Sein Leung Soo

Drilling of Titanium/CFRP/Aluminium Stacks

Following a review on the machinability of CFRP composites and multilayer stacks typically comprising metallic and composite material elements, the paper details experimental results when drilling 30 mm thick titanium/CFRP/aluminium workpiece stacks. Testing utilised a modified fractional factorial design based on an L18 Taguchi orthogonal array. This comprised four factors, three of which were at three levels and one at two levels and involved tool coating, cutting speed, feed rate and machining environment. Tools evaluated involved hardmetal and diamond coated carbide in addition to uncoated tungsten carbide drills. Response variables were principally tool wear and cutting force/torque with an end of test criteria of 300m flank wear. Peeling of the CVD diamond coating occurred within the first several holes drilled however this was not a limiting factor in terms of tool life. Principal damage occurred when drilling through the titanium (Ti-6Al-4V) rather than the aluminium (Al 7050) or CFRP (unidirectional “UD” laminates) sections. Best tool life/performance (310 drilled holes) was obtained with the more conventional uncoated carbide drills at lower cutting speed and feed rate. Typically thrust forces increased from 300 N for the first hole to ~2200 N for last hole drilled while torque values were generally below 600 N.cm for worn tools.

The Effect of Wire Electrical Discharge Machining on the Fatigue Life of Ti-6Al-2Sn-4Zr-6Mo Aerospace Alloy

The objective of this work is to investigate the dynamics of the dielectric fluid in the die sinking electrical discharge machining (EDM) process. Different methods were developed to investigate the fluid dynamics of the dielectric and particles. Both computational fluid dynamics (CFD) and experimental tests were performed. The most important achievement of this project is the improvement of the evacuation of the waste particles within the gap workpiece-electrode. An exhaustive understanding of the processes was crucial to obtain a uniform particles distribution which leads to a more efficient discharging and particle evacuation. A CFD analysis was used to figure out the characteristics for the electrode, such as its shape and dimensions and its kinematic properties. In particular, different combinations of axis Jerk, acceleration, speed and movement of the electrode were studied in detail. Different dielectric liquids were also considered. The experimental tests on full scale and increased scale models were performed at the AgieCharmilles laboratory and at the CMEFE laboratory in Geneva, in order to validate the CFD results. A test rig was built to perform study at a scale of 50:1 has been built, and a particle image velocimetry (PIV) was developed in order to study the effect of the fluid flow. The analysis of the trajectories of the waste particles inside the dielectric was performed for several configurations. The effect of the gas bubbles generated during the process is also under investigation.

The effect of peel ply layer on hole integrity when drilling carbon fibre-reinforced plastic

Fibre/matrix delamination is a common problem when cutting carbon fibre-reinforced plastic (CFRP) that can have a detrimental effect on workpiece mechanical properties and hence compromise the in-service performance of the composite part. The present paper details experimental results when drilling small holes (1.5 mm diameter) through backed and un-backed CFRP laminates using tungsten carbide twist drills to determine the effect of peel ply layers (∼100 µm thick nylon sheet attached on both sides of 3 mm thick laminates) on hole entry and exit delamination. Considerable reduction in damage relating to fuzzing, spalling, edge chipping, and delamination was observed when drilling the nylon-backed CFRP material, with scanning electron micrographs showing significantly inferior hole quality for the corresponding un-backed workpieces, particularly at hole exit.

Research on the Ultrasonic Assisted WEDM of Ti-6Al-4V

An investigation was made for application of ultrasonic vibration in electro discharge machining (EDM) and wire EDM, and only the vibration of wire was found to have been applied on WEDM. This paper studied on the effect of workpiece vibration to the WEDM process. The horizontal vibration of Ti-6Al-4V was applied on the rough cutting process of WEDM to study the variation of kerf width, machining rate and surface roughness. The results show that the workpiece vibration of WEDM has different effect with the wire vibration, which can improve the machining rate and kerf width, but increase the surface roughness.

Improving tribological and anti-bacterial properties of titanium external fixation pins through surface ceramic conversion.

In this study, an advanced ceramic conversion surface engineering technology has been applied for the first time to self-drilling Ti6Al4V external fixation pins to improve their performance in terms of biomechanical, bio-tribological and antibacterial properties. Systematic characterisation of the ceramic conversion treated Ti pins was carried out using Scanning electron microscope, X-ray diffraction, Glow-discharge optical emission spectroscopy, nano- and micro-indentation and scratching; the biomechanical and bio-tribological properties of the surface engineered Ti pins were evaluated by insertion into high density bone simulation material; and the antibacterial behaviour was assessed with Staphylococcus aureus NCTC 6571. The experimental results have demonstrated that the surfaces of Ti6Al4V external fixation pins were successfully converted into a TiO2 rutile layer (~2 μm in thickness) supported by an oxygen hardened case (~15 μm in thickness) with very good bonding due to the in-situ conversion nature. The maximum insertion force and temperature were reduced from 192N and 31.2 °C when using the untreated pins to 182N and 26.1 °C when the ceramic conversion treated pins were tested. This is mainly due to the significantly increased hardness (more than three times) and the effectively enhanced wear resistance of the cutting edge of the self-drilling Ti pins following the ceramic conversion treatment. The antibacterial tests also revealed that there was a significantly reduced number of bacteria isolated from the ceramic conversion treated pins compared to the untreated pins of around 50 % after 20 h incubation, P < 0.01 (0.0024). The results reported are encouraging and could pave the way towards high-performance anti-bacterial titanium external fixation pins with reduced pin-track infection and pin loosing.

Micro-milling of hardened AISI D2 tool steel

The paper presents an experimental investigation into the slotting of hardened AISI D2 (~62HRC) tool steel using 0.5mm diameter coated (TiAlN) tungsten carbide (WC) end mills. SEM analysis of tool morphology and coating integrity was undertaken on all tools prior to testing. Tool wear details are given based on resulting cutter diameter and slot width reduction. In addition, cutting forces are also presented together with details of workpiece burr formation. A full factorial experimental design was used with variation of cutting speed, feed rate and depth of cut, with results evaluated using analysis of variance (ANOVA) techniques. Parameter levels were chosen based on microscale milling best practice and results from preliminary testing. Main effects plots and percentage contribution ratios (PCR) are included for the main factors. Cutting speed was shown to have the greatest effect on tool wear (33% PCR). When operating at 50m/min cutting speed with a feed rate of 8µm/rev and a depth of cut of 55µm, cutter diameter showed a reduction of up to 82µm for a 520mm cut length. SEM micrographs of tool wear highlighted chipping / fracture as the primary wear mode with adhered workpiece material causing further attritious wear when machining was continued up to 2.6m cut length. All tests produced burrs on the top edges of the slots which varied in size / width to a lesser or greater degree. Under the most severe operating conditions, burr width varied from approximately 50µm to more than 220µm over the 520mm cut length. Cutting forces in general were less than 12N up to test cessation.

Measurement of Spindle Thermal Growth on a Machine Intended for Micro / Meso Scale Milling

Micro milling is gaining ground as the preferred process for the manufacture of micro/meso-scale components in conventional workpiece materials, in particular for miniature moulds and tooling inserts (~ 60HRC), for the plastics injection moulding industry. Following a brief literature review on microscale milling and associated machine tool/tooling developments, experimental results are presented in relation to spindle thermal growth for a compensated/cooled spindle operating at up to 60,000 rpm, designed to accommodate the machining of meso-scale/micro-scale components. The work involved investigation of spindle warm up and cool down rates for speeds ranging from 30,000 - 60,000 rpm and subsequently the evaluation of spindle growth using both non-contact and contact measuring systems. Growth levels of up to 16µm were detected despite active spindle cooling and the incorporation of a standard compensation algorithm within the control system. Modification to spindle acceleration and deceleration rates reduced error levels by up to 50%.

The drilling of carbon fibre composite-aluminium stacks and its effect on hole quality and integrity

The article details experimental work to evaluate tool wear, cutting forces/torque and associated hole quality/accuracy following single-shot drilling of twin layer CFRP/AA7010 stacks. A full factorial experiment was initially planned involving variation in cutting speed (60 and 120 m/min), feed rate (0.15 and 0.30 mm/rev) and drill tip geometry (double cone and flat point drills). While flank wear for the flat point drills did not exceed 40 µm even after 120 holes irrespective of operating conditions, the double cone geometry suffered catastrophic failure after only four holes at the lowest parameter combination. Therefore, the remaining tests involving the double cone drill at higher operating parameters were subsequently halted. Feed rate had a significant influence on torque in both the CFRP and Al layers, although thrust force and torque generally remained stable over the test duration. Hole diameter was typically up to 34 µm above the nominal value of 6.38 mm with corresponding out of roundness of <60 µm. Burrs were prevalent at hole exit in all tests, with an average height of ∼120 µm when drilling at the highest cutting speed–feed rate parameter combination. Similarly, the delamination factor at hole entry increased by up to 23% when operating at the higher feed rate level.

Workpiece surface integrity and productivity when cutting CFRP and GFRP composites using a CO2 laser

Following a brief literature review, results from tests involving laser cutting of carbon and glass fibre reinforced plastic (CFRP and GFRP) composites are presented. The influence of cutting speed, laser beam power and gas pressure on material removal rate (MRR), kerf width and workpiece surface integrity were investigated. Productivity was up to ~100% higher when cutting GFRP compared to CFRP, with a maximum MRR of ~8 cm3/min achieved when operating at a cutting speed of 1750 mm/min, 2500 W beam power and gas pressure of 5 bar. Charring and melting of the matrix phase was observed in both materials and similarly surface voids/cavities were evident on the CFRP and GFRP samples. Three-dimensional topographic maps also revealed the presence of grooves on the latter, which would explain the significantly higher surface roughness levels obtained (up to ~13µm Ra). Heat affected zones were visible in the majority of CFRP specimens assessed which extended to a depth of ~1.5mm (depending on the fibre orientation) while only minor damage in terms of fibre protrusion was apparent in corresponding GFRP workpieces. Kerf widths decreased with increasing cutting speed and were typically over 2 times larger in the GFRP material.