Influence of grain size and cobalt content on machinability of tungsten carbide with diamond-coated tools
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2025-01-01 |
| Journal | Materials research proceedings |
| Authors | Marco Diegel |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- Feasibility Confirmed: Demonstrated process-reliable orthogonal cutting of challenging tungsten carbide (WC) grades, including the hardest material tested (6 wt.% Co, 1743 HV30), using CVD diamond-coated tools, provided a reduced undeformed chip thickness (3 ”m) is utilized.
- Self-Sharpening Mechanism: A favorable self-sharpening effect occurs due to the unavoidable flaking of the diamond coating on the rake face, resulting in an approximate 60% reduction in the cutting edge radius and improved cutting conditions.
- Material Composition Influence: The occurrence of coating flaking (and thus self-sharpening) is delayed as the hardness of the WC material decreases (i.e., increasing grain size and cobalt content).
- Thermomechanical Load: Increasing grain size and cobalt content reduced the thermomechanical process load, which correlated with the delayed onset of the self-sharpening effect.
- Laser-Treatment Benefit: Pre-treating the tool by laser-removing the rake face coating significantly reduced the wear-related increase in force components over the cutting path, particularly beneficial for the high-hardness 6 wt.% Co carbide.
- Force Reduction: Increasing the cobalt content from 6 wt.% to 15 wt.% in fine-grain WC reduced the cutting normal force by approximately 34%, indicating that cobalt content has a greater influence on mechanical tool load than grain size.
- Thermal Safety: Measured tool temperatures remained low (100 to 200 °C), well below the 750 °C threshold for diamond graphitization, ensuring coating integrity on the flank face.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Workpiece Material 1 (CTF12D) | 6 | wt.% Co | Fine grain WC |
| Hardness (CTF12D) | 1743 | HV30 | Fine grain, 6 wt.% Co |
| Average Grain Size (CTF12D) | 0.49 | ”m | Fine grain, 6 wt.% Co |
| Workpiece Material 3 (CFG-CTM30) | 1.28 | ”m | Medium grain, 15 wt.% Co |
| Hardness (CFG-CTM30) | 1138 | HV30 | Medium grain, 15 wt.% Co |
| Diamond Coating Thickness | 15 | ”m | Microcrystalline CVD coating |
| Cutting Speed (vc) | 150 | m/min | Orthogonal cutting tests |
| Undeformed Chip Thickness (h) | 3 & 5 | ”m | Varied based on material |
| Tool Rake Angle (γ) | 0 | ° | Standard tool geometry |
| Tool Clearance Angle (α) | 12.5 | ° | Standard tool geometry |
| Tool Life Stop Criterion | 10 | m | Total cutting path (100 cuts) |
| Max Measured Tool Temperature | 200 | °C | Side surface measurement |
| Diamond Graphitization Limit | 750 | °C | Temperature threshold for diamond failure |
| Normal Force Reduction (Co increase) | 34 | % | Fine grain WC (6 wt.% Co to 15 wt.% Co) |
| Cutting Edge Radius Reduction | 60 | % | Achieved by coating flaking (self-sharpening) |
| Effective Rake Angle Increase | 20 | % | Achieved by coating flaking (self-sharpening) |
Key Methodologies
Section titled âKey Methodologiesâ- Experimental Platform: Orthogonal cutting tests were performed on a Forst RASX 2200x800x600 M / CNC vertical broaching machine, simulating defined cutting edge machining kinematics.
- Tool Preparation: Tools were manufactured from carbide substrates and coated with a 15 ”m thick microcrystalline CVD diamond layer. A subset of tools was laser-treated to remove the coating from the rake face, creating a pre-sharpened condition for comparative analysis.
- Workpiece Selection: Three commercial tungsten carbide grades (CTF12D, CTF30D, CFG-CTM30) were selected, covering fine to medium grain sizes (0.49 ”m to 1.28 ”m) and low to high cobalt contents (6 wt.% to 15 wt.%).
- Process Environment: All cutting experiments were conducted dry (without cutting fluid supply), reflecting industrial application conditions.
- Parameter Control: Cutting speed (vc) was held constant at 150 m/min. Undeformed chip thickness (h) was varied between 3 ”m and 5 ”m to investigate the transition between reliable and unreliable cutting regimes.
- Force and Wear Monitoring: Cutting force (Fc) and cutting normal force (FcN) were continuously measured using a Kistler 9257B dynamometer to track tool wear progression and the effect of coating flaking.
- Thermal Analysis: Tool temperature was monitored using a FLIR SC7500 thermography camera, focusing on the cutting edge region immediately after the cut to capture maximum thermal load.
- Microstructural and Kinematic Analysis: Chip formation was recorded using a high-speed camera (Vision Research Phantom v7.3). Tool microgeometry (cutting edge radius, K-factor) was measured using an Alicona Infinite Focus G5 before and after coating flaking.
Commercial Applications
Section titled âCommercial ApplicationsâThe findings directly support the advancement of high-efficiency machining processes for extremely hard materials used in critical industrial components.
- Die and Mold Production: Enables the efficient milling of tungsten carbide components, which are preferred for extended tool life and superior product quality in forming and stamping applications.
- Toolmaking and High-Performance Cutting Tools: Provides guidelines for selecting optimal tool design (laser-treated vs. untreated) and process parameters (chip thickness) when machining high-hardness WC grades (e.g., those with less than 8.5 wt.% Co).
- Precision Machining of Hard Materials: Offers a viable, high-productivity alternative to conventional methods like Electrical Discharge Machining (EDM) and grinding for achieving geometrically defined cutting edges on cemented carbides.
- CVD Diamond Tool Optimization: The research provides critical data on the interaction between CVD diamond coatings and WC substrates, allowing for the development of more robust and reliable diamond-coated milling tools.
- Aerospace and Automotive Manufacturing: Applicable in the production of high-stress components where materials like fine-grain, low-cobalt WC are required due to their exceptional hardness and wear resistance.
View Original Abstract
Abstract. Tungsten carbide is an important material for manufacturing cutting tools, molds and dies due to its combination of high hardness and high resistance to mechanical loads and wear. The primary restriction associated with utilizing this material pertains to the time-consuming and costly machining process, which is attributed to its low machinability. As an alternative to conventional electro discharge machining, milling with diamond-coated carbide tools has shown promising results. Due to a lack of knowledge about the interactions between the tungsten carbide to be machined, the tool and the process parameters, the economic advantages of the milling process are currently not fully utilized. In application of the cutting tools, a self-sharpening effect due to flaking of the coating on the rake face occurs, which results in improved cutting conditions. The present study addressed the prevailing knowledge cap regarding the interaction between material composition, parameter selection and tool design. Tungsten carbides with different compositions were machined in orthogonal cutting tests. In the investigations, increasing grain size and cobalt content caused a reduced thermomechanical process load and therefore delayed a favorable self-sharpening effect.