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

An investigation into the challenges of the point grinding machining process

Point grinding is an abrasive machining process that utilises miniature single layer superabrasive tools to remove material. The use of such small diameter tools offers advantages in the manufacturing of small or difficult to access complex 3D geometries, however, in their current state, these tools suffer from several critical challenges preventing their successful implementation. An investigation into the use of a typical commercially available point grinding tool for machining of hardened steel components has been carried out, with the aim of identifying the critical process challenges. The requirement for high rotational speeds, high tool deflection, variation in grit protrusion heights and bond layer thickness, accelerated tool wear, increased sensitivity to runout, zero cutting speed at tooltip and high tool loading have been identified as the main issues affecting the point grinding process. It is crucial that these challenges are correctly understood to facilitate future tool development

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

Evidence in context

xl, 434 p. ; 25 c

Assessment of thread-cutting strategies to enable damage-tolerant surfaces on an advanced Ni-based aerospace superalloy

The global aerospace manufacturing industry is continually developing higher strength superalloys to increase engine efficiency. With this comes the challenge of machining these difficult-to-cut materials at high productivity rates and increased surface accuracy requirements. Despite the technical challenges faced, it seems that the research area of thread cutting in aerospace alloys has generally been neglected. This paper addresses reports on a critical assessment of two thread-cutting methods (i.e. tapping and thread milling) applied to a ‘next-generation’ high-temperature Ni-based aerospace superalloy. Carrying out tool life tests revealed tapping (of small thread dimensions) to be particularly difficult to perform due to high and continuous friction incurred at the cutting edge—workpiece interface, which resulted in various surface anomalies (i.e. severe plastic deformation, laps). The optimized thread-milling strategies have, however, shown a significant improvement in tool life and surface integrity of the machined component. To support the understanding of the performances for the investigated thread-making methods, this paper discusses the interrelationship between the specific characteristics of thread tapping and milling sensory signals (cutting forces and torques) with further associations on the quality (metallurgical integrity and residual stresses) of threaded surfaces in notoriously difficult-to-cut Ni-based superalloys. </jats:p

Evolution of electroplated cBN tool surface texture parameters during point grinding

Point grinding is an abrasive machining process that utilises small diameter superabrasive single-layer grinding tools for accurate machining of complex 3D geometries. Due to the small nature of these tools, high wear rates and uneven wear around the tool circumference present a challenge for their successful application for finish machining of metallic components. It is, therefore, essential to monitor the surface condition of the point grinding tools, to ensure their safe and reliable operation. In this investigation, the 3D topography evolution of single-layer B126 cBN point grinding tools was characterised using focus-variation imaging. Given the wealth of information obtained using this method, a decision-matrix methodology was used to identify the most important parameters for monitoring the wear condition of the point grinding tools. Grinding trials were also performed with fixed cutting parameters and varied cutting durations up to 520mm3 of material removed to assess the evolution of the point grinding tool surfaces over time as a result of wear during grinding of hardened D2 tool steel. The best criteria for the characterisation of the surface texture of electroplated cBN point grinding tool surfaces were identified to be the average surface height (Sa), skewness (Ssk), root mean square gradient (Sdq), reduced peak height (Spk), peak material volume (Vmp) and developed interfacial area ratio (Sdr). These parameters performed best for direct measurement of point grinding tool surfaces, paving the way for the application of the imaging technique under manufacturing conditions as an on-machine monitoring method for performance assessment