Richard Hood

Radius end milling of Haynes 282 nickel based superalloy

An experimental investigation is presented involving radius end milling of a new nickel-based superalloy; Haynes 282. When operating with coated carbide inserts at high cutting speed and a feed rate of 0.1 mm/tooth, flank wear measured 213 µm after 45 min of machining. Doubling the cutting speed or feed rate typically caused a pro rata reduction in tool life. The lowest operating parameters resulted in the best surface roughness with levels ranging 0.15–0.3 µm Ra. Microhardness depth profile data was similar for all tests with a hardened region of up to ~50 HK0.05 higher than the bulk value of ~480 HK0.05, which extended to a depth of up to ~50 µm from the workpiece surface. Surface/subsurface cross-sectional micrographs showed grain deformation/damage to a maximum depth of ~15 µm.

High-speed ball nose end milling of burn-resistant titanium BuRTi alloy

Following a brief introduction to aero-engine materials and the development of BuRTi alloy (Ti-25V-15Cr-2Al-0.2C), the paper details a statistically designed machinability experiment involving high-speed ball end milling. Testing utilised 8 mm diameter AlTiN-coated carbide ball nose end mills in a Taguchi L8 fractional factorial design with six factors, each at two levels. Output measures related to tool life/wear, cutting forces, workpiece surface roughness, microstructure and microhardness. Main effect plots, tabulated ANOVA data, percentage contribution ratio (PCR) values together with graphical and SEM data are presented. Use of the lowest material removal rate, high-pressure (70 bar) cutting fluid and a workpiece orientation of 45° resulted in the longest tool life with a machining time of ~60 min; however, surface roughness was poor, and there was smeared/adhered material to a depth of 20 μm. Additionally, carbide fracture/pull-out was observed near the workpiece surfaces whereas microhardness depth profiles from sectioned, mounted and polished samples showed a moderate increase in surface hardness of ~80HK0.25 above the bulk value.

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.

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.