Physicomechanical Nature of Acoustic Emission Preceding Wire Breakage during Wire Electrical Discharge Machining (WEDM) of Advanced Cutting Tool Materials

At a Glance

Metadata	Details
Publication Date	2021-11-19
Journal	Metals
Authors	Sergey N. Grigoriev, Petr M. Pivkin, М. П. Козочкин, М. A. Volosova, Anna A. Okunkova
Institutions	Moscow State Technological University, University of Technology
Citations	35
Analysis	Full AI Review Included

Technical Analysis and Documentation: Acoustic Emission Monitoring for WEDM of Advanced Materials

Executive Summary

This research investigates the use of Acoustic Emission (AE) signals to stabilize Wire Electrical Discharge Machining (WEDM) of hard, low-thermal-conductivity materials, a critical challenge for processing advanced composites and diamond.

Core Problem Addressed: Conventional WEDM control systems fail when machining hard alloys (like WC/TiC/Co) because they cannot reliably distinguish between productive working pulses and detrimental short-circuit pulses caused by contamination, leading to wire breakage.
Diagnostic Solution: The study validates a novel diagnostic parameter, the coefficient Kf, defined as the ratio of low-frequency (1-3 kHz) to high-frequency (10-20 kHz) AE signal amplitudes.
Performance Metric: Monitoring Kf allows for adaptive control of the electrode feed rate, preventing wire breakage by timely flushing the interelectrode gap (IEG) before contamination reaches a critical level.
Quantitative Results: As contamination increased toward the critical point, the high-frequency AE amplitude dropped by >5-fold, the low-frequency amplitude increased by >2-fold, and the diagnostic coefficient Kf decreased 12-fold.
Surface Quality Correlation: The critical contamination level (low Kf) correlated directly with a 2.4-fold increase in surface residual irregularities (from 16.3 µm to 39.1 µm), indicating severe short-circuit machining damage (deep cracks and craters).
6CCVD Value Proposition: This research is directly applicable to the WEDM of MPCVD diamond (SCD and PCD), which presents an even greater thermal management challenge due to its extreme hardness and low thermal expansion. 6CCVD provides the necessary custom diamond materials and metalization required to advance this adaptive control technology.

Technical Specifications

Parameter	Value	Unit	Context
Workpiece Material	WC 88% + TiC 6% + Co 6%	%	Hard alloy composite (High heat resistance, low thermal conductivity)
Wire Electrode Material	Brass CuZn37 (Zn-coated)	N/A	AC Cut A 900
Wire Diameter	0.2	mm	Standard WEDM electrode size
Initial Surface Roughness (Ra)	16.3	µm	At low contamination (beginning of cut)
Final Surface Roughness (Ra)	39.1	µm	Just before wire breakage (critical contamination)
Roughness Increase Factor	2.4	fold	Increase in residual irregularities due to contamination
High-Frequency AE Range	10-20	kHz	Amplitude decreased >5-fold before breakage
Low-Frequency AE Range	1-3	kHz	Amplitude increased >2-fold before breakage
Diagnostic Coefficient Kf Change	12	fold	Ratio of lf/hf amplitudes decreased 12-fold (from ~15 to ~1)
Workpiece Dimensions (Max)	300 x 200 x 80	mm	Machine capacity (Agie Charmilles CUT 1000 OilTech)
Wire Tensile Strength	900	N/mm²	Property of the wire electrode

Key Methodologies

The experiment focused on monitoring the electroerosion process stability by correlating Acoustic Emission (AE) signals with the concentration of electroerosion products contaminating the dielectric fluid.

Material Processing: Hard alloy composite (WC 88%, TiC 6%, Co 6%) samples were cut using WEDM in organic oil.
Equipment: An Agie Charmilles CUT 1000 OilTech electroerosive cut-out machine was used.
Monitoring Setup: Vibroacoustic (VA) signals and discharge current were recorded simultaneously. Accelerometers and acoustic emission sensors were installed on the machine table near the workpiece/electrode zone.
Signal Acquisition: VA signals were sampled at 80 kHz. RMS amplitude values were determined for time intervals divisible by 0.01 s.
Frequency Analysis: AE signals were analyzed in octave bands with central frequencies of 1-3 kHz (low-frequency, lf) and 10-20 kHz (high-frequency, hf).
Diagnostic Parameter Calculation: The dimensionless transfer coefficient Kf was calculated as the ratio of the lf amplitude to the hf amplitude (Kf = A_lf / A_hf).
Validation: Surface quality (microtexture and residual irregularities) and wire material analysis (EDX of deposits/neck formation) were performed at the point of wire breakage to confirm the link between low Kf and catastrophic short-circuit machining.

6CCVD Solutions & Capabilities

This research demonstrates a critical need for advanced, adaptive control systems when machining materials characterized by high heat resistance and low thermal conductivity—properties inherent to MPCVD diamond. 6CCVD is uniquely positioned to supply the materials and engineering support necessary to replicate and extend this research into the realm of industrial diamond processing.

Applicable Materials for Advanced WEDM Research

The hard alloy used in this study (WC/TiC/Co) serves as a proxy for the extreme challenges encountered when machining diamond. 6CCVD offers materials optimized for electroerosion processes:

Optical Grade Single Crystal Diamond (SCD):
- Application: Ideal for high-precision, low-roughness WEDM applications (e.g., micro-cutters, optical components).
- Relevance: SCD presents the ultimate challenge in thermal management due to its extreme hardness and low thermal expansion, making the AE monitoring technique essential for stable processing.
- Thickness: Available from 0.1 µm up to 500 µm.
Polycrystalline Diamond (PCD):
- Application: Suitable for larger tool inserts and components requiring high wear resistance.
- Custom Dimensions: 6CCVD can supply PCD plates/wafers up to 125 mm in diameter, far exceeding the dimensions typically available for hard alloys.
Boron-Doped Diamond (BDD):
- Application: For researchers exploring the effect of enhanced electrical conductivity on WEDM stability.
- Relevance: BDD offers tunable conductivity, potentially allowing for optimization of the discharge localization and IEG stability, providing a variable parameter for validating the AE control system.

Customization Potential

The success of adaptive WEDM control relies on precise material preparation and geometry. 6CCVD provides comprehensive customization services:

Service	6CCVD Capability	Research Relevance
Custom Dimensions	Plates/wafers up to 125 mm (PCD); Substrates up to 10 mm thick.	Allows for scaling the AE monitoring technique to industrial-sized components and complex cutter geometries.
Polishing & Surface Finish	SCD: Ra < 1 nm. Inch-size PCD: Ra < 5 nm.	Essential for achieving the required dimensional quality and roughness (the paper noted a critical roughness of 39.1 µm before failure).
Metalization	In-house deposition of Au, Pt, Pd, Ti, W, Cu.	Critical for creating robust electrical contacts or specialized electrode layers on diamond substrates for advanced EDM/micro-EDM studies.
Laser Cutting/Shaping	Precision laser cutting services available.	Enables the creation of complex, thin-walled parts and micro-cutters, addressing the paper’s goal of machining complex-shaped cutters using thin wires.

Engineering Support

The development of adaptive control systems based on AE signals (like the Kf coefficient) requires deep expertise in material science, thermal dynamics, and electroerosion physics.

In-House PhD Team: 6CCVD’s engineering team specializes in the unique challenges of processing MPCVD diamond. We offer consultation on material selection, thermal modeling, and process optimization for similar WEDM Adaptive Control projects.
Process Extension: We can assist researchers in extending the validated AE monitoring approach to diamond, helping to define the critical Kf threshold for stable diamond WEDM, thus maximizing productivity and preventing catastrophic material failure.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

The field of applied wire electrical discharge machining (WEDM) is rapidly expanding due to rapidly increasing demand for parts made of hard-to-machine materials. Hard alloys composed of WC, TiC and Co are advanced cutting materials widely used in industry due to the excellent combination of hardness and toughness, providing them obvious advantages over other cutting materials, such as cubic boron nitride, ceramics, diamond or high-speed steel. A rational choice of the WEDM modes is extremely important to ensure the dimensional quality of the manufactured cutting inserts, while roughness of the machined surface on the cutting edge is of great importance with regards to the application of wear-resistant coatings, which increases tool life. However, the stock control systems of CNC WEDM machines, which are based on assessment of electrical parameters such as amperage and voltage, are unable to timely detect conditions at which a threat of wire breakage appears and to prevent wire breakage by stopping the electrode feed and flushing out the interelectrode gap (IEG) when hard alloys with high heat resistance and low heat conductivity, such as WC, TiC and Co composites, are being machined, due to the inability to distinguish the working pulses and pulses that expend a part of their energy heating and removing electroerosion products contaminating the working zone. In this paper, the physicomechanical nature of the WEDM of hard alloy WC 88% + TiC 6% + Co 6% was investigated, and the possibility of using acoustic emission parameters for controlling WEDM stability and productivity were explored. Acoustic emission (AE) signals were recorded in octave bands with central frequencies of 1-3 and 10-20 kHz. It was found that at the initial moment, when the dielectric fluid is virtually free of contaminants, the amplitude of the high-frequency component of the VA signal has its highest value. However, as the contamination of the working zone by electroerosion products increases, the amplitude of the high-frequency component of the AE signal decreases while the low-frequency component increases in an octave of 1-3 kHz. By the time of the wire breakage, the amplitude of the high-frequency component in the octave of 10-20 kHz had reduced by more than 5-fold, the amplitude of the low-frequency component in the octave of 1-3 kHz had increased by more than 2-fold, and their ratio, coefficient Kf, decreased by 12-fold. To evaluate the efficiency of Kf as a diagnostic parameter, the quality of the surface being machined was investigated. The analysis of residual irregularities on the surface at the electrode breakage point showed the presence of deep cracks and craters typical of short-circuit machining. It was also found that the workpiece surface was full of deposits/sticks, whose chemical composition was identical to that of the wire material. The presence of the deposits evidenced heating and melting of the wire due to the increased concentration of contaminants causing short circuits. It was also shown that the wire breakage was accompanied by the “neck” formation, which indicated simultaneous impacts of the local heating of the wire material and tensile forces. Due to the elevated temperature, the mechanical properties the wire material are quickly declining, a “neck” is being formed, and, finally, the wire breaks. At the wire breakage point, sticks/deposits of the workpiece material and electroerosion products were clearly visible, which evidenced a partial loss of the pulses’ energy on heating the electroerosion products and electrodes. A further increase in the contamination level led to short circuits and subsequent breakage of the wire electrode. It was shown that in contrast to the conventional controlling scheme, which is based on the assessment of amperage and voltage only, the analysis of VA signals clearly indicates the risk of wire breakage due to contamination of the working zone, discharge localization and subsequent short circuits. The monotonic dependence of WEDM productivity on AE parameters provides the possibility of adaptive adjustment of the wire electrode feed rate to the highest WEDM productivity at a given contamination level. As the concentration of contaminants increases, the feed rate of the wire electrode should decrease until the critical value of the diagnostic parameter Kf, at which the feed stops and the IEG flushes out, is reached. The link between the AE signals and physicomechanical nature of the WEDM of advanced cutting materials with high heat resistance and low heat conductivity in different cutting modes clearly shows that the monitoring of AE signals can be used as a main or supplementary component of control systems for CNC WEDM machines.

Tech Support

Original Source

DOI: https://doi.org/10.3390/met11111865

References

2018 - Sustainable production of micro gears combining micro reciprocated wire electrical discharge machining and precision forging [Crossref]
2012 - Diagnostic systems as basis for technological improvement [Crossref]
2018 - An inverse optimal control problem in the electrical discharge machining [Crossref]
2016 - The Control Platform for Decomposition and Synthesis of Specialized CNC Systems [Crossref]
2012 - Control of parameters of the cutting process on the basis of diagnostics of the machine tool and workpiece [Crossref]
2004 - Sensing and compensation of tool wear in milling EDM [Crossref]