Conventional and ultrasonic assisted drilling of carbon fibre reinforced polymer/Titanium alloy stacks

Drilling through Carbon Fibre Reinforced Polymer (CFRP) / Titanium (Ti) stacks is important for mechanical assembly of aircraft, however, there are concerns over rapid tool failure and part damage, which lead to reduced productivity and hole quality. Limited research has shown that Ultrasonic Assisted Drilling (UAD) has potential to improve hole quality when drilling CFRP and Ti individually. This has attracted the attention of aircraft manufacturers to evaluate the performance of UAD for CFRP/Ti stacks applications. This thesis presents three main studies of experimental work, which investigate tool wear mechanisms that govern tool life and hole quality when drilling CFRP/Ti stacks in a single continuous operation (one-shot) using carbide (WC-Co) drills. Study 1 involved conventional drilling of CFRP/Ti stacks as opposed to drilling CFRP and Ti individually using constant cutting parameters (cutting speed = 50 m/min; feed rate = 0.05 mm/rev). Study 2 and 3 investigate the effect of cutting parameters (cutting speeds = 25, 50, and 75 m/min; feed rates = 0.025, 0.05, and 0.075 mm/rev) during conventional drilling and UAD of CFRP/Ti stacks, and the performance of UAD was also evaluated. The hole quality was assessed in terms of hole diameter, CFRP damage (delamination and pull-out) and Ti surface integrity (burr formation, roughness, and hardness). The rapid tool failure when drilling CFRP/Ti stacks was found to be due to complex tool wear mechanisms, i.e. strong Ti adhesion on the abraded cutting edges and hence cutting edge fragmentation as the adhered Ti detached. Holes with inconsistent diameters, 22% – 62% more CFRP entrance delamination, 170% – 530% more CFRP pull-out and 720% higher Ti burr (after 80 holes) were observed when one-shot drilling of CFRP/Ti stacks compared to the case of drilling the materials individually (Study 1). It was shown (in Study 2 and 3) that using a low cutting speed and a high feed rate is important for better tool life during both conventional and UAD of CFRP/Ti stacks, although this does not completely resolve hole quality issues. Ti adhesion reduced, hence cutting edges wore uniformly and gradually, which resulted in a longer tool life, consistent hole diameter and reduced Ti burr when drilling CFRP/Ti stacks with reduced cutting speed and increased feed rate due to lower heat generation and contact time. However, CFRP damage when drilling CFRP/Ti stacks was not significantly affected by cutting parameters and tool wear, rather it was found to be substantially influenced by Ti chips evacuating through the CFRP. The application of UAD on CFRP/Ti stacks did not provide any significant advantage in tool wear / life compared to conventional drilling within the range of cutting parameters and cutting tool used, although there was slight improvement in hole quality. The cutting force profiles and Ti chip morphology indicated that UAD exhibited continuous cutting, i.e. the tool did not disengage from the workpiece during drilling. Even though the use of UAD was beneficial to produce more consistent hole diameter (between CFRP and Ti of the stacks), 33% lower machined Ti surface roughness and a marginal increase in Ti hardness compared to conventional drilling, there was no significant improvement in CFRP damage and Ti burr height. When drilling CFRP/Ti stacks, tool life and productivity could not be improved by the application of ultrasonic assistance using the range of parameters investigated, and finishing operations would still be needed to improve the hole quality. UAD could be more advantageous for drilling titanium individually than CFRP/Ti stacks

The effect of varying cutting speeds on tool wear during conventional and ultrasonic assisted drilling (UAD) of carbon fibre composite (CFC) and titanium alloy stacks

The application of Carbon Fibre Composite (CFC) and titanium alloys are becoming more prevalent in aerospace industry due to their high-strength-to-weight ratio. However, the drawback of these materials is poor machinability. This paper presents the potential of Ultrasonic Assisted Drilling (UAD) of CFC and titanium Ti6Al4 V stacks in delaying tool wear progression. Experiments comparing conventional and UAD were conducted using 6.1 mm diameter tungsten carbide drills, employing constant feed rate of 0.05 mm/rev and cutting speeds of 25, 50 and 75 m/min, demonstrated that the drills used in UAD underwent lower tool wear rate and thrust forces than conventional drilling

An investigation of hole quality during drilling of carbon fibre reinforced plastic and titanium (Ti6Al4V) using tungsten carbide drills

Drilling through Carbon Fibre Reinforced Plastic (CFRP) and Titanium alloy (Ti6Al4V) in a stack is crucial for mechanical assembly of aircraft and automotive parts. This paper presents an investigation of drilled CFRP/Ti stacks hole quality in comparison to when the materials were drilled separately and individually (CFRP-only and Ti-only). The drilling trials were conducted through CFRP/Ti stacks, CFRP-only and Ti-only using carbide drills (6.1 mm diameter) at a cutting speed of 50 m/min and a feed rate of 0.05 mm/rev. The quality of the drilled holes was evaluated with respect to the hole diameters, delamination and pull-out of CFRP as well as Ti burr. It was found that drilling CFRP/Ti stacks resulted in severe Ti adhesion on the cutting edges, which caused oversized drilled holes. CFRP delamination and pull-out was found to increase by 22%�62% and 170%�530% during drilling of CFRP/Ti stacks in comparison to those produced by drilling of CFRP-only. The high damage to CFRP when drilling CFRP/Ti stacks compared to drilling CFRP-only was mainly due to sharp Ti chips, which evacuated through the CFRP plate. Furthermore, it was found that Ti burr which formed at the hole exit increased as tool wear increased. This study suggested that the hole quality and tool life when drilling CFRP/Ti stacks can be improved by improving the evacuation of Ti chip and by reducing the adhesion of Ti on the cutting edges

A metrological inspection method using micro-CT for the analysis of drilled holes in CFRP and titanium stacks

This paper demonstrates a novel method that combines X-ray computed tomography (CT) and image processing for investigating two materials with significantly different densities. CT is increasingly used in industrial applications of inspecting materials and defects. The limitations of the system and data reconstruction are continuously researched so as to improve the quality of the results. One of the most common issues in CT is beam hardening, frequently experienced in multi-material scanning. The materials examined to demonstrate the method are carbon fibre reinforced polymers (CFRP) and titanium alloy Ti6Al4V, often used in combination in industry to optimise the weight to strength ratio. The assembly of the materials is usually achieved by bolting and riveting, which requires drilling through the two materials together. The machining of these materials is difficult due to their higher specific properties and as a result tool wear is always an issue. CFRPs properties depend on the nature, orientation and bond of the fibres and as a result drilling affects their service life. The results of the method ensure the quality of the drilled holes by measuring the variation of the maximum diameter, circularity, positioning of the hole and an examining the entrance delamination and exit burrs by image processing

Ultrasonic assisted machining

A commercially available DMG MORI ULTRASONIC 65 monoBLOCK machining centre was installed in WMG in 2013 and has been primarily used to machine aerospace grade materials such as carbon fibre reinforced plastic (CFRP) and titanium alloy Ti 6Al-4V (individually and stacked) and 2000 / 6000 series aluminium alloys. Rather than discuss a single set of experimental work in detail, this paper discusses some of the issues that have been encountered when applying the technique of ultrasonic assisted machining (UAM) and some of the effects that have been observed using examples from the research conducted so far to illustrate some of the more important findings

Optimization of machining parameters during milling of carbon fibre reinforced plastics

This paper investigates the optimization of cutting parameters on tool life of carbide cutting tools during milling Carbon Fibre Reinforced Plastic (CFRP) of unidirectional fiber orientation of 90°. CFRP is one form of composite material which is highly demand by manufacturing sectors due to its light weight ratio and stronger than aluminium properties. Despite of having high demand in industry, researchers face a lot of problems during machining CFRP during assemble phase. High tool wear and short tool life of cutting tool used during machining, lead to high production cost are among the factors which concern the machinist the most. This research is carried out by using spindle speed of 1000 rpm to 8700 rpm, feed rate of 500 mm/min to 1000 mm/min and depth of cut of 1 mm to 1.5 mm which will be acted as the input variable. The output response is in form of tool wear and tool life of carbide cutting tool. In this study, CNC Mazak Milling Machine with maximum spindle speed of 12000 rpm is used to mill CFRP. Box Behnken Design under Response Surface Methodology is used as Design of Experiment to generate 15 runs. The optimization of cutting parameters for 90° is analyzed using Analysis of Variance (ANOVA). It was found that, higher spindle speed, lower feed rate and lower depth of cut lead to lowest tool wear and highest tool life. ANOVA analysis recorded that feed rate is the most significant factor that influence tool life

An experimental investigation on drilling of aluminum alloy (Al 7075) using high speed steel cutting tools

Aluminum alloys 7075 have been increasingly chosen as a main part in aerospace industry due to its property which is low in weight but having high strength. This paper focuses on the effect of cutting parameters and cutting condition when drilling Aluminum alloy 7075 in terms of tool wear, surface roughness and hole circularity. The drilling was conducted using high speed steel (HSS) having point angle of 118 ° and 6 mm tool diameter. The drilling test was conducted with a constant feed rate of 0.15 mm/min, cutting speeds of 22,44 and 66 m/min, and different cutting conditions (dry and cutting fluid). Drilling Aluminum Alloy 7075 with presence of cutting fluid and low cutting speed (22 m/min) resulted in reduced tool wear by 20 to 30 % and improved surface roughness by 30 % as compared to drilling in dry condition with same cutting parameter.

Analysis of defects on machined surfaces of aluminum alloy (Al 7075) using imaging and topographical techniques

Aluminum alloys 7075 (Al 7075) are widely used for various industrial components in which machining operations are often conducted during their manufacturing process. However, the machining operations could introduce defects on the machined surfaces of the components which will be carried over and may lead to either issues in the subsequent fabrication process or failure during the products' service life. This study investigates the machined surface's defects of Al 7075 underwent drilling operations using imaging and topographical techniques which include optical microscope, scanning electron microscope and 3D surface profiler. Surface roughness was analysed with respect to the surface defects to investigate the correlation between the roughness parameters and topographical features of the machined surfaces. The defects found on the machined surfaces of Al 7075 are microcrack, adhesion, feed mark and burr. Surface roughness was found to be highly influenced by topographical features particularly feed mark. Thus, in addition to measuring the roughness, inspection through imaging and 3D topographic techniques is important for analyzing the surface characteristic in order to determine the defects, hence deducing the detailed surface features and deformation caused by the drilling operations

Ultrasonic assisted machining

A commercially available DMG MORI ULTRASONIC 65 monoBLOCK machining centre was installed in WMG in 2013 and has been primarily used to machine aerospace grade materials such as carbon fibre reinforced plastic (CFRP) and titanium alloy Ti 6Al-4V (individually and stacked) and 2000 / 6000 series aluminium alloys. Rather than discuss a single set of experimental work in detail, this paper discusses some of the issues that have been encountered when applying the technique of ultrasonic assisted machining (UAM) and some of the effects that have been observed using examples from the research conducted so far to illustrate some of the more important findings

Drilling of 7075 Aluminum Alloys

Aluminum alloy (Al 7075) has been increasingly used as structural components in automotive and aerospace industry due to their low density, high strength and good corrosion resistance compared with other metals. To manufacture and assemble the components, drilling operations are often conducted. However, Al 7075 is ductile and soft, which causes difficulty in drilling, resulting in material adhesion, high tool wear, short tool life and poor hole quality. As a result of the poor hole quality, there is a high percentage of part rejection, which can increase the manufacturing time and cost. This chapter discusses challenges and techniques to drill Al 7075 in terms of the cutting parameters and drilling conditions to prolong the tool life and achieve good hole quality. Drilling experiments on Al 7075-T6 (heat-treated) were conducted using carbide cutting tools at various cutting parameters. Reducing cutting speed and increasing feed rate resulted in reducing tool wear, whereas a reduction in surface roughness, hence improved machined surface finish, was found when both cutting speed and feed rate were reduced in drilling Al 7075-T6. Producing good hole quality is vital during the drilling process to ensure a good assembly and product service performance