D.K. Aspinwall

Review on ultrasonic machining

Ultrasonic machining is of particular interest for the cutting of non-conductive, brittle workpiece materials such as engineering ceramics. Unlike other non-traditional processes such as laser beam, and electrical discharge machining, etc., ultrasonic machining does not thermally damage the workpiece or appear to introduce significant levels of residual stress, which is important for the survival of brittle materials in service. The fundamental principles of ultrasonic machining, the material removal mechanisms involved and the effect of operating parameters on material removal rate, tool wear rate and workpiece accuracy are reviewed, with particular emphasis on the machining of engineering ceramics. The problems of producing complex 3-D shapes in ceramics are outlined.

Workpiece surface modification using electrical discharge machining

Abstract Electrical discharge machining (EDM) is a widely used process in the mould / die and aerospace industries. Following a brief summary of the process, the paper reviews published work on the deliberate surface alloying of various workpiece materials using EDM. Details are given of operations involving powder metallurgy (PM) tool electrodes and the use of powders suspended in the dielectric fluid, typically aluminium, nickel, titanium, etc. Following this, experimental results are presented on the surface alloying of AISI H13 hot work tool steel during a die sink operation using partially sintered WC / Co electrodes operating in a hydrocarbon oil dielectric. An L8 fractional factorial Taguchi experiment was used to identify the effect of key operating factors on output measures (electrode wear, workpiece surface hardness, etc.). With respect to microhardness, the percentage contribution ratios (PCR) for peak current, electrode polarity and pulse on time were ~24, 20 and 19%, respectively. Typically, changes in surface metallurgy were measured up to a depth of ~30 μm (with a higher than normal voltage of ~270 V) and an increase in the surface hardness of the recast layer from ~620 HK 0.025 up to ~1350 HK 0.025 .

Modelling of temperature and forces when orthogonally machining hardened steel

This paper initially considers heat generation in single-point metal cutting and the direct/indirect techniques employed to measure cutting temperatures. The development of analytical models of the cutting process is briefly reviewed, including more recent work involving finite element (FE) methods. Details are given of the different FE packages and formulation methods used by different researchers. Following on from this, an FE model is presented using FORGE 2® to simulate cutting forces and temperature distributions when orthogonal turning a hardened hot work die steel, AISI H13 (52HRC), with polycrystalline cubic boron nitride (PCBN) tooling. Experimental data from infrared chip surface temperature measurements and cutting force output are used to validate the model. Good correlation was obtained between experimental and modelled results for temperature; however, the FE analysis underestimated feed force results due to a lack of adequate workpiece property data and simplistic tool/chip friction assumptions.

The surface integrity of turned and ground hardened bearing steel

The surface integrity of the machined component has received greater attention over the last 20 years, especially in relation to high performance materials. Due to the inherent differences between abrasive machining and single point cutting, distinct surface texture patterns will be produced, which together with the associated metallurgical alterations can lead to substantial variations in component service performance. The paper compares several aspects of finish turning against grinding of hardened bearing steel, more specifically surface texture, microstructural alterations, changes in microhardness, residual stresses distribution and fatigue life. The findings suggest that for the operating parameters tested the microstructural alterations observed were confined to an untempered martensitic layer often followed by an overtempered martensitic layer. Compressive residual stresses were induced when turning and grinding and the best fatigue resistance was obtained when turning using PCBN cutting tools followed by turning using mixed alumina tools and finally by grinding.

Use of ceramic tools for machining nickel based alloys

Abstract The paper is the first of two dealing with the use of ceramic tool materials for the machining of nickel based alloys. While the second contribution presents the results of detailed machinability tests, involving a cross-section of current nitride, superhard and whisker reinforced ceramic tooling products, this first paper comprehensively reviews existing literature on the subject. Following an assessment of tool wear characteristics, such as depth-of-cut notching and the underlying mechanisms involved, in particular the effects of applied mechanical stress and high interface temperatures, details are given of the composition, structure, physical properties and cutting performance of various state-of-the-art ceramic tool materials. Although only recently available commercially, whisker reinforced composite tools, comprising an alumina matrix with approximately 25% by volume silicon carbide whiskers, are reported to be capable of operating at cutting speeds up to 750 m/min on some nickel based alloys.

The Machining of γ-TiAI Intermetallic Alloys

Abstract Titanium intermetallic materials are likely to play a significant role in the production of future aeroengines. The paper details the machinabilty of a range of gamma titanium aluminide (γ-TiAl) intermetallic alloys when turning, grinding, HSM, drilling, EDM and ECM. Comprehensive literature review data is augmented with experimental results for turning, turn-milling and temperature measurement when high speed milling. Despite the ability to produce crack free surfaces when grinding and HSM, turning and drilling remain problematic. Turned surfaces are in general characterised by workpiece smearing, numerous arc shaped cracks, subsurface lamellae deformation and significant strain hardening, although the use of PCD tooling and ultrasonic assisted cutting has been shown to minimise these effects.

High Speed Ball Nose End Milling of Inconel 718

Abstract High speed machining (HSM) using ball nose end mills, is attracting interest in the aerospace industry for the machining of complex 3D aerofoil surfaces in nickel based superalloys and titanium alloys. The paper initially reviews published data detailing the effect of cutter/workpiece orientation (±βfN and ±βf), tool coatings/materials and cutting environments (spray mist, high pressure flood, etc.) on tool performance. Experimental work is subsequently detailed aimed at determining the effect of cutter orientation, tool coating and cutting environment on tool life, tool wear mechanisms, cutting forces, chip formation, cutting temperature and workpiece surface roughness, when high speed ball nose end milling Inconel 718™. A horizontal downwards cutter orientation with high pressure cutting fluid (70 bar, 26 l/min) provided the best tool life with cut lengths of ∼15 m which were twice as long as when employing a dry cutting environment. Temperature measurements from the implanted thermocouple technique, indicated that high pressure cutting fluid application substantially reduced workpiece temperature from 320°C when cutting dry, to 175°C.

The application of polycrystalline diamond (PCD) tool materials when drilling and reaming aluminium based alloys including MMC

Abstract Following a review of metal matrix composite (MMC) materials and production methods, the paper outlines the development and application of PCD cutting tools. Experimental data are presented for the drilling and single blade reaming of aluminium-silicon alloys containing 7% and 13% silicon and aluminium 2618 MMC alloy reinforced with 15 vol% silicon carbide (SiC) particulate. Though initially aimed only at aerospace and defence products, MMCs have progressively moved into higher volume applications and are currently under evaluation for volume automotive components. Compared with standard hypoeutectic and hypereutectic cast aluminium-silicon alloys, tool wear when machining MMC is shown to be up to seven times more severe. Tools used for drilling include HSS-M2, diamond plated HSS, WC, TiN coated WC and PCD. Other than when using PCD drills, tool life was extremely short due to the abrasive nature of the (SiC) reinforcement. Similarly, results from PCD reaming tests confirmed that diamond tooling provides the only realistic tooling option.

Electrical discharge texturing

Abstract The paper outlines a low cost electrical discharge machining arrangement for the texturing of steel rolls used in the cold rolling of sheet steel and aluminium. The particular electrical discharge texturing (EDT) technique used affords a high degree of process control compared with more conventional methods and, therefore, optimisation of roll texture is possible through the selection of the appropriate operating variables. Sample EDT operating data are presented covering the effects of peak current and on-time relative to roll surface roughness (Ra) and peak count (Pc). Scanning electron micrographs are included of textured roll and sheet steel surfaces together with three dimensional topographical representations and a simulation of the effects of roll wear and topography transfer. Roll surface integrity data are also given which detail the structure and hardness of the white layer produced as a consequence of the EDT process.

Electrical Discharge Surface Alloying of Ti and Fe Workpiece Materials Using Refractory Powder Compact Electrodes and Cu Wire

Abstract The paper reviews the use of metal powders dispersed in the dielectric fluid and refractory PM electrodes, to initiate workpiece surface modification during EDM. Experimental work details the effects of EDM parameters (up to 270 V) on the hardness/composition of the white layer following die sink machining of AISI H13 tool steel and roll texturing of 2% Cr steel using partially sintered PM electrodes. Similar data are presented following EDM scanning and wire cutting of standard TI alloy TI-6AI-4V and a y TIAI. With AISI H13, recast layers were 5-20 μm thick and up to ∼ 1350 HK 0.025 . When machining TI-6AI-4V with WC/Co electrodes, recast microhardness was 600-2900 HK 0.025 . Wire cutting y TIAI generated porous alloyed layers up to 115 μm thick with extensive cracks and no increase in bulk hardness.