Choosing the Right Laser Welding Mode: Heat Conduction vs. Deep Fusion

Different laser radiation illuminances cause different phase changes in materials, which are reflected in the welding process as two typical welding modes: laser conduction welding and laser deep penetration welding. The heat transfer process, weld seam formation mechanism, process characteristics, and application range of the two are quite different.

Laser Conduction Welding Mode

(1) Weld Seam Formation Mechanism

During conduction welding, the laser radiation illuminance on the workpiece surface is in the range of 104~105W/cm2, and the laser energy is absorbed by a thin layer of 10~100μm on the surface. The heat on the surface is conducted to the lower layer by heat conduction. After a certain time of laser irradiation, the surface reaches a melting point, and this melting isotherm propagates towards the depth of the material.

The surface temperature continues to rise, but it can only reach the boiling point of the material at most. If the temperature is higher, the material will vaporize to form a pit, and the stable conduction welding process will be disrupted, which must be prevented. With the relative movement of the laser beam and the workpiece, a shallow and wide weld seam is formed, as shown in Figure 1-7.

From the above weld seam formation mechanism, it can be known that the melt depth obtained by heat conduction is relatively shallow, the depth-to-width ratio of the weld seam is small, generally the width of the weld seam is more than twice the melt depth, and Figure 1-8 shows the typical laser conduction welding seam cross-sectional morphology, with the weld seam shape approximating a hemisphere.

Figure 1-7 Schematic Diagram of the Laser Conduction Welding Process
Figure 1-8 Cross-sectional Morphology of a Typical Laser Conduction Weld Seam

(2) Features and Application Range of Laser Conduction Welding

The limitation of laser conduction welding is the shallow melt depth, and because the metal surface reflects high infrared laser (the absorption ratio of metal near the melting point to CO2 laser is less than 80%~90%[7]), the heat loss in the heat transfer process is also large, so the welding efficiency is low.

Its advantage is that the welding process is stable, the weld pool is calm, and it is not easy to produce various welding defects (compared to laser deep penetration welding); the welding specification is simple and easy to master. It is generally suitable for welding thin plates, especially for precision welding of micro-components, it has its unique advantages.

For example, welding of ultra-thin wall titanium boxes with a wall thickness of 0.125mm and a length of 500mm (Fig. 2-42), apart from using laser conduction welding, there is no other welding method that can replace it.

(3) Maximum Melting Depth

Using a simplified method, the maximum melt depth that the metal material surface can reach before vaporization can be calculated[4]. If the material is considered as a semi-infinite body, and a laser with radiation illuminance I uniformly irradiates the material surface, it is considered as one-dimensional heat conduction, as shown in Figure 1-9.

Figure 1-9 Heating of the Liquid Layer on the Surface of a Semi-infinite Body

Under laser irradiation, the material surface temperature rises. Once the surface temperature reaches the melting point Tm, the isothermal surface T=Tm will propagate into the material at a certain speed. The propagation speed is determined by the power density of the laser and the thermophysical properties of the solid-phase and liquid-phase materials.

Suppose the position of the isothermal surface T=Tm is X(t), the boundary condition at this time is

Where:

  • k1 – Thermal conductivity of the material in liquid phase;
  • k2 – Thermal conductivity of the material in solid phase;
  • L – Latent heat of fusion;
  • ρ – Density of the material.
  • A – Absorption ratio of laser by the material surface;
  • a – Thermal diffusivity.

Equation (1-16) holds true from the moment the material surface reaches the melting point. At this point, the elapsed heating time, tm is

Assuming the thermal conductivity and thermal diffusivity are equal in both liquid and solid phases of the material, i.e., k1=k2 and a1=a1, and for most metals, it’s approximately L/(cTm)=0.5 (where c is specific heat capacity), after solving the one-dimensional heat conduction equation, the penetration depth before melting can be calculated as

When t equals tb, the surface reaches the vaporization temperature Tb, thus we have

At this point, the maximum depth of melting is

Equation (1-20) reveals that the maximum melt depth Xm achievable in thermal conduction welding depends on the thermal physical properties of the material and the radiant exposure AI absorbed by the material surface. The larger the thermal conductivity X of the material, the greater the ratio of boiling point to melting point (Tb/Tm), hence the greater the X; whereas, the greater the absorbed radiant exposure AI, the smaller the Xm. Therefore, to increase the depth of melting while ensuring material melting, it’s essential to opt for a smaller laser radiant exposure and extend the heating time (i.e., lower welding speed).

Laser Deep Penetration Welding (Laser Keyhole) Mode

(1) Weld Seam Formation Mechanism

When the radiant exposure exceeds 106W/cm2, the material surface melts and vaporizes under the impact of the laser, and the resulting vapor recoil pressure throws out the molten material, causing a pit to form at the laser-affected site.

The beam directly acts on the pit bottom, causing further melting and vaporization of the metal, with high-pressure vapor continuously emerging from the pit bottom and jetting outwards, deepening the pit and the beam penetrates further.

This process continues, eventually forming a small hole in the liquid metal, filled with plasma formed by partial ionization due to high-temperature vapor. Also, a plasma cloud forms above the hole exit, as shown in Figure 1-10.

Figure 1-10 Schematic Diagram of the Laser Deep Penetration Welding Process

As the laser beam moves relative to the workpiece, the hole exhibits a reverse triangle shape, with the front edge slightly bent backward and the rear edge significantly inclined. The leading edge of the hole is the laser’s active area, where the temperature is high, and the vapor pressure is great, while the rear edge is relatively cooler with less vapor pressure.

Under this pressure and temperature difference, the molten liquid flows from the front end to the rear end around the hole, forming a vortex at the rear end of the hole, which eventually solidifies at the rear edge. Figure 1-11 shows the hole shape and the surrounding molten liquid flow in progress at different speeds, observed through laser ablation of glass.

Figure 1-11 The shape of the small hole in progress and the condition of the surrounding molten liquid flow at different speeds

a) v=20cm/min
b) v=80cm/min

The presence of the hole allows the laser beam energy to penetrate into the material, creating this deep and narrow weld seam. Figure 1-12 shows a typical cross-sectional morphology of a laser deep penetration welding seam.

The weld seam depth is roughly equal to the hole depth. The greater the laser radiant exposure, the deeper the hole, and the greater the weld seam depth. In high-power laser welding, the weld seam depth-to-width ratio can reach up to 12:1.

The laser radiant exposure must exceed a certain value to vaporize the material surface and trigger the keyhole effect. This value is known as the critical radiant exposure and represents the minimum radiant exposure required for laser deep penetration welding.

Figure 1-12 Typical Cross-sectional Morphology of a Laser Deep Penetration Welding Seam

It’s important to note that the plasma cloud, which accompanies the keyhole effect during laser deep penetration welding, absorbs and refracts the laser, reducing the laser radiant exposure reaching the workpiece surface. Therefore, to maintain a stable laser deep penetration welding mode throughout the welding process, the initial laser must still meet the critical radiant exposure value after attenuation by the plasma cloud.

If the selected laser radiant exposure is greater than the critical radiant exposure without the plasma cloud, but falls below the critical radiant exposure after the plasma cloud forms, such a welding process is unstable. This phenomenon is called mode-unstable welding process and must be strictly avoided. The issue of mode-unstable welding processes will be detailed in Chapter 2.

(2) Characteristics of Laser Deep Penetration Welding

Laser deep penetration welding is a typical representative of laser welding and compared to traditional welding methods, it has many unique advantages:

1) Large laser radiant exposure, small heating range, and low heat input result in minimal welding deformation and residual stress, ensuring high welding precision.

2) Fast heating and cooling speeds produce a narrow heat-affected zone and good mechanical properties of the joint.

3) Capable of welding high melting point materials that are difficult to weld by conventional methods, such as Zr, W, Mo, Nb, hard alloys, ceramics, etc.

4) Enables welding of dissimilar materials.

5) High depth-to-width ratio, fast welding speed, and high production efficiency.

6) Allows for remote or hard-to-reach welding.

7) Easy to control and automate.

8) Doesn’t require a vacuum chamber and doesn’t produce X-rays (compared to electron beam welding with the same high energy density).

In short, laser deep penetration welding is a high-quality, efficient, precise, and easily automated welding method.

Laser deep penetration welding also has some limitations:

1) The workpiece bevel processing and assembly precision requirements are high, and the spot alignment requirements are strict. Additionally, the assembly and alignment precision should not change due to interference during the welding process.

2) The welding process generates plasma and has a keyhole effect. The physical process is very complex and improper control can lead to welding defects such as keyhole-type pores, process instability, and undercuts.

3) Optical systems used for high-power lasers have aging and contamination issues. The welding process can produce phenomena like thermal lens effects, which impact the welding process and welding quality.

Based on these points, the welding parameters for laser deep penetration welding must be selected accurately, and the quality inspection and control requirements are urgent. These are the issues that will be emphasized in the following chapters.

(3) Physical Phenomena in Laser Deep Penetration Welding Process

The generation of plasma and the keyhole effect are the two most important physical phenomena occurring during the laser deep penetration welding process. Studying and analyzing these two effects form the theoretical basis of laser deep penetration welding and are the foundation for real-time detection and control of the laser deep penetration welding process.

These are the topics to be discussed in the following two sections.

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