Unmachined Surface Research Articles

Heterogeneous effect of aging temperature on the fatigue life of additively manufactured thin-walled 18Ni300 maraging steel tubular specimen

Optimizing the geometry for material savings, or heat transfer, can yield thin-walled additively manufactured parts. A wall thickness of only a few hatching spaces can be fabricated using laser powder bed fusion (LPBF). However, the unmachined surface quality and subsurface defects in the low-thickness wall put the load-bearing capacity of the manufactured part under scrutiny. This study aimed to determine the fatigue and quasi-static strengths of nominally 0.5-mm thick tubular specimens of LPBF 18Ni300 maraging steel with respect to the aging temperature (AT). The experimental tests were conducted on six batches of unmachined specimens: non-aged and age-hardened for 6 h at different temperatures, i.e., 450, 470, 490, 530, and 585 °C. The void distribution and specimen surface were characterized using X-ray micro-computed tomography. The heterogeneous effect of AT on fatigue life was recognized by a novel application of the Gaussian process for regression. The mechanical properties of a 0.5-mm thick tubular specimen were equal to or higher than those of a full cross-section specimen. Furthermore, the optimal AT to obtain the highest fatigue strength was 520 °C for low-cycle fatigue, decreasing to 480 °C for high-cycle fatigue.

The influence of surface imperfections on phosphate coating performance of nodular cast iron substrates

<p>The service performance of the nodular cast iron wheel hub was modelled by investigating the effect of surface morphology and characteristics on the phosphate coating size, morphology, and corrosion resistance properties of the zinc phosphate conversion coated cast iron substrates. The machined and unmachined surfaces as well as the coatings were examined by a scanning electron microscope coupled with an energy-dispersive X-ray spectrometer (SEM-EXD). The influence of the substrates’ surface imperfections on the phosphate coating and the subsequent corrosion resistance was assessed and rated according to the standard procedures. Surface analysis of the machined and unmachined cast iron hubs reveals the existence of graphite and foreign material inclusions on the substrate surface that impact the phosphate coating properties and resistance to corrosion. The average phosphate coating crystal size is 1.74 µm and 2.58 µm for the unmachined and machined cast iron substrates, respectively. The corrosion resistance of the coated unmachined wheel hub surfaces was rated poor and disapproved based on the application requirements. The poor corrosion resistance was ascribed to the influence of the substrates’ surface characteristics on the coating adhesion to the substrate. However, the cast surfaces should be properly shot-blasted to remove any adhere foreign materials on the as-cast skin to enhance the coating adhesion.</p>

The influence of surface imperfections on phosphate coating performance of nodular cast iron substrates

<p>The service performance of the nodular cast iron wheel hub was modelled by investigating the effect of surface morphology and characteristics on the phosphate coating size, morphology, and corrosion resistance properties of the zinc phosphate conversion coated cast iron substrates. The machined and unmachined surfaces as well as the coatings were examined by a scanning electron microscope coupled with an energy-dispersive X-ray spectrometer (SEM-EXD). The influence of the substrates’ surface imperfections on the phosphate coating and the subsequent corrosion resistance was assessed and rated according to the standard procedures. Surface analysis of the machined and unmachined cast iron hubs reveals the existence of graphite and foreign material inclusions on the substrate surface that impact the phosphate coating properties and resistance to corrosion. The average phosphate coating crystal size is 1.74 µm and 2.58 µm for the unmachined and machined cast iron substrates, respectively. The corrosion resistance of the coated unmachined wheel hub surfaces was rated poor and disapproved based on the application requirements. The poor corrosion resistance was ascribed to the influence of the substrates’ surface characteristics on the coating adhesion to the substrate. However, the cast surfaces should be properly shot-blasted to remove any adhere foreign materials on the as-cast skin to enhance the coating adhesion.</p>

Low cycle fatigue behavior of additively manufactured Haynes 282: Effect of post-processing and test temperature

This study investigated the fully reversed (R = -1), strain-controlled low cycle fatigue behavior of laser powder bed fused Haynes 282 at temperatures ranging from −195 to 871 °C and at strain amplitudes of 0.010 and 0.005 mm/mm. By comparing the fatigue behaviors of hot isostatically pressed (HIP) specimens in machined surfaces (HIP/M) and the Non-HIP ones in un-machined surfaces (Non-HIP/UM), the combined effect of surface condition and hot isostatic pressing was analyzed and discussed. The Non-HIP/UM specimens showed shorter fatigue lives compared to HIP/M ones from −195 to 649 °C, which was attributed to the presence of surface micro-notches and volumetric defects. This difference attenuated above 649 °C due to the combined effects of increased plastic deformation, which mitigated the negative impact of surface anomalies, and oxidation, which accelerated crack initiation and growth. The tests at −195 °C tended to suppress cyclic plastic deformation, especially at the lower strain amplitude, which led to improved fatigue lives. Lastly, both HIP/M and Non-HIP/UM specimens exhibited similar stress responses with respect to test temperature; at 21, 204, 760, and 871 °C, cyclic softening occurred due to shearing of γ′ precipitates by persistent slip bands, while at other temperatures, the softening was balanced or overcome by cyclic hardening due to deformation twinning (i.e., at −195 °C) and dislocation multiplication during ordinary slip (i.e., at 427 and 649 °C).

Influence of corrosion fatigue on the fatigue strength of an AlSi10MgMn high‐pressure die‐casting alloy with regard to the surface condition

AbstractThis study investigates the influence of corrosion fatigue on the fatigue strength of an AlSi10MgMn high‐pressure die‐casting alloy with regard to the surface condition. Two different surface conditions are compared. First, an unmachined surface condition with a rough surface () and second a machined and polished surface condition with a smooth, polished surface () are tested. The specimens are loaded with a cyclic bending moment at a constant stress ratio of R = 0 till failure. A 5 wt% NaCl solution enables the corrosion fatigue tests. Corrosion fatigue leads to a significant reduction of the endurable stresses along the whole load spectrum independent from the surface condition. The applied corrosive solution prevents the formation of a pronounced long‐life fatigue strength limit within the tested load cycle range. Hence, the reduction of the endurable stresses increases with the increasing number of load cycles, reaching approximately 37% at load cycles. No significant influence of the surface condition, independent of the environment (air, 5 wt% NaCl), on the fatigue behaviour is found.

QUALITY ASSESSMENT OF THE VITREOUS ENAMEL COATING APPLIED TO THE WELD JOINT

This work compares different thicknesses of vitreous enamel coating applied to welded joints. Low-carbon steel Kosmalt E300T was used as a base material. The MAG welding method was used for all samples. Metallography was performed to compare the thickness of the coating formed on the machined and unmachined surfaces of the weld. SEM analysis focused on the substrate/vitreous enamel coating interface. The results show that the high thickness of the vitreous enamel coating that occurs in unmachined weld joints significantly decreases the properties of the enamel coating. In comparison, the machining of the weld surface contributes to the quality improvement of the vitreous enamel coating.

Anisotropic micro cutting of rolled titanium alloy

In machining of rolled titanium alloy, the anisotropy in the cutting force induced by the rolling texture should be controlled to achieve high quality surface. The cutting tests on the normal direction (ND) plane of a rolled titanium alloy plate are conducted with changing the cutting direction with respect to the rolling direction. The deformation and the cutting force in micro cutting are discussed compared to those in cutting of single crystal titanium. Although the anisotropic effect on the cutting process is smaller than that of the single crystal workpiece, little deformation occurs around the boundary between the groove and the unmachined surface when cutting along the rolling direction. The material behavior is characterized by the shear plane cutting model. The shear angle and the shear stress on the shear plane depend on the cutting direction for the rolling direction.

Surface modification of cylindrical implants using electric discharge-assisted centreless turning

ABSTRACT Numerous techniques are used for the surface modification of titanium alloy. This research uses electric discharge-assisted centreless turning (EDCLT) to modify the titanium alloy surfaces used for dental implants. Experimentations were planned according to the L9 Taguchi orthogonal array design. Impulse current (IP), Pulse on time (Ton), and pulse off time (Toff) were selected as input variables, and surface roughness and hardness as output variables. The machined surface’s average surface roughness (Ra) was less than 1.90 µm. In contrast to unmachined surfaces, the machined surfaces had improved corrosion resistance and bioactivity. The average hardness of machining surfaces was found to be from 43.1 HRC to 52.8 HRC, while unmachined surfaces had an average hardness of 42.7 HRC. The machined piece with an average surface roughness (Ra) of 1.25 µm got the highest bone mineralization compared to other surface roughness.

Turning of titanium alloy with PCD tool and high-pressure cooling

Titanium alloys are difficult to cut materials due to their low thermal conductivity, which leads to intensive tool wear. The general issue is finding the best combination of cutting tool material and cutting conditions to achieve high productivity. This study used PCD cutting tool material in combination with high-pressure cooling (HPC). The main task was to find the most suitable HPC mode (various HPC settings on the rake and flank faces of the cutting tool) and intensity to reduce tool wear at a high cutting speed. Tool wear, chips, and forces were measured, and surface quality was evaluated to gain an understanding of the machining process under these particular conditions. An ANOVA test was used to determine the significance of control factors such as tool life and HPC mode and intensity. The most suitable cutting speed was 300 m/min, where a limit spiral cutting length (SCL) of 3000 m was achieved. Setting the HPC mode revealed the necessity of using the HPC on the rake face. However, the HPC on the flank face further decreased tool wear. HPC intensity should be chosen based on knowledge of the cutting process. A very intense HPC above 140 bars can lead to mechanical damage to the cutting edge or unmachined surface by chip blasting but using a 60-bar HPC can reduce tool wear similarly, without causing further damage to the cutting edge.

Study on mechanism of chip formation of grinding plasma-sprayed alumina ceramic coating

The mechanisms of ductile–brittle transition and surface/subsurface crack damage during the grinding of plasma–sprayed alumina ceramic coatings were investigated in an experiment and simulation on single diamond abrasive grain cutting. We observed that the brittle damage modes of alumina ceramic include boundary cracks, median cracks and lateral fractures. The normal force of the abrasive grain results in the initiation of median cracks, whereas the tangential force of the abrasive grain results in the propagation of median cracks in the direction of the abrasive grain cutting. Some cracks propagate downward to form machined surface cracks, whereas others propagate to the unmachined surface of the workpiece to produce brittle removal. Owing to the alternating tensile and compressive stresses, the material in contact with the top of the abrasive grain fractures continuously, forming the main morphology of the machined surface. The geometry and cutting depth of the abrasive grain have a significant influence on the ductile–brittle transition, whereas the cutting speed of the abrasive grain have no significant influence. On one hand, the stress concentration at the pore defects result in crack propagation to the deep layer; on the other hand, it reduces the local strength of the surface material, produces brittle fracturing, and interrupts crack propagation. The pores exposed on the machined surface and the broken morphology around them are important factors for reducing the surface roughness. Experimental observations show that the machined surface morphology of the alumina ceramic coating is composed of brittle fracturing, ductile cutting and plowing, cracks, original pores, and unmelted particles.

Multiphysics Simulation and Experimental Investigation on Jet Electrochemical Milling of Ti-6Al-4V Alloy

Jet electrochemical milling (jet-EC milling) is a promising technique to machine hard-to-cut metallic materials with high machining efficiency and flexibility. The process of the jet-EC milling of Ti-6Al-4V Alloy is difficult to predict due to the interaction of multiple physical fields and the formation of passivation film. In this work, a novel model is established to simulate the jet-EC milling and predict machining profile. In this model, the interactional relationships among electric field, two-phase flow field, and geometry deformation are considered using a multiphysics approach, and the breakdown process of the passivation film is involved for accurately predicting the machining results. In addition, the passivation film breakdown process of Ti-6Al-4V alloy is studied experimentally. Finally, several experiments on the jet-EC milling of Ti-6Al-4V alloy are conducted to verify the simulation results and discuss the influence of the travel rate on the material dissolution. The current density distribution on the anode surface is clarified. The proposed model is more in line with the experiments. By applying an appropriate travel rate, a sharp edge is obtained without stray corrosion as the electrolyte forms an upward reflection and the un-machined surface is free from stray corrosion due to the absence of the electrolyte.

Machining of Carbon Steel under Aqueous Environment: Investigations into Some Performance Measures

In this study, a new machining approach (aqueous machining) is applied for mill machining and its performance is compared with traditional wet machining. AISI 1020 steel is employed as the test material and Taguchi statistical methodology is implemented to analyze and compare the performance of the two machining approaches. The cutting speed, feed rate, and depth of cut were the machining parameters used for both types of machining, while the selected response variables were surface roughness and hardness. Temperature variations were also recorded in aqueous machining. Compared with wet machining, aqueous machining resulted in lower surface roughness (up to 13%) for the same operating conditions and about 14% to 16% enhancement in hardness due to the formation of finer pearlite, as revealed by the microstructure analysis. Compared to the parent unmachined surface, the hardness of machined surfaces was 24% to 31% higher in wet machining and 44% to 51% higher in aqueous machining. Another benefit of aqueous machining was the energy gain, which ranged from 718 to 8615.96 J. This amount of heat energy can be used as waste heat for preheating domestic hot water, running the organic Rankine cycle with waste heat and preheating the inlet saline water for desalination, vacuum desalination, etc. If successfully implemented in the future, this idea will provide a step towards achieving sustainable machining by saving lubricants and toxic wastes in addition to saving energy for secondary applications.

Development of a phased array ultrasound roller probe for inspection of wire + arc additive manufactured components

The wire + arc additive manufacturing (WAAM) process, which combines an electric arc as a heat source and metal wire as feedstock is proving to be very effective when producing medium to large scale metal components. The non-destructive testing (NDT) of WAAM parts, while they are being produced, provides early intervention opportunities to rectify manufacturing nonconformities and to perform in-process quality assurance of the parts. This attracts a significant amount of material, time and cost savings. Therefore, this paper presents the research, development and validation of a high temperature phased array ultrasound testing (PAUT) roller probe to perform robotically delivered in-process NDT of WAAM components. The experimental results confirm that the PAUT roller probe can endure surface temperatures up to 350 °C, can be autonomously deployed via a robotic arm and can detect 1 mm diameter flat-bottom holes located 6 mm, 9 mm and 12 mm deep under the unmachined surface of a Ti-6Al-4V WAAM calibration block.

Areal Surface Roughness Optimization of Maraging Steel Parts Produced by Hybrid Additive Manufacturing.

We report on an experimental study and statistical optimization of the surface roughness using design of experiments and the Taguchi method for parts made of 1.2709 maraging steel. We employ a hybrid additive manufacturing approach that combines additive manufacturing by selective laser melting with subtractive manufacturing using milling in an automated process within a single machine. Input parameters such as laser power, scan speed, and hatching distance have been varied in order to improve surface quality of unmachined surfaces. Cutting speed, feed per tooth, and radial depth of cut have been varied to optimize surface roughness of the milled surfaces. The surfaces of the samples were characterized using 3D profilometry. Scan speed was determined as the most important parameter for non-machined surfaces; radial depth of cut was found to be the most significant parameter for milled surfaces. Areal surface roughness could be reduced by up to 40% for unmachined samples and by 23% for milled samples as compared to the prior state of the art.

Surface quality evaluation of single crystal 4H-SiC wafer machined by hybrid laser-waterjet: Comparing with laser machining

Single crystal silicon carbide (SiC), as a 3rd generation semiconductor material, has a wide application range and development prospect, especially in some key fields such as defense and aerospace industries. It's a typical difficult-to-machine and hard-brittle material due to its high hardness and brittleness. A hybrid laser-waterjet micromachining technology was employed to implement near damage-free and high-efficient micromachining of single crystal SiC. The workpiece material is heated by laser to a temperature near but below its melting point. The material is under an extremely “softened” but still solid status. A waterjet is applied off-axially to expel the heating-softened material and cool the material to eliminate thermal damage. It combines the advantages of laser processing with those of waterjet cutting. Moreover, it's a green machining approach, which is pollution-free and recyclable. In the present study, the surface quality of single crystal 4H-SiC machined by the hybrid laser-waterjet is evaluated and compared with that machined by laser. Among the wafers respectively machined by laser, under-water laser, waterjet-assisted laser and the hybrid laser-waterjet, the one machined by the hybrid laser-waterjet is with the best surface quality. Its cut boundaries between the machined area and the unmachined area are clear, the cut edges are straight, also the cut sides are clean. On the contrary, there is severe thermal damage along the cut edges and cut sides of laser machining. The microgrooves obtained from the hybrid laser-waterjet micromachining are with V-shaped cross-sections and the transition between the cut edge and the unmachined surface is flat and smooth. The cross-section profile of the microgroove machined by laser is M-shaped and there are obvious humps along the two sides of the microgroove. The microgroove bottom is almost at the same height with the unmachined surface. Besides, there are HAZs with a width of 50–100 µm and scaly recast layers on the sides of the cut machined by laser. In contrast, there is no recast layer and HAZ on the cut side of the hybrid laser-waterjet micromachining. The texture of the cut sidewall is clear and with the typical features of the material removed under the plastic mode. The EDS results show that the oxidation behavior happens during both the hybrid laser-waterjet micromachining and laser machining. However, the content of silicon dioxide generated in the hybrid laser-waterjet micromachining process (the oxygen content is about 5%) is obviously lower than that generated in laser machining (the oxygen content is more than 30%).

Influence of pre-machining on post-machining deformation of thin-walled elements made of aluminium alloy EN AW-2024

The paper presents analysis of influence of pre-machining, consisting of removing the textured surface layer of the semi-finished product formed after plastic working, on deformation of thin-walled elements made of EN AW-2024 alloy after milling. The textured layer was removed from surface opposite to the machined surface. During tests, the influence on thin-walled elements deformation of the following machining strategies was analysed: High Speed Cutting (HSC), High Performance Cutting (HPC), and hybrid strategies HPC + HSC, HPC + CM, and HPC + CM (CM – Conventional Machining). On the basis of received results it was determined that residual stresses stored in subsurface, rolled, and unmachined surface layer have significant influence on value and character of deformations of thin-walled elements. Removing the textured surface layer formed after rolling changes character and value of the deformations of thin-walled elements. The largest difference between samples with initially removed textured layer and samples without such machining was determined for HPC + CM milling strategy. For this strategy, deformations without pre-machining were much larger than in the case of samples with removed textured surface layer.

Influence of Cutting Parameters and Tool Wear on the Surface Integrity of Cobalt-Based Stellite 6 Alloy When Machined Under a Dry Cutting Environment

The efficiency of a machining process can be measured by evaluating the quality of the machined surface and the tool wear rate. The research reported herein is mainly focused on the effect of cutting parameters and tool wear on the machined surface defects, surface roughness, deformation layer and residual stresses when dry milling Stellite 6, deposited by overlay on a carbon steel surface. The results showed that under the selected cutting conditions, abrasion, diffusion, peeling, chipping and breakage were the main tool wear mechanisms presented. Also the feed rate was the primary factor affecting the tool wear with an influence of 83%. With regard to the influence of cutting parameters on the surface roughness, the primary factors were feed rate and cutting speed with 57 and 38%, respectively. In addition, in general, as tool wear increased, the surface roughness increased and the deformation layer was found to be influenced more by the cutting parameters rather than the tool wear. Compressive residual stresses were observed in the un-machined surface, and when machining longer than 5 min, residual stress changed 100% from compression to tension. Finally, results showed that micro-crack initiation was the main mechanism for chip formation.

Fabrication and functional characterization of engineered features on pyrolytic carbon

Engineered features on pyrolytic carbon (PyC) have been reported to improve the functional performance of the bio-implants. This paper is focused on the functional characterization of micro-features created on the surface of PyC. Two different types of micro-features (wide channels and arrayed holes) have been created by micro-electrical discharge machining (micro-EDM). Two other micro-features (fine channels and micro-pillars) have been created by micromilling process. Coliform bacterial strain was isolated from a sample of water and grown on all four textured. Cell growth was carried out on an unmachined surface to see the behavior of the isolated bacterial strain on the textured/non-textured surfaces. The samples were examined under SEM before and after wash to see cell growth and cell adhesion capability of the textures. The wide channels by micro-EDM show the maximum cell growth but poor cell adhesion. 184% higher cell growth has been observed on the wide channels in comparison with unmachined surface. The fine channels by micro-milling show comparatively lower growth but the cell adhesion on this surface was found excellent. 71% cells remain unwashed after washing of the surface having fine channel textures. It means that the channels structure shows the maximum cell growth and adhesion independent of machining process.

Investigation of Nanosized-Scandia-Doped Dispenser Cathodes With Machined Surfaces

Since the cathodes with machined surfaces are required for the practical applications in some high-power vacuum electron devices, the characteristics of nanosized-scandia-doped dispenser (SDD) cathodes with machined surfaces have been investigated, especially in dc applications. In order to reopen the surface pores, which may be smeared by machining procedure, the cathode surface was washed by Ethylene Diamine Tetraacetic Acid (EDTA) solution and prolonged time was taken for the cathode activation. The emission test results show that the SDD cathodes with machined surfaces exhibit better emission performances after treatment. The space-charge-limited dc emission current density of over 20 A/cm2 at 900 °Cb has been measured with temperature compensation for electron cooling after full activation. An on-going life test with a dc load of up to 40 A/cm2 at 970 °Cb has been completed $\sim 5000$ h of operation to date. Since the emission capability can be fully restored after exposing to air and reactivating for a short time, a preheating procedure is favorable for SDD cathode with machined surface in the practical applications. Surface analysis indicates that a recrystallization of tungsten grains occurs during the prolonged heating process, leading to further reopening of the surface pores. As a result, cathode surface is covered uniformly by active elements of Ba and Sc (as well as O) as on an unmachined surface, thereby maintaining the high emission property of the SDD cathode.

Experimental investigation on electrical discharge machining of RENE80 nickel super alloy

The objective of this paper is to explore the effect of these parameters such as peak current, pulse on time and pulse off time on metal removal rate, surface roughness, white layer thickness and surface crack density in EDM of RENE 80 nickel super alloy using Taguchi method. It is found that all responses are affected mainly by peak current and pulse on time. The experimental results reveal that metal removal rate, surface roughness and white layer thickness tend to increase with increase of peak current and pulse on time whereas surface crack density tend to decrease with increase of peak current and pulse on time. Also regression models are developed for all responses and found satisfactory. Finally, the detailed analysis of surface topography has shown that there is little difference in phases of unmachined surface and machined surfaces.