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	Lomonosov Moscow State University, Moscow State Technological University
Citations	35
Analysis	Full AI Review Included

Executive Summary

This research investigates the use of Acoustic Emission (AE) signals to enhance the stability and productivity of Wire Electrical Discharge Machining (WEDM), particularly when processing advanced hard alloys with low thermal conductivity.

Problem Addressed: Conventional WEDM control systems, relying solely on amperage and voltage, fail to predict wire breakage in materials like WC/TiC/Co composites. This failure stems from the inability to distinguish true material removal pulses from contaminated pulses that waste energy heating electroerosion products.
Proposed Solution: Monitoring the Vibroacoustic (VA) signal spectrum provides an indirect, reliable measure of contamination in the Interelectrode Gap (IEG).
Key Diagnostic Parameter: The ratio of high-frequency (10-20 kHz) to low-frequency (1-3 kHz) AE components, termed Coefficient K_f, was identified as highly sensitive to contamination levels.
Quantitative Sensitivity: K_f decreased by 12-fold between stable cutting and the critical point just before wire breakage, demonstrating superior sensitivity compared to individual amplitude changes (high-frequency amplitude dropped >5-fold).
Physicomechanical Confirmation: The decrease in K_f correlates directly with discharge localization, excessive wire heating, “neck” formation, and the presence of wire material adhesions on the workpiece surface, confirming the mechanism of imminent breakage.
Control Implementation: K_f can be used for adaptive control, allowing the system to continuously adjust the wire electrode feed rate to maintain maximum productivity and initiate IEG flushing before the critical contamination threshold is reached.

Technical Specifications

Parameter	Value	Unit	Context
Workpiece Material	WC 88% + TiC 6% + Co 6%	%	Hard alloy composite
Wire Electrode Material	Brass CuZn37, Zn coated	N/A	AC Cut A 900
Wire Diameter	0.2	mm	WEDM setup
Wire Tensile Strength	900	N/mm²	Wire property
Wire Conductivity	22	% IACS	Wire property
AE High-Frequency Band (hf)	10-20	kHz	Diagnostic range (related to material removal)
AE Low-Frequency Band (lf)	1-3	kHz	Diagnostic range (related to phase transitions)
K_f Reduction (Stable to Breakage)	12	-fold	Ratio hf/lf sensitivity
hf Amplitude Reduction (Stable to Breakage)	>5	-fold	AE signal change
lf Amplitude Increase (Stable to Breakage)	>2	-fold	AE signal change
Initial Surface Irregularity (R_a)	16.3	µm	Low contamination state
Breakage Surface Irregularity (R_a)	39.1	µm	Critical contamination state (2.4-fold increase)
ADC Input Range	±10	V	Electrical monitoring system
ADC Bit Width	14	bits	Electrical monitoring system
AE Sampling Frequency	80	kHz	Vibroacoustic monitoring

Key Methodologies

The study utilized a specialized monitoring setup on a commercial WEDM machine to correlate electrical and acoustic signals with the physical state of the cutting process.

Experimental Setup: Machining was performed on a hard alloy composite (WC 88%, TiC 6%, Co 6%) using a Zn-coated brass wire (0.2 mm) on an Agie Charmilles CUT 1000 OilTech machine.
Signal Acquisition: Discharge current was monitored using a Hall sensor. Vibroacoustic (VA) signals were recorded simultaneously using an accelerometer fixed to the machine table near the workpiece.
Electrical Signal Processing: The current signal was conditioned (level shifted and amplified) to match the ±10 V input range of a 14-bit Analog-to-Digital Converter (ADC).
Acoustic Signal Processing: VA signals were sampled at 80 kHz. Low-frequency interference was removed via high-pass filtering. Root Mean Square (RMS) amplitude values were calculated for 0.01 s intervals.
Spectral Analysis: Amplitude Frequency Characteristics (AFC) were analyzed in octave bands: 1-3 kHz (low-frequency, lf) and 10-20 kHz (high-frequency, hf).
Diagnostic Parameter Definition: The transfer coefficient K_t (ratio of AE RMS amplitude to current RMS amplitude) and the diagnostic coefficient K_f (ratio of hf amplitude to lf amplitude) were calculated to assess process stability.
Failure Analysis: The wire electrode and workpiece surface were analyzed at the point of breakage. Surface quality was quantified by measuring residual irregularities (R_a). Elemental composition of deposits/adhesions was determined using EDX analysis (confirming Cu, Zn, O, Fe presence).

Commercial Applications

The findings enable significant improvements in the reliability and efficiency of WEDM, particularly for difficult-to-machine materials.

Advanced Tool Manufacturing: Essential for producing high-quality cutting inserts, dies, and complex components from hard, heat-resistant materials (e.g., cemented carbides, ceramics, refractory composites) where thermal stability is critical.
Adaptive Control Systems: Provides a robust, contamination-independent feedback loop for CNC WEDM machines, allowing for real-time adjustment of the electrode feed rate to maximize Material Removal Rate (MRR) while preventing wire breakage.
Process Reliability and Automation: Reduces machine downtime and material waste caused by unexpected wire breakage, facilitating the transition to fully automatic, “smart” EDM operations (Industry 4.0).
Micro-Machining: Crucial for maintaining stability when using thin wires required for high-precision, complex geometries, as these wires are highly susceptible to thermal failure.
Quality Control: Monitoring K_f serves as an immediate indicator of surface quality degradation (e.g., deep cracks and craters typical of short-circuit machining), allowing intervention before defects become critical.

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

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