Electrical discharge machining (EDM) of CFRP composites

Abdallah, Ramy Mohamed Abdelnaby Metwally (2020). Electrical discharge machining (EDM) of CFRP composites. University of Birmingham. Ph.D.

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Abstract

The thesis details research to evaluate the feasibility of wire electrical discharge machining (WEDM) as an alternative to conventional cutting (milling/routing/slotting) of carbon fibre reinforced plastic (CFRP) composites. A comprehensive literature review of various nonconventional machining processes including laser beam machining, abrasive water jet machining, ultrasonic machining and electrical discharge machining revealed that there was comparatively limited work on WEDM of CFRP. In order to address this gap in knowledge, 3 primary phases of experimental work were undertaken to investigate the influence of operating parameters, electrode material, workpiece fibre orientation and stacking with metallic layers when WEDM of CFRP laminates.

Phase 1 trials were aimed at studying the effect of key process parameters (open gap voltage, ignition current as well as pulse on and off times) when WEDM unidirectional CFRP plates (all plies laid up in the same orientation) using a 0.25 mm diameter zinc coated brass wire (Topas plus D). The tests were divided into 2 sub-phases, both utilising a L18 Taguchi fractional factorial experimental design with Phase 1A and 1B involving machining parallel and perpendicular to the fibre direction respectively. The main results revealed that the maximum material removal rate (MRR) achieved was higher when cutting parallel to the fibre direction (2.41 mm\(^3\)/min) as opposed to the perpendicular orientation (1.94 mm\(^3\)/min), albeit at the expense of marginally wider (3.33%) kerf widths (Wa) and higher (38.6%) workpiece surface roughness (Sa). Ignition current and pulse off time were statistically significant factors with respect to MRR for both cutting directions in addition to open voltage when machining perpendicular to fibre orientation. Conversely, open voltage was the only major factor affecting roughness.

In Phase 2, similar experimental designs were utilised when machining parallel (Phase 2A) and perpendicular (Phase 2B) to the fibre directions, but performed using a dual coated (copper and β phase) steel core wire electrode (commercially named as Compeed). Considerably higher MRR of up to 4.7 mm\(^3\)/min was observed when operating parallel to the fibres compared to corresponding results in Phase 1, while no appreciable improvement was obtained when cutting in the perpendicular direction (maximum MRR of 1.68 mm\(^3\)/min). However, a marked increase in both kerf width (up to ~ 367 μm) and surface roughness (up to 26.29 μm Sa) was evident in Phase 2A trials. Despite no signs of major defects on the kerf edges, there was visible evidence of frayed fibres, areas of redeposited resin and matrix/fibre loss on the machined surface. Pulse off time had a significant influence on workpiece roughness and recorded the highest percentage contribution ratio (PCR) of 40.55% according to the analysis of variance (ANOVA).

Preliminary trials to evaluate the influence of stacking UD-CFRP laminates between copper plates to improve workpiece conductivity and machining performance was undertaken in Phase 3A using both coated brass and steel cored wires. Surprisingly, a marginally higher MRR of 6.5 mm\(^3\)/min was obtained when machining with the Topas wire compared to 6.11 mm\(^3\)/min for the Compeed wire. This was possibly due to lower process stability when employing the Compeed wire as a result of greater resin debris in the spark gap. As wire breakage frequently occurred during the preliminary trials, the maximum values of voltage and pulse on time were reduced while pulse off time was increased in Phase 3B, where a response surface methodology (RSM) experiment was designed to study the effect of wire type, ignition current and pulse off time on MRR and surface roughness. While the Topas wire achieved a somewhat higher MRR of 5.89 mm\(^3\)/min compared to Compeed (5.35 mm\(^3\)/min), all of the factors were statistically significant at the 5% level with regard to MRR. In contrast, none of the variable parameters had a major influence on surface roughness. Notwithstanding the increased MRR, machining of the metallic-CFRP stacked configuration induced severe delamination on the top/bottom surfaces and kerf edges with frayed fibres. Results from a multi-objective optimisation analysis for MRR and roughness indicated that a current of 4 A and pulse off time of 8 μs were preferred when using the Topas wire.

In Phase 3C, multidirectional CFRP workpieces were machined using both wire electrodes, with MRR values ranging between 7.42 and 14.82 mm\(^3\)/min and from 5.29 to 13.31 mm\(^3\)/min for Topas and Compeed wires respectively. Pulse off time was the sole significant factor with a PCR of 67.76%, with a strong correlation observed between MRR, machining variables and their interactions based on a coefficient of determination (R\(^2\)) of 99.79%. The average kerf width (Wa) and Sa when machining with the Compeed wire was up to ~374 μm and 27.53 μm respectively, but which was marginally lower when using the Topas wire (~347 μm and 24.86 μm). Based on the ANOVA for kerf width (Wa), wire type and its interaction with pulse off time were significant terms, with the model for predicting Wa having a R\(^2\) of 99.88%. Although none of the variables were significant with respect to surface roughness, the corresponding model demonstrated a strong correlation between predicated Sa and experimental data with a R\(^2\) of 96.24%.

Generally, this study proved the feasibility/capability of WEDM in cutting thick CFRP composites in addition to identifying the preferred levels of key process variables and wire electrode types to achieve successful machining. While the stacking configuration of UD-CFRP using copper plates enhanced machining productivity, excessive damage was however observed. Machining parallel to workpiece fibre orientation resulted in higher MRR, Wa and Sa compared to perpendicular orientation, particularly when using the Compeed wire. However, the Topas wire revealed superior performance when machining UD-CFRP/Cu stacks and multidirectional CFRP workpieces.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):