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

International audienceS2/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 (R a and R z ), 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

S2 glass fibre reinforced epoxy composites are widely used in aeronautical applications owing to their excellent strength to weight ratio. Drilling glass fibres can be cumbersome due to their abrasive nature and poor thermal conductivity. Moreover, the use of conventional coolants is not desirable due to contamination and additional costs for cleaning the machine part. An alternative is to use environmentally friendly coolants such as liquid nitrogen (LN2) which have been previously employed in machining metals and composites. The current study investigates the effect of drilling S2 glass fibre composite in a bath of LN2. The study aims to evaluate the effect of spindle speed, feed rate and the presence of cryogenic cooling on the form and dimensional tolerances of the hole (hole size, circularity, cylindricity and perpendicularity). Design of experiments and analysis of variance (ANOVA) were used to determine the contribution of the input parameters on the analysed hole quality metrics. Results indicated that drilling S2 glass fibre in a cryogenic bath increased hole size significantly beyond the nominal hole diameter. The hole circularity and cylindricity were reduced compared to holes drilled under dry condition under all cutting parameters due to enhanced thermal stability during the drilling process. The current study aims to provide the scientific and industrial communities with the necessary knowledge on whether cryogenic bath cooling strategy provides better hole quality output compared to dry drilling and other cryogenic cooling strategies which were previously reported in the open literature

Cryogenic drilling of carbon fiber-reinforced composite (CFRP)

WOS:000491292400012In order to reduce the adverse effects on the environment and economy and to avoid health problems caused by the excessively used cutting lubrications, cryogenic machining is drawing more and more attention. In this work, a novel cryogenic machining approach was applied for drilling of carbon fiber-reinforced polymers (CFRPs). According to this approach, CFRP was dipped into the liquid nitrogen (LN2) and it was machined within the cryogenic coolant directly. Various machinability characteristics on thrust force, delamination damage, tool wear, surface roughness, and topography were compared with those obtained with dry condition. This experimental study revealed that the novel method of machining with cryogenic dipping significantly reduced tool wear and surface roughness but increased thrust force. Overall results showed that the cryogenic machining approach in this study improved the machinability of CFRP

Effects of the drill flute number on drilling of a casted AZ91 magnesium alloy

WOS: 000460131000010This experimental study aims to reveal the effect of the drill flute number on the thrust force, tool wear, tool life, chip morphology, surface hardness and microstructure in the drilling of a cast AZ91 magnesium alloy. The results showed that the cutting flute number and cutting parameters have an important effect on thrust force, tool life, wear, chip morphology, microstructure, and drilled hole hardness. When the 2-flute drill was used, less thrust force was generated during the drilling. On the other hand, less tool wear occurred with the 3-flute drill. In scanning electron microscopy (SEM) microstructural analyses, grain refinement was observed in the microstructure of the borehole surface although grain growth was also observed due to a rise in temperature. In addition, the microhardness of the borehole surface decreased as the number of holes increased, and the 2-flute drill bit provided holes with higher stiffness than the 3-flute drill bit

An experimental investigation on the influence of different surface curvatures in drilling machinability of carbon fiber reinforced plastic

WOS:000821096500001Composites used in many fields today are produced by different methods and have different surface curvatures and geometries. Thus, these materials can exhibit different behaviors in terms of machinability characteristics as well as mechanical tests. Drilling operation is an inevitable process for assembly operations of many parts, and in some cases, concave/convex composite parts must also be drilled. In this study, drilling machinability of flat, convex, and concave curved carbon fiber reinforced plastics were investigated experimentally. The machinability was evaluated considering thrust force, delamination formation, and borehole damage analysis. The experimental results demonstrate that the largest thrust force and damage occurs when drilling the flat surface, while the least ones occur when drilling the concave curved surface, in general. In addition, while the damage caused by drilling is severe on the flat surface, higher quality holes were obtained on the concave curved surface at the same cutting parameters

An experimental comprehensive analysis of drilling carbon fiber reinforced plastic tubes and comparison with carbon fiber reinforced plastic plate

WOS:000854990200001Today, carbon fiber reinforced plastic (CFRP) composites, which are the main areas of use such as aerospace and automotive, can be produced by different methods. Especially in the production of tubular composites, filament winding and roll wrapping methods are used generally, while vacuum-assisted resin transfer molding method or vacuum bagging method comes to the fore in the production of composite plates. These materials produced by different methods may exhibit different behavior in terms of machinability. In this study, the drilling machinability characteristics of CFRP tubes produced by filament winding and roll wrapping methods were investigated and compared with the CFRP plate produced with vacuum bagging and the results were presented comparatively. Experimental results showed that the composite plate generates more force and damage when drilled compared to the tubes, so its machinability is more difficult. Higher thrust force and damage occur in filament wound tube compared to roll wrapped tube. In addition, although the thrust force does not increase with the use of support in composite tubes, a significant improvement in drilling-induced delamination damage behavior is obtained. Compared to the tubes, a better borehole surface quality is obtained in the plate, while more dust chip formation is observed

The effects of stacking sequence on drilling machinability of filament wound hybrid composite pipes: Part-2 damage analysis and surface quality

WOS:000508631700060In the first part of this two-part study, filament wound hybrid composite pipes with various stacking sequences were manufactured and mechanical properties such as hardness, ring tensile strength, and burst strength were experimentally investigated. After determining mechanical properties, drilling tests were performed to research machinability characteristics. The second part of the study consists damage analysis and surface quality examination including ring test damage analysis, push-out delamination analysis, borehole damage examination and borehole surface quality. The experimental data suggested that cutting parameters, stacking sequence, and the use of back-up were impactful on the formation and propagation of various types of damages. Especially, the effect of stacking sequence was remarkable. A larger delamination area was formed in Glass-Glass-Carbon (GGC) sample after the ring tensile tests compared to Glass-Carbon-Glass (GCG) and Carbon-Glass-Glass (CGG) samples. In all cases, the utilization of back-up lead to decrease of delamination with 9-40% reduction in surface roughness. When the back-up is not used during drilling, an excessive push-out delamination occurred in all drilling tests. Moreover, CGG samples represented lower push out delamination. In addition, position of the hole depending on the winding angle plays a key role on damage formation and surface quality

The effects of stacking sequence on drilling machinability of filament wound hybrid composite pipes: Part-1 mechanical characterization and drilling tests

WOS:000518706900003In the first part of this two-part comprehensive study, mechanical properties and machinability characteristic of filament wound hybrid composite pipes with various stacking sequences of glass and carbon fibers (Glass-Carbon-Glass (GCG), Carbon-Glass-Glass (CGG), and Glass-Glass-Carbon (GGC)) were investigated experimentally. In order to determine the mechanical properties of the pipes, hardness test (Shore D), ring tensile test (ASTM D2290), and burst test (ASTM D1599) were carried out. Machinability tests were performed at various feed rates (50, 150, 250 and 350 nun/min) and spindle speeds (796, 1592, 2388 and 3184 rpm) using with and without a back-up. The results showed that stacking of the carbon layer between two glass layers (GCG) presented better performance in terms of mechanical properties and machinability characteristic. The maximum ring tensile stress of GCG specimen is 27% and 19% higher than those of GGC and CGG specimens, respectively. On the other hand, the lowest thrust forces measured during the drilling of GCG specimen while the GGC represented highest values. In addition, the use of back-up led to an increase in thrust force. The highest increase was observed in GGC sample. In GGC sample, a change in a spindle speed increased thrust force by 18-35%, while a change in feed rate increased thrust force by 20-30%

Impact of cryogenic condition and drill diameter on drilling performance of CFRP

WOS:000407700400024Machining of carbon fiber-reinforced polymer (CFRP) is a rather hard task due to the inhomogeneity and anisotropy of this material. Several defects occur in the material when CFRP is machined and machining quality deteriorates owing to these material properties. In recent years, liquid nitrogen has been considered an environmentally safe, clean, and non-toxic coolant used to cut various materials in order to enhance machinability and prevent damage during machining. In this study, a new, eco-friendly cryogenic machining technique called dipped cryogenic machining was applied for the drilling of CFRP. This experimental study investigated the effect of feed rate and drill diameter on the thrust force, delamination factor, surface quality and drill wear. Machined surfaces were analyzed in detail using a scanning electron microscope and atomic force microscope. Results indicated that the drilling of CFRP with the dipped cryogenic machining approach greatly improved machinability by reducing the surface roughness of the drilled parts and tool wear. However, it increased the thrust force and delamination factor.Commission of Scientific Research Projects of Mehmet Akif Ersoy University [0402-YL-16]This work was supported by Commission of Scientific Research Projects of Mehmet Akif Ersoy University (Project No. 0402-YL-16)

Influence of extrusion parameters on drilling machinability of AZ31 magnesium alloy

WOS:000759709900001AZ31 magnesium alloy is mainly used in aerospace applications due to its lightweight and good mechanical properties. The installation of AZ31 alloy in an aircraft usually requires machining operations such as drilling and milling for assembly purposes. The current study aims to investigate the effects of different extrusion ratios (1.77, 2.68, 3.55) on the drilling machinability of AZ31 alloys. The machinability was evaluated with thrust force, borehole surface quality, and chip characteristics. It was found that the thrust force and hole quality depend on the extrusion ratio and cutting parameters. The highest thrust force and surface roughness Ra occurred when drilling AZ31 alloy produced using the highest extrusion ratio of 3.55 in agreement with the improvement in the mechanical properties. The chip length decreased with the increase of the extrusion ratio depending on a decrease in the ductility, while SEM images showed that the AZ31 sample produced using intermediate extrusion ratio had the best hole edge quality and least burr formation