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Laser wire deposition welding (LWD) is an advanced laser welding technique primarily used to add material to the surface or volume of metal workpieces for repair, enhancement, or modification of their functionality. Here's a specialized overview of laser wire deposition welding technology:
Overview
Laser wire deposition welding technology utilizes a high-energy density laser beam to heat metal wire or powder to its melting point, simultaneously depositing and fusing the material onto the workpiece surface or volume. This technology combines the precision of laser processing with the advantages of high energy density, finding wide applications in aerospace, automotive manufacturing, mold repair, and more.
Working Principles
The operational principles of laser wire deposition welding involve several key steps:
Laser Beam Focusing: A high-energy density laser beam is focused through an optical system to concentrate its energy density into a small area.
Metal Filling Material Supply: During the welding process, metal wire or powder, controlled by a numerical control (NC) system, is continuously or intermittently supplied to the welding area.
Melting and Droplet Formation: The laser beam heats the metal filling material to its melting point, forming a molten pool, with droplet formation and flow controlled by surface tension.
Filling and Fusion: Molten droplets are selectively deposited onto the workpiece surface or volume during welding, fusing with the base material to form a uniform metal build-up layer.
Applications
Laser wire deposition welding technology finds extensive applications in several areas:
Repair and Reinforcement: Used for repairing surface defects, wear, or damage on metal parts, such as aircraft engine blades, automotive molds, etc.
Manufacturing and Additive Processes: Applied in manufacturing complex metal components or adding material to enhance functionality and performance.
Rapid Prototyping: Facilitates the rapid production of complex-shaped metal prototypes using laser wire deposition welding technology.
Advantages and Challenges
Laser wire deposition welding technology offers several advantages over traditional welding methods:
High Precision and Localized Control: Laser beam focusing enables precise control over the welding area and deposition of filling materials.
Low Heat Affected Zone (HAZ): High energy density and short welding times reduce thermal impact on surrounding materials, minimizing deformation and residual stresses.
Multi-material Adaptability: Suitable for welding and additive processes with various metal materials, including titanium alloys, nickel-based alloys, etc.
However, laser wire deposition welding technology also faces challenges such as ensuring stable droplet control, optimizing the selection of metal filling materials, and addressing cost-related factors.
In conclusion, laser wire deposition welding technology represents a promising advancement in welding and additive manufacturing, offering broad application prospects and ongoing development potential across industrial sectors.
Laser welding is predominantly a non-filler autogenous welding process in most cases. The high precision required for joint preparation and assembly limits its industrial applications to some extent. Expanding the industrial applications of laser welding and advancing its industrialization can be achieved through the adoption of laser cladding. Laser cladding can weld butt joints and thick plates with larger gaps and adjust the chemical composition of weld seams during dissimilar metal joining, significantly suppressing post-welding hot cracks. Aluminum alloys are widely used in the aerospace, automotive, and aviation industries. However, aluminum alloys have a high reflectivity to lasers, resulting in softened joints and easy generation of hot cracks during welding, which limits the application of high-power laser welding processes in the aluminum alloy processing industry. The use of filler materials can increase the absorption rate of aluminum alloys, stabilize the welding process, and improve the mechanical properties of welds for some aluminum alloys that are prone to solidification cracks.
In addition, laser cladding can be used to produce overlay coatings, where droplets are deposited one by one or overlapped on the workpiece surface to form wear-resistant or anti-wear layers, thereby improving the service life of workpieces.
I. Characteristics of Laser Cladding
The efficiency of laser cladding is similar to that of ordinary laser welding, but the use of filler wire significantly expands the application range of laser equipment, mainly in the following aspects:
(1) Solves the problem of strict requirements for workpiece clamping and assembly. Since the laser beam focuses into a spot with a diameter of a few hundred microns during welding, high precision in joint assembly and gap is required. For butt joints, non-filler autogenous welding generally allows a maximum gap of up to 10% of the plate thickness. With longer weld seams, high machining accuracy and clamping precision are required for the workpiece welding surface. The existence of gaps can cause partial laser energy to make it difficult to weld the weld seam or form a depression. Laser cladding can prevent laser energy from passing through the gap in the weld seam, relax assembly precision, and make the weld seam slightly protruding for a more aesthetically pleasing shape.
(2) Enables multi-pass welding of thick plates using lower-power lasers. The power size of existing high-power lasers limits the depth of penetration, imposing certain restrictions on applications. Generally, the welding thickness of thick plates is less than 15mm. Overcoming this limitation involves narrow gaps and multi-pass welding with filler wire, using lower heat input to achieve welding of thick plates with minimal deformation and much higher efficiency than traditional welding methods.
(3) By adjusting the composition of the welding wire, the weld seam's structural properties can be controlled, making it easier to control defects such as cracks. This is particularly advantageous for welding dissimilar materials and brittle materials.
II. Characteristics of Laser Cladding Wire Feeding
The introduction of welding wire makes the laser welding process more complex, and mastering the wire feeding characteristics of laser cladding under different welding conditions is a prerequisite for obtaining high-quality welds.
The wire feeding speed is a critical process parameter in laser cladding, and selecting the appropriate wire feeding speed can fully utilize laser energy and improve production efficiency. The wire feeding speed should be determined based on the gap size. During laser welding, almost 100% of the welding wire transitions into the weld pool. Therefore, the wire feeding speed can be calculated based on the material balance of the welding process. The height of the weld seam section and the gap between the joints rely on the filler wire filling.
According to different welding processes, the welding wire can be fed from the front or rear of the laser, and at a certain angle with the optical axis. In laser cladding, it is generally required that the welding wire be coplanar with the weld seam on the vertical plane, ensuring stable transition of molten droplets even when minor fluctuations occur during wire feeding. The straightness of the welding wire is crucial for the stability of welding, affecting the absorption of beam energy by the welding wire and the stability of the welding process. To ensure that the welding wire is precisely fed to the intersection of the optical axis and the base material, a copper tube is typically used at the end of the feeding hose to guide the welding wire, as shown in the figure. A gas tube is installed above the workpiece to blow helium or argon gas, which protects the weld pool and suppresses plasma, and for metals that are highly prone to oxidation (such as titanium alloys), a special protective cover is require
In general, an angle of 30° to 75° for wire feeding is considered suitable. The wire feeding position should ideally align with the centerline of the weld seam as much as possible. When the wire feeding position deviates 0.25mm from the centerline of the weld seam, the melting efficiency of a 2mm wire decreases by about 30%, and for 1.0mm and 1.2mm wires, the melting efficiency decreases by around 36%. Therefore, in situations where high welding requirements are needed, the best approach is to use an optical weld seam tracking system for real-time monitoring and control of the wire feeding position.
If the wire feeding speed is too fast or too slow, it can lead to excessive or insufficient accumulation of molten droplets into the weld pool. This also affects the interaction between the laser and the wire as well as the base material, thereby influencing the formation of the weld seam. Typically, the wire feeding mechanism used should include a feedback system to maintain a constant wire feeding speed throughout the welding process. The diagram below illustrates the composition schematic of a laser wire feeding welding system.
In high-precision welding processes, the amount of metal filling is required to be adjusted in real time according to changes in the weld seam bevel to ensure stable and accurate filling, thereby achieving good weld seam formation. The optimal solution is to equip a dynamic wire feeding control system that can dynamically follow changes in the weld seam bevel.
Most laser wire feeding welding processes employ cold wire techniques, where the wire is not heated prior to entering the weld pool. In such cases, a significant portion of the laser beam's energy acts upon the wire itself, inevitably reducing the welding speed. To fully utilize the laser energy, a technique known as hot wire welding can be introduced. Hot wire welding reduces the amount of laser energy consumed by the wire, effectively increasing the welding speed.
In laser hot wire welding, an additional preheating device is integrated, often using resistance heating. The electrode is directly connected to the wire feeding roller, and through a high current, the wire is rapidly heated to near-melting temperatures. As the wire reaches the edge of the weld pool, its high surface temperature requires minimal laser energy to melt it. The molten wire absorbs a significant amount of laser energy and conducts it to the base material. Compared to self-fusion welding, hot wire welding facilitates better absorption of laser energy, thereby potentially achieving higher welding speeds.
To prevent oxidation of the wire metal, the heated portion of the wire is typically kept within 3-5mm from the intersection point of the laser beam and the wire. This minimizes excessive cooling of the heated wire. In laser hot wire welding setups, there needs to be sufficient distance between the focusing lens and the workpiece for installing the heating device. Longer focal lengths are preferred despite the increase in spot diameter and decrease in power density, as they enhance depth of focus, which is advantageous for welding thick plates.
Interaction Between Laser Beam and Filler Metal
During laser wire feeding welding, the laser primarily acts on the filler wire, which fills the gap between the workpieces. Initially, the laser heats and melts the filler wire, which then fills the joint gap. With continued laser action, the base metal also melts, forming a weld pool. If the filler wire is not accurately fed to the intersection point of the laser axis and the weld seam centerline, or if the gap is too large, most of the laser energy will pass through the gap, with only a small portion being absorbed by the joint sidewalls. This results in incomplete fusion and inadequate joint formation.
The mechanism of interaction between the laser beam and the filler metal in laser wire feeding welding is complex. When the laser beam irradiates the filler wire, energy is absorbed, part of which melts the wire while another part vaporizes the filler metal. Some energy reflects off the surface of the wire, and a portion penetrates through the wire. During this process, small holes may form due to the focused point of the laser beam on the wire surface.
At the end of the wire, both solid wire and molten droplets can reflect the laser beam, with molten droplets reflecting approximately 70% of the total reflected energy of the wire. The amount of energy reflected by the wire depends on parameters such as laser beam energy, wire feeding speed, point of interaction between the laser and the wire, and energy density. At slower wire feeding speeds, the laser energy is mainly absorbed by molten droplets rather than the solid wire, which relies on heat conducted from the molten droplets for melting.
During welding, the reflection of laser energy by the wire cannot be overlooked. Changes in welding parameters and wire feeding characteristics affect the amount of laser energy reflected by the wire, necessitating control over the energy density on the workpiece surface considering the energy losses caused by wire reflection during different welding conditions. Salminen conducted measurements and analysis of laser energy reflected by the wire using thermal-sensitive paper and power meters. As depicted in the figure, the direction of reflected laser beams from CO was determined using thermal-sensitive paper and then a power meter was placed on the reflected light path to measure the energy of reflected laser beams. It was found that the process influences reflected influence greatly
Liquid filler metal has a higher absorptivity than solid metal, and the absorptivity of high-temperature metal is greater than that of low-temperature metal. Changes in process parameters determine the energy density of the laser acting on the wire per unit time, affecting the temperature and melting state of the wire, and directly influencing the absorptivity of the wire. Therefore, in the welding process, to fully utilize the laser energy, it is advisable to increase the laser power as much as possible, use short focal length lenses, and minimize the wire feed rate while meeting shaping requirements.
The direction of wire feeding affects the melting characteristics of the wire differently. Xiao Rongshi and others used high-speed cameras to capture the melting process of laser filling welding wires and proposed the following views, as shown in the diagram. In the case of wire feeding before welding, the wire is melted into the weld pool mainly through the combined effects of direct laser radiation and plasma heating. With faster wire feeding speeds, more laser energy is needed to heat the wire, leaving less energy to melt the base metal and form the keyhole. Therefore, the welding process with wire feeding before welding is less stable. In contrast, with wire feeding after welding, the heat of the weld pool can also be used to heat the wire. At low welding speeds (<3m/min), the wire can be melted relying on plasma and weld pool heat. Thus, laser energy can be directly used to heat the base metal to form stable keyholes. However, at higher wire feeding or welding speeds, relying on plasma and weld pool heat alone may not fully melt the wire. In such cases, some laser energy will also be used to melt the wire, making the welding process potentially unstable. Therefore, the different wire melting mechanisms of wire feeding before and after welding lead to differences in welding efficiency and welding process stability between the two welding methods.
Application of Laser Wire Feeding Welding Technology
4.1 Multi-pass Wire Feeding Welding Technology
The application of multi-pass wire feeding welding technology can enhance the capability of laser welding thick plates. In laser wire feeding welding, the test piece grooves are generally relatively narrow, with commonly used groove shapes including:
Step-type groove
Double-sided V-groove, as shown in the diagram.
Research indicates that for step-type grooves, the welding process is relatively stable. During welding, the plasma is pressed into the molten pool, acting as a secondary heat source, which increases the depth of fusion. This results in well-formed weld seams, but crystalline cracks may be observed at the center of the weld seam. On the other hand, the stability of the welding process is lower for double-sided V-grooves, possibly due to the increased difficulty in controlling the plasma. Although the weld seam is well-formed, only minor porosity is observed on the surface, and no crystalline cracks are found. With proper groove design and matching process parameters, satisfactory weld seams can generally be achieved with both types of grooves.
In terms of reducing heat input and controlling welding deformation, narrow-gap laser wire feeding welding methods offer greater advantages compared to traditional welding methods. Figure (a) illustrates a narrow-gap butt joint designed by Tommi Jokinen and others for welding austenitic stainless steel plates (20mm thick), primarily using a V-groove. The root thickness of the groove is 4mm and is pre-welded before welding. The groove angle ranges between 8~12°, with the groove inclination angle designed to be as small as possible under conditions ensuring complete beam incidence. Considering the groove angle variation due to shrinkage of the initial weld passes during the welding process, the beam focusing angle used in the experiment is 6°, with the minimum designed groove angle being 8°. This implies that the deformation of the groove angle during the welding process is within 2°.
The positioning and feeding speed of the wire are critical process parameters that need to be adjusted in real time based on the size of the groove. Control of the amount of wire filling must fully consider the interaction between the wire and the laser as well as the dimensions of the groove. Excessive filling metal can lead to welding defects such as incomplete fusion. In theory, the energy density is highest at the focal point. However, considering the significant difference in beam spot diameter (0.6mm) compared to wire diameter (2mm), the wire is fed approximately 2mm below the focal point to ensure complete melting of the wire. Figure (b) shows the cross-sectional shape of the weld after welding, with the main process parameters including a groove angle of 10°, laser power of 3kW, welding speed of 0.5m/min, and wire feeding speed of 4.5~6m/min.
4.2 Dissimilar Metal Welding
In dissimilar metal welding, significant differences exist in the chemical composition and structure of the base metals being joined, making it challenging to achieve satisfactory joint quality using fusion welding techniques alone. However, wire feeding welding can compensate for the limitations of fusion welding. By selecting appropriate filler wires, welding joints can possess excellent comprehensive properties. Additionally, dissimilar material welding can employ beam offset techniques to adjust heat input and control the microstructure of the welded materials.
Current research on dissimilar metal welding primarily focuses on welding dissimilar steels and steel with cast iron. Due to the substantial differences in carbon and other alloying elements, welding these materials can lead to the formation of brittle structures such as martensite or white cast iron in the weld seam, increasing welding stresses and causing weld cracking. To address this issue, it is necessary to reduce the carbon content in the weld seam using filler wires and increase the content of austenitic elements such as nickel, thereby suppressing the formation of brittle structures and promoting the formation of austenite, ferrite, or their duplex structures in the weld seam.
For example, in the welding of ferritic low-alloy steel (13CrMo44) with austenitic stainless steel (AiSi347) pipes using laser wire feeding welding, with a diameter of 43.5mm and a thickness of 4.5mm, a ∅1.2mm EniCrMo-3 nickel-based filler wire is used. During welding, when the beam is offset towards the austenitic stainless steel with an offset of 0~2.5mm, there is no significant change in the structure and properties of the weld seam, and a fully austenitic structure is obtained. However, if the offset is towards the Cr-Mo ferritic steel and exceeds 0.1mm, a duplex structure of martensite + austenite is obtained in the weld seam. The formation of martensite reduces the corrosion resistance of the weld seam.
Increasing the gap between the joints and increasing the filling amount of the filler wire, where the nickel content in the base wire reaches as high as 60%, allows beneficial supplementation of alloying elements diluted in the parent material during the welding process, facilitating the formation of single-phase austenite.