Laser cladding technology, a sophisticated surface engineering method, utilizes high-energy lasers to simultaneously melt metal powder and the substrate surface, forming metallurgically bonded alloy coatings upon rapid solidification. This advanced laser cladding process imparts excellent corrosion resistance, wear resistance, and high-temperature resistance to workpiece surfaces, making it widely adopted in various industrial applications.

In practical laser cladding applications, most industrial users aim to achieve coatings with high flatness. The flatter the surface, the less post-polishing is required, saving on metal powder material and significantly reducing production costs. Notably, the flatness of the laser cladding coating is primarily influenced by three factors: the flatness of a single cladding pass, the thickness of a single pass, and the overlap rate between adjacent cladding passes.

Impact of Spot Shape on Laser Cladding Quality

During the laser cladding process, the surface tension and wettability of the molten metal interact, leading to distinctly different melt track profiles depending on the shape of the laser spot. When using small circular spots (3mm-5mm) for laser cladding, the melt track surface typically forms a convex shape rather than the desired flat surface. Conversely, when using larger rectangular spots (10mm-30mm), factors like powder feed uniformity and spot intensity uniformity prevent the single cladding layer from achieving ideal flatness.

The Critical Role of Overlap Rate in Laser Cladding

In laser cladding, adjacent melt tracks must overlap to a certain degree, a parameter that is crucial for the final coating’s flatness. The overlap rate (R) is calculated as:
R = D/W × 100%
Where D is the overlap width and W is the single pass cladding width. If a step size d is used (the distance the laser moves forward after each pass), the overlap rate can be expressed as:
R = (W-d)/W × 100%

From the formula, it’s clear that the smaller the step size, the larger the overlap rate, meaning that the melt tracks will overlap more. In the case of rectangular spot laser cladding, the overlap rate is typically below 50%. A high overlap rate can negatively impact cladding efficiency, while a rate lower than 50% will cause fluctuations in coating thickness.

Performance Comparison of Circular vs. Rectangular Spot Sizes in Laser Cladding

Let’s assume the cladding thickness per pass is 1mm. In the process using rectangular spots, the thinnest part of the coating is around 1mm, while the thickest part can theoretically reach 2mm (slightly less in practice). Therefore, long rectangular spots may find it difficult to meet high flatness requirements for cladding coatings.

In contrast, the laser cladding process using circular spots of 3-5mm size shows a significant advantage. The principle behind this is different from that of rectangular spots: rectangular spots typically achieve the required cladding thickness through a single layer (or at most two), whereas the 3-5mm circular spot laser cladding thickness is achieved through multiple layers of overlapping.

For example, using a 5mm circular spot with a 1mm step size (80% overlap rate), to achieve a 1mm thick coating, it will require five layers of 0.2mm thick melt tracks. This multilayer stacking characteristic is a key difference between small circular spot and rectangular spot laser cladding.

Real-World Effectiveness of High-Speed Laser Cladding

By adopting the 3-5mm circular spot laser cladding process, exceptionally high flatness (below 10 microns) can be achieved. Figure 2 shows the flatness test results of coatings produced by Zhongke Zhongmei using 3-5mm circular spots, where the flatness reached an excellent Ra5-6μm level.

Conclusion: Optimizing Spot Selection for Laser Cladding Process

Based on the analysis above, a clear conclusion can be drawn: In laser cladding applications requiring high flatness, the shape and size of the laser spot are critical process parameters. The 3-5mm circular spot laser cladding process achieves the required cladding thickness through high overlap rates and multiple layer stacking, making it the best choice for high flatness coatings. For laser cladding applications focused on coating quality and subsequent processing costs, circular spots are undoubtedly the superior option.

In specific laser cladding projects, engineers should choose the spot type and process parameters based on factors like workpiece requirements, production efficiency, and economic considerations to ensure that laser cladding quality reaches its optimal state.