E.O. Ezugwu

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.

The machinability of nickel-based alloys: a review

Abstract This paper presents a detailed review of the various types of nickel-based alloys available on a commercial basis and their development including alloying additions as well as processing techniques employed to achieve specific mechanical and/or chemical properties. Problems associated with the machining of nickel-based alloys as well as tool wear and the mechanisms responsible for tool failure are identified and discussed. The integrity of the machined surfaces and tool life are the most important considerations during machining. These and other factors governing the machinability of nickel-based alloys are mainly affected by notching of the cutting tool, primarily at the depth of cut region, as well as by flank wear and insert chipping/fracture; and by failure modes caused singly or jointly by diffusion, attrition, and abrasion wear mechanisms in addition to mechanical and thermal fatigue loading of the cutting tools. Most of the failure modes can be minimised when machining in the presence of coolants and in an oxygen-rich environment. The use of a high pressure coolant supply, despite improved chip segmentation and machining in the presence of argon and nitrogen-rich environments, tend to accelerate tool wear rate leading to lower tool life. Improvements in tool performance can be achieved with an increase in the included angle and/or the nose angle of cutting tools due to the increased edge strength and tool–chip contact area plus a reduction in the approach angle. Recently developed cutting tool materials such as mixed oxide, SiC whisker reinforced alumina ceramics, sialon and multi-layer coated cemented carbide cutting tool material have all exhibited the capability to machine nickel-based alloys at higher speed conditions than those achieved with conventional cemented carbide tools.

Performance of cutting fluids during face milling of steels

Abstract An experimental investigation on the performance of an emulsion of mineral oil, semi-synthetic and synthetic cutting fluids when face milling AISI 8640 steel with coated cemented carbide tools were carried out. Dry cutting was also performed for comparison purpose. Tool life, power consumption and surface roughness were monitored during the machining trials. In order to study the cooling ability of the cutting fluids, cutting temperatures were measured during turning of AISI 1020 steels, using the tool–workpiece thermocouple method. The highest cutting temperatures were generated when machining dry, followed in a decreasing order, by the application of the synthetic, emulsion of mineral oil and semi-synthetic cutting fluids. A reverse effect was found in terms of the power consumption during machining. The best tool life was recorded when machining dry, followed, in a decreasing order by the application of synthetic and semi-synthetic cutting fluids. Comb cracking was the major failure mode of the cutting inserts during machining. Dry machining produced slightly better surface finish than machining in the presence of cutting fluid.

High speed machining of aero-engine alloys

Materials used in the manufacture of aero-engine components generally comprise of nickel and titanium base alloys. Advanced materials such as aero-engine alloys, structural ceramic and hardened steels provide serious challenges for cutting tool materials during machining due to their unique combinations of properties such as high temperature strength, hardness and chemical wear resistance. These materials are referred to as difficult-to-cut since 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 the concentration of high temperatures at the tool-workpiece and tool-chip interfaces, consequently accelerating tool wear and increasing manufacturing cost. The past decade has witnessed a radical approach to product manufacture, particularly in the developed economy, in order to remain competitive. Modern manufacturing philosophies, principles and techniques geared primarily towards reducing non value added activities and achieving step increase in product manufacture have been widely adopted. Recent advances in the machining of aero-engine alloys include dry machining at high speed conditions, the use of high pressure and/or ultra high pressure coolant supplies, minimum quantity lubrication, cryogenic machining and rotary (self-propelled) machining technique. Tool materials with improved hardness like cemented carbides (including coated carbides), ceramics, polycrystalline diamond and polycrystalline cubic boron nitride are the most frequently used for high speed machining of aero-engine alloys. These developments have resulted to significant improvement in the machining of aero-engine alloys without compromising the integrity of the machined surfaces. This paper will provide an overview on these recent developments and their application in the aerospace industry.

Finish Machining of Nickel-Base Inconel 718 Alloy with Coated Carbide Tool under Conventional and High-Pressure Coolant Supplies

Single-point turning of Inconel 718 alloy with commercially available Physical Vapour Deposition (PVD)-coated carbide tools under conventional and high-pressure coolant supplies up to 20.3 MPa was carried out. Tool life, surface roughness (Ra), tool wear, and component forces were recorded and analyzed. The test results show that acceptable surface finish and improved tool life can be achieved when machining Inconel 718 with high coolant pressures. The highest improvement in tool life (349%) was achieved when machining with 11 MPa coolant supply pressure at higher speed conditions of 60 m · min−1. Machining with coolant pressures in excess of 11 MPa at cutting speeds up to 40 m · min−1 lowered tool life more than when machining under conventional coolant flow at a feed rate of 0.1 mm · rev−1. This suggests that there is a critical coolant pressure under which the cutting tools performed better under high-pressure coolant supplies. Cutting forces increased with increasing cutting speed due probably to reactive forces introduced by the high-pressure coolant jet. Tool wear/wear rate increased gradually with prolonged machining with high coolant pressures due to improved coolant access to the cutting interface, hence lowering cutting temperature. Nose wear was the dominant tool failure mode when machining with coated carbide tools due probably to a reduction in the chip-tool and tool-workpiece contact length/area.

Wear of Coated Carbide Tools When Machining Nickel (Inconel 718) and Titanium Base (Ti-6A1-4V) Alloys

Nickel base, Inconel 718, and titanium base, IMI 318 (Ti-6A1-4V) alloys were machined with single (TiN) and multiple (TiN/TiCN/TiN) PVD coated carbide inserts at high cutting conditions in order to evaluate their performance and also to analyze their failure modes. The machining results show that coating material(s), tool geometry, machining parameters as well as material properties can singly or jointly affect tool performance and failure modes. Flank wear was the dominant failure mode at cutting speeds up to 42 m/min for the Inconel 718 alloy and up to 100 m/min for the IMI 318 alloy when machining at feed rates up to 0.25 mm/rev and depth of cut up to 2.00 mm. Excessive chipping/flaking off of tool particles and premature edge fracture also contributed to tool rejection, particularly when machining with the multi-coated inserts due to their sharp edges and instability of the machining system at higher speed conditions. The multi-layer coated tools generally produced lower flank wear rates, thus the best overall performance in terms of tool life, due to the higher resistance of the TiN/TiCN/TiN coatings to flank wear relative to the single TiN coating. Presented at the 53rd Annual Meeting In Detroit, Michigan May 17–21, 1998

Cooling ability of cutting fluids and measurement of the chip‐tool interface temperatures

Many machining researches are focused on cutting tools mainly due to the wear developed as a result of high temperatures generated that accelerate thermally related wear mechanisms, consequently reducing tool life. Cutting fluids are used in machining operations to minimize cutting temperature although there is no available indicator of their cooling ability. In this study, a method to determine the cooling ability of cutting fluids is proposed. A thermocouple technique was used to verify the chip‐tool interface temperature of various cutting fluids during turning operation. The method consists of measuring the temperature drop from 300°C up to room temperature after heating a standardised AISI 8640 workpiece and fixing it to the chuck of a lathe and with a constant spindle speed of 150 rpm the cutting fluid was applied to a specific point. The temperature was measured and registered by an infrared thermosensor with the aid of an AC/DC data acquisition board and a PC. The convective heat exchange coeffici...

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.

Behavior of Coated Carbide Tools in High Speed Machining of a Nickel Base Alloy

Multilayer TiN/TiCN/TiN and single-layer TiAIN PVD coated carbide tools were used to machine a nickel base, C-263, alloy at high-speed conditions in order to investigate their performance in terms of tool life, surface finish and component forces generated during machining. The test results show that the triple layer, TiN/TiCN/TiN, coated inserts gave longer tool life when machining at higher speed and depth of cut conditions while the single layer, TiA/N, coated inserts produced better surface finish. The feed forces recorded were generally higher than the cutting forces. This could perhaps be attributed to the adverse effect of burr formation and work hardening of the workpiece associated with prolonged machining. Analysis of the test results indicate that the difference in thermal properties and tribo-chemical behaviour of both the coating and substrate materials are the major factors influencing the tribo-contact at the tool-chip interface during machining. Wear mechanisms of the coating materials can also affect tool performance in terms of tool life, surface finish and component forces. Presented as a Society of Tribologists and Lubrication Engineers Paper at the STLE/ASME Tribology Conference in San Francisco, CA October 21–24, 2001