Hole Surface Integrity Research Articles

Influence of clean cooling medium on the hard drilling performance and machined surface integrity of 42CrMo steel

42CrMo ultra-high strength steel has high strength and fracture toughness, which is widely used in key components, such as gears, connecting rods and molds. This type of workpiece material, especially after quenching and hardening treatment, still has prominent issues with hole machining. In this work, the clean cutting media, including dry, cold air and liquid nitrogen (LN2), are selected to investigate the hard drilling performance and machined surface integrity of 42CrMo steel. The feasibility of extending tool life and improving hole machining quality is explored through clean cooling medium. The cutting force and cutting temperature are lower under cold air and LN2 conditions than that under dry cutting conditions. The tool life is extended under cold air and LN2 conditions. The low temperature brought by LN2 promotes the transformation from folded to lamellar for chip free surface. The degree of chip curling becomes larger and easier to break and discharge, which reduces the unfavorable effects of chip clogging. The thermal damage phenomenon on the hole wall surface is disappeared. The surface roughness is reduced by 37.3%, and the maximum microhardness is increased by 6.6%, which significantly improves the machined surface integrity of holes. This work demonstrates the feasibility of improving the machinability of harnessed ultra-high strength steel by LN2 cooling.

Numerical modelling of the BTA deep hole drilling process

The BTA deep hole drilling process is applied in order to produce bores with a large length to diameter ration at comparably high diameters of the bores. During drilling, first, the bore is cut by the cutting edge and shortly after that, a guide pad slides over the newly produced bore wall, burnishing the surface. These two effects condition the bore wall and alter its surface integrity depending on the process parameters. In the past, extensive destructive and non-destructive tests were conducted to obtain information on the surface integrity of the machined parts. Furthermore, a large number of in- and off-process measurements were carried out in material and cost-intensive experiments to determine values such as residual stresses or white etching layers. The presented work shows different finite element models of the BTA deep hole drilling process using continuous remeshing and, in order to reduce the calculation times, the Coupled Eulerian-Lagrangian (CEL) method as well. The results simulated based on this models comprise temperatures, process forces and surface properties that are compared to measured values from pervious works for validation. The various approaches are evaluated to determine whether they predict the bore hole surface integrity and thus support future investigations.

Effect of drilling parameters on the hole surface integrity of low alloy steel for nuclear power during BTA deep hole drilling

Boring trepanning association (BTA) deep hole drilling is widely used in machining tube sheet of steam generator. In order to get a better service integrity, the surface quality after machining is required to be higher. In this paper, the effect mechanism of BTA deep hole drilling on the integrity and quality of the machined surface layer of low alloy steel SA508Gr.3Cl.2 for nuclear power is investigated. The results show that the gradient microstructure can be obtained by BTA drilling on the surface of the inner hole, including the recrystallized layer with grain refinement and the plastic deformation layer with high-density sub-crystal structure and grain distortion. With the increase of drilling speed and feed rate, the thickness of deformation layer increases. During the BTA deep hole drilling, the proportion of low-angle grain boundaries (LAGBs) increases with the increasing depth from the machined surface. The increase of drilling speed leads to the increase of recrystallization degree and the proportion of LAGBs in the machined surface. The effect of feed rate on the proportion of LAGBs is opposite. The machined surface is characterized by regular peak and valley, and there are typical surface defects mainly involving feed marks, surface tearing, and plowing grooves. With the increase of drilling speed, the surface roughness will decrease. The effect of feed rate on surface roughness is obviously lower than that of drilling speed. With the increase of drilling speed, and feed rate, the depth of hardened layer increases gradually, which is caused by dislocation strengthening and fine grain–strengthening effect during BTA drilling process. Higher drilling speed is recommended in forming a better machined surface with a strengthening layer of a certain thickness.

Sensor-based strategies for accurate prediction of drilled hole surface integrity of CFRP/Al7075 hybrid stack

The airplane structure is often made up using carbon fiber reinforced plastic (CFRP) stacking with a metallic sheet to improve excessive load carrying and shock-absorbing capability with reduced fuel economy. It is a challenging task to drill dissimilar CFRP-Al metallic stack because the hot continuous metallic chip mutilates the hole surface in addition to drilling-induced delamination. The present work addresses the parametric effect on delamination, roughness, and circularity error of the drilled hole along with chip morphology using different drill geometry on the fabricated CFRP [0°/-45°/90°/45°]2s stack over an aluminum 7075 sheet. A drill tool dynamometer was used to capture axial thrust with associated torque variations during drilling which were further used in process monitoring. The most critical unlike interface drilling has been scrutinized using instantaneous force-torque slopes along with scanning electron micrographs. The machining time loss during CFRP→Al interface drilling has also been processed. Finally, a comparative assessment has been made on the predictability of hole quality features among various sensor-based strategies using response surface methodology. The drilled-hole surface quality was significantly improved at high spindle speed (2250 rpm), and low feed rate (0.025 mm/rev) using a low drill point angle (110°). The thrust force was found to be a prime indicator of drilled hole integrity than torque whereas peak torque and its slope in the initial stages of the drilling cycle strongly correlated with surface delamination. The top surface delamination was somewhat severe (≥ 1.6) at high drill thrust (≥ 40 N) with high torque (≥ 0.45 Nm).

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.

Evolution of hole surface integrity in successive processing using sequential milling and laser shock peening

Part's surfaces are usually produced by successive/integration machining process and the performance of the part is directly tied to the generated surface integrity. Holes are commonly used in many critical engineering structures to connect parts, whose surface primarily characterizes a high risk of fretting or contact fatigue failure. In this paper, the evolution of metallurgical and mechanical surface integrity of Ti-6Al-4V alloy hole surface which machined using multi-step milling and successive laser shock peening (LSP) treatment was investigated. Firstly, a special workpiece with various hole surface pieces was designed and four different sequential milling processes were set to conduct the milling experiments. A comprehensive analysis of the evolution of surface roughness, microstructure, microhardness, and residual stress along the sequential processing steps was carried out. The results showed that the forms of surface integrity evolution could be classified into mono-evolution and coupling evolution, which were correlated to the combined effect of the milling parameters, cutter-workpiece engagement nature, milling sequence, milling mode, and the coolant/lubricant applied strategy. Mono-evolution was predominated in material removal sequences (milling sequences), while the coupling evolution was dependent on the radial depth of cut (DoC, ~50 μm). By comparison, coupling evolution was prominent in non-material removal related sequences (LSP treatment). Due to the conformal contact nature of tool-workpiece engagement and the flood cooling strategy, the microhardness in the subsurface layer was not significantly altered in milling sequences, except that the up-milling normally produced a surface with lower hardness and the down-milling produced almost unchanged surface hardness compared with bulk material. The residual stress was a result of the coupling effect of the residual stress field introduced in the previous and current machining sequences, whose in-depth distribution could be represented as an exponential decay function. The degree of influence of the machining sequences and their interactions on the final residual stresses could be evaluated by utilizing a set of weighting coefficients. This research indicated that understanding the effect of successive processing on the surface integrity evolution could help in planning of appropriate processes to achieve desirable surface integrity.

Open hole surface integrity and its impact on fatigue performance of Al 2024-T3/Ti-6Al-4V stacks

This work studies the open hole surface integrity of Al 2024-T3/Ti-6Al-4V stacks, which have been subjected to different hole making methods, namely, conventional drilling and helical milling. The resulting hole-exit burr, surface roughness and machined surface/sub-surface microstructures have been characterized and the impact of the hole surface integrity on the fatigue performance of the machined coupons has been revealed. Results show that helical milling can effectively reduce burr formation at hole-exit and cause minimal sub-surface microstructural deformation, especially in the Ti layer. Helical milling produced holes with greater surface roughness, interestingly, the fatigue life of the helical milling coupons doubled that of conventional drilling.

Prediction of drilled hole quality in bidirectional woven carbon fiber reinforced plastic using wavelet packets of force–torque signals

Drilling of fibrous composites like carbon fiber reinforced plastics (CFRPs) is always considered a grim job to perform owing to its abrasiveness and inhomogeneous properties. Compared to the available studies, the present work addresses the parametric investigation on drilling capability of TiAlN- and TiN-coated drills over the uncoated one. The delamination factor, surface roughness, and circularity error in CFRP were considered as the quality features. The acquired thrust and torque were analyzed in the time domain as well as the time–frequency wavelet field, followed by a sensitivity analysis between decomposed original force–torque signals in different phases. Finally, an attempt was made to improve the predictability of drilled hole quality using decomposed wavelets of force and torque extended to hybrid force–torque wavelets of the best features to the developed regression models. The feed rate was more predominant than spindle speed on delamination evidenced by higher axial thrust particularly using coated tools. However, the coated drills generated more uniform torque than the uncoated drills and improved the drilled hole quality as well. The torque wavelet was a better indicator of hole surface integrity than force wavelets in which intermediate frequency band torque and low-/high-frequency band force wavelets were found to be more significant.

Investigations on the impact of the hole surface integrity on the fatigue life of a 2024‐T351 aluminum alloy drilled part

AbstractThe influence of the hole surface integrity on the fatigue life of 2024‐T351 aluminum drilled parts was investigated. Fatigue tests were conducted on open‐hole specimens, and a large campaign was carried out to characterize the hole surface integrity (roughness measurements, hardness measurements, metallographic observations, and residual stress analysis). An innovative technique, the Hole Opening Comparative Technique, was set up in order to analyze the residual stress state of the parts. In this study, strain hardening of the hole subsurface seems to be the main factor influencing the fatigue behavior, associated with a residual stress state of the part.

Surface integrity of holes machined by electrochemical discharge drilling method

Micro-hole structures are widely used in aircraft engines, but they are difficult to fabricate because of stringent machining quality requirements. Electrochemical discharge drilling (ECDD) is a hybrid machining method for fabricating small holes in nickel-based superalloys that uses low-conductivity salt solution as the working fluid. Electrical discharge and electrochemical dissolution occur simultaneously in the ECDD process. ECDD is expected to achieve micro-holes with high machining efficiency and without any recast layer, but little research has been carried out on the surface integrity. In this study, the surface morphology, element composition, residual stress, microhardness, and recast layer properties of the ECDD machined holes were analysed. The results show that the surface integrity of the holes improves by the ECDD method, and there is no recast layer, no melted debris, and no tensile residual stress. Moreover, with an increase of the working fluid conductivity from 0.005 to 3.6 mS/cm, the residual stress of the ECDD machined hole becomes compressive, the microhardness of the wall no longer decreases significantly, and the surface roughness of the wall decreases to Ra = 1.69 μm. Furthermore, the average diameter and the taper angle of ECDD machined hole increase with the increase of machining time. In addition, the entrance of the ECDD machined hole has a round corner without a sharp edge, and its radius-of-curvature is approximately 150 μm.

Monitoring of hole surface integrity in drilling of bi-directional woven carbon fiber reinforced plastic composites

Fiber-reinforced plastic is one of the top priorities lightweight materials with excellent mechanical properties for the aerospace industries in recent years. However, it is difficult to machine despite having unique properties due to its non-homogeneous and abrasive nature in alternate fiber and matrix layers. Thus, it is found to be a challenging task to drill hole on such hard-to-machine materials, which is highly essential for the development of most of the engineering structural components. The present work addresses various drilling-induced defects such as delamination, circularity error, and roughness variations in the hole surface during drilling of quasi-isotropic cross-fiber oriented bi-directional woven-type carbon fiber reinforced plastic laminate using a full factorial design of experiments for different drill geometry. The response surface methodology was considered for the regression model development, which was found to be highly significant. The machining forces with associated torque have also been acquired during drilling, which was divided and further analyzed in time domain to correlate with drilling flaws. The drilling-induced delamination was found to be higher at a high feed rate using a higher drill point angle due to substantial thrust force generation at the initial stages in the drilling cycle. However, the internal surface finish with associated circularity error was reduced for higher spindle speed with less feed rate using a low drill point angle because of low torque fluctuation at the final drilling phases. The axial thrust force was found to be a prime indicator of drilled hole surface delamination, whereas drilling torque precisely indicated internal surface roughness as well as circularity error. The global root mean square, along with a local peak of thrust and torque, both were highly essential to completely characterize the drilled hole quality.

A novel investigation study on float glass hole surface integrity & tool wear using Chemical assisted Rotary ultrasonic machining

To alleviate the alarming surface integrity defect (chipping) and tool wear issues, new machining technique has been introduced. First time, an attempt has been made by hybrid the ‘Rotary ultrasonic machining’ and ‘Chemical etching process’ that pioneer as ‘Chemical assisted Rotary ultrasonic machining’. In this work, authors have investigated a novel study to explore the float glass’s drilled hole surface integrity and tool wear. The mechanism behind the hole surface integrity and tool wear is also discussed. The results pointed that the Chemical assisted Rotary ultrasonic machining (CRUM) gives slightest chipping size (0.32 mm) using abrasive coated hollow tool of 6 mm diameter over the 5% HF acid dipped float glass specimen. However, coz of highly chemical reacting behaviour of HF acid with the tool material, it is deteriorated. Thus more research needed to be carried for the tool selection. Finally, authors recommended that the Chemical assisted Rotary ultrasonic machining is having a vibrant future.

Machining-induced surface integrity of holes drilled in lead-free brass alloy

Abstract This study presents the results of an experimental investigation on the effect of drilling process on surface integrity characteristics of lead-free brass alloy. Experimental data on dimensional accuracy and the surface quality of the holes, subsurface characteristics including microhardness and microstructure is presented and analyzed to evaluate surface integrity of machined specimens. This study demonstrates that cutting tools has strong influence on both microstructural aspects of lead-free alloy and resulting to hardening of the surface and subsurface along with deformation twinning.

Feasibility study on ultrasonic-assisted drilling of CFRP/Ti stacks by single-shot under dry condition

Carbon fiber reinforced plastic (CFRP) and titanium alloy (Ti) in a stacked structure have been widely applied in aerospace industry. Due to disparate mechanical and physical properties of CFRP and Ti, it is difficult to achieve high-quality holes and low tool wear, by drilling CFRP/Ti stacks by single-shot, even though this process exhibits high productivity in the industrial practice. In this study, for the first time, ultrasonic vibration was applied in drilling CFRP/Ti stacks by single-shot under dry condition, and its feasibility was evaluated. First, the separated cutting mode of ultrasonic-assisted drilling (UAD) was analyzed based on kinematics. Second, an experimental platform was set up, where several output parameters, during UAD of CFRP/Ti stacks, were monitored, including thrust force and torque, the interface temperature, hole accuracy, hole surface integrity, Ti chip morphologies, and tool wear conditions. Experimental results indicated that, compared to conventional drilling (CD), average thrust force and torque in UAD of CFRP decreased by 41.2 to 46.8% and 36.2 to 48.9%, respectively, and thrust force and torque in UAD of Ti decreased by 15.2 to 26.1% and 21.4 to 29.0%, respectively. Lower maximum CFRP/Ti interface temperature (353.4 °C vs. 468.1 °C) was achieved in UAD. Besides, both hole diameter accuracy and hole surface quality were significantly improved in UAD. Also, both narrow sectional ribbon Ti chips and separated Ti chips were observed in UAD, while serrated continuous ribbon Ti chips were obtained in CD. In addition, tool wear conditions in UAD were also significantly alleviated.

Investigation on chip morphology and properties in drilling aluminum and titanium stack with double cone drill

The next generation aircraft industry looks upon the multi-layered material structures. In this research work, conventional drilling technique is incorporated in assembly process for the stack material. However, in drilling process, chips play a significant role for chip tool life but the chip breaking mechanism is still unsolved, the key challenge of making high-quality hole and hole surface integrity for hole making of multi-layered material. To illustrate this issue, a series of drilling experiments were conducted on Al/Al stacks and Al/Ti Stacks with double cone drill. The objective was to examine the effect of process parameter, and influences of the double cone drill bit geometry on output response such as chip formation and chip breaking mechanism were investigated. Then, the effect of gap between Al/Al stacks and Al/Ti stacks materials respective chip formation as they flow away was probed. Surface roughness was measured by non-contact-type 3D laser scan microscope system. It was observed from the experiment that surface roughness value decreases from 4.06 to 2.59 μm with increasing cutting feed. The scanning electron microscope (SEM) analysis was carried out to study the chip morphology. Results demonstrated that upper-layer surface integrity can be improved by selecting suitable tool angles and process parameters.

Analysis of local cutting edge geometry on temperature distribution and surface integrity when dry drilling of aeronautical alloys

A suitable choice of tool geometry and tool material and the optimization of cutting conditions is the way towards dry machining, especially for difficult to machine materials. The current paper proposes an experimental work about dry drilling of AA 7075 aluminium and more particularly Ti-6Al-4V titanium alloys. Effects of complex drill geometry on thrust force and cutting temperature variations are explored. This study includes drill normal rake angle definition using a mathematical formulation based on tool CAD definition. Temperature distribution along the drill cutting edge is estimated with a specific device. Cutting forces are measured using a dynamometer table. Hole surface integrity is studied by observation of microstructures, on surface and sub-surface. Micro-hardness tests complete the measurements. Results show that temperature is much higher when drilling titanium Ti-6Al-4V than when drilling aluminium AA7075, with variations from 450 to 540 °C along the main cutting edge. Temperature variation along the cutting edge depends on cutting speed, work material and local geometry. Microstructural observations reveal that surface and subsurface are affected with grains elongation in cutting direction. Temperature and microstructural deformation increase with drilling depth.

Enhancement of electrochemical discharge machining accuracy and surface integrity using side-insulated tool electrode with diamond coating

Electrochemical discharge machining (ECDM) is an emerging non-traditional processing technology used to machine electrical non-conductive material like glass and ceramics, by using the electrochemical discharge phenomena around the tool electrode. In the general ECDM drilling process, the gas film forms and electrical discharge appears at both the tool end and the tool sidewall that were exposed to the electrolyte. The undesirable sidewall discharge enlarges the hole entrance diameter and destroys the hole surface integrity. In order to prevent the sidewall discharge, a side-insulated tool electrode with a 4 μm-thick diamond coating layer was used in ECDM micro-hole drilling. The gas film formation and electrical discharge mostly happened on the tool end due to the insulation layer on the tool sidewall. Experiments showed that, compared with the traditional tool electrode, the side-insulated tool electrode achieves a smaller hole diameter and better surface integrity without an obvious heat affected zone at the hole entrance. Furthermore, the hole diameter nearly remains the same as the machining depth increases from 50 μm to 500 μm. The side-insulated electrode has an advantage in enhancing shape accuracy by reducing the taper angle of the micro hole. When the machining depth is 600 μm, the side-insulated electrode achieves a much smaller hole taper angle () than the traditional tool electrode does ().

Surface Integrity of Holes Machined by Orbital Drilling of Composites with Single Layer Diamond Tools

Orbital drilling of composites was investigated to observe hole surface integrity in terms of surface roughness, delamination, surface burning, and geometric accuracy. Experiments were conducted with a brazed single layer diamond tool of ball-end geometry to drill 220 holes. Cutting forces and temperatures were measured. Micro-observations were made of the hole surface. The results showed that the forces increased sharply during drilling of the first 44 holes followed by a gradual rise. Part temperatures varied between 98-184°C. Surface roughness Ra varied from 13-17μm. Almost 99% of the holes were drilled without exit delamination; however, 2.3% of the holes showed entrance delamination.

Tool life and hole surface integrity studies for hole-making of Ti6Al4V alloy

With a significant growth in the use of titanium alloys in the aviation manufacturing industry, the key challenge of making high-quality holes in the aircraft assembly process needs to be addressed. In this work, case studies deploying traditional drilling and helical milling technologies are carried out to investigate the tool life and hole surface integrity for hole-making of titanium alloy. Results show that the helical milling process leads to much longer tool life, generally lower hole surface roughness, and higher hole subsurface microhardness. In addition, no plastically deformed layer or white layer has been observed in holes produced by helical milling. In contrast, a slightly softened region was always present on the drilled surface. The residual stress distributions within the hole surface, including compressive and tensile residual stress, have also been investigated in detail.

Effect of the Cutting Edge Preparation on the Surface Integrity after Dry Drilling

Nowadays, cutting edge preparation is a widely applied tool improvement method, especially when concerns tool's life. However, the effects of the tool treatments on the generated hole surface are still an ongoing investigation. The machined hole is subject to severe contact with the tool, in particular the drill corner and guide, providing an increase of friction and thus, heat generation. Consequently, the surface integrity suffers several alterations. Therefore, this study aims to understand the effect of the cutting edge preparation on the surface integrity of holes. Experiments performed with carbide tools, coated with TiAlN and three configurations of tools (sharpened, polished with abrasive brushes and treated with drag finishing surface treatment) during dry drilling, showed that the tool condition has an important effect on the surface roughness, temperature and subsurface plastic deformation.