Microstructural investigation and hole quality evaluation in S2/FM94 glass-fibre composites under dry and cryogenic conditions

Abstract

S2/FM94 glass fibre reinforced epoxy is an aerospace-grade composite currently bonded with aluminium alloys and installed in parts of the Airbus A380 fuselage. In addition to its abrasive and hard nature, S2/FM94 glass fibre is sensitive to thermal effects developed during the drilling process, and therefore using coolants becomes necessary. However, conventional oil and water-based coolants are not suitable for drilling of composites. Cryogenic coolants on the other hand are an attractive choice for machining composites and are environmentally friendly. In this study, a new environmentally friendly cryogenic cooling technique in a liquid nitrogen bath was used for the drilling of S2/FM94 glass fibre reinforced epoxy composite. The aim was to investigate the effect of drilling parameters and cryogenic cooling on cutting forces, surface roughness, hardness and delamination factor at hole entry and exit sides. The workpiece was drilled within a cryogenic bath. In this way, both cryogenic workpiece cooling and tool cooling were obtained. In addition, the drill geometry is fixed and only the cutting parameters (i.e. spindle speed and the feed rate) are varied under dry and cryogenic conditions. The results indicate that the spindle speed and cryogenic cooling had the most significant influence on the cutting forces and surface roughness parameters (Ra and Rz), while the use of cryogenic cooling had the most significant influence on increasing the hardness and size of delamination at entry and exit sides of the holes.

The effect of cryogenic machining of S2 glass fibre composite on the hole form and dimensional tolerances

“…However, a conclusion can be made here that using cryogenic bath cooling during drilling S2 glass fibre composites would results in oversized holes compared to dry conditions or other cryogenic cooling strategies. This could be attributed to the increased hardness and cutting forces when using cryogenic bath cooling as reported in a previous study on drilling S2 glass fibre composites in a cryogenic bath [55]. Another reason for increased hole size using cryogenic bath creates an extremely low temperature environment for both the cutting tool and the workpiece which restricts the rise of cutting temperatures and prevents possible thermal expansions of both tool and workpiece, i.e., cutting occurs at − 196°C or close to it.…”

“…The measurements for hole size and circularity were taken at 1 mm below and above the top and bottom surfaces of the workpiece. The two positions are named top and bottom thereafter [12] in the manuscript discussion section. Additional measurement was taken at the middle of the workpiece in order to get measurements for the hole circularity and perpendicularity.…”

“…There are a significant number of studies in the open literature on drilling glass fibre composites. A table summary on previous research was previously reported by Giasin et al [12]. Previous researchers studied the effects of cutting parameters [13][14][15][16][17], drill type, geometry and coating [15,[18][19][20] on developed cutting forces [21][22][23][24], tool wear [22,25,26] and several hole quality metrics such as delamination at exit and entry sides of the hole [18,24,27,28], surface roughness [14,27,29], hole size and circularity [30][31][32].…”

“…Other studies also reported that cryogenically treated cutting tools or using chilled air reduced delamination factor when drilling GFRP and CFRP [53,54]. After extensive review of literature, on drilling GFRP, it was found that there are no studies on the drilling of S or S2 glass fibre composites are available in the open literature except for one study conducted by Giasin et al [55]. In their study, results showed that the cutting forces and hardness increased when drilling in a bath of LN 2 compared to dry drilling, while the surface roughness was reduced.…”

Between-the-Holes Cryogenic Cooling of the Tool in Hole-Making of Ti-6Al-4V and CFRP

“…An experimental investigation carried out on rotary ultrasonic machining of CFRPs being performed in a cryogenic environment concludes that an increase in cutting speed results in a reduction in thrust force because of an abatement in the axial stress [18]. A study carried out on quantifying the effects of drilling parameters and cryogenic cooling on the performance measures of hole-making in glass fiber reinforce epoxy composites suggests that the cryogenic cooling significantly increases microstructural hardness and reduces delamination factor [19]. It is further reported that the cutting speed also needs to be fine-tuned along with the supply of a cryogenic fluid for achieving substantial reductions in surface roughness of the drilled holes.…”