implementing laser welding on ferritic stainless steel, the plasticity and toughness of the weld are higher than when using other welding methods. Compared to austenitic and martensitic stainless steel, the tendency to produce hot cracks and cold cracks when welding ferritic stainless steel with laser welding is lower.

Among stainless steels, martensitic stainless steel has poorer weldability, and the joint area is prone to develop brittle hard tissues and tends to form cold cracks. Preheating and tempering can reduce the tendency for cracking and brittleness.

Another characteristic of stainless steel laser welding is that welding thin stainless steel sheets with low-power CO2 laser welding can yield a joint with excellent appearance, smooth and aesthetically pleasing weld seam. Laser welding of stainless steel can be used for welding stainless steel pipes in nuclear power stations, nuclear fuel packages, and in chemical and other industrial sectors.

Laser Welding of Non-ferrous Metals

Laser Welding of Aluminum Alloys

Aluminum alloy laser welding is often done by deep penetration welding. The main difficulty when welding is its high reflectivity to the laser beam and its own high thermal conductivity. Aluminum is a good conductor of heat and electricity, and its high-density free electrons make it a good reflector of light, with a surface reflectivity exceeding 90% at the start of welding. That is, deep penetration welding must begin when the incoming energy is less than 10%.

This requires the use of high-power or high-performance laser beams to achieve the necessary energy density. Once a small hole is formed, its absorption rate of the light beam will rapidly increase, even reaching up to 90%, thus allowing the welding process to proceed smoothly.

When laser welding aluminum and aluminum alloys, in addition to the issue of energy density, there are three important issues to solve: porosity, hot cracking, and severe weld irregularity. Porosity is caused by changes in the solubility of hydrogen in the molten metal.

As the temperature rises, the solubility of hydrogen in aluminum increases sharply, and due to the relatively small volume of the molten pool and cooling time during laser welding, there are many pores in the weld, and voids may appear at the root during deep penetration welding, resulting in poor weld formation.

In addition, the metal surface’s oxide film will dissolve into the molten pool during the welding process, leading to porosity and weld brittleness. These oxide films can be removed by mechanical or chemical methods before welding. Aluminum alloys tend to produce hot cracks during the welding process. Crack formation is related to welding speed and cooling time and is closely related to the degree of weld protection.

The weld metal will also be oxidized or nitrided to form Al2O3 or AlN. On the one hand, Al2O3 and AlN can serve as sources for micro-crack propagation, and on the other hand, Al2O3 and AlN will cause weld contamination. Weld irregularity refers to rough weld beads, uneven fish-scale patterns, edge undercutting, and irregular root conditions.

The low vapor pressure and low surface tension of the weld cause the weld metal to become more affine to N2 and O2, leading to weld irregularity. Using Ar or He as a shielding gas can yield a smooth weld and dense fish-scale pattern, and the weld root also needs protection.

Adding filler metal when laser welding aluminum alloys can effectively prevent hot cracking and undercutting, reduce weld discontinuity, lower the requirements for joint assembly precision, and improve joint strength. During the CO2 gas laser welding and Nd:YAG laser welding process, using plasma arc and laser hybrid welding not only increases the welding speed (up to twice faster) and reduces cracking but also helps achieve a smooth weld seam.

Despite the difficulties in welding due to aluminum alloy’s strong reflection of lasers, requiring the use of high-power lasers for welding, and the need for pre-treatment of the workpiece surface and good protection measures during welding, the advantages and process flexibility of laser welding attract technicians to overcome the difficulties of aluminum alloy laser welding, greatly promoting the application of aluminum alloy laser welding in the manufacturing fields of aircraft, cars, etc.

Laser Welding of Titanium Alloys

Titanium alloys have high specific strength, good plasticity and toughness, and high corrosion resistance, making them excellent structural materials.
Titanium is chemically active and sensitive to oxidation. It is also sensitive to embrittlement caused by oxygen, hydrogen, nitrogen, and carbon atoms, so special attention should be paid to joint cleanliness and gas protection issues.

When laser welding titanium alloys, inert gas protection must be applied to both the front and back of the joint, and the gas protection range must be expanded to the temperature area of 400~500℃. Before butt welding titanium alloys, the bevel must be cleaned thoroughly, first treated with sandblasting, then cleaned chemically. Also, assembly must be precise, and joint gap must be strictly controlled.

Laser welding titanium alloys can yield satisfactory results. Ti-6Al-4V is the most widely used titanium alloy, extensively used in aerospace structure manufacturing. For 1mm thick Ti-6Al-4V sheets, using a CO2 laser welder with an output power of 4.7kW, the welding speed can exceed 15m/min.

Test results show that the welding joint is dense, free from cracks, pores, and inclusions; the joint’s yield strength and tensile strength are equivalent to the parent material; under appropriate welding parameters, the Ti-6Al-4V alloy joint has the same bending fatigue performance as the parent material. Laser welding high-temperature titanium alloys can also yield joints with good strength and plasticity.

Laser Welding of High-Temperature Alloys

Laser welding can be used to join various high-temperature alloys, including age-hardened alloys with high aluminum and titanium content that are difficult to weld using arc welding, and it can yield joints with good performance.

The laser generators used for high-temperature alloy welding are usually pulsed lasers or continuous CO2 lasers, with power ranging from 1 to 50kW. The mechanical properties of high-temperature alloy laser-welded joints are relatively high, with a strength coefficient of 90% to 100%. For example, using a 2kW fast axial flow laser to weld a 2mm thick Ni-based alloy, the optimal welding speed is 8.3mm/s.

For a 1mm thick Ni-based alloy, the optimal welding speed is 34mm/s, the weld seam grains are small, and the joint is crack-free. This is hard to achieve with conventional TIG welding.

The recommended shielding gas for laser welding is helium or a mixture of helium and a small amount of hydrogen. Using helium is costlier, but it can suppress plasma clouds and increase weld seam depth. The joint forms of high-temperature alloy laser welding are usually butt and lap joints, and the parent material thickness can reach up to 10mm, but the preparation and assembly requirements for the butt joints are high.

Laser Welding of Dissimilar Materials

Many dissimilar material connections can be accomplished using laser welding. Research shows whether dissimilar materials can be laser-welded depends on the physical properties of the two materials, such as melting point, boiling point, etc.

If the melting points and boiling points of the two materials are close, the parameter adjustment range for laser welding is larger, and the joint area can easily obtain a good structure and performance, then laser welding can be used.

Various metal combinations that can be laser-welded are shown in Table 2-7. Dissimilar metals such as copper-nickel, nickel-titanium, titanium-aluminum, and low-carbon steel-copper can all be laser-welded under certain conditions. Laser welding can also weld ceramics, glass, composite materials, etc.

When welding ceramics, preheating is required to prevent crack formation, typically preheated to 1500℃, then welded in the air. Long focal length focusing lenses are usually used, and filler wire can be added to improve joint strength. When welding metal matrix composites, brittle phases can form. These brittle phases can lead to cracks and reduce joint strength.

Although satisfactory joints can be obtained under experimental conditions, it is still in the research stage.

Table 2-7 Various Metal Combinations Suitable for Laser Welding

AlAgAuCuPdNiPtFeBeTiCrMoTeW
Al
Ag
Au
Cu
Pd
Ni
Pt
Fe
Be
Ti
Cr
Mo
Te
W
Note: ◆ denotes good weldability; ● denotes fairly good weldability; ○ denotes average weldability.

Table 2-8 Welding Parameters for Pulsed Laser Welding of Several Sets of Dissimilar Materials

Dissimilar Material CombinationsDiameter(in mm)Pulse Energy (in Joules)Pulse Width (in milliseconds)
Gold-plated Phosphor Back Copper + Aluminum Foil0. 3/0. 23.54.3
Stainless Steel + Pure Copper Foil0.145/0.082.23.6
Pure Copper Foil0.05/0.052.34.0 
Nickel-chromium Wire + Copper Sheet0.10/0.14513.4 
Nickel-chromium Wire + Stainless Steel0.10/0.1450.54.0 
Stainless Steel + Nickel-chromium Wire0.145/0.101.43.2
Silicon-aluminum Wire + Stainless Steel0.10/0.1451.43.2

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