Laser Welding of Copper - Challenges, Spatter Analysis in Copper Alloy Welding, and Technical Approach

Challenges in Welding Copper:

High Thermal Conductivity: Copper has a high thermal conductivity, which leads to rapid heat transfer to surrounding materials during welding, increasing the heat-affected zone.

High Melting Point: Copper has a melting point of approximately 1083°C, which is relatively high, requiring high-energy density lasers for effective welding.

Oxidation Issues: Copper is prone to oxidation during welding, especially at high temperatures, which can affect weld seam quality and welding stability.

Susceptibility to Hot Cracking: Copper is prone to hot cracking during welding, especially in areas with rapid cooling or stress concentration.

Analysis of Spatter in Copper Alloy Welding:

Causes of Spatter: During welding of copper alloys, due to its low thermal conductivity and high liquid viscosity, when lasers or arcs act on the surface of copper, it tends to produce a large amount of spatter.

Factors Affecting Spatter: Welding parameters (such as power, speed), welding techniques (such as welding angle, gas shielding), material surface condition (cleanliness, oxidation status), etc., all affect the generation and control of spatter.

Impact of Spatter on Weld Quality: Spatter can lead to incomplete weld joints and the formation of pores, reducing weld quality and strength.

Technology Route for Copper Alloy Welding:

Pre-treatment: Ensure the welding surface is clean, remove oxide layers, and use appropriate surface treatment methods (such as chemical cleaning, mechanical polishing).

Selecting Suitable Welding Technology: Laser welding is a commonly used high-energy density welding method, offering precise control and a smaller heat-affected zone, suitable for the high demands of copper alloy welding.

Optimize Welding Parameters: Depending on the specific alloy composition and thickness, choose appropriate laser power, focal length, welding speed, etc., to control the formation of the molten pool and heat-affected zone.

Gas Shielding: Use appropriate inert gases (such as argon) to protect the welding area, reducing oxidation and spatter formation.

Welding Process Control: Control welding speed and angle to avoid excessive heat input and welding distortion, thereby improving welding quality and stability.

In summary, laser welding of copper presents challenges that require precise welding processes and parameter control to overcome its high thermal conductivity and melting point, ensuring high-quality weld joints.

Challenges in Copper Welding

1.1 Material Characteristics of Copper — High Thermal Conductivity, Rapid Heat Dissipation

Copper, with its high thermal conductivity, results in rapid heat transfer during welding, leading to a large overall heat-affected zone in the workpiece. This presents a significant challenge in effectively fusing the materials together.

Copper boasts a high thermal conductivity of 401 W/(m*K), which causes the heat generated during welding to dissipate quickly regardless of the fusion welding process used. This often results in insufficient penetration depth (due to rapid heat conduction) and an excessively large heat-affected zone (due to extensive heat conduction area, leading to grain growth and subsequent performance degradation). Therefore, in low-energy-density welding processes such as arc welding, preheating is typically necessary. Conversely, high-energy-density welding processes such as laser and electron beam welding do not require preheating, but achieving the same penetration depth in copper as with aluminum or steel often demands higher power. This exacerbates the instability of copper welding.

Copper Material Characteristics — High Reflectivity and Drastic Fluctuations in Laser Absorption at Different States

When welding copper, typically infrared lasers (1030-1080nm) are used. Initially, only about 5% of the incident laser energy is absorbed by copper, with the remainder being reflected. Therefore, the laser welding process often requires a very high energy density laser to rapidly generate a molten pool and increase copper's absorption of the laser.

The absorption rate of liquid copper for the laser increases to around 15%. After the formation of a keyhole, the absorption rate of copper for the laser can reach approximately 55-70%.

Defects Caused by Copper Material Characteristics — Initial Voiding

Due to copper's high reflectivity and low absorption of laser energy, a phenomenon known as initial voiding often occurs at the beginning of the welding process. During this phase, there may be no visible weld bead on the copper surface. The reason behind this is that initially, the absorption rate is low, resulting in minimal heat input. The heat absorbed by copper quickly dissipates through thermal conduction. As the laser continues to irradiate, the temperature of copper gradually increases. With the rise in temperature, the absorption rate also increases. Eventually, accumulated heat begins to melt a portion of the copper, leading to a conduction weld. Subsequently, as liquid copper absorbs more laser energy, the absorption rate further increases. As a result, the heat input continues to rise, leading to the formation of a keyhole, marking the onset of deep penetration welding.

Defects Caused by Copper Material Characteristics — Spatter and Porosity Mechanism

During the laser welding process of copper, as the material temperature rises, its thermal conductivity and absorption rate both change. As shown in the figure, solid copper at room temperature has a low absorption rate, slowly increasing to around 8% at 1250K, which is only a 4% increase. Meanwhile, its thermal conductivity decreases gradually from its peak of 400 W/(mK) to around 330 W/(mK). Thus, in the solid state, pure copper maintains extremely low laser absorption and very high thermal conductivity, making laser processing extremely challenging and requiring very high laser beam power density.

However, in the narrow temperature range of 1250 to 1350K, the absorption rate of pure copper suddenly "jumps" to around 15%. At the same time, its thermal conductivity drops sharply from about 330 W/(mK) to around 160 W/(mK). This means that the optical-thermal conversion increases to twice its original rate, while the heat dissipation rate decreases by half. Consequently, under the same laser beam power density, the rate of heat accumulation in this temperature range increases several times. This is due to the significant thermodynamic changes that occur near the melting point of pure copper at approximately 1083°C (about 1356K), where the "solid-liquid" phase transition causes substantial alterations in its properties.

1. Defects Caused by Copper Material Characteristics — Spattering and Porosity

During the laser welding process of copper, several defects can arise due to its specific material properties:

• Spattering: Copper's high reflectivity and low initial absorption of laser energy contribute to spattering during the welding process. Initially, the laser energy is predominantly reflected rather than absorbed, which can lead to unstable melting and vaporization of the material. As the temperature increases and the absorption rate improves, the likelihood of spattering decreases, but it remains a challenge particularly at the start of the welding process.

• Porosity: Porosity in copper welds can occur due to several factors. One significant factor is the presence of surface contaminants or oxides, which are difficult to avoid due to copper's tendency to oxidize quickly. These contaminants can trap gas during welding, leading to the formation of pores in the weld metal. Another factor is inadequate shielding gas coverage or improper gas flow rates, which fail to protect the weld pool effectively from atmospheric gases, resulting in porosity formation.

Analysis of Defect Mechanisms in Copper Laser Welding - Spatter and Porosity

1. Mechanism of Spatter Formation in Copper Alloys

During deep penetration welding of copper alloys, the formation of spatter droplets is influenced by several forces:

• Surface Tension: Spatter droplets are affected by the surface tension of the liquid, which is the result of intermolecular attraction forces that minimize the surface area of the liquid.

• Gravity: The gravitational force acting on the liquid droplets causes them to move downward, affecting the direction and distance of spatter.

• Upward Shear Force from High-Pressure Metal Vapor inside the Keyhole: Metal vapor generated at high pressure inside the keyhole exerts an upward shear force on the liquid droplets. This pressure gradient within the keyhole contributes significantly to the formation of spatter.

Among these forces, shear force predominates in causing spatter. Typically, spatter originates from the edges of the keyhole opening, where droplets are expelled as the weld pool fluctuates. Once exposed at the keyhole, droplets are directly exposed to intense upward jets of metal vapor. This vapor applies shear force vertically, overcoming surface tension and gravity to propel droplets out of the weld pool, resulting in spatter.

Reducing spatter can be achieved by optimizing welding parameters, adjusting the laser focus position, and increasing laser power density. Additionally, effective gas shielding and proper keyhole design can help minimize spatter occurrence, improving welding quality and stability.

Key Factors in Spatter Reduction in Copper Laser Welding — Decreasing Shear Force

Effect of Metal Vapor Shear Force:Spatter is expelled from the keyhole due to the process of high-pressure metal vapor jetting. During this process, shear force must overcome the surface tension of the weld pool and the gravitational effects on spatter droplets. By reducing shear force, spatter droplets cannot overcome surface tension, thereby preventing spattering.

Source of Shear Force:Shear force originates from the periodic jetting of high-pressure metal vapor. It may be possible to reduce the intensity of metal vapor ejection by minimizing the pressure differential inside and outside the keyhole. A smaller pressure differential results in less vigorous ejection of metal vapor and consequently reduces shear force.

Why Does Metal Vapor Periodically Jet?:Despite consistent laser power and thermal input, fluctuations in the keyhole volume cause variations. Direct laser irradiation on the front wall of the keyhole causes intense vaporization of metal, creating a downward recoil pressure on the weld pool. This results in a wave-like motion of liquid metal around the weld pool towards the tail end, causing irregular fluctuations in keyhole volume. Since metal vapor is incompressible and laser input remains constant, any reduction in keyhole volume increases pressure and accelerates metal vapor ejection, thereby increasing shear force and the likelihood of spatter.

Solutions to Reduce Shear Force:

1. Keyhole Stability: Ensure keyhole stability and mitigate keyhole fluctuations (utilize various laser sources, modulation techniques).

2. Environmental Control: Consider welding in a vacuum or under controlled pressure environments to stabilize keyhole conditions.

Reducing shear force is crucial to minimizing spatter in copper laser welding processes, enhancing overall weld quality and efficiency.

Key Factors in Reducing Spatter in Copper Laser Welding — Avoiding Shear Force

Metal Vapor Shear Force:Spatter in copper laser welding occurs when high-pressure metal vapor jets out of the keyhole, overcoming the weld pool's surface tension and the gravitational pull on spatter droplets. To prevent spatter, it's crucial to ensure that droplets at the keyhole opening do not encounter metal vapor and thus do not face shear force.

Location of Spatter Formation:Spatter primarily occurs behind the keyhole and at the keyhole opening. As the weld pool oscillates, a protrusion forms above the keyhole due to the impact of metal vapor, leading to spatter formation.

Strategies to Avoid Metal Vapor:

1. Reduce Weld Pool Oscillation (Heat Source): Minimize fluctuations in the weld pool to prevent protrusions above the keyhole where it directly faces metal vapor impact.

2. Auxiliary Light Sources, Keyhole Modification: Use auxiliary light sources to widen the keyhole opening, transforming it from a vertical cylindrical shape to a "Y" shape. This modification helps divert the keyhole opening away from direct exposure to metal vapor, reducing the likelihood of spatter.

Implementing these strategies effectively reduces the impact of metal vapor shear force during copper laser welding, enhancing weld quality and stability.

Mechanism Analysis of Porosity Formation in Copper Alloys

Classification of Pores: Process Pores and Keyhole Pores

Process Pores: These are primarily related to shielding gas and material factors, typically small in size, around 50-200 micrometers.

Keyhole Pores: These are mainly caused by instability in the keyhole during the laser welding process. Keyholes are hollow structures, and if a keyhole collapses, the liquid weld pool can be engulfed by metal vapor. When the metal vapor cannot escape from the surface of the copper weld pool in time, it solidifies within the pool, forming larger diameter pores. These pores significantly affect material properties such as electrical conductivity and joint strength, making them critical to avoid.

The formation of keyhole pores is a result of complex interactions during laser welding, where maintaining keyhole stability and ensuring proper gas shielding are crucial. Effective control of these factors helps minimize porosity, ensuring high-quality welds in copper alloys.

Mechanism Analysis of Porosity Suppression in Copper Alloys

Factors Contributing to Porosity Formation:

1. Keyhole Instability: Frequent collapses at the bottom of the keyhole, entrapping gases into the keyhole.

2. Inadequate Gas Escape Time: Gases do not escape from the weld pool before solidification, leading to their entrapment within the weld seam.

Direction of Solutions:

1. Addressing Keyhole Instability:

• Options include: a. Increase Speed: Accelerate the welding speed to enlarge the keyhole opening. b. Use of Composite Heat Sources: Employ composite heat sources such as AMB (Arc-Melt-Blow) or hybrid (e.g., infrared and blue laser) to widen the keyhole opening. c. Power Modulation: Modulate laser power to stabilize the keyhole.

• Keyhole Expansion: Enlarge the keyhole to reduce the likelihood of trapping gases in the weld pool.

2. Enhancing Gas Escape Process:

• Dual-Beam Composite Heat Source: This can expand the weld pool and delay solidification.

• Swing Welding: Oscillate the welding path to enlarge the weld pool area, extend solidification time, and facilitate gas pore escape.

• Consider methods to prolong weld pool solidification and expedite gas escape:

These approaches aim to effectively control keyhole stability and gas escape dynamics, thereby minimizing porosity and enhancing the quality of welds in copper alloys.

Comparison of Copper Alloy Laser Welding Technologies

1. Single Infrared Laser Oscillation Welding

Advantages:

• Single Mode Laser: Typically with a core diameter of 14-30 micrometers. IPG lasers are commonly used in single mode applications, ranging from 1 kW to 3 kW, with some models capable of up to 6 kW. The small core diameter provides high enough laser energy density to instantaneously melt copper alloys, thus avoiding initial void welding.

• Oscillation Capabilities: The small core diameter can be combined with oscillating welding heads or galvanometer welding heads to achieve various welding trajectories. This capability enlarges the weld pool, stabilizes the keyhole, aids in gas expulsion, and enhances welding stability, reducing spatter and porosity.

• High Power Options: For higher power requirements, a 50 micrometer core diameter with a 6 kW laser is available.

Drawbacks:

• Limitations of Single Mode Products: Limited power options in the commercial market, inability to avoid keyhole collapse without oscillation, and instability at low speeds (due to small keyhole sizes). Comprehensive speed with oscillation, and limited melting depth, typically within 0.5 mm, is typically used for stacking and docking at a speed of about

Infrared vs. Short Wave Comparison

Using infrared light for welding still cannot avoid the instability in heat input caused by the dramatic changes in thermal conductivity and absorptivity of copper alloys due to solid-liquid phase transitions. Therefore, some companies have begun to explore short-wave and multi-wave solutions.

Advantages of Short Wave Welding:

• Assuming a 6 kW infrared laser as the heat source, starting from room temperature to heat pure copper with a 5% absorption rate results in an effective heating of 300W. If 450 nm blue light is used for heating with a 65% absorption rate, achieving the same heating effect would only require approximately 461W (300W ÷ 65% ≈ 461W).

• As the material reaches the solid-liquid melting point, the increase in absorption rate and decrease in thermal conductivity collectively enhance the heating effect by about 20% (reduced heat dissipation, increased absorption). This additional 20% heating effect, in the case of a 6 kW infrared laser, means an additional 1200W heating power. This can lead to an extremely unstable processing state, where accumulated heat can instantly reach the next stage of vaporization boiling point, generating copper metal vapor, causing boiling and splashing.

This illustrates how short-wave solutions can offer more stable and efficient heating for copper alloys compared to infrared, especially by mitigating the challenges posed by phase transitions and fluctuating heat input.

Blue Light Solution for Copper Laser Welding

Copper exhibits an absorption rate of over 47% for blue light lasers. Currently, blue light lasers can achieve a maximum power of 8 kW with a core diameter of around 600 μm. They are primarily used for conduction welding and cladding of high-reflectivity materials.

Characteristics of Blue Light Lasers:

• High Absorption Rate: Blue light lasers are highly absorbed by copper due to their absorption rate exceeding 47%. This makes them suitable for applications requiring efficient energy absorption.

• Direct Semiconductor Lasers: Blue light lasers are direct semiconductor lasers, which means they have poorer beam quality compared to lasers operating at smaller core diameters. Green light lasers can achieve around 50 μm and red light lasers around 14 μm, making them more suitable for intricate tasks like micro-welding and fine connections.

• Applications: Blue light lasers are mainly used for conduction welding and are suitable for joining copper alloys up to 0.5 mm in thickness. They are less suitable for applications requiring penetration welding or stacking due to their larger core diameter.

• Beam Delivery: Due to their larger core diameter, it is challenging to use scanning mirrors (galvanometers) for welding with blue light lasers. They are typically used with collimating welding heads for direct beam welding.

• Heat Affected Zone: Blue light lasers generally have a larger heat affected zone (larger spot size), which may not be as advantageous as green light or adjustable ring mode lasers for micro-connections and fine welding.

In summary, blue light lasers offer high absorption rates for copper but are best suited for applications like conduction welding and cladding where high power and efficient energy absorption are critical. For finer tasks requiring smaller spot sizes and less heat affected zones, green light and adjustable ring mode lasers may offer better performance.

Green Light Solution for Copper Alloy Laser Welding

Since 2019, green light laser products have entered the market, capable of achieving a maximum power of 3 kW, suitable for applications involving high-reflectivity materials.

Key Features of Green Light Lasers:

• Absorption Rate: Copper alloys absorb green light lasers at approximately 40%, which is nearly 10 times higher than the absorption rate of 3-5% for red light lasers by copper alloys. This enhanced absorption efficiency allows for more effective energy utilization during welding.

• Deep Penetration Welding: Green light lasers enable deep penetration welding of copper alloys. With a fiber core diameter as small as 50 μm, models like the TruDisk 3022 can focus energy more precisely on the material surface. This capability makes them suitable for remote welding applications when combined with scanning mirrors (galvanometers).

• Applications: Green light lasers are particularly effective for thermal conduction and deep penetration welding of copper alloys up to 2 mm thick. They offer a balance between power density and beam quality, enabling precise welding even in challenging scenarios.

• Current Developments: Domestic green light laser technology in China is advancing, focusing on improving specialty optical fibers and ensuring stable power output, which are critical factors for enhancing performance and reliability.

In summary, green light lasers present a significant advancement for laser welding of copper alloys, offering higher absorption rates and capabilities for deep penetration welding compared to traditional red light lasers. Their development continues to improve, especially in terms of optical fiber technology and power stability, promising further enhancements in their application range and performance.

Red-Blue Composite Laser Welding for Copper Alloys

Red-blue composite laser welding combines fiber lasers with blue lasers through an external optical path welding head. This method offers various heat source configuration models, leveraging the high absorption rate of blue light by copper alloys to quickly heat and melt the material. It also utilizes the high energy density and penetration capability of fiber lasers, along with the enhanced absorption of fiber lasers by molten copper alloys. This combination allows for the integration of thermal conduction welding and deep penetration welding, achieving stable welding processes, reducing splashing, and improving the appearance of weld seams.

Key Features:

• High Absorption Rate: Copper alloys exhibit a high absorption rate for blue light, allowing rapid heating and melting of the material.

• Energy Density and Penetration: Fiber lasers provide high energy density and penetration capabilities, enabling effective welding through various thicknesses of copper alloys.

• Combined Welding Modes: The composite of red and blue lasers allows for versatile welding modes, adapting to different requirements such as thermal conduction or deep penetration welding.

• Applications: Effective for stable processing of copper alloys up to 3 mm thick, ensuring reliable weld quality and minimizing defects.

Red-blue composite laser welding represents an advanced approach in laser welding technology, particularly suitable for copper alloys due to their specific absorption characteristics. This method not only enhances efficiency but also improves the overall quality and appearance of weld seams, meeting the demands of diverse industrial applications.