Enhanced Surface Integrity Research Articles

Comparison of machining performance of stainless steel 316L produced by selective laser melting and electron beam melting

Powder bed fusion processes based additively manufactured SS 316L components fall short of surface integrity requirements needed for optimal functional performance. Hence, machining is required to achieve dimensional accuracy and to enhance surface integrity characteristics. This research is focused on comparing the material removal performance of 316L produced by PBF-LB (laser) and PBF-EB (electron beam) in terms of tool wear and surface integrity. The results showed comparable surface topography and residual stress profiles. While the hardness profiles revealed work hardening at the surface where PBF-LB specimens being more susceptible to work hardening. The investigation also revealed differences in the progress of the tool wear when machining specimens produced with either PBF-LB or PBF-EB.

Flat diamond sliding burnishing surface roughness investigation

The sliding diamond burnishing process is one among many burnishing post-machining processes applied for surface integrity enhancement purposes. This experimental study aims to investigate the effect of burnishing parameters on the surface roughness of the product. Taguchi design was used, and the machining parameters considered were burnishing force, feed rate, and the number of passes with three levels each. C45 steel alloy workpieces were face milled with the same machining variables and their surface roughness was measured before and after the diamond sliding burnishing process. Enhanced surface roughness was achieved with increasing burnishing force and minimum feed rate and higher number of passes.

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.

Performance Study of Cryo-Treated End Mill Via Wet, Cryogenic, and Hybrid Lubri-Coolant-Milling Induced Surface Integrity of Biocompatible Mg Alloy AZ91D

The excellent biodegradability of the magnesium (Mg) alloys is gradually proving them as the potential substitutes to several biomedical implants such as chemotherapy ports or screws, which are required to be removed via secondary surgeries after a specific period of time. However, an early degradation of these alloys even before the complete healing of the damaged tissue, when exposed to the physiological atmosphere, has been limiting their full-fledged application. Some latest research articles suggest that such challenges can be effectively overcome by improving the surface integrity of Mg alloys using the sustainable manufacturing techniques, such as cryogenic machining. Recent literatures also report the outperformance of the cryogenically treated (cryo-treated) cutting tools for achieving an enhanced surface integrity. In this relation, the present research attempts to improve the surface integrity of one of the most commonly used biocompatible alloys of magnesium, known as AZ91D. For this reason, a TiAlN coated-cemented carbide end mill was used in an untreated and cryo-treated state amid wet, cryogenic, and hybrid-lubri-coolant-milling conditions. The milling and FESEM (field emission scanning electron microscopy) results showed a considerable improvement in the surface integrity in terms of an augmented surface roughness and microhardness at 56.52 m/min cutting speed with the cryo-treated end mill during hybrid-lubri-coolant-milling. At the high cutting speed hybrid-lubri-coolant-milling, the cryo-treated end mill attained 35.71% and 48.07% better surface finish than that of cryo and wet-lubri-coolant-milling, respectively. Although, the highest surface microhardness was achieved by the cryo-treated end mill amid cryo-lubri-coolant-milling, due to the poorest surface quality observed in terms of the maximum number of machining-induced cracks, the hybrid-lubri-coolant-milled surface was preferred over the cryo and wet-lubri-coolant-milled surfaces. Further, the FESEM and EDS (energy-dispersive X-ray spectroscopy) analyses confirmed the oxide layer produced by the cryo-treated end mill amid hybrid-lubri-coolant-milling, to be the thinnest (12.16 µm) and most uniform passivation layer.

Surface Integrity Enhancement of Incoloy 825 During Electric Discharge Machining

The aim of the current research work is to enhance the surface integrity of EDMed (electro-discharge machined) Incoloy 825 by suspending graphite powder particles in dielectric during electro-discharge machining (EDM). For this purpose, a specially designed and fabricated re-circulation dielectric tank was integrated with the existing EDM setup. The effect of three process variables, i.e., peak current, pulse-on time, and duty cycle was studied on EDM characteristics such as material removal rate, surface roughness (Ra), radial overcut, surface microhardness, microhardness in sub-surface regions, recast layer, phase changes and residual stress. Results showed that the machining rate increased up to 23% using powder-mixed EDM compared to conventional EDM. Significant reduction in surface roughness, white layer thickness, surface cracks and residual stress was observed while machining with graphite mixed dielectric. The addition of powder resulted in the formation of carbides and other alloy compounds on the machined surface which enhances the microhardness of surface and subsurface region.

Enhanced Surface Integrity of a Biomedical Grade Polyetheretherketone through Cryogenic Machining

Polyetheretherketone (PEEK) belongs to a group of thermoplastic polymers widely used in bioengineering applications such as trauma, orthopedic and spine implants, whose mechanical and geometrical characteristics, when put in service, must be the highest possible. However, machining operations, usually carried out for their manufacturing, can assure these characteristics uniquely when they are performed in a restricted temperature window because of the PEEK mechanical behavior change nearby its glass transition temperature.To this aim, the present paper investigates the feasibility of using cryogenic machining to enhance the surface integrity of a biomedical grade PEEK. Turning trials were carried out under dry and cryogenic machining conditions at varying feed and cutting speed. The temperature reached during the process was measured by means of a thermocouple embedded in the cutting tool, whereas the surface integrity assessed in terms of surface defects, surface roughness and hardness. Additionally, the degree of crystallinity was evaluated via Differential Scanning Calorimetry (DSC). To correlate machinability results with the PEEK mechanical behavior, tensile tests were performed in the temperatures range between -100°C and 50°C.The obtained results showed that the application of liquid nitrogen always made possible achieve an enhanced surface integrity compared to the corresponding dry condition, which represent the currently used strategy for the manufacture of biomedical polymer implants. A proper temperature control during machining assured indeed the formation of lower amounts of defects as well as reduced surface roughness, by increasing the PEEK degree of crystallinity.

Investigation of belt-finishing effect on the residual stress field through repeated scratching on rough hard-turned surface

Belt-finishing process is renowned for its high capability to enhance surface integrity through scratching of the work-piece by means of multiple abrasive grains. In this work, the axial residual stress field induced by repeated scratch tests on an initial rough hard-turned surface is studied through numerical modeling. An analytical model is adopted to calculate the penetration depth and its evolution corresponding to the fixed indentation load and the number of scratching. The effect of the initial surface roughness, the loading conditions, the number of repeated scratching, and the coefficient of friction on the residual stress field, has been studied. The results show that whatever, the parameter's values, the affected layer doesn't exceed 11 μm. Moreover, the best compressive residual stress field is obtained for the highest loading conditions, the lowest initial surface roughness and friction coefficient. The numerical results are then used as qualitative evidence to describe the residual stress field evolution during a belt-finishing process, and to propose an optimum solution, ensuring the highest surface integrity during such a process.

Synergistic effect of deep ball burnishing and HA coating on surface integrity, corrosion and immune response of biodegradable AZ31B Mg alloys.

The fast degradation and consequent loss of mechanical integrity is a major problem of biodegradable Mg alloy, which limits its clinical viability. This paper presents the influence of a synergistic approach combining deep ball burnishing and hydroxyapatite (HA) coating on biomechanical integrity, degradation and immune response of Mg alloy (AZ31B). The burnishing resulted in smooth surface topography, increased hardness from 0.87 to 1.45GPa and induced microstructural disturbances with deformation twins/twin bands, which enabled formation of a dense and compact platelet-like crystals HA coating of 110μm thickness. Compared to the untreated and burnished specimens, the burnished + HA coated surface provided remarkably higher corrosion resistance as indicated by lower corrosion current density and smaller mass loss. HA coating and surface integrity enhancement by burnishing were predominantly responsible for improved corrosion resistance. HA coating on the burnished surface exhibited hydrophilic properties and adequate bonding strength. While the modified surfaces promoted cell growth, the burnished + HA surface outperformed in exhibiting less pro-inflammatory and high anti-inflammatory cytokines, demonstrating that the treated surfaces were not posing any threat to immune cells. The findings indicate that the synergistic surface treatment can be a viable means to enhance corrosion resistance and immune response of Mg alloys implants.

Enhancing Surface Topology of Udimet®720 Superalloy through Ultrasonic Vibration-Assisted Ball Burnishing

This contribution reports the effects of an ultrasonic-vibration assisted ball burnishing process on the topological descriptors of nickel-based alloy Udimet®720. This material is of high interest for the transportation industry, and specifically for the aeronautical sector. Despite the acknowledged necessity to finish this material to achieve excelling mechanical performances of parts, surface integrity enhancement by means of plastic deformation through ball burnishing has seldom been explored in previous references so far. In this paper, different surface descriptors are used to report how the topology changes after ultrasonic-assisted ball burnishing, and how burnishing conditions influence that change. The burnishing preload and the number of passes are the only influential factors on surface change, whereas the feed velocity of the tool and the strategy reveal not to be relevant on the result. Additionally, the extent to which the process successfully modifies the objective surfaces is highly divergent depending on the original scale of the treated surface. The assistance of the process with vibrations also shows that the resulting topologies are characterized by a periodical pattern of repetitive peaks and valleys that are extended on the surface with a higher frequency in comparison to the non-assisted process, which could influence in the functional deployment of workpieces treated through it, and could deliver an advantage with regard to its non-assisted homologous process.

Influence of modern machining processes on the surface integrity of high-entropy alloys

High Entropy Alloys (HEAs) are a recent class of materials. In contrast to conventional alloys, HEAs consist of five alloying elements in equiatomic equilibrium. The high entropy effect is due, among other things, to the increased configuration entropy, which promotes solid solution formation. Many HEAs have enormous application potential due to excellent structural property combinations from very low to high temperatures. For the introduction of HEAs in real components, however, the question of the applicability of machining production technologies for component manufacture is of central importance. This has so far received little attention in global materials research. Reliable and safe processing is essential for the demand of economical component production for potential areas of application, e.g. in power plant technology. For metals, milling is the standard machining process. This article presents the results of machining analyses. It focuses on the surface integrity resulting from the milling process on a Co20Cr20Fe20Mn20Ni20-HEA. For this purpose, investigations were carried out using ball nose end milling tools for conventional milling process in comparison to an innovative hybrid process available at BAM Berlin, Ultrasonic-Assisted Milling (USAM). USAM promises a lower degradation of the surface properties due to lower loads on the workpiece surface during machining. For this purpose, basic milling parameters (cutting speed and tooth feed) were systematically varied and cutting forces were measured during the milling experiments. The subsequent analysis of these forces allows an understanding of the mechanical loads acting on the tool and component surface. These loads cause topographical, mechanical and microstructural influences on the surface and consequently on the surface integrity. For their characterization, light and scanning electron microscopy were used, and the roughness and residual stresses via X-ray diffraction were measured. The results indicate significant advantages using USAM, especially due to reduced cutting forces compared to the conventional milling process. This causes lower mechanical loads on the tool and surface, combined with lower tensile residual stresses on and below the surface, and ultimately results in a significantly enhanced surface integrity.

Surface integrity enhancement of austenitic stainless steel treated by ultrasonic burnishing with two burnishing tips

A set of ultrasonic burnishing equipment with two different burnishing tips was designed and manufactured, with which a series of experiments were performed to explore the effects of process parameters and burnishing tips on the surface integrity of austenitic stainless steel material being treated by ultrasonic burnishing (UB). Based on the experiment data, the two surface treatments, i.e. UB with ball tip and UB with roller tip, were comparatively assessed together with the other two surface machining methods of fine turning and grinding. As a further study, a microscopic FE model was built to investigate the three-dimensional transient stress and strain field inside the being treated material. It was found that parameter combination is determinative to surface finishing in UB process, and static pressure and burnishing pass are supposed to be the two most significant parameters for surface integrity of the treated sample. On the whole, roller tip is more preferable to achieve good surface enhancement than ball tip. The superposition of ultrasonic vibration leads to the dynamic change of the stress and strain field in UB, resulting in the oscillating propagation of stress wave inside the material, which gives explanation for the good performance of UB than that of conventional burnishing without ultrasonic.

Finite Element Analysis of Ball Burnishing on Ball-End Milled Surfaces Considering Their Original Topology and Residual Stress

Ball burnishing is a superfinishing operation whose objective is the enhancement of surface integrity of previously machined surfaces, hence its appropriateness to complement chip removal processes at the end of a production line. As a complex process involving plastic deformation, friction and three-dimensional interaction between solids, numerical solutions and finite element models have typically included a considerable amount of simplifications that represent the process partially. The aim of this paper is to develop a 3D numerical finite element model of the ball burnishing process including in the target workpiece real surface integrity descriptors resulting from a ball-end milled AISI 1038 surface. Specifically, its periodical topological features are used to generate the surface geometry and the residual stress tensor measured on a real workpiece is embedded in the target surface. Secondly, different models varying the effect of the coefficient of friction and the direction of application of burnishing passes with regards to the original milling direction are calculated. Results show that the resulting topology and residual stresses are independent of the burnishing direction. However, it is evident that the model outputs are highly influenced by the value of the coefficient of friction. A value of 0.15 should be implemented in order to obtain representative results through finite element models.

Milling Microchannels in Monel 400 Alloy by Wire EDM: An Experimental Analysis

This paper presents the results of an investigation on the capacity of wire electrical discharge machining (WEDM) to produce microchannels in the Nickel-based alloy, Monel 400. The main objective of the current study is to produce microchannels with desired/target geometry and acceptable surface quality. Square cross-sectional microchannels with dimensions of 500 × 500 µm were investigated. Experiments were conducted based on the one-factor-at-a-time approach for the key input WEDM process parameters, namely pulse-on time (TON), pulse-off time (TOFF), average gap voltage (VGAP), wire feed (WF), and dielectric flow rate (FR). Dimensional accuracy, machining speed, surface roughness, surface morphology, microhardness, and microstructure were analyzed to evaluate the microchannels. The minimum errors of 6% and 3% were observed in the width and depth of the microchannels, respectively. Furthermore, microchannels with enhanced surface integrity could be produced exhibiting smooth surface morphology and shallow recast layer (~0–2.55 µm).

Multiphase hydrodynamic flow finishing for surface integrity enhancement of additive manufactured internal channels

Abstract The surface finishing of internal channels for components built using additive manufacturing is a challenge. The resulting surface finish uniformity of additive manufactured internal channels (such as fuel transfer lines and cooling passages) is an issue. Therefore, we propose a novel surface finishing technique using controlled hydrodynamic multiphase flow with abrasion phenomenon to overcome the challenges in the surface finishing of additive manufactured internal channels. In this study, we performed the internal surface finishing on AlSi10Mg components manufactured by direct metal laser sintering. We investigated the surface finish potential of the proposed hydrodynamic cavitation abrasive finishing (HCAF) by varying the process parameters, namely, the hydrodynamic upstream and downstream fluid pressures, fluid temperature, abrasive concentration, and processing time. The HCAF process resulted in greater than 90 % (Ra and Rz) surface finish improvements with an acceptable thickness loss from the internal channels. We precisely mapped the surface morphology transformation at the demarcated zones over the processing time and explained the material removal mechanism. In addition, we analyzed and discussed the surface integrity of the channels in terms of the microstructure, surface hardness, and residual stress. Furthermore, we performed large-area surface topography measurements. Then, we analyzed the resulting areal surface texture parameters to determine the uniformity and flatness of the surface after internal surface finishing. Finally, we discussed the significance of using the proposed HCAF process for complex additive manufactured internal channels.

Impact of the Deionized Water on Making High Aspect Ratio Holes in the Inconel 718 Alloy with the Use of Electrical Discharge Drilling

Nickel-based superalloys are being increasingly applied to manufacture components in the aviation industry. The materials are classified as difficult-to-machine using conventional methods. Nowadays, manufacturing techniques are needed to drill high aspect ratio holes of above 20:1 (depth-to-diameter ratio) in these materials. One of the most effective methods of making high-aspect-ratio holes is electrical discharge drilling (EDD). While drilling high aspect ratio holes, a crucial issue is the flushing of the gap area and the evacuation of the erosion products. The use of deionized water as the dielectric fluid in the EDD offers a considerable potential. This paper includes an analysis of the influence of the machining parameters (pulse time, current amplitude and discharge voltage) on the process performance (drilling speed, linear tool wear, taper angle, hole’s aspect ratio, side gap thickness), during the EDD with the use of deionized water in the Inconel 718 alloy. The obtained through holes were subjected to the extended analysis. The impact of the initial working fluid temperature and pressure on the conditions of the flow through the electrode channel was also subjected to the analysis. The deionized water properties were changed by applying an initial temperature. Based on the results of an analysis of the previous research, the EDD of the through holes was performed for a pre-set initial temperature (~313.15 °K) and initial pressure of the working fluid (8 MPa) and selected process parameters. An analysis of the results indicates increasing of hole’s aspect ratio by about 15% (above 30), decreasing the side gap thickness by about 40% and enhanced surface integrity.

Experimental investigation and economic analysis of surfactant (Span-20) in powder mixed electrical discharge machining (PMEDM) of AISI D2 hardened steel

Powder mixed EDM (PMEDM) is recognized as an advanced and innovative technique with enhanced performance and limited drawbacks in comparison to conventional EDM method. This study investigates the effect of powder particle size, various powder concentrations (Cp), and surfactant concentrations (Cs) on the performance of EDM. Since the machining characteristics are highly dependent on the dielectric performances, significant attention has been directed to introduce Cr powder and Span-20 surfactant into the dielectric fluid to achieve higher productivity and enhanced surface integrity. The EDM machining was carried out on AISI D2 hardened steel through ´Plug & Plaý dielectric circulating system attached to the main machine in order to evaluate the machining performances (i.e. MRR, EWR, and Ra). Interestingly, machining performance was improved with combination of Cr powder mixed and span-20 surfactant. By comparing the performance of span-20 surfactant and micro-nano chromium, the result within selected parameters shows that the span-20 surfactant and nano-chromium is the better choice for the EDM of AISI D2 hardened steel. In the machinability studies, the EDM machining of AISI D2 hardened steel by using span-20 surfactant and nano-chromium has exhibited the excellent machining performances, which led to 45.08% MRR enhancement and 68.89% Ra enhancement comparing to micro-chromium powder and span-20 surfactant led to 35.28% MRR and 28.96% Ra. Furthermore, cost analysis revealed that the nano-Cr powder size was approximately 4 times more economical than micro-Cr powder in machining of AISI D2 hardened steel, although the price for 1 kg is quite expensive.

Surface integrity enhancement of ZL109 aluminum-silicon piston alloy employing the forward and reverse finish cutting method

The machined surface integrity has significance on fatigue life. The step-by-step feed cutting is put forward and adopted to machine ZL109 aluminum-silicon piston alloy. In addition, the forward and reverse finish cutting is also proposed to improve the machined quality and fatigue life. The result indicates that the step-by-step feed cutting can effectively avoid scratching and decrease surface roughness, bring about the compressive residual stress, and improve machined quality. Moreover, there is smaller elastic-plastic recovery and thermal expansion on the machined surface, when the step-by-step feed cutting is adopted, so that the machining dimension accuracy can be ensured perfectly. The forward and reverse finish cutting can effectively decrease the surface roughness and the tensile residual stress due to the thermal stress. So the larger the cutting thickness is, the smaller surface residual stress is, when the cutting depth is 0.15–0.30 mm in the forward and reverse finish cutting. It can improve the machined quality of ZL109 aluminum-silicon piston alloy and the fatigue life when the reasonable cutting parameter is adopted. Those results have an important practical significance.

Sequential multistage deep rolling under varied contact conditions

It is possible to induce compressive residual stress, strain hardening and reduce the surface roughness by applying deep rolling to metallic surfaces. The resulting enhancement of surface integrity is based on the mechanical impact during the deep rolling process (internal material loads). For a single contact as well as multiple contacts with identical contact conditions, a good approximation for the correlation between the process parameters and the resulting material modification was achieved in previous works. By regarding the equivalent stresses, based on Hertzian equations, the modifications by means of the residual stresses were assessed. This paper shows an approach to describe the resulting material modification when using multiple contact stages in a deep rolling process under varied contact conditions. AISI 4140 is processed. For the generation of linear deep rolling tracks resulting from multistage deep rolling, the maximum internal material load is held constant also when varying the ball diameter between the different contacts. Three different levels of internal material loads are applied with two ball diameters and three different total numbers of contacts per linear track. The analysis of the material modifications is based on XRD measured residual stresses and geometrical track descriptions.

Enhancement of surface integrity by cryogenic diamond burnishing toward the improved functional performance of the components

17-4 precipitation-hardenable (PH) stainless steel is one of the widely used materials in various applications of engineering practices owing to their excellent corrosion resistance and high strength. The components such as automotive body, aerospace compressor blades, turbine blades and molds demand higher load carrying capacity and improved fatigue strength, which is possible to achieve by surface severe plastic deformation. Diamond burnishing process is an appropriate technique to produce such components which improves the surface integrity characteristics of the material. This article presents a comprehensive examination of the surface integrity of cryogenic diamond burnished 17-4 PH stainless steel using a novel diamond burnishing tool. The impact of diamond burnishing control factors on subsurface microhardness, surface roughness, surface hardness, surface topography, residual stress and surface morphology has been analyzed. The optimal control factor setting ensures the least surface roughness of 0.03 µm by the application of one factor at a time approach. Cryogenic diamond burnished surface achieves the exceptional surface finish and the surface hardness in tool-tip of 8 mm and 6 mm, respectively. The maximum surface hardness of 413 HV was attained using 6-mm tool-tip diameter. The subsurface microhardness improvement of 2% and 4% has been observed while using a tool-tip diameter of 6 mm in contrast to 8 mm and 10 mm. Compressive residual stresses have been generated at the top surface layer of the specimen. The attained experimental results prove that cryogenic diamond burnishing can be successfully applied to 17-4 PH stainless steel to enhance its surface integrity characteristics.

On wire spark erosion machining induced surface integrity of Ni55.8Ti shape memory alloys

Ni55.8Ti shape memory alloys (SMAs) find applications in different fields of medical and engineering. In every field, surface integrity greatly affects the functional performance of shape memory alloy parts. In the present work, wire spark erosion machining of Ni55.8Ti shape memory alloys has been conducted and surface integrity parameters of the machined specimens have been evaluated. Experiments are designed using Taguchi L16 robust design of experiment technique. Effect of important process parameters, i.e. voltage, pulse-on time and pulse-off time on maximum surface roughness has been studied. Deterioration in surface integrity at various combinations of pulse-on and pulse-of time which produced high discharge energy has been observed. Scanned electron microscopic investigation, energy dispersive spectroscopy and XRD analyses, roughness measurement, and micro-hardness testing results are presented, analyzed and discussed. Optimization of process parameters resulted in surface integrity enhancement with low roughness (Rt – 7.78μm and Ra – 1.45μm) and very thin recast layer (4–6μm) along with minimum subsurface defects.