Machining Induced Surface Integrity Research Articles

On machining-induced surface integrity of Inconel 718 fabricated by powder bed fusion

Additive manufacturing (AM) introduces unique grain feature and anisotropy in metals, as well as relatively poor surface quality, so the knowledge about the formation of post-machining-induced surface integrity needs to be extended. This study aims to understand the machining phenomena in orthogonal cutting of Inconel 718 produced via powder bed fusion of metals using a laser beam system (PBF-LB/M), paying particular attention to the cutting energy, chip formation, geometrical defects, and microstructural deformation. Experiments are conducted on both wrought (W 718) and PBF-LB/M (PBF 718) Inconel 718, whereby the cutting speed and the anisotropy of the workpiece in the raw state play an important role. The results indicate that less cutting energy is consumed when cutting PBF 718 in the direction perpendicular to its build direction (BD), followed by cutting in its BD. Denser shearing-induced slip lines and relatively continuous chip morphology are found when cutting PBF 718 in its BD, moreover, the effect of cutting speed on shearing deformation of columnar grains under this condition is analysed. More importantly, this study identifies a significant geometrical defect phenomenon which is unique to PBF 718 and highly depends on the cutting speed and anisotropy of grain feature. A detailed characterisation shows that this behaviour is caused by tearing within the grains in the matrix material and not by the presence of hard inclusions, which has rarely been reported in the literature. The microstructure in the subsurface area exhibits that the density of grain boundaries in the cutting direction plays the key role in plastic deformation, as the machining process mainly causes dragging behaviour in the area below the machined surface. Thus, cutting PBF 718 in the direction perpendicular to its BD shows similar deformation layer to W 718, while cutting PBF 718 in its BD displays much stronger subsurface deformation. The obtained results greatly help to understand the cutting phenomena in post-machining of additively manufactured Ni-based components, especially the integrity for a better functional surface.

The effect of workpiece grain size on surface integrity in orthogonal cutting of Inconel 718

Inconel 718 represents a large portion of high-temperature metals in key industries, where the machining-induced surface integrity is quite important. As the different grain size results in various densities of grain boundaries in the deformed area, the surface integrity can be significantly affected. This paper studies the effect of workpiece grain size on machining-induced surface integrity when cutting Inconel 718. Workpiece materials with fine or coarse grain features are prepared with various heat treatment methods. Orthogonal cutting experiments are conducted with uncoated carbide tools under different cutting speeds (20 - 100 m/min). The cutting force results indicate that Inconel 718 with larger grain size will cause lower cutting forces and the percentage reduction approximately ranges from 20 % to 33 %. An interesting phenomenon is found that the workpiece with coarse grains exhibits large-scale geometrical defects due to tearing within the grains at low cutting speed (vc = 20 m/min). This phenomenon can be diminished if the cutting speed vc is increased to 60 m/min. In addition, the subsurface deformation layer is reduced with decreasing workpiece grain size due to the barrier effect of grain boundary on plastic deformation.

Combining turning with slide burnishing to improve surface integrity and stress corrosion resistance

The surface characteristics play a pivotal role in stress corrosion cracking (SCC) resistance and fatigue life. Strengthening and modifying the material surface properties during manufacturing is a promising approach to improving the component service performance. Therefore, this study proposes a new surface finishing and strengthening mode that combines turning with slide burnishing (TSB) with a common turning tool. The core idea is that the cutting edge of the tool removes the material surface layer as a chip while the tool corner with a large radius performs a slide burnishing process on the manufactured surface. The machining-induced surface integrity in terms of residual stress, surface roughness, micro-hardness and microstructure changes was examined, and the SCC susceptibility was tested in a strong corrosion environment. The results showed that the surfaces processed by TSB suffered lower biaxial residual stresses, higher micro-hardness and deeper hardened layer when compared with the traditional turned surfaces. Besides, it was observed the TSB processed surfaces were less susceptible to SCC initiation which could be attributed to the lower residual stress introduced by TSB. Additionally, the multi-pass TSB treated surfaces showed significant low SCC susceptibility. Consequently, this work suggests that the surface integrity and the service performance like SCC resistance can be improved by TSB.

Dosage Based Directional Minimum Quantity Fluid Application in Machining

There is an ever-increasing focus on surface integrity and machined surface quality of the machined surface and sub-surface to improve product quality, thereby improving its service life and sustainability. Machined sub-surface quality is influenced by the cutting tool system, cutting conditions and the cutting fluid system. However, past work on the influence of cutting fluid application, especially targeted application of minimal quantity fluid (MQF; varying minimum quantity of coolant and lubricant) which has never been addressed on surface integrity, is limited or non-existent. Thus, an experimental study involving face turning of AISI 1045 steel with flat-faced uncoated carbide tool was conducted with different cutting fluid combinations and at varied locations in the tool–workpiece space. This study is aimed to address machining performance effects (tool–chip contact area, cutting forces, etc.) and machining induced surface integrity (surface roughness and residual stresses). Results showed that the coolant application on the tool flank face improved the surface integrity. In addition, results emphasized the importance of targeted MQF on surface integrity and cutting tool performance, thus providing a cleaner and more efficient machining process.

Wear resistance enhancement of Inconel 718 via high-speed ultrasonic vibration cutting and associated surface integrity evaluation under high-pressure coolant supply

Machining-induced surface integrity evaluation and functional performance are considered as the most representative problems for evaluating the machining quality of Ni-based superalloys (for example, Inconel 718). Although high-speed ultrasonic vibration cutting (HUVC) has been proved to be effective for improving the machinability of Inconel 718, the surface integrity and its effect on functional performance (for example, wear resistance) are still unclear. By applying scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and white light interferometry (WLI), the surface integrity induced by HUVC and the wear resistance of the machined surface were comprehensively evaluated; the grain refinement, hardness increase, and wear mechanism of the machined surface were thoroughly studied, which were compared with the conventional cutting (CC) process. Compared with the CC process, the HUVC process achieves superior grain refinement, thicker surface plastic deformation layer, lower surface roughness Ra and Rz, higher hardness and compressive residual stress, and thicker hardening layer. The wear rate of the machined surface processed by HUVC is reduced by at most 24.01%. This study shows that the HUVC process can be potentially used to improve the wear resistance, which can effectively achieve grain refinement, surface roughness reduction, and wear resistance improvement while reducing the machining cost and process procedure.

Dry Hard Turning versus Grinding—The Influence of Machining-Induced Surface Integrity on Fatigue Performance

Dry hard turning (DHT) provides an effective process for finishing high-hardness materials. Machining-induced surface integrity has a direct impact on functional performance. This study compares the effects of the DHT and grinding processes on machining-induced surface integrity and fatigue performance of 18CrNiMo7-6 steel. The DHT and grinding experiment were carried out by using a polycrystalline cubic boron nitride tool and corundum wheel, respectively. The 3D surface morphology, surface roughness, surface residual stress, and machining accuracy of the hourglass-shaped specimen were measured. The fatigue fracture was characterized by scanning electron microscopy. The experimental results show that compared to grinding, DHT has obtained a larger surface compressive residual stress (the maximum axial and tangent residual stresses are −762.6 MPa and −442.8 MPa, respectively) and a lower surface roughness (the minimum Ra and Rq are 0.172 μm and 0.230 μm, respectively). This study is an attempt to use DHT instead of a grinding process to finish 18CrNiMo7-6 steel, providing a reference for high-quality and sustainable manufacturing of hardened steel.

Influence of sub-surface deformation induced by machining on stress corrosion cracking in lead-free brass

New stricter regulations on lead (Pb) content in brass for use in certain applications is driving the industry from traditional leaded brass towards Pb-free alloys. However, machining induced surface integrity for such Pb-free alloys and related corrosion resistance are largely unknown. Two Pb-free brass alloys, CuZn38As and CuZn21Si3P, approved for use in drinking water applications, were machined under different cutting conditions, tool geometries and tool wear states. The resulting surface integrity and sub-surface deformation was characterized using nano-indentation, scanning electron (SEM) and ion microscopy, and electron backscatter diffraction (EBSD). The materials resistance to stress corrosion cracking (SCC) was assessed by exposing the machined samples to a corrosive substance in accordance with SIS 117102. The results show that tool wear is the most influencing parameter leading to stronger sub-surface deformation. This was especially pronounced for alloy CuZn38As, where for equivalent depth of deformation, the material exhibited higher degree of work-hardening compared to the other tested alloy. Subsequently, substantial stress corrosion cracking was registered for machined CuZn38As samples.

Effect of laser shock peening on boring hole surface integrity and conformal contact fretting fatigue life of Ti-6Al-4 V alloy

The service performance of hole surface is directly tied to its machining-induced surface integrity which is usually produced by sequential operations. Due to its conformal geometry, the assembly hole has a high risk of fretting damage when connecting parts. This paper investigated the effect of laser shock peening on the surface integrity of boring hole and fretting fatigue life of Ti-6Al-4 V alloy. The results showed that the fretting fatigue life of the specimens was dramatically improved at various fatigue stress levels after LSP. The fretting fatigue life was respectively increased by 14 %, 2.27, and 9.35 folds at fatigue stress levels of high (800 MPa), intermediate (500 MPa), and low (300 MPa). In addition, the fatigue stress limit was increased by 70.4 % at the fatigue cycle of 1.0 × 107. The improvement was resulted from the beneficial surface integrity which inhibited fretting wear, and decreased short crack growth rate. Besides, the LSP could not necessarily introduce a greater compressive residual stress (CRS) but should certainly increase the penetration depth of the CRS. The LSP process partially relaxed the CRS depending on whether the existed surface residual stress reached a certain threshold, which was –450 MPa under the selected conditions in this study. This research helps understand the surface integrity evolution in successive processing, which may contribute to planning appropriate processes for desirable surface integrity.

Machining-induced surface integrity and nanocrystalline surface layers in cryogenic finishing turning of Inconel 718

In this study, the effects of cryogenic and flood cooling on the surface integrity of Inconel 718 are investigated for face turning with four selected cutting speeds of 25, 50, 75 and 100 m/min. Surface integrity of machined samples was characterized in terms of surface morphology, sub-surface microstructure, microhardness, x-ray diffraction textures, and residual stresses. While the differences between cryogenic and flood cooling were relatively limited for the majority of surface integrity metrics, a substantially increased (+80% vs flood condition) nanolayer depth was observed at the highest cutting speed of 100 m/min with cryogenic cooling. Additionally, cryogenic cooling resulted in slightly improved surface roughness and slightly increased compressive residual stress, particularly at elevated cutting speeds. Nb-rich secondary phases were detected after machining for all conditions, however, cryogenic cooling and low cutting speed led to reduced mixing of these nanocrystalized phases in the recrystallized surface layer. Based on these observations a and qualitative model for surface generation and nanocrystallization under flood and cryogenic machining conditions was proposed. Overall, the effect of cryogenic cooling on nanolayer generation was most pronounced at elevated speeds, suggesting the potential for cryogenic cooling to allow for more aggressive, yet sustainable, processing strategies with improved surface integrity.

The influence of machining induced surface integrity and residual stress on the fatigue performance of Ti-6Al-4V following polycrystalline diamond and coated cemented carbide milling

Accurate fatigue life predictions of titanium alloy components requires an understanding of how the machining affected metallurgical and micro-mechanical subsurface condition influences fatigue crack nucleation and growth. This study investigates the influence of surface integrity features generated during carbide and high-speed polycrystalline diamond machining on the fatigue behaviour of coarse and fine-grained Ti-6Al-4V. Mechanically induced compressive residual stresses, promoted by higher feed rates and the larger cutting edge radii of carbide tools, have been demonstrated to provide an overriding enhancing effect on fatigue life due to crack initiation suppression and reducing the deleterious effects of microstructural deformation and surface imperfections.

A Multiscale Study on Machining Induced Surface Integrity in Ti-6Al-4V Alloy

This study adopted a multiscale surface integrity approach (macro, micro, and sub-micro scale) to study the effects of cutting conditions and varying cutting fluid applications on the final surface state in Ti-6Al-4V alloy. Surface integrity parameters that were studied in this work are surface roughness, subsurface microhardness, residual stresses, and microstructural alterations. The results show that there is some correlation between microstructural and mechanical characteristics of machining-induced surface integrity. Also, different residual stress profiles were obtained for TMQF (Targeted Minimum Quantity Fluid) machining (compressive residual stress profile) and flood machining (tensile residual stress profile), which underlines the importance of selecting the right cutting fluid application. This work could aid in choosing the right cutting fluid application for the finish machining operation of Ti-6Al-4V alloy to obtain a desired final superior surface integrity. TMQF application conditions produced better surface integrity metrics (lower surface roughness, compressive surface residual stresses, and no undesirable microstructural damage) compared to flood fluid application while finish machining of Ti-6Al-4V titanium alloy.

Machining-induced Surface Integrity in Brass Alloys

This study presents machining process and resulting surface integrity properties of brass alloys including leaded (CW617N), low-leaded (CW511L) and lead-free (CW724R). Experimental data on cutting forces and cutting temperatures are presented to assess machining of these alloys. Surface quality, subsurface microhardness and microstructures of machined brass alloys are considered to assess surface integrity properties. Higher subsurface deformation was observed in low-leaded and lead-free brass materials compared to leaded brass materials. This present study reveals that machining process results in deformation twinning on the surface and subsurface of specimens. The role of machining parameters on performance measures and surface integrity aspects is also presented in this work. This work illustrates that machining parameters have a notable effect on measured machinability outputs. It should be noted that lead content has a strong influence on surface integrity aspects including microhardness and twinning of machined brass alloys.

Effect of cutting parameters on the generated surface integrity of hard-turned martensitic AISI 52100 bearing steel

Hard turning offers improved manufacturing efficiency but requires great control of the cutting process to achieve the required machining-induced surface integrity with respect to residual stresses, surface topography, and near-surface microstructure. This research work is focused on studying the effect of the cutting speed, feed rate, depth of cut, chamfer angle, and coolant pressure on the surface integrity after hard turning of martensitic AISI 52100 steel. The results showed that the feed rate had a significant influence on the residual stresses and the surface topography. The discontinuous mechanically induced white layer was observed at high cutting speed and high chamfer angle with increased thickness.

Machining-induced surface integrity in titanium alloy Ti-6Al-4V: An investigation of cutting edge radius and cooling/lubricating strategies

Enhanced surface integrity can increase the service life and functional performance in machined components, including the strength and fatigue life of machined parts. To study the influence of cutting edge radii and cooling/lubricating conditions on surface integrity, in this work variable cutting edge radii (less than 5 μm, 28 μm and 50 μm) and a range of cooling/lubrication conditions (Dry, MQL, LN2, Hybrid with LN2 and MQL) were applied in orthogonal cutting of Ti-6Al-4V alloy. Experimental analysis revealed that ploughing effects, which increase with increasing cutting-edge radii, and result in material being compressed into the machined surface, lead to an increase in thrust force, drive the evolution of subsurface microstructure at the cross-section which is normal to the direction of cutting speed. No obvious grain refinement layer was observed while utilizing the sharp cutting edge (<5 μm). At larger edge radii (28 and 50 μm), the thickness of grain refinement layer caused by severe plastic deformation (SPD) increased significantly. The residual stresses analysis was carried out based on investigations of selected experimental conditions. In machining with larger cutting edge radii and hybrid cooling, significant increases in the magnitude and depth of machining-induced compressive residual stress fields were also observed. Additionally, surface hardness also increased slightly with increasing cutting edge radius. Machining with cryogenic cooling (LN2) generated thinner deformation layers and larger deformation gradients, due to higher deformation stress and lower fracture strain at lower temperature conditions. Moreover, the rapid transfer heat from the cutting zone mitigated the tensile stress generating mechanisms, typically associated machining, allowing for mechanical ploughing effects to become dominant in the near-surface region of the machined workpieces.

Influence of surface integrity induced by multiple machining processes upon the fatigue performance of a nickel based superalloy

Machining operations are of key importance to the fatigue performance of nickel based superalloys due to the high thermal/mechanical loadings yielded on the machined workpiece which can significantly alter the surface integrity of the components. Therefore, understanding the influence mechanisms of machining induced surface integrity upon fatigue response is vital to determine their manufacturing processes and applications. In this respect, this paper investigates the surface integrity of nickel based superalloy subject to different mechanical and thermal loadings induced by various machining processes including conventional machining (e.g. finish and rough milling) and nonconventional machining (e.g. laser assisted milling and abrasive waterjet cutting) methods, as well as their influences upon fatigue performance and failure mechanisms. In-depth surface metallurgical and crystallographic analysis has been conducted to reveal the surface damage mechanisms, which allows the description of the machining induced mechanical and thermal alterations on the machined workpiece. Furthermore, the examination of the fractography from the fatigue specimen has been conducted, which enables the understanding of the influence mechanism of the corresponding surface defects on the fatigue crack initiation and propagation, subject to a four points bending fatigue test. While the resulted S-N curves indicate that the high cycle fatigue of machined nickel based superalloy is mainly dominated by the machining induced residual stress conditions, the surface defects from different machining processes can particularly influence fatigue crack initiation and propagation mechanisms in both the low and high cycle regimes.

Analysis of the Influence of Surface Modifications on the Fatigue Behavior of Hot Work Tool Steel Components.

Hot work tool steels (HWS) are widely used for high performance components as dies and molds in hot forging processes, where extreme process-related mechanical and thermal loads limit tool life. With the functionalizing and modification of tool surfaces with tailored surfaces, a promising approach is given to provide material flow control resulting in the efficient die filling of cavities while reducing the process forces. In terms of fatigue properties, the influence of surface modifications on surface integrity is insufficiently studied. Therefore, the potential of the machining processes of high-feed milling, micromilling and grinding with regard to the implications on the fatigue strength of components made of HWS (AISI H11) hardened to 50 ± 1 HRC was investigated. For this purpose, the machined surfaces were characterized in terms of surface topography and residual stress state to determine the surface integrity. In order to analyze the resulting fatigue behavior as a result of the machining processes, a rotating bending test was performed. The fracture surfaces were investigated using fractographic analysis to define the initiation area and to identify the source of failure. The investigations showed a significant influence of the machining-induced surface integrity and, in particular, the induced residual stress state on the fatigue properties of components made of HWS.

3D numerical modelling of turning-induced residual stresses – A two-scale approach based on equivalent thermo-mechanical loadings

Nowadays, fatigue life of engineering components is a major topic. In the literature, extensive works have been reported in order to assess the machining-induced surface integrity which is known to be a first-order parameter for the fatigue resistance. This paper lies in the framework of turning-induced residual stress prediction by presenting an up-dated method based on a two-scale approach. Previous works aimed at building a 3D hybrid method consist of modelling the turning consequences through equivalent thermo-mechanical loadings combining a finite element model with experimental tests. The new proposed global hybrid strategy keeps the same concept but has the advantage of defining the thermo-mechanical shapes more accurately thanks to local 2D orthogonal cutting models. Moreover, the calibration of the equivalent thermo-mechanical loadings is easier as only turning forces have to be measured, which makes it more accessible to end-users. Finally, the model provides results considering the effective machined surface topography instead of considering a basic flat surface. After carefully detailing each step of the numerical method, a validation case study concerning a longitudinal turning operation on a 15-5PH martensitic stainless steel is proposed. By comparing numerical results with experimental residual stresses coming from X-Ray diffraction, the new numerical method shows its superiority to predict, within few hours, much more accuratly the residual stress state induced by a real longitudinal turning operation.

Machining-induced surface integrity of Inconel 718 alloy fabricated by powder bed fusion additive manufacturing under various laser processing parameters

Parameters used in laser powder bed fusion (LPBF) process are key factors that influence the surface integrity and thus mechanical properties of Inconel 718 components fabricated by additive manufacturing (AM) technique. For this reason, various parameters with different settings through trial and error approach have used by AM community to fabricate Inconel 718 components. These AM fabricated components generally required post treatment or processing including finish machining, to enhance their surface properties. This study presents a systematic investigation on the effect of volumetric energy density, and various LPBF processing parameters including laser power, layer thickness, and scanning speed on the as-printed specimens and the surface integrity aspects, namely surface quality, porosity, and microhardness. Furthermore, using machining operations specifically finish milling on these specimens under constant machining parameters, the effect of as-printed conditions on the machinability responses including burr formation, cutting forces are evaluated accordingly. Moreover, the interrelationship between LPBF processing parameters-machining-surface and subsurface aspects are also examined. This study reveals that LPBF additive manufacturing parameters have remarkable influence on the printed Inconel 718 specimens. Results also showed that parameters including laser power, scanning speed and layer thickness also have an effect on both the machinability and final surface and subsurface properties of the AM fabricated Inconel 718 specimens. It should be also noted that there is a notable relationship between volumetric energy density and the machinability of the AM printed Inconel 718.

Turning-induced surface integrity for a fillet radius in a 316L austenitic stainless steel

Turning is a machining process extensively applied to produce revolution parts. Durability of these parts are known to depend on the turning process signature that is often referred as surface integrity. The surface integrity generated in a fillet radius has been barely studied in the literature so far, despite the well-known geometrical stress concentration factor of such singularities. Therefore this paper deals with the investigation of machining-induced surface integrity when turning a fillet radius in a 316L austenitic stainless steel. Different characterization methods are used for that purpose - SEM, EBSD, nanoindentation and X-Ray diffraction. It points out that the turning-induced consequences are not homogeneous along the machined profile. Residual stresses are strongly affected and microstructure is highly modified over a depth of 80 μm that leads to a mechanical properties gradient. It is evidenced that the average uncut chip thickness is the main governing parameter regarding surface integrity. It is also reported that deformation twins appear in the affected zone. It highlights that turning-induced microstructure evolution at a given depth is rather a consequence of severe plastic deformation at high strain rate than dynamic recrystallization. • For the first time, turning-induced surface integrity in 316L is studied along a fillet radius. • Turning-induced affected depth depends on angular position in the fillet radius. • Affected depth is twice larger for longitudinal turning than for face turning. • Microstructure evolution is correlated with average uncut chip thickness parameter and cutting edge radius. • Severe plastic deformation at room temperature or/and high strain rate governs microstructural evolution (deformation twins).

A review of surface integrity in machining of hardened steels

Abstract This paper presents a background of surface integrity in the machining of hardened steels for rolling dies, moulds, bearings, shafts, gears, etc. Hardened steels, which are normally harder than 45 HRC, are considered difficult to cut owing to their combinations of mechanical and physical properties that promote low chip–tool contact area and high forces, and in turn, result in high contact stresses and elevated temperatures. In particular, the high temperatures generated on a newly machined workpiece surface can promote poor surface integrity, such as white layers and microcracks, warping, and consequently reduced fatigue strength. Additionally, tool wear is often significant, which further exacerbates the problem of high cutting temperatures, leading to frequent rejection of hard machined parts during quality control and thus increased manufacturing costs. In this work, the background of surface integrity during machining of hardened steels is introduced, and the main challenges and their solutions from the literature are discussed based on a diverse set of case studies. Based on the state of the art, a series of conclusions and best practices are presented, in addition to an outlook on future directions in the field of machining-induced surface integrity of hardened steels.