R.C. Dewes

Surface integrity of hot work tool steel after high speed milling-experimental data and empirical models

Abstract High speed milling (HSM) using small diameter solid or indexable insert tungsten carbide end mills, with coatings such as TiAlN is now an established technology for manufacturing moulds and dies in a range of hardened tool steels. The paper presents experimental results and corresponding empirical models for workpiece surface integrity (SI) of hardened AISI H13 hot work tool steel, when HSM using solid carbide ball nose end mills coated with TiAlN. The influence of cutting speed, feed rate and workpiece angle on SI (workpiece surface roughness, microstructure, microhardness and residual stress) was studied using a full factorial experimental design with two levels of each factor. Where appropriate, empirical models were proposed in order to predict SI based on cutting parameter inputs. No significant changes in microstructure and microhardness below the machined surface were found. Within the range of cutting parameters tested, the operating parameters that gave the lowest workpiece surface roughness did not result in the most compressive residual stress distribution below the machined surface. A compromise in parameter selection is therefore necessary to achieve desired roughness and integrity.

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 .

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.

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.

Hybrid High Speed Machining (HSM): System Design and Experimental Results for Grinding/HSM and EDM/HSM

Abstract Point grinding and electrical discharge machining (EDM) are detailed on high speed machining centres. A Taguchi design was employed to evaluate operating parameters when point grinding (plunge) nickel-based superalloys with 15 mm diameter electroplated diamond and cubic boron nitride (CBN) wheels using speeds up to 60,000 rpm provided by an ancillary spindle. Material removal of up to 120 cm 3 and roughness values down to 1.2 μm Ra were obtained when machining Inconel 718. A retrofit EDM servo head unit was designed for die sinking and workpiece surface texturing. When texturing 2D and 3D workpieces, surfaces were produced with an Ra of 1-10 μm, depending on operating parameters.

Experimental Evaluation of Cutter Orientation When Ball Nose End Milling Inconel 718

Abstract High-speed machining (HSM), specifically end milling and ball end cutting, is attracting interest in the aerospace industry for the machining of complex 3D aerofoil surfaces in titanium alloys and nickel-based superalloys. Following a brief introduction on HSM and related aerospace work, the paper reviews published data on the effect of cutter/workpiece orientation, also known as engagement or tilt angle, on tool performance. Such angles are defined as ±βfN and ±βf. Experimental work is detailed on the effect of cutter orientation on tool life, cutting forces, chip formation, specific force, and workpiece surface roughness when high-speed ball end milling Inconel 718™. Dry cutting was performed using 8 mm diameter PVD-coated solid carbide cutters with the workpiece mounted at an angle of 45° from the cutter axis. A horizontal downward (-βfN) cutting orientation provided the best tool life with cut lengths ∼50% longer than for all other directions (+βfN, +βf, and –βf). Evaluation of cutting forces and associated spectrum analysis of results indicated that cutters employed in a horizontal downward direction produced the least vibration. This contributed to improved workpiece surface roughness, with typical mean values of ∼0.4 μm Ra as opposed to ∼1.25 μm Ra when machining in the vertical downward (–βf) direction.

High-speed milling of AISI H13 hot-work tool steel using polycrystalline cubic boron nitride ball-nose mills: from experimental investigations and empirical modelling to functional testing of the machined surfaces

Abstract Owing to continuous development of composite materials, the properties of polycrystalline cubic boron nitride (PCBN) can be engineered to enable their use for the manufacture of cutting edges for high-speed machining applications. This paper reports on an ‘integrative’ approach, i.e. cutting parameters—tool performance—surface integrity and residual stresses—tribological and fatigue assessment, of the high-speed milling (HSM) of AISI H13 hot-work tool steel using PCBN ball-nose end milling cutters. The experimental trials followed an experimental design (factors: cutting speed, feed per tooth, workpiece tilt angle) on which a set of key process output measures was examined: tool life, surface roughness, metallurgical integrity, microhardness, and residual stresses. Where significant variations in the output measures with the changes in the operating parameters were found, empirical models were generated; this is of particular importance for the mould/die industry to enable the prediction of critical process measures such as tool flank wear, surface roughness, and residual stresses under machined surfaces without the need for repeated measurements. It was found that only a compromise of the ‘near-best’ values of the output measures can be obtained by employing a ‘pseudo-optimal’ set of cutting parameters. Further, functional tests carried out on HSM surfaces with ‘near-best’ output measures using PCBN tools showed that superior fatigue performance (approximately 30 per cent) is expected for moulds/dies when compared with those obtained via the conventional manufacturing route (electrical discharge machining and polishing).

Grinding of Inconel 718 and Udimet 720 Using Superabrasive Grinding Points Mounted on a High Speed Machining Centre

Following a brief literature review, the paper presents initial feasibility and Taguchi data on the use of CBN and diamond mounted grinding points for the grinding of Inconel 718 and Udimet 720 nickel-based superalloys. Cutting speeds of up to 45 m/s were achieved through the use of a 60,000 rpm retrofit spindle, mounted on a high speed prismatic machining centre. With Inconel 718, tangential and normal forces were typically <30 and <80N respectively, when employing diamond abrasive at a feed rate of 2000 mm/min, depth of cut of 20μm/pass with a 10mm face width. Workpiece residual stresses were compressive, with values of up to 400 MPa in the direction of cut and microstructural damage was minimal. Subsequent L12 Taguchi experiments involving 8 factors at 2 levels, were used to identify key operating variables via main effects plots.

High Speed Milling of Hardened Steel Moulds and Press Tools

The paper outlines the use of high speed ball nose end milling for the manufacture of hardened steel moulds, dies and press tools. Information on suitable machine tools and cutting tools is given, together with relevant machinability data. The industry is characterised by large numbers of small companies (typically <50 employees) and details are given of market trends and current working practice. A case study is presented in which the traditional manufacture of a rubber mould is compared with high speed machining practice, using a spindle speed increaser, state-of-the-art coated carbide cutting tools and suitable cutter paths. Machining times were reduced from over 14 hours to less than 2 hours. Chatter occurred during high speed milling, however, component surface roughness of 1–2 μm Ra and minimal cutter wear were achieved.