Copper Laser Welding Technology - Analysis of Difficulties in Copper Laser Welding

Copper Laser Welding Technology - Analysis of 

Difficulties in Copper Laser Welding

A. Difficulties in welding copper

Since copper has a very low absorption rate of near-infrared laser at room temperature, most of the incident laser will be reflected during the welding process, resulting in severe energy loss of copper during the laser welding process and low laser energy utilization. At the same time, because copper has good thermal conductivity, its stability during laser welding is poor. The above factors make it necessary to use high-brightness lasers to obtain relatively good welding quality when using infrared laser welding of copper. However, welding problems such as poor weld formation, hot cracks, spatter, and pores cannot be avoided. These factors have greatly limited the promotion and application of copper laser welding technology.

The following is a brief explanation of the 3 major difficulties in copper laser welding：

1. High thermal conductivity, fast heat dissipation

Figure 1 Physical properties of different metals

Thermal conductivity definition: refers to the heat transferred through unit horizontal cross-sectional area per unit time when the vertical downward gradient of temperature is 1°C/m.

To put it simply: take two parallel planes 1 meter apart and with an area of 1 square meter inside the object perpendicular to the direction of heat conduction. If the temperature difference between the two planes is 1K (1°C), conduction will occur from one plane within 1 second. The heat transferred to another plane is defined as the thermal conductivity of the material, and its unit is Watts/meter·Kelvin (W/m·K).

The thermal conductivity of copper is 401W/(m*K), which is 1.7 times that of aluminum and 5 times that of steel. High thermal conductivity means that energy conduction is lost faster during the welding process.

High thermal conductivity will lead to weak welding (insufficient energy leading to insufficient penetration) and rough appearance at the macro level; at the micro level, it will cause the heat-affected zone to be too large (large conduction area will cause the grains to grow when heated, leading to performance degradation). Therefore, in welding processes with low energy density (such as arc welding), preheating is usually required. Welding processes with high energy density (laser, electron beam welding) do not need preheating, but the same melting process as aluminum and steel must be achieved. Deeper welding often requires higher power and smaller spot size, which also exacerbates the instability of copper welding quality.

2. High reflective, low absorption rate

Figure 2 Absorption rate of copper in different bands

At present, high-power lasers are mainly fiber lasers. At the same time, the localization of fiber lasers is relatively thorough and cost-effective. Using infrared lasers (band 1030-1080nm) to weld copper has a great cost advantage. However, at room temperature, only about 3 to 5% of the incident laser can be absorbed by copper in the initial stage, and the rest is reflected. This leads to the need to use a higher-brightness laser when welding copper materials. This process will intensify the welding process. instability phenomenon. Excessive laser energy reflection not only causes low energy utilization, but also causes great safety hazards to people, equipment and optical components.

3. Laser absorption rate fluctuates greatly

Figure 3 Thermal conductivity and absorptivity

As shown in "Figure 3", the thermal conductivity of pure copper at different temperatures and the absorptivity change curve of 1um band infrared laser are shown. As can be seen from the figure, the absorptivity of solid pure copper at room temperature is only 3%, and as the temperature rises to 1250K, it reaches about 8%, which is only an increase of 5 percentage points; at the same time, the thermal conductivity slowly decreases from the highest 400W/m*K to about 330W/(m*K). That is, in the solid state, pure copper maintains extremely low laser absorption rate and extremely high thermal conductivity efficiency, which makes the laser processing process extremely difficult and requires extremely high laser power density.

However, in the extremely small temperature range of 1250~1350K, the light absorption rate of pure copper suddenly "jumps" to about 15%; at the same time, its thermal conductivity also drops sharply from the original 330W/mK to about 160W/mK. . This causes the heat accumulation rate in the liquid molten pool to instantly increase several times when the laser beam power density is the same. At this time, the liquid copper absorbs a large amount of heat and the temperature further rises (above 2500°C), causing violent evaporation to form a "keyhole". Once the "keyhole effect" is formed, the incident laser is reflected and absorbed multiple times inside the keyhole, and the laser The absorption rate soars to about 60%, further aggravating the heating and evaporation of internal materials. Such huge fluctuations in heat input will cause violent fluctuations in the copper molten pool, causing "micro-explosions" and small hole collapse inside the molten pool, resulting in splashes and pores. and other defects.

B. Main defect types and formation mechanisms

1. Virtual soldering

Figure 4: Welding on the head caused by infrared laser welding of copper

As shown in "Figure 4", due to copper's high thermal conductivity and low laser absorption rate, there is often a phenomenon like this at the starting position: the starting section cannot be effectively formed. The molten pool, even without any heating marks, begins to form as the temperature gradually increases, causing "virtual welding" in the initial part.

The reason is: the initial absorption rate is low, resulting in a small heat input. The heat absorbed by the copper is quickly dispersed through thermal conduction. Under the continuous action of the laser, the temperature of the copper increases, the absorption rate increases with the increase in temperature, and the heat accumulation begins to A portion of the copper is melted, and thermal conductive welding occurs. Then the laser absorption rate of liquid copper further increases, the heat input continues to increase, keyholes begin to appear, and only then does deep penetration welding begin. This phenomenon is common when the laser spot size used is large, the power is low, or the welding speed is too fast. When the power density is high enough, deep penetration welding can be formed at the moment of laser incidence.

2. Poor weld formation

Figure 5 Poor welding seam forming effect

Because during the copper laser welding process, the laser absorption rate of copper in different states (solid 3%, liquid 15%, keyhole 60%) changes greatly, causing the molten pool to fluctuate violently during the welding process. Generally, the molten pool fluctuates as shown in the figure above. If the wave crest appears, it will cool down quickly and there will be no time to reflow the molten pool, forming a smooth transition, resulting in large appearance defects and excessive roughness.

3. Splash and pores

The use of infrared laser for linear welding of copper has an unstable process window and maximum penetration fluctuation, and is prone to defects such as spatter, melt ejection, and holes as shown in Figure 1.

Figure 6 Cu-ETP infrared welding sample

As shown in "Figure 6": the laser energy is significantly concentrated inside and below the keyhole, and eventually the inside of the keyhole surrounded by the molten pool expands excessively, increasing the In order to eliminate the instability of the keyhole, during the welding process, when the liquid metal load in the molten pool is less than the expansion pressure of the small hole, the expansion of steam at the bottom of the small hole causes the molten metal to eject, forming spatter, and the ejected molten pool area forms surface holes.

Figure 7 Schematic diagram of spatter formation mechanism

Figure 8 High-speed photography of splash formation

As shown in "Figure 8", during deep penetration welding, the sharply rising laser absorption rate (60%) causes a sharp increase in evaporation inside the molten pool, resulting in spatter, causing the keyhole to collapse, and then the laser hits the molten pool again. On the pool, the absorption rate dropped from 60% to about 20%, and then as the evaporation increased, a new keyhole was re-formed, and the laser absorption rate increased, and so on. Severe heat input fluctuations will cause periodic changes in copper thermal conduction welding and deep penetration welding, resulting in alternating depth of penetration, making the penetration uncontrollable. As shown in the figure, there will be insufficient penetration in some areas, which will also cause drastic changes in the evaporation of copper metal vapor, causing the interior of the keyhole to periodically collapse and close, forming pores and spatter.

C. Splash and pore formation mechanism

Figure 9 Copper laser welding keyhole state simulation

As shown in "Figure 9", you can clearly see the spatter and pores caused by the unstable copper welding process. These two defects are the most common in the copper laser welding process. Here’s a brief overview:

1. Splash formation mechanism

Figure 10 Schematic diagram of laser welding spatter formation mechanism

Stress analysis of splash droplets: During the copper alloy deep penetration welding process, the splash droplets are mainly affected by the upward shear force given by the surface tension of the liquid, its own gravity, and the high-pressure metal vapor in the keyhole; where the shear force is leading. Generally, splashes are mainly generated from the edge of the keyhole opening. The flying out are mainly droplets at the edge of the keyhole. As the molten pool fluctuates, once they emerge from the keyhole, they will directly face the violent eruption of metal vapor upwards and be sheared in the vertical direction. Under the action of shear force, it overcomes the surface tension and its own gravity and flies out of the molten pool, forming splash.

2. Stomatal formation mechanism

Figure 11 Schematic diagram of the formation mechanism of laser welding pores

Keyhole-type pores are mainly caused by the instability of the keyhole during the laser welding process. Since the keyhole is hollow, once the keyhole collapses as shown in e, the liquid molten pool will close the keyhole and involve the metal vapor into the molten pool. The metal vapor cannot escape from the surface of the copper molten pool in time and solidifies in the molten pool, forming pores with larger diameters in the weld.

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