Carbide products are good?for?manufacturing due to their strength and wear resistance, but pose machining challenges. Traditional grinding struggles with efficiency and precision for these materials. Picosecond laser technology offers a breakthrough with its non-contact, high-precision approach, enabling new manufacturing possibilities.

This study examines picosecond laser processing of carbide product across different energy densities, analyzing effects on surface roughness, composition and material removal to identify distinct interaction mechanisms.

 

Mechanism of Laser Processing

Laser processing may sound like futuristic sci-fi tech, but it works by focusing a laser beam to deliver energy to the workpiece, locally heating the material to melt, vaporize, or remove it—all without physical contact. Unlike traditional grinding, this method eliminates tool wear and deformation while boosting efficiency and precision. Most importantly, laser processing isn’t limited by material hardness or toughness, making it ideal for even the most challenging alloys.

Laser processing isn’t limited to just one approach. Based on pulse duration, it can be categorized into:

1.Nanosecond lasers – Like a broadsword:Longer pulses (wider heat-affected zone)?and suitable for less precision-critical applications

2.Picosecond & femtosecond lasers – Like precision engraving tools:Ultra-short pulses (minimal heat-affected zone),enable high-precision, low-damage processing, and femtosecond lasers (pulses in quadrillionths of a second) remove/modify material instantaneously with zero peripheral impact.

While femtosecond lasers offer extreme precision, their equipment costs too much. Picosecond lasers strike the ideal balance, including simplified design,stable performance,lower operating costs,and sufficient efficiency for most applications.Those points make picosecond lasers the preferred choice for processing of carbide product in industrial settings.

 

Experimental equipment and plan

The experiment used YG8 carbide product with sample dimensions of 50mm×50mm×2mm. Laser processing was conducted on a self-developed five-axis picosecond laser milling machine. The laser parameters were:

Wavelength: 1064nm

Pulse width: 10ps

Repetition rate: 2000kHz

Spot diameter: 30μm

Four energy density (ED) levels were tested:

0.45J/mm2, 1.05J/mm2, 1.65J/mm2, and 2.25J/mm2

A 10% scan overlap ratio was selected based on appropriate beam traversal speed ranges and experimental requirements, with single-pass processing (one laser scan per surface).

The energy density calculation formula is:

 

where:

P = Laser power (W)

D = Focused laser beam diameter (mm)

V? = Laser transverse scanning speed (mm/s)

Energy density variation was achieved by adjusting laser power. As shown in Figure 1, the laser followed path a for transverse motion and path b for cross-feed motion. Post-processing, samples were ultrasonically cleaned in anhydrous ethanol for 15 minutes and dried.

The three-dimensional contour morphology of the sample surface after laser processing was observed by using the product measuring instrument (Bruker Alicona Infinite Focus SL), and the surface roughness and processing depth were detected. The obtained results were all the average values of the five measurement results.

(a) Experimental apparatus

(b) Processing procedure

Figure 1 Ultrafast laser processing of carbide product

 

Experimental Results and Discussion

Surface microstructure of carbide product

When processing the surface of materials with ultrafast lasers, the energy flow density of the laser needs to reach above the ablation threshold of the material to cause ablation to the material. The surface morphologies processed by laser under different energy densities are shown in Figure 2.

(a) The energy flow density is 0.45J/mm 2

(b) The energy flow density is 1.05J/mm 2

(c) The energy flow density is 1.65J/mm 2

(d)The energy flow density is 2.25J/mm 2

Figure 2 Surface morphology of laser processing with different energy flow densities

At low energy density (0.45J/mm2), the laser-processed surface shows varied ablation morphologies due to the Gaussian beam profile. Only the central high-energy zone exceeds the ablation threshold, forming sparse LIPSS structures (Fig.2a3), while peripheral areas retain base material features (Fig.2a1). Partial Co binder removal between WC particles creates micron-scale protrusions via thermal diffusion.

Above 1.05J/mm2, stable LIPSS structures form, with periodicity determined by laser wavelength and material dielectric properties. These result from laser-induced phase transitions and resolidification. Energy variations along cross-feed directions create morphological differences.

With increasing energy density:

1.05J/mm2 produces periodic LIPSS with nano-protrusions (Fig.2b1) from phase explosion

1.65J/mm2 shows reduced protrusions and added nanoparticles (Fig.2c1)

2.25J/mm2 exhibits complete protrusion replacement by dense nanoparticles (Fig.2d1)

 

Surface Roughness of Machined Surfaces

Surface defects in laser processing are closely related to material defects and process parameters. Understanding their formation mechanisms enables better process optimization for smooth, defect-free surfaces. When laser-material interaction is thermally dominant at low energy densities (0.45J/mm2), a shallow surface melting mechanism occurs, achieving laser polishing.

As shown in Figs. 3a-b, laser-processed WC-Co surfaces (0.45J/mm2) become smoother compared to original ground surfaces with visible scratches. This improvement results from melted convex peaks flowing into adjacent valleys under capillary forces. Fig. 3f demonstrates that laser processing at 0.45J/mm2 reduces surface roughness by ~15% versus the original ground surface (Ra=0.187μm).

（a）The original grinding surface of the carbide product / (b) The energy flow density is 0.45J/mm2 /(c) The energy flow density is 1.05J/mm2

(d) The energy flow density is 1.65J/mm 2./(e) The energy flow density is 2.05J/mm 2. /(f) The influence of the energy flow density on the roughness of the machined surface

Figure 3 Three-dimensional contour morphologies of laser machined surfaces with different energy flow densities

At high energy densities (>1.05J/mm2), surface roughness increases significantly due to uniform ablation reaching WC-Co’s threshold. The molten surface develops ordered wavy microstructures (Fig.3e) from Gaussian energy distribution and thermal accumulation, causing uneven material removal. Higher energy densities intensify these laser traces, further increasing roughness.

 

Surface Elemental Changes

WC-Co composites show differential behavior:

Co binder (melting point: 1495°C) becomes unstable above 1250-1300°C

At 0.45J/mm2, EDS analysis (Fig.4) reveals preferential Co removal, with:

1.Laser-processed Co < Ground surface Co < YG8 nominal Co

2.Grinding fluid leaches Co initially

3.Low-energy laser selectively ablates Co while incompletely removing WC

(a)C?element

(b)W?element

(c) Element O

(d)Co element

Figure 4 shows the variation of elemental composition on the surface processed by laser with different energy flow densities

With increasing laser energy intensity, distinct transformations occur in the LIPSS morphology on the surface of carbide product. The uniform LIPSS distribution indicates that the ablation threshold of the WC-Co composite has been reached, resulting in stable and continuous material removal. As shown in Fig.5, the material removal depth increases progressively with higher energy density.

The laser ablation reveals compositional gradients in WC-Co along the depth direction, originating from liquid-phase sintering during manufacturing. When initial carbon content varies, cobalt migrates toward carbon during sintering, creating a cobalt gradient. EDS analysis (Fig.6a) confirms this cobalt concentration gradient from the surface to the subsurface. As laser ablation removes the cobalt-depleted surface layer and reaches cobalt-rich deeper regions, the processed surface at high energy densities shows higher overall cobalt content compared to the original ground surface.

Figure 5 The influence of energy flow density on the depth of material removal

At high energy densities, the Co binder content continuously increases, likely due to Co phase eruption and diffusion during laser processing. The thermal properties of WC particles and Co binder differ significantly: Co has lower melting (1490°C) and boiling (2927°C) points than WC (2870°C and 6000°C respectively).

Laser irradiation involves rapid heating and cooling. In the near-infrared region, WC’s absorption coefficient (~0.8) is higher than Co’s (~0.55). When irradiated, WC absorbs energy more efficiently and transfers it to surrounding Co. This heat causes structural changes in WC particle arrangements, driving molten Co to flow internally.

Due to Co’s higher thermal expansion coefficient (1.6×10??/K vs. WC’s 5.2×10??/K), the molten Co binder expands and diffuses toward the WC-Co surface. The high-temperature laser processing further promotes Co precipitation, increasing surface Co concentration.

WC-Co material removal

The influence of different energy densities varies significantly on laser processing mechanisms, surface morphology, and elemental composition changes. At low energy density (0.45J/mm2), the laser-WC-Co interaction mechanism is shown in Fig.6c, where the laser-material interaction follows a laser polishing mechanism that optimizes defects from the original ground surface. The surface morphology shows selective removal of the Co binder phase and incomplete ablation of WC forming non-uniform LIPSS structures, while the elemental composition mainly exhibits reduced Co content.

(a)Distribution of Co elements \? (b) Original grinding cross-section of WC-Co

(c) Low energy flow density WC-Co laser processing section \ (d) High energy flow density WC-Co laser processing section

Figure 6 Removal process of WC-Co material

At high energy density (1.05J/mm2), the laser-WC-Co interaction mechanism is shown in Fig.6d. The processed surface exhibits complete LIPSS structures microscopically and periodic wavy patterns macroscopically, indicating uniform melting effects with stable depth-wise material removal. Increased energy density intensifies laser traces, raising surface roughness. Elemental analysis shows higher Co content on laser-processed surfaces than original surfaces.

This occurs because depth-wise material removal reveals WC-Co’s inherent composition gradient from liquid-phase sintering. As Co-depleted surface layers are removed, underlying Co-rich regions interact with laser-induced high temperatures, causing Co precipitation that further increases surface Co content.

Both surface morphology and Co content changes significantly impact WC-Co products. Surface morphology directly affects roughness critical for quality, while Co content variations alter mechanical properties and wear resistance. Controlling laser energy density enables tailoring surface characteristics and Co distribution in WC-Co products.

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Wniosek

This study analyzes laser processing of WC-Co carbide product and discusses the relationship between laser energy density and material removal based on laser-matter interaction theory, reaching the following conclusions.

(1) Laser parameters significantly affect surface roughness. As energy density increases, material removal depth rises while surface roughness first decreases then increases.

(2) At low energy density (0.45J/mm2), the laser-material interaction mainly follows a polishing mechanism. The surface morphology shows selective Co binder removal and incomplete WC ablation forming non-uniform LIPSS structures, with reduced Co content.

(3) At high energy density (1.05-2.25J/mm2), the mechanism involves stable depth-wise material removal. The surface displays complete LIPSS structures microscopically and periodic wavy patterns macroscopically, with overall higher Co content than the original surface.