Laser Cladding

1. Introduction

Various thermal surface treatment methods exist to improve the surface properties of metallic workpieces (such as wear resistance, corrosion resistance, fatigue resistance and high-temperature resistance). Among these processes, flame spray, plasma spray, and arc welding are the most well-known techniques. The fundamental objective of these methods is to create a layer with appropriate characteristic properties on the surface. Depending on the method applied, adhesion problems, cracking, and thermal distortion may occur between the substrate and coating layer. Laser cladding technology possesses an effective approach to overcome these problems. The laser cladding process is a method that enables coating deposition on a surface through a heat source obtained via laser beam.

Laser cladding

Laser cladding is a complementary coating technology that deposits material on the surface similarly to welding techniques and competes with thermal spray methods. It is an increasingly adopted advanced alternative method used in repair and filling applications in place of plasma transfer arc welding and conventional TIG welding methods. Conventional cladding methods require high heat input to join metals together, and additional surface finishing operations after processing to achieve the final product. This results in extended production time and increased operational costs. The laser cladding process has emerged in response to growing requirements to extend product life, improve performance and efficiency. Furthermore, laser cladding can be used for repair and filling purposes, and represents an economical alternative solution for restoring part tolerances. Consequently, increased demand for laser cladding operations is expected in the industry in the coming years. The global laser cladding equipment market is experiencing continuous development and is projected to achieve growth rates exceeding 8% in 2021. Laser cladding is a surface treatment technique involving the controlled addition or deposition of material in wire or powder form onto the surface of another material. Wire or powder material fed into a focused high-energy laser beam enables deposition of the selected material on the target substrate surface.

Figure 1. Robotic laser cladding system

High laser power enables the provision of highly controlled heat input. Consequently, heating rates in the surface layer are quite high. The laser-heated surface cools rapidly on its own. As a result of high heating and cooling rates, fine grain structure and quasi-stable microstructural phases can be obtained.

Figure 2. Schematic representation of laser cladding and applications

Process Advantages:

• Can be applied with precision to any desired area. • Offers a wide range of material options that can be deposited by laser or used to fill the surface. • Creates excellent bonding between coating and surface with very low or virtually no porosity (metallurgical bond). • Minimal heat input to substrate with very narrow heat-affected zone (Low dilution less than 5%) • Low heat input reduces distortion problems, thus minimizing need for additional processing. • Easily integrated with CAD, CAM and can be adapted to CNC systems enabling automation. • High powder efficiency (90%). • High process speed (3 m/min.).

Materials:

Laser cladding process performance is directly related to proper coating material selection. Depending on the required coating properties, a wide variety of metallic or metal composite-based materials in powder or wire form can be used. Correct process and coating material selection is critically important for corrosion protection, wear resistance enhancement and high-temperature applications. Powders used in laser cladding processes range in size from 20-200 μm. Spherical form is generally preferred and production by gas atomization method is preferred. • For corrosion protection: nickel-based alloys, stainless steels, • For high-temperature applications: cobalt-based alloys, • For wear resistance: NiBSi+WC, iron-based alloys together with various carbide types.

2. Laser Cladding Process

The laser cladding process involves many parameters, with molten powder/wire being controlled deposition on the substrate surface, thus changing and improving the surface properties and behavior of the substrate. Control of critical parameters is essential for process success. Main parameters: process speed, laser power, feed rate, laser beam geometry. During laser cladding, the substrate melts to a very limited extent and no significant change occurs in the coating composition.

1. Laser Beam 2. Powder Feed 3. Workpiece 4. Coating

Figure 3. Laser cladding a. powder b. wire

The main applications of laser cladding operations are repair and refurbishment of high-value parts. Particularly turbine blades, internal combustion engine components, tools and many military-purpose parts are examples. With conventional methods (such as welding), there is considerable risk of damage to parts due to high and uncontrolled heat input. Thermal stresses reduce mechanical strength and surface quality, causing crack formation and porosity, which consequently shortens component life.

Figure 4. Cladding process parameters

3. Applications Laser cladding creates a protective coating that enhances surface functionality. Laser cladding operations can extend the life of parts subject to corrosion, wear and impact. Typical applications include: drilling drill pipes, pump components, valve parts, pneumatic cylinders, bearings, power transmission systems, extrusion parts, forging dies, corrosion-resistant pipes, engine cylinders, gear systems. Coating thicknesses can be reduced with laser cladding. Dilution is quite low and desired coating properties can be achieved throughout the required thickness. • Energy control in the cladding zone can be achieved as desired, allowing very rapid deposition without deformation. • Coating quality repeatability is quite high; desired thicknesses can be achieved with controlled process parameters. • Heat treatment can be applied to coatings. Figure 5. Steam valve subjected to significant wear at high temperature from erosive effects, subsequently repaired by laser cladding with cobalt-based superalloy.

Figure 6. Laser cladding examples

Figure 7. Comparison of microstructures

4. General Conclusion

Laser cladding is the deposition of powdered material onto metal surfaces using laser assistance to improve surface properties. It creates excellent bonding with the substrate, minimum heat-affected zone and minimal distortion. Cladding material is selected to suit the working environment conditions of the workpiece. It is preferred to provide corrosion protection, increase wear resistance and improve mechanical properties.

Figure 8. Comparison of cladding processes

  Dr. Ekrem Altuncu ETSA / TESLAB Faculty of Technology Department of Metallurgy and Materials Engineering Sakarya University  

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