Resource consumption and process performance in minimum quantity lubricated milling of tool steel

The use of cutting fluids has driven performance improvements in metal machining. However these fluids have drawbacks including resources consumed and possible negative environmental consequences. Thus the study of coolant delivery rates is important. This work investigated the use of minimum quantity lubrication (MQL) in solid carbide milling of tool steel. A tool life improvement of 60 percent was demonstrated in comparison to dry cutting. Based on measurements and calculations made, MQL consumed cutting fluid at a rate less than 5 percent of that of a typical flooding coolant system, and was a low-consumption option in terms of electrical power

The Influence of Alloy Chemistry on the Cutting Performance and Deformation Kinetics of Titanium Alloys During Turning

Machining trials were undertaken to study how alloy chemistry influences the relative cutting performance and resulting subsurface deformation for a series of commercially available titanium alloys of increasing β content. Using an experimental orthogonal machining operation, this project focuses on studying what factors influence how efficiently a cutting insert can become embedded into a workpiece and how these factors further influence the overall cutting process

Using machining force feedback to quantify grain size in beta titanium

The fluctuating forces on the cutting tool generated during machining of β processed Ti-17 alloy are shown to contain sufficient information to enable measurement of β grain size to an equivalent accuracy of standard etching methods. Three orthogonal forces were gathered, cutting force tangential to the rotation, the force in the feed (radial) direction, and the normal force in the longitudinal axis. Each individual force produced a microstructure image with a high level of contrast but in some cases did not fully highlight all features as shown in the optical image of the equivalent area. By normalising and combining the three forces into a vector, followed by noise reduction, a high-resolution image with sufficient detail to undertake grain size measurements using the linear intercept was produced. The measured grain size differed by no more than 5% with respect the grain size measured in the etched micrograph. It is believed that the forces which have a higher proportion of elastic response in their total values, i.e., the feed and normal forces, produced the higher contrast images, indicating that elastic stresses produce the highest contrast between grains and plastic strains smear out the grain to grain variation

Non-destructive detection of machining-induced white layers in ferromagnetic alloys

Machining-induced white layers are an undesirable surface integrity feature which, due to their physical properties, can have a direct effect on the in-service performance of aero-engine components. Typically, destructive methods such as cross-sectional microscopy are used during inspection to identify white layers. This is costly, both in terms of parts sacrificed and time-consumed. A non-destructive evaluation method could speed-up inspection and allow all parts to be inspected before entering service as well as throughout the component life cycle. The present work covers the quantitative characterization of machining-induced white layers in super chrome molybdenum vanadium steel through destructive methods in addition to Barkhausen noise non-destructive testing of the same surfaces. White layers formed by machining with severely worn inserts were measured to be up to 50% harder than the bulk material, possess nano-scale grains and can have an associated compressive residual stress state of up to -1800 MPa. Barkhausen noise testing was used to show that surfaces with a white layer formed by SPD could be detected by measuring shifts in the peak frequency of the Barkhausen noise signal, caused by the compressive near-surface residual stress state associated with the formation of white layers of this type

Quantitative characterization of machining-induced white layers in Ti–6Al–4V

Machining-induced white layers can affect the functional performance of engineered components, due to the resulting mechanical and microstructural properties. Destructive inspection methods such as cross-sectional microscopy are typically used to identify white layers, however, these methods are inherently costly and time-consuming. It is, therefore, desirable to detect this anomalous surface feature using non-destructive methods which requires improved knowledge around the characteristics of white layers. The present paper reports on the characterization of white layers formed during machining of Ti–6Al–4V, to aid future development of a reliable non-destructive assessment method. The microstructure of the material in the white layer was found to have a basal α-hexagonal close packed texture and there was no evidence of an α→β phase transformation during white layer formation. The white layer has a highly refined grain structure with an increased nanohardness of up to 15% compared with the bulk material. It is proposed that white layers in Ti–6Al–4V are formed by continuous dynamic recrystallization driven by severe plastic deformation during machining. According to the measured micro-mechanical properties of the white layer, suitable non-destructive testing methods are suggested for the detection of this surface feature

Titanium alloy microstructure fingerprint plots from in-process machining

Titanium alloy components require several machining stages of forged billets which are supplied in a range of annealing conditions. Generally, the machining performance is influenced by the heat treatment and changes in billet microstructures are often overlooked by tool manufacturers and machinists. Due to the non-linear strain path during primary forging, titanium alloy billets are anisotropic in nature and require ex-situ non-destructive evaluation (NDE) during the manufacturing stages to ensure excellent service performance, particularly in safety-critical aerospace components. In this study, the local analysis of the fluctuations presented in the force response during face-turning operations is directly linked to the billet heat treatment condition and presented as microstructure fingerprint plots. The evolution of cutting forces in four different billet conditions of the alpha + beta titanium alloy Ti–6Al–2Sn–4Zr–6Mo (Ti-6246) was measured. The magnitude and fluctuations in force were directly correlated to microstructural features derived from the heat treatments. In addition, local spatial high-resolution synchronization of the cutting forces was used to determine the effects of microstructure from the heterogeneous upstream forging process and subsequent heat treatment. These rapidly produced microstructure fingerprint plots are an important development step for providing manufacturers with an in-process machining NDE method: this will help to qualify material upstream prior to expensive secondary forging or finish machining stages

The effect of forging texture and machining parameters on the fatigue performance of titanium alloy disc components

The Mechanisms of fatigue failure in Ti-6Al-2Sn-4Zr-6Mo forged discs are investigated: the effects of forging and machining operations on fatigue are decoupled. A four-point bend fatigue testing approach enabled the crack initiation and propagation characteristics to be studied at multiple locations around the disc periphery. Fatigue performance variation (of ~60%) at different positions, and crack initiation and propagation behaviour were linked to the heterogeneous crystallographic texture - developed during upstream forging. Downstream machining processes were found to increase fatigue life, regardless of the cutting speed. However, circumferential fatigue heterogeneity, inherent from the forging stage was still evident even after machining

On deformation characterisation of machined surfaces and machining-induced white layers in a milled titanium alloy

Machining-induced white layers and severely deformed layers are undesirable surface integrity features which can be formed when machining high-strength aerospace alloys. An orthogonal milling process has been designed and performed to assess the impact of cutting speeds, tool wear, cutting edge radius and climb vs conventional milling on white layer formation and plastic strain distribution. The plastic deformation in the machined surface associated with the formation of white layers in Ti-6Al-4V has been quantified using micro-grids of different length scales printed using the electron beam lithography technique. It was found that white layers formed via the severe plastic deformation mechanism, at equivalent plastic strain values in excess of 1.2 and in regions of the cutting arc with the instantaneous chip thickness of less than the cutting-edge radius and ploughing and rubbing being the dominant mechanisms. The results indicated that the magnitude of the measured strains and the depth of plastically deformed material was greater at lower cutting speeds, during climb milling and when machining with a larger cutting edge radius and tool flank wear land

Destructive and non-destructive testing methods for characterization and detection of machining induced white layer: A review paper

The presence of machining-induced white layer in the near-surface of critical aeroengine alloys has a detrimental effect on the lifetime of a component. Present techniques for identifying and characterizing white layer, such as optical microscopy and hardness testing, whilst effective, are destructive, costly and time-consuming. Non-destructive testing methods may, therefore, offer improvements to the process of white layer detection. This paper discusses the formation mechanisms and the defining physical properties of machining-induced white layers before offering a comprehensive review of the current state-of-the-art in both destructive and non-destructive testing methods for detecting this anomalous surface feature

Convalescent plasma in patients admitted to hospital with COVID-19 (RECOVERY): a randomised controlled, open-label, platform trial

Background: Many patients with COVID-19 have been treated with plasma containing anti-SARS-CoV-2 antibodies. We aimed to evaluate the safety and efficacy of convalescent plasma therapy in patients admitted to hospital with COVID-19. Methods: This randomised, controlled, open-label, platform trial (Randomised Evaluation of COVID-19 Therapy [RECOVERY]) is assessing several possible treatments in patients hospitalised with COVID-19 in the UK. The trial is underway at 177 NHS hospitals from across the UK. Eligible and consenting patients were randomly assigned (1:1) to receive either usual care alone (usual care group) or usual care plus high-titre convalescent plasma (convalescent plasma group). The primary outcome was 28-day mortality, analysed on an intention-to-treat basis. The trial is registered with ISRCTN, 50189673, and ClinicalTrials.gov, NCT04381936. Findings: Between May 28, 2020, and Jan 15, 2021, 11558 (71%) of 16287 patients enrolled in RECOVERY were eligible to receive convalescent plasma and were assigned to either the convalescent plasma group or the usual care group. There was no significant difference in 28-day mortality between the two groups: 1399 (24%) of 5795 patients in the convalescent plasma group and 1408 (24%) of 5763 patients in the usual care group died within 28 days (rate ratio 1·00, 95% CI 0·93–1·07; p=0·95). The 28-day mortality rate ratio was similar in all prespecified subgroups of patients, including in those patients without detectable SARS-CoV-2 antibodies at randomisation. Allocation to convalescent plasma had no significant effect on the proportion of patients discharged from hospital within 28 days (3832 [66%] patients in the convalescent plasma group vs 3822 [66%] patients in the usual care group; rate ratio 0·99, 95% CI 0·94–1·03; p=0·57). Among those not on invasive mechanical ventilation at randomisation, there was no significant difference in the proportion of patients meeting the composite endpoint of progression to invasive mechanical ventilation or death (1568 [29%] of 5493 patients in the convalescent plasma group vs 1568 [29%] of 5448 patients in the usual care group; rate ratio 0·99, 95% CI 0·93–1·05; p=0·79). Interpretation: In patients hospitalised with COVID-19, high-titre convalescent plasma did not improve survival or other prespecified clinical outcomes. Funding: UK Research and Innovation (Medical Research Council) and National Institute of Health Research