J. Bonney

An overview of the machinability of aeroengine alloys

Abstract Advanced materials such as aeroengine alloys, structural ceramics and hardened steel provide a serious challenge for cutting tool materials during machining due to their unique combinations of properties such as high temperature strength, hardness and chemical wear resistance. Although these properties are desirable design requirements, they pose a greater challenge to manufacturing engineers due to the high temperatures and stresses generated during machining. The poor thermal conductivity of these alloys result in concentration of high temperatures at the tool–workpiece interface. This is worsened at higher cutting conditions because of the significant reduction in the strength and hardness of the cutting tool. This weakens the bonding strength of the tool substrate, thereby accelerating tool wear by mechanical (abrasion and attrition) and thermally related (diffusion and plastic deformation) mechanisms. Therefore, cutting tools used for machining aerospace materials must be able to maintain their hardness and other mechanical properties at higher cutting temperatures encountered in high speed machining. Tool materials with improved hardness like cemented carbides (including coated carbides), ceramics and cubic boron nitride (CBN) are the most frequently used for machining aeroengine alloys. Despite the superior hardness and cutting performance of CBN tools, ceramic tools are generally preferred for high speed continuous machining because of their much lower cost. Improvements in machining productivity can also be achieved with the latest machining techniques such as ramping or taper turning and rotary machining. These techniques often minimise or completely eliminate the predominant notching of the cutting tools, consequently resulting in catastrophic fracture of the entire cutting edge when machining aeroengine alloys.

High Productivity Rough Turning of Ti-6Al-4V Alloy, with Flood and High-Pressure Cooling

The performance of uncoated carbide tools when rough turning Ti-6Al-4V alloy were investigated under flood cooling and with 7 MPa coolant supply pressure. Up to twofold increase in tool life was achieved when machining at a speed of 80 m/min with a high-pressure coolant supply of 7 MPa relative to a conventional overhead coolant flow. The dominant tool failure mode(s) were maximum flank and nose wear. Higher tool wear rates were observed when machining with flood cooling due to excessive temperature generation at the cutting interfaces, which accelerated tool wear. There was evidence of plastic deformation on the machined surface after machining with both flood cooling and 7 MPa coolant supply at the higher speed conditions of 120 m/min. There was no evidence of surface hardening of the machined surfaces after machining in both coolant environments were investigated. This might be due to lower deformation forces that are unable to induce strain hardening of the machined surfaces.

The Effect of Argon-Enriched Environment in High-Speed Machining of Titanium Alloy

A major factor hindering the machinability of titanium alloys is their tendency to react with most cutting tool materials, thereby encouraging solution wear during machining. Machining in an inert environment is envisaged to minimize chemical reaction at the tool-chip and tool-workpiece interfaces when machining commercially available titanium alloys at higher cutting conditions. This article presents the results of machining trials carried out with uncoated carbide (ISO K10 grade) tools in an argon-enriched environment at cutting conditions typical of finish turning operations. Comparative trials were carried out at the same cutting conditions under conventional coolant supply. Results of the machining trials show that machining in an argon-enriched environment gave lower tool life relative to conventional coolant supply. Nose wear was the dominant tool-failure mode in all the cutting conditions investigated. Argon is a poor conductor of heat; thus, heat generated during machining tends to concentrate in the cutting region and accelerate tool wear. Argon also has poor lubrication characteristics, leading to increasing friction at the cutting interfaces during machining and an increase in cutting forces required for efficient shearing of the workpiece.

Burr formation in face milling of cast iron with different milling cutter systems

Abstract Two face milling cutter systems, both with PCBN (polycrystalline cubic boron nitride) tools, were used to study burr formation in high-speed machining of grey cast iron under various cutting conditions. Surface roughness parameters Ra and Ri, tool life (based on flank wear, VBmax) and burr formation (length of the burr, h) were recorded and used for comparing machining performance. The best performance in terms of tool life and surface roughness was obtained with the milling cutter system consisting of 24 teeth and 24 square wiper inserts. Machining with this cutter configuration produced acceptable surface roughness values, well below the rejection criterion, after machining a batch of 3000 motor blocks in addition to achieving a significant reduction in the burr length.

Tribological behaviour when face milling AISI 4140 steel with minimum quantity fluid application

Purpose – The purpose of this paper is to evaluate the performance of cutting fluid applied by minimum quantity technique when milling AISI 4140 steel with TiAlN coated cemented carbide inserts.Design/methodology/approach – The vegetable oil based cutting fluid evaluated was applied through a nozzle at the centre of the tool holder under vaporized conditions with a flow rate between 0 (dry cutting) and 200 ml/h, at 50 ml/h increments. Tool wear (based on maximum flank wear, VBmax), surface roughness parameters (Ra and Rt) and burr formation (length of burr, h) were recorded and evaluated. Scanning electron microscope images and energy dispersive X‐ray analysis of the worn tools show adhesion as the dominant wear mechanism.Findings – Encouraging tool performance was recorded when milling AISI 4140 steel due to improved lubrication and cooling at the cutting interfaces. Increase in cutting fluid flow rate improves tool life with gradual reduction of the surface roughness parameters and negligible influence ...

Evaluation of the performance of different nano-ceramic tool grades when machining nickel-base, inconel 718, alloy

High-speed machining of aerospace alloys can be enhanced by the use of advanced cutting tool materials such as nano-grain size ceramics that exhibit improved physical and mechanical properties than their micron grain counterparts. The performance of recently developed nano-grain size ceramic tool materials were evaluated when machining nickel base, Inconel 718, in terms of tool life, tool failure modes and wear mechanisms as well as component forces generated under different roughing conditions. The tools were rejected mainly due to wear on the tool nose. It is also evident that chemical compositions of the tool materials played significant role in their failure. The alumina base ceramics performed better than the silicon nitride base ceramics. Severe abrasion wear was observed on both rake and flank faces of the cutting tools while cutting forces increased with increasing cutting speed when machining with the silicon nitride base nano-ceramic tools. This is probably due to the lower superplastic flow temperature of the nitride base nano-ceramics. The alumina base ceramics are more susceptible to chipping at the cutting edge than the silicon nitride base ceramics despite their higher edge toughness.

Evaluation of cutting fluids using scratch tests and turning process

This work demonstrates that scratch test techniques can be used to provide a quick and cost effective evaluation of cutting fluids. Apparent coefficient of friction and specific energy for the scratch steel samples under several lubrication conditions provides a good indicator of cutting fluid performance. This is followed by evaluation of the surface finish and the cutting force of the ABNT NB 8640 steel with emulsion and synthetic cutting fluids, at 5% of concentrations, and neat mineral oil in the turning process. Comparative tests were carried out under dry and wet conditions. Results show that the linear scratch test was not efficient while the pendular scratch test was efficient tool in the classification of cutting fluids. The results can be transferred to conventional machining due to its dynamic nature.

Increasing productivity in high speed machining of Ti-6Al-4V alloy under high pressure coolant supply

Cemented carbide tools are the most common tool materials employed in the machining of titanium alloys. Polycrystalline Diamond (PCD) tools can substantially enhance the machining productivity. So, this study investigates the performance of PCD tools when machining Ti-6Al-4V alloy at cutting speeds up to 250 m min-1 with coolant delivered under pressures. The results show that longer tool life can be achieved when machining with high pressure coolant supplies up to 20.3 MPa compared with the conventional coolant flow. Up to three folds increase in cutting speed can be achieved when machining with uncoated cemented carbide tools relative to that currently used for manufacture in industry, while with PCD tools, up to five fold increase in cutting speed is achievable when machining with PCD tools.

Observations of Tool Life and Wear Mechanisms in High Speed Machining of Ti-6Al-4V With PCD Tools Using High Pressure Coolant Supply

Usage of titanium alloys has increased since the past 50 years despite difficulties encountered during machining. In this study PCD tools were evaluated when machining Ti-6Al-4V alloy at high speed conditions under high pressure coolant supplies. Increase in coolant pressure tend to improve tool life and minimise adhesion of the work material on the cutting tool during machining. Adhesion can be accelerated by the susceptibility of titanium alloy to galling during machining.Copyright